All-direction energy harvester based on nano/micro fibers as flexible and stretchable sensors for human motion detection

Abstract

Highly flexible smart sensors for monitoring human body motion, physiologically and biomechanically, play a paramount role for personalized healthcare. Unlike conventional silicon based devices, stretchable electronic materials are particularly desirable. Here, we demonstrated a nano/micro fibers (NMFs) based generator via a simple, cost effective method by using direct-write, near-field electrospinning (NFES) technique and polyvinylidene fluoride (PVDF) NMFs as source materials. The novelty of this paper is to propose the novel electrode configuration and device layout such that the mechanical deformation from all directions can be harvested and validated experimentally. Unlike the traditional energy harvesters can only harvest energy in a certain movement, the unique characteristic of our device shows a great potential in sustainably scavenging tiny motion without any restriction. The maximum output voltage from the four-layer stacked power generators (PGs) with serial connections reached 20 V, and the maximum output current from the parallel integration can exceed 110 nA. The PVDF NMFs was firmly glued on the skin to detect and harvest the wrist joint movement and the output voltage was ∼1 and ∼0.8 V under the wrist bending and torsional motion respectively. We believe the novel structure of our NMFs-based device is promising in the field of wearable electronics and eventually achieve all-direction flexible energy harvester with very little obstruction.

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Introduction

A self-sustained nano/microsystem serves as a wireless sensor node is a profound vision for future applications. In particular, stand-alone environmental monitoring in remote areas and ubiquitous communicating capability of smart home and factory automation are highly pursued to assemble the crucial functions of sensing, monitoring, and communicating simultaneously. The key driving forces for the recent development of nanogenerators (NGs) provide a platform for harvesting tiny amounts of mechanical energy. The ground-breaking contribution of the first piezoelectric zinc oxide (ZnO) nanowires (NWs) arrays was demonstrated to convert nanoscale mechanical energy into electrical energy.¹

Piezoelectric materials and piezoelectric effect have a long application history in sensing pressure, acceleration, strain, or force.² The traditional vibration driven energy harvesters^3–6 utilized various forms of vibrations to generate power, resonance is the key to increase the output. Another interesting application with no resonant limitation is wind-driven energy harvesters^7–9 have been reported using piezoelectric cantilevers for a self-sustained flow-sensing system. In particular, the development of piezoelectric-type NGs is progressing dramatically in recent years due to the advent of nano era. In energy harvesting principle of ZnO NGs as subject to elastic straining, the piezoelectric potential created in a NW will drive the flow of charge carriers.¹ To name few piezoelectric NGs based devices, for example, a vertically aligned ZnO NW array with a zigzag electrode,¹⁰ paired nanobrushes made of pyramid-shaped metalcoated ZnO nanotip (NTP) arrays and hexagonal-prism-shaped ZnO NW array,¹¹ radially grown ZnO around textile fibers by brushing the NWs bundles with respect to each other,¹² a laterally positioned fine ZnO wires.¹³ The integration of large numbers of NW energy harvesters into a single power source is one possibility to increase the output. The reported accomplishment recorded so far include a lateral integration of 700 rows of ZnO NWs,¹⁴ with a scalable sweeping-printing¹⁵ and dispersing the as-grown ZnO NWs onto a flat polymer film.¹⁶ Another piezoelectric material suitable for NWs configuration is PbZr_xTi1_−xO₃ (PZT) and a single array of PZT NWs had been synthesized before.¹⁷

The most recent and emerging types of NGs can be classified as triboelectric nanogenerator (TENG). A very rapid improvement with significant output has been reported in these three years and the triboelectric effect has proven to be able to scavenge irregular vibrations, sliding, and rotations.^18–21 Based on the sliding-mode and contact-mode of operation, TENG is demonstrated both energy scavenging and self-powered sensor applications.^22–24 For example, a polytetrafluoroethylene–Al based TENG was used for enhancing the photodegradation of methyl orange.²⁵ In addition, dual purposes TENG as a sustainable wind energy harvester and as a self-powered wind vector sensor system for wind speed and direction detection²⁶ as well as charging commercial electronics²⁷ were developed. A self-powered TENG enabled photodetector with 3D dendritic TiO₂ nanostructures are synthesized as the built-in UV photodetector.²⁸ A spherical three-dimensional TENG with a single electrode was proposed and constructed as an outer transparent shell and an inner polyfluoroalkoxy ball.²⁹ Human skin based TENG is a very interesting development such that both biomechanical energy can be harvested and a self-powered tactile sensor system was demonstrated.³⁰

In order to achieve a sustainable operation, an integration of nanodevices, functional components, and a power source to build self-powered nanosystem is a future direction of nanotechnology.^31,32 A huge impact of above mentioned NGs and harvesting devices rekindled the self-powered system for long distance wireless data transmission,³³ and self-sustained wireless micro-sensor nodes without using battery.³⁴ One promising route to realize self-powered system lies in the development of soft substrate^35,36 which both flexibility and stretchability are of paramount importance. Promising human-friendly devices that can be embedded into clothes, garments or even skin had been exploited for wearable devices.^37,38 One distinctive clothing-integrated devices offers an alternative for repeatable and sharable sensor³⁹ such as eyeball motion tracking paved a new way in monitoring sleeping behavior, brain activities, and any biological associated skin deformation.⁴⁰

One-dimensional (1D) nanomaterials of polymers attract a lot of attention due to very promising applications. Electrospinning provides a simple yet highly effective fabrication route to produce NFs. The working principle is to use sufficiently electrostatic forces to deform the polymer meniscus and form a Taylor cone with conical shape. Many material systems mentioned above can be electrospun continuously as a fast and efficient fabrication process of 1D nanomaterials.^41–43 Polyvinylidene fluoride (PVDF) is another alternative material for piezoelectricity and NFs with diameters ranging from 70 to 400 nm produced by electrospinning.⁴⁴ A direct-write piezoelectric PVDF NG using near-field electrospinning (NFES) was reported to convert tiny mechanical strain into electricity for energy harvesting or strain sensing applications.⁴⁵ For a single NF, very high energy conversion efficiency was reported.⁴⁶ NFES was also demonstrated a direct-write, in situ poled PVDF power generator and fully encapsulated on a flexible substrate.⁴⁷

Experiment

In this work, we introduced an innovative NMFs based PGs for effectively scavenging the intermittent biomechanical energy from various human motions such as palm and wrist movements. The novelty of this work is demonstrated via the novel electrode configuration which can harvest all-directions, in particular, biomechanical energy. Moreover, incorporating the unique properties of in situ mechanical stretching and electrical poling of NFES technique, the successful scaling-up of the piezoelectric potentials with minimal effort and relatively easy integration of PGs can be achieved.

Fig. 1(a) shows the schematic diagram of the in situ electrical poling direct-write and near-field electrospun PVDF fibers with mechanical stretching and in situ electrical poling steps. The novelty of this paper is to propose the novel electrode configuration which can accept the mechanical deformation in any direction. Electrical voltage and current superposition in the serial/parallel configuration can be easily achieved via the adjustment of the electrode numbers and gap distance. The mode of the deformation and the scaling-up voltage and current due to superposition for the electrospun NMFs are illustrated in inset such that the integrated large array energy harvesting device is readily feasible. The working principle is briefly described as follows. When mechanical deformation is induced on the substrate, tensile strain and a corresponding piezoelectric potential in the NMFs can be created, where the “±” signs indicate the polarity of the local piezoelectric potential created in situ inside the NMFs. Fig. 1(b) shows schematic of cross section of the PG, illustrating the PVDF NMFs are embedded between two flexible substrates. Fig. 1(c) shows a total of about 500 parallel NMFs were electrospun on top of the 100 metallic electrodes. The working gaps were designed at 120 μm between two electrodes. When dynamic strain/stress was applied by bending the flexible substrate, there were equivalently 50 000 active working contacts to collect charges generated from these PVDF NMFs. The as-spun PVDF NMFs have average diameters ranging from 0.9–2.3 μm. Fig. 1(d) shows the SEM images of three PVDF NMFs fabricated via direct-write NFES processing conditions. Due to the spinnability of PVDF solution, the continuous deposition of PVDF NMFs poses a severe challenge and a very narrow operating region was identified before, at the sacrifice of diameter variation of NMFs.⁴⁸ Fig. 1(e) shows the histogram of calculated diameters for the direct-write NMFs due to the trade-offs between the spinnability, continuous deposition and the controllability of NMFs diameter. The size of the fabricated PG was 25 mm × 25 mm × 0.65 mm. Encapsulation of polydimethylsiloxane (PDMS) was applied to ensure structural stability. Fig. 1(f) illustrates a photograph of the fabricated PG film, where the highly flexible and robust NMFs are demonstrated. Fig. 1(g) shows another optical image of the twisted super-flexible PG, showing the all-direction and super-flexible harvested capability. Material characterizations of electrospun PVDF nano/microfibers include FTIR (Fourier transform infrared) and DSC (differential scanning calorimetry) as well as comparison with conventional electrospinning process were presented in details in ESI Fig. S1 and S2.†

Fig. 1 The direct-write technique by means of NFES to enable the in situ mechanical stretching and simultaneously electrical poling (a) schematic diagram of the in situ electrical poling of PVDF fibers with mechanical stretching to align piezoelectric dipoles and construct PG onto a flexible substrate. The novelty lies in the design of electrode configuration such that the mechanical deformation in any direction can be harvested. (b) Schematic of cross section of the PG. (c) Optical microscope images of electrospun PVDF NMFs. (d) Enlarged SEM photomicrograph showing NMFs with diameter of 0.9, 1.6 and 2.3 μm. (e) Histogram of calculated diameters for the direct-write NMFs. (f) Photograph of the fabricated device showing highly flexible and robust NMFs. (g) Photograph of the twisted PG showing the all-direction and super-flexible harvested capability.

For the effect of flapping motion on different actuating directions and related voltage output measurements, Fig. 2(a) shows the experimental setup. The output voltage and current versus cyclic stretching-releasing deformation at a strain of 0.5% and 6 Hz can be obtained by a commercially available DC motor (RS-545SH). Voltage and current measurements were performed using an oscilloscope (model DSO1014A) and an electrochemical analyzer (model CHI611E). The induced strain can be varied via the adjustment of tip displacement while the actuating frequency can be easily tuned by the DC motor speed. The experimental results were obtained by measuring approximately 50 000 rows of NMFs arrays, respectively. Then a cyclic stretching-releasing deformation at a strain of 0.5% and 6 Hz by a linear motor was used to periodically deform the electrospun PG in different flapping directions. For example, Fig. 2(b) and (c) presents the obtained output peak voltage of ∼10 V with the corresponding output peak current of ∼40 nA when the PG was flapping by a liner motor at 0° and 90° flapping angles respectively, both flapping directions resulting in an equivalent magnitude of output power ∼0.4 μW. Fig. 2(d) presents the obtained output peak voltage of ∼15 V with the corresponding output peak current of ∼40 nA when the PG was flapping at 45° flapping angle, resulting in an increased output power of about 0.6 μW. Furthermore, the control experiment is designed and validated to exclude the triboelectric effect in ESI Fig. S3–S6.† The increased output power may be attributed to the deformation magnitude such that when flapping at 0°/90°, only two main electrodes are experiencing the strain. On the other hand, four main electrodes connect serially as two long beams can simultaneously receive strain at various magnitudes when flapping at 45°. Furthermore, the unique feature of novel electrode configuration which can harvest the mechanical deformation in any direction, which makes the energy harvesting more flexible without any limitation on movement direction. We also perform the crucial polarity test under forward and reverse connections and the purpose of the flip of polarity is to validate the signals are from authentically piezoelectric output, as details presented in ESI Fig. S7.†

Fig. 2 (a) Schematic of the experimental setup. Measured output voltage and current of the flapping PG at (b) 0° (c) 90° (d) 45° directions, respectively. Operating frequency is 6 Hz and strain is fixed at 0.5%.

Human motion enabled self-powered, wearable electronics such as walking and muscle movement are very promising and ubiquitously pursued.^49,50 Here we demonstrate the NMFs-based PG which can be easily deposited on any flexible substrate⁴⁸ at a specific location with predefined electrodes and attached onto a human palm for scavenging the human movement. The encapsulated PDMS protecting layer is employed to prevent any undesired electrical noise from direct contact of the device with human skin. Fig. 3(b) shows a firmly attached NMFs-based device on a palm and concurrently experiencing strain from the folding and releasing actions of the palm. Our NMFs-based device exhibited the voltage outputs of 6 ∼ 8 V at an angle of 180°, truthfully in-synchronized with the moving action of the human palm, as shown in Fig. 3(a). The average/error bar indicates the maximum and minimum voltages obtained at a different bent angle as plotted in Fig. 3(c). As expected, the output voltage of the PG increases rather steadily as the bending angle increases. This super-flexibility and robust NMFs device is capable of completely contracting at 180°in a repeatable cyclic motion, which exceeds the deformation limits for conventional strain sensors. Similarly, a current output appeared on the event of the folding and releasing cyclic action and resultant compressive/tensile stress is also presented in ESI Fig. S8.†

Fig. 3 Human palm-enabled PG using direct-write, in situ poled NMFs and corresponding electrical measurements. The NMFs device is direct-write on the flexible substrate with a predefined electrode pattern, fully encapsulated with PDMS and attached on the palm. Experimentally, a uniform stress is induced as the palm folds and releases at various angles during the measurements. (a) Multiple events of palm folding-releasing actions and corresponding voltage outputs of a NMFs device at a frequency of approximately 2 Hz. (b) A voltage output appeared on the event of the folding and releasing action of the palm held at an angle of ∼180° and 2 Hz. (c) Palm joint stretching property. Voltage versus angle of palm joints. Error bars: standard deviation.

According to the basic principle of superposition, the performance of output voltage/current should be proportionally enhanced by the number of electrode pairs assembled in series/parallel configurations as shown in Fig. 4(b–d). Therefore, the potential superposition gain of the as-developed PG was evaluated by integrating two and four devices respectively. The generated current of a single PG can reach 40 nA as shown in Fig. 4(a). By integrating two and four PGs in parallel connection, the generated current can exceed 60 nA and110 nA respectively. These data clearly demonstrate that the generated current can be enhanced by integrating different PGs in parallel connection modes. The reason for not exactly add up of current values can be partially attributed to the non-uniform deformation of each PG as palm induced motion and individually PG performance variation, which previously studies also presented similar results.^11,45 Voltage superposition can be similarly demonstrated and presented in ESI Fig. S9.†

Fig. 4 A largely enhanced electrical output PG device integrated in parallel for current. (a) Demonstration of the integrated PG to the feasibility of scale-up for enhancing the output current. The number of integrated layers in the parallel configurations is experimentally demonstrated to have a direct impact on the output current. Output current of PGs integrated in parallel configuration, respectively with (b) one layer, (c) two layers, and (d) four layers, at a frequency of 3 Hz.

Fig. 5 shows the responses of the PG under various applied stresses in the range of 0.5 to 2.5 MPa, for the same applied external strain of 0.5%. In the experimental setup, the cycling frequency can be directly related to the applied stress since the linear motor controls the stretch–release cycle for a given applied strain. Measurement results show that the output voltages increase proportionally to the increase of the cycling frequency (applied stress) during both the stretch and release cycles. We measured the average output of peak voltages for applied stress of 0.5, 0.75, 1, 1.25, 1.5 MPa to be approximately 2, 4, 5, 6.5 and 10 V, respectively. The increase of a pressing force acting on the NMFs will directly result in larger deformation, as well as the linear scale up of output voltages. Current frequency effect can be directly demonstrated to be proportional to the strain rate (cycling frequency) and presented in ESI Fig. S10.†

Fig. 5 The responses of the PG under various cycling frequencies ranging from 1.5 to 5 Hz for the same applied external strain of 0.5%. In the experimental setup, the cycling frequency can be directly related to the applied stress since the linear motor controls the stretch–release cycle for a given applied strain. Measurement results show that the output voltage increase proportionally to the increase of the applied stress during both the stretch and release cycles.

In order to ensure the practical applications of proposed NMFs based device, the stability of the PG is of great importance and therefore, the PG was tested continuously for 3 days at frequency of 5 Hz. Output voltage and current of a PVDF PG was experimentally operated under 5 Hz of continuous cycles of stretching and releasing for 3 days, demonstrating the stability of the PG. The PG was continuously run each day for 10 min are shown in Fig. 6(a) and (b), and only a small variation of peak output current is seen, indicating the highly stable power generation of the PG.

Fig. 6 Three days continuous output (a) voltage and (b) current of the PG operating operated at 5 Hz and strain 0.5%.

The PVDF NMFs was implemented to detect and harvest the wrist joint movement. As the wrist joint performs the cyclic bending motion in one direction and simultaneously swivel on its axis, the underlying skin also experiences the constantly varying deformation. Similar case can be applied on the twisting motion with orthogonally crossed swivel axis. In order to demonstrate the high sensitivity and high conformability of proposed super-flexible device, and PG was firmly glued on the skin with double side Scotch tape next to a human wrist, as shown in Fig. 7(a). Simply driven by bending and twisting wrist, the attached NMFs device experiences constantly compressive and tensile stress during movement and Fig. 7(b) and (c) shows the output voltage was about 1 and 0.8 V under the bending and torsional motion respectively, and the other wrist was presented in details in ESI Fig. S11.†

Fig. 7 Performance of the highly flexible PG as an active sensor under the skin and operated by human motion movement. Photographic image of the PG attached to the wrist joint; two motions of bending and twisting are demonstrated in (a) initial state, bent, and unbent to the initial state. Initial state, twisted, and unbent to the initial state. (b) Output voltage of the PG driven by bending wrist. (c) Output voltage of the PG driven by torsion wrist.

Conclusion

The novelty of this paper is to propose the novel electrode configuration which can harvest the mechanical deformation in any direction. In addition, utilizing the NFES direct-write technique to massively align piezoelectric PVDF NFs on flexible substrates have been successfully fabricated such that the structurally robust, reproducibly scalable, substrate-independent⁴⁸ and large-area PGs are experimentally validated. Moreover, incorporating the unique properties of in situ mechanical stretching and electrical poling of NFES technique can guarantee the successful scaling-up of the piezoelectric potentials with minimal effort and relatively easy integration of PGs in parallel and/or series connections for enhancement of output current and voltage. The PGs that comprised ∼50 000 rows of well-aligned PVDF NMFs are capable of producing a peak output voltage of ∼10 V and a current reaching ∼40 nA. Also demonstrated in superpositioning four-layers integrated PGs, a highly promising and feasible power source for wearable electronics can reach the maximum output voltage and current up to 20 V and 110 nA, respectively.

Finally, the demonstrated devices are capable of quantitatively detecting human motion as self-powered active sensors, extracting energy from biomechanical motions as PG and paving the foundation as the smart garment. Compared to the wearable systems commercially available, the presented devices were simple in fabrication and easily implemented to enable human-friendly devices. We believe that such devices could be functionally integrated as part of human skin and ubiquitously deployed to be beneficial to the health care applications.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra00275c

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