Silk Nanofiber‐Networked Bio‐Triboelectric Generator: Silk Bio‐TEGhome.skku.edu/~nesel/paper files/171.pdf · wileyonlinelibrary.com 1502329 / )

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Silk Nanofi ber-Networked Bio-Triboelectric Generator: Silk Bio-TEG

Hyun-Jun Kim , Jae-Hwan Kim , Ki-Woo Jun , Jong-Hun Kim , Wan-Chul Seung , Oh Hyeong Kwon , Jeong-Young Park , Sang-Woo Kim , and Il-Kwon Oh *

H.-J. Kim, J.-H. Kim, K.-W. Jun, Prof. I.-K. Oh Creative Research Initiative Center for Functionally Antagonistic Nano-Engineering Department of Mechanical Engineering Korea Advanced Institute of Science and Technology (KAIST) 291 Daehak-ro , Yuseong-gu , Daejeon 34141 , Republic of Korea E-mail: [email protected] Dr. J.-H. Kim, Prof. J.-Y. Park Center for Nanomaterials and Chemical Reactions Institute for Basic Science (IBS) Daejeon 34141 , Republic of Korea Dr. J.-H. Kim, Prof. J.-Y. Park Graduate School of EEWS Korea Advanced Institute of Science and Technology (KAIST) 291 Daehak-ro , Yuseong-gu , Daejeon 34141 , Republic of Korea W.-C. Seung, Prof. S.-W. Kim School of Advanced Materials Science and Engineering Sungkyunkwan University (SKKU) Suwon 16419 , Republic of Korea Prof. O.-H. Kwon Department of Polymer Science and Engineering Kumoh National Institute of Technology Gumi 39177 , Republic of Korea

DOI: 10.1002/aenm.201502329

equipment, and harmful materials, to be solved in the future. Above all, biodegradable and biocompatible materials, espe-cially, which are also known as eco-friendly materials, are nat-ural or synthetic materials that can function in intimate con-tact with living systems without any adverse reaction or toxicity response. [ 11,12 ] Using sustainable and eco-friendly biomaterials for electronic devices is the most important issue nowadays in view of the environment, health, and safety. [ 13 ]

Silk, one of naturally abundant and eco-friendly biomate-rials, [ 14 ] has been a common subject in various research fi elds, because it has several distinctive properties including mechani-cally high strength, [ 15 ] good biocompatibility with cell prolifera-tion, [ 16 ] and adhesion. [ 12,17 ] Silk is a natural protein fi ber that is composed mainly of fi broin and is produced by certain insect larvae to form cocoons. Silk fi bers from the Bombyx mori silk-worm have a triangular cross section with rounded corners, 5–10 µm wide and the fi broin-heavy chain is composed mostly of beta-sheets, due to a 59-mer amino acid repeat sequence with some variations. Silk is one of the strongest natural fi bers and has a durable nature over other fabrics. [ 15 ] Silk is a poor con-ductor of electricity, but susceptible to static cling. However, to the best of our knowledge, although silk is a mechanically strong and superior electronegativity biomaterial, it has not been applied to a triboelectric power generator until now.

Herein, we report for the fi rst time a silk fi broin nanofi ber-networked bio-triboelectric generator (Silk Bio-TEG). The regenerated silk fi broin fi lm, which is prepared by a simple electrospinning method, has a form of nanofi ber-networked structures that are benefi cial for triboelectric power generation because of ultrahigh surface-to-volume ratio and much rougher surfaces than bare fi lm surface. We demonstrate that the fab-ricated Silk Bio-TEG can generate a high voltage with large surface area and can be tuned for the optimal performance of power generation under various input conditions. Also, the power generation mechanism is considered by measuring the surface potential of the contact materials with Kelvin probe force microscopy (KPFM). Present Silk Bio-TEG shows excep-tionally durable energy harvesting performance due to high mechanical stiffness and strength of silk fi bers and the fracture tolerant behavior of silk fi broin networks.

The mechanism of electric power generation in TEG has been explained by the coupling effects of contact electrifi ca-tion in inner circuit and electrostatic induction in outer cir-cuit. [ 5 ] When two friction materials are put in contact with each other, the one that has the higher tendency to gain an electron will be negatively charged relatively. [ 18 ] Silk Bio-TEG consists of silk fi broin nanofi ber-networked fi lm, polyimide (PI) fi lm, and aluminum foil as a current collector as shown in Figure 1 .

With the growing environmental problems resulting from greenhouse gas emissions of fossil fuels and the continuous increase of energy prices resulting from eventual fossil fuel depletion, interest in renewable energy, as well as in enhance-ment of energy effi ciency, is growing. [ 1 ] Most of all, energy har-vesting, which is the process of capturing small amounts of energy that would otherwise be lost as movement, vibration, or heat, and accumulating and storing such energy for later use, has been attracting public attention. [ 2 ] Especially energy harvesting using mechanical energy sources such as piezoelec-tric generator, electromagnetic generator, etc., has been widely studied because it has great technical potential for use in self-powered system. [ 3 ] Recently, as a new type of mechanical energy harvesting, a triboelectric generator (TEG) that converts the external mechanical energy into electricity occurring between two frictive contact surfaces of dissimilar materials was sug-gested by Wang and co-workers. [ 4 ]

The TEGs have been intensively investigated and developed by Wang and co-workers, [ 4–6 ] and further research is going on to enhance power generation with increased charge surface area, [ 7,8 ] metal nanoparticle decoration, [ 9 ] or organic material etching, [ 6,10 ] etc. However, to utilize the TEG as an eco-friendly and more feasible energy harvester in practical life, there are remaining issues, such as complex fabrication steps, expensive

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According to the triboelectric series, PI has an electronegativity higher than that of silk. [ 19 ] So, the PI fi lm gains electrons and is negatively charged, whereas the silk fi lm loses electrons and is positively charged when the two materials are put in contact. In our study, the surface potential of PI and silk was charac-terized using KPFM to understand the correlation between the triboelectricity and the surface potential. KPFM is used to measure the contact potential difference (CPD) between the conductive tip and the sample, ( V tip − V sample ), by nullifying the CPD-induced capacitive force [ 20,21 ] (see the detailed results in the Supporting Information). Many theories and review papers assume that electron states exist within the large band gaps of insulators and that these states are localized at the sur-faces. [ 21,22 ] When the insulators with different energy states at the surface are contacted, the quasi-work functions difference between the insulators can drive the electron transfer. [ 21,22 ] The measurement of surface potential difference between PI and silk allows us to attribute the electron transfer effect as the possible mechanism of the tribocharging to some extent. That is, when the two insulators come to contact, the electron can depart from the fi lled state of silk to the unoccupied state of PI due to the surface potential difference, charging the PI surface negatively. The proposed Silk Bio-TEG operates in contact and separation modes. Initially, there is no charge on the surface of the contacting fi lms, which are in the shape of an arch. When a vertical compressive force generated by a mechanical shaker is applied to the top of the Silk Bio-TEG, the silk fi lm and the PI fi lm are brought in contact and rub together, resulting in charged states of positive and negative, respectively. Once the compressive force is released, the Silk Bio-TEG will return to its original shape due to the elasticity and fl exibility of the fi lms.

At that time, the opposite charges of the friction parts quickly become separated by an air gap, forming a dipole moment; then, an electric potential difference is created between the two electrodes. [ 23 ] As a result, electrons start to fl ow from the top electrode (of the negatively charged PI fi lm) to the bottom electrode (of the positively charged silk fi lm) until the accumu-lated charges reach an equilibrium state; however, there is no electron fl ow between the fi lms due to the nature of the insu-lator. [ 4 ] When the compressive force is reapplied to the device, the dipole moment is decreased. Then, the electric potential dif-ference starts to drop, resulting in the generation of electron fl ow in a reverse direction, from the bottom electrode to the top electrode, in a positive signal direction.

The silk from B. mori silkworm is made of protein-based fi bers that are composed of peptide bonds of various amino acids, [ 24 ] which result in the formation of the secondary protein structures like α -helix and β -pleated sheet structures. [ 25 ] Since these protein structures are basically linked with hydrogen bonding [ 24 ] between the adjacent peptide blocks as shown in Figure 1 a, silk is known for having unique mechanical proper-ties and interesting biological functionality. [ 26 ] After removing sericin protein, which is known to cause some biocompat-ibility problems, [ 12 ] the chemo-mechanical properties of the two regenerated silk fi broin fi lms were analyzed. The morphologies of the two regenerated silk fi broin fi lms, which are respectively made by electrospinning and casting processes, were observed by the fi eld emission scanning electron microscope (SEM), with result shown in Figure 2 a,b. It can be seen that the electrospun silk fi broin nanofi bers, which have diameters in a range of about 100–200 nm, are entangled with one another. The cast silk fi lm, however, has a smooth surface with little unevenness.



Figure 1. Fabrication steps for electrospun silk fi broin membrane and concept of triboelectric energy harvesting. a) Schematic of the electrospun silk fi broin preparation and electrospun silk fi broin nanofi ber-networked fi lm. b) Schematic of the triboelectric electric energy generation process.

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When some nanofi bers in the electrospun silk fi lm are damaged, the network-structured electrospun silk fi lm may maintain its ability to be charged because the other nanofi bers can be charged instead of the damaged nanofi bers, resulting in a kind of fracture tolerant behavior. Moreover, the electrospun silk nanofi ber-networked fi lm has a specifi c surface area higher than that of cast silk fi lm, resulting in a large amount of charge.

As illustrated by the results shown in Figure 2 c, the chemical bonds of the electrospun silk fi lm and the cast silk fi lm were further investigated by the fourier-transform infrared spectros-copy (FT-IR) analysis, a common and powerful tool for the pro-tein conformation analysis. Basically, the silk fi broin fi lm has characteristic absorption bands at 3300 cm −1 , corresponding to the O–H stretching vibration, and the various amide groups (amide I, II, III), which are attributed to the secondary structure of silk (α -helix and β-pleated sheet). [ 17,25,27 ] Interestingly, due to its regular chemical structures, the electrospun silk fi lm shows peaks sharper than those of the cast fi lm; this discrepancy is the result of external forces such as the shear force across the cap-illary needle and the coulombic force due to the applied high electric fi eld during the electrospinning process. [ 28 ]

The X-ray diffraction (XRD) data in Figure 2 d shows the crystallinity characterization of the electrospun silk fi lm and of the cast silk fi lm, properly corresponding to the results of FT-IR. Due to the relatively high intensity signal, the electro-spun silk fi lm exhibits much higher crystallinity than that of the cast silk fi lm, though the same peaks referring to the sec-

ondary structure of protein (α -helix and β-pleated sheet), based on hydrogen bonding of silk, simultaneously appeared. [ 29 ] Compared with other materials that have been developed and featured in published articles, it can be clearly seen that the pre-sent electrospun silk fi lm is well-synthesized and structurally stable due to its regular chemical bonding and crystallinity. [ 30 ]

Since the mechanical energy from a practical environment quite varies and is irregular, it is necessary to study the relation-ship between the output of Silk Bio-TEG and the various condi-tions. The output signal of the Silk Bio-TEG was performed by periodically pressing and releasing by a mechanical shaker with controlled pulse signals; input forces applied to the Silk Bio-TEG were measured using a force sensor (see also Figure S2, Supporting Information). By controlling the pulse magnitude, frequency, and pulse width, which are variations of the input pulse signal, the output of the Silk Bio-TEG was compared in a variable environment. The condition for the basic input pulse signal by the peak voltage of 2 V is corresponding to an input force of 16.8 N under excitation frequency of 1 Hz and pulse width of 10 ms, as shown in Figure S2 (Supporting Informa-tion). The wire linked to the top electrode of the PI fi lm side was connected to the (+) cable of all the measurement devices. So, if there is electron fl ow from the outside to the wire, the output signal will be positive, in the forward direction. With reference to the schematic illustration of the Silk Bio-TEG mechanism, shown in Figure 1 b, a positive voltage signal was generated when the Silk Bio-TEG was pressed to put it into



Figure 2. Morphological and chemical analyses of electrospun and cast silk membranes. FE-SEM images of regenerated silk fi broins: a) Electrospun silk and b) cast silk. c) FT-IR spectra and d) XRD patterns of regenerated silk fi broins.

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close contact with the two sides, whereas a negative voltage signal in a reverse direction was generated under the release of the pressing force to the Silk Bio-TEG. In order to investi-gate the effect of the electrospun membranes in TEG, two Silk Bio-TEGs were assembled with regenerated silk fi broin fi lms by electrospinning and casting processes; the same PI fi lm was used on the other sides. Then, the open-circuit voltages of the two Silk Bio-TEGs were compared under the basic condition, with results shown in Figure 3 a. The peak-voltage of the elec-trospun Silk Bio-TEG is about 1.5 times higher than that of the cast Silk Bio-TEG. There is one possible explanation for this result: well-networked silk fi broin fi lm made by electrospin-ning process has a larger surface area than that of the just cast silk fi broin fi lm, even though the basic material is the same as silk fi broin. Several reports have shown a similar tendency of increasing output voltage with large surface area of friction parts. [ 7,10,31 ] As mentioned earlier, the output voltage perfor-mance of the Silk Bio-TEG is infl uenced by the input condition with the pulse magnitude, frequency, and pulse width. First, diverse input forces from 16.8 to 51.2 N were applied to the Silk Bio-TEG, and the open-circuit voltages were found to gener-ally increase with the pulse magnitude, as shown in Figure 3 b. Above the specifi c levels of the input forces, however, output voltages were not always found to increase. And, the differen-tial of the output voltage at various input forces is smaller in the process of releasing than in the process of pressing because the Silk Bio-TEG returns to its original shape due to structural elasticity rather than due to the input force by the shaker. [ 5,7 ]

Second, we tested the Silk Bio-TEG at different frequencies from 1 to 10 Hz without any change of remaining basic condi-tion. As shown in Figure 3 c, the open-circuit voltage increases slightly with the increasing frequency. A potential cause of this phenomenon is that positively and negatively charged states on the friction parts are less naturally neutralized under the high frequency deformation, resulting in more accumulation on the electrodes. [ 4 ] Finally, we only changed the pulse width from 10 to 500 ms in input basic pulse signal, and carefully plotted each signal with a coincidence of time for the forward signal to compare the backward signal. Additionally, to distinguish the signal direction, two light-emitting diodes (LEDs) in par-allel connection reversely were linked to the Silk Bio-TEG, as indicated in Figure 3 d (see also Figures S3 and S4, Supporting Information). Owing to the characteristics of the pulse signal, the rise and fall times of these signals were found to increase slightly with larger pulse width. In other words, the open-cir-cuit voltages of the Silk Bio-TEG reach higher values with fast straining rate at the same frequency.

Furthermore, it is important to check to determine how the new Silk Bio-TEG can be made useful in actual life. Power and accumulation tests, which can show the feasibility of this device for proper applications, were performed. In practice, the level of electric power depends on the load in a system. Therefore, as an external load, the Silk Bio-TEG was linked to electrical resistors; then, the electric output was measured in order to examine the effective electric power of the Silk Bio-TEG. With an increase of the resistance, the instantaneous maximum voltage through



Figure 3. Generated output voltages under various input conditions in open circuit systems. a) Open-circuit voltages of triboelectric nanogenerator using two kinds of regenerated silk fi broin, each under basic condition of 16.8 N input force, 1 Hz frequency, and 10 ms pulse width. Performance characterization of the electrospun silk-based triboelectric nanogenerator under different experimental conditions of pulse signal input: b) input power, c) frequency, and d) pulse width. Inset of (d) shows the parallel LED turn-on/off with forward or backward signal each.

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the resistor generally increases and saturates to the open-circuit voltage when the resistance is infi nitely large, while the cur-rent across the resistor follows an opposite trend as shown in Figure 4 a. Additionally, the triboelectric surface charge density, which is an important parameter of triboelectric generators, was calculated by integrating a current signal over time [ 5,32 ] (see also Figure S5, Supporting Information). Consequently, it appears that the instantaneous power output ( W = V 2 peak / R ) [ 33 ] reaches its maximum value (about 4.3 mW m −2 ) at the resist-ance of 5 MΩ under the basic input pulse signal as shown in Figure 4 b. In addition, one more process is required for the Silk Bio-TEG to be used as an actual power source. Through the rectifi cation of the alternating output, the direct output can be stored in energy storage devices such as capacitors and bat-teries. Figure 4 c shows that the generated alternating output can be transferred to the direct output in the same direction under the basic input pulse signal. To demonstrate the capa-bility of the direct power source, two parallel connections to 20 commercial LED bulbs were utilized. And then we compared

the performance of fl ashing LEDs with or without rectifying bridge circuit under the input signal of 500 ms pulse width (see Movie S1, Supporting Information). As shown in Figure 4 d, the accumulated voltage across a single capacitor was meas-ured after the rectifi cation at the different pressing frequency. With reference to Figure 3 c, it is clearly apparent that repeated pressing can make the accumulated voltage high due to the increase of the output density. Further, the stability and dura-bility test over 25 000 cycles was also conducted to confi rm the sustainability of the Silk Bio-TEG, as shown in Figure 4 e,f. The Silk Bio-TEG shows exceptionally durable and reliable energy harvesting performance because of mechanically super-strong material properties of silk fi bers and fracture tolerant behavior of nanofi ber-networks.

In summary, we have successfully demonstrated an eco-friendly silk fi broin bio-triboelectric generator (Silk Bio-TEG) based on an electrospun nanofi ber-networked fi lm. By using a simple and cost-effective electrospinning method, the power generation of the electrospun Silk Bio-TEG was improved due



Figure 4. Performance characteristics of the Silk Bio-TEG: Dependence of a) the voltage output and b) the power output on the external load resistance under the basic condition of pulse signal input. c) Rectifi ed voltage signal under the basic condition of pulse signal input. d) The accumulated voltage across a single capacitor (10 µF) charged by the Silk Bio-TEG with variation of pulse frequency. e) Comparing the stability and durability results of two kinds of regenerated silk fi broin under basic input pulse signal; peak voltage. f) The stability and durability test of the Silk Bio-TEG; whole signal.

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to a much higher surface-to-volume ratio and much rougher surfaces compared to those characteristics of a smooth cast silk fi lm. The peak-voltage of the electrospun Silk Bio-TEG was about 1.5 times higher than that of the cast Silk Bio-TEG. The triboelectric surface charge density and the instantaneous elec-tric power of the electrospun Silk Bio-TEG were measured up to 1.86 µC m −2 and 4.3 mW m −2 , respectively, at the electric resist-ance of 5 MΩ; also, after simply rectifying the signal under the basic input pulse signal, it was possible to rapidly charge a capacitor to 2 V using this generator within 5 min. Most interestingly, the Silk Bio-TEG shows exceptionally durable and reliable energy harvesting performances owing to mechani-cally super-strong material properties of silk fi bers and fracture tolerant behavior of nanofi ber-networks. Using an eco-friendly and sustainable biomaterial and simple fabrication process, this Silk Bio-TEG has great potential for self-powered systems even under environments of harsh vibration and in living organisms with no harmful effects.

Supporting Information Supporting Information is available from the Wiley Online Library or from the author.

Acknowledgements This work was partially supported by Creative Research Initiative Program (2015R1A3A2028975) funded by National Research Foundation of Korea (NRF).

Received: November 22, 2015 Revised: December 9, 2015

Published online:

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