Characterizing Mechanical and Cell Proliferation Properties of Poly(vinyl Alcohol) and Resorcinol Diphenyl Phosphate Clay Blend Towards Brain Aneurysm Modeling

It has been shown that physicians have found it very beneficial in simulating complex surgeries on 3-D printed model analogues before performing the actual procedure [1]. 3-D modeling provides far more detail and allows for dynamic modeling of fluid flow through venous structures. Brain aneurysms are an excellent example where this approach has been advantageous due to its complex 3-D branching and anomalous cerebral spinal fluid circulation. Synthesizing these structures directly from micro-CT scans and integrating them into a realistic flow system can offer significant advantages. Furthermore, if trials of medication or materials for obduration are being developed, in vivo experiments are required. Replicating human-like aneurysms in animal models presents significant challenges, making the incorporation of printed aneurysm models into animal analogues a more desirable approach. The success of these models depends on its ability to stimulate the nature of surrounding blood vessels, with Poly(vinyl alcohol) (PVA) hydrogels emerging as a potential material.<br/><br/>In this study, we present PVA and PVA-methacrylate (MA) hydrogels formulations that simulate the mechanical properties found in native vasculature and where the mechanical properties could be controlled via different cross linking mechanisms. We showed that PVA’s lack of cell adhesiveness could be addressed with the addition of Resorcinol Diphenyl Phosphate (RDP) coated clays, and we experimented with different strategies to enable formation of complex vessel geometries.<br/><br/>Cell adhesion and proliferation of Sodium Hydroxide (NaOH) and Sodium Trimetaphosphate (STMP) crosslinked PVA hydrogels were tested, where 0%, 1% and 5% RDP clay were blended into a 10% PVA hydrogel, plated with HUVEC-EGFP at a density of 8000 cells/cm2 and conducted Alamar blue assays on days, 1, 3, and 5. Results indicate a lack of cell adhesion in pure 10% PVA hydrogels; but increased cell adhesion and proliferation was observed with increasing concentration of RDP clay.<br/><br/>To assess its mechanical properties, PVA hydrogel sheets were formed, cut into strips and disks, placed and measured on an Instron Universal Mechanical Tester and Kinexus Pro + Rheometer (Netzsch). For the 0%, 1%, and 5% RDP concentrations no statistically significant difference for both Young’s Modulus (~19kPa) and Shear Modulus (~55kPa) was observed.<br/><br/>In order to synthesize the vascular grafts, two processes were developed. In the first process, 20% PVA-MA was radically crosslinked with Ammonium Persulfate (APS) and Tetramethylethylenediamine (TEMED), inserted into the vessel mold from the bottom up.<br/><br/>The second process involved dip-coating and spinning dry 10% NaOH/STMP crosslinked PVA on a single axis spinner. A systematic study of spin rate for the dip coating of the PVA was conducted between 49.5 rpm and 68 rpm, where 60 rpm proved optimal in uniformity and human-like mechanics. A model was proposed where the heightened drying speed of the PVA solution surrounding the wax rod decreased fluid motion thereby increasing uniformity.<br/><br/>The vessels were imaged under a Keyence VHX-7000 microscope, where we obtained mean diameters for composites with 10% PVA and RDP close to the desired value of 2000 µm. The burst pressures though were dependent on RDP content, decreasing below physiological values at a loading of 5% RDP clay. Vascular grafts created with 20% PVA-MA were more uniform and had a shorter production time, but 10% PVA vascular grafts exhibit stronger mechanical properties. Currently, we are examining the implementation of dual-crosslinking, integrating both methodologies to maximize mechanical properties and production efficiency.<br/><br/>The cell adhesion and proliferation properties of the PVA/RDP clay blends along with our novel synthesization methods demonstrate great promise towards eventually modeling complex brain aneurysms.<br/><br/>This research was supported by the Louis Morin Charitable Trust.<br/><br/>[1] Lan, Q, et al. (2016): 434-42.