Journal of Bio- and Tribo-Corrosion. December , Cite as. Bio-inspired tribology is an interdisciplinary field of science where scientists and engineers seek to investigate and incorporate tribological properties encountered in biological beings into engineering applications. In this paper, bio-inspired tribological research that are speculated to have a huge impact on tribological applications have been reviewed. These research involve 1 investigations related to replication of lubricin found in synovial fluids of mammalian joints which have super-low friction values that can be utilized in IC engines, 2 surface replication concerning to superhydrophobic properties of gecko skin which is seen to have anti-wetting and self-cleaning properties, 3 friction-reducing shark skin through specialized nanoparticle coatings that is seen to give a different perspective on surface texturing, 4 new techniques, such as soft lithography to replicate surfaces of lotus leaf and air lubrication phenomenon inspired by emperor penguins that is being applied to propel boats, ships, and torpedoes faster by reducing skin friction underwater.
Further, an investigation in self-healing materials inspired from pitcher plant that has led to the innovation of self-healing and slippery liquid-infused porous surfaces has been discussed. These research works reviewed not only provide a deep insight into the current advances in bio-inspired tribology but also helps understand the plausibility of the research applications in the future and the practicality of innovations possible.
Over the last decade, Biomimetics, more commonly known as Bio-inspired science, has become a very popular field of interest, inspiring innovation in many engineering applications more so in tribology and material science. Understandably, this research is however still in its infancy, mostly because it is not easy to comprehend and implement the working of many biological ideas into engineering applications. Although the theory of evolution may not be evident and accepted by many, it is possible to agree on one thing that nature has its own way of learning to make sure there is a sustainable progress of all life forms.
The image of the Egyptian God Khensu with wings left illustrates the age-old fantasy of humans of being able to fly [ 2 ]. It is only very recently that the concept of learning from nature is being adopted by researchers. He presented it as a study involving copying, imitating, and learning from biology [ 3 ].
Later in , Otto. Schmitt coined the science and engineering behind this as Biomimetics [ 4 ]. The term Biomimetics is derived from bios, meaning life, and mimesis, meaning to imitate. It is literally the science of mimicking nature biomimicry to incorporate the mechanisms and capabilities behind a biological phenomenon or be it for an engineering application. In the current era where crises in resources, energy, and environmental conditions are being seen around the world, tribology is expected to play a major role to strive toward application of eco-friendly technology.
There is so much scope for biotribological applications in engineering that deal with the development of functional adhesives inspired from gecko feet; novel mechanical attachment devices inspired by insect attachment pads onto the plants; 3-D micro-electromechanical systems inspired by diatom hinges and their interlocking devices; stain-resistant paints, shoe or windshield coatings inspired by plant surfaces concerning their self-cleaning and anti-wetting properties [ 5 ]; bioengineered Synovial Fluids SF for low-speed lubrication applications inspired from native SF from SF joints in mammals [ 6 ]; are amongst a few.
Biomimetic materials are also usually environmentally friendly, since they are a natural part of the ecosystem. For this reason, the biomimetic approach in tribology is particularly promising [ 7 ]. The present study discusses latest ongoing researches concerning adaptation of technology from biological beings which serve as an inspiration for tribologists and material scientists to move away from the conventional methods of material design and look for new innovative techniques that yield better results. The applications discussed in this study shows endeavors of researchers to make sense of incomprehensible tribological phenomenon of nature and their attempts to mimic it, keeping in sight its limitations.
While, in most cases, it is not possible to directly borrow solutions from nature and to apply them in engineering, it is often possible to take biological systems as a starting point and a source of inspiration for engineering design. Some of the best and most intriguing studies concerning tribology which are expected to have a high impact with their engineering applications are discussed in this section.
With preceding seven decades of lubricant technology research, most of the lubricants in use today for engineering applications are oil-based lubricants relying on chemical additives. These lubricants are expected to serve their basic tribological function like protection from wear, maintain required level of friction, remove heat from the system, carry away impurities, and wear debris.
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The pivotal function that serves these purposes is providing boundary lubrication which is defined by asperity contact of tribocouples and the tribochemical reactions that occur as a result of this contact. Clockwise from top - right : Ball and socket joint, Condyloid joint, Plane joint, Saddle joint, Hinge joint, and Pivot joint; b Structure of synovial joint in the Knee [ 8 ]. Hyaluronate HA , Glycoprotein lubricin , Glycosaminoglycans such as chondroitinsulfate, chondroitinsulfate, and keratan sulfate in.
Schmidt et al.
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In the study, synovial fluid was shown to provide lower friction levels compared to phosphate buffered saline PBS in a cartilage rotating against cartilage model under boundary lubrication conditions. Synovial fluid was shown to have qualities which protected cartilage from wear that were lost when digested with trypsin. It was also shown that an intact articulating surface played an equally important role as the intrinsic mechanical properties of cartilage tissue had an important step in determining its wear resistance [ 18 ].
Polished borosilicate glass surfaces and Hydroxyethyl methacrylate HEMA lenses were used as the tribo-pair for analysis. For the basic glycoproteic gel solution, coefficient of friction was found to be 0. Gel-in solution test was conducted for the composition 2 where large accumulation of fluorescent details outside of contact was found because of the expulsion of lipid vesicles, but there is no variation in friction coefficient value 0. But for the Gel-out solution test of composition 3 friction coefficient was found quite higher 0. Phosphate buffered saline abbreviated PBS is a buffer solution commonly used in biological research.
It is a water-based salt solution containing disodium hydrogen phosphate, sodium chloride, and, in some formulations, potassium chloride and potassium dihydrogen phosphate. The osmolarity and ion concentrations of the solutions match those of the human body. Bovine osteochondral explant was used as the specimen material on which Pin-on-disk test was conducted at normal load varied between 0. The friction coefficient was found to decrease with increasing contact pressure and decreasing equilibrium time. No beneficial effect of lipid bilayer was observed for implant joints. The essential function of a balanced salt solution is to maintain pH and osmotic balance as well as provide your cells with water and essential inorganic ions.
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Solutions most commonly include sodium, potassium, calcium, magnesium, and chloride. Friction coefficient ranges from 0. It was found that if bovine serum albumin BSA is added to other lubricants, then the friction coefficient becomes less in case of metallic surfaces because polymeric film transfer does not occur in metallic surface, but in case of alumina, the polymeric film transfer is much more intense when the BSA is added to solutions.
As a result, the coefficient of friction is quite higher on alumina surfaces. Human serum albumin is the version of serum albumin found in human blood. It is the most abundant protein in human blood plasma; it constitutes about half of serum protein. It is produced in the liver.
This lubricant renders more hydrophobic surfaces which adsorb denatured proteins and increases friction forces, but thicker, denser films are formed by the adsorption of native proteins on more hydrophilic surfaces, which has the potential to reduce lubricated friction.
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HA is a major component of synovial fluid in diarthrodial joints and is believed to be at least partially responsible for the excellent biolubrication of articular cartilage due to its remarkable viscoelastic properties. HA imparts stiffness and resiliency due to the well-known entropic elasticity of polyelectrolytes. A coefficient of 0. Overall, the results show that one can covalently graft HA to a supported membrane surface, but that this does not impart excellent lubrication or sufficient wear properties.
But now, it is clear that synovial fluid in native joints functions as a biomechanical lubricant, lowering the friction and wear of articulating cartilage in synovial joints. A bioengineered SF recapitulating the properties of native SF has been found to be beneficial in tissue engineering of articular cartilage and synovial joints for the treatment of arthritis. This appropriate lubricating environment may be critical to maintain the low-friction, low-wear properties of articulating cartilage surfaces undergoing joint-like motion in bioreactors.
Frictional properties of cartilage against different counterface materials. Morrel et al. To add to this, Katta et al. For biotribological applications, the most common area of interest for researchers, is the impact of engineered synovial fluid lubricants as applied for engineering applications which can be enormous. Hill [ 19 ] has shown that the effective lubrication of SF in these joints is expressed by a low friction in the range of 0. Even in the same working conditions as the synovial joints, this kind of low friction values cannot be achieved with all the advancements made in boundary lubrication technology.
Lubricity Chart comparing mammalian joint friction against friction obtained in IC engine tribological system [ 19 ]. Secondly, the most important bit is the durability aspect of lubricants, mammals do not go around getting their synovial joint fluids replaced from time to time! This equates to nearly 1 million loading cycles per year and more than 75 million loading cycles over a lifetime. Whereas a good IC engine lubricant for passenger car needs to be replaced every , miles with a total loading cycle of million.
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There are more biological systems that exhibit even greater durability cycles than that of synovial joints like the heart valve leaflets which can operate efficiently for up to 5 billion cycles [ 21 ]. But now, the question arises what function of synovial joints can actually be mimicked?
It is not possible to ignore the fact that the tribological conditions in terms of load, temperature, speed, and other environmental conditions vastly vary in the synovial joints when compared to IC engines or any other application for that matter. It is important to note that, it is the functionality that there is some degree of similarity and a potential for biomimicry exists. Many tribologists and researchers are working toward better understanding of this functionality.
Gregory et al. It has been observed that it is this lubricin in the synovial fluid that provides boundary lubrication and helps avoid the cell and protein adhesion to achieve a near frictionless joint motion. Investigations into lubricin are also being made by Alexandra et al.
Recently, many such research concerning role of lubricin and boundary layer lubrication have been analyzed with respect to the lubrication and wear mechanisms observed in synovial joints and articular joints in general [ 24 , 25 , 26 , 27 , 28 , 29 , 30 , 31 ].