Take a close look at your bank card or passport, and you will probably see a hologram, used to make the document harder to fake. The surface of the hologram is minutely textured, embossed with tiny bumps that are the code for an image. When light waves bounce off the surface, they reconstruct the image so that it appears to float in mid air. Now, researchers in Germany are applying the same principle with sound waves. They 3D-print a piece of plastic to create a sonic "lens" that shapes sound waves emerging from an underwater speaker so that they form an acoustic "landscape". This could be used to move cells around in a dish or even to treat diseases inside the human body. Laura Brooks heard how it works from inventor Andrew Mark...
Andrew - What we're trying to achieve is controlled shaping of the sound field. We'd like to have particular areas that are high intensity and other areas that are low intensity, and we do that through what we call an "acoustic hologram." The idea is that we have a transducer and a transducer is basically a speaker. It projects a sound wave through the hologram and the hologram acts effectively like a lens to modify that sound wave so that in the far field or downstream of the hologram we have a well defined shaped sound field.
Laura - So can we think of it as something like a landscape that has hills and valleys of sound, effectively?
Andrew - That's exactly right. So the hologram itself is a piece of 3D printed plastic and it has tomography to it. So certain pixels within the hologram are higher or taller than others and because the speed of sound within the hologram is different than within the water in which it's immersed, the waves have different phase when they leave the holographic plate.
Laura - And what kinds of patterns are you making and what would you use them for?
Andrew - So one of the things that we try to do is create a two dimensional image downstream from the hologram and we can shape that image into the shape of a dove, for instance. And we use that to do particle collection so we have many, many small microscopic particles that collect into the sound field into the shape of the dove.
Laura - So you're drawing pictures then with these sound landscapes?
Andrew - Exactly right!
Laura - That's really impressive. What else can you do with them?
Andrew - So one of the other things that we can do is we can change the arrangement of the hologram to project sound upwards towards the surface of the water. So the hologram is under water and it projects sound waves up to where the water meets the air. And where there's higher sound intensity we get crests forming in the surface of the water and, if you put particles onto the crests, then the particles are trapped there. So one of the nice things that we can do is shape the profile of these crests so that they don't just have simple points but they're instead linear tracks for instance.
And the other nice thing is, because we have such complexity in this hologram, we can actually build a phase gradient into the track and the phase gradient serves to actually push the particles. So now we have a track that the particles are confined to and a phase gradient that pushed them along the track. We can make these tracks into shapes like, for instance, letters or we can do rings and we can have multiple rings. And, in some cases, we can have particles that move in opposite directions depending on the direction of the phase gradient along these rings.
Laura - So you can actually move things around without even touching them - just using these sound fields?
Andrew - Exactly.
Laura - So you've talked about assembling these tiny particles but what could this be used for in terms of practical applications?
Andrew - One of the things we have in mind is to use it for either therapeutic or diagnostic medical energy. So one of the applications that ultrasound is used for right now is for either ablation or for thermotherapy where it's used to heat things deep inside the human body.
We can imagine a scenario where a doctor takes an image of the patient's body - a particular patient. Figures out what the best way or the best distribution of sound field inside the patient's body is, graphs a hologram by 3D printing that is specific to that patient, and then uses that hologram to heat or ablate in a way that's particular to that patient's needs.