**3.4. A pendulum-like motion of nanofiber gel actuator synchronized with external periodic pH oscillation**

In this study, in order to drive the nanofiber gel actuator in response to the external pH changes, we selected the pH responsive poly(AAc) (PAAc) as a main polymer chain. The PAAc is protonated when the pH is below the pKa. When the pH of the solution is below pKa, the nanofiber gel collapses due to hydrogen bonding among the polymer chains. On the other hand, when the pH is above the pKa, the PAAc polymer chain is ionized, that is, the solubility of the polymer chain changes from a hydrophobic to a hydrophilic nature. As a result, the nanofiber gel expands because of the electrostatic repulsion force among the charged PAAc polymer chains. However, the fiber gel that consists of the only PAAc polymer chain, finally dissolves into the aqueous solution because the gel does not have the cross-linkage among the polymer chains into the nanofiber. In order to avoid the nanofiber gel dissolving, especially when it is above the pKa, we adopted the tBMA domain into the PAAc as a cross-linkage and a solubility control site, due to the hydrophobic interaction among the tBMA in the nanofiber. As a result, the poly(AAc-*co*-nBMA) nanofiber gel does not dissolve in the aqueous solution.

Figure 20 shows distributions of diameter of the poly(AAc-*co*-nBMA) nanofibers electrospun at two flow rates (2.0 mL/h and 1.0 mL/h) (See Figure 21). In general, the fiber diameters depend on the flow rate. In our experiment, the average diameter at the flow rate 2.0 mL/h (302 nm) was thicker than at 1.0 mL/h (233 nm).

Reproduced with permission.)

**external periodic pH oscillation** 

not dissolve in the aqueous solution.

(302 nm) was thicker than at 1.0 mL/h (233 nm).

**Figure 19.** Spatiotemporal diagram constructed from sequential images of the acid propagation. The extracted horizontal lines correspond to the black bar in Fig. 18 (a). (Reprinted ref. 58, Copyright *IEEE*.

In this study, in order to drive the nanofiber gel actuator in response to the external pH changes, we selected the pH responsive poly(AAc) (PAAc) as a main polymer chain. The PAAc is protonated when the pH is below the pKa. When the pH of the solution is below pKa, the nanofiber gel collapses due to hydrogen bonding among the polymer chains. On the other hand, when the pH is above the pKa, the PAAc polymer chain is ionized, that is, the solubility of the polymer chain changes from a hydrophobic to a hydrophilic nature. As a result, the nanofiber gel expands because of the electrostatic repulsion force among the charged PAAc polymer chains. However, the fiber gel that consists of the only PAAc polymer chain, finally dissolves into the aqueous solution because the gel does not have the cross-linkage among the polymer chains into the nanofiber. In order to avoid the nanofiber gel dissolving, especially when it is above the pKa, we adopted the tBMA domain into the PAAc as a cross-linkage and a solubility control site, due to the hydrophobic interaction among the tBMA in the nanofiber. As a result, the poly(AAc-*co*-nBMA) nanofiber gel does

Figure 20 shows distributions of diameter of the poly(AAc-*co*-nBMA) nanofibers electrospun at two flow rates (2.0 mL/h and 1.0 mL/h) (See Figure 21). In general, the fiber diameters depend on the flow rate. In our experiment, the average diameter at the flow rate 2.0 mL/h

**3.4. A pendulum-like motion of nanofiber gel actuator synchronized with** 

**Figure 20.** (**a**) An electrospun fiber sheet. (**b**) The SEM image of nanofibers at flow rate 1.0 mL/h. (**c**) The SEM image of nanofibers at flow rate 2.0 mL/h. (**d**) Distribution of fiber diameters of poly(AAc-*co*nBMA) nanofibers electrospun at different flow rates.

**Figure 21.** The method of introducing the anisotropic structure into the nanofiber gel. (**a**) Electrospinning at a flow rate of 2.0 mL/hour (sprayed for 30 minutes). (**b**) Electrospinning at a flow rate of 1.0 mL/hour (sprayed for 60 minutes). (**c**) Drying in 50 °C over night. (**d**) Cutting into 15 mm × 3 mm × 200 μm.

In order to drive the nanofibrous gel actuator synchronized with autonomous pH oscillation, we focused on the Landolt pH-oscillator, based on a bromated/ sulfite/ ferrocyanide reaction discovered by Edblom *et al.* [39,40]. This reaction causes the autonomous cyclic pH changes with a wide range at room temperature. The reaction has many reaction steps, so we estimated the main reactions as follow [41].

$$3BrO\_3^- + 3HSO\_3^- + H^+ \to Br^- + 3SO\_4^{2-} + 4H^+ \tag{3}$$

$$\text{BrO}\_3^- + 6\text{Fe(CN)}\_6^{4-} + 6\text{H}^+ \rightarrow \text{Br}^- + 6\text{Fe(CN)}\_6^{3-} + 3\text{H}\_2\text{O} \tag{4}$$

In Process (1), H2SO3 is oxidized by bromate, and ferrocyanide is oxidized by bromate in Process (2). In the above two processes, the hydrogen ions produced and consumed at comparable rates. Therefore, in this reaction, the pH oscillation takes place in the CSTR. Figure 22 shows the experimental set up of the CSTR. The CSTR was constructed by using

four peristaltic pumps in order to feed four solutions of potassium bromated, sodium sulfite, potassium ferrocyanide and sulfuric acid. Moreover, this system had one more peristaltic pump to drain the excess solution. The degree of changing the pH range (amplitude) and period of the oscillating reaction can be controlled by changing the feed concentration, flow rate and solution temperature.

**Figure 22.** Continuous monitoring system for oscillation: monitoring both the medium pH and the gel motion in a CSTR.

Figure 23 shows a motion of the nanofiber gel actuator. The bending and stretching motions of the gel actuator synchronized with the pH oscillating reaction. As shown in Figure 23, we defined R as the length between two edges of the gel. Figure 24 shows the trajectory of the nanofiber gel strip. As shown in Figure 24, the gel strip caused the pendulum-like motion. As the external pH is below the pKa, the nanofiber gel stretches because of the deswelling originating from the hydrogen bonding (1→3). Next, when the pH is above the pKa, the gel bends because of the swelling originating from the repulsive force among the anionic polymer chains (4→6).

Figure 25 shows the temporal changes of R of the gel strip and the external pH, respectively. The range of the pH oscillation based on a bromate/sulfite/ferrocyanide reaction was 3.1 < pH < 7.2, and the period was about 20 min. When the external pH changes periodically, the R of the gel strip cyclic changes synchronized with the external pH change. As shown in Figure 25, when the pH sharply decreased, the R of the gel strip starts to increase because the gel collapsed. Next, when the pH increased rapidly, R gradually decreased, due to the gel actuator swelling originating from the repulsive force of AAc domain in the polymer chain. That is because the gel has different rates at swelling and deswelling. In general, the swelling motion of the gel is slower than the deswelling motion. Therefore, when the gel actuator bended, the R value gradually decreased.

336 Smart Actuation and Sensing Systems – Recent Advances and Future Challenges

concentration, flow rate and solution temperature.

motion in a CSTR.

polymer chains (4→6).

four peristaltic pumps in order to feed four solutions of potassium bromated, sodium sulfite, potassium ferrocyanide and sulfuric acid. Moreover, this system had one more peristaltic pump to drain the excess solution. The degree of changing the pH range (amplitude) and period of the oscillating reaction can be controlled by changing the feed

**Figure 22.** Continuous monitoring system for oscillation: monitoring both the medium pH and the gel

Figure 23 shows a motion of the nanofiber gel actuator. The bending and stretching motions of the gel actuator synchronized with the pH oscillating reaction. As shown in Figure 23, we defined R as the length between two edges of the gel. Figure 24 shows the trajectory of the nanofiber gel strip. As shown in Figure 24, the gel strip caused the pendulum-like motion. As the external pH is below the pKa, the nanofiber gel stretches because of the deswelling originating from the hydrogen bonding (1→3). Next, when the pH is above the pKa, the gel bends because of the swelling originating from the repulsive force among the anionic

Figure 25 shows the temporal changes of R of the gel strip and the external pH, respectively. The range of the pH oscillation based on a bromate/sulfite/ferrocyanide reaction was 3.1 < pH < 7.2, and the period was about 20 min. When the external pH changes periodically, the R of the gel strip cyclic changes synchronized with the external pH change. As shown in Figure 25, when the pH sharply decreased, the R of the gel strip starts to increase because the gel collapsed. Next, when the pH increased rapidly, R gradually decreased, due to the

**Figure 23.** Periodical pendulum motion of poly(AAc-*co*-nBMA) nanofiber gel.

**Figure 24.** Trajectory of the tip of the gel relative to its attachment position during pH oscillation.

**Figure 25.** The time series of pH (discontinuous line) and R (solid line).
