Contact lens biofuel cell tested in a synthetic tear solution

Self-adhesive wearable poly (vinyl alcohol)-based hybrid biofuel cell powered by human bio-fluids

Advancement of wearable microelectronics demands their power source with continuous energy supply, skin-integration and miniaturization. In light of poly (vinyl alcohol) (PVA) hydrogel with nontoxicity, good biocompatibility and low cost, an advanced wearable PVA-based hybrid biofuel cells (HBFCs) with high self-adhesiveness was developed. Through the reaction between PVA molecules and succinic anhydride (SAA), the carboxylated PVA (PVA/SAA) was obtained, and by incorporation with PDA as crosslinker, the self-adhesive PVA/SAA-DA hydrogel electrolytes formed by dual covalent and hydrogen bonding. With increasing SAA and PDA content, the pore size decreased, and a uniform and dense network formed, endowing the hydrogel with a relatively high absorption capacity of PBS solution of lactate as cell fuel. Meanwhile the various functional groups of hydrogel, including catechol, quinone, amino and hydroxyl groups, contributed to impressive tissue adhesion strength against pigskin under dry and wet conditions. The PVA/SAA-DA hydrogel displayed high conductive property, and the integrated PVA-based HBFC generated open circuit voltage of 0.50 V and maximum power density of 128.76 μW/cm2 in 20 mM lactate solution, which was optimized to be 0.57 V/224.85 μW/cm2 when the pore size was enlarged. The power retention reached above 70% in one week, showing long-term stability of HBFC. The PVA-based HBFC was further adhered to human skin without extra adhesive tapes to scavenge human sweat as biofuel, and the maximum power density reached 85.34 μW/cm2, while by connected with a DC-DC converter, the HBFC could power watch, exhibiting promising application potentials as wearable electronic device to provide bioelectricity.

Biofluid-Activated Biofuel Cells, Batteries, and Supercapacitors: A Comprehensive Review

Recent developments in wearable and implanted devices have resulted in numerous, unprecedented capabilities that generate increasingly detailed information about a user's health or provide targeted therapy. However, options for powering such systems remain limited to conventional batteries which are large and have toxic components and as such are not suitable for close integration with the human body. This work provides an in-depth overview of biofluid-activated electrochemical energy devices, an emerging class of energy sources judiciously designed for biomedical applications. These unconventional energy devices are composed of biocompatible materials that harness the inherent chemistries of various biofluids to produce useable electrical energy. This work covers examples of such biofluid-activated energy devices in the form of biofuel cells, batteries, and supercapacitors. Advances in materials, design engineering, and biotechnology that form the basis for high-performance, biofluid-activated energy devices are discussed. Innovations in hybrid manufacturing and heterogeneous integration of device components to maximize power output are also included. Finally, key challenges and future scopes of this nascent field are provided.

Multifunctional MOF-Derived Au, Co-Doped Porous Carbon Electrode for a Wearable Sweat Energy Harvesting-Storage Hybrid System

As an efficient alternative for harnessing the energy from human's biofluid, a wearable energy harvesting-storage hybrid supercapacitor-biofuel cell (SC-BFC) microfluidic system is established with one multifunctional electrode. The electrode integrates metal-organic framework (MOF) derived carbon nanoarrays with embedded Au, Co nanoparticles on a flexible substrate, and is used for the symmetric supercapacitor as well as the enzyme nanocarriers of the biofuel cell. The electrochemical performance of the proposed electrode is evaluated, and the corresponding working mechanism is studied in depth according to the cyclic voltammetry and density functional theory calculation. The multiplexed microfluidic system is designed to pump and store natural sweat to maintain the continuous biofuel supply in the hybrid SC-BFC system. The biofuel cell module harvests electricity from lactate in sweat, and the symmetric supercapacitor module accommodates the bioelectricity for subsequent utilization. A numerical model is developed to validate the normal operation in poor and rich sweat under variable situations for the microfluidic system. One single SC-BFC unit can be self-charged to ≈0.8 V with superior mechanical durability in on-body testing, as well as energy and power values of 7.2 mJ and 80.3 µW, respectively. It illustrates the promising scenery of energy harvesting-storage hybrid microfluidic system.

Recent advances in enzymatic biofuel cells enabled by innovative materials and techniques

The past few decades have seen increasingly rapid advances in the field of sustainable energy technologies. As a new bio- and eco-friendly energy source, enzymatic biofuel cells (EBFCs) have garnered significant research interest due to their capacity to power implantable bioelectronics, portable devices, and biosensors by utilizing biomass as fuel under mild circumstances. Nonetheless, numerous obstacles impeded the commercialization of EBFCs, including their relatively modest power output and poor long-term stability of enzymes. To depict the current progress of EBFC and address the challenges it faces, this review traces back the evolution of EBFC and focuses on contemporary advances such as newly emerged multi or single enzyme systems, various porous framework-enzyme composites techniques, and innovative applications. Besides emphasizing current achievements in this field, from our perspective part we also introduced novel electrode and cell design for highly effective EBFC fabrication. We believe this review will assist readers in comprehending the basic research and applications of EBFCs as well as potentially spark interdisciplinary collaboration for addressing the pressing issues in this field.

Safe, Durable, and Sustainable Self-Powered Smart Contact Lenses

Smart contact lenses have the potential to serve as noninvasive healthcare devices or virtual displays. However, their implementation is limited by the lack of suitable power sources for microelectronic devices. This Article demonstrates smart contact lenses with fully embedded glucose fuel cells that are safe, flexible, and durable against deformations. These fuel cells produced stable power throughout the day or during intermittent use after storage for weeks. When the lenses were exposed to 0.05 mM glucose solution, a steady-state maximum power density of 4.4 μW/cm² was achieved by optimizing the chemistry and porous structure of the fuel cell components. Additionally, even after bending the lenses in half 100 times, the fuel cell performance was maintained without any mechanical failure. Lastly, when the fuel cells were connected to electroresponsive hydrogel capacitors, we could clearly distinguish between the tear glucose levels under normal and diabetic conditions through the naked eye.

A Ceramic-Electrolyte Glucose Fuel Cell for Implantable Electronics

Next-generation implantable devices such as sensors, drug-delivery systems, and electroceuticals require efficient, reliable, and highly miniaturized power sources. Existing power sources such as the Li-I₂ pacemaker battery exhibit limited scale-down potential without sacrificing capacity, and therefore, alternatives are needed to power miniaturized implants. This work shows that ceramic electrolytes can be used in potentially implantable glucose fuel cells with unprecedented miniaturization. Specifically, a ceramic glucose fuel cell-based on the proton-conducting electrolyte ceria-that is composed of a freestanding membrane of thickness below 400 nm and fully integrated into silicon for easy integration into bioelectronics is demonstrated. In contrast to polymeric membranes, all materials used are highly temperature stable, making thermal sterilization for implantation trivial. A peak power density of 43 µW cm^-2 , and an unusually high statistical verification of successful fabrication and electrochemical function across 150 devices for open-circuit voltage and 12 devices for power density, enabled by a specifically designed testing apparatus and protocol, is demonstrated. The findings demonstrate that ceramic-based micro-glucose-fuel-cells constitute the smallest potentially implantable power sources to date and are viable options to power the next generation of highly miniaturized implantable medical devices.

Smart Chemical Engineering-Based Lightweight and Miniaturized Attachable Systems for Advanced Drug Delivery and Diagnostics

Smart attachable systems have attracted much attention owing to their capabilities in terms of body performance evaluation, disease diagnostics, and drug delivery. Recent advances in chemical and engineering techniques provide many opportunities to improve device fabrication and applications owing to the advantages of being lightweight and easy to control as well as their battery absence and functional diversity. This review highlights the latest developments in the field of chemical engineering-based lightweight and miniaturized attachable systems, which are mainly inspired by the natural world. Their applications for real-time monitoring, point-of-care sampling, biomarker detection, and controlled release are discussed thoroughly with respect to specific products/prototypes. The perspectives of the field, including persistence guarantee, burden reduction, and personality improvement, are also discussed. It is believed that chemical engineering-based lightweight and miniaturized attachable systems have good potential in both clinical and industrial fields, indicating a large potential to improve human lives in the near future.

Powering smart contact lenses for continuous health monitoring: Recent advancements and future challenges

As the tear is noninvasively and continuously available, it has been turned into a convenient biological interface as a wearable medical device for out-of-hospital and self-monitoring applications. Recent progress in integrated circuits (ICs) and biosensors coupled with wireless data communication techniques have led to the implementation of smart contact lenses that can continuously sample tear fluid, analyze physiological conditions, and wirelessly transmit data to an electronic device such as smartphone, which can send data to relevant healthcare units. Continuous analyte monitoring is one of the significant characteristics of wearable biosensors. However, despite several advantages over other on-skin wearable medical devices, batteries cannot be incorporated on smart contact lenses for continuous electrical power supply due to the limited area. Herein, we review the progress of power delivery techniques of smart contact lenses for the first time. Different approaches, including wireless power transmission (WPT), biofuel cells, supercapacitors, flexible batteries, wired connections, and hybrid methods, are thoroughly discussed to understand the principles of self-sustainable contact lens biosensors comprehensively. Additionally, recent progress in contact lens biosensors is reviewed in detail, thereby providing the prospects for further developments of smart contact lenses as a common biosensing platform for various disease monitoring and diagnostic applications.

Flexible and Stretchable Enzymatic Biofuel Cell with High Performance Enabled by Textile Electrodes and Polymer Hydrogel Electrolyte

Biofuel cells with good biocompatibility are promising to be used as the power source for flexible and wearable bioelectronics. We here report a type of highly flexible and stretchable biofuel cells, which are enabled by textile electrodes of graphene/carbon nanotubes (G/CNTs) composite and polymer hydrogel electrolyte. The CNT array covalently grown from a graphene layer not only can be served as a conducting substrate to immobilize enzyme molecules but also can provide efficient charge transport channels between the enzyme and graphene electrode. As a result, the developed biofuel cells deliver a high open-circuit voltage of 0.65 V and output power density of 64.2 μW cm^-2, which are much higher than previously reported results. Benefiting from the unique textile structure of electrodes and the polymer hydrogel electrolyte, the biofuel cells exhibit high retention of power density after 400 bending cycles and even stretched to a high strain of 60%.

Biorefinery of galacturonic acid using a biofuel cell as a reactor

A reactor based on an enzymatic biofuel cell (an EBFC reactor) was constructed to simultaneously generate electricity and chemical products from biomass.

Organic Bioelectronic Devices for Metabolite Sensing

Electrochemical detection of metabolites is essential for early diagnosis and continuous monitoring of a variety of health conditions. This review focuses on organic electronic material-based metabolite sensors and highlights their potential to tackle critical challenges associated with metabolite detection. We provide an overview of the distinct classes of organic electronic materials and biorecognition units used in metabolite sensors, explain the different detection strategies developed to date, and identify the advantages and drawbacks of each technology. We then benchmark state-of-the-art organic electronic metabolite sensors by categorizing them based on their application area (in vitro, body-interfaced, in vivo, and cell-interfaced). Finally, we share our perspective on using organic bioelectronic materials for metabolite sensing and address the current challenges for the devices and progress to come.

Energy Autonomous Sweat-Based Wearable Systems

The continuous operation of wearable electronics demands reliable sources of energy, currently met through Li-ion batteries and various energy harvesters. These solutions are being used out of necessity despite potential safety issues and unsustainable environmental impact. Safe and sustainable energy sources can boost the use of wearables systems in diverse applications such as health monitoring, prosthetics, and sports. In this regard, sweat- and sweat-equivalent-based studies have attracted tremendous attention through the demonstration of energy-generating biofuel cells, promising power densities as high as 3.5 mW cm^-2 , storage using sweat-electrolyte-based supercapacitors with energy and power densities of 1.36 Wh kg^-1 and 329.70 W kg^-1 , respectively, and sweat-activated batteries with an impressive energy density of 67 Ah kg^-1 . A combination of these energy generating, and storage devices can lead to fully energy-autonomous wearables capable of providing sustainable power in the µW to mW range, which is sufficient to operate both sensing and communication devices. Here, a comprehensive review covering these advances, addressing future challenges and potential solutions related to fully energy-autonomous wearables is presented, with emphasis on sweat-based energy storage and energy generation elements along with sweat-based sensors as applications.

Advances in Biosensors for Continuous Glucose Monitoring Towards Wearables

Continuous glucose monitors (CGMs) for the non-invasive monitoring of diabetes are constantly being developed and improved. Although there are multiple biosensing platforms for monitoring glucose available on the market, there is still a strong need to enhance their precision, repeatability, wearability, and accessibility to end-users. Biosensing technologies are being increasingly explored that use different bodily fluids such as sweat and tear fluid, etc., that can be calibrated to and therefore used to measure blood glucose concentrations accurately. To improve the wearability of these devices, exploring different fluids as testing mediums is essential and opens the door to various implants and wearables that in turn have the potential to be less inhibiting to the wearer. Recent developments have surfaced in the form of contact lenses or mouthguards for instance. Challenges still present themselves in the form of sensitivity, especially at very high or low glucose concentrations, which is critical for a diabetic person to monitor. This review summarises advances in wearable glucose biosensors over the past 5 years, comparing the different types as well as the fluid they use to detect glucose, including the CGMs currently available on the market. Perspectives on the development of wearables for glucose biosensing are discussed.

Ratiometric Fluorescent Biosensors for Glucose and Lactate Using an Oxygen-Sensing Membrane

In this study, ratiometric fluorescent glucose and lactate biosensors were developed using a ratiometric fluorescent oxygen-sensing membrane immobilized with glucose oxidase (GOD) or lactate oxidase (LOX). Herein, the ratiometric fluorescent oxygen-sensing membrane was fabricated with the ratio of two emission wavelengths of platinum meso-tetra (pentafluorophenyl) porphyrin (PtP) doped in polystyrene particles and coumarin 6 (C6) captured into silica particles. The operation mechanism of the sensing membranes was based on (i) the fluorescence quenching effect of the PtP dye by oxygen molecules, and (ii) the consumption of oxygen levels in the glucose or lactate oxidation reactions under the catalysis of GOD or LOX. The ratiometric fluorescent glucose-sensing membrane showed high sensitivity to glucose in the range of 0.1–2 mM, with a limit of detection (LOD) of 0.031 mM, whereas the ratiometric fluorescent lactate-sensing membrane showed the linear detection range of 0.1–0.8 mM, with an LOD of 0.06 mM. These sensing membranes also showed good selectivity, fast reversibility, and stability over long-term use. They were applied to detect glucose and lactate in artificial human serum, and they provided reliable measurement results.

Continuous Non-Invasive Glucose Monitoring via Contact Lenses: Current Approaches and Future Perspectives

Diabetes mellitus (DM) is a chronic disease that must be carefully managed to prevent serious complications such as cardiovascular disease, retinopathy, nephropathy and neuropathy. Self-monitoring of blood glucose is a crucial tool for managing diabetes and, at present, all relevant procedures are invasive while they only provide periodic measurements. The pain and measurement intermittency associated with invasive techniques resulted in the exploration of painless, continuous, and non-invasive techniques of glucose measurement that would facilitate intensive management. The focus of this review paper is the existing solutions for continuous non-invasive glucose monitoring via contact lenses (CLs) and to carry out a detailed, qualitative, and comparative analysis to inform prospective researchers on viable pathways. Direct glucose monitoring via CLs is contingent on the detection of biomarkers present in the lacrimal fluid. In this review, emphasis is given on two types of sensors: a graphene-AgNW hybrid sensor and an amperometric sensor. Both sensors can detect the presence of glucose in the lacrimal fluid by using the enzyme, glucose oxidase. Additionally, this review covers fabrication procedures for CL biosensors. Ever since Google published the first glucose monitoring embedded system on a CL, CL biosensors have been considered state-of-the-art in the medical device research and development industry. The CL not only has to have a sensory system, it must also have an embedded integrated circuit (IC) for readout and wireless communication. Moreover, to retain mobility and ease of use of the CLs used for continuous glucose monitoring, the power supply to the solid-state IC on such CLs must be wireless. Currently, there are four methods of powering CLs: utilizing solar energy, via a biofuel cell, or by inductive or radiofrequency (RF) power. Although, there are many limitations associated with each method, the limitations common to all, are safety restrictions and CL size limitations. Bearing this in mind, RF power has received most of the attention in reported literature, whereas solar power has received the least attention in the literature. CLs seem a very promising target for cutting edge biotechnological applications of diagnostic, prognostic and therapeutic relevance.

Holographic contact lens display that provides focusable images for eyes

In this paper, we propose a holographic image generation technique for contact lens displays. The proposed technique employs a phase-only spatial light modulator (SLM), a holographic optical element (HOE) backlight, and a polarizer. The proposed holographic technique can generate 3D images apart from the contact lens displays. Therefore, the eyes can focus on the 3D images while simultaneously observing the real scene through the phase-only SLM and the HOE backlight, which provides see-through capability. A bench-top experimental system was constructed to verify the far-distance image generation capability and see-through function.

Integrated contact lens sensor system based on multifunctional ultrathin MoS2 transistors

Smart contact lenses attract extensive interests due to their capability of directly monitoring physiological and ambient information. However, previous demonstrations usually lacked efficient sensor modalities, facile fabrication process, mechanical stability, or biocompatibility. Here, we demonstrate a flexible approach for fabrication of multifunctional smart contact lenses with an ultrathin MoS₂ transistors-based serpentine mesh sensor system. The integrated sensor systems contain a photodetector for receiving optical information, a glucose sensor for monitoring glucose level directly from tear fluid, and a temperature sensor for diagnosing potential corneal disease. Unlike traditional sensors and circuit chips sandwiched in the lens substrate, this serpentine mesh sensor system can be directly mounted onto the lenses and maintain direct contact with tears, delivering high detection sensitivity, while being mechanically robust and not interfering with either blinking or vision. Furthermore, the in vitro cytotoxicity tests reveal good biocompatibility, thus holding promise as next-generation soft electronics for healthcare and medical applications.

Fiber-crafted biofuel cell bracelet for wearable electronics

Wearable devices that generate power using sweat have garnered much attention in the field of skin electronics. These devices require high performance with a small volume and low production rate of sweat by living organisms. Here we demonstrate a high-power biofuel cell bracelet based on the lactate in human sweat. The biofuel cell was developed by using a lactate oxidase/osmium-based mediator/carbon nanotube fiber for lactate oxidation and a bilirubin oxidase/carbon nanotube fiber for oxygen reduction; the fibers were woven into a hydrophilic supportive textile for sweat storage. The storage textile was sandwiched between a hydrophobic textile for sweat absorption from the skin and a hydrophilic textile for water evaporation to improve sweat collection. The performance of the layered cell was 74 μW at 0.39 V in 20 mM artificial sweat lactate, and its performance was maintained at over 80% for 12 h. Furthermore, we demonstrated a series-connection between anode/cathode fibers by tying them up to wrap the bracelet-type biofuel cell on the wrist. The booster six-cell bracelet generated power at 2.0 V that is sufficient for operating digital wrist watches.

Recent Advances in the Direct Electron Transfer-Enabled Enzymatic Fuel Cells

Direct electron transfer (DET), which requires no mediator to shuttle electrons from enzyme active site to the electrode surface, minimizes complexity caused by the mediator and can further enable miniaturization for biocompatible and implantable devices. However, because the redox cofactors are typically deeply embedded in the protein matrix of the enzymes, electrons generated from oxidation reaction cannot easily transfer to the electrode surface. In this review, methods to improve the DET rate for enhancement of enzymatic fuel cell performances are summarized, with a focus on the more recent works (past 10 years). Finally, progress on the application of DET-enabled EFC to some biomedical and implantable devices are reported.

Non-Invasive Blood Glucose Monitoring Technology: A Review

In recent years, with the rise of global diabetes, a growing number of subjects are suffering from pain and infections caused by the invasive nature of mainstream commercial glucose meters. Non-invasive blood glucose monitoring technology has become an international research topic and a new method which could bring relief to a vast number of patients. This paper reviews the research progress and major challenges of non-invasive blood glucose detection technology in recent years, and divides it into three categories: optics, microwave and electrochemistry, based on the detection principle. The technology covers medical, materials, optics, electromagnetic wave, chemistry, biology, computational science and other related fields. The advantages and limitations of non-invasive and invasive technologies as well as electrochemistry and optics in non-invasives are compared horizontally in this paper. In addition, the current research achievements and limitations of non-invasive electrochemical glucose sensing systems in continuous monitoring, point-of-care and clinical settings are highlighted, so as to discuss the development tendency in future research. With the rapid development of wearable technology and transdermal biosensors, non-invasive blood glucose monitoring will become more efficient, affordable, robust, and more competitive on the market.

Enzymatic Glucose-Based Bio-batteries: Bioenergy to Fuel Next-Generation Devices

This article consists of a review of the main concepts and paradigms established in the field of biological fuel cells or biofuel cells. The aim is to provide an overview of the current panorama, basic concepts, and methodologies used in the field of enzymatic biofuel cells, as well as the applications of these bio-systems in flexible electronics and implantable or portable devices. Finally, the challenges needing to be addressed in the development of biofuel cells capable of supplying power to small size devices with applications in areas related to health and well-being or next-generation portable devices are analyzed. The aim of this study is to contribute to biofuel cell technology development; this is a multidisciplinary topic about which review articles related to different scientific areas, from Materials Science to technology applications, can be found. With this article, the authors intend to reach a wide readership in order to spread biofuel cell technology for different scientific profiles and boost new contributions and developments to overcome future challenges.

Site-Directed Mutagenesis of Multicopper Oxidase from Hyperthermophilic Archaea for High-Voltage Biofuel Cells

Enzymes from hyperthermophilic archaea are potential candidates for industrial use because of their superior pH, thermal, and long-term stability, and are expected to improve the long-term stability of biofuel cells (BFCs). However, the reported multicopper oxidase (MCO) from hyperthermophilic archaea has lower redox potential than MCOs from other organisms, which leads to a decrease in the cell voltage of BFCs. In this study, we attempted to positively shift the redox potential of the MCO from hyperthermophilic archaeon Pyrobaculum aerophilum (McoP). Mutations (M470L and M470F) were introduced into the axial ligand of the T1 copper atom of McoP, and the enzymatic chemistry and redox potentials were compared with that of the parent (M470). The redox potentials of M470L and M470F shifted positively by about 0.07 V compared with that of M470. In addition, the catalytic activity of the mutants towards 2,2'-azino-bis(3-ethylbenzothiazoline-6-sulphonic acid) (ABTS) increased 1.2-1.3-fold. The thermal stability of the mutants and the electrocatalytic performance for O₂ reduction of M470F was slightly reduced compared with that of M470. This research provides useful enzymes for application as biocathode catalysts for high-voltage BFCs.

Soft Materials for Wearable/Flexible Electrochemical Energy Conversion, Storage, and Biosensor Devices

Next-generation wearable technology needs portable flexible energy storage, conversion, and biosensor devices that can be worn on soft and curved surfaces. The conformal integration of these devices requires the use of soft, flexible, light materials, and substrates with similar mechanical properties as well as high performances. In this review, we have collected and discussed the remarkable research contributions of recent years, focusing the attention on the development and arrangement of soft and flexible materials (electrodes, electrolytes, substrates) that allowed traditional power sources and sensors to become viable and compatible with wearable electronics, preserving or improving their conventional performances.

Biofuel-powered soft electronic skin with multiplexed and wireless sensing for human-machine interfaces

Existing electronic skin (e-skin) sensing platforms are equipped to monitor physical parameters using power from batteries or near-field communication. For e-skins to be applied in the next generation of robotics and medical devices, they must operate wirelessly and be self-powered. However, despite recent efforts to harvest energy from the human body, self-powered e-skin with the ability to perform biosensing with Bluetooth communication are limited because of lack of a continuous energy source and limited power efficiency. Here, we report a flexible and fully perspiration-powered integrated electronic skin (PPES) for multiplexed metabolic sensing in situ. The battery-free e-skin contains multimodal sensors and highly efficient lactate biofuel cells that use a unique integration of zero- to three-dimensional nanomaterials to achieve high power intensity and long-term stability. The PPES delivered a record-breaking power density of 3.5 milliwatt-centimeter-2 for biofuel cells in untreated human body fluids (human sweat) and displayed a very stable performance during a 60-hour continuous operation. It selectively monitored key metabolic analytes (e.g., urea, NH4 +, glucose, and pH) and the skin temperature during prolonged physical activities and wirelessly transmitted the data to the user interface using Bluetooth. The PPES was also able to monitor muscle contraction and work as a human-machine interface for human- prosthesis walking.

Flexible Electrochemical Bioelectronics: The Rise of In Situ Bioanalysis

The amalgamation of flexible electronics in biological systems has shaped the way health and medicine are administered. The growing field of flexible electrochemical bioelectronics enables the in situ quantification of a variety of chemical constituents present in the human body and holds great promise for personalized health monitoring owing to its unique advantages such as inherent wearability, high sensitivity, high selectivity, and low cost. It represents a promising alternative to probe biomarkers in the human body in a simpler method compared to conventional instrumental analytical techniques. Various bioanalytical technologies are employed in flexible electrochemical bioelectronics, including ion-selective potentiometry, enzymatic amperometry, potential sweep voltammetry, field-effect transistors, affinity-based biosensing, as well as biofuel cells. Recent key innovations in flexible electrochemical bioelectronics from electrochemical sensing modalities, materials, systems, fabrication, to applications are summarized and highlighted. The challenges and opportunities in this field moving forward toward future preventive and personalized medicine devices are also discussed.

A Lactate/Oxygen Biofuel Cell: The Coupled Lactate Oxidase Anode and PGM-Free Fe-N-C Cathode

The rapid development of both wearable and implantable biofuel cells has triggered more and more attention on the lactate biofuel cell. The novel lactate/oxygen biofuel cell (L/O-BFC) with the direct electron transfer (DET)-type lactate oxidase (LOx) anode and the platinum group metal (PGM)-free Fe-N-C cathode is designed and constructed in this paper. In such a reasonable design, the surface-controlled direct two-electron electrochemical reaction of the lactate oxidase was determined by cyclic voltammetry (CV) on the carbon nanotube (CNT) modified electrode with favorable high electrochemical active surface area and electronic conductivity. Additionally, the biosensor based on DET-type LOx modified electrode impressively presented linear response to lactate with different concentrations from 0.000 mM to 12.300 mM. In particular, the apparent Michealis-constant (K_M^app) calculated as 0.140 mM clearly indicates that LOx on CNT has strong affinity to the substrate lactate. Meanwhile, 4e^- transfer oxygen reduction reaction (ORR) was proven to take place on the Fe-N-C catalysts inthe 0.1 M PBS system, indicating the advantage by using the Fe-N-C catalysts at the cathode of L/O-BFC. Last but not least, the L/O-BFC with the direct electron transfer (DET)-type lactate oxidase(LOx) anode and the Fe-N-C cathode produced an superior open circuit potential (OCP) of 0.264 V and a maximum output power density (OPD) of 24.430 μW cm^-2 in O₂ saturated 95.020 mM lactate solution. The above results will not only bring about significant interest in developing a DET-type biofuel cell, but also offer guiding direction to explore novel catalyst materials for the biofuel cell. This work enriches the research content and may push developments of the implantable and wearable biofuel cell forward.

A sensitive H2O2 biosensor based on carbon nanotubes/tetrathiafulvalene and its application in detecting NADH

Reduced nicotinamide adenine dinucleotide (NADH) plays a pivotal role in the electron-transfer chain of biological system. Analysis of many biological markers is based on the detection of the enzymatically generated NADH. In this paper, a sensitive hydrogen peroxide (H₂O₂) biosensor, fabricated by carbon nanotubes (CNTs)/tetrathiafulvalene (TTF)/horseradish peroxidase (HRP), was applied for detecting the NADH in a buffer containing methylene blue (MB) at low operating potential of - 0.3 V (vs. Ag/AgCl). Since the NADH could be oxidized by MB to release H₂O₂, the electrochemical biosensor enables to detect the NADH in the MB buffer. And the low working potential made the biosensor avoid the interference from other electroactive substances. Linear response ranges from 10 μM to 790 μM, with a sensitivity of 4.76 μA mM^-1 and a detection limit of 1.53 μM were obtained under the optimum conditions. The proposed sensor provided a promising approach for sensitively detecting the NADH.

Bio-Integrated Wearable Systems: A Comprehensive Review

Bio-integrated wearable systems can measure a broad range of biophysical, biochemical, and environmental signals to provide critical insights into overall health status and to quantify human performance. Recent advances in material science, chemical analysis techniques, device designs, and assembly methods form the foundations for a uniquely differentiated type of wearable technology, characterized by noninvasive, intimate integration with the soft, curved, time-dynamic surfaces of the body. This review summarizes the latest advances in this emerging field of "bio-integrated" technologies in a comprehensive manner that connects fundamental developments in chemistry, material science, and engineering with sensing technologies that have the potential for widespread deployment and societal benefit in human health care. An introduction to the chemistries and materials for the active components of these systems contextualizes essential design considerations for sensors and associated platforms that appear in following sections. The subsequent content highlights the most advanced biosensors, classified according to their ability to capture biophysical, biochemical, and environmental information. Additional sections feature schemes for electrically powering these sensors and strategies for achieving fully integrated, wireless systems. The review concludes with an overview of key remaining challenges and a summary of opportunities where advances in materials chemistry will be critically important for continued progress.

Wearable biofuel cells based on the classification of enzyme for high power outputs and lifetimes

Wearable enzymatic biofuel cells would be the most prospective fuel cells for wearable devices because of their low cost, compactness and flexibility. As the high specificity and catalytic properties of enzymes, enzymatic biofuel cells (EBFCs) catalyze the fuel associated with the redox reaction and get electrical energy. Available biofuels such as glucose, lactate and pyruvate can be harvested from biofluids of sweat, tears and blood, which afford cells a favorable use in implantable and wearable devices. However, the development of wearable enzymatic biofuel cells requires significant improvements on the power density and enzymes lifetime. In this paper, some new advances in improving the performance of wearable enzymatic biofuel cells are reviewed based on the bioanode and biocathode by classifying single-enzyme and multi-enzyme catalysis system. Thereinto, the bioanode usually contains oxidases and dehydrogenases as catalyst, and the biocathode utilizes the catalysis of multi-copper oxidases (MCOs) in the single system. For further enhancing the power density, efforts to develop multi-enzyme catalysis strategies are discussed in bioanode and biocathode respectively. Moreover, some potential technologies in recent years, such as carbon nanodots, CNT sponges and mixed operational/storage electrode are summarized owing to notable efficiency and the capability of enhancing electron transfer on the electrode. Finally, major challenges and future prospects are discussed for the high power output, stable and practical wearable enzymatic biofuel cells.

Contact-Lens Biosensors

Rapid diagnosis and screening of diseases have become increasingly important in predictive and preventive medicine as they improve patient treatment strategies and reduce cost as well as burden on our healthcare system. In this regard, wearable devices are emerging as effective and reliable point-of-care diagnostics that can allow users to monitor their health at home. These wrist-worn, head-mounted, smart-textile, or smart-patches devices can offer valuable information on the conditions of patients as a non-invasive form of monitoring. However, they are significantly limited in monitoring physiological signals and biomechanics, and, mostly, rely on the physical attributes. Recently, developed wearable devices utilize body fluids, such as sweat, saliva, or skin interstitial fluid, and electrochemical interactions to allow continuous physiological condition and disease monitoring for users. Among them, tear fluid has been widely utilized in the investigation of ocular diseases, diabetes, and even cancers, because of its easy accessibility, lower complexity, and minimal invasiveness. By determining the concentration change of analytes within the tear fluid, it would be possible to identify disease progression and allow patient-oriented therapies. Considering the emerging trend of tear-based biosensing technology, this review article aims to focus on an overview of the tear fluid as a detection medium for certain diseases, such as ocular disorders, diabetes, and cancer. In addition, the rise and application of minimally invasive detection and monitoring via integrated contact lens biosensors will also be addressed, in regards to their practicality and current developmental progress.

Nanoporous Gold-Based Biofuel Cells on Contact Lenses

A lactate/O₂ enzymatic biofuel cell (EBFC) was prepared as a potential power source for wearable microelectronic devices. Mechanically stable and flexible nanoporous gold (NPG) electrodes were prepared using an electrochemical dealloying method consisting of a pre-anodization process and a subsequent electrochemical cleaning step. Bioanodes were prepared by the electrodeposition of an Os polymer and Pediococcus sp. lactate oxidase onto the NPG electrode. The electrocatalytic response to lactate could be tuned by adjusting the deposition time. Bilirubin oxidase from Myrothecium verrucaria was covalently attached to a diazonium-modified NPG surface. A flexible EBFC was prepared by placing the electrodes between two commercially available contact lenses to avoid direct contact with the eye. When tested in air-equilibrated artificial tear solutions (3 mM lactate), a maximum power density of 1.7 ± 0.1 μW cm^-2 and an open-circuit voltage of 380 ± 28 mV were obtained, values slightly lower than those obtained in phosphate buffer solution (2.4 ± 0.2 μW cm^-2 and 455 ± 21 mV, respectively). The decrease was mainly attributed to interference from ascorbate. After 5.5 h of operation, the EBFC retained 20% of the initial power output.

Enzymatic Fuel Cells: Towards Self-Powered Implantable and Wearable Diagnostics

With the rapid progress in nanotechnology and microengineering, point-of-care and personalised healthcare, based on wearable and implantable diagnostics, is becoming a reality. Enzymatic fuel cells (EFCs) hold great potential as a sustainable means to power such devices by using physiological fluids as the fuel. This review summarises the fundamental operation of EFCs and discusses the most recent advances for their use as implantable and wearable self-powered sensors.

Fuel-independent and membrane-less self-charging biosupercapacitor

A fuel-independent self-charging biosupercapacitor consisting of an enzymatic biocathode and a bioelectrode employing supercapacitive features of immobilized myoglobin is described.