Xiaohao Ma , Zhengfan Jiang and Yuanjing Lin ,

1Schoolof Microelectronics, SouthernUniversity of Scienceand Technology, Shenzhen 518055, China

2EngineeringResearch Center of IntegratedCircuits for Next-GenerationCommunications, Ministry of Education, SouthernUniversity of Scienceand Technology, Shenzhen 518055, China

Abstract: With the growing market of wearable devices for smart sensing and personalized healthcare applications, energy storage devices that ensure stable power supply and can be constructed in flexible platforms have attracted tremendous research interests. A variety of active materials and fabrication strategies of flexible energy storage devices have been intensively studied in recent years, especially for integrated self-powered systems and biosensing. A series of materials and applications for flexible energy storage devices have been studied in recent years. In this review, the commonly adopted fabrication methods of flexible energy storage devices are introduced. Besides, recent advances in integrating these energy devices into flexible selfpowered systems are presented. Furthermore, the applications of flexible energy storage devices for biosensing are summarized. Finally, the prospects and challenges of the self-powered sensing system for wearable electronics are discussed.

Key words: flexible electronics; energy storage devices; self-powered systems; wearable bioelectronics

1.Introduction

The explosive development of the internet of things (IoT)has promoted the progress of portable electronic products.Smartphones,notebookcomputers, and electronicwatches are expected to be continuously revolutionized with desirable form factors including miniaturization, flexibility, and lightweight. Meanwhile, with the increasing demand for personalized healthcare and remote diagnostics, wearable bioelectronics such as smart wristbands and intelligent glasses have come into vogue. These devices are connected to the internet and combined with various software applications to enable userstoperceive and monitor theirhealth status andsurroundings. Research efforts on bioelectronics have been intensivelydevotedto thefieldof bio-medicine, IoTs and health monitoring with the rapidprogressionof electronictechnology[1−3].

However, one of the challenges lies in the novel flexible energy storage devices, without which the smooth functionalization of various flexible electronics cannot be guaranteed. Conventional energy storage devices are rigid and robust. When bendingandfolding, it iseasy to causecracks oncollectors,affectelectrochemicalperformance, and even lead to electrical short-circuit,resultingin serious safety problems. Therefore,flexible energy storage devices that can withstand mechanical deformation and retain their electrochemical properties have become a research hotspot. Impressive progress has been achieved in developing energy storage devices in a variety of flexible formats with research efforts in material engineering,device structural design, andsystem integration[4−7].The practical requirementsofwearableelectronicsput forwardthe demands on self-poweredintegrated systems,which convert clean energy into electricity and support system power supply without external charging. For instance,photovoltaicdeviceshave beenadoptedto collect solar energy, thermoelectronics for thermal energy, triboelectric and piezoelectric generators for mechanical energy, respectively.

In this review, we mainly focus on the recent research progress of flexible energy storage devices(e.g.,batteries andsupercapacitors), self-powered systems, and their applications in integrated wearable bioelectronics, as shown inFig. 1.First, anoverview of commonly adoptedmethods forfabricating flexible energy devices will be provided. They are classifiedas chemical methodsandphysicalmethods, which will be discussed in Section 2. Moreover, representative reports on self-powered systems based onflexible energydevices will be introduced in Section 3, including their working principles and novelties. In Section 4, biosensing devices’ physiological and physical signal detection that can be integrated with flexible energy devices are summarized. Discussion on future perspectivesof flexible energy storage devices will be included in Section 5.

Fig. 1.(Coloronline) The fabricationmethodsandenergysourcesforflexibleenergystorage devices andtheirapplications in wearablebiosensing[8–15].

2.Fabrication methods

The commonly adopted fabrication methods for flexible energy storagedevicesare summarized inthissection. Generally, material synthesis and device fabrication can be categorizedintochemicalmethodsand physicalmethods,respectively. The morphology and properties of active materials with energy storage capabilitycan bemodifiedvia various approaches. As for chemical methods, the morphology modificationof the activematerial canbe achieved by changingthe reaction conditions, leading to optimizing the device performance. Besides, it has higher compatibility with complicated micro-nanostructures. Physicalmethods, such as coating and sputtering, provide approaches for material loading and device construction on flexible substrates, usually in the form of thin films[16].

2.1.Chemical method

Hydrothermal synthesis:Hydrothermal synthesis refers to a methodof preparing materialsby dissolvingand recrystallizing the powder with water as the solvent in a sealed vessel.Inrecent years,it is reportedthat metal-organicframeworks,nanostructures, hybrids of organic polymers, metal oxides,and other substances can all be formed on the substrate by hydrothermalsynthesis[17−19]. Asshown inFig. 2(a), Wu and coworkers utilized a liquid phase reaction to primarily grow dodecahedral 2-methylimidazole cobalt(ZIF-67)crystal oncarbon cloth. Then the ZIF-67 crystal is gradually transformed into a hollow manganese dioxide (MnO2) polyhedron through ahydrothermal reaction[20].Whentested, theaqueousbattery showed a high reversible capacity of 263.9 mA·h/g after 300 cycles, andits performancefarexceeded thatof thecommercial MnO2electrode. Furthermore, different nanomaterials such as nanospheres,nanocomposite films and nanoneedle arrays are frequently fabricated as anodes and cathodes via hydrothermal synthesis[21−23]. The reaction conditions of the hydrothermalsynthesis are relatively facile and are highly controllable to ensure the stability of the synthesis.Forinstance, Wang’s group summarized controlledhydrothermal synthesis of lithium iron phosphate (LiFePO4) cathode for LIBs,and the LiFePO4nanorods exhibitedahigh capacity of 155 mA·h/g at 0.5 C and sustainable capacitance retention of 80%[24].

Electrochemical deposition:By applying an electric potential in an ionic solution to trigger a reduction or oxidation reaction, alayer of desired materials canbedirectlydeposited onto the conductive substrate. Generally, it relies on a threeelectrode system with the working electrode, the counter electrode, and the referenceelectrode. The uniqueadvantages of electrochemical deposition include: (1) During the metal reduction process, the potential difference, solution concentration,and ambienttemperature canbeadjusted tocontrol themorphology of the product. (2) The electrolyte can be mixed with variousmetal saltsolvents,whichenables composite active materials. (3) The reaction conditions are relatively mild, and the synthesis can be completed at room temperature and atmosphericpressure. Therefore,it iswidely used for flexible electrodes functionalization with metals, metal oxides and polymers.For instance,Fig. 2(b) illustrates that metal nanoparticle layers were assembled on insulating paper via layerby-layer metal electrodeposition to prepare conductive paper, whichretainedthe porous structureofthe original paper and delivered an area capacitance of 811 mF/cm2[10]. In Singh’s work, transparent core-shell MnO2and gold (Au)coatednanofibernetworkelectrodes werefabricated by electrodeposition, which showed mechanical flexibility, high energydensity, high transparency, and long-termcycling stability[25]. A flexible and transparent supercapacitor was further developed and delivered a high energy density of 0.14μW·h/cm2, alongwith a higharealcapacitance of 2.07 mF/cm2.

Microwave-assisted synthesis:Microwave-assisted synthe-sis achieves material deposition via chemicalreactionswithin the electromagnetically heated electrolytes. It can be appliedto the synthesisof porous materials, inorganiccomplexes, nanocrystalline particles, organic compounds and so on[26−30]. Compared with conventional hydrothermal synthesis, microwave passes through the materialand provides energy. Thus, the heating process is rapid and uniform, which enables fast and energy-efficient synthesis. Nonetheless, it is relatively challenging to precisely control the reaction temperature and form factors of the produced materials. Jeon’s group endowed pristinecarboncloth withlarge surfacearea,porosity and high specific capacitance via microwave-assisted functionalization. The supercapacitor based on carbon cloth delivered a maximum energy density of 64μW·h/cm2at a power density of 1mW/cm2with long-term cycling[31].Fig.2(c)illustrates the exploration of advancednickel-ironbased catalysts for rechargeable zinc-air batteries via microwave-assisted synthesis[32]. As a result, four flexible zincair batteries were connected in seriesto power amobile phoneandshowedhighcyclicstability underdifferentbending states.

Fig. 2.(Coloronline) Chemical methodsfor flexible energystorage devices fabrication. (a) Two-stephydrothermal synthesis of MnO2nanosheetassembled hollow polyhedronson carboncloth[20]. (b)Metal-likeconductivepaperelectrodesbased onAunanoparticleassembly followed by nickelelectroplating[10]. (c) A microwave-assistedrapid sysnthesis of nickel-iron-basedcatalysts for rechargeable zinc-air battery[32]. (d) Synthesis of 3Dnanofiber electrodeviaCVD[36].

Chemical vapor deposition (CVD):When two or more gaseousrawmaterialsare mixedinto a reaction chamber, the chemical reaction can be triggered to form materials on the substrate surface. It isthe mostwidelyusedtechnology in the semiconductor industry for the synthesis of materials such as Si nanofiber, graphene, and carbon nanotube(CNT)[33−35].Asshown inFig. 2(d),Zhang’sgroup fabricated the three-dimensional (3D) nanofiber electrode via CVD. It provided relativelyhigh controllability, and the as-fabricated supercapacitors exhibited high capacitance stability of 96.5%[36]. In addition, Zang and his coworkers manufactured novel3D CNT-graphene and 3DCNT-CNTnetworkstructures as electrodes, which showed enhanced energy density for supercapacitors[37]. Besides,nanotube andnanofiber films prepared by CVD methods have been widely adopted as anodes for lithium-ion batteries (LIBs)[33,38]. For instance, Yanget al. presentedan aprotic lithium-oxygenbattery witha high discharge capacity of 11512.4 mA·h/g and long cycle life of 130 cycles[39]. CVDprovides aninexpensiveand facile methodto fabricate functionalizedflexible materials for energy storage applications without the use of special or toxic atmosphere, whichmakes it popular in industry.

2.2.Physical method

Coating:A slurry consisting of active electrode powder with conductiveadditives and bindersis prepared and smeared onto the substrate. Post-annealing or drying is normal. Itis oneof the most frequentlyadopted methodsto fabricate flexible electrodes. For instance, Manjakkal and coworkers designed a sweat-activated flexible supercapacitor using the coating method[40]. The valuable device exhibited energy and power densities of 1.36 W·h/kg and 329.7 W/kg, respectively. Besides, Dai's group utilized the coating method to fabricate supercapacitors based on activated carbon. The devices showed significantly improved cyclic stabilityand ratecapability[41]. Fig. 3(a)shows that flexible CNT-based cathodes with controllable thicknesses aresuccessfully fabricated via a facile blade-coating method, and the pouch cell shows impressive cyclic stabilityunderbothbalanced state and bent state[42].

Infiltration:Infiltration as a mild and low-cost material loadingprocess has been widely employedfor porousmaterials,such as woven cloth, paper, or sponges. Thin films of materialscan form onthe surfaceof the substrate.For instance, polyaniline (PANI) has been loaded on the functionalized carbon cloth inthedilutedsolutionvia infiltration[20,30].It then served as a flexible electrode in a symmetrical supercapacitor,which presented the areacapacitance of 350.8 F/g andhigh capacitance retention of 90.8% for 10 000 cycles. In Song’s work, a lithium-sulfur-infiltratedcatholytewas efficiently infiltrated into the carbon cloth during cell fabrication[43]. This carbon cloth cathode was reported with a high areal capacity of 3.2mA·h/cm2for 200 cycleswithasulfur loadingof 6 mg/cm2, and the battery cells remained functioning even after 300factitiousbending cycles. Moreover, Kimet al. fabricated all-solid-state LIBsusing electrolyte-infiltrated polyimide film, which exhibitedpromising electrochemicalperformance(146mA·h/g)as well asexcellentthermal stability (up to~400 °C)[44]. Fig. 3(b) illustrates the fabrication of the sulfide solid electrolytemembrane byinfiltrating electrospunpolyimide nanowires.Infiltration shows itscompatibility and scalability for manufacturing composite electrodesona large scale.

Fig. 3. (Color online) Physical methods for flexible energy storage devices fabrication. (a) The coating process to achieve flexible CNT-based cathodes[42]. (b) Infiltration of electrospun porous polyimide nanowires for sulfide solid elelctrolyte membranes[44]. (c) Fabrication of graphite/Si hybrid electrode via sputtering[46]. (d) Schematic of the 3D printed interdigital electrodes for micro-supercapacitors[11].

Sputtering:Sputter deposition involves ejecting material vapor from a target source onto the substrate. Although the process is of high costand relies on complex equipment,it can well control the thickness and quality of the thin films.For instance,pseudocapacitive materials can be conformally coated on self-supported CNT-aligned films by magnetron sputtering tofabricate fiber-shapedsupercapacitors[45]. Such self-supportedmaterials deposited bymagnetron sputtering can be widely used for wearable bioelectronics. Besides, it is capable of materials loading on substrates with micro/nanostructures. Fig.3(c)illustrated thetwo-step sputtering method to prepare graphite/silicon hybrid electrodes on 3D current collectors forflexible batteries[46].A remarkableenhancement of 19.7% on specific capacity was obtained, and the overall capacity of 108 mA·h/g at 0.5 C was achieved.

3D printing:3D printing is an additive manufacturing method to construct 3D objects in a layer-by-layer manner following the designed 3D model. Therefore, it can fabricate 3D objects with attractive geometric characteristics based on a variety of materials, including metals, plastics or composite materials, etc.[11,47,48]. Orangi and coworkers adopted 3D printing to construct electrode patterns with conductive ink[11].Fig. 3(d)shows the schematic of printing interdigital electrodes, and the height of the printed metal carbides and nitrides (Mxene) ink electrodes can be increased by printing additional layers. The printed solid-state micro-supercapacitors exhibited excellent electrochemical performance with a surface capacitanceof1035mF/cm2. Inaddition,a variety of high-quality batteries, including LIBs, nickel-iron batteries,iron (III) periodate and zinc batteries, have shown distinct advantages via 3Dprinting[49−51].For instance, Lee’sgroup fabricated yarn-type LIBs by direct ink writing-based 3D printing technology[52]. The technology facilitates its integration into commercialfabrics suchas woolen glovesand develops a new strategy for next-generation smart fabrics.

Fig. 4. (Color online) Self-powered systems consists of flexible energy storage devices and energy harvestingcomponents. (a)Schematic of a printable self-poweredsystem consists of solar cells, supercapacitorsand gas sensor[60].(b)Design of a thermocell for harvesting body heatand chargingsupercapacitors[63]. (c)Self-powered clothconsists of TENG,supercapacitor and wearable sensor[69]. (d)An all-solid-state self-poweredsystemwithhighperformancePENGusinga particular mesoporous film[73].

3.Self-powered systems

A self-poweredsystemis defined asa system that operates by utilizing the ambient energy presenting in the system environment without external charging[53]. Especially for wearableelectronics,theself-power capabilitycan elongate the device duration time and reduce the recharging frequency[54,55]. However, energy harvesting and conversion from theenvironment mightresult in fluctuations in power output, which might fail to fulfill the requirements of sustainable power supply forelectronicdevices. To tackle this challenge, the integration of flexible energy storage devices, such as batteries or supercapacitors, can serve as energy buffers andrelease thestored energy when required.Here,theadvances of flexible energy storage devices integrated with energyharvestingcomponents, including solar cells, thermal cells and mechanical nanogenerators will be summarized.

3.1.Solar energy

Solar energy is an ancient energy source with universal harmlessness and long-term sustainability. The design of flexible power supplies that integrate batteries andamorphous silicon solar modules have been frequently reported. Among these batteries,LIBs, zinc-ion batteriesandaluminum-airbatteries are commonly used to power health-monitoring devices[56−59]. Ostfledet al. designed a self-powered system that combines a photovoltaic (PV) module and a LIB composedofa printed graphite-based anodeanda lithiumoxide cathode[56].By selectinga load dutycycle appropriately,the system could balance the current between the battery and the PV module to maintain a constant charge. Finally, the battery was used in a pulse oximeter after 600 mechanical bending cycles,andits capacity remained atleast 90%, proving its effectivenessas a power source for wearablesensing systems. The system canbe integrated intowearabledevices such as jacket sleeves, bags and travel mugs with excellent flexibility.

Similarly, solar energy can also be stored in supercapacitorsas shown inFig.4(a)[60]. Fan’s groupsuccessfullyrealized a printable andflexible self-powered systemutilizingsolar energy to drive a gassensor. Thesolarenergycanalsobe stored in supercapacitors and released to ensure continuous operation when interruption from illumination variation occurs. Donget al. also manufactured and described a printable dye-sensitizedsolarcell(DSC) integratedwith a supercapacitor[61].Under sunlight exposure,the supercapacitorswere charged by the current generated in DSC and then discharged with a capacitance of 0.14 mF/cm2. The electrical performance was tested outdoors under different extreme bending conditions. The stable data highlights the device’s performance undervariousextreme mechanicalloadconditions and paves theway forfuture highly flexible integratedenergy systems.

3.2.Thermal energy

Thermal energycanbe collected around our environment and from human bodies as well. By setting a temperature difference between the edges of two semiconductors of different properties, it generates a direct-current voltage at both ends. Thermoelectric modules can be used as nanogenerators based on the Seebeck coefficient, which are electrically in seriesand thermallyin paralleltofunction uninterruptedly 24 h aday[62].

Body thermal energy is one of the easily accessed energy sources for wearable bioelectronics and has attracted increasing research interest. Fig. 4(b) illustrates a thermocell for harvesting body heat and canprovideconstant power forsupercapacitorsreportedby Liuandcoworkers[63].Itutilized the temperature gradient betweenthe two surfacesofanobject,namely, the thermoelectric effect (TE), to create a typical TE module. Connected with multiple p–n cells in series, sandwich structures of n-type and p-type semiconductors were successfully matchedinthese thermocells, andan incremental potentialto 0.34 V at ΔT=10 K was obtained. Furthermore,asupercapacitor was charged by collecting body heat and then was applied to illuminate green light-emitting-diodes (LEDs).Wang’s group fabricated a LIB for simultaneously stored the harvested thermal, mechanical and solar energies[64]. Therefore,stableelectrical energy at comparable outputpower can beachievedcompared with self-powered systemsconsisting of only active harvesters such as triboelectric or piezoelectric generators.

3.3.Mechanical energy

Triboelectric nanogenerator (TENG):The triboelectric nanogenerators convert mechanical energy based on the combination of the triboelectric effect and electrostatic induction. Various micro/nanostructured materials have been adopted to realize TENG-supercapacitorsystemssuchasthin films, foams,soft rubbers and fabrics[8,65−69].Fig. 4(c)shows a self-charging power textile in one piece of cloth with excellent mechanical flexibility reported by Puet al.[69]. It consists of three functional units: the TENG fabric for energy harvesting, the superca-pacitor fabric for energy storageandwearable electronics for sensing. The self-charged powertextileachieved highcapacitance (72.1mF/cm2) and stable cycling performances (96% for 10 000cycles).However,the system showeda limitationof unstable energyoutput anda shortoperationaltime. Chunet al. proposed an all-in-oneself-powered system for touch sensingwith flexibleand transparent electrodes[70]. The TENGcomponent couldharvest high electricpower (1.45 mW/cm2with 17kPa)whentouched. The supercapacitor exhibits a high capacitance of 3.83μF/cm2and stable performance without degrading theultrahigh transparency(77.4%). Meanwhile, the tactilesensorcoulddetectnon-contact/contact touches by measuring the capacitance change.

Piezoelectric nanogenerator (PENG):Piezoelectric components can convert slight vibration or strain differences into electrical energy and then power downstream circuits. Nanostructured and flexible materials have been frequently used to fabricate PENG inwearable applications[71,72]. For instance,Heet al. fabricated an all-solid-state self-powered systemusing mesoporous films to generate piezoelectric effect as shown in Fig. 4(d). The self-powered system was charged by compressive deformation, and the overall capacity is 0.118μA·h within 240 s. Such self-powered module based on PENG successfully powered up asmartwatch, sportswristband and arrayofLEDs,respectively,indicatingits potential applications for self-sustainable wearable electronics[73].

In addition, the development of hybrid nanogenerators with high electrochemical stability has received much attention in recent years[3]. For instance, a self-arched nanogenerator (SANG)witha combination of triboelectric and piezoelectriceffectsis designed forreal-time monitoringof pulse waveform by Li’s group[74]. This unique device structure and mixed effect of triboelectric and piezoelectric contribute effectively to SANG's stability and sensitivity. Moreover, Jianget al.demonstratedafree-rotating hybridnanogeneratorconstituted by a triboelectric andanelectromagnetic generator.It canprovidepowerforcalculators, wireless temperature sensors and even charging mobile phones[15]. The integration of energy storage devices with hybrid nanogenerators significantly contributes to the development of bioelectronics with desirable self-power capability.

4.Flexible energy storage module for sensing applications

A variety of flexible energy storage devices charged by different self-powered systems were reviewed, and they could be further integrated for sensing applications. In addition, wirelesswearable bioelectronics hasgained tremendous attraction due to its potential for non-invasive health monitoring[75]. The integration of the wireless data transmission module in wearable biosensing systems enables real-time analysis of target analytes levels and remote monitoring[76−78].While itposes higherrequirementsfor the powersupply module in wearable electronics. This section will introduce the sensing applications for flexible energy storage devices, includingphysiological andphysical signaldetection.And Table 1has summarized recent flexible energy storage devices integrated with sensing systems, and their superior performanceis also involved.

Table 1. Summary of recent flexible energy storage devices integrated with sensing systems.

4.1.Physiological signal detection

Sodium sensors:The sodiumionis indispensableinour daily life, and it is essential for human healthcare. Typically,clinical sodiummonitoring relies oninstrumentslike inductively coupled plasma mass spectrometry. With the advancement of sensor manufacturing methods, sweat sodium can be detected withhigh sensitiveion sensors. Sodiumsensors can be embedded in a flexible system and integrated with signal processing circuits. Fig. 5(a) shows a flexible sodium sensing patch that can be self-powered[78]. In this system,Dahiya’s group carried out a stable sodium measurement from perspirationon theskin. The perspirationalso servedas a biofuelfor energy supply. The measured sensitivity ofNa+is 55.5 ± 0.3 mV per decade when the entire device is under continuous movement, whichindicates asafe andsustainable perspective for wearable bioelectronics. Nevertheless, other power supply methods, such as human movement, are also fulfilled inpracticalapplications[88,89]. Since high energy consumption is a critical problem when biosensors are attached to the human body, efficient energy supply from human motion represents a fascinating approachtosustainably driving wearable bioelectronics.

Fig. 5. (Color online) Physiological sensingsystems integrated withflexible energy storagedevices. (a)Anall-in-one, andflexible self-poweredsodium sensing patch withwireless data transmission[78]. (b) A self-poweredsmartwatchfor non-invasive sweat glucose monitoring[92]. (c) Schematicofaself-powered system withpH sensor and itsperformance under dynamicbending conditions[79]. (d)Aself-powered wristband thatcan power upLED asanindicator of gas detection[60].

Glucose sensors: Glucose levels in human fluids are significant health state indicator,especiallyfordiabetes diagnostics.Therefore, wearable devices for glucose monitoring have longbeen aresearch focus. Interestingly, somematerialscan besimultaneously utilizedfor excellentglucose-sensing performance as well as energy devices[90,91]. Ngo and coworkers adopted nickeloxide andgraphitic carbonnitride hybrid nanostructure for electrode fabrication for both the glucose sensorandsymmetrical supercapacitor.Ahighglucosesensitivity of 5.387mA/(mM·cm2) was obtained, andan energy density of 1.06 kW/kg was also observed. However, it is relatively challenging to monitor the sweat glucose because it is less concentratedand othercomposites in sweat could affect the sensingaccuracy. Fig. 5(b)illustratesaself-powered smartwatch for specifically non-invasive glucose monitoring[92]. In this work, Zhaoet al. proposed a Zinc-MnO2battery that servedasa flexible energy storage device. It retained91.86%of the initialcapacitanceafter 1000 cycles of charging/discharging at 1.8 A/g while the self-powered system was charged up to 6.0 V within 1 h under bright sunlight. Apart from batteries, flexiblesupercapacitorscanalso provide energyfor glucosesensors.Sunet al.reported thata flexiblesupercapacitor could power enzyme-free biosensors, and it also had a high sensitivity for glucose detection (592μA/mM)[82]. However,long-term reliableglucose sensorsremaina challenge for real-time biosensing applications.

pH sensors:As the pH values inside human body are relatively stable, pH sensors are more commonly adopted in sweatsensing. For instance, Manjakkal’s group invented a sweat-activated batterythatsimultaneouslymonitoredheart rate, sweat chloride and sweat pH[79]. Also, a single pH sensor was proposed with a flexible graphene foam supercapacitor.Fig.5(c)shows the 3D schematic ofaself-powered system witha pH sensor andits performance under dynamic bending conditions. The self-charged supercapacitor exhibited a capacitance of 38 mF/cm2and an energy density of 3.4μW·h/cm2. Theelectrochemical and supercapacitiveperformanceindicateditsfurtherapplications, suchas multi-sensing e-skin for human healthcare monitoring.

Gas sensors:A gassensor can convert a certain gas volume fraction suchas the composition andconcentrationinto a corresponding electrical signal that can be further analyzed. Gas sensors are generally classified according to the detection of differentchemicals,such as hydrogen, oxygen, ammonia gas, nitrogendioxide,sulfur dioxide, etc.[93−96]. They have wide applications from personalized healthcare to environmental monitoring. A variety of strategies for scalable device manufacturing and facile systemintegration have showngreat potential forwearabledevices.Linet al. fabricated a fully integrated sensing system powered by supercapacitors on plastic substrates[60]. Fig. 5(d) demonstrates that the as-fabricated self-powered wristband can power LEDs as anindicator of gas detection. In particular, the flexible supercapacitors provided an area capacitance of 12.9 mF/cm2, which could power the high-sensitivity tin oxide gas sensor at room temperature. Besides,micro-electromechanical sensing systems with significantly reduceddevice volume couldenable a higher sensor array density. It also allows the integration with rechargeable and flexible energy devices to achieve reliableand repeatable device performance.Benedictet al. proposeda gasmappingsystem with awirelesscommunication module to track the global pollution levels by continuous and remote-controllable gas sensing[97]. It is believed with flexible energy devices, thesensing systemwill havepracticalapplications for wearable biosensingandenvironment protection as well.

Fig. 6. (Color online) Physical sensing systems integrated with flexible energy storage devices. (a) A screen-printed flexible solid-state supercapacitor forself-poweredpulse sensing[106]. (b) Schematicillustrationofan integratedself-poweredtactilesensor[114].(c) Thestructuredesign of the FBGsensorforin-situtemperaturemeasurement[117]. (d) Integrationof thedual-modestrainsensorand supercapacitoron adeformablesubstrate[126].

Humidity sensor:Humidity sensors have been investigatedforindustrial applicationswhile attracting increasing research interestforbiosensing. Various sensing materials are proposed for high-performance humidity sensing systems, including metal oxides, carbonmaterials, polymers, cellulose,etc.[98−102]. Pereira and coworkers fabricateda polymerhumidity sensor and a printedbattery made of lithium iron phosphate to form an all-printed smart label[103]. The printed humidity sensor had a linear response with a sensitivity of 0.004% relative humidity. However, the printed battery cannot support long-term power supply at this stage. It was recently reported that an all-solid-state flexible capacitor provided a higher capacitance to drive a humidity sensor[104]. This textile supercapacitor exhibited a specific capacitance of 8.01 F/g and a highcyclability of 5000 cycles. Without additionalcharging, it can drive commercial high-power sensors for 47min.

4.2.Physical signal detection

Pulse sensors: Pulse sensors can detect the pressure change generated during arterial pulsation and convert it into an electrical signal that can be observed in a straightforward manner. Self-sustaining powerpacks havebeen demonstrated topower the pressuresensorand monitor human physiological signals[105]. InXu’swork,photovaltaicdevices and supercapacitors wereintegratedinto an assembled power pack. Itprovided stable power output for pulse sensing regardless of the sunlight fluctuation, demonstrating its potential applications in future wearable electronics. In addition, Rajendranet al. proposed a screen-printed flexible solidstate supercapacitor for self-powered pulse sensing systems as shown inFig. 6(a). The system demonstrated excellent mechanical stability with and serpentine independent interconnections[106]. The supercapacitor based on cured CNT electrodes showed anareal capacitanceof 62 mF/cm2at an applied current density of 0.19 mA/cm2, and onlyadecrement of 10% duringstretching was observed. Thepulsesensorwas successfullypowered even atlowintensity in outdoorsports activities, provingits feasibility in personalizedhealth monitoring. Yu and coworkers designed graphene-based electrodes for both supercapacitors andpressure sensors in a physiological signal sensing system[107]. Apart from supercapacitors, flexible batteries also have been demonstrated to power a wearable pulse sensor[108]. Liet al. reported a flexible solid-state zinc ion battery with desirable operational safety. It deliveredapowerdensityof148.2 mW/cm2andexcellent cycle stability(maintainedat97%after 1000 cycles). Itsuccessfully replaced thecommercial battery packfora smartwatch and supported pulse sensing.

Tactile sensors: With the development of microelectronic technology and the emergence of various organic materials, a variety of tactile sensors have been proposed for strain and pressure sensing. They play an increasingly important role inwearabledevicesfor artificial intelligentbody monitoring[109]. Typicalsubstrates for tactilesensorsfabrication include hydrogels, papers andtextiles[110−112]. Dai’sgroup reportedthe fabrication ofunique sheath-core yarn toconstruct devices with high energy storage capacity and a highly multifunctional sensing system[80]. The integrated electronic effectively monitored human body motions including different ranges of stress deformations. It exhibited an ultrahigh strain sensing range (0–350%) as well as an excellent capacitance of 761.2 mF/cm2at the scanningrate of 1 mV/s. Shitet al. reported a self-poweredcapacitive sensorcould be usedon the skin to sense thehumanheartbeat, pulse andvoice[113].Besides, a physical stimulus that may cause a change in physical size or dielectric constant will introduce a measurable change in capacitance. Fig. 6(b) shows a self-powered flex sensor integrated with and its sensing responses to different bending angles proposed by Xu’s group[114]. The system used Mxene nanosheets to fabricate flexible supercapacitors. The sensing response current increasedwith the bendingdegrees. Such self-powered and wearablesystems can be widelyadopted in human-machine interfacesand biological monitoring.

Temperature sensors:According to the measurement method,temperature sensors can bedividedinto contact andnon-contactsensing.While temperature sensors are now widely usedin industrial and agricultural life, reports on the combinationofflexible temperature sensors and energystorage devices forwearableapplications are rare[115].Hong's group reportedasmartskin temperature patch that could fit the skinconformallyand continuously monitor biological dataforassessingphysical condition[116]. This work showsthe potential for smart patches tomonitornon-febrileconditions in the communityaswell as non-invasively monitor individuals’ body temperature, even in real-life situations. In addition,Seemaand coworkersdesigned afiber Bragg grating(FBG)sensor forin-situ temperature measurement, integratedwith flexible planar supercapacitors intoaself-powered device[117].Fig. 6(c)showsthe structure design oftheFBGsensor. The bendinganglechangesaccording to themeasuredtemperature inthe surrounding environment.

Opto-sensors:Photodetectors can detect the conductivityof the irradiatedmaterialchangesdue toradiation, and it is widely used in the range of visible spectrum, infrared spectrum and UV spectrum[118−120]. Applications and the powerful functions of fibrous photodetectors have been reported.Yildirimet al. reported a self-charging photodetector based on nanowires[121]. The nanowire can receive light energy and light information simultaneously. In other words, the nanowire serves as a nanogenerator to power itself and a photodetector to sense light signals. Recently, Zong’s group invented a high-performance flexible supercapacitor based on cobalt selenide (CoSe2) and CNT film[122]. It generated a stable output voltage of 1.8 V and a high energy density of 0.25 mW·h/cm2. In addition, a CoSe2/CNT-based photodetector was powered up, and fast response was observed under different wavelengths.

As discussed above, wearable sensors play an essential role in physiological and physical signal-detecting fields, and they can record continuous signals integrated with selfpowered systems. Multifunctional sensing systems, including chemical and physical sensing, have also become a popular research area[123−125]. Park and coworkers fabricated an integrated system of physical sensors and a supercapacitor on a wearable substrate using liquid-metal interconnections. As Fig. 6(d)shows, dual-mode and a strain sensor are integrated with a flexible supercapacitor and attached to the hand’s skin[126]. This study used microporous polypyrrolecoated graphene foam as the single functional material for the fabrication of active multifunctional sensors. Overall, such multifunctional sensing systems with sustainable power supplies have made exciting progress. It provides promising strategies to design a self-powered sensing system with considerations on safety,stability and biologicalconformality to fulfill the requirements for health monitoring applications.

5.Challenges and prospects

In recent years, rapid research advances have been achieved in flexible electrochemical energy storage, and many of them are adopted in commercially available products. The latest development on the integration of flexible energy storage devices into wearable bioelectronics is introduced in this review. The technology on material engineering and flexible device fabrication attract tremendous research interests, reflecting the urgent demand for flexible energy devices with desirable characteristics to power wearable sensing systems.

However, there still exist challenges on flexible energy storage devicesforpracticalapplications. Firstly, one of the critical issues for flexible devices lies in mechanical stability. While energy devices can be successfully constructed on various flexible platforms, most conductive current collectors and active materialsfor energy storage are intrinsically rigid. Therefore,mechanical interference such as bending, twisting and stretching could possibly introduce cracks within layers and device delamination.It will undoubtedly lead topoor deviceperformance during charging and dischargingandeven raise safety concerns, such as organic electrolyte leakage. To tackle this challenge, innovation on flexible materials, structural device designs, andreliable approaches forback-end packagingare expected.Secondly,as the volume of the energy device keeps reducing to fulfill the requirements on device miniaturization, especially for wearable applications, its energy storage capacitydecreases significantly. It couldresult ininadequate powersupplyduring long-term operation. Especially for wearable biosensing applications that aim at real-time and longterm monitoring, its practical applications could be limited. In order to achieve the competitive energy capacity of flexible energy storagedevices compared withtheircounterparts in rigid formats, several strategies have been proposed. For instance, the electrodes can be texturized with micro/nanostructures to increasechargestorage andfacilitate ion transfer to improvetheenergy density. Thirdly,for wearable biosensing applications, biocompatibility is one of the crucial considerations. The materials utilized for energy devices fabrication shouldbe nontoxicand nonirritatingtohuman skin. The flexible substrates and packing materials are also expected to provide attractive form factors such as air permeability and moisture conductivity, so as to improve the comfortability for wearing.In summary, it is believed that flexible energystorage would be developed and revolutionized with high mechanical and electrochemical stability, which is essential for their further integration with wearable bioelectronics for applications inpersonalizedhealthcareandrobotics.

Acknowledgements

This work was supported by the Engineering Research Center of Integrated Circuits forNext-GenerationCommunications Grant (Y01796303) and Southern University of Science and Technology Grant (Y01796108, Y01796208).