Hydrogels are hydrophilic, three-dimensional networks that are able to absorb large quantities of water or biological fluids, and thus have the potential to be used as prime candidates for biosensors, drug delivery vectors, and carriers or matrices for cells in tissue engineering. In this critical review article, advantages of the hydrogels that overcome the limitations from other types of biomaterials will be discussed. Hydrogels, depending on their chemical composition, are responsive to various stimuli including heating, pH, light, and chemicals. Two swelling mechanisms will be discussed to give a detailed understanding of how the structure parameters affect swelling properties, followed by the gelation mechanism and mesh size calculation. Hydrogels prepared from natural materials such as polysaccharides and polypeptides, along with different types of synthetic hydrogels from the recent reported literature, will be discussed in detail. Finally, attention will be given to biomedical applications of different kinds of hydrogels including cell culture, self-healing, and drug delivery.

We report an investigation of triplet-triplet annihilation upconversion (TTA-UC) based on polymeric emitters with tunable inter-chromophore distances. Poly[(9-anthrylmethyl methacrylate)-co-(methyl methacrylate)] (poly(AnMMA-co-MMA)) with different percentages of AnMMA was synthesized using reversible addition-fragmentation chain transfer (RAFT) polymerization, and used as an emitter in association with platinum octaethylporphyrin as a sensitizer to form TTA-UC systems. It is observed that the TTA-UC intensity first increases with increasing AnMMA percentage in the polymers then decreases, and ultimately disappears, upon further increasing the AnMMA percentage. The results shed light on the key factors affecting TTA-UC in polymers, and have implications for the design of polymer-based TTA-UC systems.

The first self-immolative polymer (SIP) nanofiber membrane is demonstrated in this report, in which the immolation can be triggered by external stimulus. Electrospun SIP/polyacrylonitrile (PAN) fibers provide depolymerization that is ∼25 times quicker and more responsive (i.e., immolation) than that of a cast film in the triggering condition. Depolymerization of SIP in the SIP/PAN blended fiber membrane results in the transition of the surface properties from hydrophobic (∼110°) to hygroscopic (∼0°). Triggered release of encapsulated functional molecules was demonstrated using coaxially electrospun fiber membrane made of a SIP/PAN blend sheath and polyvinylpyrrolidone/dye core. Coaxial fibers with the SIP/PAN sheath provide minimal release of the encapsulated material in nontriggering solution, while it releases the encapsulated material instantly when the triggering condition is met. Its versatility has been strengthened compared to that of non-SIP coaxial fibers that provide no triggering reaction by external stimulus.

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