Mechanical Optimization of an Arteriovenous Malformation Embolization Material: A Predictive Model Analysis

In vitro characteristics of a gelling PEGDA-QT polymer system with model drug release for cerebral aneurysm embolization

A liquid-to-solid gelling polymer system, such as the poly(ethylene glycol) diacrylate-pentaerythritol tetrakis (3-mercaptopropionate) (PEGDA-QT) system, can fill cerebral aneurysms more completely than current embolization materials, reducing the likelihood of aneurysm recurrence. PEGDA-QT gels were formulated using PEGDA of different molecular weights (PEGDA575 and PEGDA700 ), and their characteristics were examined in vitro. Experiments examined gel time, mass change, crosslink integrity, cytotoxicity, and protein release capabilities. In general, PEGDA575 -QT gels were more hydrophobic, requiring an initiating solution with a higher pH (pH 9.5) to achieve a gel time comparable to PEGDA700 -QT gels, which used an initiating solution at pH 9.19. The mass change and crosslink integrity of gels were analyzed over time after gels were submerged in 150 mM phosphate buffered saline. After 380 days, PEGDA575 -QT gels achieved a maximum mass increase of 72% due to water uptake, while PEGDA700 -QT gels doubled their initial mass (100% increase) by 165 days. Compression tests showed that PEGDA700 -QT gels hydrolyzed more quickly than PEGDA575 -QT gels. Cytotoxicity assays showed that in general, PEGDA575 -QT negatively affected cell growth, while PEGDA700 -QT gels promoted cell viability. Sustained, controlled release of lysozyme, a 14.3 kDa protein, was achieved over an 8-week period when loaded into PEGDA700 -QT gels, but PEGDA575 -QT gels did not show sustained release. These studies show that although they are similar in composition, these PEGDA-QT gel formulations behave considerably differently. Although PEGDA700 -QT gels swell more and degrade faster than PEGDA575 -QT gels, their cytocompatibility and protein release characteristics may prove to be more beneficial for in vivo aneurysm treatment. © 2013 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater, 2013.

In vitro delivery, cytotoxicity, swelling, and degradation behavior of a liquid-to-solid gelling polymer system for cerebral aneurysm embolization

This study examines the in vitro characteristics of a crosslinking polymer system for cerebral aneurysm embolization. The polymeric material is composed of poly(propylene glycol)diacrylate (PPODA) and pentaerythritol tetrakis(3-mercaptopropionate) (QT), formulated with the liquid contrast agents Conray™ or Omnipaque™ 300. The PPODA-QT system was tested for delivery feasibility through mock delivery into a model aneurysm. Cytotoxicity was performed by exposing 3T3 cells to gel formulations, followed by a cell viability assay. Swelling was measured on samples submerged in 150 mM phosphate buffered saline at 37 or 50°C. The same samples underwent compression testing to assess degradation, characterized by reduction in Young's modulus over time. The PPODA-QT system was easily deliverable to mock aneurysms. Cytotoxicity results indicated that Conray-formulated gels are initially less toxic than Omnipaque-formulated gels, but show greater susceptibility to swelling and degradation over time. In general, these experiments represented more challenging conditions than would be present in vivo, and therefore, reported results are likely overestimations of in vivo outcomes. However, these results highlight potential issues with each PPODA-QT formulation. Given the desired outcome of neointimal tissue growth over the polymer material, initial cytotoxicity may be more important than long-term factors when choosing an optimal formulation for aneurysm embolization.

Controlling delivery properties of a waterborne,in-situ-forming biomaterial

This study details efforts to transition an in-situ-gelling polymer for endovascular embolization from the bench-top to preclinical cerebral arteriovenous malformation animal model studies. The in-situ-forming gel is based on waterborne, reverse emulsion materials. For controlled embolization of vascular defects, it is crucial to understand the delivery properties of an in-situ-forming gel. Directing a liquid into a small cavity requires both precise control of the fluid flow, and depends upon minimal variability in the materials behavior. A 2(3) factorial experiment performed in the laboratory revealed that temperature, mixing time, and buffer strength are all significant factors affecting the gelation time of the specific system studied. All three factors were also seen to reduce the standard deviation on the gel times. Changing the temperature from 21.3 to 37.0 degrees C reduced the cross population variability from 6.0 +/- 3.3 min to 3.4 +/- 1.6 min. At 30-s premixing, the protocol produced an average gel time of 5.3 +/- 3.0 min, which was reduced to 3.3 +/- 1.2 min with 90 s. Finally, a 50 mM buffer solution provided a gel time of 6.5 +/- 3.2 min, which was reduced to 2.95 +/- 0.6 min at 100 mM. Viscosity data was analyzed to suggest a model for injection volume and viscosity.

Gel strength and solution viscosity of temperature-sensitive, in-situ-gelling polymers for endovascular embolization

The goal of this work was to investigate the relationship of the gel strength and stiffness (at 37 degrees C) to solution viscosity (at 25 degrees C) in poly(N-isopropylacrylamide-co-acrylic acid) solutions with regard to acid content, molecular weight and solution concentration. It was hypothesized that the gel strength could be maximized while minimizing the increase in solution viscosity. If so, there would be motivation to investigate these materials for arteriovenous malformation embolization. The co-polymers were synthesized with 0-2 mol% content of acrylic acid (AAc) in benzene, dioxane, THF, 50:50 benzene/dioxane, or 50:50 dioxane/THF to obtain polymers of different molecular weight. The polymers were characterized for molecular weight by GPC/light scattering, for acrylic acid content by acid titration, for lower critical solution temperature by differential scanning calorimetry, and for solution viscosity (at 25 degrees C) and gel strength (at 37 degrees C) by rheometry. Solutions of lower-molecular-weight polymers were shown to have lower viscosities while possessing higher strengths as gels than the highest manageable concentrations of higher-molecular-weight polymers. This work demonstrates that the mechanical properties of poly(N-isopropylacrylamide-co-acrylic acid) can be increased while minimizing the increase in solution viscosity.