# Polymers and Polymer Nanocomposites for Cancer Therapy +
## Polymer Hydrogels and Nanogels
Nanogels possess the features of hydrogels and nanoparticles. They have attracted much attention in the last two decades due to the combined properties of nanotechnology and crosslinking gel-like materials. They have shown great biocompatibility and biodegradability in both in vitro and in vivo studies and have been widely used in biomedical science in the areas of imaging, diagnosis, drug delivery, tissue engineering, biosensors, etc. Their sizes, shapes, charges, and surface chemistries can be easily tuned, which offers nanogels many advantages as nanocarriers in the delivery of small molecules, proteins, genes, and other therapies [102]. Nanogels are three-dimensional network nanostructures composed of functional polymers. Their size is in the range of 10-1000 nm. They present numerous advantages, such as water solubility, high water content, and, as already mentioned, biocompatibility [103]. Due to their porosity, nanogels are able to encapsulate drugs and release them in a controlled manner. Internal and external factors such as pH, magnetic field, temperature, enzyme, and light can stimulate the response of nanogels and are useful in the controlled release of bioactive agents [104]. One article has reviewed related subjects such as nanogel assimilation in nanomedicine, features and their advantages, and recent progress in nanogels as drug carriers [105]. Nanogels can also be defined as highly crosslinked nanosized hydrogels ranging from 20 to 200 nm. Nanogels have been developed to deliver drugs to treat a range of diseases, including cancer, autoimmune disorders, and neurodegenerative conditions. They can be administered through various routes, including oral, pulmonary, nasal, parenteral, and intraocular routes [106]. Copolymers containing PEG, maleimide, and pendant hydroxyl groups as side chains are often used to produce nanogels [107]. The shift from mono- to dual- and then to multistimuli-responsive polymers, including hydro- and nanogels, has increased confidence in their ability to deliver drugs intracellularly while overcoming the problems associated with mono- and dual-responsive products. The sensitivity of these macromolecules to multiple stimuli makes them quite versatile and increases their applicability [108]. Polymer nanogels are able to efficiently encapsulate therapeutics of diverse sizes and natures through simple mechanisms. One review analyzed the use of extracellular pH-targeting for nanogel nSiRNA delivery and PEGylated nanogels for improved cancer therapies [109]. Semi-interpenetrating nanohybrid hydrogels made of poly(N-isopropylacrylamide)/CS/MWCNTs were prepared. They showed temperature and pH dual-responsiveness, and a hydrogel containing MWCNTs-COOH possessed high swelling ratios. The researchers considered that these designed nanogels could be used for the size-specific target delivery of anticancer drugs [110]. A positively charged nanogel was synthesized to inhibit the proliferation of breast cancer cell lines. A model drug, 10-hydroxycampotthecin (HCPT), was trapped in the core via facile diffusion to form CS/HCPT. Its characteristics were evaluated by assessing particle size, drug loading quantity, and loading efficiency. Furthermore, the cell internalization, cytotoxicity, and apoptosis of CS/HCPT were also investigated in vitro. The research indicated that the positively charged CS-based nanogels could be used as a drug delivery agent [111]. A thermo/pH/magnetic triple-sensitive nanogel based on poly(N-isopropyl acrylamide), poly(N-dimethylaminoethyl methacrylate), and magnetic graphene oxide was synthesized. In vitro cytotoxicity studies showed that the obtained nanogel had low cytotoxicity and good biocompatibility and could reduce the cytotoxicity of DOX [112]. An approach was presented for the synthesis of thermoresponsive and pH-sensitive magnetic nanogels by using the biodegradable starch maleate as a crosslinker and a magnetic nanoparticle stabilizer. Chemically modified starch maleate produced highly stable Fe\({}_{3}\)O\({}_{4}\) magnetic nanoparticle aqueous dispersion.