Constitutive modeling of compressible type-I collagen hydrogels

Collagen hydrogels have been used ubiquitously as engineering biomaterials with a biphasic network of fibrillar collagen and aqueous-filled voids that contribute to a complex, compressible, and nonlinear mechanical behavior-not well captured within the infinitesimal strain theory. In this study, type-I collagen, processed from a bovine corium, was fabricated into disks at 2, 3, and 4% (w/w) and exposed to 0, 10(5), 10(5), and 10(7) microjoules of ultraviolet light or enzymatic degradation via matrix metalloproteinase-2. Fully hydrated gels were subjected to unconfined, aqueous, compression testing with experimental data modeled within a continuum mechanics framework by employing the uncommon Blatz-Ko material model for porous elastic materials and a nonlinear form of the Poisson's ratio. From the Generalized form, the Special Blatz-Ko, compressible Neo-Hookean, and incompressible Mooney-Rivlin models were derived and the best-fit material parameters reported for each. The average root-mean-squared (RMS) error for the General (RMS. 0.13 0.07) and Special Blatz-Ko (RMS= 0.13 0.07) were lower than the Neo-Hookean (RMS= 0.23 0.10) and Mooney-Rivlin (RMS= 0.18 0.08) models. We conclude that, with a single fitted-parameter, the Special Blatz-Ko sufficiently captured the salient features of collagen hydrogel compression over most examined formulations and treatments. (C) 2018 IPEM. Published by Elsevier Ltd. All rights reserved.