Computational and experimental comparison on the effects of flow-induced compression on the permeability of collagen gels

Collagen is a naturally occurring polymer and is popular in tissue engineering due to its high biocompatibility, ubiquity throughout the body, and its porous nature. The transport properties of collagen help dictate the de-livery of nutrients to tissues, and the mechanical properties can help improve the function of engineered tissues. The objective of this study is to investigate experimentally the change in permeability as collagen gels undergo flow-induced compression and compare these results with model predictions using a finite element model. We developed a horizontal apparatus to measure the hydraulic permeability of collagen gels undergoing flow-induced compression. The permeability of 1.98 mg/mL, 3.5 mg/mL, and 5 mg/mL collagen Type I rat tail hydrogels were determined experimentally by tracking the pressure drop across the gels as water flowed through the samples, which simultaneously compressed them under pressure. The Holmes-Mow model was used to fit the permeability as the gels underwent compression. A finite element model was created using FEBio to estimate the Young's modulus of collagen gels at the macroscopic level by fitting the experimental pressure vs. the compressive stretch ratio to the model. Our results suggest that the initial permeability of collagen gels decreased with increasing concentration, as expected. However, gels with a lower initial concentration compressed to a greater degree, resulting in smaller final permeabilities once fully compressed. Taken together, our work suggests that the treatment of a collagen gel as an isotropic, elastic material is sufficient to model its transport properties on a macroscopic level but is inadequate if more localized transport properties, which are dependent on network architecture (such as collagen alignment or inhomogeneous densification), are required.