Photographs and schematic illustrations of the hydrogel as it changes from the sol form (left) at low pH to a gel form (center) at pH 12 and eventually a compact solid hydrogel (right) on further aging.
© 2010 ACS
Hydrogels are a unique class of cross-linked polymer that can hold a large amount of water. They are the material of choice for making sensors, wound dressings, contact lenses and a myriad of products with important engineering and biomedical applications. Scientists have developed a diverse range of hydrogels with tunable properties that can be activated or terminated through light, pH or temperature changes. However, there is one critical property they have yet to gain control: gelation—the process that converts the liquid ‘sol’ phase to the gel phase.
Ye Liu and co-workers at the A*STAR Institute of Materials Research and Engineering have now developed a hydrogel with a gelation process that can be activated, terminated and reinitiated in a precise manner through pH changes. The hydrogel-forming material was synthesized using a one-pot, two-step polymerization approach. The researchers mixed aminoethyl piperazine (AEPZ) with N,N’-bis(acryloyl)cystamine (BAC) at a molar ratio of 1:2 to yield terminal vinyl groups, and then introduced polyethylene glycol (PEG) to the mixture to give a solution of a branched polymer called poly(BAC2-AEPZ1)-PEG, or BAP.
Under highly alkaline conditions (pH 12), the BAP solution formed a watery hydrogel that became more viscous over time. Aging the hydrogel for 24 hours or longer caused the BAP to fall out of solution and change into a compact solid hydrogel (see image). The researchers were able to interrupt this gelation process by lowering the pH of the solution, and reinitiate the process by increasing the pH back to 12. The aged compact hydrogel has good stability and is able to maintain its shape in solution for over half a year.
Using nuclear magnetic resonance spectroscopy to analyze the BAP molecules, the researchers found that at high pH, large numbers of BAP molecules aggregate together to form micelle-like core–shell structures. This process is accompanied by thiol–disulfide exchange between BAP molecules, which results in the formation of a stable hydrogel with chemical cross-linkages. As this reaction no longer proceeds below pH 7, the gelation can be controlled easily by changing the pH.
The hydrogel is biocompatible, biodegradable and could easily be made into various shapes and sizes. The researchers believe their approach could be used for the design, synthesis and self-assembly of hydrogels with novel properties. “It is difficult to control the degree of cross-linkages using other approaches once the polymerization has started,” says Liu. “To the best of my knowledge, we are the first to have this control in a hydrogel system.”
The A*STAR-affiliated researchers contributing to this research are from the Institute of Materials Research and Engineering.
