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

G. Haugstad, K. Wormuth
University of Minnesota,
United States

Keywords: tribology, thin films, swelling, hydration, polymer


Atomic force microscopy (AFM) is well-known as a tool for imaging surface topography down to subnanometer scales on all forms of matter, utilizing a scanning stylus (“tip”). The method is applicable to biomolecules adsorbed to substrates as well as surfaces of biocompatible synthetic materials such as polymers. Moreover such systems can be probed by AFM under aqueous immersion. A common challenge for the AFM analyst, however, is minimizing the interaction between the scanning tip and the surface of interest. If the interaction is too strong, material can be displaced or torn, which in common application is detrimental to achieving good images and useful metrological data such as surface roughness. But the irreversible perturbation of a sample material by the AFM tip also can be utilized to the analyst’s advantage. Indeed such “nanomanipulation” with an AFM tip can be considered one of its three principal strengths (along with imaging and quantitative property analysis). In this presentation we firstly describe how one can raster scan a material with AFM so as to shear-displace the topmost portion, then reduce the tip-sample interaction force to image and analyze the result. We provide brief examples in the context of measuring dry film thickness and probing cohesive strength, primarily on organic/polymeric/bio materials but also on mica and gold (i.e., common substrates). To aid understanding we include practical settings for AFM users to implement the method and time-elapsed images (“movies”) during abrasive scanning. We go into further detail on one system: 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. 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 commonly available, but generally underutilized, AFM measurement modes. Two-dimensional abrasive scanning (“contact mode” at high force) was used to bare a small region of the substrate, but followed by force-distance curve mapping (“force volume” mode) over the modified and surrounding region. We demonstrate that conventional topographic imaging of this region is an erroneous means of quantifying film thickness, and thus film swelling, because of substantial mechanical compliance atop the swollen polymer; whereas force-distance mapping determines this compliance, and furthermore reveals interesting chain-molecule behavior. We describe custom software algorithms for more advanced and quantitative analyses of these phenomena, and in the process achieve the goal of measuring swollen film thickness with reasonable precision and statistics. We also discuss the translation of these methods to the industrial analytical lab.