Novel Volumetric Mapping of 3D Nanomechanical Heterogeneities in Gelatin, Collagen and Polypeptide Hydrogels and Films

Porous and fibrous hydrogels and films are promising scaffolds for tissue engineering and drug delivery. To assess their nanovolumetric mechanical properties, far-field longitudinal ultrasonic excitation and simultaneous 3D optical mapping are employed to detect down to picometer scale displacement vectors throughout a uniformly loaded gel or film using up to thousands of fluorescent markers as beacons. This allows local distortions within the hydrogel, and even incorporated cells, to be volumetrically mapped and the force distribution to even be back-calculated. Results are reported for a range of gelatin, collagen, and polypeptide compositions, as well as applied acoustic power and load duration, and compared to macroscopic storage and loss moduli determined by rheology. Spatial variations in local magnitudes and vectorial directionality, as well as elastic recovery, are also visualized and statistically analyzed by specimen and depth. Collectively, this approach reveals 3D nanomechanical heterogeneities resulting from local differences in hydrogel cross-linking, density, hydration, and/or cellular mechanical structures and active adhesion.