The Long and Lucky Road to Continuous Molecular Monitoring: A Personal History of Electrochemical Aptamer-Based Sensors

Conspectus “Continuous molecular monitoring”, the ability to measure specific molecules in vivo, in real time, and at high frequency, could transform biomedical science and clinical practice. It could, for example, expose physiological dynamics that current “sample-then-analyze” paradigms obscure and improve the personalization of medical treatment, such as via feedback-controlled dosing that responds automatically to a patient’s changing physiology. Unlocking the full potential of continuous molecular monitoring requires a reagentless, reversible sensing mechanism both selective enough to deploy in the body and general enough to detect almost any analyte, a combination that had long proven elusive. Enzymatic sensors, for example, including the continuous glucose monitor, often work well in vivo but are limited to the small number of analytes for which suitably transforming enzymes exist. “Affinity biosensors”, in contrast, do not require the chemical transformation of their target, rendering them far more general. However, these have historically relied on adsorption to generate a signal (e.g., binding-induced changes in mass), a mechanism that performs poorly in unprocessed biofluids in vitro, much less in vivo. In this Account, I describe the near-30-year effort required to create and bring to clinical trials the first affinity biosensor architecture that performs well in vivo: electrochemical aptamer-based (EAB) sensors. To achieve this, EAB sensors use the binding-induced folding of an electrode-attached aptamer to alter the electron transfer kinetics of an attached redox reporter, a biomimetic signaling mechanism that is reagentless, spontaneously reversible, highly selective, and thus supports the real-time, in vivo measurement of almost any analyte for which a suitable aptamer is available. The road to EAB sensors began in the late 1990s with coincidentally convergent evidence suggesting that binding-induced folding is a near-universal property of destabilized proteins. Through the years-long contributions of many creative collaborators, this ultimately evolved into a nucleic acid-based, electrochemical biosensor architecture of broad utility. However, while even the first examples worked well in blood serum in vitro, EAB sensors drift when placed in the body, a problem ultimately solved by an engineering insight: the strongly frequency-dependent signaling of EAB sensors makes it possible to use them as their own drift-correcting reference. Leveraging this, in 2017, we demonstrated the first real-time, multihour in vivo monitoring of circulating drugs, an advance since extended to a wide range of analytes, to week-long in vivo operation and to feedback-controlled drug delivery. Given these unique measurement and control capabilities, EAB sensors can answer scientific questions and solve clinical problems that other technologies cannot. Seconds-resolved EAB sensor measurements are beginning to reveal, for example, previously unrecognized regulatory and clearance mechanisms and to reshape our understanding of drug and metabolite transport within the body, and the recent successful clinical trial of wearable EAB sensor “patches” supporting real-time measurements of the antibiotic vancomycin suggests the technology could similarly advance the measurement of drugs and biomarkers in healthcare. In short, I believe the continuous molecular monitoring that EAB sensors enable is poised to revolutionize our understanding of physiology and our ability to detect, monitor, and treat disease.