Bamboo-inspired tubular scaffolds with functional gradients

Abstract

Bamboo achieves its mechanical efficiency in bending and compression, meaning mechanical performance per unit mass, due to its hierarchical structure. As an orthotropic tube with a higher strength and stiffness parallel to the tube axis and with a density and property gradient across the tube wall, in which fiber bundles are embedded in a porous matrix, the bamboo culm is both stiffer and stronger in bending and less prone to ovalization and catastrophic failure than an orthotropic tube without property gradients would be. Few engineered materials exist that emulate bamboo's mechanical efficiency. The results of the study presented here demonstrate that freeze casting (ice templating) is a manufacturing process with which bamboo-inspired tubular scaffolds with property gradients across the tube wall can be custom-made. A highly aligned, honeycomb-like porosity is generated by ice crystal growth opposite to the direction of heat flow. Using a core-shell mold, the microstructure of the tube wall material, such as the pore size, geometry, and alignment, is defined by the mold materials' properties and applied cooling conditions. These also allow to custom-design the desired property gradient across the section. Further customization of the tube gradient structure and properties is possible through the deposition of additional layers on the freeze-cast scaffolds. Characterizing the pore structures of the tubes using X-ray microtomography, pore morphology and property gradients can be analyzed and correlated to both the processing conditions and the resulting mechanical properties determined in three-point bending, longitudinal and radial compression. The resulting fundamental structure-property-processing correlations support the custom design of tubular scaffolds that are ideally suited for applications that range from conduits for peripheral nerve repair to ureteral stents.