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Viscoelastic Modeling of Porous Matrices Used In Tissue Engineering

Rahul D. Mirani and Sundararajan V. Madihally. School of Chemical Engineering, Oklahoma State University, 423 Engineering North, Stillwater, OK 74078

Many naturally obtained matrices such as small intestinal sub mucosa and synthetic matrices are utilized in tissue regeneration to support the in growth of cells to be colonized. Recent advances have shown the elastic property of the porous structure play a significant role in cell colonization. However, mechanical properties of the porous structures are not well understood, particularly the viscoelastic properties. In this study we explore the viscoelastic properties of both the natural matrix SIS and synthetic matrices made of chitosan/gelatin supported by 50:50 PLGA layer. First, the uniaxial tensile tests done on the composite in hydrated medium at 37C at a controlled cross head speed of 10 mm per minute and a fixed load of 100 N show that it can be elongated up to nearly 400 % of its initial length, nearly 8 times more than SIS.

In order to understand the behavior of the composite material under relaxation, stress relaxation tests are performed. The specimen is subjected to a predetermined amount of increasing ramp strain inputs, consisting of a loading part wherein the strain is applied at the rate of 3.125 % per second for 16 s and a relaxation part where the material is allowed to relax for 100 seconds. Strain rates ranging from 0.9375 % to 3.125 % for two different relaxation times of 60 s and 100 s were used for the test. Similar experiments are performed on the SIS and the output stress relaxation behavior was compared. A lower range of the strain rate had to be used for the SIS because it fails at the higher ranges. It was also observed that the SIS has a higher maximum stress value which shows that it accumulates more stress than the composite. Also for a similar time range the composite relaxed to a much greater extent than the SIS and accumulated less stress. Thus the composite was able to relax more naturally as compared to the SIS with less deformation in the long run. To quantify the relaxation behavior of the matrices, the Quasi Linear Viscoelastic (QLV) model is used and the data obtained from the model is compared to the experimentally obtained stress values for both the loading as well as the relaxation part of the stress relaxation curve. The QLV model is modified to incorporate the fact that the loading was done in a finite amount of time and not instantaneously as assumed by Fung YC. The data obtained from feeding the model equation in MAT LAB matched the experimentally obtained data with an average deviation of 3%. Thus the QLV model is successful in quantifying the nonlinear behavior of the composite.