Engineering a Reverse Thermo-Responsive Injectable Liquid Embolic Agent for Brain Aneurysm Treatment

Current treatments for brain aneurysms, such as metallic coiling, suffer from drawbacks including mass effect symptoms and coil compaction. A new method uses F88 Pluronic polymer Functionalized with dimethacrylate groups (F88DMA), the polymer crosslinks, which forms stable soft coil structures to embolize aneurysms, eventually degrading along with the dilated vessel. This project aimed to engineer an injectable liquid embolic agent (LEA) from F88DMA with the appropriate mechanical, chemical, and biological properties for aneurysm treatment.<br/>To determine the fundamental properties of the uncrosslinked gels, rheological analysis was conducted. At approximately 24.6°C, viscosity increase indicated micelle formation and differential scanning calorimetry confirmed this micelle formation at the same temperature. A negative relationship between concentration of F88DMA and average micellization temperature was established.<br/>Crosslinking with 1% ammonium persulfate (APS) and 3.5% tetramethylethylenediamine (TEMED) was performed to analyze the crosslinked gel mechanics. Rheological analysis revealed a linear correlation, with an R2=0.94 where lower concentration of F88DMA led to longer crosslinking times after adding APS and TEMED. Therefore, linear regression can be used to guide selection of F88DMA concentration based on desired crosslinking time.<br/>Swelling experiments were conducted on crosslinked F88DMA samples to investigate the effects of concentration, Iohexol addition, static vs. flow conditions, and crosslinking temperatures. Mass swelling ratios were determined, and rheology was performed to correlate shear modulus with time. Initially, all gels swelled rapidly for three hours, then contracted over several days. Higher F88DMA concentrations led to greater swelling, while higher Iohexol concentrations reduced swelling. A combination of high Iohexol and low F88DMA caused the gels to contract over time. Gels exposed to flowing saline (simulating blood flow) and those crosslinked below the micellization temperature showed reduced swelling. A reduced level of swelling would enhance safety by minimizing the risk of aneurysm rupture.<br/>To mimic cell adhesion in blood vessels, F88DMA hydrogels were immersed in a fibrinogen solution before plating human umbilical vascular endothelial cells, which were then observed over several days. Cell adhesion and proliferation were quantitatively measured using Alamar Blue assay, while EVOS microscopy provided detailed imaging. The fibrinogen-treated hydrogels exhibited significantly more properly-shaped, adherent cells and supported higher proliferation, with an average proliferation ratio of 4.54 on day 7, compared to 1.16 for the untreated gels. These results suggest that fibrinogen enhances cell compatibility and proliferation on polymer gels, making them promising for aneurysm treatment.<br/>A stable procedure was developed to test the injectability of the polymer by attaching a syringe to a catheter, and injecting it into a model aneurysm. The F88DMA solution crosslinked through the catheter and formed a strand-like gel when exiting. The impact of Iohexol on F88DMA in the catheter was recorded as well. Blood flow was tested using water in the model to depict how the gel would act over time under the realistic setting of a blood vessel. The results showed consistent gel strands inside the aneurysm, indicating a successful procedure and suggesting that this method may be an effective and sustainable treatment for brain aneurysms. Future work will involve research on the long-term sustainability of the polymer coils in vivo.<br/>We gratefully acknowledge funding from the Louis Morin Charitable Trust.