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Control of Skin Potential by Triboelectrifi cation with Ferroelectric Polymers

Ju-Hyuck Lee , Ronan Hinchet , Tae Yun Kim , Hanjun Ryu , Wanchul Seung , Hong-Joon Yoon , and Sang-Woo Kim *

J.-H. Lee, T. Y. Kim, Prof. S.-W. Kim SKKU Advanced Institute of Nanotechnology (SAINT) Center for Human Interface Nanotechnology (HINT) Sungkyunkwan University (SKKU) Suwon 440-746 , Republic of Korea E-mail: kimsw1@skku.edu Dr. R. Hinchet, H. Ryu, W. Seung, H.-J. Yoon, Prof. S.-W. Kim School of Advanced Materials Science and Engineering Sungkyunkwan University (SKKU) Suwon 440-746 , Republic of Korea

DOI: 10.1002/adma.201502463

performance of TENGs. In this paper, we found a new way to reduce our skin positive potential and increase the quantity of negative charges inside our body without directly connecting with the earth ground. This innovation is based on contact elec-trifi cation and ferroelectric effects and occurs by simple friction between our skin and particular ferroelectric materials that can be easily used in our daily life environment. In addition, the development of highly positive triboelectric materials over skin could also enhance the performances of TENGs.

As shown in the triboelectric series ( Figure 1 a), the most positive triboelectric material besides air is human skin, as reported several times. [ 5–9 ] Indeed, human skin is easily posi-tively charged during friction with other materials. However, our experimental results show that by polarizing a ferroelectric material, it is possible to change its triboelectric properties to be higher than skin. In our experiments, mouse skin and highly crystalline ferroelectric polymers of poly(vinylidenefl uoride- co -trifl uoroethylene) (P(VDF-TrFE)) thin fi lm (thickness = 8–9 µm) were used to observe their triboelectric series relations. Thus, we demonstrated that when human skin rubs P(VDF-TrFE), negative charges can be generated on the skin and the number of positive charges can be decreased depending on the β-P(VDF-TrFE) polarization. This means that some polarized ferroelectric materials can have higher positive triboelectric properties than human skin (Figure 1 b).

Mouse skin has been used as a skin model in studies of skin electrical properties due to its layered structure which is analogous to that of human skin, as both are composed of an epidermis, dermis, and outermost keratin layers (Figure 1 c and Figure S1 in the Supporting Information). [ 19,20 ] The native mouse skin surface potential was measured using a Kelvin probe force microscope (KPFM), [ 21,22 ] exhibiting an average potential of 216 mV (Figure 1 d). The formation of β-phase P(VDF-TrFE), which has ferroelectric properties, in the P(VDF-TrFE) thin fi lm was confi rmed by X-ray diffraction, [ 23,24 ] Fourier transform infrared spectroscopy (FTIR), [ 25 ] and polarization–electric-fi eld ( P – E ) hysteresis loops measurements (Figure S2, Supporting Information). [ 26 ] The β-P(VDF-TrFE) thin fi lm was negatively polarized by using an electrical poling process in order to attract positive charges when rubbing skin. To explore the β-P(VDF-TrFE), surface charge dynamic at nanoscale, pie-zoelectric force microscopy (PFM), [ 27,28 ] and KPFM [ 29 ] meas-urements were performed. The PFM phase image (Figure 1 e) clearly shows a high contrast between the negatively and positively polarized β-P(VDF-TrFE), with an average phase of −90° corresponding to a negatively oriented polarization in our measurements (Figure S3, Supporting Information). The

Contact electrifi cation, or triboelectrifi cation, is a well-known effect. Despite the fact that much research has been done, its mechanism is still not very well understood. [ 1–3 ] However, the order of the triboelectric series has been established based on experimental results and allowed to predict which material will become positively and negatively charged after contact or fric-tion. [ 4,5 ] The human body is easily positively charged by contact electrifi cation between the skin and other materials such as clothes and shoes. This is because the human skin holds the highest positive position in the triboelectric series, [ 6–9 ] whereby the human body easily loses negative charges and receives posi-tive charges by friction with the other surrounding materials. It has been reported that electrons in the body neutralize reactive oxygen species, which help the body’s immune and infl amma-tory responses. [ 10,11 ] Therefore, the presence of these positive static charges on the skin surface is a major problem affecting the feeling, appearance, and health of skin. [ 12,13 ] Through direct contact or through animal skins used as footwear or sleeping mats, the ground’s abundant free electrons can enter the body, which is electrically conductive. [ 13 ] However, due to our modern lifestyle, humans are being increasingly separated from the pri-mordial fl ow of the Earth’s electrons.

Moreover, recently, triboelectric nanogenerators (TENGs) based on the contact electrifi cation effect received a great atten-tion due to their high output power density, low cost, and easy fabrication process. Until now, most of the reports were related to the surface patterning, device structure, and their applications. [ 14,15 ] Only few improvements have been brought to negative triboelectric materials such as fl uorinated poly-dimethylsiloxane, [ 16 ] plasma surface treated polytetrafl uoro-ethylene (PTFE), [ 17 ] and fl uorinated ethylene propylene. [ 18 ] In complementary, the development of new and performant posi-tive triboelectric materials is necessary to further enhance the

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KPFM image (Figure 1 f) is in good agreement with the PFM results and shows a good contrast between the negatively and positively polarized β-P(VDF-TrFE) with a surface potential of +2.23 V compared to the platinum (Pt) atomic force microscope (AFM) tip, for the negative polarized region.

Using a single electrode TENG, its output voltage was meas-ured when contacting skin with other triboelectric materials (P(VDF-TrFE), nylon, and PTFE; Figure 2 ). When pushing on the skin/β-P(VDF-TrFE) TENG, a negative output voltage was generated and when releasing, a positive voltage was produced (Figure 2 a). On the other hand, when pushing on the skin/nylon and skin/PTFE TENGs, a positive output voltage was generated fi rst and then a negative voltage was produced while releasing (Figure 2 b,c). The output voltage direction between the electrode and the ground revealed that PTFE, which is one of the most negative triboelectric materials, acted as a negative triboelectric material compared to skin. Even nylon, which is quite a high positive triboelectric material, acted as a negative triboelectric material compared to skin. However, the nega-tively polarized β-P(VDF-TrFE) acted as a positive triboelectric material compared to skin, which means that β-P(VDF-TrFE) has higher positive triboelectric properties than skin, which has never been reported before (Figures S4 and S5, Supporting Information). [ 30,31 ] The output voltage and current of a single electrode TENG using negatively polarized β-P(VDF-TrFE) as a triboelectric substrate and a human fi nger as a freestanding triboelectric layer exhibit the same result as that using the mouse skin. When the human fi nger touched the negatively polarized β-P(VDF-TrFE) surface, a negative output voltage and current were measured fi rst and then a positive output voltage and current were measured when separated (Figure S6,

Supporting Information). This demonstrates that negatively polarized β-P(VDF-TrFE) behaves the same way when it is touching human skin or mouse skin and confi rms that skin is lower than negatively polarized β-P(VDF-TrFE) in the tribo-electric series.

To investigate the triboelectric properties of β-P(VDF-TrFE) depending on its polarization direction and intensity, we measured the output voltage generated by a single electrode TENG, rubbed using mouse skin, depending on the polariza-tion direction and intensity of the β-P(VDF-TrFE) used as a triboelectric layer ( Figure 3 ). Using nonpolarized β-P(VDF-TrFE), a positive output voltage was generated fi rst when the skin touched the β-P(VDF-TrFE), exhibiting the lower tribo-electric properties of nonpolarized β-P(VDF-TrFE) compared to skin (Figure 3 a). As the poling electric fi eld increased negatively, the output voltages of the TENG increased due to the lower triboelectric properties of the positively polarized β-P(VDF-TrFE) compared to skin (Figure 3 b–d). On the other hand, as the poling electric fi eld was increased positively, the output voltage decreased and switched to the opposite direc-tion when the poling electric fi eld was lower than +50 MV m −1 (Figure 3 e–g). This transition poling electric fi eld value is in good agreement with the coercive fi eld value E c = 47 MV m −1 from the P – E curve results (Figure S2c, Supporting Infor-mation). This corresponds to the value where the negatively polarized β-P(VDF-TrFE) has the same triboelectric properties as those of skin. This demonstrates that the triboelectric prop-erties of ferroelectric materials depend on their polarization direction and intensity.

Figure 4 shows the surface potential difference of the skin compared to the Pt AFM tip before and after friction with

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Figure 1. a) Triboelectric series of common materials with highlighted position of skin, nylon, platinum, and PTFE. b) Illustration of the triboelectric contact between a polarized ferroelectric material over a fi nger. i) Before the fi nger touches the polarized surface of ferroelectric materials; ii) when the fi nger touches the surface; iii) when the fi nger and the ferroelectric material are separated. c) Mouse skin has been used as an animal skin model instead of human skin due to its analog layered structure composed of an epidermis, dermis, and outermost keratin layers. d) Surface potential distribution of native mouse skin measured by KPFM. e) PFM phase image of the β-P(VDF-TrFE) thin fi lm after the generation of negatively and positively polarized domains by poling process. Inset shows the polarization direction of β-P(VDF-TrFE) thin fi lm. f) KPFM image of the surface potential distribution of negatively and positively polarized β-P(VDF-TrFE).

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PTFE, nylon, and negatively polarized β-P(VDF-TrFE). [ 1,11,32,33 ] The native mouse skin shows a surface potential of +216 mV (Figure 4 a), revealing that the energy level of the skin is higher than that of Pt. Thus, considering their surface poten-tial, skin is a higher triboelectric material than Pt. After fric-tion with PTFE and nylon, the mouse skin shows a surface potential decrease of +95 mV and +136 mV, respectively (Figure 4 b,c), because nylon and PTFE have lower triboelec-tric properties than skin. Therefore, during the friction with nylon and PTFE, skin loses negative charges and receives posi-tive charges, resulting in a decrease of its energy level and a reduction of the surface potential differences between skin and Pt. On the other hand, after friction with negatively polarized β-P(VDF-TrFE), the surface potential of skin was +285 mV, an increase of +69 mV compared to native skin (Figure 4 d). Thus, the energy level of the skin increased compared to Pt, while the skin surface loses positive charges and receives negative charges, confi rming the higher positive triboelectric proper-ties of negatively polarized β-P(VDF-TrFE) compared to skin. The density of charge accumulated on the skin after friction ( Table 1 ) can be obtained using the KPFM results and the simple capacitance equation

ε εΔ= Δ = Δ /0 skin

Q

A

C V

AV d (1)

where Δ Q is the transferred charge quantity during the fric-tion, C is the capacitance of the skin, Δ V is the KPFM surface potential difference before and after friction, ε 0 is the vacuum permittivity, ε skin = 6.04 is the skin dielectric constant, [ 34 ] A is the rubbed surface, and d = 180 µm is the skin thickness. These charge density results are reasonable and of the same order as that of the typical values generated and measured in the litera-ture of TENGs. [ 28 ] Finally, AFM measurements show that the surface roughness of skin did not signifi cantly change after friction and had a negligible effect on the surface potential vari-ations (Figure S7, Supporting Information).

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Figure 2. The output pulse voltage polarity order of the triboelectric nanogenerator depends on the relative position of the two materials in contact in the triboelectric series. a) A negative output voltage pulse is produced when the skin and negatively polarized β-P(VDF-TrFE) are in contact. A positive output voltage pulse is then generated when the skin and material are separated. b) A positive output voltage pulse is generated when skin and nylon are in contact and a negative output voltage pulse is generated when they are separated. c) A positive output voltage pulse is generated when skin and PTFE are in contact and a negative output voltage pulse is then generated when they are separated.

Table 1. Transferred charge density on skin by friction with PTFE, nylon, and β-P(VDF-TrFE).

Friction materials with skin PTFE Nylon P(VDF-TrFE)

Transferred charge density [µC m −2 ] 35.9 23.8 −20.5

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In conclusion, we have demonstrated that polarized ferro-electric materials can show higher positive triboelectric prop-erties than skin. Therefore, when they come into contact with human skin, they supply negative charges to skin and reduce its overall positive charge. It was found that the negatively polarized β-P(VDF-TrFE) acted as a positive triboelectric mate-rial compared to skin, which means that β-P(VDF-TrFE) has higher positive triboelectric properties than skin and could improve TENG’s performances. KPFM measurement results show that skin is negatively charged after rubbing with nega-tively polarized β-P(VDF-TrFE), while skin is positively charged after rubbing with PTFE and nylon. Our study provides new way to reduce our skin positive potential and increase the quantity of negative charges inside our body by simple friction between our skin and particular ferroelectric materials that can be easily used in our daily life environment.

Experimental Section Materials Preparation : The materials used in experiments discussed

here were P(VDF-TrFE) (70/30) copolymer resins in powder (purchased from Piezotech, France), PTFE (Ilwoong platech., PTFE sheet, 300 µm), Nylon (Kolon, nylon 6–6, 15 µm), and mouse skin (ICR, male, 8 weeks old, 180 µm). The P(VDF-TrFE) solution was prepared by dissolving P(VDF-TrFE) in N , N -dimethylformamide (DMF) solvent. A solution of P(VDF-TrFE) (20 wt%) dissolved in DMF solvent was spun on the indium tin oxide (ITO)-coated polyethylene naphthalate (PEN) substrate to form a 8–9 µm thick P(VDF-TrFE) thin fi lm and then dried at 60 °C for 10 min to remove the DMF solvent. This layer was then maintained at 140 °C for 2 h and then cooled at room temperature. Two P(VDF-TrFE)-coated ITO/PEN substrates were stacked for the poling process. The electrical poling process was carried out by applying an electric fi eld of 100 MV m −1 for 30 min between the two ITO electrodes. An ICR male mouse of 8 weeks old was sacrifi ced by overdosing it with a 6:4:90 ratio of a 250 cc zoltetil:rompun:saline solution. The dorsal skin was immediately excised, subcutaneous tissues were scraped off with scissors, and the hair was

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Figure 3. a) Without poling process, positive output voltage is observed when fi nger touched the surface of the P(VDF-TrFE) fi lm. b–d) When the nega-tive electric fi elds are applied to the device, the output voltage is enhanced as the electric fi eld increases. e–g) When a positive electric fi eld is applied to the device, the output voltage direction is reversed over 50 MV m −1 .

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shaved carefully with a shaver. The skin was gently cleaned with a 10% (v/v) soap solution, rinsed with tap water for 30 s, and leveled on the table to dry in an ambient condition overnight (22 °C, relative humidity (RH) of 35%–40%) (Figure S1, Supporting Information).

Characterization of Materials : All experiments were performed under ambient conditions (typically, a temperature of ≈22 °C and RH of 35%–40%). All materials were fi rst washed with ethanol and then dried to ensure electroneutrality. The synthesized piezoelectric polymers underwent structural and crystallographic characterizations using XRD (Bruker D8 DISCOVER) with Cu KR radiation ( λ = 1.54 Å). Fourier transform infrared (FTIR) measurements were performed for the structural investigation of the β phase of P(VDF-TrFE) fi lms. A Tektronix DPO 3052 Digital Phosphor Oscilloscope and a low-noise current preamplifi er (model no. SR570, Stanford Research Systems, Inc.) were used for electrical measurements. KPFM measurements were performed using Park systems XE-100 with Pt/Cr-coated silicon tips (tip radius <25 nm, force constant 3 N m −1 , and resonance frequency of 75 kHz). 10 µm × 10 µm size KPFM images were

scanned at a scanning speed of 0.3 Hz, and set at a point of 13 nm from a sample in atmospheric pressure at room temperature. 2 V AC voltages were applied from a rock-in amplifi er with a phase of −90° and frequency of 17 kHz.

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

Acknowledgements J.-H.L. and R.H. contributed equally to this work. This work was fi nancially supported by the Basic Science Research Program (2015R1A2A1A05001851, 2009–0083540) through the National Research

Figure 4. a) The average surface potential of native mouse skin is 216 mV. b) The surface potential of skin decreased by Δ V = 121 mV to an average potential of 95 mV after friction with PTFE. c) The surface potential of skin decreased by Δ V = 80 mV to an average potential of 136 mV after friction with nylon. d) However, the surface potential of the skin increased by Δ V = 69 mV to an average potential of 285 mV after its friction with negatively poled β-P(VDF-TrFE). According to these results, the order of all these materials in the triboelectric series, from the more positive to the negative, is as follows: negatively poled β-P(VDF-TrFE), skin, nylon, and PTFE.

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Foundation (NRF) of Korea Grant funded by the Ministry of Science, ICT & Future Planning. This study was reviewed and approved by the Institutional Animal Care and Use Committee (IACUC) of Sungkyunkwan University School of Medicine (SUSM). SUSM is an Association for Assessment and Accreditation of Laboratory Animal Care International (AAALAC International) accredited facility and abide by the Institute of Laboratory Animal Resources (ILAR) guide.

Received: May 22, 2015 Revised: July 10, 2015

Published online: August 20, 2015

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