Use of multiple unconfined compression for control of collagen gel scaffold density and mechanical properties

Collagen gel is a poroelastic/biphasic system consisting of a fibrillar loose lattice structure filled with > 99% fluid. Its mechanical behaviour is governed by the inherent viscoelasticity of the fibrils, and their interaction with the fluid. This study investigated the underlying mechanisms of plastic compression (PC), a recently introduced technique for the production of dense collagen matrices for tissue engineering. Unconfined compressive loading results in the rapid expulsion of the fluid phase to produce scaffolds with improved mechanical properties potentially suitable for direct implantation and suturing. The controllability of the PC, as a single or multi-stage process was investigated in terms of fluid loss, remaining protein concentration, and morphological characteristics. Time dependent analysis, and quasi-static mechanical (compressive and tensile) properties of hyper-hydrated and PC collagen, produced by single (SC) and double (DC) compression, were also investigated on the non-covalently cross-linked gels. Under unconfined compressive creep, the behaviour of hyper hydrated gel was dictated by the fluid movement relative to the solid (i.e. poroelasticity) with negligible recovery upon load removal. Similar behaviour was achieved in multiple compressed gels; however, these progressively dense matrices displayed an instantaneous recovery that was in line with the increase in fibrillar collagen concentration. Under tension, where the mechanical response of the gels is dominated by the fibrils, there was significant increase in both break strength and modulus with increasing fibril concentration due to multiple compression as DC provided greater opportunity for physical interaction between the nano-sized fibrils.