(203d) Assessing Viscoelastic Properties of Chitosan Scaffolds and Unifying with the Cyclical Tests

Abstract

With the availability of different natural and synthetic materials that can be introduced to human body, regenerating defective tissues outside the body has attracted significant interest.  Three dimensional scaffolds can be prepared from both synthetic and natural materials that are i) compatible with the human body, ii) bio-degradable and iii) supportive of reparative cell colonization.  Apart from being bio-compatible, tissue engineering scaffolds should have highly porous structures in order to aid biological activities and be mechanically strong to withstand the stresses and strains in the body.  Since many tissues in the body exhibit viscous (like fluids) and elastic (like solids) behavior, prepared materials should have similar characteristics.  Chitosan-based scaffolds have garnered significant consideration due to various advantages including cost, availability and biocompatibility.  However, the viscoelastic characteristics of chitosan scaffolds have not been completely understood.

The objective of this study was to evaluate and model the viscoelastic characteristics of chitosan and chitosan-gelatin scaffolds prepared using freeze-drying.  To validate the model, cyclical properties were predicted using the model and experiments were performed on the scaffolds.  Scaffolds were frozen at different temperatures and freeze dried, similar to our previous publications.  Using the scaffolds, uniaxial tensile properties were evaluated under physiological conditions (hydrated in Phosphate Buffered Saline (PBS) at 37°C) at a constant strain rate of 2.5 percent/s.  Based on the linear region of tensile stress vs. strain plot, ramp and hold type of stress relaxation tests were performed for four successive cycles.  Experiments were also performed by changing the ramp rates.  Changes in the microstructure of the tested samples were evaluated using a stereo microscope.  The micrographs of scaffolds after relaxation experiments showed orientation of pores suggesting the retention of the stretched state even after many hours of relaxation.  Based on these observations, a model containing i) an hyperelastic spring (containing two parameters) and ii) retain pseudo components (containing three parameters) were developed in Visual Basic accessed through MS Excel.  Also, leap-frogging method of optimization of parameters was used.  Since one retain pseudo component could not predict the experimental stress relaxation, another component was added. 

The model fitted the experimental stress-strain behavior of chitosan with an SSD of 1.792 and that of chitosan-gelatin with SSD of 0.933.  Using the model parameters, cyclical behavior of chitosan and chitosan-gelatin were developed.  Further, cyclical experiments were performed and compared to the model predictions.  These results showed the model could be used to predict the cyclical behavior under the tested strain rates.  When experiments were performed at very slow strain rates (5 %/min) than the model predictions (150%/min), larger discrepancy was observed.  In summary, the pseudo-component modeling approach can accept any strain history, including sequential strain-and-hold stage, and provide good fits to experimental data.  This approach can be extended to other biomaterial scaffolds to characterize the viscoelastic behavior.