Novel nanocomposites of gold nanoparticle and poly(4-vinylpyridine) (Au@PVP) were fabricated through surface-initiated atom-transfer radical polymerization (SI-ATRP) at ambient conditions. The citratestabilized gold nanoparticles were first modified by a disulfide initiator for ATRP initiation, and the following polymerization of 4-vinylpyridine (4VP) occurred on the surface of gold particles. The assembled Au@PVP nanocomposites are pH-responsive because of the pyridyl groups, which are facially protonated and positively charged. At low pH (<3.2), the polymer chains attached on gold nanoparticles are expanded by electrostatic forces, and the polymer layer is loosely swelled; hence, the Au@PVP composite particle displays a comatulid-like nanostructure in 3-D AFM images. However, at a relatively high pH (>3.2), the polymer chains shrink and wrap around the gold particle surface, which results in the aggregation of gold nanoparticles with a thin shrunken polymer layer under TEM observation. Such assembled Au@PVP nanocomposites as a smart supporter can entrap transition metal ions by their efficient coordinating segments, and subsequently, the metal ions can be reduced in situ to construct novel bimetallic nanocomposites, which are regarded as intelligent catalysts with environmental stimuli activity.

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Poly(N-isopropylacrylamide) (PNIPAM)-based thermosensitive hydrogels demonstrate great potential in biomedical applications. However, they have inherent drawbacks such as low mechanical strength, limited drug loading capacity and low biodegradability. Formulating PNIPAM with other functional components to form composited hydrogels is an effective strategy to make up for these deficiencies, which can greatly benefit their practical applications. This review seeks to provide a comprehensive observation about the PNIPAM-based composite hydrogels for biomedical applications so as to guide related research. It covers the general principles from the materials choice to the hybridization strategies as well as the performance improvement by focusing on several application areas including drug delivery, tissue engineering and wound dressing. The most effective strategies include incorporation of functional inorganic nanoparticles or self-assembled structures to give composite hydrogels and linking PNIPAM with other polymer blocks of unique properties to produce copolymeric hydrogels, which can improve the properties of the hydrogels by enhancing the mechanical strength, giving higher biocompatibility and biodegradability, introducing multi-stimuli responsibility, enabling higher drug loading capacity as well as controlled release. These aspects will be of great help for promoting the development of PNIPAM-based composite materials for biomedical applications.