If you need a material that can literally be changed to suit you over time, look no further.
Metamaterials – meaning “beyond matter” – are engineered materials with properties not found in nature. This gives them unique scope to work outside of the realms of “normal” acquired materials.
One such example has recently been reported in Nature by Tian Chen, of École Polytechnique Fédérale de Lausanne, Switzerland, who designed a metamaterial that can be reprogrammed to have different mechanical properties after it is already made.
“I wondered if there was a way to change the internal geometry of a material’s structure after it’s been created,” says Chen. “The idea was to develop a single material that can display a range of physical properties, like stiffness and strength, so that materials don’t have to be replaced each time.
“For example, when you twist your ankle, you initially have to wear a stiff splint to hold the ankle in place. Then as it heals, you can switch to a more flexible one. Today you have to replace the entire splint, but the hope is that one day, a single material can serve both functions.”
The material is made of small mechanical bits, called m-bits, that are reminiscent of computer bits.
In a hard drive, tiny pieces of digital information can be stored as bits. Magnetic bits can be programmed to switch between the values of 0 and 1, or on/off, by magnetising them in different directions to confer binary information. That binary code can be controlled by an external electromagnetic circuit, which changes the direction of those bits to recode the hard driver with a new memory.
So, if you’re storing your favourite song on a hard drive, the direction of those bits change based on the code that is imparted, and the digital properties of the hard-drive are altered to include the memory of how to play your song.
This principal is somewhat like Chen’s material, except that he used mechanical units instead. His m-bits are made of silicone and magnetic powder and have a unique shape that allows each individual cell to move between a compressed and decompressed state. These two states act as the programmable binary code, like computer bits.
This essentially means that the material can contain a memory about what it is supposed to be.
“You can activate and deactivate individual cells by applying a magnetic field. That modifies the internal state of the metamaterial, and consequently its mechanical properties,” says Chen.
The property that can be altered in this way is the stiffness of the material. When the cells are switched on by the magnetic field, the material is stiff; when they’re switched off, the material is more flexible.
If that isn’t incredible enough, its possible to program various combinations of the on/off cells to provide a range of flexibility, basically whenever it’s needed.
This is the first report that shows both programmed memory and physical change imparted by bits in a single material.
This extraordinary combination of computer science and mechanical engineering strives to find the sweet spot between static material and machine. This unlocks potential materials that might be used in a plethora of useful items, from prosthetics to aeronautics to shock absorption in orthopaedic shoes.
A few things need to be sorted out before it reaches the usable stage, though.
“We could design a method for creating 3D structures, since what we’ve done so far is only in 2D,” says Pedro Reis, the leader of Chen’s lab at École Polytechnique Fédérale de Lausanne . “Or we could shrink the scale to make even smaller metamaterials.”
Regardless, the ability to program the memory of materials so they’ll change properties is a very exciting development indeed.
Dr Deborah Devis is a science journalist at The Royal Institution of Australia.
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