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Viscoelastic Surfaces and interfaces inversely to
classical laws of friction @ Tribology
P. Tomar ( pankaj_12343@rediffmail.com )
IGDTUW
Article
Keywords: Adhesion, Lubrication, Amphiphilicity, Viscoelasticity, Friction coecient
Posted Date: November 10th, 2023
DOI: https://doi.org/10.21203/rs.3.rs-3576731/v1
License: This work is licensed under a Creative Commons Attribution 4.0 International License.
Read Full License
Additional Declarations: No competing interests reported.
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Abstract
Tribology of bio and bio-inspired interface is useful for assessment of friction, lubrication, and wear of
interacting surfaces. The ultra-low friction coecient (~ 0.01 or less) of soft biological interface in sliding
condition is due to the impact of surface chemistry and biomechanical diffusion. The biomimicry of soft
hydrogels from hydrophobic macromolecules with supramolecular interaction of hydration shells is
emerging for designing of amphiphilic substrates. The viscoelasticity is the inherent mechanical property
of soft biological matter/hydrogels for performance under loadings.
1. Introduction
The word ‘tribology’ was coined almost 450 years after the death of Leonardo da Vinci (1452–1519) for
explaining the fundamental of friction, lubrication and wear at sliding or rubbing mechanical interface
[1–2]. Amontons' laws of friction 1699 had ensured a proportionality between friction force and the
applied load and the friction force is independent of apparent area of contact is a well-known in the eld
of tribology or friction “laws” invalid in many practical situations [3]. The systematically breaks down of
Amontons' law for an elastic object experiencing a friction force for assessment of the macroscopic
static friction coecient corresponds to the onset of bulk sliding of the object decreases as system
length increases [4]. The involvement of real area of contact in prediction of friction at tribological
interface is rst time included during second half of twenty century by Bowden & Tabor in published
literature [5–6]. The contact mechanics of friction of the engineering sciences are indispensable for the
designing of energy ecient surface for interpretation of friction for the development of new products
and technologies [7–8]. The inuence of surface energy between elastic contact are derived for the
contact size and the force of adhesion between two spherical solid surfaces by experiments on the
contact of rubber and gelatine spheres [9]. The rubbing interfaces have been expressed in brief
retrospectively for transformation of rigid substrates to viscoelastic soft matter with a milestone of
elastic deformation at dry polymeric interface [10]. Friction, lubrication, and Wear regulation are
ubiquitous in daily life from human thermodynamics to engine lubrication for the achievement of
mechanical eciency.
Nature is evolving heterogeneous surfaces, interfaces, and interphases across millions of years of life on
land such as hydrophilic, hydrophobic, amphiphilic, and oleophobic for synchronization of friction in a
conscious environment [11]. Bio-inspired tribology is an effective way for synchronization of
environmental drag at engineered substrates for promotion of sustainability and advancement of
scientic innovation as a function of state variables [12–14]. The bio-inspired surfaces mimicry is
resulted in rationalization of energy losses due to tribological processes at mechanical contacts, friction,
and drag reduction, design and development of anti-wear and anti-adhesion surfaces for harmony of
sustainable development in reducing emissions of greenhouse gases [15]. The surface chemistry of the
gas–solid, liquid–solid, and solid–solid interface under reactive environment creates a boundary between
interfacial materials and environment for tribological performance in biomedical, anticorrosion coatings,
and nanotechnology at the molecular level [16]. The interdependence of surface roughness, adhesion,
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stick-slip interlocking of asperities, and environment are the fundamental factors in explaining the
frictional behavior of human skin for condition monitoring of skin-personal protective equipment (PPE)
interaction to prevent facial tissue injury [17–19]. The SARS-CoV-2-pandemic has threatened life in urban
cities for promotion of N95 face masks as ubiquitous factors affecting human breathing, stick-slip soft
tribology, resistance to a re-breathing volume of exhaled CO2 for advancement of sustainability [20]. The
study of friction, lubrication, and wear (Fig.1) is related to bio-inspired science for surface functionalities.
2. Soft matter & Biomaterials
A uid mosaic macrostructure of the proteins/lipids of biological membranes from thermodynamics
orientation is a heterogeneous/amphipathic molecule arranged with the ionic and polar groups
protruding from the membrane into the hydro phase, the nonpolar groups ane in the hydrophobic part
of the membrane [21]. The molecular organization of biological membranes may be predicted including
membrane transport and a few aspects of membrane biochemistry [22]. Amphiphilic membrane
possesses a hydrophilic (lipophobic) head and a hydrophobic (lipophilic) tail consisting of one or two
hydrocarbon chains whereas the hydrophilic head is in aqueous solution, the hydrophobic tail tends to
minimize contact with aqueous solution [23]. The uid and mechanical properties of biological
membranes enable biological functions from membrane tension in the assessment of amphiphilic
exudation, biomechanical diffusion, and surface energy [24–25]. The uid-mosaic model of a liquid-like
plasma membrane can ow in response to tension gradients and membrane diffusion resists local
physiochemical change due to the action-reaction hypothesis [26]. The connective tissues of cartilage
and corneal stroma are fundamentally hydrogels of brous collagen, proteoglycans, and more than 70
percent water molecules in providing tribological behavior of low friction at soft tissues [27]. The
tribological sliding interfaces of soft aqueous gels in the body including cell membranes, pleura, cartilage,
and the eye provide a natural defence against applied loads for biomimicry 7.5 wt% polyacrylamide, 0.3
wt% N, N′-methylenebisacrylamide provide constant contact pressures measurements [28]. The
fundamental factor responsible for the tribological performance of bio and bio-inspired surfaces,
interfaces, and interphases has been expressed (Table1) for explaining the mechanics and mechanisms
of rubbing contacts. The biomimicry of biological uid mosaic macrostructure may provide a mechanism
and mechanics of hydrogels or soft matter.
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Table 1
The mechanics and mechanism of biotribology in explaining ultra-low friction coecient due to
biomechanical diffusion, high friction of human skin due to viscoelasticity with roughness, and the
involvement of surface chemistry for residual tribological boundaries [21–28]
Synergy Nomenclature
Mechanical tension of interacting surfaces regulates the integrity of bio membrane
such as self-organization, soft matter viscoelastic properties, and a rationalized
friction behaviour of interfaces
Surface
tension
Shearing for ow dynamics in cellular microenvironment for sensing biomechanical
factors through mechanosensitive channels
Shearing
Mechanical stiffness of soft membrane less than 500 kPa due to amphiphilic
macrostructure of extracellular matrix for providing kinematics
Stiffness
Human body is a thermodynamic heat engine producing mechanical work in an
environment of source and sink for inclusion of mechanical loadings
(Static/Dynamic) as per the assessment of mechanical eciency (An indirect
measure of tribology performance)
Mechanical
loadings
Muscles contracts in pursual of human kinematics during gait or running for
providing effective lubrication of rubbing contacts with stick-slip external
boundaries during locomotion
Lubrication
Fluid-mosaic macromolecules restore hydration shells or water molecules in a
supramolecular interaction of hydrophobic molecules for bioinspiration of soft
hydrogels
Hydration
The polyvinyl alcohol (PVA) hydrogel tribological testing is carried out in a ball-in-socket using a
pendulum hip simulator causing up to 98% reduction in friction coecient compared to conventional
mechanical socket joints of metal/ceramic heads with UHMWPE cup [29]. The polyethylene glycol (PEG)
lubrication for intra-articular has the potential to prevent wear of UHMWPE by mixing with synovial uid
for the durability of knee joint prostheses [30]. The cell membrane-inspired phospholipid polymers'
usefulness of medical devices composed of 2-methacryloyloxyethyl phosphorylcholine (MPC) units has
been preferred in the preparation of an articial cell membrane structure at the surface of articial and
biological systems [31]. Hydration lubrication in the amphiphilic membrane of poly(2-
methacryloyloxyethyl phosphorylcholine-co-n-butyl methacrylate) or poly(MPC-co-BMA) is resulted in a
reduced frictional force for surface coating for catheter applications for smoother catheterization [32].
The physiological needs of the ocular surface for the evolution rst hydrogel or poly-
hydroxyethylmethacrylate (pHEMA) had been developed by Wichterle followed by a few advanced
hydrogels including oxygen permeability in using siloxane and uorosiloxane based hydrogels [33–35].
Polydimethylsiloxane (PDMS) is a soft hydrogel that provides biocompatibility, transparency, and
mechanical properties in potential applications such as tissue engineering, ocular tribology, microuidics,
exible devices, and many others [36–38]. The socioeconomic and biomedical demand for ophthalmic
treatments for modulation of conventional polymethyl methacrylate (PMMA) to advanced contact lenses
is a now rapidly evolving biomaterial for applications such as drug delivery for designing and
manufacturing technologies [39]. The design of stimuli-responsive hydrogels and soft biomaterials have
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recently created scientic advancement by improvement of technological potential for regulating soft
matter useful for biomedical applications and multi-responsiveness [40–43]. The tribological
performance of soft matter and biomaterials are viable in engineering domain for designing and
manufacturing of biocompatible materials.
3. Joint tribology
The friction, lubrication, and adsorption at articular cartilage is a complex phenomenon of a
heterogeneous supramolecular chemistry, mechanical loading, diversity, ageing, and a materials-energy
balance for preventing cartilage-cartilage adhesion [44–46]. The lubricin is a mucinous glycoprotein
initially known as supercial zone protein, a product of the supercial zone chondrocytes,
biomechanically diffused in synovial joints to form a nano coating over the cartilage surface, prevent
cartilage adhesion, and providing boundary lubrication or act as boundary lubricant [47]. The high molar
mass hyaluronan (HA) presence in this synovial uid provides viscosity for quasi-mechanical loading or
impact resistance and protect cartilage in daily life [48]. The in-situ presence of phospholipids in synovial
ingredients is due to the evolution of hydrophilic–hydrophobic macrostructure for a given phospholipid
bilayer so it leads to a lamellar-repulsive mechanism in stable low-friction conditions [49]. The synovial
uid viscosity decreased by 58% with age and 38% with body mass index, the shear-thinning index
increased by 40% and 7%, respectively as adhesion energy correlation to the coecient of friction
increased by 172% with age and by 234% with body mass index, and the interface adhesion properties of
synovial uid support the relationship with physicochemical properties [50]. The surface active
hyaluronan molecules with phosphatidylcholine lipids present in joints to form a boundary lubricating
layer create a tribology in providing the lubrication of synovial joints from the hydration lubrication [51].
The biotribology from friction, lubrication, and adsorption at articular cartilage is inuenced by synovial
uid digestion or degradation due to lean mechanical work in anticipation of human body as a
thermodynamic heat engine (Table2) for effective materials and energy balance.
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Table 2
The joint tribology of synovial uid due to the heterogeneity of Lubricin/HA/Phospholipid, mechanical
factors, aging, diversity of biology, and improper life style in assessment of moderate risk domain
expressed by virtual survey of enlisted references for advancement of sustainable pattern of life [44–51]
Synergy Factors
Lubricin (230–280 kDa), Spercial zone protein or SZP (345 kDa), hyalunoric acid or
HA (3–7 MDa), and phospholipid biochemical environment of cartilage interface in
providing ultra-low friction
Biochemistry
Human locomotion evolves mechanical stress in a domain of boundary lubrication,
mixed lubrication, and hydrodynamic lubrication regimes as a function of state
variables
Mechanical
stress
Degradation of surface topography and lowering surface tension of tribological
contacts for lean biological lubrication
Aging
Diversity may be a little factor as Gender serving in a culture restricting mechanical
work per day for assessment of risk tribological domain
Diversity
Building up of body mass index due to lean mechanical work, lower net mechanical
eciency, and polluted environment inuencing fuel oxidation
Work-life
imbalance
Metals, esters, ceramics, and synthetic polymers are used in biomedical surgery of
articulating joints such as CoCrMo, titanium alloys, ultra-high weight polyethylene
(UHMWPE), PLGA, and hydroxyapatite porous ceramics
Biomaterials
A kinetic friction coecient is being invariant with state variables to implement articular cartilage-on-
cartilage lubrication test for validation of the friction properties on sliding velocity, axial load, and time,
and establish conditions on boundary lubrication [52]. Titanium alloys and CoCrMo alloys used for hip
and knee endoprostheses due to thermomechanical and physicochemical integrity for further studies of
biolm formation during aseptic and septic loosening [53–54]. Ultra-High Molecular Weight Polyethylene
(UHMWPE) is used for tribological performance to provide durable implants in biomedical applications
due to biocompatibility, ductility, and high wear resistance preferred in recent decades [55]. The denition
of the biodegradable and bioabsorbable are used in the tissue engineering to discuss the rationale
function as well as physicochemical properties of polymer scaffolds [56]. The biodegradable polymers
useful for orthopedic biomaterials applications have secured the history for last ve decades based on
poly(lactides) and poly(glycolides) such as poly lactic acid (PLA) or poly lactic-co-glycolic acid (PLGA)
among the most attractive polymers [57]. The biomaterials have been advanced decade to decades
through three different generations, rst generation for bioinert materials, second generation for
biodegradable materials, and third generation for materials designed to stimulate responses at the
molecular level based on metals, ceramics and polymers preferred in orthopaedic applications [58]. The
topography and chemistry of bioimplant surfaces help in understanding the biological interactions or
develop novel implant nano surface for biological interactive properties to regeneration of tissues,
osteoblast functions, and bone scientic role in orthopaedic implants [59–60]. Bioengineering aims to
restore biological functions to retain or modify damaged tissues in terms of key variables viz. Economy,
Biocompatibility, and Mechanical properties.
Page 7/17
4. Friction coecient
The SARS-CoV-2 pandemic outbreak was a self-isolation for transforming work-life balance such as
enhancement of mechanical work in gait for achievement of rational mechanical eciency. The human
locomotion in gait is for providing stick-slip friction during human kinematics and mechanical
interlocking of skin texture on soft technological ubiquitous in daily life. The crushing of asperities peaks
under applied or created loadings evolves sticking and slipping boundaries termed stick-slip friction for
soft tribology [61–63]. The surface, interface, and interphase of foot with engineered substates provide
in
situ
friction by sticking with slipping boundaries in environment [64]. The surface cracking of biological
matter at foot of skin ssures have been briey concluded not only by a function of environment but also
inuenced by impact of dynamic mechanical loadings. The evolution of surface tension under
mechanical loadings in gait, brittleness of soft matter due to scarcity of hydration lubrication across bio
membrane, and exceeding mechanical stress above safe boundary quoted a biomechanical ground under
tribological rubbing for rupture of surface as a mechanism of skin ssures. The friction coecient for
materials in tribological contacts against skin is strongly inuenced by the selection of the materials such
as natural or synthetic, mechanical parameters, ambient humidity, and skin hydration regime. The friction,
lubrication, and wear at the tribological interface depend on the state variables and interphase
environment [65–69]. The heterogeneity of tribological interfaces (Table3) for prediction of friction
coecients have been expressed from a diverse academic survey of biological and bio-inspired surfaces.
Table 3. The coecient of friction (COF) of soft surfaces, interfaces, and interphases expressed from a
virtual survey of academic content listed with paper from ultra-low friction to stick-slip realities of
biological and bio-inspired materials [12, 14, 18-19, 27-28, 44, 64-65, 67-68, 70]
Page 8/17
Interphases COF
Healthy joint articulation in boundary lubrication regime due to the few layers of
lubricin/PRG4 coat over soft biological matter, biomechanical diffusion, and hydration
lubrication
~0.001
Mean value of kinetic friction of phosphate buffered saline (PBS) and synovial uid (SF)
for mimicry biological lubrication 0.014
to
0.072
The slippery catsh mucus secreted at the tribological surface and environment to protect
from predators, ease to move faster due to the ultra-low coecient of friction (COF) of sh
skin
0.005
to
0.007
The friction coecient broad range of the sodium methacrylate NaMA, poly N-isopropyl-
acrylamide pNIPAM or DMAEMA hydrogels in neutral and acidic surroundings 0.05 to
1.2
The lower bound values of friction coecients, dependence on the load, and the inuence
of chemical structure, surface properties of the opposing substrates, and the measurement
conditions soft gel
0.001
to
0.0001
Load independent friction coecient of “Gemini” contacts for hydrogel sample of 7.5 wt%
polyacrylamide, 0.3 wt% N,N′-methylenebisacrylamide in self-mated tribological
conguration
~0.006
Hyaluroran (HA) expressed from avidin–biotin chemistry, and the surfaces incubated with
dipalmitoylphosphatidylcholine (DPPC)for preparation of HA-DPPC liposomes
resemblance of synovial uid for tribological testing
0.007
to
0.0006
The friction characteristics of human skin in sticking and sliding contacts expressed from
physiochemical and environmental factors at epidermis incorporating adhesion and
viscoelastic deformation of substrates
0.27 to
0.79
Endocarp and epicarp of Banana skin friction at oor surface during sliding conditions
from polysaccharide follicular gel as an environment at the interphase of tribological
testing
~0.07
Polyacrylamide hydrogels: Ammonium persulfate (APS) and N,N,N′N′-
tetramethylethylenediamine (TEMED) and 2-hydroxy-4′-(2-hydroxyethoxy)-2-
methylpropiophenone (Irgacure 2959) in sliding contact with glass hemispherical probes
0.07 to
0.002
Si3N4 ball and sulfonated polyether ether ketone (SPEEK) disk friction coecient smaller
than the polyether ether ketone (PEEK) and polystyrene sulfonate(NaPS) composite disc
with 10 wt % and 20 wt % NaPS
0.05 to
0.07
5. Biomechanical cracks repairing
Human body is a thermodynamic heat engine in pursual of mechanical work for effective
functionalization of rubbing biological interface over engineered surfaces. The mechanical loading and
mechanical work in daily life evolve surface fracture due to enhancement of skin brittleness, lubrication
scarcity, and unforeseen biomechanical factors. The biotribology of skin surface at sticking and slipping
zone of foot is ubiquitous in daily life for achievement of net mechanical eciency above resting.
Mechanochemical and environmental factors contribute for cracking of skin surface as a function of
environmental moisture, direct rubbing contact during locomotion, and loss of skin hydration ions by
Page 9/17
mechanical work in addition to genetic and aging [71]. Adults human skin is covering ~ 2 m2 of apparent
area of contact in defeating reactive forces of nature consciousness with viscoelastic mechanical
properties. Skin biomechanics is fundamentally physiochemical properties of “Epidermis” and “Dermis”
for providing a defence mechanism against environmental reactions, injury, ultraviolet radiation, and loss
of skin integrity during crude mechanical work or improper life-style [72]. Skin hydration, real area of
contact, and mechanical properties altogether resolves sticking and slipping attributes of tribology for
underlying mechanism of asperities contacts [73–75]. Skin ssures (Fig.2 & Fig.3) are the surface
fracture due to evaporation of skin hydration due to increase of brittleness and surface tension of
epidermis evolve cracking as a function of diversity, genetic, aging, environmental factors, and
mechanical loading.
6. Conclusions
Bio and bio-inspired science from nature lead to innovative new technologies of energy-ecient materials
with remarkable surface properties, biocompatibility, and functionalities owing to their complex
composite structures. The materials and energy balance of soft matter, soft matter over hard substrates,
and bioabsorption of biomaterials have been promoted by researchers for the last ve decades. Tribology
of interacting surfaces, interfaces, and interphases have been included primarily from soft viscoelastic
substrates with a brieng for heterogeneous systems as;
To understand the mechanical loadings on the facemask skin affected by the characteristics of the
PPE in terms of its material, geometric and interfacial properties for covid#19 health workers in
preventing injuries
Net mechanical eciency for interpretation of hydration lubrication conditions of starved or stick-slip
surfaces, interfaces, and interphases in an interacting environment of soft matter with engineered
surface
Diversity, aging, gender, and improper lifestyle for explaining synovial digestion for joint pain for
explaining underlying mechanisms of biotribology as implicit factors affecting lubrication
boundaries
Biomechanical factors such as mechanical loadings, degradation of the interface, and
environmental factor as the human body is a thermodynamic engine for the conversion of
mechanical work in a conscious umbrella of the biosphere
Biomaterials for socioeconomic resilience e.g., esters, ethers, metals, polymers, and ceramics for the
advancement of a hundred billion Dollars industries
The ultra-low coecient of friction COF ~0.01 or less of soft matter is predicted from the
amphiphilicity of membrane for restoring uid, diffusion of uid under surface tension, and
heterogeneity of rubbing environment
Virtual assessment of the moderate risk domain of lean joint lubrication by researching the available
literature on friction, lubrication, and materials-energy balance
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The swelling of biological membranes, surface tension due to restoring energy of muscles, and
conservation of mass and energy principle altogether explain the complexities of underlying
mechanisms and mechanics
The biomaterials industries have crossed hundred Billon Dollars for designing and manufacturing
biocompatible systems.
The stick-slip friction of human skin is a surface reaction of exposed interface with a conscious umbrella
of Nature for evolving surface topography of macro, micro, and nanodomains. The anisotropy and
heterogeneity have been found in the shearing phenomenon from molecular friction, seismology, and
mechanical work in gait at slipping and sticking oscillatory footwear soft contacts.
Declarations
Acknowledgment
The author may like to acknowledge Dr. Saroj Sehrawat, Paras Hospital, Gurugram, Haryana, India in
addition to JioFibre of Reliance Industries for providing a cyber facility useful in virtual re-searching of
academic content
Author contribution
The author wrote a paper on the achievement of performance indicators
Ethics declaration
No human subjects are involved in the writing of the paper
Funding resources
No funding resources are available
Data availability statement
Expressed data is borrowed from enlisted references
Conict of Interests
None conict of interests to declare
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Figures
Figure 1
Soft matter of heterogeneous and anisotropic biology, tribology, functionalities, and bio-inspired science
for modulation of innovation from conventional rubbing at mechanical substrates to cell adhesion and
manifold applications of biotribology e.g., Synovial lubrication, Ocular Tribology, Oral Tribology, Skin
tribology, and vascular drag
Figure 2
Page 17/17
(a) Skin ssures evolve from mechanical loading due to evaporation of skin hydration by environmental
reactions (t=0) at rubbing stick-slip contact of biological skin with mechanical substrate in open
environment of nature consciousness; (b) Repaired skin surface by inclusion of hydrophilic interface at
the interphase of foot skin with soft mechanical substate predicted from the skin hydration loss
resistance by biocompatible thermal shield (t=15 months)
Figure 3
The illustration of tribological repairing of cracked biomechanical skin surface evolved from mechanical
loading, brittleness, and environmental absorption of skin hydration for promotion of surface science and
advancement of biotribology of stick-slip domain ubiquitous in daily life
