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=ASIA6AP2G7AKDF5JF6PB%2F20260119%2Feu-central-1%2Fs3%2Faws4_request&X-Amz-Date=20260119T005941Z&X-Amz-Expires=3600&X-Amz-Security-Token=IQoJb3JpZ2luX2VjEMD%2F%2F%2F%2F%2F%2F%2F%2F%2F%2FwEaDGV1LWNlbnRyYWwtMSJHMEUCIQC0GfehIo8DqHQ%2FmxkW1EXVyynU4FmxMc0SVxIl50rxgwIgFNkWHWYCdAVQq5zu7i9%2Fdxp81MI6jg%2FDtlOA5ezYL5EqxgUIif%2F%2F%2F%2F%2F%2F%2F%2F%2F%2FARACGgw5NjMxMzQyODk5NDAiDLkJXZnuMNPNvWhT1SqaBQBTrRLeYqt8hWxliwVEaOmCdUj0aLwZdYjT4TwoFRzJH7u54vb9Wb%2B4BEfVvlmUUWFY652kviXD%2BnMUmOrwCdPuH1XwhAzg%2FdGrB169BjMAIgpKP4L77RUBW9vIC31xp31ZuSRGCUh1%2FZRptCIfHLc6z0NNmRge0jwY0Xt4gL7Csb8iaBe4mMdet8XTAHGOoQdxYsIdjjEXP5Wfhw2%2BOz8HxcvfBeewOv8HPx5sWuOu1kyVtZT5h7Dm1r5k2SuTyjN1JijwFHpNI97mRiqFcKk9IMbiz5PrAgPxX4rD8JCZbLTs38TkR2G%2Bu6%2BgOfIt6Mm0dtQOhSWY88pTAwMtWjvctOs08Jnrk%2F8VcOkdvScH4U593i7alYm5IgYpjPhaVvq9SGU9%2FYrWGZglvpVBOmt3BnHZkg%2BAfxY436XqUA3jj5AlHqhEpxm2yN211Kow%2BwEGt2nB2%2BHiRJseaGOq2shDth3Gb%2F6FA%2FzLoukWBDgQT5LzLshmfYf0jGZnsFTzhRCC7Z83tyMktAGsB6F03wE3TbelCIzQ%2B8QZJT5s9a9f9yKjdduFeqCUcCbxJ6zcAqv797tl0Vysn6UoDh41SNP7umqjpPYDdn75WK0ZmIvwUaIak%2BRM5J87gCryyIlCOIve3%2F%2Bo9Jo%2B35e7qGP2Vz84goHVR508ue709SBFllYKcaFPK9SCYS2b%2FTqBLZuQQMupweE3pGa0hP9D2tCrYV7%2BifCitudAPlGv%2F7WBi9M%2FbQKadD12z5HqLRBHDtLt5tBaEba29YjUotB4OrGr4fyE%2Bp%2BjghObIoqCPyQjZLEnIsUpHvq%2BwdQr3OX9cOWgOquhkQEevIW%2B78gTGS4XySYOetyZP9nJtinexkLpkUur5P7iBxw5FSzDhzCX2rXLBjqxAVU1mE87yWPMViGe1f3HG6idxGvDQILEUh%2BnPTQHfW3rcLR7sc4eplwnbuaPEGXQ8MiVVWqx5LXYdQvS3hH5bcqCnLgtX%2Fup6q3uBj2qh0dhKc%2ByUnuZOXBvx2HCMXugacBrit3rubfAo0Pv43Z6RS12d%2F6ZXwT8Tw%2FDlPSJZ%2FK8Xy6iGfZXkComuDBEmc12lxbgNy8HuUNPYasSSBe5IGCp%2FCFLee4TwMK1x%2B2pcxW4sA%3D%3D&X-Amz-Signature=cbb67dd6d41ee25d3b1bfd40fcd3cc13f00a634252be8f8c98b10c0b50ea7275&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=ASIA6AP2G7AKDF5JF6PB%2F20260119%2Feu-central-1%2Fs3%2Faws4_request&X-Amz-Date=20260119T005941Z&X-Amz-Expires=3600&X-Amz-Security-Token=IQoJb3JpZ2luX2VjEMD%2F%2F%2F%2F%2F%2F%2F%2F%2F%2FwEaDGV1LWNlbnRyYWwtMSJHMEUCIQC0GfehIo8DqHQ%2FmxkW1EXVyynU4FmxMc0SVxIl50rxgwIgFNkWHWYCdAVQq5zu7i9%2Fdxp81MI6jg%2FDtlOA5ezYL5EqxgUIif%2F%2F%2F%2F%2F%2F%2F%2F%2F%2FARACGgw5NjMxMzQyODk5NDAiDLkJXZnuMNPNvWhT1SqaBQBTrRLeYqt8hWxliwVEaOmCdUj0aLwZdYjT4TwoFRzJH7u54vb9Wb%2B4BEfVvlmUUWFY652kviXD%2BnMUmOrwCdPuH1XwhAzg%2FdGrB169BjMAIgpKP4L77RUBW9vIC31xp31ZuSRGCUh1%2FZRptCIfHLc6z0NNmRge0jwY0Xt4gL7Csb8iaBe4mMdet8XTAHGOoQdxYsIdjjEXP5Wfhw2%2BOz8HxcvfBeewOv8HPx5sWuOu1kyVtZT5h7Dm1r5k2SuTyjN1JijwFHpNI97mRiqFcKk9IMbiz5PrAgPxX4rD8JCZbLTs38TkR2G%2Bu6%2BgOfIt6Mm0dtQOhSWY88pTAwMtWjvctOs08Jnrk%2F8VcOkdvScH4U593i7alYm5IgYpjPhaVvq9SGU9%2FYrWGZglvpVBOmt3BnHZkg%2BAfxY436XqUA3jj5AlHqhEpxm2yN211Kow%2BwEGt2nB2%2BHiRJseaGOq2shDth3Gb%2F6FA%2FzLoukWBDgQT5LzLshmfYf0jGZnsFTzhRCC7Z83tyMktAGsB6F03wE3TbelCIzQ%2B8QZJT5s9a9f9yKjdduFeqCUcCbxJ6zcAqv797tl0Vysn6UoDh41SNP7umqjpPYDdn75WK0ZmIvwUaIak%2BRM5J87gCryyIlCOIve3%2F%2Bo9Jo%2B35e7qGP2Vz84goHVR508ue709SBFllYKcaFPK9SCYS2b%2FTqBLZuQQMupweE3pGa0hP9D2tCrYV7%2BifCitudAPlGv%2F7WBi9M%2FbQKadD12z5HqLRBHDtLt5tBaEba29YjUotB4OrGr4fyE%2Bp%2BjghObIoqCPyQjZLEnIsUpHvq%2BwdQr3OX9cOWgOquhkQEevIW%2B78gTGS4XySYOetyZP9nJtinexkLpkUur5P7iBxw5FSzDhzCX2rXLBjqxAVU1mE87yWPMViGe1f3HG6idxGvDQILEUh%2BnPTQHfW3rcLR7sc4eplwnbuaPEGXQ8MiVVWqx5LXYdQvS3hH5bcqCnLgtX%2Fup6q3uBj2qh0dhKc%2ByUnuZOXBvx2HCMXugacBrit3rubfAo0Pv43Z6RS12d%2F6ZXwT8Tw%2FDlPSJZ%2FK8Xy6iGfZXkComuDBEmc12lxbgNy8HuUNPYasSSBe5IGCp%2FCFLee4TwMK1x%2B2pcxW4sA%3D%3D&X-Amz-Signature=afea66402d1a3fbf6936ffdfaa3dcc5afbb2cfc50a1592a45c8cd6367122cd71&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=ASIA6AP2G7AKDF5JF6PB%2F20260119%2Feu-central-1%2Fs3%2Faws4_request&X-Amz-Date=20260119T005941Z&X-Amz-Expires=3600&X-Amz-Security-Token=IQoJb3JpZ2luX2VjEMD%2F%2F%2F%2F%2F%2F%2F%2F%2F%2FwEaDGV1LWNlbnRyYWwtMSJHMEUCIQC0GfehIo8DqHQ%2FmxkW1EXVyynU4FmxMc0SVxIl50rxgwIgFNkWHWYCdAVQq5zu7i9%2Fdxp81MI6jg%2FDtlOA5ezYL5EqxgUIif%2F%2F%2F%2F%2F%2F%2F%2F%2F%2FARACGgw5NjMxMzQyODk5NDAiDLkJXZnuMNPNvWhT1SqaBQBTrRLeYqt8hWxliwVEaOmCdUj0aLwZdYjT4TwoFRzJH7u54vb9Wb%2B4BEfVvlmUUWFY652kviXD%2BnMUmOrwCdPuH1XwhAzg%2FdGrB169BjMAIgpKP4L77RUBW9vIC31xp31ZuSRGCUh1%2FZRptCIfHLc6z0NNmRge0jwY0Xt4gL7Csb8iaBe4mMdet8XTAHGOoQdxYsIdjjEXP5Wfhw2%2BOz8HxcvfBeewOv8HPx5sWuOu1kyVtZT5h7Dm1r5k2SuTyjN1JijwFHpNI97mRiqFcKk9IMbiz5PrAgPxX4rD8JCZbLTs38TkR2G%2Bu6%2BgOfIt6Mm0dtQOhSWY88pTAwMtWjvctOs08Jnrk%2F8VcOkdvScH4U593i7alYm5IgYpjPhaVvq9SGU9%2FYrWGZglvpVBOmt3BnHZkg%2BAfxY436XqUA3jj5AlHqhEpxm2yN211Kow%2BwEGt2nB2%2BHiRJseaGOq2shDth3Gb%2F6FA%2FzLoukWBDgQT5LzLshmfYf0jGZnsFTzhRCC7Z83tyMktAGsB6F03wE3TbelCIzQ%2B8QZJT5s9a9f9yKjdduFeqCUcCbxJ6zcAqv797tl0Vysn6UoDh41SNP7umqjpPYDdn75WK0ZmIvwUaIak%2BRM5J87gCryyIlCOIve3%2F%2Bo9Jo%2B35e7qGP2Vz84goHVR508ue709SBFllYKcaFPK9SCYS2b%2FTqBLZuQQMupweE3pGa0hP9D2tCrYV7%2BifCitudAPlGv%2F7WBi9M%2FbQKadD12z5HqLRBHDtLt5tBaEba29YjUotB4OrGr4fyE%2Bp%2BjghObIoqCPyQjZLEnIsUpHvq%2BwdQr3OX9cOWgOquhkQEevIW%2B78gTGS4XySYOetyZP9nJtinexkLpkUur5P7iBxw5FSzDhzCX2rXLBjqxAVU1mE87yWPMViGe1f3HG6idxGvDQILEUh%2BnPTQHfW3rcLR7sc4eplwnbuaPEGXQ8MiVVWqx5LXYdQvS3hH5bcqCnLgtX%2Fup6q3uBj2qh0dhKc%2ByUnuZOXBvx2HCMXugacBrit3rubfAo0Pv43Z6RS12d%2F6ZXwT8Tw%2FDlPSJZ%2FK8Xy6iGfZXkComuDBEmc12lxbgNy8HuUNPYasSSBe5IGCp%2FCFLee4TwMK1x%2B2pcxW4sA%3D%3D&X-Amz-Signature=c5797cae869d0bbf60e3cbda9bda5cfe6623ead6dc8aba2e95a6f685a5f15b5f&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 | ||