Real‐Time Multi‐Ion Detection in the Sweat Concentration Range Enabled by Flexible, Printed, and Microfluidics‐Integrated Organic Transistor Arrays

Abstract

working conditions. [12-15] More precisely, in human sweat, the concentration of Na + changes between 10 × 10 −3-100 × 10 −3 m, it is sweat rate-dependent and associated with dehydration. [3,14,16] K + concentration level ranges between 1 × 10 −3-18.5 × 10 −3 m with a sweat rate-independent partitioning. [3,16] Also, pH can strongly vary between 3-8 units, [3] with changes associated with dehydration and muscle fatigue. [14,15] For the next generation of wearable ion sensors, key requirements include mechanical flexibility, simple array patterning for multi-parametric analysis, and microfluidics integration for continuous sampling. [17-23] Conventionally, selective ion measurements are performed using potentiometric two-electrode systems, in which the potential drop between an Ion Selective Membrane (ISM) and a reference electrode is measured. [21,24,25] However, standard potentiometric sensors are difficult to integrate into an array configuration in a microfluidic platform, due to their high output impedance, [18] and the difficult miniaturization of a stable reference electrode. [26-28] Organic electrochemical transistors (OECTs) are an interesting alternative to conventional potentiometric sensors, overcoming some of these limitations. [29] The OECTs are three-terminal devices (drain, source, and gate), with the source and the drain electrodes connected by a conducting polymeric channel. The organic channel is based on conjugated polymer-polyelectrolyte blends, such as the mainly used poly(3,4-ethylene dioxythiophene):polystyrene sulfonate (PEDOT:PSS). [22,30-32] This active material enables mixed ionic and electronic charges interaction, with ionic conduction provided by the PSS polyelectrolyte chains and electronic conduction by nanometric-sized PEDOT crystallites. [33-35] In the presence of an electrolyte and once a positive gate voltage is applied, the dissolved cations are injected into the PEDOT:PSS channel. The cations compensate electrostatically the sulfonate anions of the PSS phase, subsequently lowering the drain current (hole de-doping) in the bulk of the layer. [2,29] This technology, without the need for a reference electrode, allows a facile miniaturization. [28,36] Moreover, the mechanical flexibility of the PEDOT:PSS channel, [22,37] the compatibility with digital manufacturing such as inkjet printing, [38-41] and the very low output impedance, [18] make OECTs promising candidates for Organic electrochemical transistors (OECTs) show remarkable promise as biosensors, thanks to their high signal amplification, simple architecture, and the intrinsic flexibility of the organic material. Despite these properties, their use for real-time sensing in complex biological fluids, such as human sweat, is strongly limited due to the lack of cross-sensitivity and selectivity studies and the use of rigid and bulky device configurations. Here, the development of a novel flexible microfluidics-integrated platform with an array of printed ion-selective OECTs enables multi-ion detection in a wearable fashion. T...

PEDOT:Tosylate‐Polyamine‐Based Organic Electrochemical Transistors for High‐Performance Bioelectronics

“…In the last years, the use of OECTs for sensing applications has shown enormous progress, achieving the successful detection of a variety of biologically relevant analytes. [39,[54][55][56] However, it has been stated that the pH sensitivity of PEDOT:TOS is rather low, [57,58] which hampers its use in pH-based sensing www.advelectronicmat.de devices. The same has been observed for PEDOT:PSS, as the sulfonate moieties function as the counterion in the complex and, moreover, they show a pK a around 2.…”

Engineering Materials for Electrochemical Sweat Sensing

“…On the other hand, continuous development of ion sensitive field effect transistor (ISFET) and organic electrochemical transistor (OECT) also provides an alternative detection and regulation strategy for electrochemical sweat sensing. [ 146–150 ] ISFET and OECT could be rationally introduced to flexibly control the electrode potential for improving selectivity. From the sensing principles and materials, there is also potential interdependence between the represent biomarkers and temperature.…”

Printed Electrochemical Biosensors: Opportunities and Metrological Challenges

“…Their concentration, ranging from µM to mM, can be quantified both in blood and sweat by potentiometric detection due to their intrinsic charges. The state-of-the-art sensing capabilities for potassium, sodium, and pH are ≈10 µA dec −1 [220]. Traditionally, using non printed devices, or with commercially available Screen Printed Electrodes (SPEs), the selectivity is ensured by adopting selective membrane.…”