In vitro models of soft tissue damage by implant-associated frictional shear stresses

Silicone elastomer medical implants are ubiquitous in medicine, particularly for breast augmentation. However, when these devices are placed within the body, disruption of the natural biological interfaces occurs, which significantly changes the native energy-dissipation mechanisms of living systems. These new interfaces can introduce non-physiological contact pressures and tribological conditions that provoke inflammation and soft tissue damage. Despite their significance, the biotribological properties of implant-tissue and implant-extracellular matrix (ECM) interfaces remain poorly understood. Here, we developed an in vitro model of soft tissue damage using a custom-built in situ biotribometer mounted onto a confocal microscope. Sections of commercially-available silicone breast implants with distinct and clinically relevant surface roughness (Ra=0.2±0.03μm, 2.7±0.6μm, and 32±7.0μm) were mounted to spherically-capped hydrogel probes and slid against collagen-coated hydrogel surfaces as well as healthy breast epithelial (MCF10A) cell monolayers to model implant-ECM and implant-tissue interfaces. In contrast to the “smooth” silicone implants (Ra100  Pa), which led to greater collagen removal and cell rupture/delamination. Our studies may provide insights into post-implantation tribological interactions between silicone breast implants and soft tissues

A Simple Contact Mechanics Model for Highly Strained Aqueous Surface Gels

Abstract Background Soft, biological, and bio-inspired materials are often compositionally heterogeneous and structurally anisotropic, and they frequently feature graded or layered organizations. This design complexity enables exceptional ranges in properties and performance yet complicates a fundamental understanding of the contact mechanics. Recent studies of soft gel layers have relied on Hertzian or Winkler foundation (“bed-of-springs”) models to characterize the mechanics but have found neither satisfactory. Objective The contact mechanics of soft gel layers are not yet fully understood. The aim of this work is to develop a simple contact mechanics model tailored for compositionally-graded materials with soft surface layers under high strains and deformations. Methods Concepts from polymer physics, fluid draining, and Winkler foundation mechanics are combined to develop a simple contact mechanics model which relates the applied normal force to the probe radius of curvature, elastic modulus, and thickness of soft surface layers subjected to high strains. Results This simple model was evaluated with two examples of graded surface gel layers spanning multiple length-scales, including commercially available contact lenses and stratified hydrogels. The model captures the nonlinear contact mechanics of highly strained soft aqueous gel layers more closely than either Hertz or Winkler foundation theory while simultaneously enabling a prediction for the thickness of the surface gel layer. Conclusion These results indicate that this simple model can adequately characterize the contact mechanics of highly strained soft aqueous gel layers

A Simple Contact Mechanics Model for Highly Strained Aqueous Surface Gels

Abstract Background Soft, biological, and bio-inspired materials are often compositionally heterogeneous and structurally anisotropic, and they frequently feature graded or layered organizations. This design complexity enables exceptional ranges in properties and performance yet complicates a fundamental understanding of the contact mechanics. Recent studies of soft gel layers have relied on Hertzian or Winkler foundation (“bed-of-springs”) models to characterize the mechanics but have found neither satisfactory. Objective The contact mechanics of soft gel layers are not yet fully understood. The aim of this work is to develop a simple contact mechanics model tailored for compositionally-graded materials with soft surface layers under high strains and deformations. Methods Concepts from polymer physics, fluid draining, and Winkler foundation mechanics are combined to develop a simple contact mechanics model which relates the applied normal force to the probe radius of curvature, elastic modulus, and thickness of soft surface layers subjected to high strains. Results This simple model was evaluated with two examples of graded surface gel layers spanning multiple length-scales, including commercially available contact lenses and stratified hydrogels. The model captures the nonlinear contact mechanics of highly strained soft aqueous gel layers more closely than either Hertz or Winkler foundation theory while simultaneously enabling a prediction for the thickness of the surface gel layer. Conclusion These results indicate that this simple model can adequately characterize the contact mechanics of highly strained soft aqueous gel layers

Normal Load Scaling of Friction in Gemini Hydrogels

Health and physiology are critically dependent on the ability of soft, permeable, and aqueous materials (e.g. cartilage, cells, and extracellular matrix) to provide lubrication over a wide range of speeds and contact stresses. Living cells and tissues present tremendous handling and experimental challenges for fundamental biotribology studies. Synthetic high water content hydrogels, designed to share similar mechanical and transport properties of biomaterials, can provide fundamental insights into the basic dissipative mechanisms associated with aqueous lubrication. Recent studies investigating the response of self-mated (Gemini) hydrogels to a wide range of sliding speeds under constant load conditions revealed transitions in friction behavior that may be associated with polymer relaxation time and contact time for a surface mesh during sliding (mesh size divided by the sliding speed). Here, the extent to which contact pressure and contact area affect hydrogel friction behavior was explored by changing the applied load over two orders of magnitude (0.1–20 mN) and the sliding speed over four orders of magnitude (10 μm/s–100 mm/s). Oscillating pin-on-disk microtribological experiments were performed in ultrapure water for Gemini polyacrylamide hydrogels (average mesh size ~7 nm). Friction coefficient decreased across all ranges of sliding speed with increasing applied load, consistent with predictions of contact area scaling non-linearly with applied load and pressure-independent surface shear stresses. The contact area for Gemini hydrogel interfaces under these conditions has been shown to follow Hertzian contact mechanics theory, and supports the scaling of friction coefficient in the speed-independent regime that follows μ ~ F n −1/3