Summary. Electrochemical, aptamer-based (EAB) sensors are a minimally invasive technology already shown to support seconds-resolved, real-time, in vivo molecular measurements irrespective of the chemical reactivity of its targets. That is, EAB sensors are the only real-time molecular measurement technology that is simultaneously (1) generalizable to drugs and biomarkers and (2) demonstrated to work in situ in the body. In support of these claims, as preliminary results we have reported the first ever seconds- (and even sub-second) resolved, in-vivo measurements of multiple drugs and protein biomarkers. These measurements, which employ 75-µm-diameter sensors placed in situ in the blood (jugular) and tissues (brain, muscle) of awake, freely moving rats, achieve clinically relevant accuracy, precision and specificity over the course of hours. A remaining technological limitation, however, still precludes the further clinical testing of EAB sensors: after more than after ~5 h in vivo, aptamer degradation begins to reduce EAB precision significantly. The R21 project described here is focused on solving this problem. Specifically, here we propose the introduction of degradation-resistant, non-natural XNA aptamers into the EAB platform as a means achieving multi-day in vivo molecular measurements. Successful conclusion of this work would set the stage for R01-scale projects aimed at ascertaining the clinical value of long-duration EAB measurements in, for example, the feedback-controlled delivery of narrow-therapeutic-index antibiotics over multi-day treatment courses and the real-time monitoring of diagnostic cytokines in the ICU over the multi-day clinical course of infections, such as in sepsis and COVID-19.