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The development of wearable biosensors for continuous noninvasive monitoring of target biomarkers is limited to assays of a single sampled biofluid. An example of simultaneous noninvasive sampling and analysis of two different biofluids using a single wearable epidermal platform is demonstrated here. The concept is successfully realized through sweat stimulation (via transdermal pilocarpine delivery) at an anode, alongside extraction of interstitial fluid (ISF) at a cathode. The system thus allows on‐demand, controlled sampling of the two epidermal biofluids at the same time, at two physically separate locations (on the same flexible platform) containing different electrochemical biosensors for monitoring the corresponding biomarkers. Such a dual biofluid sampling and analysis concept is implemented using a cost‐effective screen‐printing technique with body‐compliant temporary tattoo materials and conformal wireless readout circuits to enable real‐time measurement of biomarkers in the sampled epidermal biofluids. The performance of the developed wearable device is demonstrated by measuring sweat‐alcohol and ISF‐glucose in human subjects consuming food and alcoholic drinks. The different compositions of sweat and ISF with good correlations of their chemical constituents to their blood levels make the developed platform extremely attractive for enhancing the power and scope of next‐generation noninvasive epidermal biosensing systems.

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Despite tremendous recent efforts, noninvasive sweat monitoring is still far from delivering its early analytical promise. Here, we describe a flexible epidermal microfluidic detection platform fabricated through hybridization of lithographic and screen-printed technologies, for efficient and fast sweat sampling and continuous, real-time electrochemical monitoring of glucose and lactate levels. This soft, skin-mounted device judiciously merges lab-on-a-chip and electrochemical detection technologies, integrated with a miniaturized flexible electronic board for real-time wireless data transmission to a mobile device. Modeling of the device design and sweat flow conditions allowed optimization of the sampling process and the microchannel layout for achieving attractive fluid dynamics and rapid filling of the detection reservoir (within 8 min from starting exercise). The wearable microdevice thus enabled efficient natural sweat pumping to the electrochemical detection chamber containing the enzyme-modified electrode transducers. The fabricated device can be easily mounted on the epidermis without hindrance to the wearer and displays resiliency against continuous mechanical deformation expected from such epidermal wear. Amperometric biosensing of lactate and glucose from the rapidly generated sweat, using the corresponding immobilized oxidase enzymes, was wirelessly monitored during cycling activity of different healthy subjects. This ability to monitor sweat glucose levels introduces new possibilities for effective diabetes management, while similar lactate monitoring paves the way for new wearable fitness applications. The new epidermal microfluidic electrochemical detection strategy represents an attractive alternative to recently reported colorimetric sweat-monitoring methods, and hence holds considerable promise for practical fitness or health monitoring applications.

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Conspectus In this Account, we detail recent progress in wearable bioelectronic devices and discuss the future challenges and prospects of on-body noninvasive bioelectronic systems. Bioelectronics is a fast-growing interdisciplinary research field that involves interfacing biomaterials with electronics, covering an array of biodevices, encompassing biofuel cells, biosensors, ingestibles, and implantables. In particular, enzyme-based bioelectronics, built on diverse biocatalytic reactions, offers distinct advantages and represents a centerpiece of wearable biodevices. Such wearable bioelectronic devices predominately rely on oxidoreductase enzymes and have already demonstrated considerable promise for on-body applications ranging from highly selective noninvasive biomarker monitoring to epidermal energy harvesting. These systems can thus greatly increase the analytical capability of wearable devices from the ubiquitous monitoring of mobility and vital signs, toward the noninvasive analysis of important chemical biomarkers. Wearable enzyme electrodes offer exciting opportunities to a variety of areas, spanning from healthcare, sport, to the environment or defense. These include real-time noninvasive detection of biomarkers in biofluids (such as sweat, saliva, interstitial fluid and tears), and the monitoring of environmental pollutants and security threats in the immediate surrounding of the wearer. Furthermore, the interface of enzymes with conducting flexible electrode materials can be exploited for developing biofuel cells, which rely on the bioelectrocatalytic oxidation of biological fuels, such as lactate or glucose, for energy harvesting applications. Crucial for such successful application of enzymatic bioelectronics is deep knowledge of enzyme electron-transfer kinetics, enzyme stability, and enzyme immobilization strategies. Such understanding is critical for establishing efficient electrical contacting between the redox enzymes and the conducting electrode supports, which is of fundamental interest for the development of robust and efficient bioelectronic platforms. Furthermore, stretchable and flexible bioelectronic platforms, with mechanical properties similar to those of biological tissues, are essential for handling the rigors of on-body operation. As such, special attention must be given to changes in the behavior of enzymes due to the uncontrolled conditions of on-body operation (including diverse outdoor activities and different biofluids), for maintaining the attractive performance that these bioelectronics devices display in controlled laboratory settings. Therefore, a focus of this Account is on interfacing biocatalytic layers onto wearable electronic devices for creating efficient and stable on-body electrochemical biosensors and biofuel cells. With proper attention to key challenges and by leveraging the advantages of biocatalysis, electrochemistry, and flexible electronics, wearable bioelectronic devices could have a tremendous impact on diverse biomedical, fitness, and defense fields.

Biocatalyst immobilization onto carbon-based nanosupports has been implemented in a variety of applications ranging from biosensing to biotransformation and from decontamination to energy storage. However, retaining enzyme functionality at carbon-based nanosupports was challenged by the non-specific attachment of the enzyme as well as by the enzyme-enzyme interactions at this interface shown to lead to loss of enzyme activity. Herein, we present a systematic study of the interplay reactions that take place upon immobilization of three pure enzymes namely soybean peroxidase, chloroperoxidase, and glucose oxidase at carbon-based nanosupport interfaces. The immobilization conditions involved both single and multipoint single-type enzyme attachment onto single and multi-walled carbon nanotubes and graphene oxide nanomaterials with properties determined by Fourier transform infrared spectroscopy (FTIR), energy dispersive X-ray analysis (EDX), scanning electron microscopy (SEM), and atomic force microscopy (AFM). Our analysis showed that the different surface properties of the enzymes as determined by their molecular mapping and size work synergistically with the carbon-based nanosupports physico-chemical properties (i.e., surface chemistry, charge and aspect ratios) to influence enzyme catalytic behavior and activity at nanointerfaces. Knowledge gained from these studies can be used to optimize enzyme-nanosupport symbiotic reactions to provide robust enzyme-based systems with optimum functionality to be used for fermentation, biosensors, or biofuel applications.

This work describes a wireless wearable ring-based multiplexed chemical sensor platform for rapid electrochemical monitoring of explosive and nerve-agent threats in vapor and liquid phases. The ring-based sensor system consists of two parts: a set of printed electrochemical sensors and a miniaturized electronic interface, based on a battery-powered stamp-size potentiostat, for signal processing and wireless transmission of data. A wide range of electrochemical capabilities have thus been fully integrated into a 3D printed compact ring structure, toward performing fast square-wave voltammetry and chronoamperometric analyses, along with interchangeable screen-printed sensing electrodes for the rapid detection of different chemical threats. High analytical performance is demonstrated despite the remarkable miniaturization and integration of the ring system. The attractive capabilities of the wearable sensor ring system have been demonstrated for sensitive and rapid voltammetric and amperometric monitoring of nitroaromatic and peroxide explosives, respectively, along with amperometric biosensing of organophosphate (OP) nerve agents. Such ability of the miniaturized wearable sensor ring platform to simultaneously detect multiple chemical threats in both liquid and vapor phases and alert the wearer of such hazards offers considerable promise for meeting the demands of diverse defense and security scenarios.