This article has been accepted for publication in a future issue of this journal, but has not been fully edited. Content may change prior to final publication. Citation information: DOI 10.1109/LRA.2017.2717082, IEEE Robotics
and Automation Letters
IEEE ROBOTICS AND AUTOMATION LETTERS. PREPRINT VERSION. ACCEPTED JUNE, 20171

Surface Texture of Deformable Robotic
Fingertips for a Stable Grasp under both Dry and
Wet Conditions
Kaori Mizushima1, Toshihiro Nishimura1, Student Member, IEEE, Yosuke Suzuki2, Member, IEEE
Tokuo Tsuji2, Member, IEEE, and Tetsuyou Watanabe2, Member, IEEE
Abstract— This study investigates the effect of the surface
texture of soft deformable fingertips on the maximum resistible
force under dry and wet conditions, and proposes a new hybrid
structure that provides a stable grasp under both conditions. One
definition of stable grasp is the capability of balancing a large
external force or moment while grasping. For soft fingertips, boththe friction and surface deformation contribute to the stability.
Therefore, we investigate the maximum resistible force, which isdefined as the maximum tangential force at which the fingertip
can maintain contact when applying and increasing thetangential/shear force. We investigate the slit textures withprimitive patterns and demonstrate that the non-pattern
performs the best under dry conditions, whereas the horizontal
slit pattern performs the best under wet (oily) conditions. Basedon this, a concentric hybrid texture of the two patterns is proposed,
and its effectiveness is verified by a grasping test.
Index Terms— End effectors, Friction, Grasping, Lubrication,
Rough surfaces, Surface textureR

I. INTRODUCTIONOBOTS are entering the human environment and thedemand for them to substitute manual daily work isincreasing. The robotic hand is the key tool to conduct the work,and there are numerous requirements. One important
requirement that has not yet been extensively explored is thegrasping under wet conditions. Note that in this paper, wet isdefined as “wet with water, oil, or other types of liquids.” Water,oil, and other liquids are media that are used daily, and thus thegrasping of objects under wet conditions is common in daily
life. However, conventional robotic hands implicitly assume
application under dry conditions, with the exception of a few
studies [1]–[3]. This study challenges the stable grasping of
Manuscript received: February, 15, 2017; Revised May, 10, 2017; Accepted
June, 1, 2017.
This paper was recommended for publication by Editor Han Ding uponevaluation of the Associate Editor and Reviewers’ comments. This work waspartly supported by JSPS KAKENHI Grant Number 16H04298.
1
K. Mizushima and T. Nishimura are with the Graduated school of Naturalscience and Technology, Kanazawa University, Kakuma-machi, Kanazawa,
9201192 Japan (e-mail: k_mizushima@stu.kanazawa-u.ac.jp).
2
Y. Suzuki, T. Tsuji, and T. Watanabe are with the Faculty of MechanicalEngineering, Institute of Science and Engineering, Kanazawa University,
Kakuma-machi, Kanazawa, 9201192 Japan (e-mail: te-watanabe@ieee.org).
Digital Object Identifier (DOI): see top of this page.

objects irrespective of surface conditions, focusing on thesurface structure of robotic fingertips.
A. Soft surface
Robotic hands with soft surfaces are becoming more
attractive [2], [4]–[8]. Fluids are easily adaptable to external
environments, thus, they have thus been utilized to compose
fingertips. In particular, pneumatic systems are often used [9]–[19]. Liquids are also candidate materials for soft fingers [20]–[24]. The soft surface provides a low contact impact, safeinteraction between the object and environment, high friction,
and adaptation to the object shape. These benefits are attributed
to the surface deformation, which is also critical to developing afingertip surface structure to cope with both dry and wet
conditions. The surface deformation can change the surface
structure and then facilitate the embedding of multiple
functions on the surface. In particular, the deformation is higher
in fluid fingertips composed of liquids [20]–[23] than thosecomposed of the other materials such as air or silicone rubber.
With this in mind, this study focused on soft/fluid robotic
fingertips.
One definition of stable grasp is the capability of balancing
large external forces or moments during grasping. In soft
fingertips, both friction and surface deformation contribute to
the stability. Therefore, we define the maximum resistible force
as the maximum tangential force at which the fingertip canmaintain contact when applying and increasing thetangential/shear force [25], [26]. Friction on soft surfaces andthe effects of surface deformation have been investigated by
Kao et al. [27]–[31], Hirai et al. [32] [33], Ciocarlie et al. [34],and Watanabe et al. [25], [26], [35]. However, theseinvestigations were conducted for dry conditions. Kitchen
robots are an example of coping with wet conditions [1], [36]–[39], as are robotic hands for underwater manipulation [2] [3].
The IRT Research Initiative developed a kitchen robot that
could wash dishes [1], where a thick glove prevented therobotic hand from becoming wet. Other kitchen robots utilized
kitchen tools and avoided the necessity of dealing directly withwet conditions. However, grasping oily foodstuffs or dishes
and stowing wet cups are common in daily life, and grasping
objects under wet conditions is worthy of investigation.
Robotic hands that function underwater have been developed[2] [3], but the grasping forces were provided by fluid pressure

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This article has been accepted for publication in a future issue of this journal, but has not been fully edited. Content may change prior to final publication. Citation information: DOI 10.1109/LRA.2017.2717082, IEEE Robotics
and Automation Letters
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IEEE ROBOTICS AND AUTOMATION LETTERS. PREPRINT VERSION. ACCEPTED JUNE, 2017control, and the effects of friction and the deformation during
grasping were not investigated.
B. Surface structure to avoid slippage
Some robots have a special surface to avoid slippage.
Dadkhah et al. developed a gripper using an electrostatic andgecko-like adhesive [40]. Hawkes et al. developed a thin film
resembling the leg surface of a gecko [41] and utilized the film
to construct a gripper. However, wet conditions were notconsidered.
Friction under wet conditions has been investigated in thetribology field. Tsipenyuk et al. [42] and Varenberg et al. [43]showed that the hexagonal surface increases drainage
performance and improves friction on surfaces under oily
conditions. Drotlef et al. [44] developed a material with anuneven surface resembling a frog leg to increase friction under
wet conditions. It is well-known that commonly-used shoesoles and car tires have grooves to drain rainwater and increase
friction under wet conditions [45], [46]. As pointed out in [47],[48], even in the tribology field, no research has been
conducted providing a design criterion for surface
patterns/textures, and the individual design issue must be basedon empirical knowledge. Thus, in this study, we experimentallyinvestigate the surface patterns to produce a stable grasp underboth wet and dry conditions.
C. Contribution
This study aims to develop a novel surface texture for the
deformable fluid fingertip [20]–[23] (Fig. 1), which provides
high resistible forces under both dry and wet conditions. The
main contributions of this study are as follows:• We investigated the effect of the surface textures on theresistible force and showed which textures provide a large
resistible force under dry and wet conditions.
• Based on the investigation, a novel surface texture thatprovides large resistible forces under both dry and wet
conditions is presented.
The rest of this paper is organized as follows. The subsequent
section presents the results of the preliminary experiments.
Based on these results, the investigations of the slit texture
patterns are then described. Based on the investigation, hybrid
texture patterns are proposed and evaluated. Lastly, thesummary is presented.

area was 35 mm 35 mm and the texture was provided by asilicone sealant (Hapio seal pro, Kanpe Hapio), which deforms
according to the expansion and contraction of the rubber film.
The mold for the texture pattern was built by a 3D printer(Stratasys uPrintSE). After placing the mold on the rubber film,
the silicone sealant was applied. This was the fabrication
process.
III. INVESTIGATION OF PRIMITIVE TEXTURE PATTERNS
We investigated the effect of primitive texture patterns on the
maximum resistible force. Fig. 3 displays the investigatedtexture patterns. The following primitive patterns wereselected: non-pattern, vertical and horizontal grooves, anddot-pattern. Here, the terms “vertical” and “horizontal” refer tothe patterns constructed by lines parallel and perpendicular tothe loading direction, respectively. The interval distance
between grooves or dots was set to 1 mm to correspond to theresolution of the 3D printer. The target object grasped was asilicon cube with an edge of 35 mm, mass of 30.4 g, andhardness of 50 (Shore A). The flat surface was selected because
it is suitable for maintaining a uniform wet condition in thegrasp area and for the investigation of resistible force.

Fig. 1. Fluid fingertip
with surface texture

Fig. 3.

Fig. 2.

Basic structure of the fluid fingertip

Surface texture patterns for the investigation

II. BASIC STRUCTURE OF FINGERTIP
A. Fluid fingertip
In this study, the fluid fingertip illustrated in Fig. 2 wasutilized, and a surface texture was embedded and examined.
The fingertip base is a rounded square with a width of 45 mm,with two holes connected to a pump and pressure sensor(Keyence AP-12S). A nitrile rubber film covers the base andwas filled with a compressive fluid, chain saw oil (ISO VG100).
The pressure sensor senses the fluid pressure, which iscontrolled by the pump.
B. Surface texture
A silicone texture was coated on the rubber film. The coatedFig. 4.

Schematic view of experimental setup for resistible force test

A. Experimental setup and method
Fig. 4 shows the experimental setup when utilizing the fluid
fingertips. The resistible force was investigated by applying a
vertical load while grasping the target object. One fingertip was
fixed by the vise, while the other was fixed to the handmade
parallel-beam structure-based load cell [21] (allowable load: 10
N; resolution: 0.01 N) attached to the horizontal automatic
positioning stage (Oriental motor ELSM2XF030K). The force
gauge (IMADA DS2-50N) was fixed to the vertical automatic
positioning stage (IMADA MX2-500N) that moves in thevertical direction to apply a load on the grasped target object.

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This article has been accepted for publication in a future issue of this journal, but has not been fully edited. Content may change prior to final publication. Citation information: DOI 10.1109/LRA.2017.2717082, IEEE Robotics
and Automation Letters
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MIZUSHIMA et al.: SURFACE TEXTURE OF DEFORMABLE ROBOTIC FINGERTIPS FOR A STABLE GRASP UNDER BOTH DRY AND WET CONDITIONS

C. Fluid fingertip
The surface of a fluid fingertip can deform. It is thus
expected that the deformation can increase the contact area andresistible force. The deformation can also increase or decrease
the grooves on the textures, the effect on the resistible force of
which is unclear. This was investigated in this study. The
experiment was conducted using fluid fingertips with the sametexture patterns at the surface. The fluid pressure was 4 kPa.
The results are presented in Fig. 8.

5
Non‐pattern
Vertical grooves
Horizontal grooves
Dot‐pattern

4
3
2
1
0

Dry

Oily

Fig. 5. Maximum resistible
force when grasping a silicone
cube by the rigid and flat plates
under dry and oily conditions

Fig. 7.

Maximum
resistible force [N]Maximum resistible force [N]

B. Rigid and flat plate fingertip
To observe the effect of the texture patterns only on friction,
we conducted a preliminary experiment where rigid and flat
plates were utilized for the fingertips instead of the fluid
fingertips shown in Fig. 4. Because there is no surface
deformation, the resistible force is identical to the frictional
force.
Fig. 5 displays the results. Under the dry condition, thenon-pattern provided the largest maximum resistible force,
while the other patterns provided smaller forces. In contrast,
under the oily condition, the non-pattern provided the smallest
maximum resistible force. It is observed that the oily condition
decreased the friction and the texture patterns reduced thisdecrease.
Fig. 6 shows the relationship between the apparent contact
area and maximum resistible force. The apparent area is aconvex area of 30 mm 30 mm, which is depicted by the red
area in Fig. 7. The maximum resistible force is observed toincrease with the apparent contact area. We considered that theoily condition provides a fluid film at the contact area,
decreasing the friction, but the texture could reduce the friction
decrease because of the grooves. The grooves could reduce thethickness of the fluid film at the convex area of the texture. The
decrease in fluid film thickness corresponds to the increase infriction from Newton’s law of viscosity.

Under the dry condition, we obtained the same tendency as
the previous experimental results; the maximum resistible force
increased with the apparent contact area. However, thedifference in maximum resistible force between the non-pattern
and the other patterns was large. The horizontal-grooves anddot patterns have cavities/hollows or breaks in the loading
direction. A local large bending deformation occurs because of
the small second moment of area, and the resistible force thus
decreases [25], [26]. Therefore, the resistible forces for the twopatterns were small.
Under the oily condition, the fingertips with the non-patternfailed to grasp, while the fingertips with the other patterns
succeeded, although the maximum resistible force was small.
Slippage was prevented by the textures.

Maximum resistible force [N]First, the object was placed on a stage between the fingertips,
and the horizontal automatic positioning stage was controlled
such that the object was grasped with a grasping force of 1 N.
The stage under the object was removed after grasping. The
vertical automatic positioning stage for loading was controlled
such that the force gauge could contact the grasped object witha load of 0 N. This was the initial state. From the initial state,
the vertical load was increased by moving the vertical stage
slowly (at 50 mm/min) until the object dropped. The maximum
value of the load is referred to as the maximum resistible force
in the given experimental condition. The experiment was
carried out four times for each texture pattern. Dry and oily
conditions were examined. Water was not utilized, because its
viscosity is too low to maintain uniform and constantconditions. It should be noted that from elastohydrodynamic
lubrication (EHL) theory (see Appendix), the oily condition(thickness of the oil film) can be kept constant if the supplying
oil is sufficient and the grasping force is constant. To produce
the dry condition, we cleaned the surface with ethanol andwaited until the ethanol volatilized. Namely, the dry and oily
conditions were kept constant through all investigations.

5

Non‐pattern
Vertical grooves
Horizontal grooves
Dot‐pattern

4
3
2
1
0
0

250

500

750

1000

Apparent contact area [mm2]Fig. 6. Relationship between theapparent contact area and maximum
resistible force when grasping asilicone cube by the rigid and flat
plates under dry conditions

Schematic view of the apparent contact area for each texture
pattern10

Non‐pattern
Vertical grooves
Horizontal grooves
Dot‐pattern

9
8
7
6

※ND：No Data

5
4
3
2
1
0

ND

Dry

Oily

Fig. 8. Maximum resistible force
when grasping a silicone cube by thefluid fingertips under dry and oily
conditions

Fig. 9. Arrangement change byincreasing fluid pressure

IV. INVESTIGATION OF SLIT TEXTURE
The preliminary experiments suggested that
 Under the dry condition, the maximum resistible force
increased with the apparent contact area.
 Under the oily condition, the drainage effect of the surface
texture pattern is useful for grasping.
 The increases in both apparent contact area and drainage
ability are preferable for stable grasping under bothconditions.

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This article has been accepted for publication in a future issue of this journal, but has not been fully edited. Content may change prior to final publication. Citation information: DOI 10.1109/LRA.2017.2717082, IEEE Robotics
and Automation Letters
IEEE ROBOTICS AND AUTOMATION LETTERS. PREPRINT VERSION. ACCEPTED JUNE, 2017

A. Effects of fluid pressure and slit pattern
The fluid pressure of the fluid fingertips affects the stiffnessand deformability, and thus the arrangement or pitch of the
texture pattern. If the fluid pressure is low and, therefore,
fingertip stiffness, surface extension, or bulge are small, the slit
expansion will be small and the apparent contact area will belarge. Then, a large resistible force is expected under the dry
condition; however, a small resistible force is expected under
the oily condition. If the fluid pressure is high, antipodal results
are expected. Therefore, we investigated the effects of the fluidpressure as well as the slit patterns. In order to preserve thedeformability function of the fluid fingertip, a low thickness ofthe silicone sealant is required. Thus, the range of changeable
thicknesses is narrow, and the thickness was set to (constant) 1
mm. If the depth of the slits is shorter than the thickness, thedeformability of the fluid fingertip cannot be utilized and thedrainage effect can be too low. Thus, the depth was set to thesame value as the thickness. The interval between the slits was
set to 2 mm. The available range of fluid pressure was 2–8 kPa.
Fluid pressures of 2, 4, 6, and 8 kPa were then investigated.
Fig. 11 presents the results. First, the results with a fluidpressure of 4 kPa were compared with the results shown in Fig.
8. The non-pattern exhibited a similar performance. The
remaining slit patterns had larger resistible forces than thegroove patterns under both conditions. These results indicatethat the slit textures exhibit the drainage ability.
The comparison of the patterns under dry conditions
indicates that the non-pattern texture provided the largest
maximum resistible forces, followed by the vertical slit texture.
There is no break in the loading direction in these patterns,
providing large contact areas. In contrast, there are breaks in theloading direction in the horizontal and grid slit patterns. The
load increased the deformation of the texture pattern and thebreaks in the loading direction, and decreased the contact area,
which corresponds to a decrease in resistible force. Fig. 12 (a)shows the schematic view of the hypothesis of the phenomenon
where the contact area was limited to that around the corners of
the breaks.
Under oily conditions, the grasp failed with the non-patterntexture, and the drainage effect of the slit texture patterns was
confirmed. For a fluid pressure of 2 kPa, the horizontal slits
provided an extremely high maximum resistible force. The grid
texture also provided a large maximum resistible force. It isconsidered that no break resulted from oil lubrication (whichindicates a large R in (1) of the Appendix: EHL theory), and theresistible force decreased. The breaks were effective at
reducing the lubrication (which indicates small R in (1)),
particularly the breaks in the loading direction. As shown in Fig.
12 (b), it is considered that the corners of the breaks reduce theoil layer and the contact area is similar to that under the dry
condition.
The lowest fluid pressure (2 kPa) performed the best underboth conditions. At low fluid pressures, the deformation is large
and large contact areas were obtained. The large slit expansion
due to the high fluid pressure indicates a large R in (1), whichcorresponds to a low friction. These are considered the reasonsfor the best performance at the lowest fluid pressure.

Fig. 10. Slit texture patterns
12

Non‐pattern
Horizontal slits

11
10

Vertical slits
Grid

9
8
7
6
5
4
3
2
1
0

2 kPa

4 kPa

6 kPa

8 kPa

Maximum resistible force [N]To achieve a stable grasp regardless of the condition, atexture pattern that provides a large apparent contact area andreduces the oil lubrication effects on the resistible force ispreferable. Here, we focus on the deformation of the fluid
fingertip. The elastic film deforms by the change in fluidpressure or external load. This deformation can change theroughness. Based on this, a slit texture pattern was selected (seeFig. 9). The increase in fluid pressure expands the slit and thegroove texture is obtained, as shown in Fig. 9. The apparent
contact area is larger than the textures with grooves (Fig. 3). A
slit pattern was fabricated by cutting the silicone sealant, after
coating the rubber film with the silicone sealant. The primitive
slit texture patterns displayed in Fig. 10 were selected andinvestigated for easy fabrication and to understand the effects
of different slit directions.
It is noted that the fluid fingertip provides uniform contactpressure, and the pressure is very low. Thus, an increase in thegrasping force is less effective at increasing the ratio of actual
to apparent contact area (actual contact area is proportional tofrictional force) than increasing just the apparent contact area.
If the grasping force is changed, the obtained values of the
maximum resistible force will be different, whereas the relative
trend in the differences between the different conditions will besimilar. We thus conducted the investigations while fixing thegrasping force value to 1 N.

Maximum resistible force [N]412

Non‐pattern
Horizontal slits

11
10

Vertical slits
Grid※ND：No Data

9
8
7
6
5
4
3
2
1
0

ND

ND

2 kPa

(a) Dry condition

4 kPa

ND

6 kPa

ND

8 kPa

(b) Oily condition

Fig. 11. Maximum resistible force for various fluid pressures and various
slit texture patterns(a) Dry condition(b) Oily condition
Fig. 12. Hypothesis of local surface deformation for the horizontal and grid
slit texture patterns

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This article has been accepted for publication in a future issue of this journal, but has not been fully edited. Content may change prior to final publication. Citation information: DOI 10.1109/LRA.2017.2717082, IEEE Robotics
and Automation Letters
MIZUSHIMA et al.: SURFACE TEXTURE OF DEFORMABLE ROBOTIC FINGERTIPS FOR A STABLE GRASP UNDER BOTH DRY AND WET CONDITIONS(a) Before applying load(b) After applying load

Fig. 14. Contact area of the non-pattern before and after applying a load
under the dry conditionchanged with the loading. In contrast, at the horizontal slits (Fig.
15), the upper edge area of each segment maintained contact
only after applying the load, which confirms the hypothesis
shown in Fig. 12 (a). Figs. 16 and 17 present the results under
the oily condition, where the black area corresponds to thecontact area. With the non-pattern (Fig. 16), the contact area
was covered by the oil film, and no clear contact area wasobserved when loading. With the horizontal slits (Fig. 17), oilstands (brown colored area) was observed at the breaks/slits,
and the contact area around the upper edge of each segment
between the slits expanded downwards with the increase inloading. These results confirm the hypothesis shown in Fig. 12(b).
Maximum resistible force [N]Fig. 13. Schematic view of the experimental setup for observing thecontact area

5

8
7
6

1.0 mm
2.0 mm
3.0 mm

1.5 mm
2.5 mm
3.5 mm

5
4
3
2
1
0

Dry

Oily

Fig. 18. Maximum resistible force for various slit intervals; the horizontal
slit texture pattern with a fluid pressure of 2 kPa was evaluated under dry andoily conditions(a) Before applying load(b) After applying load

Fig. 15. Contact area of the horizontal slits before and after applying aload under the dry condition(a) Before applying load(b) After applying load

Fig. 16. Contact area of the non-pattern before and after applying a load
under the oily condition; the oil is colored in red(a) Before applying load(b) After applying load

Fig. 17. Contact area of the horizontal slits before and after applying aload under the oily condition; the oil is colored in red.

B. Observation of contact area
To confirm the hypothesis shown in Fig. 12, the contact area
was observed. Fig. 13 shows the experimental setup. The
fingertip with the non-pattern and horizontal slit texture
patterns was pressed against a transparent acrylic plate with aload of 1 N and fluid pressure of 2 kPa. The load was controlled
by the same system as that shown in Fig. 4. The contact area
was observed by a microscope (Keyence VHX-2000). We
investigated dry and oily conditions.
Figs. 14 and 15 present the results under the dry condition
where the black area corresponds to the contact area. With thenon-pattern (Fig. 14), the component of the contact area hardly

C. Effect of slit interval
The effect of the slit interval was evaluated using thefingertip with the horizontal slit texture pattern and a fluidpressure of 2 kPa, which was the most effective for drainage, as
shown in Fig. 11 (b). The range of evaluated intervals was 1.0–
3.5 mm. The remaining experimental conditions were as
described in section IV.A. Fig. 18 shows these results. The
maximum resistible force increased with the decrease ininterval under both conditions. As described in the previous
subsection, under the dry condition, the main contact area is theupper edge area of each segment and the increase in contact
area increases the friction when the contact pressure is low. If
applying EHL theory (see Appendix) to the cases for the oily
condition, the interval corresponds to the equivalent contact
radius R, and the other parameters are regarded as constant. A
small R produces a thin fluid film, which leads to a large
friction. It should be noted that if the interval is too small, theslit segment will not be able to resist the tangential shear
force/stress, because of the low rigidity/stiffness in the bending,thus, it flexes and the friction decreases [25], [26]. Therefore,
the performance at the interval of 1.0 mm was lower than
expected, especially under the oily condition. The otherinteresting point is that the maximum resistible force under theoily condition is larger than that for the dry condition, except
for the interval of 1.0 mm. In summary, the interval of 1.5 mm
performed the best in the range investigated. The interval
should be minimized within the range that the segment canmaintain its structure and not flex.
V. HYBRID SLIT TEXTURE PATTERN
To summarize the aforementioned investigations, Under the dry condition, the non-pattern performed the best.

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IEEE ROBOTICS AND AUTOMATION LETTERS. PREPRINT VERSION. ACCEPTED JUNE, 2017

 Under the oily condition, the horizontal slits performed thebest, and the maximum resistible force was larger than that
under the dry condition.
The slit interval of 1.5 mm and fluid pressure of 2 kPa
performed the best.
The maximum resistible force for the horizontal slits under
the dry condition was approximately 40% of that for the
non-pattern, which is too small. Therefore, we considered ahybrid texture pattern of the non-pattern and horizontal slits fora stable grasp under both dry and oily conditions. To determine
the structure, we reinvestigated the fingertip deformation in theexperiment shown in Fig. 4. In this case, we focused on the sideview (Fig. 19) and investigated how the interval of thehorizontal slits changed with the loading under the oily
condition. Following loading, the slits around the upper area
expanded, while the slits around the lower area compressed.
This change in size increased the area of the oil stands and R in(1) (given in the Appendix), and provided the increase in fluid
film thickness leading to the decrease in friction at the upperarea. The lower area improved at gripping because of antipodal
reasons. Therefore, the slits around the upper area did not
perform well.

Maximum resistible force [N]Fig. 19. Side view of the object grasped by the fingertip with the horizontal
slit texture patterns after applying a load under the oily condition;
investigation of how the slit interval changed with loading

Fig. 20. Fluid fingertip with ahybrid texture pattern of
non-pattern and horizontal slits

12
11
10
9
8
7
6
5
4
3
2
1
0

Non‐pattern
Horizontal 2.0 mm
Hybrid 2.0 mm

Horizontal 1.5 mm
Hybrid 1.5 mm※ND：No Data

ND

Dry

Oily

Fig. 21. Maximum resistible force for the
hybrid texture pattern considering a single
loading direction

A. Considering a single loading direction
Based on this, we propose the new slit texture pattern shownin Fig. 20 for a stable grasp under both dry and oily conditions.
It is a hybrid structure of the non-pattern in the upper area andhorizontal slit pattern in the lower area.
The effectiveness of the proposed hybrid texture pattern wasinvestigated by the same experiment shown in Fig. 4.
Considering the effect of the slit interval, we investigated thecases when the slit interval was 1.5 and 2.0 mm, which
demonstrated the best and second-best performances,respectively. For comparison, the non-pattern and horizontal
slits were also investigated. Fig. 21 shows the results where thehybrid texture with the slit interval of 2.0 mm showed thehigher performance of both the conditions. Under the oily
condition, similar performances were achieved with the

exception of the non-pattern. Under the dry condition, thehybrid texture with a slit interval of 2.0 mm exhibited anintermediate performance between the non-pattern andhorizontal slits. In contrast, at the interval of 1.5 mm, theperformance was nearly half of the one at the non-patterntexture. The non-pattern area for the hybrid texture is half of all
the area, and the contribution of the non-pattern area on theperformance was considered close to half of the performancefor the non-pattern texture. It is then considered that at theinterval of 1.5 mm, the slit part did not perform well for an
increase in resistible force, because of the low rigidity/stiffness
in the bending resulting from the small interval, indicated by
the analysis of the results in Fig. 18. The stress at the lower partwas larger than that at the upper part [50], and, therefore, theslit area with a relatively large interval was required for a good
performance in the hybrid texture.
B. Concentric hybrid texture pattern
The hybrid texture shown in Fig. 20 can only accommodate asingle loading direction. For real tasks, the grasping posture canbe changed by a robotic arm, and then the loads in arbitrary
directions should be considered. Here, we proposed theconcentric hybrid texture pattern displayed in Fig. 22 so that thepattern could be a local hybrid non-pattern and horizontal
pattern in any arbitrary direction on the contact surface. It is ahybrid structure of the non-pattern in the center area andcircular slits in the outer area.
The following fabrication procedure was used. First, a flat
surface made of silicone sealant was molded on apolypropylene sheet. Next, the surface was slit by a compass
cutter. Following this, a rubber film (fingertip surface) was
bonded to the slit surface. After solidification, the sheet wasremoved and the surface texture was obtained.
The effectiveness of the new texture was examined. The
diameter of the texture area was 35 mm, and the diameter of the
non-pattern area was set to 15 mm such that the contact area
includes the outer concentric slits while grasping the target
object (the silicone cube). Let ΔR be the slit interval for theouter concentric slits. We investigated the performances forvarious ΔR (1.0, 1.5, 2.0, 2.5, 3.0 mm). Fig. 23 shows theresults where the non-pattern texture was fabricated by thesame method as the concentric hybrid texture pattern. The
maximum resistible force increased with the slit interval under
the dry condition, while the force decreased under the oily
condition. At the interval of 1.0 mm, the performance under theoily condition was better than that under the dry condition. The
result indicates that the concentric texture performed well. At
the interval of 1.5 mm, the performance was inverted; theperformance under the oily condition was lower than that under
the dry condition. A similar trend was observed with the results
for the intervals of 1.5 and 2.0 mm in the case of the liner hybrid
pattern, as exhibited in Fig. 21. The results indicate that achange in arrangement to the concentric texture caused it toperform well (an increase in resistible force) in all directions,although the required slit interval for good performance wasdifferent. It should be noted that the fabrication process ofpressing the textures produced smoother surfaces, which led to2377-3766 (c) 2016 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications_standards/publications/rights/index.html for more information.

This article has been accepted for publication in a future issue of this journal, but has not been fully edited. Content may change prior to final publication. Citation information: DOI 10.1109/LRA.2017.2717082, IEEE Robotics
and Automation Letters
MIZUSHIMA et al.: SURFACE TEXTURE OF DEFORMABLE ROBOTIC FINGERTIPS FOR A STABLE GRASP UNDER BOTH DRY AND WET CONDITIONS

increases in friction and rigidity/stiffness in bending. The
different values were then observed with the results in Figs. 21
and 23, especially under the dry condition.

Maximum resistible force [N]C. Grasping test
To see the effectiveness of the concentric hybrid texture in
actual object grasping, a grasping test was conducted. The
target objects presented in Fig. 24 were selected. Considering
the actual situations, the target objects could be handled under
wet or oily conditions. It should be noted that silken tofu was
normally graspable by conventional fluid fingertips [20]–[23],but after leaving the tofu on a table for a few hours, the water
exuded and the friction decreased. Then, the conventional fluid
fingertips failed to grasp the tofu under the wet condition. Here,
we investigated the grasping under the wet condition. The othertarget objects were investigated under dry and oily conditions.
We also investigated various slit intervals of 1.0–3.0 mm. Table
1 summarizes the success rate of the grasping. The objects were
grasped successfully under the dry conditions. The smallintervals of 1.0 and 1.5 mm performed well under bothconditions. The other intervals showed the performanceassociated with the results shown in Fig. 23. The results
confirmed the validity of the concentric hybrid texture pattern.

Fig. 22. Concentric hybrid
texture pattern

30※ND：No Data25
20

Non‐pattern
1.0 mm
1.5 mm
2.0 mm
2.5 mm
3.0 mm

15
10
5

ND

0

Oily

Dry

Fig. 23. Maximum resistible force for theconcentric hybrid texture patternFig. 24. Target objects for grasping test

ObjectSilkenTofu

Size
353 mm3
35 g

mm
Bearing Φ35
40 g
mm
Ramekin Φ52
43 g

TABLE I
SUCCESS RATE OF GRASPING
Condition 1.0 mm 1.5 mm 2.0 mm 2.5 mm 3.0 mm
Wet

4/4

4/4

3/4

3/4

1/4

Dry
Oily
Dry
Oily

4/4
4/4
4/4
4/4

4/4
4/4
4/4
4/4

4/4
4/4
4/4
4/4

4/4
3/4
4/4
4/4

4/4
3/4
4/4
3/4

was postulated in some previous studies [25] [26]. This isrelated to the friction and fingertip deformation. Stable grasp
was defined as the capability of balancing large external forces
or moments during grasping. The texture patterns were
evaluated by the maximum resistible force. To attain a large
contact area that facilitates a large resistible force, as suggestedby the preliminary experimental results, we considered slit
texture patterns. Under the dry condition, the non-pattern
performed the best, whereas under the wet (oily) condition, thehorizontal slit pattern performed the best. Additionally, for thehorizontal slit pattern, the maximum resistible force under theoily condition was larger than that under the dry condition. A
slit interval of 1.5 mm and fluid pressure of 2 kPa showed thebest performance. Based on these results, the concentric hybrid
texture pattern was proposed for stable grasping under the dry
and wet conditions. The grasping test showed the effectivenessof the proposed pattern. Note that only the primary patterns
were investigated in this study. The optimization of the surface
pattern including the investigation of irregular intervals andshapes under various conditions and the investigations for theother types of fingertips will be studied in the future.
APPENDIX: ELASTOHYDRODYNAMIC LUBRICATION
A fluid film at the contact area can provide lubrication. If thecontact area is elastically deformable, the lubrication becomes
an elastohydrodynamic lubrication (EHL) [49], [50]. As thetargeted surface patterns were elastic, we introduced EHL
theory. Each segment of the patterns had a width with respect tothe sliding direction of the fluid, then, the Ertel–Grubin model[50], [51] was adopted assuming line contact (contact between
a flat surface and cylinder):
/
/
/
/
(1)
1.95
where is the film thickness, and are the parameters of
the Barus equation modeling the relationship between viscosityand pressure, is the sliding velocity, is the equivalent
Young’s modulus for two materials constructing the contact,is the width of the contact area, is the load (grasping force),
and is the equivalent contact radius. The equation was derived
by combining the Reynolds equation representinghydrodynamics, Barus equation, and Hertz theory representing
the contact between elastic bodies. It should be noted that from
Newton’s law of viscosity, friction increases with the decreasein.
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In this study, we investigated the effect of surface texture
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hybrid texture patterns with stable grasping under bothconditions. The maximum resistible force is the maximum
tangential force at which the fingertip can maintain contact
when applying and increasing the tangential/shear force, which

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2377-3766 (c) 2016 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications_standards/publications/rights/index.html for more information.

This article has been accepted for publication in a future issue of this journal, but has not been fully edited. Content may change prior to final publication. Citation information: DOI 10.1109/LRA.2017.2717082, IEEE Robotics
and Automation Letters
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