**3.1. Mechanism of buoyancy control**

### *3.1.1. Gas generation by water electrolysis*

When an electric current is applied between positively and negatively charged electrodes in ionized water, a chemical reaction occurs, in which pure oxygen and hydrogen gases are produced. Hydrogen gas appears at the cathode electrode (3) and oxygen gas appears at the anode electrode (4). Equation (5) shows the net reaction (Shaaban, 1994).

$$2\text{H}\_2\text{O}\_2 + 2e^- \rightarrow \text{H}\_2 + 2\text{OH}^-\tag{3}$$

$$2\,\text{4OH}^{\cdot}\to\text{O}\_{2} + 2\,\text{H}\_{2}\text{O} + 4e^{\cdot}\tag{4}$$

$$\text{2H}\_2\text{O}(l) \xrightarrow{current} \text{O}\_2\text{(g)} + \text{2H}\_2\text{(g)}\tag{5}$$

Two thin gold plates, each approximately 2 cm by 2 cm, are used as electrodes (Fig. 17). Typically, the two plates are placed 1-2 cm apart to allow the current to travel through the water. Most electrolysis experiments are performed in ionic solutions, which are usually prepared by adding salt, acid, or base. However, adding electrolytes is not feasible in this case because the device must be able to generate gas in regular tap water.

**Figure 17.** Gold electrodes and a sample of an ionic polymer-metal composite.

Electrolysis in tap water is much slower because of the limited number of ions present. In order to enhance the electrolysis process, an IPMC (150 μm thick) is placed directly in between the electrodes (Fig. 17). In this study, the IPMC is used only as a medium to contain electrolytes and provide channels for ion movement in the electrolysis process. By allowing the current to flow through the electrodes and IPMC, oxygen and hydrogen gases are generated. These gases are collected in a gas chamber, displacing the water in the artificial bladder. Because the gas mixture has a much lower density than that of water, the device becomes more positively buoyant. Care must be taken because the mixture contains a proportion of constituents such that it can burn instantly when ignited.

#### *3.1.2. Gas releasing mechanism by solenoid valve*

The mechanism to control the release of gas, and thereby depth, uses a two-way solenoid valve (Hargraves Tech. Corp, Part # 75M06U2.A005S). This valve is closed in its nonactuated state and is suitable for the size and power constraints of this proof-of-concept design. This particular valve can be opened by applying 6 VDC voltage and has a power consumption and mass of 0.5 W and 5 g, respectively. When the valve is actuated, the gas formed during electrolysis escapes from the device and water supersedes. As a result, the density of the device increases, causing it to become more negatively buoyant. When the valve is closed, water cannot enter through a bottom opening because the pressure inside the device is equal to that of outside. Thus the device is able to maintain the same depth by Ionic Polymer-Metal Composite Artificial Muscles in Bio-Inspired Engineering Research: Underwater Propulsion 239

having a constant buoyant force. In order to travel up or downwards, gas is either produced or released in a controlled manner.
