Engineering physical structure in biomimetic collagen scaffolds: strategies for regulating cell behavior

Tissue engineering has traditionally relied on the use of scaffolds as inert, durable materials for seeded cells to re-grow. However, a paradigm shift in the role of scaffolds has become necessary towards bio-functional ‘devices’ which directly mimic native tissue matrix. It is key to control extracellular matrix remodelling and tissue structure by building control cues into the initial cell support matrix. The aim of this study was to test the effect and predictability of physical cues, engineered into 3D native collagen scaffolds by a cell-independent fabrication method, Plastic Compression (PC). Our findings indicated that fluid expulsion during collagen hydrogel compression produced anisotropic structuring and could be modelled as an ultrafiltration process. A groove/ridge topography engineered on collagen scaffolds through pattern-template-embossing influenced endothelial cell attachment/orientation and keratinocyte stratification in culture. Matrix collagen density and stiffness were directly related to its hydration level and could be controlled by limiting the extent of compression. Human-Dermal-Fibroblast (HDF) proliferation was proportional to matrix stiffness. In addition HDFs, seeded evenly within a PC collagen stiffness gradient, migrated and accumulated at the stiff end after 6 days. Bi-layer collagen matrices underwent cell-mediated integration, but despite higher cell migration across the interface in compliant than in stiff matrices at 24hrs, there was no significant difference in interface adhesive strength at 1 week. Core O2 tension in 3D spiral constructs directly correlated with total cell number along the diffusion path, i.e. consumption path length. This model was used to engineer local cell-mediated hypoxia in 3D constructs to generate populations of hypoxia-induced-signalling cells which produced angiogenic factor protein cascades. This in turn induced directed, functional micro-vascular ingrowth in vitro and in vivo. These data indicate how directing physical cues can be built into the structure of biomimetic, tissue-like scaffolds. This helps to understand intricate cell-matrix behaviours without reliance on complex biological control mechanisms and points the way to using simple physical cues for tissue formation