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A schematic drawing of the layered structure of human skin. The figure depicts both a projection view and a skin cross-sectional isometric view of the various skin layers The exuvium surface geometry of shed snake epidermis does not differ from that of a live animal [42, 66, 67]. Therefore, the shed skin of snakes reflects the frictional response of the live animal. Furthermore, the shed skin reflects the metrological surface and textural features of the live animal. Data for human skin entailed analysis and characterization of the skin of a group of volunteers. The groups were as follows: Caucasian women ages 30 to 40 years, Young Caucasian boys (4-6 years old), and African boys (5-8 years old). The main purpose of the study is the cross-correlation of friction response of the skin types examined. The detailed metrological comparison between the two skin types is therefore out of the scope of this presentation. However, to provide a generalized view of the metrological structure and differences between the two skin types we included representative data. This data reflect the general trends of the roughness features of each surface. The sites chosen for presentation differ due to the nature of function of the species (human VS snake). For snakes the ventral parts are principally friction sites. They are used in locomotion and gripping. For humans not all the sites are used for locomotion or gripping, yet most of the sites undergo friction be it permanently or occasionally. As such we elected to sample human skin metrology through the analysis of roughness of a generalized site rather than a specialized one (until the time of a comprehensive study between the locomotry cites in both skin types i.e. ventral sites
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A comparative study of frictional response of shed
snakeskin and human skin
H.A. Abdel-Aal, M. El Mansori, H. Zahouani
To cite this version:
H.A. Abdel-Aal, M. El Mansori, H. Zahouani. A comparative study of frictional response
of shed snakeskin and human skin. Wear, Elsevier, 2017, 376-377 (Part A), pp.281-294.
�10.1016/j.wear.2016.12.055�. �hal-02416726�
A comparative Study of Frictional Response of Shed Snakeskin and Human
H. A. Abdel-Aal1,*, M. El Mansori2, H. Zahouani3
1. Drexel University, 3141 Chestnut St., Philadelphia, PA-USA, 19104,
* corresponding author:
2. MSMP-EA7350, Ecole Nationale Supérieure d’Arts et Métiers, 2 Cours des Arts et Métiers, Aix En
Provence Cedex 1, France
3. Laboratoire de Tribology et Dynamique des Systèmes, UMR CNRS 5513, ENI Saint Etienne - Ecole
Centrale de Lyon .36 Avenue Guy de Collongue, 69131 Ecully cedex. France
Skin in biological systems, including humans, perform several synchronized tasks (mechanical,
protective, tactile, sensory, etc.). Tribological function is among skin tasks and may determine
the survivability of many species. Cross comparison of tribological functional traits of skin of
different species, albeit interesting, is rarely encountered, if at all exists, in tribology literature.
One interesting example is that of snake and human skins. This skin pair was the subject of
many studies for transdermal drug delivery. Results in that context concluded that snakeskin
is highly compatible with human skin despite apparent differences in surface structure and
topology. The reported compatibility raises curious question of whether there exists frictional
or tribological compatibility between the two skins and if so, under what conditions, and which
context. In this work, we report, for the first time in open literature, results of a comprehensive
comparative investigation of shed snakeskin and human skin with respect to tribological
behaviour. To this end, we compared the frictional response of shed skin obtained from P.
regius and human skin from different anatomical sites, gender, and age. The results imply that,
in essence, the mechanisms governing the friction response of human skin are common to snake
skin despite difference in chemical composition and apparent surface structure. In particular,
both skin types display sensitivity to hysteresis and adhesive dissipation. Human skin,
however, being more sensitive to hysteresis than snakeskin. One interesting finding of the
study is that the ratio of the coefficients of friction for snake and human skin, when sliding on
the same interface, depends on the reciprocal of their respective moduli of elasticity.
Areal Real area of contact
E Modulus of Elasticity Nm-2
E* Composite Modulus of Elasticity Nm-2
EH Modulus of Elasticity for human skin Nm-2
Es Modulus of Elasticity for snakeskin Nm-2
EcComposite modulus of elasticity
Fad Adhesive component of friction force
Ffr Friction force
Fint Interfacial component of friction force
Fdef Deformation component of friction force
P Contact force
R Composite contact radius in Hertzian contact theory
Rq Mean arithmetic value of profile roughness (µm)
Rsradius of asperity
Sa Aerial Mean arithmetic value of roughness (µm)
Sku Profile Kurtosis parameter
Sq Root mean square average of the roughness profile
Ssk Profile skewness parameter
W Normal Load
SEM Scan Electron Microscopy
COF Coefficients of friction
RMS Root Mean Square
TBH Trailing Body Half
WLI White light Interferograms
Greek symbols
Shear strain
Poisson's ratio
B Coefficient of friction in backward motion for
F Coefficient of friction in forward motion for snakeskin
Dry Coefficient of friction in dry sliding for human skin
Wet Coefficient of friction in wet sliding for human skin
H Human skin
SB snake backward direction
SF snake forward direction
1. Introduction
Skin in vertebrates and inter-vertebrates manifest complex composition. It comprises arrays
of collagen fibers arranged in various patterns. Arrays of collagen and elastin are common in
skin of many species (human, worms, fish, etc.,). Collagen fibers assume several shapes
(straight, convoluted, or crimps). They are arranged in patterns of various degree of
randomness [1] Despite sizeable variation in structural patterns, the stress strain curve of almost
all skin types is of a universal form (the so-called J-shaped stress-strain curve) [2].
Mai and Atkins [3] analyzed the energetics of the J-curve at each interval of its evolution during
tensile tests. They observed that within the early stages of this type of stress-strain curve, strain
is almost independent of stress. This implied the lack of shear connection in the particular
material. Lack of shear connection prevents the concentration of energy into the path of a
putative crack. Lack of shear connection, equivalent to lack of shear stiffness in anisotropic
solids, is the origin of high tear resistance of skin [3].
Friction is an interfacial phenomenon within which a shearing force performs work on an
effective volume of two complying materials. This effective volume includes the surface layer
of the skin as well as several sublayers that support the normal complying load. Dissipation of
the friction-induced-work takes place within the sub-layers along a path that depends on the
pattern of the fiber-elastin matrix comprising the skin. The universal behavior of the stress
strain curve for the skin implies also a universal behavioral trend in shear loading and thereby
in friction. That is one may anticipate that the mechanics of accommodation frictional loads
in skin are universal regardless of the species.
In the past three decades or so, considerable work that probe friction behavior of human skin
took place. The general motivation of these efforts varied between cosmetics [4-8], healing of
burn and wounds [9-13], prosthesis [14-16], and haptics [17-21] among other things.
Understanding the sense of touch and reaction of skin to fabrics was another major motivation
[22-27]. Recently the problem of developing synthetic skin assumed considerable momentum.
Several works that compare the friction of human skin to several synthetic skins started to
emerge in the literature [28-34]. Tribology literature however still lacks studies where the
friction behavior of skin from different species is cross-correlated. In fact, few studies that
consider skin of animals or reptiles is not frequently encountered to start with. Of the existing
studies, only few animal skin have been investigated (e.g. few reptiles, porcine and rat [35-
42]). Cross correlation of human skin performance to performance of other skin types is a very
active topic within transdermal drug delivery.
A primary objective in the design and optimization of dermal or transdermal drugs is to
understand the mechanics of “in-vivo” performance. When the drug is designed for humans,
it is essential evaluate percutaneous absorption of molecules. The best prohibit experimentation
with human subjects within the initial development stage. A challenge, therefore, arises since
the option at this stage is to find a plausible correlation between “ex-vivo”, animal and human
studies for prediction of percutaneous absorption in humans [43]. Consequently, considerable
investigations took place within the past four decades to assess the permeability of many
biomaterials in comparison to human skin. The list includes, primates, porcine, rodents, guinea
pigs and snakeskin (with porcine skin being frequently showing many similarities to human
skin [44-51]).
Higuchi and Kans [52] were the first to propose shed snakeskin as a barrier membrane in-vitro
permeation studies. Following their lead, several researchers incorporated shed snakeskins in
their experimental protocols. Skin from several snake species, Elaphe obsolete [53-57], have
been investigated. Haigh et al. [58] investigated the effect of species, sites and body regions
of the shed snakeskin on measurements and their relation to actual performance of human skin.
Haigh reported good correlation to human skin. He suggested the use of shed snakeskin as a
model membrane for permeation studies despite anatomical differences and chemical
compositions [54, 55, 59].
Trans-dermal diffusion is a time-dependent phenomenon that initiates at the skin surface (i.e.
at the level of the micro-topography (roughness)). The process starts by the diffusing substance
attempting to occupy the void space between the roughness features of the target surface.
Roughness features (or micro-topographical features on the skin) have no regular or uniform
geometry. Spacing between, roughness features, volume occupied by an individual feature,
and shape are all different on any surface. These parameters affect the path and time of
diffusion through affecting the resistance to initially filling the void space. It follows that the
manner the roughness features branch to occupy their respective volume in space will
determine the void volume available for the diffusing substance to occupy, and thereby initial
resistance to diffusion. That is, the layout of the micro topography features on the surface of
the skin maps initial resistance to diffusion. Compatibility of diffusion between two surfaces
(or skin types) therefore should originate from common features in the branching of roughness.
One difficulty encountered in identifying potential common metrological features is the
appearance of the surface of both skin types. Abdel-aal [60] avoided this difficulty by
considering the fractal structure of both skin types. That is by focusing on the growth of
topographical features in space rather than on the statistical variation within the topography of
the two surfaces. The analysis, thus, focused on identifying the relationship between form and
volume in space then relating the findings to particular metrological features (i.e., the fractal
description of the two skin types). Examination of exuviae of some 45 snake species and
comparison to human skin verified that both skin types, despite displaying different surface
topological features, share a narrow band of fractal dimensions (2.55 D 2.6). Sharing the
fractal dimension explained the time compatibility of snakeskin to human skin observed in
permeability experiments.
Sharing a common form of a stress-starin curve and a fractal dimension points at possible
generalized tribological features between human and snake types (with potential generalization
to skin in general). This work therefore aims at probing those shared aspects of the tribological
response of human and snakeskin.
2. Background
2.1. Structure of snake skin
The skin of snakes has two principal layers: the “dermis” and the “epidermis”. The dermis is
the deeper layer of the two main layers. It comprises connective tissue containing a rich supply
of blood vessels and nerves. The epidermis is composed of “strata” that contain closely packed
cells. The strata consist in seven layers that form an outer protective coating of the body [61].
The first of the seven layers forming the epidermis (figure 1) is the “stratum germinativum”.
This is the deepest layer lining cells having the capacity for rapid division. Following this layer,
six additional layers form each “epidermal generation”, the old and the new skin layers. These
are: the clear layer and the lacunar layer, which matures in the old skin layer as the new skin is
growing beneath;.
Beta ( )-layer
Beta ( )-layer
mesos layer
mesos layer
alpha layer
alpha layer
lacunar tissue
clear layer
outer generation
inner generation
Figure 1 Generalized epidermis of a squamate reptile.
the alpha (layer, the mesos layer and the beta (-layer. The alpha (, the mesos layer
and the beta (-layer consist of cells that become keratinized with the production of two types
of keratin ( and keratin). The keratinization process continuously transforms such layers
into a hard protective layer. Finally, there is the “oberhautchen” layer, which forms the toughest
outer most layer of keratinized dead skin cells. . The oberhautchen layer of the ventral side is
also the layer that directly interacts with the substratum during locomotion. Additionally, it is
the layer containing the micro-ornamentation of the skin (both the dorsal and ventral).
Snakes, like other reptiles, have a skin covered in scales of various shapes and sizes. Scales are
formed by the differentiation of the snake's underlying skin or epidermis. Each scale has an
outer surface and an inner surface. A snake hatches with a fixed number of scales. The scales
do not increase in number as the snake matures nor do they reduce in number over time. The
scales however grow larger and may change shape with each molt. In snakes cell division, in
the “stratum germinativum”, occurs periodically [62], and leads to the replacement of all the
layers above the area where cell division takes place. The reptile, therefore, grows a second
skin underneath the old skin, and then “sheds” the old one. About two weeks before the reptile
sheds its skin, the cells in the stratum germinativum begin active growth and a second set of
layers form slowly underneath the old ones. Following such a process, the cells in the lowest
layers of the old skin, the clear and the lacunar layers, and the Oberhautchen layer of the skin
below undergo a final maturation and a so called “shedding complex forms”. Fluid is exuded
and forms a thin liquid layer between them. Enzymes, in this fluid, break down the connections
between the two layers, the old skin lifts and the reptile actively removes it.
2.2 Structure of human Skin
The human skin is a soft biomaterial that manifests a sophisticated hierarchical structure [63].
Similar to reptilian skin, human skin encases the entire human body and its internal organs. It
is the organ of the largest surface area in a human body (covers around 1.6-2 m2 in adults and
accounts for approximately 16% of a person’s weight [64]. The skin is composed of three
anatomically distinct functional layers (figure 2): the epidermis, the dermis and the
A human epidermal layer represents the main barrier between the body and its surroundings.
This layer performs the principal skin protective actions (i.e., its function is similar to the
oberhautchen layer in a snake). The thickness of a human epidermis ranges between 0.027 and
0.15 mm depending on the particular body cite examined. The epidermis mostly comprises
dead cells. These originate within the lower regions then are pushed by newly generated cells.
The outermost layer of the epidermis is the “stratum corneum”, which provides primary
protection for the skin. This region, only 10–20 μm thick, acts as the primary barrier to the
percutaneous absorption of compounds as well as to water loss. Underlying the stratum
corneum is the viable epidermis (50–100 μm thick), which is responsible for generation of the
stratum corneum. The dermis (1–2 mm thick) is directly adjacent to the epidermis and provides
the mechanical support for the skin. The viable epidermis is a stratified epithelium consisting
of basal, spinous and granular cell layers. Each layer is defined by position, shape, morphology
and state of differentiation of keratinocytes. The keratinocyte synthesize and express numerous
different structural proteins and lipids during their maturation. The last sequences of the
keratinocyte differentiation result in their transformation into chemically and physically
resistant cornified squames of the stratum corneum, called corneocytes. In this context,
keratinization for protective functionality, the dynamics of the epidermis in humans is similar
to the alpha () mesos, and the beta () layers in snakeskin.
The second layer within the human skin is the “dermis”. This is a layer of variable thickness
(between 0.6-3 mm). It comprises dense, irregular connective tissue, nerve, and blood vessels.
A human dermal layer comprises two sub layers: the papillary and the reticular dermis. The
dermis confers firmness, high elasticity/ resilience, tensile strength and tear resistance to the
skin [65]. The “hypodermis” is the innermost layer of the skin. It contains over 50% of the
total body fat. It is composed of loose elastic and adipose tissues and has two main functions:
firstly as a strong connection layer between the skin and the muscles or bones beneath, and
secondly for thermal insulation purposes. It also serves as a protective layer by absorbing
impacts from the outside that could result in internal damage for nerve endings and blood
3. Materials and methods
The current study examined shed skin from Royal Python (P. regius). Table 1 presents a
summary of species taxonomy and major dimensional features
Table 1 summary of species taxonomy and major dimensional features
P. regius
Family Pythonidae
Subfamily Python
Genus P.regius
Length (cm) 150
Number of ventral scales 208
Ratio of length to maximum diameter 10.2
Mass (Kg) 1.3
Average area of ventral scale mm2 102.35
Maximum length of fibrils (m) 1
Maximum ventral scale aspect ratio 3.142
Minimum Ventral scale aspect ratio 1.75
General features of the species were described elsewhere [39]. Figure 3 meanwhile, details the
micro features of the skin on the dorsal and ventral sides.
dermal papilla
sweat pores stratum corneum
stratum spinosum
stratum basale
Figure 2: A schematic drawing of the layered structure of human skin. The figure depicts both
a projection view and a skin cross-sectional isometric view of the various skin layers
The exuvium surface geometry of shed snake epidermis does not differ from that of a live
animal [42, 66, 67]. Therefore, the shed skin of snakes reflects the frictional response of the
live animal. Furthermore, the shed skin reflects the metrological surface and textural features
of the live animal.
Data for human skin entailed analysis and characterization of the skin of a group of volunteers.
The groups were as follows: Caucasian women ages 30 to 40 years, Young Caucasian boys (4-
6 years old), and African boys (5-8 years old).
The main purpose of the study is the cross-correlation of friction response of the skin types
examined. The detailed metrological comparison between the two skin types is therefore out
of the scope of this presentation. However, to provide a generalized view of the metrological
structure and differences between the two skin types we included representative data. This
data reflect the general trends of the roughness features of each surface. The sites chosen for
presentation differ due to the nature of function of the species (human VS snake). For snakes
the ventral parts are principally friction sites. They are used in locomotion and gripping. For
humans not all the sites are used for locomotion or gripping, yet most of the sites undergo
friction be it permanently or occasionally. As such we elected to sample human skin metrology
through the analysis of roughness of a generalized site rather than a specialized one (until the
time of a comprehensive study between the locomotry cites in both skin types i.e. ventral sites
in snakes and underfoot and inner palm (for example in humans). Evaluation of metrological
texture parameters utilised a white light interferometer (WYKO 3300 3D automated optical
profiler system). Analysis of all resulting White light..
light skin
dark skin
ventral scale
Figure 3. General appearance of the P. Regius and SEM details of the three skin colors of the
species: Dorsal skin light colored, Dorsal skin dark colored and the Ventral skin. All
observations were performed on a JEOL JSM-5510LV SEM using an acceleration
voltage that ranged between 4 kv V 6 Kv).
Interferograms, WLI, to extract the surface parameters used two software packages: Vision ®v.
3.6 and Mountains® v 6.0.
To determine the metrological features of shed snakeskin, we identified 16 regions on the hide
of each of the studied snakes. Each of the examined spots comprised a section that is
approximately 2.5 cm wide. Five interferograms were recorded for each of the dorsal and the
ventral sides of the chosen region on the skin and these were analyzed to extract the surface
texture parameters. Sample representative results of the metrological parameters for shed
snakeskin and human skin are presented in figures 4 and 5. Each of the figures depicts six
plots. The first, labeled a, shows the original-raw- white light interferogram of the particular
skin sample. Following this plot, the figure labeled b presents the surface isotropy plot. The
last four plots, labeled c through f, present the roughness profile extraction of the skin sample
in four directions (AE-PE, LL-RL, AR-PL, AL-PR).
010 20 30 40 50 60 70 80 90 100 µm
010 20 30 40 50 60 70 80 90 µ
010 20 30 40 50 60 70 80 90 100 µm
010 20 30 40 50 60 70 80 90 100 µm
Figure 4 Metrological data extraction for shed python skin. Figure 4-a, original white light
interferogram, b) surface isotropy plot, c) roughness profile extraction in the anterior
posterior direction (AE-PE), d) roughness profile extraction in the lateral direction
(LL-RL), e) roughness profile extraction in the diagonal direction (AL-PR), and f)
roughness profile extraction in the diagonal direction (AR-PL).
00.1 0.2 0.3 0.4 0. 5 0.6 0.7 0.8 0. 9 mm
00.2 0.4 0. 6 0.8 11. 2 mm
00.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 11.1 1.2 1.3 1.4 1.5 mm
Figure 5 Metrological data extraction for Human skin. Figure 4-a, original white light
interferogram, b) surface isotropy plot, c) roughness profile extraction in the anterior
posterior direction (AE-PE), d) roughness profile extraction in the lateral direction
(LL-RL), e) roughness profile extraction in the diagonal direction (AL-PR), and f)
roughness profile extraction in the diagonal direction (AR-PL).
It is noted that in general the texture of both skin types does not show significant dominant
angle (for both skis the so called isotropy index Str falls in the interval 0.21 Str 0.6 for
python skin and 0.2 Str 0.261 for human skin).
Roughness profiles, however, differ by direction. Table 2, which presents the so-called Aerial
parameters of the skin support this remark. Observe for example that the root mean square,
and the mean value of the roughness, Sa and Sq, for snakeskin represent a small fraction of the
values pertaining to human skin. More interesting is the comparison of the Kurtosis and
Skewness, Sku and Ssk parameters of the skin types.
The kurtoses for snakeskin and for the woman group are almost equal numerically and imply
a Gaussian distribution of the roughness. Young human skin (Caucasian and African)
manifests high values. The skewness of snakeskin is positive and is opposite in sign to
skewness of human skin. The combination of positive kurtosis-positive skewness (snakeskin)
implies a surface dominated by peaks, whereas a positive-negative combination implies a
surface dominated by valleys. This arrangement is not universal in snakes but depends on the
habitual function of the particular species [39]. In case of P. regius it facilitates ventral arboreal
Table 2 Summary values of extracted metrological parameters for snakeskin and human skin.
Parameter Snake Woman A_Boy C_Boy
Sa m 0.293 25.2 16.1 24.7
Sq m 0.373 31.7 21.4 33.3
Sku 3.37 3.36 6.16 5.70
Ssk 0.322 -0.773 -0.706 -0.574
2.3 Friction measurements
2.3.1 Snake skin
Frictional behavior of the shed snakeskin was characterized by a bio-tribometer [68]. The
active data collection element in the device comprises a thin nitrocellulose spherical
membrane; 40 mm in diameter, with approximate thickness 0.2 mm. Table 3 provides a
summary of the properties of the probe and the experimental conditions.
In a typical experimental run, the skin remains stationary and the tribo-probe moves with an
average speed of 20 mm/s using a normal force of 0.5 (±0.05) N. The skin used in
measurements consisted of 100 mm long patches taken from four locations on the ventral side
of the shed skin. Skin samples did not receive any chemical or physical treatment.
Table 3 summary of contact probe characteristics and experimental conditions
Geometry Sphere
material nitrocellulose
Thickness 0.2 mm
Diameter 40 mm
Mechanical Properties
Young’s Modulus 1 Gpa
Surface roughness
Ra 4 μm
Rz 31 μm
Experimental conditions
Sliding speed 20 mm/s
Nominal contact force 0.5 (±0.05) N
To mimic the effect of the snake body we ran a series of experiments with a flexible silicone-
padding cushioning the skin. Before starting an experiment, the particular skin patch was
placed on a rectangular elastic pad of dimensions length L= 100 mm, width W=75 mm and of
approximate thickness 4 mm. The pad is made of silicone rubber (Silflo®™, Flexico
Developments Ltd., Potters Bar, UK). Table 4 provides a summary of the pad material
properties. Measurement of the friction forces proceeded along the two major body axes: the
anterior-posterior axis (AE-PE), and the lateral axis (LL-RL). Measurements were recorded in
the forward and backward directions along both axis (i.e, going from the AE to the PE and
converse and from the LL side to the RL and converse).
Table 4: Summary of geometric dimensions and mechanical properties of elastic pads used to
cushion skin in experiments.
Geometry Rectangle
Length (mm) x Width (mm) 200 x 100
Material Silicone Rubber (Silflo ®, Flexico
Developments Potters Bar, UK)
Young’s Modulus (E) MPa 2 @20 C
Poissons Ratio 0.3
Stiffness (K) 300N/m
2.3.2 Human skin
Friction of human skin, in theory, is a function of hydration, anatomical site, age, gender, and
race. Many researchers investigated the effects of these factors. Cua and Elsner [69,70]
pointed the significant influence of anatomical site on the COF. Influence of race, age, and
gender, however, remains inconclusive. Elsner [70] and Asserin [71], for example, did not
observe age related differences when examining the friction of the volar forearm. Cue et al.
did not report significant differences in friction of skin based on gender. Manuskiatti et al [72]
examined roughness and friction of human skin as a function of race (Black VS Caucasian).
Again, these researchers were not able to isolate a significant race-based influence. In addition,
to the knowledge of the authors, to date, reports that indicate the existence of frictional
anisotropy in human skin are non-existent within open literature. Given such a background,
our experimental protocol did not entail provisions to investigate the effect of race, age or
gender induced influences on frictional behavior of human skin. Rather, our procedure focused
on measuring the COF of test subjects only as a function of variation in anatomical site and
All friction measurements for human skin were obtained using the same tribo-probe used for
measuring snakeskin under the same experimental conditions (load and sliding speed).
The sites used for friction measurements comprised the principal sites within the human body
(legs, Knee, face, arms). In this study only repeatable and consistent data are reported. The
condition of repeatability confined site data to those shown in what follows.
4. Results
The skin responds distinctly to the combination of surface geometry, material property, and the
mechanical contact parameters of the substratum. Frictional response of the skin, therefore, is
a complex phenomenon that depends not only on the process parameters, but also on the
individual interaction between these parameters. This complexity reflects on the manner and
scope of presenting the results of any investigation of skin friction. Bearing this in mind,
presentation of results in this work will not cover every aspect of the tribology of snake and
human skins. Rather within the following sub-sections, we report on those findings that are
most relevant to the comparison process (which is the main subject of this work). The sequence
of presentation starts with exposition of the data obtained for snakeskin followed by presenting
data of human skin.
4.1. Snake Skin
Figure 6 presents a collective plot that traces the repeatability of COF values obtained. Circles
represent the COF obtained in each measurement run. Square symbols denote average values
along with associated standard deviation of data. The data shown in the plot depict
measurements obtained in dry and wet sliding modes of the leading half of the Python ventral
skin. Measurements proceeded along two principal directions. The first, termed here as forward
motion, represents motion in straight line along the anteroposterior axis of the reptile, however,
going toward the cranial end of the reptile). Moving toward the caudal end of the reptile along
the anteroposterior axis is termed here as backward motion.
The COF in backward motion is greater than the COF obtained in forward motion (i.e., B>F),
in each of the friction modes wet and dry, which manifests frictional anisotropy. This result is
consistent with the findings of several authors [38, 73-78]. The anisotropy in friction of snakes
partially emanates from the geometrical structure of the individual micro-fibrils [79].
Measurements in wet mode for forward motion, however, yield friction coefficient values that
are higher than those obtained in dry mode. In backward motion, however, values of the COF
are lower in case of wet skin. In both cases, wet and dry, the forward motion COF is less than
that in backward motion.
Figure 7 presents a collective plot of the COF obtained in various experimental runs, and the
average value, for the dorsal and ventral sides of the leading half of the Python (mid-section to
head region). The plot compares friction in the forward and backward directions for the dorsal
and ventral sides in dry sliding. The dorsal scales show higher COF than the ventral scales.
Such a result is consistent with the findings of Berthe et al [78], who confirmed the higher
friction of the dorsal scales of C. hortulanus (Amazon tree boa).
Wet measurements
Dry measurements
Forward motion
Forward motion
Backward motion
Backward motion
Python regius
Figure 6 Variation and scatter of COF measurements for skin of the P. regius in dry and wet
sliding conditions, for different experimental runs, obtained by the tribo-acoustic
probe method. Forward motion indicates sliding in AE-PE direction (tail to head);
Backward motion implies motion from head to tail (PE-AE). Circles indicate values
obtained in different measurement runs, whereas hexagonal symbols denote average
values along with their respective standard deviation.
Measurements also manifest the frictional anisotropy between forward and backward motion
reflected in figure 6. Figure 8 provides a comparative bar plot of the average “static” COF for
P. regius in forward and backward motion. The numerical values of the bars represent the
statistical mean of the COF obtained from all spots on the hide.
4.2. Human skin
Figure 9, presents a comparative summary of measurements obtained for human skin. The
figure presents two plots (each comparing the COF of the skin in a particular sliding mode).
Figure 9-a, depicts a bar plot of data obtained in dry sliding mode, whereas figure 9-b, plots
data obtained in wet mode. In obtaining wet mode measurements, we made no provisions to
determine skin moisture content. The procedure entailed rubbing the skin spot of interest with
a moist paper towel, then applying the measuring device. As such, values of the COF presented
as figure 9-b are to be considered as general indicators of the behavior.
The plot generally confirms the variation of the COF with anatomical site (both in wet as well
as in dry measurements). Values of the COF vary considerably between locations. The COF
value for the knee, for example, is roughly four times the COF value for the check in dry mode.
In all, COF measurements obtained in wet mode imply higher values than measurements
Dorsal front half
Ventral front half
forward motion
forward motion
bac kwa rd mo tion
bac kwa rd mo tion
Python regius
Figure 7 Scatter and comparison of the COF values obtained for skin located at the dorsal and
ventral sides of the front half of a P. regius. Measurements are depicted for Forward
(tail-to-head) motion and the converse. Circles indicate values obtained in different
measurement runs, whereas hexagonal symbols denote average values along with
their respective standard deviation.
Front Half Trailing Half
f Fricti
Figure 8 Comparison between the average values of the static ventral COF obtained for the
P. regius.
obtained in dry mode. Due to the statistical constraints set in this work, a one-to-one
comparison between the COF for the same anatomical location is not possible. As such, the
following observations are to be accepted with caution.
For the cheek, the COF in dry mode is almost one third of the COF value obtained in wet mode.
The difference between the COF values for dorsal forearm, in wet and dry friction, is not as
pronounced as in the case of the cheek (value of the wet COF is still higher than that in dry
sliding). So that, while wet measurements are consistently higher than dry measurements, the
data do not allow the identification of a unique relationship that correlates wet and dry COF
values. This is due to the limited scope of the investigation. Never the less, the data reflect
dependence of the COF for human skin on the anatomical cite examined. One also has to keep
in mind that hydration in this study was imposed on the skin using water for which human skin
has a complex tribological response. The forces developed in water-based lubrication of the
skin depend on the topography of the anatomical site and on the thickness of the water meniscus
developing on the skin. The thickness of the water meniscus determines whether water would
induce a lubricating effect or, alternatively, it will induce an additional adhesive component to
the COF. Data presented in figure 9 imply that for the experimental conditions examined in
work, water induces adhesion to the COF. This adhesive contribution leads to a higher COF in
wet sliding than in dry sliding (i.e., wet>dry).
oefficient of Friction (
Woman 40 years
Dorsal forearm
Volar Fore arm
Dry Measurements
Volar Forearm
Index Fi nger
Dorsal Forearm
wet measurements
Figure 9 Comparison between local COF of human skin for dry and hydrated conditions.
It is interesting to note that the index finger and the volar forearm manifest significantly high
frictional resistance in the wet mode (wet >1). In addition to the presence of moisture, such a
high COF may originate from the unique deformation mechanics of skin of that particular
location, where topography and stiffness play considerable role in generation of frictional
To address the limitations induced by repeatability of data we include tables 5 and 6. The tables
present summary COF values of human skin extracted from literature. Table 5 provides
frictional data for human skin against different materials in dynamic and static modes. Table
6, meanwhile presents frictional data as a function of skin condition (i.e. wet or dry).
Comparison of values plotted in figure 9, with those of tables six and seven reveals very good
Table 5: COF data for human skin, from different anatomical sites, in dry friction against
different materials in dynamic and static modes
Dynamic Measurements Static Measurements Reference
location material-subs D substrate S
forehead Teflon 0.373
upper arm 0.33
volar forearm 0.46
dorsal forearm 0.455
Post aurcular 0.161 [69]
palm 0.57
Average 0.38 Teflon 0.2 [79]
polyethylene 0.3
polyethylene 0.5-0.6
glass 0.4 glass
s. steel 0.2-0.4
Ruby 0.7
Glass 0.3-0.4
forearm teflon 0.48 [70]
Post forearm 0.43
Ant. forearm 0.46
dorsum of hand 0.47
Silicone 0.61
Cotton Sock
Pelite 0.45
Table 6 COF data for human skin, from different anatomical sites, in dry friction against
different materials in dynamic and static modes as a function of moisture (wet or dry)
Skin area Skin condition COF Probe/material Reference
Calf Dry (untreated) 0.5 PE sphere [80]
Wet (sweating) 1.0
Dry (cleaned) 0.18–0.72 PE sphere [83]
Forearm Dry (normal) 1.63 Steel sphere [84]
0.41 Steel slider [85]
0.9 Gold cylinder [86]
Dry (untreated) 0.45–0.65 Copper cylinder [87]
Dry (cleaned) 0.24 Steel wires [88]
0.37–0.8 Copper cylinder [83]
Wet (occluded) 1.0 Copper cylinder [87]
Wet (hydrated) 0.71 Steel slider [85]
Cheek Dry (normal) 0.12–0.22 Teflon wheel [89]
Dry (washed) 0.3–0.85 Aluminum ring, PTFE ring [90]
Wet (humid
climate) 0.925–2.1 Aluminum ring, PTFE ring [90]
Index finger Dry (untreated) 0.49 Glass [91]
0.7 Rubber glove [92]
Dry (cleaned) 0.6–1.75 Glass, paper, steel [93]
Dry (washed) 0.38 Paper [94]
Dry (washed) 0.63–1.1 Polyester sheets [95]
Cotton, Polyester, rayon, [96]
Wet (water) 1.2 Rubber glove [92]
5. Discussion
In discussing the results obtained in this work, for both types of skin, we first present the
obtained values of the coefficients of friction. Thereafter, we analyze the findings to predict
the tribological behavior and identify common as well as opposite traits.
5.1. Frictional behavior
Figure 10, presents a summary bar plot that collects all the measured COF values for human
and P. regius skins in dry friction. The plot presents the COF for human skin by anatomical
site. For Python skin, labeled P, the figure presents three COF values. These are COF for the
front half of the ventral skin (FH), the trailing half of the ventral skin (TH), and the average
values for the Dorsal side (D). For example, the bar labeled FHF represents the value of the
COF in forward motion averaged over the front ventral half of the reptile. Similarly, the label
(THB) stands for the COF in backward motion averaged over the trailing half of the reptile.
The COF values reflect wide variation. In general, a unified trend that governs the data is not
easy to identify. However, individually, some anatomical sites may share COF values. For
example, the COF for the cheek and calf of the human skin are close to the COF value of the
ventral front half (FHF), and that for the dorsal side of the Python in backward motion (DB).
The friction of dry skin differs from that of wet skin. The main distinction is that friction of
dry skin, as a first approximation, resembles the friction of solids where the COF does not
depend on the apparent area of contact. In such a case the surface roughness of the particular
anatomical site and the load, are the only factors influencing friction. Dependence on external
load affects the real area of contact, which in turn depends on the roughness of the particular
site within the body. Moreover, the skin being essentially a viscoelastic material will deform
according to the stiffness of the contact between the acoustic probe and the underlying cushion
under the skin. To verify this assumption we performed an extra set of experiments where the
thickness of the pad underneath the skin varied from 4mm to 6 mm resulting in an increase in
the stiffness of the contact from 300 N/m to 500 N/m. The COF was recorded for both cases,
high stiffness and low stiffness, and the results are shown in figure 11. Values plotted in the
figure are average values in the sense of being the average of the values obtained on both halves
of the ventral side of the reptile. For comparison, we include the values for the human calf and
cheek since they are the anatomical sites of which sliding resulted in a COF close to that of the
snakeskin. The plot depicts values for the COF in forward and in backward motion. The data
show that higher stiffness of the underlying cushion to the skin results in lower COF. This
result points at the effect of contact stiffness on skin friction. Consequently, the local stiffness
of an anatomical site will influence the frictional response of the skin.
Dorsal fore arm
Volar Forearm
Dry Friction
Figure 10 Comparison of COF of human skin, from two zones (calf and cheek), with the COF
of python skin (in SF and SB sliding directions).
5.2. Effect of hysteresis
Deformation of skin and subsurface tissue during friction can contribute to the friction
coefficient in the form of viscoelastic hysteresis or ploughing [4]. The contribution to
hysteresis increases with normal load and contact pressure. Contact pressure, in turn, is a
function of the surface roughness of the skin. So in order to check which of the skin types is
more likely to entertain larger hysteresis contribution, it is of interest to evaluate, at least,
qualitatively, which skin type develops a higher contact pressure under the action of the same
nominal normal load. For such a purpose, we invoke the well known Greenwood-Williamson
contact model [97].
Human Skin
Python K= 300 N/m
Python K=500 N/m
Figure 11 Effect of the stiffness of the contact between the acoustic probe and the underlying
cushion under the skin on the COF of reptilian skin (P. regius). Values for human
skin is included for comparison.
The model assumes that the surface roughness has a constant radius (Rs) and that the heights
of the roughness elements follow a Gaussian distribution. For elastic deformation of the
roughness elements, the contact load over a circular contact spot (similar to that established
due to the contact between the acoustic probe and the skin) s given by:
 
Pd AE R Z d ZdZ
For Gaussian distribution, there is no closed form solution for equation (1). Shi and Polycarpou
[98], showed that an approximation to the Gaussian distribution assumed in equation (1) is an
exponential roughness height distribution of the form,
Where C and λ are constants of values 17 and 3 respectively. Adopting the distribution of
equation two simplifies the integration and therefore allows a closed form solution of equation
one. Accordingly, for an exponential height distribution and elastic contact conditions, the
contact force will be given by,
 
 
Equation 3 emphasizes the influence of the roughness of the skin surface on the magnitude and
distribution of the pressure acting on the skin in dry friction. This equation may be rearranged
to express the contact pressure explicitly in terms of standard surface parameters. To this
effect, for Hertzian contact conditions, we write
Pd d
where is the density of the roughness. As such, to determine which of the skins, human or
reptilian, is more likely to be affected by hysteresis, it is sufficient to develop the ratio between
the contact pressures affecting the skins under the same nominal load, and assuming common
asperity radii Rs, viz,
exp 3
Nd R Ed
Nd R E R R
  
 
  
Here the subscripts H and P denote human and snake skin respectively. Two ratios control the
outcome of equation (5). The first is the ratio of the root mean roughness RqH/RqP that is a
metrological function and is rooted in the topography of each of the skin surfaces. The second
ratio meanwhile is the ratio of the composite moduli of the elasticity E*H/E*P. This implies
that at least within the context of our simplifying assumptions the outcome of equation (5) is
determined by the level of interaction of the surface structure and the mechanical properties of
the particular type of skin. Referring to table (2) and observing the values of the Rq roughness
parameter we note that the most influential term in equation five is the ration qH
is in the
order 23 qH
Measuring the elastic modulus of human skin was a subject of many studies. The values quoted
in literature reflect dependency on contact scale, location, and state of hydration. Van
Kinelenburg [99] introduced a unified equation that predicts the effective modulus for human
skin taking into consideration most of the results reported in literature and likely scale of
contact. Using such equation the effective modulus for human skin falls in the interval 1 Mpa
EH 5 Mpa. For snakeskin, however, few studies have attempted to measure the elastic
modulus for different species. Rivera [100] reported that the elastic modulus for the common
garter snake, Thamnophis sirtalis, ranges between 6 Esn 12 MPa. Using this value along
with that for human skin in equation (5) results in a contact pressure for human skin that is at
least 10 times that of snakeskin for the same separation distance. This implies that under the
same external loads and geometry of contact probe the human skin is more influenced by
hysteresis than snakeskin.
5.3. Coefficient of Friction
For design purposes, it is essential to quantify the COF regardless of the anatomical site. That
is, it is necessary to work with an average value that represents the friction coefficient of the
skin. Therefore, we evaluated the overall average of all measurements performed on reptilian
skin (i.e., measurements for both snakes used) and compared them to the average coefficient
for human skin.
Figure 12 presents a plot of these average quantities. Figure 12-a presents a plot of the average
of the measurements obtained in the current work, whereas figure 12-b depicts a plot of the
average of the values obtained in the current work and those values reported in literature
(summarized in tables 5 and 6).The first plot, figure 12-a, depicts the overall average of the
COF of the skin in dry friction based on measurements obtained in the current work. The plot
comprises three bars. The first, labelled human skin represents the average of the COF
measurements obtained for all anatomical sites examined. Two bars are included for the skin
of the snakes. The first (in light blue) represents the average value for the forward COF (f) for
both snake species, and all sites examined on the skin. The second bar meanwhile, represents
the average value, for the COF in backward motion (). The values imply that the COF for
human skin is considerably higher than the overall value for the snakeskin (almost four folds).
Such a trend is consistent with the order of magnitude of the average heights of the roughness.
Recall that the values of the roughness parameters for human skin are higher than the
parameters of snakeskin.
Snake skin bak. Motion
Huma n Skin
Dry Measurements
f Fric
Dry measurements
Human Skin
Snake skin Forward Motion
Snake skin bak. Motion
Dyna mi c C oeffi ci ent of Fri ction
Figure 12 Plot of the average COF for the skins examined in the current work for wet and dry
friction (a) plot of the average of the measurements obtained in the current work,
(b) depicts a plot of the average of the values obtained in the current work and those
values reported in literature (summarized in tables 6 and 7).
5.4. Dry friction mechanisms
Dry friction of the skin may be studied by means of Bowden and Tabor two-term model [101].
The model considers the force of friction developed in rubbing of dry skin a superposition of
two contributions: an interfacial component, Fint, and a deformation component Fdef. This
yields, the total friction force as,
int def
FF (6)
The deformation component has its origin in the incomplete recovery of energy dissipated due
to deformation of the skin. For many cases involving the friction of polymers and thin films
of biomaterials, the deformation component may be ignored and only the interfacial
contribution is to be solely considered [102]. The origin of the interfacial component is the
dissipation of energy due to rupture of intermittent junctions formed between the two sliding
interfaces and deformation of roughness elements. Therefore, we write the interfacial
contribution of the friction force as [103, 104] in the linear form
ad real
 (7)
Where is the interfacial shear stress and real is the real area of contact between skin and
substrate. The adhesion force Fad is a function of two contributions, the surface energy of the
counter faces and the area over which the adhesive bonds do form [105-107].
As a first approximation, one may model skin behaviour in friction as an elastic deformable
material of which interaction with the sliding counter face follows Hertzian theory. It follows
that the area of contact Areal in equation (7) is calculated from,
real c
 (8)
Where R is the radius of the contact probe, W is the applied normal load and Ec is the composite
or reduced Young’s Modulus given by,
skin substrt
 (9)
Considering the rigidity of the probe (substrate) compared to the skin, the second term in
equation (9) becomes zero. Consequently, the real area of contact Areal is expressed as,
Where K is a constant, and the Poisson’s ratio and the modulus of elasticity are those of the
skin. Recognizing that the COF is the ratio of the friction force to the normal load acting on
the skin, we may write
 
Under the assumption of elastic behaviour the shear stress in equation (11) may be expressed
Now consider two skin samples the first is a human sample, H, and the second is that of a
snake, denoted by s. If both samples slide under the same shear stress, and against the same
interface, then the ratio of their shear strain, again within the elastic solid approximation, takes
the form,
Using equations (11) and (13) the ratio of the COF for the human skin to the COF of snakeskin
simplifies to,
 
Recognizing that the magnitude of the second term in equation (14) is around unity, we deduce
the main influence on the ratio of the friction coefficients as,
 (15)
Equation (15) indicates that the ratio of friction coefficients in dry mode is proportional to the
ratio of the moduli of elasticity of the skins. As such, using the values previously used in
equation 5 yields the maximum approximate value of the ratio of friction coefficients /Sn
as 4.5. Such a value is very close to the ratio of the coefficients calculated from figure (12-a)
(/SnF 4.5 and /SnB 3).
It is important to note that the comparison of COF performed to note that the comparison of
COF performed in this work is intended as an initial exploration. The behavior of skin in
friction, regardless of the species, is very complex. The complexity drives from the internal
makeup of the skin and the mechanical behavior of the components or layers. As seen in
section two, the composition of both human and snake skin entails layers of essentially different
composition (chemically and mechanically). These layers are inhomogeneous with respect to
the building blocks (fibers, keratin structures, filaments, etc.). The individual components also
have different orientations. This complexity of construction definitely reflects on the
tribological behavior of the skin and for sure reflects on the mechanical properties of the skin.
Each layer of the skin has its own mechanical properties. Detailed analysis of friction response
of human skin for example [108, 109] showed that the outer layer of the skin, the stratum
corneum, has a dominant influence. This layer, however, does not act individually but supports
the loads through assistance from the underlying strata of the skin (each contributing according
to the mechanical properties).
Behavior of snakeskin is not drastically different as the outer layer, the Oberhautchen, contains
the topographical features of the skin, whence it dominates friction. Similar to human skin, it
supports loads in conjunction with the underlying layers of the skin and the musculature
involved. In all, the detailed picture concerning the response of skin to friction does not lend
itself to simple analysis. As such, in this work we sought a generalized framework that seeks
the features common in behavior of the two skin types.
The analysis presented here mainly dealt with dry sliding. For sliding in presence of moisture,
other effects have to be considered (e.g. surface energy of the skin). Many of the necessary
parameters are not uniquely characterized for human skin. The literature is full of different,
sometimes contradicting, values for properties of human skin in wet state. For snakeskin, the
situation is more undefined. There is a complete vacuum when mechanical properties are
considered. However, some experimental work, which is in preparation and performed in the
first author’s lab, indicates similarity in behavior between human skin and snakeskin when
sliding in wet mode. In particular, the COF for wet snakeskin increases, compared to the dry
COF, in the presence of moisture which is similar to the behavior of wet human skin reported
elsewhere [67-81].
The COF model presented in this work has its origin in Hertzian contact mechanics. One way
of modelling the contact mechanics involved is that of studying a punch, which represents the
roughness feature of the substrate, initially penetrating through the skin. The mechanical
response of the skin surface will depend on the depth that an asperity penetrates through the
skin. Hill [110], motivated by early experimental and theoretical works [111-115], developed
a so-called similarity solution for rigid indentation, that which models the examined contact
situation. He showed that the overall solution of the contact mechanics problem, whence the
normal and tangential forces resulting could be derived from a single solution given that
appropriate scaling is adopted. That is the geometry, stress and strain fields throughout the
indentation process are derived from a power law relationship [115-117].
The conclusions reached herein should be accepted only in the context of the experimental
condition prevailing in the current work. We examined human skin with all compositional
layers involved in friction (despite the apparent notion that only the superficial layer, the
“dermis”, is in direct contact with the triboprobe. Snake shedskin samples, however, comprised
only the outer layers of the skin without the sublayers that typically support the skin in a life
reptile. Such a situation affects the deformation and viscoelastic dissipation contributions to
friction of both skins.
The lateral friction force in the two-term model invoked in this work is in essence a
superposition of an adhesive component and a deformation component. The adhesion
component is a consequence of interfacial shear and the deformation component originates
from the energy dissipated by subsurface viscoelastic deformation especially at the front of the
probe. The subsurface deformation also entails viscoelastic stretching, or hysteresis, in the
lateral direction (i.e. lateral skin stretching). The stretching is to be noted takes place within
all subsurface supporting layers of the skin. Naturally, within the constraints of the current
work, dissipation within the human skin samples involved those supporting layers since the
measurements were performed in situ. For snakeskin, however, dissipation did not involve
such layers. That is, lateral and tangential dissipation in case of snakeskin was not activated.
This situation indicates a discrepancy between the amounts of energy dissipated due to
deformation in each of the examined skins. The exact effect of this discrepancy on the accuracy
of the numerical values of the COF ratio was not attempted here. However, it is estimated that
this effect will not have a considerable effect on the trend reflected in the current work for two
reasons. The first is that the deformation contribution to the COF is typically negligible (in the
order of 0.04 to 0.06 [102]). The second meanwhile has to do with the high fractal dimension
of both skins D 2.55 [60], which indicate that the behavior of the material in friction is
dominated by the mechanical properties of the surface features rather than those of the bulk of
the material.
The primary motivation of this study is to explore the similarities in friction response of various
skin types. This work, therefore, introduced, for the first time in tribology literature, a cross-
correlation study of the behaviour of two different skin types (human and snake) in dry friction.
The premise of the work originates from the similarity of the stress-strain curve obtained in
tensile testing of skin across species (i.e. the so-called J-type curve) and from the similarity of
the building blocks of skin across species (fibres and elastin). Combined to the well-
documented influence of mechanical properties on tribological behaviour of materials, the
existence of a common stress-strain curve may suggest the existence of a generalized
framework that can describe, and then predict, the friction behaviour of skin in general.
The analysis invoked the conventional two-term friction model to correlate the friction
coefficient of human skin to that of snakeskin. In doing so we have introduced the simplifying
assumption of elastic behaviour as a first approximation. Within such frame of reference the
ratio of the COF, snake to human, correlated to the inverse ratio of the respective moduli raised
to a fraction exponent.
The results, albeit preliminary and under simplifying conditions, imply that dominant
mechanisms in dry friction of both skins are similar. In particular, the dominance of the moduli
of elasticity implies the principal importance of frictional energy dissipation-accumulation for
structural integrity of the skin building blocks.
Future work will be directed toward the comparison of wet friction performance for both types
of skin and to widen the scope of investigation to include in vitro reptilian skin measurements
and skin from other species.
[1] Vincent JF. Structural biomaterials. Princeton University Press; 2012.
[2] Wainwright SA. Mechanical design in organisms. Princeton University Press; 1982.
[3] Mai YW, Atkins AG. Further comments on J-shaped stress-strain curves and the crack
resistance of biological materials. Journal of physics D: Applied physics. 1989 Jan
[4] Dowson D. Tribology and the skin surface. Bioengineering of the skin: Skin surface
imaging and analysis. 1996 Dec 3:159-79.
[5] Timm K, Myant C, Spikes HA, Grunze M. Particulate lubricants in cosmetic
applications. Tribology International. 2011 Nov 30;44(12):1695-703.
[6] Abramovits W, Gonzalez-Serva A. Sebum, cosmetics, and skin care. Dermatologic
clinics. 2000 Oct 1;18(4):617-20.
[7] Bhushan B, Tang W. Surface, tribological, and mechanical characterization of synthetic
skins for tribological applications in cosmetic science. Journal of applied polymer
science. 2011 Jun 5;120(5):2881-90.
[8] Bhushan B, Wei G, Haddad P. Friction and wear studies of human hair and skin. Wear.
2005 Aug 31;259(7):1012-21.
[9] Korinko A, Yurick A. Maintaining skin integrity during radiation therapy. AJN The
American Journal of Nursing. 1997 Feb 1;97(2):40-4.
[10] Herst PM. Protecting the radiationdamaged skin from friction: a mini review. Journal
of medical radiation sciences. 2014 Jun 1;61(2):119-25.
[11] Rogachefsky AS, Taylor JS. Professional sports: Skin disorders in athletes.
InHandbook of Occupational Dermatology 2000 (pp. 1072-1083). Springer Berlin
[12] Zhang M, Mak AF. In vivo friction properties of human skin. Prosthetics and orthotics
international. 1999 Jan 1;23(2):135-41.
[13] Derler S, Preiswerk M, Rotaru GM, Kaiser JP, Rossi RM. Friction mechanisms and
abrasion of the human finger pad in contact with rough surfaces. Tribology
International. 2015 Sep 30;89:119-27.
[14] Mak AF, Zhang M, Boone DA. State-of-the-art research in lower-limb prosthetic
biomechanics-socket interface: a review. Journal of rehabilitation research and
development. 2001 Mar 1;38(2):161.
[15] Zhang M, Roberts C. Development of a nonlinear finite element model for analysis of
stump/socket interface stresses in below-knee amputee. WIT Transactions on
Biomedicine and Health. 1970 Jan 1;1.
[16] Filep R, Arotaritei D, Turnea M, Ilea M, Rotariu M, Budacu C. A viscoelastic model
for skin of stump in transtibial prosthesis. Proceedings in Manufacturing Systems. 2016
Mar 1;11(1):35.
[17] Derler S, Gerhardt LC. Tribology of skin: review and analysis of experimental results
for the friction coefficient of human skin. Tribology Letters. 2012 Jan 1;45(1):1-27.
[18] Schreiner S, Rechberger M, Bertling J. Haptic perception of friction—correlating
friction measurements of skin against polymer surfaces with subjective evaluations of
the surfaces' grip. Tribology International. 2013 Jul 31;63:21-8.
[19] Tang W, Chen N, Zhang J, Chen S, Ge S, Zhu H, Zhang S, Yang H. Characterization
of tactile perception and optimal exploration movement. Tribology Letters. 2015 May
[20] Darden MA, Schwartz CJ. Skin tribology phenomena associated with reading braille
print: The influence of cell patterns and skin behavior on coefficient of friction. Wear.
2015 Jun 30;332:734-41.
[21] Yoon WJ, Hwang WY, Perry JC. Study on effects of effects of surface properties in
haptic perception of virtual curvature. International Journal of Computer Applications
in Technology. 2016;53(3):236-43.
[22] Gerhardt LC, Strässle V, Lenz A, Spencer ND, Derler S. Influence of epidermal
hydration on the friction of human skin against textiles. Journal of The Royal Society
Interface. 2008 Nov 6;5(28):1317-28.
[23] Hendriks CP, Franklin SE. Influence of surface roughness, material and climate
conditions on the friction of human skin. Tribology Letters. 2010 Feb 1;37:361-73.
[24] Derler S, Huber R, Feuz HP, Hadad M. Influence of surface microstructure on the
sliding friction of plantar skin against hard substrates. Wear. 2009 Jun 15;267:1281-8.
[25] Guerra C, Schwartz CJ. Investigation of the influence of textiles and surface treatments
on blistering using a novel simulant. Skin Research and Technology. 2012 Feb
[26] Soneda T, Nakano K. Investigation of vibrotactile sensation of human fingerpads by
observation of contact zones. Tribology International. 2010 Feb 28;43(1):210-7.
[27] Frackiewicz-Kaczmarek J, Psikuta A, Bueno MA, Rossi RM. Air gap thickness and
contact area in undershirts with various moisture contents: influence of garment fit,
fabric structure and fiber composition. Textile Research Journal. 2015 Mar
[28] Van Der Heide E, Lossie CM, Van Bommel KJ, Reinders SA, Lenting HB.
Experimental investigation of a polymer coating in sliding contact with skin-equivalent
silicone rubber in an aqueous environment. Tribology Transactions. 2010 Oct
[29] Cottenden DJ, Cottenden AM. A study of friction mechanisms between a surrogate skin
(Lorica soft) and nonwoven fabrics. Journal of the mechanical behavior of biomedical
materials. 2013 Dec 31;28:410-26.
[30] Bostan LE, Taylor ZA, Carré MJ, MacNeil S, Franklin SE, Lewis R. A comparison of
friction behaviour for ex vivo human, tissue engineered and synthetic skin. Tribology
International. 2016 Jul 28.
[31] Gerhardt LC, Schiller A, Müller B, Spencer ND, Derler S. Fabrication, characterisation
and tribological investigation of artificial skin surface lipid films. Tribology letters.
2009 May 1;34(2):81-93.
[32] Wang Z, Jiang F, Zhang Y, You Y, Wang Z, Guan Z. Bioinspired Design of
Nanostructured Elastomers with Cross-Linked Soft Matrix Grafting on the Oriented
Rigid Nanofibers To Mimic Mechanical Properties of Human Skin. ACS nano. 2014
Dec 31;9(1):271-8.
[33] Nachman M, Franklin SE. Artificial Skin Model simulating dry and moist in vivo
human skin friction and deformation behaviour. Tribology International. 2016 May
[34] H. Shirado, Y. Nonomura and T. Maeno: Development of artificial skin having human
skin-like texture (realization and evaluation of human skin-like texture by emulating
surface shape pattern and elastic structure), Trans. Jpn. Soc. Mech. Eng. C, 73-726,
541/546 (2007)
[35] Chen S, Bhushan B. Nanomechanical and nanotribological characterization of two
synthetic skins with and without skin cream treatment using atomic force microscopy.
Journal of colloid and interface science. 2013 May 15;398:247-54.
[36] Lijie HF, Pan LH. Study on Tribological Behaviors of Porcine Serum in Water-soluble
Polyalkylene Glycol and Water. Lubrication Engineering. 2009;1:025.
[37] Xiao H, Ariyasinghe N, He X, Liang H. Tribological evaluation of porcine skin.
Colloids and Surfaces B: Biointerfaces. 2014 Apr 1;116:734-8.
[38] Benz MJ, Kovalev AE, Gorb SN. Anisotropic frictional properties in snakes. InSPIE
Smart Structures and Materials+ Nondestructive Evaluation and Health Monitoring
2012 Apr 26 (pp. 83390X-83390X). International Society for Optics and Photonics.
[39] Abdel-Aal HA, El Mansori M, Mezghani S. Multi-scale investigation of surface
topography of ball python (Python regius) shed skin in comparison to human skin.
Tribology letters. 2010 Mar 1;37(3):517-27.
[40] Abdel-Aal HA, Vargiolu R, Zahouani H, El Mansori M. A study on the frictional
response of reptilian shed skin. Journal of Physics: Conference Series 2011 (Vol. 311,
No. 1, p. 012016). IOP Publishing.
[41] Abdel-Aal HA, El Mansori M. Characterization of load bearing metrological
parameters in reptilian exuviae in comparison to precision-finished cylinder liner
surfaces. Surface Topography: Metrology and Properties. 2014 Oct 23;2(4):045002.
[42] Klein MC, Gorb SN. Ultrastructure and wear patterns of the ventral epidermis of four
snake species (Squamata, Serpentes). Zoology. 2014 Oct 31;117(5):295-314.
[43] Godin B , Touitou E , 2007. Transdermal skin delivery: predictions for humans from
in vivo, ex vivo and animal models Adv Drug Deliv Rev 59(11), 1152-61
[44] G.M. Gray, H.J. Yardley, 1975. Lipid compositions of cells isolated from pig, human,
and rat epidermis, J. Lipid Res. 16, 434–440.
[45] U. Jacobi, M. Kaiser, R. Toll, S. Mangelsdorf, H. Audring, N. Otberg, W. Sterry,
Lademann 2007. Porcine ear skin: an in vitro model for human skin, Skin Res. Technol.
13, 19–24.
[46] I.P. Dick, R.C. Scott, 1992. Pig ear skin as an in-vitro model for human skin
permeability, J. Pharm. Pharmacol. 44, 640–645.
[47] F. Muhammad, J.D. Brooks, J.E. Riviere, 2004Comparative mixture effects of JP-
8(100) additives on the dermal absorption and disposition of jet fuel hydrocarbons in
different membrane model systems, Toxicol. Lett. 150, 351–365.
[48] Lin SY, Hou SJ, Hsu TH, Yeh FL., 1992. Comparisons of different animal skins with
human skin in drug percutaneous penetration studies. Methods Find Exp Clin
Pharmacol. Oct;14(8), 645-54.
[49] F. Netzlaff, U.F. Schaefer, C.M. Lehr, P. Meiers, J. Stahl, M. Kietzmann, F. Niedorf,
2006. Comparison of bovine udder skin with human and porcine skin in percutaneous
permeation experiments, Altern. Lab. Anim. 34, 499–513.
[50] Sandby-Moller, J., Poulsen, T., H.C. Wulf, Epidermal thickness at different body sites:
relationship to age, gender, pigmentation, blood content, skin type and smoking habits,
Acta Derm.-Venereol. 83 (2003) 410–413.
[51] Simon, G.A., Maibach, H.I., 2000. The pig as an experimental animal model of
percutaneous permeation in man: qualitative and quantitative observations an overview,
Skin Pharmacol. Appl. Skin Physiol. 13, 229–234.
[52] Higuchi, T., L. Kans, 1988. Method for in vitro determination of transdermal
absorption. United States Patent, Patent number 4771004, 13 Sep.
[53] Itoh, T., J. Xia, R. Magavi, T. Nishisata, J.H. Rytting, 1990a. Use of shed skin as a
model membrane for invitro percutaneous penetration studies: comparison with human
skin. Pharm. Res., 7, 10421047.
[54] Bhattachar, S.N., J.H. Rytting, T. Itoh, T. Nishihata, 1992. The effects of complexation
with hydrogenated phospholipid on the transport of salicylic acid, diclofenac and
indomethacin across snake stratum corneum. Int. J. Pharm., 79: 263-271.
[55] Bhatt, P.P., J.H. Rytting, E.M. Topp, 1991. Influence of azone and lauryl alcohol on
the transport of acetaminophen and ibuprofen through shed snake skin.Int. J. Pharm.,
72: 219-226.
[56] Harada, K., T. Murakami, E. Kawasaka, Y. Higashi, S. Yamanoyo and N. Yata, , 1993.
In vitro permeability to salicylic acid of human, rodent and shed snake skin. J. Pharm.
Pharmacol., 45, 414418.
[57] Pongjanyakul, T., S. Prakongpan, S. Panomsuk, S. Puttipipatkhachorn and A. Priprem,
2002. Shed king cobra and cobra skins as model membranes for in-vitro nicotine
permeation studies. J. Pharm. Pharmacol., 54, 13451350.
[58] Haigh, J.M., E. Beyssac, L. Chanet and J.M. Aiache, 1998. In vitro permeation of
progesterone from a gel through the shed skin of three different snake species. Int. J.
Pharm., 170, 151-156.
[59] Turunen, T.M., S. Buyuktimkin, N. Buyuktimkin, A. Urtti, P. Paronen and J.H. Rytting,
Enhancer delivery of 5-fluorouracil through shed snake skin by two transdermal
penetration enhancers. 1993. Int. J. Pharm. 92, 8995.
[60] Abdel-Aal, H. A., El Mansori, M. The Fractal Structure of the Ventral Scales in Legless
Reptiles, 15th International Conference on Metrology and Properties of Engineering
Surfaces, March 2-5, 2015, UNC Charlotte, Charlotte, North Carolina, USA
[61] Rossi, J.V. 1996. Dermatology. In: Reptile Medicine and Surgery. Douglas R. Mader,
editor. WB Saunders, NY. 104 -117.
[62] Alexander, N.J., Parakkal. P.F., 1969. Formation of a- and ß-type keratin in lizard
epidermis during the molting cycle. Cell and Tissue Research (Historical Archive)101,
[63] Subramanyan, K., Misra, M., Mukherjee, S., Ananthapadmanabhan,, K.: Advances in
the materials science of skin: a composite structure with multiple functions. MRS Bull.
32(10), 770–778 (2007)
[64] Agache, P., Humbert, P. 2004. Measuring the Skin—Non-Invasive Investigations,
Physiology, Normal Constants, 1st edn. Springer- Verlag, Berlin
[65] Elsner, P.: What textile engineers should know about the human skin. Curr. Probl.
Dermatol. 31, 24–34 (2003)
[66] Klein, M-C. G, Deuschle, J.,Gorb S. N.,, 2010. Material properties of the skin of the
Kenyan sand boa Gongylophis colubrinus (Squamata, Boidae). J. of Comp. Phys. A:
Neuroethology, Sensory, Neural, and Behav. Phys., 196: 659-68.
[67] Klein, M-C. G., Gorb, S N., Epidermis architecture and material properties of the skin
of four snake species J. R. Soc. Interface rsif 20120479; 2012,
doi:10.1098/rsif.2012.0479 1742-5662
[68] Pailler-Mattei, C., Nicoli, S., Pirot, E., Vargiolu, R. and Zahouani, H., “A new approach
to describe the skin surface physical properties in vivo”, Colloid Surf; 68: 200–206,
[69] Cua, A. B., Wilheim, K. P. and Maibach, H. I. 1990. Friction Properties of Human Skin:
Relation to Age, Sex and Anatomical Region, Stratum Corneum Hydration and
Transepidermal Water Loss,Brit. J. Dermatol., 123, 473-479.
[70] Elsner, P., Wilhelm, D. and Maibach, H. I., 1990, Friction Properties of Human
Forearm and Vulvar Skin: Influence of Age and Correlation with Transepidermal Water
Loss and Capacitance.Dermatologica, 181, 88-91.
[71] Asserin J., Zahouani H., Humbert Ph. Couturaud V., Mougin D., 2000. Measurement
of the friction coefficient of the human skin in vivo Quantification of the cutaneous
smoothness; Colloids and Surfaces B: Biointerfaces 19:1-12.
[72] Manuskiatti WD, Schwindt DA, Maibach HI. Influence of age, anatomic site and race
on skin roughness and scaliness. Dermatology. 1998 Jun 26;196(4):401-7.
[73] Berthé, R. A., Westhoff, G., Bleckmann, H., Gorb, S. N., 2009. Surface structure and
frictional properties of the skin of the Amazon tree boa Corallus hortulanus (Squamata
Boidae) J. Comp. Phys. A 195: 311–318.
[74] Shafiei, M., Alpas, A. T., 2008. Fabrication of biotextured nanocrystalline nickel films
for the reduction and control of friction. Mat. Sc. and Eng.: C; 28: 1340-1346.
[75] Shafiei, M., Alpas, A. T., 2009 Nanocrystalline nickel films with lotus leaf texture for
superhydrophobic and low friction surfaces App. Surface Science 256: 710-719.
[76] Abdel-Aal, H. A., Vargiolu, R., Zahouani, H., El Mansori, M., 2012. Preliminary
investigation of the frictional response of reptilian shed skin Wear 290–291, 51–60
[77] Berthé, R. A., Westhoff, G., Bleckmann, H., Gorb, S. N., 2009. Surface structure and
frictional properties of the skin of the Amazon tree boa Corallus hortulanus (Squamata
Boidae) J. Comp. Phys. A 195: 311–318.
[78] Abdel-Aal, H. A., 2013. On Surface Structure and Friction Regulation in Reptilian
Locomotion, JMBBM 22:115-135, DOI 10.1016/j.jmbbm.2012.09.014
[79] Comaish, S. and Bottoms, E. 1971. The Skin and Friction: Deviations from Amonton’s
Laws, and the Effects of Hydration and Lubrication, Brit. J. Dermatol., 84, 37-43.
[80] Naylor, P. F. D. (1955), “The Skin Surface and Friction,” Bri. J. Dermatol., 67, 239-
[81] Prall, J. K. (1973), “Instrumental Evaluation of the Effects of Cosmetic Products on
Skin Surfaces with Particular Reference to Smoothness,” J. Soc. Cosmet. Chem., 24,
[82] Johnson, S. A., Gorman, D. M., Adams, M. J. and Briscoe, B. J. (1993), “The Friction
and lubrication of Human Stratum Corneum,” Thin Films in Tribology, Eds.
Dowson,D. et al., Proc. 19th Leeds-Lyon Symp. on Trib., Elsevier Science Publishers,
B. V., 663-672.
[83] Li, W., Kong, M., Liu, X.D., Zhou, Z.R.: Tribological behavior of scar skin and
prosthetic skin in vivo. Tribol. Int. 41(7), 640–647 (2008)
[84] Elleuch, K., Elleuch, R., Zahouani, H.: Comparison of elastic and tactile behavior of
human skin and elastomeric materials through tribological tests. Polym. Eng. Sci.
46(12), 1715–1720 (2006)
[85] Gupta, A.B., Haldar, B., Bhattacharya, M.: A simple device for measuring skin friction.
Ind. J. Dermatol. 40(3), 116–121 (1995)
[86] Nakajima, K., Narasaka, H.: Evaluation of skin surface associated with morphology
and coefficient of friction. Int. J. Cosmet. Sci 15, 135–151 (1993)
[87] Sivamani, R.K., Goodman, J., Gitis, N.V., Maibach, H.I.: Coefficient of friction:
tribological studies in man-an overview. Skin Res. Technol. 9(3), 227-234 (2003)
[88] Egawa, M., Hirao, T. and Takahashi, M., “In vivo estimation of Stratum corneum
thickness from water concentration profiles obtained with Raman spectroscopy”, Acta
Derm Venereol; 87: 4–8, 2007.
[89] Christensen, M.S., Nacht, S., Packman, E.W. 1983: Facial oiliness and dryness:
correlation between instrumental measurements and self-assessment. J. Soc. Cosmet.
Chem. 34(5), 241–253
[90] Hendriks, C. P. and Franklin, S. E., “Influence of Surface Roughness, Material and
Climate Conditions on the Friction of Human Skin”, Tribol Lett; 37: 361-375, 2010.
[91] Ramalho, A., Silva, C. L., Pais, A. A. C. C. and Sousa, J. J. S., “In-vivo friction study
of human skin: influence of moisturizers on different anatomical sites”, Wear; 263:
1044–1049, 2007.
[92] Roberts, A. D. and Brackley, C. A. Friction of surgeons' gloves. J. Phys.: Appl. Phys.,
1992, 25, A28–A32. 37
[93] Gee, M.G., Tomlins, P., Calver, A., Darling, R.H., Rides, M.: A new friction
measurement system for the frictional component of touch. Wear 259(7–12), 1437–
1442 (2005)
[94] Skedung, L., Danerlo¨v, K., Olofsson, U., Aikala, M., Niemi, K., Kettle, J., Rutland,
M.: Finger friction measurements on coated and uncoated printing papers. Tribol. Lett.
37(2), 389–399 (2010)
[95] Childs, T.H.C., Henson, B., 2007. Human tactile perception of screen printed surfaces:
self-report and contact mechanics experiments. Proc. Inst. Mech. Eng. J. 221(J3), 427–
[96] Savescu, A.V., Latash, M.L., Zatsiorsky, V.M.: A technique to determine friction at the
fingertips. J. Appl. Biomech. 24(1), 43–50 (2008)
[97] Greenwood JA, Williamson JB. Contact of nominally flat surfaces. In Proceedings of
the Royal Society of London A: Mathematical, Physical and Engineering Sciences 1966
Dec 6 (Vol. 295, No. 1442, pp. 300-319). The Royal Society.
[98] Shi X, Polycarpou AA. Measurement and modeling of normal contact stiffness and
contact damping at the meso scale. Journal of vibration and acoustics. 2005 Feb
[99] van Kuilenburg J, Masen MA, van der Heide E. Contact modelling of human skin: what
value to use for the modulus of elasticity?. Proceedings of the institution of mechanical
engineers, Part J: Journal of Engineering Tribology. 2013 Apr 1;227(4):349-61.
[100] Rivera G, Savitzky A H., Hinkley J. A., Mechanical properties of the integument of the
common gartersnake, Thamnophis sirtalis (Serpentes: Colubridae), J Exp Biol 2005
208:2913-2922. ; doi:10.1242/jeb.01715
[101] Bowden, F. P. and Tabor, D., Friction and Lubrication of Solids, Oxford University
Press, London, 1954.
[102] Adams, M. J., Briscoe, B. J. and Johnson, S. A., “Friction and lubrication of human
skin”, Tribology Letter ; 26; 3, 2007.
[103] Johnson, S. A., Gorman, D. M., Adams, M. J. and Briscoe, B. J. (1993), “The Friction
and lubrication of Human Stratum Corneum,” Thin Films in Tribology, Eds.
Dowson,D. et al., Proc. 19th Leeds-Lyon Symp. on Trib., Elsevier Science Publishers,
B. V., 663-672.
[104] Han, H. Y., Shimada, A. and Kawamura, S., “Analysis of friction on human fingers and
design of artificial fingers”, In IEEE International Conference on Robotics and
Automation, Minneapolis, Minnesota; 3061–3066, 1996.
[105] Tomlinson, S. E., Lewis, R. and Carré, M .J., “The effect of normal force and roughness
on friction in human finger contact”, Wear; 267: 1311-1318, 2009.
[106] Mossel, W. P. and Roosen, C. P. G., “Friction and the skin”, Contemporary
Ergonomics; 1: 353-358, 1994.
[107] Tang, W., Ge, S. R., Zhu, H., Cao, X. C. and Li, N., “The influence of normal load and
sliding speed on frictional properties of skin”, Journal of bionic engineering; 5: 33-38,
[108] Zahouani, H., Boyer, G., Pailler-Mattei, C., Tkaya, M. B. and Vargiolu, R., “Effect of
human aging on skin rheology and tribology”, Wear; 271: 2364-2369, 2011.
[109] Pailler-Mattei, C., Guerret-Piécourt, C., Zahouani, H. and Nicoli, S., “Interpretation of
the human skin biotribological behaviour after tape stripping”, J R Soc Interface; 8:
934-941, 2011.
[110] Hill, R., Stor˚akers, B. & Zdunek, A. B. 1989 A theoretical study of the Brinell
hardness test. Proc. R. Soc. Lond. A436, 301–330.
[111] Meyer, E. 1908 Untersuchungen ¨uber Harteprufung und Harte. Z. Ver. Deutche Ing.
52, 645– 654.
[112] O’Neill, H. 1944 The signicance of tensile and other mechanical test properties of
metals. Proc. Inst. Mech. Engineers 151, 116–130.
[113] Tabor, D. 1951 The hardness of metals. Oxford University Press.
[114] Norbury, A. L. & Samuel, T. 1928 The recovery and sinking-in or piling-up of
material in the Brinell test, and the eects of these factors on the correlation of the
Brinell with certain other hardness tests. J. Iron Steel Institute 117, 673–687
[115] Timothy, S. P., Pearson, J. M. & Hutchings, I. M. 1987 The contact pressure
distribution during plastic compression of lead spheres. Int. J. Mech. Sci. 29, 713–
[116] Bower, A. F., Fleck, N. A., Needleman, A. & Ogbonna, N. 1993 Indentation of power
law creeping solids. Proc. R. Soc. Lond. A441, 97–124
[117] Biwa, S. & Stor˚akers, B. 1995 An analysis of fully plastic Brinell indentation. J.
Mech. Phys. Solids 43, 1303–1334.
... The tribological properties of snake scales have attracted significant attention during the last decades [3,[10][11][12][13][14][15][16][17][18][19]. The analysis of the topography of ventral scales often reveals micron-sized fibril structures oriented from head to tail [20][21][22]. ...
... Figure 1 shows examples for three snakes living on different continents. These micro-fibrils are commonly considered as an important component for the control of friction and wear during snake sliding [11,16,23,24]. Some studies reported anisotropic friction performance of snake skin [11,16,25] while only a few studies acknowledged the correlation between microstructure of the scales and the respective frictional properties [3,20,24,26]. ...
... These micro-fibrils are commonly considered as an important component for the control of friction and wear during snake sliding [11,16,23,24]. Some studies reported anisotropic friction performance of snake skin [11,16,25] while only a few studies acknowledged the correlation between microstructure of the scales and the respective frictional properties [3,20,24,26]. Investigations on the geometrical patterns and dimensions of snake scales [20,[26][27][28] revealed that the frictional anisotropy on ventral snake scales is caused by the asymmetric geometry of the micro-fibril ends. ...
Full-text available
Ventral scales of most snakes feature micron-sized fibril structures with nanoscale steps oriented towards the snakes' tail. We examined these structures by microtribometry as well as atomic force microscopy (AFM) and observed that the nanoscale steps of the micro-fibrils cause a frictional anisotropy which varies along the snake's body in dependence of the height of the nanoscale steps. A significant frictional behaviour is detected when a sharp AFM tip scans the nanoscale steps up or down. Larger friction peaks appear during upward scans (tail to head direction) while considerably lower peaks are observed for downward scans (head to tail direction). This effect causes a frictional anisotropy on the nanoscale, i.e., friction along the head to tail direction is lower than in opposite direction. The overall effect increases linearly with the step height of the micro-fibrils. Although the step heights are different for each snake, the general step height distribution along the body of the examined snakes follows a common pattern. The frictional anisotropy, induced by the step height distribution, is largest close to the tail, intermediate in the middle, and lower close to the head. This common distribution of frictional anisotropy suggests that snakes even optimized nanoscale features like the height of micro-fibrils through evolution in order to achieve optimal friction performance for locomotion. Finally, ventral snake scales are replicated by imprinting their micro-fibril structures into a polymer. As the natural prototype, the artificial surface exhibits frictional anisotropy in dependence of the respective step height. This feature is of high interest for the design of tribological surfaces with artificial frictional anisotropy.
... It operates physiologically in the large deformation regime and shows a highly nonlinear stress-strain response. The frictional behavior of human skin at the tissue scale (in the order of cm) is affected by several variables including age [25,26], anatomical region [27,28,29], contact material [30], type of contact [5,31], environmental conditions [6,26,28], and wetness in the tissue [32,16]. However, to discriminate between the relative contributions of these factors, it is advantageous to zoom in to the mesoscopic scale, in the order of hundreds of m to a few mm. ...
... Previous experimental studies have shown that the interaction of the human skin with different materials and surfaces can show coefficient of friction over a wide range, between 0.059 and 3.7 [101,43,102]. Although our analysis does not represent the interaction between the human skin and particular materials, values of =0.25 have been reported for the interaction between calf skin and nitrocellulose [29], =0.22 for the contact between forearm skin and stainless steel [103], =0.22 between forearm and polypropylene and between different body zones and Teflon [27], and =0.19 between forearm skin and steel [43] among other similar values to the local coefficient of friction considered here. Previous work considering rigid indenters and soft substrate showed that the reaction forces on the indenter are not just due to the local friction but that an asymmetric deformation field on the length scale of the indenter can contribute a significant deformation component [33,34,85]. ...
Full-text available
Human skin enables interaction with diverse materials every day and at all times. The ability to grasp objects, feel textures, and perceive the environment depends on the mechanical behavior, complex structure, and microscale topography of human skin. At the same time, abrasive interactions, such as sometimes occur with prostheses or textiles, can damage the skin and impair its function. Previous theoretical and computational efforts have shown that skin’s surface topography or microrelief, is crucial for its tribological behavior. However, current understanding is limited to adult surface profiles and simplified two-dimensional simulations. Yet, the skin has a rich set of features in three dimensions, and the geometry of skin is known to change with aging. Here we create a numerical model of a dynamic indentation test to elucidate the effect of changes in microscale topography with aging on the skin’s response under indentation and sliding contact with a spherical indenter. We create three different microrelief geometries representative of different ages based on experimental reports from the literature. We perform the indentation and sliding steps, and calculate the normal and tangential forces on the indenter as it moves in three distinct directions based on the characteristic skin lines. The model also evaluates the effect of varying the material parameters. Our results show that the microscale topography of the skin in three dimensions, together with the mechanical behavior of the skin layers, lead to distinctive trends on the stress and strain distribution. The major finding is the increasing role of anisotropy which emerges from the geometric changes seen with aging.
... The concept of surface texturing is inspired by nature. It has been found that due to surface roughness morphology of body, the organisms such as snakes, sharks and beetles have excellent anti-adhesion, anti-drag and anti-wear functions [7][8][9]. In order to take advantage of surface texturing, many researchers have applied to various mechanical surfaces, which were in relative motion. ...
Full-text available
To investigate the friction and wear characteristics, the impact of texture and un-texture on tribological performance of AISI 52100 bearing steel and 316 L stainless steel sliding pair was evaluated under lubricated conditions. The square shaped protrusions having texture height of 5 μm and area density 0.1 were fabricated on bearing steel through chemical etching process by varying texture distribution such as radial, uniform and zigzag. The pin on disc sliding tests were conducted at normal loads of 10 N, 20 N and 30 N; and sliding speeds of 0.5 m/s, 1 m/s and 1.5 m/s. The results show that the textured specimen has reduced the friction coefficient and specific wear rate than that of un-textured sample. As for as texture’s distribution’s concern, zig-zag pattern has the advantageous effect on friction and wear reduction, followed by specimen with uniform pattern and radial pattern. Furthermore, a normal load of 10 N and a sliding speed of 1.5 m/s have exhibited the best influence and enhanced the tribological performance by capturing wear debris and increasing the hydrodynamic effects at the contact. Scanning Electron Microscopic (SEM) analysis shows that the textured specimen can reasonably reduce the abrasive wear and improve the frictional and wear performance than un-textured specimen.
... In addition, some researchers have also focused on the surface morphology of snakes [184] and water snail [185] etc., to develop surface textures with similar structures to reduce friction. Human joints have excellent lubrication and a very low friction coefficient at high loads [186]. ...
Full-text available
With the development of green tribology in the shipping industry, the application of water lubrication gradually replaces oil lubrication in stern bearings and thrust bearings. In terms of large-scale and high-speed ships, water-lubricated bearings with high performance are more strictly required. However, due to the lubricating medium, water-lubricated bearings have many problems such as friction, wear, vibration, noise, etc. This review focuses on the performance of marine water-lubricated bearings and their failure prevention mechanism. Furthermore, the research of marine water-lubricated bearings is reviewed by discussing its lubrication principle, test technology, friction and wear mechanism, and friction noise generation mechanism. The performance enhancement methods have been overviewed from structure optimization and material modification. Finally, the potential problems and the perspective of water-lubricated bearings are given in detail.
... Over billions of years of evolution, a large number of organisms generally have excellent functions of adhesion reduction, drag reduction and wear resistance [20,21]. Wang et al. [22] investigated the in uences of smooth and bio-inspired surfaces on tribological properties of the carbon-ber-reinforced polyetheretherketone (CFRPEEK) coupled with stainless steel 316L under natural seawater lubrication. ...
Full-text available
Carbon fibre reinforced polymer (CFRP) composites are widely used in high-tech industries like the automobile and aerospace sectors, but the wear resistance of tools is one of the most significant restrictions in machining CFRP. In this study, bionic cutting tools based on surface microstructures of blood clams were fabricated to improve the dry cutting performance of CFRP. Three types of bionic cutting tools with different size parameters were applied in dry cutting test. The three-axis cutting forces and the cutting temperature of the processed workpiece were measured. The average friction coefficient between the rake face and the chip was calculated, and the morphology of the tools wear were observed. The results showed that bionic microstructures with appropriate size parameters can extremely improve the cutting performance of CFRP for tool in turning.
Water-lubricated bearing has become the development trend in the future because of its economy and environmental friendliness. The poor friction performance under low speed and heavy load seriously limits the popularization and application of water-lubricated bearings. Learning from nature, the phenomenon of low friction and wear in nature has aroused great interest of scientists, and a lot of research has been carried out from mechanism analysis to bionic application. In this review, our purpose is to provide guiding methods and analysis basis for the bionic design and theoretical research of anti-friction and anti-wear of water-lubricated bearings. The development of water-lubricated bearing materials are described. Some typical examples of natural friction reduction and drag reduction are introduced in detail, and several representative preparation methods are listed. Finally, the application status of bionic tribology in water-lubricated bearings is summarized, and the future development direction of water-lubricated bearings is prospected.
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Antioxidants of natural origin are used in medicines and cosmetics with several benefits, such as: photoprotective action, anti-aging, moisturizing and anti-pollutant. The human epidermis has an important barrier effect and limited anti-oxidative capacity, so studies with the epidermis is essential. Shed snakeskin (SS) is composed of the stratum corneum and provide a barrier like human stratum corneum. This alternative does not show a tendency to microbiological degradation and can be considered ecologically correct. This study intends to present, in an innovative way, the Electron Paramagnetic Resonance spectroscopy (EPR) and The Forster Resonance Energy Transfer (FRET) were employed to evaluate the natural antioxidant substances (Resveratrol/ RES 3.0 w/w and Ferulic acid/ FA 1.0 w/w) associated with organic sunscreens ingredients (Ethylhexyl Methoxycinnamate/ EHMC 10.0%w/w and Butyl Methoxydibenzoylmethano/ BMBM 5.0%w/w in a photoprotective emulsion (PB). Furthermore, the use of SS seedlings as a possible alternative to the use of human or animal skin ex-vivo. RES and FA can absorb the energy emitted by the EHMC in FRET, preventing the passage through the triplet state, favoring the photostability of this sunscreen, the same not ocorred with the BMBM. Antioxidant activity of the photoprotective formulations was evaluated in vitro by the percentual inhibition of the radical 2,2-diphenyl-1-picrihydrazyl (DPPH•). The antioxidant activity with RES, 97.0% inhibition of DPPH• in the PB, was higher than PB + FA (91.0%), however the concentration of RES in PB was higher than FA. The sample SS + PB + FA was the one with the lowest number of free radicals after irradiation, which corroborated the high percentage of radical inhibition in vitro and it was the better association with the photoprotective formulation.
Skin tribology is complex and in situ behaviour of skin varies considerably between test subjects. The main influencing factor, elasticity, varies due to structural and moisture differences. To find a more reliable test platform, for the first time, synthetic and biological (tissue engineered) substitutes were compared to ex vivo skin, epidermis and dermis. Friction initially increased with rising hydration, before decreasing beyond a threshold for all samples. Friction for Synthetic skin and dermis increased at a similar rate to the other samples, but from a different starting point, and friction dropped at lower hydration. Tissue engineered skin could provide a reliable test platform, but the synthetic skin could only be used if the offset in the data is accounted for.
Stiffness and friction are regarded as important haptic cues for designers of haptic virtual environments. One virtual concave curvature discrimination test based on randomised block design with three factors, radius, stiffness and friction, has been designed and performed to understand human perceptual performance. Randomised block design with three factors can separately distinguish the subject random effect from main effects. In the experiment, subjects compare two virtual sequential concave surfaces and identify the surface having the higher curvature. The statistical results based on Analysis of Variance (ANOVA), regression tree and logistic regression indicate that variations in curvature discrimination may be slightly attributable to varying virtual haptic stiffness between 0.3 N/mm and 0.7 N/mm. Also, a clear interaction in radius and friction coefficient between 0 and 0.2 is found. While the experimental results are limited by some factors, the authors believe that the resulting effects of haptic surface properties may serve to improve the fidelity of medical virtual reality simulators.
In vivo friction and indentation deformation experiments were carried out using the human volar forearm of a healthy 29 year old Caucasian woman and compared with various synthetic materials in order to select materials and develop a new moisture-sensitive artificial skin model (ASM). Analogous to human skin the final ASM comprised two different layers: a relatively stiff hydrophilic moisture-absorbing top layer representing the epidermis and a very soft under-layer representing the dermis and hypodermis. The friction and deformation behaviour of the new ASM was comparable to human skin when tested under dry and moist skin conditions. This development has potential for use as a test-bed in the development of devices that interact with the skin in a mechanical way.
In vivo measurements are reported of the lubrication imparted by water, various aqueous media, and some hydrophobic oils, to a glass probe sliding on the inner human forearm. The primary aim of the paper is to show that the tribology of the human skin surface may be satisfactorily interpreted on the basis of precedents developed for the rationalization of frictional data for elastomers and thermoplastics, and selected data are interpreted on this basis. Water itself plasticises the stratum corneum and, consequently, has profound effects on the frictional behaviour. The effective lubrication observed in surfactant solutions can be explained in terms of electrostatic double-layer repulsion between charged layers adsorbed on the contacting surfaces. Viscosity effects associated with conventional hydrodynamic lubrication are demonstrated using topically applied silicone oils.
The heat and water vapor transport in clothing results from fabric properties, air layers enclosed in the garment and environmental conditions. Accumulation of moisture in clothing layers intensifies this transport because of the change in thermal properties of wet fabrics and the size of air gap thickness and the contact area. This paper presents distribution of air gaps and contact areas in relation to various moisture contents in typical undershirts confectioned from fabrics with contrasting affinity to moisture. The effect of the undershirt fit, body region, fabric structure and fiber type is also discussed. The air gap thickness and the contact area were determined using three-dimensional scanning and the post-processing technique. The results show that the influence of the moisture content on the sought parameters noticeably varied among body regions and was related to the regional fit of the clothing. This variation was larger in cotton than polyester undershirts or those containing spandex, but the direct relevance of the fabric structure was not clear. Although influence of the moisture content was found, the magnitude of the air gap thickness and the contact area resulted mainly from the garment fit.