Abrasive scanning with AFM on biocompatible polymers: Measuring dry vs. swollen film thickness and probing properties

In this study we exemplify raster-scan abrasion with an AFM probe at high force (in "contact mode") to shear-displace relative soft (organic, polymeric) films on hard substrates, and nonperturbatively analyze the results at lower force. We delve into extra detail on Polyacrylamide films chemisorbed on silanized glass surfaces via ultraviolet-initiated attachment and crosslinking. This ultrathin film system is a technological platform for DNA microassays; more generally, thicker Polyacrylamide films are added as lubricious coatings on medical devices, [surmodics] One problem of interest to those engineering such films is measuring the degree of film swelling under aqueous immersion (and its relation to processing parameters). We present a methodology for this measurement using force-distance curve mapping over the scan-abraded and surrounding region. We demonstrate that conventional topographic imaging (i.e., under setpoint feedback) of this region is an erroneous means of quantifying swollen film thickness because of substantial mechanical compliance atop the swollen polymer; whereas force-distance mapping accounts for this compliance, and furthermore reveals interesting macromolecular behavior. We exemplify some algorithms for more advanced and quantitative analyses of these datasets.