The spatial ultrasound modulator, which is capable of generating dynamic complex acoustic fields, now opens up the possibility to project and update much more sophisticated ultrasound patterns. “This technique could be useful, for instance, if one wants to project ultrasound only onto a small targeted region and adjust the exposed area carefully, as is desirable in medical applications”, says Zhichao Ma, the first author of the study which appears in the journal Nature Communications.
The CMOS chip consists of 10,000 individually addressable electrodes covered in a conductive fluid. When an electrode receives a signal a current flows and a microbubble is formed at the electrode by electrolysis. Since it only takes a very thin layer of air to effectively block ultrasound in water, each microbubble behaves as a local sound blocker. By creating bubbles at some pixels and not at others, the CMOS chip acts as a mask limiting the regions through which the sound can pass. This mask can be shaped in the way of a computed digital hologram, and modulate the transmitted acoustic wave into any desired acoustic image. Even more so the bubbles on the chip can be wiped away and recreated to form the next holographic image and frame by frame an ultrasonic movie.
The researchers show that potential applications of their new device include manipulating small objects using acoustic forces controlled by the fields they project. In one demonstration, they pattern polymer particles into different shapes that change with time. Beyond small particles, it is also possible to arrange living cells with acoustic holograms, as the researchers from the Micro Nano and Molecular Systems Lab have demonstrated previously.