As wearables become more popular, researchers are devising new devices that may play a critical role in monitoring health. The future of biomedical wearables, however, first requires advancements in biosensing and human-device compatibility.
To this end, researchers have honed in on a type of transistor called the organic electrochemical transistor (OECT) that has shown notable biosensing capabilities. A transdisciplinary group from Northwestern University recently announced significant strides in developing OECT technology using a vertical transistor architecture.
The electrochemical transistor is based on a new kind of electronic polymer and a vertical architecture. Image courtesy of Northwestern University
What Are Organic Electrochemical Transistors?
An OECT operates by modulating the conductivity of organic semiconductor materials by applying an external electric field or chemical potential.
In general, OECTs consist of three major parts: an active layer, a gate electrode, and source and drain electrodes. The active layer, which is normally made of organic semiconducting material, is responsible for the device’s electric properties. The gate electrode controls current flow through the active layer, and the source and drain electrodes receive the applied voltage and measure out the resulting current. In an OECT, all three of the electrodes are in contact with the active layer.
The schematic of an OECT sensor. Image courtesy of Frontiers
In many cases, the gate terminal of an OECT connects to an electrolyte solution, which can be used to introduce ions into the organic semiconductor material. In these applications, the OECT uses an electrolyte solution to control the ion concentration in the semiconductor material and, thus, the electrical conductivity. These ions can then be used to modulate the conductivity of the material and control the flow of current between the source and drain terminals.
In this way, OECTs can be used as a switch or a detector.
Why OECTs Appeal in Biometric Monitoring
There are a number of reasons why OECTs are being investigated so heavily for bio-wearables.
OECTs have a number of favorable operating characteristics, such as low driving voltages, low power consumption, and high transconductance (in other words, a high change in output current for a small change in input voltage). Because of these characteristics, OECTs have promise in biomedical applications where it is necessary to detect and amplify small biological signals, such as neurotransmitters or enzymes, and convert them into measurable electrical signals.
OECTs in biomedical applications. Image courtesy of Frontiers
Additionally, because they are made from organic materials, OECTs are considerably flexible and biocompatible compared to inorganic transistors. This allows OECTs to fit into a number of form factors and locations where traditional electronics may not be feasible, such as implantable medical devices.
Yet, despite their promise, there are still challenges in developing OECTs. In particular, the poor electron-transporting properties of OECTs have historically made them much more performant as p-type transistors than n-type transistors. To date, n-type OECTs have been shown to have approximately 1,000 times lower transconductance and current density than their n-type counterparts.
Before OECTs can find use in any application, it is necessary to develop n-type and p-type variants that perform comparably.
Northwestern Researchers Develop Vertical OECT
This week, a team of researchers from Northwestern University announced a new, high-performance, and balanced OECT variant.
According to the researchers, they developed OECTs that share equal performance between n-type and p-type variants thanks to two major developments: a new kind of electronic polymer and a vertical architecture.
As described in their paper in Nature, the researchers created a new electronic polymer that blends redox-active semiconducting polymers with a redox-inactive photocurable and/or photopatternable polymer. The result was an ion-permeable semiconducting channel in the OECT that exhibited p-type and n-type mobilities of (3.33 ± 0.27) × 10−3 cm2 V−1 s−1 and (3.06 ± 0.61) × 10−3 cm2 V−1 s−1, respectively.
The vertical architecture of the OECT. Image courtesy of Nature
Additionally, the team employed a vertical architecture in which the contacts are layered on top of each other. The researchers claim that this vertical structure led to greater cycling stability in the device and the ability to stack transistors, allowing for denser integration.
According to the team, this is the first demonstration of a “truly vertical” OECT. The electrochemical transistor prototype is also compatible with both water and blood, allowing it to amplify signals while measuring a heartbeat, the levels of potassium and sodium in blood, and eye motion during sleep disorder studies.
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