Strain sensing was demonstrated by utilizing electrically conductive silicon-carbide (β-SiC) fabricated by femtosecond-laser-based direct modification of polydimethylsiloxane (PDMS). Depending on the laser scanning direction used for the fabrication procedure, the fabricated structures showed different sensitivity to strain and this difference was discussed by observing the surface morphology at various bending radii using scanning electron microscopy (SEM). The change in electrical conductance at the flat state after repeated bending was also investigated. Furthermore, preliminary demonstration of human motion sensing was performed using the fabricated structures. The presented method will open doors to novel electronic device applications using PDMS.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
Flexible and stretchable strain sensors have attracted considerable amount of attention, owing to their potential novel applications including electronic skins (e-skins) , smart textiles , and health monitors . A piezoresistive strain sensor, which is one of the most investigated strain sensors, is, in many cases, composed of electrically conductive materials and a flexible and/or stretchable substrate. Such strain sensors have been demonstrated by combining a wide range of electrically conductive materials, including metals in the form of liquids , nanowires , nanoparticles , and thin-films  and carbon-based materials, including carbon black , carbon nanotubes , and graphene , with elastic substrates, such as polydimethylsiloxane (PDMS) , polyimide (PI) , and natural rubber . Various fabrication methods have been developed to fabricate such composite structures for strain sensing, including transferring methods , printing technology [15,16], and liquid-phase mixing . Although these methods enable the fabrication of strain sensors with promising performances, such as high sensitivity and high repeatability, simpler and faster methods that are capable of high-resolution patterning would considerably improve production-throughput and scalability.
Laser-based fabrication methods are being proposed as a compelling method to fabricate electrically conductive structures on elastic substrates for flexible electronics, including strain sensors. Noncontact, spatially selective, and high-resolution patterning of structures can be provided with laser processing. In addition, laser processing can allow for fewer process steps, since it can simultaneously synthesize and pattern conductive materials. Tian et al. reported the fabrication of a graphene/polyethylene terephthalate (PET) strain sensor by laser-based reduction of graphene oxide (GO) using an infrared continuous-wave (CW) laser . They also patterned the graphene structures into the form of micro-ribbons by scanning the laser beam, to enhance the gauge factor of the sensor. Laser-based modification of polymers has also been recognized as a simple but effective method to fabricate electrically conductive structures on flexible polymeric substrates. By laser irradiation, the polymers can be selectively modified into electrically conductive structures by photo-thermal and/or photo-chemical effects. As a pioneering work, Lin et al. reported a novel one-step method to fabricate and pattern electrically conductive carbonaceous structures on the surface of PI using an infrared CO2 laser (10.6 µm) . The formation of the structures is attributable to the pyrolysis of PI by the high localized temperatures resulting from the laser irradiation. As for the application of this method, flexible strain sensors were fabricated using the laser-modified PI sheets . Moreover, stretchable strain sensors were achieved by transferring the electrically conductive structures fabricated on PI, onto the surface of a PDMS substrate . Recently, Gao et al. fabricated electrically conductive SiC structures on the surface of an elastomer substrate, Ecoflex, by thermal-degradation using a CW laser (532 nm) for strain sensing . Nakajima et al. reported the fabrication of electrically conductive structures composed of crystalline silicon-carbide (β-SiC) on the surface of PDMS using a femtosecond laser (522 nm) . Since PDMS has high biocompatibility and optical transparency, in combination with high flexibility and elasticity, versatile applications as wearable devices would be realized.
In this paper, simple fabrication of a strain sensor is demonstrated by femtosecond-laser-based-modification of a widely used elastomer, PDMS. Electrically conductive structures were fabricated between electrodes by direct modification of PDMS, and the effect of bending on the electrical conductance of the fabricated structures was investigated. The change in electrical conductance with bending depended on the laser scanning direction used for fabrication, and the structures showed anisotropic property to applied strain. This difference was discussed using surface morphology observations conducted by scanning electron microscopy (SEM). The effect of repeated bending on the electrical conductance of the structures fabricated with different laser scanning directions was also assessed by subjecting the structures to a series of bending cycles. Furthermore, sensing of human motion was demonstrated using electrically conductive structures fabricated on PDMS.
2. Materials and methods
2.1 Material preparation
Since flexible substrates with high degrees of bending were required, thin sheets of PDMS were prepared. Liquid photo-curable PDMS (KER-4690A/B, Shin-Etsu Chemical Co., Ltd., Japan) was mixed with hexane at a 10:1 ratio and 3.0 ml of the mixture was poured into a rectangular mold with base dimensions of 3.5 cm x 6.5 cm. The uncured PDMS mixture was first de-gassed in a vacuum chamber for approximately 30 min. Following this, the mixture was placed under an UV-lamp with a wavelength of 365 nm for 30 min. at room temperature, to prepare flexible PDMS sheets with a thickness of approximately 680 µm.
2.2 Laser irradiation
Laser pulses with a central wavelength of 522 nm, the second harmonic wave of a 1045-nm femtosecond laser (High Q-2, Spectra-Physics, USA), 192-fs pulse length, and 63 MHz repetition rate was used for the irradiation process. Femtosecond laser pulses were focused with a 20x objective lens with a numerical aperture (NA) of 0.4 (Olympus, Japan), and irradiated onto the bottom surface of the PDMS in ambient conditions. Before irradiation, the PDMS sheets were cut and placed onto a cover glass with 140-µm-air-spacing. Spacings were implemented to exclude the effects of the cover glass on the fabrication process, as well as prevent damage to the structures due to adhesion to the coverglass when removing from the coverglass. The beam diameter at the focal point was assumed to be ∼1.6 µm, according to the formula d = 1.22λ/NA; where d is the beam diameter, λ is the laser wavelength. Laser fluence of 0.12 J/cm2 was used for the fabrication process. A 3-axis (xyz) translation stage was utilized to raster scan the PDMS in the xy-plane (8 mm in the x-direction and 3 mm in the y-direction). As shown in Fig. 1, two 8 mm x 3 mm structures, S1 and S2, were fabricated. For S1, femtosecond laser pulses were raster scanned in the longitudinal direction (8 mm direction), as shown in Fig. 1(a). For S2, femtosecond laser pulses were raster scanned in the transversal direction (3 mm direction), as shown in Fig. 1(b). Scan speed of 2 mm/s was used for the fabrication process. Adjacent lines were sufficiently overlapped to ensure full modification of the scanned area. Line spacing of 25 µm was used for the fabrication process. The fabrication process was monitored in real time using a CMOS camera (Thorlabs, USA).
The surface morphology as well as the cross sections of the fabricated structures were observed by SEM (Inspect F50, FEI, USA). The surface morphology of the fabricated structures at the bent state was observed by placing the PDMS sheets with the fabricated structures onto curved polyvinyl chloride (PVC) platforms.
Gold film electrodes, with an approximate thickness of 72 nm, were fabricated on both ends of the fabricated structures, with a 2 mm overlap by ion sputtering, as shown in Fig. 1(c), to facilitate the electrical-conductance-measurement procedure. For the electrical-conductance-measurement procedures, probes were placed onto the gold electrodes, fabricated on both ends of the structure, and the electrical conductance was measured by two-probe method in the range of 0 to 10 V in 0.1 V steps using a digital source meter (2401, Keithley, USA). Electrical conductance of the fabricated structures at various bending radii were also measured by two-probe method by placing the PDMS sheets with the fabricated structures onto curved PVC platforms with various radii (159 mm, 134 mm, 108 mm, 83 mm, 57 mm, 38 mm, 35 mm, 19 mm). Gold electrodes were also bent to various radii and the electrical conductance was measured at each radius to ensure the change in electrical conductance measured was not due to the deformation of the electrodes. Bending cycle tests were also performed on the fabricated structures by bending the structures to a certain bending radius (134 mm, 83 mm, 35 mm) and then releasing the structures back to the flat state. After each release, the electrical conductance of the fabricated structures was measured at the flat state. Additionally, a simple demonstration of strain sensing was performed by attaching a green LED to a power supply using the fabricated structures, S1. The fabricated structures were bent to 159 mm, 83 mm, 38 mm, and 11 mm bending radii from the flat state. An optical image of the LED was taken at each bending radius. Furthermore, the fabricated structure was attached to a nylon glove at the index-finger joint, and a green LED was attached to one end. Optical images of the LED were taken at unstrained and strained states.
3. Results and discussion
3.1 SEM observation of the fabricated structures
Laser power of 150 mW and scan speed of 2 mm/s was used for the fabrication process, based on our preliminary experiments for optimization of parameters for highest electrical conductivity (data not shown), and the width of a single line fabricated with these parameters was measured to be ∼100 µm. The width of the line was significantly larger than the beam diameter at the focal point, which is attributable to the diffusion of thermal energy, resulting from the high repetition rate. Femtosecond laser pulses were raster scanned, with a scan-to-scan interval of 25 µm, in an 8 mm x 3 mm area on a PDMS-thin-sheet. The femtosecond-laser-pulse irradiated areas were modified from optically transparent PDMS into black structures. The formation of the black structures by femtosecond laser irradiation coincides with the results of Nakajima et al., in which the structures were identified to be composed of β-SiC . Figure 2(a) shows a SEM image of the surface morphology of the fabricated structures. Unidirectional and periodic micro-grooves that correspond to the direction and the scan-to-scan interval of the laser scanning were observed on the surface. Additionally, bridges that connect adjacent grooves were observed on the surface. The repetition rate was 63 MHz for the fabrication, which could induce heat accumulation during laser scanning, resulting in localized melting by succeeding laser pulses. The fabricated structures were cut perpendicular and parallel to the laser scanning direction to observe the cross-sections. Figure 2(b) and (c) are SEM images of cross-sections when cut perpendicular and parallel to the laser scanning direction, respectively. The depth of the modification in PDMS was measured to be approximately 60 µm from the surface, based on the SEM images of the cross-sections. Periodic grooves with distances comparable to the scan-to-scan interval, 25 µm, were observed from the cross-section when cut perpendicular to the laser scanning direction (Fig. 2(b)). On the contrary, no obvious grooves were observed from the cross-section of the structures when cut parallel to the laser scanning direction, and the fabricated structures seemed continuous (Fig. 2(c)). A hole-like structure, with a thin-layer on top, can be observed in Fig. 2(b) and on the right side of Fig. 2(c). The top thin-layer is probably the bridge structures or the melted structures of adjacent apexes of the grooves due to thermal effects, while leaving a space underneath.
3.2 Electrical conductance measurements of the fabricated structures
As explained in the Materials and methods section, two 8 mm x 3 mm structures were fabricated with different scanning directions, S1 and S2, and the electrical conductance was measured. Figure 3(a) shows the current-voltage curve of the fabricated structures at the flat state. The electrical current increased linearly with the applied voltage for both S1 and S2. The average resistances of the structures were calculated to be approximately 1.2 kΩ for S1, where the laser scanning direction was in the longitudinal direction (8 mm), and 13.9 kΩ for S2, where the laser scanning direction as in the transverse direction (3 mm). The difference in electrical resistance may be attributed to the directions of the grooves and the electrical current. The electrical current could flow perpendicular to the periodic grooves through a less connected area as well as bridges between adjacent grooves in the case of S2, which resulted in the lower conductance compared to the case of S1.
Bending tests were performed on the fabricated structures, for both S1 and S2, by measuring the electrical conductance when bent to various radii. The 8 mm x 3 mm structures were bent in the longitudinal direction, and at each bending radius, the electrical conductance of the structures was measured. Figure 3(b) shows the relative change in average electrical resistances when the structures were bent to various bending radii. For both S1 and S2, the electrical resistances increased with an increase in applied strain, i.e. smaller bending radii. S1 showed a gradual change in electrical resistance for bending radii greater than 38 mm. However, when S1 was bent to smaller bending radii (< 38 mm), the electrical resistance increased drastically to more than tenfold. In contrast, S2 showed significantly smaller changes in electrical resistances with applied strain, and no significant increase in electrical resistance was observed at small bending radii, as was observed for S1. The change in electrical conductance was significant when strain was applied parallel to the laser scanning direction, S1, however was almost negligible when applied perpendicular to the laser scanning direction, S2. This result indicates that the fabricated structures show directional sensitivity to strain depending on the relationship between scanning direction used for the fabrication process and the direction of applied strain. Since the conductance only changes when the structures are bent parallel to the laser scanning direction used for the fabrication process, by combining multiple layers of the conductive structures, each fabricated with different laser scanning directions, a strain sensor capable of multi-directional strain sensing would be achieved.
3.3 Observation of surface morphology evolution with applied strain
To discuss the change in the electrical conductance with bending, the surface of the fabricated structures was observed via SEM. SEM images of the surface morphology evolution for S1 and S2, are shown in Fig. 4. Note that the same location of the same samples was observed, for S1 and S2 respectively. At larger bending radii, 134 mm and 83 mm, no obvious differences, from the surface at the flat state, were observed for both S1 (Fig. 4(a) and (b)) and S2 (Fig. 4(f) and (g), respectively). At smaller bending radii, 38 mm and 19 mm, the generation of large cracks perpendicular to the laser scanning direction were observed on the surface of S1, Fig. 4(c) and (d), respectively. On the contrary, no generation of such cracks were observed on the surface of S2. The large increase in electrical resistance observed for S1, Fig. 3(b), is attributable to the generation of the large cracks perpendicular to the laser scanning direction, due to the excessive strain. No significant increases in electrical resistance was observed for S2 at smaller bending radii, since the fabricated structures were not significantly damaged with the application of large strains. The enhancement of mechanical flexibility and the prevention of fracturing of materials by surface texturing have been previously reported [24,25]. The existence of periodic grooves perpendicular to the bending direction for S2 lowers the local bending stiffness, and thus enhancing the mechanical flexibility of the material.
The effect on repeated bending on the electrical conductance of the structures was evaluated by subjecting S1 and S2 to bending cycles at different bending radii, 134 mm, 83 mm, and 35 mm (Fig. 5). A total of 10 bending cycles were performed on the structures to discuss the difference in the effects of repeated bending on the electrical conductance of the structures fabricated with different laser scanning directions. For bending cycles conducted with a bending radius of 134 mm, no apparent change in electrical conductance was measured after 10 cycles (R/R0<2%), for both S1 and S2. After 10 bending cycles with a bending radius of 83 mm and 35 mm, the electrical conductance of S1 at the flat state increased by 20∼30%, while S2 showed no significant changes in electrical conductance at the flat state, after 10 bending cycles. Compared to the results in Fig. 4, the change in electrical conductance is correlated to the generation of cracks. For S1, generation of large cracks was observed on the surface at a bending radius of 83 mm (Fig. 4(c)) and 35 mm (Fig. 4(d)), and with repeated bending it is assumed that additional generation and propagation of cracks occurred, resulting in further destruction of the fabricated structures, decreasing the electrical conductance with further cycles. However, the electrical resistance at the flat state only increased gradually for S1, and no significant increasing was measured. This is hypothesized to be because when the structures are returned to the original position, although the cracks are still existent, separated structures return to a contacted-state, without significant distortion (Fig. 4(e)).
3.4 Application of the fabricated structures for strain sensing
Figure 6(a)-(e) shows optical images of a LED, which is connected to a power supply. Considering S1 showed higher electrical conductance and higher sensitivity to applied strain than S2, S1 was used for the experiments. The brightness of the green LED changes when the fabricated structures were bent to various radii. At the flat state, the LED is the brightest, since the electrical conductance is the highest for this experiment (Fig. 6(a)). However, as the bending radius is decreased, and in turn the electrical conductance decreased, the brightness of the LED decreases (Fig. 6(b)-(d)). At a bending radius of 11 mm, no light from the LED could be visibly confirmed since the electrical conductance decreased significantly due to excessive bending (Fig. 6(e)). As a preliminary demonstration of strain sensing, S1 was attached to the joint of the index finger, and a LED was attached to visibly monitor the motion of the joint (Fig. 6(f)). When the joint is at the non-strained state, the LED is brightly lit (Fig. 6(g)). However, even with a limited strain, ∼2° bending, the brightness of the LED changed, and highly sensitive detection of motion was observed (Fig. 6(h)).
The electrically conductive structures, fabricated by direct modification of PDMS, showed changes in electrical conductance when bent. The change in electrical conductance deferred depending on the laser scanning direction, used for the fabrication procedure. SEM images of the surface morphology at various bending radii revealed the generation of large cracks, perpendicular to the bending direction, on the surface. The change in electrical conductance when bent is attributable to the generation of such cracks on the surface. Additionally, bending cycle tests performed on the fabricated structures showed gradual decrease in electrical conductance of the structures when returned to the flat state, indicating additional generation and/or propagation of cracks, depending on the laser scanning direction. A preliminary demonstration of strain sensing was performed using the fabricated structures, and the results showed that the structures fabricated by femtosecond-laser-based modification of PDMS could be applied for strain sensing. As a future outlook, by focusing femtosecond laser pulses inside bulk PDMS, arbitrary three-dimensional electrically conductive structures could also be fabricated inside PDMS without additional integration-process steps. The presented method will open doors to novel electronic device applications using PDMS.
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