Tribol Lett (2017) 65:133
DOI 10.1007/s11249-017-0916-7

Author's version. Original one is available at https://link.springer.com/article/10.1007%
2Fs11249-017-0916-7. DOI: 10.1007/s11249-017-0916-7

ORIGINAL PAPER

Characteristics of Dummy Skin Contact Mechanics During
Developing Process of Skin Abrasion Trauma
Xuewei Mao1 • Yoji Yamada1 • Yasuhiro Akiyama1 • Shogo Okamoto1

Received: 18 May 2017 / Accepted: 22 August 2017
Ó Springer Science+Business Media, LLC 2017

Abstract This study focuses mainly on potential skinabrasion risks that are likely to be caused by the interactive
force on a contact surface between a human body and aphysical assistant robot’s cuff, particularly when theserobots are misused. In this study, we develop a novel
method for evaluating skin contact states in order to estimate abrasion risks. First, a simplified cuff model isdesigned to conduct abrasion tests on a piece of dummyskin. The skin is pasted onto a dummy’s leg to simulate thehuman skin’s viscoelastic properties. Then, the edge condition of the contact surface is measured while the cuffmodel slides on the dummy skin. We conduct an analysisof the interactive force and displacement between the cuffmodel and dummy skin and several principal influential
factors are examined during the evolution of abrasion-induced skin damage. These results are applied to evaluate
the contact conditions between the robot’s cuff and humanskin, which makes a significant contribution to estimating
the risk of abrasion injuries for a physical assistant robot
user.
Keywords Contact mechanics  Biotribology  Safety Wear mechanism

This work was supported by AMED (Japan Agency for Medical
Research and Development).
& Xuewei Mao
xueweimaomao@gmail.com
1

Academy for Safety Intelligence, Department of Mechanical
Science and Engineering, Graduate School of Engineering,
Nagoya University, Furo-cho, Chikusaku, Nagoya 464-8603,
Japan

1 Introduction
Skin surface abrasion damage is typically caused by anencounter with an abrasive surface [1]. Although it appears
as relatively slight skin damage, abrasion trauma is usuallymore painful and unbearable than a deeper cut, as a resultof the millions of nerve endings that are exposed without
skin protection [2]. For example, it was reported that selfhealing took more than twelve weeks in the case of a hip
scratch test, which was considered well tolerated [3]. For
the safe utilization of physical assistant robots, it is necessary to design the cuff part of the robot to prevent users
from suffering abrasion traumas.
Stress concentration around the edge of a robot fixation
cuff is considered to be the primary cause of skin abrasion
trauma [4, 5]. The mechanism of scratch-induced damage
to viscoelastic material has been investigated in various
manners [6, 7], such as the evolution map [6] or scratch
resistance [8]. It is widely accepted that the stress concentration around the scratching tip and pileup substrate are
essential contributing factors to the development of scratch
damage [6, 9]. Therefore, particular attention has been paid
to stress concentration, to clarify the human skin abrasion
damage process. However, few studies focusing on abrasion trauma have been conducted on the human body
directly, because this involves inflicting abrasion sufferingon human subjects [3].
To avoid the ethical controversy of human abrasion
tests, Jee et al. [10] performed an in vitro investigation ofshearing damage on porcine skin, and demonstrate how thenormal load, scratching speed, and number of scratch
cycles influence the skin sample friction coefficient.
However, neither the external load nor scratch time on the
microscale examined by Jee et al. is useful for providing
information that enables us to establish practical safety

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guidance for physical assistant robot use, which instead
requires analysis of the macroscopic human–robot contact
mechanics. Furthermore, it is crucial to utilize experimental subjects with relatively stable abrasion-induced
damage characteristics, so that appropriate criteria can be
determined efficiently during abrasion tests to develop a
reliable method for evaluating human–robot contact states.
This is evident based on the fact that the results of in vitro
or in vivo tests on skin friction injuries were observed tovary with experimental subject individualities [11, 12].
In a previous study on blistering, which is another typeof skin friction trauma, Xing et al. [13] reduced the disturbances of subject individualities by using a dynamic
nonlinear finite-element model to replicate the skin structure. It is difficult for such a computer model to obtain
detailed information, such as the effects of repeated abrasion on the skin surface during the abrasion injury development. However, variations in skin properties and thegradually increasing degree of abrasion damage play animportant role in establishing an estimation method for
robot user abrasion risks. Guerra et al. developed a synthetic skin simulant platform to investigate the mechanics
of skin friction blisters and the effect of different surface
treatments on the skin damage [14, 15], and Chimata
et al. [16] further improved the skin simulant to a two-layer
elastomeric model for investigating the influences of thenormal load on the skin blister generation. Instead ofsimulation by computer, this human skin substitute wasutilized to exhibit skin friction damage after a certain
number of reciprocating cycles [14, 16]. Although there
are an insufficient number of studies in terms of the contact
mechanics, it can be concluded from previous blisterstudies that variations in skin mechanical properties may beobserved during the development of abrasion-induced
damage on a synthetic skin simulant [14, 16].
For our study, we selected a distinctive type of manufactured multilayer skin [17] as a substitute for an in vivo
human skin subject. As exhibited in Tang et al.’s
study [19], the near-surface deformation under scratches ofpolypropylene blends was similar to that of porcine skinspecimens observed in previous in vitro tests [10]. Because
the dummy skin applied in our study was also composed ofpolymers, it is considered to be feasible to substitute theskin samples under abrasion. The dummy skin properties
showed hardly any individuality variations, and a wrinkle
with a certain length value was invariably observed after
the dummy skin was damaged by repetitive abrasion
motions. In comparison with the previously tested humanor animal materials [11, 12], our experimental subject
provided a more stable abrasion damage degree criterion.
Moreover, the dummy skin and human skin exhibit similarities in terms of the friction coefficient [17, 18], which isa critical property for human friction injury [20, 21], as123

Tribol Lett (2017) 65:133

well as viscoelasticity [17], which has a significant influence on human skin friction deformation components[22, 23]. For the abrasion motion on the dummy skinsurface, a cuff model equipped with a group of force sensors was controlled by a manipulator and repeated unidirectional abrasion was conducted along a preset route.
During the repetitive abrasion motion, cuff–skin contact
surface friction behavior was monitored, and a significant
clue of irreversible abrasion damage was observed byanalyzing the decreasing tendency of the friction coefficient. As explained in the previous in vitro study [12], it
was nearly impossible to obtain an invariable initial skinfriction damage time point effectively by using porcine
skin. However, if it can be verified that the development
process of skin damage can be indirectly obtained byobserving the varying tendency of the contact mechanics,
the major obstacle of using porcine skin as a substitute will
be removed. Our skin simulant can then be utilized for
performing a reliable safety validation test after matchingits characteristics to those of porcine skin.
According to previous studies on scratch-induced damages on polymer, the indenter size [7, 8], tip velocity [7],specimen mechanical properties [7, 8], and externalloads [6–8] have a significant effect on the severity of thedamage. However, only the external load, abrasion velocity, and interaction frequency are noticeably affected bydifferent usage of physical assistant robots. Therefore, in
our study, we mainly focused on these three physicalparameters and compared their influences on the abrasion
damage development process on the dummy skin.
Although the abrasion times were rarely mentioned in theconventional mechanical model of abrasion-induced damage, considering the possibility of applying our study in thepractical robot use, the number of abrasion times waschosen as a criterion in the establishment of an analyticalmodel for the abrasion damage mechanism regarding thecuff–skin contact state.
In this study, we developed an experimental method forevaluating robot cuff–human skin contact states. The
remainder of this paper is organized as follows. Section 2
provides a description of the entire experimental setup anddescribes the main features of the dummy skin and robot
cuff model. In Sect. 3, the abrasion motion and adhesion
test method used in the experiments are presented, followed by an explanation of all of the tested cuff–skincontact states. Section 4 introduces the unique characteristics of abrasion damage on the dummy skin surface andsummarizes the variation principle of the mechanicalproperties during the abrasion damage development. The
abrasion times’ effects on the adhesion of dummy skinsuperficial layer were exhibited, and the varying tribological properties were also analyzed under different abrasion
conditions. By comparing the influences of different

Tribol Lett (2017) 65:133

physical parameters on the abrasion damage development,
our evaluation method for the contact states is verified to befeasible for making a considerable contribution to estimating the potential severity of skin abrasion trauma during physical assistant robot use.

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Taking both the general shape and softness of therobot’s cuff into consideration, the cuff model is designed
as a flat punch with a rounded edge. Several force sensors
were fixed to the cuff model in order to achieve cuff–skininteraction monitoring.
2.3 Main Experimental Apparatus

2 Experimental Setup
2.1 Experimental Material
In order to gain high repeatability of an evaluation method,
the properties of the experimental subject are required to berelatively stable. For this reason, we selected a special kindof dummy skin created in our laboratory [17] to substitutein vivo skin samples in this study. The main material types
and structure of the dummy skin are shown in Fig. 1. The
dummy skin’s properties are similar to that of human skin,particularly in terms of viscoelasticity and the friction
coefficient, which play an important role in the abrasion
damage process. The thickness of the dummy skin’s
superficial layer is approximately 30 lm. This value isroughly equal to the total thickness of the stratum corneum
and upper layers of epidermis [24], which typically constitute the part that is damaged during a skin scratch
injury [25, 26]. According to Jee et al.’s scratch study [10],the superficial layer is first separated from the layer
underneath during the initial skin scratch damage stage.
Because the superficial layer may also first be separated
from the bottom gel of the dummy skin when it is damaged
by abrasion motions, the dummy skin is regarded as an
appropriate substitute for human skin in the abrasion test.

During the abrasion tests, the cuff model was moved by amanipulator (MOTOMAN-MH3F, Yaskawa Electric Corporation, Japan) on a dummy thigh model with a piece ofdummy skin, which was composed of a human-skin-gelsheet (H0-10, EXSEAL, Co., Japan) and film dressing tape
(H24R10, Nitoms, Inc., Japan), pasted onto it. The cuffmodel was connected to the manipulator’s end-effector,
with a 6-axis force-moment sensor (IFS-67M25A50-140ANA, NITTA, Japan) between the two. Figure 2 shows themain experimental apparatus. The cuff model consisted ofa small fixing box, and sensor box base and surface (madewith a uPrint SE Plus 3D Printer, Stratasys Ltd., and using
ABSP430 XL model (black) as printing material). The
components were assembled in the order shown in Fig. 3,
from left to right.
2.4 Sensors for Monitoring Cuff–Skin Interaction
According to Giannakopoulos’s study [9] on stress concentration caused by a rounded flat punch similar to ourcuff model, the contact surface edge is the most hazardous
area when the cuff conducts abrasion on the skin. Therefore, we focus mainly on the edge of the cuff model, which

2.2 Simplified Cuff Model
Due to the viscoelasticity and the variability of geometricalprofile of human skin, the actual contact condition between
the cuff of a physical assistant robot and human body is toocomplex to be evaluated directly. It is therefore necessary
to design a simplified cuff model to simulate the contact
condition and make it easier to monitor the interactionchange between the cuff model and skin.

Fig. 1 Main material type and structure of dummy skinFig. 2 Main experimental apparatus of abrasion test

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Fig. 3 Main components ofsensor box and relative
positionsis the small fixing box shown in Fig. 3. Three 3-axis force
sensors (one USL06-H5-500N-C and two USL08-H61KN-C, Tec Gihan, Japan) were fixed onto the sensor box
base with the small fixing box. Since the upper edge of thesmall fixing box will first make contact with the dummyskin and then conduct abrasion the skin during the test, it isnecessary to ensure its size accuracy. Therefore, this small
fixing box was manufactured with aluminum, and its
rounded edge radius was measured using an industrial
microscope (OLS4100, Olympus Corp., Japan).
The other parts of the sensor box are mainly for balancing the force and torques that may be generated whenthe entire sensor box contacts the dummy skin, so that we
can obtain the interactive force on the edge in a real sense.
The relative positions between these sensors and the sensor
box base are shown in Fig. 3.

Japan). The microscope was operated at an accelerating
voltage of 5 kV.
In addition, the adhesion force of the dummy skinsamples after three, five, and eight times of abrasion was
also examined by a tension and compression testingmachine (SV-52NA, IMADA SEISAKUSHO CO., LTD,
Japan). Before the adhesion test, the sample width was cut
into 10 mm which contains the main damaged area, and thesuperficial film was pre-peeled from one end of the sample
strip, until it was less than 5 mm from the damaged area.
Then, the pre-peeled end was fixed and lifted by the headpart of the machine at a speed of 20 mm/min. The entire
superficial layer was continued to be peeled while thepeeling force and displacement of the fixing head wererecorded simultaneously.
3.2 Experimental Condition

3 Experimental Design
3.1 Experimental Method
During the abrasion test, the manipulator end tip, equippedwith the cuff model, was first moved to the bottom of thedummy leg, then conducts abrasion on the skin surface
from the bottom to the top, and finally left the dummy skin.
This motion is referred to as one abrasion time. Within one
trial, the abrasion motion was repeated a preset number oftimes, such as one, two, or four. Between trials, theappearance of the skin surface was observed and a picture
taken. The trial was repeated in one test until astable phenotype was found on the dummy skin surface.
Crucial phynotypes showing the gradually increasing
abrasion damage severity were repetitively tested and,
respectively, examined using field emission scanningelectron microscopy (FESEM) (SPG-724, JEOL Ltd.,

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Because human skin viscoelasticity can be simulated using
polymer materials, studies focusing on scratch-induced
damage to polymers were included as important references
when we decided on the parameters to be examined in theevaluation of contact states. In their study on scratch-induced polymer damage, Jiang et al. [6] claimed that the
von Mises stress or normal load was directly related to theseverity of damage. For a rounded punch, the maximum
tensile stress around the cuff edge is mainly determined by
the normal and tangential loads [9]. In addition, the edge
radius and specimen thickness may affect the maximum
stress calculation method [9]. ‘‘Scratch hardness,’’ which is
the most commonly used parameter for evaluating material
scratch performance, is also calculated by the normalload [7, 8]. Moreover, the projected load bearing area plays
an important role in scratch hardness calculation [7, 8].
Pelletier et al.’s study demonstrated that the projected load
bearing area was mainly related to the scratch indentation

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depth, material properties, and indenter shape and tip
velocity [8]. Although the indenter size, tip velocity,
specimen properties, and external loads all affect thescratch-induced damage severity, only the external loads
and tip velocity vary frequently when a patient uses aphysical assistant robot. Since the indentation depth is also
dependent on the normal load during the abrasion test, thenormal load was chosen to examine the effects of external
forces on abrasion damage. Therefore, in our study, we firstexamined the influence of the normal force and tip velocity
on the abrasion-induced dummy skin damage. Furthermore, considering the viscoelasticity of skin, stress relaxation plays an important role in the abrasion-induced
damage process. Since this parameter is directly related torobot utilization frequency, stress relaxation was also taken
into account in the abrasion test. By changing the abrasion
times in every trial, we were able to investigate the effects
of abrasion frequency. In total, seven different abrasion
conditions were tested in our experiments, the details of
which are shown in Table 1, where the abrasion frequency
refers to the abrasion times in every trial.

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Fig. 4 Surface appearance of dummy skin after abrasion

4 Results and Discussion
4.1 Abrasion-Induced Damage on Dummy Skin
After several times of abrasion were conducted on the
dummy skin, a white wrinkle appeared on its surface,which was the position first contacted by cuff edge. Figure 4 illustrates the stable appearance of the wrinkle, andFig. 5 provides more detailed information using FESEM. If
a relatively small number of times of abrasion were conducted on the dummy skin, this wrinkle disappeared within
1 min after the cuff model left the skin surface. However,
the wrinkle’s disappearance became slower with increasing
abrasion times. According to our experimental observations, if the length of this wrinkle is maintained at greaterthan or equal to 10 mm during the interval between two
continuous trial times, its disappearance will be sufficiently
slow and its length almost stable. Therefore, when thewrinkle length was greater than or equal to 10 mm before
beginning the next trial, the abrasion times already conducted were recorded and regarded as a criterion forevaluating the abrasion condition hazards.
Table 1 Tested abrasion
conditions

Fig. 5 Scanning electron micrograph of surface appearance ofdummy skin after abrasion tests

4.2 Variation of Normal Force and Tangential
Force During One Abrasion Time
Under condition 1 of Table 1, the normal and tangential
forces measured during one abrasion time were combined
into Fig. 6. All the data were processed with a 50th -order
low-pass filter with a cutoff frequency of 1 Hz, while thefirst 50 were deleted to ensure accuracy. It can be seenfrom Fig. 6 that the entire process can be separated into
four stages. In the first stage, the tangential force gradually
increases, while the normal force remains relatively stable.

Order of condition

1

2

3

4

5

6

7

Indentation depth (mm)

3

2

1

2

2

2

2

Tip velocity (mm/s)

2.7

2.7

2.7

5.4

1.35

2.7

2.7

Abrasion frequency (abrasion times/trial)

1

1

1

1

1

2

4

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abrasion time, and the ratio between this value and its
corresponding normal force was calculated and regarded as
the characteristic point of one abrasion time. An entire test
consisted of a group of repetitive trials, and one trial was
repeated under a fixed condition until a stable wrinkle was
generated on the dummy skin surface. The characteristic
points for all abrasion times were calculated, and two outof three points were selected for comparison with the firsttrial.
4.3.1 Statistical Analysis

Fig. 6 Normal and tangential force in one abrasion time undercondition 1

Accompanied with the cuff tangential displacement in theabrasion direction, the increasing tangential force appears
to be caused by the increasing piled-up dummy skin in
front of the cuff edge, as shown in Fig. 7. At the end of thefirst stage, a new, shorter stage appears, in which both thenormal force and the ratio between the normal and tangential forces (friction coefficient) remain relatively stable.
Thereafter, both the normal and tangential forces begin to
decrease sharply and finally become zero after the cuff
leaves the skin surface entirely.
4.3 Variation of Normal Force and Tangential
Force During One Test
To identify the variation in the normal and tangential forces with increasing abrasion times, a relatively
stable characteristic within one abrasion time is necessary
to enable location of the same point in every abrasion. As
per the phenomenon described in the previous section, theratio between the normal and tangential forces remains
relatively stable in the second stage. Therefore, the highest
tangential force was selected from the second stage in one

Fig. 7 Dummy skin deformation during abrasion test

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Statistically significant differences between measurements
were determined according to a post hoc one-way varianceanalysis, which was conducted using the Tukey–Kramer
method. The null hypothesis was applied to determine
whether the normalized mean ratio value was statisticallydifferent from the first abrasion time for different abrasion
times. The null hypothesis was rejected for p\0:01 [significance level a = 0.01]. Trends were determined byconsidering the number of measurements [n ¼ 6] and statistically significant differences [p\0:01]. The results of
the ratio comparison for the six tests are shown in Fig. 8,
where the error bars represent the standard deviation aboveand below the corresponding mean value.
4.3.2 Variance of the Tangential/Normal Force Ratio
During one test time, the ratio between the tangential andnormal forces decreased gradually until the end of the test.
Small fluctuations were also determined during this process; however, after five abrasion times, the ratio decreased
significantly, and the decreasing trend became more
remarkable with increasing abrasion times. To obtain

Fig. 8 Comparison of ratio between normal and tangential forces
during entire abrasion test

Tribol Lett (2017) 65:133

further details for the decreasing process, a similar varianceanalysis was specifically conducted for the first 11 abrasion
times in the six tests, the result of which is shown in Fig. 9.
In this case, it can be seen more clearly that after sevenabrasion times, the ratio decreases significantly, and thetrend becomes more remarkable with increasing abrasion
times. The variances of the normal and tangential forces
during the 11 abrasion times are also compared in Fig. 10.
The normal force exhibited no distinct varying tendencywith increasing abrasion times, while the tangential force
and the ratio between the forces gradually decreased.
A similar varying tendency was observed for the friction
coefficient in the previous scratch study using porcine
skin [10]. Fluctuations in the friction coefficient wereregarded as sequences of adhesive failure of the skin’s
stratum corneum layer in Jee et al.’s investigation [10].
The adhesive failure was also demonstrated as the main
failure mode in the previous friction blister study conducted on a skin simulant model [16]. Therefore, the significant decrease in the ratio between the tangential andnormal forces was considered to be caused by the adhesive
delamination of the dummy skin superficial layer.
Since the friction force per unit area typically remains
constant, we conclude that the decreasing tendency of thetangential force is caused by a decreased net contact area
between the cuff model and dummy skin. As shown in
Fig. 4, the skin superficial layer delamination will gradually generate wrinkles on the dummy skin surface. Considering that the decreasing tendency appears when the
dummy skin is irreversibly damaged, it is likely that theskin superficial layer delamination, which generates wrinkles on the surface, simultaneously causes a decrease incontact area and declining ratio between the tangential andnormal forces.

Fig. 9 Comparison of ratio between normal and tangential forces
during the first 11 trials

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Fig. 10 Comparison of maximum tangential force and its corresponding normal force for each trial in the first 11 repetitions

4.3.3 Skin Surface Morphology Around the Significantly
Decreasing Point
By comparing the skin surface morphologies for different
abrasion times, as shown in Fig. 11, it can be seen that an
obvious damage dot appears after five abrasion times. Thisdot indicates irreversible damage generated on the skinsurface, the separation at the superficial layer–skin substrate interface. Because five abrasion times can also causea significant decrease in the ratio between the tangential
and normal forces, this can be regarded as a warning ofirreversible skin abrasion damage.
4.4 Influences of Abrasion Times on the Adhesion
Work of the Dummy Skin Superficial Layer
After we finished two abrasion tests, the two pieces ofdummy skin samples with a wrinkle on it were cut into two
strips 10 mm wide. For each skin sample, one more strip
was also taken as a control sample from the area where no
contact happens between the cuff and the skin during theabrasion test. The four strips were preprocessed by themethod described in Sect. 3.1 and peeled by the tension
and compression testing machine. The appearance of thesample before adhesion test is shown in Fig. 12. The
peeling force and the displacement recorded during thepeeling test are plotted in Fig. 13.
As shown in Fig. 13, the force for peeling the superficial
film of the samples after abrasion is much lower than thatof the samples without abrasion. When it is around thewrinkle position, i.e., the displacement is between 0 and
5 mm, the peeling force significantly decreases once,
which indicates that the abrasion damage appearing as the

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Fig. 11 Scanning electron micrographs of dummy skin surface morphologies for different abrasion times

Fig. 12 Preprocessed sample for adhesion test

Fig. 14 Peeling force and displacement during adhesion test for skinsamples after different times of abrasion

the peel force per unit width of the film F and the peelangel /W  Fð1  cos /Þ

Fig. 13 Peeling force and displacement during adhesion test for skinsamples after entire abrasion tests

adhesive delamination apparently reduces the adhesion of
the dummy skin superficial film.
In order to investigate the detailed relationship between
the abrasion times and the remained adhesion, the superficial film adhesion of the dummy skin samples was further
examined after three different times of abrasion. The
peeling force and the displacement recorded in the tests are
summarized in Fig. 14. It also appears that the more timesof abrasion the dummy skin suffers, the less force is needed
to peel the skin superficial layer.
As demonstrated in Wei’s previous study [27], the workof adhesion (energy per unit area) W can be calculated with

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ð1Þ

It should be noted that the above calculation was started
from the point when the pre-peeled part was sufficiently
stretched in each of the adhesion test to exclude the elastic
energy as much as possible. The calculated work ofadhesion for different samples is summarized in Table 2.
The relationship between the adhesion test results and thepossible analytical model of friction trauma mechanics will
be discussed later in Sect. 4.5.

Table 2 Work of adhesion after different times of abrasion
Times of abrasion

3

5

8

Work of adhesion (kJ/m2 )

8.8

5.9

2.9

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4.5 Influences of Different Parameterson the Dummy Skin Abrasion Damage

W¼

To confirm the effects of the various physical parameters
on abrasion-induced damage, a series of experiments wereconducted using three different sets of conditions, normalload levels, abrasion velocities, and abrasion frequencies.
Figure 15 shows the variances of the normal and tangential
forces in one abrasion time under different conditions.
The different indentation depths applied in the abrasion
tests resulted in different normal loads exerted on the
dummy skin. Figure 16 shows the normal force values at
the maximum tangential force point (TFmax) for the different depths. Because the dummy leg stood stably during
the test, the time of the four stages increases with anincreased indentation depth and normal force. For theseries of tests conducted using different abrasion velocities,
as shown in Fig. 15a, the time of one abrasion motion
decreases with increased velocity, which is caused by theconstant abrasion distance. In contrast, the normal force
level barely changes among the tests with the three different velocities and abrasion frequencies, which are shownin Fig. 15b, c.
For the tests conducted under different conditions, threefigures were drawn to show how the respective physicalparameters affected the abrasion times required to generatea wrinkle with a stable length (that is, abrasion times until
the test end), which are summarized in Fig. 17. A post hoc
one-way variance analysis was also conducted to compare
the difference influences. The analysis method is similar tothe one described in Sect. 4.3.1, and the results are shownin the same figure.
From Fig. 17, the abrasion frequency appears to be
insignificant to the abrasion damage inside the tested
range. The one-way variance analysis conducted todetermine the effect of abrasion frequency on abrasion
damage severity provided a similar result, namely that the
influence of the tested abrasion frequencies showed few
theoretically significant differences. In contrast, theseverity of abrasion damage increases with the increase in
the indentation depth and normal load and decreases with
the increase in the tip velocity when the abrasion distancemaintains constant.
As claimed in Blees et al.’s study [28], the critical load
Lcrit of debonding the sol–gel film from the polypropylene
substrate is proportional to the square root of the work ofadhesion W
Lcrit ¼

2
p scrit dcrit;
4 lcritsince the work of adhesion can be calculated asð2Þs2crit
t;
2E

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ð3Þ

where dcrit ; lcrit ; scrit are the critical width of scratch damage, friction coefficient and shear stress, and E and t are theYoung’s modulus and thickness of the coating film.
Since dcrit almost maintained constant under the same
condition, Lcrit lcrit should be proportional to the square rootof the work of adhesion, i.e., the tangential load should be
proportional to the square root of the work of adhesion. If
the abrasion times can be verified to be inversely proportional to the corresponding square root of the work ofadhesion, the tangential load should also be inversely
proportional to the abrasion times for generating a certain
degree of abrasion damage.
Based on the results in Table 2, the relationship between
the abrasion times (AT) and the square root of the corresponding adhesion work W yields a least-squares fit of theform
pﬃﬃﬃﬃﬃ
ð4Þ
W ¼ 10:44 AT 1
The R2 value of the fit is 0.78, which indicates that thesquare root of the adhesion work is reasonably considered
to be inversely proportional to the abrasion times.
According to the above hypothesis, the relationshipbetween the tangential load and the abrasion times until thetest end is exhibited with the trend lines in Fig. 18, where x
is the number of the abrasion times until the test end, and yis the maximum tangential force in one time of abrasion.
The R2 value of the trend line is also shown in the same
figure.
The relative higher R2 values shown in Fig. 18 confirm
the consistency between the analytical model establishedwith our experimental system and the conventional scratchmechanical model. Therefore, the influences of the significant parameter on the skin abrasion damage can be
determined with the analytical model, and our method isverified to be feasible for evaluating the skin–cuff contact
state and estimating the severity of abrasion injury.

5 Conclusions and Future WorkIn this study, we have established a novel method forevaluating contact states between a physical assistantrobot’s cuff and human skin. To realize efficient evaluation, a simplified cuff model was firstly designed to exert aload on the contact surface during the abrasion test. The
variances of the normal force, tangential force, and friction
coefficient were observed during the abrasion test, and asignificant clue for irreversible abrasion damage wasdetermined by comparing the varying tendencies of thefriction coefficient. Based on the abrasion damage

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b Fig. 15 Normal and tangential forces in one abrasion time withdifferent a tip velocities (conditions 2, 4, and 5 in Table 1),b indentation depths (conditions 1, 2, and 3 in Table 1) or c abrasion
frequencies (conditions 2, 6 and 7 in Table 1)

Fig. 18 Relationship between tangential load and abrasion times until
test end

Fig. 16 Comparison of normal force values at TFmax during one
abrasion time with different indentation depths

mechanism and actual use of physical assistant robots,
three physical factors were regarded as having the mostsignificant effect on skin abrasion trauma. The different
influences of these physical factors on the abrasion damage
were subsequently determined with abrasion times in ananalytical model, which provides a feasible way for theevaluation of human skin–robot cuff contact state andestimation of abrasion damage severity.
In the future, we would like to verify that the clue of
irreversible dummy skin abrasion damage can also act as awarning for initial human skin abrasion trauma. The
evaluation method for the skin–cuff contact condition will
be further improved by matching the dummy skin contact
mechanics with those of in vitro skin samples, so that theabrasion times can be applied as a reliable criterion in theevaluation directly contributing to the safety validation ofphysical assistant robots.

Fig. 17 Influences of different parameters on abrasion times until test end

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Page 12 of 12

Acknowledgements This work was supported by AMED (Japan
Agency for Medical Research and Development). The authors would
also like to thank Mr. Akihiro Kato, Nagoya University, and Mr.
Xudong Hao, University of Michigan, for their technical support for
cuff–dummy skin interactive force data collection.

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