(Invited) New Flexible Electrodes for Electrochemical Analysis: A Step Forward Towards Wearable Sensors
Authors: Pozo-Ayuso Diego F. , Castaño-Álvarez Mario , Fernandez-la-Villa Ana 
Published: February 2020

Electrochemical sensors present many advantages to develop wearable sensors and to be applied for real samples as at the point-of-care using. They are easy to use, low cost and above all, they allow easy integration with the electronics and different parts of the system. However, there are several problems still to be solved in order to carry out the complete integration of these systems. Currently flexible electronics is already a reality, so, the manufacture of the sensor itself on a flexible substrate without losing any of its relevant functionalities takes on paramount importance. In this sense, printing technologies offer a suitable solution for the development of flexible electrodes as well as electronics to use them. Therefore, these technologies open the door to the development of a whole new range of flexible devices for many applications in biosensing, Point-of-Care (PoC) analysis and wearable systems.

In this work, the fabrication process of carbon-based electrochemical sensors on flexible plastic substrates has been optimized. Thus, different flexible substrates; including several materials, thickness and color; have been evaluated for printing the new sensors. These sensors have been printed using different carbon and silver ink formulations with several mesh screens (stainless steel, polyester…). The selection of the ink formulations and mesh affects to the final surface morphology as well as the shelf-life and performance of electrodes. In the same way, a critical step in the fabrication of flexible sensors has been the selection of the drying conditions. This step enables the curing of the inks by removing the organic solvents, but without affecting to the flexible substrates. In most of the cases, the plastic substrates show a low glass transition temperature. Therefore, the drying time and temperature as well as the methodologies (box oven, conveyor dryer…) have been essential for avoiding some physical change in the substrate and electrode surface. As a flexible substrate is used, the final electrode structures should also show stretchable features.

A microscopy and electrochemical characterization has been carried out with the new flexible carbon-based electrodes. The final sensor consists of a multilayer structure integrating the working (WE), reference (RE) and counter (CE) electrode as well as a dielectric layer on the same bendable substrate (
Figure 1A
). Thus, the surface morphology of the electrodes was observed by using scanning (SEM) and transmission (TEM) electron microscopy in order to check the effect of the substrate, printing conditions and ink compositions. The performance of new electrodes has been also evaluated by using different electrochemical techniques such cyclic voltammetry (CV), amperometry and electrochemical impedance spectroscopy (EIS), in static, and specially, dynamic conditions.

The new flexible sensors were integrated into a basic flow analysis system by using a modular platform (
Figure 1D
). This platform enables the interfacing between the sensor and the other components of the FIA system; peristaltic pump, injection valve and electrochemical station (
Figure 1 B and C
). A portable All-in-One electrochemical station with built-in EIS analyzer (
Figure 1 E
) was used for performing the electrochemical measurements. The performance of the new flexible electrochemical sensors has been evaluated with different benchmark redox compounds in the FIA system.

This work has been supported by EU (MANUNET) and IDEPA (IDE/2017/000124)

Figure 1