J. Linnabary, J. Twiddy, S. Swaminathan, M. Daniele
North Carolina State University, University of North Carolina at Chapel Hill,
United States
Keywords: electrochemistry, impedimetric, biosensor, wearable, discrete, miniaturized, impedance spectroscopy, edge sensing, point of care
Summary:
Electrochemical impedance spectroscopy (EIS) is a versatile sensing technique used for materials characterization, analysis of batteries and other electrochemical systems, and sensors targeting human health. EIS has been used in a variety of biomedical contexts, including detection of antibiotics and other medications, sensing of biomarkers related to various physiological and pathophysiological states, blood components analysis, and detection of pathogens. Beyond analyte detection, EIS has also enabled mechanical sensing of blood pressure, tomographic imaging of tissue, and quantification of cells in culture. The ability of EIS to facilitate label-free detection of a range of targets has stimulated interest in applying the technique to wearable, point-of-care, point-of-patient, and edge sensing applications in which clinical resources and infrastructure are limited; however, conventional benchtop EIS systems are expensive, power-hungry, and large, rendering them unsuitable for mobile use cases. To address this limitation, our group and others have developed portable, miniaturized EIS systems, in an attempt to improve the field-readiness of this technology; “minimalist” systems targeting single-frequency impedimetric sensing show particular promise for use in this role. Nevertheless, several features intrinsic to EIS as a technique – such as the requirement for precise sine wave generation or computationally-intensive Fourier analysis, depending on the approach used – ultimately constrain the versatility and efficiency of these systems, highlighting a need for lower-power and less-expensive alternatives. In response to this unmet need, we propose a system employing a simpler, time-domain impedimetric measurement to facilitate minimalist edge sensing with minimal hardware requirements. Our technique assumes a Randles-like electrode response, and replaces sinusoidal perturbation of the sample with a digital stepwise perturbation. By analyzing the time-domain response of the electrochemical cell, our system bypasses the need for either sinusoidal perturbation or frequency-domain analysis common to conventional EIS sensing. As a result of this simplification, we are able to provide binary analysis of samples using a hardware scheme that is extremely quick, compact, inexpensive, and low power compared to existing EIS systems targeting mobile use, enabling widespread dissemination outside of the clinic for purposes of identifying patients requiring more accurate secondary screening. To demonstrate the utility of this technique, we have created a proof-of-concept prototype that performs impedimetric sensing using only four digital I/O pins, three comparators, and a single transimpedance amplifier. To minimize power consumption and enable supercapacitor-based operation, our device rapidly acquires and stores impedance data to a passive NFC tag before powering down, allowing data to be exfiltrated and processed by an external device such as a standard smartphone. Ongoing characterization and testing of our prototype suggests that this represents a viable approach for future impedimetric edge sensing in resource-constrained environments, opening the door to a new range of biosensing applications in the near future.