Figure 1.

Stretching of collagen fibrils and analysis of indentation data. Fibrils deposited on a polydimethylsiloxane (PDMS) foil are shown in (a). When the foil is stretched macroscopically, the fibrils are strained along longitudinally, leading to the straightening of disordered domains and breaking of cross-links. AFM imaging and nanoindentation are performed at different strains to determine the stiffness of fibrils at a resolution sufficient to resolve overlaps and gaps. A simplified model of nanoindentation is shown in (b). The AFM tip represents a hard indenter that penetrates a small distance into the sample, thereby probing a convolution of the radial compression stiffness and the longitudinal tensile stiffness of the fibril. The prestraining of the fibrils longitudinally on the foil (a) has the same effect as increasing the longitudinal tensile force but not the radial compression force (b). The slope of the top quarter of the unloading branch, S₁, of the indentation force-versus-distance curve (c) provides the indentation stiffness. The “zero-distance point,” where the AFM tip just touches the surface, is defined by the inflection point of the unloading curve. Determining the indentation stiffness at different prestrains of the foil (d) effectively probes the longitudinal stress-strain curve of the fibrils at a much greater strain range than possible by nanoindentation alone. An optical microscopy image of a collagen fibril on the PDMS foil is shown in (e). Foil stretching is shown in (f): (i) shows the foil before stretching in which fibrils are deposited, (ii) shows the manual stretching of foil, and (iii) shows the adhesive tape keeping the foil stretched. To see this figure in color, go online.