ABSTRACT: Dielectric elastomer actuators have recently been used to drive loudspeakers andacoustic absorbers. So far, these acoustic devices are opaque due to use of metallic or carbon-grease compliant electrodes. A transparent device of acoustic-absorber is desirable for large-areal installation to glass window or roof. There were reports of transparent compliant electrode based on ionic hydrogel, which however does not last long when water evaporates. This paper investigates the use of transparent conductive polymer and its printing to make a transparent acoustic absorber. We formulated the aqueous ink with improved ink’s wettability to the elastomeric substrate. In addition, we optimized the droplet spacing to form a continuous electrode coating. The ink-jet printing enables the hassle-free patterning of transparent compliant electrodesto make a micro-perforated dielectric elastomer actuator. Testing shows that this transparentmembrane DEA can produce a maximum voltage-induced radial expansion of 20%; whereas, a transparent perforated-membrane DEA can size down the holes by 15%, for tuning the acoustic resonant frequency.
Dielectric elastomer actuators have recently been used to drive loudspeakers (R. Heydt, R.Kornbluh et al. 1998, Keplinger, Sun et al. 2013) and acoustic absorbers (Lu, Shrestha et al.2017). They can be either a solid membrane or a perforated one. A solid-membrane-type actuatorchanges their axial tension upon voltage activation. In turn, it vibrates out-of-plane for soundgeneration. A perforated-membrane-type actuator can change their perforation size and tensionupon voltage-activation. It can tune the resonant frequency of Helmholtz resonator and shift itsoptimal absorption spectrum, which is broader in bandwidth than the membrane-type absorber.
A dielectric elastomer actuator (DEA) is a soft capacitor that generates a large deformation u the voltage activation. They consist of two compliant electrodes sandwiching a dielectric elastomer membrane. When a high voltage (V) is applied between the electrodes it induces a Maxwell stress
across the dielectric elastomer membrane of thickness t, relativepermittivity and is the vacuum permittivity (Ronald E. Pelrine, Roy D. Kornbluh et al. 1998).
INK-JET PRINTING OF TRANSPARENT AND STRETCHABLE ELECTRODES FOR DIELECTRIC ELASTOMER ACTUATOR
MILAN SHRESTHA*Singapore Centre for 3D printing, School of Mechanical and Aerospace Engineering, Nanyang
Technological University, Singapore 639798
ZHENBO LUTemasek Laboratories, National University of Singapore, Singapore 117411,
GIH KEONG LAUSchool of Mechanical and Aerospace Engineering, Nanyang Technologica,l University,
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liquids as electrodes. As they are liquid-based conductors, prolonged exposure to air can dry and degrade their properties. Moreover, a voltage above a certain limit can cause the ions to cross the dielectric interface. This degrades dielectric property of the elastomer. Therefore, transparent DEA-based devices need an easy pattering method and the stable transparent electrode.
We report the fabrication of a transparent DEA using inkjet printing technology. Transparent and stretchable poly(3,4-ethylene dioxythiophene) polystyrene sulfonate (PEDOT:PSS) (Eom, Senthilarasu et al. 2009, Lau and Shrestha 2017) electrode is printed. PEDOT:PSS is obtained in the form of water-based suspension. However, a uniform coating of aqueous suspension is impossible on acrylate elastomer due to uneven spread on the low hydrophilic surface (see Figure 3(a)). To resolve this problem, a surfactant (Triton-x100) was added to the aqueous conductive ink which greatly improves the ink’s wettability to the elastomeric substrate(Vosgueritchian, Lipomi et al. 2012, Yoon and Khang 2016). Inkjet printing technology simplified the electrode patterning steps by eliminating masking, sputtering and wrinkling processes (see Figure 2(right)). Besides, we optimized the drop spacing parameter of the inkjet printer to obtain a sufficiently transparent and conductive film.Upon drying it forms a stable DEA.
Figure 2. Fabrication steps of an MPDEA; (left) previously used steps to make MPDEA with the wrinkled gold thin film; (right) current fabrication with inkjet printing of PEDOT:PSS electrode.
MATERIALS AND METHODS
Material
Figure 3. (a) Poor wettability of pristine PEDOT:PSS suspension on VHB and hence formed non-uniform films. Mixing surfactant Triton-x100 enhanced the wettability and enables to form a uniform film. (b) Table showing various proportions of water and Triton-x100 and corresponding optical and electrical property.
(a) Pre-stretching of VHB
(f) Partial release of pre-stretch to wrinkle the thin- film electrodes
(b) Teflon stencil masking
(c) DC magnetron Sputtering
Gold thin film electrode
(d) Mask removal
(e) Laser drilling to make pores
Repeat the electrode patterning on the other side of VHB
(a) Pre-stretching of VHB
(b) Inkjet Printing of PEDOT:PSS thin film
(e) Laser drilling to make pores
Repeat the electrode patterning on the other side of VHB
Previous fabrication steps Current fabrication steps
11.5°
PEDOT:PSS withTriton-x100
44.54°
Pristine PEDOT:PSS suspension
Wet
ting
on V
HB
su
bstra
tePr
inte
d on
VH
B
Subs
trate
PEDOT:PSS/ Triton-x/
Water (wt%)
90% / 10% /
0%
61.3% / 6.8%/ 31.8%
40.78% / 4.5% / 54.72%
26.75% / 3% /
70.25%
Resistance (k ) 1.152 2.3746 4.4372 5.5416
Forms thin film on VHB Yes Yes Yes Yes
Transmittance @550nm (%) 84 89 91 89
(a) (b)
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RESULT AND DISCUSSION
In an inkjet printing system, the drop spacing determines the fraction of the surface covered by the thin film and its thickness (see Figure 5(b)). An optimum surface coverage can be expected if the drop spacings are close to the half diameter of the disc. While printing, each drop formed a disc of 38.10±0.61 m diameter and 0.27±0.008 m thickness. Therefore, we examined drop spacings of 10 m, 15 m, and 20 m. All three drop spacing could form a continuous line when printed along printer head scanning direction. However, the continuous film was only formed with 10 m and 15 m drop spacing. The reason was the lower resolution of the stage moving perpendicular to the printer head. Hence, the resistance of the printed films varied when measured parallel and perpendicular to the printer scanning direction. The thin films with drop spacing of 15 m and 20 m are highly transparent and sufficiently conductive. Hence these drop spacings are chosen for basic DEA fabrication (see Figure 5(d) and (e)).
DEAs with circular electrodes printed at drop spacing of 15 m and 20 m on 3 times pre-stretched VHB4910 substrates are tested. The high voltage applied between the sandwiching electrodes caused its active diameter to increase. Figure 6(a) shows a DEA with 15 m drop spacing can produce larger diameter change. In the actuation plots, the initial exponentially increasing strain indicates stretching of the electrode without losing conductivity. After stretching more than 10% in diameter, the samples with 20 m drop spacing shows a rapid drop in actuation rate. This is the indication of the electrode fracture and reduction in electrode area coverage. Meanwhile, the sample with 15 m drop spacing shows a reliable operation for more than 15% increase in diameter. Based on this observation, a 15 m drop spacing is a better choice to make a DEA.
Figure 5. Resolution test and effect of drop spacing on geometrical, optical and electrical properties of films formed by DMP 2381. (a) 3D morphology of a drop, line and a film of PEDOT:PSS formed by inkjet printing; Effect of drop spacing on (b) width and thickness of a printed line, (d) sheet resistance of a printed film and (e) transparency of the film.
MPDEAs with single and two layer of VHB is investigated. The patterned electrodes were printed with a drop spacing of 15 m. The electrodes of a single layer MPDEA are exposed to air. The dielectric breakdown of the air occurs above an applied voltage of 4kV through the holes. This limits their activation voltage (see Figure 6(b)). Meanwhile, the positive electrode of a two-layered MPDEA is isolated from air. It could be activated to a voltage above 5.5kV and reduce the hole diameter by more than 15%. This proves that this transparent MPDEA can produce a shift in the absorption spectrum similar to the device demonstrated by (Lu, Shrestha et al. 2017).
44.5
55.5
0 100 200Scan
hei
ght (
m)
Scan length ( m)
2.7
2.9
3.1
0 75 150Scan
hei
ght (
m)
Scan length ( m)
0.91
1.11.2
0 25 50 75Scan
hei
ght (
m)
Scan length ( m)
0.2
0.24
0.28
4045505560
10 15 20
Line
Thi
ckne
ss (
m)
Line
wid
th (
m)
Drop Spacing ( m)
020406080
100
400 500 600 700
Inlin
eTr
ansm
ittan
ce
(%)
Wavelength (nm)
10um Drop scapce15um Drop space20um Drop space0
48
1216
10 15 20
Shee
t R
esis
tanc
e(k
/)
Drop Spacing ( m)
Perpendicular to print directionParallel to print direction
(a)
(c)(b) (d)
Printed droplet Printed line Printed film
a
a’
b
cc’a-a’ b-b’ c-c’b’
b
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Figure 6. (a) Testing of basic DEAs with PEDOT:PSS printed at different drop spacing. Voltage is applied to observe the actuation strain produced. (b) A small prototype of the MPDEA device with printed PEDOT:PSS thin film as an electrode. Upon voltage activation, the active area expands and the hole diameter shrinks.
CONCLUSION
This report presented a transparent DEA that can be used to make a loudspeaker and an MPDEA device. The devices were made with inkjet printed PEDOT:PSS thin film electrode. Use of the inkjet printing technology simplified the fabrication process of the device compared to previously used methods. A basic DEA prototype fabricated by this method showed expansion stain of more than 20%. Meanwhile, an MPDEA prototype could tune the hole size similar to the previously reported devices. Using the printing technology has made these devices more easily fabricable at industrial scale. Further work can be done to improve the uniformity and transparency of the devices.
ACKNOWLEDGMENTS This research was supported by Singapore Millennium Foundation, funded by Temasek Trust. The first author, Mr. M. Shrestha, is grateful to Singapore Centre for 3D Printing (SC3DP), Nanyang Technological University, Singapore for supporting his Ph.D. scholarship.
REFERENCE Eom, S. H., et al. (2009). "Polymer solar cells based on inkjet-printed PEDOT:PSS layer."
987. Lau, G.-K. and M. Shrestha (2017). "Ink-Jet Printing of Micro-Electro-Mechanical Systems
(MEMS)." Micromachines 8(7): 194. Lu, Z., et al. (2017). "Electrically tunable and broader-band sound absorption by using micro-
perforated dielectric elastomer actuator." Applied Physics Letters 110(18): 182901. R. Heydt, et al. (1998). "Design and Performance of an Electrostrictive-Polymer-Film Acoustic
Actuator." Journal of Sound and Vibration 215(2): 297-311. Ronald E. Pelrine, et al. (1998). "Electrostriction of polymer dielectrics with compliant electrodes
as a means of actuation." Sensors and Actuators A: Physical 64: 77-85. Vosgueritchian, M., et al. (2012). "Highly conductive and transparent PEDOT: PSS films with a
fluorosurfactant for stretchable and flexible transparent electrodes." Advanced Functional Materials 22(2): 421-428.
Yoon, S.-S. and D.-Y. Khang (2016). "Roles of Nonionic Surfactant Additives in PEDOT:PSS Thin Films." The Journal of Physical Chemistry C 120(51): 29525-29532.
Chee Kai Chua, Wai Yee Yeong, Ming Jen Tan, Erjia Liu and Shu Beng Tor (Eds.)
