As a new strategy for improved urinary drainage in parallel to the potential for additional functions such as drug release and self-removal, highly porous chitosan stents are manufactured by radial, bi-directional freeze-casting. Inserting the porous stent in a silicone tube to emulate its placement in the ureter shows that it is shape conforming and remains safely positioned in place, also during flow tests, including those performed in a peristaltic pump. Cyclic compression tests on fullyhydrated porous stents reveal high stent resilience and close to full elastic recovery upon unloading. The drainage performance of the chitosan stent is evaluated, using effective viscosity in addition to volumetric flow and flux; the porous stent's performance is compared to that of the straight portion of a commercial 8 Fr double-J stent which possesses, in its otherwise solid tube wall, regularly spaced holes along its length. Both the porous and the 8 Fr stent show higher effective viscosities when tested in the silicone tube. The performance of the porous stent improves considerably more (47.5%) than that of the 8 Fr stent (30.6%) upon removal from the tube, illustrating the effectiveness of the radially aligned porosity for drainage. We conclude that the newly-developed porous chitosan ureteral stent merits further in vitro and in vivo assessment of its promise as an alternative and complement to currently available medical devices.

A novel freeze-cast porous chitosan conduit for peripheral nerve repair with highly-aligned, double layered porosity, which provides the ideal mechanical and chemical properties was designed, manufactured, and assessed in vivo. Efficacies of the conduit and the control inverted nerve autograft were evaluated in bridging 10-mm Lewis rat sciatic nerve gap at 12 weeks post-implantation. Biocompatibility and regenerative efficacy of the porous chitosan conduit were evaluated through the histomorphometric analysis of longitudinal and transverse sections. The porous chitosan conduit was found to have promising regenerative characteristics, promoting the desired neovascularization, and axonal ingrowth and alignment through a combination of structural, mechanical and chemical cues.

Few systematic structure-property-processing correlations for directionally freeze-cast biopolymer scaffolds are reported. Such correlations are critical to enable scaffold design with attractive structural and mechanical cues in vivo. This study focuses on freeze-cast collagen scaffolds with three different applied cooling rates (10, 1, and 0.1°C/min) and two freezing directions (longitudinal and radial). A semi-automated approach for structural characterization of fully hydrated scaffolds by confocal microscopy is developed to facilitate an objective quantification and comparison of structural features. Additionally, scanning electron microscopy, and compression testing are performed longitudinally and transversely. Structural and mechanical properties are determined on dry and fully hydrated scaffolds. Longitudinally-frozen scaffold have aligned and regular pores while those in radially-frozen ones exhibit greater variations in pore geometry and alignment. Lamellar spacing, pore area, and cell wall thickness increase with decreasing cooling rate: in longitudinally-frozen scaffolds from 25 μm to 83.5 μm, 814 μm 2 to 8,452 μm 2 , and 4.21 μm to 10.4 μm, and in radially-frozen ones, from 69 μm to 116 μm, 7,679 μm 2 to 25,670 μm 2 , and 6.18 μm to 13.6 μm, respectively. Both longitudinally-and radially-frozen scaffolds possess higher mechanical property values, when loaded parallel rather than perpendicular to the ice-crystal growth direction. Modulus and yield strength range from 779-4,700 kPa and 38-137 kPa, respectively, as a function of cooling rate and freezing direction. Collated, the correlations obtained in this study enable the custom-design of freeze-cast collagen scaffolds, which are ideally suited for a large variety of tissue regeneration applications.

Despite considerable recent interest in microand nanofibrillated cellulose as constituents of lightweight structures and scaffolds for applications that range from thermal insulation to filtration, few systematic studies have been reported to date on structure−property-processing correlations in freeze-cast chitosan−nanocellulose composite scaffolds, in general, and their application in tissue regeneration, in particular. Reported in this study are the effects of the addition of plant-derived nanocellulose fibrils (CNF), crystals (CNCs), or a blend of the two (CNB) to the biopolymer chitosan on the structure and properties of the resulting composites. Chitosan−nanocellulose composite scaffolds were freeze-cast at 10 and 1 °C/min, and their microstructures were quantified in both the dry and fully hydrated states using scanning electron and confocal microscopy, respectively. The modulus, yield strength, and toughness (work to 60% strain) were determined in compression parallel and the modulus also perpendicular to the freezing direction to quantify anisotropy. Observed were the preferential alignments of CNCs and/or fibrils parallel to the freezing direction. Additionally, observed was the self-assembly of the nanocellulose into microstruts and microbridges between adjacent cell walls (lamellae), features that affected the mechanical properties of the scaffolds. When freeze-cast at 1 °C/min, chitosan−CNF scaffolds had the highest modulus, yield strength, toughness, and smallest anisotropy ratio, followed by chitosan and the composites made with the nanocellulose blend, and that with crystalline cellulose. These results illustrate that the nanocellulose additions homogenize the mechanical properties of the scaffold through cell-wall material self-assembly, on the one hand, and add architectural features such as bridges and pillars, on the other. The latter transfer loads and enable the scaffolds to resist deformation also perpendicular to the freezing direction. The observed property profile and the materials' proven biocompatibility highlight the promise of chitosan−nanocellulose composites for a large range of applications, including those for biomedical implants and devices.

Tapered nanopillars with various cross sections, including cone-shaped, stepwise, and pencil-like structures (300 nm in diameter at the base of the pillars and 1.1 μm in height), are prepared from epoxy resin templated by nanoporous anodic aluminum oxide (AAO) membranes. The effect of pillar geometry on the shear adhesion behavior of these nanopillar arrays is investigated via sliding experiments in a nanoindentation system. In a previous study of arrays with the same geometry, it was shown that cone-shaped nanopillars exhibit the highest adhesion under normal loading while stepwise and pencil-like nanopillars exhibit lower normal adhesion strength due to significant deformation of the pillars that occurs with increasing indentation depth. Contrary to the previous studies, here, we show that pencil-like nanopillars exhibit the highest shear adhesion strength at all indentation depths among three types of nanopillar arrays and that the shear adhesion increases with greater indentation depth due to the higher bending stiffness and closer packing of the pencil-like nanopillar array. Finite element simulations are used to elucidate the deformation of the pillars during the sliding experiments and agree with the nanoindentation-based sliding measurements. The experiments and finite element simulations together demonstrate that the shape of the nanopillars plays a key role in shear adhesion and that the mechanism is quite different from that of adhesion under normal loading.