Soft nanocomposites: Nanoparticles to tune gel properties

Gel nanocomposites offer a simple yet powerful concept towards material design, with the combination of nanoparticles functionality (here: luminescence) with the 'soft' properties of a gel matrix (here: shape retention and mechanical support). The demand for new, soft materials with bespoke physical and biological characteristics and functionality has fuelled the research into nanocomposite hydrogels. 'Soft' nanocomposites - nanoparticles within a hydrated, polymeric gel matrix - offer a simple, yet versatile, platform for the design of materials with specific - and tunable - properties. Indeed, the 'soft' properties of the matrix can be combined with the inherent functionality of the nanoparticles (drug loading, antimicrobial, light refraction etc.) or give rise to altogether new characteristics (toughness, optical properties, self-healing etc.) evolved from the synergistic interaction of the polymer chains with the particles. In this review, we report the evolution and achievements of nanocomposite gels, with a focus on mechanisms and structure. The review is therefore structured around the properties resulting from the gel/nanoparticle association, rather than a classification based on applications or specific types of polymer or nanoparticles. How can nanoparticles tune mechanical, optical, biological properties or impart stimuli-responsiveness to a polymer gel matrix - and how is this behaviour linked to the underlying structure?