# Radiation-Assisted Synthesis of Polymer-Based Nanomaterials
## 6 Controlling Radiation-Induced Grafting on Nanoscale
### Cell Sheet Harvesting  
Polymeric nanostructures find a wide range of applications in biomedicine. Drug delivery, gene carriers, bioimaging and tissue engineering and regenerative medicine are the applications with the most pronounced effects [80]. The important role of \"nanogels\" was already discussed in terms of their use as drug delivery platforms and gene carriers in previous sections. Recent developments on the synthesis of Quantum Dots clearly showed their potential in better bioimaging. In this section, how radiation-induced grafting in nanoscale contributes to the development of thermoresponsive cell culture surfaces is discussed.  
At 37 \({}^{\circ}\)C, poly(_N_-isopropyl acrylamide) (PNiPAAm) grafted cell culture surface typically made of polystyrene is hydrophobic and allows cells to adhere and proliferate on modified surface. The lower critical solution temperature of PNiPAAm is around 32 \({}^{\circ}\)C which makes it thermoresponsive and it becomes hydrophilic below this temperature. Therefore, at temperatures below 32 \({}^{\circ}\)C, cells grown on the surface detach themselves from hydrophilic surface spontaneously. Based on this approach, cell sheet engineering technology was developed by [PERSON] and his team and has become an established method of harvesting cell sheets noninvasively [81; 82].  
The thickness of the grafted PNiPAAm layer, which should be controlled within the 100 nm range, is one of the key parameters affecting hydration/dehydration phenomena, hence, cell sheet recovery [83]. This is where nanostructuring of polymers comes into play. A rapid cell sheet recovery is essential for clinical applications. Slow hydration of the PNiPAAm layer is the bottleneck of this process. In order to accelerate cell sheet recovery, several solutions were attempted such as using porous PS substrate and the incorporation of PEG into the PNiPAAm matrix. A better solution was found by first grafting a hydrophilic monomer, acrylamide, as the basal plane on PS then continued with grafting of N-isopropylacrylamide as the top layer to accelerate hydration PNiPAAm in the graft layer [84]. Thus, a double polymeric layer was formed like an interpenetrating network structure. The solution we proposed to this problem, however, is more systematic. First, a very thin layer of PAAm was grafted from the e-beam-irradiated PS surface via RAFT mechanism. Reactive RAFT moieties at PAAm chain ends were used to extend these chains by PNiPAAm chains growing to desired lengths, as shown schematically in Figure 3. Ellipsometric measurements revealed that after constructing a PAAm layer of \(\sim\)58 nm, the thickness of top PNiPAAm layer could be controlled in 23-90 nm range as a function of duration of grafting [85]. Water contact angle measurements made below and above the lower critical solution temperature of PNiPAAm, namely 32 \({}^{\circ}\)C, revealed a perfect control of hydrophobic/hydrophilic balance in the graft layer, which enhanced the rate of hydration of PNiPAAm.  
It has been elaborated that PNiPAAm-based thermoresponsive surfaces provide new types of biomedical applications. These nanomanufactured surfaces have been developed for separating and purifying pharmaceutical proteins and therapeutic cells. Successful biomedical applications of PNiPAAm-modified surfaces were discussed in a recent review [86]. Thermoresponsive chromatography has been considered as a new separation and analysis tool.  
Figure 3: Modification of PS cell culture surface by consecutive grafting of PAAm and PNiPAAm via RAFT mechanism. Reprinted with permission from Ref. [85]. Copyright 2021 Elsevier. (i) E-beam irradiation of PS dish surface; (ii) Acrylamide grafting from the surface via RAFT agent CMDT; (iii) Chain extension by copolymerization with PNiPAAm.