Soft Robots: 3D Printing Artificial Muscles

Using a standard filament 3D printer, artificial robot muscles can be directly printed. These air powered muscles can be used in all kinds of robots or powered prostheses.

Intro pic shows a six muscle flower robot activated. Such a simple robot could be used to indicate air pressure, temperature or as a protective enclosure that opens up. Pic 2 shows it closed.

This instructable shows how to make the flower muscles and a pull muscle which is detailed in step 7.

Previously I have made artificial muscles using a 3D printer by printing molds and casting silicone in them. See here:https://www.instructables.com/id/3d-Print-An-Artifi...
That is a fairly involved method and does not produce the same precision as directly 3d printing.

This is a more direct method where the muscles themselves are directly 3D printed using a flexible filament called Ninjaflex. The printing pattern of a 3D filament printer leaves many microscopic holes that will not hold air pressure. So, after printing, the muscles are dip-coated in an flexible elastomeric glue to seal the holes. This allows them to hold air pressure of 22 PSI or higher.

I used a Makerbot Replicator 2 to make these muscles, but other filament printers that can print Ninjaflex can be used.

This is still at the early stage of experimentation, but it shows great promise for producing muscles that can replace servos and gear-motors in robots. This will make them considerably less expensive and when used in soft robots, make them more human friendly to touch and be around.

A Stereo-lithography printer using flexible resins, would make it possible to produce smaller and more precise types of muscles and robot skin that a filament printer cannot. More intricate air channels and unusual shapes that do not need supports could also be printed. A flower robot like this could be printed assembled in one piece.

Step 1 pic shows the flower opening when all six muscles are pressurized at 22 PSI.

All of these muscles were designed using a free program: 123D Design.

To begin the design, the basic shape is drawn and then extruded as a solid.

It is then hollowed out to create walls that are .026" thick. Size the solid to produce an internal air channel around .002" thick. This wall thickness produces 2 shell walls that will fuse together solidly. This minimal thickness works well for a Makerbot Replicator 2 printer. For larger and more robust muscles a wall thickness of .053" will produce 4 shell walls. For a different printer, you will have to experiment with different wall thicknesses that work with your particular settings.

a square nozzle with a square hole for tubing is attached and the muscle is done.

Makerbot Replicator 2 or other filament 3D printer

Ninjaflex filament.
Available from: http://www.3dsupplyworld.com/flexible/

MEK solvent available from most hardware stores.

Loctite fabric glue available from Walmart or Amazon. I tried a few dozen glues, paints and sealers, and the Loctite glue was the only one that would fuse well to the Ninjaflex and remain flexible. It also glues well to vinyl tubing and acrylic materials.

Mason Jar. Mason jars work well for holding solvents and solvent glues and paints. If you reverse the lid then the metal will be against glass and the rubber seal won't dissolve.

1/8"vinyl tubing and fittings-available from: http://www.usplastic.com

.25" clear acrylic tubing available from: http://www.usplastic.com
This is for the flower stem, but it could be printed in PLA instead.

Flower muscles are the smallest volume of muscle that can produce a large amount of movement. They could be used as indicators of pressure, temperature or other parameters. They can also be used to latch moving pieces or to open and close protective enclosures. Controlled as separate petals they could be used to indicate numbers. They could also be used to create facial expressions on a robot head.

The muscles and muscle hub and petals were printed in Ninjaflex with a Makerbot Replicator 2 using the following settings:

No supports
Infill: 100 per cent
Shells: 2
Layer Height: .2mm
Temp: 225 C
Speed Extruding: 30 mm/s
Speed Traveling: 150 mm/s

Step 4 pic shows a single printed and sealed muscle relaxed.

Pic two shows the muscle pressurized.

After printing a short piece of vinyl tubing is inserted into the nozzle. Then, the artificial muscle is dipped in a sealant and allowed to dry for 2 hours. After drying, it is dipped again and then allowed to dry overnight. This will usually create a permanent seal. Occasionally, after pressure testing the muscle underwater, leak points will be revealed which can be sealed with a dab of glue.

This should be done under a vent hood or outside with a fan to blow the solvent fumes away.

The sealant is composed of Loctite Fabric Glue that is thinned with 10 to 20 per cent MEK solvent by volume. The MEK slightly dissolves the Ninjaflex resulting in an extremely good fusion of glue and Ninjaflex. Adding more MEK will tend to swell the Ninjaflex and make some muscle types hard to seal.

After six muscles have been sealed with a vinyl tube in the nozzle, they should be tested under pressure. Once they are leak free they are glued to the petals as in the step 6 pic.

The hub should be dipped twice in sealant and left to dry overnight before gluing the muscles to the holes in the hub.

Finally a .25 clear acrylic or PLA printed tube (sealed) should be glued to the hub and once dry the whole flower tested under pressure in water.

Artificial muscles that push are in many ways more efficient than muscles that pull as they do not require a shell around them or tendons to work. There are however, many places where a pull muscle with tendons is required.

ZIG-Zag Muscles
The zig zag muscles are the best design I have found so far for getting maximum expansion from a minimal resting volume of muscle.

The step 7 pic shows the muscle lifting a one pound weight. This muscle has an expanded volume of 2.5 cu. in. and can lift more than 4 pounds.

Pic 2 shows the muscle relaxed.
Pic 3 shows the components of the pull muscle

These muscles could be scaled up considerably for more power.

Experimental Muscle Types
Step 8 pic shows just a few of the muscle types I have experimented with. I have barely scratched the suface of what is possible in the 3D printing of artificial muscles. Pic 2 shows the muscles pressurized. The largest blue elbow muscle is 1" x 1" x 3" long.

The Future Of Robotics
I believe that the future of robotics, if it is to become more affordable and useful, will involve the 3d printing of soft robot muscles and skin teamed up with 3d printed stiff bones and shells. This will minimize the number of expensive servos and gear-motors that will necessary. It should eventually be possible to print the muscles, bones, and skin of a robot in one print.

3D Printing Forms For Casting Muscles
If you want to cast muscles in 3D printed forms using silicone or other flexible materials see here:

https://www.instructables.com/id/3d-Print-An-Artifi...

3D Printing Servo Controlled Valves
You can 3D print your own valves that are easy to control with a microcontroller:
https://www.instructables.com/id/3D-Print-Make-Serv...