The nanoscale role of water at tribological interfaces on lubricious biomedical coatings is explored in AFM measurements. Our purview includes both normal and shear force methods, versus humidity and under full aqueous immersion. On polyacrylamide coatings (PAm) we analyze dramatic changes in the shape of force-distance curves as a function of relative humidity (RH). Among other metrics, we find a maximum (rather than monotonically increasing) pull-off force from PAm versus relative humidity, in the mid-80 to low-90 RH percentage range, assigned to a glass-rubber transition and akin to loss modulus as a function of temperature. We further explore this transition as a function of coating thickness (confinement). In sliding contact we correspondingly record a maximum in both friction force and the propensity for shear-induced plastic deformation (“rippling”). On polyvinyl pyrrolidone (PVP) coatings we similarly explore the humidity dependence of the glass-rubber transition but for different degrees of UV-activated crosslinking (cure). We find a 2-3% variation in the critical RH for a glass-rubber transition over a range of UV cure times, equivalent to a ~10 C shift in Tg. Normal force measurements on gel-state PVP under aqueous immersion reveal a &lt;1 kPa to &gt;100 kPa variation in elastic modulus versus crosslinking from 0-50 seconds UV cure, and corresponding differences in coating swelling. Friction force imaging with a colloid probe (to avoid tearing) further explores this curing dependence, revealing single macromolecular (Langevin) shear-extension forces for 0-5 second UV cure that are absent at higher levels of cure. We relatedly discuss our developments in data analysis algorithms to enable the measurement science.

Humidity-dependent and aqueous-immersion AFM: Nanotribological analysis of biomedical coatings

2019 Fall National Meeting

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