There are various options for the inner tubing used in making McKibben actuators. Below are some rough guidelines on the different options and their advantages/disadvantages.
Balloons
Party balloons are good for quick prototyping as they are easy to buy and modify. However, they are not very reliable. Also, as they are made of latex, they cannot be cast in silicone (latex inhibits silicone curing) and can trigger allergic reactions in some people, which is a limitation when it comes to medical applications.
Another option is custom-made balloons of the kind used in medical procedures (i.e. stent placement). These are more reliable, but also much more expensive.
Bought tubing
Purchasing rubber tubing makes fabrication much easier since it's just off the shelf.
However, there are fewer options -- for geometry, selection is limited to standard diameters/sizes, and typically it is difficult to get low-modulus tubing (the softest tubings on McMaster are still too stiff for use in McKibbens). This means that actuators made with bought tubings generally require higher pressure.
When selecting off-the-shelf tubing/choosing a supplier, it's important to look at shore scale of durometer, wall thickness, and modulus at the % strain range that you will be working in (generally <100%).
Some supplier options:
- Kent Elastomer Products, Inc. -- low durometer latex
- Specialty Silicone Fabricators (SSF) -- low durometer silicone
Self-cast tubing
With self-casting, it is possible to make tubing with much lower durometers than is commercially available, which enables very low threshold pressures. It is also possible to make the whole actuator monolithic (e.g. end caps) and lends itself to casting in a matrix of the same material. However, self-cast tubing is time-consuming to make and has lower yields (can get up to 80% with experience).
There are several methods of self-casting tubing, two of which are described in this documentation:
1) 3-D printed mold in 2 halves
Pros:
- Can make tubing of any ID & OD desired
- Demolding is easy
Cons:
- 3-D printing can get expensive
- Molds are hard to clean and material is fragile (alignment posts tend to snap off)
- Messy, lots of leakage, wasted elastomer
- Long rods bend, causing off-center inner cavity and non-uniform wall thickness
- Resulting tubing has “web” on side
2) Molds made using off-the-shelf pipes
Pros:
- Works well for small, thin-walled tubing
- Easy to make many in parallel
- Mold materials are cheap
Cons:
- Does not work well for longer tubes
- Making the molds requires more manual work (compared to 3D printing)
- Tube outer diameter determined by the inner diameter of off-the-shelf plastic tubing, which comes in discrete sizes (so limited customization)