Examining the intricate electrical signaling properties of the nervous system is made possible by the interface between electronics and neural tissue. Additionally, neural circuitry can be modulated by implanted electronic devices to prevent or treat a variety of diseases. Sadly, there is a basic mismatch between soft tissues and stiff electronic substrates that puts fragile biological systems at risk of harm.
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The way Swedish researchers have solved this mismatch is by implanting electrodes inside the body. Linköping, Lund, and Gothenburg University researchers have developed a method that produces soft, substrate-free conducting materials inside live tissue by using the molecules found in the body as triggers.
An injectable solution composed of an intricate mixture of molecular precursors serves as the foundation for the method, which was detailed in Science. This gel includes an organic monomer called ETE-COONa, along with crosslinkers, oxidase enzymes (lactate oxidase (LOx) or glucose oxidase (GOx)), and horseradish peroxidase embedded in a polymer matrix. Following injection, the enzymes cause the organic monomer to polymerize into a stable, soft conducting gel by breaking down endogenous metabolites (such as glucose or lactate) in the tissue.
We have been attempting to develop electronics that imitate biology for a number of decades. According to a press release from Magnus Berggren of Linköping University, “We now let biology create the electronics for us.”
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Fabrication of in vivo electrodes:
By injecting the cocktail gels into live, anesthetized zebrafish, Berggren and associates confirmed the enzyme-triggered polymerization process. Gels that were injected into the tailfins of zebrafish polymerized in vivo, giving the fin cavities their characteristic dark color. Both lactate and glucose functioned as efficient catalysts; lactate induced faster polymerization, probably because zebrafish tissues contain higher concentrations of lactate than glucose.
Roger Olsson from Lund University led the team that injected the cocktail including LOx, the oxidase enzyme, into the anesthetized zebrafish brains. Dark blue polymer was found in the dissected brain slices, suggesting polymerization, which the researchers verified with UV-vis absorption spectroscopy.
They used gold microelectrode arrays (MEAs) to support the brain slices and recorded electrical activity in areas of the slices that had dark patches. Applying a voltage between -0.5 and 0.5 V resulted in a linear current through dehydrated brain slices. Comparable tissue samples showed a lower current than this one.
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Crucially, it seemed that the gels based on LOx were safe. The zebrafish exhibited typical swimming behavior three days following gel polymerization in the brain, and there were no indications of tissue damage at the injection site.
Polymerization was not induced in the zebrafish brains by injecting cocktail gels based on GOx. Zebrafish brains are known to have high lactate and low glucose concentrations, much like those of humans, so this poor performance was not surprising.
Additionally, the scientists submerged the removed zebrafish hearts in cocktail gels made of GOx or LOx. Dark blue lines were seen on the surface of the hearts for both gels, demonstrating that polymerization was induced by both lactate and glucose. When a linear voltage sweep was applied, hearts that had been taken out of the gel and integrated with MEAs showed a linear current response—a behavior that was absent from control samples.
These results show that soft electronics can be developed in a variety of biological tissues and settings through the production of electronic conductors powered by endogenous metabolites. The gels were injected into samples of beef, pork, chicken, and tofu by the researchers in order to verify this. The absence or low concentration of the necessary metabolites prevented them from seeing polymerization in the plant-based tofu, but they did see it in the other tissues.
Finally, using the straightforward and accessible nervous system of medicinal leeches, the scientists injected the gel into these organisms to explore the prospect of developing recording and stimulating electrodes for neuroscience applications. They demonstrated how in-situ polymerization of LOx-based cocktails allowed them to interface nerve tissue with gold electrodes on a small, flexible probe.(Source: PhysicsWorld)