Scanning force microscopy of gelatin films in the dry, swollen and redried states

We employ scanning force microscopy (SFM) to investigate films of the complex biopolymer gelatin. We distinguish different phases of gelatin via frictional force signatures. A low-friction minority phase is substantially present in thin dry films prepared from low-concentration aqueous solutions. Frictional force is measured as a function of tip speed comparatively on the different phases and related to the degree of "crystallinity" (triple-helical content) in the different film regions. The mechanical properties and surface forces are characterized in force vs. distance measurements on thick, water-swollen gelatin films prepared with and without covalent crosslinking agents. SFM imaging of redried thick films reveal "craters", which were absent in the original film. These craters are increasingly prevalent as a function of increased drying rate; an anisotropic distribution results from extremely fast directional drying. In some cases craters are observed only along boundaries between dissimilar friction domains, suggesting that interfacial stresses between domains may yield the formation of crater defects. Topographic SFM images display substantial reticulation in redried films containing a peptide coupler crosslinking agent.