Sustainable Energy Generation from Piezoelectric Biomaterial for Non

Research Article pubs.acs.org/journal/ascecg

Sustainable Energy Generation from Piezoelectric Biomaterial for Noninvasive Physiological Signal Monitoring Sujoy Kumar Ghosh and Dipankar Mandal* Organic Nano-Piezoelectric Device Laboratory, Department of Physics, Jadavpur University, 188 Raja SC Mallick Road, Kolkata 700032, India S Supporting Information *

ABSTRACT: Sustainable development of self-powered wearable electronics relies on bioinspired piezoelectric materials which could transduce minute deformation of human skin. In this work, fish-skin-based sustainable energy production is discussed in light of structure−property correlation. The developed energy harvester acts as a sensor that interacts with human body parts to monitor real-time physiological signals. Self-assembled collagen nanofibrils comprising fish skin show stable crystalline structure and possess nonlinear electrostriction effect without any electrical poling treatment. A fishskin-based nanogenerator (FSKNG)/pressure sensor is ultrasensitive (sensitivity ∼27 mV N−1) and highly durable (over 75 000 cycles) and possesses very fast response time (∼4.9 ms). Importantly, in response to the external pressure (∼1.8 MPa), FSKNG generates open-circuit voltage, Voc ∼ 2 V, and shortcircuit current, Isc ∼ 20 nA, due to inherent piezoelectric effect. The magnitude of the generated power (∼0.75 mW m−2) turns out to be the working mode of low-power electronics (such as blue light-emitting diodes); therefore, it substitutes the requirement of an external power supply to drive a pressure sensor, as well. KEYWORDS: fish skin, collagen, piezoelectric nanogenerator, wearable sensor, sustainable energy, physiological signal monitoring

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INTRODUCTION Flexible pressure sensors are indispensable for the development of human interactive artificial electronic skin (e-skin).1 In recent years, several promising routes toward the improvement of piezoresistive,2 capacitive,3 triboelectric,4 and piezoelectric5 pressure sensors with sensitivity on par with human skin have been demonstrated. Among them, the piezoelectric transduction mechanism is a suitable choice because of its superior sensitivity, fast response time, and higher durability.1,5 However, development of a biocompatible and nontoxic multifunctional sensor satisfying the requirements of ultrahigh mechanosensitivity, flexibility, and durability remains a challenge. In general, an electronic sensor is operated by a certain amount of electrical energy which transfers the physiological signal to a central processing unit regardless of the type of sensor used (passive or active).1,5 In order to avoid the joule heating effect during power supply to a sensor by complex electrical wirings from a separate power source, a permanent solution is necessary. The piezotronic effect of an energy-harvesting device, called nanogenerator (NG), is an emerging nanotechnology that transduces universally available mechanical actuation into electrical energy.6 It is becoming more feasible for NG to act as a pressure sensor, which could allow future biomedical prostheses and robots to naturally interact with human body parts and the environment. Over the decades, imitation and inspiration from nature continuously © 2017 American Chemical Society

improve the sensitivity of e-skin. The capabilities of animal and insect skins also inspire more functions to device performance. For example, mimicking the function of chameleons’ skin was utilized to distinguish the applied pressure directly by capturing the change in device colors.7 Inspired by cephalopod skin, optoelectronic systems can help adaptive camouflage in military applications.8 Recently, electric eel skin inspired super stretchable e-skin has been developed.4 In nature, fish skin also senses extremely small variations of environmental mechanical vibration. It is composed of collagen nanofibrils, which is a biocompatible and biodegradable piezoelectric polymer. The presense of hydrogen bonding within the polypeptide chains leads to the piezoelectricity in the collagen nanofibrils.9,10 In order to orientate the dipoles, generally, electrical poling is employed. However, with substantial power expenditure, electrical breakdown failure associated with the electric poling restricts the use of piezoelectric materials in certain specific applications where desirable shapes like robotic interface, skin sensors, etc. are required.9,10 Thus, self-poled natural piezoelectric materials have attracted considerable attention due to their higher sensitivity with nontoxicity, biocompatibility, and flexibility.9−13 This is the radical advanceReceived: May 23, 2017 Revised: August 5, 2017 Published: September 4, 2017 8836

DOI: 10.1021/acssuschemeng.7b01617 ACS Sustainable Chem. Eng. 2017, 5, 8836−8843

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ACS Sustainable Chemistry & Engineering

Figure 1. (a) (i) Illustration of the fabrication process for fish-skin-based nanogenerator (FSKNG) from the raw fish skin (FSK) and (ii) digital photograph of the FSKNG showing the flexibility (scale bar ∼20 mm). The characteristics of FSK collagen is shown by (b) FE-SEM image with the histogram profile of D-periodicity in the inset, (c) FT-IR spectra, (d) dielectric spectra (frequency range of 100 Hz to 1 MHz), (e) piezoelectric longitudinal strain amplitude (S) response as a function of applied electric field (E), and (f) schematic illustration of dipolar orientation in collagen nanofibrils.

ment toward practical use of biocompatible sensors for noninvasive and continuous monitoring of human physiological signals. To the best of our knowledge, fish skin has not been demonstrated as a wearable biosensor sensor until now. Here, we present a self-powered wearable bioinspired piezoelectric pressure sensor (i.e., biopiezoelectric pressure sensor) utilizing the unprecedented piezoelectric properties of fish skin (FSK). It possesses excellent sensitivity (∼27 mV N−1) and fast response time (∼4.9 ms) to the external contact force. The developed fish-skin-based nanogenerator (FSKNG) produces 200 mV of open-circuit voltage (Voc) and 2 nA of short-circuit current (Isc) with long-term stability (75 000 cycles) under 7.5 N of contact force. In addition, it generates large Voc ∼ 2 V and Isc ∼ 20 nA under higher external pressure (∼1.8 MPa), which is further used to drive a light-emitting diode (LED) by serially connecting two FSKNGs. As a proof-

of-concept of a self-powered wearable healthcare monitoring device, it could precisely detect real-time human physiological signals such as arterial pulses, vocal cord vibration, and gentle wrist movements without any external power supply.

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EXPERIMENTAL SECTION

Preparation of Fish Skin. Biowaste fish skins (FSKs) from Catla catla fish were collected from the nearby fish processing market. Then, those were washed thoroughly with deionized water followed by descaling and demineralization process. For the demineralization process, a solvent system was prepared by mixing of NaOH pellet (Merck, India) with deionized water, and then 1.0 M NaCl (Merck, India), 0.05 M Tris HCl (Merck, India), and 20.0 mM EDTA (Merck, India) were mixed together to wash the skin. Finally, demineralized fish skin was achieved by stirring the skin in a solution of 0.5 M EDTA. The average thickness of the used FSK was 250 ± 30 μm. 8837


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single α-chain along the triple helix structure of the FSK collagen. Therefore, due to stable crystalline structure with rich hydrogen bonding network of the polypeptide chains, FSK possesses superior dielectric property (dielectric constant, εr ∼ 50, and loss tangent, tan δ ∼ 0.6 at 1 kHz) (Figure 1d) compared to other available biopolymers.14 It is mainly influenced by trapping of free charges between fibers and air pores, known as Maxwell−Wagner−Sillars interfacial polarization effect.14 In addition, other secondary processes may also contribute to εr due to the crystallinity, ionic movements, and hydrogen bonds bound to the protein structure.9,14 The nonlinear nature of εr with frequency is attributed to the ferroelectric behavior due to the relation between dielectric relaxation and polarization.14,15 The polarization (P) versus electric field (E) hysteresis loop measured at 100 Hz shows that FSK possesses the remnant polarization (Pr) of 0.12 μC cm−2 and coercive field (Ec) of 22 kV cm−1 (Supporting Information, Figure S2). The presence of Pr is of interest to achieve a high piezoelectric activity for energy-harvesting applications. In addition, electric-field-induced strain amplitude behavior (i.e., converse piezoelectricity) in the FSK was studied by symmetrical butterfly-shaped longitudinal strain amplitude (Sa) response under electric field (Figure 1e). Evidently, the origin of piezoelectricity in the FSK collagen nanofibrils is the nonlinear electrostriction effect biased by the spontaneous polarization which is originated from the polar −CONH hydrogen bonding motifs as molecular dipoles.16 Basically, type I collagen present in FSK is composed of three polypeptide chains (α-chain). Each of them, such as two identical α1(I) chains and a different α2(I) chain, consists of repeated triplet amino acid motif sequences of Gly (glycine)−X−Y, in which X and Y are generally proline (Pro) and hydroxyproline (Hyp), respectively.9,10 The possible explanantion for arising longitudinal Sa in FSK collagen nanofibrils is that application of electric-field-induced compressive Maxwell strain on the compact and ordered collagen nanofibrils causes neighboring α-helices on the collagen surface to rub against each other, which possesses an intrapolypeptide −CONH hydrogen bonding motif. That leads to the deformation of the triple helical structure, and new electric dipole moments are generated.10,12 Another suggested mechanism is that, as collagen possesses C6 symmetry, the compressive deformation could break this symmetry and subsequently create new charges on the top and bottom surfaces of the FSK.10,12 In this case, the amplification of these effects has been made by the presence of an interpolypeptide −CONH hydrogen bonding motif being formed between the polypeptide chains of neighboring αhelices in the highly oriented collagen crystal (Figure 1f). Additionally, the enhancement of the piezoelectric response in the FSK can be considered as cooperative electromechanical mutual interaction among the adjacent collagen nanofibrils.9,10 Thus, it is quite expected and desirable that the FSK shows longitudinal piezoelectric effect. The electrostrictive relation between Sa and P as

Electrical Measurements. The dielectric and ferroelectric properties of the FSK were measured using a precision impedance analyzer (Wayne Kerr, 6500B) and a ferroelectric testing system (P-LC100 V, Radiant Technology Precision), respectively. Nanogenerator Fabrication. To fabricate the robust FSKNG, FSK was laminated by polydimethylsiloxane (PDMS) (Sylgard, 184 silicone elastomer) layer, where 20 μm of layer thickness was kept fixed. The encapsulation layer was prepared by mixing curing agent in 10:1 ratio and degassed followed by curing in an oven at 60 °C for 30 min. Material Characterizations. The detailed surface morphologies were investigated by field emission scanning electron microscopy (FESEM; FEI, INSPECT F50) operated at an acceleration voltage of 20 kV. The structural and crystallographic features of FSK were studied by Fourier transform infrared (FT-IR) spectroscopy (TENSOR II, Bruker) and X-ray diffractometry in the 10−60° range (Bruker, D8 Advance). The d33 value of FSK was measured by d33 meter (Piezotest, PM300) under the constant applied force of 0.25 N and frequency at 110 Hz in HI/d33 mode (where capacitance, C, measurement of the material was inactive) after calibrating the equipment in the identical condition with a piezometer reference sample (PZT) having a d33 value of 350 pC/N. The output voltage and current from the FSKNG were recorded using National Instruments (NI) DAQ board (USB 6000) via online interface with PC with sampling rate of 1000 samples/s and a picoammeter (Keithley 6485), respectively. The vertical force was recorded by a calibrated three-axial force pressure sensor (FlexiForce A201).

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RESULTS AND DISCUSSION In order to make a biopiezoelectric sensor, biowaste fish skin (FSK) was collected from the food processing market. Then, it was descaled followed by a demineralization process. A small portion (20 mm × 18 mm) of the demineralized FSK (thickness ∼250 ± 30 μm) was cut, and electrodes were made on either side (effective area ∼138 mm2) by painting silver paste. Then, electrical output leads were connected on both side of the electrodes. The packaging of the FSKNG was made by encapsulating the FSK into the PDMS elastomeric layer that provided high structural stability, including durability and robustness. The entire fabrication procedure is illustrated schematically (Figure 1a(i)), including the original image of the FSKNG (Figure 1a(ii)). The presence of collagen nanofibrils with D-periodicity (∼50 nm) in the FSK is visualized in the FESEM image (Figure 1b). The banded pattern in collagen nanofibrils demonstrates native helical conformation of polypeptide chains. The strong presence of polypeptide chains is further evidenced from the vibrational bands of amide I (1633 cm−1), amide II (1539 cm−1), and amide III (1235 cm−1) in the FT-IR spectra (Figure 1c). The absorption ratio (∼1.06) between amide III and δ (CH2) (1453 cm−1) confirms the triple helical structure of collagen nanofibrils. In addition, the firm hydrogen bonding network between the polypeptide chains is appreciated from the amide A band (3294 cm−1). The free N−H stretching vibration generally takes place in the range of 3400−3440 cm−1. As the −NH group of the peptide chain is engaged with a hydrogen bond, the vibrational band shifts toward lower frequencies (i.e., 3294 cm−1). Furthermore, the deconvolution of the X-ray diffraction pattern separates the entire crystallographic feature of FSK collagen (Supporting Information, Figure S1). The broad diffraction pattern around 2θ ∼ 22.8° represents the amorphous part. The deconvoluted peaks at 2θ ∼ 32, 40.5, and 46.5° determine the linear translational length (d ∼ 0.279 nm) per amino acid and the axial translation length in amino acid residues in the N (d ∼ 0.222 nm) and C (d ∼ 0.195 nm) terminal telopeptides in a

Sa = QP 2

(1)

evaluates the magnitude of charge-related longitudinal electrostrictive coefficient, Q ∼ 1.54 m4/C2, from the slope of the Sa versus P2 plot (Supporting Information, Figure S3), which is significantly higher than that of typical inorganic piezoelectric materials.16 Furthermore, out-of-plane charge coefficient measurement shows that FSK possesses piezoelectric charge 8838


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Figure 2. (a) Output voltage response under several contact forces. The dotted line is a linear regression giving the sensitivity of ∼27 mV N−1. (b) Response of the FSKNG induced by the lightweight (∼58 mg) insect, butterfly. (c) Frequency-dependent output voltage and current. (d) Output voltages in forward and reverse connections. (e) Stability test over 75 000 cycles. (f) Simulated piezopotential and (g) dependence of output voltage and power density (inset shows equivalent circuit diagram) to external load resistances under constant contact force of 7.5 N (or stress amplitude of 0.3 MPa). (h) Output voltage and current responses and (i) variation of output voltage and power density (equivalent circuit diagram is shown in the inset) as a function of variable external resistances under hand punching. The illumination of LED by serially connected two FSKNGs under hand punching is shown in the inset.

coefficient, d33 ∼ −3 pC/N (Supporting Information, Figure S4). Owing to superior piezoelectric effect, the pressure sensing properties of FSKNG were realized by impacting periodical compressive contact force (F) using a custom-built pressure imparting system (Supporting Information, Figures S5 and S6). It is interesting to note that generated open-circuit voltage (Voc) varies linearly with the increase of contact force (F) at a constant frequency of 5 Hz (Figure 2a). Quantitatively, the mechanosensitivity can be defined as Sm =

ΔVoc ΔF

implies that the FSKNG is extremely suitable for detecting minute mechanical pressure impacts. According to the linear graph between Voc and F presented in Figure 2a, the loading of butterfly produces 0.26 N of contact force, which might be considered as the detection limit of the sensor. It is important to note that though FSKNG produces output voltages over a wide range of frequencies, we have chosen 5 Hz frequency of mechanical vibration in order to evaluate the sensitivity because FSKNG generates maximum Voc ∼ 200 mV at 5 Hz of frequency and the corresponding short-circuit current (Isc) is 2.5 nA (Figure 2c). Additionally, it possesses a very fast response time, τ ∼ 4.9 ms (Supporting Information, Figure S7), which is significantly higher than that of previously reported sensors.1,2 It is worth noting that generated Voc and Isc both varied at various frequencies ranging from 1 to 10 Hz in response to the constant external compressive force, F ∼ 7.5 N (i.e., stress amplitude, σa ∼ 0.3 MPa) due to the variation of applied strain rate (Figure 2c).17−19 The magnitude of generated charge (Q) under the applied contact force, F ∼

(2)

where ΔVoc and ΔF are the differences of Voc and F, respectively. The force-dependent linear variation of Voc yields sensitivity of FSKNG as 27 mV N−1. Most importantly, relying on a unique bioinspired structure, the ultrasensitivity of the FSKNG was distinguished by its rapid response upon loading− unloading of a butterfly (58 mg) repeatedly (Figure 2b). It 8839


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Figure 3. Time-dependent current responses during (a) repeated bending−releasing cycles of the wrist, (b) swallowing motions (one signal is enlarged in the inset), (c) repeated coughing actions, (d) speaking of different words, such as “Hi”, “Hello”, “Nature”, and “Energy” with the STFT processed 3D spectrogram in the lower inset, (e) monitoring radial artery and (f) carotid artery pulses with enlarged view of one cycle in the insets. The digital photographs of the physiological signal monitoring procedures by the biosensor are given in the insets of respective images.

output from the PDMS-based control device (where FSK was not used) has been also measured where no reliable output has been appreciated (Supporting Information, Figure S9). It further ensures that the output signals from FSKNG are associated with the piezoelectric properties of FSK. It is important to note that the generation of electricity is quite stable without any degradation over longer endurance even after 3 months, such as 75 000 cycles (Figure 2e and Supporting Information, Figure S10) at 5 Hz frequency under F ∼ 7.5 N. Thus, FSKNG is highly suitable for longterm energy supply. The basic mechanism of electricity generation from the FSKNG is very simple. For example, an external mechanical vibration induces piezoelectric potential (i.e., piezopotential) by the direct piezoelectric effect, and the electrons move via external load (as FSK is a dielectric material) to neutralize the potential difference between the top and bottom electrodes. Thus, the positive peak voltage and current pulses are generated. Once vibration is released, the induced piezopotential rapidly vanishes and the accumulated electrons on the electrode flow back to the opposite electrode. Thus, negative peak voltage and current pulses have been observed. In order to further understand the electromechanical

7.5 N, could be calculated by the integration of a current peak, that is, using Q=

∫ Iscdt

(3)

It is evaluated as Q ∼ 26.9 pC (Supporting Information, Figure S8). Thus, the estimated magnitude of piezoelectric coefficient is given by

|d33| =

Q F

(4)

that leads to the value of 3.5 pC N−1.9,12,13 This value is in close agreement to our experimental observation. Figure 2d illustrates alternating electric signals, which is composed of positive and negative peak of Voc, attributed to the repeated compression−release cycle of contact force. As a consequence of direct piezoelectric effect, the FSKNG shows identical amplitude of Voc with reversed polarization in reverse connection (Figure 2d). This is known as switching polarity test. It suggests that the electrical output signals are attributed to the FSKNG and not from any contact electrification between the measurement setup and sensor. Furthermore, the electrical 8840


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ACS Sustainable Chemistry & Engineering behavior of the FSKNG, finite element method (FEM)-based simulation has been performed using COMSOL multiphysics software (Figure 2f). In the simulation, it was assumed that applied pressure is uniformly distributed and holds at constant stress amplitude of 0.3 MPa. The computational simulation follows the classical piezoelectric theory. According to piezoelectric theory, the generated piezoelectric voltage Voc = σag3jL

and releasing motions of the wrist led to measurable output signal (i.e., current) because of the fact that the fish-skin-based biopiezoelectric sensor mimics the stretching and shrinking motions of human skin. Repeated bending results in positive piezopotential across the device, which generates a positive peak, and releasing results reverse piezopotential and thus generate a negative peak amplitude (Figure 3a). As a matter of fact, a tensile strain is developed in the FSKNG during repeated bending and releasing motions of the wrist. In this case, bending of the device into an arc shape generates the tensile strain along the thickness direction of the device (εy, parallel to the dipole orientation), which can be written as εy = L/2r = 0.42%, where r = 300 mm is the bending radius and L is the thickness (∼250 μm).15,19 On the other hand, the strain developed along the length direction of device (perpendicular to the preferential dipole orientation) is εx ∼ 0.88%, obtained from the relation of Poisson’s ratio, v = |εx/εy| = 2.1 of collagen nanofibrils.25 This result includes the static tactile sensing ability of FSKNG during human activities. Furthermore, the FSKNG-based sensor is also utilized for monitoring subtle physiological signals, such as the swallowing movements of the human throat, vocal cord vibration during coughing, specific phonation recognition, and artery pulses. The motion of the laryngeal prominence (the Adam’s apple) of a healthy young man was detected by fixing the sensor to the throat (Figure 3b). It could precisely identify the closing and opening psychological feature of the glottis during swallowing (inset of Figure 3b).26 Thus, the biopiezoelectric sensor might be useful in breath monitoring for the early detection of sudden infant death syndrome (SIDS).27 In addition, the biopiezoelectric sensor exhibited high accuracy in sensing the vocal cord vibration during repeated coughing actions (Figure 3c). Thus, the pressure sensor could be used to diagnose a patient’s damaged vocal cords and chronic obstructive pulmonary disease (COPD), such as asthma, chronic bronchitis, etc., by assessing output signals of coughing action.26 Furthermore, the FSKNG responds well during specific phonation of the wearer, such as “Hi”, “Hello”, “Nature”, and “Energy” (Figure 3d). In order to understand the physiological signals, short-time Fourier transform (STFT) processed 3D spectrograms of the acoustic profile of each word is shown in the inset of Figure 3d. The frequency distribution of the words “Hi”, “Hello”, “Nature”, and “Energy” consist of one, two, two, and three peaks, respectively, which represent the syllable of the corresponding words. Thus, it could noninvasively monitor the difference of epidermis deformation and muscle movement around the throat during phonation and may be suitable for developing a speech pattern/ voice recognition system. To date, a most important social concern is the prevalence of cardiovascular diseases (CVD) for premature death. Thus, early detection and regular monitoring of arterial blood pressure and heart rate are significant in a noninvasive rational strategy. Our wearable pressure sensor was attached to an adult human wrist, just above the radial artery (inset of Figure 3e) as it is generally used in arterial tonometry and sphygmography.28,29 The frequency and amplitude of the wrist pulses readily read out in real time (each peak denotes one pulse) under normal rest condition (∼74 bpm (beats per minute)) (Figure 3e). The enlarged view of one signal shows typical radial artery pulse waveforms with three clearly distinguishable determinants (inset of Figure 3e). In this case, P0 was diastolic pressure, and typical pressure waveforms include three peaks, namely, early systolic peak pressure (P1(t1): percussion wave (P-wave)), late systolic augmentation

(5)

where g3j is the piezoelectric voltage coefficient and L is the thickness. Thus g 3j =

d 3j εr ε0

= 6.8 × 10−3 Vm/N

(6)

where d3j is the piezoelectric charge constant of induced polarization in direction 3 (Z-axis) of a three-dimensional coordinate system; the subscript ‘j’ denotes direction 1 (X-axis) or 3 (Z-axis) of induced strain.10,15,17−19 The experimentally observed piezopotential (200 mV) was slightly lower than the simulated result (500 mV). This deviation seems to be due to the voltage drop resulting from internal leakage paths and charge loss in the metal−insulator−metal (MIM) structure.10,15,17−19 Furthermore, the effective power output of FSKNG was obtained by measuring the voltage output (VL) as a function of the external load resistors (RL) ranging from 1 to 40 MΩ under the repeating σa ∼ 0.3 MPa (Figure 2g). The instantaneous voltage drop across the resistances gradually increases with the change of RL accordingly and saturates at infinitely high resistance (∼40 MΩ) like Voc. The effective output power density (P) of the FSKNG is calculated by P=

V2 1 × L A RL

(7) 2

where A is the effective contact area (∼25 mm ). The variation of P with RL demostrates a high-output power density of 46 μW m−2 at the RL of 35 MΩ. The output power of the FSKNG is very much superior to that of the previously reported bioinspired piezoelectric nanogenerators, which reports the power output in the nW level.12,20,21 As a matter of fact, FSKNG attains instantaneous piezoelectric energy conversion efficiency of 1.3% (Supporting Information, discussion S1, Figure S11). In addition, it has been observed that FSKNG produces a large Voc ∼ 2 V and Isc ∼ 20 nA by repeated hand punching (σa ∼ 1.8 MPa) (Figure 2h). Thus, single FSKNG produces maximum output power density of 0.75 mW m−2 at the RL of 35 MΩ (Figure 2i), which is also much higher than that of the previously reported high-performance nanogenerators.13,22−24 Therefore, the performance of FSKNG compared to other piezoelectric pressure sensors and nanogenerators was found to be superior in terms of maximum output power (Supporting Information, Table S1). In order to meet the working mode of low-power consumer electronic components, the output power was modulated by serially integrating two FSKNGs of the same polarity and similar electromechanical responses. In this case, generated Voc ∼ 4 V (Supporting Information, Figure S12) directly turns on the blue LED (inset of Figure 2i). Therefore, developed FSKNG was further used as a selfpowered wearable pressure sensor for monitoring human physiological signals. It was attached to the human wrist by adhesive tape (inset of Figure 3a). Interestingly, the bending 8841



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shoulder (P2(t2): tidal wave (T-wave)), and diastolic pulse waveform (P3(t3): diastolic wave (D-wave)) normally appearing in the diastole region.28,29 The characteristic pulse pressure shape (PPS) was developed from superposition of ejected blood wave by the left ventricular contraction and the reflected blood wave from the lower limb. Using the data in Figure 3e, one can derive three of the most commonly used parameters such as the time delay between the first and second peaks, Δτ = t2 − t1, and the radial augmentation index, AIr = P2/P1, which was used as an index for vascular aging related to arterial stiffness diagnosis and increased pulse wave velocity (PWV). In this case, the average AIr was 0.52 (Δτ ∼ 0.31 s) for normal rest condition. The deduced AIr and Δτ values under normal rest conditions are compatible with a person in their late twenties, as reported by Nichols via tonometry.29 In addition, when the biopiezoelectric sensor was placed directly onto the neck over the carotid artery, it also records the real time in situ arterial pulse wave (Figure 3f). Inset shows the enlarged view of one cycle for detailed clinical information. A typical characteristic pulse wave shape was obtained with three clearly distinguishable determinants, such as systolic peak (PS), point of inflection (Pi), and dicrotic wave (PD), which are known to be, respectively, resulting from the blood ejected from the left ventricle, the reflected pulse wave, and the ejected blood back to the left ventricle. The pulse wave shape is significantly related to the physiological condition of the human cardiovascular system, which can be quantified by two of the most commonly used parameters: the augmentation index AIx(%) =± −1

PS − Pi = PP

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b01617. Additional information on electrostrictive coefficient, d33 measurement, design of the custom-built pressure imparting system, response time calculation, evaluated charge value, output voltage from the PDMS-based device (without fish skin), calculation of piezoelectric energy conversion efficiency, output voltage from serially connected two FSKNGs (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] or [email protected]. Tel: +91-8336-017243. Fax: +91-33-2413-8917. ORCID

Dipankar Mandal: 0000-0001-5216-6721 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by a grant from the Science and Engineering Research Board (SERB/1759/201415), Government of India. Authors are also grateful to DST, Government of India, for the development of the instrumental facilities under FIST-II programme and for awarding INSPIRE fellowship (IF130865) to Mr. Sujoy Kumar Ghosh. We acknowledge Mr. Samiran Garain for his skillful FE-SEM operation, and Mr. Anirban Biswas for developing the Labview program for real-time data acquisition. We also acknowledge Dr. Shrabanee Sen (Scientist, Sensor and Actuator Division, Central Glass and Ceramic Research Institute (CSIR)) for fruitful discussion on d33 study.

−16%, and the reflection index RI = h/Δt =

5.6 m s , where h is the subject height, Δt is the time delay between PS and PD, whereas PP is the absolute pulse wave magnitude. For the young man with better arterial compliance capability, the pulse wave is spread through the arteries at a lower velocity, and also the reflected wave arrives back to the aorta after the late systole, resulting in a negative AIx and a smaller RI. Thus, the FSKNG-based self-powered biopiezoelectric sensor successfully monitors human physiological signals irrespective of any parts of the human body.

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REFERENCES

(1) Zang, Y.; Zhang, F.; Di, C.; Zhu, D. Advances of Flexible Pressure Sensors Toward Artificial Intelligence and Health Care Applications. Mater. Horiz. 2015, 2, 140−156. (2) Wang, X.; Gu, Y.; Xiong, Z.; Cui, Z.; Zhang, T. Silk-Molded Flexible, Ultrasensitive, and Highly Stable Electronic Skin for Monitoring Human Physiological Signals. Adv. Mater. 2014, 26, 1336−1342. (3) Metzger, C.; Fleisch, E.; Meyer, J.; Dansachmuller, M.; Graz, I.; Kaltenbrunner, M.; Keplinger, C.; Schwodiauer, R.; Bauer, S. FlexibleFoam-Based Capacitive Sensor Arrays For Object Detection at Low Cost. Appl. Phys. Lett. 2008, 92, 013506. (4) Lai, Y.; Deng, J.; Niu, S.; Peng, W.; Wu, C.; Liu, R.; Wen, Z.; Wang, Z. L. Electric Eel-Skin-Inspired Mechanically Durable and Super-Stretchable Nanogenerator for Deformable Power Source and Fully Autonomous Conformable Electronic-Skin Applications. Adv. Mater. 2016, 28, 10024−10032. (5) Lee, T.; Jang, W. S.; Lee, E.; Kim, Y. S.; Wang, Z. L.; Baik, H. K.; Myoung, J. M. Ultrathin Self-Powered Artificial Skin. Energy Environ. Sci. 2014, 7, 3994−3999. (6) Wang, Z. L.; Song, J. Piezoelectric Nanogenerators Based on Zinc Oxide Nanowire Arrays. Science 2006, 312, 242−246. (7) Chou, H. H.; Nguyen, A.; Chortos, A.; To, J. W.; Lu, C.; Mei, J.; Kurosawa, T.; Bae, W. G.; Tok, J. B. H.; Bao, Z. A Chameleon-Inspired Stretchable Electronic Skin With Interactive Colour Changing Controlled By Tactile Sensing. Nat. Commun. 2015, 6, 8011. (8) Yu, C.; Li, Y.; Zhang, X.; Huang, X.; Malyarchuk, V.; Wang, S.; Shi, Y.; Gao, L.; Su, Y.; Zhang, Y.; et al. Adaptive Optoelectronic

CONCLUSIONS

In summary, a human interactive self-powered wearable biopiezoelectric pressure sensor has been developed with the aid of waste byproduct, fish skin. The structure−performance correlation of the sensor has been explored in the light of underlying physics of the piezoelectric properties (longitudinal charge coefficient, d33 ∼ 3 pCN−1) of the FSK collagen nanofibrils. In addition, the FSKNG behaves as an ultrasensitive, higly durable, and fast responding pressure sensor that successfully monitors real-time human physiological signals. Furthermore, the FSKNG generates sufficient output power to operate consumer electronics under human perception. The electricity generation capability of the FSK-based pressure sensor, harvesting the biomechanical energy with longer endurance, makes it valuable for a range of applications in continuous health care monitoring and clinical medicine apart from its wide range of applications in the field of low-power portable electronics. 8842


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