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Received 19 February 2024, accepted 7 March 2024, date of publication 12 March 2024, date of current version 20 March 2024.
Digital Object Identifier 10.1109/ACCESS.2024.3376407
Development and Testing of Novel Soft
Sleeve Actuators
MOHAMMED ABBOODI , (Member, IEEE), AND MARC DOUMIT
Department of Mechanical Engineering, University of Ottawa, Ottawa, ON K1N 6N5, Canada
Corresponding author: Mohammed Abboodi (mabbo103@uottawa.ca)
ABSTRACT The field of soft actuators and robotics has garnered considerable attention in recent years,
driven by their distinct properties to adapt to diverse environments and enable secure and engaging
interactions with humans. While current literature highlights a significant body of work on various soft
actuators, it is noteworthy that the concept of soft sleeve actuation remains unexplored, as it has not yet
been proposed. The concept of soft sleeve actuation represents a significant leap forward in the field
of robotics, heralding tremendous potential for diverse applications, particularly for wearable robotics.
This paper introduces a novel soft sleeve actuation mechanism, encompassing the development of two
actuators capable of generating linear and bending motion. These actuators are lightweight and capable of
generating considerable force and motion. Using Fused Deposition Modeling technology, a comprehensive
fabrication framework was adopted to overcome manufacturing variability and fabricate high-quality airtight
actuators. The mechanical performance of the proposed soft sleeve actuators (SLA) was investigated through
a custom-built experimental testing setup. The impact of geometric parameters and material stiffness on the
behavior of the developed actuators is studied and discussed.
INDEX TERMS Soft bending actuator, soft sleeve actuator, soft linear actuator, soft exoskeleton, soft robotic.
I. INTRODUCTION
The burgeoning field of soft robotics marks a paradigm shift
in the design and application of robotic systems, particularly
in the field of wearable technology [1]. Central to this inno-
vation are soft actuators, which promise to revolutionize how
machines interact with and assist the human body [2]. Unlike
their rigid counterparts, soft actuators offer a harmonious
blend of flexibility, adaptability, and compatibility with the
anatomical framework of the user, making them ideal for a
diverse array of applications, from medical rehabilitation to
enhancing the mobility of daily activities [3].Among these
applications, soft wearable exoskeletons emerge as a par-
ticularly promising area. These exoskeletons, crafted from
flexible materials and seamlessly integrated with the body
through soft interfaces [4], epitomize the advancement in soft
robotics. They are designed to minimize movement obstruc-
tion and enhance safe user interaction, aligning closely with
the natural biomechanics of the human body [3]. However,
The associate editor coordinating the review of this manuscript and
approving it for publication was Yangmin Li .
despite these advancements, the development of actuation
mechanisms that are soft, lightweight, wearable, powerful,
and energy-efficient presents a substantial challenge [5]. This
ongoing quest underlines a critical focus in soft robotics:
to achieve the optimal balance of these attributes in actua-
tors, a task that remains at the forefront of current research
efforts [6].
Among the existing potential smart actuators, Shape Mem-
ory Alloy (SMA) actuators can produce significant forces
but encounter limitations due to their slow actuation speed
and limited recoverable deformation [7]. Other promising
actuators are Twisted and Coiled Polymer (TCP) actuators.
Like SMA, a salient issue is their relatively slow relaxation
time, which most often demands a cooling system to restore
their initial state [8]. Moreover, given that wearable devices
interact closely with the human body, the excessive heat
generation and high operating temperature for SMA and TCP
actuators can lead to discomfort or potential harm to user’s
skin. Therefore, ensuring user safety and thermal comfort
emerges as a critical challenge in the realm of wearable
technology.
VOLUME 12, 2024
2024 The Authors. This work is licensed under a Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 License.
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M. Abboodi, M. Doumit: Development and Testing of Novel Soft Sleeve Actuators
Electroactive polymer (EAP) actuators, renowned for
inducing large deformations under an electric field, come
with a unique set of challenges. Achieving such deformations
typically demands high voltages or hermetic encapsulation,
problematic in wearable devices [9]. Particularly for wear-
ables intended for direct skin contact, the high operating
voltages of EAPs pose substantial safety risks. Also, despite
the quick response times, the low power-to-volume ratio of
EAP actuators limits their usefulness.
Cable-driven soft actuators operate on the principle of
transducing a motor’s mechanical force, conveyed through a
cable, into motion within a pliable and soft structure. How-
ever, the integration of conventional heavy rigid elements
like motors, cables, and bearings compromises the intrinsic
flexibility of these actuators, thus expanding their physical
profile [10]. This enlargement could affect wearer comfort,
a critical factor in wearable technologies. Additionally, the
semi-rigid cable conduit from the actuation unit may obstruct
user motion. Inherent issues include efficient force trans-
fer [11], friction, backlash [10], actuation precision, and
potential skin-fabric slippage [12]. Their ability to provide
high forces or swift movements is further limited, potentially
restricting their utility in certain wearable applications [12].
Soft pneumatic actuators are acknowledged as a predom-
inant technology in the discipline of soft robotics, owed
in part to many advantageous properties. These include
their lightweight and rapid response times, factors that
significantly expedite their implementation in various appli-
cations [5], [13], [14]. Moreover, their capacity for substantial
deformation, inherent safety features, and exceptional power-
to-weight ratio underscore their efficacy [14]. Coupled with
their low production costs, these characteristics highlight
the growing prevalence of soft pneumatic actuators in this
field [5], [12], [15]. While the intrinsic advantages of soft
pneumatic actuators are undeniable, their utilization within
wearable devices is accompanied by notable challenges and
constraints. This is observed in McKibben muscles [12],
which are characterized by an outstanding force-to-weight
ratio. However, to generate a desired force output, the muscles
necessitate a large diameter [12]. Additionally, to ensure
functional efficacy, the anchor of the actuator’s point load
must be securely established on the user [16].
Another illustration pertains to fiber-reinforced actuators,
renowned for their versatility in generating diverse motions,
thereby finding utility in a range of upper limb wear-
able devices [17], [18]. The intricate structure and distinct
material characteristics of fiber-reinforced actuators impose
constraints on attaining the requisite precision and accuracy
for certain wearable applications [12]. Despite their merito-
rious attributes, PneuNet actuators bear inherent limitations.
Primarily, they exhibit reduced force generation and consid-
erable energy dissipation due to undesired radial expansion,
commonly referred to as the ballooning effect [15]. Further-
more, the generation of complex motion patterns poses a
significant challenge. The final category in the spectrum of
pneumatic actuators is represented by Fabric-Based Inflatable
and Textile actuators. Despite their lightweight construction,
superior force generation, and simplified production proce-
dures, these actuators face various challenges. These include
mechanical fragility, layer delamination hindering fluidic
chamber creation, and slippage between fabric layers and the
human skin [12].
The bellows soft actuator, a preeminent member of the
soft pneumatic actuator family, has its origins in the early
20th century, initially functioning as a pneumatic jack. These
devices are distinguished by their corrugated walls, avail-
able in a multitude of geometric designs [19], which are
adept at converting pneumatic pressure into kinetic motion.
A variety of studies, such as those denoted in [20] and [21],
have incorporated origami-inspired geometries to refine the
design of pneumatic bellows. Despite these advancements,
there has been a challenge [22], [23] in fully integrating these
actuators with the human body; they have been relegated to
serving as external power sources for wearable devices, thus
limiting their portability. The integration of bellows within
textiles, as explored in [4], typically results in a consider-
able dissipation of actuation energy through the stretching of
interfacing layers, thereby compromising their stability and
activity. In essence, the efficacy of bellows and soft pneu-
matic actuators, in general, is contingent upon the textiles
to which they are secured. The optimal strategy to reduce
this dependency is through the redesign of the actuator into
a sleeve-like configuration, which could be applied directly
over the body, enabling the application of forces that align
with the natural movements of the musculoskeletal system.
In wearable soft robotics, the deformation of interface lay-
ers, such as sleeves or braces, under actuator-generated force
can impact both device performance and user comfort. This
deformation diverts energy, reducing the net force transmitted
to the user and potentially affecting the fit and comfort of the
device [4]. To attain optimal performance and user comfort in
wearable devices, it is vital to devise innovative approaches.
One such approach could be the use of ‘soft sleeve actuators’
to diminish the dependence on interface layers or transmis-
sion mechanisms. This method would substantially enhance
the device’s effectiveness and improve wearer comfort.
While great advancement has occurred in the field of
smart and soft actuators in the last few decades, no sleeve
and potentially wearable actuator has been proposed. This
research introduces an advancement in the field of soft actu-
ation by presenting the first soft sleeve actuator (SLA) that
can be potentially adapted for wearable applications such
as an exoskeleton. Unlike traditional approaches that rely
on braces, belts, or textile layers, the proposed actuator is
a self-contained wearable device that can be applied to the
user’s limb, ensuring stability and comfort.
In addition to their role in wearable technologies, the pro-
posed soft sleeve actuators (SLAs) exhibit potential in the
field of precision engineering. Although piezoelectric actua-
tors (PEAs) are prevalent in micromanipulation systems [24],
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M. Abboodi, M. Doumit: Development and Testing of Novel Soft Sleeve Actuators
FIGURE 1. Linear concept evolution (a) initial concept (b) ring support concept (c) letteral support concept (d) Linear SLA 3d representation
(e) geometrical parameters of linear SLA (Fh) - Fold height, (Fw) Fold width, (β) Fold Angle, and (T) wall thickness (f) tie restraining layer (g) sectional
view of Linear SLA.
a variety of soft actuation mechanisms are increasingly being
explored for their unique advantages. Notable among these
are shape memory alloys [25], dielectric elastomers [26], and
Electro active polymer [27], each contributing distinct prop-
erties to the field. Diverging from the traditional rigid-link
mechanisms typically employed in precision engineering,
SLAs introduce a paradigm shift by harnessing the intrinsic
flexibility of soft materials. This design allows for motion
generation through material deformation, thereby negating
the need for lubrication and substantially mitigating issues
related to friction.
II. METHOD
A. CONCEPTUALIZATION
1) LINEAR SLA DESIGN
The linear SLA was originally designed based on a triangular
bellows mechanism, as shown in Figure 1 (a). This concept
entailed two V-shaped walls, namely an internal and external
wall. The actuator is pressurized through the top air chan-
nel, yielding to an expansion of the structure. This bellows
mechanism, although effective for conventional soft actua-
tors [19], [23], [28], proved untenable for soft sleeve actuation
mechanisms. Upon inflation of the SLA, undesirable motions
occurred, namely, the contraction of the internal wall and the
expansion of the external wall, resulting in an alteration of
the actuator’s shape from circular to elliptical as shown in
figure 2(d) and the failure of producing any meaningful linear
motion. This challenge has yet to be addressed in the existing
literature, given the absence of any proposed sleeve actuation
mechanism. Consequently, three distinct methodologies were
devised to surmount this challenge.
One approach to prevent the contraction and expansion of
the internal and external walls, respectively, involved the use
of a rigid frame. Thus, V-shaped rings were developed and
attached to the valleys of the interior and exterior walls as
shown in Figure 1(b). While this solution achieved a desired
linear motion and effectively averted internal wall contrac-
tion, the inherent rigidity introduced by the rigid frame
compromised the actuator’s flexibility and consequently its
ability to achieve considerable linear motion. An extension
rate of less than 10% was achieved.
To improve these performances, the external rigid frame
solution was replaced with internal soft lateral tie-restraining
layers, connecting the interior and exterior walls along the
length of the sleeve except for the first and last layer allow-
ing airflow throughout the sleeve, as shown in Figure 1(c).
This design approach significantly enhanced the flexibility
of the original design while mitigating radial expansion and
contraction. This actuator design demonstrated linear motion,
capable of extending by more than 15%.
To enhance further the linear displacement of the actu-
ator, a folded mechanism was introduced, as shown in
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M. Abboodi, M. Doumit: Development and Testing of Novel Soft Sleeve Actuators
FIGURE 2. (a) contraction motion (b) extension motion (c) bending motion (d) simple triangular bellows without tie-restraining layers (e) linear
actuator without tie restraining layers (f) bending actuator without tie- restraining layers frontal view.(g) bending actuator without tie-restraining
layers side view.
Figure 1(d-g). This approach involved the development of
soft corrugated walls, integrated into the structural frame-
work. Unlike previous designs which expand their framework
when pressurized (e.g. bellow), this design unfolds its struc-
ture when pressurized (e.g. origami). The primary function
of its walls is to convert pneumatic pressure into mechanical
power, enabling significant forces and motions. longitudinal
tie-restraining layers were implemented, which served as a
connection between the interior and exterior walls. The air
channel network comprises multiple longitudinal channels
that span the entire length of the actuator, establishing a con-
nection at the bottom end through the bottom lateral channel
and at the top end via the upper lateral channel. The adop-
tion of a triangular origami configuration was necessitated
by manufacturing constraints, as alternative shapes proved
arduous to fabricate without support materials.
The final linear sleeve actuator design significantly aug-
mented the actuator’s performances, achieving an extension
rate of 80% and generating a contraction force of 210 N. This
was achieved using a sleeve prototype fabricated from TPU
(Thermoplastic Polyurethane) with a hardness of 85 Shore
A. Notably, the actuator enables a minimum step motion
of 100 micrometers. The sleeve maintains a total thickness
of 8mm, an inner diameter of 30 mm, a length of 80mm,
weighing 150 grams, and operates under a gauge pressure of
200 kPa. Furthermore, using a vacuum pressure of 95 Kpa,
the actuator achieved a 30% contraction rate and an expansion
force of 51 N. Figure 2(a) presents the linear SLA in its orig-
inal and compressed state, establishing a visual reference for
its deformation under negative pressure. Figure 2(b), in con-
trast, displays the actuator in both its original and extended
state, highlighting the extent of its elongation capability.
2) BENDING SLA DESIGN
The design of the Bending SLA posed a challenge due to
its intricate motion along a cylindrical configuration. Out of
the numerous conceptual prototypes devised, only two have
ultimately emerged successful. The bending second concept
stands as one of the successful models,
incorporating a blend of traditional bellows with lateral tie
restraining layers, as illustrated in Figure 3(b). Although this
design demonstrated great torque generation capabilities, it is
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FIGURE 3. Bending concept evolution (a) initial bending concept (b) Bending second concept (c) Bending SLA 3d representation (d) geometrical
parameters of bending SLA (e) tie-restraining layer (f) section view of bending SLA.
hindered by a restricted bending angle, measuring less than
40◦. Furthermore, the actuator exhibited notable rigidity.
Alternatively, the subsequent design employs a triangular
folding mechanism paired with a longitudinal tie restraint
layer, both of which were introduced and developed in the
preceding section. The design utilizes custom constrained
layers to induce the desired bending motion. The design
of the bending actuator is depicted in Figure 3 (c-f). This
bi-directional bending actuator surpasses the coveted 70◦
bending angle on both sides, amounting to a total of 140◦,
while exhibiting a force generation capacity of 38 N as shown
in Figure 9(h). Additionally, the actuator features precise
control capabilities, with a minimum step angle resolution
of 0.01 degree, ensuring fine-tuned operational movements.
This was achieved using a sleeve.
prototype fabricated from TPU (Thermoplastic Polyurethane)
with a hardness of 85 Shore A. The sleeve maintains a total
thickness of 8mm, inner diameter of 30 mm, length of 80mm,
weighing 150 g, and at gauge pressure of 200 kPa.
The operational principles underlying the soft pneumatic
bellows bending actuators in both the second concept and the
final concept rely on the principle of differential inflation.
Both designs employ a dual-chamber system, sealed and
delineated by constrained layers, as depicted in figure 3(b)
and 3(c). Upon the application of air pressure, the actuator
exhibits a bending motion induced by the unequal expansion
of its chambers. The constrained layers act as a regulatory
mechanism that limits the expansion on one side, hence
yielding the actuator to bend towards the side with restricted
inflation. Bending SLA boasts an enhanced design over bend-
ing concept two by offering increased flexibility and a much
smaller internal cavity. This design efficiency contributes to
a more responsive and energy-efficient actuator. In contrast,
the operational principle of the first concept, as shown in
figure 3(a), diverges significantly. It lacks constrained layers,
and the bending motion is generated from the differential
expansion between the corrugated walls and the flat walls.
This design results in an actuator that is less flexible, as its
rigidity is derived from the internal flat wall. Figure 2(c)
provides a visual representation of the Bending SLA actuator,
displaying it in its initial, undeformed state alongside its
bending state when subjected to pneumatic pressure.
B. MANUFACTURING
The initial fabrication attempt of the SLAs incorporated con-
ventional manufacturing methodologies, utilizing molding
techniques. However, these methods encountered substantial
challenges when creating intricately thin-walled structures
and hermetically sealed chambers. To overcome this chal-
lenge, three-dimensional printing technologies were adopted
to enable the production of the SLA. Among the various 3D
printing techniques attempted, Fused Deposition Modeling
(FDM) stands out as the preference. This approach involves
the precise deposition of thermoplastic filament, layer by
layer, through an extruder. For this, the Ultimaker S3 Bowden
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M. Abboodi, M. Doumit: Development and Testing of Novel Soft Sleeve Actuators
tube printer was selected. A comprehensive framework was
developed to address the challenges unique to Bowden tube
printers and ensure their compatibility to produce soft pneu-
matic actuators.
One critical parameter encountered affecting the printing
process of flexible materials is humidity. To ensure successful
and high-quality prints, the framework focuses on imple-
menting two measures. Firstly, a dehumidifier is used to
reduce humidity levels below 25% in the workspace, then
a filament dryer with silica gel packets is employed. The
filament went through a 5-hour drying process at 50◦C before
used in printing.
In addition to humidity control, two key modifications
were implemented to address challenges related to fila-
ment movement. Firstly, upgrading the Bowden tube to a
smaller inner diameter made of PTFE helps prevent filament
compression and buckling. Secondly, specific adjustments
are identified to prevent filament stretching and tangling
around the extruder gears. Disabling the retraction setting
ensures print integrity and high-quality results by minimiz-
ing filament entanglement and under-extrusion. Additionally,
reducing the extruder tension to the lowest possible set-
ting helps minimize filament stretching and tangling. The
impact of support material on actuator airtightness was
also investigated. Using soluble materials as support proved
unsatisfactory due to stringing and irregular blob forma-
tion. In contrast, utilizing TPU as support material improved
actuator quality; however, it led to air leakage. Therefore,
avoiding support material is crucial for pneumatic actua-
tors with desired airtight properties. Overhangs in flexible
materials require precise engineering with essential support
structures. The soft sleeve mechanisms are designed with
these necessary support structures, addressing the challenge
of overhangs in flexible materials. This involves ensur-
ing secure layer-by-layer printing while acknowledging that
these measures alone are insufficient. To effectively mitigate
issues such as drooping and curling, the optimization of
printing speed and temperature is imperative. Furthermore,
maintaining print core cleanliness is crucial.
Finally, optimizing printing settings plays a vital role
in achieving airtightness and high-pressure resilience in
pneumatic actuators. Key parameters, including printing
speed, layer height, printing temperature, cooling speed, and
flow rate, were fine-tuned. An optimal printing speed of
15mm/s was determined to balance print quality, airtight-
ness, and manufacturing time. A layer height of 0.1mm is
recommended to achieve optimal hermeticity and mechan-
ical properties. The printing temperature of 240◦C ensures
proper bonding between layers, while moderate cooling
speed prevents sagging and promotes layer adhesion. The
flow parameter varies based on the material type, with values
of 125% and 135% for TPU 95A and TPU 85A, respectively.
C. EXPERIMENTAL SETUP
To evaluate the performance of the proposed sleeve actuators,
multiple experimental setups as shown in Figure 4 have been
devised to evaluate the forces, displacements, and bending
angles generated by the actuators. The primary experimental
setup depicted in Figure 4 (a) consists of a custom aluminum
structure with the following components, (1) graphical user
interface (2) data acquisition system (3) power supply (4)
electric circuit (5) vacuum sensor (6) air buffer tank (7) force
measurement unit for bending actuator (8) 4k camera (9)
force measurement unit for linear actuator (10) Fixture (11)
Solenoid Valves and (12) pressure transducers.
To assess the isometric forces generated by the linear sleeve
actuator, the experimental setup illustrated in Figure 4(b) was
employed. This setup involved securing the top soft actuator
proximal cap to a rigid fixture, while enabling unrestricted
movement at the opposite end of the actuator. To enable
the measurement of the actuator’s force, a force measure-
ment unit (FMU) was developed, as shown in Figure 4(b).
The FMU transmits the load from the actuator’s terminal
point to the extremity of a load cell (FC2231, TE Con-
nectivity). A pressure transducer (61CP, Texas Instruments)
was employed to monitor the internal pressure within the
actuator. The acquisition and processing of sensor data were
accomplished through the implementation of the National
Instrument’s (NI) data acquisition system. Concurrently, the
monitoring and control of the hardware components were
executed by means of a graphical user interface (GUI).
For the concentric contraction test, the FMU was modified
to enable quantification of the force generated during contrac-
tion motion. Simultaneously, the vacuum sensor (ADP5111,
Panasonic) was employed to monitor the internal negative
pressure of the actuator.
To determine extension and contraction rates, a modified
experimental setup was employed, mirroring the previous
force measurement methodology with an exception. The
FMU was replaced by a Linear Variable Differential Trans-
former (LVDT) for precise length measurements. The LVDT
was connected to the actuator’s free end through a fabricated
3D coupling unit.
To study the behavior of the bending actuator, an experi-
mental setup, as illustrated in Figure 4(c), was devised. While
sharing most hardware components with the linear exper-
imental setup, this configuration features a bespoke FMU
capable of accommodating the bending motion. Notably, the
force at the actuator’s tip is captured by connecting it to the
free end of the FMU via a flexible steel cable. A force sensor
(FC22-10) is employed to measure the force at the tip of the
actuator.
To measure the bending angle, a testing protocol was
employed based on the methodologies presented in [29], [30],
and [31]. This experimental investigation involved a setup
where the soft actuator’s proximal end, featuring an air inlet,
was securely clamped within an actuator fixture. Meanwhile,
the distal end of the actuator remained unrestricted, allowing
for bending motion. To capture and analyze the actuator’s
motion accurately, a high-speed 4k camera (GoPro Hero
11) was utilized. The camera was aligned with a checkered
background to enhance measurement precision. To process
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M. Abboodi, M. Doumit: Development and Testing of Novel Soft Sleeve Actuators
FIGURE 4. Schematic overview of the experimental setup components and configurations: (a) Primary components of the experimental setup (1)
graphical user interface (2) data acquisition system (3) power supply (4) electric circuit (5) vacuum sensor (6) air buffer tank (7) force measurement unit
for bending actuator (8) 4k camera (9) force measurement unit for linear actuator (10) Fixture (11) Solenoid valves (12) pressure transducers. (b) linear
experimental setup (c) experimental setup for bending motion. (d) Stiffness experimental setup.
the recorded video footage and calculate the bending angle,
an open-source software Kinovea was used similarly to other
studies [29], [31]
To measure the stiffness of the actuator, an experimental
setup as depicted in Figure 4(d) was developed. This setup
involved the integration of the sleeve linear actuator with
an Instron machine through a bespoke actuator design. The
objective of this setup was to study the actuator’s stiffness as
a function of its displacement and internal pressure. To this
end, a compressor and a buffer tank were utilized, providing
a stabilized air pressure source for the actuator. Concurrently,
a pressure sensor was deployed to accurately measure the
internal pressure of the actuator. Within this context, experi-
ments were conducted using five distinct constant pressures:
25, 50, 75, 100, and 125 KPa. To maintain these pressures
constant within the actuator, industrial pressure regulator
(QBX pressure control valve, Proportion-Air, Inc.) were used.
III. RESULTS
A. LINEAR SLA
For the proposed linear SLA, its ability to generate force and
linear motion was tested and evaluated under diverse geomet-
rical parameters and material stiffness. To analyze the effect
of each parameter, a series of models were constructed, tested
and evaluated. Table 1 details these models, specifying the
geometrical parameters (Figure 1e) as diameter (r), actuator
length (l), fold height (fh), fold width (fw), fold angle (β),
number of restraining layers (nr), thickness of restraining
layers (rt), wall thickness (wt), and shore hardness (sh).
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FIGURE 5. Impact of geometrical parameters and material Stiffness on generated force of linear SLA in extension motion (a) effect of fold angle
(b) effect of material stiffness (c) effect of the tie restraining layers thickness (d) effect of wall thickness (e) effect of fold width (f) effect of the number
of tie-restraining layers.
TABLE 1. Linear actuator models (all dimension in mm).
1) FORCE GENERATION RESULTS
The influence of geometric parameters and material stiffness
on the ability of the linear SAL to generate force was exam-
ined during extension upon application of positive pressure
and during contraction under negative pressure. For exten-
sion, experimental setup as shown in Figure 4(b) was used and
the actuator was pressurized up to 200 kPa. The contraction
phase was evaluated using the same experimental setups and
by applying a vacuum pressure of up to 95 kPa. During these
tests, each parameter was altered individually, allowing the
understanding of their effects on actuator force performance.
The subsequent subsections provide a detailed analysis of
these parameter changes.
a: EFFECT OF FOLD ANGLE
Fold angle (β), a critical geometrical parameter, is portrayed
as the internal angle at the juncture of two converging inclined
sides of a fold. As shown in Figure 5(a), an increase in the fold
angle from 30◦ to 40◦ inversely impacts the resultant force,
inducing a decrease from 130 N to 101 N. Similarly, the actu-
ator’s response during contraction movements follows this
pattern. Demonstrated in Figure 6(a), augmenting the fold
angle from 30◦ to 40◦ leads to a decreased force generation,
from 24 N to 14.5 N.
b: EFFECT OF MATERIAL STIFFNESS
The materials used in the fabrication of the actuators
included two distinct grades of thermoplastic polyurethane:
the flexible TPU 85A (Ninjaflex) versus the compara-
tively less flexible TPU 95A (Ultimaker). An increase
in stiffness from 85A to 95A resulted in a decreased
peak force during extension, from 130 N to 89.9 N,
as shown in Figure 5(b). Conversely, during contraction
testing, a stiffness increase from 85A to 95A led to an
increase in force, from 24 N to 33 N, as demonstrated
in Figure 6(b).
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FIGURE 6. Impact of geometrical parameters and material stiffness on generated force of linear SLA in contraction motion (a) effect of fold angle
(b) effect of material stiffness (c) effect of wall thickness (d) effect of the tie restraining layers thickness (e) effect of fold width. (f) effect of the number
of tie-restraining layers.
c: EFFECT OF TIE-RESTRAINING LAYERS THICKNESS
The thickness of tie-restraining layers (tr) is pivotal in
ensuring these layers possess the capability to withstand
high pressure, thereby preserving structural integrity. During
the extension movement, as shown in Figure 5(c), increas-
ing the layer thickness from 0.4 mm to 0.8 mm leads
to a decrease in force generation, from 110 N to 95 N.
In contrast, the experiment detailed in Figure 6(d) demon-
strates that changing the thickness of these layers in the
same range has a negligible impact on force output during
contraction.
d: EFFECT OF THE NUMBER OF TIE RESTRAINING LAYERS
The number of tie-restraining layers (nr) exerts a signif-
icant impact on the durability of these layers and effec-
tively reduces the ballooning effect within the actuator
structure. Figure 5(f) clearly shows that during the exten-
sion phase, increasing the number of restraining layers
from 14 to 18 leads to a substantial reduction in force,
from 191 N to 86 N. A parallel observation is made in
the contraction motion. Figure 6(f) indicates an increase
in the number of restraining layers inversely affects the
force exerted. This is demonstrated by a decrease in force
from 35 N to 20 N as the layer count goes from 14 to
18 layers.
e: EFFECT OF WALL THICKNESS
The wall thickness (wt) refers to the thickness of the walls
constituting the folds of the linear SLA, a dimension that
substantially influences the actuator’s flexibility. As demon-
strated in Figure 5(d), increasing the wall thickness from
1.2 mm to 1.6 mm results in an increased force output, from
110.5 N to 130.2 N during extension movements. Corre-
spondingly, during contraction movements, a thicker wall
corresponds to an amplified force, evidenced by the data
presented in Figure 6(c), where a marginal increment in wall
thickness of 0.4 mm considerably increases the force from
27.5 N to 36.5 N.
f: EFFECT FOLD WIDTH
Fold width (fw) is the horizontal measurement between the
peaks and valleys of the corrugations of the linear SLA when
it is not inflated. A notable finding is that decreasing the
fold width from 12 mm to 4 mm significantly enhances the
force generated during extension. This is clearly illustrated
in Figure 5(e), where such a reduction in fold width results
in a force increase from 130 N to 214.5 N. In the context of
contraction motion, a similar trend is observed. A decrease
in fold width markedly boosts the force output, as shown in
Figure 6(e). Here, reducing the fold width from 12 mm to
VOLUME 12, 2024 40003
M. Abboodi, M. Doumit: Development and Testing of Novel Soft Sleeve Actuators
FIGURE 7. Impact of geometrical parameters and material stiffness on displacement of Linear SLA in extension motion (a) effect of fold angle (b) effect
of material stiffness (c) effect of the tie restraining layers thickness (d) effect of wall thickness (e) effect of fold width.
8mm leads to a doubling of the force generated, from 24 N to
51.5 N.
2) LINEAR DISPLACEMENT RESULTS
This section discusses the testing and results of the lin-
ear displacement characteristics of the linear SLA. The
evaluation encompasses both extension and contraction
motions, employing a similar experimental setup as shown
in Figure 4(b). For extension, the linear SLA underwent
pressurization to 200 kPa, whereas for contraction, a vacuum
pressure of 95 kPa was applied. In the following sections,
a detailed analysis is presented, delving into how variations
in geometric parameters and the material stiffness of the
actuator affect the SLA displacement capabilities.
a: EFFECT OF FOLD ANGLE
As demonstrated in Figure 7(a), an increment in the fold angle
from 30◦ to 40◦ inversely affects the extension length.
Specifically, this increase in fold angle is associated with
a decrease in extension displacement, from 52.7 mm to
39.3 mm. Similarly, the impact of fold angle on contraction
displacement exhibits a comparable pattern. As evidenced by
Figure 8(a), where an increase in the fold angle from 30◦ to
40◦ results in a decrease in contraction displacement from
11 mm to 6 mm.
b: EFFECT OF MATERIAL STIFFNESS
In Figure 7(b), experimental outcomes for two prototypes
with distinct material stiffness values, 85A and 95A, are
presented. These results show that an elevation in material
stiffness results in a substantial decrease in displacement
during extension, with measurements showing a reduction
from 52.7 mm to 13.4 mm. Correspondingly, during the
contraction phase, as illustrated in Figure 8(b), an increase
in material stiffness from 85A to 95A is observed to diminish
the displacement from 11 mm to 4.3 mm.
c: EFFECT OF RESTRAINING LAYERS THICKNESS
The influence of an increase in the thickness of the actuator’s
layers on its displacement during both extension and con-
traction was analyzed. Specifically, an increment in thickness
from 0.4 mm to 0.8 mm was observed to notably affect the
actuator’s performance. In the extension phase, as illustrated
in Figure 7(c), this increase in thickness led to a decrease in
the extension displacement from 30 mm to 18 mm. Similarly,
in the contraction phase, increasing the layer thickness by
the same margin resulted in a reduction in contraction, from
7 mm to 5 mm as show in Figure 8(c).
d: EFFECT OF WALL THICKNESS
An increase in wall thickness from 1.2 mm to 1.6 mm
has been found to significantly influence the displace-
ment of the actuator. As depicted in Figure 7(d), this
increase in wall thickness leads to a reduction in the exten-
sion displacement, from 30 mm to 20 mm. Similarly, the
contraction displacement is affected by this change in thick-
ness, showing a decrease from 7 mm to 6 mm as shown
in Figure 8(d).
40004 VOLUME 12, 2024
M. Abboodi, M. Doumit: Development and Testing of Novel Soft Sleeve Actuators
FIGURE 8. Impact of geometrical parameters and material stiffness on displacement of linear SLA in contraction motion (a) effect of fold angle (b) effect
of material stiffness (c) effect of the tie-restraining layers thickness (d) effect of wall thickness (e) effect of fold width.
e: EFFECT OF FOLD WIDTH
The influence of fold width is observed to vary distinctly
between these two operational modes. In terms of extension,
increasing the fold width from 8 mm to 12 mm results in a
decrease in the extension rate. This is quantitatively demon-
strated in Figure 7(e), where the extension distance dimin-
ishes from 53 mm to 43 mm with the increased fold width.
This trend underscores the inverse relationship between fold
width and extension displacement. Conversely, the contrac-
tion motion exhibits a contrasting behavior. As shown in
Figure 8(e), a decrease in fold width leads to an enhance-
ment in the contraction rate. A notable instance of this
effect is observed when a reduction in fold width results
in an increase in the contraction distance, from 11 mm
to 19 mm.
B. BENDING SLA
This section is devoted to the experimental outcomes for
bending SLA, with a particular emphasis on assessing
the influence of geometric parameters, as demonstrated in
Figure 3(d-f), and material stiffness on the generated forces
and bending angles. In the testing phase, each bending actua-
tor underwent pressurization to 200 kPa, following which the
bending angles and resultant forces at the tip of actuator were
measured. The investigative framework employs a diverse set
of models, each detailed in Table 2.
These models, distinct in their geometric and material com-
positions, collectively encompass a broad spectrum of design
TABLE 2. Bending actuator models (all dimension in mm).
possibilities. The testing of these models was conducted in
a controlled experimental setting, as depicted in Figure 4(c).
The subsequent section will provide an in-depth exploration
of the experimental results.
1) EFFECT OF FOLD ANGLE
Figure 9(a) shows that increasing the fold angle from 30◦
to 45◦ leads to a reduction in force from 12.3 N to 10.8 N,
signifying an inverse correlation between fold angle and force
generation. In terms of bending angle, Figure 10(a) indicates
that the same increase in fold angle results in a decrease in
bending angle from 22◦ to 9.2◦. These observations collec-
tively emphasize a consistent inverse relationship between an
increase in fold angle and the reduction in bending perfor-
mance.
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M. Abboodi, M. Doumit: Development and Testing of Novel Soft Sleeve Actuators
FIGURE 9. Impact of geometrical parameters and material stiffness on generated force of bending SLA. (a) effect of fold angle (b) effect of material
stiffness (c) effect of wall thickness (d) effect of the number of tie-restraining layers (e) effect of the tie restraining layers thickness (f) effect of
constrained layers (g)effect of fold width. (h) generated forces.
2) EFFECT OF MATERIAL STIFFNESS
The experimental results shown in Figure 9(b) demonstrate
that increasing stiffness from 85A Ha to 95A marginally
elevates the force, from 12.3 N to 14 N. This finding indi-
cates a subtle yet positive influence of stiffness on force
output. In contrast, bending angle results, as presented in
Figure 10(c), reveal a pronounced decrease from 22◦ to 12◦
with higher material stiffness. This underscores a significant
inverse relationship between material stiffness and bending
flexibility.
3) EFFECT OF WALL THICKNESS
Figure 9(c) reveals that an increase in wall thickness from
1.2mm to 1.6mm results in enhanced force generation, with
force values rising from 12.3 N to 16 N. This increase
suggests that greater wall thickness contributes positively
to the force output. Conversely, Figure 10(d) indicates an
inverse effect on bending performance. The same increase in
wall thickness, from 1.2mm to 1.6mm, leads to a decreased
bending angle, moving from 22◦ to 17.3◦. These findings
highlight that while thicker walls augment force generation,
they concurrently reduce bending flexibility.
4) EFFECT OF THE NUMBER OF TIE-RESTRAINING LAYERS
Testing reveals that decreasing the tie-restraining layers
from 18 to 10 leads to an increase in force from 12.3 N to
23.5 N, as shown in Figure 9(d), indicating that fewer layers
enhance force output. Concurrently, Figure 10(f) displays that
this reduction in layers also results in an increased bending
40006 VOLUME 12, 2024
M. Abboodi, M. Doumit: Development and Testing of Novel Soft Sleeve Actuators
FIGURE 10. Impact of geometrical parameters and material stiffness on bending angle of bending SLA (a) effect of fold angle (b) effect of constrained
layers (c) effect of material stiffness (d) effect of wall thickness (e) effect of the tie restraining layers thickness (f) effect of the number of tie-restraining
layers (g) improve the bending angle by reducing wall thickness (h) effect of fold width.
angle from 22◦ to 44◦, suggesting an augmented bending
capability with fewer restraining layers.
5) EFFECT OF THE THICKNESS OF TIE-RESTRAINING LAYERS
Increasing layer thickness by 0.4mm reduces force from
12.3 N to 9.3 N, as Figure 9(e) shows, indicating a negative
correlation with force generation. Concurrently, Figure 10(e)
demonstrates a decrease in bending angle from 22◦ to 12◦ due
to the same thickness increment, underscoring its detrimental
impact on bending performance.
6) EFFECT OF CONSTRAINED LAYERS
Figure 9(f) delineates that an augmentation in constrain lay-
ers’ thickness from 1.6 mm to 3.2 mm has a negligible
impact on the forces generated by the actuator. Conversely,
Figure 10(b) reveals that this same increase in thickness
results in a slight decrease in bending angle, from 22◦ to 18◦,
thereby indicating a more pronounced effect on the actuator’s
bending capacity.
7) EFFECT OF FOLD WIDTH
The data in Figure 9(g) indicates that expanding the fold
width from 8mm to 12mm leads to an increase in force
by 3N. This finding suggests that wider folds contribute to
enhanced force generation. Regarding bending performance,
Figure 10(h) demonstrates that the same alteration in fold
width results in an increase in bending angle from 56◦ to 61◦.
This outcome illustrates that increased fold width not only
VOLUME 12, 2024 40007
M. Abboodi, M. Doumit: Development and Testing of Novel Soft Sleeve Actuators
FIGURE 11. (a) Expermental and fitted force-displacement data
(b) Variation of actuator stiffness with displacement under different
pressure conditions.
enhances force output but also positively affects the actuator’s
bending range.
C. ACTUATOR STIFFNESS
This experiment aims to quantify the stiffness of the actuator.
Model L5 was selected for testing under five distinct pressure
conditions: 0 kPa, 25 kPa, 50 kPa, 75 kPa, and 100 kPa,
utilizing the experimental setup depicted in Figure 4(d) and
detailed in Section II. In this setup, one end of the actuator
was affixed to a fixture, and the other end was connected
to a coupling unit, which in turn was attached to the Instron
head. Subsequently, the Instron head achieved a displacement
of 40 mm at a speed of 50 mm/min, with data collection
facilitated through LabVIEW.
The results, presented in Figure 11(a), depict the actuator’s
response across five pressure settings. Increasing the internal
pressure of the actuator leads to a rise in the force required
to induce displacement. The SLA demonstrates a nonlinear
force-displacement relationship where a polynomial model
was used to approximate the force-displacement curve.
Further analysis on SLA stiffness, delineated as the force-
to-displacement ratio, was conducted over eight discrete
measurement intervals from 0 to 40 mm, in increments of
5 mm. This analysis highlighted the stiffness’s dependence
on internal pressure variations. Notably, at 0 kPa, stiffness
measured in the 0 to 5 mm displacement range stood at
1716.42 N/m, increasing significantly to 4087.55 N/m at
125 kPa for the same displacement range. This trend demon-
strates a direct correlation between pressure incrementation
and stiffness enhancement within the actuator. Figure 11(b)
graphically underscores the pressure-stiffness relationship,
showcasing the critical influence of internal pressure adjust-
ments on the actuator’s stiffness.
IV. DISCUSSION
The experimental results from the preceding section highlight
the performance of the Linear and Bending SLA as a function
of the material properties and the geometrical parameters of
the actuator. This greatly contributes to an in-depth under-
standing of the mechanical operation of the proposed SLA
and how each identified parameter contributes and impacts
its mechanical performances. More specifically, a significant
correlation was observed between material stiffness and the
flexibility of the actuator, impacting the displacement in both
linear and bending SLA. It was noted that increasing material
stiffness beyond 85 Shore A impairs the flexibility of the
actuator. Moreover, the Fold Angle is identified as a critical
parameter for SLA performance. A reduction in this angle is
associated with a significant enhancement in actuator ability
to achieve displacement and force generation for both linear
and bending SLA. The smallest Fold Angle utilized in this
research, constrained by manufacturing limitations, was 30◦.
Wall thickness is another influential factor. It was found that
a minimum thickness of three lines is essential to ensure the
airtightness of the sleeve. While an increase in wall thickness
can reduce the ballooning effect of the actuator, thus slightly
enhancing force production, it concurrently decreases the
actuator’s flexibility and diminishes both displacement in
linear SLA and bending angle in bending SLA. Additionally,
for the Fold Width parameter, decreasing its value results
in an increase in the forces generated in both bending and
linear SLA. However, this reduction also leads to a decrease
in extension displacement and bending angle, and a reduction
in contraction displacement. The thickness of constrained
layers, exclusive to bending SLA and instrumental in the
transformation from linear to bending motion, was found to
have a minor impact on generated forces and bending angle.
The importance of Tie-restraining layers in maintaining the
structural integrity of the actuator is underscored. Absent
these layers, the actuator would assume a balloon-like form,
as illustrated in figure 2(d-g). It was discovered that increas-
ing either the number or thickness of these layers leads to a
reduction in generated forces, linear displacement, and bend-
ing angle. The experimental results indicate that a restraining
layer thickness of 0.4 mm is suitable for pressures up to
200 kPa for both linear and bending SLA; for applications
exceeding this pressure, the thickness should be increased
accordingly. The optimal number of restraining layers for
40008 VOLUME 12, 2024
M. Abboodi, M. Doumit: Development and Testing of Novel Soft Sleeve Actuators
better performance was found to be 10 for bending SLA and
14 for linear SLA.
The development of SLA presents a promising actuation
technology. These actuators can be further configured in
series, combining linear and bending motions, or aligning
multiple bending actuators, to achieve a versatile range of
motions. The sleeve structure of these actuators lends itself to
a myriad of practical applications. In the domain of wearable
technology, sleeve actuators offer significant benefits. The
SLA are self-contained and can be conform directly to the
wearer’s limb. This direct application promotes both stability
and comfort. These devices function akin to a sleeve, aug-
menting the user’s muscular actions, thereby offering support
and amplification of movement. Furthermore, the internal
pressure within these SLA can be modulated to alter their
stiffness. This adaptability renders them suitable for creating
variable-stiffness structures, such as robotic grips or smart
textiles that respond to changing conditions or tasks.
V. LIMITATIONS AND FURTHER DEVELOPMENT
The existing methodology for producing these actuators
presents certain limitations, particularly concerning the rigid-
ity of materials used, which cannot currently be reduced
below 85 Ha. Exploring methods to diminish this material
stiffness could augment the energy efficiency of the actuators
without undermining their operational capacity. Addition-
ally, the lowest achievable fold angle in the present setup is
restricted to 30◦ due to inherent manufacturing constraints.
A decrease in this angle would proportionally increase the
actuator’s displacement performance.
In addition, the future work should focus on incorporating
finite element analysis (FEA) and numerical simulations to
enhance the understanding and optimization of the SLA.
These simulations would provide insights into the actua-
tor’s behavior under various load conditions, enabling a
comprehensive understanding of its structural integrity and
operational limits. Additionally, numerical analyses can be
employed to optimize the design parameters, thereby improv-
ing the efficiency and applicability of these actuators.
This research paper primarily focused on the design, fabri-
cation, and mechanical evaluation of a novel SLA. For future
research, the focus will shift towards adapting the SLA to
mobility assistive devices. This entails adjusting the SLA
geometry to suit human anatomical specifications. Another
area for future development is the integration of sensory
feedback systems within the SLA design. This would allow
for real-time monitoring and adjustments of actuator behav-
ior, enhancing the user experience and safety in assistive
applications.
VI. CONCLUSION
In conclusion, soft sleeve actuators hold significant potential
for various applications in wearable robotics. However, the
development of these actuation mechanisms remains unex-
plored. To address this research gap, this paper presents a
novel bidirectional actuator consisting of linear and bending
capabilities. The linear SLA demonstrated notable perfor-
mance, generating over 200 N with 80% extension rate
at 200 Kpa of pneumatic pressure. It also achieved more
than 50 N with a 30% contraction rate at a vacuum pressure
of 95 kPa. On the other hand, the bending actuator exhibited
the ability to generate over 38 N and achieve bending angles
of over 140◦ in both directions (±70◦).
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MOHAMMED ABBOODI (Member, IEEE)
received the B.Sc. degree (Hons.) from the Uni-
versity of Baghdad, Iraq, in 2008, and the M.Sc.
degree from the University of Central Florida,
FL, USA, in 2014. He is currently pursuing the
Ph.D. degree in mechanical engineering with the
University of Ottawa, Canada. His research inter-
ests include soft robotics, wearable devices, and
bio-inspired soft robotics.
MARC DOUMIT received the B.S. degree in
mechanical engineering, the M.Eng. degree in
engineering management, and the Ph.D. degree
in mechanical engineering from the University of
Ottawa, Canada, in 2000, 2003, and 2009, respec-
tively. He is currently an Associate Professor in
mechanical engineering with the University of
Ottawa. His research interests include smart actu-
ators, soft actuators, exoskeletons, prosthetic and
orthotic limbs, and bio-inspired designs.
40010 VOLUME 12, 2024

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Novel 3D-Printed Soft Linear and Bending Actuators

  • 1. Received 19 February 2024, accepted 7 March 2024, date of publication 12 March 2024, date of current version 20 March 2024. Digital Object Identifier 10.1109/ACCESS.2024.3376407 Development and Testing of Novel Soft Sleeve Actuators MOHAMMED ABBOODI , (Member, IEEE), AND MARC DOUMIT Department of Mechanical Engineering, University of Ottawa, Ottawa, ON K1N 6N5, Canada Corresponding author: Mohammed Abboodi (mabbo103@uottawa.ca) ABSTRACT The field of soft actuators and robotics has garnered considerable attention in recent years, driven by their distinct properties to adapt to diverse environments and enable secure and engaging interactions with humans. While current literature highlights a significant body of work on various soft actuators, it is noteworthy that the concept of soft sleeve actuation remains unexplored, as it has not yet been proposed. The concept of soft sleeve actuation represents a significant leap forward in the field of robotics, heralding tremendous potential for diverse applications, particularly for wearable robotics. This paper introduces a novel soft sleeve actuation mechanism, encompassing the development of two actuators capable of generating linear and bending motion. These actuators are lightweight and capable of generating considerable force and motion. Using Fused Deposition Modeling technology, a comprehensive fabrication framework was adopted to overcome manufacturing variability and fabricate high-quality airtight actuators. The mechanical performance of the proposed soft sleeve actuators (SLA) was investigated through a custom-built experimental testing setup. The impact of geometric parameters and material stiffness on the behavior of the developed actuators is studied and discussed. INDEX TERMS Soft bending actuator, soft sleeve actuator, soft linear actuator, soft exoskeleton, soft robotic. I. INTRODUCTION The burgeoning field of soft robotics marks a paradigm shift in the design and application of robotic systems, particularly in the field of wearable technology [1]. Central to this inno- vation are soft actuators, which promise to revolutionize how machines interact with and assist the human body [2]. Unlike their rigid counterparts, soft actuators offer a harmonious blend of flexibility, adaptability, and compatibility with the anatomical framework of the user, making them ideal for a diverse array of applications, from medical rehabilitation to enhancing the mobility of daily activities [3].Among these applications, soft wearable exoskeletons emerge as a par- ticularly promising area. These exoskeletons, crafted from flexible materials and seamlessly integrated with the body through soft interfaces [4], epitomize the advancement in soft robotics. They are designed to minimize movement obstruc- tion and enhance safe user interaction, aligning closely with the natural biomechanics of the human body [3]. However, The associate editor coordinating the review of this manuscript and approving it for publication was Yangmin Li . despite these advancements, the development of actuation mechanisms that are soft, lightweight, wearable, powerful, and energy-efficient presents a substantial challenge [5]. This ongoing quest underlines a critical focus in soft robotics: to achieve the optimal balance of these attributes in actua- tors, a task that remains at the forefront of current research efforts [6]. Among the existing potential smart actuators, Shape Mem- ory Alloy (SMA) actuators can produce significant forces but encounter limitations due to their slow actuation speed and limited recoverable deformation [7]. Other promising actuators are Twisted and Coiled Polymer (TCP) actuators. Like SMA, a salient issue is their relatively slow relaxation time, which most often demands a cooling system to restore their initial state [8]. Moreover, given that wearable devices interact closely with the human body, the excessive heat generation and high operating temperature for SMA and TCP actuators can lead to discomfort or potential harm to user’s skin. Therefore, ensuring user safety and thermal comfort emerges as a critical challenge in the realm of wearable technology. VOLUME 12, 2024 2024 The Authors. This work is licensed under a Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 License. For more information, see https://creativecommons.org/licenses/by-nc-nd/4.0/ 39995
  • 2. M. Abboodi, M. Doumit: Development and Testing of Novel Soft Sleeve Actuators Electroactive polymer (EAP) actuators, renowned for inducing large deformations under an electric field, come with a unique set of challenges. Achieving such deformations typically demands high voltages or hermetic encapsulation, problematic in wearable devices [9]. Particularly for wear- ables intended for direct skin contact, the high operating voltages of EAPs pose substantial safety risks. Also, despite the quick response times, the low power-to-volume ratio of EAP actuators limits their usefulness. Cable-driven soft actuators operate on the principle of transducing a motor’s mechanical force, conveyed through a cable, into motion within a pliable and soft structure. How- ever, the integration of conventional heavy rigid elements like motors, cables, and bearings compromises the intrinsic flexibility of these actuators, thus expanding their physical profile [10]. This enlargement could affect wearer comfort, a critical factor in wearable technologies. Additionally, the semi-rigid cable conduit from the actuation unit may obstruct user motion. Inherent issues include efficient force trans- fer [11], friction, backlash [10], actuation precision, and potential skin-fabric slippage [12]. Their ability to provide high forces or swift movements is further limited, potentially restricting their utility in certain wearable applications [12]. Soft pneumatic actuators are acknowledged as a predom- inant technology in the discipline of soft robotics, owed in part to many advantageous properties. These include their lightweight and rapid response times, factors that significantly expedite their implementation in various appli- cations [5], [13], [14]. Moreover, their capacity for substantial deformation, inherent safety features, and exceptional power- to-weight ratio underscore their efficacy [14]. Coupled with their low production costs, these characteristics highlight the growing prevalence of soft pneumatic actuators in this field [5], [12], [15]. While the intrinsic advantages of soft pneumatic actuators are undeniable, their utilization within wearable devices is accompanied by notable challenges and constraints. This is observed in McKibben muscles [12], which are characterized by an outstanding force-to-weight ratio. However, to generate a desired force output, the muscles necessitate a large diameter [12]. Additionally, to ensure functional efficacy, the anchor of the actuator’s point load must be securely established on the user [16]. Another illustration pertains to fiber-reinforced actuators, renowned for their versatility in generating diverse motions, thereby finding utility in a range of upper limb wear- able devices [17], [18]. The intricate structure and distinct material characteristics of fiber-reinforced actuators impose constraints on attaining the requisite precision and accuracy for certain wearable applications [12]. Despite their merito- rious attributes, PneuNet actuators bear inherent limitations. Primarily, they exhibit reduced force generation and consid- erable energy dissipation due to undesired radial expansion, commonly referred to as the ballooning effect [15]. Further- more, the generation of complex motion patterns poses a significant challenge. The final category in the spectrum of pneumatic actuators is represented by Fabric-Based Inflatable and Textile actuators. Despite their lightweight construction, superior force generation, and simplified production proce- dures, these actuators face various challenges. These include mechanical fragility, layer delamination hindering fluidic chamber creation, and slippage between fabric layers and the human skin [12]. The bellows soft actuator, a preeminent member of the soft pneumatic actuator family, has its origins in the early 20th century, initially functioning as a pneumatic jack. These devices are distinguished by their corrugated walls, avail- able in a multitude of geometric designs [19], which are adept at converting pneumatic pressure into kinetic motion. A variety of studies, such as those denoted in [20] and [21], have incorporated origami-inspired geometries to refine the design of pneumatic bellows. Despite these advancements, there has been a challenge [22], [23] in fully integrating these actuators with the human body; they have been relegated to serving as external power sources for wearable devices, thus limiting their portability. The integration of bellows within textiles, as explored in [4], typically results in a consider- able dissipation of actuation energy through the stretching of interfacing layers, thereby compromising their stability and activity. In essence, the efficacy of bellows and soft pneu- matic actuators, in general, is contingent upon the textiles to which they are secured. The optimal strategy to reduce this dependency is through the redesign of the actuator into a sleeve-like configuration, which could be applied directly over the body, enabling the application of forces that align with the natural movements of the musculoskeletal system. In wearable soft robotics, the deformation of interface lay- ers, such as sleeves or braces, under actuator-generated force can impact both device performance and user comfort. This deformation diverts energy, reducing the net force transmitted to the user and potentially affecting the fit and comfort of the device [4]. To attain optimal performance and user comfort in wearable devices, it is vital to devise innovative approaches. One such approach could be the use of ‘soft sleeve actuators’ to diminish the dependence on interface layers or transmis- sion mechanisms. This method would substantially enhance the device’s effectiveness and improve wearer comfort. While great advancement has occurred in the field of smart and soft actuators in the last few decades, no sleeve and potentially wearable actuator has been proposed. This research introduces an advancement in the field of soft actu- ation by presenting the first soft sleeve actuator (SLA) that can be potentially adapted for wearable applications such as an exoskeleton. Unlike traditional approaches that rely on braces, belts, or textile layers, the proposed actuator is a self-contained wearable device that can be applied to the user’s limb, ensuring stability and comfort. In addition to their role in wearable technologies, the pro- posed soft sleeve actuators (SLAs) exhibit potential in the field of precision engineering. Although piezoelectric actua- tors (PEAs) are prevalent in micromanipulation systems [24], 39996 VOLUME 12, 2024
  • 3. M. Abboodi, M. Doumit: Development and Testing of Novel Soft Sleeve Actuators FIGURE 1. Linear concept evolution (a) initial concept (b) ring support concept (c) letteral support concept (d) Linear SLA 3d representation (e) geometrical parameters of linear SLA (Fh) - Fold height, (Fw) Fold width, (β) Fold Angle, and (T) wall thickness (f) tie restraining layer (g) sectional view of Linear SLA. a variety of soft actuation mechanisms are increasingly being explored for their unique advantages. Notable among these are shape memory alloys [25], dielectric elastomers [26], and Electro active polymer [27], each contributing distinct prop- erties to the field. Diverging from the traditional rigid-link mechanisms typically employed in precision engineering, SLAs introduce a paradigm shift by harnessing the intrinsic flexibility of soft materials. This design allows for motion generation through material deformation, thereby negating the need for lubrication and substantially mitigating issues related to friction. II. METHOD A. CONCEPTUALIZATION 1) LINEAR SLA DESIGN The linear SLA was originally designed based on a triangular bellows mechanism, as shown in Figure 1 (a). This concept entailed two V-shaped walls, namely an internal and external wall. The actuator is pressurized through the top air chan- nel, yielding to an expansion of the structure. This bellows mechanism, although effective for conventional soft actua- tors [19], [23], [28], proved untenable for soft sleeve actuation mechanisms. Upon inflation of the SLA, undesirable motions occurred, namely, the contraction of the internal wall and the expansion of the external wall, resulting in an alteration of the actuator’s shape from circular to elliptical as shown in figure 2(d) and the failure of producing any meaningful linear motion. This challenge has yet to be addressed in the existing literature, given the absence of any proposed sleeve actuation mechanism. Consequently, three distinct methodologies were devised to surmount this challenge. One approach to prevent the contraction and expansion of the internal and external walls, respectively, involved the use of a rigid frame. Thus, V-shaped rings were developed and attached to the valleys of the interior and exterior walls as shown in Figure 1(b). While this solution achieved a desired linear motion and effectively averted internal wall contrac- tion, the inherent rigidity introduced by the rigid frame compromised the actuator’s flexibility and consequently its ability to achieve considerable linear motion. An extension rate of less than 10% was achieved. To improve these performances, the external rigid frame solution was replaced with internal soft lateral tie-restraining layers, connecting the interior and exterior walls along the length of the sleeve except for the first and last layer allow- ing airflow throughout the sleeve, as shown in Figure 1(c). This design approach significantly enhanced the flexibility of the original design while mitigating radial expansion and contraction. This actuator design demonstrated linear motion, capable of extending by more than 15%. To enhance further the linear displacement of the actu- ator, a folded mechanism was introduced, as shown in VOLUME 12, 2024 39997
  • 4. M. Abboodi, M. Doumit: Development and Testing of Novel Soft Sleeve Actuators FIGURE 2. (a) contraction motion (b) extension motion (c) bending motion (d) simple triangular bellows without tie-restraining layers (e) linear actuator without tie restraining layers (f) bending actuator without tie- restraining layers frontal view.(g) bending actuator without tie-restraining layers side view. Figure 1(d-g). This approach involved the development of soft corrugated walls, integrated into the structural frame- work. Unlike previous designs which expand their framework when pressurized (e.g. bellow), this design unfolds its struc- ture when pressurized (e.g. origami). The primary function of its walls is to convert pneumatic pressure into mechanical power, enabling significant forces and motions. longitudinal tie-restraining layers were implemented, which served as a connection between the interior and exterior walls. The air channel network comprises multiple longitudinal channels that span the entire length of the actuator, establishing a con- nection at the bottom end through the bottom lateral channel and at the top end via the upper lateral channel. The adop- tion of a triangular origami configuration was necessitated by manufacturing constraints, as alternative shapes proved arduous to fabricate without support materials. The final linear sleeve actuator design significantly aug- mented the actuator’s performances, achieving an extension rate of 80% and generating a contraction force of 210 N. This was achieved using a sleeve prototype fabricated from TPU (Thermoplastic Polyurethane) with a hardness of 85 Shore A. Notably, the actuator enables a minimum step motion of 100 micrometers. The sleeve maintains a total thickness of 8mm, an inner diameter of 30 mm, a length of 80mm, weighing 150 grams, and operates under a gauge pressure of 200 kPa. Furthermore, using a vacuum pressure of 95 Kpa, the actuator achieved a 30% contraction rate and an expansion force of 51 N. Figure 2(a) presents the linear SLA in its orig- inal and compressed state, establishing a visual reference for its deformation under negative pressure. Figure 2(b), in con- trast, displays the actuator in both its original and extended state, highlighting the extent of its elongation capability. 2) BENDING SLA DESIGN The design of the Bending SLA posed a challenge due to its intricate motion along a cylindrical configuration. Out of the numerous conceptual prototypes devised, only two have ultimately emerged successful. The bending second concept stands as one of the successful models, incorporating a blend of traditional bellows with lateral tie restraining layers, as illustrated in Figure 3(b). Although this design demonstrated great torque generation capabilities, it is 39998 VOLUME 12, 2024
  • 5. M. Abboodi, M. Doumit: Development and Testing of Novel Soft Sleeve Actuators FIGURE 3. Bending concept evolution (a) initial bending concept (b) Bending second concept (c) Bending SLA 3d representation (d) geometrical parameters of bending SLA (e) tie-restraining layer (f) section view of bending SLA. hindered by a restricted bending angle, measuring less than 40◦. Furthermore, the actuator exhibited notable rigidity. Alternatively, the subsequent design employs a triangular folding mechanism paired with a longitudinal tie restraint layer, both of which were introduced and developed in the preceding section. The design utilizes custom constrained layers to induce the desired bending motion. The design of the bending actuator is depicted in Figure 3 (c-f). This bi-directional bending actuator surpasses the coveted 70◦ bending angle on both sides, amounting to a total of 140◦, while exhibiting a force generation capacity of 38 N as shown in Figure 9(h). Additionally, the actuator features precise control capabilities, with a minimum step angle resolution of 0.01 degree, ensuring fine-tuned operational movements. This was achieved using a sleeve. prototype fabricated from TPU (Thermoplastic Polyurethane) with a hardness of 85 Shore A. The sleeve maintains a total thickness of 8mm, inner diameter of 30 mm, length of 80mm, weighing 150 g, and at gauge pressure of 200 kPa. The operational principles underlying the soft pneumatic bellows bending actuators in both the second concept and the final concept rely on the principle of differential inflation. Both designs employ a dual-chamber system, sealed and delineated by constrained layers, as depicted in figure 3(b) and 3(c). Upon the application of air pressure, the actuator exhibits a bending motion induced by the unequal expansion of its chambers. The constrained layers act as a regulatory mechanism that limits the expansion on one side, hence yielding the actuator to bend towards the side with restricted inflation. Bending SLA boasts an enhanced design over bend- ing concept two by offering increased flexibility and a much smaller internal cavity. This design efficiency contributes to a more responsive and energy-efficient actuator. In contrast, the operational principle of the first concept, as shown in figure 3(a), diverges significantly. It lacks constrained layers, and the bending motion is generated from the differential expansion between the corrugated walls and the flat walls. This design results in an actuator that is less flexible, as its rigidity is derived from the internal flat wall. Figure 2(c) provides a visual representation of the Bending SLA actuator, displaying it in its initial, undeformed state alongside its bending state when subjected to pneumatic pressure. B. MANUFACTURING The initial fabrication attempt of the SLAs incorporated con- ventional manufacturing methodologies, utilizing molding techniques. However, these methods encountered substantial challenges when creating intricately thin-walled structures and hermetically sealed chambers. To overcome this chal- lenge, three-dimensional printing technologies were adopted to enable the production of the SLA. Among the various 3D printing techniques attempted, Fused Deposition Modeling (FDM) stands out as the preference. This approach involves the precise deposition of thermoplastic filament, layer by layer, through an extruder. For this, the Ultimaker S3 Bowden VOLUME 12, 2024 39999
  • 6. M. Abboodi, M. Doumit: Development and Testing of Novel Soft Sleeve Actuators tube printer was selected. A comprehensive framework was developed to address the challenges unique to Bowden tube printers and ensure their compatibility to produce soft pneu- matic actuators. One critical parameter encountered affecting the printing process of flexible materials is humidity. To ensure successful and high-quality prints, the framework focuses on imple- menting two measures. Firstly, a dehumidifier is used to reduce humidity levels below 25% in the workspace, then a filament dryer with silica gel packets is employed. The filament went through a 5-hour drying process at 50◦C before used in printing. In addition to humidity control, two key modifications were implemented to address challenges related to fila- ment movement. Firstly, upgrading the Bowden tube to a smaller inner diameter made of PTFE helps prevent filament compression and buckling. Secondly, specific adjustments are identified to prevent filament stretching and tangling around the extruder gears. Disabling the retraction setting ensures print integrity and high-quality results by minimiz- ing filament entanglement and under-extrusion. Additionally, reducing the extruder tension to the lowest possible set- ting helps minimize filament stretching and tangling. The impact of support material on actuator airtightness was also investigated. Using soluble materials as support proved unsatisfactory due to stringing and irregular blob forma- tion. In contrast, utilizing TPU as support material improved actuator quality; however, it led to air leakage. Therefore, avoiding support material is crucial for pneumatic actua- tors with desired airtight properties. Overhangs in flexible materials require precise engineering with essential support structures. The soft sleeve mechanisms are designed with these necessary support structures, addressing the challenge of overhangs in flexible materials. This involves ensur- ing secure layer-by-layer printing while acknowledging that these measures alone are insufficient. To effectively mitigate issues such as drooping and curling, the optimization of printing speed and temperature is imperative. Furthermore, maintaining print core cleanliness is crucial. Finally, optimizing printing settings plays a vital role in achieving airtightness and high-pressure resilience in pneumatic actuators. Key parameters, including printing speed, layer height, printing temperature, cooling speed, and flow rate, were fine-tuned. An optimal printing speed of 15mm/s was determined to balance print quality, airtight- ness, and manufacturing time. A layer height of 0.1mm is recommended to achieve optimal hermeticity and mechan- ical properties. The printing temperature of 240◦C ensures proper bonding between layers, while moderate cooling speed prevents sagging and promotes layer adhesion. The flow parameter varies based on the material type, with values of 125% and 135% for TPU 95A and TPU 85A, respectively. C. EXPERIMENTAL SETUP To evaluate the performance of the proposed sleeve actuators, multiple experimental setups as shown in Figure 4 have been devised to evaluate the forces, displacements, and bending angles generated by the actuators. The primary experimental setup depicted in Figure 4 (a) consists of a custom aluminum structure with the following components, (1) graphical user interface (2) data acquisition system (3) power supply (4) electric circuit (5) vacuum sensor (6) air buffer tank (7) force measurement unit for bending actuator (8) 4k camera (9) force measurement unit for linear actuator (10) Fixture (11) Solenoid Valves and (12) pressure transducers. To assess the isometric forces generated by the linear sleeve actuator, the experimental setup illustrated in Figure 4(b) was employed. This setup involved securing the top soft actuator proximal cap to a rigid fixture, while enabling unrestricted movement at the opposite end of the actuator. To enable the measurement of the actuator’s force, a force measure- ment unit (FMU) was developed, as shown in Figure 4(b). The FMU transmits the load from the actuator’s terminal point to the extremity of a load cell (FC2231, TE Con- nectivity). A pressure transducer (61CP, Texas Instruments) was employed to monitor the internal pressure within the actuator. The acquisition and processing of sensor data were accomplished through the implementation of the National Instrument’s (NI) data acquisition system. Concurrently, the monitoring and control of the hardware components were executed by means of a graphical user interface (GUI). For the concentric contraction test, the FMU was modified to enable quantification of the force generated during contrac- tion motion. Simultaneously, the vacuum sensor (ADP5111, Panasonic) was employed to monitor the internal negative pressure of the actuator. To determine extension and contraction rates, a modified experimental setup was employed, mirroring the previous force measurement methodology with an exception. The FMU was replaced by a Linear Variable Differential Trans- former (LVDT) for precise length measurements. The LVDT was connected to the actuator’s free end through a fabricated 3D coupling unit. To study the behavior of the bending actuator, an experi- mental setup, as illustrated in Figure 4(c), was devised. While sharing most hardware components with the linear exper- imental setup, this configuration features a bespoke FMU capable of accommodating the bending motion. Notably, the force at the actuator’s tip is captured by connecting it to the free end of the FMU via a flexible steel cable. A force sensor (FC22-10) is employed to measure the force at the tip of the actuator. To measure the bending angle, a testing protocol was employed based on the methodologies presented in [29], [30], and [31]. This experimental investigation involved a setup where the soft actuator’s proximal end, featuring an air inlet, was securely clamped within an actuator fixture. Meanwhile, the distal end of the actuator remained unrestricted, allowing for bending motion. To capture and analyze the actuator’s motion accurately, a high-speed 4k camera (GoPro Hero 11) was utilized. The camera was aligned with a checkered background to enhance measurement precision. To process 40000 VOLUME 12, 2024
  • 7. M. Abboodi, M. Doumit: Development and Testing of Novel Soft Sleeve Actuators FIGURE 4. Schematic overview of the experimental setup components and configurations: (a) Primary components of the experimental setup (1) graphical user interface (2) data acquisition system (3) power supply (4) electric circuit (5) vacuum sensor (6) air buffer tank (7) force measurement unit for bending actuator (8) 4k camera (9) force measurement unit for linear actuator (10) Fixture (11) Solenoid valves (12) pressure transducers. (b) linear experimental setup (c) experimental setup for bending motion. (d) Stiffness experimental setup. the recorded video footage and calculate the bending angle, an open-source software Kinovea was used similarly to other studies [29], [31] To measure the stiffness of the actuator, an experimental setup as depicted in Figure 4(d) was developed. This setup involved the integration of the sleeve linear actuator with an Instron machine through a bespoke actuator design. The objective of this setup was to study the actuator’s stiffness as a function of its displacement and internal pressure. To this end, a compressor and a buffer tank were utilized, providing a stabilized air pressure source for the actuator. Concurrently, a pressure sensor was deployed to accurately measure the internal pressure of the actuator. Within this context, experi- ments were conducted using five distinct constant pressures: 25, 50, 75, 100, and 125 KPa. To maintain these pressures constant within the actuator, industrial pressure regulator (QBX pressure control valve, Proportion-Air, Inc.) were used. III. RESULTS A. LINEAR SLA For the proposed linear SLA, its ability to generate force and linear motion was tested and evaluated under diverse geomet- rical parameters and material stiffness. To analyze the effect of each parameter, a series of models were constructed, tested and evaluated. Table 1 details these models, specifying the geometrical parameters (Figure 1e) as diameter (r), actuator length (l), fold height (fh), fold width (fw), fold angle (β), number of restraining layers (nr), thickness of restraining layers (rt), wall thickness (wt), and shore hardness (sh). VOLUME 12, 2024 40001
  • 8. M. Abboodi, M. Doumit: Development and Testing of Novel Soft Sleeve Actuators FIGURE 5. Impact of geometrical parameters and material Stiffness on generated force of linear SLA in extension motion (a) effect of fold angle (b) effect of material stiffness (c) effect of the tie restraining layers thickness (d) effect of wall thickness (e) effect of fold width (f) effect of the number of tie-restraining layers. TABLE 1. Linear actuator models (all dimension in mm). 1) FORCE GENERATION RESULTS The influence of geometric parameters and material stiffness on the ability of the linear SAL to generate force was exam- ined during extension upon application of positive pressure and during contraction under negative pressure. For exten- sion, experimental setup as shown in Figure 4(b) was used and the actuator was pressurized up to 200 kPa. The contraction phase was evaluated using the same experimental setups and by applying a vacuum pressure of up to 95 kPa. During these tests, each parameter was altered individually, allowing the understanding of their effects on actuator force performance. The subsequent subsections provide a detailed analysis of these parameter changes. a: EFFECT OF FOLD ANGLE Fold angle (β), a critical geometrical parameter, is portrayed as the internal angle at the juncture of two converging inclined sides of a fold. As shown in Figure 5(a), an increase in the fold angle from 30◦ to 40◦ inversely impacts the resultant force, inducing a decrease from 130 N to 101 N. Similarly, the actu- ator’s response during contraction movements follows this pattern. Demonstrated in Figure 6(a), augmenting the fold angle from 30◦ to 40◦ leads to a decreased force generation, from 24 N to 14.5 N. b: EFFECT OF MATERIAL STIFFNESS The materials used in the fabrication of the actuators included two distinct grades of thermoplastic polyurethane: the flexible TPU 85A (Ninjaflex) versus the compara- tively less flexible TPU 95A (Ultimaker). An increase in stiffness from 85A to 95A resulted in a decreased peak force during extension, from 130 N to 89.9 N, as shown in Figure 5(b). Conversely, during contraction testing, a stiffness increase from 85A to 95A led to an increase in force, from 24 N to 33 N, as demonstrated in Figure 6(b). 40002 VOLUME 12, 2024
  • 9. M. Abboodi, M. Doumit: Development and Testing of Novel Soft Sleeve Actuators FIGURE 6. Impact of geometrical parameters and material stiffness on generated force of linear SLA in contraction motion (a) effect of fold angle (b) effect of material stiffness (c) effect of wall thickness (d) effect of the tie restraining layers thickness (e) effect of fold width. (f) effect of the number of tie-restraining layers. c: EFFECT OF TIE-RESTRAINING LAYERS THICKNESS The thickness of tie-restraining layers (tr) is pivotal in ensuring these layers possess the capability to withstand high pressure, thereby preserving structural integrity. During the extension movement, as shown in Figure 5(c), increas- ing the layer thickness from 0.4 mm to 0.8 mm leads to a decrease in force generation, from 110 N to 95 N. In contrast, the experiment detailed in Figure 6(d) demon- strates that changing the thickness of these layers in the same range has a negligible impact on force output during contraction. d: EFFECT OF THE NUMBER OF TIE RESTRAINING LAYERS The number of tie-restraining layers (nr) exerts a signif- icant impact on the durability of these layers and effec- tively reduces the ballooning effect within the actuator structure. Figure 5(f) clearly shows that during the exten- sion phase, increasing the number of restraining layers from 14 to 18 leads to a substantial reduction in force, from 191 N to 86 N. A parallel observation is made in the contraction motion. Figure 6(f) indicates an increase in the number of restraining layers inversely affects the force exerted. This is demonstrated by a decrease in force from 35 N to 20 N as the layer count goes from 14 to 18 layers. e: EFFECT OF WALL THICKNESS The wall thickness (wt) refers to the thickness of the walls constituting the folds of the linear SLA, a dimension that substantially influences the actuator’s flexibility. As demon- strated in Figure 5(d), increasing the wall thickness from 1.2 mm to 1.6 mm results in an increased force output, from 110.5 N to 130.2 N during extension movements. Corre- spondingly, during contraction movements, a thicker wall corresponds to an amplified force, evidenced by the data presented in Figure 6(c), where a marginal increment in wall thickness of 0.4 mm considerably increases the force from 27.5 N to 36.5 N. f: EFFECT FOLD WIDTH Fold width (fw) is the horizontal measurement between the peaks and valleys of the corrugations of the linear SLA when it is not inflated. A notable finding is that decreasing the fold width from 12 mm to 4 mm significantly enhances the force generated during extension. This is clearly illustrated in Figure 5(e), where such a reduction in fold width results in a force increase from 130 N to 214.5 N. In the context of contraction motion, a similar trend is observed. A decrease in fold width markedly boosts the force output, as shown in Figure 6(e). Here, reducing the fold width from 12 mm to VOLUME 12, 2024 40003
  • 10. M. Abboodi, M. Doumit: Development and Testing of Novel Soft Sleeve Actuators FIGURE 7. Impact of geometrical parameters and material stiffness on displacement of Linear SLA in extension motion (a) effect of fold angle (b) effect of material stiffness (c) effect of the tie restraining layers thickness (d) effect of wall thickness (e) effect of fold width. 8mm leads to a doubling of the force generated, from 24 N to 51.5 N. 2) LINEAR DISPLACEMENT RESULTS This section discusses the testing and results of the lin- ear displacement characteristics of the linear SLA. The evaluation encompasses both extension and contraction motions, employing a similar experimental setup as shown in Figure 4(b). For extension, the linear SLA underwent pressurization to 200 kPa, whereas for contraction, a vacuum pressure of 95 kPa was applied. In the following sections, a detailed analysis is presented, delving into how variations in geometric parameters and the material stiffness of the actuator affect the SLA displacement capabilities. a: EFFECT OF FOLD ANGLE As demonstrated in Figure 7(a), an increment in the fold angle from 30◦ to 40◦ inversely affects the extension length. Specifically, this increase in fold angle is associated with a decrease in extension displacement, from 52.7 mm to 39.3 mm. Similarly, the impact of fold angle on contraction displacement exhibits a comparable pattern. As evidenced by Figure 8(a), where an increase in the fold angle from 30◦ to 40◦ results in a decrease in contraction displacement from 11 mm to 6 mm. b: EFFECT OF MATERIAL STIFFNESS In Figure 7(b), experimental outcomes for two prototypes with distinct material stiffness values, 85A and 95A, are presented. These results show that an elevation in material stiffness results in a substantial decrease in displacement during extension, with measurements showing a reduction from 52.7 mm to 13.4 mm. Correspondingly, during the contraction phase, as illustrated in Figure 8(b), an increase in material stiffness from 85A to 95A is observed to diminish the displacement from 11 mm to 4.3 mm. c: EFFECT OF RESTRAINING LAYERS THICKNESS The influence of an increase in the thickness of the actuator’s layers on its displacement during both extension and con- traction was analyzed. Specifically, an increment in thickness from 0.4 mm to 0.8 mm was observed to notably affect the actuator’s performance. In the extension phase, as illustrated in Figure 7(c), this increase in thickness led to a decrease in the extension displacement from 30 mm to 18 mm. Similarly, in the contraction phase, increasing the layer thickness by the same margin resulted in a reduction in contraction, from 7 mm to 5 mm as show in Figure 8(c). d: EFFECT OF WALL THICKNESS An increase in wall thickness from 1.2 mm to 1.6 mm has been found to significantly influence the displace- ment of the actuator. As depicted in Figure 7(d), this increase in wall thickness leads to a reduction in the exten- sion displacement, from 30 mm to 20 mm. Similarly, the contraction displacement is affected by this change in thick- ness, showing a decrease from 7 mm to 6 mm as shown in Figure 8(d). 40004 VOLUME 12, 2024
  • 11. M. Abboodi, M. Doumit: Development and Testing of Novel Soft Sleeve Actuators FIGURE 8. Impact of geometrical parameters and material stiffness on displacement of linear SLA in contraction motion (a) effect of fold angle (b) effect of material stiffness (c) effect of the tie-restraining layers thickness (d) effect of wall thickness (e) effect of fold width. e: EFFECT OF FOLD WIDTH The influence of fold width is observed to vary distinctly between these two operational modes. In terms of extension, increasing the fold width from 8 mm to 12 mm results in a decrease in the extension rate. This is quantitatively demon- strated in Figure 7(e), where the extension distance dimin- ishes from 53 mm to 43 mm with the increased fold width. This trend underscores the inverse relationship between fold width and extension displacement. Conversely, the contrac- tion motion exhibits a contrasting behavior. As shown in Figure 8(e), a decrease in fold width leads to an enhance- ment in the contraction rate. A notable instance of this effect is observed when a reduction in fold width results in an increase in the contraction distance, from 11 mm to 19 mm. B. BENDING SLA This section is devoted to the experimental outcomes for bending SLA, with a particular emphasis on assessing the influence of geometric parameters, as demonstrated in Figure 3(d-f), and material stiffness on the generated forces and bending angles. In the testing phase, each bending actua- tor underwent pressurization to 200 kPa, following which the bending angles and resultant forces at the tip of actuator were measured. The investigative framework employs a diverse set of models, each detailed in Table 2. These models, distinct in their geometric and material com- positions, collectively encompass a broad spectrum of design TABLE 2. Bending actuator models (all dimension in mm). possibilities. The testing of these models was conducted in a controlled experimental setting, as depicted in Figure 4(c). The subsequent section will provide an in-depth exploration of the experimental results. 1) EFFECT OF FOLD ANGLE Figure 9(a) shows that increasing the fold angle from 30◦ to 45◦ leads to a reduction in force from 12.3 N to 10.8 N, signifying an inverse correlation between fold angle and force generation. In terms of bending angle, Figure 10(a) indicates that the same increase in fold angle results in a decrease in bending angle from 22◦ to 9.2◦. These observations collec- tively emphasize a consistent inverse relationship between an increase in fold angle and the reduction in bending perfor- mance. VOLUME 12, 2024 40005
  • 12. M. Abboodi, M. Doumit: Development and Testing of Novel Soft Sleeve Actuators FIGURE 9. Impact of geometrical parameters and material stiffness on generated force of bending SLA. (a) effect of fold angle (b) effect of material stiffness (c) effect of wall thickness (d) effect of the number of tie-restraining layers (e) effect of the tie restraining layers thickness (f) effect of constrained layers (g)effect of fold width. (h) generated forces. 2) EFFECT OF MATERIAL STIFFNESS The experimental results shown in Figure 9(b) demonstrate that increasing stiffness from 85A Ha to 95A marginally elevates the force, from 12.3 N to 14 N. This finding indi- cates a subtle yet positive influence of stiffness on force output. In contrast, bending angle results, as presented in Figure 10(c), reveal a pronounced decrease from 22◦ to 12◦ with higher material stiffness. This underscores a significant inverse relationship between material stiffness and bending flexibility. 3) EFFECT OF WALL THICKNESS Figure 9(c) reveals that an increase in wall thickness from 1.2mm to 1.6mm results in enhanced force generation, with force values rising from 12.3 N to 16 N. This increase suggests that greater wall thickness contributes positively to the force output. Conversely, Figure 10(d) indicates an inverse effect on bending performance. The same increase in wall thickness, from 1.2mm to 1.6mm, leads to a decreased bending angle, moving from 22◦ to 17.3◦. These findings highlight that while thicker walls augment force generation, they concurrently reduce bending flexibility. 4) EFFECT OF THE NUMBER OF TIE-RESTRAINING LAYERS Testing reveals that decreasing the tie-restraining layers from 18 to 10 leads to an increase in force from 12.3 N to 23.5 N, as shown in Figure 9(d), indicating that fewer layers enhance force output. Concurrently, Figure 10(f) displays that this reduction in layers also results in an increased bending 40006 VOLUME 12, 2024
  • 13. M. Abboodi, M. Doumit: Development and Testing of Novel Soft Sleeve Actuators FIGURE 10. Impact of geometrical parameters and material stiffness on bending angle of bending SLA (a) effect of fold angle (b) effect of constrained layers (c) effect of material stiffness (d) effect of wall thickness (e) effect of the tie restraining layers thickness (f) effect of the number of tie-restraining layers (g) improve the bending angle by reducing wall thickness (h) effect of fold width. angle from 22◦ to 44◦, suggesting an augmented bending capability with fewer restraining layers. 5) EFFECT OF THE THICKNESS OF TIE-RESTRAINING LAYERS Increasing layer thickness by 0.4mm reduces force from 12.3 N to 9.3 N, as Figure 9(e) shows, indicating a negative correlation with force generation. Concurrently, Figure 10(e) demonstrates a decrease in bending angle from 22◦ to 12◦ due to the same thickness increment, underscoring its detrimental impact on bending performance. 6) EFFECT OF CONSTRAINED LAYERS Figure 9(f) delineates that an augmentation in constrain lay- ers’ thickness from 1.6 mm to 3.2 mm has a negligible impact on the forces generated by the actuator. Conversely, Figure 10(b) reveals that this same increase in thickness results in a slight decrease in bending angle, from 22◦ to 18◦, thereby indicating a more pronounced effect on the actuator’s bending capacity. 7) EFFECT OF FOLD WIDTH The data in Figure 9(g) indicates that expanding the fold width from 8mm to 12mm leads to an increase in force by 3N. This finding suggests that wider folds contribute to enhanced force generation. Regarding bending performance, Figure 10(h) demonstrates that the same alteration in fold width results in an increase in bending angle from 56◦ to 61◦. This outcome illustrates that increased fold width not only VOLUME 12, 2024 40007
  • 14. M. Abboodi, M. Doumit: Development and Testing of Novel Soft Sleeve Actuators FIGURE 11. (a) Expermental and fitted force-displacement data (b) Variation of actuator stiffness with displacement under different pressure conditions. enhances force output but also positively affects the actuator’s bending range. C. ACTUATOR STIFFNESS This experiment aims to quantify the stiffness of the actuator. Model L5 was selected for testing under five distinct pressure conditions: 0 kPa, 25 kPa, 50 kPa, 75 kPa, and 100 kPa, utilizing the experimental setup depicted in Figure 4(d) and detailed in Section II. In this setup, one end of the actuator was affixed to a fixture, and the other end was connected to a coupling unit, which in turn was attached to the Instron head. Subsequently, the Instron head achieved a displacement of 40 mm at a speed of 50 mm/min, with data collection facilitated through LabVIEW. The results, presented in Figure 11(a), depict the actuator’s response across five pressure settings. Increasing the internal pressure of the actuator leads to a rise in the force required to induce displacement. The SLA demonstrates a nonlinear force-displacement relationship where a polynomial model was used to approximate the force-displacement curve. Further analysis on SLA stiffness, delineated as the force- to-displacement ratio, was conducted over eight discrete measurement intervals from 0 to 40 mm, in increments of 5 mm. This analysis highlighted the stiffness’s dependence on internal pressure variations. Notably, at 0 kPa, stiffness measured in the 0 to 5 mm displacement range stood at 1716.42 N/m, increasing significantly to 4087.55 N/m at 125 kPa for the same displacement range. This trend demon- strates a direct correlation between pressure incrementation and stiffness enhancement within the actuator. Figure 11(b) graphically underscores the pressure-stiffness relationship, showcasing the critical influence of internal pressure adjust- ments on the actuator’s stiffness. IV. DISCUSSION The experimental results from the preceding section highlight the performance of the Linear and Bending SLA as a function of the material properties and the geometrical parameters of the actuator. This greatly contributes to an in-depth under- standing of the mechanical operation of the proposed SLA and how each identified parameter contributes and impacts its mechanical performances. More specifically, a significant correlation was observed between material stiffness and the flexibility of the actuator, impacting the displacement in both linear and bending SLA. It was noted that increasing material stiffness beyond 85 Shore A impairs the flexibility of the actuator. Moreover, the Fold Angle is identified as a critical parameter for SLA performance. A reduction in this angle is associated with a significant enhancement in actuator ability to achieve displacement and force generation for both linear and bending SLA. The smallest Fold Angle utilized in this research, constrained by manufacturing limitations, was 30◦. Wall thickness is another influential factor. It was found that a minimum thickness of three lines is essential to ensure the airtightness of the sleeve. While an increase in wall thickness can reduce the ballooning effect of the actuator, thus slightly enhancing force production, it concurrently decreases the actuator’s flexibility and diminishes both displacement in linear SLA and bending angle in bending SLA. Additionally, for the Fold Width parameter, decreasing its value results in an increase in the forces generated in both bending and linear SLA. However, this reduction also leads to a decrease in extension displacement and bending angle, and a reduction in contraction displacement. The thickness of constrained layers, exclusive to bending SLA and instrumental in the transformation from linear to bending motion, was found to have a minor impact on generated forces and bending angle. The importance of Tie-restraining layers in maintaining the structural integrity of the actuator is underscored. Absent these layers, the actuator would assume a balloon-like form, as illustrated in figure 2(d-g). It was discovered that increas- ing either the number or thickness of these layers leads to a reduction in generated forces, linear displacement, and bend- ing angle. The experimental results indicate that a restraining layer thickness of 0.4 mm is suitable for pressures up to 200 kPa for both linear and bending SLA; for applications exceeding this pressure, the thickness should be increased accordingly. The optimal number of restraining layers for 40008 VOLUME 12, 2024
  • 15. M. Abboodi, M. Doumit: Development and Testing of Novel Soft Sleeve Actuators better performance was found to be 10 for bending SLA and 14 for linear SLA. The development of SLA presents a promising actuation technology. These actuators can be further configured in series, combining linear and bending motions, or aligning multiple bending actuators, to achieve a versatile range of motions. The sleeve structure of these actuators lends itself to a myriad of practical applications. In the domain of wearable technology, sleeve actuators offer significant benefits. The SLA are self-contained and can be conform directly to the wearer’s limb. This direct application promotes both stability and comfort. These devices function akin to a sleeve, aug- menting the user’s muscular actions, thereby offering support and amplification of movement. Furthermore, the internal pressure within these SLA can be modulated to alter their stiffness. This adaptability renders them suitable for creating variable-stiffness structures, such as robotic grips or smart textiles that respond to changing conditions or tasks. V. LIMITATIONS AND FURTHER DEVELOPMENT The existing methodology for producing these actuators presents certain limitations, particularly concerning the rigid- ity of materials used, which cannot currently be reduced below 85 Ha. Exploring methods to diminish this material stiffness could augment the energy efficiency of the actuators without undermining their operational capacity. Addition- ally, the lowest achievable fold angle in the present setup is restricted to 30◦ due to inherent manufacturing constraints. A decrease in this angle would proportionally increase the actuator’s displacement performance. In addition, the future work should focus on incorporating finite element analysis (FEA) and numerical simulations to enhance the understanding and optimization of the SLA. These simulations would provide insights into the actua- tor’s behavior under various load conditions, enabling a comprehensive understanding of its structural integrity and operational limits. Additionally, numerical analyses can be employed to optimize the design parameters, thereby improv- ing the efficiency and applicability of these actuators. This research paper primarily focused on the design, fabri- cation, and mechanical evaluation of a novel SLA. For future research, the focus will shift towards adapting the SLA to mobility assistive devices. This entails adjusting the SLA geometry to suit human anatomical specifications. Another area for future development is the integration of sensory feedback systems within the SLA design. This would allow for real-time monitoring and adjustments of actuator behav- ior, enhancing the user experience and safety in assistive applications. VI. CONCLUSION In conclusion, soft sleeve actuators hold significant potential for various applications in wearable robotics. However, the development of these actuation mechanisms remains unex- plored. To address this research gap, this paper presents a novel bidirectional actuator consisting of linear and bending capabilities. The linear SLA demonstrated notable perfor- mance, generating over 200 N with 80% extension rate at 200 Kpa of pneumatic pressure. It also achieved more than 50 N with a 30% contraction rate at a vacuum pressure of 95 kPa. On the other hand, the bending actuator exhibited the ability to generate over 38 N and achieve bending angles of over 140◦ in both directions (±70◦). REFERENCES [1] N. El-Atab, R. B. Mishra, F. Al-Modaf, L. Joharji, A. A. Alsharif, H. Alamoudi, M. Diaz, N. Qaiser, and M. M. Hussain, ‘‘Soft actuators for soft robotic applications: A review,’’ Adv. Intell. Syst., vol. 2, no. 10, Oct. 2020, doi: 10.1002/aisy.202000128. [2] H. Xiong and X. 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