[Author's version]
ADVANCED ROBOTICS, 2017https://doi.org/10.1080/01691864.2017.1318716

FULL PAPER

Safety verification method for preventing friction blisters during utilization ofphysical assistant robots
Xuewei Maoa

, Yoji Yamadaa , Yasuhiro Akiyamaa , Shogo Okamotoa and Kengo Yoshidaba Department of Mechanical Science and Engineering, Graduate School of Engineering, Nagoya University, Nagoya, Japan; b Reactor

Maintenance and Repair Division, Mihama Nuclear Power Plant, Kansai Electric Power Co., Inc., Mihama, Japan

ABSTRACTARTICLE HISTORYThis study is mainly concerned with the potential hazards of interactive friction force on a human–robot contact surface, which may result in various skin traumas, such as friction blisters. In this study,
a novel safety verification method is developed for helping physical assistant robot users to avoid
this hazardous condition. First, based on the microscopic structure of the epidermis–dermis layer andthe characteristics of blisters on porcine skin, we prove that the porcine skin, instead of the humanskin, is feasible as an innovative experimental material for generating friction blisters. Second, theconditions for generating blisters on porcine skin are confirmed using a series of adaptive timegradient tests. Considering the properties of the censored results, we clarify an inherently safe region
in the tangential traction–time relationship, which is based on a nonparametric estimation of thesurvival function on the subjects. After a condition-matching calculation, this region for the porcine
skin is further verified to be consistent with that for the human skin. Third, the safety of the ‘stand-up
and sit-down’ motion of a physical assistant robot user is successfully validated by applying thisverification method, which finally proves its practical usability with regard to the possible physicalstress hazard.

Received 4 October 2016
Revised 21 January 2017
Accepted 4 April 2017

1. Introduction
Physical assistant robots, which are directly attached tothe human bodies, are expected to provide various opportunities of creating assisting technologies for eﬃciently
transferring power to the human musculoskeletal system[1]. Consequently, physical assistant robots have foundincreasingly wide applications in various ﬁelds [2] fromindustrial environments [3] to rehabilitation practices[4,5]; however, because of the close human–robot contact, misalignment between the human anatomical joints
and the exoskeleton of robot may exert continuous undesired friction force on the human body [6,7], whichbecomes a potential hazard for the users [8]. Considering
the results of the risk assessment, we concentrate on
the hazards of friction force, which are unique to thephysical assistant robot operation, such as the one causedby anatomical mismatch.
According to ISO 13482 [8], the measurement of
hazardous physical stress is required for validating thesafety of devices in question for the physical assistantrobots, particularly for the restraint type of physical assistant robots. So far, several studies have contributed
signiﬁcantly to the ﬁndings for the safety veriﬁcation ofhuman–robot coexistence systems. Regarding relatively
CONTACT Xuewei Mao

mao.xuewei@e.mbox.nagoya-u.ac.jp© 2017 Taylor & Francis and The Robotics Society of JapanKEYWORDS

Assistive robotics;
human–robot contacting;friction; health and safety;
life estimation

light mechanical impact, Yamada et al. ﬁrst investigatedthe limit of human pain tolerance through a biomechanical approach [9,10], and Saito et al. tried to determine thelimit by a static measurement [11]. Further, a human–robot contact limit at a whole-body scale was intensively
surveyed and provided under static conditions [12], andHaddadin et al. directly related the medical observationof injury to the physical input parameters of the motionof the robot and paved the way for generating safe robot
motions with more general terms [13].
Considering the biomechanical aspect, there have notbeen any reported studies regarding the dependence oftangential component on the harm severity, whereas thehazards of normal force have been considered seriously
using the existing safety assessment standard formachinery. However, the friction force exerted by thebrace parts of the physical assistant robots can lead toconsiderably uncomfortable feelings for the users [14]. It
has also been observed that the friction force is the maincause of skin friction traumas such as friction blisters andabrasions [15], particularly for patients with poor healthcondition. In a clinical observation, Okado et al. observedthat patients with diabetes or liver cirrhosis obtained
friction blister trauma or skin ulcer more easily than2

X. MAO ET AL.

other users of physical assistant robots [16]. Our study
focuses on the grossly neglected friction blister injuries
and tries to ﬁll the gap in the existing safety veriﬁcation
methodology by mainly investigating the eﬀect of thefriction force on human–robot contact surfaces.
As is evident from some published investigations, theexperimental studies of friction blisters have been conducted on living subjects such as humans [17] or animals[18], which inevitably inﬂict unwanted pain on thesubjects [17]. To avoid any possible ethical controversy,
Xing et al. employed a dynamic non-linear ﬁnite-element
model for replicating the blister-characterized structure[19]. Using an algorithm for the outer covering and roofof the blister, the model can qualitatively evaluate theeﬀects of loading environments and contacting materials.
In addition, several studies have developed other kinds ofsimulation models consisting of multiple layers for investigating the relationship among surface treatment, skincondition, and damage caused by friction blisters [20,21];
however, they still contain less detailed information suchas the eﬀects of diﬀerent friction force values on the actual
blister generation time and cannot act as a standard for acertain safety validation test. In our study, fresh porcine
skin was selected as a novel substitute for living subjects torelieve them from the suﬀering caused by friction blisters.
As validated in previous studies, porcine tissue is a wellsuited substitute for human skin [22,23]. Moreover, considering the individualities of experimental subjects, theresults obtained using porcine skin can conform better tothe results obtained under real human conditions, which
diﬀer even in one group of people and are diﬃcult to be
simulated using a single ﬁxed model.
From several experimental studies on friction blisters,
we can argue that there are several factors aﬀecting thedegree of damage caused by friction blisters, such as theloading mode [24], skin surface conditions [15,24], andthe individualities of diﬀerent skin types [15]. A credible
safety validation should be conducted under the same
condition as that of the target condition [25]. Therefore, the results of the friction blisters obtained fromprevious tests, which were performed under diﬀerent
environmental conditions, cannot be directly used in oursafety veriﬁcation test, which focuses on a new condition– the utilization of physical assistant robots. During thiscondition, if the robots are excessively used, the undesired
friction force exerted by the robot cuﬀs may cause blisters
on the surface of the human skin contacting with the
cuﬀs. Therefore, experimental results obtained under thecondition of rubbing over long periods of time are morepractical for our purpose of applying them in the daily
use of robots. In addition, considering that the contact
state always varies with diﬀerent ways of using the robots,
we selected a rheometer for performing rubbing tests

with controlled tangential traction (friction force per unit
area) and time. As the tangential traction is unique underdiﬀerent contact states, the experimental condition of ametal head rubbing a piece of porcine skin facilitates thesafety veriﬁcation for various human skin–cuﬀ contact
states. Based on this method, the skin damage condition
during the utilization of a physical assistant robot can be
simulated accurately under the application of repetitive
interactive forces between the skin of the user and the
cuﬀ of the robot.
In this study, a novel safety veriﬁcation method isdeveloped for helping the users of physical assistantrobots to avoid possible skin friction blisters. This paper isorganized as follows: Section 2 provides a description of
the entire experimental setup; the section also describes
the main processing techniques used for the experimental material before and after the test. In Section 3, theloading principle used in our experiments is ﬁrst presented, followed by an explanation of the adaptive testing
method. Using this specially designed method, the possible blister generation time under a certain tangential traction (friction force per unit area) value can be eﬃciently
conﬁrmed even when there is no obvious macroscopic
symptom on the skin surface. Section 4 brieﬂy introduces
the preprocessing process for microscopic analysis andthen summarizes the unique characteristics of the friction
blisters formed on fresh porcine skin, which is basedon the observation results. Considering that most of theexperimental results obtained from the microscopic observation are censored data, an inherently safe condition
region for the tangential traction–time relationship is
clariﬁed in this section by conducting a nonparametric estimation of the survival function on the subjects.
Furthermore, Section 5 enhances the calculation of theinherently safe threshold to make it more appropriate forthe condition of human blister generation. In Section 6,
this novel safety veriﬁcation method is applied to thesafety validation of human–robot interaction during a‘stand-up and sit-down’ motion, and the feasibility ofthe method is conﬁrmed by the agreement between our
experimental result inferences and microscopic observation results.

2. Experimental material and method
2.1. Experimental material
As a well-suited substitute for human skin, the physiological and biological properties of porcine skin are similar
to those of human skin [18], particularly in the epidermis
and dermis layers, which are closely related to the generation of friction blisters [15,24]. In the structural aspect,
the epidermal thickness of the porcine skin and the ratio
of it to the dermal thickness are both similar to those of

ADVANCED ROBOTICS

the human skin [23,26]. The histological appearance ofthe porcine skin is demonstrated to be similar to thatof the human skin, including the epidermis keratinous
protein fractions [27], stratum corneum’s variable ﬁlament density and areas of cell overlapping [28], andthe dermal architecture of collagen ﬁbers and ﬁber bundles [26], which can signiﬁcantly aﬀect the friction blister
generation. Moreover, it is conﬁrmed that the epidermal–
dermal junction of the porcine skin resembles that of thehuman skin [28]. There are also several studies detailing the similarity of the mechanical properties between
human skin and porcine skin, among which, their similarity under dynamic tensile loading helps most in
exhibiting the feasibility of porcine skin acting as a substitute for human skin friction blister tests [29]. Therefore,
the fresh porcine skin excised from the anterior shank
was used as the experimental material. It was delivered
to our laboratory within 16 h below a temperature of
4 ◦ C. So as to shape it more similar to human skin, we
shaved and cleaned its surface and removed the excessive
underneath fat.
2.2. Experimental setup
In most studies about friction blisters [15,30], friction
force is regarded as one of the most signiﬁcant factors forgenerating blisters. In our experiments, friction force wasalso selected as the representative direction (tangential
direction) of force component, which was applied foremulating the repetitive rubbing motion of the cuﬀ ofa physical assistant robot on the skin surface. As theforce eﬀects on blisters are also related to the actual
contacting area, our study utilized the friction force perunit area as tangential traction. To achieve a horizontal
reciprocating rubbing action under controlled tangential
traction, the Discovery Hybrid Rheometer (DHR-2-NA,
TA Instruments, US) was used for performing oscillatingrotational rubbing on the porcine skin with high calibration accuracy (0.1 Pa). During the experiment, therevolving geometry of the DHR acted as a rubbing head
and the tangential traction exerted in the skin surface
oscillated with a preset frequency and constant amplitude. The normal force was approximately 5 N, and thereason for selecting this value is shown later.
To avoid undesired displacement of the experimental
material, a ﬁxation frame was used for clamping the
porcine skin on the plate of the rheometer. A sheet ofsandpaper (grit size: 40) was attached between the skinand the plate for a ﬁrmer ﬁxation, as shown in Figure 1.
After every rubbing test, the porcine skin sample was
immersed sequentially in 10% formalin solution, 20%
sucrose, and 30% sucrose with phosphate-buﬀered saline
for 8 h. Before histological sectioning, the tissue samples

3

Figure 1. Main experimental apparatus of rubbing test.

were manually cut into ﬁne cubes and mixed with optimal
cutting temperature (O.C.T.) compound for fast freezing
with dry ice. Subsequently, they were placed in a cryostat(CM 3050 S, Leica Biosystems, Germany) and sectioned
at 10 μm after precooling for 30 min. To detect thepresence of the skin structure, the histological sections of
the samples were stained using the normal H&E staining
technique, which stains the basophilic structures in blue
using hematoxylin and the acidophilic structures in red
using eosin. After dehydration with a descending arrayof ethanol and xylene, the sections were mounted andobserved under a microscope.
As the porcine skin sample after one test was sectioned
and examined immediately, every rubbing test needed
one new piece of fresh porcine skin, i.e. there were no
tested porcine skin samples continuously applied in thesubsequent rubbing tests.

3. Experimental design
The distribution of tangential traction on the surface ofthe skin sample under the rubbing head can be calculated
as follows:τ (r) = ηγ̇ (r) = η


r
=η ·r
H
H

(1)

where η is the viscosity, H is the thickness, and γ̇ is theshear rate of the porcine skin ﬁrst conﬁrmed by DHR.
Moreover,  is the angular velocity measured during thetest, and r is the distance from the contact center [31].
Using the contact area between the rubbing head andthe tested porcine skin A, the torque of the rubbing head,T, can be calculated as follows:



(2)
T = dA · τ (r) · r = η · r 2 dA
H
As it can also be measured using DHR in real time, thetangential traction distributed on the porcine skin surface
can also be calculated as follows:4

X. MAO ET AL.

τ (r) =

2T
·r
πR4

(3)

where R is the radius of the contact area.
According to the above equations, we can accurately
ﬁx a certain tangential traction value exerted to a target area of the porcine skin surface contacting with therubbing head of DHR.
With regard to the individuality of porcine skin pieces,
it is not suﬃciently reliable to conduct only one testfor one tangential traction–time point for obtaining theveriﬁcation data. In addition, as the blister generation
cannot be directly observed from the appearance of theskin surface, it is not suﬃciently eﬃcient to use the observation of the sections acquiring from a series of ﬁxed
time-gradient tests for approaching the possible initialtime of blister generation. To solve these two problems,
we designed the test method as follows.
First, for one series of experiments under the same tangential traction value, several diﬀerent rubbing durations
were tested with a time gradient of t, and for every
tangential traction–time condition, one set of tests was
conducted with n pieces of porcine skin. Second, under
one tangential traction value condition, the rubbing tests
were started from a relatively long time. The rubbing time
for the next step, Tk+1 , was decided on the basis of theresult of the last step that was obtained using the H&E
staining method. Moreover, to avoid the unnecessary
tests conducted on areas where no blisters can be generated or all samples can be heavily damaged, it is essential
to conduct most of the tests around the condition areasthat are worth testing, i.e. under the conditions where
about half of the samples can generate blisters, and thisarea will be investigated in the near future. Therefore,
the rubbing time of the next test was decided according
to the number of blister samples after the last test, bk . If
most of the samples develop blisters after a test, a shorter
rubbing time will be selected for the next step; otherwise,
the rubbing time will be increased.
If n is an odd number, Tk+1 can be calculated asfollows:n+1
Tk + ( n+1
2 − bk )t if bk < 2
Tk+1 =(4)n−1
Tk + ( 2 − bk )t otherwise
Similarly, if n is an even number, Tk+1 can be calculated as follows:n− bk t(5)
Tk+1 = Tk +2
As 3 is the smallest number that can help us identify
whether a tangential traction–time condition can generate a blister more easily, we ﬁxed n as 3, i.e. threepieces of porcine skin were tested under one tangential

Figure 2. Intact stratum spinosum of porcine skin before tests(indicated by an arrow).

traction–rubbing time condition. Moreover, a series of
rubbing tests under one tangential traction value wascontinued until three sets of tests under three diﬀerent lengths of rubbing time conditions were observed
to be ended with three diﬀerent incidences of blistersamples, 33%, 67%, and 100%. Therefore, a series of rubbing tests under one tangential traction value condition
contained at least three sets of tests, which rubbed ninepieces of porcine skin under three diﬀerent time durations of rubbing conditions. In addition, the average valueof the previous two rubbing time values might be selected
for avoiding the unnecessary repetition test. A moredetailed and general experimental procedure ﬂowchart ispresented in Appendix 1.

4. Experimental results and discussions
4.1. Feasibility analysis of porcine skin fordeveloping experimental friction blisters
Because of the eﬀect of stress concentration [32], thetangential traction increased sharply when it approached
the rim of the surface of the sample, and this part of theskin was typically badly worn. For obtaining relatively
stable results, we focused on the condition of the skinlocated 0.5R from the center of the rubbing area. The
appearance of the microscopic porcine skin structure
before tests and the appearance of the samples, rubbed
under the tangential traction of 32 kPa for 1200 s, areshown in Figures 2 and 3, respectively.
From Figure 2, it can be clearly observed that theskin epidermal structure was maintained intact before
the tests. Similar to the human skin [27], the epidermis of
the porcine skin also includes the stratum corneum, stratum granulosum, stratum spinosum, and epidermal basal
layer. As claimed by the previous studies of experimental
friction blisters on human skin [24], the friction force
exerted on the skin surface can be transmitted throughthe stratum corneum and granulosum of the epidermis.
Subsequently, the stratum spinosum can be degenerated
and some clefts can be produced between the stratum

ADVANCED ROBOTICS

5

the friction blister generation, which proves that our
new experimental model is still eﬀective to be applied toinvestigate the generation condition of friction blisters.
4.2. Censored properties of current results

Figure 3. Intra-epidermal clefts of porcine skin after test indicatedby arrows.

Figure 4. Intact stratum spinosum of porcine skin after test
(indicated by arrow).

granulosum and the basal layer. Within a short periodof time, those clefts are ﬁlled with free ﬂuid from thedermis layer of the skin and macroscopic friction blisters
are developed.
Unlike the sample without any rubbing test in
Figure 2 and the sample after a shorter rubbing test (900 s)
under the same tangential traction in Figure 4, the sample shown in Figure 3 developed severe splits in thestratum spinosum layer. According to the study summarized by Knapik and his colleagues [15], the cavities
or clefts of friction blisters produced on human skinare invariably observed at the same intra-epidermal layers, which include the stratum corneum composing theblister roof, the stratum granulosum, and a segment of
the stratum spinosum that is traumatically degenerated
during the blister generation. As the clefts observed after
our rubbing tests were also located in the same intraepidermal layers and were composed with the intact stratum corneum and stratum granulosum similar to thehuman friction blisters, we regarded the clefts shown in
Figure 3 as an essential evidence of the initial stage of theexperimental friction blisters in our tests.
Considering the biophysical property changes in thein vitro experiment compared to the in vivo experiment,
the available ﬂuid left in the skin after excision was insuﬃcient for ﬁlling the clefts and raising the blister roof.
Therefore, the macroscopic blisters were diﬃcult to be
developed on the porcine skin used in our tests. However,those intro-epidermal clefts act well as an indicator of

After one set of rubbing tests under the same tangential
traction–rubbing time condition, the number of piecesof skin that obtained blisters was observed under the
microscope. However, the results we observed after rubbing tests were all censored data [33], and it is significantly diﬃcult to obtain the exact time condition of
the initial blister generation. For example, if we observe
one piece of porcine skin that developed blisters after arubbing test, then the only information it can provide us
is that the blisters were generated before this observation
time; however, no one can determine when it was initiallygenerated. Such results are regarded as left-censored data.
In contrast, if a piece of skin did not develop blisters after
a rubbing test, we can infer that it will develop blisters in
the future but we still cannot determine the exact time.
Such results are called right-censored data.
We conducted experiments for each set of the porcine
skin consisting of three pieces, and the results of all the
rubbing tests are summarized with the tangential traction and time as shown in Figure 5. The percentages ofgenerating friction blisters vary: 100%, 67%, and 33%,
respectively, from the left to the right in Figure 5.
4.3. Inherently safe thresholds to prevent friction
blisters on porcine skin
To estimate the probability of the blister generation on
the porcine skin based on the current results, a survival
analysis technique for the censored data was selected sothat this special state can be taken into consideration [33].
In general, the diﬀerent groups of the three samples
tested under diﬀerent rubbing time values have similar
properties with regard to blister generation. For most of
the tangential traction values tested in our experiments,
the second piece of porcine skin used less than 50 s fordeveloping blisters after the ﬁrst piece of skin developed
blisters. If we considered the severest state, this piece ofskin might develop blisters just after the shortest observation time; i.e. if we did not stop the rubbing testfor observation, this piece of skin would develop blisters
at the previous stopping time point. As the applicationof the survival analysis technique requires at least one
datum that is not censored, we assumed that only one
piece of skin developed blisters exactly at the observation time, instead of one sample developing blisters after
this time. Similarly, for almost all the results, the testin which two blisters were generated is only 50 s or

6

X. MAO ET AL.

Figure 5. Experimental results after rubbing tests.

100 s shorter than the one in which three blisters weregenerated under the same tangential traction. Therefore,a similar assumption was considered for the results inwhich two samples developed blisters; i.e. one more pieceof skin developed blisters exactly at the observation time
point, instead of one sample developing blisters after thistime. According to the above assumptions, the original
experimental results can be converted to the ones listed
in Table 1.
Under these assumptions, if we consider a grid of time
points 0 = t0 < t1 < t2 < · · · < tm as the rubbing time wetested previously, di as the number of samples developing
blisters at time ti , Yi as the number of samples at risk attime ti , ri as the number of samples developing no blisters
at time ti , ci as the number of samples developing blisters
before time ti , and j as the order of only the time points
with left-censored data, then the survival functions fordiﬀerent tangential traction values can be estimated basedon the following algorithm [33]:Step 0: Compute an initial estimate of the survival function, S0 (tj ), at each tj based on the product-limit estimatorwhile ignoring the left-censored samples,


di
1−
Ŝ0 (tj ) =
Yi
t ≤t
i

(6)

j

where i = 1, 2, 3, . . . ; j = 1, 2, 3, . . . .
Step (K) - 1: Using the current estimate of the survival
function, SK−1 (tj ), at each tj , estimate pji for i ≤ j :pji =SK−1 (ti−1 ) − SK−1 (ti )1 − SK−1 (tj )(7)where K = 1, 2, 3, . . . .
Step (K) - 2: Use pji for estimating the number of samples
developing blisters at time ti :md̂i = di +cj pjij=1(8)where m is the total number of left-censored samples.
Step (K) - 3: Calculate the usual product-limit estimator
based on d̂i and ri at ti , while ignoring the left-censored
samples. If this estimate, SK (t), is close to SK−1 (t) for all
ti , stop this procedure; if not, increase K by 1 and go backto Step (K) - 1 and repeat the subsequent steps.
Based on the Ŝ(t) estimated above, the mean time
for the blister generation restricted to the interval [0, τ ],where τ is the time required for obtaining three blistersamples, can be estimated using
μ̂τ =

 τ
0

Ŝ(t)dt(9)

The expected value of the deviation of the blister generation probability, σ̂ , can also be computed using
n

σ̂τ =

pi · (ti − μ̂τ )2

(10)

i=1
i−1
pk , p0 = 0 and i = 1, 2, 3, . . ..
where pi = 1 − Si − k=0
Combining the estimated mean value and variance ofthe blister generation time condition, a time point forevery tangential traction value, such as μ̂τ − 3σ̂τ , can becalculated with a relatively less blister generation probability. This kind of points is deﬁned as the inherently safe
tangential traction–time conditions in this paper, and thepractical method of deﬁning inherently safe conditions
for the usage of robots is discussed in the later part.
To provide robot users with more options, three kindsof inherently safe time conditions indicating three diﬀerent levels of safety are computed by μ̂τ − 2σ̂τ , μ̂τ − 3σ̂τand μ̂τ −4σ̂τ , and summarized in Table 2. The inherently
safe points and all the time points tested under one tangential traction value, 30 kPa, are shown in Figure 6, fromwhich, it can be deduced that the possibility of obtaining
blisters under the inherently safe condition is eﬀectively
reduced.
Three regression curves were drawn from the three
types of inherently safe time points. They were regarded

ADVANCED ROBOTICS

7

Table 1. Converted experimental results based on assumptions.
Tangential traction [kPa]Group oftested samples

Incidence of samples/ Survival times [s]

42

40

36

34

32

30

1st 3 of 9 samples

33%
33%
33%

(0, 300)
300[300, +∞)

(0, 450)450[450, +∞)

(0, 500)
500[500, +∞)

(0, 450)450[450, +∞)

(0, 950)950[950, +∞)

(0, 1400)
1400[1400, +∞)2nd 3 of 9 samples

67%
33%

(0, 350)350(0, 500)500(0, 550)550(0, 500)500(0, 1000)1000(0, 1600)
1600

3rd 3 of 9 samples

0%
100%

[350, +∞)
(0, 400)[500, +∞)
(0, 600)[550, +∞)
(0, 600)[500, +∞)
(0, 600)[1000, +∞)(0, 1100)[1600, +∞)(0, 2000)

Table 2. Inherently safe conditions to prevent friction blisters on porcine skin.
Tangential traction [kPa]τ [s]μ̂τ [s]σ̂τ [s]μ̂τ − 2σ̂τ [s]μ̂τ − 3σ̂τ [s]μ̂τ − 4σ̂τ [s]

42
40
36
34
32
30

400
600
600
600
1100
2000

322.92472.92522.92472.92972.921491.66

24.91
24.91
24.91
24.91
24.91
99.65

273.1
423.1
473.1
423.1
923.1
1292.36248.19398.19448.19398.19898.191192.71223.28373.28423.28373.28873.281093.06

Figure 7. Inherently safe condition to prevent friction blisters on
porcine skin.
Figure 6. Comparison between the inherently safe time condition
and tested time condition.

as three types of inherently safe thresholds for exhibiting aunique characteristic of the porcine skin friction blisters
in the tangential traction–time relationship (Figure 7).
Moreover, the equations of these thresholds are shownin the same ﬁgure, where the vertical axis, y, represents
the tangential traction condition and the horizontal axis,
x, represents the time condition. It should be noted thatthe three inherently safe thresholds shown in Figure 7
only act as an example for three possible types of theinherently safe conditions. The inherently safe conditions

with the type of μ̂τ − α σ̂τ can help the users of robots
to avoid friction blisters; however, the real number α
should be decided according to the various speciﬁc social
backgrounds.

5. Feasibility confirmation of porcine skin forsafety verification
To conﬁrm the feasibility of using porcine skin as anexperimental model for developing friction blisters, thecharacteristics of blister generation should match withthe characteristics obtained by the studies conducted on

8

X. MAO ET AL.

normal human skin, such as the one performed by Naylor [24]. The normal force set in Naylor’s study was also
5 N [24], and the tested area on the human body was alsosimilar to our tested area; therefore, it is possible for us tocombine the results of human skin tests with ours. However, in our rubbing tests, the tangential traction was set as
the parameter for analyzing the interaction between therubbing head and the porcine skin surface, which was different from Naylor’s results described with the compression and average friction force only [24]. To compare theblister generation characteristics, a condition-matchingcalculation was applied for converting the parameters
used in the Naylor’s study to ours.
5.1. Matching diﬀerent experimental criteria
As Naylor applied a small hemisphere head for rubbing [34], the tangential traction value tested in theirstudies can be approximately calculated based on the
Amontons’ law [32]3μd Pq(r) =
2πa2√
a2 − r 2a(11)

where μd is the dynamic friction coeﬃcient between thehuman skin and the rubbing head, P is the total load
compressing the skin surface, and a is the radius of thecontact circle. According to Hertz theory [32], a can becalculated as follows:

a=

3PR
4E ∗

1
3

(12)

where R is the relative curvatures. The principal curvatureof the two surfaces contacting with each other, R1 and R2 ,are used for calculating R with the following equation.
1
1
1
+
=
R
R1
R2

(13)

In addition, using the elastic moduli of the two surfaces
contacting with each other, E1 and E2 , and Poisson’s
ratios, ν1 and ν2 , E ∗ can be calculated as follows:
1 − ν12 1 − ν22
1
=
+
E∗
E1
E2

(14)

The rubbing head used in the Naylor’s study was fabricated using polythene. As its elastic modulus is considerably larger than that of the skin, we can ignore its
contribution to E ∗ . With regard to the tested humanskin area, which was located in about the middle thirdof the anterior surface of the tibia [24], the Poisson’s
ratio and the elastic modulus used in the above equations

were assumed to be 0.45 and 130 kPa, respectively, [35].
Based on this condition-matching calculation, the friction blister generation conditions tested in the Naylor’s
study can be ﬁnally described with the rubbing time andits corresponding tangential traction values. Moreover,as the highest tangential traction in the center of thecontacting circle played a dominant role in generating
blisters, the p(0) was considered as a representative value
in the tangential direction.
5.2. Feasibility analysis of porcine skin for safety
veriﬁcation test
To provide a more appropriate inherently safe threshold for human blister generation state, we calculated theinherently safe points from the Naylor’s results [24] andused them as a reference for improving the previous
threshold. As the tangential traction gradient in our studyis 2 kPa, the tangential traction values tested in the Naylor’s study were also separated into several groups witha diﬀerence of 2 kPa. The subjects tested in one groupwere regarded as being tested under the medial tangential
traction of this group, and then, their inherently safe
points, μ̂τ −2σ̂τ , μ̂τ −3σ̂τ , and μ̂τ −4σ̂τ , were computed
using the similar method described in Section 4.3. The
results of this calculation are summarized in Table B1 ofAppendix 2.
As the negative values in Table B1 had no practicalsigniﬁcance, these negative values were ignored. Only
the positive values were added to the inherently safe
points obtained in Section 4.3 for calculating an enhanced
inherently safe threshold. In addition, the inherently safe
points calculated using the excessively concentrated data(with σ̂τ < 5) were also removed from this enhanced
operation because they contributed less to the statistical
distribution. As the tangential traction applied on the skinof the users of the robots is usually not signiﬁcantly large,
this time we only focused on the area of lower tangential
traction. The three enhanced inherently safe thresholds
are shown in Figure 8. From Figure 8, it can be concluded
that the three diﬀerent thresholds are almost similar toeach other.
Therefore, the medial threshold calculated according
to the inherently safe points of μ̂τ − 3σ̂τ was selected
as an example of the inherently safe threshold and usedin the following discussion. As noted before, the real
number α in μ̂τ − α σ̂τ should be selected adequately
in the community where the safety veriﬁcation data areused considering other factors such as social background.
To conﬁrm the diﬀerences between the enhanced
threshold and the previous one, we combined the curves
for these two thresholds in Figure 9. As shown in theﬁgure, the inherently safe points calculated using part ofADVANCED ROBOTICS

Figure 8. Inherently safe threshold improved by combining
porcine skin’s and human skin’s test results.

Figure 9. Comparison of two kinds of inherently safe thresholds.

the human test results do not change the entire thresholdconsiderably. The agreement between these two thresholds again proves that it is feasible to apply our novel
method for determining the inherently safe threshold forhuman users.

5.3. Analysis of inﬂuence of outlier on ﬁnal result
Considering that our study mainly focuses on the practical operation of physical assistant robots, a more detailed
discussion is essential for the points in the relatively

9

Figure 10. Comparison of different thresholds for analyzing
outlier’s influence.

longtime region in Figure 9. After the regression curve
was drawn from the inherently safe points and was regarded as the inherently safe threshold, one point (398.19
s–34 kPa) constituting a small inconsistency with thiscurve, in the long time, was observed.
For analyzing the inﬂuence of this outlier on the entire
threshold, a new inherently safe threshold was calculated
from the same group of safe points but ignored the outlier
point. The new threshold was compared with the previous one, and the combined threshold curves are shownin Figure 10. As 1600 s was the longest rubbing time
under which at least one no-blister result can still beacquired, more attention was paid to the time interval thatis appropriately 1600 s; however, there was only a small
diﬀerence, 0.68 kPa at maximum, between these two
thresholds, and the percentage of this diﬀerence to theentire corresponding tangential traction range was only
2%. Therefore, the inﬂuence of the outlier is regarded tobe suﬃciently small and can be eliminated. Moreover, the
new threshold computed without the outlier is applied tothe ﬁnal safety veriﬁcation.

6. Safety validation test using porcine skin andmanipulator
Owing to the diﬀerence in the contact states between thestainless steel plate–porcine skin in Figure 1 and the cuﬀ–
human skin of the physical assistant robot, it is essential
to conﬁrm the feasibility of the characteristics obtainedin Section 5. Two series of safety validation tests were
performed. These tests focused on the ‘stand-up and sitdown’ motion of a human subject, which had the ability
to exhibit a general function of a physical assistant robot.

10

X. MAO ET AL.

Figure 11. Experimental setup of preliminary application.

6.1. Experimental design of safety validation
6.1.1. For original ‘stand-up and sit-down’ motion
In our previous studies [36], the original cuﬀ–skininteractive force was ﬁrstly measured on human subjects
wearing physical assistant robots. After equipping a contact sensing function by implementing iterative feedback
tuning (IFT) control, a manipulator exerted the interactive force repetitively on a piece of dummy skin using a
cuﬀ mounted on its end tip [36]. This manipulator was
also utilized for performing our safety validation tests.
Moreover, in the ﬁrst series of the validation tests, themanipulator rubbed the porcine skin by repeating theactual human skin–robot interactive force, which wasmeasured during the ‘stand-up and sit-down’ motion ofa human subject, on the skin surface.
6.1.2. For relatively higher human–robot interactive
force
Considering the hazards of the relatively higher part in
the entire human–robot interactive force, it is essential
to ensure the feasibility of our safety veriﬁcation method
under higher friction force. Therefore, the original human–robot interactive force was increased by 1.3 times, and a
2.5 s part with the highest force value in the middle wasrepeated on porcine skin in the second series of safetyvalidation tests.

6.2. Experimental setup of safety validation
Figure 11 shows the main part of the experimental
apparatus used in the validation test, where the cuﬀ ofa physical assistant robot is simulated using a piece ofurethane foam and the structure of fat and muscle are
simulated by pasting the polyurethane gel (whose hardness, as measured using the ASKER Durometer Type C, is0) on the dummy leg. The porcine skin was ﬁnally pasted
on the gel after the same preprocessing method describedin Section 2.1 and the multiple-layer structure was ﬁxed
using tapes and bands.

Figure 12. Samples’ positions for safety validation.

6.3. Experimental result inferences
6.3.1. For original ‘stand-up and sit-down’ motion
In the ﬁrst series of validation tests, three pieces of intact
porcine skin were tested under the same interactive force
as the one measured during the ‘stand-up and sit-down’motion of a robot user. According to previous studiesof the normal rehabilitation training program, patientsusually need 100 repetitions at most for enhancing their‘stand-up and sit-down’ motion every day [37,38]. To
conduct suﬃciently long and convincing tests, each of
the test lasted for 1800 s. Moreover, the human–robot
interactive force of one ‘stand-up and sit-down’ motion
was repeated for more than 200 times.
As the smallest contact area between the porcine skinand the cuﬀ was approximately 12×10−4 m2 and thehighest friction force exerted on the porcine skin was 27.7
N, the highest tangential traction exerted on the porcine
skin was approximately 23 kPa. As shown in Figure 10,
the experimental condition, 1800 s–23 kPa, is located in
the inherently safe region and the tested skin should be
relatively safe in less than 7500 s. Even if the highest
tangential traction is exerted on the porcine skin for theentire 1800 s, there should be few blisters observed in theskin samples. Therefore, we supposed that the porcine
skin would remain intact after the rubbing test.
6.3.2. For relatively higher part of human–robot
interactive force
After the original force value was increased by 1.3 times,
the highest tangential traction exerted on the porcine skinbecame 30 kPa. As shown in Figure 10, the experimental
condition, 1800 s–30 kPa, is located in the hazardous
region and there may be friction blisters generated in
the tested porcine skin. Two other durations, 1000 s and2600 s, were also considered for conﬁrming the feasibilityof our safety veriﬁcation method under diﬀerent conditions, which should, respectively, acquire intact porcine

ADVANCED ROBOTICS

11

Figure 13. Appearances of three porcine skin samples after first series of safe validation tests (arrows indicate intact stratum spinosum).

Figure 14. Appearances of nine porcine skin samples after second series of safe validation tests (arrows indicate friction blisters or
damaged skin surface).

skin and damaged samples according to our veriﬁcation
results in Figure 10.
6.4. Safety validation test results
The positions of samples used for observation after thetests are shown in Figure 12. The area of the porcine skinsurface under rubbing is indicated by a red rectangle in
Figure 12. Three samples were considered, each from thetop, medium, and bottom parts of the skin–cuﬀ contact
area after every validation test. The purple rectangles
show the examined areas. Each of them contains a small
additional control part, which did not contact the cuﬀ
during the entire test. Moreover, the part, which is located
far from the skin–cuﬀ contact area and marked by a green
circle shown in Figure 12, was considered for providing
a control sample and for ensuring the intact condition of
the original skin.

12

X. MAO ET AL.

6.4.1. For original ‘stand-up and sit-down’ motion
Using the same method described in Sections 2.1 and4, the porcine skin samples after validation tests were
processed and examined. After the ﬁrst series of tests, noblister was observed in the skin samples. As shown in
Figure 13, the appearances of the skin samples selectedfrom three positions are almost similar to each other, witha slight stratum corneum avulsion but an intact stratum
spinosum. This result indicates that the duration timeof 1800 s ‘stand-up and sit-down’ motion is suﬃciently
short for the users of the robot system with the cuﬀ under
our experiments to accept it.

considering the subject individualities. An inherently safe
condition region for the tangential traction–time relationship was then estimated by a nonparametric survival
analysis technique, which eﬃciently used the experimental results containing the censored data. After performing
a condition-matching calculation, it was observed thatthe inherently safe condition for the porcine skin was
similar to that for the human skin. This ﬁnding further
veriﬁed the feasibility of our novel safety veriﬁcation
method for the human users. Our novel method was thenused for performing safety validation tests focusing on
the ‘stand-up and sit-down’ motion of a physical assistantrobot user. Based on the safety veriﬁcation results, correct
inferences were made for the experimental results, which
conﬁrmed that our novel method is suﬃciently practical
for the safety validation of the physical assistant robots
with possible physical stress hazards.
Considering that the daily usage of a physical assistant robot typically exerts relatively low tangential traction on the skin of the users, in this study, we provide
more attention to the low tangential traction area. In the
future, more tests will be performed on porcine skin at
higher tangential traction values for further enhancing
the inherently safe threshold.

6.4.2. For relatively higher part of human–robot
interactive force
The results of the second series of safety validation tests
are shown in Figure 14. From Figure 14(a), it can beclearly observed that the porcine skin samples after the1000 s test still maintain intact epidermis layers, without
considering from where the sample is selected. However,
several friction blisters can be observed in the epidermis
layer from 1800 s as indicated by the arrows in Figure
14(b). After 2600 s tests, the epidermis layers of the tested
samples were heavily damaged or even removed from theskin surface as shown in Figure 14(c).
The second series of safety validation tests eﬀectively
demonstrates that the inherently safe condition summarized using our veriﬁcation method described in Section 5 is suﬃciently feasible for helping the robot users
to avoid blister injuries, even though the experimental
environment in the veriﬁcation method is diﬀerent fromthe environment of the actual robot usage. Moreover,
all these safety validation test results are consistent with
our inference for relatively longer time of rubbing tests,
which cannot be evidently concluded from the Naylor’s
study. The validation tests conﬁrm not only the feasibilityof our novel safety veriﬁcation method for the actual
human–robot contact state, but also its practicability forthe relatively longtime utilization of physical assistantrobots.

Acknowledgements

7. Conclusions

This work was supported by NEDO (New Energy and Industrial Technology Development Organization) and AMED
(Japan Agency for Medical Research and Development) [grant
number 16he1202004h0004].

A novel safety veriﬁcation method was established in thisstudy for preventing friction traumas on the skin of physical assistant robot users. To the best of our knowledge,
we ﬁrst conﬁrmed the feasibility of using porcine skin as
a substitute material for the experimental friction blister
generation by performing a series of time-gradient rubbing tests. We also identiﬁed its unique properties withreference to previous results for human skin. In addition,
we used an adaptive repetitive testing method for eﬃciently clarifying the blister generation condition while

The authors would also like to thank Dr. Cota Nabeshima,
Cyberdyne Inc., for discussing the possible injury in the framework of contact safety associated with physical assistant robots.
The authors are also grateful to Prof. Susumu Hara, Nagoya
University, for his helpful advice of data analysis improvement,
and Mr. Ishiguro Kenji, Nagoya University, for his technicalsupport for human–robot data collection.

Disclosure statement
No potential conﬂict of interest was reported by the authors.

Funding

Notes on contributors
Xuewei Mao received the BE and MS degree in Measurement
and Control Technology and Instrument in 2010 and 2013,
respectively, from the Graduate School of Precision Instrument
and Opto-electronics Engineering, Tianjin University, China.
Since 2013, she has been a PhD candidate at Nagoya University,
Japan. Her main areas of research interests are mechanicalsafety and human-robot interaction.

ADVANCED ROBOTICS

Yoji Yamada received a doctor degree from Tokyo Institute ofTechnology in 1990. He had been with Toyota Technological
Institute, Nagoya, Japan, since 1983 and became an associate
professor in the Graduate School of the Institute. In 2004,
he joined Intelligent Systems Research Institute of National
Institute of Advanced Industrial and Science Technology(AIST). In 2008, he moved to the Department of Mechanical Science and Engineering, Graduate School of Engineering,
Nagoya University as a professor. He is a member of Robotics
Society of Japan, and his current research interests include
safety and intelligence technology in human/machine systems,
their robotic sensing and control.
Yasuhiro Akiyama received the BE degree in Engineering from
Tokyo Institute of Technology, Tokyo, Japan, in 2006, and the
MS and the PhD degree in Engineering from the Universityof Tokyo, Tokyo, Japan, in 2008 and 2011, respectively. Since
2016, he has been an assistant professor at the Graduate Schoolof Engineering, Nagoya University, Japan. His main areas ofresearch interests are mechanical safety, humanrobot interaction, and manned space mission.
Shogo Okamoto received MS and PhD degrees in Information
Sciences in 2007 and 2010, respectively, from the Graduate
School of Information Sciences, Tohoku University. Since 2016,he has been an associate professor at the Graduate School ofEngineering, Nagoya University. His research interests include
haptics and human-assistive technology.
Kengo Yoshida received the BE and MS degree in Mechanoinformatics and Systems in 2013 and 2015, respectively, from
Nagoya University, Nagoya, Japan.

ORCID
Xuewei Mao

http://orcid.org/0000-0002-2300-5723

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Appendix 1
This appendix provides a more detailed and general introduction
to the experimental procedure presented in Section 3. The main
experimental parameters and experimental procedure ﬂowchart
are listed in Table A1 and shown in Figure A1, respectively, where
Tbn and bk are initially set as 0.

Table A1. Main parameters of improved experimental method.
k
Tk
Tbn
bk
t
Testi
Q

order of test
rubbing duration for kth test
rubbing duration developing blisters in n samples
number of samples with blisters generated obtained in kth test
gradient of time
estimation of initial time when blister is generated
k
k
k



(bi − 1) +(bi − 2) +(bi − 3)
Q=
i=1

i=1

i=1

ADVANCED ROBOTICS

15

Appendix 2
The inherently safe points for human skin tested in Naylor’s study [24] were computed using the similar method described in Section 4.3.
The results of this calculation are summarized in Table B1.

Table B1. Inherently safe conditions against friction blister generation on human skin.
Tangential traction [kPa]τ [s]μ̂τ [s]σ̂τ [s]μ̂τ − 2σ̂τ [s]μ̂τ − 3σ̂τ [s]μ̂τ − 4σ̂τ [s]

78
75
73
71
69
67
65
63
61
59
57
55
53
51

102
76
108
123
165
153
192
195
237
294
255
264
354
414

96
75.5
87
88.5
111.43118.2
129.86128.32145.5
153.73159217.8
227.57264.75

4.9
0.5
20.05
34.5
25.78
25.29
37.08
38.91
52.09
57.80
48.73
56.01
78.16
96.24

86.2
74.5
46.9
19.5
59.87
67.62
55.7
50.5
41.32
38.13
61.54
105.78
71.25
72.27

81.3
74
26.85
-15
34.09
42.33
18.62
11.59
-10.77-19.67
12.81
49.77
-6.91
-23.97

76.4
73.5
6.8
-49.5
8.31
17.04
-18.46-27.32-62.86-77.47-35.92-6.24-85.07-120.21