Letter pubs.acs.org/NanoLett
Frequency-Multiplication High-Output Triboelectric Nanogenerator for Sustainably Powering Biomedical Microsystems Xiao-Sheng Zhang,† Meng-Di Han,† Ren-Xin Wang, Fu-Yun Zhu, Zhi-Hong Li, Wei Wang, and Hai-Xia Zhang* National Key Laboratory of Nano/Micro Fabrication Technology, Peking University, Beijing, China S Supporting Information *
ABSTRACT: An attractive method to response the current energy crisis and produce sustainable nonpolluting power source is harvesting energy from our living environment. However, the energy in our living environment always exists in low-frequency form, which is very difficult to be utilized directly. Here, we demonstrated a novel sandwich-shape triboelectric nanogenerator to convert low-frequency mechanical energy to electric energy with double frequency. An aluminum film was placed between two polydimethylsiloxane (PDMS) membranes to realize frequency multiplication by twice contact electrifications within one cycle of external force. The working mechanism was studied by finite element simulation. Additionally, the well-designed micro/nano dual-scale structures (i.e., pyramids and V-shape grooves) fabricated atop PDMS surface was employed to enhance the device performance. The output peak voltage, current density, and energy volume density achieved 465 V, 13.4 μA/cm2, and 53.4 mW/cm3, respectively. This novel nanogenerator was systematically investigated and also demonstrated as a reliable power source, which can be directly used to not only lighten five commercial light-emitting diodes (LEDs) but also drive an implantable 3-D microelectrode array for neural prosthesis without any energy storage unit or rectification circuit. This is the first demonstration of the nanogenerator for directly driving biomedical microsystems, which extends the application fields of the nanogenerator and drives it closer to practical applications. KEYWORDS: Triboelectric nanogenerator, frequency multiplication, biomedical application, self-powered system
A
Additionally, for some applications, especially in biomedical fields, a higher output voltage than the previous work10 is demanded. In this work, we present a sandwich-shape triboelectric nanogenerator, which can produce frequency-multiplication output signal due to twice contact electrifications between twopiece polymer membranes and middle metal film within one cycle of external force. Moreover, micro/nano dual-scale structures are employed to strengthen the output performance of triboelectric nanogenerator, and ultrahigh voltage and high energy density are reached. Experiments. Figure 1 shows the schematic view of structural design and fabrication process flowchart of the sandwich-shape triboelectric nanogenerator. An aluminum film with a thickness of 20 μm is fixed with elastic tape between two surface-micro/nanostructured PDMS film with the thickness of 450 μm, which constitutes the effective contact electrification sandwich-shape structure. A 125 μm PET/ITO thin film was fabricated atop the PDMS film, in which PET layer was bent to be arch-shape to enhance the output,10 and the ITO layer was
s an attractive vision for the future, researchers have tried to realize a real integrated micro/nanosystem, which can work continually, independently, and effectively.1,2 The significant challenge to realize this desire is to establish a sustainable energy supplying source.3 An attractive approach to response this challenge and produce sustainable green power is harvesting energy from our living environment.4,5 The nanogenerator converting mechanical energy to electric energy based on the triboelectric effect has been proved to be an effective approach by Wang’s group.6,7 Microstructures, which actually increase the roughness, have been introduced to enhance the output of triboelectric nanogenerator in the previous work.8,9 Subsequently, an arch-shaped triboelectric nanogeneator10 with outstanding performance was presented, whose output voltage and current density achieved 230 V and 15.5 μA/cm2, respectively. However, the energy in our living environment always exists in low-frequency form. The previous nanogenerators, including piezoelectric type and so on,11 can only produce output with the same frequency as that of the external force due to the only one contact electrification between two layers in one cycle. For some micro/nanosystems that require energy supplying source with high frequency, a frequency converter is necessary, which will complicate the system and consume extra energy. © 2013 American Chemical Society
Received: December 10, 2012 Revised: January 19, 2013 Published: February 5, 2013 1168
dx.doi.org/10.1021/nl3045684 | Nano Lett. 2013, 13, 1168−1172
Nano Letters
Letter
based mold15 (Figure 1b ). More experimental details have been given in Figure S1 and S2 in the Supporting Information file. Figure 2b,c show the micro/nano dual-scale PDMS surfaces with pyramids array and V-shape grooves, respectively. The open-circuit voltage and short-circuit current of this sandwich-shape triboelectric nanogenerator were measured via a digital oscilloscope (RIGOL DS1102E). A vibration system (Video 1 in the Supporting Information file), including a waveform generator (RIGOL DG1022), a power amplifier (SINOCERA YE5871A), and a vibration platform (SINOCERA), was assembled to apply a cycled compressive force with controllable frequency. Results and Discussion. Working Principle. The working principle of this sandwich-shape nanogenerator was studied by finite element simulation (COMSOL). At the origin state, no electric potential exists between Al and PDMS as shown in Figure 3a . By applying a periodic compressive force, Al and PDMS will rub twice during one cycle due to the specially designed sandwich-shape structure, thus doubling the frequency of the output. When the device is pressed, the surfaces of Al and PDMS are charged with the same surface density10 shown in Figure 3a . As the force is removed, Al first separates from the bottom PDMS layer, and the potential difference between Al and PDMS will drive the electrons to flow through an external load (Figure 3a ). Soon afterward, the top PDMS layer separates from Al, generating a current flow with the same direction (Figure 3a ) until the device reaches electrical equilibrium (Figure 3a ). When the generator is pressed again, the redistributed charges will build a reversed potential, thus driving electrons to flow at the opposite direction. Similarly, the friction between the top PDMS layer and Al gives the first peak output (Figure 3a ), and the rub of Al and bottom PDMS layer produces the second peak output (Figure 3a ). It is worth mentioning that the downward peaks may be overlapped because of the rapid applied force. Additionally, the separating sequence of the layers fabricated by other materials could happen either way even for the same sandwich-shape structure, which depends on the mechanical property.16 Figure 3b shows the comparison between the output signal and electrocardiography (ECG) in one cycle. Two upward peaks with unequal amplitude can be clearly observed in the output signal, which is similar to the QRS complex and T wave in ECG.17 By changing the gap between each layer, the interval between the upward peaks can be modulated to simulate the waveform of ECG, showing the potential application for the artificial component of heart. Frequency Effect. The performance of this sandwich-shape triboelectric nanogenerator with surface-nanostructured PDMS under the compressive force at different frequencies was investigated, as is shown in Figure 4. As the external force frequency increases from 1 to 5 Hz, the open-circuit voltage increases from 120 to 320 V due to external electrons flowing to reach equilibrium in a shorter time.10,18 Then, the opencircuit voltage keeps constant while the frequency increases from 5 to 7 Hz. However, the open-circuit voltage decreases to 218 V under 10 Hz external force due to the under-releasing of the sandwich-shape nanogenerator. If the circle of the compressive force is too short (i.e., too high frequency), the nanogenerator cannot recover to the original position before the next force impact. This mechanism also induces the trend of the frequency-multiplication output signal, whose peak output voltage is marked with red dashed line (Figure 4). The
Figure 1. Schematic view of (a) structural design and (b) fabrication process flowchart of the sandwich-shape triboelectric nanogenerator. Fabrication of microstructures on silicon substrate by the combination of photolithography and KOH wet etching. Fabrication of high-dense nanostructures atop microstructures using the improved DRIE process to form Si micro/nano dual-scale structures. Fabrication of PDMS film with micro/nano dual-scale structures replicated from silicon mold.
used as the electrode to accumulate electric charges. Figure 2a shows the scanning electron microscope (SEM) images of the
Figure 2. Scanning electron microscope (SEM) images of (a) the cross-view of sandwich-shape triboelectric nanogenerator, and the 40°tilted view of micro/nano dual-scale PDMS surface with (b) pyramid array and (c) V-shape grooves, respectively.
sandwich-shape triboelectric nanogenerator. The total size of the sandwich-shape triboelectric nanogenerator is 2 cm × 4 cm. To enlarge the friction area and further enhance the output performance, micro/nano dual-scale structures were introduced to the PDMS surface. First, well-designed Si-based microstructures (i.e., inverted pyramid array and V-shape grooves) were fabricated by using photolithography and KOH wet etching (Figure 1b ). Second, Si-based micro/nano dualscale structures was realized by using an improved DRIE process12−14 to fabricate high-dense nanotips atop microstructures (Figure 1b ). Finally, PDMS film with micro/ nano dual-scale structures was replicated from the above Si1169
dx.doi.org/10.1021/nl3045684 | Nano Lett. 2013, 13, 1168−1172
Nano Letters
Letter
Figure 3. Working mechanism of the sandwich-shape triboelectric nanogenerator. (a) Schematic diagram showing the electrical potential and working principle of the sandwich-shape triboelectric nanogenerator. (b) Comparison of the output signal and electrocardiography in one cycle. The upper left corner shows the photograph of the device.
especially relevant to the frequency effect. When the size dimension of nanogenerator decreases to 2 cm × 2 cm, its output performance does not decline as the frequency of the external force increases even as high as 10 Hz. Structure Effect. The effect of structural geometric morphology on the output performance of sandwich-shape triboelectric nanogenerator has also been studied, as is shown in Figure 5. In brief, the open-circuit voltage and short-circuit current are enhanced via increasing the interface roughness of the friction surfaces. It has been demonstrated that patterned material surface with microstructures can enhance the output performance in previous works.8−10 Here, this work introduced micro/nano dual-scale structures, which increases the effective roughness than pure microstructures or nanostructures, to strengthen the triboelectric nanogenerator performance. The advantages of the dual-scale structure have been demonstrated by comparing the output performance of micro/nano dual-scale structure (Figure 5e,g) with nanostructure (Figure 5c) and microstructure (Figure 5e,f). Therefore, the micro/nano dualscale PDMS film with V-shape grooves (Figure 5e) enlarged the open-circuit voltage and the short-circuit current by 61.4% and 118% than that of flat PDMS film (Figure 5b), respectively. Moreover, the micro/nano dual-scale PDMS film with pyramid
Figure 4. Characterization of the output performance of the sandwichshape nanogenerator with nanostructured PDMS film under external forces with different frequencies in the range of 1−10 Hz.
maximum peak voltage of the frequency-multiplication output reached 62 V under 3 Hz compressive force. It is worth mentioning that the size dimension of the nanogenerator is 1170
dx.doi.org/10.1021/nl3045684 | Nano Lett. 2013, 13, 1168−1172
Nano Letters
Letter
Figure 5. Characterization of the output performance of the sandwich-shape triboelectric nanogenerator with different structures, including (a) flat PET film: (a1) 2 cm × 2 cm or (a2) 2 cm × 4 cm, (b) flat PDMS film, (c) surface-nanostructured PDMS film, (d) PDMS film with micro V-shape grooves, (e) PDMS film with micro/nano dual-scale V-shape grooves, (f) PDMS film with micro pyramid arrays, (e) PDMS film with micro/nano dual-scale pyramid arrays.
Figure 6. Applications of the sandwich-shape triboelectric nanogenerator for electric device and biomedical system. (a) Five paralleled commercial LEDs were directly lightened without any energy storage device or rectification circuit. (b) Implantable 3-D microelectrode array (i,ii) for neural prosthesis was directly driven, and the current reached 88 μA (iii).
arrays (Figure 5g) showed more attractive performance, which further enhanced the voltage and the current by 100% and 157% than that of flat PDMS film (Figure 5b), respectively.
The measured output waveform of micro/nano dual-scale pyramids has been given in Figure S3 in the Supporting Information file. However, the microscale morphology is the 1171
dx.doi.org/10.1021/nl3045684 | Nano Lett. 2013, 13, 1168−1172
Nano Letters
Letter
Author Contributions
dominant factor according to the small difference of output performance compared with micro/nano dual-scale structure. Applications. The output performance of this novel triboelectric nanogenerator is remarkable with the highest peak voltage of 465 V and current of 107.5 μA, due to the archshape sandwich structure and the micro/nano dual-scale structures. This output is large enough to directly drive many micro/nanosystems, even some commercial electronic devices. Figure 6a shows that five paralleled LEDs were directly lightened without any energy storage unit or rectification circuit. Figure 6a and show the photos of LEDs driven by the frequency-multiplication output (small peak) and the main output (large peak) during one compressive force cycle. More details are shown in Video 2 in the Supporting Information file. This sandwich-shape triboelectric nanogenerator has been demonstrated to drive an implantable 3-D microelectrode array for neural prosthesis (Figure 6b ) without additional circuits. The 3-D silicon-tip array with 20 tips was fabricated by the combination of anisotropic and isotropic etching, and then the sharp tip-shape was transferred to the Parylene C film as a flexible and biocompatible substrate. The patterned platinum electrodes with an intermediate layer of gold were realized via an aluminum-photoresist dual-layer lift-off process.19,20 Phosphate buffered saline (PBS), made of 4.0 g of NaCl, 0.1 g of KCl, 0.12 g of KH2PO4, 1.82 g of Na2HPO4, and 1000 mL of water, was chosen as the artificial physiological environment for the test. The measured current reached 88 μA, as is shown in Figure 6b . At this stage, there are still several challenges, such as flexibility, compatibility, and integration, to overcome. However, this novel generator has shown an attractive application future. For example, it can be placed on human arms or even between two eyelids21 to harvest low-frequency mechanical energy and convert to frequency-multiplication high-output electric energy, which can be used directly to drive biomedical micro/nanosystems (Video 3 in the Supporting Information file). Conclusion. This sandwich-shape triboelectric nanogenerator shows outstanding performance to harvest low-frequency mechanical energy and generate frequency-multiplication highoutput electric energy. The novel frequency-multiplication property makes it attractive for high-frequency micro/nanosystems. The reliable high-output performance has been demonstrated by lightening five-paralleled commercial LEDs and driving an implantable 3-D microelectrode array for neural prosthesis without additional components, which is also the first demonstration of the nanogenerator for driving biomedical microsystem directly. This sandwich-shape triboelectric nanogenerator shows attractive potential applications in micro/ nanosystems, especially in biomedical fields.
■
ASSOCIATED CONTENT
■
AUTHOR INFORMATION
†
These two authors contributed equally to this work.
Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS The authors would like to thank colleagues of the institute of microelectronics for helpful discussions and assistance in the experiments. This work is supported by the National Natural Science Foundation of China (Grand No. 91023045 and No. 61176103), National Ph.D. Foundation Project (20110001110103), and Key Laboratory Fund (No. 9140C790103110C7903).
■
REFERENCES
(1) Wang, Z. L. Adv. Mater. 2012, 24, 280−285. (2) Wang, Z. L. Angew. Chem., Int. Ed. 2012, 51, 11700−11721. (3) Wang, Z. L. Nanogenerators for Self-Powered Devices and Systems; SMARTech digital repository, Georgia Institute of Technology: Atlanta, GA, 2011. (4) Lewis, N. S. Science 2007, 315, 798−801. (5) Himmel, M. E. Science 2007, 316, 982−982. (6) Fan, F. R.; Tian, Z. Q.; Wang, Z. L. Nano Energy 2012, 1, 328− 334. (7) Yang, Y.; Lin, L.; Zhang, Y.; Jing, Q. S.; Hou, T. C.; Wang, Z. L. ACS Nano 2012, 6, 10378−10383. (8) Fan, F. R.; Lin, L.; Zhu, G.; Wu, W. Z.; Zhang, R.; Wang, Z. L. Nano Lett. 2012, 12, 2109−3114. (9) Zhu, G.; Pan, C. F.; Guo, W. X.; Chen, C. Y.; Zhou, Y. S.; Yu, R. M.; Wang, Z. L. Nano Lett. 2012, 12, 4960−4965. (10) Wang, S. H.; Lin, L.; Wang, Z. L. Nano Lett. 2012, 12, 6339− 6346. (11) Xu, S.; Qin, Y.; Xu, C.; Wei, Y. G.; Yang, R. S.; Wang, Z. L. Nat. Nanotechnol. 2010, 5, 366−373. (12) Sun, G. Y.; Gao, T. L.; Zhao, X.; Zhang, H. X. J. Micromech. Microeng. 2010, 20, 075028. (13) Zhang, X. S.; Di, Q. L.; Zhu, F. Y.; Sun, G. Y.; Zhang, H. X. Micro Nano Lett. 2011, 6, 947−950. Zhang, X.; Di, Q.; Zhu, F.; Sun, G.; Zhang, H. J. Nanosci. Nanotechnol. 2013, 13, 1539−1542. (14) Zhang, X. S.; Zhu, F. Y.; Sun, G. Y.; Zhang, H. X. Sci. China Ser. E Tech. Sci. 2012, 55, 3395−3400. (15) Peter, N. J.; Zhang, X. S.; Chu, S. G.; Zhu, F. Y.; Seidel, H.; Zhang, H. X. Appl. Phys. Lett. 2012, 101, 221601. (16) Rebeiz, G. M. RF MEMS: Theory, design, and technology; John Wiley & Sons: New York, 2003; pp 21−86. (17) Hurst, J. W. Circulation 1998, 18, 1937−1942. (18) Gu, L.; Cui, N. Y.; Cheng, L.; Xu, Q.; Bai, S.; Yuan, M. M.; Wu, W. W.; Liu, J. M.; Zhao, Y.; Ma, F.; Qin, Y.; Wang, Z. L. Nano Lett. 2013, 13, 91−94. (19) Wang, R. X.; Zhao, W.; Wang, W.; Li, Z. H. J. Microelectromech. Syst. 2012, 21, 1084−1089. (20) Wang, R. X.; Huang, X. J.; Liu, G. F.; Wang, W.; Dong, F. T.; Li, Z. H. J. Microelectromech. Syst. 2010, 19, 367−374. (21) Lee, S.; Bae, S. H; Lin, L.; Yang, Y.; Park, C.; Kim, S. W.; Cha, S. N.; Kim, H.; Park, Y. J.; Wang, Z. L. Adv. Funct. Mater. 2012, DOI: 10.1002/adfm.201202867.
S Supporting Information *
Videos 1−3: Video 1 shows the test system. Video 2 shows five paralleled LEDs directly driven by pressing a sandwich-shape triboelectric nanogenerator. Video 3 shows an implantable 3-D microelectrode array driven by the sandwich-shape triboelectric nanogenerator. This material is available free of charge via the Internet at http://pubs.acs.org. Corresponding Author
*E-mail:
[email protected]. 1172
dx.doi.org/10.1021/nl3045684 | Nano Lett. 2013, 13, 1168−1172
