Development of a Low Cost, Low-Force Mechanical Testing Device and Its Application in Measuring Mechanical Properties of Polyelectrolyte Capsules for Engineered Tissue Assembly

Tissue engineering muscle is a promising approach to restoring function in cases of volumetric loss due to cancer or trauma. We previously developed polylelectrolyte GAG-chitosan microcapsules as a platform for modular tissue engineering. We subsequently modified the capsule technology to generate directly extruded polyelectrolyte hollow fibers encapsulating mesenchymal stromal cells, and proposed to evaluate their utility in engineering skeletal muscle tissue. However, the hollow fibers require significant enhancement to their mechanical properties before deployment into the muscle problem. In this study, we explored how the addition of photocrosslinkable acrylate groups to chitosan affected the mechanical properties of the GAG-chitosan polyelectrolyte membranes found in the microcapsules and hollow fibers. A convenient method of quantifying the membrane mechanical properties and the effects of photocrosslinking involves measuring the burst strength of spherical polyelectrolyte capsules. To generate capsules, 0.5 mL of 1.5 wt% carboxymethylcellulose/4 wt% chondroitin 4-sulfate solution was dispensed dropwise into a rapidly stirred solution of glycidyl-methacrylate modified chitosan (GMA-chitosan) to form microcapsules 3-4 mm in diameter. Microcapsule membranes were subsequently exposed to long-wave UV light to induce photocrosslinking of the GMA moieties. Since the costs of low-load mechanical testing systems are prohibitive, a suitable system was designed and assembled in house from commercially available components. Specifically, a 100 g load cell and a 10 cm stroke linear actuator were purchased, together with manufacturer-supplied USB interface boards. Microcapsule rupture strength was measured following a modified standard. Microcapsules 4 mm in diameter were prepared, with the microcapsule placed exactly in the middle of the load cell plate. Each capsule specimen was compressed by the linear actuator. Failure occurred under uniaxial compression at a controlled strain rate. The rupture strength of the capsule was then calculated from the maximum load recorded by the load cell. Results to date indicate that this system is able to measure capsule rupture forces with a high degree of precision. Derivatization of chitosan with GMA was found to significantly increase capsule rupture strength even without any photocrosslinking of the GMA groups. Additionally, rupture force pre-crosslinking was found to increase with the degree of GMA substitution. These observations suggest that hydrophobic interactions between non-crosslinked methacrylate groups may be playing a significant role in strengthening capsule membranes. Microcapsules formed with GMA-chitosan at a 5% degree of substitution ruptured at 1.72 g before crosslinking compared to 7.98 g after crosslinking. Interestingly, GMA-chitosan capsules with a 15% degree of substitution ruptured at 23.77 g pre-crosslinking, but showed reduced strength (12.06 g) after photocrosslinking. The testing system is currently being used to evaluate the detailed effects of degree of GMA substitution on the burst strength in order to optimize capsule membrane properties.