# State-of-All-the-Art and Prospective Hydrogel-Based Transdermal Drug Delivery Systems
## 3 Fundamentals of Hydrogels
### Types of Hydrogels
#### 3.2.1 Classification Based on Source Origin  
Hydrogels can consist of natural or synthetic materials, depending on their composition. Hydrogels of natural origin are derived from non-synthetic sources, making them inherently biocompatible and bioactive [29]. Synthetic hydrogels are fabricated through chemical polymerization methods, offering enhanced durability, strength, and water absorption capacity when greater robustness is desired [29]. Hydrogels of natural origin are primarily sourced from materials such as cellulose, chitosan, hyaluronic acid, gelatin, and others [31]. They have high biocompatibility and biodegradability, low immunotoxicity, and good cytocompatibility. However, they have poor mechanical strength and low reproducibility [32]. Artificial hydrogels are created through the polymerization of monomers, such as poly-2-hydroxyethyl methacrylate (PHEMA), polyethylene glycol (PEG), polyvinyl alcohol (PVA), and polyacrylamide (PMMA) [1]. These substances exhibit stability and possess considerable mechanical strength. Additionally, in terms of composition, hydrogels can be further categorized into homopolymer hydrogels, copolymeric hydrogels, and multipolymer interpenetrating hydrogels. Homopolymeric hydrogels are derived from single monomers. Identical monomers are subjected to repeated polymerization to form a homopolymer. The crosslinks formed during polymerization are interlinked with the \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Type of Composition** & **Synthetic Polymer Hydrogel** & **Natural Polymer Hydrogel** & **Natural\& Synthetic Hydrogel** \\ \hline \hline \multirow{8}{*}{\begin{tabular}{c} \(\bullet\) \\ \(\bullet\) \\ \(\bullet\) \\ \(\bullet\) \\ \end{tabular} } & \multirow{8}{*}{\begin{tabular}{c} Polysaccharides (chitin, chitosan, cellulose, gums, alginate, carrageenan) \\ \end{tabular} } & \multirow{8}{*}{\begin{tabular}{c} Semi-IPN chitosan-poly \\ (acrylamide-co-ethylene \\ oxide) hydrogel [26] \\ \end{tabular} } \\  & & & & & \\ \cline{1-1}  & & & & & \\ \cline{1-1}  & & & & & \\ \cline{1-1}  & & & & & \\ \cline{1-1}  & & & & & \\ \cline{1-1}  & & & & & \\ \hline \hline \multirow{8}{*}{\begin{tabular}{c} Advantages \\ \end{tabular} } & \multirow{8}{*}{\begin{tabular}{c} \(\bullet\) \\ \(\bullet\) \\ \(\bullet\) \\ \(\bullet\) \\ \end{tabular} } & \multirow{8}{*}{\begin{tabular}{c} Prolong shelf life. \\ Easy to modify \\ Enhanced strength and water \\ capacity [29] \\ \end{tabular} } & \multirow{8}{*}{\begin{tabular}{c} Outstanding \\ Biocompatibility \\ \end{tabular} } & \multirow{8}{*}{\begin{tabular}{c} \(\bullet\) \\ \(\bullet\) \\ \(\bullet\) \\ \end{tabular} } & \multirow{8}{*}{\begin{tabular}{c} Combines features of both \\ synthetic and natural \\ adaptable physical, \\ electrical, and chemical \\ properties tailored to suit \\ the study’s requirements. \\ Enhanced mechanical \\ strength [29] \\ \end{tabular} } \\ \hline \multirow{2}{*}{\begin{tabular}{c} Disadvantage \\ \end{tabular} } & \multirow{2}{*}{\begin{tabular}{c} \(\bullet\) \\ \(\bullet\) \\ \(\bullet\) \\ \end{tabular} } & \multirow{2}{*}{\
\begin{tabular}{c} Lacks in vivo compatibility. \\ Reduced degradation rate. [29] \\ \end{tabular} } & \multirow{2}{*}{
\begin{tabular}{c} Low mechanical strength. \\ Chances of disease \\ spread from naturally \\ derived materials. [29] \\ \end{tabular} } \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Various types of Hydrogel compositions. swelling properties of homopolymer hydrogels. Instances of homopolymer hydrogels encompass hydrogels made from polyacrylamide (PAAm), polyethylene glycol (PEG), and poly(N-isopropylacrylamide) (PNIPAAm) [30]. Copolymeric hydrogels are formed by copolymerization of two or more monomers. They can be modified to have unique properties and functionalities based on the selection of monomers. Poly (acrylic acid)-co-poly(ethylene glycol) (PAA-co-PEG) is a copolymer composed of acrylic acid and ethylene glycol. Its swelling behavior is pH-sensitive, attributed to the inclusion of acrylic acid [30].