*2.3.4. Experimental apparatus for causing the CT reaction in the pH-responsive tubular gel*

The feeding solutions were stored in three separated reservoirs containing an alkaline sodium chlorite solution ([NaClO2] = 1.2 × 10<sup>−</sup>1 M, [NaOH] = 1.5 × 10<sup>−</sup>4 M), an alkaline potassium tetrathionate solution ([K2S4O6] = 3.0 × 10<sup>−</sup>2 M, [NaOH] = 1.5 × 10<sup>−</sup>4 M), and a sodium hydroxide solution ([NaOH] = 1.3 × 10<sup>−</sup>2 M). The solutions were pumped by peristaltic pumps (ATTO SJ-1211L) and premixed by a magnetic stirrer (AS-ONE CT-3A) before entering the tubular gel. The flows of chlorite and tetrathionate solutions were maintained at 30 mL/h, and the mixed solution flowed into the tubular gel by a peristaltic pump (ATTO SJ-1211H). The tubular gel was soaked in the mixed CT solution for 30 min in advance. After entering the flow of the mixed solution, an acid perturbation was applied by touching the gel at a particular spot with a paper soaked in a 1 M H2SO4 solution. The behavior of the tubular gel was observed by using a microscope equipped with a CCD camera controlled by a computer. Methyl red, a color indicator that changes from yellow (basic) to red (acidic), was added to the solutions to directly visualize the propagation of acid regions. Fig. 3 shows the schematic illustration of experimental apparatus.

**Figure 3.** Schematic illustration of experimental apparatus for causing the CT reaction in the pHresponsive tubular gel. (Reprinted ref. 58, Copyright *IEEE*. Reproduced with permission.)
