Figure 2.

Hybrid and synthetic hydrogels used for creating stiffness gradient. (A) Gradient generation by syringe pumps. Reprinted with permission from [110]. (B) A moving photomask to create different degrees of photopolymerization. Reprinted with permission from [125]. (C) Microstructured hydrogel layer (bottom) and superficial gel layer (top) to create a mechanical interface with different stiffness levels. Reprinted with permission from [91]. (D) Microfluidic-based gradient generator was used for creating a gradient of bis-acrylamide that led to a gradient of crosslinking density upon UV irradiation. Reprinted with permission from [95]. (E) PDMS was cured with a temperature gradient, leading to a gradient of crosslinking. Reprinted with permission from [114]. (F) PVA was repeatedly dipped into liquid nitrogen to induce a gradient of physical crosslinking. Reprinted with permission from [115].