A printed and self-powered test strip for digital conductivity measurement of low volume liquid samples

In this paper, we present a smart implementation of a self-powered device able to analyze the ionic conductivity of low volume liquid samples. The device sensing core consists of a stack of two single use paper-based batteries that operate as conductivity sensors. To operate the system in a self-powered mode a resistive element is connected to the battery stack so its output voltage is directly related to the conductivity of the electrolyte to be analyzed. The output voltage signal is managed by a flexible hybrid circuit fabricated using inkjet printing that requires a minimal number of discrete off-the-shelf electronic components to function. The energy delivered by the battery allows displaying the result to the user by means of screen-printed electrochromic displays. The sensing battery has been calibrated and tested using NaCl samples at different conductivities. As a practical application, a final device able to assess the suitability for crop irrigation of water samples has been designed, fabricated and validated with real water samples coming from different sources. However, its applicability expands to conductivity measurement of any biological sample.