Organic Semiconductor Devices for Bioelectronics
April 2, 2009
Daniel Bernards, Postdoctoral Scholar
University of California, San Francisco
Since the discovery of highly conductive polymers, a wide range of polymeric and small molecule semiconductors have been developed for devices including transistors, photovoltaics and light emitting diodes. Organic semiconductors have many attractive properties, such as low cost, ease of processing, synthetically tunable properties, compatibility with a variety of substrates, and covalent integration of chemical and biological functionalities. Sensors have recently emerged as a promising application of organic electronics since they benefit from the advantages of organics (e.g. processing and cost) and are not significantly impacted by the usual drawbacks (e.g. performance and lifetime). Furthermore, increased development of biocompatible organic semiconductors and devices functional in aqueous environments make interfacing organic electronics with biology an attractive application.
Distinct from the majority of contemporary electronics, organic semiconductors have the unique ability to sustain mixed conduction of ionic and electronic charge at room temperature. In addition to conventional devices, a subset of organic electronic devices exhibit novel properties due to mixed conduction, such as electrically switchable surface properties, pumping of ions, and release of therapeutic molecules. A prime example of a mixed conductor device is the organic electrochemical transistor, which is analogous to a field-effect transistor with the dielectric replaced by an ionically conducting electrolyte. While its current-voltage characteristics are similar to a depletion mode transistor, these devices are electrochemical in nature unlike the electrostatic behavior of field-effect transistors. Given mixed conduction in these devices, it is natural to incorporate electrochemical or ionic elements for sensing: two examples are enzyme-based sensing and ion channel-based sensing. For enzymatic sensors, sensing is mediated by enzymatic degradation of an analyte (e.g. glucose oxidase for sensing glucose). With appropriate enzyme selection, this class of devices is capable of sensing a variety of analytes over a wide range of concentrations. Combining analytical models from electrochemistry and device physics, it is possible to model these enzymatic sensors and consequently optimize device design. A second sensor type utilizes lipid bilayers and ion channels, where device response is dictated by the behavior of the incorporated ion channels. This sensing mechanism has tremendous potential and is limited only by the stability of the lipid bilayer and the type of ion channels utilized. Interfacing organic electronics and biology has been established through biosensor development and is at the forefront of an effort to integrate electronics and biology. Future applications of organic semiconductors in bioelectronics, from fundamental tools to advanced therapeutic devices, will be discussed.
Daniel Bernards is currently a postdoctoral scholar at the University of California at San Francisco working in the research group of Tejal Desai with projects focused on nanostructured materials for drug delivery. He did his doctoral work in the Department of Materials Science and Engineering at Cornell University in the research group of George Malliaras. His graduate research focused on the interplay between ionic and electronic charge in a variety of organic semiconductor devices, including light emitting devices, photovoltaics, electrochemical transistors, and biosensors. He is the recipient of a National Defense Science and Engineering Graduate Fellowship.