In situ mechanical and molecular investigations of collagen/apatite biomimetic composites combining Raman spectroscopy and stress-strain analysis

We report the design, fabrication and application of a novel micro-electromechanical device coupled to a confocal Raman microscope that enables in situ molecular investigations of micro-fibers under uniaxial tensile load. This device allows for the mechanical study of micro-fibers with diameters in the range between 10 and 100 lm and lengths of several hundred micrometers. By exerting forces in the mN range, the device enables an important force range to be accessed between that of atomic force microscopy and macroscopic stress-strain measurement devices. The load is varied using a stiffness-calibrated glass micro-needle driven by a piezo-translator during simultaneous Raman microscopy imaging. The method enables experiments probing the molecular response of micro-fibers to external stress. This set-up was applied to biomimetic non-mineralized and mineralized collagen micro-fibers revealing that above 30% mineralization the proline-related Raman band shows a pronounced response to stress, which is not observed in non-mineralized collagen. This molecular response coincides with a strong increase in the Young’s modulus from 0.5 to 6 GPa for 0% and 70% mineralized collagen, respectively. Our results are consistent with a progressive interlocking of the collagen triple-helices by apatite nanocrystals as the degree of mineralization increases.