Tweezers are not just for plucking eyebrows and removing splinters, they can be used to arrange small objects. Scientists have been using a type of tweezers based on pairs of laser beams to move cells and other microparticles. Optical tweezers opened up biological studies with microarrays, tissue engineering, and regenerative medicine.

Now, a US team [Huang et al., DOI: 10.1039/b910595f] has turned off the lights and is making waves with acoustic tweezers instead. They explain that acoustic tweezers have several advantages, allowing them to manipulate and pattern cells and microparticles irrespective of the shape, size, charge or polarity of those particles.

The acoustic tweezers device is smaller than a dime and so could fit on to a chip produced using standard micro-manufacturing techniques. It also uses 500 000 times less energy than optical tweezer techniques, which means it is non-invasive as there is no burst of heat to damage or kill living cells in a sample.

The researchers built their acoustic tweezers by affixing a transducer to a piezoelectric chip surface, which can be vibrated using an alternating electrical current to produce a sound. Sample cells are held in a tiny amount of liquid in microchannels fabricated on the chip.

To control the freely moving cells in the sample, the team uses the device to setup a standing wave of sound using two sound sources facing each other emitting sound at the same wavelength. The system creates points of destructive (troughs) and constructive (peaks) interference between the two sound waves. The wave pressure of the peaks gently nudges any particles in the sample into the low pressure troughs.

This allows the team to move a cell or nanoparticle by fine-tuning the two sounds and thus the precise position of the peaks and troughs in the sample. Two parallel sound sources facing each other will produce a series of lines whereas perpendicular sound sources results in a checkerboard pattern.

The researchers have demonstrated proof of principle using microscopic Dragon Green fluorescent polystyrene beads and bovine red blood cells, as well as bacterial cells of Escherichia coli. “We can pattern most cells or particles in a few seconds,” explains Huang, “The results verify the versatility of our technique as the two groups of cells differ significantly in both shape and size.”

The team adds that acoustic tweezers could be useful to biologists, chemists, engineers, and materials scientists alike.