Rutgers team invents “4D printing” method for shape-shifting hydrogels

Rutgers engineers have designed a new 4D printing approach that involves printing a 3D object with a hydrogel (water-containing gel) that changes shape over time when temperatures change.

A tiny chess king, 3D-printed with a temperature-responsive hydrogel, in cold water. It contains 73 percent water but remains solid. Photo: Daehoon Han/Rutgers University–New Brunswick

The team was able to demonstrate fast, scalable, high-resolution 3D printing of hydrogels, which remain solid and retain their shape despite containing water.

The smart gel could provide structural rigidity in organs such as the lungs, and can contain small molecules like water or drugs to be transported in the body and released. It could also create a new area of soft robotics, and enable new applications in flexible sensors and actuators, biomedical devices and platforms or scaffolds for cells to grow, explains said Howon Lee, senior author of a new study and assistant professor in the Department of Mechanical and Aerospace Engineering at Rutgers University–New Brunswick.

“We added another dimension to it, and this is the first time anybody has done it on this scale,” Lee adds. “They’re flexible, shape-morphing materials. I like to call them smart materials.”

Engineers at Rutgers–New Brunswick and the New Jersey Institute of Technology worked with a hydrogel that has been used for decades in devices that generate motion and biomedical applications such as scaffolds for cells to grow on. But hydrogel manufacturing has relied heavily on conventional, two-dimensional methods such as molding and lithography.

The engineers used a lithography-based technique that involves printing layers of a special resin to build a 3D object. The resin consists of the hydrogel, a chemical that acts as a binder, another chemical that facilitates bonding when light hits it and a dye that controls light penetration.

In temperatures below 32 degrees Celsius, the hydrogel absorbs more water and swells in size. When temperatures exceed 32 degrees Celsius, the hydrogel begins to expel water and shrinks. The objects they can create with the hydrogel range from the width of a human hair to several millimeters long. The engineers also found that they can grow one area of a 3D-printed object – creating and programming motion – by changing temperatures.

“If you have full control of the shape, then you can program its function,” Lee said.

The study was published in Scientific Reports, with lead author Daehoon Han, a doctoral student in the Department of Mechanical and Aerospace Engineering at Rutgers–New Brunswick. Co-authors include Zhaocheng Lu, another doctoral student, and Shawn A. Chester, an assistant professor at New Jersey Institute of Technology.

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