A team of researchers at Princeton University in New Jersey have developed a new type of recyclable hydrogel that can be cut and moulded into different forms.
Hydrogels, flexible networks of polymer chains suffused with water, possess numerous useful properties, including softness, elasticity and biocompatibility. Accordingly, the squishy materials have already found widespread use as contact lenses and wound dressings, but they also hold great promise as drug delivery systems and for use in agriculture and food packaging, among other applications.
Unfortunately, conventional hydrogels pose environmental pollution problems because they can’t be effectively recycled or reprocessed; they also degrade after long-term use. According to the researchers, these limitations derive from the materials’ structure. Conventional hydrogels rely on chemical bonds for their firmness and ability to soak up water and other solvents. On a chemical level, these bonds are cross-linked, meaning that bonds form among different polymer molecules within the hydrogel. This cross-linking, characteristic of resins undergoing curing or rubber undergoing vulcanisation, gives hydrogels both flexibility and strength, but it also makes them extremely difficult to separate into components for recycling.
Xiaohui Xu (pictured below, at right), a postdoctoral researcher, previously worked with hydrogels for use in water purification and wondered if she could create a more environmentally sustainable hydrogel. She and her colleagues took a new approach to building hydrogels. Rather than relying on chemical bonds to connect different polymers, the researchers decided to harness phase separation, a familiar phenomenon in which mixed liquids, such as oil and water, separate into components.
‘Hydrogels offer tremendous societal benefit, but their lack of sustainability has loomed as a significant issue,’ said Xu. ‘In this study, we have shown how taking advantage of phase separation can lead to new kinds of hydrogels that are durable and recyclable and still have good mechanical properties.’
To make the new hydrogel, the researchers formulated polymers with a complex and varying relation to water. The researchers created polymers that are water-loving in some sections of their chain and water-repelling in other sections. When they added water to the polymer mix, parts of the polymers absorbed the water, while other parts repelled it. This tension gave the hydrogel its structural strength.
Because the strength is based on physical characteristics rather than chemical bonds among the polymers, as in conventional hydrogels, recycling the hydrogel into its component polymers is relatively easy, as is repeatedly dehydrating and rehydrating the material.
What’s more, Xu said, the process allows engineers to tailor the characteristics of a hydrogel by adjusting the components of the polymers. The research team took advantage of this to create a hydrogel that could be cut and moulded into any desired shape. For the research, Xu made an octopus.
‘Xiaohui used phase separation as a way to control the morphology, and ultimately, the properties of these hydrogel materials,’ said Rodney Priestley, the Pomeroy and Betty Perry Smith professor of chemical and biological engineering and dean of the Graduate School at Princeton. ‘This work demonstrates an environmentally friendly approach for making tough and reusable hydrogels.’
The Princeton researchers put the new hydrogel through its paces, testing its stability in extreme acidic and alkaline conditions, and in air and water. Across the board, the hydrogel held up and performed as expected.
With additional testing and development, the novel material could make existing and emerging applications for hydrogels – such as artificial muscles and soft robots for safe operation around humans – more sustainable and environmentally friendly.
‘Our general approach for recyclable hydrogels could help expand their applications in all kinds of areas,’ said Xu. ‘The benefits of hydrogels can now be better realised without the environmental costs.’
Xu said that in the long term, the research team will explore whether hydrogels could eventually serve as a substitute for plastics in many applications. If that’s the case, advanced hydrogels could become a recyclable solution to the problem of plastic pollution.
The research has been published in the Journal of the American Chemical Society Au.