Abstract
1. Introduction
The explosive development of the internet of things (IoT) has promoted the progress of portable electronic products. Smartphones, notebook computers, and electronic watches 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 users to perceive and monitor their health status and surroundings. Research efforts on bioelectronics have been intensively devoted to the field of bio-medicine, IoTs and health monitoring with the rapid progression of electronic technology[
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 bending and folding, it is easy to cause cracks on collectors, affect electrochemical performance, and even lead to electrical short-circuit, resulting in 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, and system integration[
In this review, we mainly focus on the recent research progress of flexible energy storage devices (e.g., batteries and supercapacitors), self-powered systems, and their applications in integrated wearable bioelectronics, as shown in Fig. 1. First, an overview of commonly adopted methods for fabricating flexible energy devices will be provided. They are classified as chemical methods and physical methods, which will be discussed in Section 2. Moreover, representative reports on self-powered systems based on flexible energy devices 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 perspectives of flexible energy storage devices will be included in Section 5.
Figure 1.(Color online) The fabrication methods and energy sources for flexible energy storage devices and their applications in wearable biosensing[
2. Fabrication methods
The commonly adopted fabrication methods for flexible energy storage devices are summarized in this section. Generally, material synthesis and device fabrication can be categorized into chemical methods and physical methods, respectively. The morphology and properties of active materials with energy storage capability can be modified via various approaches. As for chemical methods, the morphology modification of the active material can be achieved by changing the reaction conditions, leading to optimizing the device performance. Besides, it has higher compatibility with complicated micro-nanostructures. Physical methods, such as coating and sputtering, provide approaches for material loading and device construction on flexible substrates, usually in the form of thin films[
2.1. Chemical method
Hydrothermal synthesis: Hydrothermal synthesis refers to a method of preparing materials by dissolving and recrystallizing the powder with water as the solvent in a sealed vessel. In recent years, it is reported that metal-organic frameworks, nanostructures, hybrids of organic polymers, metal oxides, and other substances can all be formed on the substrate by hydrothermal synthesis[
Figure 2.(Color online) Chemical methods for flexible energy storage devices fabrication. (a) Two-step hydrothermal synthesis of MnO2 nanosheet-assembled hollow polyhedrons on carbon cloth[
Electrochemical deposition: By applying an electric potential in an ionic solution to trigger a reduction or oxidation reaction, a layer of desired materials can be directly deposited onto the conductive substrate. Generally, it relies on a three-electrode system with the working electrode, the counter electrode, and the reference electrode. The unique advantages of electrochemical deposition include: (1) During the metal reduction process, the potential difference, solution concentration, and ambient temperature can be adjusted to control the morphology of the product. (2) The electrolyte can be mixed with various metal salt solvents, which enables composite active materials. (3) The reaction conditions are relatively mild, and the synthesis can be completed at room temperature and atmospheric pressure. Therefore, it is widely 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 layer-by-layer metal electrodeposition to prepare conductive paper, which retained the porous structure of the original paper and delivered an area capacitance of 811 mF/cm2[
Microwave-assisted synthesis: Microwave-assisted synthesis achieves material deposition via chemical reactions within the electromagnetically heated electrolytes. It can be applied to the synthesis of porous materials, inorganic complexes, nanocrystalline particles, organic compounds and so on[
Chemical vapor deposition (CVD): When two or more gaseous raw materials are mixed into a reaction chamber, the chemical reaction can be triggered to form materials on the substrate surface. It is the most widely used technology in the semiconductor industry for the synthesis of materials such as Si nanofiber, graphene, and carbon nanotube (CNT)[
2.2. Physical method
Coating: A slurry consisting of active electrode powder with conductive additives and binders is prepared and smeared onto the substrate. Post-annealing or drying is normal. It is one of the most frequently adopted methods to fabricate flexible electrodes. For instance, Manjakkal and coworkers designed a sweat-activated flexible supercapacitor using the coating method[
Figure 3.(Color online) Physical methods for flexible energy storage devices fabrication. (a) The coating process to achieve flexible CNT-based cathodes[
Infiltration: Infiltration as a mild and low-cost material loading process has been widely employed for porous materials, such as woven cloth, paper, or sponges. Thin films of materials can form on the surface of the substrate. For instance, polyaniline (PANI) has been loaded on the functionalized carbon cloth in the diluted solution via infiltration[
Sputtering: Sputter deposition involves ejecting material vapor from a target source onto the substrate. Although the process is of high cost and 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 to fabricate fiber-shaped supercapacitors[
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.[
3. Self-powered systems
A self-powered system is defined as a system that operates by utilizing the ambient energy presenting in the system environment without external charging[
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 and amorphous silicon solar modules have been frequently reported. Among these batteries, LIBs, zinc-ion batteries and aluminum-air batteries are commonly used to power health-monitoring devices[
Similarly, solar energy can also be stored in supercapacitors as shown in Fig. 4(a)[
Figure 4.(Color online) Self-powered systems consists of flexible energy storage devices and energy harvesting components. (a) Schematic of a printable self-powered system consists of solar cells, supercapacitors and gas sensor[
3.2. Thermal energy
Thermal energy can be 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 series and thermally in parallel to function uninterruptedly 24 h a day[
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 can provide constant power for supercapacitors reported by Liu and coworkers[
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-supercapacitor systems such as thin films, foams, soft rubbers and fabrics[
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 in wearable applications[
In addition, the development of hybrid nanogenerators with high electrochemical stability has received much attention in recent years[
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, wireless wearable bioelectronics has gained tremendous attraction due to its potential for non-invasive health monitoring[
4.1. Physiological signal detection
Sodium sensors: The sodium ion is indispensable in our daily life, and it is essential for human healthcare. Typically, clinical sodium monitoring relies on instruments like inductively coupled plasma mass spectrometry. With the advancement of sensor manufacturing methods, sweat sodium can be detected with high sensitive ion sensors. Sodium sensors 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[
Figure 5.(Color online) Physiological sensing systems integrated with flexible energy storage devices. (a) An all-in-one, and flexible self-powered sodium sensing patch with wireless data transmission[
Glucose sensors: Glucose levels in human fluids are significant health state indicator, especially for diabetes diagnostics. Therefore, wearable devices for glucose monitoring have long been a research focus. Interestingly, some materials can be simultaneously utilized for excellent glucose-sensing performance as well as energy devices[
pH sensors: As the pH values inside human body are relatively stable, pH sensors are more commonly adopted in sweat sensing. For instance, Manjakkal’s group invented a sweat-activated battery that simultaneously monitored heart rate, sweat chloride and sweat pH[
Gas sensors: A gas sensor can convert a certain gas volume fraction such as the composition and concentration into a corresponding electrical signal that can be further analyzed. Gas sensors are generally classified according to the detection of different chemicals, such as hydrogen, oxygen, ammonia gas, nitrogen dioxide, sulfur dioxide, etc.[
Humidity sensor: Humidity sensors have been investigated for industrial applications while attracting increasing research interest for biosensing. Various sensing materials are proposed for high-performance humidity sensing systems, including metal oxides, carbon materials, polymers, cellulose, etc.[
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 power packs have been demonstrated to power the pressure sensor and monitor human physiological signals[
Figure 6.(Color online) Physical sensing systems integrated with flexible energy storage devices. (a) A screen-printed flexible solid-state supercapacitor for self-powered 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 in wearable devices for artificial intelligent body monitoring[
Temperature sensors: According to the measurement method, temperature sensors can be divided into contact and non-contact sensing. While temperature sensors are now widely used in industrial and agricultural life, reports on the combination of flexible temperature sensors and energy storage devices for wearable applications are rare[
Opto-sensors: Photodetectors can detect the conductivity of the irradiated material changes due to radiation, and it is widely used in the range of visible spectrum, infrared spectrum and UV spectrum[
As discussed above, wearable sensors play an essential role in physiological and physical signal-detecting fields, and they can record continuous signals integrated with self-powered systems. Multifunctional sensing systems, including chemical and physical sensing, have also become a popular research area[
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 devices for practical applications. 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 materials for 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 to poor device performance during charging and discharging and even raise safety concerns, such as organic electrolyte leakage. To tackle this challenge, innovation on flexible materials, structural device designs, and reliable approaches for back-end packaging are 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 capacity decreases significantly. It could result in inadequate power supply during long-term operation. Especially for wearable biosensing applications that aim at real-time and long-term monitoring, its practical applications could be limited. In order to achieve the competitive energy capacity of flexible energy storage devices compared with their counterparts in rigid formats, several strategies have been proposed. For instance, the electrodes can be texturized with micro/nanostructures to increase charge storage and facilitate ion transfer to improve the energy density. Thirdly, for wearable biosensing applications, biocompatibility is one of the crucial considerations. The materials utilized for energy devices fabrication should be nontoxic and nonirritating to human 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 energy storage would be developed and revolutionized with high mechanical and electrochemical stability, which is essential for their further integration with wearable bioelectronics for applications in personalized healthcare and robotics.
Acknowledgements
This work was supported by the Engineering Research Center of Integrated Circuits for Next-Generation Communications Grant (Y01796303) and Southern University of Science and Technology Grant (Y01796108, Y01796208).
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