Challenge 3 - Non-linear Systems

The tribology of non-linear materials captures some exciting and interdisciplinary work that brings together rheology, biology and advanced materials science. The origins of industrial tribology lay in the understanding of hard-on-hard contacts; friction of contacting surfaces in this case albeit multifactorial involves properties of materials relating to the modulus, asperity distribution, ductility, reactivity (chemical effects). In non-linear materials (‘soft tribology’) the application areas cover a diverse range from polymer engineering, food, oral tribology, occular tribology, pharmaceutical/cosmetic products and tissue tribology related to a number of applications across medicine. In this challenge we are limiting the work to the behaviour of natural materials and the importance of understanding friction across the length scales in such systems. The challenge links to the importance of medical interventions and understanding how tissue/device interfaces behave from a tribological standpoint. The team between Sheffield and Leeds have complementary expertise including:

  • Mechanobiology of cells; understanding how the mechanical stimulation of cells affects cell-cell, cell-matrix adhesion, gene expression and protein localisation (Perrault1/Armes)
  • Complex rheology across multiple length scales; numerical and experimental simulations (Wilson/Kapur/Armes)
  • Microfluidic sensing and measurement of droplets and other two phase systems (Kapur/Armes)
  • Tissue damage induced by surgical instruments and catheter devices (Neville/Bryant)
  • Particle adhesion and small scale rheology (Ghadiri/Kapur)
  • Synthesis of cell-containing mucin-inspired materials and poroelastic systems (Armes2/Bryant))

The tribology of non-linear and ‘soft’ biological systems is a flourishing area. Traditionally, research in this area has been highly controversial, focussed around understanding the lubrication mechanisms of articular cartilage. Significant advances have been made in the recent years from both a lubrication and biochemical point of view3. However the tribology community is yet to bring these theories together. The fundamental principles of tribology, mainly friction, have been further on applied to cellular matter including cell mono-layers, live soft tissues and skin. Observations in these natural systems has led to an explosion of interest in bio-inspired gels and polymer functionalised surfaces. Hydrogel and polymer brush technologies are been extensively researched to 1) understand the origins of super-lubricity and 2) to develop engineering systems capable of imparting superior wear and friction as observed in nature. The “non-linear systems” challenge in this PG overlaps into two other areas; reactive surfaces (linking to tribo-induced biochemical changes) and extreme interfaces (phase changes and extreme derformations) as shown in the figure above. The two projects (3 and 6) in this challenge cover the mesoscopic/macroscopic scale and the microscale.

The natural synovial joint, the colon and other biological systems can represent very efficient tribological systems with ultra-low friction coefficients and in many cases low or zero wear. Things can go wrong with biological system and can lead to medical intervention; when components are replaced. Often a soft, poroelastic and viscoelastic system is replaced by a hard-hard contact with a lubricant between. The artificial hip joint is a good example and one that has received much attention. To understand, in a tribological sense, how the lubrication of articular cartilage occurs and how this might be mimicked in an engineering system is the challenge presented here. It is clear that the friction values reached in joints can only be reached if some of the load bearing mechanism operates through hydrodynamic mechanisms, whether this be via a fluid film or through the expulsion or pressurisation of fluid from the porous matrix. The low friction contact between the opposing surfaces appears to be of equal  importance. Considering the speeds that are typical for joint movement, the fluid film between the rubbing surfaces is not achieved due to hydrodynamic principles but as a result of interaction of the fluid with the lubricated material, in this case the porous articular cartilage4. Therefore, a key aspect of the effective lubrication in the synovial joint is the interaction between the material being lubricated and the fluid which ensures pressurised lubricant between the rubbing surfaces. It appears that nature uses a “system solution” which involves both the material to be lubricated and the lubricant – in contrast to the existing engineering approach where low friction and wear is usually achieved by separately improving the material and lubricants. The hierarchical structures of natural materials, in this case of articular cartilage but also of materials on other articulating systems in nature such as beetle head articulation5, are of great importance to ensure the desired interaction with the lubricating fluid. In this project we will synthesise porous structures and assess their lubrication in conditions where the interactions between the lubricating fluid and the solid surfaces that make up the porous material are varied. We combine the functionalization of polymer surfaces expertise in the project with the tribology expertise in terms of experimental and computational simulations of deformable materials.

Friction between endothelial surfaces and medical devices (stents, catheters, surgical instruments) can lead to adverse reactions with tissue and associated problems (e.g. clots, vessel damage, blockage, tissue rupture). Strategies aimed at reducing friction have explored the tissue/device interface but have mainly focused on the damage of the hard surface rather than the impact on the biological tissue. For graspers in laparoscopic surgery the damage to tissue once a critical traction force was achieved was demonstrated6. Soft tissues are low multi-moduli, biphasic poro-elastic systems. Tribological contacts encompass a range of classical elastic/plastic and rheological processes resulting in highly non-linear behaviour. Straddling the boundaries between rheology and tribology is necessary if friction at a cellular level is to be understood. The input of mechanical work to a cellular system imposes deformation (either according to fluid or solid or combined constitutive equations) but also, as importantly imposes alterations in cell function. Mechanical activation of cellular function has been studied but until now the loop has not been closed. We do not currently understand how that adaptation of cell function (through up- and down- regulation of protein expression) can affect the cell-cell tribological reactions. This challenge is addressed here.



  • Prof. Roger Lewis – Challenge Leader
  • Prof. Nik Kapur – Challenge Leader
  • Dr. Michael Bryant
  • Prof. Mojtaba Ghadiri
  • Dr. Mark Wilson
  • Dr. Cecile Wilson
  • Prof. Mark Rainforth
  • Prof. Steve Armes
  • Dr. Matt Carré

Postdoctoral Staff

  • Dr. Abdullah Azam – Project 3
  • Dr. Raman Maiti  – Project 6

1S Barreto, C M Perrault, D Lacroix, Structural finite element analysis to explain cell mechanics variability, Journal of the Mechanical Behaviour of Biomedical Materials, 38 (2014).
2I Canton, N J Warren, A Chahal, K Amps, A Wood, R Weightman, E Wang, H Moore, S P Armes, Mucin-inspired thermoresponsive synthetic hydrogels induce human pluripotent stem cells and human embryos, ACS Cent. Sci., 2016, 2 (2), pp 65–74.
3J Klein, Hydration Lubrication, Friction, March 2013, Volume 1, Issue 1, pp 1–23.
4Neville et al. IMechE Part C: J. Mechanical Engineering Science, Vol 221, 10, pp 1223-1230, 2007.
5Gorb et al. The Journal of Experimental Biology, Vol 209, pp. 722-730, 2006.
6L Hunter, PhD Thesis, 2015, University of Leeds