Researchers at the University of Wollongong, Australia have created a 3D printer-compatible hydrogel that is mechanically tough and able to repeatedly change shape in response to water temperature. The scientists have demonstrated the technology by 3D-printing an autonomous water valve, but the material could also be used to create soft robots, custom designed sensors and self-assembling macrostructures.

The aim of so-called "4D printing" is to extend additive manufacturing to the dimension of time. The idea is to create 3D-printed objects using special materials that are sensitive to heat, water or pressure that can autonomously change shape in very specific, purposeful ways in response to environmental conditions, long after they’ve come out of the printer. In some cases, the objects can even revert back to their original shape.

Examples of 4D printing have included simple self-assembling bodies that fold together when baked, polymers that bend into shape in response to water, heat or pressure, and smart strands inspired by self-assembling nanostructures. Admittedly, 4D printing is far from practical in its current iteration, but the technology is very young and will likely take big steps forward as 3D printing becomes more accessible.

According to Professor Marc in het Panhuis and team, current 4D printing applications can produce some very radical changes in shape, but they often take a long time to respond, the materials lose mechanical strength as they bend, and the shape-shifting is only reversible only up to a point. The scientists have therefore developed a special hydrogel that goes in a very different direction, generating quick, reversible and mechanically reliable changes in response to changes in water temperature.

The researchers created a material that uses a tough ionic covalent entanglement (ICE) gel as the active material in concentrations of up to 20 percent. ICE gels consist of two polymer networks that rely on crosslinked ionic and covalent chemical bonds. The unique structure toughens the material and prevents microscopic cracks from propagating throughout its structure, avoiding catastrophic mechanical failures.

Beyond a critical temperature of about 35° C (95° F), the gel quickly loses a great part of its water content and shrinks down in volume by nearly 50 percent. Panhuis and colleagues used this phenomenon to 3D-print a valve that closes when exposed to hot water, blocking 99 percent of water flow, and then reliably opens once water temperature drops. Unlike a standard 3D-printed material, the gel morphs without human intervention and can repeatedly open and close without straining.

It’s early days yet for this technology, but shape-shifting materials like this one (whether they respond to water temperature, heat or pressure) could lead to big advances in construction, making it easier to assemble materials in extreme environments like deep space without relying on humans or expensive, elaborate robots. And if future 3D printers can shrink to the molecular scale, the medical applications could be truly revolutionary.

The study is described in the latest issue of the journal Macromolecular Rapid Communications.