Piezoelectricity goes viral
by Samantha Cheung
With an ever-increasing number of devices that require electricity to operate, wouldn’t it be amazing to charge small electronics, like your smart phone, by the simple action of walking across campus? Scientists are exploring this approach using piezoelectrics, materials that convert the mechanical forces involved in movement to electrical energy. Recently, researchers in Lawrence Berkeley National Laboratory’s Physical Biosciences Division, led by Professor Seung-Wuk Lee of the Department of Bioengineering at UC Berkeley, have found that electricity can be extracted from a surprising new piezoelectric: viruses.
Piezoelectricity is a nuanced but incredibly practical mechanism for energy conversion. In a battery, stored chemical energy can be converted into electrical potential energy, creating a voltage difference across the battery’s terminals. If the battery is connected to a cell phone or car, an electric current can rapidly and efficiently carry this energy away for other uses. Similarly, piezoelectrics develop a voltage difference with the application of mechanical stress, allowing them to convert kinetic energy into electrical energy. This effect happens because ions or molecules in the material are polarized, having a separation between positive and negative charges. Normally, these polarized constituents cancel each other out, and the material as a whole remains unchanged. When the piezoelectric is distorted, however, the overall symmetry is lost and the internal polarization comes to the surface. Because of this, one can generate an electrical voltage in these materials—much like that in a battery—just by squeezing, stretching, or twisting them. This microscopic effect makes piezoelectrics practical for a number of applications and they are currently used in a variety of common devices. For example, piezoelectric crystals in microphones convert vibrations created by sound to small electrical signals that can be amplified and projected to an audience.
Lee and his team of researchers found that certain viruses have piezoelectric capabilities. Viruses are biological molecules composed of a shell of proteins (known as “coat proteins”) that surrounds genetic material (DNA or RNA) containing the instructions for their own replication. The three-dimensional structure of the protein shell can vary greatly from one type of virus to another. Knowing that many organized biological molecules like DNA and proteins exhibit piezoelectric properties, Lee’s team predicted that, based on its structure, a virus known as M13 bacteriophage might also be a piezoelectric. The team observed that M13 has polarized coat proteins and a structure that ensures the polarization does not cancel out. In the presence of an electric field, materials can be tested for their piezoelectric potential by measuring changes in shape due to the applied electric field (known as the converse piezoelectric effect). Using piezoresponse force microscopy, a high-resolution method of measuring atomic-level changes in material structure under an applied electric field, the researchers confirmed the piezoelectric properties of M13 virus (YouTube).
Lee’s team next hypothesized that they could increase the strength of the piezoelectric effect by increasing the separation of positive and negative charges in the virus coat proteins. This is where the genius of developing viruses as piezoelectric materials comes to light. Because a virus contains the very genetic material that encodes its own coat protein, genetic engineering allows one to directly tune and optimize that structural shell. To create a larger piezoelectric effect, they engineered the addition of four negatively charged amino acids, the building blocks of proteins, to the negative terminus of the coat protein. This increased the charge separation, and ultimately the virus’s piezoelectric effect. Furthermore, aligning many genetically engineered M13 viruses to form multiple layers of film strengthened the piezoelectric effect of the M13 material.
Fabrication of thin films of M13 is simple and inexpensive. “An overnight bacterial culture infected with M13 can create millions of copies of the virus,” Lee says. While traditional piezoelectric materials are difficult to engineer, viral piezoelectric films are straightforward to synthesize: the M13 virus has an elongated shape allowing for alignment in a specific direction. The viruses then become the “basic building blocks to induce electric generation,” said Lee, in stark contrast to currently used piezoelectric materials that require expensive, complex, and labor-intensive procedures to assemble. In addition, virus-based piezoelectric devices provide a safer alternative to commonly used piezoelectric materials, which contain toxic compounds such as lead, nickel, and zinc oxide.
The final proof-of-concept for virus piezoelectrics was to demonstrate that multilayered films of M13 could power a small electrical device (YouTube). Lining the virus films with gold electrodes, the team connected a small liquid crystal display (LCD) to the device. Pressing on the piezoelectric system with a finger generated about 400 millivolts of electricity, enough to power the LCD screen. “This is the first biomaterial that has generated enough electricity to turn on and off electric devices,” said Lee.
While this technology cannot yet provide enough energy to power most personal electronics, it is rapidly improving. Since this study, Lee and colleagues are closer to developing viral piezoelectrics capable of powering small electronic devices; the team has already developed enhanced viral material that can provide 20 times more power than the original experiment. At this rate of progress, it may not be long before we dance to the tune of virus-powered iPods, running off the energy of our own gyrations and never plugging into a wall outlet again.