Test and tune: evaluating, adjusting and optimising the stiffness of hydrogels to influence cell fate

The use of bioequivalent hydrogels in tissue engineering (TE) is enabling 3D tissue-like scaffolds capable of reproducing the sophistication of natural cell–matrix interactions. Alongside the common concerns of chemical function, it is crucial that hydrogels have suitable mechanical properties, particularly stiffness, to create a complete biomimetic environment for cell development. Non-covalent biocompatible hydrogels are often too soft, while stiffer, covalently crosslinked materials may have challenging microenvironments in which porosity and residual chemicals can be problematic. If the potential of hydrogel-based TE to be realised, design strategies need to be carefully considered to achieve desirable end-use biomechanical properties. This review is intended for a cross-disciplinary readership; we discuss recent successes in bioengineering hydrogel stiffness, where materials that are responsive to cell inputs are used to explore the relationship between substrate stiffness and cellular fate commitment. We discuss the most popular measurements for mechanical studies, and outline optimal substrate stiffness for different cells. We summarise recent advanced studies on tuning stiffness and highlight future challenges challenges to address.<p></p>