(323a) Influence of Porous Architecture On Relaxation and Cyclical Properties of Biodegradable Polymers

Promising novel solutions to restore, maintain, or enhance tissue function or a whole organ is regenerating tissues using biodegradable structures onto which cells attach, populate, and synthesize new tissue.  The biodegradable structures from various animal tissues such as skin, bladder, fat and intestine have seen clinical usage due to the advantage of premade architecture, which is conducive for tissue regeneration.  However, manipulating these architectures to grow other tissues has shown many obstacles.  Hence, synthesizing matrixes using various materials and processes such as electrospinning, freeze drying, and salt leaching techniques have been considered.  Three dimensional scaffolds are 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 using different natural and synthetic polymers has attracted significant interest.  In addition to showing bioactivity, scaffolds should have i) high porous structures to aid cell in-growth and mechanically withstand the stresses and strains in the body.  Biological tissues exhibit viscous (like fluids) and elastic (like solids) behavior, hence, prepared materials should have similar characteristics. 

Previously we have reported on the stress relaxation characteristics of of poly-lactic-co-glycolic acid (PLGA) films [1], Polycaprolactone (PCL) films [2] and chitosan, chitosan-gelatin porous structures [3] formed by freeze-drying.  We have also modeled some of the behavior using quazi-linear viscoelastic model, and pseudocomponent models.  The objective of this study was to evaluate and model the effect of porous architecture in these materials.  For this purpose, we prepared PCL scaffolds by salt leaching technique and chitosan, chitosan-gelatin films using air drying technique.  First, uniaxial tensile properties were evaluated under physiological conditions (hydrated in phosphate buffered saline at 37 °C). From the estimated break strain, the limit of strain per ramp was calculated and stretched.  The ramp-and-hold type of stress relaxation test was performed for five successive stages.

We developed two models (i) containing a hyper-elastic spring (containing two parameters) and (ii) retaining pseudo-components (containing three parameters) in Visual Basic Applications accessed through MS Excel.  The models were used to fit the experimental stress-relaxation data and parameters were obtained to understand the influence of porous architecture.  To validate the utility of the models, obtained parameters were used to predict cyclic behaviors, which were compared independently run cyclical experimental results.  These results showed the model could be used to predict the cyclical behavior under the tested strain rates. 

[1]  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.

[2]  Pok SW, Wallace KN, Madihally SV. In vitro Characterization of Self Assembled Polycaprolactone Matrices Generated in Aqueous Media.Acta Biomaterialia,6(3):1061-1068. 2010.

[3]  Ratakonda S, Manimegalai, Rhinehart RR, Madihally SV. Assessing Viscoelastic Properties of Chitosan Scaffolds and Unifying with the Cyclical Tests. Acta Biomaterialia. 8(4):1566-75. 2012.