10.1177@0954411919878306

Original Article
Proc IMechE Part H:
J Engineering in Medicine
1–11
Ó IMechE 2019
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DOI: 10.1177/0954411919878306
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Improving the surface energy of
titanium implants by the creation of
hierarchical textures on the surface
via three-dimensional elliptical
vibration turning for enhanced
osseointegration
Sayed Ali Sajjady, Mohammad Lotfi, Saeid Amini,
Hamidreza Toutounchi and Alireza Bagheri Bami
Abstract
An increase in bone-implant contact and an increase in surface hydrophilicity are the two important factors involved in
improving osseointegration. Therefore, three-dimensional elliptical vibration turning method is applied to increase the
hydrophilicity of titanium surface by the generation of hierarchical nano- and micro-textures. That being the case, face
turning process at different cutting conditions is carried out in this research. Surface roughness and the contact angle of
water drops with machined surfaces were selected to be measured for the analysis of surface hydrophilicity. The results
show that an additional surface area can be achieved by the generation of micro- or nano-textures, resulting in a lower
contact angle. Furthermore, intermittent movement of cutting tool in vibration cutting causes the process to be more
stable, achieving the desired range of surface roughness.
Keywords
Titanium alloy, dental implant, osseointegration, wettability, surface roughness
Date received: 18 May 2019; accepted: 2 September 2019
Introduction
Dental implants are now widely accepted by dentists
and patients throughout the world as a suitable replace-
ment for the lost teeth.1
The efficiency of current
implants is positively reported; however, the attempts
are still being kept on by researchers to improve it
more.2
An increase in bone-implant contact (osseointegra-
tion) plays a decisive role in the short- and long-term
sustainability of dental implants.3
The previous
researches concluded that the osseointegration can be
improved by an increase in the surface hydrophilicity
of the implants where the bone-implant contact
increases.4,5
Two approaches are considered to increase
the surface hydrophilicity. The first one is macroscale
optimization (the overall shapes of the implants are
considered)6–8
and the second one is microscale optimi-
zation (the surface parameters are considered).9–13
Accordingly, different works showed that the
sustainability of dental implants increased by an incre-
ment in surface roughness.14,15
However, an excessive
increment in surface roughness can increase ionic leak-
age.16
The proper value is around 2 mm,17
because
this size of surface roughness maximizes the bone-
implant interlocking.18,19
In addition to that, nanoscale
setting is another issue. Although most of the available
implants are manufactured with the surface roughness
in microscale, it is proved that the important factors
involved in osseointegration are related to nanoscale
setting.20,21
Department of Manufacturing, Faculty of Mechanical Engineering,
University of Kashan, Kashan, Iran
Corresponding author:
Saeid Amini, Department of Manufacturing, Faculty of Mechanical
Engineering, University of Kashan, Kashan, Iran.
Email: amini.s@kashanu.ac.ir

In general, hydrophilicity is related to two para-
meters. (1) The surface energy is affected by the chemi-
cal composition of the surface. Note that the materials
with the higher surface energy have more hydrophilic
quality, because, a higher energy is needed to break
down the inter-molecular bonds between the liquid and
the contact surface. (2) Surface topography is coupled
with surface roughness, because a direct relation exists
between surface roughness and contact angle (hydro-
philicity) with osseointegration.22,23
A surface is called hydrophilic when the contact
angle of water droplets is less than 90°(u1 908).24
By
the creation of micro-dimples on a hydrophilic surface,
it becomes more hydrophilic, which means the contact
angle is reduced (u2 u1).25
Furthermore, if a hierarch-
ical roughness (micro and nano) is created on a hydro-
philic surface, its quality increases highly and is called
as super-hydrophilic surface (u’08).26
The most com-
mon methods to produce micro-roughness on the sur-
face of dental implants are plasma spray, grit blasting,
acid etching, and anodization.27–29
In this research, it is tried to create micro-dimples on
the surface of titanium alloy (Ti-6Al-4V) by using ellip-
tical vibration turning process. As this process causes
micro-dimples to be formed regularly (with a particular
pattern),30,31
it is expected that bone growth and micro-
bonds are to be isotropic in different directions due to
the reduction of anisotropic wettability.32
This event is
the novelty of this method compared to other surface
preparation methods. Accordingly, face turning process
is conducted on the specimens in two manners of con-
ventional turning (CT) and three-dimensional elliptical
vibration turning (3D-EVT). Full-factorial approach is
used to design the experiments. To estimate the wett-
ability of the machined surfaces, the contact angles of
the droplets are measured. At the end, the effect of each
particular parameter on wettability and anisotropic
wettability of machined surfaces is investigated by using
analysis of variance (ANOVA) in Minitab software.
Experimental preparation
Setup and ultrasonic equipment
The experimental setup is shown in Figure 1. In this
study, a novel horn is used to convert the linear vibra-
tion generated by piezoelectric (PZT) to 3D vibration
(longitudinal vibration in z-direction and bending
vibrations in x and y directions). In fact, linear vibra-
tion reaches three beams (1, 2, and 3) after passing
through the cylindrical section of the horn.33,34
Beam 1 transfers the vibration in longitudinal mode
to the tip of the cutting tool (z-direction).
Beams 2 and 3 convert the linear vibration generated
by PZTs to the bending form when it is passed through
each of them.
The cutting insert is made of tungsten carbide and
the workpiece material is a titanium alloy (Ti-6Al-4V).
Most of the dental implants are made up of Ti-6Al-
4V alloy, which has higher fatigue resistance and yield
strength compared to pure titanium.35
The chemical
composition of the surface of the titanium implants is a
factor affecting the hydrophilicity. In many reports, it
has been highlighted that implants with hydrophilic sur-
faces are more effective when interacting with biological
tissues, cells, and fluids.36,37
The other effective factors on hydrophilicity are sur-
face topography and surface roughness. As the osseoin-
tegration is the case, surface roughness is listed in three
categories.28
The first one is macroscale (from several
Figure 1. The experimental setup.
2 Proc IMechE Part H: J Engineering in Medicine 00(0)

10 mm to several millimeters). It is related to the general
shape of dental implants.
The second category is microscale (from 1 to 10 mm),
which consists of peaks and valleys that exist on the sur-
faces. This kind of surface roughness maximizes the
interlocking of bone and implant. Pratap and Patra
indicated that semi-hemispherical micro-dimpled sur-
face is the most appropriate one among different micro-
textured surfaces for hip joint prosthesis due to lower
friction and higher hydrophilicity.38
In total, the second
type of surface roughness is the most effective type on
the osseointegration.39,40
The nanoscale of surface roughness is the third one
which affects the rate of osseointegration.28
In this
research, 3D-EVT is applied to generate the texture on
the surface of Ti-6Al-4V alloy. Figure 2 shows a sample
of surface topography textured by 3D-EVT operation.
It is seen that a hierarchical structure is generated on
the surface, including all three above-mentioned cate-
gories. In particular, feed channels, micro-dimples (pro-
duced by 3D elliptical vibrations), and nano-attributes
(produced by the overlapping of micro-dimples) belong
to the macro-, micro-, and nanoscales, respectively.
Moreover, it is ascertained from Figure 2 that the gen-
erated micro-dimples have hemispherical/elliptical
shape. Therefore, it is proved that 3D-EVT process can
efficiently be used as a method of producing hierarchi-
cal textures on the surface of dental implants where the
secondary processes required to reach the desired sur-
face attributes are eliminated. It is clear that different
shape of micro-dimples could be generated if vibratory
parameters (such as amplitude) are changed.30
Experimental design and measurement
Full-factorial design is used to investigate the wettabil-
ity of the surfaces. With respect to Table 1, three levels
were selected for spindle speed and feed rate where the
depth of cut was constant.
To measure the surface roughness in feed (Raf
) and
cutting speed (Rac
) directions, a roughness tester is
applied (Mahr device with 2.5 mm cutoff length and
0.5 mm/s tracing velocity).41
To evaluate the surface
textures, a vision measuring microscope (VMM) is
used.
To measure the contact angle of the droplets, a CA-
ES10 measurement device is utilized (applying sessile
droplet method; seen in Figure 3).42,43
By the imple-
mentation of half angle method, contact angle is
obtained based on equation (1)
u = 2arctan
H
L

ð1Þ
where the radius and height of the droplet are L and H,
respectively. Measurements are carried out in two direc-
tions (feed (uf) and cutting speed (uc)) to investigate the
Figure 2. Hierarchical surface topography generated by 3D-EVT process (N = 500 r/min and f = 0.24 mm/rev).
Table 1. The cutting conditions (spindle speed (N), feed rate
(f), and depth of cut (ap)).
Cutting parameters Unit Value
N r/min 355, 500, 710
f mm/rev 0.08, 0.16, 0.24
ap mm 0.2
Sajjady et al. 3

contact angle. The anisotropic wettability (Du) is esti-
mated by using equation (2)
Du = uf À uc ð2Þ
More hydrophilic surface is observed by the lower
value of Du, resulting in bone growth and isotropic
micro-bonds in different directions.
As known, hydrophilicity is the tendency of the
liquid to spread on a solid surface. Based on Young
theory, the angle between (horizontal) solid surface and
tangential line on the liquid–vapor interface is called
real contact angle (uReal) (Figure 4). In equation (3), R
and uIdeal are the roughness ratio (equation (4)) and the
angle between the liquid and the solid surfaces,
respectively
cos uReal = R cos uIdeal ð3Þ
The additional surface area generated by the texture
is calculated by Sdr parameter
R = 1 +
Sdr
100
ð4Þ
Sdr =
Texture surface area À Cross sectional area
Cross sectional area
ð5Þ
The micro- or nano-textures cause the Sdr parameter
to be increased, resulting in an increment in roughness
ratio (R). Consequently, the real contact angle (uReal) is
reduced (an increment in hydrophilicity).
Mean summit curvature (SSC) is another effective
parameter on hydrophilicity. It is related to the peaks
and valleys of the surface. It means that the smoother
surface gives the higher wettability. In the following,
the influence of parameters on surface roughness and
hydrophilicity (which in turn affects osseointegration)
is discussed, and finally the effect of these parameters
on anisotropic wettability (Du) (which in turn affects
the isotropy of bone growth and micro-bonds in differ-
ent directions) is studied.
Results and discussion
The effective parameters on roughness and
hydrophilicity
To evaluate the effect of input parameters (cutting
operation, feed rate, and spindle speed) on surface
roughness and average contact angle (wettability),
ANOVA was applied using Minitab software. Tables 2
and 3 represent the percentage of contribution for each
particular parameter on surface roughness (Raf
) and
average contact angle (uf) in feed direction. It is ascer-
tained from the tables that feed rate with 38.04% effec-
tiveness has the most effect on Raf
where spindle speed
and type of cutting process are in the next levels, respec-
tively. These results are repeated for the average contact
angle, as the feed rate with 39.55% effectiveness has the
most impact.
Furthermore, Tables 4 and 5 show the percentage of
contribution for each particular parameter on surface
roughness (Rac
) and average contact angle (uc) in cut-
ting speed direction. It is seen that spindle speed with
51.47% effectiveness has the most effect on Rac
where
type of cutting process and feed rate are in the next lev-
els, respectively. The spindle speed with 45.82% effec-
tiveness has the most impact on the average contact
angle.
Effect of the process on surface roughness and
hydrophilicity. As it is given in Figures 5 and 6, the type
and effect of cutting process (CT and 3D-EVT) are
evaluated. It is extracted that adding 3D elliptical ultra-
sonic vibration to cutting process of Ti-6Al-4V caused
surface roughness to be reduced reaching the desired
range for better osseointegration. A decrement in sur-
face roughness can be explained by work hardening of
Ti-6Al-4V alloy and the lower temperature in tool-chip
Figure 3. The measurement device of contact angle.
Figure 4. Contact angle of water droplet on the (a) ideal and
(b) real surfaces.

cutting zone. These two parameters result in the lower
formation of built-up edge, which causes the tool cut-
ting edge to be kept sharp. Note that harmonic motion
of cutting tool in vibration cutting produces cooling
cycle during each disengagement time which reduces
the temperature.44,45
It is known that sharpness of cut-
ting edge generates thinner deformed chip where shear-
ing action is happened easier.46
In such a condition,
lower surface roughness is produced. Besides, harmonic
motion of cutting insert causes the cutting forces to be
reduced in vibration cutting, resulting in more stable
cutting process.47–49
In addition, 3D-EVT method
changes the natural deformation of the chip. Therefore,
it prevents from the generation of long snarled chips
(seen in CT) which twist around the workpiece reduc-
ing the surface quality. Figure 7 illustrates the chips.
Figure 8 shows the droplets after dropping on the
surfaces using contact angle measurement device. With
the help of this test, the hydrophilicity of machined sur-
faces can be defined accurately. In this figure, the
results of contact angles in three levels of cutting condi-
tions, which is proportional to Figure 10(a), are illu-
strated. The complete results of cutting angles are
shown in Figures 9 and 10 by graphs. In these two fig-
ures, the surface hydrophilicity is compared by the
analysis of contact angles in two directions: feed (uf)
and cutting speed (uc). It is seen that 3D-EVT method
properly reduced the contact angles. It means that this
Table 2. ANOVA results of surface roughness in feed direction (Raf ).
Source df Sequential SS Adjusted MS F p Contribution (%)
Types of cutting process 1 0.4487 0.4487 0.44 0.501 10.53
f (mm/rev) 2 1.6204 0.8102 8.01 0.006 38.04
N (r/min) 2 1.3768 0.6884 6.81 0.011 32.32
Error 12 0.8137 0.1011 – – 19.11
Total 17 4.2596 – – – 100
ANOVA: analysis of variance; df: degrees of freedom; SS: sum of squares; MS: mean of squares.
Table 3. ANOVA results of average contact angle in feed direction (uf ).
f (mm/rev) 2 875.64 437.82 22.37 0.000 39.55
N (r/min) 2 600.48 308.24 15.75 0.001 27.15
Error 12 234.89 19.57 – – 10.59
Total 17 2213.80 – – – 100
Table 4. ANOVA results of surface roughness in cutting speed direction (Rac).
f (mm/rev) 2 0.14424 0.0721 0.52 0.606 10.34
N (r/min) 2 0.71829 0.3591 8.47 0.005 51.47
Error 12 0.20859 0.0343 – – 14.94
Total 17 1.39546 – – – 100
Table 5. ANOVA results of average contact angle in feed direction (uc).
f (mm/rev) 2 424.09 212.05 13.77 0.001 24.7
N (r/min) 2 786.63 393.31 32.05 0.000 45.82
Error 12 121.37 15.39 – – 7.07
Total 17 1716.82 – – – 100
Sajjady et al. 5

method causes titanium alloy to be more hydrophilic,
which results in better osseointegration. As mentioned
in the ‘‘Experimental preparation’’ section, the addi-
tional surface areas generated by micro- or nano-
textures increase the Sdr and finally the R parameters.
Therefore, it is shown that 3D-EVT is able to increase
these parameters causing contact angle decreases.
For better explanations, Figure 11 is prepared to
show the surface of experimental specimens after
machining process. The illustrations are taken by
VMM device. This figure is represented in different
feed values where spindle speed is at the middle during
CT and 3D-EVT. It is clearly seen that the 3D elliptical
ultrasonic vibration eliminates the usual peaks and val-
leys generated on the surfaces during CT. It proves that
Figure 5. Surface roughness in feed direction (Raf
): (a) at different feed rates (when spindle speed = 355 r/min) and (b) at different
spindle speeds (when feed rate = 0.24 mm/rev).
Figure 6. Surface roughness in cutting speed direction (Rac
): (a) at different feed rates (when spindle speed = 355 r/min) and (b) at
different spindle speeds (when feed rate = 0.24 mm/rev).
Figure 7. Generated chips during (a) CTand (b) 3D-EVT.

why the surface roughness is reduced during 3D-EVT.
Furthermore, micro- or nano-textures are seen in the
microscopic illustrations, which indicate that additional
surface area is produced.
Effect of cutting parameters on surface roughness and
hydrophilicity. Figures 5(a) and 6(a) show the effect of
feed variations on surface roughness obtained during
CT and 3D-EVT in two directions. Accordingly, a
direct relation exists, in a way that, by a decrease in
feed value, the surface roughness decreases in both
directions (Raf
and Rac
), while there is a reverse relation
between surface roughness and spindle speed. In accor-
dance with Figures 5(b) and 6(b), surface roughness is
reduced in both directions by an increase in the spindle
speed.
Apart from the variation effects of cutting para-
meters on average contact angles are also investigated
in Figures 9 and 10. It is concluded that contact angle
has a reverse relation with feed rate and spindle speed
in both directions (uf and uc). In particular, the depth
of feed channels increases by an increase in feed values
causing the spread of more dimples on the surface.
Therefore, an increment in Sdr factor increases surface
tension that exists between droplets and machined sur-
face. In this condition, cutting angle is reduced (wett-
ability increases), which positively affects the
osseointegration. By variation of spindle speed, SSC fac-
tor increases (mean summit curvature). An increment in
this factor indicates that the surface is smoother, which
helps the wettability of the surface to be more.
The effective parameters on anisotropic wettability
Based on ANOVA, effect of input parameters on aniso-
tropic wettability (Du) is investigated. Table 6 shows
that type of cutting process with 63.22% contribution is
Figure 8. The average contact angles in the direction of feed (uf ) and cutting speed (uc): (a) f = 0.08 mm/rev, (b) f =0.16 mm/rev, (c)
f =0.24 mm/rev, and (d) the directions shown on a specimen.
Sajjady et al. 7

the most effective factor on anisotropic wettability. In
other words, 3D elliptical vibrations cause the cutting
process to be changed, significantly, where the effects of
feed and spindle speed are very low.
As the tool vibration is carried out in all three
directions, the generated dimples are semi-spherical in
which a regular surface is generated at different direc-
tions. Thus, the anisotropic wettability is reduced
compared to the surfaces produced by CT method
(Du = uf À uc) . As mentioned earlier, the isotropy of
bone growth and osseointegration increase by a
decrease in anisotropic wettability. The comparison of
Du is given in Figure 12.
Concluding remarks
In this work, 3D-EVT method was used to generate
hierarchical structure (nano- or micro-textures), which
increases the surface hydrophilicity of titanium alloy
(Ti-6Al-4V). Accordingly, the effect of cutting para-
meters (during CT and 3D-EVT processes) on surface
roughness and contact angle has been analyzed. The
main achievements are as follows:
1. The 3D-EVT process can be used as a method for
the creation of micro- or nano-textures on the implant
surfaces to obtain all desired features. By using 3D-
EVT method, the secondary processes required to
reach the desired surface attributes are eliminated.
2. The 3D elliptical vibration causes the surface
roughness to be reduced obtaining the proper
range needed for an appropriate osseointegration.
It was due to intermittent movement of cutting
tool in vibration cutting which causes the cutting
forces to decrease, resulting in the more stable cut-
ting process.
Figure 10. Contact angle in cutting speed direction (uc): (a) at different feed rates (when spindle speed = 355 r/min) and (b) at
different spindle speeds (when feed rate = 0.24 mm/rev).
Figure 9. Contact angle in feed direction (uf ): (a) at different feed rates (when spindle speed = 355 r/min) and (b) at different
spindle speeds (when feed rate = 0.24 mm/rev).

Figure 11. Surface topographies generated during CTand 3D-EVT.
Table 6. ANOVA results of anisotropic wettability (Du).
f (mm/rev) 2 31.409 15.705 3.27 0.073 16.78
N (r/min) 2 12.672 6.336 0.76 0.489 6.77
Error 12 24.774 3.731 – – 13.23
Total 17 187.185 – – – 100
Sajjady et al. 9

3. The additional surface area achieved by generating
micro- or nano-textures increases Sdr and R para-
meters. Therefore, it was proved that 3D-EVT is
able to increase these parameters causing contact
angle decreases. This condition increases hydrophi-
licity in both feed and cutting speed directions.
4. Generation of regular and integrated textures on
the surfaces during 3D-EVT causes the anisotropic
wettability to reduce compared to the surfaces pro-
duced by CT method. As a result, the isotropy of
bone growth and osseointegration are enhanced.
Consequently, it can be noted that 3D-EVT method
is able to increase the osseointegration by the genera-
tion of additional surface area coupled with the reduc-
tion of surface roughness.
Declaration of conflicting interests
The author(s) declared no potential conflicts of interest
with respect to the research, authorship, and/or publi-
cation of this article.
Funding
The author(s) received no financial support for the
research, authorship, and/or publication of this article.
ORCID iD
Alireza Bagheri Bami https://orcid.org/0000-0002-
3077-5099
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