# Water Driven Soft Actuator
## 1 Introduction  
Soft actuators and soft robots are actively studied due to its safety and flexibility in recent years [1]. There are many studies on soft robots aiming to be used in real environments. Soft robots have useful features that are used in real environments, such as high compatibility and safety with the environment [2, 3]. Soft robots can enter a narrow space where a hard robot cannot enter as they have flexible structures and can change its own shape due to their softness [4]. It is also hard to damage even if it receives a strong impact, such as falling unlike hard robots [5]. Soft actuators and soft robots are expected to be used in various situations. There are many approaches to achieve soft actuators.  
An approach to achieve soft actuation is to employ air pressure. Pneumatic actuator is a well-known actuator using the mechanical constraints. McKibben actuator is an old example of this approach [6, 7]. It is composed of rubber tube and non-stretchable mesh and shrinks about 25%. It is used in various scenes, such as worm-like robot [8], quadruped robot [9], humanoid [10, 11], and infant robot [12], due to its safeness. Some soft robots are also driven by jamming and pneumatic actuator [13, 14]. Soft actuators driven by water pressure are also reported [15]. The response speeds of these actuators are relatively fast. However, these soft actuators require hard pumps as the external equipment to be controlled and these hard devices prevent the robot from becoming completely soft.  
Some soft actuators using chemical reaction are also reported. There are many kinds of soft actuators that swell and shrink with various stimuli from external environments. They are categorized as stimuli-responsive polymers (or composite) and they become increasingly important. They are driven by various stimuli, such as temperature [16, 17, 18], pH [19, 20], electric field [21], and light [22]. These types of actuators can swell and shrink depending on the external stimuli without using mechanical parts, such as motors, drive shafts, gears, battery, and so on. We can find many mechanically deformable materials that are composed of stimuli-responsive polymers. The survey on stimuli-responsive polymers on actuation shows some examples [23], such as self-folding liquidcrystal elastomer-based actuators, carbon-based nanomaterials, nafion-based actuators, and so on. It categorized them into liquid crystal elastomer-based actuators, dielectric-elastomer-based actuators and ionic-polymer-based actuators. The developed soft actuator is composed of the particle of sodium polyacrylate and nylon pouch. Although sodium polyacrylate is an ionic-polymer and inside the device, the actuation of the device is due to the mechanical structure of the actuator. The proposed actuator is very simple and has some good points compared to existing actuators. The actuator is a kind of water driven soft actuator, and swells and shrinks depending on the amount of the water flow. In spite of its simple implementation, the response time of swell and shrink is very fast like mechanical approaches. The device is a fully soft actuator with safe and human-friendly materials, unlike some soft actuators with dangerous materials. It is expected that passive scale changeable spiles depending on the water flow and actuators for robots driven by the sea tide and rain are potential applications of the proposed devices. The rests of the paper are organized as follows. In Section 2, the chemical feature of the sodium polyacrylate is briefly explained and the design of the proposed actuator is introduced. The mechanism of the swell and shrink of the proposed actuator is also discussed. In Section 3, some of the experimental results are shown to clarify the response speed, the duration rate and the force of the proposed actuator. Conclusions follow in Section 4.