Mechanical and Optical Characterization of a Tissue Surrogate Polymer Gel

Abstract

Hydrogels and polymer gels are widely used for simulating the mechanical response of human or animal tissue to understand the effects of trauma on organs, design body armor, detect tumors robotically, simulate surgical procedures and test the effectiveness of firearms. Polymer gels are preferred to hydrogels for environmental stability, low cost, and ease of altering sample composition, shape and dimensions and do not have moisture evaporation issues. Motivated by these, a transparent polymer gel is studied in this work as a tissue simulant.

The composition of the polymer gel is first assessed using FTIR spectroscopy. This is followed by mechanical characterization studies comprising of tension, compression and shear tests at different crosshead speeds to evaluate strain rate dependency. Uniaxial tension tests are conducted to evaluate stress-strain responses and failure properties. Highly compliant nature of the gel, however, demands special specimen sizing and end tabbing techniques to prevent premature specimen damage. Digital Image Correlation method is used for measuring two orthogonal strains during tensile tests. The stress-strain data are modeled as Mooney-Rivlin, Neo-Hookean and Yeoh hyperelastic materials. Compression tests are performed as well on cylindrical samples for completeness. The gel elastic modulus is found to increase modestly (103 to 141 kPa) over strain rates 0.0004 to 0.04 /sec whereas the Poisson’s ratio is in the range 0.46 to 0.48. However, both failure stress and failure strain increased by nearly 100% in the same range of strain rates, the latter being unexpected relative to conventional engineering materials. Shear tests are also conducted to obtain the corresponding shear moduli and to independently compare with the ones from tension tests.

Optical transparency of the polymer gel is a feature useful for visualization and quantification of mechanical fields using full-field optical techniques. Accordingly, its elasto-optical constant that accounts for the combined density changes and Poisson effects is quantified using Digital Gradient Sensing (DGS) method by measuring angular deflections of light rays propagating through a diametrically compressed circular gel disk. The elasto-optical constants ranged from -1.47 to -1.18 mm2/N over the strain rates considered. The feasibility of studying stress concentration effects using DGS is demonstrated by performing experimental and numerical simulations on a classical Flamant loading configuration.

Lastly, preliminary work towards understanding the mechanics of needle-tissue surrogate gel interaction is carried out at two different insertion/retraction rates. Significant strain rate effects and hysteresis are observed both in terms of measured stress gradients and global force measurements. During needle insertion, forces increased monotonically and are compressive. During retraction, however, forces not only decreased but turned tensile before becoming zero. Non-uniform stress distribution in the vicinity of the needle with a high concentration of stresses is evident from the optical measurements.