Innovative materials mimic biological compartmentalization for soft robotics applications

Living organisms often store chemicals in tiny compartments, using particles and vesicles to maintain high concentrations of reagents. Inspired by nature’s use of compartmentalization, NCCR Bio-Inspired Materials researchers set out to create materials that offer similar efficiency and adaptability — an approach that could find applications in soft robotics, rehabilitation devices, and more.

The team, led by NCCR PI Esther Amstad — associate professor of materials science and engineering at the École Polytechnique Fédérale de Lausanne (EPFL), initially focused on hydrogels. Hydrogels are water-rich materials that tend to dry out over time, making them unsuitable for robotics that are exposed to air. So, the researchers turned to elastomers, rubber-like materials that do not retain moisture. The challenge, however, was that elastomers lack the ability to locally control flexibility and stiffness, which is crucial for creating elastomer-based mechanical joints and moving robots.

To overcome this limitation, the team developed a unique method for turning elastomers into tiny microparticles. This process involves creating drops of elastomer ingredients in the form of an oil-in-water emulsion. When these droplets are exposed to UV light, a chemical reaction occurs, transforming them into solid microparticles. These particles can then be combined with additional elastomer precursors to create a paste that can be 3D printed. After printing, another UV-triggered reaction solidifies the material, forming a second network that links the microparticles together.

By adjusting the composition of the microparticles and the second elastomer network, it is possible to create materials that range from soft and flexible to stiff and strong. Unlike traditional elastomers, the mechanical properties of these materials — called Double Network Granular Elastomers (DNGEs) — can be changed at will. “We can locally vary mechanics, and this allows us to print materials that deform in predefined fashions,” Amstad says.

DNGEs could be stretched up to 1000% without breaking, the team found. Different composition of the microparticles also resulted in varying degrees of stretchability and energy absorption. For example, when the researchers dropped a ball onto both a soft and a stiff DNGE, the soft version absorbed almost all of the ball’s energy, while the stiff version made it rebound. The researchers published their findings in the journal Advanced Materials.

The potential applications for these materials are vast, including in soft robotics, wearable devices, and prosthetics such as mechanical joints that allow movement in certain directions while restricting unwanted motion, Amstad says.

She also notes that study first author Eva Baur, a PhD student in her lab, has undergone extensive training in entrepreneurship and is looking to launch a spin-off company to commercialize these innovative materials. The initiative, Amstad says, highlights the support Baur received from the broader NCCR Bio-Inspired Materials network, which provided her with diverse perspectives and encouragement throughout her doctoral studies.

Reference: Baur, E.; Tiberghien, B.; Amstad, E. 3D Printing of Double Network Granular Elastomers with Locally Varying Mechanical Properties. Advanced Materials 2024, 36 (23), 2313189. https://doi.org/10.1002/adma.202313189.