pH sensitive Laponite/alginate hybrid hydrogels: swelling behaviour and release mechanism

Stimuli-sensitive block copolymer hydrogels, which are reversible polymer networks formed by physical interactions and exhibit a sol-gel phase-transition in response to external stimuli, have great potential in biomedical and pharmaceutical applications, especially in site-specific controlled drug-delivery systems. The drug may be mixed with a polymer solution in vitro and the drug-loaded hydrogel can form in situ after the in vivo administration, such as injection; therefore, stimuli-sensitive block copolymer hydrogels have many advantages, such as simple drug formulation and administration procedures, no organic solvent, site-specificity, a sustained drug release behavior, less systemic toxicity and ability to deliver both hydrophilic and hydrophobic drugs. Among the stimuli in the biomedical applications, temperature and pH are the most popular physical and chemical stimuli, respectively. The temperature- and/or pH-sensitive block copolymer hydrogels for biomedical applications have been extensively developed in the past decade. This review focuses on recent development of the preparation and application for drug delivery of the block copolymer hydrogels that respond to temperature, pH or both stimuli, including poly(N-substituted acrylamide)-based block copolymers, poloxamers and their derivatives, poly(ethylene glycol)-polyester block copolymers, polyelectrolyte-based block copolymers and the polyelectrolyte-modified thermo-sensitive block copolymers. In addition, the hydrogels based on other stimuli-sensitive block copolymers are discussed.

Present and future applications of alginates are mainly linked to the most striking feature of the alginate molecule; i.e. a sol/gel transition in the presence of multivalent cations, e.g. Ca2+, almost independent on temperature. These very mild conditions, combined with the fact that alginates are highly characterised and understood both in the liquid and in the gel phase, makes this biopolymer unique compared to other gelling polysaccharides. Only pectins resemble alginate in the sol/gel transition behaviour, but this system can hardly be said to be as well characterised and understood as the alginates. The properties of alginate solutions and gels suggest biomedical and pharmaceutical uses. In this paper, the question of the specifications required by a polymer for applications in some biomedical areas will be discussed.