All-fiber pyro- and piezo-electric nanogenerator for IoT based self-powered health-care monitoring

An all-fiber pyro- and piezo-electric nanogenerator for IoT based self-powered health-care monitoring has been presented.


Introduction
Recently, "Internet of Things (IoT)" gained considerable attention in numerous applications, in remote health care monitoring systems, specially in infectious disease (e.g. pneumonia, . 1,2,3 An ultra-sensitive pressure-temperature dual functional pressure sensor is the main component of the stat-of-the-art IoT based remote health care system. It is used to collect the physiological signals from different parts of our body that behave as a transducer by converting an applied force and temperature fluctuations into an electrical signal, or other perceived signal output. 4,5 These sensors can be adhered into our regularly used textiles or may be attached on the human skin for real-time monitoring of human health, activities and physiological signal measurements. In real-life application the textile based self-powered sensor is an ideal candidate as it is ultra-flexible, light weight and amenable to any surface of the body as well as no external bias is required to operate it. In this scenario, piezoelectric and pyroelectric materials are ideal candidate as it required no external bias to operate. Conventionally, very highly sensitive pressure sensors are witnessed by piezo-resistive and capacitive mechanism which needs external bias electric field. This essential requirement restricts the convenient utilization and integration of the sensors with the wireless communications systems. In terms of mechanical energy harvesting approach triboelectric nanogenerators are ideal candidate for higher power output generations 6 , but piezoelectric nanogenerators are essential device for higher stability and lower pressure sensing application, specifically physiological signal monitoring. To date, despite of brittle (low toughness) and rigid (high stiffness) in nature, a large number of inorganic and lead based materials have been applied to fabricate piezo-and pyro-electric nanogenerators. 7 In contrast the organic polymer based piezoelectric and pyroelectric materials are of particular interest towards electronic skin because of their high compliance and flexibility, light weight nature and bio-compatibility. PVDF is one of the most favorable ferroelectric polymer classes. 8 5 Among the five crystallographic forms =O " P " Q " R and S>" the P + is most suitable form for piezoelectric and pyroelectric properties of PVDF. There are several well-known methods available to attain high P + content, including the electrical poling method, where high electric field (typically 100 kV/mm) is applied. 9 The electrical poling strategies can be overcome using the electrospinning method where the introduction of the P + and the in situ alignment of dipoles occur concurrently due to the nano-confinement effect in electrospun nanofiber. In addition, the electrospinning process enables the preparation of flexible and ultra-light piezoelectric membranes, making them more applicable in flexible self-powered sensors for human health detection or monitoring. However, the performance of electrospun PVDF nanofiber remains relatively low due to the lack of suitable device engineering, which leads to a small output power when used in energy harvesting applications. In this scenario, a large number of researchers have used different types of filler, especially; MWCNTs to improve the performance or efficiency of PVDF electrospun fiber based nanogenerators. [10][11][12] The addition of MWCNTs to the PVDF significantly improved output responses for nanogenerators. From a materials point of view, the feasibility of MWCNTs as a filler material lies in its structural appearance, which made up of different coaxial cylinders composed of a single layer of graphene around the cylindrical inner hole. It is noticed that a huge numbers of articles are reported on PVDF nanofibers based nanogenerators, but temperature-pressure dual functional all-fiber based wearable sensor was hardly reported. The metal foil or metal-coated thin film-based electrodes for charge collection both are having limitations in the device lifetime under the prolonged cyclic stress. 13 There were few more limitations found which includes the poor fatigue resistance causes early failure of the metal foils, huge mismatch of Young's modulus of the metal coating electrode and the active thin film. However, due to prolonged period of application, loss of mechanical integrity and electric connectivity occurred. Now all these issues are being 6 avoided from the present state of devices by adopting an efficient and durable all-fiber nanogenerator of a three-layered structure where both piezo-and pyro-electric active constituent and electrodes are composed of adaptable and soft fiber arrays.
In this work, a notable enhancement of electrocactive phase content in electrospun PVDF nanofiber has been attained by synergistic effect of interfacial interactions of MWCNTs with PVDF chain and further mechanical stretching in the course of collection of fiber by high speed rotating collector. As a result very high piezoelectric/pyroelectric coefficient, figure of merit and energy harvesting output power with high power conversion efficiency has been achieved. A highly sensitive all-fiber nanogenerator is designed with PVDF-MWCNTs nanofibers which could capture very low level temperature fluctuation and possesses very high pressure sensitivity. With such effective performance, an IoT based wireless health care system is developed through which the physiological signals are possible to transfer to the smart phone, indicating a promising way of real time remote health care monitoring.

Results and discussion:
All-fiber Nanogenerator/Sensor Fabrication Strategy: The copper (Cu)-nickel (Ni) plated interlocked micro-fiber based polyester fabric was used as top and bottom electrodes to fabricate the piezoelectric and pyroelectric nanogenerator (PPNG). Conducting copper wires were attached on both electrodes and finally the three layered structure PPNG was covered with a PDMS layer. The schematic diagram of the allfiber nanogenerator is shown in Fig. 1a. The generated electrospun nanofibers are shown in Fig. 1a(i). The all-fiber nanogenerator consists of PVDF-MWCNT electrospun nanofibers ( Fig. 1a(ii)) sandwiched between interlocking micro-fiber arrays of conducting fabric as charge collecting electrodes ( Fig. 1a(iii)). The device was further encapsulated with PDMS for protection from environment and to ensure compact structure of the device. The diameter of the interlocked micro-fiber of the conducting electrode was of 15 W%< The excellent 7 flexibility and conformability of the generated electrospun nanofibers to human finger is shown in the Fig. 1b.The digital image of fabricated original PPNG (shown in Fig. S1, ESI).
Thus, the PPNG fabricated by the active piezoelectric and pyroelectric component with the electrodes, all are composed of fibers.

Structural and Electrical Properties
The FE-SEM image with the corresponding histogram profiles of PVDF-MWCNT nanofibers is shown in Fig. 2a. The fiber diameter distribution plot is presented in the inset of Fig. 2a. It was observed that the randomly oriented nanofibers are formed and the average diameters of the fibers are ranging from 40-165 nm, while the maximum number of fibers possesses a diameter of ~ 85 nm. During the formation of fibers i.e., electrospinning process, the molecular dipoles of PVDF nanofibers are preferentially oriented in the out-of-plane direction since the nanofibers and polymer chains are aligned within the plane of the collector. 14 This is due to the simultaneous application of electric field and mechanical stretching leading to polymer jet elongation, whipping and in situ poling. 15 During electrospinning of the MWCNTs mixed PVDF solution, the conductive MWCNTs can produce inductive charges on its surface when the external electric field is applied, which results to a greater Coulomb force during the electrospinning process. This effect helps to improve piezoelectric and pyroelectric properties of electrospun PVDF-MWCNTs nanofibers due to excellent interfacial interaction between the PVDF polymer chain and MWCNT surface charges at nanoscopic scale. Therefore, a synergistic effect occurred between interfacial interaction and mechanical stretching during collection of fiber by high speed rotating collector during crystallization of fibers. As a result, the content of electroactive P phase was much higher in PVDF-MWCNT nanofibers than that of the neat (pure) PVDF nanofibers and this is evident from the FT-IR plot, see cm -1 respectively). 9,16 However, the PVDF-MWCNT nanofibers contain only polar P and Q phases and no O + peak were available in spectrum, as seen in the inset of Fig. 2b. This implies that the conversion of non-polar phases =O + > into polar phases =P " Q + > in PVDF-MWCNT nanofibers in comparison to neat PVDF nanofibers by electrospinning process. Eventually, it is found that both the nanofibers having 841 cm -1 vibrational band, which presenting the P and Q + concurrently.
Assuming that the infrared absorption follows the Beer-Lambert law and relative proportion of electroactive P " Q + phases (defined as F EA ) is calculated using equation (1) as, where, A 762 and A 841 represent the absorbencies at 762 and 841 cm -1 respectively, K 762 =6.1×10 4 cm 2 mol -1 and K 841 =7.7×10 4 cm 2 mol -1 representing the absorbance coefficient corresponding to wave numbers. 9,17,18 Therefore, the calculated F EA of neat PVDF nanofibers is F EA ~ 67 %, whereas in the case of PVDF-MWCNT nanofibers, F EA ~ 87 %.
Additionally, Fig. 2c shows the curve-deconvoluted XRD patterns of the PVDF-MWCNT and Neat PVDF nanofibers. As evident, the diffraction peak at 3Z of 19 Fig. 2b and Fig. 2c), which is due to the addition of MWCNT in the PVDF matrix. The conversion of non-polar to polar phase is carried way due to two factors: and are the summation of integral area of crystalline peaks and amorphous halo + , *-+ , .-respectively. Due to addition of MWCNT, the total crstallinity of PVDF-MWCNT nanofibers is higher (59%) than that of neat PVDF (53%). The crystallization along with thermal stabilization of P + in PVDF-MWCNTs was evaluated by differential scanning calorimetry (DSC) study, as shown in Fig. 2d.
It can be seen that both the melting peaks (t m ) of two types of nanofibers indicate the existence of P Q + < The endothermic melting peak of the P and Q + is found in PVDF-MWCNT nanofibers at (~ 168 °C) and (~179 °C) respectively. 16 , [19][20][21] For the neat PVDF nanofibers, it is found that the melting peak of OJP + is (~ 160 °C). 16,[19][20][21] As a result, both the P Q + are considered more stable thermally in the PVDF-MWCNT nanofibers with respect to neat PVDF nanofibers. As a consequence, the crystallization temperature (t c ) of PVDF-MWCNT nanofibers (~ 148 °C) was also enhanced with respect to the neat PVDF (~ 141 °C). This is due to the MWCNTs acting as a nucleating agent and hinders the movement of polymer chain segments. Moreover, the induced polar P phase is stabilised during rapid crystallization and not relaxed back to non-polar O + via thermal motion. As a result, the formation of the crystalline P + is irreversible in the PVDF-MWCNT nanofibers and not relaxed back to O + via thermal motion. 22 Noteworthy that in the case of free standing film sample containing of gamma phase, the melting points shows relatively much higher side. 19 In contrast, electrospun samples act a bit differently it may be because of air-permeable spongy like behaviour that leading to lower melting temperature than the right film specimen. In addition, from the DSC data the total degree of crystallinity =\ ct ) of the nanofibers was calculated using, where, represents the melting enthalpy, is the melting enthalpy of the 100 % 01 . 01 0 . crystalline PVDF (~ 104.7 Jg -1 ) and is the weight fraction of MWCNT. 22 The value of 2 ) */ =40% and 33% for PVDF-MWCNT and neat PVDF nanofibers respectively. In overall, the origin of improved electroactive phase content and improved degree of crystallinity and higher thermal stabilization is the synergistic effect. Therefore, it is concluded that the degree of crystallinity of PVDF-MWCNT nanofibers is improved due to the presence of MWCNT.
From materials point of view, it is very evident that PVDF-MWCNTs nanofibers are proven to be an excellent material for the application of piezo-and pyro-electric nanogenerator. Of particular interest, interfacial interaction between the MWCNT NPs and PVDF dipoles is schematically shown in Fig. 2e. Here, MWCNT mainly acts as a nucleating agent. During the electrospinning process, charge is accumulated at the interfacial boundaries between MWCNT and non-polar O phase and ferroelectric P phase. 23 In addition, there might be present dipolar intermolecular interaction between G # of MWCNT and -CH 2 -dipoles of PVDF chain. So, inter-conversion of O to P phase near the MWCNT surface can be described by the charge accumulation at the interphase and dipolar interaction. Moreover, the induced polar P phase is stabilised during rapid crystallization and not relaxed back to nonpolar O + via any thermal motion.

Pyroelectric Energy Harvesting
The harvesting of thermal energy from the all-fiber PPNG is obtained by pyroelectric effect.
The temperature fluctuations is maintained using an IR light source placed on the top surface of the PPNG. The pyroelectric energy harvesting performance of the PPNG is shown in is the rate of temperature change (K/s) and is the pyroelectric [24][25][26] This enhancement of I sc was accomplished by a thermally induced piezoelectric coupling effect, as the output performance is proportional to I and dT/dt. The available maximum power density (P) of the PPNG is estimated to be ~12nWm -2 when temperature difference between the electrodes exist I ~ 14.3 K. Using equation 4, the estimated pyroelectric coefficient of the PPNG is 60 nCm -2 K -1 , which is almost fifteen ( times higher than the neat PVDF fiber (4 nCm -2 K -1 ). 24 The I sc and V oc are found to be linearly proportional to dT/dt and I profiles, respectively (Fig. 3e,f) in reliability with theoretical paradigm. These linear characteristics suggest the potential of the PPNG as a self-powered pyroelectric temperature sensor for future practical applications especially in various infectious diseases. Fig. 3g shows the expanded view of positive current pulse signal of the rectangular red dotted marked cycle of Fig. 3b. It has been observed that the current increases from zero to its peak value within ~1.48 s at positive side while in negative side it takes ~ 1.54 s (inset of Fig. 3g). As soon as the heat source is removed, the positive output current is returned back to a minimum value. Fig. 3h represents the "I sc vs T" curve where it is observed that the current pulse is decreasing exponentially with a time constant value of 2.60 s. This decaying process corresponds to a temperature reduction of the PPNG. The value of the reset time for the PPNG is estimated as the time required recovering to 1/e or (37%) and is approximately 2.60 s. So, it is noteworthy to mention that the PPNG can be used as a good temperature sensor due to its reduced response time and reset time than the previously reported thermoelectric sensor. 27 The better pyroelectric performance in PVDF-MWCNT nanofibers is ascribed to the infrared transparency (~ 5%) 28 and improve absorption property of electromagnetic energy by MWCNT which generated rapid heating in the nanofibers. When the device is exposed under heat (i.e. dT/dt > 0) the short circuited current (I sc ) gets its positive peak and a negative peak  Table S1 (ESI). It has been observed that in spite of lower pyroelectric co-efficient of PVDF-MWCNT nanofibers in many cases the output performance, in terms of output voltage and current is much higher than that of previous reports.

Piezoelectric Energy Harvesting
To 13 35 V respectively which shows more than 90% improvement in output voltage under magnitude of compressive stress b a =2 kPa over the neat PVDF based nanogenerator (see Fig.   S2, ESI). In addition, the press and release response of single peak from PPNG is shown in the right lower inset of Fig. 4a and it is found that the PPNG shows ultra-fast response time of ~10 ms. It is important to note that generated V oc varies almost linearly with the increase of b a (right upper inset of Fig. 4a) which is consistent with the piezoelectric theory. ) and other various state-of-the-art PVDF-based nanofibres. 22 The 9.7 × 10 12 1 improvement in piezoelectric coefficient with the addition of MWCNTs is due to high To determine the input mechanical energy (E mec ) we have to consider the total axial deformation of the PPNG under 6 kPa stress as, . Here, S is the developed axial L = @ × L strain and is the total thickness of the device. The generated axial strain is given by, E 9 @ 16 Thus, the total input mechanical energy per cycle is calculated by, < . * = × 9 = 0.63 × J where F is the applied force. 10 6 The instantaneous piezoelectric energy conversion efficiency of the PPNG can now be written as, . = < = * < . * × 100 = 19.3% Finally it is found that the instantaneous piezoelectric energy conversion efficiency of the PPNG is superior to that of previously reported works. 22,41 A comparison of device materials, electrode materials and percentage of piezoelectric energy conversion efficiency (% of ) the PPNG with the reported nanogenerators is shown in Table S3, (ESI) where in many cases the % of our developed device is much higher than that of previous works.
A summary or comparison of device materials, electrode materials, and output performance of the PPNG with the reported nanogenerators is shown in Table S4, (ESI) where in many cases the output power of our developed device is much higher than that of previous works.
Importantly, previous research on MWCNT reinforced PVDF composites didn't explore the temperature and pressure dual functionality in the device form which is the one of main focus of this research. Table S5, (ESI) clearly indicates the uniqueness of this work in terms of both thermal and mechanical energy harvesting over previous research.

Healthcare Monitoring System
In this section PPNG is being used as a self-powered sensor. Here PPNG was attached on the human skin to detect slight movement of the palm, wrist, arm muscle, leg muscle, throat (coughing and laughing situation) and different voltage responses are shown in the Fig. 5a-f and corresponding digital picture during the test is shown in the inset. The responding positive and negative voltages generated by palm and wrist bending and releasing are shown in Fig. 5a,b which are mainly caused by the deformation of epidermis during wrist movements. Fig. 5c,d are shown the output voltage response to the arm and leg muscle movement. Furthermore, the PPNG could transduce the vocal cord vibrations (Fig. 5e,f) to the coughing and laughing situation when the device is attached to the throat. Fig. 5g,h presented the Fast Fourier Transform (FFT) of the coughing and laughing signal (shown in Fig. 5e,f). The FFT treated cough acoustic frequency spectrum 200-380 Hz and laughing acoustic frequency spectrum 126-424 Hz is presented respectively which is good agreement with the healthy people coughing and laughing sound. 22,[42][43] Therefore, the PPNG can be used as self-powered sensor to monitor and separate virus infected patients from normal humans. It is noteworthy to mention that the PPNG can be used for self-powered humanmotion detection as it exhibited a good repeatability and high responsivity to strain variation caused by the joint/muscle movements without external power supply. Also this sensory information from skin attachable/wearable PPNG is beneficial for body movement analysis during sports activities and shows the potential of using as epidermal device for skin motion monitoring. 44

Remote Health Monitoring
Here we have demonstrated a real time practical application of remote health monitoring system which is very much important for continuous monitoring of both viruses suspected and infected patients reducing the risks of caregivers being exposed to the virus. We have System (RTOS). The clock speed is 80MHz. also it has a 802.11 b/g/n and frequency range is 2.4G-2.5G (2400M-2483.5M) system. With the help of MCU, we performed analog to digital conversion of PPNG/sensor data value for processing. The waveforms are processed and corresponding data are sent to a local server made by using ESP8266A HTTP protocol. The patient or doctor can easily check the report to access on this network through Wi-Fi/Bluetooth using their smart phones and laptops. Fig. 6c shows the real time practical circuit which shows the PPNG/sensor response under repeated finger touch in the smart phone screen through the local server for IoT based remote health care monitoring system and Fig. 6d shows the plotted sensor output graph.
Using this system clinician can monitor several physiological signals such as, heartbeat, respiration rate, body temperature, coughing signal and so on of the virus infectious/suspected patients even in quarantine situation round the clock wirelessly without the direct physical contact to the patient which simply prevent further spread of the virus. In future, envisioned strategy through non-invasive piezo-and pyro-electric-based wearable sensors will be taking place for infectious diseases and in vivo body implantable. 45,46

Conclusions
In summary, we have developed a single device platform by all-fiber nanogenerator for harvesting both the mechanical and thermal energy using PVDF-MWCNTs electrospun nanofibers as active piezo-and pyro-electric component respectively. It has been observed that the content of electroactive phases, degree of crystallinity and thermal stabilization of P ). In addition to that the PPNG shows ultra-33 /~22 9.7 × 10 12 1 fast response time of ~10 ms, high electrical throughput (output voltage 35 V and maximum power density ~ 34 µW.cm -2 ) and power conversion efficiency ( ).

~19.3%
As a result, the nanogenerator drives several consumer electronics components such as, commercial capacitors and LEDs by simple human finger imparting. In addition, we have also developed a human health care monitoring system where our developed sensor can distinguish different muscle vibration signal from different parts of our body. Finally, a wireless health care system for remote health monitoring using IoT and a Bluetooth module with an Arduino Uno is developed by which a clinician can monitor several physiological signals of the virus infected/suspected patients round the clock wirelessly without the direct physical contact to the patient which simply prevents further spread of the virus. This type of healthcare system is urgently needed for the frontline health staffs during current pandemic.
In overall, the integrated device design, portable feature, higher thermal and mechanical sensing capabilities of our developed all-fiber device could find the possible applications in artificial electronic skin, prosthetic limbs, rehabilitation programs and other artificial intelligence applications in near future to monitor dynamic tactile and strain information.

Fabrication of all fibers PPNG
The details of the fabrication process of our proposed all fibers PPNG has been discussed in section "All-Fiber Nanogenerator/Sensor Fabrication Strategy", under results and discussion part. The detail of the device structure is shown in Fig. 1 .

Conflict of interest
The authors declare no competing financial interest.