Possible “4D” printing approach arrives

Chess piece 3D printed with hydrogel (Credit: Daehoon Han/Rutgers University–New Brunswick)

Engineers from Rutgers University have invented a “4D printing” method 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 4D printing approach here involves printing a 3D object with a hydrogel (water-containing gel) that changes shape over time when temperatures change,” says Howon Lee, senior author of a new study and assistant professor in the department of mechanical and aerospace engineering at Rutgers University.

The study, published in Scientific Reports, demonstrates that a fast, scalable, high-resolution 3D printing of hydrogels will remain solid and retain their shape despite containing water. Examples of hydrogels in modern life include in Jell-O, contact lenses, diapers and the human body.

“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,” says Lee. “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 full potential of this smart hydrogel has not been unleashed until now. We added another dimension to it, and this is the first time anybody has done it on this scale. They’re flexible, shape-morphing materials. I like to call them smart materials.”

Engineers at Rutgers and the New Jersey Institute of Technology have been working 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. This hydrogel manufacturing has traditionally relied heavily on conventional, two-dimensional methods such as molding and lithography.

In the study, the engineers used a lithography-based technique that’s 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, and another chemical facilitates bonding when light hits it with a dye that controls light penetration.

The engineers have learned to control how much the hydrogel will shrink and grow. If temperatures sink below 32 degrees Celsius (about 90 degrees Fahrenheit), the hydrogel will absorb more water and swell in size. Meanwhile temperatures that exceed 32 degrees Celsius will begin to expel water and shrink the hydrogel. Thus far, the objects the researchers have been able to create within the hydrogel range from the width of a human hair to several millimeters long. They have also discovered that they can control the motion of these objects by interceding with the temperature.

“If you have full control of the shape, then you can program its function,” adds Lee. “I think that’s the power of 3D printing of shape-shifting material. You can apply this principle almost everywhere.”

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