All living organisms have systems that can link multiple signals to manage tasks. For example, humans have systems of organs that can combine inputs from sight, hearing, and smell in a fear response that can cause our hearts to speed up. This ability—called complex signal integration—is not found in artificial systems. This new study demonstrates a pathway for simple, soft artificial materials to use multiple signals from external sources to produce distinct responses. Scientists discovered that, when exposed to the sequential addition of acid and metal ions, a type of material called a hydrogel polymer responded with unexpected gel expansion, contraction, and color change.
Hydrogels are 3D materials that can absorb large amounts of liquid. Scientists have developed many kinds of hydrogels that respond in specific ways to chemical signals. This new research creates the possibility of artificial materials with abilities similar to biological materials. By programming how hydrogels respond and behave, scientists could design hydrogels for complex applications. For example, these future materials may be able to move and navigate on their own. Hydrogels could even to tune themselves over time and space to modify the rate of chemical reactions.
In biological organisms, materials convert stimuli into chemical reactions, changes in shape, and other responses. These dynamic responses are what make living things adaptable. In living materials, these cascading reactions can often outlast the stimulus itself. The longer cascades enable the response to be coupled to a subsequent stimulus. In synthetic hydrogels, by contrast, the cascade of reactions ends almost instantly. This means that hydrogels have fast responses, but do not integrate multiple signals separated across time. Materials scientists are working to mimic this ability to integrate multiple signals in artificial materials.
In this research, scientists sought to form longer-lifetime response complexes in a hydrogel and use those longer responses to produce unique reactions in the hydrogel. Scientists previously had not considered that hydrogels could do more than simply shrink and expand in phase with specific stimuli. The experiment involved sending sequential stimuli (in the form of copper or acid) to a widely used hydrogel. Adding either copper or acid independently shrinks the gel. However, sending the copper or acid stimuli sequentially caused the hydrogel to respond with a cascading series of energy conversion reactions in the form of brief bursts of competing localized chemical and mechanical energy flows. The reaction occurs when the addition of acid displaces copper already bound to the gel more quickly than the copper can diffuse out of the gel, leading to a temporary influx of water. This triggers traveling waves of elastic expansion and contraction through the gel. Having the acid move more slowly through the gel allows the displaced copper to create traveling color waves as the copper rebinds to the gel ahead of the acid front. The scientists developed a theoretical model that fully predicts these phenomena. This model allows researchers to program reactions with specific chemistries. It also enables researchers to explore other such pathways for converting energy introduced into a material by outside forces into productive work.
This research was supported by the Department of Energy Office of Science, Basic Energy Sciences. The principal investigator also thanks the Netherlands Organization for Scientific Research and the Ministry of Education, Culture and Science for financial support.
Korevaar, P.A, et al., “Non-equilibrium signal integration in hydrogels” Nat. Commun. 11(1): 38620 (Jan 2020). [DOI: 10.1038/s41467-019-14114-0]