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Abstract
We experimentally demonstrate the fabrication of silver/polydimethylsiloxane (PDMS) composite microline structures based on the simultaneous induction of the photoreduction of silver ions and the photopolymerization of PDMS using a femtosecond laser. We show that the fabricated line structures exhibit electrical conductivity and that the resistance of the structures increases when they are subjected to mechanical force. Demonstration of sensitivity to an external mechanical force obtained by exploiting flexible composite structures fabricated by a femtosecond laser is reported for the first time. The present technique using a femtosecond laser is promising for the fabrication of flexible optical/electrical devices.
© 2017 Optical Society of America
1. Introduction
Polydimethylsiloxane (PDMS) has been widely utilized in microfluidic devices [1], in microlenses [2], and as a sponge for oil absorption [3] owing to its high optical transparency, mechanical flexibility as well as elasticity, and inertness. PDMS is also an attractive soft material for integration with metal in the emerging fields of flexible optics/electronics for applications such as stretchable displays [4] and flexible pressure sensors [5]. It is often used as a flexible substrate on the surface of which metal structures, typically gold and silver, are patterned using microfabrication techniques including lift-off processes and printing/stamping techniques [6–8]. Fabrication of PDMS microtubes containing a liquid metal alloy has also been reported [9]. A few papers recently reported optical/electrical functionalization of PDMS by mixing it with metals or carbon materials [10–12]. For example, PDMS films with doped metal nanoparticles were fabricated by immersing the PDMS films in gold or silver ion solutions [10] or by polymerizing a mixture of PDMS and gold or silver ions [11] to obtain metal/PDMS composite films having plasmonic optical properties. Electrically conductive carbon black/PDMS composite films were fabricated by polymerizing a mixture of carbon black and PDMS [12]. There have also been a few investigations of micro-/nanoshaping of optically/electrically functionalized PDMS. Functionalized PDMS structures were precisely shaped by molding [12] and wet etching [13]; however, most of the reported shaping techniques were based on two-dimensional patterning.
Laser additive manufacturing is an attractive technique for fabrication of three-dimensional (3D) arbitrary complex structures [14]. Note that the femtosecond laser is a powerful tool for additive manufacturing based on photoreduction of metal ions and photopolymerization. There have been various reports on photoreduction-based fabrication of metal structures, including gold or silver micro-/nanostructures, not only on glass and polymer substrates [15–18] but also inside host materials [19, 20]. On the other hand, photopolymerization-based fabrication of polymer microstructures such as SU-8 [21] and protein [22] has been reported not only on glass substrates but also inside a glass channel [23]. However, despite the versatility of PDMS, only a few papers have reported fabrication of PDMS micro-/nanostructures based on photopolymerization using a femtosecond laser [24–27], e.g., a report on fabrication of tunable PDMS microlenses by Lu et al. [27].
Fabrication of polymer microstructures with doped metal nanostructures based on photopolymerization using a femtosecond laser has been reported recently for optical/electrical applications in order to integrate soft materials with metals. For example, microstructures of gold-nanorod-doped polymers including polyacrylamide [28], bovine serum albumin [29], and methyl methacrylate [30] have been fabricated by femtosecond laser irradiation. More interestingly, metal/polymer composite structures have been fabricated by simultaneous induction of photoreduction and photopolymerization using femtosecond laser irradiation [31–35]. For example, Shukla et al. reported a pioneering study on the fabrication of gold/SU-8 composite line structures having plasmonic optical properties and electrical conductivity [31]. Blasco et al. reported fabrication of gold/acrylate-functionalized poly(ethylene glycol) derivative (PEG-triacry) composite bridge structures and demonstrated that the electrical conductivity varied with the mixture ratio of the components [32]. Sun et al. reported fabrication of silver/silk fibroin composite line structures showing electrical conductivity [33]. Nakamura et al. reported fabrication of electrically conductive gold/SU-8 composite line structures [34]. To the best of our knowledge, however, sensitivity to an external mechanical force obtained by exploiting flexible composite structures fabricated by a femtosecond laser has not been reported.
In this study, we demonstrate fabrication of metal/PDMS composite structures based on simultaneous induction of photoreduction and photopolymerization using a femtosecond laser. We experimentally show that irradiation of a mixture of silver ions and PDMS by a femtosecond laser results in fabrication of composite line structures that have silver in the center and are covered with PDMS. The electrical conductivity of the fabricated line structures is also confirmed. Moreover, we demonstrate the sensitivities of fabricated structures to change in resistance induced by an external mechanical force by exploiting their flexibility and elasticity.
2. Materials and methods
Photocurable PDMS (KER-4690A/B) was provided by Shin-Etsu Chemical Co., Ltd. (Japan). PDMS (1 mg) was degassed under vacuum and mixed with silver benzoate (Sigma-Aldrich, USA) in hexane (1 μl, 2 mg/ml) with a magnetic stirrer for 10 min. The mixture was sandwiched between two 150-μm-thick cover glasses to obtain a uniform height of the mixture and was placed on an xyz translation stage. Figure 1 shows the experimental setup for fabrication of composite line structures by laser irradiation. Femtosecond laser (HighQ-2, Spectra-Physics, USA) oscillating laser pulses of central wavelength of 522 nm, pulse duration of 192 fs, and repetition rate of 63 MHz was used. The beam diameter of the laser pulses was expanded using a pair of plano-convex lenses, and the pulses were focused by an objective lens (numerical aperture (N.A.) 0.4, Olympus, Japan). The focal point in the z direction was set on the boundary between cover glass 1 and the mixture using the translation stage. Focused laser pulses were scanned in the x and y directions at a scanning speed of 2 mm/s. Fabrication process was monitored in real time using a CMOS camera in the optical setup shown in Fig. 1. After laser irradiation, the unsolidified portion of the mixture was removed using tetrahydrofuran (THF), and the fabricated structures were observed by optical microscopy and scanning electron microscopy (SEM). Elemental analysis of the fabricated structures was conducted using energy-dispersive X-ray spectroscopy (EDX) with an electron accelerating voltage of 15 kV.
Fig. 1 Optical setup for fabrication of composite line structures by laser irradiation with in situ monitoring.
Figure 2 shows the experimental procedure for conductivity measurement of the fabricated composite line structures. An 8-nm-thick gold thin film was deposited on a cover glass by ion sputtering, as shown in Fig. 2(a). Laser ablation was performed by scanning the focused femtosecond laser pulses with the same oscillator to obtain an approximately 90-μm-wide gap in the gold film to prepare two gold electrodes (Fig. 2(b)). The laser power for laser ablation was 120 mW. No conductivity was obtained between the two gold electrodes before the line structure was fabricated, which was confirmed by two-terminal measurement. To fabricate the line structure across the two gold electrodes, laser pulses were used to scan the mixture sandwiched between the cover-glass-deposited gold electrodes and a cover glass without the electrodes (Fig. 2(c)). Current–voltage curves of the line structures were obtained by two-terminal measurement using a digital source meter (2401, Keithley, USA) (Fig. 2(d)). For the measurement, the voltage was varied from 0 to 5 V. To investigate the dependence of the conductivity of the line structure on external mechanical forces, temporal profiles of the resistance were obtained while the fabricated structure was subjected to air-blowing for 5 s. The distance between the air-blowing nozzle and the fabricated structure was fixed to be 5 mm. Air-blowing was repeated three times at 30 s intervals (Fig. 2(e)). Temporal profiles of the resistance of the line structure were measured using a source measure unit (2401, Keithley, USA). The pressure produced by the blowing air was separately measured to be 3 kPa at the place of the line structure by using a digital pressure sensor (GP-M001, Keyence).
Fig. 2 Experimental procedure for electrical conductivity measurement of a fabricated composite line structure. (a) Ion sputtering of an 8-nm-thick gold thin film on a cover glass. (b) Preparation of gold electrodes. Laser ablation was carried out to obtain approximately 90-μm-wide gap of the gold film. (c) Fabrication of a line structure across the two gold electrodes. (d) Electrical conductivity measurement of the line structure. (e) Temporal profile of the resistance of the line structure when 5-second-air-blowing applied.
3. Results and discussion
3.1 Demonstration of the fabrication of a silver/PDMS composite line structure
Figure 3(a) shows an optical microscope image of a structure fabricated by femtosecond laser irradiation of a mixture of silver benzoate in hexane and PDMS (“the mixture” hereafter) deposited on a cover glass. Laser pulses were focused on the boundary between cover glass 1 and the mixture shown in Fig. 1. A line structure with an average line width of 23.3 μm was observed along the scanning direction. The line structure was observed at a focal depth above the surface of the cover glass, which indicates that the line structure was not obtained by laser ablation of the surface of the cover glass, which is also confirmed by the SEM image of the fabricated structure (Fig. 3(b)). The unsolidified portion of the mixture was removed by THF before the observation; therefore, the line structure remained on the surface of the cover glass after fabrication by femtosecond laser irradiation. Since a PDMS film containing silver benzoate has little linear optical absorption at 522 nm [11], the fabrication of the structure is mainly attributable to two-photon absorption. Dark structures are distributed in the center of the line structure, whereas translucent structures are clearly visible on the boundary of the line structures. This result suggests that femtosecond laser irradiation of the mixture induced both reduction of silver ions and polymerization of PDMS; i.e., a line structure composed of silver and translucent PDMS was fabricated.
Fig. 3 (a) Optical microscope image of a structure fabricated by femtosecond laser irradiation of a mixture of PDMS and silver benzoate in hexane. Laser power was 60 mW. Scanning speed was 2 mm/s. Number of laser scans was 5. Scale bar indicates 25 μm. (b) SEM image of the fabricated structure shown in (a). Scale bar indicates 25 μm.
Figure 4 shows EDX spectra, which reveal that the obtained line structure was composed of silver and PDMS. Figure 4(a) shows the EDX spectrum of the center of the line structure fabricated by femtosecond laser irradiation. Not only a Ag signal but also Si and O signals were detected. Figure 4(b) shows the EDX spectrum at a position with no line structure in the same sample, in other words, a position where laser irradiation did not occur. No Ag signal was detected, whereas signals of Na, Al, and K were detected in addition to signals of Si and O. These results demonstrate that a line structure that contains silver was fabricated by femtosecond laser irradiation. The Na, Al, and K signals were attributable to the components of the borosilicate glass used as a cover glass. The low signals of the Na, Al, and K from the line structure (Fig. 4(a)) were attributable to a small amount of characteristic X-rays generated on the glass surface because the line structure probably attenuated the electron beam penetrating the glass surface. The line structure could attenuate the generated characteristic X-rays propagating to the detector. This could also be confirmed by the EDX spectrum of a PDMS line structure without silver ions which was fabricated by femtosecond laser irradiation (Fig. 4(c)). The Si signal appears in Figs. 4(a)-(c). On the line structures, the Si signal originating from the cover glass should be decreased as similar to the Na, Al, and K signals; however, the result in Fig. 4(a) and (c) shows a significant Si signal. These results indicate that the certain Si signal detected in Figs. 4(a) and (c) were attributable to polymerized PDMS, which is consistent with the fact that the line structure contains translucent structures, as shown in Fig. 3(a). From the above results, the line structure fabricated by laser irradiation was found to be a composite line structure containing silver and PDMS.
Fig. 4 EDX spectra of (a) line structure fabricated by femtosecond laser irradiation of the mixture, (b) a position that the laser did not irradiated in the same sample, and (c) PDMS line structure fabricated without silver ions by femtosecond laser irradiation.
3.2 Fabrication of silver/PDMS composite line structures under different irradiation conditions
Figure 5(a) shows optical microscope images of silver/PDMS composite line structures fabricated by femtosecond laser irradiation with different numbers of laser scans. With 1 scan, dark structures were distributed intermittently along the scanning direction. The dark structures are probably aggregation mainly composed of the formed silver micro/nano particles. With 5 scans, dark structures with a larger line width were obtained (Fig. 3(a)). Multiple scans probably increased reduction of silver ions inside and/or around the line structure because of the increase in optical absorption by the fabricated composite structures. Although the laser wavelength of 522 nm is not in resonance to the absorption peak of silver nanoparticles [11] which could be formed inside a PDMS film, slight increase in optical absorption could induce heat effect as well as plasmonic (off-resonance) electric field enhancement, which resulted in the enhancement of photoreduction and photopolymerization. These phenomena could increase the line widths of dark structures and translucent structures. With 10 and 20 scans, uniformity of the fabricated structures increased, which was probably because optical absorption to the fabricated line structures became gradually stable. Dark structures with line widths comparable to the result obtained with 5 scans were fabricated in the center of the line structure and were surrounded by translucent structures. It is possible that, in addition to direct optical polymerization of PDMS, the polymerization was enhanced by the above phenomena induced by the fabricated silver nanostructures. Enhancement of polymerization might also occur by oxidizing species such as 1O2 produced by femtosecond laser irradiation as similar to the case of previous studies [33, 36].
Fig. 5 Optical microscope images of silver/PDMS composite line structures fabricated by femtosecond laser irradiation of the mixture fabricated using different laser parameters. Scanning speed was 2 mm/s. (a) 1, 10, or 20 scans were used. Laser power was 60 mW. (b) Laser power of 50, 70, or 80 mW was used. Number of laser scans was 10. All the scale bars in the figures indicate 25 μm.
Figure 5(b) shows optical microscope images of line structures obtained at different laser powers. The line widths of the structures increased with increasing laser power because a higher laser power increases the area at which the laser intensity exceeds the thresholds for photoreduction and photopolymerization in the irradiated beam area for a Gaussian optical intensity. The line widths of the line structures fabricated in this study were thicker than 10 μm. When a laser pulse at a wavelength λ of 522 nm is focused on the cover glass with an objective lens of N.A. = 0.4, the theoretical beam diameter d is ~1.6 μm according to the formula d = 1.22λ/N.A. The line widths obtained experimentally were thicker than the calculated beam diameter. A possible explanation is that photoreduction and photopolymerization could occur away from the center if the local laser intensity exceeded their thresholds. In addition, because a high-repetition-rate femtosecond laser was used in this study, heat accumulation is not negligible.
3.3 Conductivity measurement of silver/PDMS composite line structures
Figure 6(e) shows an optical microscope image of a silver/PDMS composite line structure fabricated by femtosecond laser irradiation of the mixture deposited on a cover glass with two gold electrodes. The current–voltage curve of the line structure is shown in Fig. 6(a). The average line width of the line structure was 33.9 μm (Fig. 6(e)), which was 9.4 μm larger than the average width of the line structure fabricated on the cover glass without electrodes under the same laser irradiation condition (Fig. 5(b)). The increase in the line width can be explained by enhanced reduction of silver ions and polymerization of PDMS due to heat generated by thermal relaxation of the laser energy absorbed by the gold electrodes. The line width of the line structure is constant on both the electrodes and the gap, where there is no gold electrode. This result can probably be explained by the fact that the increase in optical absorption by the composite structures fabricated on the gold electrodes induced more heating of the surrounding area, which resulted in the larger line width. As shown in Fig. 6(e), the current increased linearly with increasing applied voltage. When a linear approximation was applied to the obtained current–voltage curve, the resistance R of the line structure was calculated to be 28.8 kΩ. If the cross section of the line structure is assumed to be a half circle with a diameter comparable to the measured line width of the line structure, the resistivity ρ is calculated to be 5.9 × 10−1 Ωm, assuming a measured distance between the electrodes of 87.9 μm and a measured line width of 33.9 μm. This resistivity is much larger than that of bulk silver [37]; however, it is notable that the resistivity is comparable to the resistivity of polymer/gold composite structures fabricated by femtosecond laser irradiation (before annealing) by other groups [32]. As shown in Fig. 6(e), the formed silver seems to be connected inside the composite line structure, but it is possible that a small amount of PDMS exists, as well as spaces, between the silver structures.
Fig. 6 Conductivity measurement of a silver/PDMS composite line structure fabricated by femtosecond laser irradiation of the mixture deposited on a cover glass with two gold electrodes. (a) Current–voltage curve of the fabricated structure shown in (e). (b) Resistances of composite line structures fabricated by femtosecond laser irradiation of the mixture with different numbers of scans. Laser power was 60 mW. Scanning speed was 2 mm/s. (c)–(f) Corresponding optical microscope images of the line structures with different numbers of scans: (c) 2, (d) 4, (e) 10, and (f) 20 scans. All the scale bars in the figures indicate 50 μm.
Figure 6(b) shows the resistances of the composite line structures fabricated by femtosecond laser irradiation of the mixture with different numbers of scans. Figures 6(c)–(f) show the corresponding optical microscope images of the line structures fabricated using different numbers of scans. The resistance of the line structure fabricated using 2 scans was 100.2 kΩ, whereas it decreased to 48.8 kΩ when the scan number increased to 4. The resistance decreased further for 10 and 20 scans. This is because increasing the number of scans induced increased photoreduction of silver ions, and the silver density of the line structure could be increased. In addition, multiple laser scans of existing composite structures could improve the connection between silver structures inside the line structures because of the thermal effect of the laser scans. Such thermal annealing might have contributed to improving the conductivity of polymer/metal composite structures, as in the case reported by Blasco et al. [32]. In addition, the lower resistance with increasing number of laser scans is possibly attributable to the increase of PDMS layers as well.
3.4 Sensing of external mechanical forces by the fabricated silver/PDMS composite line structures
Figure 7 shows temporal resistance profiles of the silver/PDMS composite line structures when they were subjected to air-blowing. The horizontal axis shows elapsed time, and the vertical axis shows the ratio of the measured resistance R to the initial resistance R0. The line structures were fabricated using 4 (a), 10 (b), and 20 scans (c). Air-blowing was applied to the line structures for 5 s at 30 s intervals, as indicated by blue areas in the figures. During the first application of air-blowing to the line structure fabricated using 4 scans, the resistance increased by ~9% (Fig. 7(a)). The increase in the resistance of the line structure is attributable to bending due to the air-blowing followed by slight stretching of the line structure and rearrangement of silver structures inside the structure. Amjadi et al. reported that the resistance of a stretched polymer containing metal nanostructures increased because of increasing number of disconnections between metal nanostructures inside the polymer [38]. The resistance increase by bending the composite structures in the present study can also be explained as similar to the previous report. Immediately after air-blowing stopped, the resistance decreased significantly, probably because the line structure that was bent returned to its initial straight shape. The resistance became constant ~5 s after air-blowing stopped, but the resistance was slightly larger than the initial value. This is probably because the structure bent by air-blowing did not completely return to its initial straight shape. During the second and third applications of air-blowing, an increase in the resistance as similar to the first application was observed, indicating that the resistance of the line structure showed reproducible responses to air-blowings. The resistance increase was ~11% during the second air-blowing, which was larger than that during the first air-blowing. This was attributed to further bending of the line structure by later applications of air-blowing. These results show that we demonstrated sensing of external mechanical forces using the resistance increase of the line structure and that reproducible sensing is possible. Note that we could not observe significant bending of the fabricated structures subjected to air-blowing with observation with a digital microscope. In other words, the change in resistance of the fabricated structure is highly sensitive.
Fig. 7 (a)–(c) Temporal resistance profiles of the silver/PDMS composite line structures during air-blowing. Laser power was 80 mW. Scanning speed was 2 mm/s. Different numbers of laser scans were used: (a) 4, (b) 10, and (c) 20 scans. Air-blowing was applied to the line structures for 5 s at 30 s intervals, as indicated by blue areas in the figures. R0 indicates the initial resistances of the fabricated structures. (d) Average values of the maximum measured R/R0 obtained during each application of air-blowing to samples fabricated using different numbers of laser scans.
A resistance increase during air-blowing was also observed for the line structures fabricated using 10 (Fig. 7(b)) and 20 scans (Fig. 7(c)). For the structures fabricated using 10 and 20 scans, the resistance increase during the second and third applications of air-blowing was equivalent to that during the first air-blowing, unlike the result for the sample fabricated using 4 scans. The resistance of the line structures fabricated using 10 and 20 scans showed response for sensing an external mechanical force with higher reproducibility than the line structures fabricated using 4 scans. Increasing the number of scans might increase the robustness of the structures. Figure 7(d) shows the average values of the maximum measured R/R0 obtained during each application of air-blowing to samples fabricated using different numbers of laser scans. The resistance increase was ~10% for the sample fabricated using 4 scans, whereas it was ~5% for the 10-scan sample and ~6% for the 20-scan sample. This result shows that increasing the number of scans resulted in less bending of the line structures when they were subjected to air-blowing because the robustness of the line structures increased as the amount of silver reduction increased with a larger number of scans. A PDMS microtube containing a liquid metal alloy has reportedly been used to sense air-blowing [9]. In that paper, the deformation of the microtube by air-blowing at 50–100 kPa increased the resistance by ~2% [9]. The resistance of the silver/PDMS composite line structures fabricated in this study increased by ~5–10% when they were subjected to air-blowing at 3 kPa, indicating high sensitivity to external mechanical forces.
4. Conclusion
We demonstrated fabrication of silver/PDMS composite microline structures by femtosecond laser irradiation of a mixture of silver benzoate in hexane and PDMS, which was confirmed by optical microscopy, SEM, and EDX. This result was attributable to simultaneous induction of photoreduction and photopolymerization by the femtosecond laser. The fabricated line structures exhibited electrical conductivity. Moreover, we showed that the resistance of the fabricated line structures increased when they were subjected to air-blowing, which was attributable to mechanical bending of the structures. To the best of our knowledge, this study is the first report of sensitivity to a mechanical force obtained by exploiting flexible composite structures fabricated by a femtosecond laser. Fabrication of silver/PDMS composite microstructures by additive manufacturing using a femtosecond laser is promising for fabricating flexible and elastic composite microstructures in the emerging fields of flexible optics/electronics.
Funding
This work was partially supported by a grant from The Amada Foundation and by JSPS and DAAD under the Japan-Germany Research Cooperative Program.
Acknowledgments
Y. Nakajima is grateful for a Grant-in-Aid for Research Fellow of the Japan Society for the Promotion of Science (JSPS). PDMS used in this study was provided by Shin-Etsu Chemical Co., Ltd.
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