From macroscopic mechanics to cell-effective stiffness within highly aligned macroporous collagen scaffolds

In the design of macroporous biomaterial scaffolds, attention is payed predominantly to the readily accessible macroscopic mechanical properties rather than to the mechanical properties experienced by the cells adhering to the material. However, the direct cell mechanical environment has been shown to be of special relevance for biological processes such as proliferation, differentiation and extracellular matrix formation both in vitro and in vivo. In this study we investigated how individual architectural features of highly aligned macroporous collagen scaffolds contribute to its mechanical properties on the macroscopic vs. the microscopic scale. Scaffolds were produced by controlled freezing and freeze-drying, a method frequently used for manufacturing of macroporous biomaterials. The individual architectural features of the biomaterial were carefully characterized to develop a finite element model (FE-model) that finally provided insights in the relation between the biomaterial's mechanical properties on the macro-scale and the properties on the micro-scale, as experienced by adhering cells. FE-models were validated by experimental characterization of the scaffolds, both on the macroscopic and the microscopic level, using mechanical compression testing and atomic force microscopy. As a result, a so-called cell-effective stiffness of these non-trivial scaffold architectures could be predicted for the first time. A linear dependency between the macroscopic scaffold stiffness and the cell-effective stiffness was found, with the latter being consistently higher by a factor of 6.4 ± 0.6. The relevance of the cell-effective stiffness in controlling progenitor cell differentiation was confirmed in vitro. The obtained information about the cell-effective stiffness is of particular relevance for the early stages of tissue regeneration, when the cells first populate and interact with the biomaterial. Beyond the specific biomaterial investigated here, the introduced method is transferable to other complex biomaterial architectures. Design-optimization in 3D macroporous scaffolds that are based on a deeper understanding of the mechanical environment provided to the cells will help to enhance biomaterial-based tissue regeneration approaches.