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1. Introduction
In the past decades, there has been a significant increase in the interest in using printing technologies as techniques for the development of low-cost, large-area electronic systems. These provide a wide range of advantages compared to traditional techniques of fabrication of silicon-based electronics. One of the most important advantages is the possibility of working with flexible substrates such as plastic that has allowed the manufacture of sensors at low cost. The combination of additive fabrication techniques (inkjet, gravure, serigraphy, etc.) and roll-to-roll fabrication can lead to the production of ultra-low-cost devices (FP7 Observatory, 2010) and what is more, it has allowed the development of new types of sensors with particularly relevant features for being used in bioengineering applications.
Regarding bioelectrical surface recordings, they are usually performed by unipolar or bipolar disc electrodes, even though they entail the serious disadvantage of having poor spatial resolution. Laplacian potential on the body surface can be estimated by means of concentric ring electrodes (He and Cohen, 1992, Lu and Tarjan, 2002 and Besio et al. , 2006). It has been proved that bioelectrical surface recordings performed with such concentric ring electrodes improve spatial resolution compared to unipolar or bipolar disc electrodes. However, concentric ring electrodes have commonly been implemented on rigid substrates. Therefore, they do not adapt to the body surface curvature, which provokes discomfort to the patient and a poor contact that affects the signal quality. An active sensor based on a tripolar concentric ring electrode in bipolar configuration (TCB) implemented using screen-printing technology on a flexible substrate for obtaining high-spatial-resolution electrocardiogram (ECG) recording has already been developed. However, to further improve the spatial resolution achieved with the TCB electrode, a tripolar concentric ring electrode (TCE) design is needed (Prats-Boluda et al. , 2012).
At present, there are a several different printing techniques used in the fabrication of electronic devices, sensors or electrodes, which have their origin in graphic arts and can be used in the fabrication of these devices. The most widely used printing technologies in industry or research are screen-printing, inkjet printing, stamping/nanoimprinting and gravure printing. Obviously, there are many more techniques, but they are not usually applied in the fabrication of electronic systems. It is worthwhile to highlight the advantages and disadvantages of...