Industry Insights

KEY CONCEPTS
· Biological interfaces often involve complex interactions between dissimilar materials under a range of pressures and speeds.
· Replicating real-world biosystems using laboratory testing or modeling remains difficult.
· Experts in various fields must communicate with each other and understand each other’s tools and testing methods.

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Meet the Presenter
This article is based on an Industry Insights presentation titled Biotribology: Current Directions and Challenges by STLE member Dr. Marina Ruths on Nov. 16, 2021. Industry Insights is a monthly virtual event series available for free to STLE members. These events provide a forum for members to collaborate with STLE industry peers and an expert on a specific topic who gives a brief presentation, then facilitates conversation by asking discussion questions and providing actionable takeaways. For more information, click here.

Dr. Marina Ruths has a doctorate in chemistry from the University of California, Santa Barbara. She did postdoctoral work at the University of Illinois at Urbana-Champaign and at the Max Planck Institute for Polymer Research in Mainz, Germany, and is now a professor of chemistry at the University of Massachusetts Lowell. Her research focuses on direct measurements of interfacial forces and friction of polymers and polyelectrolytes, self-assembled structures and surfaces with different roughness, including model systems for biological surfaces. You can reach her at [e-mail].

Marina Ruths

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Biotribology is a rapidly growing, highly multidisciplinary field. Biological systems are notoriously difficult to study because of their complex structures and the variety of environments in which they function. In addition, human bodies are much less standardized than the mass-produced mechanical parts that tribologists often study. The mechanical properties¹ of biological materials and structures commonly fall outside the ranges studied with traditional tribometers, and laboratory testing and modeling of tissue samples under realistic conditions is very difficult. 

Biotribological systems
In living systems, including human joints, skin, eyes and teeth, tribology studies focus on friction, adhesion and surface textures. Researchers seek to understand how these systems work, why they fail and what the important mechanisms are. Isolating and studying the many processes involved is an ongoing challenge because they occur simultaneously, and they interact in complex ways. Human bodies come in a wide range of shapes and sizes, and no two are exactly alike. Thus, joints and implants can experience very different loads and contact pressures.

Tribologists seek to understand biological systems² as they are in the real world—under normal or abnormal conditions. They must consider interactions between soft and hard materials (bones and muscles, for example) operating under a range of pressures and speeds, which can change suddenly (e.g., a sprinter taking off from the starting block or a baseball player running, then sliding the last several feet toward home base). 

Developing test methodologies and protocols that produce results relevant to the behavior of living systems remains challenging. What does a “friction coefficient” mean in a biological context? What pressure range is appropriate for working with cartilage samples?³ What parameters are most relevant in the context of understanding durability and wear resistance for biological samples? Often, researchers don’t know exact conditions (e.g., contact pressures and loads) in situ because of difficulties in measuring these quantities precisely, and they must rely on estimates and models. It can be difficult to establish the most relevant range of test conditions. And sometimes, biological systems adapt during testing by reversibly deforming or adjusting the amount of lubricant⁴ at the interface.

Researchers can study living systems using cadavers or reclaimed implants, but these are imperfect means at best, since they offer a static representation of the system at a certain point in time, usually after the implant (or human) has ceased to function. In vivo studies can provide baseline information on normally functioning processes and tissues, or they can observe the effects of disease or injury in real time. Ideally, these tests are noninvasive and nondestructive, both to avoid harming the individual and to interfere as little as possible with the system being observed.

Systems and materials that mimic those found in nature can help researchers understand how they function, and they provide an important means of solving practical applications problems in the development of medical devices and therapeutic materials. One ongoing research challenge is applying biomimetic approaches to solving mechanical problems and discovering whether and how these approaches can be applied to other types of tribosystems.

Model systems
Physical analogues⁵ or computational and analytical⁶ models provide valuable information in situations where testing in situ is impractical or unethical. Models must provide enough complexity to produce realistic results, but be simple enough to meet laboratory and computational resource limitations and to isolate and quantify the relevant factors contributing to observations of interest.

Modelers can test the effects of modifying natural systems, repairing damage or mimicking systems from other organisms to solve problems in human bodies—the wear and adhesion resistance of pangolin scales, to cite just one example. One problem with physical models that mimic other systems is that they sometimes provide a wealth of information on the model systems themselves, but limited understanding of the real-world systems they were intended to emulate.

Modelers can test the effects of modifying natural systems, repairing damage or mimicking systems from other organisms to solve problems in human bodies—for example, the wear and adhesion resistance of pangolin scales. Courtesy of Valerius Tygart / CC-BY-SA-3.0.

Constructing realistic models involves reproducing the multicomponent systems within the body. Bodily fluids (e.g., blood, synovial fluid) are aqueous solutions containing a host of ionic species that interact in ways that are difficult to control or emulate.⁷ The contact mechanics between soft and hard tissues is complex, involving layered materials, various modes of sliding and complex surface textures and topographies.

Tribological phenomena in living systems occur over a wide range of length scales, from chemical reactions at the atomic scale to crack formation at the macro scale. Because these processes interact with each other, modelers must find ways to interlink models across a hierarchy of domains.

Medical devices and therapeutics
Studies of living systems and models provide basic insights into how tribological processes work in the body, but they also support the development of therapies and devices to correct, repair or replace damaged or diseased tissues and structures. Medical device developers are concerned with durable, wear-resistant materials—especially for joint replacements—and the associated lubricants and coatings. 

Predicting the properties of new materials and surface treatments is one of the grand challenges in this area. Devices and other materials⁸ designed to function within a human body must restore functionality to injured or diseased organs and tissues. They must be compatible with the surrounding tissues, and they must be durable, so as to avoid frequent replacements and to avoid the release of wear particles or chemical species into the surrounding tissues.

Many biological systems use a mobile surface layer of water as a lubricant. This is of particular concern where metal implants, which may corrode, are involved. Recent research on hydrogels has produced surface structures that allow a degree of control over the amount of water and charged species at the interface, which, in turn, allows control over the friction coefficients. These hydrogels have about the same consistency as a contact lens, so they can be handled without disintegrating. Hydrogels are not amenable to tensile testing, but indentation tests can be done.

Haptics—the surface properties that affect our sense of touch—is another area of development. Humans gather a wide variety of information through our fingertips: the characteristic feel of leather versus sandpaper, surface patterns⁹ (e.g., Braille text) or the electrostatic vibration feedback from a cell phone’s fingerprint reader. Surface patterns, textures and coatings can mimic their biological counterparts or incorporate something completely new to manage asperities and control deformation. Various types of surface structures (e.g., polymer brushes¹⁰ or “lotus leaf” structures) hold lubricants differently and exhibit various degrees of hydrophobicity.

Various types of surface structures, such as lotus leaf structures, hold lubricants differently and exhibit various degrees of hydrophobicity. Courtesy of William Thielicke / CC-BY-SA-4.0 International.

In some cases, even very successful devices are not completely understood. Device testing, especially in the early stages, must occur outside the body, using instruments and testing conditions that produce relevant information. Tests must be done on healthy and diseased tissues, which requires access to samples and some means of comparing samples from various sources. In addition, these samples must maintain their structural integrity outside the organism from which they came. This can be especially challenging for soft, layered or multiphasic materials. One typical problem for tribology researchers is that data from real-time monitoring of patients going about their daily activities is scarce to nonexistent. Tribologists must often rely on data from unused medical devices and from devices retrieved from patients’ bodies after the devices fail or the patient dies. 

Other active areas of research include self-healing and self-lubricating materials, sustainable sourcing (especially for lubricants and coatings) and materials containing encapsulated drugs, crack-sealing polymers or lubricants that can be released over time in a controlled fashion. One concern for materials that release chemical species into the body is determining when these species must be classified as drugs, which requires meeting stringent government regulations. 

Implant durability, over a span of several decades, is becoming increasingly important. Patient demand is evolving from restoration of basic mobility to preservation of active lifestyles, including athletic activity. Thus, patients are demanding joint replacements at an earlier age, with the expectation that they will hold up well into old age. Finding ways to test materials and devices for this type of long-term durability is a challenge.

Interdisciplinary communication
Success in each of these areas requires biologists, tribologists, device designers, computer modelers and others to communicate across disciplinary lines. Researchers in various fields must understand each other’s tools and testing methods—what are they measuring, and what information is contained in the test results? Researchers also must understand what laboratory and computer models tell them, and how this information can be applied to real-world problems.

REFERENCES
1. Hart, S.M., McGhee, E.O., Urueña, J.M., Levings, P.P., Eikenberry, S.S., Schaller, M.A., Pitenis, A.A. and Sawyer, W.G. (2020), “Surface gel layers reduce shear stress and damage of corneal epithelial cells, Tribology Letters, 68, 106. Available here.
2. Zhou, Z.R. and Jin, Z.M. (2015), “Biotribology: Recent progresses and future perspectives,” Biosurface and Biotribology, 6 (1), pp. 3-24. Available here.
3. Farnham, M.S. and Price, C. (2020), “Translational cartilage tribology: How close are we to physiologically relevant benchtop articular cartilage testing?” TLT, 76 (2). Available here.
4. Farnham, M.S., Larson, R.E., Burris, D.L. and Price, C. (2020), “Effects of mechanical injury on the tribological rehydration and lubrication of articular cartilage,” Journal of the Mechanical Behavior of Biomedical Materials, 101, 103422. Available here.
5. Nečas, D., Vrbka, M., Marian, M., Rothammer, B., Tremmel, S., Wartzack, S., Galandáková, A., Gallo, J., Wimmer, M.A., Křupka, I. and Hartl, M. (2021), “Towards the understanding of lubrication mechanisms in total knee replacements – Part I: Experimental investigations,” Tribology International, 156, 106874. Available here.
6. Chau, A.L., Cavanaugh, M.K., Chen, Y.-T. and Pitenis, A.A. (2021), “A simple contact mechanics model for highly strained aqueous surface gels,” Experimental Mechanics, 61, pp. 699-703. Available here.
7. Shoaib, T., Yuh, C., Wimmer, M.A., Schmid, T.M. and Espinosa-Marzal, R.M. (2020), “Nanoscale insight into the degradation mechanisms of the cartilage articulating surface preceding OA,” Biomaterials Science, 14, pp. 3944-3955. Available here.
8. McGuire, N. (2018), “The body mechanical,” TLT, 74 (6), pp. 52-64. Available here.
9. Jin, R., Skedung, L., Cazeneuve, C., Chang, J.C., Rutland, M.W., Ruths, M. and Luengo, G.S. (2020), “Bioinspired self-assembled 3D patterned polymer textures as skin coatings models: Tribology and tactile behavior,” Biotribology, 24, 100151. Available here.
10. Yu, J., Jackson, N.E., Xu, X., Morgenstern, Y., Kaufman, Y., Ruths, M., de Pablo, J.J. and Tirrell, M. (2018), “Multivalent counterions diminish the lubricity of polyelectrolyte brushes,” Science, 360 (6396), pp. 1434-1438. Available here.

Nancy McGuire is a freelance writer based in Albuquerque, N.M. You can contact her at [e-mail].