Introduction and problem: Ultrasound is widespread with applications ranging from medical imaging and industrial inspection to therapy and tissue ablation in surgery. While sophisticated technologies exist to detect and analyze ultrasound, the generation and projection of ultrasound is still fairly basic. Compared to optical systems, where one can use million-pixel light projectors (and spatial light modulators), ultrasonic systems typically consist of a few sound generators that can steer or focus a beam, but not project changeable patterns and images. For surgical applications, for instance, it is important to have a projection system that can be used to dynamically generate complex beam shapes (e.g. bent or elliptical or multi-focus beams). The challenge is that the sound speed cannot be easily modified in most materials, unlike in optics where the refractive index within a liquid crystal display can be changed using an electric field. The state of the art, therefore, uses as many ultrasound generators (transducers) as is technically possible. This is at best a few hundred, but is far too few to create complex fields.

Our invention: While most materials show little variation in their ultrasound response, even a 20 micron air bubble can effectively block ultrasound transmission in water. Our idea is to generate microbubble patterns and use these to realize an ultrasound projector. We worked with our collaborators at IMS Chips in Stuttgart to implement spatial amplitude control of an ultrasound wave with a CMOS chip. The chip has 10,000 electrodes that can be individually activated to generate a microbubble locally. Hence, each electrode functions as a pixel in this projector that can be locally blocked or unblocked, as shown in the Figure. The pattern of bubbles modifies the wavefront of the ultrasound wave passing through the chip. We were thus able to realize the first dynamic high resolution projector for ultrasound and the work has appeared in Nature Comm. 11, 4537 (2020) [].

Discussion and Outlook: We have won a competitive Max Planck—Fraunhofer grant to develop new ultrasound technologies. Our ultrasound modulator permits complex acoustic images to be projected in water and to be changed on-the-fly. Because ultrasound means pressure, we can also generate sophisticated pressure patterns and manipulate objects like cells. We are working on a robust implementation with even higher resolution and simpler control. Our goal is to be able to dynamically project high power ultrasound fields for robotic control and medical applications.

More information: https://pf.is.mpg.de/publications/2020ma