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Migration of cells along the right direction is of paramount importance in a number of in vivo circumstances such as immune response, embryonic developments, morphogenesis, and healing of wounds and scars. While it has been known for a while that spatial gradients in chemical cues guide the direction of cell migration, the significance of the gradient in mechanical cues, such as stiffness of extracellular matrices (ECMs), in directed migration of cells has only recently emerged. With advances in synthetic chemistry, micro-fabrication techniques, and methods to characterize mechanical properties at a length scale even smaller than a single cell, synthetic ECMs with spatially controlled stiffness have been created with variations in design parameters. Since then, the synthetic ECMs have served as platforms to study the migratory behaviors of cells in the presence of the stiffness gradient of ECM and also as scaffolds for the regeneration of tissues. In this review, we highlight recent studies in cell migration directed by the stiffness gradient, called durotaxis, and discuss the mechanisms of durotaxis. We also summarize general methods and design principles to create synthetic ECMs with the stiffness gradients and, finally, conclude by discussing current limitations and future directions of synthetic ECMs for the study of durotaxis and the scaffold for tissue engineering.

The miscibility of poly(ether sulfone), PES, with various hydrophilic copolymers containing 1-vinylpyrrolidone or vinyl alcohol as a repeat unit was explored. Among these blends, PES formed homogeneous mixtures with poly(1-vinylpyrrolidone-co-styrene) copolymers, P(VP-S), containing VP from 59 to 92 wt %. Miscible PES blends with P(VP-S) copolymers underwent phase separation on heating caused by lower critical solution temperature type phase behavior. The phase separation temperature of miscible blends first increases gradually with VP content, goes through a broad maximum centered at about 77 wt % VP, and then decreases just prior to the limiting content of VP for miscibility with PES. The calculated interaction energies of PES/P(VP-S) blends indicated that miscibility of these blends stemmed from the intramolecular repulsion between VP and styrene.

Owing to its superior water absorption capacity, superabsorbent polymer (SAP) based on a poly (acrylic acid) network is extensively used in industrial products such as diapers, wound dressing, or surgical pads. However, because SAP does not degrade naturally, a massive amount of non-degradable waste is discarded daily, posing serious environmental problems. Considering that diapers are the most widely used end-product of SAP, we created one that is degradable by a human urinary enzyme. We chose three enzyme candidates, all of which have substrates that were modified with polymerizable groups to be examined for cleavable crosslinkers of SAP. We found that the urokinase-type plasminogen activator (uPA) substrate, end-modified with acrylamide groups at sufficient distances from the enzymatic cleavage site, can be successfully used as a cleavable crosslinker of SAP. The resulting SAP slowly degraded over several days in the aqueous solution containing uPA at a physiological concentration found in human urine and became shapeless in ~30 days.