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Tribology International 42 (2009) 1575–1581

Contents lists available at ScienceDirect

Tribology International

0301-67

doi:10.1

� Corr

E-m

T.H.C.Ch

journal homepage: www.elsevier.com/locate/triboint

Developing an artificial fingertip with human friction properties

Fei Shao �, Tom H.C. Childs, Brian Henson

Mechanical Engineering, University of Leeds, Leeds LS2 9JT, UK

a r t i c l e i n f o

Article history:

Received 3 September 2008

Received in revised form

6 February 2009

Accepted 9 February 2009Available online 20 February 2009

Keywords:

Artificial fingertip

Finite element (FE) model

Friction

9X/$ - see front matter & 2009 Elsevier Ltd. A

016/j.triboint.2009.02.005

esponding author. Tel.: +441133432215; fax

ail addresses: F.Shao@leeds.ac.uk (F. Shao),

ilds@leeds.ac.uk (T.H.C. Childs), B.Henson@le

a b s t r a c t

This paper describes the construction and use of an artificial fingertip with a visco-elastic core, a skin

and finger print surface roughness, and the initial development of a virtual model, to mimic the

structure, the shape, the softness and the friction properties of real fingertips to apply to human feeling

studies. The mechanical (including friction) properties of different artificial fingertips were tested and

analyzed. The results show that the pure silicone artificial fingertip had different friction characteristics

than a real fingertip. However, when the softness of the multi-layer artificial fingertip is closer to the

real fingertip, the friction properties are similar to a real fingertip. The required properties for a 2D

model to mimic a 3D fingertip are being developed.

& 2009 Elsevier Ltd. All rights reserved.

1. Introduction

Touch, being one of the human senses is probably taken forgranted by many people. Humans, consciously or not, make ajudgment about how a surface is felt and whether they liked thisfeel or not. This subjective judgment has been recognized as onekey factor to win or lose customers in the future for industriessuch as packaging, textiles, and furniture, where personal taste ontouch–feel perception will be a main purchase criterion [1].

One important factor that influences tactile perception isfriction properties of the skin [2]. However, human skin varies indifferent conditions. Dowson [3], Sivamani et al. [4], Adams et al.[5] and Tomlinson et al. [6] have reviewed current knowledge onthe tribology of human skin. The frictional properties of skin arerelated with skin hydration, lipid films and surface structure andcould be influenced by age and anatomical site. This complicatesthe task of gaining reliable touch–feel data from real people. Also,real people’s touch may be difficult to control repeatedly inexperimental sessions. There would be benefit if a bench markartificial fingertip and/or virtual model could be created withsimilar friction properties to a real human fingertip for tactileperception measurement and interpretation. This paper is a stepin that direction.

To investigate alternative materials which have similar frictionproperty to a real finger, Ramkumar [7,8], Derler et al. [2] and Liuet al. [9] investigated the friction properties of polyvinylsiloxane,brass, steel, rubber polyurethane and silicone materials. Derlerconcluded that a polyurethane coated polyamide fleece with a

ll rights reserved.

+44113 34 32150.

eds.ac.uk (B. Henson).

surface structure similar to that of skin showed the bestcorrespondence with human skin under dry conditions.

On the other hand, in robotics and haptics, contact mechanicsand friction of the finger have been studied in order to mimiceffective gripping and manipulation experiences. Early researcheswere mainly conducted considering simplified hemisphericalfingertips made of homogeneous material [10,11]. The achievedresults cannot be directly applied in all those cases where the softlayers present limited thickness and have close interaction withother structural elements of the biologically-inspired roboticfinger, like an external skin layer with different elastic propertiesor a rigid external shell mimicking the nail or an internal rigidcore reproducing the phalangeal bone.

Many authors have focused on a better knowledge of themechanical behaviour of the biological finger or developed andtested biologically inspired fingertip structures. Shimoga andGoldenberg [12] introduced a multiple layer artificial fingertipmodel which consisted of an external elastic cover and gel for thefiller material. Han [13,14] studied the friction and the stiffness ofartificial fingers in comparison with human ones. Murakami andHasegawa [15] tested a fingertip equipped with a soft elastic coverand a hard nail. Kao et al. [16] applied the robotic stiffness modelsto human grasping analysis. Kinoshita et al. [17] investigated theeffects of tangential torques and tangential force on the minimumnormal force required to prevent slip. Nakazawa et al. [18]examined the impedance characteristics of human fingertips inthe tangential direction. Tiezzi et al. [19] reported on a roboticfinger with a suitable material, a polyurethane gel, which shows asoftness quite close to the human skin, a nonlinear viscoelasticbehaviour and other promising features. Since these artificialfingertips reported in the literature were used for robots,their compliance and high friction were more focused on andmultiple layer models were not applied widely. Their mechanical

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Skin layer

Soft tissue

10mm

a

b

F. Shao et al. / Tribology International 42 (2009) 1575–15811576

properties such as softness and friction are still quite differentfrom the real fingertip [2,20].

For tactile sensation research, Phillips and Johnson [21] andSrinivasan [22] proposed ‘continuum’ fingertip virtual models wherethe skin and subcutaneous tissues were represented by homo-geneous, isotropic, and incompressible elastic media. The continuumfingertip models predict the stress and strain distributions withinthe tissues, and thus the response profiles of the receptors within theskin tissue. Maeno et al. [23] developed a 2D model that representsthe external geometry of the finger as an ellipse. It had multipleinterior skin layers including the epidermis, dermis, and subcuta-neous tissue. In their model, finger prints were also simulated. Themodel was developed to compare how the calculated stress andstrain measured near all four types of mechanoreceptors matchedwith the response characteristics observed for each mechanorecep-tive type to unique stimuli. Wu et al. [24–26] developed two-dimensional FE models of fingertips, which included the mostimportant anatomical structures: soft tissue, nail, and bone. The skinwas considered as hyperelastic and viscous and the subcutaneoustissue was modeled using a sponge-like media. In 2006, a moresophisticated (3D) finite element (FE) model for the fingertip wasdeveloped by Wu et al. [27] for static and dynamic studies.

The aim of the present study is trying to construct an artificialfingertip and virtual model which can mimic the structure, theshape, the softness and the friction properties of real fingertipssufficiently to be useful for tactile measurements and interpreta-tions for tactile feelings studies. Whereas previous work has beenaimed at the functionality of fingertips, studies of how peopleexplore surfaces to develop judgments about them, which iswhere this work is leading, not much has developed.

Hard back

Fig. 1. (a) Black fingertip made of pure silicone and (b) multi-layer artificial

fingertip.

2. Materials and methods

2.1. Construction of the artificial fingertip

The basic structure of skin consists of the epidermis as thesurface layer. Beneath this there is the dermis, followed by thesubcutaneous tissue. In a first trial, to mimic only the shape of areal fingertip including the detail features such as finger prints,101RF silicone rubber (cured hardness: 30 Shore A) from Microsetwas used, firstly to make a soft mould of a real fingertip, then asthe fingertip shape itself, cast from the mould. The fingertip madeof this pure silicone rubber is shown in Fig. 1(a). It has a muchhigher Young’s modulus than a real fingertip.

To improve the softness of the artificial fingertip, multiple layermodels were then constructed as shown in Fig. 1(b). Proprietarymanufacture was carried out by a specialist prosthetic effectscompany (see Acknowledgements). The outer layer is an encap-sulated silicone with a thin acrylic layer to represent skin. Theinner layer is a combination of silicone gel base and elastomer(cured hardness: 8 Shore A) to represent soft tissue. The higherthe proportion of silicone gel the more hysteretic is the artificialfingertip. In this study two types of multi-layer artificial fingertipwere made. One, named later in this paper the elastic artificialfingertip, has as its inner layer a gel base with an equal proportion(100%) of elastomer. The other, named the hysteresis artificialfingertip, has as its inner layer a gel base with a 60% proportion ofelastomer. To support the fingertip and simulate the bone, there isa hard core at the back of the artificial fingertip.

2.2. Physical experiments

As shown in Fig. 2, a friction measurement system wasdesigned to evaluate the friction behaviour of the artificial

fingertips. The testing system consisted of a two-axis load cell(MiniDyn: Multicomponent dynamometer Type 9256C2, Kistler),an X–Z motion table (Series 1000 Cross Roller, Motion link), anartificial fingertip, a controller and a PC. The artificial fingertipswere fixed to the motion table and were slid over the testingsurfaces.

Fifteen samples were used to evaluate the friction behaviour ofthe artificial fingertips. They included five types of materials: (1)surface 1 was a smooth stainless steel (2) surfaces 2 and 3 weremade of acrylonitrile butadiene styrene (ABS) (smooth andtextured) (3) surfaces 4–8 were cardboards; (4) surfaces 9–13were thin film and paper; and (5) surfaces 14–15 were laminateboards. The 15 samples cover a good variety of textures withdifferent roughness, hardness, surface finishes. The experimentswere carried out by sliding the artificial fingertips over the 15surfaces with a normal force of 1 N70.1 and a sliding speed of10 mm/s. These are typical of exploratory human touch contacts.All experiments were carried out in dry conditions, but in somecases talcum powder was lightly rubbed into the artificialfinger surface, using a sponge applicator. The friction of realfingertips sliding on the surfaces was also measured, as acomparison. Five people were asked to wash their hands insoap and water and then dry them. Since friction coefficient canbe dependant on loading force, they then slid their indexfingers over the surfaces, from side to side, keeping the load at

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Frame

Sample holder

Artificial fingertip

X

Motion control

Load cell (Friction force)

Load cell (normal force)

Sample X

Z

L shape plate Z

Vertical movement

Horizontal movement

Artificial fingertipForce table

Fig. 2. (a) Schematic diagram of the friction measurement system and (b) friction

measurement by using artificial fingertip.

Bone

Nail

Ridge

Test sample

Dermis

Epidermis

Soft tissue

Fig. 3. FE model of the fingertip (a) structure view and (b) meshed view.

Table 1The mechanical properties of the fingertip model.

Young’s modulus (kPa) Poisson’s ratio

Initial Modified

Plate 2�106 2�106 0.3

Bone 17�106 17�106 0.3

Soft tissue 34 24 0.4

Epidermis 136 80 0.48

Dermis 80 50 0.48

Nail 17�104 17�104 0.3

F. Shao et al. / Tribology International 42 (2009) 1575–1581 1577

around 1 N, and the sliding velocity around 10 mm/s to obtaincomparable data.

In addition to the main sliding experiments the rig (Fig. 2) wasalso used to measure the compliance of the artificial tips. A tipwas loaded on to and unloaded from a flat rigid surface in theabsence of sliding, up to a maximum Z load of 1 N with speed of1 mm/s. These were compared to measurements with realfingertips. A cradle was constructed, mounted on the motiontable, into which a person placed his/her index finger, in the sameorientation as the artificial finger. It was then driven against andfrom the counterface in the same way as an artificial fingertip. 12people (6 males, 6 females, aged 18–45) volunteered for this.

Two subsidiary tests are also reported here, to study separateeffects of adhesive and hysteresis (deformation) friction. Theinfluence of load on friction force, on smooth and rough surfaces,was studied with a talcum-powdered hysteresis artificial finger-tip. Talcum powder was used to reduce adhesive friction, relatedto tests without it. In a different type of sliding test, elastic andhysteresis artificial fingertips were slid over a smooth steel ball(diameter 20 mm). In this case, the Z-displacement of thefingertips were held constant, so that the normal load againstthe ball varied as a fingertip passed over it. The Z-displacementwas adjusted so that the maximum load was 1 N. Loading andunloading occurred in 2 s, similarly to the loading time in thenormal loading experiment. The different rates of loadingand unloading of both the normal and the friction forces werestudied.

2.3. FE simulation

A multi-layered 2D FE fingertip model was created, as shownin Fig. 3, using the commercial FE software package ABAQUS/CAEversion 6.6. The dimensions of the fingertip are 20 mm wide,14 mm high, assumed to be representative of the index finger of atypical male subject. The cross-sections of the fingertip and thebone are assumed to be elliptical. The fingertip is assumed to becomposed of epidermis, dermis, subcutaneous tissue, bone, andnail. The thicknessess of the epidermis and dermis layers wereassumed as 0.7 mm [28] and 0.8 mm [29]. To mesh the fingertipmodel, 4-node bilinear plane stress quadrilateral and 3-nodelinear plane stress triangle elements were used. In the contactarea, fine mesh was applied with element size of 0.05 mm andcoarse mesh was applied on the other part of the model with

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1.41.6

Real fingertipHysteresis artificial fingertipElastic artificial fingertip

F. Shao et al. / Tribology International 42 (2009) 1575–15811578

element size of 0.1–0.5 mm. In total, 29 213 elements wereused in this simulation. Material properties used in the modelare in Table 1.

00.20.4

0.60.8

11.2

Displacement (mm)

Load

ing

(N) Silicone artificial fingertip

2.01.510.50

Fig. 5. The compliance and hysteresis of different artificial fingertips and real

fingertips.

0

0.1

0.2

0.3

0.4

0.5

0.6

0Normal force (N)

Fric

tion

forc

e (N

)

Smooth ABS tile

Textured ABS tile

10.80.60.40.2

Fig. 6. Friction dependence on load for the powdered hysteresis fingertip.

3. Results

3.1. Physical experiments

The friction coefficients from the sliding friction tests areshown in Fig. 4. This has been split into two parts to aid clarity ofpresentation. In Fig. 4a, the elastic fingertip clearly follows thebehaviour of the real fingertip. The silicone fingertip does not. InFig. 4b both the elastic and hysteresis fingertip follow realfingertip behaviour. Talcum powder on the hysteresis fingertipgreatly changes its behaviour. This indicates the huge effect ofadhesive friction.

The compliance measurements are shown in Fig. 5. Theydemonstrate the similar compliance of the real and artificialfingertips and the matching of the hysteresis of the hysteresis tipto a real tip.

The subsidiary test results are shown in Figs. 6–8. Friction forceversus load in sliding tests of talcum-powdered hysteresisartificial fingertips against surfaces 2 and 3 (smooth and texturedABS) are shown in Fig. 6. The tests on sliding over a steel ball areshown in Figs. 7 and 8. Fig. 7 shows the raw data of the variationwith sliding distance of normal and friction forces, for both theelastic and hysteresis artificial fingertips. An asymmetry is seenbetween the loading and unloading parts of the curves. This isfocused on in Fig. 8 in which the unloading portions of the Fig. 7curves are re-potted, folded back about the points of maximumforce, better to enable a comparison to be made with the loading

00.2Fr

ictio

n co

effic

ient

(µ)

0.40.60.8

11.21.41.61.8

Fric

tion

coef

ficie

nt (µ

)

Silicone fingertipElastic fingertipReal fingertip

00.20.40.60.8

11.21.41.61.8

Sample number

Powdered hysteresis fingertipElastic fingertipHysteresis fingertipReal fingertip

151050

Sample number151050

Fig. 4. Friction coefficients of the 15 samples with fingertips (a) silicone, elastic

and real and (b) elastic, real, powdered and unpowdered hysteresis types.

-0.2

0

0.2

0.4

0.6

0.8

1

1.2

0

Displacement (mm)

Forc

e (N

)

Friction forceNormal force

-0.2

0

0.2

0.4

0.6

0.8

1

1.2

Forc

e (N

)

Friction force Normal force

2015105

0

Displacement (mm)

2015105

Fig. 7. Dependence of forces on sliding displacement for (a) the elastic and (b) the

hysteresis artificial fingertip.

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Fig. 8. A comparison between loading and unloading results for (a) the elastic and

(b) the hysteresis artificial fingertip.

Fig. 9. The stress distribution of the FE fingertip model (out-of-plane width 1 mm).

0

0.02

0.04

0.06

0.08

0.1

0.12

0.14

0Displacement (mm)

Forc

e pe

r uni

t wid

th (N

/mm

)

2.521.510.5

Fig. 10. The loading test result of FE fingertip model.

405060708090

100110120

0.5Displacement (mm)

Effe

ctiv

e Y

oung

's M

odul

us(k

Pa)

Initial FE modelRea lfingertipModified FE model

1.91.71.51.31.10.90.7

Fig. 11. The softness calibration of the FE fingertip model.

F. Shao et al. / Tribology International 42 (2009) 1575–1581 1579

portions at the same displacement from the maximum forcepoints.

3.2. FE simulation

To test the compliance of the FE fingertip model, the FEfingertip model was loaded against a flat surface (taken to besteel) with a normal displacement of 2 mm as shown in Fig. 9. Atthe start, the model’s elastic properties were those marked ‘initial’in Table 1. They are taken from the literature and previous work byone of the authors [23,30]. The final mechanical properties for theFE simulation were those marked ‘modified’ in Table 1. Fig. 10shows the calculated variation of normal load per unit width ofthe model with increasing displacement. Comparing Fig. 10 with

Fig. 5, it is seen that although the displacement scales are thesame, the loads in the two figures are approximately 10-folddifferent. This is in part because Fig. 5 shows experimental resultsfrom a 3D real finger, whereas Fig. 10 is from a 2D simulation. Thetwo are not immediately comparable.

Fig. 11 does make a comparison, in terms of an effectiveYoung’s modulus. For the loading of an elastic sphere on to a flat,Hertz theory gives the displacement (or approach of distantpoints) d, in terms of load W, sphere of radius R, Young’s modulusE and Poisson’s ratio n (when the flat is effectively rigid) as in Eq.(1a) [31]. This can be re-arranged to an expression for E, in termsof a measured d and W, Eq. (1b). In Fig. 11 the effective Young’smodulus for a real fingertip is the value of E obtained from the realfingertip values of W and d from Fig. 5. In the 2D case, Fig. 10, theapproach of distant points has been taken to be as in Eq. (2a), withsymbols as before, except that in addition d is the distancebetween the lower edge of the bone and the contact surface and a

is the half-width of the contact [32]. From this an effective E canbe obtained from measurements of W, d and a, as in Eq. (2b).These give rise to the initial and modified FE model values in Fig.11. The initial values were obtained from Fig. 10 which, as alreadystated, results from the initial choice of model material propertiesin Table 1. These properties were modified, to produce changedversions of Fig. 10 and new estimates of E from Eq. (2b), untilagreement was obtained with the estimates of E from Eq. (1b) andFig. 5. Table 1’s modified values are the values that gave thatagreement.

d ¼9

16

W2ð1� u2Þ

RE2

!1=3

(1a)

E ¼ 0:75W1� n2

Rd3

� �1=2

(1b)

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F. Shao et al. / Tribology International 42 (2009) 1575–15811580

d ¼Wð1� n2Þ

pEf2 lnð2d=aÞ � u=ð1� nÞg (2a)

E ¼Wð1� n2Þ

pdf2 lnð2d=aÞ � u=ð1� nÞg (2b)

4. Discussion

By selection of proportions of a silicone gel and elastomer corematerial, and encasing it with a silicone elastomer and acrylicskin, an artificial fingertip can be created with realistic compli-ance behaviour. Fig. 5 demonstrates that the fingertip described as‘hysteresis artificial’ has both the same compliance and samehysteresis as a real finger (the real finger data is an average from12 participants). The ‘elastic artificial’ fingertip is slightly less andthe ‘silicone artificial fingertip’ is much less compliant than thereal ones.

Sliding tests (Figs. 4a and b) show that the both the hysteresisand elastic artificial fingertips mimic real behaviour, as far as theirsliding friction coefficients are concerned, more than the siliconetip. In Fig. 4a, which compares the friction coefficients of theelastic artificial, silicone artificial and real fingertips, not only arethe friction coefficients of the silicone tip sliding over the 15different surfaces on average approximately twice those for thereal finger, the variation of friction coefficient from surface tosurface is different. On the other hand, the variation of frictionfrom surface to surface for the elastic artificial fingertip closelymatches that of the real one, although the levels of friction arehigher, particularly sliding against surfaces 5 and 9.

Surfaces 5 and 9 were glossy and smooth. This, with thegenerally high level of friction coefficient, points to adhesivefriction rather than hysteresis or deformation friction beingdominant. This agrees with the conclusion of Asserin [33]. Itsuggests that matching the real areas of contact between the realand artificial tips is most important. Of course, the level ofadhesive shear stress should also be matched. Fig. 4b looks morecarefully at differences between the elastic and hysteresis artificialfingertips, and also at the effect of reducing adhesive shear stresswith a trace of talcum powder. The friction coefficients of theelastic and hysteresis artificial fingertips are almost identical. Insome cases the elastic tip gives a higher friction coefficient thanthe hysteresis one, in others that is reversed. Introducing talcumpowder particularly reduces the highest friction coefficientsobserved on surfaces 5 and 9 and generally almost to the pointof not following the surface to surface variations of the real tip.Fig. 6 further looks at the behaviour of the powdered hysteresistip, sliding on surfaces 2 and 3. The variation of friction force withload is almost linear (in agreement with many other studies[4,14,33,34]. The larger slope (higher friction coefficient) for thetextured surface 2 than the smooth surface 3 is not likely to be alarger deformation (surface roughness) effect, because of themagnitude of the difference. It is more likely to be due to adifference of adhesive shear stress between the two sets of tests.Control of the level of adhesive shear is identified as of mostdifficulty in developing the artificial finger test. It is also probablythe cause of the small discrepancy in the trends in the frictionshown in Fig. 4.

The sliding friction tests (Fig. 4) may suggest, through thesimilar friction coefficients of the elastic and hysteresis tips, thatadjusting the level of hysteresis to real levels may not beimportant in developing an artificial fingertip for touch–feelstudies. However, hysteresis will affect the internal stresses,beneath the skin and at the site of the mechano-receptors. Thedifferent deformations between elastic and hysteretic fingers arehighlighted in sliding at constant normal displacement past a

steel ball (Figs. 7 and 8). Fig. 8 particularly shows hysteresis inboth the normal and friction components of force. It remains to betested whether this represents the behaviour of real fingers too.And the virtual fingertip model (Figs. 3 and 9) does not yet includevisco-elastic behaviour.

The purpose of developing the virtual fingertip model is tostudy sub-surface deformations at the sites of mechanoreceptors,following Maeno et al. [23]. In that work it is not clear how the 2Dmodel was calibrated against 3D reality. This has continued to berecognized as a problem. It has been pointed out, for example, thata typical fingertip contact on a flat surface may be 10 mm wide.Thus the loads from a 2D calculation in which out-of-plane widthis taken as 1 mm might be multiplied by 10 to recover actualloads, at least at a semi-quantitative level [35]. This approachwould work here. The peak loads in Fig. 10 are approximately onetenth those in Fig. 5. Another approach has been to normalizeforces and compare only the shapes of the load–displacementcurves [36]. The present paper’s comparison of simulation andreal contacts through a consideration of elastic contact mechanics,Eqs. (1) and (2), is believed to be novel. It leads to a modification(Table 1) of the model’s soft tissue, epidermis and dermis elasticmoduli from their initially assumed values. Initial studies ofinternal stresses during sliding the virtual finger over roughsurfaces are in progress.

5. Conclusions

Artificial fingertips have been made consisting of an encapsu-lated silicone and acrylic surface skin around a silicone gel andelastomer mix core. The compliance and hysteresis of the tip havebeen varied by varying the proportions of gel and elastomer in thecore. A good fit to real fingertip characteristics has been achieved.In dry sliding friction tests on a wide range of some 15 differentsurfaces the variation in friction coefficient from surface to surfacehas also been matched to real fingertip sliding tests. Adhesivefriction dominates, as has been demonstrated by lightly dusting atip with talcum powder. Control of adhesive friction shear stress isthe main issue in developing the artificial finger sliding test. Avirtual model fingertip is under development to study sub-surfacedeformations, at the sites of mechano-receptors, during sliding onrough surfaces. Its soft tissue, epidermis and dermis elasticmaterial properties have been calibrated against the propertiesof a real finger by a novel method.

Acknowledgements

This work was funded by the UK’s EPSRC (EP/D060079/1) andthe EC (NEST 043157), and supported by MacDermid Autotype Ltd.The views expressed here are the authors’ and not those of the EC.The multi-layer artificial fingertips were made by Mike Stringer ofHybrid Enterprises Ltd.

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