Foam actuators are typically made of 3 components:
- an open-celled elastomer foam
- a strain-limiting layer
a nonporous sealing layer
The elastomer foam can be custom fabricated or purchased as as finished product from a manufacturer. The key property is that the foam be open-celled (that is, contains an interconnected pore network) so that fluid may be transported throughout.
Custom foams can be made using any variety of foam-forming processes; however we had the most success using either a lost salt process or castable polyurethane foams.
The lost salt process involves mixing salt particles into a liquid elastomer (that is, before the elastomer has cured). After the elastomer has cured, the salt is removed leaving a porous elastomer. Castable polyurethane foams generate pores without any additives. These materials can be purchased as a two-part liquid systems. Once the two parts have been thoroughly mixed, they can be cast into a mold. As the polyurethane cures, it generate gas which forms the pores within the final cured elastomer.
Because foams of many different materials can be formed through the lost salt process, this method allows greater control of the foam mechanical properties and porosity. Polyurethane foams can be easier and faster to use as they require less processing.
Similar to other soft actuators, foam actuators require some form of regional strain constraint to direct the motion of actuation. The strain-limiting materials can often be embedded with the foam or the sealing layer.
Several materials can be used to locally limit strain including plastic mesh, wound fibers, or even just a stiffer elastomer. So long as the strain limiting material forms a region with a higher stiffness than the rest of the actuator, its motion upon inflation will be directed. This is explained in more detail below.
The final component of the foam actuator is the sealing layer. The seal should be a nonporous rubber and serves to make the actuator airtight (often, with the exception of an inlet port for pressurization). For foams made using a lost salt process, the seal can be composed of the same elastomer as the foam ensuring chemical compatibility between the materials.
The location and orientation of the strain-limiting layer directly determines the type of the resulting actuator. For example, a bending rectangular actuator is formed when a plastic mesh is embedded on one surface of the sealing layer. An extending cylindrical actuator is formed when the mesh is embedded in circumferential strips around the foam.