Nanocarriers providing spatiotemporal control of drug release contribute to reducing toxicity and improving therapeutic efficacy of a drug. On the other hand, nanocarriers face unique challenges in controlling drug release kinetics, due to the large surface area per volume ratio and the short diffusion distance. To develop nanocarriers with desirable release kinetics for target applications, it is important to understand the mechanisms by which a carrier retains and releases a drug, the effects of composition and morphology of the carrier on the drug release kinetics, and current techniques for preparation and modification of nanocarriers. This review provides an overview of drug release mechanisms and various nanocarriers with a specific emphasis on approaches to control the drug release kinetics.

BackgroundInjectable hydrogels have been extensively researched for the use as scaffolds or as carriers of therapeutic agents such as drugs, cells, proteins, and bioactive molecules in the treatment of diseases and cancers and the repair and regeneration of tissues. It is because they have the injectability with minimal invasiveness and usability for irregularly shaped sites, in addition to typical advantages of conventional hydrogels such as biocompatibility, permeability to oxygen and nutrient, properties similar to the characteristics of the native extracellular matrix, and porous structure allowing therapeutic agents to be loaded.Main bodyIn this article, recent studies of injectable hydrogel systems applicable for therapeutic agent delivery, disease/cancer therapy, and tissue engineering have reviewed in terms of the various factors physically and chemically contributing to sol-gel transition via which gels have been formed. The various factors are as follows: several different non-covalent interactions resulting in physical crosslinking (the electrostatic interactions (e.g., the ionic and hydrogen bonds), hydrophobic interactions, π-interactions, and van der Waals forces), in-situ chemical reactions inducing chemical crosslinking (the Diels Alder click reactions, Michael reactions, Schiff base reactions, or enzyme-or photo-mediated reactions), and external stimuli (temperatures, pHs, lights, electric/magnetic fields, ultrasounds, or biomolecular species (e.g., enzyme)). Finally, their applications with accompanying therapeutic agents and notable properties used were reviewed as well.ConclusionInjectable hydrogels, of which network morphology and properties could be tuned, have shown to control the load and release of therapeutic agents, consequently producing significant therapeutic efficacy. Accordingly, they are believed to be successful and promising biomaterials as scaffolds and carriers of therapeutic agents for disease and cancer therapy and tissue engineering.

We have used molecular modeling of both random and blocky hydrogel networks of poly (N-vinyl-2-pyrrolidone-co-2-hydroxyethyl methacrylate) with VP:HEMA=37:13 composition to investigate the effect of the monomeric sequence on the mechanical properties. The degrees of monomer sequence randomness for the random and the blocky copolymers were 1.170 and 0.104, respectively, and the degree of polymerization was set as 50. The equilibrated density of the dry gel network was 0.968+/-0.007 and 0.911+/-0.007 g/cm3 for the random and the blocky sequences, respectively. In the partially hydrated state with 10 wt % water content, the effect of the monomeric sequence causes more distinct differences in density of 1.004+/-0.007 and 0.916+/-0.009 g/cm3 for the random and the blocky copolymer network, respectively. We observed that in such networks, the water molecules are associated more closely with the N-vinyl-2-pyrrolidone than with the hydroxyethyl methacrylate moieties, which is consistent with results from quantum mechanical solvation free energy calculations. By simulating a compressive deformation of the dry gels up to 80% strain, we found that the random sequence network develops higher stress levels than the blocky network. We also found that stress reduction occurs in the random sequence network due to the hydration, which is not evident in the blocky sequence network. This difference in stress reduction between the random and the blocky sequence networks is due to the difference in the structural rearrangement of monomers in the presence of water during deformation. The random sequence network is able to undergo much more efficient rearrangement of HEMA units than in the blocky sequence network.

The magnetorheological (MR) elastomer as a hard and soft hybrid functional material, a composite material consisting of magnetic hard particles embedded in elastomeric soft matrix, is a branch of MR materials that are functional smart materials rapidly responding to external magnetic fields. These tunable properties of MR elastomers facilitate a variety of applications. In this brief review paper, in addition to general information on the MR elastomers, recent research not only on a wide variety of MR elastomeric systems focusing on various magnetic particles, elastomeric matrices, additives and particle modification methods, but also on their characteristics including MR properties from dynamic oscillation tests is covered along with their mechanical properties such as the Payne effect, tensile strength and engineering applications.