Silk transistors could revolutionize wearable electronics

In your smartphone’s microprocessor chips, more than 15 billion tiny transistors are packed, comprised of materials like silicon, metals such as gold and copper, and insulators. These transistors convert electric current into 1s and 0s to process and store information. The materials used in these transistors are inorganic, originating from rocks and metals.

However, imagine if we could make these fundamental electronic components biologically responsive, capable of adapting to the environment like living tissue.

This remarkable feat was achieved by a team at Tufts University’s Silklab when they replaced the insulating material in transistors with biological silk. Their findings were documented in Advanced Materials.

Silk fibroin, the structural protein found in silk fibers, can be precisely deposited onto surfaces and easily customized with various chemical and biological molecules to alter its characteristics. This modified silk can effectively detect and respond to a wide array of substances from the environment or the human body.

The team’s initial prototype used these hybrid transistors with silk fibroin to create an exceptionally sensitive and rapid breath sensor, capable of detecting changes in humidity.

By further enhancing the silk layer in these transistors, it becomes possible for these devices to detect cardiovascular and pulmonary diseases, sleep apnea, or even measure carbon dioxide levels and other gases and molecules in the breath, providing valuable diagnostic information. When applied to blood plasma, these devices could potentially offer insights into oxygen levels, glucose levels, circulating antibodies, and more.

Before developing these hybrid transistors, the Silklab, led by Fiorenzo Omenetto, the Frank C. Doble Professor of engineering, had already used fibroin to create bioactive inks for fabrics. These innovative inks are capable of sensing environmental and bodily changes, leading to inventions such as sensing tattoos that can be placed under the skin or on the teeth for health and diet monitoring, and sensors that can be printed on any surface to detect pathogens, including the virus responsible for COVID-19.

How it works

At its core, a transistor functions as a basic electrical switch. It features an incoming and outgoing metal lead, with a semiconductor material sandwiched in between. The term “semiconductor” is apt, as it only conducts electricity under specific conditions.

This electronic gatekeeper contains another crucial element: the gate itself, set apart from everything else by an insulator. The gate operates as the “key” to regulate the transistor’s on and off states. It activates the on-mode when a particular threshold voltage generates an electric field across the insulator, spurring electron movement within the semiconductor and enabling the flow of current between the leads.

In the realm of biological hybrid transistors, a silk layer takes on the role of the insulator. When this silk material absorbs moisture, it behaves like a gel that carries embedded ions, electrically charged molecules. The gate, in this case, triggers the on-state by rearranging these ions within the silk gel. The ability to alter the ionic composition within the silk results in changes in transistor operation, allowing it to be triggered by a range of gate values between zero and one.

This innovation opens the door to circuits that can process information not limited to the binary levels of digital computing but can handle variable information, similar to analog computing. Such variations stem from the manipulation of the silk insulator’s contents. Fiorenzo Omenetto explains this as an avenue to integrate biology into modern microprocessors, where the most powerful biological computer, the brain, processes data through variable chemical and electrical signals.

The primary challenge in constructing hybrid biological transistors lay in achieving nanoscale silk processing, reaching down to 10 nanometers or less than 1/10,000th the width of a human hair.

According to Beom Joon Kim, a postdoctoral researcher at the School of Engineering, having surmounted this challenge, we can now fabricate hybrid transistors using the same processes employed in commercial chip manufacturing. This means the potential to manufacture billions of these transistors, harnessing capabilities available today.

With a multitude of transistor nodes, whose connections can be reconfigured by biological processes within the silk, we enter a realm where microprocessors could mimic the neural networks of artificial intelligence. This suggests the possibility of integrated circuits that can self-train, adapt to environmental cues, and directly store memory within the transistors, bypassing the need for separate storage solutions.

While we’re yet to create devices that can detect and respond to more complex biological states or delve into large-scale analog and neuromorphic computing, Fiorenzo Omenetto holds a positive outlook on the future. He envisions a novel interface between electronics and biology, anticipating numerous important discoveries and applications to emerge in the days to come.

Source: Tufts University

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