| Progressive and irreversible retinal diseases are among the primary causes of blindness in the United States, attacking the cells in the eye that transform environmental light into neural signals for the optic pathway. Medical implants designed to restore visual function to afflicted patients can cause mechanical stress and ultimately damage to the host tissues. Research shows that an accurate understanding of the mechanical properties of the biological tissues can reduce damage and lead to designs with improved safety and efficacy. Prior studies on the mechanical properties of biological tissues show characterization of these materials can be affected by environmental, length-scale, time, mounting, stiffness, size, viscoelastic, and methodological conditions.;Using porcine sclera tissue, the effects of environmental, time, and mounting conditions are evaluated when using nanoindentation. Quasi-static tests are used to measure reduced modulus during extended exposure to phosphate-buffered saline (PBS), as well as the chemical and mechanical analysis of mounting the sample to a solid substrate using cyanoacrylate. The less destructive nature of nanoindentation tests allows for variance of tests within a single sample to be compared to the variance between samples. The results indicate that the environmental, time, and mounting conditions can be controlled for using modified nanoindentation procedures for biological samples and are in line with averages modulus values from previous studies but with increased precision.;By using the quasi-static and dynamic characterization capabilities of the nanoindentation setup, the additional stiffness and viscoelastic variables are measured. Different quasi-static control methods were evaluated along with maximum load parameters and produced no significant difference in reported reduced modulus values. Dynamic characterization tests varied frequency and quasi-static load, showing that the agar could be modeled as a linearly elastic material. The effects of sample stiffness were evaluated by testing both the quasi-static and dynamic mechanical properties of different concentration agar samples, ranging from 0.5% to 5.0%. The dynamic nanoindentation protocol showed some sensitivity to sample stiffness, but characterization remained consistently applicable to soft biological materials.;Comparative experiments were performed on both 0.5% and 5.0% agar as well as porcine eye tissue samples using published dynamic macrocompression standards. By comparing these new tests to those obtained with nanoindentation, the effects due to length-scale, stiffness, size, viscoelastic, and methodological conditions are evaluated. Both testing methodologies can be adapted for the environmental and mounting conditions, but the limitations of standardized macro-scale tests are explored. The factors affecting mechanical characterization of soft and thin viscoelastic biological materials are researched and a comprehensive protocol is presented. This work produces material mechanical properties for use in improving future medical implant designs on a wide variety of biological tissue and materials. |
