(5, 10, 11) The components commonly used to prepare hydrogel composites for biomaterial use include cellulose, hyaluronic acid, chitosan, polyethylene glycol, collagen, poly(acrylamide) (PAAm) and hydroxyapatite. (3, 7, 8) Hydrogel composites contain a second phase, (9) which are often nanoparticles, dispersed within the hydrogel with the aim of providing additional functionality and/or improved mechanical properties. Hydrog els containing particles (i.e., hydrogel composites) have attracted considerable attention in the biomaterials science literature (1-6) because of their potential for applications involving tissue scaffolds, soft tissue repair and drug delivery. The hydrogel composites may have potential application as enteric gels or for intra-articular drug delivery. The hydrogel composites showed dual action pH-triggered softening with simultaneous drug release which occurred without a volume increase. The composite gels had low cytotoxicity as evidenced by Live/Dead and MTT assays. CHPs containing a model drug were used to demonstrate pH-triggered release from PAAm/CHP and the release kinetics obeyed Fickian diffusion. The CHPs fragmented at pH values greater than the p K a of the particles, and this process decreased the gel modulus to values similar to that of the parent PAAm hydrogel. Inclusion of the CHPs within the gel composites increased the modulus in a tunable manner. These data showed the CHPs were dispersed throughout the PAAm network. The PAAm/CHP composite morphology was probed with optical microscopy, CLSM and SEM. The CHPs were constructed from poly(methyl methacrylate- co-methacrylic acid), which is a pH-responsive copolymer. The CHPs were prepared using a scalable (mainly) water-based method and had a bowl-like morphology that was comparable to that of red blood cells. In this study hydrogel composites are investigated that contain sacrificial pH-responsive collapsed hollow particles (CHPs) entrapped within a poly(acrylamide) (PAAm) network.
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