Spherically capped membrane probes for low contact pressure tribology

Low contact pressure measurements are needed for biotribology studies on cells, cell layers, and tissues. Such studies typically require low forces to achieve kPa range contact pressures, but such methods frequently come at the expense of contact area and, in turn, relevance. In seeking lower contact pressures, the corresponding contact areas can become too small to be physiologically relevant or important. The recently developed method creates probes that are thin spherical shells, specifically designed to create low contact pressures. These polyethylene glycol and polyacrylamide hydrogel probes were polymerized with embedded fluorescent nanoparticles, and indentation experiments were performed in situ on a confocal microscope to determine the deformation mechanics. The indentation load versus contact width behavior revealed that this design has the ability to achieve pressures in the kPa range at macroscopic contact areas that are independent of the applied load (i.e., the area grows linearly with the applied load, leading to a constant contact pressure).