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Universities of Leeds, Sheffield and York
This is an author produced version of a paper published in Proceedings of the
Institution of Mechanical Engineers, Part C: Journal of Mechanical
Engineering Science.
White Rose Research Online URL for this paper:
Published paper
Neville, A., Morina, A., Liskiewicz, T. and Yan, Y. (2007) Synovial joint lubrication
– does nature teach more effective engineering lubrication strategies?
Proceedings of the Institution of Mechanical Engineers, Part C: Journal of
Mechanical Engineering Science, 221 (10). pp. 1223-1230. ISSN 0954-4062
White Rose Research Online
[email protected]
Synovial Joint Lubrication – does nature teach more effective engineering
lubrication strategies?
A. Neville, A. Morina, T. Liskiewicz, Y. Yan
Institute for Engineering Thermofluids, Surfaces and Interfaces (iETSI),
School of Mechanical Engineering,
University of Leeds,
LS 2 9JT
[email protected]
Nature shows numerous examples of systems which show energy efficiency, elegance in
their design and optimum use of materials. Biomimetics is an emerging field of research
in engineering and successes have been documented in the diverse fields of robotics,
mechanics, materials engineering and many more. To date little biomimetics research
has been directed towards tribology in terms of transferring technologies from biological
systems into engineering applications. The potential for biomimicry has been recognised
in terms of replicating natural lubricants but this system reviews the potential for
mimicking the synovial joint as an efficient and durable tribological system for potential
engineering systems. The use of materials and the integration of materials technology
and fluid/surface interactions are central to the discussion.
Lubrication technology has for the last 6 decades relied to a great extent on chemical
additives added to oils to provide important tribological functionality, namely wear
protection and friction reduction. In the boundary lubrication regime, the system is
characterized by intimate contact between the asperities of the tribocouple and the
tribochemical reactions that occur as a result of this contact are crucial for enabling the
lubricant to become functional.
Radical changes in lubricant additive technology are being forced on formulators,
primarily through changes in legislation, and because of this there is a need for
alternative approaches towards effective lubrication or more efficient tribological systems.
Incremental steps are being made to get towards environmentally-acceptable solutions to
achieve target CO2 emissions, alongside retained engine performance but these are
unlikely to deliver any more than an incremental move to keep in line with the shifting
targets imposed by government. As examples:
The level of P has progressively decreased from 0.12wt% in 1993 for ILSAC
(International Lubricant Standardization and Approval Committee) GF-1 oils, to
0.1wt% in 1996 and 2001 (GF-2 & 3) and it will be further reduced to 0.08wt%
when GF-4 is introduced.
The CHON concept (lubricating engines with only Carbon, Hydrogen, Oxygen
and Nitrogen) has been introduced by formulators and some progress in this
respect has been made
Alternative additives, some based on e.g B, are being introduced to replace some
of the functionality of P. Future legislation on B-containing compounds is not yet
Engineers are increasingly turning to nature for effective solutions to some of the most
challenging technological problems – perhaps because nature provides systems which are
energy efficient, normally elegant and durable. The field of biomimetics has made an
impact in the fields of robotics, biomedical devices, sensors and several others.
In tribology there has been progress made towards using lubricants derived from nature
[1-4] and replication of natural lubricants [5,6]; the work being driven by the need to
provide “green” lubrication strategies. Also, in tribology there has been a lot of effort
expended in the last decades to understand fully how synovial joints work and the field of
biotribology is enormous and growing. However, the drive in biotribology is to enable
more efficient replacement joints to be developed. Both the understanding of the natural
joint and the understanding of how the artificial joint operates attract much attention from
In this paper the focus is to consider biomimetics, and in particular mimicking the
synovial joint, as a means of making advances towards more effective engineering
tribological systems – this is in contrast to consideration given to the joint in biotribology
where the main focus is to produce more effective joint replacements.
The main difference between man-made and natural lubricants is that the former are
usually “oil-based” while the latter are “water-based” systems.
Use of water for
lubrication, instead of oil, has many benefits (most notably environmental) and nature is a
great tutor to show us how to reach this. In the field of effective natural lubrication, one
of the most striking examples of the possibility for bioinspiration is the inspiration from
the lubrication of mammalian joints. The effective lubrication in this system is expressed
by the low friction which is found to be in range of 0.002-0.006. Considering the low
speeds involved, this friction is much lower than would be expected using existing
technologies. To illustrate this difference, Figure 1 shows the friction values reached in
mammalian joints compared to the friction values that are reached in tribological systems
(e.g. in the internal combustion engine).
Mammalian joints
Figure 1. Friction obtained in mammalian joints in scale with the friction obtained in
internal combustion engine tribological systems [7].
The tribological characteristics, friction and wear, of a system are a result of many
parameters involved in the contact. Just to mention few, these are: contact pressure,
speed, viscosity of the lubricant, roughness of the materials in contact, physical properties
of the materials in contact etc. Table 1 shows the tribological parameters in joints in
comparison with engine components.
0.03-0.3 m/s
15 m/s
0-25 m/s
7 m/s
Max 18 MPa
60 MPa
10 MPa
Max 1 GPa
25-40 °C
120-150 °C
125-200 °C
~ 100°C
Synovial fluid –
Oil – when in
Oil – when in
Oil – when in
EHD thixotropic
EHD thixotropic
EHD thixotropic
Table 1. Tribological parameters in joints and comparison with the ones in engine components.
It would appear that the friction values reached in joints can only be reached if there is
hydrodynamic (full film) lubrication as is the case when there is a thick film between the
rubbing surfaces and the frictional response is dominated by the bulk viscosity of the
liquid. However, the slow speeds that characterize joint movement suggests that it is
extremely difficult for a film to be formed and maintained. At these low speeds, in
engineering systems, boundary lubrication will occur; this being the regime where the
thickness of the lubricating fluid film is smaller than the height of the asperities and
effective lubrication is provided by tribochemical films formed on the asperities. This
implies one of two things: hydrodynamic lubrication is achieved as a result of the
interaction between the articular cartilage and the synovial fluid or that there is an
amazingly effective boundary lubricant present in synovial fluid. It could however, be a
combination of both. If boundary lubrication is achieved then the friction values suggest
that boundary films formed in joints are much superior (i.e. lower friction) to their
synthetic counterparts.
Despite extensive research being done in analysing the
lubrication system of the joints, there is still a great debate about the definite
mechanism(s) of lubrication in synovial joints [1,8].
Potential Benefits from Successful Mimicking of Synovial Joint Mechanisms
The purpose of this paper is to critically assess the mechanism of lubrication in this very
effective lubricated system and assess the possibility of development of synthetic
materials and engineering systems that would mimic the lubrication in joints.
Before continuing in this respect it is important to consider what benefits (in terms of fuel
economy and emissions reduction) could be realized should a successful attempt to
mimic the synovial joint be forthcoming.
Quantitatively what are we to achieve if we can attain the drastic reductions in friction
coefficient shown schematically in Figure 1? Fuel economy and frictional losses in
internal combustion engines are inextricably linked and so the major benefit in reducing
friction is to increase fuel economy. Of course to perform robust calculations on the
increases in fuel economy requires a complete analysis of the vehicle dynamics and
engine efficiency but a reduction of the coefficient of friction from the typical
engineering boundary lubrication values (assumed to be in the order of 0.12) to the
typical values seen in the synovial joint (estimated to be 0.004 [9]). It is also the case that
should lubrication be based on the concepts of the synovial joint that the liquid lubricant
phase will have lower viscosity – benefiting fuel economy in the hydrodynamic regime.
Lower viscosity oils SAE 20 and SAE 10 lead to increases in fuel economy of 3 and
4.4% respectively [10]. For CO2 emission reduction achievable annual targets for these
lubricants would be 67kg and 97kg respectively (from a total of approximately 2250kg
It is clear that the potential offering of a significant increase in fuel economy is the main
incentive to mimick the synovial joint but of course there are other major considerations
in relation to durability which would be paramount should lubricant viscosity be reduced.
In this respect it is useful to compare the durability of natural systems (e.g synovial joint)
and the internal combustion engine (e.g in a passenger car) and the durable “life” of each.
The synovial joint can operate efficiently in the vast majority of cases for over 75 years
(exhibiting extremely low friction and wear). This equates to more than 1 million
loading cycles per year [8] and more than 75 million over the lifetime.
If this is
compared with the average IC engine in a passenger car over 100,000 miles the total
number of cycles would be 220 million. Other biological systems (involving tribological
action) can demonstrate durability for several orders of magnitude greater numbers of
cycles than the synovial joint – the heart valve leaflets being one such system [8] which
can operate effectively for up to 5 billion cycles.
What Do We Have to Mimic?
Central to being able to mimick the synovial joint functionality is a thorough
understanding of its tribology and from this developing design concepts that will enable
the biological system to be replicated for technological applications. One point should be
clarified at this stage – it has been discussed previously in this paper that the tribological
conditions in terms of load, temperature, speed etc are all vastly different in the synovial
joint and an IC engine (or indeed other technological applications in tribology). It is in
the functionality that the similarities exist and that the potential for biomimcry exists.
Translation of biological materials into the technological application in this case is
unfeasible and so the major challenges are (i) designing the materials which will function
as the components of the synovial joint do under tribological conditions (ii) assembling a
“system” to perform in the required tribological environment but display drastically
lower friction coefficients comparable with the synovial joint and with the required
durability. This is not trivial and the first discussion towards this is embedded in the
remainder of the paper following a discussion of how the synovial joint works.
The major elements of the natural synovial joint, shown in Figure 2, are [8]:
The underlying bone
The articular cartilage (in the knee, the meniscus)
The synovial fluid and
The tissues that constrain and articulate the joint, the ligaments, tendons and
soft tissue capsule.
Figure 2. Synovial joint capsule
Articular Cartilage
Articular cartilage (AC) is a soft porous composite material, white with smooth and shiny
surface. The main constituents of AC are collagen, proteoglycans and water. Collagen
and proteoglycans in the cartilage form interpenetrating networks that create a strong
solid matrix. The pores containing the water constituent have been estimated to have a
diameter of
Collagen represents 50-75% of the dry weight of AC while
proteoglycans 15-30% [11,12]. There is around 80% water in the cartilage. AC is
aneural, meaning that there is no blood supply and alymphatic [11], encouraging the
possibility of building an artificial structure that would mimic AC structure and
The cartilage is bonded to the bone, Figure 3, and behaves as a thin-layer cushion contact
[8]. As shown in Figure 3, the articular cartilage is reported to have three layers [13]:
small closely packed fibres parallel to the surface
an intermediate layer with an open mesh work of S-shaped fibres approx. 900nm
in diameter. It is suggested that they were arranged in this manner to allow
deformation for energy absorption.
in the deep zones of the cartilage there were large fibres (1400nm diameter)
arranged radially and running into the subchondral bone.
Collagen and water
Figure 3. Internal structure of the articular cartilage and schematic representation of
proteoglycans, collagen and water concentration with depth [13].
As can be seen in Figure 3 the collagen fibre orientation varies with depth from the
articular surface. At the articular surface the collagen fibres are orientated parallel to the
surface, in the middle zone the orientation is at an angle to the surface and in the deep
zone the collagen fibres have orientation perpendicular to the bone interface, with the
fibres extending into the bone for effective anchorage. Bone and AC are materials with
high and low modulus of elasticity and their junction is a good example [14] of bonding
materials with different mechanical properties; something not commonly seen in
tribological applications.
The major non-collagenous components of the solid phase of AC are proteoglycan
macromolecules. The concentration of proteoglycans is lowest near the AC surface and
increases with depth. The proteoglycan macromolecules consist of a protein core in
which 50-100 glycosaminoglycans chains (chondroitin sulphate and keratan sulphate) are
bonded to form a bottlebrush-like structure. These structures are then aggregated to a
backbone of hyaluronic acid, Figure 4, to form a macromolecule with a weight up to 200
million and a length of approximately 2 µm [12,15].
Figure 4. Proteoglycan structures aggregated to a backbone of hyaluronic acid [12].
Proteoglycans are negatively charged and attract the hydrogen atoms (Figure 5) of the
water molecules, hydrating the zone where there are proteoglycans. Absorption of water
from the synovial joints results in swelling of the collagen fibrils [11,12].
Figure 5. Schematic structure of the articular cartilage.
The compressive properties of cartilage are provided partly by the proteoglycans that
resist compression because glycosaminoglycans chains repulse each other due to their
negative charges [11].
These characteristics of proteoglycans, water attraction and
repulsion from each other, provide the viscoelastic properties of articular cartilage, very
important properties for effective lubrication.
Synovial fluid and its rheological properties
Critical to the successful long-term tribological function of synovial joints, besides the
mechanical properties of AC, is also the nature of the synovial fluid. Synovial fluid is
clear to yellowish and is stringy. It resembles egg white, and it is this resemblance that
gives joints their name, synovia, meaning “egg white”.
Synovial fluid is essentially a dialysate of blood plasma with chief constituent being
water and containing:
1. long chain protein molecules
2. hyaluronic acid and
3. phospholipids.
In the existing tribological systems, for good full film lubrication the rheological
properties of the lubricating fluid are of great importance. Synovial fluid at different
pressures shows no significant change of viscosity (Figure 6a) a factor that proves to be a
critical difference when comparing to the positive pressure-viscosity coefficients in
engineering oils [16]. An increase of shear causes shear thinning (Figure 6b) and the
decrease of viscosity with increased shear rate [17].
Figure 6. Rheological properties of synovial fluid, a) pressure-viscosity [16] and b)
shear-thinning properties [17].
Another important property of synovial fluid is the stress increase in time during steady
shear – rheopexy [18]. This property is attributed to protein aggregation and it is thought
to strongly enhance the viscoelastic character of synovial fluid.
Synovial joint lubrication mode
The results of the experimental work done by Krishnan et al. [19] suggest that interstitial
fluid pressurization is a primary mechanism in the regulation of the friction response of
articular cartilage. By supporting the majority of the load transmitted across the contact
interface, the interstitial pressurization reduces the load supported by the contacting
collagen-proteoglycan matrix and opposing surface, considerably reducing the frictional
force relative to the total contact force. However, McCutchen pointed out that the
problem with the fluid-film models is that, if anyone stands for 30 min, all fluid is going
to be “squeezed out” from between the load-bearing articular surfaces of the knee.
However, in the healthy people, the knee joint is still perfectly lubricated the moment
movement starts. This suggests that boundary lubrication could also occur. In the work
of Hills [7] the effects of the components of synovial fluid ingredients are reviewed.
Hyaluronic acid has for many years been considered to be the main lubricant, but it has a
negligible load bearing capacity; the load bearing of hyaluronic acid is shown to be only
0.4 kg/cm2, which is very low when compared with 3 kg/cm2 for the normal load on the
adult knee joint when standing. Hyaluronic acid has an important role in retaining water.
The centrifugation of the synovial fluid resulted in its separation into two layers:
a “hyaluronate” layer and
a “proteinaceous” layer.
Results from friction tests have pointed to the protein (lubricin MW 227500) as the vital
load bearing ingredient residing in the “proteinaceous” and not in the “hyaluronate” layer.
11 percent of Lubricin was found to be surface-active phospholipid (SAPL). 86% of
these macromolecules are characterised but of the remaining 14%, 12% was subsequently
identified as Surface Active Phospholipids (SAPL), raising the issue whether lubricin is
the lubricant per se or whether it simply acts as the macromolecular water-soluble carrier
for these small (Molecular Weight approx 734) surfactant molecules that are otherwise
very insoluble in water [5].
The lubrication system in the joint and moving sites in vivo appears to consist of a fluid
in contact with sliding surfaces coated with an oligolamellar lining of Surface Active
PhosphoLipids (SAPL).
As the outermost layer, this lining provides boundary
lubrication and imparts the hydrophobicity characteristics of these surfaces when rinsed
free of synovial fluid, which appears to contain a wetting agent to promote hydrodynamic
lubrication. Thus, the fluid film provides lubrication wherever it can support the load but,
with physiological velocities being so low by engineering criteria, SAPL would appear to
play a major role as a boundary lubricant, especially in load bearing joints [5,20,21]
although this theory is challenged by other researchers [22]. The capability of SAPL to
act as a boundary lubricant was first recognised in the thoraic cavity, in which frictionless
sliding of the lungs is needed to reduce the work of breathing [23]. Researchers have
speculated that SAPL is the boundary lubricant found wherever tissues need to slide over
each other, also acting as an antistick agent [23].
Moving Towards a Technological Solution
Key to being able to use biomimetics principles for improving tribological design is the
realization that it is the functionality that is to be mimicked. Changing the tribological
“system” to mimick the synovial joint is potentially the most fruitful way forward – using
all the attributes of what is an energy efficient and durable tribological system to develop
new technological designs. In Table 2 some ideas on mimcry and the main constituents
are presented. This is an initial attempt as predicting the way forward for biomimetics in
lubrication/Tribology and outlines some of the most attractive areas of research that could
be exploited. There is great potential for development of lubrication strategies involving
(a) porous materials (b) aqueous functional fluids (c) deformable solids and design of a
system using biomimetic principles will require serious studies in all of these areas.
Elements of the
Frictional System
enable interstitial
flow of fluid
metallic and
polymer cellular
structure deformation
for energy adsorption
soft metals,
polymers, rubber
fluid attraction
fluid swallow under
load removal
limit the structure
compaction under
the load by collapse
of the upper layer
controlled structure
with variable
upper layer more
permeable to exude
the fluid faster
Viscosity of synovial
fluid does not change
significantly with
Broad range of
fluids – potential
for water
Low friction
surface film
Reduce friction when
the fluid film breaks
down and there is
surface contact
Articular cartilage
Synovial fluid
SAPL (?)
Hyaluronan – resist
shear flow and strain
linearly with time
when a stress is
Mimic System
High elasticity
and porous
lubricated by
water based
Any low friction
film formation
Additives who
having viscoelastic
properties. (May
not for the
application of
water lubrication)
Table 2. Mimicry of the functionality of the synovial joint – some ideas for use of
biomimetic principles in tribology
Concluding comments
The unique functionality of the synovial joint has been described in an attempt to draw
out some of the potential for biomimetic-based design in tribology using an appreciation
of the synovial joint as a “system”. The paper discusses potential strategies to mimic the
functionality of the synovial joint and identifies fruitful research areas to achieve the
potential gains from a biomimetic approach in tribology.
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