The secret of nylon muscles

By twisting a polymer cord up tight, you can vastly improve its strength. A team of researchers in the USA have worked out how much stronger this makes nylon. They also found a way to use the material in artificial muscles.

Old phone cables come in a helix shape: a single twist around the length of the cord. Many materials can be twisted like this. If you twist a rubber band, the same happens. If you continue twisting, the rubber contracts into coils, a double-twist in the material.

Coiling nylon in the same way increases its load-bearing capacity. This is what researchers at the University of Texas in Dallas did. Using polythene fishing line, the scientists tested the breaking strength of the coiled nylon and were surprised at the results. A nylon cord of around ten times the thickness of a human hair suspended over 7 kg.

Now comes the interesting part. It is well-known that thermoplasts, such as nylon and polyethylene, shrink in length when heated, while expanding in width. So when you take a strong, coiled cord and heat it, it contracts, like a muscle does.

When the material is cooled, it returns to its original length. This reversible process is the key to the researchers’ proposal for nylon muscles. Using hot and cold water, the team made the artificial muscle contract and expand many times over, lifting weights far heavier than previously thought possible.

Better yet, similar materials, such as shape memory polymers or carbon nanotubes, are very costly. Nylon is an extremely common and very cheap polymer.

The team is now working on complex coiling systems. This is to make muscles that can contract by about half, or 2,5 times as much as human muscles do.

Alternating hot and cold water isn’t always practical. But it is easy to imagine coating the nylon with a heat absorbing material, making it react to sunlight, for instance. Equally, wearable textiles could adjust to climate conditions, keeping the user warm.

Watch a video of the artificial muscle here .

Images and information via National Geographic.