Microstructural mechanics of collagen gels in confined compression: Poroelasticity, viscoelasticity, and collapse

Background: Collagen gels are important as platforms,for in vitro study of cell behavior and as prototypical bioartificial tissues, but their mechanical behavior, particularly, on the microscopic scale, is still poorly understood. Method of Approach: Collagen gels were studied in step (10% strain in 0.05 s) and ramp (0.1%/s strain rate for 100 s) confined compression. Real-time birefringence mapping gave the local collagen concentration and orientation along with piston stress. Variations in the retardation allowed material-point tracking and qualitative determination of the strain distribution. Results: Ramp tests showed classical poroelastic behavior: compression near the piston and relaxation to a uniform state. Step tests, however, showed an irreversibly collapsed region near the piston. Conclusions: Our results suggest that interstitial flow and fibril bending at crosslinks are the dominant mechanical processes during compression, and that fibril bending is reversible before collapse.