**5.4 Characterizations**

The actuator characterization system that utilizes a balance to measure the expansion and contraction ratios under the load stress has already been described in Fig. 2.4. The PPy actuator was used as the working electrode in the lithium bis(trifluoromethansulfonyl)imide (LiTFSI) electrolyte solution of 1 mol.dm-3, and both of the PPy actuator ends were

Polypyrrole Soft Actuators 183

Fig. 5.5. Time dependences of the displacement (length change) of the actuators as measured with different weights of (a) 0.05 N, (b) 0.2 N, and (c) 0.5 N for the initial period of 800

seconds. Those weights correspond to 0.07, 0.3 and 0.76 MPa, respectively.

Fig. 5.3. Optical microscope image of the fabricated actuator taken from the backside (substrate side) of the actuator. The Au film in the plane PPy area was peeled off, while the Au film covering the silicon micro spring was not peeled off.

Fig. 5.4. Electron microscope (SEM) images of the PPy actuator observed from the surface (electrolyte solution side) of the micro spring (a) and backside (substrate side) of the micro spring (b).

suspended by metal clips. The size of the moving part of the PPy actuators was 7 mm in width, 8.3 mm in length, and 91 µm in thickness. The PPy actuator exhibited the expansion and contraction motions under the alternating potential with the triangular wave shape applied between the PPy actuator and the counter electrode. The potential voltage difference between the PPy actuator and the electrolyte solution was monitored using an Ag/AgCl reference electrode. The peak values of the potential voltage were -1 and +1 V, and the potential sweep rate was 10 mVs-1. The extension and contraction of the PPy actuator was measured by monitoring the displacement of the weight position using the laser displacement sensor. An arbitrary load stress was applied on the PPy actuator by putting weights on the saucer of the balance. The weights used here were 0.05, 0.2, and 0.5 N. The load stresses for these weights correspond to 0.07, 0.3, and 0.76 MPa, respectively.

Figure 5.5. shows the time dependences of the displacement (length change) of the actuators as measured with different weights of (a) 0.05 N, (b) 0.2 N, and (c) 0.5 N for the initial period of 800 seconds. Those weights correspond to 0.07, 0.3 and 0.76 MPa, respectively. The displacement vs. time curve for the eight of 0.05 N [Fig. 5.5a] seems to consist of two components of the electrochemical strain caused by the anion doping/dedoping processes and the creeping strain possibly due to the swelling of the PPy film. Here, the peak height was subtracted by the creeping strain and as shown in Fig. 5.5a, and the subtracted peak

Fig. 5.3. Optical microscope image of the fabricated actuator taken from the backside (substrate side) of the actuator. The Au film in the plane PPy area was peeled off, while the

Fig. 5.4. Electron microscope (SEM) images of the PPy actuator observed from the surface (electrolyte solution side) of the micro spring (a) and backside (substrate side) of the micro

suspended by metal clips. The size of the moving part of the PPy actuators was 7 mm in width, 8.3 mm in length, and 91 µm in thickness. The PPy actuator exhibited the expansion and contraction motions under the alternating potential with the triangular wave shape applied between the PPy actuator and the counter electrode. The potential voltage difference between the PPy actuator and the electrolyte solution was monitored using an Ag/AgCl reference electrode. The peak values of the potential voltage were -1 and +1 V, and the potential sweep rate was 10 mVs-1. The extension and contraction of the PPy actuator was measured by monitoring the displacement of the weight position using the laser displacement sensor. An arbitrary load stress was applied on the PPy actuator by putting weights on the saucer of the balance. The weights used here were 0.05, 0.2, and 0.5 N. The load stresses for these weights

Figure 5.5. shows the time dependences of the displacement (length change) of the actuators as measured with different weights of (a) 0.05 N, (b) 0.2 N, and (c) 0.5 N for the initial period of 800 seconds. Those weights correspond to 0.07, 0.3 and 0.76 MPa, respectively. The displacement vs. time curve for the eight of 0.05 N [Fig. 5.5a] seems to consist of two components of the electrochemical strain caused by the anion doping/dedoping processes and the creeping strain possibly due to the swelling of the PPy film. Here, the peak height was subtracted by the creeping strain and as shown in Fig. 5.5a, and the subtracted peak

Au film covering the silicon micro spring was not peeled off.

correspond to 0.07, 0.3, and 0.76 MPa, respectively.

spring (b).

Fig. 5.5. Time dependences of the displacement (length change) of the actuators as measured with different weights of (a) 0.05 N, (b) 0.2 N, and (c) 0.5 N for the initial period of 800 seconds. Those weights correspond to 0.07, 0.3 and 0.76 MPa, respectively.

Polypyrrole Soft Actuators 185

2005a, 2005b, Sendai et al., 2009). Although the actuator investigated here exhibited a slow response, the generating stress of the order of 0.3 MPa is relatively large as an actuating mechanism for MEMS actuation. The PPy actuator requires an electrolyte solution during actuation. Therefore, some protecting film such as an artificial skin to cover the electrolyte

Two kinds of PPy thin film actuators with or without the silicon MEMS microspring were fabricated and compared. The polypyrrole thin films with the thickness of 91 m were deposited by galvanostatic electropolymerization of a polypyrrole thin film using a methyl benzoate electrolyte solution of tetra-n-butylammonium bis(trifluoromethansulfonyl)imide (TBATFSI). One of the actuators was inserted with the silicon MEMS microspring with the length of 15 mm, the width of 0.5 mm, and the thickness of 60 m. The MEMS PPy actuator exhibited nearly 12% of the electrochemical strain under the load of 0.2 N in a water solution of an electrolyte, lithium bis-trifluoromethane sulphonyl imide (LiTFSI) at the bias sweep rate of 10 mVs-1 in the voltage range between –1 and 1V. The load stress was approximately 0.3 MPa. Although the performances of the MEMS actuators showed some degradation compared to the PPy actuator without the MEMS microspring, the MEMS PPy actuator may be beneficial to drive MEMS structures, which require a large strain and a large stress with a

The author express heartfelt applications to his collaborators, Mr. Hiroyuki Katsumata, Mr. Takayuki Fujiya, Mr. Daiki Hoshino, Mr. Tsuyoshi Morita, Mr. Yutaka Chida, Mr. Zongfan Duan, Mr. Yutaro Suzuki, Mr. Shou Ogihara, Mr. Syota Kaihatsu, Mr. Masahiro Higashi, and other students. The staffs of the Micro Functional Device Research Center are also

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*Japan Society of Mechanical Engineers, C62(596),* 1384.

Hara, S., Zama, T., Takashima, W., & Kaneto K. (2004). *Polym. J., 36*, 933.

Hara, S., Zama, T., Takashima, W., & Kaneto, K. (2004). *J. Mater. Chem.,* 14, 1516.

Hara, S., Zama, T., Takashima, W., & Kaneto, K. (2005). *Synthetic Metals, 149,* 199. Hara, S., Zama, T., Takashima, W., & KANETO, K. (2005). *Smart Mater. Struct., 14,* 1501.

Hara, S., Zama, T., Takashima, W., & Kaneto, K. (2006). *Synth. Met., 156*, 351.

Han G., & Shi, G. (2004). *Sensors and Actuators, B 99,* 525.

solution surrounding the PPy actuator may be needed for actual MEMS applications.

**5.5 Conclusion** 

low voltage actuation.

appreciated.

**7. References** 

**6. Acknowledgements** 

*519,* 115.

2724.

Baughman, R.H. (1996). *Synth. Met., 78,* 339.

height value was divided by the initial length of the actuator to define the electrochemical strain. In this case of the weight of 0.05 N (0.07 MPa), the electrochemical strains of the PPy actuators with or without the silicon microspring are nearly identical. In contrast, when the weight increased up to 0.2 N (0.3 MPa), the creeping strain of the PPy actuator with the silicon microspring increased notably, while the electrochemical strains of two actuators are nearly identical as seen in Fig. 5.5b. When the 0.5 N (0.76 MPa) was applied, both of the actuators showed the smaller electrochemical strains of approximately 7-8%. In addition, the PPy actuator with the microspring torn off while the PPy actuator without the microspring did not tear off. The optical microscope observation of the torn off actuator showed cracks at the spring/PPy interface. The reason for the reduced electrochemical strain in the high load stress is not clear. The stressed polymer networks might possibly block the volume expansion caused by penetration of anions.

Figure 5.6. shows the repeated operation of the actuator for 15 times. The displacement of the actuators gradually increased due to creeping, while the electrochemical strains of them slightly decreased. The actuator with the microspring continued to have the electrochemical strain of 13%, while the actuator without the microspring continued to have that of 15%. The creeping effect of the PPy actuator with the silicon microspring was larger than the PPy actuator without the silicon microspring. Although the PPy actuator with the silicon microspring had slightly degraded performances compared to the PPy actuators without the silicon microspring, it will be beneficial for MEMS applications because of its large stress and strain.

Fig. 5.6. Relationships between the displacement of the actuators and time under the repeated operation of the actuator for 15 times. The load weight was 0.2 N that corresponds to 0.3 MPa stress.

The length of the actuator continues to increase as time elapses. This behavior looks similar to the creeping effect in metal deformation processes. Zama and others recently reported on their detailed study for this creeping effect of PPy actuators, and concluded that this elongation could be recovered by releasing the stress during the deformation (Zama et al.,

height value was divided by the initial length of the actuator to define the electrochemical strain. In this case of the weight of 0.05 N (0.07 MPa), the electrochemical strains of the PPy actuators with or without the silicon microspring are nearly identical. In contrast, when the weight increased up to 0.2 N (0.3 MPa), the creeping strain of the PPy actuator with the silicon microspring increased notably, while the electrochemical strains of two actuators are nearly identical as seen in Fig. 5.5b. When the 0.5 N (0.76 MPa) was applied, both of the actuators showed the smaller electrochemical strains of approximately 7-8%. In addition, the PPy actuator with the microspring torn off while the PPy actuator without the microspring did not tear off. The optical microscope observation of the torn off actuator showed cracks at the spring/PPy interface. The reason for the reduced electrochemical strain in the high load stress is not clear. The stressed polymer networks might possibly block the volume

Figure 5.6. shows the repeated operation of the actuator for 15 times. The displacement of the actuators gradually increased due to creeping, while the electrochemical strains of them slightly decreased. The actuator with the microspring continued to have the electrochemical strain of 13%, while the actuator without the microspring continued to have that of 15%. The creeping effect of the PPy actuator with the silicon microspring was larger than the PPy actuator without the silicon microspring. Although the PPy actuator with the silicon microspring had slightly degraded performances compared to the PPy actuators without the silicon microspring, it will be beneficial for MEMS applications because of its large stress

Fig. 5.6. Relationships between the displacement of the actuators and time under the repeated operation of the actuator for 15 times. The load weight was 0.2 N that corresponds

The length of the actuator continues to increase as time elapses. This behavior looks similar to the creeping effect in metal deformation processes. Zama and others recently reported on their detailed study for this creeping effect of PPy actuators, and concluded that this elongation could be recovered by releasing the stress during the deformation (Zama et al.,

expansion caused by penetration of anions.

and strain.

to 0.3 MPa stress.

2005a, 2005b, Sendai et al., 2009). Although the actuator investigated here exhibited a slow response, the generating stress of the order of 0.3 MPa is relatively large as an actuating mechanism for MEMS actuation. The PPy actuator requires an electrolyte solution during actuation. Therefore, some protecting film such as an artificial skin to cover the electrolyte solution surrounding the PPy actuator may be needed for actual MEMS applications.
