Poro-viscoelastic models applied to porous gelatin scaffolds

Materials and Methods: Gelatin type A samples were made from porcine skin (Sigma-Aldrich, St-Louis, MO, USA). Samples were prepared by dissolving gelatin in deionized water stirred at 50°C for 1h. The solution was poured into Petri dishes and let jellify for 12 hrs at 25°C.Two cooling steps were used to freeze the samples: 6h at 4°C and then 8h at -20°C. Samples were then freeze-dried to create the porous matrix[3]. Viscoelastic and poro-elastic behaviors manifest themselves at different time-scales during relaxation and were studied using a method proposed by Wang[4]. This method relies on unconfined compression relaxation tests on disk-shaped samples of various sizes. For relaxation tests, a 25 mm diameter compression plate was attached to a 150 g load cell mounted on a Biomomentum Mach 1 (Laval, Qc, Canada) testing apparatus. Compressions were followed by a relaxation time as long as 12 hours (43200s). All tests were performed at room temperature in Phosphate Buffered Saline. Simulations were done with Simulink (Matlab) to assess strain-stress relationships using Neo-Hookean and Mooney-Rivlin models. Results and Discussion: Figure 1 shows a typical scanning electronic microscope (SEM) image of the porous structure of the hydrogel. Poro-elasticity analysis was used to study the migration of fluids within a porous matrix. By mechanical compression, fluid diffusion was induced through the gels from the center to the outmost layer. Figure 2 shows mechanical responses of porous gelatin samples of different sizes. For comparison purposes, collagen hydrogel relaxation (viscoelastic only) was added to show the behavior difference between a poro-elastic sample and a viscoelastic sample. The mechanical properties of porous gelatin specimens at different concentrations and pore sizes were investigated. The presence of poro-elastic behavior was detected with the use of a simple method based on curve fitting techniques.

Conclusion: The method described in this work presents new techniques to assess the viscoelasticity and poro-elasticity of hydrogels. While the contribution of poro-elasticty relative to viscoelasticity might be negligible for some hydrogels, it constitutes a crucial part of mechanical behavior for other hydrogels like porous gelatin.