Microneedle Aptamer-Based Sensors for Continuous, Real-Time Therapeutic Drug Monitoring

A novel strategy for therapeutic drug monitoring: application of biosensors to quantify antimicrobials in biological matrices

Over the past few years, therapeutic drug monitoring (TDM) has gained practical significance in antimicrobial precision therapy. Yet two categories of mainstream TDM techniques (chromatographic analysis and immunoassays) that are widely adopted nowadays retain certain inherent limitations. The use of biosensors, an innovative strategy for rapid evaluation of antimicrobial concentrations in biological samples, enables the implementation of point-of-care testing (POCT) and continuous monitoring, which may circumvent the constraints of conventional TDM and provide strong technological support for individualized antimicrobial treatment. This comprehensive review summarizes the investigations that have harnessed biosensors to detect antimicrobial drugs in biological matrices, provides insights into the performance and characteristics of each sensing form, and explores the feasibility of translating them into clinical practice. Furthermore, the future trends and obstacles to achieving POCT and continuous monitoring are discussed. More efforts are necessary to address the four key 'appropriateness' challenges to deploy biosensors in clinical practice, paving the way for personalized antimicrobial stewardship.

Long-Term In Vivo Molecular Monitoring Using Aptamer-Graphene Microtransistors

Long-term, real-time molecular monitoring in complex biological environments is critical for our ability to understand, prevent, diagnose, and manage human diseases. Aptamer-based electrochemical biosensors possess the promise due to their generalizability and a high degree of selectivity. Nevertheless, the operation of existing aptamer-based biosensors in vivo is limited to a few hours. Here, we report a first-generation long-term in vivo molecular monitoring platform, named aptamer-graphene microtransistors (AGMs). The AGM incorporates a layer of pyrene-(polyethylene glycol)5-alcohol and DNase inhibitor-doped polyacrylamide hydrogel coating to reduce biofouling and aptamer degradation. As a demonstration of function and generalizability, the AGM achieves the detection of biomolecules such as dopamine and serotonin in undiluted whole blood at 37 °C for 11 days. Furthermore, the AGM successfully captures optically evoked dopamine release in vivo in mice for over one week and demonstrates the capability to monitor behaviorally-induced endogenous dopamine release even after eight days of implantation in freely moving mice. The results reported in this work establish the potential for chronic aptamer-based molecular monitoring platforms, and thus serve as a new benchmark for molecular monitoring using aptamer-based technology.

Electrochemical Affinity Biosensors: Pervasive Devices with Exciting Alliances and Horizons Ahead

Electrochemical affinity biosensors are evolving at breakneck speed, strengthening and colonizing more and more niches and drawing unimaginable roadmaps that increasingly make them protagonists of our daily lives. They achieve this by combining their intrinsic attributes with those acquired by leveraging the significant advances that occurred in (nano)materials technology, bio(nano)materials and nature-inspired receptors, gene editing and amplification technologies, and signal detection and processing techniques. The aim of this Perspective is to provide, with the support of recent representative and illustrative literature, an updated and critical view of the repertoire of opportunities, innovations, and applications offered by electrochemical affinity biosensors fueled by the key alliances indicated. In addition, the imminent challenges that these biodevices must face and the new directions in which they are envisioned as key players are discussed.

Communication Protocols Integrating Wearables, Ingestibles, and Implantables for Closed-Loop Therapies

Body-conformal sensors and tissue interfacing robotic therapeutics enable the real-time monitoring and treatment of diabetes, wound healing, and other critical conditions. By integrating sensors and drug delivery devices, scientists and engineers have developed closed-loop drug delivery systems with on-demand therapeutic capabilities to provide just-in-time treatments that correspond to chemical, electrical, and physical signals of a target morbidity. To enable closed-loop functionality in vivo, engineers utilize various low-power means of communication that reduce the size of implants by orders of magnitude, increase device lifetime from hours to months, and ensure the secure high-speed transfer of data. In this review, we highlight how communication protocols used to integrate sensors and drug delivery devices, such as radio frequency communication (e.g., Bluetooth, near-field communication), in-body communication, and ultrasound, enable improved treatment outcomes.

Real-Time Monitoring of Antibiotics in the Critically Ill Using Biosensors

By ensuring optimal dosing, therapeutic drug monitoring (TDM) improves outcomes in critically ill patients by maximizing effectiveness while minimizing toxicity. Current methods for measuring plasma drug concentrations, however, can be challenging, time-consuming, and slow to return an answer, limiting the extent to which TDM is used to optimize drug exposure. A potentially promising solution to this dilemma is provided by biosensors, molecular sensing devices that employ biorecognition elements to recognize and quantify their target molecules rapidly and in a single step. This paper reviews the current state of the art for biosensors regarding their application to TDM of antibiotics in the critically ill, both as ex vivo point-of-care devices supporting single timepoint measurements and in vivo devices supporting continuous real-time monitoring in situ in the body. This paper also discusses the clinical development of biosensors for TDM, including regulatory challenges and the need for standardized performance evaluation. We conclude by arguing that, through precise and real-time monitoring of antibiotics, the application of biosensors in TDM holds great promise for enhancing the optimization of drug exposure in critically ill patients, offering the potential for improved outcomes.

Proximity-Driven DNA Nanosensors

In proximity-driven sensing, interactions between a probe and an analyte produce a detectable signal by causing a change in distance of two probe components or signaling moieties. By interfacing such systems with DNA-based nanostructures, platforms that are highly sensitive, specific, and programmable can be designed. In this Perspective, we delineate the advantages of using DNA building blocks in proximity-driven nanosensors and provide an overview of recent progress in the field, from sensors that rapidly detect pesticides in food to probes that identify rare cancer cells in blood. We also discuss current challenges and identify key areas that need further development.

A Facile Method for the Fabrication of the Microneedle Electrode and Its Application in the Enzymatic Determination of Glutamate

Herein, a simple method has been used in the fabrication of a microneedle electrode (MNE). To do this, firstly, a commercial self-dissolving microneedle patch has been used to make a hard-polydimethylsiloxane-based micro-pore mold (MPM). Then, the pores of the MPM were filled with the conductive platinum (Pt) paste and cured in an oven. Afterward, the MNE made of platinum (Pt-MNE) was characterized using cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), and scanning electron microscopy (SEM). To prove the electrochemical applicability of the Pt-MNE, the glutamate oxidase enzyme was immobilized on the surface of the electrode, to detect glutamate, using the cyclic voltammetry (CV) and chronoamperometry (CA) methods. The obtained results demonstrated that the fabricated biosensor could detect a glutamate concentration in the range of 10-150 µM. The limits of detection (LODs) (three standard deviations of the blank/slope) were also calculated to be 0.25 µM and 0.41 µM, using CV and CA, respectively. Furthermore, the Michaelis-Menten constant (KMapp) of the biosensor was calculated to be 296.48 µM using a CA method. The proposed biosensor was finally applied, to detect the glutamate concentration in human serum samples. The presented method for the fabrication of the mold signifies a step further toward the fabrication of a microneedle electrode.

Survey of oligoethylene glycol-based self-assembled monolayers on electrochemical aptamer-based sensor in biological fluids

The ability to monitor levels of endogenous markers and clearance profiles of drugs and their metabolites can improve the quality of biomedical research and precision with which therapies are individualized. Towards this end, electrochemical aptamer-based (EAB) sensors have been developed that support the real-time monitoring of specific analytes in vivo with clinically relevant specificity and sensitivity. A challenge associated with the in vivo deployment of EAB sensors, however, is how to manage the signal drift which, although correctable, ultimately leads to unacceptably low signal-to-noise ratios, limiting the measurement duration. Motivated by the correction of signal drift, in this paper, we have explored the use of oligoethylene glycol (OEG), a widely employed antifouling coating, to reduce the signal drift in EAB sensors. Counter to expectations, however, when challenged in 37 °C whole blood in vitro, EAB sensors employing OEG-modified self-assembled monolayers exhibit both greater drift and reduced signal gain, compared with those employ a simple, hydroxyl-terminated monolayer. On the other hand, when EAB sensor was prepared with a mix monolayer using MCH and lipoamido OEG 2 alcohol, reduced signal noise was observed compared to the same sensor prepared with MCH presumably due to improved SAM construction. These results suggest broader exploration of antifouling materials will be required to improve the signal drift of EAB sensors.

Revolutionizing Precision Medicine: Exploring Wearable Sensors for Therapeutic Drug Monitoring and Personalized Therapy

Precision medicine, particularly therapeutic drug monitoring (TDM), is essential for optimizing drug dosage and minimizing toxicity. However, current TDM methods have limitations, including the need for skilled operators, patient discomfort, and the inability to monitor dynamic drug level changes. In recent years, wearable sensors have emerged as a promising solution for drug monitoring. These sensors offer real-time and continuous measurement of drug concentrations in biofluids, enabling personalized medicine and reducing the risk of toxicity. This review provides an overview of drugs detectable by wearable sensors and explores biosensing technologies that can enable drug monitoring in the future. It presents a comparative analysis of multiple biosensing technologies and evaluates their strengths and limitations for integration into wearable detection systems. The promising capabilities of wearable sensors for real-time and continuous drug monitoring offer revolutionary advancements in diagnostic tools, supporting personalized medicine and optimal therapeutic effects. Wearable sensors are poised to become essential components of healthcare systems, catering to the diverse needs of patients and reducing healthcare costs.

Continuous molecular monitoring of human dermal interstitial fluid with microneedle-enabled electrochemical aptamer sensors

The ability to continually collect diagnostic information from the body during daily activity has revolutionized the monitoring of health and disease. Much of this monitoring, however, has been of physical "vital signs", with the monitoring of molecular markers having been limited to glucose, primarily due to the lack of other medically relevant molecules for which continuous measurements are possible in bodily fluids. Electrochemical aptamer sensors, however, have a recent history of successful in vivo demonstrations in rat animal models. Herein, we present the first report of real-time human molecular data collected using such sensors, successfully demonstrating their ability to measure the concentration of phenylalanine in dermal interstitial fluid after an oral bolus. To achieve this, we used a device that employs three hollow microneedles to couple the interstitial fluid to an ex vivo, phenylalanine-detecting sensor. The resulting architecture achieves good precision over the physiological concentration range and clinically relevant, 20 min lag times. By also demonstrating 90 days dry room-temperature shelf storage, the reported work also reaches another important milestone in moving such sensors to the clinic. While the devices demonstrated are not without remaining challenges, the results at minimum provide a simple method by which aptamer sensors can be quickly moved into human subjects for testing.

Flexible and Wearable Biosensors for Monitoring Health Conditions

Flexible and wearable biosensors have received tremendous attention over the past decade owing to their great potential applications in the field of health and medicine. Wearable biosensors serve as an ideal platform for real-time and continuous health monitoring, which exhibit unique properties such as self-powered, lightweight, low cost, high flexibility, detection convenience, and great conformability. This review introduces the recent research progress in wearable biosensors. First of all, the biological fluids often detected by wearable biosensors are proposed. Then, the existing micro-nanofabrication technologies and basic characteristics of wearable biosensors are summarized. Then, their application manners and information processing are also highlighted in the paper. Massive cutting-edge research examples are introduced such as wearable physiological pressure sensors, wearable sweat sensors, and wearable self-powered biosensors. As a significant content, the detection mechanism of these sensors was detailed with examples to help readers understand this area. Finally, the current challenges and future perspectives are proposed to push this research area forward and expand practical applications in the future.

Perspective-The Feasibility of Continuous Protein Monitoring in Interstitial Fluid

Real-time continuous monitoring of proteins in-vivo holds great potential for personalized medical applications. Unfortunately, a prominent knowledge gap exists in the fundamental biology regarding protein transfer and correlation between interstitial fluid and blood. Additionally, technological sensing will require affinity-based platforms that cannot be robustly protected in-vivo and will therefore be challenged in sensitivity, longevity, and fouling over multi-day to week timelines. Here we use electrochemical aptamer sensors as a model system to discuss further research necessary to achieve continuous protein sensing.

Microneedle electrochemical aptamer-based sensing: Real-time small molecule measurements using sensor-embedded, commercially-available stainless steel microneedles

Microneedle sensors could enable minimally-invasive, continuous molecular monitoring - informing on disease status and treatment in real-time. Wearable sensors for pharmaceuticals, for example, would create opportunities for treatments personalized to individual pharmacokinetics. Here, we demonstrate a commercial-off-the-shelf (COTS) approach for microneedle sensing using an electrochemical aptamer-based sensor that detects the high-toxicity antibiotic, vancomycin. Wearable monitoring of vancomycin could improve patient care by allowing targeted drug dosing within its narrow clinical window of safety and efficacy. To produce sensors, we miniaturize the electrochemical aptamer-based sensors to a microelectrode format, and embed them within stainless steel microneedles (sourced from commercial insulin pen needles). The microneedle sensors achieve quantitative measurements in body-temperature undiluted blood. Further, the sensors effectively maintain electrochemical signal within porcine skin. This COTS approach requires no cleanroom fabrication or specialized equipment, and produces individually-addressable, sterilizable microneedle sensors capable of easily penetrating the skin. In the future, this approach could be adapted for multiplexed detection, enabling real-time monitoring of a range of biomarkers.

A tight squeeze: geometric effects on the performance of three-electrode electrochemical-aptamer based sensors in constrained, in vivo placements

Electrochemical, aptamer-based (EAB) sensors are the first molecular monitoring technology that is (1) based on receptor binding and not the reactivity of the target, rendering it fairly general, and (2) able to support high-frequency, real-time measurements in situ in the living body. To date, EAB-derived in vivo measurements have largely been performed using three electrodes (working, reference, counter) bundled together within a catheter for insertion into the rat jugular. Exploring this architecture, here we show that the placement of these electrodes inside or outside of the lumen of the catheter significantly impacts sensor performance. Specifically, we find that retaining the counter electrode within the catheter increases the resistance between it and the working electrode, increasing the capacitive background. In contrast, extending the counter electrode outside the lumen of the catheter reduces this effect, significantly enhancing the signal-to-noise of intravenous molecular measurements. Exploring counter electrode geometries further, we find that they need not be larger than the working electrode. Putting these observations together, we have developed a new intravenous EAB architecture that achieves improved performance while remaining short enough to safely emplace in the rat jugular. These findings, though explored here with EAB sensors may prove important for the design of many electrochemical biosensors.

Continuous Measurement of Lactate Concentration in Human Subjects through Direct Electron Transfer from Enzymes to Microneedle Electrodes

Microneedle lactate sensors may be used to continuously measure lactate concentration in the interstitial fluid in a minimally invasive and pain-free manner. First- and second-generation enzymatic sensors produce a redox-active product that is electrochemically sensed at the electrode surface. Direct electron transfer enzymes produce electrons directly as the product of enzymatic action; in this study, a direct electron transfer enzyme specific to lactate has been immobilized onto a microneedle surface to create lactate-sensing devices that function at low applied voltages (0.2 V). These devices have been validated in a small study of human volunteers; lactate concentrations were raised and lowered through physical exercise and subsequent rest. Lactazyme microneedle devices show good agreement with concurrently obtained and analyzed serum lactate levels.

Explaining the Decay of Nucleic Acid-Based Sensors under Continuous Voltammetric Interrogation

Nucleic acid-based electrochemical sensors (NBEs) can support continuous and highly selective molecular monitoring in biological fluids, both in vitro and in vivo, via affinity-based interactions. Such interactions afford a sensing versatility that is not supported by strategies that depend on target-specific reactivity. Thus, NBEs have significantly expanded the scope of molecules that can be monitored continuously in biological systems. However, the technology is limited by the lability of the thiol-based monolayers employed for sensor fabrication. Seeking to understand the main drivers of monolayer degradation, we studied four possible mechanisms of NBE decay: (i) passive desorption of monolayer elements in undisturbed sensors, (ii) voltage-induced desorption under continuous voltammetric interrogation, (iii) competitive displacement by thiolated molecules naturally present in biofluids like serum, and (iv) protein binding. Our results indicate that voltage-induced desorption of monolayer elements is the main mechanism by which NBEs decay in phosphate-buffered saline. This degradation can be overcome by using a voltage window contained between -0.2 and 0.2 V vs Ag|AgCl, reported for the first time in this work, where electrochemical oxygen reduction and surface gold oxidation cannot occur. This result underscores the need for chemically stable redox reporters with more positive reduction potentials than the benchmark methylene blue and the ability to cycle thousands of times between redox states to support continuous sensing for long periods. Additionally, in biofluids, the rate of sensor decay is further accelerated by the presence of thiolated small molecules like cysteine and glutathione, which can competitively displace monolayer elements even in the absence of voltage-induced damage. We hope that this work will serve as a framework to inspire future development of novel sensor interfaces aiming to eliminate the mechanisms of signal decay in NBEs.

Microneedle-based transdermal detection and sensing devices

Microneedles have been expected for the construction of next-generation biosensors towards personalization, digitization, and intellectualization due to their metrics of minimal invasiveness, high integration, and favorable biocompatibility. Herein, an overview of state-of-the-art microneedle-based detection and sensing systems is presented. First, the designs of microneedle devices based on extraction mechanisms are concluded, corresponding to different geometries and materials of microneedles. Second, the targets of equipment-assisted microneedle detections are summarized, as well as the objective significance, revealing the current performance and potential scenarios of these microneedles. Third, the trend towards highly integrated sensors is elaborated by emphasizing the sensing principles (colorimetric, fluorometric and electronic manner). Finally, the key challenges to be tackled and the perspectives on future development are discussed.

Biosensors for healthcare: current and future perspectives

Biosensors are utilized in several different fields, including medicine, food, and the environment; in this review, we examine recent developments in biosensors for healthcare. These involve three distinct types of biosensor: biosensors for in vitro diagnosis with blood, saliva, or urine samples; continuous monitoring biosensors (CMBs); and wearable biosensors. Biosensors for in vitro diagnosis have seen a significant expansion recently, with newly reported clustered regularly interspaced short palindromic repeats (CRISPR)/Cas methodologies and improvements to many established integrated biosensor devices, including lateral flow assays (LFAs) and microfluidic/electrochemical paper-based analytical devices (μPADs/ePADs). We conclude with a discussion of two novel groups of biosensors that have drawn great attention recently, continuous monitoring and wearable biosensors, as well as with perspectives on the commercialization and future of biosensors.

On-Site Therapeutic Drug Monitoring of Paracetamol Analgesic in Non-Invasively Collected Saliva for Personalized Medicine

Paracetamol or acetaminophen is the main non-opioid analgesic recommended for mild pain by the World Health Organization (WHO) analgesic ladder. However, the high levels used of paracetamol are associated with the hepatotoxicity and nephrotoxicity caused by accumulation of toxic metabolites. The sensor is produced on a polyester substrate containing a full electrochemical device with working, auxiliary, and reference electrodes in which, guiding personalized medicine solutions are not reported. Temporal paracetamol profiles in human saliva are performed with the subject taking different amounts of commercial analgesic pills. The variation of saliva paracetamol levels is demonstrated to be interference free from electroactive interfering species and human saliva constituents. In addition, the sensor displays to be useful as a disposable device for the fast detection of paracetamol in untreated raw saliva following pill intake. The maximum concentration (C_max ) and half-life time (t_1/2 ) for paracetamol are 143.27 µm and 110 min. The results demonstrate the potential of a simple strategy with electrochemical devices for noninvasive personalized therapy toward guiding drug interventions through tracking of active substance, detecting, and correcting insufficiency of absorption to meet individual needs avoiding overdoses, side effects, and intoxication.

Epidermal Wearable Biosensors for Monitoring Biomarkers of Chronic Disease in Sweat

Biological information detection technology is mainly used for the detection of physiological and biochemical parameters closely related to human tissues and organ lesions, such as biomarkers. This technology has important value in the clinical diagnosis and treatment of chronic diseases in their early stages. Wearable biosensors can be integrated with the Internet of Things and Big Data to realize the detection, transmission, storage, and comprehensive analysis of human physiological and biochemical information. This technology has extremely wide applications and considerable market prospects in frontier fields including personal health monitoring, chronic disease diagnosis and management, and home medical care. In this review, we systematically summarized the sweat biomarkers, introduced the sweat extraction and collection methods, and discussed the application and development of epidermal wearable biosensors for monitoring biomarkers in sweat in preclinical research in recent years. In addition, the current challenges and development prospects in this field were discussed.

Functional microneedles for wearable electronics

With an ideal comfort level, sensitivity, reliability, and user-friendliness, wearable sensors are making great contributions to daily health care, nursing care, early disease discovery, and body monitoring. Some wearable sensors are imparted with hierarchical and uneven microstructures, such as microneedle structures, which not only facilitate the access to multiple bio-analysts in the human body but also improve the abilities to detect feeble body signals. In this paper, we present the promising applications and latest progress of functional microneedles in wearable sensors. We begin by discussing the roles of microneedles as sensing units, including how the signals are captured, converted, and transmitted. We also introduce the microneedle-like structures as power units, which depend on triboelectric or piezoelectric effects, etc. Finally, we summarize the cutting-edge applications of microneedle-based wearable sensors in biophysical signal monitoring and biochemical analyte detection, and provide critical thinking on their future perspectives.

Ionic Strength and Hybridization Position near Gold Electrodes Can Significantly Improve Kinetics in DNA-Based Electrochemical Sensors

A variety of electrochemical (EC) biosensors play critical roles in disease diagnostics. More recently, DNA-based EC sensors have been established as promising for detecting a wide range of analyte classes. Since most of these sensors rely on the high specificity of DNA hybridization for analyte binding or structural control, it is crucial to understand the kinetics of hybridization at the electrode surface. In this work, we have used methylene blue-labeled DNA strands to monitor the kinetics of DNA hybridization at the electrode surface with square-wave voltammetry. By varying the position of the double-stranded DNA segment relative to the electrode surface as well as the bulk solution's ionic strength (0.125-1.00 M), we observed significant interferences with DNA hybridization closer to the surface, with more substantial interference at lower ionic strength. As a demonstration of the effect, toehold-mediated strand displacement reactions were slowed and diminished close to the surface, while strategic placement of the DNA binding site improved reaction rates and yields. This work manifests that both the salt concentration and DNA hybridization site relative to the electrode are important factors to consider when designing DNA-based EC sensors that measure hybridization directly at the electrode surface.

Skin Interstitial Fluid-Based SERS Tags Labeled Microneedles for Tracking of Peritonitis Progression and Treatment Effect

Skin interstitial fluid (ISF)-based microneedle (MN) sensing has recently exhibited wide promise for the minimally invasive and painless diagnosis of diseases. However, it is still a great challenge to diagnose more disease types due to the limited in situ sensing techniques and insufficient ISF biomarker sources. Herein, ISF is employed to pioneer the tracking of acute peritonitis progression via surface-enhanced Raman scattering (SERS) tags labeled MNs patch technique. Densely deposited core-satellite gold nanoparticles and 3-mercaptophenylboronic acid as a Raman reporter enable the developed MNs patch with high sensitivity and selectivity in the determination of H₂O₂, an indicator of peritonitis development. Importantly, the MNs patch not only reliably tracks the different states of peritonitis but also evaluates the efficacy of drugs in the treatment of peritonitis, as evidenced by the altered SERS signal consistent with plasma pro-inflammatory factor (TNF-α) and peritoneum pathological manifestations. Interestingly, the major source of H₂O₂ in ISF of acute peritonitis investigated may not be through conventional blood capillary filtration pathway. This work provides a new route and technique for the early diagnosis of acute peritonitis and the evaluation of drug therapy effects. The developed MNs patch is promising to serve as a universal sensing tool to greatly enrich the variety and prospect of ISF-based disease diagnosis.

Tailoring electrode surface charge to achieve discrimination and quantification of chemically similar small molecules with electrochemical aptamers

Electrochemical biosensors based on structure-switching aptamers offer many advantages because they can operate directly in complex samples and offer the potential to integrate with miniaturized electronics. Unfortunately, these biosensors often suffer from cross-reactivity problems when measuring a target in samples containing other chemically similar molecules, such as precursors or metabolites. While some progress has been made in selecting highly specific aptamers, the discovery of these reagents remains slow and costly. In this work, we demonstrate a novel strategy to distinguish molecules with miniscule difference in chemical composition (such as a single hydroxyl group) - with cross reactive aptamer probes - by tuning the charge state of the surface on which the aptamer probes are immobilized. As an exemplar, we show that our strategy can distinguish between DOX and many structurally similar analytes, including its primary metabolite doxorubicinol (DOXol). We then demonstrate the ability to accurately quantify mixtures of these two molecules based on their differential response to sensors with different surface-charge properties. We believe this methodology is general and can be extended to a broad range of applications.

Optimization of Vancomycin Aptamer Sequence Length Increases the Sensitivity of Electrochemical, Aptamer-Based Sensors In Vivo

The measurement of serum vancomycin levels at the clinic is critical to optimizing dosing given the narrow therapeutic window of this antibiotic. Current approaches to quantitate serum vancomycin levels are based on immunoassays, which are multistep methods requiring extensive processing of patient samples. As an alternative, vancomycin-binding electrochemical, aptamer-based sensors (E-ABs) were developed to simplify the workflow of vancomycin monitoring. E-ABs enable the instantaneous measurement of serum vancomycin concentrations without the need for sample dilution or other processing steps. However, the originally reported vancomycin-binding E-ABs had a dissociation constant of 45 μM, which is approximately 1 order of magnitude higher than the recommended trough concentrations of vancomycin measured in patients. This limited sensitivity hinders the ability of E-ABs to accurately support vancomycin monitoring. To overcome this problem, here we sought to optimize the length of the vancomycin-binding aptamer sequence to enable a broader dynamic range in the E-AB platform. Our results demonstrate, via isothermal calorimetry and E-AB calibrations in undiluted serum, that superior affinity and near-equal sensor gain in vitro can be achieved using a one-base-pair-longer aptamer than the truncated sequence originally reported. We validate the impact of the improved binding affinity in vivo by monitoring vancomycin levels in the brain cortex of live mice following intravenous administration. While the original sequence fails to resolve vancomycin concentrations from baseline noise (SNR = 1.03), our newly reported sequence provides an SNR of 1.62 at the same dose.

Molecular Imprinted Polymers on Microneedle Arrays for Point of Care Transdermal Sampling and Sensing of Inflammatory Biomarkers

The skin interstitial fluid (ISF) contains biomarkers that complement other biofluids such as blood, sweat, saliva, and urine. It can be sampled in a minimally invasive manner and used either for point of care testing or real time, continuous monitoring of analytes, the latter using microneedle arrays. The analytes present in the skin ISF are indicative of both systemic and local (i.e., skin) physiology. In this paper, we describe combining microneedle technology with molecularly imprinted polymers to demonstrate the potential of transdermal electrochemical sensing. The molecularly imprinted polymer employed here is easy to produce; it can be thought of as plastic antibody. Its synthesis is scalable, and the resulting sensor has a short measurement time (6 min), with high accuracy and a low limit of detection. It provides the requisite specificity to detect the proinflammatory cytokine IL-6. IL-6 is present in the skin ISF with other cytokines and is implicated in many clinical states including neurodegenerative diseases and fatal pneumonia from SARSCoV 2. The ability to mass produce microneedle arrays and plastic antibodies will allow for low-cost transdermal sensing devices. The transdermal sensors were able to detect IL-6 at concentrations as low as 1 pg/mL in artificial skin ISF, indicating its utility for routine point of care, bloodless measurements in simpler settings, worldwide.

Real-Time, In Vivo Molecular Monitoring Using Electrochemical Aptamer Based Sensors: Opportunities and Challenges

The continuous, real-time measurement of specific molecules in situ in the body would greatly improve our ability to understand, diagnose, and treat disease. The vast majority of continuous molecular sensing technologies, however, either (1) rely on the chemical or enzymatic reactivity of their targets, sharply limiting their scope, or (2) have never been shown (and likely will never be shown) to operate in the complex environments found in vivo. Against this background, here we review electrochemical aptamer-based (EAB) sensors, an electrochemical approach to real-time molecular monitoring that has now seen 15 years of academic development. The strengths of the EAB platform are significant: to date it is the only molecular measurement technology that (1) functions independently of the chemical reactivity of its targets, and is thus general, and (2) supports in vivo measurements. Specifically, using EAB sensors we, and others, have already reported the real-time, seconds-resolved measurements of multiple, unrelated drugs and metabolites in situ in the veins and tissues of live animals. Against these strengths, we detail the platform's remaining weaknesses, which include still limited measurement duration (hours, rather than the more desirable days) and the difficulty in obtaining sufficiently high performance aptamers against new targets, before then detailing promising approaches overcoming these hurdles. Finally, we close by exploring the opportunities we believe this potentially revolutionary technology (as well as a few, possibly competing, technologies) will create for both researchers and clinicians.

Wearable microneedle-based electrochemical aptamer biosensing for precision dosing of drugs with narrow therapeutic windows

Therapeutic drug monitoring is essential for dosing pharmaceuticals with narrow therapeutic windows. Nevertheless, standard methods are imprecise and involve invasive/resource-intensive procedures with long turnaround times. Overcoming these limitations, we present a microneedle-based electrochemical aptamer biosensing patch (μNEAB-patch) that minimally invasively probes the interstitial fluid (ISF) and renders correlated, continuous, and real-time measurements of the circulating drugs' pharmacokinetics. The μNEAB-patch is created following an introduced low-cost fabrication scheme, which transforms a shortened clinical-grade needle into a high-quality gold nanoparticle-based substrate for robust aptamer immobilization and efficient electrochemical signal retrieval. This enables the reliable in vivo detection of a wide library of ISF analytes-especially those with nonexistent natural recognition elements. Accordingly, we developed μNEABs targeting various drugs, including antibiotics with narrow therapeutic windows (tobramycin and vancomycin). Through in vivo animal studies, we demonstrated the strong correlation between the ISF/circulating drug levels and the device's potential clinical use for timely prediction of total drug exposure.

Voltammetry Peak Tracking for Longer-Lasting and Reference-Electrode-Free Electrochemical Biosensors

Electrochemical aptamer-based sensors offer reagent-free and continuous analyte measurement but often suffer from poor longevity and potential drift even with a robust 3-electrode system. Presented here is a simple, software-enabled approach that tracks the redox-reporter peak in an electrochemical aptamer-based sensor and uses the measurement of redox peak potential to reduce the scanning window to a partial measure of redox-peak-height vs. baseline (~10X reduction in voltage range). This same measurement further creates a virtual reference standard in buffered biofluids such as blood and interstitial fluid, thereby eliminating the effects of potential drift and the need for a reference electrode. The software intelligently tracks voltammogram peak potential via the inflection points of the rising and falling slopes of the measured redox peak. Peak-tracking-derived partial scanning was validated over several days and minimized electrochemically induced signal loss to <5%. Furthermore, the peak-tracking approach was shown to be robust against confounding effects such as fouling. From an applied perspective in creating wearable biosensors, the peak-tracking approach further enables use of a single implanted working electrode, while the counter/reference-electrode may utilize a simple gel-pad electrode on the surface of the skin, compared to implanting working, counter, and reference electrodes conventionally used for stability and reliability but is also costly and invasive. Cumulatively, peak-tracking provides multiple leaps forward required for practical molecular monitoring by extending sensor longevity, eliminating potential drift, simplifying biosensor device construction, and in vivo placement for any redox-mediated sensor that forms parabolic-like data.

3D-Printed Microneedles for Point-of-Care Biosensing Applications

Microneedles (MNs) are an emerging technology for user-friendly and minimally invasive injection, offering less pain and lower tissue damage in comparison to conventional needles. With their ability to extract body fluids, MNs are among the convenient candidates for developing biosensing setups, where target molecules/biomarkers are detected by the biosensor using the sample collected with the MNs. Herein, we discuss the 3D printing of microneedle arrays (MNAs) toward enabling point-of-care (POC) biosensing applications.