Amay J. Bandodkar

Non-invasive wearable electrochemical sensors: a review

Wearable sensors have garnered considerable recent interest owing to their tremendous promise for a plethora of applications. Yet the absence of reliable non-invasive chemical sensors has greatly hindered progress in the area of on-body sensing. Electrochemical sensors offer considerable promise as wearable chemical sensors that are suitable for diverse applications owing to their high performance, inherent miniaturization, and low cost. A wide range of wearable electrochemical sensors and biosensors has been developed for real-time non-invasive monitoring of electrolytes and metabolites in sweat, tears, or saliva as indicators of a wearers health status. With continued innovation and attention to key challenges, such non-invasive electrochemical sensors and biosensors are expected to open up new exciting avenues in the field of wearable wireless sensing devices and body-sensor networks, and thus find considerable use in a wide range of personal health-care monitoring applications, as well as in sport and military applications.

Electrochemical tattoo biosensors for real-time noninvasive lactate monitoring in human perspiration.

The present work describes the first example of real-time noninvasive lactate sensing in human perspiration during exercise events using a flexible printed temporary-transfer tattoo electrochemical biosensor that conforms to the wearers skin. The new skin-worn enzymatic biosensor exhibits chemical selectivity toward lactate with linearity up to 20 mM and demonstrates resiliency against continuous mechanical deformation expected from epidermal wear. The device was applied successfully to human subjects for real-time continuous monitoring of sweat lactate dynamics during prolonged cycling exercise. The resulting temporal lactate profiles reflect changes in the production of sweat lactate upon varying the exercise intensity. Such skin-worn metabolite biosensors could lead to useful insights into physical performance and overall physiological status, hence offering considerable promise for diverse sport, military, and biomedical applications.

Tattoo-Based Noninvasive Glucose Monitoring: A Proof-of-Concept Study

We present a proof-of-concept demonstration of an all-printed temporary tattoo-based glucose sensor for noninvasive glycemic monitoring. The sensor represents the first example of an easy-to-wear flexible tattoo-based epidermal diagnostic device combining reverse iontophoretic extraction of interstitial glucose and an enzyme-based amperometric biosensor. In-vitro studies reveal the tattoo sensors linear response toward physiologically relevant glucose levels with negligible interferences from common coexisting electroactive species. The iontophoretic-biosensing tattoo platform is reduced to practice by applying the device on human subjects and monitoring variations in glycemic levels due to food consumption. Correlation of the sensor response with that of a commercial glucose meter underscores the promise of the tattoo sensor to detect glucose levels in a noninvasive fashion. Control on-body experiments demonstrate the importance of the reverse iontophoresis operation and validate the sensor specificity. This preliminary investigation indicates that the tattoo-based iontophoresis-sensor platform holds considerable promise for efficient diabetes management and can be extended toward noninvasive monitoring of other physiologically relevant analytes present in the interstitial fluid.

Epidermal tattoo potentiometric sodium sensors with wireless signal transduction for continuous non-invasive sweat monitoring

This article describes the fabrication, characterization and application of an epidermal temporary-transfer tattoo-based potentiometric sensor, coupled with a miniaturized wearable wireless transceiver, for real-time monitoring of sodium in the human perspiration. Sodium excreted during perspiration is an excellent marker for electrolyte imbalance and provides valuable information regarding an individuals physical and mental wellbeing. The realization of the new skin-worn non-invasive tattoo-like sensing device has been realized by amalgamating several state-of-the-art thick film, laser printing, solid-state potentiometry, fluidics and wireless technologies. The resulting tattoo-based potentiometric sodium sensor displays a rapid near-Nernstian response with negligible carryover effects, and good resiliency against various mechanical deformations experienced by the human epidermis. On-body testing of the tattoo sensor coupled to a wireless transceiver during exercise activity demonstrated its ability to continuously monitor sweat sodium dynamics. The real-time sweat sodium concentration was transmitted wirelessly via a body-worn transceiver from the sodium tattoo sensor to a notebook while the subjects perspired on a stationary cycle. The favorable analytical performance along with the wearable nature of the wireless transceiver makes the new epidermal potentiometric sensing system attractive for continuous monitoring the sodium dynamics in human perspiration during diverse activities relevant to the healthcare, fitness, military, healthcare and skin-care domains.

A wearable chemical–electrophysiological hybrid biosensing system for real-time health and fitness monitoring

Flexible, wearable sensing devices can yield important information about the underlying physiology of a human subject for applications in real-time health and fitness monitoring. Despite significant progress in the fabrication of flexible biosensors that naturally comply with the epidermis, most designs measure only a small number of physical or electrophysiological parameters, and neglect the rich chemical information available from biomarkers. Here, we introduce a skin-worn wearable hybrid sensing system that offers simultaneous real-time monitoring of a biochemical (lactate) and an electrophysiological signal (electrocardiogram), for more comprehensive fitness monitoring than from physical or electrophysiological sensors alone. The two sensing modalities, comprising a three-electrode amperometric lactate biosensor and a bipolar electrocardiogram sensor, are co-fabricated on a flexible substrate and mounted on the skin. Human experiments reveal that physiochemistry and electrophysiology can be measured simultaneously with negligible cross-talk, enabling a new class of hybrid sensing devices.

Epidermal Biofuel Cells: Energy Harvesting from Human Perspiration†

The healthcare industry has recently experienced a major paradigm shift towards wearable biomedical devices. Such devices have the ability to monitor vital physiological parameters, such as heart rate or blood pressure. Particular recent attention has been directed towards skin-worn electronic devices fabricated by novel hybrid techniques for the measurement of these vital signs. Despite dramatic technological advances, further progress in the arena of on-body biomedical devices has been hindered by the lack of effective wearable power sources able to scavenge sufficient energy from the wearer. Major efforts have thus been directed towards the identification of a suitable wearable power source that offers conformal integration with the wearer s body. This activity has resulted in the development of flexible thin-film batteries, piezoelectric nanogenerators, wearable solar cells, mircosupercapacitors, and endocochlear-potential-based biobatteries. Nevertheless, new body-worn conformal power sources able to extract biochemical energy from the wearer s body (and his/her epidermis, in particular) are still highly desired. Herein we demonstrate the ability to generate substantial levels of electrical power from human perspiration in a noninvasive and continuous fashion through the use of epidermal biofuel cells based on temporary transfer tattoos (tBFCs). Enzymatic BFCs have attracted considerable interest owing to their ability to generate power from the bioelectrocatalytic reaction of common chemicals and metabolites, such as glucose and alcohol, under physiological conditions. Recent efforts resulted in implantable glucose BFCs that can generate significant power densities in small animals, such as snails, insects, and rats. However, there are no reports on harvesting the chemical energy from a human in connection with the rapidly developing field of wearable electronics. The successful development of non-invasive tBFCs requires the judicious integration of new manufacturing processes and advanced surface functionalization for efficient power generation from lactate present in the wearer s perspiration. The development of the tBFC builds on our recent introduction of epidermal electrochemical sensors. The two electrode constituents of the new wearable tBFC were designed in the shape of “UC” (acronym for “University of California”; Figure 1; see Figure S1 in the Supporting

Highly Stretchable Fully-Printed CNT-Based Electrochemical Sensors and Biofuel Cells: Combining Intrinsic and Design-Induced Stretchability

We present the first example of an all-printed, inexpensive, highly stretchable CNT-based electrochemical sensor and biofuel cell array. The synergistic effect of utilizing specially tailored screen printable stretchable inks that combine the attractive electrical and mechanical properties of CNTs with the elastomeric properties of polyurethane as a binder along with a judiciously designed free-standing serpentine pattern enables the printed device to possess two degrees of stretchability. Owing to these synergistic design and nanomaterial-based ink effects, the device withstands extremely large levels of strains (up to 500% strain) with negligible effect on its structural integrity and performance. This represents the highest stretchability offered by a printed device reported to date. Extensive electrochemical characterization of the printed device reveal that repeated stretching, torsional twisting, and indenting stress has negligible impact on its electrochemical properties. The wide-range applicability of this platform to realize highly stretchable CNT-based electrochemical sensors and biofuel cells has been demonstrated by fabricating and characterizing potentiometric ammonium sensor, amperometric enzyme-based glucose sensor, enzymatic glucose biofuel cell, and self-powered biosensor. Highly stretchable printable multianalyte sensor, multifuel biofuel cell, or any combination thereof can thus be realized using the printed CNT array. Such combination of intrinsically stretchable printed nanomaterial-based electrodes and strain-enduring design patterns holds considerable promise for creating an attractive class of inexpensive multifunctional, highly stretchable printed devices that satisfy the requirements of diverse healthcare and energy fields wherein resilience toward extreme mechanical deformations is mandatory.

All-printed stretchable electrochemical devices.

The fabrication and characterization of all-printed, inexpensive, stretchable electrochemical devices is described. These devices are based on specially engineered inks that can withstand severe tensile strain, as high as 100%, without any significant effect on their electrochemical properties. Such stretchable electrochemical devices should be attractive for diverse sensing and energy applications.

Wearable textile biofuel cells for powering electronics

The fabrication and performance of a wearable biofuel cell printed directly onto textile substrates are reported. The textile biofuel cell utilizes physiologically produced sweat lactate as the fuel to generate electrical energy, producing up to 100 μW cm−2 at 0.34 V during in vitro experimentation, even after repeated bending stress. Furthermore, the wearable and flexible biofuel cell can be easily integrated with a portable energy storage device for on-demand powering of wearable electronics. To validate energy harvesting, the biofuel cell is integrated into a headband and a wristband, and with the help of an on-board DC/DC converter, extracts energy from perspiring human subjects for direct powering of an LED or a digital watch. Convenient incorporation and removal from a variety of garments are achieved by printing the biofuel cell on a detachable care label. Such textile-based non-invasive biofuel cells can be expected to serve in the future as the power unit for wearable electronics and biomedical devices.

A stretchable and screen-printed electrochemical sensor for glucose determination in human perspiration

Here we present two types of all-printable, highly stretchable, and inexpensive devices based on platinum (Pt)-decorated graphite for glucose determination in physiological fluids. Said devices are: a non-enzymatic sensor and an enzymatic biosensor, the latter showing promising results. Glucose has been quantified by measuring hydrogen peroxide (H2O2) reduction by chronoamperometry at -0.35V (vs pseudo-Ag/AgCl) using glucose oxidase immobilized on Pt-decorated graphite. The sensor performs well for the quantification of glucose in phosphate buffer solution (0.25M PBS, pH 7.0), with a linear range between 0 mM and 0.9mM, high sensitivity and selectivity, and a low limit of detection (LOD). Thus, it provides an alternative non-invasive and on-body quantification of glucose levels in human perspiration. This biosensor has been successfully applied on real human perspiration samples and results also show a significant correlation between glucose concentration in perspiration and glucose concentration in blood measured by a commercial glucose meter.