Mapped AFM force spectroscopy on gels: methodological and interpretive developments

We present aqueous-immersion force-distance analysis on fibrin gels and polyvinyl pyrrolidone (PVP) gel-phase coatings. Initial applications were mechanical matching (fibrin to tissue) and lubricity/durability assessment (PVP). In both cases we discovered (a) a need for extensive mapping (tens or hundreds of microns) to survey spatial heterogeneities in both mechanics and topography as well as their correlations; and (b) widely ranging modulus values both among gel samples (~0.1-100 kPa) and across individual gels (one order of magnitude). On the softest PVP gel coatings colloid microprobes were used instead of sharp AFM tips to allow a shear contact (friction characterization).
In developing data analytics, we find that vendor algorithms for modulus mapping require modification (here in matlab) in order to: (1) demarcate solid body mechanics (Hertzian) from steric forces within each force curve as a function of distance, and/or treat Poissons ratio as variable with gel compression; (2) account for local surface curvature and thereby system radius of curvature (for colloid probe); (3) invoke criteria for rejecting anomalous force-distance measurements (e.g., sites of fibril adhesion) prior to compiling distribution functions of spatially surveyed modulus measurements.