Abstract
Nanogels—internally crosslinked macromolecules—have a growing palette of potential applications, including as drug, gene or radioisotope nanocarriers and as in vivo signaling molecules in modern diagnostics and therapy. This has triggered considerable interest in developing new methods for their synthesis. The procedure based on intramolecular crosslinking of polymer radicals generated by pulses of ionizing radiation has many advantages. The substrates needed are usually simple biocompatible polymers and water. This eliminates the use of monomers, chemical crosslinking agents, initiators, surfactants, etc., thus limiting potential problems with the biocompatibility of products. This review summarizes the basics of this method, providing background information on relevant aspects of polymer solution thermodynamics, radiolysis of aqueous solutions, generation and reactions of polymer radicals, and the non-trivial kinetics and mechanism of crosslinking, focusing on the main factors influencing the outcomes of the radiation synthesis of nanogels: molecular weight of the starting polymer, its concentration, irradiation mode, absorbed dose of ionizing radiation and temperature. The most important techniques used to perform the synthesis, to study the kinetics and mechanism of the involved reactions, and to assess the physicochemical properties of the formed nanogels are presented. Two select important cases, the synthesis of nanogels based on polyvinylpyrrolidone (PVP) and/or poly(acrylic acid) (PAA), are discussed in more detail. Examples of recent application studies on radiation-synthesized PVP and PAA nanogels in transporting drugs across the blood–brain barrier and as targeted radioisotope carriers in nanoradiotherapy are briefly described.
Highlights
The typical half-time observed in our pulse radiolysis studies is perhaps a tenth of this value. These results indicate that intramolecular second order kinetics is appropriate for the analysis of the pulse radiolysis measurements
The most characteristic feature of this process is a decrease in the macromolecule’s radius of gyration (Rg ) even when the average molecular weight may remain consistent or increase
Since both Rg and weight-average molecular weight can be determined simultaneously by the multi-angle static laser light scattering technique, MALLS, and Mw is determined by this technique in an absolute way, MALLS can be considered the method of choice for tracking nanogel formation
Summary
Nanogels (NGs) consist of internally crosslinked macromolecules with solvent filling the spaces between the polymer segments. NGs have gathered significant interest over the past decade for their applications as nanocarriers for drug delivery, tissue engineering, biomedical implants, and gene therapy. Due to their good biocompatibility, high stability, hydrophilic and drug loading properties, nanogels can make ideal nanocarriers. The hydrophilic nature of some NGs is due to the presence of polar groups along their polymer chains (-OH, -COOH, -CONH2 , etc.). The properties of nanogels can be controlled by varying the type and concentration of polymer used in synthesis [1].
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