Video: Triggered by electricity or light, the thermoresponsive hydrogel with an integrated graphene network can carry out movement tasks. © Margarethe Hauck
Strong yet supple: Inspired by natural muscles, researchers have transformed hydrogels into powerful motor skills that can be activated by electrical current or heat radiation. The material, which consists mainly of water, was equipped with fine tube structures. They enable rapid changes in water content and therefore volume to carry out movement tasks. The scientists say that the novel concept could now advance developments in soft robotics.
Hard metal and rigid plastics have long shaped the development of robotics – but for some time now a technology has found its way there that is based on the rather gentle concepts and materials of nature: Developments in soft robotics enable applications that conventional systems with do not offer their rigid structures. Because they can handle sensitive objects or move flexibly and non-destructively in complex environments. As a result, they have a wide range of potential applications, for example in industry or medicine. To move these systems, units are used that take on the function of muscles. There are already concepts based on pneumatic or hydraulic effects. A research team led by the Christian-Albrechts-Universität zu Kiel (CAU), on the other hand, is dedicated to the potential of hydrogels as motor units in soft robotics.
These are structures based on polymer materials that can absorb enormous amounts of water. They are similar to the structure and consistency of body tissues such as muscles. In principle, thermoresponsive hydrogels can also be excited to repeat contractions. From a temperature of 32 degrees Celsius, they release water and thus reduce their volume by up to 90 percent. When the temperature drops, the hydrogel then reabsorbs the water and returns to its original size. “Basically, these hydrogels already have the potential to function like a human muscle,” says lead author Margarethe Hauck from the CAU. However, the previous hydrogels are not suitable for use as soft drive elements for new types of soft robots. This is because the process of volume change is much too slow for most practical applications.
Fine canals integrated
In order to speed up the process of changing the water content and make it controllable, the materials researchers have now integrated a network of tiny tubes into their hydrogels. The micro-lines are formed by zinc oxide and have a thin graphene coating. “Our approach follows nature’s example,” says senior author Fabian Schütt from the CAU. “Plants and animals have networked, hierarchically structured canal systems for effective material and fluid transport, such as the capillary system in humans. “We can also use this principle to improve the properties of soft materials,” says Schütt.
As the researchers explain, the water can flow very effectively out of or into the hydrogel through the interconnected tubes, thus enabling the volume to be changed quickly. The graphene coating of the fine network also makes it electrically and thermally conductive. In this way, the researchers can effectively heat the hydrogel with electricity or radiation and thus control the change in volume at the touch of a button. “This means that the material can be reduced and enlarged much faster than before,” says co-first author Lena Saure from the CAU. Schütt adds: “This is a crucial aspect when it comes to the practical application of such soft actuators. We are now also working on further accelerating this process”.
Can be activated by electricity or light
The team has already been able to demonstrate the effectiveness of the concept through example applications. For example, a gripper made of this material can grab an object by activating it using light energy, lift it and then release it again when it cools down. “This makes it clear that the material can also be controlled wirelessly and very flexibly,” says Saure. The developers emphasize that the hydrogel is also generally characterized by its adaptability: a different structure of the inner tubes or a higher or lower concentration of graphene changes the reaction times or the forces exerted.
“For the first time we can move a hydrogel with both speed and force. Together with its responsive properties, with which it reacts independently to external stimuli, this brings us a decisive step closer to intelligent, high-performance materials for soft robotics,” says co-author Rainer Adelung from the CAU. Due to the tissue-like properties of the hydrogel, the researchers see application potential in the medical field, for example in assisted surgery or in artificial tissue construction.
Source: Christian-Albrechts-University of Kiel, specialist article: Specialist article: Advanced Materials doi: 10.1002/adma.202302816