Finite Networks of Infinite Capabilities: Nanogels

Abstract

Hydrogels are three-dimensionally cross-linked polymeric networks of natural or synthetic origin swollen by the solvent (i.e. water) in which they are dissolved. The polymers exhibit high water absorbent capacities (over 90% weight of water in the composite). When the size of the hydrogel networks is in the range of nanometers, they are called nanogels. The term “nanogels” was introduced in 1999 by Vinogradov and co-workers [1, 2] to define the swollen chemically cross-linked networks of cationic and neutral polymers such as branched PEG-cl-PEI made from Polyethylenemine (PEI) and poly(ethylene glycol) (PEG), initially designed for the delivery of antisense oligonucleotides. However, Sunamoto and co-workers [3] six years before described the phenomenon of the self-assembly of cholesterol-modified polysaccharides, which resulted in the formation of swollen hydrogels of nanoscale size. Hydrogels, in general, and nanogels, in particular, are similar to living cells and are unique systems that are distinctly different from rigid nanoparticles, flexible macromolecules, micelles, vesicles and soft components. Living cells contain multiple compartmentalized organelles surrounded by membranes that perform distinct functions to maintain cell physiology. The construction of multi-compartmental systems to perform distinct biochemical reactions in one pot, as in living cellular systems, has attracted the attention of many research groups [4-8]. Compared to Pickering emulsions and functional polymeric micelles which even though opportunely manipulated to form distinguished spatial compartments to optimize incompatible tandem reactions [9, 10] present the challenge of bio-compatibilities, nanogels exhibit reliable mechanical stability and biocompatibility making them not only promising for the construction of multi-compartmental systems, but also widely applicable in the biomedical industry as discussed by Nita et al. [11] in their recent review entitled “Polymeric Nanogels with applicability in the biomedical field”. Compared to comprehensive and specific review articles in the same field [12-16], the review by Nita et al. [11] has the relevant characteristic of focusing on recent patents literature carefully divided according to their domain of applicability: drug delivery systems, inhibition of tumor cells for the release of chemotherapeutic compounds, vaccines, tissue engineering reconstruction, contact lens and contrast agents, imaging and theranostic applications (Fig. 1). Among these biomedical applications, the area of drug delivery