|Abstract:||Biological tribosystems are excellent examples of nature leveraging soft matter properties to achieve exceptional lubrication for prolonged periods of activity. In these systems, lubrication is provided by sparsely crosslinked, polymeric surface layers imbibed with an aqueous lubricant. A prominent biological tribosystem is the articular cartilage, an avascular tissue consisting of an extracellular matrix made of collagen fibrils and proteoglycans, with a small number of chondrocyte cells. However, in this tissue, there exists a gradient in the orientation of the collagen fibers and water content as a function of the distance from the bone, which emphasizes the importance of the microstructure in cartilage’s functionality, i.e. a load-bearing tissue that maintains low friction and wear. In fact, recent studies have shown that the cartilage’s articulating surface comprises of a network of highly hydrated mucins, polysaccharides, glycoproteins, and phospholipids, which play a key role in maintaining low friction in boundary lubrication. This has been evident in studies performed on multiple other biphasic, non-biological hydrogels as well, where a prominent effect of the interfacial microstructure is observed on their mechanical and tribological properties. Yet, not only there is a lack of knowledge but also wide discrepancy about the fundamental underlying mechanisms relating the dynamic and static frictional dissipation to the microstructure of these materials. Conversely, this fundamental gap in knowledge also limits progress in the design of functional replacements, based on hydrogel-like materials.
Our aim was to not only advance the existing knowledge about the frictional dissipation of hydrogels, by precisely correlating the role of microstructure to the tribological performance, but also, to establish design principles that can help combat some of the existing challenges related to their application as tribological biomaterials. In light of this, the doctoral work presented here has achieved the following specific goals:
I. Studied, modeled and quantified influence of the microstructure, crosslinking degree and stiffness of the polymer on the dynamic and static frictional response
II. Scrutinized the relation between friction force and interfacial rheology of hydrogels
III. Elucidated the pathways of network formation in double network hydrogels which lead to enhanced mechanical and frictional response
IV. Scrutinized mechanical and tribological response of biological hydrogels in physiologically relevant conditions
By combining powerful state-of-the-art experimental techniques such as the Dynamic Light Scattering (DLS), Atomic Force Microscopy (AFM) and extended surface forces apparatus (SFA), we have demonstrated that the main mechanisms behind the frictional dissipation of hydrogels arise directly from their biphasic nature – the polymeric network and the imbibing fluid. In the context of dynamic friction, the viscous-adhesive model developed here quantifies the hydrogel’s frictional response by considering an interplay of adhesive and viscous dissipation directly arising from the hydrogel’s microstructure. The model accounts for confinement effects, poroelastic deformation, and the influence of the polymer on the viscous friction force, and helps reconcile seemingly contradictory models proposed previously. The adhesive contribution was modeled as a combination of reversible, transient adhesive bonds between the hydrogel and the countersurface and the poroelastic deformation of the hydrogel during shear, while the role of viscous dissipation was revealed to be directly related to the rheological performance of the hydrogel’s interface. In the latter, the polymer and imbibed fluid, both dictated viscous dissipation. Scrutiny of the rheological behavior of hydrogel thin films in tandem with nanotribology was conducted to show that the effective viscosity measured in rheology agrees with the friction behavior, although it is not sufficient to capture the rich frictional response of hydrogels as a function of sliding velocity. In the context of static friction, the combined effects of microstructure, interfacial shear stresses, interfacial ageing, and temperature were all tied together into a conceptual phase diagram for the static friction of hydrogels. Feasibility of the models developed for the dynamic and static friction was validated by extending the concepts to other hydrogel systems such as physically crosslinked agarose and cartilage, thereby demonstrating the universality of the proposed mechanisms for biphasic soft materials.
The study was further extended to DN hydrogels and biological hydrogels. Systematic investigations of the DN hydrogels comprising of agarose and polyacrylamide hydrogels as independent, interpenetrating networks revealed the design limitations of achieving high strength and high lubricity, simultaneously. Lastly, the novel experimental study on the gel-like surface of the articular cartilage was conducted as a direct application of this research. The graded response of the cartilage’s gel-like articulating surface in elevated calcium concentrations was traced back to changes in the surface and sub-surface microstructure, which was reported to subsequently modulate the mechanical and tribological response of the material.
In summary, through its collective experimental studies and comprehensive models, this doctoral work provides the basic framework to understand lubrication mechanisms of hydrogel-like materials in light of their microstructure. Furthermore, it also helps provide the basic design principles for fabricating hydrogels capable of achieving low friction coefficients and augmented wear resistance through the precise control of their microstructure. Lastly, the novel methodologies and protocols stemming from this dissertation open up previously unexplored research avenues and hence can influence diverse areas of inquiries, not only limited to biolubrication and biomedical applications but soft robotics and microelectromechanical devices.