Your phone relies on tiny sensors made of silicon, but soon, they might be made of wood waste instead.
Microelectromechanical systems (MEMS) contain tiny moving parts that can be used to detect things like acceleration and mass extremely precisely. They are found in small devices like smart phones and can also be used as part of microphones and displays.
Almost all MEMS are made of silicon and creating them requires temperatures around 1000掳C and potentially hazardous chemicals like hydrofluoric acid.
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Instead of silicon, Virginia Davis at Auburn University in Alabama and her colleagues figured out how to make these minuscule machines out of cellulose crystals extracted from discarded wood pulp.
Knock on wood
They built their MEMS by depositing layers of several different materials on top of one another, with a layer of cellulose nanocrystals in the middle, and then etching it into the right shape.
The process doesn鈥檛 require high temperatures or lots of energy, and the chemicals used are relatively harmless. The whole thing is also biodegradable.
鈥淭he cellulose layer is like a film of tiny toothpicks,鈥� says Davis. By changing the direction the toothpicks are pointing, they could change the strength and flexibility of the final device.
For example, some of the MEMS involve strips of material that extend like diving boards to measure mass. Those strips need to be strongest along their long side, so the crystals are arranged to all line up parallel to that side.
These new MEMS are only about one third as strong as silicon ones, but the ability to change their properties by tweaking the different layers and the arrangement of the crystals means that they can be customised for various applications in ways that silicon cannot, says Davis.
One use for the wooden MEMS could be detecting water contaminants by measuring their added weight in the liquid. They could also be integrated into biodegradable electronics.
鈥淚t won鈥檛 replace all the silicon devices out there, but we see it filling a need that silicon can鈥檛 because of the cost and intensity of producing silicon devices,鈥� says Davis.
ACS Applied Materials & Interfaces
