**5. Comparison of polypyrrole organic thin film actuators with or without silicon microspring**

#### **5.1 Introduction**

Recently, Ding et al. reported a new type of PPy actuator that had a tubular geometry and a helical wire interconnect (Ding et al., 2003). The actuator was fabricated by forming a PPy

increased the porous spacing. This will make the anions more easily penetrate into the PPy

Fig. 4.5. Sweep rate dependences of electrochemical strain rates for PPy actuators in LiTFSI

The influences of the potential sweep rate on electrochemical and creep strains PPy actuators in aqueous LiTFSI electrolyte solutions with 2-propanol concentrations of 0 and 20% were compared under the load stress of 0.3 MPa. The electrochemical strain rates in the 0 and 20% 2-propanol solution were found to be approximately 0.1%s-1 and 0.25%s-1, respectively, and they are nearly independent of the sweep rate between 10 and 25 mVs-1. These results suggest that the TFSI anion penetration into the PPy films might be limited by the interactions between the electrolyte and the PPy surface, and the reduced viscosity in the LiTFSI electrolyte solution containing 2-propanol possibly enhances the doping and dedoping of TFST anions along with the swelling effect of the PPy film by 2-propanol. The creep rate was more rapidly increased as time elapsed in the electrolyte solution containing

20% 2-propanol, which was due to the swelling of PPy film in the electrolyte.

**5. Comparison of polypyrrole organic thin film actuators with or without** 

Recently, Ding et al. reported a new type of PPy actuator that had a tubular geometry and a helical wire interconnect (Ding et al., 2003). The actuator was fabricated by forming a PPy

solutions with 2-propanol concentrations of 0 (water) and 20%.

**4.3 Conclusion** 

**silicon microspring** 

**5.1 Introduction** 

film, which resulted in the increase of the deformation of the PPy actuators.

film on a platinum wire (125 m in diameter) that was wrapped around with thinner (25 m in diameter) platinum wire. A maximum strain of 5% and the response of 10%s-1 were achieved. A similar structure to minimize the response time of PPy actuators was also reported by Hara et al. (Hara et al., 2003, 2004a, 2004b). They deposited a PPy film on a tungsten helical coil (250 m in diameter) made of a tungsten wire with a diameter of 30 m. This fibrous PPy actuator exhibited a strain of 11.6 % under the load of 0.2 N. The tungsten wire also helped to reduce the potential drop within the PPy for the improved performance. Another trial to increase the extension and contraction ratios employing a corrugated PPy structure has been reported (Morita et al., 2010), and a bimorph structure of a PPy actuator for more uniform bending has also been reported (Chida et al., 2010).

In contrast, we considered applying these PPy soft actuator techniques to the actuation of the very small mechanisms used in silicon microelectromechanical systems (MEMS). To the best of our knowledge, there have been very few applications of these techniques to the actuation of small MEMS mechanisms (Chida et al., 2010, Guo et al., 1996, Hutchison et al., 2000). In this section, two kinds of PPy thin film actuators with or without a silicon MEMS microspring were fabricated and compared.

#### **5.2 Linear actuator design**

Figure 1 describes the designed actuator. The silicon microspring has a width of 0.5 mm, length of 15 mm, and thickness of 60 m. The microspring consists of silicon wires having a cross-section of 10 x 60 m2. The surface of this microspring is covered by a 91 m-thick PPy

Fig. 5.1. Design of the PPy-driven silicon linear actuator; (a) corresponds to designed silicon micro spring, and (b) corresponds to the designed linear actuator driven by PPy expansion and contraction motions.

Polypyrrole Soft Actuators 181

The space between the microsprings was not filled with the PPy film. This may mean that the polypyrrole film on the microspring does not contribute the extension and contraction of the silicon actuator. Therefore, it was determined that the actuation was realized utilizing the extension and contraction of the plane PPy film deposited beside the microspring.

Fig. 5.2. Cross- sectionally described linear actuator fabrication processes

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

**5.4 Characterizations** 

film of length and width 15 mm and 7mm, respectively. The PPy film covering the microspring shrinks and expands along with the PPy film beside the microspring, which causes the actuation of the microspring. The top and bottom parts of the actuator were clipped with metal electrodes. As a result, the working area of the actuator was 8.3 mm in length and 7 mm in width. A PPy actuator with the same dimensions without the silicon MEMS micro spring was also fabricated for comparison.

### **5.3 Actuator fabrication processes**

Figure 5.2. describes the fabrication processes of the microactuator. First, an extremely thin (60 m) silicon film was anodically bonded to a glass substrate of 1.5 mm in thickness (a). Next, the silicon microspring pattern was photolithographycally defined on the silicon film, followed by silicon etching using an inductively coupled plasma (ICP) dry etcher (b). The silicon microspring was released by immersing the structure in a buffered HF solution (c). The microspring was placed on an acrylic board, and a 100 nm-thick Au film was sputter deposited on the whole surface of the microspring and the acrylic board (d). Finally, the PPy film with a thickness of approximately 91 m was electrochemically deposited on the whole surface of the sputtered Au film, and the structure was peeled off from the acrylic board in acetone (e).

The polymerization was done using a computer-controlled potentio-galvanostat (Hokuto Denko HZ-5000). A counter electrode (Ti), a reference electrode (Ag/AgCl), and a working electrode (Au) were immersed into a solvent containing pyrrole monomers and an electrolyte, and the potential voltage was controlled to keep a constant current between the counter electrode and the working electrode of the Au surface covering the silicon microspring during the PPy polymerisation. The electropolymerization of PPy was done in a methyl benzoate solution with a volume of 50 ml in which pyrrole monomers with a concentration of 0.25 mol.dm-3 and the electrolyte tetra-n-butylammonium bis(trifluoromethansulfonyl)imide (TBATFSI) with a concentration of 0.2 mol.dm-3 are dissolved. The polymerization was done at a constant current density of 0.2 mA.cm-2 for 4 h at room temperature. The PPy film deposited on the Au surface was peeled off, and cut into the actuator dimension of 7 x 15 mm2 as shown in Fig. 5.3b. The PPy actuator without the Si microspring of 7 x 15 mm2 was also cut from the same PPy film. The thickness of the plane part of the PPy film was measured to be approximately 91 m using a micrometer. The measured electro conductivity of the PPy film was approximately 12 Scm-1. In the previous publications, fairly large strain and fast response of PPy actuators, that were polymerized electrochemically with TBATFSI at the temperature of –10 oC, were reported (Hara et al., 2005a, 2004b). Therefore, the PPy actuators fabricated using (TBATFSI) were focused in this research.

Figure 5.3. shows the 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 unintentionally peeled off from the PPy film during the actuator peeling off process in acetone, while the Au film covering the silicon microspring was not peeled off. Figure 5.4a. and 5.4b. show scanning electron microscope (SEM) images of the PPy actuator observed from the surface of the micro spring (electrolyte solution side) and the backside of the micro spring, respectively. It was indicated that the PPy film almost covered the silicon spring surface, and it had a rugged surface structure. Similar rugged structures were observed in the image taken from the backside. It is believed that this sponge-like structure is the origin of the large expansion and contraction ratios during the ion doping and dedoping processes.

film of length and width 15 mm and 7mm, respectively. The PPy film covering the microspring shrinks and expands along with the PPy film beside the microspring, which causes the actuation of the microspring. The top and bottom parts of the actuator were clipped with metal electrodes. As a result, the working area of the actuator was 8.3 mm in length and 7 mm in width. A PPy actuator with the same dimensions without the silicon

Figure 5.2. describes the fabrication processes of the microactuator. First, an extremely thin (60 m) silicon film was anodically bonded to a glass substrate of 1.5 mm in thickness (a). Next, the silicon microspring pattern was photolithographycally defined on the silicon film, followed by silicon etching using an inductively coupled plasma (ICP) dry etcher (b). The silicon microspring was released by immersing the structure in a buffered HF solution (c). The microspring was placed on an acrylic board, and a 100 nm-thick Au film was sputter deposited on the whole surface of the microspring and the acrylic board (d). Finally, the PPy film with a thickness of approximately 91 m was electrochemically deposited on the whole surface of the sputtered Au film, and the structure was peeled off from the acrylic

The polymerization was done using a computer-controlled potentio-galvanostat (Hokuto Denko HZ-5000). A counter electrode (Ti), a reference electrode (Ag/AgCl), and a working electrode (Au) were immersed into a solvent containing pyrrole monomers and an electrolyte, and the potential voltage was controlled to keep a constant current between the counter electrode and the working electrode of the Au surface covering the silicon microspring during the PPy polymerisation. The electropolymerization of PPy was done in a methyl benzoate solution with a volume of 50 ml in which pyrrole monomers with a concentration of 0.25 mol.dm-3 and the electrolyte tetra-n-butylammonium bis(trifluoromethansulfonyl)imide (TBATFSI) with a concentration of 0.2 mol.dm-3 are dissolved. The polymerization was done at a constant current density of 0.2 mA.cm-2 for 4 h at room temperature. The PPy film deposited on the Au surface was peeled off, and cut into the actuator dimension of 7 x 15 mm2 as shown in Fig. 5.3b. The PPy actuator without the Si microspring of 7 x 15 mm2 was also cut from the same PPy film. The thickness of the plane part of the PPy film was measured to be approximately 91 m using a micrometer. The measured electro conductivity of the PPy film was approximately 12 Scm-1. In the previous publications, fairly large strain and fast response of PPy actuators, that were polymerized electrochemically with TBATFSI at the temperature of –10 oC, were reported (Hara et al., 2005a, 2004b). Therefore, the PPy actuators fabricated using

Figure 5.3. shows the 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 unintentionally peeled off from the PPy film during the actuator peeling off process in acetone, while the Au film covering the silicon microspring was not peeled off. Figure 5.4a. and 5.4b. show scanning electron microscope (SEM) images of the PPy actuator observed from the surface of the micro spring (electrolyte solution side) and the backside of the micro spring, respectively. It was indicated that the PPy film almost covered the silicon spring surface, and it had a rugged surface structure. Similar rugged structures were observed in the image taken from the backside. It is believed that this sponge-like structure is the origin of the large expansion and contraction ratios during the ion doping and dedoping processes.

MEMS micro spring was also fabricated for comparison.

**5.3 Actuator fabrication processes** 

(TBATFSI) were focused in this research.

board in acetone (e).

The space between the microsprings was not filled with the PPy film. This may mean that the polypyrrole film on the microspring does not contribute the extension and contraction of the silicon actuator. Therefore, it was determined that the actuation was realized utilizing the extension and contraction of the plane PPy film deposited beside the microspring.
