When you flex a muscle in your arm, it’s not the muscle that’s shrinking, but the cells that comprise it. Each cell lining the muscle’s length has its own limited contractile ability, one driven by tiny motor and ratchet proteins that crumple up a cell and refuse to let it relax. Rather than being a single continuous muscular action, your bicep contracts as a result of millions of tiny, discreet contractions all working in concert. That being the case, it’s always been something of a misnomer to refer to robot muscles as muscles, per-se — despite being visually similar, they are really just servos, or hydraulic pumps, or whatever else. This week marks the first proof of concept for an attempt to change that, to make usefully strong artificial muscles that actually work like muscles — with all the advantages and disadvantages that come with it.
The research comes from the US Department of Energy’s Berkeley Lab, and it focuses on an amazing material called vanadium dioxide. It’s already used widely in electronics and other industries, mostly because it has an amazing ability to switch between being an electrical insulator and conductor, based purely on temperature.
At exactly 67 degrees Celsius, the crystal structure undergoes a phase shift that allows the free flow of electrons through the system — and it also physically deforms the metal. That physical change occurs with incredible force — as seen in the video below, a coil of vanadium dioxide can launch objects 50 times heavier than itself over a distance five times its length within 60 milliseconds.
The robo-myosin was able to undergo over a million cycles without degrading, snapping back and forth in its basic contractile cycle. Since this cycle is controlled by heat, the researchers came up with two methods of heat delivery. To cause all the units to contract as one, a tiny heating pad can be used to change the overall temperature. The better solution is passing an electrical current down the filament, since it uses less energy per contraction and allows targeted heating of individual motors.
The advantages of this model of muscle action are numerous. Think about the fact that, despite being made from relatively weak constituents like protein and fat chains, has only now been surpassed by technology. Of course, a bulky, pneumatic Atlas muscle could outperform both this and a human in terms of raw strength, but not while remaining so light and compact, nor is it so instantaneously responsive as a system based on micro-meter movements. The actual energy costs per unit strength are unknown right now, but has the potential to be extremely efficient, relative to current solutions.
Many a modern robot engineer would love to be able to create directed mechanical force by simply sending a current down a vanadium filament. Bear in mind that this is just an early proof of concept for a much more robust system down the road. What this paper outlines is the basic diving force behind robot muscle contraction — a huge achievement, but a far cry from having actually created such a muscle. And don’t take the phrase “robot muscle” too literally, either, as there are plenty of less anthropomorphic applications for this technology. Future research will focus on using this force in a directed, useful way.
Research paper — 10.1002/adma.201304064: Powerful, Multifunctional Torsional Micromuscles Activated by Phase Transition