Rutgers Engineers Develop 4D Printing Method for Shape-Shifting Hydrogels

Whenever I hear the term shape-shifting, I always picture some scary creature from a horror movie that’s able to change its form instantly to capture victims and is *probably* not real. But in reality, 3D printed shape-shifting structures can be used to make better bone implants, expandable robots that can switch from hard to soft, and even pasta that morphs into shapes when cooked and can free up packaging space. Engineers from Rutgers University-New Brunswick have created a 4D printing method for a shape-shifting smart gel that could one day be used to develop soft robots and targeted drug delivery.

When 3D printed objects can move and change shape of their own volition, this technically makes them 4D printed.

According to Howon Lee, an assistant professor in the university’s Department of Mechanical and Aerospace Engineering (MAE), the team’s 4D printing technique prints a 3D object with a hydrogel that morphs over time when the temperature increases or decreases, similar to the 4D printing work by researchers at the Georgia Institute of Technology, Singapore University of Technology and Design, and Xi’an Jiaotong University.

Lee is the senior author of a new study, titled “Micro 3D Printing of a Temperature-Responsive Hydrogel Using Projection Micro-Stereolithography,” that was just published in Scientific Reports; MAE doctoral student Daehoon Han is the lead author, and co-authors include doctoral student Zhaocheng Lu and Shawn A. Chester, an assistant professor at the New Jersey Institute of Technology.

The abstract reads, “Poly(N-isopropylacrylamide) (PNIPAAm), a temperature responsive hydrogel, has been extensively studied in various fields of science and engineering. However, manufacturing of PNIPAAm has been heavily relying on conventional methods such as molding and lithography techniques that are inherently limited to a two-dimensional (2D) space. Here we report the three-dimensional (3D) printing of PNIPAAm using a high-resolution digital additive manufacturing technique, projection micro-stereolithography (PμSL). Control of the temperature dependent deformation of 3D printed PNIPAAm is achieved by controlling manufacturing process parameters as well as polymer resin composition. Also demonstrated is a sequential deformation of a 3D printed PNIPAAm structure by selective incorporation of ionic monomer that shifts the swelling transition temperature of PNIPAAm.”

The study shows the team’s scalable method of fast, high-resolution hydrogel 3D printing. Researchers have long been interested in stimuli-responsive hydrogels that exhibit chemical or physical changes in response to environmental conditions, and as you can see in the above video, the materials, which contain water, stay solid and hold their shape, even while shape-shifting.

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

In the study, the team used temperature-responsive PNIPAAm hydrogel, which has been used for decades in motion-generating devices and biomedical applications like scaffolds.

While hydrogels are most often manufactured using traditional 2D methods like molding, the Rutgers engineers used an inexpensive lithography-based technique, called projection microstereolithography (PμSL), that can quickly print a range of materials into 3D shapes out of a resin made up of the PNIPAAm hydrogel, a dye that controls light penetration, a binding chemical, and a chemical that facilitates bonding when it’s exposed to light.

The research team learned how to use hot and cold temperatures to control the hydrogel’s shrinkage and growth. When temperatures exceed 32°C, the material expels water and shrinks, but when the temperature drops below this, it absorbs water and expands.

“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,” Lee said. “They’re flexible, shape-morphing materials. I like to call them smart materials.”

At their smallest, the 4D printed hydrogel objects the researchers created are the width of a human hair, but can measure up to several millimeters in length. In addition, the team also discovered that by changing temperatures, they can grow just one area of a 3D printed object.

Their 4D printed smart hydrogel could be used in many applications, such as soft robotics, tissue engineering, flexible actuators and sensors, and biomedical devices. It could even offer structural rigidity in human organs, like the lungs, and scientists could insert small molecules, like drugs or water, into the material, which could transported within the human body and released at the proper point.

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[Source/Images: Rutgers]