Fig. 1
![Examples of chemically (a–c) and physically (d–f) crosslinked hydrogels [7]](https://sciendo-parsed.s3.eu-central-1.amazonaws.com/6471db0b215d2f6c89db3264/j_ftee-2022-0024_fig_001.jpg?X-Amz-Algorithm=AWS4-HMAC-SHA256&X-Amz-Content-Sha256=UNSIGNED-PAYLOAD&X-Amz-Credential=ASIA6AP2G7AKL2Y7WIZC%2F20260305%2Feu-central-1%2Fs3%2Faws4_request&X-Amz-Date=20260305T224923Z&X-Amz-Expires=3600&X-Amz-Security-Token=IQoJb3JpZ2luX2VjEA4aDGV1LWNlbnRyYWwtMSJIMEYCIQCvkutjJqAI%2BOtw%2FVa7CPrK2sgqSovc7QTqwFBXJ5VjlwIhAJcpa0Gf%2B9CJ7fm1UmJI%2FR7PIp1pjxGx4E7NuGCLwUO5KsYFCNf%2F%2F%2F%2F%2F%2F%2F%2F%2F%2FwEQAhoMOTYzMTM0Mjg5OTQwIgyy3tRNoypwe8WkTQoqmgV1yJ8eHHP4VzrhebJ9hYS2Eid%2BY0TfKLydMNNFx4eM2k9ABC7YfxGDjDY3aPYLflIGrci6Ltu24CNc1CjDn99c2Mpx5QpFUoUbZ2eiq1TaihBPTjY0K%2FP04XI9XR8AYZFXkXfb%2BQI3Q%2BFmHyAsoEjxk2OmA0IvrkQDYxXleGrecM4t2GgjPQyu7AlffpLAOhMdiWJa59nZT7rLDYDbedvSFDNQsXAm9UIyn%2ByAwVwupg2vGo0D6tETkE8sCYmbn5sAKg5MJUnVD59%2FmgGrcbwDQk4xBvwGeU94ssoMZHMMBwYzre7lCDS65GrTXdJKMev6j1OeA5u4SVSKKzfIwVnISCxnqLHiq211H3Oe4jE%2FbO615VWuSVcHLMCckjS8qIviJ2e966IJ%2BaGGp8%2BVVDIXGNkcWDu6jmO3yxtBvCv0G5vTXNE%2BOcjYPnXXluCTCylmk5sChNJ43k%2BSwZjGRa2y%2BFGBy2VjSqWwuGCCdrnJ7dv%2FyEuR0iU5Hkp7cZqur1o9xJ2Dk5RPP9mOaF0v4vByuP8eNUGsg%2F%2BKCQuZsi145n8D2srQo0j%2BGQ6VCLKV35r3sjvCuqLYowJ4he9fisObDFCKLYuAqc%2FKK5Gye6DywQAbHq6QAtEk%2BaTnkemGBKfXA%2BizLqdpW9DeVYYosVdqwDY0J0Gru97MT3iWIf7PeEXE1pVBYuSb9qh8LK35iOUM84DYhGfYDNufvNo6lc4jjrivd6TSv24hdrjzUp%2FZH4pGBt%2Bd4dCI6Fe3UyT1EtBtd0%2BRz4dE7uwPH%2Fzt3Cihg%2FxjIA9BdLVMVgziH4yB4XsvQUyIyOI%2B122NyLVul8lsNHvylytrlrnkWY%2FplwlDZ26S2nrc20OxX3G4m02GjMPY7kdjAnxp5w0w1fWnzQY6sAFnVzZpAPfXiLGX2nc%2FW17F800C2zoPEAmUhnZsVZ%2BMiFokhis75RPmIoScfPjHfas9YCqKZ0vW6SXatMdvWNbCX6w9x36rsq86tmOUSZMwDP3s%2B1NW9wvht95zVk3fQ%2FPtK%2F8QnIL2jW1dW79W2EfGnn6Idiw%2B%2FqKEpcPFlD2Ytmd3bDFbln1UcNodNNjSiLOQfKPoIELVteGNz7VeKquRJds05MHc3GhcnvGyxrjn1Q%3D%3D&X-Amz-Signature=ffb1c6d796d9a7fffd2c2d928eb9ca21579a2d6fdad8d6a498c4d1ee3f1cc66d&X-Amz-SignedHeaders=host&x-amz-checksum-mode=ENABLED&x-id=GetObject)
Fig. 2
![Structure of collagen (A) and keratin (B) [39, 51]](https://sciendo-parsed.s3.eu-central-1.amazonaws.com/6471db0b215d2f6c89db3264/j_ftee-2022-0024_fig_002.jpg?X-Amz-Algorithm=AWS4-HMAC-SHA256&X-Amz-Content-Sha256=UNSIGNED-PAYLOAD&X-Amz-Credential=ASIA6AP2G7AKL2Y7WIZC%2F20260305%2Feu-central-1%2Fs3%2Faws4_request&X-Amz-Date=20260305T224923Z&X-Amz-Expires=3600&X-Amz-Security-Token=IQoJb3JpZ2luX2VjEA4aDGV1LWNlbnRyYWwtMSJIMEYCIQCvkutjJqAI%2BOtw%2FVa7CPrK2sgqSovc7QTqwFBXJ5VjlwIhAJcpa0Gf%2B9CJ7fm1UmJI%2FR7PIp1pjxGx4E7NuGCLwUO5KsYFCNf%2F%2F%2F%2F%2F%2F%2F%2F%2F%2FwEQAhoMOTYzMTM0Mjg5OTQwIgyy3tRNoypwe8WkTQoqmgV1yJ8eHHP4VzrhebJ9hYS2Eid%2BY0TfKLydMNNFx4eM2k9ABC7YfxGDjDY3aPYLflIGrci6Ltu24CNc1CjDn99c2Mpx5QpFUoUbZ2eiq1TaihBPTjY0K%2FP04XI9XR8AYZFXkXfb%2BQI3Q%2BFmHyAsoEjxk2OmA0IvrkQDYxXleGrecM4t2GgjPQyu7AlffpLAOhMdiWJa59nZT7rLDYDbedvSFDNQsXAm9UIyn%2ByAwVwupg2vGo0D6tETkE8sCYmbn5sAKg5MJUnVD59%2FmgGrcbwDQk4xBvwGeU94ssoMZHMMBwYzre7lCDS65GrTXdJKMev6j1OeA5u4SVSKKzfIwVnISCxnqLHiq211H3Oe4jE%2FbO615VWuSVcHLMCckjS8qIviJ2e966IJ%2BaGGp8%2BVVDIXGNkcWDu6jmO3yxtBvCv0G5vTXNE%2BOcjYPnXXluCTCylmk5sChNJ43k%2BSwZjGRa2y%2BFGBy2VjSqWwuGCCdrnJ7dv%2FyEuR0iU5Hkp7cZqur1o9xJ2Dk5RPP9mOaF0v4vByuP8eNUGsg%2F%2BKCQuZsi145n8D2srQo0j%2BGQ6VCLKV35r3sjvCuqLYowJ4he9fisObDFCKLYuAqc%2FKK5Gye6DywQAbHq6QAtEk%2BaTnkemGBKfXA%2BizLqdpW9DeVYYosVdqwDY0J0Gru97MT3iWIf7PeEXE1pVBYuSb9qh8LK35iOUM84DYhGfYDNufvNo6lc4jjrivd6TSv24hdrjzUp%2FZH4pGBt%2Bd4dCI6Fe3UyT1EtBtd0%2BRz4dE7uwPH%2Fzt3Cihg%2FxjIA9BdLVMVgziH4yB4XsvQUyIyOI%2B122NyLVul8lsNHvylytrlrnkWY%2FplwlDZ26S2nrc20OxX3G4m02GjMPY7kdjAnxp5w0w1fWnzQY6sAFnVzZpAPfXiLGX2nc%2FW17F800C2zoPEAmUhnZsVZ%2BMiFokhis75RPmIoScfPjHfas9YCqKZ0vW6SXatMdvWNbCX6w9x36rsq86tmOUSZMwDP3s%2B1NW9wvht95zVk3fQ%2FPtK%2F8QnIL2jW1dW79W2EfGnn6Idiw%2B%2FqKEpcPFlD2Ytmd3bDFbln1UcNodNNjSiLOQfKPoIELVteGNz7VeKquRJds05MHc3GhcnvGyxrjn1Q%3D%3D&X-Amz-Signature=41f013661d74f9d05c2f4b304a7a6111ad6bfdf574ea6143ba3bf27e09950fe1&X-Amz-SignedHeaders=host&x-amz-checksum-mode=ENABLED&x-id=GetObject)
Fig. 3
![Fabrication scheme of collagen/PNIPAAm hydrogel [67]](https://sciendo-parsed.s3.eu-central-1.amazonaws.com/6471db0b215d2f6c89db3264/j_ftee-2022-0024_fig_003.jpg?X-Amz-Algorithm=AWS4-HMAC-SHA256&X-Amz-Content-Sha256=UNSIGNED-PAYLOAD&X-Amz-Credential=ASIA6AP2G7AKL2Y7WIZC%2F20260305%2Feu-central-1%2Fs3%2Faws4_request&X-Amz-Date=20260305T224923Z&X-Amz-Expires=3600&X-Amz-Security-Token=IQoJb3JpZ2luX2VjEA4aDGV1LWNlbnRyYWwtMSJIMEYCIQCvkutjJqAI%2BOtw%2FVa7CPrK2sgqSovc7QTqwFBXJ5VjlwIhAJcpa0Gf%2B9CJ7fm1UmJI%2FR7PIp1pjxGx4E7NuGCLwUO5KsYFCNf%2F%2F%2F%2F%2F%2F%2F%2F%2F%2FwEQAhoMOTYzMTM0Mjg5OTQwIgyy3tRNoypwe8WkTQoqmgV1yJ8eHHP4VzrhebJ9hYS2Eid%2BY0TfKLydMNNFx4eM2k9ABC7YfxGDjDY3aPYLflIGrci6Ltu24CNc1CjDn99c2Mpx5QpFUoUbZ2eiq1TaihBPTjY0K%2FP04XI9XR8AYZFXkXfb%2BQI3Q%2BFmHyAsoEjxk2OmA0IvrkQDYxXleGrecM4t2GgjPQyu7AlffpLAOhMdiWJa59nZT7rLDYDbedvSFDNQsXAm9UIyn%2ByAwVwupg2vGo0D6tETkE8sCYmbn5sAKg5MJUnVD59%2FmgGrcbwDQk4xBvwGeU94ssoMZHMMBwYzre7lCDS65GrTXdJKMev6j1OeA5u4SVSKKzfIwVnISCxnqLHiq211H3Oe4jE%2FbO615VWuSVcHLMCckjS8qIviJ2e966IJ%2BaGGp8%2BVVDIXGNkcWDu6jmO3yxtBvCv0G5vTXNE%2BOcjYPnXXluCTCylmk5sChNJ43k%2BSwZjGRa2y%2BFGBy2VjSqWwuGCCdrnJ7dv%2FyEuR0iU5Hkp7cZqur1o9xJ2Dk5RPP9mOaF0v4vByuP8eNUGsg%2F%2BKCQuZsi145n8D2srQo0j%2BGQ6VCLKV35r3sjvCuqLYowJ4he9fisObDFCKLYuAqc%2FKK5Gye6DywQAbHq6QAtEk%2BaTnkemGBKfXA%2BizLqdpW9DeVYYosVdqwDY0J0Gru97MT3iWIf7PeEXE1pVBYuSb9qh8LK35iOUM84DYhGfYDNufvNo6lc4jjrivd6TSv24hdrjzUp%2FZH4pGBt%2Bd4dCI6Fe3UyT1EtBtd0%2BRz4dE7uwPH%2Fzt3Cihg%2FxjIA9BdLVMVgziH4yB4XsvQUyIyOI%2B122NyLVul8lsNHvylytrlrnkWY%2FplwlDZ26S2nrc20OxX3G4m02GjMPY7kdjAnxp5w0w1fWnzQY6sAFnVzZpAPfXiLGX2nc%2FW17F800C2zoPEAmUhnZsVZ%2BMiFokhis75RPmIoScfPjHfas9YCqKZ0vW6SXatMdvWNbCX6w9x36rsq86tmOUSZMwDP3s%2B1NW9wvht95zVk3fQ%2FPtK%2F8QnIL2jW1dW79W2EfGnn6Idiw%2B%2FqKEpcPFlD2Ytmd3bDFbln1UcNodNNjSiLOQfKPoIELVteGNz7VeKquRJds05MHc3GhcnvGyxrjn1Q%3D%3D&X-Amz-Signature=93424df60813c3f9ada82c687c42cc5c3d719527ec7ad89c803bcbd8b0a0c32d&X-Amz-SignedHeaders=host&x-amz-checksum-mode=ENABLED&x-id=GetObject)
Classification of hydrogel types based on the network nature
| Type of hydrogel | Examples | References |
|---|---|---|
| Homopolymer | Poly(2-hydroxyethyl methacrylate) | [12] |
| 2-Hydroxyethyl methacrylate (HEMA) | [13] | |
| Poly(vinyl alcohol) (PVA) | [14] | |
| Polyacrylamide (PAAm) | [15] | |
| Polyetylene glycol (PEG) | [16] | |
| Copolymer | Methacrylic acid (MAA) | [17] |
| Carboxymethyl cellulose (CMC) | ||
| Polyvinylpyrrolidone (PVP) | ||
| Interpenetrating network | Poly(N-isopropylacrylamide) (PNIPAAm) | [18] |
| Polyacrylamide-grafted-gum ghatti (PAAm-g-GG) and sodium alginate | [19] | |
| Polyacrylamide-grafted-tamarind seed polysaccharide (PAAm-g-TSP) | ||
| Poli(vinyl alcohol)/chitosan | [20] | |
| Hyaluronic acid–methylcellulose (HA-MC) | [21] | |
| Double-network hydrogels | Poly(2-acrylamido-2-methylpropanesulfonic acid) (PAMPS) as the first network and polyacrylamide as the second network | [22] |
| Poly(2-acrylamido-2methylpropanesulfonic acid) as the first network and poly(N, N-dimethylacrylamide) (DMAA) or polyzwitterionic gels as the second network |
Sub-classification of responsive hydrogels [23]
| Chemically responsive | Physically responsive | Biochemically responsive |
|---|---|---|
| pH | Temperature | Antigens responsive |
| Glucose | Electricity light | Enzymes responsive |
| Oxidant | Pressure | Ligands responsive |
| Solvent composition | Sound and magnetic field | |
| Ions and specific molecular recognition events | ||