Schrödinger's Kitten

Irreverent Science for Everyone

Sunday 26 April 2015

My First Ultrasonic Trapping Device

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In my ongoing studies of making Small Shit That Does Shit™ (AKA functional nanomaterials) I have created a device that uses shaped, high frequency soundwaves to move around things a micrometer across (0.001 millimetres) without touching them. You could call it an audio-vibratory, physio-molecular transport device.

You mean...

It's something I've been working on for quite some time, since October at least. Basically, it's four crystals arranged in a square. When you put a voltage across them, they change shape: this is called piezoelectric behaviour. When we change the voltage going through them a couple of million times a second when they are in water, this expansion and shrinkage pulses ultrasonic waves through the water around them.

Since they are arranged in a square, these waves end up heading towards each other, and when they combine they create a standing wave pattern. This pattern of energy in the water is made of variations in water pressure and velocity, and if we put particles into it, they find and stay at low-energy places for them. Ultrasonic waves have wavelengths of hundreds of micrometers, and the patterns they make also have that sort of spacing and features: so this device can hold them with micrometer precision without needing to poke them at all. You just put them in water and let them find where they want to be, and if you arrange the frequency of the vibrations right, that will be, as if by magic, where you want them to be too. If you want to move them, just adjust the vibrations, the pattern changes and the particles move.

I didn't invent this technique, I just made an example of this device in a slightly new way: lots of people have made and used ultrasonic manipulators already.


Lots of things are microns wide — human cells, flour, viruses, a few wavelengths of light, entire organisms — and it would be kind of nice to move them around without touching them. Some of them are hard to grab on to in any way, some are delicate, some need to be held in place while you work freely all around them, some have other things going on around them that you don't want to interrupt.

Some things that have been done with ultrasonic manipulation:1

  • Holding cells still while viruses that have been stuffed with artificial loads are introduced into them, so we can see what the loads do (Lee et al, Gene Therapy, 2005 — paper currently offline, hope it comes back)
  • Detecting parasites in blood (Reboud et al., PNAS, 2012)
  • Holding small light-bending crystals in perfect alignment while plastic sets around them, to create a weird material that bends light like nothing in nature (Mitri and Sinha, 2011 IEEE International Ultrasonics Symposium).
  • Laying down a pattern of cells, so other cells can be added on top and a complex structure created (Gesellchen et al., Lab on a chip, 2014). Stem cells let us grow new cells with the same genetics as the patient, but in order to make organs or tissues they need to be in the right pattern and shape, which is still a major issue for medicine. Since only 1 in 4 people get the organs they need (are you on the organ donor register?) that's a big one to crack.

There's also the really interesting possibility of holding things in one dimension with acoustic trapping (using sound waves, as described above) and manipulating it in other dimensions using other ways. In this way you can get precise 3D control and transfer particles between their preferred resting places in the pattern. A PhD student at Bristol, Phil Bassindale, who helped me and my lab partner a lot on this project works on this. He did a really cool experiment to see how strongly the particles are held in acoustic traps by measuring how much they resisted being dragged away from their sites with a laser.

So what did you do?

With my lab partner, Noha, I developed an idea from Phil to make a device from a printed circuit board and laser cut acrylic pieces. We hoped that having a PCB base instead of loose wires would be flatter, letting the waves move freely, and also be less fiddly in use and quicker to make. It was! Our biggest problem was that the joins on the coaxial cables we'd made were unreliable — that's more a problem with us than them. And here they are!

The finished schematic: the cross in the middle holds 4 piezoelectric transducers

The device itself

At the end of this project I was jumping up and down with excitement to have produced this picture:

It's aligned! ALIGNED!

Then I realised research is basically about becoming fiercely concerned and obsessed about something anyone normal would regard as ridiculous. Still, who needs social acceptance when you've got a 112 micron spaced grid of particles?

To give an idea of how research actually progresses, I also kept a rolling list of things that caused problems during our project. You will note very few of them are science or engineering related. Many should be obvious, but hey, if we knew what we were doing it wouldn't be called research.

If you're interested in this topic, the group is the Ultrasonics and Non-destructive Testing at the department of Engineering, University of Bristol. One of my supervisors for this project, Professor Bruce Drinkwater, fielded an exhibit, 'Ultrasonic Waves' at last year's Royal Society Summer Science Exhibition. The other, Dr Adrian Barnes, studies material order and disorder.

Additional thanks to the Bristol Hackspace mailing list for tips on cutting holes in PCBs safely, and Tom Kennedy, University of Bristol technician in Physics, for help with making our first PCBs.

I did this project as an 'exploratory training assignment' as part of my first year at the Bristol Centre for Functional Nanomaterials Centre for Doctoral Training. I am funded for this and three future years of PhD research by the Engineering and Physical Sciences Research Council. I am very grateful for this.

1. I'm really sorry if you can't see these papers and are interested in them, it is flipping awful that public money funds studies that are then not visible. Open access is the way forward.

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