Airic robot aarm developed by Festo AG uses air muscles controlled by tiny piezo proportional valves, 2007. —Credit: Festo AG & Co. KG
Airic robot aarm developed by Festo AG uses air muscles controlled by tiny piezo proportional valves, 2007. —Credit: Festo AG & Co. KG

Engineers and scientists often create wonderful, constructive devices that make life better for people who are sick and disabled. Nearly as often, they design and build weapons and implements of warfare. And once in a while, the same person does both.

Joseph Laws McKibben was an important figure in World War II’s Manhattan Project. A nuclear physicist from the University of Wisconsin, Dr. McKibben was on the team that researched the properties of the tamper, the device that controlled the speed and power of the atomic bomb’s chain reaction.

In addition to being a theoretician, McKibben was a hands-on sort of scientist. On July 16, 1945, he pushed the button that set off the first ever A-bomb — code-named Trinity — in the New Mexico desert. Early that morning, McKibben made the final electrical connections to the explosives that would initiate the chain reaction in the bomb. Then he hopped into his jeep and drove six miles to a concrete bunker where the countdown to HHour was underway. There, McKibben threw the switch to initiate the final control sequence that detonated the bomb.

— Credit: Globe Photos

Six years later, in 1952, McKibben’s daughter, Karan, was stricken with polio. Paralyzed from the neck down, she was confined to an iron lung for a time. Dr. McKibben felt that he could use his engineering skills to improve the quality of her life. So, working with the doctors at the rehabilitation center where she was living, he began researching ways to give polio patients some control over their fingers.

McKibben studied the existing hydraulic, electric, and pneumatic methods of moving paralyzed arms, and he became intrigued by one that he felt had particular promise. A few years earlier, a German scientist had prototyped a clever pneumatic gadget operated by compressed gas. The device consisted of a flexible bladder that could be filled with carbon dioxide. The bulging bladder closely simulated the natural motion of human muscles. Could this idea, McKibben wondered, allow paralyzed fingers to work once again?

— Credit: Globe Photos

Within a few years McKibben’s team developed the gadget now known as the air muscle, or braided pneumatic actuator. Dr. McKibben placed it next to his daughter’s paralyzed forearm and attached it to her thumb and first and second fingers with splints. When she operated a lever by shrugging her shoulder, gas flowed into a tube, causing a contraction that drew the paralyzed fingers together. At the next touch of the lever, the plastic tube deflated and her fingers relaxed.

McKibben’s air muscle has become an important component used by roboticists and biomedical engineers, thanks to its high force-to-weight ratio, flexible structure, and low manufacturing costs. A typical modern air muscle consists of a flexible rubber tube or bladder inside a polymer mesh sleeve that’s braided in a helical weave. When the bladder is inflated, the mesh expands in width but simultaneously contracts in length. This shortens the muscle, so anything attached to the ends of the muscle is pulled together. The muscle contracts smoothly and with a surprising amount of pulling force.

You can easily construct an air muscle from hardware-store parts. It’s a great example of using basic pneumatic principles to make devices for controlling motion. It’s also a testament to Joe McKibben’s versatile ability to engineer solutions to different sorts of problems.

— Illo Credit: Damien Scogin

Project Steps

Insert the bolt

Insert the bolt into one end of the silicone rubber tubing. It should fit snugly.

Insert the rubber tubing

Insert the rubber tubing all the way through the braided sleeve. It may take quite a bit of wiggling.

Insert the valve

Push the middle port of the barbed 3-way valve fitting into the open end of the rubber tubing as far as possible.

Place the hose clamps

Place one hose clamp over the sleeve, tube, and barb, and tighten securely. Place the other hose clamp over the sleeve, tube, and bolt, and again, tighten securely so it is airtight. This completes the air muscle.

Make the air line

Now make your air line. Cut a 6″ length of the polyethylene tubing. Insert one of the remaining barbed ports of the 3-way valve. If it’s too tight, try warming the tubing in hot water first. Secure the connection with a hose clamp.

Prepare the other end

Insert the barb of the ¼” barbed to ¼” NPTF male connector into the other end of the polyethylene tube. Secure with a hose clamp.

Make the connection

Finally, connect the female end of the ¼” NPTF quick-connect hose coupling to the male end of the ¼” NPTF to barbed connector, using pipe compound or teflon tape between the male and female connectors. Your air muscle is now ready for use.

Use It

There are many ways to mount your air muscle. The easiest is to use wire to attach one end of the muscle to a support and the other end to something you want to move. By using levers and pulleys, you can obtain very sophisticated movements for robotics, prosthetics, and automation projects.

  1. Wear safety glasses whenever you work with compressed air. Connect the air muscle to a highpressure air source such as a compressor or air tank. The higher the pressure, the more the air muscle will contract — but too much pressure may split the tube.
  2. Connect your air source to the air hose coupling.
  3. Open the 3-way valve so that the air muscle fills with air. As it fills, the rubber tube expands but is constrained by the mesh sleeve, causing the air muscle to contract. Moving the valve handle in the other direction exhausts the air from the muscle, allowing it to relax.