Studying the mechanical properties of bone at different length scales allows for an improved understanding of its complex hierarchical structure and the correlation thereof with mechanical quantities like bone stiffness, strength and toughness. Porosity, anisotropy as well as inhomogeneity of the bone material need to be considered when investigating individual bone lamellae or interlamellar interfaces at the microscale. Whereas mechanical tests are used to obtain mechanical parameters of bone material, corroborating information for the structure-function relationships might be gained by correlating the orientation of collagen fibrils with the mechanical properties. The intensity of the Amide I (C=O stretch vibration of peptide bond) band changes depending on the polarization angle of the incident Raman laser light. Hence, the orientation of collagen fibrils can be determined based on non-destructive, label-free Raman spectroscopy. As a proof-of-principle, an osteon consisting of concentrically oriented layers of collagen fibrils with different orientation was imaged with Raman spectroscopy. A Raman spectrum is recorded at each pixel of the Raman map with different incident light polarizations. Raman spectra of one pixel (A) are shown representatively for the 7 investigated polarization angles (-90°, -60°, -30°, 0°, 30°, 60°, 90°). A fit of the integrated Amide I band changing at different laser polarization angles (C), allows to calculate the orientation of collagen fibrils at each pixel of the Raman map (D). The result was further verified by SHG (second harmonic generation) imaging of the same sample area (B).
Cooperation between Prof. Bernhard Lendl (CTA – Environmental Analytics, Process Analytics and Sensors; TU Wien) and Prof. Philipp Thurner (Institute of Lightweight Design and Structural Biomechanics; TU Wien)
2-photon polymerization (2PP) is a 3D-printing technology that allows fabrication of objects at the micro- and nanoscale. Long-wave laser light is tightly focused into a resin to locally induce polymerization. Software-based laser beam-guidance allows to fabricate objects with a resolution in the nanoscale. Its unrivaled spatial resolution when compared to conventional additive manufacturing technologies allows the fabrication of highly detailed 3D-geometries. This renders 2PP of great interest for the production of biocompatible and -degradable structures e.g. for tissue engineering purposes in the medical sector. The efficiency of a material composition can be determined by comparing the required light dosage for different writing speeds and exposure intensities. This is usually done in a quantitative way where the first observable features are used as an indicator for the necessary light dosage to induce polymerization. In order to correlate experimental results with theoretical calculations, Raman spectroscopy was used to determine the conversion rate of double to single bond carbon linkage (C=C to C-C) during the polymerization process of ETA:TTA (Ethoxylated trimethylolpropane triacrylate – trimethylolpropane triacrylate) for varying laser powers and exposure times. This ratio was then correlated to the crosslinking density of the fabricated geometry. The results not only allowed to study the minimum dosage required for observable polymerization, but also the effect of higher light dosage on the material. As can be seen in Figure 1, an increase in laser power from 12 to 40 mW for a given exposure time leads to a significantly decreased C=C stretch vibration intensity in the according Raman spectrum due to the polymerization-dependent conversion of C=C to C-C. This decrease can be correlated to the crosslinking density of the fabricated structure.
Cooperation between Prof. Bernhard Lendl (CTA – Environmental Analytics, Process Analytics and Sensors; TU Wien) and Associate Prof. Aleksandr Ovsianikov (Additive Manufacturing Technologies; TU Wien)