A “4D printing” method was developed for a smart gel that could lead to the development of living structures in human organs and tissues, soft robots, and targeted drug delivery.
The approach involves printing a 3D object with a hydrogel (water-containing gel) that changes shape over time when temperatures change. The hydrogels 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.
The hydrogel has been used for decades in devices that generate motion, and in biomedical scaffolds; however, hydrogel manufacturing has relied heavily on conventional, two-dimensional methods such as molding and lithography.
The new method uses a lithography-based technique that is fast, inexpensive, and can print a wide range of materials into a 3D shape. It 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 °C (about 90 °F), the hydrogel absorbs more water and swells in size. When temperatures exceed 32 °C, the hydrogel begins to expel water and shrinks. The objects that can be created with the hydrogel range from the width of a human hair to several millimeters long. One area of a 3D-printed object can be grown — creating and programming motion — by changing temperatures.