Wearable Bioelectronics: Enzyme-Based Body-Worn Electronic

Article pubs.acs.org/accounts

Cite This: Acc. Chem. Res. XXXX, XXX, XXX−XXX

Wearable Bioelectronics: Enzyme-Based Body-Worn Electronic Devices Published as part of the Accounts of Chemical Research special issue “Wearable Bioelectronics: Chemistry, Materials, Devices, and Systems”. Jayoung Kim,†,⊥ Itthipon Jeerapan,†,⊥ Juliane R. Sempionatto,†,⊥ Abbas Barfidokht,† Rupesh K. Mishra,† Alan S. Campbell,† Lee J. Hubble,†,‡ and Joseph Wang*,† Acc. Chem. Res. Downloaded from pubs.acs.org by UNIV OF SOUTH DAKOTA on 11/07/18. For personal use only.

†

Department of NanoEngineering, University of California San Diego, La Jolla, California 92093, United States CSIRO Manufacturing, Lindfield, New South Wales 2070, Australia

‡

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.

1. INTRODUCTION The development of bioelectronics involves interfacing biomaterials with electronic devices, with the biological components assembled on a conductive material that transduces biological interactions into readable electronic signals.1 Such coupling has been transformative and serves as the centerpiece for new idea generation with the most representative bioelectronic devices including biosensors, biofuel cells (BFCs), and bioreactors. Electrochemical biosensors, based on enzyme recognition reactions, represent the oldest and most common bioelectronic devices.2 Such biosensing applications rely on the remarkable specificity of © XXXX American Chemical Society

enzymatic reactions toward the corresponding substrate, and on reaction rates proportional to substrate concentration.3 Furthermore, the interface of enzymes with conducting electrode materials can be exploited to develop BFCs.4 These bioelectronic devices rely on the bioelectrocatalytic oxidation of biological fuels, such as lactate or glucose, for energy harvesting applications. Crucial to the successful application of enzymatic bioelectronics is deep knowledge of the enzyme electron-transfer kinetics, enzyme stability, and Received: September 7, 2018

A

DOI: 10.1021/acs.accounts.8b00451 Acc. Chem. Res. XXXX, XXX, XXX−XXX

Article

Accounts of Chemical Research

Figure 1. (A) Conceptual illustration of wearable bioelectronics integrated with the human body, consisting of a biosensor, electronic device, energy source, and display. Schematics of wearable enzyme-based bioelectronics, including (B) biosensor, showing biocatalytic reactions on the transducer surface, (C) BFC illustrating bioanode and biocathode, and (D) self-powered biosensor utilizing BFCs that generate power proportional to the analyte concentration.

monitoring of environmental pollutants and security threats in the surroundings of the wearer.10 Wearable energy-harvesting BFCs represent another important example of bioelectronic systems. Such devices can also be configured to serve as selfpowered biosensors. Wearable bioelectronic devices are commonly in close contact with the body, and hence require the development of body-compliant platforms with the necessary flexibility and stretchability.11,12 The wearability of the integrated bioelectronics can also be limited by the size and weight of power sources. Existing power sources can hinder the wearer's activity and require further miniaturization and flexibility. Energy harvesting based on body-compliant BFCs represents an attractive prospect in this direction. Our group has been working on developing wearable bioelectronic systems based on extensive knowledge gained over three decades on developing conventional bioelectronic devices. Transitioning traditional bioelectronic technologies to wearable bioelectronic devices requires special attention to the challenges associated with the realization of efficient bioelectrocatalytic reactions on our body. Further attention is required for operating such enzyme-based devices under uncontrolled conditions (e.g., common outdoor activities) without compromising the performance that these bioelec-

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 bioelectronic platforms. The resulting enzyme-based bioelectronic systems can be used for a multitude of important applications ranging from continuous monitoring of blood glucose in diabetes patients5 to alerting against dangerous chemical threats.6 The recent development of wearable bioelectronics has generated tremendous interests (Figure 1). This Account highlights recent progress on wearable bioelectronics focused on biocatalytic layers. Bioelectronic systems has recently expanded the scope of wearable devices, such as wristbands, wristwatches, and smartphones, from the monitoring of mobility and vital signs (e.g., steps, heart rate),7 toward the noninvasive analysis of important chemical biomarkers. Wearable enzyme electrodes thus offer exciting opportunities to a variety of areas, spanning from healthcare,8 sports,9 to the environment or defense.10 In practice, such devices allow (1) real-time noninvasive detection of biomarkers in biofluids, such as sweat, saliva, interstitial fluid (ISF), and tears, to yield health or fitness-relevant information that is traditionally obtained through blood analysis; (2) the B


Article


are subject to drawbacks, such as oxygen fluctuations or deficiency and potential electroactive interferences. Due to the low fluctuating oxygen concentrations in biofluids,20,24 oxidasebased biocatalytic layers should be designed carefully.13 In addition, when the system utilizes the oxidation of H2O2 for amperometric signal transduction, a high operating potential may result in undesired oxidation of electroactive interfering compounds present in biofluids (e.g., ascorbic or uric acid). Such interference effects can be mitigated by utilizing permselective coatings, such as cellulose acetate,25 Nafion,10,26 poly(phenylenediamine) (PPD),20,27 or polypyrrole.28 Lowering the operating potential is another route for alleviating electroactive interferences. Prussian Blue (PB), considered as an artificial peroxidase, can be used as electrode material or modifier, leading to low-potential reduction of H2O2 that minimizes electroactive interferences.29 Thus, most of our demonstrated wearable biosensors employ PB-based electrode transducers.5,9,16,24 However, this signal transduction approach exhibits oxygen dependency challenges. Sensitivity to fluctuating O2 concentrations can be addressed by utilizing a redox mediator, acting as an electron-shuttling agent between the enzymatic active center and the electrode, as common in second generation biosensors.30 Certain target analytes reside in low concentrations, which require highly sensitive detection systems. Since the device sensitivity is strongly related to the electron transfer between the target redox substance and the electrode surface,2 electron transfer mediators (e.g., tetrathiafulvalene (TTF)) or conductive nanomaterials (e.g., gold nanoparticles) can be used for obtaining fast rates of electron transfer.12,15,31 The mediated electron transfer approach is commonly used for enhancing the power density of wearable BFC applications. Attention should also be given to potential mediator leaching and related toxicity. This issue can be further addressed by introducing direct mediator-free electron transfer systems into wearable BFCs.32 The catalytic activity of enzymes is highly sensitive to operating conditions (e.g., temperature and pH). Fluctuations in these parameters, expected during various operations of wearable devices, can greatly compromise the reproducibility and stability of a wearable biosensor or of the power generation efficiency of an on-body BFC. For instance, each biofluid has its own physiological pH range (e.g., human sweat33 from 3 to 6.5 and tears34 from 6.5 to 7.6). Significant sweat pH fluctuations35 can lead to changes in enzymatic activity, ultimately affecting the performance of wearable enzyme electrodes. Additionally, variations in biofluid composition can lead to differences in ionic strength and viscosity, influencing bioelectronic device performance. Some of the above challenges can be partially addressed through efficient enzyme immobilization onto the surface. Such surface immobilization can enhance enzyme stability, leading to consistent biocatalytic activity under extreme conditions. Fluctuations in the response due to pH or temperature changes can be corrected through integrated temperature and pH feedback sensors.36 Surface biofouling issues should also be carefully considered in the design of wearable bioelectronics. Biofoulingsurface passivation due to adsorption of biomolecules (e.g., proteins)can result from prolonged operation of the wearable device in complex biofluid matrices.20,37−39 Biofouling can have severe deleterious effect on the sensor sensitivity and stability. Biofouling can be minimized by covering the

tronic devices display in controlled laboratory settings. Therefore, this Account focuses on how to interface biocatalytic layers onto wearable electronic devices for creating efficient and stable on-body electrochemical biosensors and BFCs. It is not a comprehensive Account but aims to describe representative examples of wearable bioelectronic systems from the authors’ laboratory toward addressing the above challenges and enabling diverse monitoring and energy-harvesting applications.

2. INTERFACING BIOCATALYTIC LAYER WITH WEARABLE BIOELECTRONICS Creating effective wearable bioelectronics requires the introduction of new capabilities, including high-quality enzyme-electronic interfaces with controlled biocatalytic properties, mechanical flexibility, and large-area processing of electronic films. Mechanical flexibility, essential for bioelectronic devices that contact the skin or soft tissue, has been the subject of numerous reviews on wearable devices.13,14 Accordingly, major attention is given here to the realization of efficient enzymatic reactions while operating wearable bioelectronic devices in uncontrolled outdoor or indoor settings. Here, we will focus on biosensing and BFC applications of different enzyme-based wearable bioelectronic platforms. Such wearable systems are commonly based on using the enzymatic reaction of the substrate (analyte or fuel) to generate electrical signal or bioenergy (Figure 1B and C, respectively). The key factor to consider in adapting biocatalytic reactions to wearable bioelectronic applications is the enzyme-electronics interface. The performance of these wearable devices depends strongly on the electron transfer process from enzyme to electrode, which is dictated primarily by the enzyme immobilization strategy. The enzyme immobilization route depends on the specific applications of wearable bioelectronics. In the case of wearable BFCs, effective wiring of enzyme to electrode leads to increased power densities while maintaining high enzyme activity. Different strategies have been used for facilitating electron transfer with enzymes and optimum conditions for maximum enzyme activity. Wearable biosensors are being developed to alert the wearers on unexpected changes in the levels of chemical markers in his/her biofluids or surrounding environment.10,15−19 The performance of these sensing devices, and particularly their accuracy, strongly depends on the integrated enzymatic recognition layer.15,20 High selectivity toward the target analyte can be realized by coupling the specific enzyme recognition layer with permselective coatings that reject coexisting electroactive interferences.21 Common enzyme immobilization strategies22 should thus be tailored to meet the specific requirements of the individual wearable bioelectronic systems and corresponding biofluid.13 The first consideration in establishing effective biocatalytic layers for wearable electronics is the choice of the enzyme that reacts with the target analyte or fuel. For example, owing to the importance of diabetes, the most widely studied substrate has been glucose, which is recognized selectively by glucose oxidase (GOx) or glucose dehydrogenase. However, the latter requires a cofactor (e.g., nicotine adenine dinucleotide) which is not readily immobilized or regenerated, hence hindering wearable applications.23 Oxidase enzymes have thus been preferred in wearable bioelectronics. Oxidase-based biocatalytic layers offer high sensitivity toward the target analyte, yet C


Article


Figure 2. Examples of biofuel cells (BFCs). Images and schematics illustrating: (A) Tattoo-based BFC. Adapted with permission from ref 43. Copyright 2013 John Wiley & Sons. (B) Highly stretchable BFC. Adapted with permission from ref 12. Copyright 2016 American Chemical Society. (C) Stretchable electronic skin-based island-bridge BFC. Adapted with permission from ref 46. Copyright 2017 The Royal Society of Chemistry. (D) Stretchable textile-based BFC as self-powered sensors. Adapted with permission from ref 47. Copyright 2016 The Royal Society of Chemistry. (E) Microneedle-based self-powered glucose sensors. Adapted with permission from ref 49. Copyright 2014 Elsevier. (F) Natural extract-based BFC. Adapted with permission from ref 51. Copyright 2018 The Royal Society of Chemistry.

3. WEARABLE ELECTRONIC DEVICES

biocatalytic layer with protective coatings, excluding the fouling agents from the surface or by entrapping the enzymes within an electropolymerized polymer. The surface modification can also be used for extending the dynamic range of wearable enzyme electrodes. Biosensors limited by the kinetics of the enzymatic reaction may suffer from a narrow dynamic range. The linear range of these devices can be extended by creating a diffusion-limiting layer that modifies analyte access. However, such coating may also increase the response time and lower the sensitivity.

3.1. Wearable Bioenergy Harvesting Devices: Biofuel Cells

The tremendous progress in wearable devices has been hindered by the lack of anatomically compliant power sources.40 Enzymatic BFCs, harvesting usable energy from metabolites present in biofluids, represent an attractive route to address this roadblock.41 Innovative efforts over the past decade have led to the demonstration of implantable BFCs in various organisms.42 However, the challenges faced by implantable BFCs have led to recent explorations of nonD


Article


Figure 3. Examples of wearable biosensor devices. Images and schematics illustrating: (A) First example of a screen-printed noninvasive glucose biosensor based on a tattoo-like iontophoretic system. Adapted with permission from ref 16. Copyright 2015 American Chemical Society. (B) Alcohol tattoo biosensor integrated with electronic board. Adapted with permission from ref 17. Copyright 2016 American Chemical Society. (C) Dual biofluid sampling tattoo devices for simultaneous detection of ISF glucose and sweat alcohol. Adapted with permission from ref 52. Copyright 2018 John Wiley & Sons. (D) Hybrid sensing system for monitoring sweat lactate and electrophysiological signals. Adapted with permission from ref 9. Copyright 2016 Nature. (E) Sweat sampling microfluidic platform for detecting lactate and glucose. Adapted with permission from ref 55. Copyright 2017 American Chemical Society. (F) Microneedle biosensor platform for continuous subcutaneous alcohol monitoring. Adapted with permission from ref 27. Copyright 2017 Elsevier. (G) Bandage-based sensor platform for detecting the melanoma cancer biomarker tyrosinase. Adapted with permission from ref 53. Copyright 2018 John Wiley & Sons. (H) Salivary uric acid mouthguard biosensor integrated with wireless amperometric circuit board. Adapted with permission from ref 20. Copyright 2015 Elsevier. (I) “Lab-on-a-glove” based fingertip biosensing of organophosphate nerve agents. Adapted with permission from ref 10. Copyright 2017 American Chemical Society.

tattoo, was employed for extracting bioenergy from sweat lactate during exercise.43 This device relied on the oxidation of the lactate biofuel, catalyzed by lactate oxidase (LOx) and mediated by TTF. The electron shuttling between LOx and the electrode was enhanced through an electron−donor− acceptor TTF/CNT interface. Noninvasive operation, dem-

invasive wearable BFCs that harvest energy from biofuels present in bofluids.41 An early platform merging enzymatic BFCs with wearable technologies offered an attractive route for harvesting energy from human perspiration (Figure 2A). This first epidermal BFC, based on a bioanode and cathode printed on a temporary E


Article


mediators. This system has the potential to be further applied as a self-powered wearable ethanol biosensing platform, in particular for highly biocompatible applications.50

onstrated by attaching the tattoo BFC on human subjects during exercise, enabled power densities up to 70 μW·cm−2. This concept was demonstrated further on textile substrates and could power a light emitting diode with an integrated DC/ DC converter.44 Epidermal BFCs can experience severe mechanical stress during practical applications leading to deteriorated performance, due to cracking of the conductive support, which anchors the enzyme. To address this challenge, we have engineered screen-printable stretchable inks that incorporate the electrochemical and mechanical properties of CNTs with the stretchable properties of a polyurethane binder, along with a free-standing serpentine design (Figure 2B).12 Coupling these stress-enduring inks and the serpentine configuration offers additional degrees of stretchability, representing the first platform for stretchable enzymatic BFCs. Enhancing the power density attained by wearable BFCs requires optimization of the amount of incorporated enzyme, mediator, and conductive components. Compressed bioanode pellets have been developed to boost the loading levels of these components.45 Since this approach may compromise the mechanical properties, efforts have been made to create stretchable island-bridge structures combining the high enzyme loading of pellet islands with stretchable serpentine bridge interconnects.46 Such unique architecture resulted in a soft bioelectronic skin for scavenging relatively high energy from human perspiration, leading to a power density of 1.2 mW· cm−2 (Figure 2C). Such skin-worn stretchable BFCs provided ∼1 mW power during exercise, sufficient to power electronic devices. In addition, we described the first example of highly stretchable textile-based BFCs, that can be used as selfpowered sensors, extracting electrical power/signal from sweat glucose and lactate (Figure 2D).47 Scavenged bioenergy from the wearer’s sweat itself can thus directly determine the metabolite levels, minimizing the need for external energy sources. Unlike common BFCs based on O2-reduction cathodes, that may fail under anaerobic conditions, the textile BFC relied on a Ag2O/Ag cathode. Such a cathode does not depend on the oxygen reduction, allowing power generation under fluctuating O2 concentrations. Alternatively, it is possible to use O2-rich cathodes for operation under severe O2-deficit conditions.48 A minimally invasive microneedle-based BFC was demonstrated by integrating enzyme carbon pastes into hollow microneedles (Figure 2E).49 This system displayed glucose level-dependent power outputs toward utility in self-powered continuous glucose monitoring without limitation of batterylife. In addition to its high selectivity against electroactive interferences, carbon paste offers favorable stability and storage-life as the enzymes are confined in hydrophobic carbon paste matrixes that minimize the protein mobility.50 Such a minimally invasive system requires critical assessment of the biocompatibility. Many BFCs rely on redox mediators that shuttle electrons between the enzyme active site and electrode. The use of mediators raises several concerns for wearable devices, including safety and instability (due to leaching and toxicity). An alternative “green” approach, based on the use of edible electrode components, including mushroom/plant extracts as biocatalytic BFC systems, was described recently (Figure 2F).51 The system relies on ethanol-oxidation and oxygenreduction biocatalytic systems coupled with coexisting natural

3.2. Wearable Biosensor Devices

Wearable biosensors provide direct detection of target biomarkers16,20,52−54 in whole biofluids such as sweat,55 saliva,24 and ISF,27 yielding useful bioanalytical information. In the following section, we discuss practical on-body sensing applications of such bioelectronic systems. Particular attention has been given to epidermal monitoring of sweat and ISF. We introduced the application of temporary tattoos as wearable bioelectronic sensing platforms.6,15,16,56 Considering the major needs for monitoring glucose in diabetics, we introduced a tattoo platform for measuring glucose, integrating iontophoretic ISF extraction with amperometric glucose biosensing (Figure 3A). The latter was demonstrated by immobilization of GOx on PB-carbon electrode. The PB transducer acted as an “artificial peroxidase”, offering high selectivity toward the H2O2 product of the GOx reaction.16 An alcohol tattoo biosensor was also developed for alcohol monitoring in stimulated sweat (Figure 3B).17 The alcohol oxidase (AOx) enzyme was placed at the anodic iontophoretic electrode to measure alcohol in sweat induced by iontophoretic delivery of the pilocarpine drug. Furthermore, we integrated the alcohol and glucose tattoos into a single platform, capable of sampling sweat and ISF simultaneously for dual fluid multianalyte detection. (Figure 3C).52 The advantages of selective epidermal biosensing devices have been widely illustrated for the detection of sweat lactate in connection to immobilized LOx. We first demonstrated a sweat lactate biosensor on a tattoo platform15 and further developed a hybrid healthcare sensor by coupling a physical electrocardiogram sensor with the sweat lactate biosensor (Figure 3D).9 LOx was immobilized on the PB electrode surface using chitosan, that entrapped the enzyme and absorbed the anionic lactate.57 The enzymatically generated peroxide was detected at the PB transducer (Figure 3D). This Chem-Phys patch thus offered simultaneous monitoring of lactate and heart rate during exercise. The sweat pH while exercising is lower than chemically induced sweat, which may cause a challenge for the enzymatic lactate detection. The active site of LOx, can be protonated at pH < 5; therefore, the natural sweat acidity may decrease the LOx activity.58 Enzyme-based sweat analysis can benefit from the integration of a microfluidic system. Such epidermal fluidic devices offer continuous transport of fresh sweat over the biocatalytic detector, eliminating errors due to potential sweat mixing, carry over, or contamination effects.55 Figure 3E displays a recent example of sweat glucose and lactate monitoring using a bioelectronic fluidic system, relying on the natural sweat pumping, offering reproducible and efficient sweat sample transport over the corresponding enzyme flow detectors, connected to four sweat fluidic inlets. Minimally invasive microneedle devices allow continuous accessibility to ISF.59 The levels of molecules in ISF commonly display good correlation with their blood concentrations due to the diffusional equilibrium between ISF and blood vessels.49 For example, a subcutaneous alcohol microneedle sensor was demonstrated by immobilizing AOx on a Pt wire working electrode (Figure 3F).27 Subsequently, the electrode was modified with a permselective PPD layer, along with a Nafion F


Article


expanding the scope of wearable bioelectronics beyond energy-harvesting systems and noninvasive biosensors to new self-powered biosensors (without a powering device) or enzyme-logic biocomputing devices (that facilitate decisionmaking). The latter can be further integrated with a feedback loop toward autonomous optimal therapeutic intervention.60 Currently, most of the developed wearable BFCs are based on epidermal platforms using sweat lactate as fuel. Other biofuels (e.g., urea, ascorbic acid) and biofluids (e.g., tears) should be explored for future energy harvesting efforts. A stable fuel supply is also essential for efficient power harvesting. This challenge can be addressed by integrating microfluidics to provide continuously flowing biofluids. The major stability challenge can be mitigated by engineering biocatalysts and enzyme−electronic interfaces with greater degree of resistance. Further power generation enhancement can be achieved by integrating enzymatic cascade reactions.61 BFC systems can also be further utilized in connection with biocomputing logicgate systems involving multiple enzymes leading to reversible On/Off switching of the power output.42 Wearable enzyme-based electrochemical sensors have been demonstrated on diverse (head to toe) wearable platforms and in connection to different biofluids, environments, and analytes. The most important requirement for wearable biosensors is the data accuracy which requires reproducible and stable enzyme activity. Variations of enzymatic response from changing operational conditions (pH, temperature) can be corrected by simultaneous monitoring of those factors and data processing. Improved accuracy has thus been demonstrated in several studies.36,14 Furthermore, coupling these noninvasive biosensors with microfluidic devices is expected to enhance further on-body measurements of dynamically changing biomarker signals. While a wide range of prototypes has been demonstrated, wider acceptance and clinical translation of wearable biosensors will require extensive largepopulation validation studies. As wearable bioelectronics systems merge biomaterials, electronics, and wearables, comprehensive expertise of multiple fields is necessary toward further progress. With continued multidisciplinary efforts and innovative solutions to key challenges, the field of wearable bioelectronics is expected to open exciting opportunities for monitoring human physiology, and to have a tremendous impact on diverse biomedical, fitness, and defense fields.

layer. H2O2, generated by the AOx ethanol oxidation, was selectively transported through the size-exclusion PPD film and detected at the Pt electrode. Anionic electroactive interferences were rejected by the outer Nafion layer. Innovative strategies for expanding the scope of detectable biomarkers have led to wearable biosensors for disease screening based on monitoring enzymes directly. Unlike most wearable biosensors that rely on immobilized enzymes, these devices are based on the immobilization of the corresponding substrates. Recently, we introduced a bandage biosensor for detecting the tyrosinase (TYR) enzyme, a melanoma cancer biomarker (Figure 3G).53 The new bandage device relied on immobilizing the TYR’s substrate catechol (CAT). In the presence of TYR, CAT is oxidized to benzoquinone (BQ), which is detected electrochemically at −0.25 V (vs Ag/AgCl) to yield reduction current proportional to TYR levels. We envisioned that such wearable sensors will offer new possibilities of point-of-care screening for skin cancer. Saliva is another accessible biofluid with considerable promise for wearable biomonitoring applications. While saliva can offer rich healthcare-related information, it contains large amounts of proteins and other macromolecules, leading to rapid fouling of in-mouth biosensors. Our group introduced a mouthguard enzymatic biosensing platform for continuous monitoring of salivary lactate or uric acid.20,24 Figure 3H illustrates an uric acid biosensor mounted on a mouthguard platform. Selectivity was achieved by immobilization of uricase enzyme on the PB-carbon electrode. Subsequently, PPD was electrodeposited on the biocatalytic layer to minimize biofouling. Such protective action of PPD allowed direct detection of uric acid in undiluted saliva. Biocompatibility (of the sensor and supporting electronics) represents another major challenge for in-mouth sensing owing to prolonged operation in the oral cavity. Besides monitoring biomarkers, wearable bioelectronics have been developed for monitoring chemical threats in the surrounding environment.10,54 For example, we developed the first wearable biosensor on a glove platform (Figure 3I) for the fingertip detection of organophosphorus nerve agents. This flexible and stretchable “lab-on-a-glove” integrated the collector and sensing fingers for mechanical sample accumulation and enzymatic (organophosphorus hydrolase) electrochemical analysis, respectively, with a conductive hydrogel utilized to complete the electrochemical cell once the two fingers were brought together for measurement.

■

AUTHOR INFORMATION

Corresponding Author

4. CONCLUSIONS AND OUTLOOK In this Account, we have described enzyme-based wearable bioelectronics, focusing on biosensor and BFC strategies and applications introduced primarily in the authors’ laboratories. Such devices require special attention to the enzyme− electronic interface and to several considerations related to wearable applications, such as mechanical properties (flexibility and stretchability), operational stability in different biofluids and under changing conditions (e.g., pH, temperature), biofouling, selectivity, and low target concentrations. Keeping these requirements in mind, our group has developed a variety of wearable biosensing and BFC devices. Although tremendous endeavors have been made in wearable bioelectronics, the resulting devices are mostly at the proof-of-concept prototyping stage, with significant attention given to glucose monitoring devices for diabetes. There is tremendous potential for

*E-mail: [email protected]. ORCID

Itthipon Jeerapan: 0000-0001-8016-6411 Joseph Wang: 0000-0002-4921-9674 Author Contributions ⊥

J.K., I.J., and J.R.S.: Equal contribution. All authors have contributed to the manuscript and have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest. Biographies Jayoung Kim is a postdoc in the Department of NanoEngineering at the University of California San Diego (UCSD). She received her G


Article


(7) Amjadi, M.; Kyung, K.-U.; Park, I.; Sitti, M. Stretchable, SkinMountable, and Wearable Strain Sensors and Their Potential Applications: A Review. Adv. Funct. Mater. 2016, 26, 1678−1698. (8) Yang, Y.; Gao, W. Wearable and Flexible Electronics for Continuous Molecular Monitoring. Chem. Soc. Rev. 2018, DOI: 10.1039/C7CS00730B. (9) Imani, S.; Bandodkar, A. J.; Mohan, A. M. V.; Kumar, R.; Yu, S.; Wang, J.; Mercier, P. P. A Wearable Chemical-electrophysiological Hybrid Biosensing System for Real-Time Health and Fitness Monitoring. Nat. Commun. 2016, 7, 11650. (10) Mishra, R. K.; Hubble, L. J.; Martín, A.; Kumar, R.; Barfidokht, A.; Kim, J.; Musameh, M. M.; Kyratzis, I. L.; Wang, J. Wearable Flexible and Stretchable Glove Biosensor for On-Site Detection of Organophosphorus Chemical Threats. ACS Sensors 2017, 2, 553− 561. (11) Bandodkar, A. J.; Nuñez-Flores, R.; Jia, W.; Wang, J. All-Printed Stretchable Electrochemical Devices. Adv. Mater. 2015, 27, 3060− 3065. (12) Bandodkar, A. J.; Jeerapan, I.; You, J.-M.; Nuñez-Flores, R.; Wang, J. Highly Stretchable Fully-Printed CNT-Based Electrochemical Sensors and Biofuel Cells: Combining Intrinsic and Design-Induced Stretchability. Nano Lett. 2016, 16, 721−727. (13) Bandodkar, A. J.; Jeerapan, I.; Wang, J. Wearable Chemical Sensors: Present Challenges and Future Prospects. ACS Sensors 2016, 1, 464−482. (14) Bandodkar, A. J.; Wang, J. Non-Invasive Wearable Electrochemical Sensors: A Review. Trends Biotechnol. 2014, 32, 363−371. (15) Jia, W.; Bandodkar, A. J.; Valdés-Ramírez, G.; Windmiller, J. R.; Yang, Z.; Ramírez, J.; Chan, G.; Wang, J. Electrochemical Tattoo Biosensors for Real-Time Noninvasive Lactate Monitoring in Human Perspiration. Anal. Chem. 2013, 85, 6553−6560. (16) Bandodkar, A. J.; Jia, W.; Yardımcı, C.; Wang, X.; Ramirez, J.; Wang, J. Tattoo-Based Noninvasive Glucose Monitoring: A Proof-ofConcept Study. Anal. Chem. 2015, 87, 394−398. (17) Kim, J.; Jeerapan, I.; Imani, S.; Cho, T. N.; Bandodkar, A.; Cinti, S.; Mercier, P. P.; Wang, J. Noninvasive Alcohol Monitoring Using a Wearable Tattoo-Based Iontophoretic-Biosensing System. ACS Sensors 2016, 1, 1011−1019. (18) Sempionatto, J. R.; Mishra, R. K.; Martín, A.; Tang, G.; Nakagawa, T.; Lu, X.; Campbell, A. S.; Lyu, K. M.; Wang, J. Wearable Ring-Based Sensing Platform for Detecting Chemical Threats. ACS Sensors 2017, 2, 1531−1538. (19) Sempionatto, J. R.; Nakagawa, T.; Pavinatto, A.; Mensah, S. T.; Imani, S.; Mercier, P.; Wang, J. Eyeglasses Based Wireless Electrolyte and Metabolite Sensor Platform. Lab Chip 2017, 17, 1834−1842. (20) Kim, J.; Imani, S.; de Araujo, W. R.; Warchall, J.; ValdésRamírez, G.; Paixão, T. R. L. C.; Mercier, P. P.; Wang, J. Wearable Salivary Uric Acid Mouthguard Biosensor with Integrated Wireless Electronics. Biosens. Bioelectron. 2015, 74, 1061−1068. (21) Barlett, P. N.; Cooper, J. M. A Review of the Immobilization of Enzymes in Electropolymerized Films. J. Electroanal. Chem. 1993, 362, 1−12. (22) Sassolas, A.; Blum, L. J.; Leca-Bouvier, B. D. Immobilization Strategies to Develop Enzymatic Biosensors. Biotechnol. Adv. 2012, 30, 489−511. (23) Wang, J. Electrochemical Glucose Biosensors. In Electrochemical Sensors, Biosensors and their Biomedical Applications; Elsevier, 2008; pp 57−69. (24) Kim, J.; Valdés-Ramírez, G.; Bandodkar, A. J.; Jia, W.; Martinez, A. G.; Ramírez, J.; Mercier, P.; Wang, J. Non-Invasive Mouthguard Biosensor for Continuous Salivary Monitoring of Metabolites. Analyst 2014, 139, 1632−1636. (25) Cardwell, T. J.; Cattrall, R. W.; Deady, L. W.; Dorkos, M.; O’Connell, G. R. A Fast Response Membrane-Based pH Indicator Optode. Talanta 1993, 40, 765−768. (26) Mishra, R. K.; Vinu Mohan, A. M.; Soto, F.; Chrostowski, R.; Wang, J. A Microneedle Biosensor for Minimally-Invasive Transdermal Detection of Nerve Agents. Analyst 2017, 142, 918−924.

Ph.D. in Material Science and Engineering from UCSD under Prof. Wang’s supervision. Her research interest is focused on developing wearable biosensors for healthcare applications. Itthipon Jeerapan is a Ph.D. candidate in the Department of Nanoengineering at the UCSD. His current research interests involve wearable bioelectronics and advanced materials for energy, biomedical, and electrochemical applications. Juliane R. Sempionatto is a Ph.D. student in the Department of Nanoengineering at the UCSD. Currently, she is focused on developing wearable electrochemical sensors and wearable platforms for sports and health-care applications. Abbas Barfidokht received a Ph.D. degree in chemistry from UNSW Australia and currently works as a postdoc at the Nanobioelectronics Laboratory of Prof. Wang at UCSD and is focusing on wearable bioelectronic devices. Rupesh K. Mishra received his Ph.D. from BITS, Pilani, India and currently works as a postdoc in Prof. Wang’s group at UCSD. He develops wearable sensors for defense and security monitoring, food safety, and biomedical and environmental analysis based on various wearable platforms. Alan S. Campbell is a postdoc in the Department of NanoEngineering at the UCSD. He received a Ph.D. from Carnegie Mellon University. His current research is focused on wearable alcohol monitoring systems. Lee J. Hubble is a Senior Research Scientist at CSIRO, Australia and a Visiting Scholar at UCSD. He received his Ph.D. in Chemistry from the University of Western Australia. His research interests include hybrid nanomaterials for sensing applications, point-of-care diagnostics, and wearable chemical sensors. Joseph Wang is a Distinguished Professor, SAIC Endowed Chair and Chair of the Department of Nanoengineering at UCSD. He held the Regents Professorship at NMSU and served as the Director of the Center for Bioelectronics at ASU. His scientific interests focus on nanomachines, bioelectronics, biosensors, wearable devices, and bionanotechnology.

■

ACKNOWLEDGMENTS Financial support from The Defense Threat Reduction Agency Joint Science and Technology Office for Chemical and Biological Defense (HDTRA1-14-1-0064) and the UCSD Center of Wearable Sensors is acknowledged. I.J., J.R.S., and A.S.C. acknowledge fellowships from the Thai DPST, CNPq., and NIH NIAAA T32 (training grant AA013525), respectively. L.J.H. acknowledges travel support from the ResearchPlus Julius Career Award by CSIRO.

■

REFERENCES

(1) Katz, E. Implantable Bioelectronics, 1st ed.; Wiley, Weinheim, 2014. (2) Wang, J. Analytical Electrochemistry, 3rd ed.; Wiley: Hoboken, NJ, 2006. (3) Newman, J. D.; Setford, S. J. Enzymatic Biosensors. Mol. Biotechnol. 2006, 32, 249−268. (4) Willner, I.; Yan, Y.-M.; Willner, B.; Tel-Vered, R. Integrated Enzyme-Based Biofuel Cells-A Review. Fuel Cells 2009, 9, 7−24. (5) Kim, J.; Campbell, A. S.; Wang, J. Wearable Non-Invasive Epidermal Glucose Sensors: A Review. Talanta 2018, 177, 163−170. (6) Mishra, R. K.; Barfidokht, A.; Karajic, A.; Sempionatto, J. R.; Wang, J.; Wang, J. Wearable Potentiometric Tattoo Biosensor for OnBody Detection of G-Type Nerve Agents Simulants. Sens. Actuators, B 2018, 273, 966−972. H


Article

Accounts of Chemical Research (27) Mohan, A. M. V.; Windmiller, J. R.; Mishra, R. K.; Wang, J. Continuous Minimally-Invasive Alcohol Monitoring Using Microneedle Sensor Arrays. Biosens. Bioelectron. 2017, 91, 574−579. (28) Foulds, N. C.; Lowe, C. R. Immobilization of Glucose Oxidase in Ferrocene-Modified Pyrrole Polymers. Anal. Chem. 1988, 60, 2473−2478. (29) Karyakin, A. A.; Gitelmacher, O. V.; Karyakina, E. E. Prussian Blue-Based First-Generation Biosensor. A Sensitive Amperometric Electrode for Glucose. Anal. Chem. 1995, 67, 2419−2423. (30) Wang, J. Glucose biosensors: 40 years of advances and challenges. Electroanalysis 2001, 13, 983−988. (31) Gao, F.; Viry, L.; Maugey, M.; Poulin, P.; Mano, N. Engineering Hybrid Nanotube Wires for High-Power Biofuel Cells. Nat. Commun. 2010, 1, 1−7. (32) Cooney, M. J.; Svoboda, V.; Lau, C.; Martin, G.; Minteer, S. D. Enzyme Catalysed Biofuel Cells. Energy Environ. Sci. 2008, 1, 320− 337. (33) Matousek, J. L.; Campbell, K. L. A Comparative Review of Cutaneous pH. Vet. Dermatol. 2002, 13, 293−300. (34) Abelson, M. B.; Udell, I. J.; Weston, J. H. Normal Human Tear pH by Direct Measurement. Arch. Ophthalmol. (Chicago, IL, U. S.) 1981, 99, 301. (35) Guinovart, T.; Bandodkar, A. J.; Windmiller, J. R.; Andrade, F. J.; Wang, J. A Potentiometric Tattoo Sensor for Monitoring Ammonium in Sweat. Analyst 2013, 138, 7031−7038. (36) Gao, W.; Emaminejad, S.; Nyein, H. Y. Y.; Challa, S.; Chen, K.; Peck, A.; Fahad, H. M.; Ota, H.; Shiraki, H.; Kiriya, D.; Lien, D. H.; Brooks, G. A.; Davis, R. W.; Javey, A. Fully Integrated Wearable Sensor Arrays for Multiplexed in situ Perspiration Analysis. Nature 2016, 529, 509−514. (37) Duwensee, H.; Vazquezalvarez, T.; Flechsig, G.; Wang, J. Thermally Induced Electrode Protection against Biofouling. Talanta 2009, 77, 1757−1760. (38) Wisniewski, N.; Moussy, F.; Reichert, W. Characterization of implantable biosensor membrane biofouling. Fresenius J. Anal. Chem. 2000, 366, 611−621. (39) Zhang, H.; Chiao, M. Anti-Fouling Coatings of Poly(Dimethylsiloxane) Devices for Biological and Biomedical Applications. J. Med. Biol. Eng. 2015, 35, 143−155. (40) Stoppa, M.; Chiolerio, A. Wearable Electronics and Smart Textiles: A Critical Review. Sensors 2014, 14, 11957−11992. (41) Bandodkar, A. J.; Wang, J. Wearable Biofuel Cells: A Review. Electroanalysis 2016, 28, 1188−1200. (42) Katz, E.; MacVittie, K. Implanted Biofuel Cells Operating in Vivo − Methods, Applications and Perspectives − Feature Article. Energy Environ. Sci. 2013, 6, 2791−2803. (43) Jia, W.; Valdés-Ramírez, G.; Bandodkar, A. J.; Windmiller, J. R.; Wang, J. Epidermal Biofuel Cells: Energy Harvesting from Human Perspiration. Angew. Chem., Int. Ed. 2013, 52, 7233−7236. (44) Berchmans, S.; Bandodkar, A. J.; Jia, W.; Ramírez, J.; Meng, Y. S.; Wang, J. An Epidermal Alkaline Rechargeable Ag−Zn Printable Tattoo Battery for Wearable Electronics. J. Mater. Chem. A 2014, 2, 15788−15795. (45) Zebda, A.; Gondran, C.; Cinquin, P.; Cosnier, S. Glucose Biofuel Cell Construction Based on Enzyme, Graphite Particle and Redox Mediator Compression. Sens. Actuators, B 2012, 173, 760−764. (46) Bandodkar, A. J.; You, J.-M.; Kim, N.-H.; Gu, Y.; Kumar, R.; Mohan, A. M. V.; Kurniawan, J.; Imani, S.; Nakagawa, T.; Parish, B.; Parthasarathy, M.; Mercier, P. P.; Xu, S.; Wang, J. Soft, Stretchable, High Power Density Electronic Skin-Based Biofuel Cells for Scavenging Energy from Human Sweat. Energy Environ. Sci. 2017, 10, 1581−1589. (47) Jeerapan, I.; Sempionatto, J. R.; Pavinatto, A.; You, J.-M.; Wang, J. Stretchable Biofuel Cells as Wearable Textile-Based SelfPowered Sensors. J. Mater. Chem. A 2016, 4, 18342−18353. (48) Jeerapan, I.; Sempionatto, J. R.; You, J.-M.; Wang, J. Enzymatic glucose/oxygen biofuel cells: Use of oxygen-rich cathodes for operation under severe oxygen-deficit conditions. Biosens. Bioelectron. 2018, 122, 284−289.

(49) Valdeś-Ramírez, G.; Li, Y.-C.; Kim, J.; Jia, W.; Bandodkar, A. J.; Nuñez-Flores, R.; Miller, P. R.; Wu, S.-Y.; Narayan, R.; Windmiller, J. R.; Polsky, R.; Wang, J. Microneedle-Based Self-Powered Glucose Sensor. Electrochem. Commun. 2014, 47, 58−62. (50) Wang, J.; Liu, J.; Cepra, G. Thermal Stabilization of Enzymes Immobilized within Carbon Paste Electrodes. Anal. Chem. 1997, 69, 3124−3127. (51) Jeerapan, I.; Ciui, B.; Martin, I.; Cristea, C.; Sandulescu, R.; Wang, J. Fully Edible Biofuel Cells. J. Mater. Chem. B 2018, 6, 3571− 3578. (52) Kim, J.; Sempionatto, J. R.; Imani, S.; Hartel, M. C.; Barfidokht, A.; Tang, G.; Campbell, A. S.; Mercier, P. P.; Wang, J. Simultaneous Monitoring of Sweat and Interstitial Fluid Using a Single Wearable Biosensor Platform. Adv. Sci. 2018, 5, 1800880. (53) Ciui, B.; Martin, A.; Mishra, R. K.; Brunetti, B.; Nakagawa, T.; Dawkins, T. J.; Lyu, M.; Cristea, C.; Sandulescu, R.; Wang, J. Wearable Wireless Tyrosinase Bandage and Microneedle Sensors: Toward Melanoma Screening. Adv. Healthcare Mater. 2018, 7, 1701264. (54) Mishra, R. K.; Martín, A.; Nakagawa, T.; Barfidokht, A.; Lu, X.; Sempionatto, J. R.; Lyu, K. M.; Karajic, A.; Musameh, M. M.; Kyratzis, I. L.; Wang, J. Detection of Vapor-Phase Organophosphate Threats Using Wearable Conformable Integrated Epidermal and Textile Wireless Biosensor Systems. Biosens. Bioelectron. 2018, 101, 227−234. (55) Martín, A.; Kim, J.; Kurniawan, J. F.; Sempionatto, J. R.; Moreto, J. R.; Tang, G.; Campbell, A. S.; Shin, A.; Lee, M. Y.; Liu, X.; Wang, J. Epidermal Microfluidic Electrochemical Detection System: Enhanced Sweat Sampling and Metabolite Detection. ACS Sensors 2017, 2, 1860−1868. (56) Windmiller, J. R.; Bandodkar, A. J.; Valdés-Ramírez, G.; Parkhomovsky, S.; Martinez, A. G.; Wang, J. Electrochemical Sensing Based on Printable Temporary Transfer Tattoos. Chem. Commun. 2012, 48, 6794. (57) Ahmadi, F.; Oveisi, Z.; Samani, S. M.; Amoozgar, Z. Chitosan Based Hydrogels: Characteristics and Pharmaceutical Applications. Res. Pharm. Sci. 2015, 10, 1−16. (58) Muller, F. Chemistry and Biochemistry of Flavoenzymes: Volume II; CRC Press, 2017. (59) Mukerjee, E. V.; Collins, S. D.; Isseroff, R. R.; Smith, R. L. Microneedle Array for Transdermal Biological Fluid Extraction and in Situ Analysis. Sens. Actuators, A 2004, 114, 267−275. (60) Zhou, M.; Zhou, N.; Kuralay, F.; Windmiller, J. R.; Parkhomovsky, S.; Valdés-Ramírez, G.; Katz, E.; Wang, J. A SelfPowered “Sense-Act-Treat” System That is Based on a Biofuel Cell and Controlled by Boolean Logic. Angew. Chem., Int. Ed. 2012, 51, 2686−2689. (61) Van Nguyen, K.; Giroud, F.; Minteer, S. D. Improved Bioelectrocatalytic Oxidation of Sucrose in a Biofuel Cell with an Enzyme Cascade Assembled on a DNA Scaffold. J. Electrochem. Soc. 2014, 161, H930−H933.

I


Wearable Bioelectronics: Enzyme-Based Body-Worn Electronic

Recommend Documents