Just In: Sugar Transistors
Christopher Zou
Researchers at Columbia University have engineered a biocompatible transistor using sorbitol, a type of sweetener you find in your gum.
From electro-modular epilepsy treatment to Neuralink’s fantastic brain-computer interfaces, all forms of neurotechnology involve gathering and sending signals to the nervous system. These signals can be simplified to electron flows and we’ve developed instruments, such as EEG machines, to analyze them outside of the body. Less developed — but nevertheless of great significance — is how we might link machines of metal physically to human neurons, which are biological constructs.
This is why biocompatible transistors are such an important revelation. What are transistors? Why have they been so difficult to work with in biological contexts? What was the discovery and what comes next? Read on to find out.
Transistors
Electrical engineering students may laugh at me, but for the sake of simplicity, let’s assume that transistors are electronic building blocks that can amplify, receive, and send electric current. By way of their ability to turn circuits “on” or “off,” they also store binary information. Consequently, complex sequences of transistors can represent logic flows and underwrite computing processes in the device you’re reading this on; neurons fulfill similar roles in our nervous system and underwrite the brain you’re reading this with.
For visual learners, here’s a picture of a Field Effect Transistor, or FET, on which the breakthrough bio-transistors were based:
In an OFF state, electrons wish to flow from the source to the drain, but are denied by the gate in between; in an ON state, a positive charge opens the gate and creates a thin channel that permits electron flow. The basic mechanics are similar to other transistors, but FETs collect cleaner signal data and require very slight power input to switch states. Thus, they permit powerful analysis at scales appropriate to the body.
The Bio-Combability Challenge
We fabricate ordinary transistors using silicon, whose semi-conductivity allows it to assume the properties treated above when treated or “doped” correctly. However, because we wish to manipulate signals within living systems, these transistors must be bio-compatible. Silicon transistors mix poorly with humans’ internal environments: water and complex pH changes lead to corrosion and short-circuiting, and waterproofing techniques make for bulky circuits. Moreover, traditional semimetal transistors have rigid surfaces that can damage surrounding tissue.
To combat these challenges, researchers have been at work since at least 2012, when researchers at Harvard and MIT pioneered a 3D tissue scaffold made of epoxy. The scaffolds of transistors and porous material encouraged cell growth in and around them, such that it became difficult to tell where the “tissue began and the electronics ended.” Yet other researchers, including those at Columbia, were unsatisfied with the engineered tissue’s computing speed.
Internal-Ion-Gated Organic Electrochemical Transistors
As a result, Professors Dion Khodagholy and Jennifer Gelinas of Columbia University have created a new device based on the field effect transistor. It is waterproof, biocompatible, and fast enough to allow real-time analysis of nervous electrical impulses. Similar in structure to the figure above, the bulk of the transistor is composed of gold, an inert metal. The bulk of Khodagholy’s innovation, however, is in the channel through which a charge carrier — electrons in ordinary transistors — typically flows to transmit information.
Indeed, Khodagholy’s team first decided to discard the notion of using electrons as charge carriers, relying on ions, or charged particles, instead. They tested the transistor with calcium and potassium ions, which are actually those used by human neurons. Subsequently, Khodagholy’s team engineered a channel out of a flexible, biocompatible, and conductive polymer called poly(3,4-ethylene dioxythiophene) polystyrene sulfonate, incorporating the novel chemical d-sorbitol (the aforementioned sweetener, a form of which is found in chewing gum) in its structure.
By incorporating sorbitol in the channel structure, the channel itself draws water and ions from the transistors’ surroundings, effectively increasing the number of ions inside it and forming an ion bridge. Khodagholy and his team found that this vastly reduced the time necessary for ion crossing, improving computing speed, as TechExplore reported, by an “order of magnitude.”
Where next?
Khodagholy and his team have rigorously demonstrated their device’s capabilities, and Gelinas already anticipates strong applicability in medicine: in a press release, she expressed the transistor’s potential for precise diagnoses and electro-modulation therapy.
In the long term, however, biocompatible transistors may empower real-time analysis of brain activity. Should further research confirm and scale such biocompatible transistors, Khodagholy’s breakthrough could very well be the first steps toward a true brain-computer interface, providing the impetus for singularity-scale dreams of cyborgs and knowledge downloads.
For a list of references, please email ntabnewsletter@gmail.com
Christopher Zou leads the Publications Division of Neurotech@Berkeley with Divya Rodrigues. He studies neurobiology and computer science.
