We discuss atomic force microscopy modalities applied to lubricious polyvinyl pyrrolidone gel coatings. The character of this work is methods development, wherein different AFM surface tracking modes and property sensitive measurements under full aqueous immersion are explored and developed. In much of our work, particularly on kPa-regime modulus gels (cases of none-to-low crosslinking), the sharp AFM tip is replaced with a colloid microprobe (radius 3.3 um); whereas we find that gels with moduli on the order of 10’s of kPa and higher can be mechanically probed and even topographically imaged using standard sharp AFM tips (radius ~10 nm). We explore differences in spatial resolution on these latter gels with both types of probe. In the process we discover that defect structures seen in the initial dry state on sponge-deposited / UV-cured coatings – dubbed craters, pinholes, fissures, wrinkles and cheetah spots – tend to “heal” upon hydration to gel state, and dramatically more so for lesser crosslinking. Beyond rich and diverse coating morphologies, normal and shear force AFM perturbations yield variable response. Shear-force perturbation and measurements reveal marked differences in the initial dry state per the degree of crosslinking. We further explore both sliding friction and force-distance curves in the gel state. In all cases these spatially mapped measurements – up to several tens of microns in lateral scale – reveal crosslinking-dependent responses and heterogeneity. In the process we develop shear modulation (pinned, viscoelastic) measurements ancillary to both contact mode imaging and force-distance mapping. Given an emerging appreciation of the importance of nano- to micro-scale structures and behavior to the interaction between medical device surfaces and the body, we believe that the insights afforded by diverse AFM methods merit an increasing role in understanding and predicting the performance of lubricious coatings.

AFM methods for aqueous probing of lubricious gel coatings for medical devices

253rd National Meeting (2017)

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