With their sights set on flying, Orville and Wilbur Wright spent the first few years of the 20th century perfecting lift-generating wings for gliders. Then, on 17 December 1903, the brothers progressed from gliding to sustained flight with the Wright Flyer, which flew for 37 meters along the beach in Kill Devil Hills, North Carolina, in 12 seconds. Unlike their gliders, the Flyer had propellers to generate forward thrust. More than a century later, planes still use the same flight strategy: Their wings generate lift, and propellers or turbines produce aerodynamic thrust by pushing air backward.
Last month Steven Barrett and coworkers at MIT demonstrated a new way of flying. Their plane is powered by electroaerodynamics, using electric fields to generate airflow. The 5-meter-wide aircraft covered about 45 meters in less than 10 seconds, limited by the length of the indoor track where it was tested. Using an electric field to move air is fundamentally different from using propellers or turbines because it doesn’t require any moving parts. It is also an improvement over a ramjet, which is also devoid of moving parts but requires the aircraft to already be traveling at high speed.
The first solid-state flight is part of a larger trend. For many applications, researchers are developing solid-state counterparts to traditional machines. Moving parts make machines less efficient because friction between the parts generates heat, so they may require lubrication and cooling systems. The parts also wear out and need to be replaced—not always an easy task, especially when it comes to space hardware. Solid-state machines can represent a more mechanically robust alternative by providing the same functions, such as pumping, switching, and actuating, without the issues introduced by moving parts.
A shift to solid state
The transition from hard disk drives (HDDs) to solid-state drives (SSDs) for computing over the past few decades exemplifies the benefits of the shift to solid state. HDDs use rapidly spinning disks to store data and have a mechanical arm for reading and writing information on those disks. With no mechanical parts, SSDs are more shock resistant. They are also quieter, produce no mechanical vibrations, and use less power while providing faster access to data. Although HDDs are still preferred for some applications—they are cheaper and often have higher capacity—they are often passed over for their smaller, more efficient solid-state counterparts.
Flight with solid-state propulsion has similar benefits. Instead of using loud, maintenance-needy propellers, the aircraft sustains an electric potential between a wire and an airfoil to generate thrust. Because the wire is so much smaller than the airfoil, the electric field around it is stronger, and the surrounding air becomes ionized. The charges accelerated by the field toward the collecting electrode drag neutral molecules along with them, which creates airflow.
The MIT researchers propose a solid-state replacement for a conventional propulsion system, but they have not yet produced a solid-state plane. Their current model would still require hinged flaps on the wings and tail of the plane for steering. Control of those parts on traditional planes has shifted from mechanical to electronic using a system known as fly-by-wire, but the parts themselves still move. Barrett eventually hopes to accomplish solid-state steering by shaping electric fields.
Although electroaerodynamic passenger jets are likely to stay in the realm of science fiction, the newly realized flight mechanism could prove useful for smaller aircraft that require less thrust. And, like the SSD, it has the added benefit of being quiet. As drones are increasingly being considered for applications such as delivery, law enforcement, and environmental monitoring, their contribution to noise pollution is a concern. Small electroaerodynamic aircraft, which are already being developed, avoid the additional ambient noise that would otherwise be generated.
Tiny machines
The development of miniature solid-state aircraft points to an important engineering benefit of solid-state machines: their scalability. In his 1959 lecture “There’s Plenty of Room at the Bottom,” Richard Feynman described the untapped potential of miniaturizing technology, from nanoscale machines to the idea of “swallowing the doctor.” Making and assembling moving parts for miniature and microscopic machines can be extremely difficult. A machine that can perform the same function without any moving parts circumvents that technical hurdle. To that end, researchers have been developing new solid-state techniques for performing machines’ tasks.
Micropumps, first built in the 1970s and then developed over the following two decades, used to be traditional machines; they used the motion of tiny mechanical parts, like pistons and valves, to control fluid flow. The devices were difficult to fabricate and prone to failure. In the 1990s researchers made the first solid-state micropump, which was based on electrohydrodynamics, the same principle that the MIT group used to generate airflow but in liquid.
Electrokinetic pumping that drives flow using an electric field has been a key component in the development of compact, portable “lab-on-a-chip” devices that can perform lab tests with tiny volumes of fluid. In a nod to those devices, Anette Hosoi’s group at MIT recently created the “tree-on-a-chip,” a new type of solid-state micropump driven by osmotic pressure instead of electric fields. Their design was inspired by trees, which generate large pressure differences to transport sugar-rich fluid over long distances without any active mechanism for doing so.
The ability to controllably deform and direct forces—as a door hinge, a finger, or a contracting muscle does—is essential for many machines. Early attempts at microscopic actuators to perform such functions were microgrippers that used electrostatics to draw two microfabricated pincers together, but such systems have limited applications because generating even small displacements requires high voltages.
More recently, research has been directed toward developing bimorph actuators. The thin, two-layered structures are made of materials whose mechanical response to an environmental stimulus, such as voltage or pH, is different. When one material expands or contracts while the other does not, the flat sheet bends accordingly. With microscopic machines in mind, Itai Cohen, Paul McEuen, and coworkers at Cornell University harnessed the technology to create three-dimensional structures that fold and unfold in response to environmental changes. By fixing rigid panels to certain parts of a bimorph sheet, they directed the sheet to fold into cubes, tetrahedrons, helices, and more.
Macroscopic solid-state actuators are now also of interest to the robotics community for generating motion and gripping. Hod Lipson and coworkers at Columbia University have created a soft actuator that works as an artificial muscle by permeating a silicone elastomeric network with ethanol. The material contracts like a flexing muscle when a voltage is applied and returns to its extended size once the voltage is removed. When used in a bimorph geometry, like the Cornell group’s micromachines, the artificial muscle mimics a hand that can be used to grab objects.
The past few decades have seen the development of solid-state machines that are quiet, efficient, and more robust than their traditional analogues. Such devices are more readily scalable than machines with moving parts and have therefore enabled advances in microtechnology. The realization of solid-state flight is one more step toward what Arthur C. Clarke describes in his 1956 novel The City and the Stars as the technological ideal for the future: a world in which “no machine may contain any moving parts.”
