This time-lapse photo series compares a hydrogel with fuel added (middle row) with a control hydrogel to which nothing has been added (top row), over a three-week period (504 hours). The bottom row shows a hydrogel that changes shape independently when two different types of fuel are added. Image: TU Delft.Researchers at Delft University of Technology in the Netherlands have discovered a new process that uses fuel to control non-living materials, replicating similar processes that occur inside living cells. The reaction cycle can easily be applied to a wide range of materials and its rate can be controlled – a breakthrough in the emerging field of such reactions.
This discovery is a step towards soft robotics – soft machines that can sense what is happening in their environment and respond accordingly. The researchers report their work in a paper in Nature Communications.
Chemist Rienk Eelkema and his group try to mimic nature, specifically the chemical reactions in living cells that provide the fuel for the cell. But the toolbox of reactions that can drive non-living materials in the same way is limited.
“Up to now, there are only about five types of reactions that are widely used by researchers,” says Eelkema. “Those reactions have two major drawbacks: their rate is difficult to control and they only work on a specific set of molecules.” Together with PhD candidate Benjamin Klemm, Eelkema found a new type of reaction whose rate can be effectively controlled and which also works on a wide range of materials.
“The essence of the reaction cycle is that it can switch between an uncharged and a charged particle by adding a chemical fuel to it,” Eelkema explains. “This allows us to charge materials and thus modify the structures of those materials, because equal charges repel each other and different charges attract each other. The type and amount of fuel determines the reaction rate, and therefore how long a charge and thus a given structure exists.”
As a demonstration, the researchers used their reaction cycle to charge a hydrogel. Because these charges repelled each other, the gel began swelling in size.
This cycle of chemical reactions could be useful for building soft robots. “Soft robots do already exist – for example, microparticles controlled externally through magnetic or electric fields,” says Eelkema. “But ultimately you’d want a robot to be able to control itself: to see for itself where it is and what is happening and then respond accordingly. You can program our cycle into a particle in advance, then leave it alone, and it performs its function independently as soon as it encounters a signal to do so.”
Eelkema’s next step is to link this process to the environment by adding signal processing to it. “For example, a polymer particle could contain some components of such a cycle. When it encounters the last part of the reaction, the cycle is completed, serving as a signal to disintegrate or swell up, for example.”
The cells in humans and other organisms need energy for a variety of functions: to move, to sense that something is happening and to divide. “This is also the reason why we humans need to eat,” says Eelkema. “That linking of energy to function takes place through chemical reactions and is what defines living beings. It enables cells to control when and where structures are formed or processes take place, locally and for a limited time.”
In contrast, non-living materials can exist forever and function without an energy supply. Until a decade ago, there were no processes that could use a chemical fuel to drive interactions in non-living materials. “We introduced that here in Delft, along with a few other places, and since then the field has exploded,” says Eelkema.
This story is adapted from material from Delft University of Technology, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.