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Cambridge Ultrasonics
Cambridge, UK
Consultancy service in physics, electronics, maths & ultrasonics

Cambridge Ultrasonics

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Above - printed circuit board with 10 lock-in amplifiers to perform impedance measurements on single white blood cells at 10 frequencies simultaneously.

Above - photograph of a biological liquid with cylindrical ultrasonic standing waves in it. Red cells are collecting in ring patterns at the nodes of the standing waves.

Micro-fluidics

Impedance of cells

A start-up business in the USA wanted to develop new products for analysing blood cells by measuring the electrical impedance of individual white cells. A microfluidic system was required with supporting electronics. Cambridge Ultrasonics was asked to develop the electronic part. Our design used ten lock-in amplifiers all on one circuit board. Each lock-in amplifier was tuned to a frequency chosen by the operator. Although Cambridge Ultrasonics' tasks were restricted to the electronics, the requirements for accurate measurements meant that we started to influence the micro-fluidic design. The design was patented by the client.

Marshalling cells

Cambridge Ultrasonics has worked with several clients on how to use ultrasound to marshal or manipulate micron sized particles, including biological cells. Ultrasound applies a force on particles due to radiation pressure that can be used to marshal the particles. Drag force limits the speed that the particle moves, generally so that particle speed is much less than the speed of the ultrasonic waves. In a standing wave, two sets of waves travel in opposite directions and interfere. Any particles in the standing wave pattern coalesce at nodal points, where there is destructive interference and a potential well of radiation pressure. Standing waves of specific patterns occur at specific resonance frequencies that depend upon the speed of sound in the fluid and the dimensions of the container holding the fluid and particles. The pattern changes as the frequency changes. The lower the frequency the fewer the number of nodes. With our long experience in visualization we were able to adapt our visualization equipment to observe standing wave patterns under a microscope.

One way that particles can be marshalled is by electronically switching between resonance frequencies - this makes the particles jump from one nodal point to another nodal point. Particles jump to the nearest node. Away from a resonance and at certain frequencies it is possible to cause mixing of particles and re-circulating flows of differing rotational speed can be established. Large particles cannot follow tightly curving streamlines so a rotational flow can be used to separate particles according to size.

If the intensity of the ultrasound is increased then the particles can be processed, for example lysing of biological cells.

Ultrasound can be used in a variety of ways to marshal and manipulate micron sized particles and the beauty of the method is that the manipulation is all controlled by frequency and amplitude changes of ultrasound, which in turn can be controlled by software. The method therefore lends itself to automation.