Microneedle-based devices for minimally invasive diagnostics

Today, one of the main goals of medical applications is to improve therapies’ effectiveness while reducing the costs and patients’ pain. The golden standard for medical diagnosis is based on blood sampling, given its cost-effectiveness and efficiency. However, some diseases such as diabetes require continuous monitoring of particular blood parameters, making blood sampling difficult and extremely painful. Moreover, the continuous glucose monitoring (CGM) devices available on the market are quite invasive since they rely on hypodermic needles about 5mm long. This work focuses on the development and characterization of a glucose biosensor based on microneedles (uNs) offering a promising alternative to the state-of-the-art CGM devices, thanks to its minimal invasiveness. The reduced needles length, in the range of hundreds of µm, allows to access the dermal interstitial fluid (ISF). ISF represents one of the best candidates for glucose detection in human body thanks to its richness in biomarkers and its easy access. The proposed glucose biosensor employs a 3-electrode configuration and operates via amperometric detection. To properly work, amperometric sensors need stable and reliable Reference Electrode (RE) and Working Electrode (WE). The project is divided in two main parts. The first part of this work deeply investigates the miniaturized WE features. In particular, the WE is functionalized with different layers to obtain the best chemistries’ combination to maximize the glucose sensitivity and to reduce the activation of competitive compounds. A layer-by-layer approach is followed, to carefully identify and isolate the characteristics of each layer. The WE in then validated exploiting Chronoamperometry (cAMP) and Cyclic Voltammetry (CV). The second part of this study investigates the RE stability. The miniaturization of the RE is one of the main challenges to be faced during the production of microsensors. The miniaturized RE, called quasi-RE (q-RE), is directly exposed to the analyte solution making it susceptible to deterioration followed by possible toxicity and unwanted potential drift. This work focuses on Iridium Oxide (IrOx) based q-RE, which is a promising material due to its biocompatibility and mechanical stability. A first activation step is performed through CV, followed by long-term Open Circuit Potential (OCP) measurement to assess stability. However, IrOx showed variability in stability, leading to the investigation of screen-printed silver/silver chloride (Ag/AgCl) RE as an alternative material. In conclusion, this work demonstrates how the deposition of a semipermeable membrane significantly attenuates the influence of competitive compounds on the WE surface, reducing their contribution to the detected current by about 90%. In addition, the Ag/AgCl q-RE exhibites OCP stability comparable to commercial sensors after approximately one week of measurement. Nevertheless, IrOx remains a strong candidate for q-RE fabrication, according to its chemical nature. The presented work allows to trace a route for an alternative solution to existing CMG devices. Further in-vitro and in-vivo studies represent the next step that will allow to achieve a stable, market-ready device.