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The mechanical properties of collagen fibrils depend on the amount and thedistribution of water molecules within the fibrils. Here, we use atomic forcemicroscopy (AFM) to study the effect of hydration on the viscoelasticproperties of reconstituted type I collagen fibrils in air with controlledrelative humidity. With the same AFM tip, we investigate the same area of acollagen fibril with two different force spectroscopy methods: force-distance(FD) and amplitude-phase-distance (APD) measurements. This allows us toseparate the contributions of the fibril's viscoelastic response and thecapillary force to the tip-sample interaction. A water bridge forms between thetip apex and the surface, causing an attractive capillary force, which is themain contribution to the energy dissipated from the tip to the specimen indynamic AFM. The force hysteresis in the FD measurements and the tipindentation of only 2 nm in the APD measurements show that the hydratedcollagen fibril is a viscoelastic solid. The mechanical properties of the gapregions and the overlap regions in the fibril's D-band pattern differ only inthe top 2 nm but not in the fibril's bulk. We attribute this to the reducednumber of intermolecular crosslinks in the reconstituted collagen fibril. Thepresented methodology allows the mechanical surface properties of hydratedcollagenous tissues and biomaterials to be studied with unprecedented detail onthe nanometer scale.

Unraveling capillary interaction and viscoelastic response in atomic force microscopy of hydrated collagen fibrils