Table 1. Various Materials and Methods Used to Engineer Varied Stiffness Gradients.
| materials | preparation methods | stiffness range | ref |
|---|---|---|---|
| PVA hydrogel | LN2-contacting gradual freezing and thawing method | 1–24 kPa | (38) |
| PVA/HA hydrogel | 20–200 kPa | (147) | |
| poly(allylamine hydrochloride) and poly (acrylic acid) | controlled dipping into a cross-linking solution | 0.5–110 MPa | (39) |
| PEG diacrylate hydrogels | photopolymerization using a sliding mask | 2–100 kPa | (150) |
| polyacrylamide | 2.5–11 kPa | (151) | |
| HA hydrogel system | 3–100 kPa | (152) | |
| acrylamide/bis-acrylamide | 1–240 kPa | (153) | |
| PDMS | temperature gradients during cross-linking | 190 kPa–3.1 MPa | (148) |
| PEG hydrogels | graded exposure to UV | 7–32 kPa | (159) |
| PDMS | shielded plasma oxidation with a mask | 6–89 MPa | (146) |