As the liquid moves through the microchannel, the particles are collected in a levitating cluster while the surrounding medium is washed away. Particles in the micrometer size range, such as cells, are large enough to be instantly captured in the acoustic field. Smaller particles, such as bacteria or extracellular vesicles, can be captured if the acoustic trap is pre-loaded with microparticles that act as seed particles, scattering the sound field and creating secondary forces.
While the sample is held in the acoustic trap, it will be surrounded by vortices of rapidly moving liquid. This phenomenon is called acoustic streaming and enables rapid reactions to be performed in the acoustic trapping platform. Processing steps such as staining, labelling and other binding assays can be performed with greatly reduced incubation time.
In the AcouTrap, the acoustic trap is mounted as the tip of a pipetting robot.
The acoustic trapping chip consists of a glass capillary, less than half a millimeter in height and a few centimeters long, that is coupled to a small piezoelectric transducer. An electrical signal is applied to the piezoelectric transducer to generate ultrasonic vibrations. The frequency is matched to the height of the channel to create a standing wave between top and bottom walls. This creates a strong, localized acoustic field that generates multidimensional acoustic forces. The forces depend on the concentration and size of the particles and their density and compressibility in relation to the surrounding liquid. Particles that are large and/or dense enough will be focused into the pressure node. As the inter-particle distances decrease, forces scatter among the particles, giving rise to an additional secondary force that moves the particles closer together. As a result, the particles are trapped in a cluster, levitating in the capillary above the transducer.
The strong acoustic field will also give rise to acoustic streaming, with rapidly moving vortices around the trapping site. For particles below 2 µm, streaming is the dominating force, which is why these particles cannot be directly trapped. However, this is overcome by preloading the trap with larger particles. The larger particles act as seed particles, scattering the sound wave to create secondary forces strong enough to capture the smaller particles. These act as seed particles, scattering the sound wave and creating secondary forces strong enough to capture the smaller particles. When the electrical signal to the transducer is turned off, the cluster is instantly released and the particles will dissociate from each other.