Independent manipulation of multiple microparticles using circular ultrasonic arrays

The ability to trap and manipulate objects on the micro-metre scale has attracted interest in the biosciences and for micro-fabrication. While optical tweezers are well established, analogous systems using ultrasound are in their relative infancy. However, the use of ultrasonic manipulation offers the ability to work with larger objects than optical systems, including agglomerates of biological cells. Previously the authors have demonstrated trapping and manipulation of microparticles in two dimensions using Bessel-function acoustic pressure fields. The objective of the work reported here was to increase functionality by manipulating multiple particles independently and increasing the working volume within which this manipulation can take place.

For small dense particles in water, a standing acoustic field forces the particles away from antinodes in the pressure field and towards nodes. This can be exploited by generating fields with nodes surrounded by regions of higher pressure amplitude to form traps. A device capable of producing arbitrary fields in a fluid filled chamber, and updating them in real time, allows dexterous manipulation of these traps. Arrays allow generation of arbitrary fields over a region determined by the pitch of the elements, with aliasing degrading control outside this region. A circular array forming the periphery of the fluid chamber results in a region of control whose radius is proportional to the number of elements and the wavelength in the fluid, λ. For a previous array with 16 elements, this radius was 0.9λ; for the 64-element device reported here it is 3.7λ.

The array was fabricated from an electroded piezoelectric ring which was diced into 64 elements, with both absorbing backing and anti-reflective matching layers. The device is operated at f = 2.4 MHz, the thickness extensional mode of the elements, to maximise pressure amplitudes. This corresponds to λ = 0.62 mm in water. By varying the amplitude and phase of the signal applied to each element, it is possible to generate various traps in the fluid-filled chamber.

Bessel functions are a form of solution to the wave equation that allows circular traps to be produced. A first-order Bessel function was generated by incrementing the phase delay of each element, such that the total phase shift around the ring was δφ = 2π, and a 45 μm diameter polystyrene microsphere was trapped. The trap was moved within the central 3.7λ (2.3 mm) radius region by applying further phase delays. Multiple traps were generated by applying a linear superposition of the excitation signals required for each individual trap. With this method,