Sensing in Textiles
Micro-Electro-Mechanical Systems is a technology that in its most general form can be
defined as miniaturized mechanical and electro-mechanical elements that are made
using the techniques of micro fabrication.
MEMS are made up of components between 1 to 100 micrometres in size (i.e. 0.001 to
0.1 mm), and MEMS devices generally range in size from 20 micrometres (20 millionths
of a metre) to a millimetre (i.e. 0.02 to 1.0 mm).
Because of the large surface area to volume ratio of MEMS, surface effects such as
electrostatics and wetting dominate over volume effects such as inertia or thermal mass.
Micro machines are divided into two functional groups: the sensors and the
A sensor is defined as a device that provides a usable electrical output signal
in response to a signal. When a sensor is integrated with signal processing
circuits in a single package (usually a polysilicon chip), it is referred to as an
integrated sensor or smart sensor.
An actuator is a device that converts an electrical signal, which may be taken
from a sensor to an action.
A transducer is considered as a device that transforms one form of signal or
energy into another form. Therefore, the term transducer can be used to
include both sensors and actuators.
Smart sensors have all the electronic integrated in a MEMS structure. A photo
of a silicon wafer with one hundred microstructures.
Principles Used in Sensors
Physical principles or effects grouped according to the six forms of physical energy.
Advantages of MEMS devices
•The function is replicated numerous times giving a higher accuracy to the
•Due to the replications, failure of some sensors would not affect the system
performance. Such system is usually referred to as an array of sensors.
• A small device interferes less with the environment that it is trying to
measure when it is of a smaller size.
• They can be placed in small places where traditional macro devices could
• A higher precision is achieved with actuators. Motions of micrometer range
are precisely achievable.
The micromechanical components are fabricated using compatible
"micromachining" processes that selectively etch away parts of the silicon wafer or
add new structural layers to form the mechanical and electromechanical devices.
Typical MEMS are miniature sensors and actuators.
Approaches to integrate the MEMS on textiles .
The first approach is trying to develop yarn-like electronics and transducers using
existing and new flexible materials in order to stitch up the sensors on the textile
directly, which may result in limited sensing capabilities and computation capabilities.
The other approach is trying to design and fabricate the silicon-based flexible sensors
with MEMS technology and then sew the flexible sensors array on the textile.
A MEMS device in general is rigid so that it cannot be bent. However, if MEMS devices
of the rigidity of these small size are fabricated on a flexible substrate in the form of
isolated islands, one flexible silicon sensor skin is then obtained The flexible substrate is
patterned with metal wires that are to be used as interconnects between the MEMS and
processing circuits. Then the intelligent textiles are obtained by sewing up this flexible
silicon sensor skin on the fabric.
In order to achieve flexible
skin, there are two approaches
to fabricate MEMS devices on
the flexible substrate: (1) First
we fabricate MEMS devices
processes" after depositing a
layer of polymer on a fourinch wafer, and make MEMS
devices isolate each other and
obtain flexible silicon sensor
skin promptly to strip off the
coating polymer on the wafer.
(2) Second, here first we deposit a
layer of polymer on front side of
the wafer after fabricating MEMS
devices using "bulk subtractive
processes", then corrode the
reverse side of the wafer to make
MEMS devices form the detached
islands in isolation each other, later
deposit a layer of polymer and
obtain the base skin of flexible
silicon promptly on the wafer
reverse side again.
The bulk subtractive processes is more practical and cost effective.
The principle of the thermo resistive transducer is that the resistance changes
according to material heat change and the resistance (R) of the material can be
calculated according to the following formula:
is resistance coefficient of the material, L is the length of the material, and A is
the area of the material. For being compatible with MEMS, we have chosen the p-Doped
silicon as the resistance material, and the resistance coefficient of the p-Doped silicon can be
calculated according to the following formula:
where p is the carrier concentration, q is charge on electron, and µp is the hole mobility.
MEMS Fabrication on Fabrics
Fabrics present a very different substrate compared with a silicon wafer
– Rough, uneven surface with pilosity (hairiness).
– Flexible and elastic
– Suitable for low temperature processing
– Limited compatibility with solvents and chemicals
To use standard printing techniques to deposit a range of custom inks in order to realise
freestanding mechanical structures coupled with active films for sensing and actuating.
Also known as thick-film printing, this is normally used in the fabrication
of hybridised circuits and in the manufacture of semiconductor packages.
Non contact direct printing onto substrate, used for fabrics and electronics
Printed MEMS Process
Sacrificial layer requirements:
Easily removable without damaging fabric or other layers.
Structural layer requirements
Suitable mechanical/functional properties.
Case Study: Strain Gauge
Exploits the piezoresistive effect: the resistance of a printed film changes as it is
strained (stretched) due to a change in the resistivity of the material. Useful for
sensing movement, forces and strains.
Silver electrodes printed using a low temperature polymer silver paste.
Piezoresistive paste is based on graphite.
Cured at 120-1250C
Ink types required
•Simple heater is a current carrying conductive element.
•Existing heaters integrated in textiles by weaving or knitting.
•Woven approach limited by direction of warp and weft.
•Knitted solution requires specialist equipment .
Heated car seat element(BMW)
Overcomes surface roughness and pilosity of fabric substrate providing a continuous
planar surface for subsequent printed layers.
Cross-section SEM micrograph of 4
screen printed interface layers on
polyester cotton fabric
Heater has three layers: Interface, Conductor and Encapsulation.
Interface layer improves heater performance but increases fabric coverage to ~40% still below limit of 50%.
Piezoelectric materials expand when subject to an electrical field, similarly they produce
an electrical charge when strained.
Ideal material for sensing and actuating applications.
Piezoelectric material sandwiched between electrodes.
Polarising voltage required after printing to make the piezoelectric active.
Cured at temperatures below 150 oC.
Textile-based (MEMS) Accelerometer for Pelvic Tilt Mesurement
An accelerometer is a device that measures
proper acceleration (in relativity theory, proper
acceleration is the physical acceleration
experienced by an object. It is thus acceleration
relative to a free fall, or inertial, observer who is
momentarily at rest relative to the object being
measured. Gravitation therefore does not cause
proper acceleration, since gravity acts upon the
inertial observer that any proper acceleration
must depart from (accelerate from). A corollary
is that all inertial observers always have a proper
acceleration of zero. The proper acceleration
measured by an accelerometer is not necessarily
the coordinate acceleration (rate of change of
velocity). Instead, the accelerometer sees the
acceleration associated with the phenomenon of
weight experienced by any test mass at rest in the
frame of the accelerometer device.
Micro Electro Mechanical System (MEMS) accelerometer is an electro-mechanical
device that measure acceleration force exerted on it. The development of textilesbased MEMS for pelvic tilt measurement is an effort to reduce the cost in medical
The piezoresistive effect describes change in the electrical resistivity of a
semiconductor or metal when mechanical strain is applied. In contrast to the
piezoelectric effect, the piezoresistive effect only causes a change in electrical
resistance, not in electric potential.
The accelerometer sensor is designed as a cantilever beam structure with suspended
mass at one end.
(a)Schematic drawing of accelerometer design. (b) Close-up drawing on conductive section of accelerometer.
(c) Actual photo of textile cantilever accelerometer.
1. Textile-based accelerometer provides an alternative to the costly and hazardous
radiographic measurement of pelvic tilt.
2. The flexibility of textile structure makes it more advantageous to conform to body
contour than rigid digital inclinometer and more accurate than indirect
4. Textile material is relatively low-cost, flexible, lightweight, readily
available, environmental friendly and easy to use.
Other research areas and future scope
Monitoring warp end tension and breaks during fabric formation. A custom designed
micro machine sensor has been designed is being fabricated. It will replace the off shelves
sensors currently used to measure warp tension.
Manipulating and aligning micro fibres in up to 6 axes is the first step towards a micro
weaving machine. Future work could absolutely be the fabrication of this micro weaving
1. Rakesh B. Katragadda, Yong Xu, A novel intelligent textile technology based
on silicon flexible skins, ECE Department, Wayne State University, Detroit, MI
2. S Beeby, M J Tudor, R Torah, K Yang, Y Wei, MICROFLEX Project: MEMS on
New Emerging Smart Textiles/Flexibles, Electronics and Computer Science, University of
3. Maozhou Meng, Yong Xu, Honghai Zhang, and Sheng Liu, Intelligent Textiles Based on
MEMS Technology, Division ofMOEMS, Wuhan National Laboratory for Optoelectronics
and Institute of Microsystems, Huazhong University of Science and Technology
1037 Luo Yu Road, Wuhan, Hubei 430074, China and Electrical and Computer
Engineering, Wayne State University, Detroit, Michigan, USA .
4. Nik Nur Zuliyana Mohd. Rajdia, Azam Ahmad Bakira, Syaidah Md. Saleha, and Dedy
Wicaksonoa, Textile-based Micro Electro Mechanical System (MEMS) Accelerometer for
Pelvic Tilt Mesurement, International Symposium on Robotics and Intelligent Sensors
2012 (IRIS 2012).
5. S Beeby, R Torah, K Yang, Y Wei, J Tudor, MICROFLEX Project - Microtechnology in
Smart Fabrics, Electronics and Computer Science, University of Southampton.