Fast Response, High‐Power Tunable Ultrathin Soft Actuator by Functional Piezoelectric Material Composite for Haptic Device Application

Recently, self‐driven soft robotics based on biomimetics, capable of mimicking biological motion, has attracted attention. Soft actuators using intrinsically soft organic materials are expected to be applied to haptic devices, artificial muscles, and micropumps. Ferroelectric polymers can aid in the realization of such soft actuators. However, actuators using such materials encounter problems in terms of the response frequency to an applied voltage. In this study, a soft actuator is fabricated by a printing process using a unique composite material comprising P(VDF‐TrFE), nano‐carbon material (single‐walled carbon nanotubes (SWCNT) and graphene oxide (GO)), and conductive polymer. To characterize the actuator using a minimum substrate thickness of 25 µm, hysteresis curves in the ferroelectric properties and driving characteristics according to the applied frequency are clarified. In addition, the mechanical life of the actuator under continuous voltage sweep is clarified considering it as a mechanical property. Subsequently, a simple haptics system is constructed using the fabricated actuators, and a human‐sensitive actuator demonstration system is constructed wherein the phase of the sweep frequency is variable.


Introduction
Recently, self-driven soft robotics based on biomimetics, which can mimic biological motion, has attracted attention. [1][2][3][4][5] In particular, achieving flexible motion in soft robotics using conventional motor-driven mechanisms based on the electromagnetic induction of inorganic materials is challenging. www.advelectronicmat.de of thinner films, lowers costs, and reduces processing temperatures. Furthermore, on application of an electric field, the piezoelectric material deforms because of the inverse piezoelectric effect. Actuators that utilize this effect are referred to as piezoelectric polymer actuators, such as PVDF. Alique et al. reported a poly(vinylidenedifluoride-trifluoroethylene) [P(VDF-TrFE)] device as a multifunctional platform with inkjet-printed silver electrodes. [17] Hao et al. reported a polymeric composite material-based solvent-based dual-responsive actuator. [18] However, actuators using P(VDF-TrFE) encounter several problems in device fabrication, such as the requirement of a low-voltage drive to provide the high voltage required to drive the actuator. Furthermore, achieving a high-speed response of 10 Hz or higher is challenging owing to the dependence on the stiffness of the device substrate. K. Y. Cho et al. used P(VDF-TrFE) and single-walled carbon nanotubes (SWCNT) while working on thin-film actuators with PVDF copolymers. [19] However, the maximum frequency was ≈10 Hz, which is an issue for the future. Thus, the reduction of substrate stiffness and the improvement of drivability of the actuator device using PVDF copolymer can facilitate an actuator capable of achieving highquality and high-frequency characteristics.
This study fabricated soft actuators via a printing process using a unique composite material comprising P(VDF-TrFE), nanocarbon material (single-walled carbon nanotubes (SWCNTs)), conductive polymers, and graphene oxide (GO). To characterize the actuator using a minimum substrate thickness of 25 µm, hysteresis curves in the ferroelectric properties and driving characteristics according to the applied frequency were clarified. Further, the mechanical life of the actuator under continuous voltage sweep was also clarified as a mechanical property. Subsequently, a simple haptics system has been constructed using the fabricated actuators, and a human-sensitive actuator demonstration system has been constructed wherein the phase of the sweep frequency is variable. Figure 1a shows the appearance of the fabricated device. The soft actuator comprised top and bottom electrodes on a thinfilm substrate and an active layer composed of the piezoelectric polymer P(VDF-TrFE) and nanocarbon material (SWCNT). The electrodes were composed of a composite material of PEDOT:PSS and GO. Similarly, a composite material of P(VDF-TrFE) and SWCNT was used as the active layer. Figure 1b shows a magnified view of a single soft actuator. Figure 1c shows the cross-sectional structure of the fabricated device. Figure 1d shows the original material system for the under and upper electrodes and active layer. In particular, by blending nanocarbon materials, the electrical and mechanical characteristics of the actuator can be improved. In Figure 1e, the real measurement system for the displacement of the proposed device is illustrated. Figure 1f shows the method used to measure the displacement in this study, which was calculated as the difference between the maximum and minimum displacement values (Δx mm).

Results and Discussion
Here, the characterization focused on the effect of the blending of SWCNT and GO on the electric characteristics of the actuator. Figure 2a,b shows the hysteresis curves of the device fabricated in this study and the device using pure P(VDF-TrFE) and blended nanocarbon materials of SWCNT and GO, respectively. The residual polarization value (Pr) was defined as the polarization value when the electric field was www.advelectronicmat.de 0 mV m −1 . The Pr value of the pure and proposed devices were 7.56 and 10.1 µC cm −2 , respectively, thus indicating an improvement. In this study, addition of nanocarbon materials improves the crystallization of P(VDF-TrFE). This phenomenon is derived from the alignment of the crystal face [110/200] of P(VDF-TrFE) orientation due to hydrogen bonding between the hydroxyl groups on the nanocarbon surface and P(VDF-TrFE). [20] Figure 2c,d shows the results of cross-sectional profiles and thickness measurements. The thicknesses of the active layer and electrode were 5000 and 500 nm, respectively. No change in the macroscopic surface was observed because of the blending effect. In Figure 2e-h, we showed microscopic surface morphology of active layer and electrode measured by atomic force microscopy (AFM). As shown in the Figures, the active layers and electrode layers with the addition of Nanocarbon materials exhibited minute morphological changes of less than 0.5 µm (minute morphological change in the thin film surface). Further, the root-mean-square (RMS) surface roughness of the active layers in Pure P(VDF-TrFE) and the blended P(VDF-TrFE) were 35.4 and 43.3 nm, respectively. The RMS of the electrode layer were 15.7 and 15.0 nm, respectively. Figure 2i shows the conductivity of the electrode layer. The addition of GO to PEDOT:PSS, which also has conductive properties, improved the conductivity of the electrode layer. In Figure S1 (Supporting Information), detailed characteristics of the PEDOT: PSS electrode are presented. Figure 2j,k shows the XRD spectrum of the active layer, thereby confirming that the addition of SWCNT to P(VDF-TrFE) improved the crystallinity. This may be attributed to the interaction between the functional groups of P(VDF-TrFE) and the SWCNTs, which increased the crystallinity of the β-phase compared to previous reports. [21] In particular, the crystallinity was enhanced by hydrogen bonding between P(VDF-TrFE) and GO. Figure 2l shows the IR spectra. A β-phase peak was observed at ≈850 cm −1 . [22] Thus, it can be concluded that the proposed actuator can secure good drivability as a piezo-type device. Figure 3a shows the displacement of each device under the driving conditions: applied frequency and electric field of 1 Hz

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and 50 mV m −1 , respectively. The displacement of the pure P(VDF-TrFE) and proposed actuators were 0.8 and 1.5 mm, respectively, the latter being approximately twice as large. In particular, the actuator with a substrate thickness of 25 µm exhibited the highest drivability, implying that the lower the substrate stiffness, the better the drivability of the actuator. Figure 3b shows the amount of displacement for substrate thickness and applied electric field, and Figure 3c shows the relationship between substrate thickness and maximum displacement. For all substrates, the displacement increased with increase in the electric field. However, the amount of displacement remained constant above an electric field of 50 mV m −1 other than 25 µm thickness. Figure 3d shows the relationship between the amount of displacement and the applied frequency for each device. The displacement of the proposed actuator was observed to increase at the applied frequency of 50 Hz or lower. In addition, the displacement tended to be much larger for the applied frequency range of ≈5-20 Hz. This may be owing to the dependence on the improvement of ferroelectric properties, which also improved the frequency characteristics. Figure 3e shows the amount of displacement for substrate film thickness and applied electric field. For all substrates, the displacement decreased with increase in the frequency. Figure 3f shows the correlation between applied frequency and measured frequency. The pure P (VDF-TrFE) actuator was driven up to an applied frequency of 50 Hz and could not be driven further above 60 Hz. In contrast, the proposed actuator could be driven up to an applied frequency of 100 Hz. The displacement results of pure P(VDF-TrFE) and nanocarbon blend actuators for substrate thickness of 50 µm are shown in Figure S2 (Supporting Information). Figure 3g shows the mechanical lifetime for each device. Figure 3h shows the mechanical lifetime at different  Figure S3 (Supporting Information). Thus, a constant driving frequency can be obtained regardless of the thickness of the substrate with respect to the applied frequency.
The displacement performance of the proposed soft actuator were compared with those of recently reported studies, as shown in Figure 4. [23−37] In terms of high response-ability (over 1 Hz), the proposed P(VDF-TrFE) and nanocarbon compositebased soft actuators exhibited superior performance. In particular, achieving high-speed actuation of 10 Hz or higher and a large actuation of 1 mm or greater has been a challenging task, even for soft actuators that use piezo-based materials. Here we compare the parameters of displacement and frequency in previously reported high-performance devices with our actuators. Dias et al. reported ionic liquid-based bending actuators by tailored interaction with the polar fluorinated polymer matrix [25] Moreover, Kim et al. reported ionic polymer-metal composite actuators for biomimetic undulatory tadpole robot. [29] These devices produced a large displacement of over 3.5 mm and a high-speed drive of 10 Hz, approximately. Our actuator is the first organic soft actuator that can simultaneously achieve such a large displacement and high-speed drive. In addition, this study overcame these difficulties by optimizing the novel material composition and device structure. In addition to the performance, the fabrication method is also an important aspect. The development of soft actuators has been mainly focused on application areas such as robotics and artificial muscles. Therefore, they should satisfy the requirements of low cost, scalability, and mass fabrication. Indeed, recent studies have offered interesting results and advancements in perfor-mance. However, actuators developed using printed fabrication methods and expensive materials cannot be promoted in applications where the cost is an important factor. In addition, in actual applications, the device is integrated with circuitry; therefore, easy patterning, good uniformity, and reproducibility of the device should be considered.
Finally, the applicability of the proposed actuator to haptic devices was demonstrated. A sponge made of melamine resin was mounted on the tip of the actuator as a sensor amplification material. Subsequently, a sensitivity test was conducted for tactile sensation when driven at 5 and 10 Hz (Figure 5). In this demonstration, the substrate thickness of the soft actuator used was 25 µm (please see Figure 3e for detailed characteristics.). When a sponge touched a human finger, the mechanical stimulation was recorded, and the number of times was graphed for each frequency. Figure 5a shows a schematic of the haptic experiment. It is a platform wherein the voltage was turned ON and touched the human finger. Figure 5b summarizes the applied voltage, displacement of the actuator, and number of mechanical stimuli. Stimulations were confirmed three and five times at 5 and 10 Hz, respectively. However, at frequencies above 10 Hz, although a clear mechanical stimulus was felt, the number of times could not be counted. These results indicate the high adaptability of the high-performance soft actuators to haptic devices system. The drivability of the proposed actuator at each frequency and video of the actual sensitivity test is shown in Video S1 (Supporting Information).

Conclusion
This study fabricated high-performance soft actuators using composite ferroelectric materials via screen-printing. The changes in the actuator properties were evaluated by varying the applied electric field, frequency, and substrate film thickness. Consequently, the relationship between ferroelectricity, crystallinity, conductivity, applied electric field and frequency, and drivability and lifetime of the soft actuators obtained from P(VDF-TrFE) with SWCNTs and PEDOT:PSS with GO were clarified. The addition of SWCNTs resulted in a displacement approximately twice as large as that of the pure device, and the displacement improved in the applied frequency range of 5-20 Hz. In addition, the device was proven capable of being driven at high frequencies, although the displacement was small. Furthermore, the mechanical life of the device was much longer than that of the pure device when considering the number of cycles.

Experimental Section
A Polyethylenenaphthalate (PEN) film (Panac Corporation) was used as the substrate. The substrate thicknesses were 25, 50, 100, and 125 µm. Next, the substrate surface was chemically modified using a plasma treatment system (RIE-PC300, SAMCO Corporation). The surface treatment conditions were as follows: applied power, 100 W; surface treatment time, 60 s; gas flow, 30 sccm; and oxygen gas for plasma generation. Oxygen gas was used for plasma generation to improve film formation in the electrode material. Further, a solution of GO in NMP was mixed with PEDOT:PSS (Clevios SV4 STAB, Heraeus) at 1.0 wt.% and stirred for 10 min. Subsequently, the bottom electrode was deposited www.advelectronicmat.de on the PEN film via screen printing and annealed at 150 °C for 30 min. The thickness of the electrode film was 500 nm. Thereafter, P(VDF-TrFE) (FC 25, Arkema-Piezotech) was prepared in NMP at 12 wt.%, and then SWCNTs were added at 0.075 wt.% and stirred at 60 °C for 8 h. Weight average molecular weights of P(VDF-TrFE) was 440 kg mol −1 . Ratio of VDF and TrFE was 75:25 (wt.%). Before printing, the solution was passed through a filter with a pore diameter of 1 µm. Subsequently, the solution for the active layer was deposited on the bottom electrode via screen printing and annealed at 135 °C for 1 h. The thickness of the drive layer obtained was 5000 nm. The top electrode was fabricated by annealing at 135 °C for 30 min using the same solution as that used for the bottom electrode. In addition, the hysteresis curves representing the drivability of the fabricated actuators were measured at 1 Hz with an electric field of ±100 mV m −1 . A laser displacement meter (Keyence CL-3000) was used for the drivability measurements. Further, to drive the change, the voltage was swept from −100 to 100 mV m −1 at several frequencies. Consequently, for the mechanical life measurements, the response displacement of a constant sweep of ±50 mV m −1 at 25 Hz was measured continuously.

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