The progress in the development of wearable devices in the last decade has been of great interest and importance in scientific research. In the last decade, one of the most critical research areas in wearable devices has been developing and manufacturing flexible electrochemical sensors capable of detecting and monitoring analytes in body fluids such as sweat, tears, and saliva. This interest arises since the traditional method for monitoring most analytes related to diseases or health conditions is carried out through blood analysis. This traditional analysis method of analytes in the blood is painful; it is invasive sampling and more expensive than sweat analysis. In recent years, a variety of wearables have been developed to analyze analytes in sweat through electrochemical and optical interfaces. However, the problem with developing these sweat biosensors has been limited to monitoring a small number of biomarkers such as lactate, glucose, electrolytes, and pH.

Our goal is the development of a flexible biosensor capable of real-time detection and monitoring neuropeptides and neurotransmitters in human sweat. One of the molecules of great interest for our project is neuropeptide Y (NPY). Studies have shown that its excessive presence or absence serves the prognosis of various neurodegenerative diseases and conditions such as Parkinson's disease, epilepsy, depression, anxiety, insomnia, obesity, and recently has been found that NPY could be associated with cardiovascular problems. The development of a device capable of detecting and monitoring the various levels of NPY on sweat could enhance and facilitate treatment and diagnostic for different diseases. These levels can be quantified through electrochemical analysis by testing aptamer-modified silver flexible electrodes to amplify the electrode's selectivity by enabling NPY adsorption to the surface of the working electrode.

Our flexible sensor consists of paper-based microfluidics that serves as our system's passive flow method and silver inkjet electrodes printed on the PET surface. Microfluidic systems can be characterized as active or passive, depending on the force applied to the sample or flow. We can perform minimal flow or particles in a minimum amount of liquid or a sample flow with paper-based microfluidics systems. Among the passive microfluidic systems, paper-based microfluidics, also known as micro-pads, are among the most promising methods because they are easy to manufacture and use, inexpensive, and have low sample volume. Furthermore, micro-pads are the most widely used and suitable for point-of-care testing thanks to their ability to move the flow through capillarity and flexibility. We performed electrochemical characterization on our silver inkjet electrodes through cyclic voltammetry and electrochemical impedance spectroscopy (EIS). Since sweat is a complex fluid, it is necessary to modify the working electrode's surface to give the electrode's selectivity towards the molecule to be analyzed or detected. To achieve the detection for NPY, we first add carboxyl groups to the surface of the working electrode, which will serve as an anchor for the specific aptamer for NPY. We use different buffer solutions like artificial cerebrospinal fluid, phosphate buffer solutions, and artificial sweat. We decreased the oxidation of the surface of our electrodes by changing the buffer and modifying the surface for greater selectivity towards NPY. In conclusion, these electrodes can be an innovation in monitoring non-invasive diseases and, therefore, painless.