Development of a novel test method for skin safety
verification of physical assistant robots*
Xuewei Mao, Yoji Yamada, Yasuhiro Akiyama, Shogo Okamoto and Kengo Yoshida
Department of Mechanical Science and Engineering
Nagoya University
Nagoya, Japan 464-8603
Email: xueweimaomao@gmail.com{yamada-yoji, akiyama-yasuhiro, okamoto-shogo}@mech.nagoya-u.ac.jp
yoshida.kengo@e.mbox.nagoya-u.ac.jp

Abstract—The study mainly concerns a novel blister generation method for the establishment of a safety veriﬁcation test.
This test system focuses on the skin friction trauma, which has
been found in the operators of physical assistant robots due totheir misuses or overlong operating, and can even deteriorate
into serious dermatopathya without preventing and prompt
treatment. In the present study, ﬁrst, we prove that blisters
can be generated in the porcine skin. With the histologicalsection of the porcine skin getting micro blisters, the symbols
of porcine skin’s blister in the dermis-epidermis structure are
identiﬁed with those of human blisters, and summarized for
applying this substitutive material to help researchers avoid
the ethical controversy of adopting human subjects. Second,the characteristics of blister generation in the porcine skinwere further conﬁrmed with a series of time-gradient tests.
After a condition-matching calculation, these characteristics
are veriﬁed to play a role on extending that of human blister inprevious studies and we integrate our results with the previous
human blister generation data and clarify an inherently safe
condition region in the shear stress - time relationship. Third,with this novel method, we successfully complete a safetyvalidation test on a ”stand up and sit down” activity of auser wearing a physical assistant robot, which ﬁnally proves
our ﬁndings are of practical usability in regard to a possible
physical stress hazard.

I.

I NTRODUCTION

When physical assistant robots are worn by their users
during the assistance, the close physical human-robot interaction will help the robot realize a more effective transfer of
power to the human musculoskeletal system [1], and assist
the user’s movements in a way similar to the natural one.
However, it has been reported in some studies that the skinwounds such as friction blisters are likely to be generated in
the use of the robots’ assistance with such a close physicalinteraction, e.g. for a wearer suffering cirrhosis of the liver[2].
It is pointed out in ISO 13482 [3] that hazards due to thephysical stress may result in injuries, and its measurement
is listed as one of the items that are requested for safety
verification and validation. Some studies make achievements*This work was supported by NEDO (New Energy and Industrial
Technology Development Organization)c
978-1-4799-1808-9/15/$31.00 2015
IEEE

on the safety issues of relatively slight injuries concerning
the human-robot coexisting system, such as the human pain
tolerance limit tested by a biomechanical method [4] ortruly static measurement [5], the safe contact realization withefficient human-robot system [6], and more general limit for
static and dynamic human-robot contact [7]; however, they
only focus on the normal stress issues and little attention has
been paid on the hazards of shear stress by the existing safetyassessment standard of machinery. By now, there has been noreports on the safety verification test method against the skinwounds mainly caused by shear stress, such as friction blister. Therefore, in this study, we focus on establishing a safety
verification mainly related to the blisters generated by thecontinuous friction on the human-robot contacting surfacing.
Considering the fact that the experimental subjects have tosuffer hurts during the friction blister test, a replacement of
the human subject is needed to avoid this ethical controversy.
Several special models, dynamic non-linear finite-element
model [8] and synthetic skin simulant platform approach [9],have already been built to simulate the skin blister generation
process. They can help to estimate the influence of textiles,
surface treatment, loading frequency and other factors on
blister qualitatively. But if we want to establish a reliablesafety standard against friction blister, a new substitute muchcloser to the human condition is still needed for analyzing
blister generation conditions quantitatively. In this study,
porcine skin is chosen as the experimental subject because of
its similar physiological and biological property with humanbeing skin from epidermis to dermis [10]. Together with theremaining individuality of different experimental subjects,
porcine skin can supply more reliable information than thoseexisting blister simulants.
Although several experiments were conducted to study
the friction genesis under various skin exposure conditions[11]–[13], none of them is similar to the actual operation
situation of a physical assistant robot. Thus, no previous
studies can help us set up a particular safety verification for
it. Since the generation of friction blister can be affectedby various factors, such as interactive force combination[11], environmental condition [11], [12] and individual skincharacteristics [13], [14], it is necessary for us to firstly
repeat the interactive force between a physical assistant319

robot’s cuff and its wearer’s skin during an actual operation.
By conducting tests on experimental subjects under therubbing conditions similar to the actual situation, we canpossibly provide a more accurate skin trauma which is also
close to the actual one.
In this study, we develop a novel blister generation
method for establishing a safety verification test system
focusing on skin friction trauma. First, Section II describes
the entire experimental setup, which can control the rubbing conditions applied on experimental subjects with highaccuracy, and reliably confirm the porcine skin’s epidermisdermis structure after test. Second, Section III details theexperimental design and proposes an efficient way to test
the effects of various shear stress-time conditions under asituation that the porcine skin cannot generate macroscopic
blisters on its surface. Based on the summarization of ourresults, the symbols of porcine skin’s blister in the epidermis
layer are characterized and an inherently safe condition
region in the stress-time relationship is clarified. Then,
Section V demonstrates that this verified safe region canalso be applied on the safety verification for human users.
This novel test method’s feasibility is finally proved by apreliminary application of a safety validation for a ”standup and sit-down” motion of a physical assistant robot’s
operators.
II.

E XPERIMENTAL M ATERIAL AND M ETHOD

The porcine skin as experimental subject was excisedfrom the anterior part of a pig’s shank shortly after its death,and delivered to our lab within sixteen hours under 4 degree
centigrade. Before the experiments, the skin surface wasshaved by a scissor, cleaned by alcohol, and the excessive
fat underneath was also cut off. In our study, the representative direction (shear direction) of force component isselected for emulating the skin surface rubbing phenomena.
For conducting a horizontal reciprocating rubbing action,
Discovery Hybrid Rheometer (DHR) (TA Instrument Co.)is used to exert oscillating rotational rubbing on the porcine
skin with high calibration accuracy. In order to fix the skin on
DHR and avoid the unexpected movement during rubbing,the porcine skin for the test was extended and then firmly
clamped between the plate with a sheet of sandpaper (40#)
attached and a fixation frame as shown in Fig.1. After acertain period of rubbing under oscillating shear force, thestructure of the porcine skin will be examined by serial
sectioning at intervals of 10 μ m and the micro-section isanalyzed by haematoxylin and eosin (H & E) and the stained
sections are observed with a micro-scope sequentially.
III.

E XPERIMENTAL D ESIGN

The shear stress distribution of the porcine skin surface
under the stainless steel plate of DHR can be calculated
according to skin’s viscosity η , shear rate γ̇ , angular velocity
Ω, skin’s thickness H and the distance from the detected
point to the center of the contact circle r [15]

τ (r) = η γ̇ (r) = η

320

Ωr
Ω
= η ·rH
H(1)

Fig. 1. Diagram of porcine skin fixation and main part of experimental
apparatus

The torque of the rubbing head, T, can be measured by
DHR and calculated with the contact area A


Ω
(2)
T = dA · τ (r) · r = η · r2 dA
H
Therefore, the shear stress on the surface under rubbing
can be finally calculated as
2T
·r
π R4
where R is the radius of contact area.

τ (r) =

(3)

Taking the individuality of porcine skin into consideration, only one trial for one shear stress-time point is notreliable enough to be used as verification data. Besides,
fixing time gradient is too stiff to quickly approach thepossible time region for initial blister generation. Therefore,
an improved experimental method was designed to solve theabove problems in a more efficient way.
First, since 3 is the least number which can help us
identify whether it is easier to generate blister under specificconditions, we tested 3 pieces of porcine skin at every
stress-time point. Second, for each stress value, a seriesof time points were tested to confirm the blister generation
conditions. During this process, we gradually modified therubbing time test by test. In order to avoid testing too many
stress-time points which are too easy or too hard to generate
blisters, the modification of rubbing time for the next test
is based on the previous results. For example, if all 3 or
2 of 3 samples get blisters after rubbing, the next step’s
rubbing time will be shorten by 100s or 50s; otherwise, itwill be lengthened by 100s or 50s when 0 or 1 of 3 samples
get blisters. Besides, some special principles for modifying
rubbing time were also applied to avoid repeated tests. The
detailed experimental procedure flow (e.g. 10 time points
tested under one shear stress value) and its main parameters
are shown in Fig. 2 and Table I, inside of which, T bn andbk are set as 0 at first.
IV.

E XPERIMENTAL R ESULTS AND D ISCUSSION

A. Feasibility of porcine skin as an experimental friction
blister model
After every rubbing test, the experimental subject was
fixed with 10% formalin for 8h, and then 20 % sucrose and2015 IEEE International Conference on Rehabilitation Robotics (ICORR)

Fig. 2.

Experimental procedure flow chart

30% sucrose for 8 hours sequentially and sectioned witha cryostat (Leica Microsystems Co.) at 10 μ m. The skinstructure presence was then detected by H & E staining.
The section of original porcine skin without any rubbing
and the microscopic appearance of porcine skin after the
first rubbing experiment are shown in Fig. 3 and 4.
It can be seen from Fig. 3 that the main epidermal
structure of porcine skin, including the stratum corneum,
stratum granulosum, stratum spinosum, and epidermal basal
layer, keeps intact before rubbing test. As supposed in theprevious experimental blister studies on normal human skin,friction force can be transmitted through ”stratum corneum
and granulosum” of epidermis, degenerate stratum spinosum,
and finally produce clefts between stratum granulosum andbasal layer. Once being filled up with fluid from dermis,
those clefts will develop into friction blister rapidly [11],[12]. Compared with the section of original porcine skinshown in Fig. 3 and the section of the porcine skin after
900s of rubbing under shear stress value 3.2 × 104 Pa shownin Fig. 5, it can be observed clearly that there is severe
split generated in the stratum spinosum layer of the porcine
skin which was rubbed under shear stress value 3.2 × 104 Pa
for 1200s (Fig. 4). According to Marion’s study [12], theblister cleft or cavity always appears at the same intraepidermal layer, with the blister roof composed of thestratum corneum, stratum granulosum, and a segment of
traumatically degenerated stratum spinosum. Therefore, thecleft in Fig. 4 with intact stratum granulosum and stratum
TABLE I.

Fig. 3.
Appearance of original porcine skin(intact stratum spinosum
indicated by arrow)

Fig. 4. Intra-epidermal clefts (indicated by arrow) after 1200s of rubbing
under shear stress value 3.2 × 104 Pa

M AIN PARAMETER OF IMPROVED EXPERIMENTAL
METHOD

k
Tk
Tbn
bk
Δt
Testi

the order of trial
the rubbing time of kth trial
the rubbing time of getting n samples with blister
the number of blister sample got in kth trial
time gradientestimation of initial blister generation time

Fig. 5. Intact stratum spinosum (indicated by arrow) after 900s of rubbing
under shear stress value 3.2 × 104 Pa

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TABLE II.

T HE RESULTS OF EXPERIMENTS TESTED WITH THE
IMPROVED EXPERIMENTAL METHOD

Shear stress [104 Pa]

Rubbing time [s]Number ofblister-samples
after rubbing

4.2

300
350
400

1
2
3

4.0

450
500
600

1
2
3

3.6

500
550
600

1
2
3

3.4

450
500
600
700

1
2
3
3

3.2

950
1000
1100
1200

1
2
3
3

3.0

1400
1600
2000

1
2
3

corneum, can be regarded as an evidence of the first stageof blister generation. Because of the biophysical propertychange in vitro compared with in vivo experiment, little fluid
remains to fill those clefts and raise the stratum granulosum
and corneum as obvious blister roof. Even though it isdifficult to get macroscopic blister on dead porcine skin with
our test, the cleavage indicating blister generation is also able
to prove the effectiveness of this new experimental model inobtaining the friction blister generation time. This kind ofcleavage is regarded as the symbol of blister generation in
the following experiments.
B. Results of the Experiment Conducted with Improved
MethodThe first shear stress value tested with improved experimental method is 4.2 × 104 Pa and the first rubbing time forthis shear stress was set as 400s and all of the 3 samples got
blisters and one of them was heavily destroyed. Based on
the action selection principle in Section III, the next rubbing
time was set as 300s, which was 100s less than the lasttime value. This time, only one sample generated blisters
while the other two samples’ epidermis still kept intact.
Therefore, the next rubbing time was set as 350s, which
was 50s longer than the last one. On the contrary, only one
sample kept intact after the third step and the other two evenlost their epidermis. Until now, with only 3 tests, the time
points around the most worth being tested points were bothtested, which further verified the efficiency and feasibility of
the improved experimental method.
Five more shear stress values were also tested with thesame improved experimental method. The results of thisseries of experiment were summarized by Table II.

322

C. Calculation of Inherently Safe Condition Thresholdagainst Blister Generation on Porcine Skin
Since we tested 3 samples for every shear stress-time
point, the points generating 1, 2 or 3 blister-samples can be
regarded as the conditions which can cause blisters in 33.3%,
66.7% or 100% of experimental subjects respectively. Supposing that the rubbing time is normally distributed as blister
generation conditions for a certain shear stress, a rubbing
time point, which can only cause blister in less than 0.13%
of subjects, can be calculated according to the ”3σ -rule”,
and such a time point can be regarded as an inherently safe
point against blister generation under a certain shear stressvalue. All the safe points for the tested shear stress values
were calculated respectively and summarized in Table III.
V.

I DENTIFICATION OF B LISTER G ENERATION
E XPERIMENTAL C ONDITIONS FOR S AFETYV ERIFICATION

In order to confirm the feasibility of porcine skin, thecharacteristics of blister generation should match with thoseobtained in the previous human friction blister test, which
was conducted by Naylor [11]. In our experimental model,
we set shear stress as a criterion of the interactive force
between the porcine skin and the rubbing head. In the caseof human skin test reported in the Naylor’s study, only
the compression and average friction force were describedin detail. Therefore, a condition-matching calculation isnecessary for us to convert the friction force in Naylor’s
study to the corresponding shear stress for comparing their
blister generation characteristic.
A. Matching the Experimental Conditions
Since the rubbing head applied in the Naylor’s study isa small hemisphere [16], the tested shear stress value can becalculated approximately based on Hertz theory [17]√3μ P a2 − r2
τ (r) =
(6)2π a2awhere P is the total load compressing the skin surface, μis friction coefficient, and a is the radius of contact circle,
which can be calculated by the following formula
a=(

3PR 1
)3
4E ∗

(7)

where R is the relative curvature, which can be calculatedby the principal curvature of the two contact surfaces, R1
and R2
1
1
1
+
(8)
=
R R1 R2
TABLE III.

I NHERENTLY SAFE CONDITION AGAINST FRICTION
BLISTER ON PORCINE SKIN

Stress [104 Pa]

4.2

4

3.6

3.4

3.2

3

Time [s]

227.6

329.5

427.6

329.5

829.5

918.5

2015 IEEE International Conference on Rehabilitation Robotics (ICORR)E ∗ can be calculated by the elasticity modulus of the two
surfaces in contact with each other, E1 and E2 , and Poisson’s
ratio, ν1 and ν2
1 − ν12 1 − ν22
1
=
+
∗
E
E1
E2

(9)

The rubbing head in Naylor’s study was made by polythene, whose elasticity modulus is much higher than that ofhuman skin, and thus its contribution to E ∗ can be ignored.
Considering that the tested human skin area is approximately
the middle third of the anterior surface of the tibia [11],the elasticity modulus and Poisson’s ratio of human skinare assumed to be 130kPa and 0.45, respectively [18]. Afterthe above calculation, we can finally obtain the relationshipbetween the rubbing time required to produce blisters andthe corresponding shear stress in Naylor’s study.
B. Analysis of Porcine Skin’s Feasibility as a Reliable Modelfor Safety Verification Test
We calculate to draw a regression curve from the inherently safe time points, regard it as an inherently safe
threshold, and combine it with the human subject’s blister
generation conditions to produce a new shear stress-timecharacteristic for blister generation, which is shown in Fig.
6. It can be seen that the threshold is located in the safety
region left of and below the points representing humanblister generation. We can claim from the results that it isfeasible to utilize porcine skin as a reliable safe verification
test model to confirm that a certain operation action issafe enough to avoid generating friction blisters. However,more appropriate calculation for the inherently safe time
points and drawing a threshold line may require further
consideration because there is one point which is not so
consistent with the threshold line.

We focused on a human subject’s ”stand-up and sit-down”
motion which is considered to exhibit a longest level ofrelative displacement between the cuff and the skin surfaceof the subject. According to the previous study of ourgroup [19], the interaction force between the robot’s cuff
and the user’s thigh skin can be exerted repeatedly by amanipulator with high accuracy at the endtip of which acuff with a contact sensing function is mounted. We choose
this apparatus to realize a safety validation test.
A. Experimental Setup for Preliminary Application
The experimental setup of this application is shown in
Fig. 7, where the urethane is used to simulate the cuff
material of a physical assistant robot, the polyurethane gel(hardness is 0 by use of ASKER Durometer Type C) ispasted on the dummy leg. The porcine skin, after the same
preprocessing introduced in Section II, is pasted and fixed
on the gel layer with tape and band.
B. Estimation of Experimental Results
During the series of experiments, three pieces of porcine
skin were tested for safety validation of a cuff-human skinsurface system, and every test lasted for 1800s. Since thesmallest contact area between urethane and porcine skin
is about 12×10−4 m2 and the largest shear force appliedon porcine skin is 27.7N, the highest shear stress appliedon porcine skin is calculated approximately as 2.3×104 Pa.
According to the inherently safety region shown in Fig. 6,
no blister can be generated on porcine skin inside of 3350s
under the highest shear stress. Therefore, the porcine skinshould keep intact after this rubbing test.
C. Experimental Results for Safety Validation

After confirming the feasibility of using porcine skinfor a safety verification test, we conducted an experimentvalidating a cuff attachment in the use of a wearable robot.

The safety validation test were repeated three times.
After every test, three samples were respectively taken fromthe higher, middle and lower parts where the porcine skin
was contacted with the urethane-made cuff shown by the
red rectangular part in Fig.8. Every sample taken from thepurple rectangular part in Fig.8 contains a small additional
area where the skin wasn’t be rubbed during the test. One
more sample was also taken from the uncontacted part shownby the green circle in Fig.8, which is located far from therubbing area. All of these samples were processed withthe same method introduced in Section V, but not a single

Fig. 6. Combination of porcine skin’s inherently safe conditions and humansubject’s blister generation conditions

Fig. 7. Experimental setup of preliminary application with porcine skin’ssafe verification modelVI.

P RELIMINARY A PPLICATION OF P ORCINE S KIN FOR
S AFETY VALIDATION WITH M ANIPULATOR

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323

Nagoya University, for his helpful advice of data analysis
improvement, and Mr. Ishiguro Kenji, Nagoya University,
for his technical support for human-robot data collection.
R EFERENCES

Fig. 8.

Sample position for safety validation

blister was found inside of them, which indicates that 1800sof repeated operation of ”stand-up and sit-down” are safeenough under the special conditions [19]. The actual test
result is in agreement with our estimation of the safety region
for a larger number of repeating times that was not evident
based upon the original blister generation conditions in the
Naylor’s study.
VII.

C ONCLUSIONS AND F UTURE W ORK

In the study, a novel blister generation method wasdeveloped for establishing a safety verification test, whichmainly focuses on the skin trauma for the user of physicalassistant robots during their operation. By a series of rubbing
experiments on the porcine skin with Discovery Hybrid
Rheometer, we first confirmed and summarized the possibility and special characteristics of friction blister generationon the porcine skin. Besides, an improved experimental
method was designed to conduct a series of repeated tests
which can rapidly approach to the time range really worth
testing and clarify the initial blister generation time moreaccurately. After a calculation matching the human skin’s
blister generation conditions with ours, it was revealed thatthe inherently safe threshold for the porcine skin’s blister
generation was located far from the hazardous conditions
for human skin’s blister generation, which further proves
the feasibility of our experimental system. With this novel
method, we conducted a safety validation test on a ”stand up
and sit down” motion of a user wearing a physical assistantrobot. The consistency between the estimation and actual
results of this test finally proves our findings are practicalenough for safety validation of physical assistant robots witha possible physical stress hazard.
Since now the way of estimating the inherently safe
conditions in our study is only based on an generally
accepted assumption that the rubbing time acting as a kindof blister generation conditions is normally distributed, inthe future, the calculation method for the inherently safe
threshold will be reconsidered.

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ACKNOWLEDGMENT
The authors would like to thank Dr. Cota Nabeshima, Cyberdyne Inc., for discussing the possible injury in the framework of contact safety associated with physical assistantrobots. The authors are also grateful to Prof. Susumu Hara,

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2015 IEEE International Conference on Rehabilitation Robotics (ICORR)