Human bones are flexoelectric. When microfractures occur during exercise (A), the behavior of cells (B) that form new bone (C) is dictated by the fracture’s flexoelectric field.

By Dave DeFusco

A Katz School of Science and Health paper that devises a theoretical framework for studying the quantum origins of flexoelectricity, a phenomenon in which electric charges scatter within a material after it has been bent, has been published in the Journal of Applied Physics.

Flexoelectricity occurs when an ordinary object, which is normally uncharged, is significantly changed in size and shape—or deformed—by mechanical or other forces. The result is that positive and negative charge regions inside the object are rearranged due to the migration of electrons.

“The net charge is still zero,” said Dr. Fredy Zypman, professor and chair of the M.A. in Physics program, who authored the article . “But the fact that you can separate the charges will have an effect via the electric field induced by that dissociation.”

Most people have experienced charge migration on a dry winter day when they feel a shock at the touch of a metallic doorknob. The spark is a violent manifestation of a fast electric charge transfer. While charge relocation is ubiquitous in nature, its manifestation is usually, unlike the spark, subtle to common observation. In the case of flexoelectricity, the effect is so well concealed that it shows up in extremely tiny objects that can only be monitored with very delicate instrumentation.

Flexoelectricity is present in all materials, but it’s virtually imperceptible in everyday events. Still, it’s prevalent at submicroscopic scales. Researchers have recently become more interested in experimentally validating this phenomenon, which has been a point of speculation since the 1950s, as the trend toward the miniaturization of electronics and microchips has accelerated.

“Flexoelectricity has tantalizing implications in a wide range of areas, ranging from biotechnology to energy harvesting,” said Dr. Zypman. “Most important, all materials display this property, making the menu of flexoelectric materials unlimited in practice, a most desirable quality in scientific, engineering and commercial spheres.”

Flexoelectric materials can be used as sensors and actuators at the nanometer scale; by comparison, a sheet of paper is about 100,000 nanometers thick. Sensors can monitor small movements by direct measurement of the voltage created by a charge separation. Conversely, in an actuator, the input is a voltage—or charge—and the output is motion. It could be linear motion, said Dr. Zypman, or angular motion, like a twist.

“These two aspects have vast practical applications,” he said. “For example, a submicroscopic autonomous robot—a micro-cyborg—can use actuators to move and sensors to decide how to move.”

These flexoelectric sensors can also be used to monitor the structural integrity of a building by measuring its vibrations with great precision. Pacemakers implanted in human hearts and utilizing lithium batteries could instead be self-powered as natural movement generates electrical power. Human bones are flexoelectric. When microfractures occur during exercise, the behavior of cells that form new bone is dictated by the fracture’s flexoelectric field.

“While flexoelectricity is a property of all materials, it is necessary to continue finding unambiguous foundational connections between theory and experiments to be able to assess real systems,” said Dr. Zypman. “This is important as applications of flexoelectricity in nanodevices grow. It is still a very exciting work in progress.”