9 - POSS-based stimuli-responsive nanocomposite hydrogels for biomedical applications

Abstract

Hydrogels are a class of smart materials consisting of hydrophilic molecules arranged in a three-dimensional network with permanent or transient junction points. The hydrophilic nature of the hydrogels enables absorption/elution of water or aqueous solutions. Therefore hydrogels are finding applications in drug delivery systems, scaffolds, wound dressing, vision care, super absorbents, contact lenses and intraocular lenses, and filtration/separation devices. This chapter discusses multifunctional, smart hydrogels, based on the macromer polyhedral oligomeric silsesquioxanes (POSS). Permanent molecular networks are formed by crosslinking reaction, using UV radiation or temperature with appropriate initiators, forming covalent bonds. Physical hydrogels consist of unique intermolecular POSS associations forming transient crosslinking sites, and these vary from van der Waals interactions to supramolecular assemblies. Hence, the chemical hydrogels have a permanent structure whereas the physical hydrogels are quite versatile as the “crosslinking” sites in the 3D networks are not permanent. A series of nanocomposite hydrogels based on poly-N-isopropyl acrylamide (PNIPAm), polyethylene glycol (PEG), and the macromer POSS will be discussed. The physical hydrogels of PNIPAm–POSS exhibit self-healing, liquid-like behavior under large amplitude shear and rubber-like behavior immediately after cessation of shear. Furthermore, the physical hydrogels can exhibit thermally activated two-way shape memory (SM) behavior, and the lower-critical solution temperature (LCST) being the only stimulus. The SM process involved swelling-deswelling effectively opening opportunities for drug delivery. Finally, POSS modulated the shear elastic modulus G′ and LCST; therefore the PNIPAm–POSS copolymers open opportunities for reusable, thermo-formable SM hydrogels with tuned mechanical modulus and activation temperature. Chemically crosslinked PEG–POSS networks also exhibit SM behavior and are therefore suitable for biomedical smart devices, where the crystalline nature of PEG plays a predominant role in the SM properties whereas POSS tunes the mechanical properties, controls the degree of swelling, and enhances the antibacterial behavior of PEG hydrogels.