Influence of Mesh Size, Type of Crosslinking, Polymer Stiffness and Interfacial Rheology on the Frictional Characteristics of Hydrogels

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This grant will support research that will advance the knowledge related to a structure-property relation for hydrogels, promoting both the progress of science and advancing national health. Hydrogels are a class of soft materials, composed of crosslinked polymeric networks that remarkably swell in water, which closely simulate many natural biological tissues, like mucous gel layers. Biological lubrication taking place in respiratory and gastrointestinal tracts, in the oral cavity and in the eyes, relies on the low friction coefficients provided by these mucous gel layers. Despite significant research effort, there is still disagreement about the origin of such low friction, which hinders the design of biomaterials capable of achieving efficient biolubrication to date. This project will thus deliver the needed knowledge of the relation between hydrogel's properties and frictional response. Thereby, it will enable to establish design principles of hydrogels for targeted biomedical applications capable of achieving low friction coefficients, and augmented wear resistance, and therefore, results from this research will benefit the U.S. society. This research involves several disciplines including materials chemistry and science, and tribology. The multi-disciplinary approach will help broaden participation of underrepresented groups in research and positively impact engineering education.In order to fill the outlined knowledge gap, two major lines of research will be followed: one comparing physically and chemically crosslinked hydrogels with a wide range of crosslinking degrees, and a second one that will study the influence of polymer chain semi-flexibility. To evaluate lubrication mechanisms, friction force measurements will be carried out on well-characterized hydrogels (with a wide range of compositions) by colloidal probe Atomic Force Microscopy and with a Surface Forces Apparatus as a function of load and sliding velocity (spanning over five orders of magnitude) to probe microscopic phenomena with different relaxation times. The experimental effort will provide insight into the effects of mesh size, type of crosslinking, polymer stiffness, as well as aging of the interface, on the frictional characteristics. The research team will also conduct dynamic shear-force measurements to scrutinize the relation between friction force and interfacial rheology of hydrogels. The knowledge generated by this research will afford to modulate the frictional response of hydrogels through the precise control of their microstructure.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.