Sakai et al. Robomech J (2018) 5:20
https://doi.org/10.1186/s40648-018-0117-7

Open Access

RESEARCH ARTICLE

Evaluation of effects caused by individual
differences in human shape that affect the safe
utilization of wearable robots
Yuma Sakai†, Yasuhiro Akiyama*† , Yoji Yamada and Shogo Okamoto

Abstract
When using a wearable robot, the interaction force applied to the area of contact may cause skin injuries over time.
Therefore, validating the contact safety of wearable robots is important for their practical application. Because previ‑
ous studies indicated that the repetitive share stress of the wearable robots increased the risk of blister generation,
analysis of stress distribution, which is affected by the contact state, was viewed as very important. However, theeffect of variability of the shape of the human body on the shear stress applied to the contact area is rarely analyzed,
even though uneven contact between human tissue and the robotic cuff can cause stress concentration. In thisstudy, a system for safety verification and validation of the robotic cuff was developed, and the interaction force
exerted on the contact area of a variably shaped human tissue dummy when rubbed by a robotic cuff was measured.
As a result, the maximum interaction force occurred when the robotic cuff moved over the convex part of the surface.
Furthermore, the magnitude of the interaction force corresponded to the gradient. Thus, the shear stress increased
by approximately 10% as the height of the iliac spine, which originally mimicked the anterior superior iliac spine,changed by 1 mm. This variability suggests that the stress concentration caused by the unevenness of human tissue
plays an important role in the risk of blister.
Keywords: Wearable robot, Contact safety, Indevidual difference, Skin shape, Stress concentration
Background
Recently, the use of wearable robots has been on theincrease in society. For example, the usage of such robots
has expanded from rehabilitation in hospitals [1] to dailyliving assistance in society [2, 3]. Accordingly, the safe
usage of such robots is now being discussed with several
manufacturers proposing safety test methods for theirwearable robots [4, 5]. Further, the international standard IS0-13482:2014 [6], which was enacted for the safe
usage of personal care robots, also determines the safety
requirements and the safety test methods.
Because a wearable robot is directly attached tothe human body and exerts assistive torque and force
through the contact area, regardless of the part to which*Correspondence: akiyama‑yasuhiro@mech.nagoya‑u.ac.jp†
Yuma Sakai, Yasuhiro Akiyama contributed equally to this work
Department of Mechanical Science and Engineering, Nagoya University,
Furo‑cho, Chikusa‑ku, Nagoya 464‑8603, Japan

it is attached [7–10], the contact safety of the wearable
robot has to be considered carefully. In particular, blister
generation around the contact area is a major concern
from the viewpoint of contact safety because skin blisters
occur under repetitive rubbing [11, 12]. To assess the riskof blister generation, it is important to estimate the interaction force at the contact area because the magnitudeof the shear force critically affects the endurance time ofblister generation [13, 14]. Thus, in previous studies, theinteraction force exerted at the contact area of the upperlimb [15] and the lower-limb [16] during human–robot
interaction was measured.
To examine the contact safety, meaning by estimating the risk of blister generation at the contact surface
between human skin surface and a cuff of a physicalassistant robot, we previously proposed the safety test
process shown in Fig. 1. In this process, first, the relative motion between the robotic cuff and human skinand the interaction force exerted at the contact area

© The Author(s) 2018. This article is distributed under the terms of the Creative Commons Attribution 4.0 International License(http://creat​iveco​mmons​.org/licen​ses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium,provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license,
and indicate if changes were made.

Sakai et al. Robomech J (2018) 5:20

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Robot

Air pressure
Fig. 1 Wounds risk evaluation process [19]of the robotic cuff is measured [17, 18]. Then, the relative motion and interaction force are reproduced by
a 6-degree of freedom manipulator on dummy tissue [19]. The risk of blister generation when using awearable robot is then assessed by turning the shear
force applied to the dummy tissue into a thresholdcurve of the safety [14], which is drawn as the relationship between the shear force and endurance time.
However, the interaction force, which strongly affects
blister generation, does not become an even distribution because of the unevenness of the shape of the contact area. In particular, the surface shape and physicalparameters such as stiffness of the human body vary
among individuals [20, 21] because of the variance ofmusculoskeletal frame and amount of fat. However, theeffect of such contact area unevenness was not considered sufficiently. Because such uneven contact causes
stress concentration owing to the distribution of theform resistance and friction force [22], the risk of blister generation under uneven contact increases compared to even contact. Thus, estimation of the effectof variance of the surface shape of the human body on
the shear force applied to the contact area is necessary
for accurate assessment of the risk of blister generation
due to human–robot interaction.
To measure the shear force exerted during the surface rubbing motion on various surface shapes, a device
whose surface shape could be deformed to a certain
extent was developed. Further, the surface deformability of the mechanism used for this device was validated
by our group [23]. However, changes in the reaction
force exerted on the contact surface caused by differences in contact surface shape were not measured andevaluated. Consequently, by rubbing the surface generated by the device using a manipulator, as shown in
Fig. 2, the effect of surface unevenness, which differs
among individuals, on the shear force was determinedin this study. First, the design of the surface deformation device was described. Then, after evaluation of thephysical parameters of the device, the surface shapes,
which consist of the baseline shape obtained from an

Dummy skin(freely deformable)

Fig. 2 Safety test with variable shape dummy

actual human and shapes that slightly differ from thebaseline shape, were determined. Then, the shear force
exerted on the robotic cuff was measured via a rubbing test using this device and the manipulator. Thisis the newly developed system which was designed toexperimentally test and evaluate the effect of the shapeof robotic cuff and skin surface on the contact force forthe safety verification and validation of the robotic cuff.
Thus, this study constitutes the first step in the evaluation of the effect of individual differences in people’s
shapes on the safety design of wearable robots.

Three‑dimensional (3D) skin‑shape reproduction
systemSkin‑shape reproduction device

A 3D skin-shape reproduction device, shown in Fig. 3,
was previously developed [23]. It consists of 18 DC
motors (RE30, Maxon Motor, Switzerland), 18 cablesaligned in a grid, and a silicon rubber sheet. Shape reproduction proceeds as follows: First, the deformable rubber sheet is inflated by air injected from the bottom of
the deformable area, with inflation of the rubber sheet
restricted by adding tension to each cable by winding
them up using the motors. A variety of surface shapes
can be reproduced by adjusting the length of each cable.
In our previous study, it was confirmed that this device
can reproduce a variety of deformation patterns [23].

Fig. 3 Upper part of the variable shape human tissue dummySakai et al. Robomech J (2018) 5:20

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Fig. 5 Example of human shape (pelvis) [24]Fig. 4 Skin-shape reproduction process

Shape reproduction process

Skin-shape reproduction is achieved via the process
shown in Fig. 4. First, an objective shape, which corresponds to the deformation part of the human dummy,
is obtained. Next, the length of each cable is calculated
as the length of the pathways, which connect the endsof each cable, on the surface of the objective shape. The
calculated lengths become the target value of each cable.
Then, after initial pressure and pretension are applied
to the device, the tension of the cables is optimized toachieve the target length.

Fig. 6 Human shape reproduction area and coordinates

Validity of generated surface shape and elasticity
as a human dummy
Reproduction of human pelvis

The validity of the surface shape of the device was tested
by comparing the surface shape of the device and the target shape obtained from a human. In this study, the partaround the anterior superior iliac spine (ASIS), in which
the bone is pointed, as shown in Fig. 5, was set to thetarget shape. Fig. 6 shows the reproduction area and thereference coordinates. The reproduction area is definedas the area surrounded by the red line considering thewound risk and the configuration or the general wearable
robot. An example of the shape reproduction is shown in
Fig. 7. The figure shows dummy skin made from urethanegel, which mimics the characteristics of human tissue,
fixed onto a rubber sheet. Thus, the bumps of the surface

Fig. 7 Shape reproduced with dummy skinof the membrane formed by cables does not affect thesurface shape of the device. The values for the mechanicalparameters of the device were set as displayed in Table 1.
The size of the surface and the distances between cables
were designed to set in a convex of the human body.

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Table 1 Conditions of the shape reproduction parametervaluesYoung’s modulus (rubber sheet)

5.4 MPa

Young’s modulus (cable)

1.5 GPa

Dummy skin hardness

0 (Asker C)

Poisson’s ratio

0.5

Rubber sheet thickness

2.0 mmDummy skin thickness

3.0 mmDeformation area (x-direction)

90 mm

Deformation area (y-direction)

90 mm

Number of cables (x-direction)

9

Number of cables (y-direction)

9Interval of cables

10 mm

Confirmation of shape reproducibility

For the validation check of the shape reproduced using
the device, the displacement and curvature of the device
and target shape were compared in Figs. 8 and 9. The
shape of the device was measured using a 3D scanner.
In the figures, the comparison was conducted at y =Fig. 9 Curvature comparison results. a Curvature comparison (y = 5),b curvature comparison (x = 5)5 mm and x = 5 mm, where the curvature was largest.
The graphs show that the device reproduced the featureof the target shape-in this case, the local bulge. Thus, inthis study, the reproduced shape was determined as thebaseline shape. Then, shapes slightly deformed from thebaseline shape were used to evaluate the effect of the difference of unevenness.
Elasticity of the deformed surface

Fig. 8 Displacement comparison results. a Displacement comparison(y = 5), b displacement comparison (x = 5)

Because the device comprised the musculoskeletal system and surface tissue of a human, the deformed surface
has to reproduce the mechanical properties of humanbone, muscle, and fat. Thus, the elasticity in the normal direction and the shear modulus of the device were
measured.
The elasticity in the normal direction was measured
using the manipulator shown in Fig. 10. In this method,
the contactor attached to the robotic cuff was pushed
into the reproduced body. Then, the reaction force
applied to the contactor was measured using a force sensor. In this experiment, the area of the contactor was
64 mm2 and the indentation force was measured up to adepth of 5 mm. As a result, the normal stress per indentation depth was calculated as 64.0 kPa/mm. AlthoughSakai et al. Robomech J (2018) 5:20

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Fig. 11 Transverse modulus evaluation of the reproduced shape

Fig. 10 Measurment of the elasticity in normal direction of the
reproduced shapethe elasticity in the normal direction around ASIS was
not reported previously, the rigidity of the relaxed muscle
with a large deformation was estimated at approximately
5.5 kPa/mm using the strain–stress diagram [25]. Thissuggested that the device was much stiffer than the soft
tissue of a human and was closer to that of the part withthe thin tissue supported by bone.
The transverse elasticity was also measured, as shownin Fig. 11. In this experiment, the robotic cuff was moved
along the shear direction using the manipulator, which
was preloaded to avoid slippage. The reaction force in theshear direction was measured with a force sensor. The
area of the contactor was approximately 1750 mm2 andthe deformation in the shear direction was up to 5 mm.
The shear modulus was calculated by dividing the shear
stress by the shear strain. The shear strain was calculatedby dividing the motion distance of the cuff by the thickness of the deformable part of the device. The shear modulus of the device, 7.48 kPa, was relatively close to that ofhuman muscle, 5.40 kPa [26].

Effect of surface shape variability on interaction
force
Evaluation process using the surface deformation device

The evaluation of the effect of surface shape variability on
the interaction force exerted during rubbing motions was
conducted using the surface deformation device and themanipulator. As mentioned above, the baseline shape wasdesigned from the 3D shape data around ASIS, acquired
from a human. Then, surface shapes with slightly different gradients from the baseline shape, were designed
from the baseline shape. The interaction force when the
robotic cuff rubbed these surfaces using the manipulator
was measured and compared to each other.

Fig. 12 Target shapes for the experiment

Design of the surface shape variance

It was hypothesized that the shear force on human skinincreases when the cuff is over the convex because oflarger form resistance and friction. In this study, threeshapes, plotted in Fig. 12, were used to evaluate the effectof shape variance. These shapes were designed by modifying the ASIS ( x = 5 mm and y = −10 mm) part of thebaseline shape to increase or decrease its height by ±1 mm, as shown in Fig. 12.
Experimental protocol

Because the interaction force exerted when rubbing
on the uneven surface is affected by the normal forceof the robotic cuff, the normal force of the manipulator was controlled to 20 N, which was the value exerted
at the cuff when using a wearable robot in the previous
study [17]. The surface of the robotic cuff was covered by
a 40 mm × 40 mm × 15 mm sponge sheet, determinedby considering the softness of the cuff used in commercial wearable robots. In this study, initially, the center ofthe robotic cuff was set at x = 5 mm and the front edge
was set at y = −15 mm. Then, the robotic cuff rubbed
the surface along the y-direction, keeping the force inthe z-direction constant, as shown in Fig. 13. DuringSakai et al. Robomech J (2018) 5:20

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Fig. 14 Interaction force pattern in the shear direction

Table 2 Maximum
in the experiments
Shape

Fig. 13 The rubbing motion using the manipulator

the rubbing motion, the position of the robotic cuff andinteraction force exerted on it was measured. This process was repeated 10 times for each condition.
Result of experiment

The recorded shear force on the different surface shapes
is shown in Fig. 14. This result suggests that the interaction force in the shear direction increases with gradient.
As hypothesized, this is probably caused by the increaseof the form resistance as the gradient increases. Consequently, the interaction forces increase when the robotic
cuff moved over the convex part of the surface.
The maximum shear forces and stresses observed in
the experiments are shown in Table 2. The maximum
shear stress changed approximately 10–20% even when
the height changed 1 mm, as shown in Fig. 12.

Discussion
Relationship between surface shape and interaction force

The exerted shear force shown in Fig. 14 and Table 2 suggests that there is a relationship between the pattern ofunevenness and the interaction force. First, the interaction forces among the different surface shapes did not
differ until around y = −8 mm, although the shape under
the robotic cuff differed, as shown in Fig. 12. This wasprobably because the effect of the variance of the surface
shape was absorbed by the deformation of the spongeof the robotic cuff. The shear modulus may also haveshear

forcesandstresses

Maximum shear force Maximum(N)
shear stress(kPa)

Gradient (large)

23.5

14.7

Actual human shape

21.7

13.6

Gradient (small)

18.9

11.8

contributed to this easing. Then, the interaction force
started to show the difference among gradient values, as
stated above.
When the front edge of the robotic cuff reachedapproximately y = 5 mm, the increase rate of the interaction force started to decline. In this phase, the partial
contact of the robotic cuff, a part of which separated fromthe surface of the device, was observed. Because the normal force of the robotic cuff is controlled to a constantvalue, the robotic cuff moved upward when getting overthe convex. Finally, the interaction force of the baseline
shape and large gradient shape started to decrease after
y = 10 mm. Although the shear force did not increase,the partial contact probably causes stress concentration,
which could not be observed even using this measurement system.
Because the target of this study was to identify theincrease of the wounds risk caused by the variance of surface shape when using the wearable robot, some featuresof the contact state between the robot and human, suchas the shape and stiffness of the robotic cuff, viscoelasticity of the wearer’s tissue, and the loading pattern of therobotic cuff on the surface, were simplified. Thus, more
precise experimentation and analysis is required for practical validation testing of the wearable robot. However,the necessity of considering the surface shape and its variance based on the human were expounded in this study.

Sakai et al. Robomech J (2018) 5:20

Effect of surface shape variance from the viewpointof contact safety

When evaluating the risk that appears after several exposures to the hazard, the method using the threshold curveis commonly used. By using the threshold curve, theendurance time, which is the threshold time of acceptable risk, can be estimated under specific loading conditions. Thus, using the curve that evaluates the wounds
risk caused by continuous rubbing [27], the endurance
time under various surface shapes was estimated andcompared.
The shear force exerted in rubbing motions increases
with the normal force. According to previous study [17],the normal force exerted when using a wearable robot
seldom exceeded 50 N. Thus, in this section, considering the severe condition when using the wearable robot, arubbing motion under 50 N of normal force was assumed.
In addition, because of the limitation of the specificationof the device, it was difficult to keep the surface shape
under such a large compression force. Thus, the shear
force under the normal force of 50 N was estimated byincreasing the pattern of the interaction force observedin the experiment mentioned above proportionally.
The estimated maximum shear stress and the endurance time under such a condition area displayed inFig. 15 and Table 3. The risk curve displayed here wasobtained from the rubbing test using porcine skin andhuman skin [27]. These results suggest that the endurance time significantly changes even when the surface
shape changes only slightly. In addition, the risk curveused in this study did not include the effect of local stressconcentration. As mentioned above, it suggests that thestress concentration occurs over the large convex. Moreover, the existence of stress concentration was verified
by visualizing the pressure distribution on the surfaceof the device. A tactile sensor sheet (I-SCAN, Nitta Corporation, Japan), which could measure the normal force

Fig. 15 Threshold curve for blister generation [27]Page 7 of 8

Table 3 Change of safety condition caused by the changeof shapes under the 50 N indentation force
Shape

Maximum shear
stress (kPa)

Endurance time (s)Gradient (large)

36.8

453

Actual human shape

33.9

676

Gradient (small)

29.5

1.32 × 10 3

exerted on square cells lined on the surface, was used.
By putting this flexible sheet on the uneven surface, theconcentration of normal force was roughly observed via arubbing test. The observed force concentration made theendurance time short, as shown in the risk curve.
For the safety design of the cuff of the wearable robot,
the increase and concentration of shear force around theconvex will cause a problem. Thus, it seems better to use
maximum pressure instead of mean value when evaluating the contact force under the robotic cuff. Furthermore,
the shape of robotic cuff should probably be designed todecrease the pressure exerted around the body convex.
One option is to support the interaction force with a flatarea of the body surface. In addition, the contact between
flat cuff and body convex such as the situation of thisstudy was not suitable from the view of contact safety.
Thus, the area of robotic cuff, which contacts the body
convex, should have sufficient indentation to avoid force
concentration. However, it should be noted that the index
to evaluate the body convex, which could be used to estimate the range of individuality, was not determined yet.

Conclusion
The effect of surface shape variance of the human bodyon the interaction force at the contact area when using
a wearable robot was estimated in this study. For thispurpose, a device that can mimic the unevenness andvariance of the human shape, was developed. Then, theperformance of the device was confirmed by comparing the characteristics of the device to that of an actual
human. Because of the greater unevenness and individuality of the surface shape, the experiment, which mimicked the contact between the human body and the cuffof the wearable robot, targeted the ASIS of the human.
The result of the rubbing test using the device and therobotic cuff attached to the manipulator suggests that theinteraction force applied to the surface changed approximately 10% as the height at the iliac spine changed±1 mm. Furthermore, this difference probably affects theendurance time, which is an index of the contact safety,
significantly under the severe condition. Thus, this study
suggests that considering the individual difference in theSakai et al. Robomech J (2018) 5:20

characteristics of human tissue and surface is important
for the safety design of a wearable robot.
Authors’ contributions
YS and YA designed and developed the device and control algorithm.
Experimental protocol and analysis were planed and proceeded by them.
YY proposed the idea of device and drew grand design of this study. He
contributed to improve the validity of experimental condition and analysis.
SO contributed to design experiment and analysis method for validating thedevice. All authors read and approved the final manuscript.
Acknowledgements
This study is based on the results of research and development of Japan
Agency for Medical Research and Development (Project to Promote theDevelopment and Introduction of Robotic Devices for Nursing Care).
Competing interests
The authors declare that they have no competing interests.

Publisher’s NoteSpringer Nature remains neutral with regard to jurisdictional claims in pub‑lished maps and institutional affiliations.
Received: 20 April 2018 Accepted: 24 August 2018

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