![]() We see this admittedly crude initial step as a simple example of how models of controlled ion/molecule transport, an approach with widespread utility in biological systems, can be combined with principles from electronic devices to realize ionic amplification or switching of chemical systems. The nanopore itself provides a confined transport space, which enhances the effects of space charge from counter ions. Chemical selectivity at the level of counter ion charge (positive/negative) is achieved in nanopores with excess surface charge or in thin polymer films 15. While abiotic systems cannot presently approach this level of sophistication, we can provide simplified, layered designs to construct synaptic cleft inspired devices. This process-selective release (e.g., from vesicle fusion at the presynaptic terminal), confined transport of ions in space (in the synaptic cleft), and selective uptake (e.g., a specific ion channel receptor in the postsynaptic cleft), provides an ultimate example of what can be realized with gated and selective transport of ions. In principle ionic amplification is observed in the classic biological example of spatially and chemically complex ion transport that occurs in the synaptic cleft, where signaling molecules, driven by a propagating action potential, are released from the presynaptic terminal, diffuse across the synaptic cleft, and are taken up by receptors on the postsynaptic terminal 14. Such a system would be fundamentally different in operation from OECT based devices and may find utility when the chemical identity of the output is important, and when that output has to be delivered to a specific part of the circuit. It would be of great interest however to design circuits that amplify small ionic currents such that the output is ionic as well. OECTs enable natural communication between ionic devices-so-called iontronics-and electronics but due to electronic output, they do not offer amplification in an ionic manner. Organic electrochemical transistors have found numerous applications, which include printed integrated circuits, biological interfaces, and neuromorphic devices. OECTs offer large amplifications, because small ionic signals are amplified into a large electronic current 13. In OECTs injection of ions from solution leads to modulation of conductivity of a semiconductor or conducting polymer. Among ionic circuits reported, those that utilize organic electrochemical transistors (OECTs) have attracted significant interest from both fundamental and applied perspectives due to the high amplifications and speeds of operation they offer 11, 12. ![]() Logic gates composed of microfluidic ionic transistors have also been designed 10. In another example, ionic diodes based on rectifying nanopores and microchannels were connected into logic gates 7, 8, 9. Connecting four such channels into a circuit known from electronics as a bridge rectifier enabled changing alternating current into direct current. One of the first ionic circuits was composed of four biological α-hemolysin channels, where individual channels had been chemically modified so that they functioned as ionic diodes 6. In the signal transduction of sound, for example, hair cells of the cochlea mechanically transduce sound waves into ion currents by opening cochlear ion channels to ionic transport open channels allow millions of ions to pass through per second, which leads to signal generation (in the form of a change in transmembrane potential), and is ultimately detected and processed by the brain 5.Įxamples of man-made ionic circuits have already been reported. This transmembrane ionic transport is often the first step in a biological amplification process, which enables sensing external stimuli including light, sound, and odor. The key players in physiological processes are biological channels in a cell membrane that facilitate exchange of ions and molecules, for instance between the intracellular and extracellular spaces in cells and tissues. On the other hand, the physiological processes of living organisms rely on another type of circuit, which is entirely ionic and functions in an aqueous environment 3, 4. ICs consist of electronic components such as diodes and transistors, and allow for signal manipulation and amplification. Integrated circuits (ICs) revolutionized our lives, and are ubiquitous in virtually all electronic devices including cell phones, computers, and pacemakers 1, 2.
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