Hit certain crystalline materials with a jolt of electricity and they will change shape. Squeeze them and they will jolt you right back. Scientists have used these so-called piezoelectrics for decades in ultrasound medical imaging; the materials are so sensitive that they can pick up the motion of sound waves moving through tissue. Now, researchers have come up with a simple new way to make potent transparent piezoelectrics, which could lead to improved medical imagers, invisible robots, and touch screens that power themselves.
Piezoelectrics are made up of either myriad tiny crystallites or single crystals of a variety of materials including ceramics and polymers. In both cases, a mix of atoms arrange themselves into a simple crystalline unit—typically the size of a handful of atoms—that’s repeated over and over. Inside each of these building blocks, the atoms are arranged in a so-called electric dipole, with more positive charges on one side and more negative charges on the other.
Applying pressure to these materials can subtly alter the position of atoms, enough to rearrange the charges and produce an electric voltage. Applying an electric voltage does the opposite, forcing the material to expand in one direction and contract in another.
That makes piezoelectrics extremely useful for a wide variety of applications, says Sri-Rajasekhar Kothapalli, a biomedical engineer at Pennsylvania State University, University Park, who was not affiliated with the current research. Piezoelectric devices, he notes, are a part of everything from cigarette lighters and push-button ignition switches on barbeques to precision motors on scanning force microscopes.
They’re also essential for photoacoustic imaging, which uses a piezoelectric device called a transducer to detect the ultrasound waves soft tissues emit as they absorb light from a laser. Different molecules—from hemoglobin to melanin—absorb different frequencies, so doctors can image many kinds of tissues to detect health problems. However, opaque transducers cast a slight shadow, which means the tissue immediately underneath them cannot be imaged. To get around the problem, researchers have created transducers using transparent piezoelectrics, but so far these materials have been too weak to solve all their imaging challenges.
A few years ago, researchers in Japan came up with a better way to make transparent piezoelectrics. Their chosen material, lead magnesium niobate lead titanate (PMN-PT), was a ferroelectric, which naturally harbors electric dipoles. Researchers had previously turned these materials into piezoelectrics by exposing them to a DC electric current, the sort made by batteries. But the Japanese team found that exposing them to AC currents—the sort fed to homes and businesses—produced stronger piezoelectricity. “It’s like shaking the crystal back and forth,” says Long-Qing Chen, a computational materials scientist at Penn State. This shaking could double a crystal’s piezoelectricity, the Japanese team reported in a 2011 patent.
In the new study, Chen’s group joined that of Fei Li, an electronic materials expert at Xi’an Jiaotong University, to replicate the work. Initially they only saw PMN-PT’s piezoelectric values rise by 20% to 40%. “That’s still a lot,” Chen says. But by using computer simulations, they were able to sort out a set of rules for how to improve the material’s piezo performance with AC fields.
Normally PMN-PT is opaque, as the separate groups of dipoles scatter light in all directions. Using their AC currents, Chen’s team caused the dipoles to align, and with the help of some heating and polishing, turned the material transparent and gave it a piezoelectric force 50 times more powerful than common transparent piezoelectrics, they report today in Nature.
That improved performance could lead to more sensitive photoacoustic imaging devices, which could aid doctors in everything from breast cancer and melanoma detection to the tracking of blood flow for the treatment of vascular diseases, Kothapalli says. Chen and his colleagues report that the advance could also inspire transparent actuators for invisible robotics and screens that power themselves when touched.
The work could also drop the cost of a wide range of piezoelectric devices, says Amar Bhalla, an expert at the University of Texas, San Antonio, who was not involved with the research. Typically, the best devices are made from single crystalline materials. However, Bhalla says, “Single crystals are difficult and expensive to grow.” The new AC technique allows researchers to grow high-quality piezoelectrics from polycrystalline materials that are cheap and easy to craft, he says. And that could give a jolt to an already burgeoning industry.