Digital microbubble array creates ultrasound holograms

September 14, 2020 // By Nick Flaherty
Digital microbubble array creates ultrasound holograms
A team at the Max Planck Institute for Intelligent Systems has developed a chip that can produce a series of ultrasound holograms that can be used to build microparticles

Researchers in Germany have developed a dynamic massive spatial ultrasound modulator (SUM) that can focus acoustic signals into holograms for building microparticles.

The team at the Max Planck Institute for Intelligent Systems, and Institut für Mikroelektronik in Stuttgart developed an array that produces microbubbles on a 0.8um CMOS process to emulate the digital mirror devices used in video projector. An array of 10,000 digitally addressable microbubble pixels on the CMOS chip surface within 12 seconds through water electrolysis. Between frames, the SUM surface is mechanically reset, which creates the first high-resolution animation of sequential acoustic images. The SUM can be used to assemble microparticles into complex shapes.

The acoustic impedance mismatch between gas and liquid means a thin layer of air in liquid can effectively stop ultrasound, even when its thickness is less than the acoustic wavelength. This means a microbubble can serve as a local sound blocker. A pattern of microbubbles in the path of an ultrasound wave therefore create a corresponding amplitude pattern on the wavefront of the acoustic field.

Patterning a large number of microbubbles enables the on-demand shaping of an acoustic field’s amplitude distribution.

The SUM device architecture consists of a CMOS chip placed on top of an acoustic transducer. A liquid film of electrolyte is sandwiched between the chip surface and a conveyor film. The CMOS chip surface has 10,000 individually addressable electrodes on a 70 x 70 μm gold pads in a 100 x 100µm grid.

Positioned next to the chip is a copper electrode, which serves as the anode. A switchable DC power supply provides a potential difference between the copper electrode (+5 V) and the 10,000 gold electrode pads of the CMOS chip. Once the DC power is switched to a CMOS pixel, the electrolysis of the surrounding water solution generates hydrogen and oxygen gas, respectively, at the gold and copper electrodes. As we will see below, the current is controlled to define the size of the microbubbles.


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