AFM Multimode Imaging and Nanoindetation Method for Assessing Collagen Nanoscale Thin Films Heterogeneity

Atomic Force Microscopy (AFM) operates in a variety of modes and techniques. AFM can offer a wide range of information, from topography to mechanical properties of surfaces and interfaces, including those of biomaterials and scaffolds. In this paper it was sought to gain insights of structural and mechanical heterogeneity of collagen fibers in thin films by combining AFM multimode imaging, including phase imaging, with quantitative measurements through nanoindetation. Due to its filamentous shape and its associative properties collagen type I is a very promising molecule for the development of nanostructures, scaffolds in tissue engineering and nanobiomaterials Since collagen based materials nanotopography and mechanical properties can influence cellbiomaterial/ scaffold interactions it is of crucial importance to characterize its surface and heterogeneity in the nanoscale. Among the different collagen-based biomaterials, collagen thin films are of great interest since they possess unique properties and can be used for forming novel biomaterials or for covering non-biological surfaces in order to offer them biocompatibility. The results demonstrated that the overlap and gap region on collagen fibers (D-periodicity) yield a significant phase contrast, due to different mechanical properties. In addition, phase contrast was also demonstrated in kinks (areas where collagen fibers changes abruptly direction) which provides evidence that collagen fiber shell and core possess different properties. The quantitative measurements with nanoindentation method confirmed the heterogeneity of collagen fibers D-periodicity since overlapping zones were characterized by a higher Young modulus (0.7 GPa) than the gap zones (0.46 GPa). The correlation between the heterogeneous structure and the mechanical properties of collagen fibrils in thin films will enable the design and development of biomaterials and tissue scaffolds with improved properties, as well as it will enable the investigation of cell response on different nanoscale features.