Ultrasound is widely used as a diagnostic tool in both medicine and materials science and can also be used therapeutically. Cancer tumors often are treated with high-power ultrasound that destroys the cancer cells by specific heating of the diseased tissue. In order to avoid damaging healthy tissue, the sound pressure profile must be precisely shaped.
The Spatial Ultrasound Modulator (SUM) projector flexibly modulates three-dimensional ultrasound fields with comparatively little technical effort. Dynamic sound pressure profiles can thus be generated with higher resolution and sound pressure than the current technology allows, enabling ultrasound profiles to be tailored to individual patients.
Conventional methods vary sound fields with several individual sound sources, the waves of which can be superimposed and shifted against each other. But because the individual sound sources cannot be miniaturized at will, the resolution of these sound pressure profiles is limited to 1,000 pixels. The sound transmitters are then so small that the sound pressure is sufficient for diagnostic but not therapeutic purposes.
With the new technology, the researchers first generate an ultrasonic wave and then modulate its sound pressure profile independently, essentially killing two birds with one stone. This allows use of much more powerful ultrasonic transducers. Thanks to a chip with 10,000 pixels that modulates the ultrasonic wave, a much finer-resolved profile can be generated.
In order to modulate the sound pressure profile, different acoustic properties of water and air are utilized. While an ultrasonic wave passes through a liquid unhindered, it is completely reflected by air bubbles. The researchers thus constructed a chip the size of a thumbnail on which they can produce hydrogen bubbles by electrolysis (i.e. the splitting of water into oxygen and hydrogen with electricity) on 10,000 electrodes in a thin water film. The electrodes each have an edge length of less than a tenth of a millimeter and can be controlled individually.
If an ultrasonic wave is sent through the chip with a transducer, it passes through the chip unhindered. But as soon as the sound wave hits the water with the hydrogen bubbles, it continues to travel only through the liquid. Like a mask, this creates a sound pressure profile with cutouts at the points where the air bubbles are located. To form a different sound profile, the researchers first wipe the hydrogen bubbles away from the chip and then generate gas bubbles in a new pattern.
The researchers demonstrated how precisely and variably the new projector for ultrasound works by writing the alphabet in a kind of picture show of sound pressure profiles. To make the letters visible, they caught micro-particles in the various sound pressure profiles. Depending on the sound pattern, the particles arranged themselves into the individual letters.
The technique could be used not only for diagnostic and therapeutic purposes but also in biomedical laboratories.