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Different methods for preparing stiffness gradients. (A) Schematic representations displaying the formation of PVA hydrogel with stiffness gradient by the freeze–thaw method. Reprinted with permission from ref (38). Copyright 2015 Elsevier, Ltd. (B) Schematic preparation process for PDMS stiffness gradients via a temperature gradient during curing. Reprinted with permission from ref (148). Copyright 2012 Elsevier, Ltd.
