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

A newly created 4D printing method for a smart gel could lead to the development of "living" structures in human organs and tissues, soft robots, and targeted drug delivery.

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.

The 4D printing approach involves printing a 3D object with a hydrogel (water-containing gel) that changes shape over time when temperatures change. The study demonstrates fast, scalable, high-resolution 3D printing of hydrogels, which remain solid and retain their shape despite containing water.

Engineers 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.