Introduction: Injectable, adhesive hydrogels are attractive materials for drug delivery applications due to good tissue integration, biocompatibility, and tunable degradation/release. Adhesive hydrogels based on oxidized polysaccharides and polyamidoamine (PAMAM) dendrimers offer high tissue-specific interaction[1] but require further modification for use in challenging loading environments like articular cartilage. Additionally, precursor solutions readily flow under gravity or into surrounding fluids during injection. To allow implantation in the wet arthroscopic environment, we have exfoliated aluminosilicate nanoplatelets (NPs) to create a nanocomposite hydrogel with tunable rheology and mechanics.
Materials and Methods: Aluminosilicate NPs were used: one with ~20 nm lateral dimension to control solution rheology, and one with ~200 nm lateral dimension to control gel mechanics. NPs were dispersed in macromer solutions of dextran aldehyde and PAMAM; dispersion was characterized by low angle x-ray diffraction (XRD) and cryo-TEM. Hydrogels were polymerized by mixing the two solutions at varying polymer and NP concentrations. Mechanical properties were evaluated by static compression to high strains (50+%). Rheology of precursor solutions was evaluated to determine yield stress and shear thinning properties. Cytocompatibility was evaluated through elution and direct contact assays with C3H10T1/2 cells by live/dead and Alamar Blue metabolic assay. Cartilage defects in bovine knees were filled ex vivo to optimize handling.
Results and Discussion: NPs were successfully dispersed into the polymer precursor solutions, with 20 nm NPs dispersing best in oxidized dextran and 200 nm NPs dispersing best in PAMAM dendrimer (Fig1A-B). Better dispersion was observed by XRD and TEM for higher polymer and lower NP concentrations, suggesting a strong effect of polymer-platelet interactions in driving dispersion. Mechanical testing showed a strong dose response of increasing compressive modulus with increasing loading of 200 nm NPs, but >7% filler resulted in unchanged or inferior properties (Fig1C, * p<0.05). This suggests that good dispersion provides an upper limit on mechanical improvements, which matches well with theoretical limits in the literature[2]. Comparisons to 200 nm particulate controls of hydroxyapatite did not show as large improvements in mechanics, underlining the importance of filler shape and polymer-filler interactions. Rheological measurements showed that addition of 20 nm NPs up to 6% resulted in dramatic increases in yield stress, turning a low viscosity polymer solution into a Bingham fluid with shear thinning properties (Fig1D, significance between 20 nm NPs, 200 nm NPs, and nanoparticulate controls). Biocompatibility testing indicated the material was well-tolerated by C3H10T1/2 cells (Fig1E). Pilot ex vivo testing in bovine cartilage defects demonstrated optimal handling conditions.
Conclusions: Enhancing adhesive hydrogels with exfoliated nanoplatelet fillers dramatically improves mechanics and rheology in a manner dependent on dispersion – a process driven by processing conditions and polymer-filler interactions. The effects of the fillers were dependent on NP concentration and aspect ratio. The material is cytocompatible, injectable, and adhesive, and represents a promising material for drug delivery and tissue engineering in the arthroscopic environment.
References:
[1] Artzi N, Shazly T, Baker AB, Bon A, Edelman ER. Aldehyde-Amine Chemistry Enables Modulated Biosealants with Tissue-Specific Adhesion. Advanced materials (Deerfield Beach, Fla). 2009;21(0):3399-3403. doi:10.1002/adma.200900340.
[2] Okada, Akane, and Arimitsu Usuki. "Twenty Years of Polymer‐Clay Nanocomposites." Macromolecular Materials and Engineering 291.12 (2006): 1449-1476.