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| Type of stimulation | Examples | Features and mechanisms | Application directions | References |
|
| Enzyme | MMP | Under the catalysis of specific enzymes, the hydrogels show the characteristics of configuration change or fracture and promote the formation or degradation of hydrogels | Bone defect; myocardial infarction; diabetes mellitus; spinal cord injury; oral drug controlled release; cancer; wound healing | [199, 200] |
| Phosphatase | [201, 202] |
| Trypsin | [203] |
| Tyrosinase | [204] |
|
| Temperature | Poly (n-isopropylacrylamide) | It usually contains hydrophilic and hydrophobic groups, which can undergo phase transformation at an undetermined temperature so that the morphology of hydrogels changes with the change of swelling degree | Injectable hydrogels; drug delivery; breast cancer; perforation of digestive tract | [205, 206] |
|
| Light | Polymers containing acrylics/coumarin functional groups | Photocrosslinked hydrogels can undergo addition polymerization or cyclization under the combined action of photoinitiator | Hydrophobic drug delivery; hydrogel crosslinking; development of far-infrared light-responsive hydrogel materials; tissue adhesion | [207, 208] |
| Polymers containing the o-nitrobenzyl groups and their derivatives | Photodegradable hydrogels usually contain groups that break under light irradiation |
|
| Magnetic | The composite hydrogels containing magnetic nanoparticles | When no magnetic field is applied, the drug is released through free diffusion. When the magnetic field is applied, the magnetic particles in the hydrogels aggregate and shrink the 3D network structure of the hydrogels, and the porosity decreases rapidly, resulting in the binding of drug molecules in the hydrogels, and the drug release rate decreases. When the field turns off again, the hydrogels return to original state, increasing the drug’s release rate | Diabetes mellitus; cancer; bone defect; articular cartilage injury; arthritis | [209–211] |
|
| Shear force | Natural organic-based hydrogels | It is usually formed by high polymer with dynamic imine bond crosslinking. Spherical structure with layered fiber network, shear thinning, and self-healing properties | Injectable hydrogels; drug delivery; cancer; myocardial infarction; ligament rupture; osteoarthritis; articular cartilage injury; gastrointestinal perforation; limb ischemia | [212–216] |
|
| Ultrasonic | Hydrogels with Diels–Alder bonding bonds | When ultrasound is focused, these connections undergo a reverse reaction, reassembling the hydrogels, and releasing the model protein of internal load. Increasing the amplitude and time of focused ultrasound can improve the rate of protein release | Integration of diagnosis and treatment; synergistic therapeutic ultrasound improves therapeutic effect | [217] |
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