One-of-a-kind tool lets scientists manipulate cells without touching them

When studying the spread of cancer or the behavior of a virus like the one that causes COVID-19, the irony is that working with these noxious pathogens requires gentleness. Particularly in the case of COVID, particles don’t fare well when they come into contact with surfaces. To observe and move a live virus, methods that don’t make physical contact will keep these destructive but tiny subjects alive longer, leaving more time to study them.

Zhenhua Tian, ​​an assistant professor at Virginia Tech, has developed an approach that uses two types of energy to move microparticles, offering new applications in diagnosing, treating and preventing the spread of disease.

By combining acoustic waves with electric fields, Tian and his team have created a new microscopic device capable of precisely capturing and manipulating small particles without contact. Their work was published in Scientific progress.

“This is the first study of its kind,” Tian said. “Others, including my team, have demonstrated how acoustic waves can control particles. Electric fields have also been widely used to do the same thing. This is the first study that combines the two.”

Wave upon wave

Tian and his research team combined two forms of energy for this novel technique. The first is acoustic energy using standing waves. These sounds are outside the range of sounds that humans can hear and are designed to propagate along the flat surfaces of small chips equipped with tiny acoustic emitters.

Sound energy moves like the energy of an earthquake, but on a much smaller scale. Although invisible, ultrasonic waves are commonly visualized as mountains and valleys. The frequency at which this up and down pattern occurs is called its frequency.

For this project, Tian’s team emitted two waves of different frequencies, which crossed each other. When the waves crossed, their overlap created grid-like energy patterns composed of multiple energy valleys, called acoustic wells.

When microparticles encounter these wells, they are trapped and settle in the center. That’s where they remain, protected enough to be studied, until another force is introduced. Electric fields provide the second force.

Electric fields in acoustic wells

To control the movements of particles in acoustic wells, another type of energy is needed: this is where electric fields come in.

Where acoustic waves travel through mountains and valleys, electric fields can be generated using microscopic electrodes. By superimposing an electric field on an acoustic well, Tian’s team was able to precisely move the microparticles trapped in the well. Instead of remaining immobilized in the center of the well, the force induced by the electric field allows them to move around the center of the well.

Additionally, the crossing of acoustic waves creates multiple wells, resulting in an almost invisible egg-crate shape. The wells are distributed in a grid pattern, allowing multiple particles to be captured at once. When electric fields are used to move the particles inside, each particle can be controlled separately, opening the way for complex motion.

Working in the waves

The applications of this technology are numerous. Researchers working at the microscopic scale will have new possibilities to carry out their work, and the ability to manipulate delicate materials, such as viral particles, opens up new perspectives.

“We envision using this method to solve many different problems,” Tian said.

“Researchers who want to precisely manipulate cells and test how they interact with each other, such as the interactions between white blood cells and cancer cells, can do so without the cells touching each other. If you want to study how a virus like COVID invades a cell, you could apply this method to bring them closer together or control their distance.”