Moving hydrogels have potential for soft robotics and artificial muscles

Texas team develops hydrogel films that can be programmed to expand and shrink to perform complex three-dimensional movements and functions, similar to muscles

artificial muscle

The field of soft robotics is developing fast, as it is of great interest to important industries like agriculture and food handling. Developing materials that can change their shape in a similar way to natural muscles also has great potential for medical applications such as prosthetics and tissue engineering. However, such materials have proved difficult to design. Materials scientists and engineers at the University of Texas at Arlington (UTA) are now reporting that they have developed a process by which two-dimensional hydrogels – essentially, thin films – can be programmed to expand and shrink in a way that applies force to their surface, which forces them into complex three-dimensional shapes and motions.

Kyungsuk Yum and his PhD student, Amirali Nojoomi, studied the behaviour of continuously deformable soft tissue such as muscles in nature, to see whether any of the natural properties could be mimicked to create dynamic 3D structures. Yum hit upon an approach using temperature responsive hydrogels whose rates of swelling and shrinking our non-homogeneous: that is, some parts shrink more than others. In a paper in Nature Communications Yum and Nojoomi explain how they developed a digital light 4D printing method (the three linear dimensions plus time) to form such materials.

The printing method uses materials which polymerise in response to light, and the regional temperature response difference is encoded by using two different types of compound in the initial mix which cross-link the strands of the hydrogel precursor as the polymer forms. The amount of cross-linking is controlled by the time the precursor mixture is exposed to light. Yum refers to this as ‘phototunability’, and explains in the paper that this provides “a flexible means to encode the hydrogels with spatially and temporally controlled growth (swelling and shrinking), which can be used to program the formation of 3D structures and their motions.

The initial mix consists of three different compounds: N-isopropylacrylamide (NIPAm, which forms the bulk of the hydrogel polymer), N,N′-methylene bisacrylamide (BIS; a short-chain crosslinker), and poly(ethylene glycol) diacrylate (PEGDA; a long-chain crosslinker). Hydrogels linked by BIS swell and shrink at different rates to those linked by PEGDA, and using both gives the hydrogel its phototunability.

Different zones within the hydrogels are structured with varying degrees of cross-linking using short and long links, enabling programmable motion

Yum has also developed design rules for the hydrogel to print it in a modular fashion to create even more complex bio-inspired structures which have sequential motions programmed into them. This allows them to move through space, while also giving him the ability to control the speed at which the structures change shape. This has allowed him to create a form which moves like a stingray swimming through the ocean. “Unlike traditional additive manufacturing, our digital light 4D printing method allows us to print multiple, custom-designed 3D structures simultaneously. Most importantly, our method is very fast, taking less than 60 seconds to print, and thus highly scalable.”

Commenting on the research, Stathis Meletis, chair of UTA’s materials science and engineering department, who was not involved in the work, said: “Dr Yum’s approach to creating programmable 3D structures has the potential to open many new avenues in bioinspired robotics and tissue engineering. The speed with which his approach can be applied, as well as its scalability, makes it a unique tool for future research and applications.”