Producing stronger and tougher hydrogels 

From the soft fibrous tissue found around our joints to the gluey threads that mussels secrete to stick to rocks, many natural materials can stretch, bend and bear weights thanks to the particular way in which they are assembled. Now, NCCR Bio-Inspired Materials researchers have developed a 3D-printing approach that mimics the assembly process of natural materials to produce stronger and tougher hydrogels. These hydrogels could find applications in soft robotics, for example for making medical tools that can better navigate the tortuous architecture of blood vessels.

Hydrogels are water-loving polymers that can absorb and retain water: they’re found in everyday products such as disposable diapers as well as in several medical applications including wound dressing. Hydrogels used in biomedicine are biocompatible, but they are too weak to bear weights. On the other hand, hydrogels used in material science are stiff but also very brittle, so they can’t be printed into complex shapes.

To produce hydrogels that are soft and strong at the same time, NCCR Principal Investigator Prof. Esther Amstad and her team at Lausanne’s Federal Institute of Technology (EPFL) set out to develop a novel 3D-printing approach inspired by nature.

Nature produces many of its tough materials from tiny compartments filled with reagents that merge together. For example, the adhesive threads that allow marine mussels to firmly hold onto rocks in oceans are produced from compartments filled with the protein collagen. These vesicles are released on demand and self-assemble into threads in the foot of the mussel.

In contrast, researchers usually mix reagents in bulk to produce synthetic materials — like cooks combine ingredients in the kitchen. “This method is simple, cheap and easy to scale-up, but it doesn’t allow us to control the local composition of the material or to process it into complex 3D shapes,” says Amstad. To overcome this hurdle, her team designed hydrogels that are composed of microgel grains that assemble together. This approach has the advantage that each grain can carry different reagents, so researchers can easily vary the material’s local composition.

First, the team converted a reagent-filled liquid drop into a hydrogel. They then exploited the sponge-like property of hydrogel particles to load a second reagent — a component of a glue — inside the particles. Next, the researchers concentrated the particles so that they started touching. As soon as the particles touched each other, they no longer behaved as a liquid. Instead, they became a paste that could be 3D-printed and solidified once it exited the nozzle.

Using this approach, the researchers printed different objects, including a hydrogel strip the size of an SD card that could bear a weight of one kilogram. Further tests showed that a hydrogel cylinder produced with the new approach could be compressed and crushed without breaking. “We even stood upon it,” Amstad says. The findings were published in the journal Advanced Functional Materials.

Stronger and tougher hydrogels could be used to make soft grippers such as prosthetic hands, for example, or tools for introducing stents in narrowed vessels to restore the blood flow. To insert stents, doctors need tools that are soft enough to navigate arteries and veins, but also stiff so that stents can be placed correctly, Amstad says.

Reference: Hirsch, M.; Charlet, A.; Amstad, E. 3D printing of strong and tough double network granular hydrogels, Adv. Funct. Mater., 2020, 31, 2005929. 

Text by Giorgia Guglielmi