Flexible electronics technology dramatically changes the capability of sensors, which allows us to detect human biological signals not only on the skin but also inside the human body. Wearable sensors that stick to the skin surface can detect various biomechanical movements, biological signals, and byproducts such as useful elements from sweat and body temperature. On the other hand, implantable sensors directly or indirectly contact with biological components inside the body, such as tissue, organs, or muscles, to support or treat bodily functions or problems. With the development of these advanced sensors, we can live together with a huge number of sensors in the future. Toward body sensor networks that can be fully implanted in the future, sustainable energy sources that support the operation of sensors as well as the development of materials that enable long-term use inside the body remain challenges. In this review, we first summarize various state-of-the-art sensors in terms of flexible and wearable platforms. Additionally, we review the recent research trends of energy harvesters in mechanical and thermal energy conversion into useful electricity for the operation of the sensors. Furthermore, we cover recent studies in the aspect of materials for implantable sensors. Finally, we discuss future direction of the sensors that may enable implanted body sensor networks in the body.
With the development of flexible materials and cutting-edge electronic technology, sensors have evolved into various forms that sense biological information of human. Flexible electronics technology enables sensors to stick on the human skin or to implant inside the human body. For instance, wearable and stretchable electronics can be affixed anywhere in the body and record information such as body temperature, heart rate, and blood pressure.1–4 The information allows people not only to check their body condition but also to accumulate physiological information, improving the quality of daily life. Furthermore, flexible and soft electronics transform bulky and packaged biomedical devices into implantable and biocompatible devices, showing promising prospects for clinical applications. For instance, advanced bioelectronics with flexible and soft platforms are developed not only to record biological signals but also to stimulate and control the organs or the nervous systems.5–8 Flexible and soft neural prostheses provide connection between the human nervous systems and devices or robotic systems to replace or enhance sensory, motor, and cognitive modalities of the human.9–11
To power the wearable and implantable electronics sustainably, another research trend is to develop energy harvesters with wearable and implantable forms that scavenge human body energy into useful electrical energy.12–16 This may overcome the next critical challenges from the aspect of sustainable and long-term use of such devices: (i) reliable light-weight power source with sound output power; (ii) bulky size of bioelectronics when the implants integrate with many active components (such as amplifiers for neural recording and stimulators for neural stimulation); and (iii) additional electronics placed outside the body to process complicated signal, providing feedback signal and regulating the closed loop system. Furthermore, these devices can be used as sensors or energy sources to stimulate organs or the nervous system.17–19
This paradigm shifting from stiff electronics to flexible electronics continuously requires to be smaller, lighter, higher capacities but should also be composed of biocompatible materials, especially in implantable applications. Advanced 2D materials as well as biodegradable materials are applied to implantable sensors and energy harvesters to meet these requirements.20–23 The revolutionary changes in these flexible electronics have opened a new era and may amplify combined with flow of Internet of Things (IoT) and Artificial Intelligence (AI).
This review summarizes state-of-the-art wearable and implantable devices, from sensors and stimulators to energy harvesters. Furthermore, recent development trends of biodegradable devices and 2D material-based devices are analyzed. Subsequently, discussions on recent research and future prospect are provided.
II. WEARABLE AND IMPLANTABLE DEVICES
Significant transition from typical electronics to flexible electronics started with materials and mechanic and design strategies to match the epidermis.24,25 This e-skin has demonstrated promising results for various applications such as human-machine interface, prosthetics, and medicine.6 These devices have capability of mimicking the skin’s ability to sense and generate biomimetic signals, which can be embedded on robotic systems [Fig. 1(a)].26 Furthermore, a soft, conformal class of device records physiological mechano-acoustic signals from the skin, capable of heart murmur detection in a series of cardiac patients for advanced clinical diagnostics [Fig. 1(b)].27 In addition, wearable sweat sensors detect physiologically relevant information for non-invasive health monitoring [Fig. 1(c)].2 This shows that personalized healthcare is about to realize soon.
The development of implantable devices has started with the need to support or active body functions by electrically stimulating organ or muscles. For instance, a pacemaker is invented and implanted in the human body to support heart function.28 Also, deep brain stimulation (DBS) is suggested and demonstrated to activate or inhibit brain signals for neurological disease such as Parkinson’s disease, essential tremor, dystonia, chronic pain, and major depression.29,30 Those have still been used in clinical trials, but there is still limited battery capacity that requires to be replaced, making patient burden in terms of cost and the risk of surgery. Furthermore, the rigid body of implantable devices and interface has the limitation of mechanical mismatch between the object and tissues, causing serious medical problems.31 Advances in flexible and soft electronics enable conformal interface with soft tissue, organ, and muscles, providing several strategies of therapeutic effect. For instance, flexible drug delivery microdevice (f-DDM) delivers timely drug administrations and maintenance of effective dose to maximize curing effects with minimal side effects.32 This device wirelessly receives power and is flexible enough to be implantable on the curved cerebral cortex, delivering two different chemicals or prevention of seizure activity using an anti-epileptic drug [Fig. 1(d)]. Furthermore, flexible and soft electronics are applied to a retinal prosthesis. For instance, a foldable and photovoltaic wide-field epiretinal prosthesis is developed, capable of stimulating wireless retinal ganglion cells.33 The fabricated polydimethylsiloxane (PDMS)-interface and of the dome-shaped PDMS enable to match with the curvature of the eye [Fig. 1(e)]. To solve the battery issue in these applications, wireless approaches are demonstrated.
Developing energy harvesters is one of the alternatives to meet powering themes. For instance, combination of wearable energy harvesters and implantable neural interface enable to directly modulate muscle without battery [Figs. 1(f) and 1(g)].17–19 This technology may be able to apply muscle stimulation combining with muscle electrodes once the total output generated will be increased more [Fig. 1(h)]. In addition, photosynthetic bioelectronic sensors are developed to show that the ability to sense not only touch stimuli but also low-intensity ultraviolet radiation and generate an electrical power toward self-powered E-skin.34 This convergence research has been accelerated, showing tremendous possibilities for various applications.
III. WEARABLE AND IMPLANTABLE ENERGY HARVESTERS
Along the past few years, wearable and implantable energy harvesters have been extensively investigated and developed as a promising technology over the current energy sources—batteries. Wearable energy harvesters are mainly developed according to piezoelectric, thermoelectric, and triboelectric mechanisms. Piezoelectric mechanism is operating under the direct piezoelectric effect to convert applied mechanical energy into electrical energy using piezoelectric materials, including various material forms such as piezoelectric particles,35–38 piezoelectric nanowires,39–41 piezoelectric ribbons,42,43 and piezoelectric thin films.44–46 Thermoelectric mechanism is based on Seebeck effect of thermoelectric materials to harvest thermal energy in the form of temperature gradient, e.g., temperature difference between human body and environment.47,48 Triboelectric mechanism is based on the coupling of triboelectrification (i.e., contact electrification) of two dissimilar materials and the subsequent electrostatic induction of generated charges. Triboelectric nanogenerator (TENG), since its first emergence in 2012, has been widely developed in diverse configurations employing traditional polymer materials49–61 and emerging fabric materials62–67 for wearable and implantable applications.
Various developed wearable energy harvesters are summarized and depicted in Fig. 2. As depicted in Fig. 2(a), a flexible piezoelectric nanocomposite generator is demonstrated by using Pb(ZrxTi1−x)O3 (PZT) particles with excellent piezoelectricity to achieve high output performance.38 The PZT particles, multiwalled carbon nanotubes (MW-CNTs) as dispenser, and polydimethylsiloxane (PDMS) are mixed together to form the piezoelectric nanocomposite (p-NC), which is further transferred onto indium tin oxide (ITO)-coated polyethylene terephthalate (PET) substrates. The output voltage and current of a 3 cm × 3 cm generator are 10 V and 1.3 µA under bending, respectively. Using a bar-coating technique, a larger generator of 30 cm × 30 cm can be fabricated with an output of 100 V and 10 µA under human motions. Another flexible high-output nanogenerator (HONG) based on piezoelectric ZnO nanowires is reported on a flexible polyimide substrate [Fig. 2(b)].39 The single layer device can achieve an open circuit voltage of 2.03 V and a peak power density of ∼11 mW/cm3, showing great potentials of using piezoelectric nanowire based generators for self-powered systems. Piezoelectric PZT thin film can also be fabricated on a flexible substrate through a large-area laser lift-off (LLO) process [Fig. 2(c)].46 The demonstrated flexible nanogenerator is able to achieve an excellent output performance of ∼200 V and ∼150 µA/cm2 under mechanical deformations, due to the high-quality piezoelectric thin film transferred by the LLO process.
Except for mechanical energy, thermal energy is another widely available energy source in wearable applications due to the temperature difference between the human body and the ambient. A flexible thermoelectric power generator is developed using screen-printing and laser multiscanning lift-off technique, which consists of p-type Bi0.3Sb1.7Te3 inorganic film and n-type Bi2Se0.3Te2.7 inorganic film [Fig. 2(d)].48 When 72 thermoelectric couples (p-type and n-type) are connected in series, a power density of 4.78 mW/cm2 can be achieved at a temperature difference of 25 °C.
Due to the simple device configuration/process, wide range of material choice, cost effectiveness, and high output performance, TENGs are widely adopted for energy harvesting and self-powered sensing. For instance, a biological-cells-inspired energy harvesting skin is developed based on a soft and stretchable TENG [Fig. 2(e)].53 The cellular structures of physiological saline (electrode) are interconnected and encapsulated by soft silicone rubber (triboelectric layer), with an ultrahigh stretchability of up to 600%. The energy harvesting skin can be mounted on various body parts (e.g., neck, arm, wrist, finger, palm, and foot.) to scavenge diversified biomechanical energy from human motions. A maximum power density of ∼1.16 mW/cm2 can be generated from the device at 2.3 Hz operation frequency. Another stretchable electronic skin (e-skin) is proposed to detect and harvest various mechanical stimuli, which is the first stretchable energy harvesting e-skin at the time [Fig. 2(f)].54 In order to detect and differentiate normal pressure, strain, or bending, capacitance change and film resistance change of the e-skin are measured simultaneously. Meanwhile, the e-skin is able to harvest mechanical stimuli with an output of around tens of volts and tens of μA/cm2. The integrated energy harvesting and tactile sensing features enables the e-skin for a large variety of applications in robotics and self-sustainable systems.
Recently, fabric or textile based TENGs are developing rapidly due to their unique natures of wearable, lightweight, washable, and breathable. A wearable power-textile is demonstrated through the integration of fabric based TENG and fiber-shaped dye-sensitized solar cells (FDSSCs) to scavenge energy from human motions and solar light simultaneously [Fig. 2(g)].67 The grating-structure TENG is fabricated by laser-scribing masking and Ni plating, with an optimized power density of 0.32 mW/m2 under a 0.75 m/s sliding motion. At the same time, the FDSSCs can achieve an average power conversion efficiency, open circuit voltage, and short circuit current density of 6%, 0.6 V, and 10.6 mA/cm2, respectively.
Development of implantable energy sources is as important as wearable energy sources, which can be realized based on two major approaches. The first approach is to scavenge the movements of internal organs, e.g., the heart, lung, and diaphragm, by using piezoelectric45,68–73 or triboelectric12,72,74–78 devices. The second approach is energy delivery to transfer energy from external sources to implantable devices through inductive power transfer79–81 or acoustic energy transfer.82–85
To harvest the biomechanical energy inside a rat, an implantable ZnO single-wire generator (SWG) is demonstrated [Fig. 2(h)].69 The piezoelectric ZnO nanowire is fabricated by a physical-vapor deposition technique, with a diameter of 100-800 nm and a length of 100-500 µm. Triggered by the heartbeat motions of a live rat, the SWG can generate an average voltage and current of 3 mV and 30 pA, respectively. This research opens up the possibility to harvest the biomechanical energy from the in vivo environment, e.g., heartbeat, muscle stretching, blood pressure, breath, and other vibrations. Another piezoelectric energy harvester using the PZT ribbon is reported for scavenging the biomechanical energy from the motions of the heart, lung, and diaphragm [Fig. 2(i)].70 The PZT ribbons are transferred to a flexible polyimide substrate through a PDMS stamp. For the in vivo measurement, different animal models with organ size similar to human have been adopted to show the feasibility of the device. When attached on a bovine heart, the device is able to produce an output voltage of around 4 V. Besides, high-performance piezoelectric thin film, i.e., (1 − x)Pb(Mg1/3Nb2/3)O3-(x)Pb(Zr,Ti)O3 (PMN-PZT), is developed with biocompatibility to enable in vivo self-powered wireless data transmission [Fig. 2(j)].71 Triggered by porcine heartbeat motions, the energy harvester can generate an open-circuit voltage and short-circuit current of 17.8 V and 1.75 µA (higher by a factor of 4.45 and 17.5 than previously reported in vivo piezoelectric energy harvesters), respectively. Due to the excellent output performance, wireless data transmission powered by the porcine heartbeat driven an energy harvester is successfully demonstrated.
Except for directly scavenging biomechanical energy from internal organ motions, energy delivery through acoustic energy transfer is another promising technique to enable more reliable and miniaturized implantable energy harvesters. A miniaturized and broadband piezoelectric ultrasonic energy harvester (PUEH) based on a PZT diaphragm array is proposed as a reliable implantable energy source [Fig. 2(k)].85,86 Through adjusting ultrasound frequency within the broad bandwidth, standing wave induced power loss can be minimized for a wide range of implanted depths. In the underwater test with 1 cm separation between the ultrasound transmitter and the PUEH, output power density can be improved from 0.59 µW/cm2 to 3.75 µW/cm2 with frequency adjustment, when the input ultrasound intensity is 1 mW/cm2.
TENGs, due to the flexible nature and ultra-wide range of materials, are also adopted for implantable energy harvesting after biocompatible and waterproof packaging. An encapsulated arc-shaped implantable TENG (iTENG) is reported for in vivo harvesting biomechanical energy from heartbeats [Fig. 2(l)].76 Driven by porcine heartbeats, the iTENG can generate an open circuit voltage of 14 V and a short circuit current of 5 µA. Long-term stability of the iTENG is also investigated by continuous output generation for over 72 h in an active animal model. Because of the superior in vivo performance, self-powered wireless transmission of electrical signals associated with the heartbeat is successfully achieved for real-time heartbeat monitoring. In addition, another TENG based self-powered implantable triboelectric active sensor (iTEAS) is proposed to achieve in vivo continuous and real-time monitoring [Fig. 2(m)].77 Driven by the heartbeat and breathing motions of a living swine, the iTEAS can produce an open circuit voltage of ∼10 V and a short circuit current of ∼4 µA. In addition, the iTEAS is able to continuously monitor the physiological and pathological signs, including heartbeat rate, respiratory rate, and arrhythmia.
IV. NEURAL INTERFACE FOR THE CENTRAL NERVOUS SYSTEM (CNS) AND PERIPHERAL NERVOUS SYSTEM (PNS)
Despite of extensive research on the brain, the mystery of the brain has not yet been fully understood, and it is a research area that needs to be further explored. Brain neural interfaces are important tools to provide a variety of information of neurological characteristics to neuroscientists and doctors. Neural signals from the brain have been referred to as electroencephalogram (EEG) when recorded from the scalp, as electrocorticography (ECoG) when recorded on the cortical surface, and as the local field potential (LFP; also known as micro-depth or intracranial EEG) when recorded by a small size electrode in the brain.87 Such signals were used in various applications ranging from medical to engineering fields, such as an epilepsy surgery using ECoG88 and brain computer interface applications using EEG.89,90 Due to the limitations of EEG and ECoG that cannot obtain signals inside the brain,91 multi-electrode arrays (MEAs) were designed and demonstrated to be implanted inside the brain using microelectromechanical systems (MEMS) technologies. The MEA such as an Utah array and Michigan probe implanted inside the brain investigated various neural characteristics of cortical and sensory areas in the brain [Fig. 3(a)].91,92
Advances in flexible and soft electronics have allowed neural interfaces to better adapt to soft tissue as well as to apply to a variety of platforms like optoelectronic based interfaces. It enables to demonstrate new approaches for the central nervous system (CNS). For instance, Shin et al. developed flexible near-field wireless optoelectronic implants for optogenetic control in a variety of behavioral apparatuses [Fig. 3(b)].93 Furthermore, bi-directional neuromodulation was achieved by combining ECoG recording and optical stimulation on small, freely behaving animals.94 Syringe-implanted mesh electronics showed stable long-term mapping and modulation of brain activity at the single-neuron level [Fig. 3(c)].95 In addition, flexible microneedle electrodes enabled to attach a curved surface of rat brain to provide recording capability [Fig. 3(d)].96 E-dura implant was also developed to insert in the spinal subdural space of rats [Fig. 3(e)].97 A magnetic approach was developed to stimulate neurons with less energy as well.98
The peripheral nervous system (PNS) is a kind of pathway of neural communications that delivers commands from a central headquarters to branches. Depending on applications, neural interfaces are required to record or stimulate neural signals. For instance, advanced neuroprostheses require neural interfaces capable of the high quality of recording from efferent signals to record intention of any movements as well as capable of reliable stimulation to provide sensory feedback to the brain.99,100 For this application, a relatively non-invasive cuff electrode and a flat interface nerve electrode (FINE) were developed and demonstrated to provide sensory perceptions [Figs. 4(a) and 4(b)].10 Intrafascicular approaches such as a transverse intrafascicular multichannel electrode (TIME) was implanted on an ulnar nerve [Fig. 4(c)]11 as well as a thin film-longitudinal intrafascicular electrode (tf-LIFE) was implanted on the median nerve to provide feedback control [Fig. 4(d)].101 A 3D spiked ulatraflexible neural (SUN) interface was demonstrated for decoding peripheral nerve sensory information [Fig. 4(e)].100 Flexible and soft electronics can be applied for regenerative electrodes that allow us to interface a high number of nerve fibers for bidirectional communication between a nerve and a prosthetic device or system.102,103 For instance, complete elastomer-based microchannel electrode implant was developed and was interfaced to highly flexible wires with conductive paste [Fig. 4(f)].104 Advances in flexible and soft electronics also provide various platforms combining novel technology with neural interfaces. For instance, a neural dust was suggested and demonstrated for neural recording of a sciatic nerve in an anesthetized rat using an ultrasound approach [Fig. 4(g)].105 This shows that neuromodulation does not need to be a confined electrical approach and can be applied using a variety of energies.
By stimulating or modulating peripheral neural signals, bioelectronics enables us to control bodily functions or treat diseases. For instance, the autonomic nervous system or visceral nervous system is targeted to require advanced version of bioelectronics and neural interfaces.99,106,107 Two aspects would be considered; reliable and long-term use interfaces and power issues with the interfaces. Accordingly, a wireless cuff was suggested, combining reliable cuff interface and mid-field transfer. This modulated a vagus nerve to control heart functions [Fig. 4(h)].108 In addition, neural signals from the glossopharyngeal and vagus nerves were chronically recorded using carbon nanotube (CNT) yarn electrodes [Fig. 4(i)].109 An advanced form of carbon nanotube fibers (CNTfs)-laden devices was implanted on the tracheal syringeal nerve for recording.5 Microchannels were implanted on the dorsal surface of the S1-S2 spinal cord for feedback control of bladder functions [Fig. 4(j)]110 A flexible neural clip interface was suggested and stimulated vagus nerves, branch of sciatic nerves, and bladder pelvic nerves. In addition, a wireless neural clip achieved wireless modulation of bladder functions [Fig. 4(k)].7 Novel approaches were demonstrated previously, and recently one of the research trends of the neural interface is to combine with neural tissue or biodegradable materials for long-term use or removing additional surgery.21,111,112
V. BIODEGRADABLE ENERGY HARVESTERS
For implantable energy harvesters, biodegradability is of great importance especially in short-term application where no further surgical procedure is required to take out the implantable devices. Over the past few years, a few energy harvesters have been demonstrated with great biodegradability for implantable applications.113–118 For example, a biodegradable nanogenerator is demonstrated using lead-free piezoelectric nanoparticles and silk fibroin composite for powering implantable devices [Fig. 5(a)].113 Ag nanowires and polyvinylpyrrolidone are also added to the nanoparticle-silk composite, with the function of enhancing the dispersion of the nanoparticles and preventing Ag nanowires from connecting, respectively. Triggered by a foot stepping motion, a maximum output voltage of 2.2 V and a current density of 0.12 µA/cm2 can be achieved. The water-soluble property of the composite film can be controlled with the glycerol content for up to two days lifetime. Another biodegradable TENG (BD-TENG) with multilayer structure of biodegradable polymers and resorbable metals is proposed for short-term implantable energy harvesting [Fig. 5(b)].114 The open circuit voltage and short circuit current of the BD-TENG can reach up to ∼40 V and ∼1 µA, respectively. Through adopting different materials, tunable output performance and degradation lifetime can be achieved. After completion of the working cycle, the BD-TENG is able to be degraded and resorbed inside the animal body without causing any adverse long-term effects.
As depicted in Fig. 5(c), various natural materials with complete bioabsorbability are investigated as the triboelectric materials for biodegradable TENGs.115 The positions of the five natural materials in “triboelectric series” are measured and ranked, providing guidance for material selection of biodegradable TENGs. Different levels of output performance can be achieved by the combination of these natural materials. When egg white (EW) and rice paper (RP) are adopted as triboelectric materials, the fabricated TENG can produce a maximum voltage, current, and power density of 55 V, 0.6 µA, and 21.6 mW/m2, respectively. After modification of the silk fibroin encapsulation film, the operation lifetime can be tuned from days to weeks. Another fully biodegradable TENG is demonstrated using a gelatin film and electrospun polylactic acid (PLA) nanofiber as the triboelectric materials [Fig. 5(d)].116 Through optimizing the contact surfaces and thickness of the gelatin and PLA films, a maximum output voltage, short circuit current density, and power density of 500 V, 10.6 mA/m2, and 5 W/m2 can be achieved, respectively, with a device size of 4 cm × 4 cm. The biodegradable characterization of the device shows that all the materials can be completely degraded in water for about 40 days.
VI. 2D MATERIALS BASED ENERGY HARVESTERS
Recently, two-dimensional (2D) materials have attracted increasing research effort due to their unique property of a single layer of atoms. Various piezoelectric, thermoelectric, and triboelectric energy harvesters have been developed based on the emerging 2D materials, such as MoS2,119–123 graphene,41,124–128 and black phosphorus.129
In 2014, the piezoelectric property of MoS2 is first reported with experimental measurements showing that output can be generated from the cyclic stretching and releasing of MoS2 flake with an odd number of atomic layers but not an even number of atomic layers [Fig. 6(a)].120 The results indicate that single-layer MoS2 has strong intrinsic piezoelectric response because of the broken inversion symmetry, while the bilayers and bulk crystals are non-piezoelectric due to the centrosymmetric structure. A single monolayer MoS2 flake can generate a peak output voltage of 15 mV and a current of 20 pA under 0.53% strain. The corresponding power density and the mechanical-to-electrical energy conversion efficiency are 2 mW/m2 and 5.08%, respectively. Later on, the unique directional dependent piezoelectric effect of chemical vapor deposition (CVD)-grown monolayer MoS2 flake is investigated [Fig. 6(b)].121 The experimental results show that the piezoelectric coefficient (d11) exhibits different values along different directions, 3.78 pm/V along the armchair direction and 1.38 pm/V along the zigzag direction. Correspondingly, the developed flexible piezoelectric nanogenerator with MoS2 in the armchair direction shows about two times higher output performance than that with MoS2 in the zigzag direction under the same strain.
Furthermore, the thermoelectric nanogenerator using MoS2/graphene nanocomposite is developed for thermal energy harvesting and temperature sensing.122 The nanogenerator based on MoS2/graphene nanocomposite exhibits improved thermoelectric performance compared to that only based on graphene or MoS2. Under a temperature gradient of −35 K, the MoS2/graphene based nanogenerator can generate an output voltage of −0.73 mV and a power density of 8.8 nW/cm2. MoS2 can also be adopted to enhance the output performance of TENG as shown in Fig. 6(c), where a monolayer MoS2 is introduced in the negative friction layer as a triboelectric electron acceptor layer.123 As a result, the fabricated TENG with the MoS2 layer can produce a peak power density up to 25.7 W/m2, which is 120 times higher than that without the MoS2 layer. The great enhancement of the triboelectric output performance is due to the efficient capture of triboelectric electrons in the MoS2 layer.
Graphene is another widely investigated 2D material. A conformal graphene based TENG is demonstrated with a total thickness of less than 2.4 mm and good adhesion ability on the human skin [Fig. 6(d)].124 With an optimized nanostructured and fluorinated treatment, the peak output power of the device can be increased to 130 mW. When adhered conformally on the human body, the TENG is able to generate output in contact with various fabric materials, including nylon, silk, cotton, and latex. The TENG in contact latex shows best output performance due to both the increased effective contact area and electron affinity difference. Another CVD-grown graphene based flexible transparent TENG is reported [Fig. 6(e)].126 The TENG with monolayer (1L) graphene can produce a voltage of 5 V and a current density of 500 nA/cm2. For randomly stacked graphene TENG, the generated voltage and current density decrease with the number of layers. For regularly stacked graphene TENG, the generated voltage and current density can be increased to 9 V and 1.2 µA/cm2, respectively.
VII. SUMMARY AND OUTLOOKS
From the research trends of flexible electronics technology, using diversified flexible electronics can create sensors, actuators, stimulators, energy harvesters, and/or integrated systems, for wide applications ranging from wearable to implantable electronics. Direct stimulation using energy harvesters and neural interfaces is still in its infant level, but it has great potentials with the rapid development of conceptual paradigm shift, materials, and devices. We can imagine that any mechanical forces from outside or inside the body can be used for sensors130 and stimulators.17,19 Wireless neural interfaces will be applicable to other novel peripheral nerves not only for bioelectronics medicines but also for peripheral neuromodulation.7,108 Combination of an active neural interface and energy harvesting may construct a body sensor network (BSN) inside the body that is investigated for healthcare and the gaming. With the parallel development of BSN versus IoT and AI concept, new paradigm changes may open up in the near future.
The authors would like to acknowledge the financial support from following research grants: HIFES Seed Funding-2017-01 (Grant No. R-263-501-012-133) “Hybrid Integration of Flexible Power Source and Pressure Sensors” at the National University of Singapore; Grant No. NRF-CRP10-2012-01 “Peripheral Nerve Prostheses: A Paradigm Shift in Restoring Dexterous Limb Function”; Grant No. NRF-CRP8-2011-01 “Self-powered body sensor for disease management and prevention-orientated healthcare” from the National Research Foundation (NRF), Singapore; Faculty Research Committee (FRC) (Grant No. R-263-000-B56-112) “Thermoelectric Power Generator (TEG) Based Self-Powered ECG Plaster—System Integration (Part 3)” at the National University of Singapore; Agency for Science, Technology and Research (ASTAR), Singapore and Narodowe Centrum Badań i Rozwoju (NCBR), Poland Joint (Grant No. R-263-000-C91-305) “Chip-Scale MEMS Micro-Spectrometer for Monitoring Harsh Industrial Gases”; The DGIST Start-up Fund Program of the Ministry of Science and No. ICT 2018090020.