# Polymers and Polymer Nanocomposites for Cancer Therapy +
## Polymer Hydrogels and Nanogels  
The hydrophilic functional groups of polymer 3D networks become hydrated in aqueous media or in biological fluids, forming the hydrogels. Extensive application of these products in industrial and environmental areas is considered to be of prime importance. Hydrogels have specific characteristics that make them applicable in some areas of medicine, such as sensors, tissue repair, drug carriers, and many others. Their main limitations are poor mechanical properties [86]. Hydrogels can be classified based on source (natural or synthetic), type of polymer (homopolymer, copolymer, multipolymer), morphology (amorphous, semicrystalline, crystalline), type of crosslinking, physical appearance, and electrical charge [87]. Hydrogels can be synthesized with or without crosslinking components. There are different techniques to obtain hydrogels without crosslinking agents, such as the following:Through H bonds (example: PAA with PEG);  
- From an amphiphilic graft and a block copolymer;  
- With PLA and PEG;  
- With poly(butylene terephthalate) and PEG;  
- With hydrophobized polysaccharides and others [88]. Hydrogels are thermodynamically compatible with water, which allows them to swell. They are widely used in the pharmaceutical and medical fields, such as in biosensors, materials for contact lenses, synthetic skin, and lining for hearts. They can be used for 3D cell cultures and as drug delivery carriers. Hydrogels are efficient candidates for controlled release, bioadhesives, and/or targeted drug delivery, as they are able to encapsulate biomacromolecules, including proteins, DNA, and hydrophilic and hydrophobic drugs. Hydrogel-based platforms can be used in different ways for oral, rectal, ocular, epidermal, and subcutaneous applications [89]. Some hydrogels are formed by smart polymers, which are able to adjust their properties in response to factors such as temperature, pH, and ionic strength. Among the smart polymers, thermoresponsive poly(N-isopropylacrylamide) is very important because of its well-defined structure and its temperature response, which is close to human body temperature: it can also be fine-tuned [90]. Hydrogels based on biopolymers (polysaccharides, proteins) are of great interest in drug delivery and tissue engineering. Cellulose, CS, and collagen are probably the most important, since they are the most abundant on earth and in the human body (collagen). Cellulose derivatives such as methylcellulose are ideal for the preparation of thermoresponsive hydrogels. The cationic character of CS favors the formation of gels through electrostatic interactions with sodium sulfate as a precipitant. Various hydrogels can be obtained from collagen, gelatin, and peptides [91]. The increase of the stability of CS in solution is very important for its application. This problem can be solved through hydrogelation. A CS hydrogel was readily prepared through the addition of NaOH solution to a CS acetic acid solution. In a wet state, the CS hydrogel was stable and could maintain its MW for long periods of time at room temperature. The CS hydrogel could be readily dissolved with the addition of organic acids [92]. The efficiency of an Na/CS/HAP hydrogel as a drug carrier with DOX for liver cancer was studied. DOX release, the effect of its concentration, as well as the amount of HAP in the drug release were investigated. The results showed that the drug release was pH-sensitive and that the best release was at pH 5 [93]. CS gel has fragility in some of its applications, and therefore it needs modification. One physicochemical change has been done by crosslinking sodium tripolyphosphate. In order to improve CS-based hydrogel properties, CS derivatives have been synthetized. The functional amino groups on CS macromolecules helped the polymer to participate in chemical reactions, which led to derivatives with improved characteristics such as mucoadhesion, high drug loading, and the ability to transfer genes.