Kai Tao, Zhensheng Chen, Haiping Yi, Ruirong Zhang, Qiang Shen, Jin Wu, Lihua Tang, Kangqi Fan, Yongqing Fu, Jianmin Miao, Weizheng Yuan?

ABSTRACT Flexible, compact, lightweight and sustainable power sources are indispensable for modern wearable and personal electron?ics and small?unmanned aerial vehicles (UAVs). Hierarchical hon?eycomb has the unique merits of compact mesostructures, excellent energy absorption properties and considerable weight to strength ratios. Herein, a honeycomb?inspired triboelectric nanogenerator (h?TENG) is proposed for biomechanical and UAV morphing wing energy har?vesting based on contact triboelectrification wavy surface of cellular honeycomb structure. The wavy surface comprises a multilayered thin film structure (combining polyethylene terephthalate, silver nanowires and fluorinated ethylene propylene) fabricated through high?temperature thermoplastic molding and wafer?level bonding process. With superior synchronization of large amounts of energy generation units with honeycomb cells, the manufactured h?TENG prototype produces the maximum instantaneous open?circuit voltage, short?circuit current and output power of 1207 V, 68.5 μA and 12.4 mW, respectively, corresponding to a remarkable peak power density of 0.275 mW cm?3 (or 2.48 mW g?1) under hand pressing excitations. Attributed to the excellent elastic property of self?rebounding honeycomb structure, the flexible and transparent h?TENG can be easily pressed, bent and integrated into shoes for real?time insole plantar pressure mapping. The lightweight and compact h?TENG is further installed into a morphing wing of small UAVs for efficiently converting the flapping energy of ailerons into electricity for the first time. This research demonstrates this new conceptualizing single h?TENG device’s versatility and viability for broad?range real?world application scenarios.

KEYWORDS Honeycomb?inspired structure; Morphing wing energy harvesting; Electret power generation; Triboelectric nanogenerator; Self?powered insole pressure mapping

1 Introduction

Recent advances in Internet of Things (IoT) have witnessed the rapid development of intelligent sensor nodes with characteristics of low power consumption, flexibility and sustainability [1—3]. Conventional power supply technolo?gies with electrochemical batteries or power adapters will not be the preferred choice due to their limited lifespan, complicated maintenance and environmental unfriendli?ness. Energy harvesting systems as self?sustained power sources are capable of harvesting and transforming unused ambient energy into electricity. Such energy harvesters provide alternatives to those conventional electrochemical batteries and could pave an important step for the realiza?tion of self?autonomous operation and intelligent sensing [4—10].

There are four major transduction mechanisms for con?verting mechanical energy into electrical energy, namely piezoelectric [11—13], electromagnetic/magnetostrictive [14—19], electrostatic [20—23] and triboelectric [24—30]. Tri?boelectric nanogenerators (TENGs) were first proposed by Professor Zhonglin Wang’s group in 2012. It is based on the coupling of contact electrification and electrostatic induc?tion, where electron migration is created when two materials with dissimilar electron attraction are contacted. Redistribu?tion of electrons takes place when two friction materials contact/separate each other under mechanical excitations. Electromagnetic/magnetostrictive transduction mechanism is based on Faraday’s law of induction with changing of magnetic flux in a closed coil, either by relative movement (electromagnetic) or mechanical strain (magnetostrictive). The electrostatic mechanism is based on variable capacitor structures with a constant voltage/charge bias. The bias can be provided either by pre?charged electrets or by an exter?nal power source. Piezoelectric harvesters usually operate on stretching or compressing piezoelectric materials, such as piezoelectric polymers and piezoelectric ceramics, thus transforming mechanical strain or stress into electricity due to the piezoelectric effect [31—35]. Among these mecha?nisms, the triboelectric nanogenerator presents numerous advantages in terms of high output voltage and efficiency, ease of structural design and fabrication, diverse material selection, low?cost and broad low?frequency application sce?narios such as human motion and aircraft morphing wing oscillation [25, 26, 36—38].

To improve the performance of TENGs, numerous efforts have been made in various aspects, including mul?tilayer?stacked structures [39—52], nanostructured surface modifications [53—55], contact material synthesis [56, 57], optimization of surface charge density [58—60], advanced mechanical design [61—65], novel manufacture approaches and power management circuit design [66—68]. Among these, multilayer?stacked TENG has been regarded as one of the most effective and simple methods to maximize the instantaneous output power by synchronizing all the layers of TENGs. Both contact triboelectrification and electro?static induction can be significantly enhanced by multi?plying contact area variations and capacitance variation within a confined space. Table S1 summarizes the state?of?the?art TENGs with multilayered structures for improving the performance of TENGs in various aspects, including substrate material, device volume, weight and area, pri?mary structure, connected method, etc. However, these approaches still encounter some significant challenges:

? To sustain periodic contacts and separation operations, most multilayered or arch?shaped TENGs require auxil?iary architectures to provide elastic support, making the whole device rigid, heavy?weight and bulky. Therefore, they may have difficulties in integrating with wearable or biomechanical microsystems [45, 46].

? The charge circulation of TENG is mainly driven by capacitance variation of electrostatic induction. The maximum power generation occurs only when the two electrodes are at a very close position, where the capacitance variation reaches its maximum value. This indicates that the power generation efficiency per unit volume is not high if the initial stage’s air gap is too large. Therefore, a small air gap in the initial stage is often required for high output performance and high?power density [39, 48, 69].

? The output power of TENGs is quadratically proportional to the triboelectric surface charge density. The electro?static induction of TENGs is severely affected by the lim?ited surface charge density, which only relies on contact triboelectrification. Therefore, the charge trap capability is not fully exploited, except contact triboelectrification processes [58].

? Current TENGs with rigid and burdensome architectures usually restrict their operation modes, making it non?adaptable to excitation vibrations with irregular, random and low?frequency characteristics, such as morphing wing motions.

Herein, we propose a honeycomb?inspired TENG with multilayer?wavy?structured fluorinated ethylene propyl?ene (FEP) electret thin films to address the above issues (Fig. 1a). Nanostructured pillars were created on the FEP thin film surface by an ion beam etching process to increase the contact area during triboelectrification (Fig. 1b). Trans?parent polyethylene terephthalate (PET) film synthesized with conductive silver nanowires (AgNWs) is employed as a flexible substrate to sustain cycling pressing and releasing operations. The scanning electron microscope (SEM) image of the AgNWs electrode film’s surface morphology shows that the AgNWs film surface roughness is within a range of ～ 82 to 78 nm (Fig. 1c). Such a high surface roughness and many defects between AgNWs and FEP electret are ben?eficial for capturing more electrical charges, which are ben?eficial for the performance of the h?TENGs. Furthermore, this newly proposed h?TENG device exhibits several unique advantages:

Fig. 1 a Schematic structure of the proposed hierarchical honeycomb?structured electret/triboelectric nanogenerator. b, c Architecture of single arch?shaped deformable energy generation unit within honeycomb cells: SEM images of the FEP thin film with a nanostructured surface (top layer) and surface morphology of the fabricated AgNWs electrode film (bottom layer). d, e The proposed h?TENG operates at bending mode and compression mode. f, g A schematic illustration of the power generation mechanism of the morphing wing by replacing the honeycomb cellular mesostructure with the lightweight and compact h?TENGs skin. h The proposed h?TENGs can be integrated into the insole for real?time plantar pressure mapping applications

? Stacked multilayer h?TENGs can be conveniently con?structed through high?temperature thermoplastic mold?ing and wafer?level bonding of multilayer FEP/PET/FEP composites to form honeycomb structures. These fac?ile and low?cost formation processes are applicable for future manufacture with a mass production capability.

? The elastic nature of honeycomb cellular mesostructures and PET/AgNWs substrate endows the h?TENG device with excellent resilient and self?rebounding properties. Therefore, no auxiliary resilient supports are required, making the whole device compact, flexible, transparent and lightweight.

? The porous honeycomb architecture has divided the sizeable hollow space into numerous energy generation cells. The overall capacitance variations can be signifi?cantly amplified by synergizing all the energy generation cells with small air gaps, making the proposed h?TENG extremely sensitive to external perturbations.

? The honeycomb?structured TENG proposed in this research adopts a novel 3?D hierarchical structure based on multilayer PET/AgNWs/FEP composites compared with the previous TENGs with in?plane honeycomb pat?terns [70—72]. These parallel?connected and deformable units can significantly boost capacitance variations and enhance the overall performance of the device. Both the coupling of contact electrification and electrostatic induction can be multiple improved within a confined space.

? Corona discharging process is further utilized to maxi?mize the surface charge density of the thin film electret. Therefore, high energy output can be readily obtained by full use of electret thin films’ charge trap capability.

The fabricated h?TENG has such properties of ultra?lightweight, long?term durability and excellent flexibility. Therefore this newly proposed device can be integrated into unmanned aerial vehicles (UAVs) for harvesting kinetic energy from morphing wing motions. Figure 1d, e show the two operation modes of the proposed h?TENG: bending mode and compression mode. Figure 1f, g show schematic illustrations of the morphing wing’s power generation mech?anism by replacing the honeycomb cellular mesostructure within UAVs with the proposed lightweight and compact h?TENGs skin. The h?TENG device can be further integrated into the insole for real?time plantar pressure mapping appli?cations (Fig. 1h). An instantaneous open?circuit voltage of 1207 V and a short?circuit current of 68.5 μA are generated by gently tapping the h?TENG by hand. As a real?time self?powered tactile sensor, the 5 × 5 h?TENG array is capable of lighting up a 10 × 10 LED display panel for instantaneous pressure mapping, which will be deliberated in the subse?quent sections.

2 Experimental Sections

2.1 Synthesis of AgNWs on PET Surface

PET thin film (Hefei Vigon Material Technology Co., Ltd) with a thickness of 30 μm is adopted as the substrate mate?rial. The PET thin film is firstly treated with alginate for 2 min and then brushed with AgNWs using a rolling/press?ing process to achieve good conductivity. After solidifying AgNWs coated PET film at room temperature, CaCl2and chlorine chloride are introduced to enhance the bonding of the AgNWs onto the surface of PET thin films. Finally, 120?nm?thick conductive AgNWs thin film is successfully synthesized on the surface of PET to form transparent PET/AgNWs composite, which is used as both the conductive electrode and elastic substrate for the h?TENG device.

2.2 High?Temperature Molding of PET/AgNWs/FEP Composite

A vacuum hot?pressing process is applied to bond 50?μm?thick FEP electret thin film (Goodfellow Cambridge Co., Ltd) onto the surface of PET/AgNWs film to achieve a good contact and adhesion. The PET/AgNWs/FEP com?posite is then pressed together using a complementary wavy?shaped iron alloy mold and silica gel at a tempera?ture of 80 °C and an applied force of 1 kN for 10 min. The complementary iron (Fe—Ni) alloy template with a size of 5 × 3 × 4 cm3is used for the molding process. The silica gel is used to protect AgNWs, thus maintains a good conduct?ibility during the hot pressing.

2.3 Assembly of the h?TENG Device

The ionized charges are pre?implanted into the wavy?struc?tured electret composites of PET/AgNWs/FEP through a corona discharging process under a ? 4 kV voltage for 10 min. The wavy?structured PET/AgNWs/FEP composite is then turned over and positioned onto the flat surface of the ethoxyline?resin?coated wafer. Therefore, only the backside nodal line of the wavy structure is coated with ethoxylate resin. Finally, the multilayered honeycomb structures and h?TENG devices are fabricated through a layer?by?layer stacking process of PET/AgNWs/FEP composites.

2.4 Measurement of the h?TENG Device

A high?impedance data acquisition system (DAQ, NI USB?6289 M series, USA, impedance＞10 GΩ) was used in recording the electrical signal. The fabricated h?TENG was sandwiched with two parallel plates. The top plate is fixed to a 3D positioning stage, which controls the h?TENG pro?totype’s height. The bottom plate is mounted on the top of the electromagnetic vibrator, which can provide sinusoidal vibration excitation at tunable amplitudes and frequencies. The capacitance of h?TENG is measured using a precision LCR meter (Applent AT811, CN). In the mechanical and electrical property characterization, the frequency of exci?tation acceleration and load resistance are set at 20 Hz and 500 MΩ, respectively.

3 Fabrication and Working Principle

3.1 Fabrication of h?TENG Device

Figure 2a—h schematically illustrate the fabrication process of the h?TENG device. PET thin film is adopted as the sub?strate material since it has the excellent flexibility and trans?parency. The PET thin film is firstly treated with alginate (Fig. 2a) and then brushed with AgNWs (Fig. 2b). CaCl2and chlorine chloride are introduced to enhance the adhesion between AgNWs and PET thin films (Fig. 2c). The Cl?in the chlorine chloride peels off the stabilizer on the surface of the AgNWs and then induces a ‘welding effect’ of AgNWs, thus reduce the contact resistance. In the meanwhile, the Ca2+in the CaCl2reacts with alginate on the substrate to form an alginate gel, which improves the adhesion between the AgNWs and substrate. Therefore, transparent PET/AgNWs composite is successfully synthesized as the elastic substrate for the h?TENG device. Figure 2d shows wavy?structured PET/AgNWs/FEP composites are formed through a vacuum hot?pressing process since the PET thin film has excellent thermo?plasticity with a low Young’s modulus of only 2.7 GPa. The period and amplitude of the wavy sheet can be controlled by adjusting the iron alloy template param?eters, as shown in Fig. S1. The silica gel is used to protect AgNWs, thus maintains a good conductibility during the hot pressing.

The setup of vacuum hot?pressing machine and PET deformation properties under different temperatures are shown in Figs. S2a and S2b, respectively. To maintain a good conductivity of AgNWs/PET composites, the mold?ing temperature should be adjusted below the glass transi?tion temperature of PET to achieve a good stability of wavy structures while maintaining moderate stress. Figure S2c shows the stability characterization results of electret surface potentials and wavy amplitude variations of PET/AgNWs/FEP composites after continuous operation of ～ 10,000 cycles. Results show that a proper elasticity of PET and a reasonable charge stability of electrets can be achieved when the molding temperature is controlled at around 80 °C. This guarantees a good elasticity of the wavy?shaped PET, regard?less of various forces applied during the assembly process.

The FEP thin film, applied as the electrets, can provide an electric field for years through quasi?permanent electric charges or dipole polarization generated after the discharg?ing process (Fig. 2f). An ion beam etching process is applied to create a nanostructured surface of FEP thin film for increasing the contact area during triboelectrification. The corona discharging process can maximize the charge density of FEP electret thin films for improving the performances of both the contact electrification and non?contact electrostatic induction. Figure 2g—j show schematic illustration and opti?cal photographs of the h?TENG device fabricated through a layer?by?layer stacking process of wavy PET/AgNWs/FEP composites. The green sheet in Fig. 2h is h?TENG substrate material, which is made by flexible printed circuit board (FPCB) technology. FPCB is utilized for wiring connec?tion and integration of TENG arrays. Figure S3 shows that good transparency is still maintained after stacking several PET/AgNWs/FEP layers together. The electrical resistance of PET/AgNWs is examined using a four?probe electrical resistivity measuring technique, and results show that the wavy?shaped PET/AgNWs still maintains 95% conductiv?ity of the planar PET/AgNWs sheet. The PET/AgNWs thin film’s original square resistance is around 38 Ω sq?1and increased to 40 Ω sq?1after the vacuum hot?pressing pro?cess. The fabricated h?TENG is capable of integrating into wearable electronics for biomechanical energy harvesting and self?powered sensing applications.

Fig. 2 Fabrication processes and working mechanisms of the proposed h?TENG: a PET thin film is treated with alginate for 2 min. b AgNWs are pressed onto the surface of PET by rolling process. c CaCl2 treatment is used to ‘weld’ the AgNWs onto the surface of PET thin film. d FEP electret thin film is bonded onto PET/AgNWs film surface with ethoxylate resin. e A single?wavy structure is formed using iron alloy templates through high?temperature molding process. f A corona discharging process is used to implant charge into FEP electret thin films. g Honeycomb structures are assembled by stacking multilayered PET/AgNWs/FEP composite together. h–j schematic illustration and optical photographs of the fabricated h?TENG device. k Capacitance values change from 20 to 480 pF when the height of h?TENG changes from 3 cm to 3 mm with stacking triple?layer wavy structure. l Electric field and charge redistribution within four stages of h?TENG in a compress?release cycle: I Initial stage of original state. II Electrostatic induction by electrets when two electrodes get close to each other under external compressive force. III Contact triboelectrification when two electrodes get intimate contact with each other. IV Charge flows back when releasing

3.2 Working Mechanism

The porous honeycomb architecture has separated the h?TENG into a parallel?connected array of arch?shaped deformable capacitors. These parallel?connected and deformable energy generation units are beneficial for boost?ing the overall power output and increasing the whole h?TENG device’s flexibility. For the electrostatic induction or triboelectrification, the maximum output power is gener?ated when the two electrodes are at their closest positions. Theoretically, the amount of capacitance variation when the distance of two electrodes is changed from 1 mm to 1 μm is about ten times higher than when the corresponding distance is changed from 1 cm to 1 mm. It indicates that by dividing a large hollow structure into many small and parallel?con?nected capacitors, the overall performance of the h?TENG is enhanced significantly, especially when the displacement of the electrode is comparatively tiny. For example, results shown in Fig. 2k demonstrates that the capacitance value is increased from 20 to 480 pF when the height of the h?TENG is decreased from 30 to 3 mm with a stacked triple?layer wavy structure. In contrast, the capacitance is only increased from 8.2 to 76 pF for a single?layer system under the same condition. Therefore, due to these two strategies’ synergistic effects, a high performance of energy harvesting can be read?ily obtained with the proposed h?TENG device.

Finite element method (FEM) via COMSOL Multiphys?ics software is employed to understand the electric field and potential redistribution during the compress?release cycle of the h?TENG device. Figure 2l presents a cross?section view of the geometrical layout and electric field distribution of a two?layer structured h?TENG device. In the initial state, the electric field is established between the FEP layer and the counter electrode. The net charges and dipoles that existed in the FEP form a constant bias surface potential when the counter electrode is grounded (Stage I). When the distance of two electrodes of the arch?shape energy conversion unit is decreased due to the applied external compress force, elec?trostatic induction is generated by the pre?implanted charges in the FEP electrets. It drives the induced charges flowing in the external circuit (Stage II). The amount of redistributed charges are kept increasing until the two electrodes of the arch?shaped energy conversion unit are intimately contacted with each other, when the triboelectrification effect occurs. At this stage, surplus charge is created due to different work functions of two dissimilar contact surfaces (Stage III). After the external force is removed, the arch?shaped energy conver?sion unit of the h?TENG recovers to its original shape due to the internal self?bounding ability of the PET material, which results in the induced charges flowing back to the counter electrode. Finally, when the generator is compressed again, a periodic current is generated by pressing and releasing the process intermittently (Stage IV). In this way, both the elec?tret?based electrostatic induction and contact triboelectrifica?tion are engaged during the charge circulation process, lead?ing to a superior charge synchronization within this unique honeycomb multilayered structure.

4 Results and Discussion

4.1 Mechanical and Electrical Property Characterization

Various mechanical and electrical parameters of the h?TENGs are investigated in order to optimize their output performance, including the initial wavelength, wave ampli?tude, honeycomb height and layer, external acceleration excitation and load resistance. Mechanical properties of the proposed h?TENG are highly dependent on the ther?moplastic process of wavy?structured PET/AgNWs/FEP composites. Therefore, the wavelength and amplitude of wavy structures need to be optimized and controlled by iron alloy templates’ mold parameters.

Figure 3a shows the output voltage waveforms of the wavy h?TENGs with a constant wave amplitude of 4 mm and different wavelengths ranging from 10 to 20 mm. It can be seen that with the wavelength increased from 10 to 20 mm, and the output voltage is decreased from 80 to 35 V. The decrease of output voltage is mainly due to the slower capacitance variations and smaller contact surface area. Figure 3b shows the output voltage waveforms of h?TENG with a constant wave wavelength of 15 mm and different amplitudes of 2, 4 and 6 mm. It is observed that with higher wave amplitude, the output voltage is increased from 43 to 92 V under the same excitation. This is because a larger capacitance variation can be generated with a larger air gap. Therefore, the wavelength decrease and height increase are beneficial for improving the output performance of the h?TENGs. However, due to fabrication restrictions and conduction problems associated with substrates with larger curvatures, the rational values with a wavelength of 15 mm and an amplitude of 2 mm are adopted in the later design of the honeycomb structure. Figure 3c demonstrates that the output voltage is amplified by 2.7 times when the layers of wavy PET/AgNWs/FEP thin films are increased from one to three. It indicates that increasing the power generation lay?ers is the most effective method to improve the performance of the h?TENG. Figure 3d—f show the corresponding output currents of h?TENGs with different wavelengths, amplitudes and layers of wavy PET/AgNWs/FEP composites, demon?strating the similar characteristics and trends with that of output voltages.

Fig. 3 Optimizations of mechanical and electrical parameters for the h?TENG: a Output voltage waveforms of h?TENG at a constant wave amplitude of 4 mm and different wavelengths ranging from 10 to 20 mm. b Output voltage waveforms of h?TENG with a constant wave wave?length of 15 mm and different amplitudes of 2 mm, 4 mm, 6 mm. c Output voltage waveforms of h?TENGs with from a single?layer wavy thin films to triple?layer films. d—f The corresponding output currents of h?TENG with different wavelengths, amplitudes and layers of wavy PET/AgNWs/FEP composites. g, h Output current and power optimizations with varying accelerations of excitation and load resistances. i Output power optimizations with different layers and load resistances

4.2 Power Generation with Human Motion

The flexible and lightweight h?TENG is convenient for wearable or human motion energy harvesting applications. Figure 4a shows the derivations of output voltages and cur?rents of a single?layer h?TENG device with different load resistances under the human palm’s pressing excitation. It is seen that the short?circuit current and open?circuit voltage can reach up to 68 μA and 1123 V, respectively. By a sim?ple finger pressing, the h?TENG can efficiently lighten up a 75 × 55 mm2LCD with a logo of ‘NPU’ (Fig. 4b). To obtain a better visualization of power generation demonstration, we connect a Bennet’s doubler conditioning circuit for driving the external electronics (Fig. 4c). The Bennet’s circuit is capable of rectifying output AC signals from the h?TENG device to unidirectional DC signal, then effectively boosting the voltages and storing the energy into the capacitor. With the aid of Bennet’s circuit, the 75 × 55 mm2LCD can be lightened up for 83 s by simply hand pressing the h?TENG device ten times, as shown in Video S1. After that, the LCD brightness is gradually reduced.

Fig. 4 a Output voltages and currents of h?TENG with different load resistances by hand tapping. b Lighten up 75 × 55 mm2 LCD with h?TENG by finger touching. c Bennet’s doubler conditioning circuit for driving external electronics. d Output power optimization at different load resist?ances with hand tapping. e Light intensity of globe bulb up to 43 lx obtained by continuously tapping the h?TENG for ten times with a buckled ruler

The output power optimization at different load resist?ances with simply hand tapping is shown in Fig. 4d. Trig?gered by gentle hand tapping, the fabricated h?TENG prototype produces instantaneous open?circuit voltage, short?circuit current and output power, with their maximum readings of up to 1207 V, 68.5 μA and 12.4 mW, respec?tively, corresponding to a remarkable peak power density of 0.275 mW cm?3(or 2.48 mW g?1) using the hand tap?ping. Based on the inset Figure’s output waveform shown in Fig. 4d, the restoring time of the h?TENG takes only ～ 5 ms. The excellent responsivity of the h?TENG is suitable for self?powered sensor for instantaneous sensing applications.

Figure 4e shows the photograph of a globe bulb that is powered by the h?TENG. The h?TENG device is squeezed for ten times with a buckled ruler. The generated power is firstly stored in a 47 μF capacitor through the Bennet’s cir?cuit and then discharged to lighten up the globe bulb. The intensity of light is characterized using a digital lux meter (GM 1020). The results show that the light intensity can be increased up to 43 lx, which is able to use to lighten up a printing paper, as shown in the inset of Fig. 4e. Figure S4 in the supplementary information also demonstrates that the flexible h?TENG can be integrated into a smart foam mat, collecting kinetic energy from a person’s repeated stepping and jumping exercises.

4.3 Self?powered Sensing Demonstration

The output characteristics of h?TENG are further stud?ied with continuous periodic excitations under different accelerations. With an excitation frequency of 20 Hz and an external load resistance of 10 MΩ, the time?domain waveforms and output voltage magnitudes versus differ?ent excitation accelerations in the range of 0.3 ～ 2.1 g are shown in Fig. 5a, b, respectively. It is found that the out?put performance is in a quasi?linear relationship with the excitation accelerations below 1.6 g. It demonstrates the applicability of h?TENG for a self?powered accelerometer or force sensor in this acceleration range. After that, the stiffness of the h?TENG becomes significantly higher with enhanced rebounding ability and responsivity due to the decreased plasticity of the PET thin films.

Fig. 5 a, b Output voltage waveforms and magnitudes versus different excitation accelerations in the range of 0.3 ～ 2.1 g. c Output voltages of proposed h?TENG under continuous operations for ～ 75,000 cycles at an excitation acceleration of 5 g and a frequency of 30 Hz. d—f Output sig?nal characteristics of proposed h?TENG by human motions, such as finger touching, bending and tapping. g Snapshot of real?time insole plantar pressure mapping using an array of h?TENGs during human walking. h Lighten up 10 × 10 LED display panel by 5 × 5 h?TENG array for instan?taneous hand force mapping, when the whole palm, one finger or two fingers touch the generator array

To investigate the stability and long?term durability, the h?TENG device is further characterized by continuous opera?tion for ～ 75,000 cycles at an excitation acceleration of 5 g and a frequency of 30 Hz, with the tapping motion generated using a mechanical shaker. According to the obtained results shown in Fig. 5c, there is no noticeable performance deg?radation of the h?TENG, reflecting its ultra?robustness and good stability after such a long?term operation. The excel?lent performance of the proposed h?TENG should be attrib?uted to the excellent mechanical properties of honeycomb structures and the stable thermo?plasticity of PET/AgNWs/FEP composites.

Due to the excellent elastic property and self?rebounding honeycomb structure of the h?TENG, the flexible and trans?parent device can be easily pressed, bent and integrated into wearable devices. Figures 5d—f demonstrate output signals of the proposed h?TENG by human motions, such as finger touching, bending and tapping. Due to different frequencies and deformation characteristics, the output voltage signals demonstrate significant differences in their output voltage amplitudes, frequencies and waveforms. During bending experiments, two hands were used to hold the h?TENG to compress and bend the device to the degree of 90° and then release it at a slower frequency of 2 Hz to simulate the daily hand bending movements. The hand bending of h?TENG generates the lowest frequency but with the most symmetri?cal waveforms. While tapping, two fingers at the frequency of 6 Hz were applied on the h?TENG to simulate the state of the fabricated device being tapped during self?powered sens?ing applications. The finger tapping motion generates the lowest output voltages, whereas the hand pressing motion produces the highest output voltages but with the most asymmetrical waveforms. The experimental results indicate that different human motions can be distinguished using the proposed flexible h?TENG.

Insole plantar pressure mapping is critical for sport injury prevention, foot ulceration prediction and biomechanics information acquisition, etc. The above experiments have already demonstrated that the proposed h?TENG device has an excellent self?restoring ability and a fast response. Therefore, it has a great potential to be used for self?powered insole plantar pressure mapping and tactile imaging.

In order to integrate the h?TENG into the insole, the vol?ume of each h?TENG unit is decreased to 20 × 15 × 10 mm3. The insole substrate is made from a flexible printed circuit board (FPCB) with its integrated circuit design and wire connection pads. Firstly, an array of ten h?TENG devices are bonded to the FPCB and wired to the connection pad onsite. The output signal is connected to a signal acquisi?tion card (NI?USB6289). After that, the whole device was installed into the tester’s shoes. Secondly, the outputs of the multi?arched h?TENGs were acquired by a total of 10 integrated circuits on the signal acquisition card. The 20 analog input channels were real?time sequential scanned by the customized data real?time processing and graphics display procedure based on LabVIEW Software, which is able to normalize the output signal and visualize the output magnitudes with different colors. Thirdly, the output signals were transferred into the graphical data to display the walk?ing tester’s foot pressure distributions. Figure 5g shows an example of a snapshot of real?time insole plantar pressure mapping by using an array of h?TENGs during the walking of a person. The real?time video of insole plantar pressure mapping during the person’s walking is shown in Video S2. It can be clearly seen that the person is walking with these smart insoles. The pressure distribution of the insole can be real?time displayed on the computer screen with various color differences.

A square shape of 5 × 5 array of h?TENG devices is fur?ther integrated with a white FPCB board for tactile hand pressure mapping. On the back of the PCB, a 10 × 10 LED lamp array has been applied, and each h?TENG is linked to four series?connected LED lamps. The LEDs at different locations can be lightened up when the hand touches differ?ent positions of the device. Figure 5h demonstrates that the back LED has been lightened when the whole palm, one fin?ger, or two fingers touch the generator array, respectively. It can be clearly observed that the LED lights can be lightened up when different hand excitations are applied to the vari?ous locations, reflecting the ability of the cellular h?TENGs for self?powered and real?time touch imaging applications.

4.4 Aircraft Morphing Wing Energy Harvesting Demonstration

Recently, there is a rapid growth of unmanned aerial vehi?cles (UAVs) in various applications ranging from military security and surveillance to civil infrastructure inspection, mainly due to their advantages such as high mobility, low maintenance cost and ease of deployment, etc. (Fig. 6a). Recent advances in morphing UAVs demonstrate great supe?riorities compared to those traditional rigid UAVs, such as reduced weight without transmission mechanisms, better aerodynamic performance for hingeless and contoured con?trol surfaces, and multiple missions with single morphing aircraft systems.

Fig. 6 a Morphing wing of the unmanned aerial vehicle (UAV). b A schematic view of the cross?section of the morphing wing, where honey?comb cellular mesostructure is replaced by light and compact h?TENGs skin. c A digital photograph of the fabricated morphing wing, which is controlled by a motor through non?conformal contact for adjusting the flap angle. d—f Output voltage waveforms of h?TENG at various excita?tion frequencies under different wing deflected angles of 10°, 20°, 30°. g Output currents of h?TENG with different wing deflected angles at a frequency of 2 Hz. h Output power comparisons of h?TENG with different wing deflected angles at a frequency of 2 Hz. i Comparisons of output power generation of morphing wings with unidirectional deflection and bidirectional motion at an excitation frequency of 3 Hz and a deflected angle of 20°

One of the critical challenges of morphing UAVs is the small and lightweight fuselage, which restricts the battery size and capacity, and limits the UAVs flight time and range. One promising strategy is to harvest energy from aircraft during flights and convert it into electricity, and the kinetic energy generated from wing vibrations would be an ideal choice. Meanwhile, honeycomb cellular mesostructures have been extensively used in these UAVs since the structures have excellent energy absorption properties and considerable weight to strength ratios. In this work, the lightweight and compact h?TENG skin is fabricated and installed into mor?phing wings to replace honeycomb cellular mesostructure for converting kinetic energy to electricity (Fig. 6b). This work is the first study to integrate the TENG device into UAV morphing wing energy harvesting application.

Figure 6c shows the fabricated morphing wing integrated with the h?TENG device. The dimensions of flap length and depth of the morphing wing are 300 and 40 mm, respec?tively. The size and parameters of our UAV are mainly based on the NACA Airfoil Series. Detailed dimensions of the profile are further calculated by the design parameters of NACA 0012 Airfoil. For morphing wing technology, 1 Hz of the wing actuation is often used during cruising. However, during the taking off, rapid wing maneuver and landing period, the aircraft needs a quicker actuating of the morphing wing (usually between 2 and 3 Hz). The motion and deflection of the morphing wing is controlled using a steering engine connected with a crank axle. The angle (θ) between the center chord line of the rigid part and the center chord of the tail (flexible wing) is a crucial control factor for calibration. In the experiment, the angles (θ) and excited frequencies(f) are varied to beθ= 10°, 20°, 30° andf= 1 Hz, 2 Hz, 3 Hz, respectively. Figures 6d—f show the obtained output voltage waveforms of h?TENG at differ?ent excitation frequencies under different deflected angles, respectively. Video S3 shows the dynamic demonstration of h?TENG power generation from the oscillations of the mor?phing wing. The generated power can continuously lighten up dozens of LEDs. Figure 6g, h show the comparisons of output voltage waveforms and output powers when the mor?phing wings are deflected into different angles, respectively. It can be seen that the maximum output power of 275 μW is achieved at the deflected angle of 30° and a load resist?ance of 21 MΩ. Figure 6i shows the comparisons of power generation of morphing wings with unidirectional deflection and bidirectional motions at a continuous excitation of 3 Hz and a deflected angle of 20°, indicating that the output volt?age can be further amplified by four times with bidirectional continuous operation.

5 Conclusions

In this work, we have demonstrated a hierarchical honey?comb structure?inspired electret/triboelectric nanogenerator for applications in self?powered insole plantar pressure map?ping and aircraft morphing wing energy harvesting. The pro?posed h?TENG has been successfully fabricated by thermo?plastic molding and wafer?level bonding with multilayered wavy PET/AgNWs/FEP thin films. The porous honeycomb architecture of h?TENG has divided the large and single hol?low space into numerous energy generation cells, increasing the overall capacitance changes even at tiny displacement and amplifying the output performance. A corona discharg?ing process has been further utilized to maximize the thin film electret’s surface charge density. Therefore, excellent energy conversion performance has been readily obtained due to the synergized merits of these two strategies. With hand tapping excitations, an instantaneous open?circuit volt?age and output power of up to 1207 V and 12.4 mW has been achieved, respectively, corresponding to a remarkable peak power density of 0.275 mW cm?3and 2.48 mW g?1.

The unique honeycomb?inspired architecture and elas?tic material property make the h?TENG device capable of self?rebounding to its original state without any auxiliary resilient supports, making the whole device simple, light?weight and compact. Therefore, the flexible and transparent h?TENG has been further applied to wearable devices for real?time insole plantar pressure mapping and instantane?ous tactile imaging. Further investigation has demonstrated its applicability of replacing the honeycomb cellular meso?structure of UAVs for converting flapping wing motions into electricity for the first time. The results of this research have demonstrated the versatility and viability of proposed h?TENG to realize a variety of self?powered and energy har?vesting applications.

Authors’ ContributionsKT, JW, and WY: Conceptualization, Supervision, Funding acquisition and Writing?original draft; ZC, HY, RZ and QS: Experiment, Device fabrication and characteriza?tion; LT, KF, JM and YF: Formal analysis, Data curation, Writing?review and editing. All authors provided input into the paper.

FundingThis research is supported by the National Natural Sci?ence Foundation of China Grant (Nos. 51705429 & 61801525), the Fundamental Research Funds for the Central Universities, Guang?dong Natural Science Funds Grant (2018A030313400), Space Sci?ence and Technology Foundation, 111 Project No. B13044, UK Engineering and Physical Sciences Research Council (EPSRC) for support under grant EP/P018998/1, Newton Mobility Grant (IE161019) through Royal Society.

Open AccessThis article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Com?mons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Com?mons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http:// creat iveco mmons. org/ licen ses/ by/4. 0/.

Supplementary InformationThe online version contains supplementary material available at https:// doi. org/ 10. 1007/ s40820? 021? 00644?0.