# Radiation-Assisted Synthesis of Polymer-Based Nanomaterials
## 4 Polymer-Based Nanomaterials
### Nanogels  
Polymeric nanogels are nanosized hydrogel particles prepared by physical or chemical crosslinking of hydrophilic or amphiphilic polymers which can be ionic or nonionic. The size of nanogels ranges from 20-200 nm. Nanogels can be prepared by one of these methods [37]: (1) physical self-assembly of interactive hydrophilic polymers, (2) polymerization of monomers in micro or nano environment, (3) crosslinking of preformed polymers or (4) template assisted fabrication of nanoparticles. Covalent crosslinking of preformed polymers exposed to ionizing radiation was first proposed by [PERSON] [38] and elaboratedby [PERSON] and [PERSON] in the years to follow in many publications that laid the foundations of radiation synthesis of nanogels [39]. This method provides unique opportunities, as the synthesized nanogels would be free from residual monomers, crosslinking agents or surfactants, thus eliminating tedious purification steps. By working with dilute polymer solutions, the formation of intermolecular crosslinks between polymer coils can be reduced in favor of intramolecular crosslinks within a single coil with resultant nanogels. Crosslinks are formed by combination of polymer-bound radicals generated either by the direct action of radiation or via the OH radical attacks abstracting hydrogen from polymer chains abundantly produced by the radiolysis of water. In order to avoid formation of intermolecular crosslinks, concentration of polymer chains must be kept at minimum, which results with the absorption of radiation by the water of the medium. OH radicals abundantly generated in water will attack polymer chains with possible reactions depicted schematically in Figure 2. By taking the necessary precautions, most of these reactions can be suppressed in favor of the desired one. In fact, non-radiation methods such as electrochemistry [40] and Fenton reactions [41] can also be used in the generation of OH radicals in aqueous environment to abstract hydrogen atoms from polymer backbones to initiate intramolecular crosslinking for the synthesis of nanogels. The conditions for the radiation synthesis of nanogels are fulfilled when working in very low polymer concentrations and using high dose rates [38]. The control of particle size of nanogels is of great importance as they affect the blood circulation time, viscosity and drug loading capacity. Particles with diameters ranging from 70-200 nm are known to show the maximum bioavailability [42]. The molecular weight and concentration of polymer and total dose and dose rate are the factors affecting size of nanogels. The prospect of radiation synthesis of nanogels from aqueous polymer solution was elaborated in a very recent review [43]. Current techniques used in designing nanogels with a future outlook was the focus of another review highlighting the upgrades in the methodologies, microfluidics and 3D printing [44]. In the following two sections, radiation formation of PVP and PAA nanogels is discussed, as these are the water-soluble polymers most frequently studied by different groups. * PVP nanogels The synthesis of PVP nanogels by radiation has been widely investigated as PVP is an FDA-approved polymer for medical applications. Formation of intramolecular crosslinks within a PVP coil by using ionizing radiation was first proposed by [PERSON] [38], who also developed the idea of radiation synthesis of PVP-based hydrogel wound dressings. A review published in 2014 collects results of the early studies on radiation synthesis of PVP nanogels [45]. During the past decade, the groups led by [PERSON] from Italy and [PERSON] from Egypt intensified their research on various aspects of radiation synthesis and applications of PVP nanogels. The formation of primary amino groups and carboxyl groups useful for bioconjugation were followed as a function of dose [46]. By working with both N\({}_{2}\)O and air-saturated conditions, the impact of oxygen with the consequences of H\({}_{2}\)O\({}_{2}\) formation on nanogel properties was investigated.