(485s) A Novel Pseudo-Component Model to Assess the Viscoelastic Behavior of Scaffolds Used in Tissue Engineering

Most biological tissues and scaffolds used in tissue regeneration display time-dependent and load-history-dependent mechanical behavior.  Soft tissues such as muscles, tendons, ligaments, fascia, nerves, fibrous tissues, fat, blood vessels, and synovial membranes display nonlinear material behavior and are found to exhibit viscoelastic character.  Further, porous polymeric biodegradable structures utilized in tissue regeneration also show viscoelastic behavior.  A number of models have been developed to assess the nonlinear viscoelastic behavior of soft materials.  The most common phenomenological model has been the Quasi Linear Viscoelastic (QLV) model, introduced by Fung and later modified by many others according to comply with specific test or material requirements.  The QLV theory assumes that the stress relaxation behavior of soft tissues can be expressed as a convolution integral of a strain independent reduced relaxation function and a nonlinear instantaneous stress function resulting from a ramp strain.  However, several aspects of these approaches (idealizations, assumptions, truncations, constitutive models), render them inapplicable to the nonlinear, nonstationary, and confounding aspects of the viscoelastic deformation and relaxation mechanisms of biological tissue structures.  

This work proposes a pseudo-component modeling approach, and a numerical method to assess the non-linear viscoelastic stress relaxation character.  The pseudo-component model proposed in this work does not seek to model each component and/or the exact internal structure.  By contrast, this model attributes the overall viscoelastic response to several parallel pseudo-components, sharing the same external deformation, additively combining stress forces and relaxing at individual rates.  The use of a numerical method to solve the model eliminates the linear and truncation assumptions of an analytical model.  Pseudo components have a nonlinear instantaneous stress-strain relation, and a first-order relaxation rate toward a fractionof their maximum individual stress.

The model was tested using a composite scaffold made of chitosan/gelatin porous structures reinforced with 50:50 PLGA membranes [1, 2].  The experimental protocol consists of subjecting the composite to increasing ?ramp and hold? type of stress relaxation tests under physiological conditions (hydrated in Phosphate Buffered Saline (PBS) at 37°C) for four successive cycles.  Experiments were also performed by changing the ramp rates.  Results from one test set are shown in the figure.

[1]  Lawrence BJ, Maase EL, Link H-K, Madihally SV. Multilayer Composite Scaffolds with Properties Similar Small Intestinal Submucosa. J. Biomedical Materials Research-Part A. 88A(3):434-43. 2009.

[2]  Mirani RD, Pratt J, Iyer P, Madihally SV.  Influence of Nanoetching on the Stress Relaxation Properties of Composite Matrices. Biomaterials. 30(5):703-710.  2009.