Flexible Printed Reference Electrodes for Electrochemical Applications

Abstract

conventional glass-based REs (GREs) in emerging wearable systems. Besides being nonflexible, the GREs are fragile, and it is difficult to miniaturize and integrate them for multisensor applications. [4][5][6] The periodic filling of liquid KCl electrolyte needed in GREs also limits their use in continuous monitoring of health and water quality. [7,8] These limitations of current REs, along with the huge potential of flexible and wearable systems, [5,9] are driving the research toward miniaturized and flexible/wearable REs. Currently, there is no commercial RE that could meet the requirements for flexible electrochemical studies.In terms of the state of the art, various types of REs (Figure 1) have been explored for electrochemical applications. These include standard hydrogen electrode and saturated calomel electrodes (Hg|Hg 2 Cl 2 ), [10] Ag|AgCl, [11,12] and Cu|CuSO 4 . [13] Among these, the Ag|AgCl-based REs are common and more attractive because of easy fabrication, stable electrode potential, and their nontoxicity. [10,14,15] Conventional REs are fabricated on glass using AgCl coated Ag metal wire in a saturated KCl or enriched Cl − ion solution. [8,10,16] However, as discussed earlier, due to rigidity of glass, it is difficult to use GREs in emerging application such as wearables' systems. For this reason, Ag|AgCl-based thin [17,18] and thick film [19][20][21][22] REs have also been explored recently. Although both these types of REs have been shown to have good potential for wearable and disposable devices, they still have several limitations. For example, being a quasi-RE (i.e., absence of KCl layer), the majority of thin film REs exhibit fluctuations in the open circuit potential (OCP) while reacting with Cl − ions. [20] The unstable potential due to the decomposition of AgCl layer over a period of time is detrimental to the long-term operation of thin film REs. The absence of KCl in thin film REs also increases the issue of cross-sensitivity [23] and for this reason thick film RE have been explored. The thick film REs use KCl paste or polymer-KCl composites over AgCl electrode,. [20,21] A common issue with thick films REs is their short lifetime due to rapid decay of KCl layer. [20,21,24] Further, the polymer-based KCl layer in these REs require long setting-up time (more than 2 h) prior to use. These issues have been overcome in the current work through development of flexible reference electrode (FRE) based on thick film Ag|AgCl|KCl.The thick film Ag|AgCl|KCl-based FRE reported here has been developed with screen printing technique, which is known as a cost-effective electrode fabrication method. [25,26] The performance of FRE has been evaluated through electrochemical This work presents screen-printed thick film reference electrodes (REs) on a polyethylene terephthalate substrate. The ion-conducting channel of presented screen-printed thick film Ag|AgCl|KCl electrodes are made from a composite of glass-KCl powder. With this new formulation the REs exhibit negligible variation in the potential (±4 mV)...

Flexible potentiometric pH sensors for wearable systems

“…135 Moreover, repeated bending and stretching of the tattoo-based sensors exhibited minimal effect on mechanical integrity and potentiometric behaviour. Neither bending (180 ) nor stretching (10%) strains impacted the response of the tattoo-based sensor. In addition, the sensors exhibited no apparent memory effect and could rapidly measure dynamic pH changes.…”

“…167,178,179 The decay of the glass-KCl composite electrode in solution is very low and hence the fabricated thick lm REs can show lifetime of over 2 years. 179,180 The exible REs developed with this composite material can also exhibit excellent performance (Fig. 3b), 180 with negligible variation in potential (AE4 mV) under different bending states (radius 3, 5, and 7 mm).…”

“…179,180 The exible REs developed with this composite material can also exhibit excellent performance (Fig. 3b), 180 with negligible variation in potential (AE4 mV) under different bending states (radius 3, 5, and 7 mm). 180 For the large-scale fabrication of exible REs, novel methods such as solid junction membrane based on a polyacrylate inkjet ink could be explored.…”

A Wearable Supercapacitor Based on Conductive PEDOT:PSS‐Coated Cloth and a Sweat Electrolyte

“…The capacitance with variation of frequency was measured by using the expression reported in previous works using imaginary component of the complex impedance data. [ 30,33 ] The capacitance of SC for varying frequencies is shown inset of Figure 3b. The measurement reveals that PEDOT:PSS‐coated‐cloth‐based SC with sweat electrolyte shows capacitance of 59 mF (capacitance density of 9.8 F g −1 ) at a frequency of 10 mHz.…”

Stretchable Sensors for Nanomolar Glucose Detection

“…Wearable technology has been widely developed with increasing interest in continuous physiological monitoring during daily activities 1–4. More insight into the user's health state can be obtained at molecular level with analysis of biomarkers present in sweat, such as glucose, ethanol, lactic acid, ions, cortisol, and small proteins 5–7.…”