Design of Organic Electrochemical Transistors for Bioelectronic Circuits


Bioelectronic devices at the biotic/abiotic interface face a number of key challenges that include device degradation when exposed to biological fluid, their elicited immune response due to mechanical mismatch, and poor signal transduction. Organic electronic materials and their devices, such as organic electrochemical transistors (OECTs) address these shortcomings. They can efficiently transduce ionic signals into electronic currents within the bulk of the device owing to their mixed ionic/electronic transport characteristics. In addition, their operational stability in water, soft mechanical properties (compared to inorganic metals and semiconductors), and diverse fabrication routes have made them attractive choices for biological applications. While single OECTs sensors have made significant contributions to bioelectronics, their integration into more complex circuitry has been limited. The creation of OECT or hybrid circuitry can expand the functionality of bioelectronic devices, allowing for on-site signal amplification or data processing at the interface. Limited circuit implementation arises from shortcomings of current organic mixed ionic/electronic conducting (OMIEC) materials, which often lack stability in oxygenated environments, have low electronic mobilities and relatively slow response times preventing their use in many applications. Moreover, traditional device form factors and fabrication methods hinder the use of multiple OMIECs in one circuit and limit the compactness of the OECTs, which is crucial for future highly localized or high-density recording applications. In this work, I explore new OMIECs, OECT geometries, and fabrication methods to address current shortcomings of state-of-the-art OECTs and show how these materials and approaches can advance OECT circuitry, particularly complementary circuits, for biological applications. First, I characterize the electronic properties of novel OMIECs to assess their potential for use in OECT-based circuitry. I explore hole-transporting (p-type) and electron-transporting (n-type) OMIECs that are optimized for enhanced stability in oxygenated environments, n-type OMIECs optimized for enhanced electronic mobility, and even small molecules and 2-dimensional polymers for their potential to enable new classes of high performance soft active materials. This work shows systematic synthetic design criteria can be developed to create high performing OMIECs for OECT-based circuits. I then explore a new OECT geometry and configuration that includes two vertical OECTs (vOECTs) arranged opposite to one another such that they share a single sensing site. With this compact device geometry, I demonstrate the first ambipolar OECT-based inverter and show its unique use in an analog application (voltage-to-voltage amplification) to record electrophysiological signals. This concept allows for direct amplification of biosignals at the biotic/abiotic interface without sacrificing device footprint and does not rely on the less desirable current output of single OECT sensors. Next, by taking advantage of OMIEC’s bulk properties I demonstrate a new device fabrication paradigm for OECT circuits with the potential to simplify manufacturing. The fabrication method results in self-aligned laser-cut OECTs which are compatible with flexible substrates and can be used to create complementary circuits. Lastly, I discuss future directions for new fabrication methods and compact OECT concepts for next generation sensors and circuits.

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