Dynamic single-beam acoustic tweezers for microparticle manipulation

Abstract

The development of modern technologies in medicine and manufacturing demand increasingly accurate and small scale positioning and manipulation of microparticles - acoustics provides a solution. Acoustical tweezers hold promise for dexterous tasks ranging from sub-micron to millimetre scales and demonstrate five orders-of-magnitude higher forces per unit input power than achieved with their optical counterpart. This thesis presents the development of single-beam ultrasonic transducers operating in the Rayleigh regime, along with acoustic field-forming techniques; hereby providing capable tools with which researchers can handle small particles, such as cells or drug capsules. The applications include bio-medical assays, micromanipulation and assembly of electrical components and additive manufacturing techniques. The technique is contactless and does not result in excessive heating; thus tasks can be performed in-vivo or within micro fluidic apparatus without contaminating or damaging sensitive living cells. The single-beam transducers described in this thesis have been designed to generate an acoustic trap-a 3D radiation potential minima-inwater. Single-beam devices, which can be classified as a transducer or array emitting sound in a dominant direction, are necessary when the application only facilitates access from one side, for example flow devices under a microscope, inside the human body or within complex structures. The ultrasound field is generated by combining a holographic phase signature and a focus, produced by piezo electric elements and a 3D-printed lens respectively, to modulate a monolithic coherent source. Numerical and analytical models are used to optimise the lenses profiles and predict the acoustic forces on a particle. A single-beam transducer using a phase signature termed a twin-trap is demonstrated manipulating particles in 3D, producing maximum forces of 0.08 µN on 300 µm polystyrene beads. A time-domain modulation technique- switching between different phase signatures-is developed to enable tailored force profiles. The individual phase signatures (such as the twin-trap) do not provide equal forces along all axes, but by rapidly switching between the different traps faster than the time constant associated with particle motion more uniform forces on all axes can be achieved. The multiplexing generates a dynamic force profile on microparticles to counter changing external forces or induce stress as required. The work has demonstrated a simple and compact transducer design capable of generating stable 3D acoustic traps that can hold particles against gravity and translate them through water. Its reduced complexity compared to array transducers represents a significant simplification over previous particle manipulation systems and promotes future miniaturisation. The design of a device capable of applying omnidirectional and dynamic forcing on cells, as well as the low cost fabrication techniques, will facilitate innovation in tissue culture, 3D bioprinting and microscopic robots. This work improves the performance and understanding of ultrasonic trapping devices, and it is hoped this will promote a broader range of acoustic manipulation applications across the scientific community.

Date of Award	24 Mar 2020
Original language	English
Awarding Institution	University of Bristol
Supervisor	Bruce W Drinkwater (Supervisor)