As 3D printing continues to revolutionize manufacturing, researchers have decided that three dimensions are not enough, and so the concept of 4D printing has begun to emerge. These four-dimensional objects are still built layer by layer in a 3D printer. But given time – the fourth dimension – these devices can automatically morph into a different shape, and thereby even change their function.
So far, researchers have developed devices using materials that are actuated by water or heat. This is significant, since the structures are ready as soon as you pick them up from the printer. However, up until now, the prototypes developed were slow, severely limited in the amount of times they could be used, and weak, since they relied on a bending motion in a flexible material.
Professor Marc in het Panhuis and PhD student Shannon Bakarich are set to change all that. The University of Wollongong researchers are the first to use a process whereby four different materials were printed simultaneously. The hydrogels used by the team consist of a network of poly N-isopropylacrylamide (PNIPAAm) and alginate. Alginate is a salt of alginic acid that is commonly found in seaweed and algae. Among other things, it is used as a thickener in food. PNIPAAm consists of two polymer networks entangled in one another. This gives the material strength and durability. When cracks form in one network, the other network bridges the gaps and so prevents greater damage.
The dual-network structure is not unique to PNIPAAm. However, the researchers used PNIPAAm since it exhibits a large change in volume at a critical temperature of about 32-35° Celsius (90-95° F). This change in volume is caused by a transition of the polymers from a collapsed globule state to an expanded coil state. When the temperature goes down, the polymers collapse back into globules.
The researchers combined thin sections of PNIPAAm with traditional materials. This allowed them to create a design capable of relatively fast linear motion, much like the contraction of a muscle. Best of all, this process is reversible. The transition can be actuated by different stimuli, depending on the hydrogels used.
Using PNIAAm, the researchers have developed a functioning valve that responds to the temperature of the water surrounding it. “It’s an autonomous valve,” says Panhuis in a statement. “There’s no input necessary other than water.” An autonomous device like this is valuable in medical soft robotics. As soon as the surrounding water reaches a certain temperature, the polymer strands inside the hydrogel change their shape. The large change in volume in the hydrogel causes a strong linear motion, which closes the valve.
Combining smart materials and 3D printing in this way offers an exciting method of creating custom designs of small autonomous devices. “The cool thing about it is, it’s a working, functioning device that you just pick up from the printer,” Panhuis said. Maybe we will one day even be able to print our own self-assembling structures and soft robots.
by Bauke Stelma | May 24, 2015 at 9:30 am