50 MHz etched silicon holographic ultrasonic tweezers: force calibration and cell patterning

Acoustic tweezers can achieve non-contact manipulation of particles and cells. Compared with other cell assembly techniques such as dielectrophoresis and surface patterning, acoustic tweezers have superior non-contact and biological safety properties, and do not require sample pre-treatment. They have great potential for application in the field of biomedicine. Moreover, holographic acoustic tweezers have greater spatial flexibility. However, problems such as the design and manufacturing of micrometer-scale acoustic tweezers and the characterization of high-frequency acoustic fields have hindered the development of holographic acoustic tweezers towards higher precision. In this study, a 50 MHz holographic ultrasonic tweezer was demonstrated using low-attenuation silicon lenses. The holographic lens was fabricated via deep silicon etching. Phase distributions were optimized using the iterative angular spectrum approach (IASA), with field reconstruction fidelity validated by finite element analysis (FEA). By integrating the lens with a 50 MHz LiNbO₃ transducer, a two-point focusing ultrasonic field was achieved. The key point is that, in order to address the issue of the insufficiency of methods for characterizing and calibrating high-frequency acoustic fields, a force sensor based on micropipette was employed to locally calibrate the acoustic radiation force, revealing a linear dependence on excitation voltage. Finally, the device's biocompatibility and manipulation capability were demonstrated through two patterning modes of microspheres and yeast cells. The latter was found to be more difficult to capture acoustically than microsphere. This work represents one of the highest-frequency holographic tweezers reported to date, providing a robust platform for precise, programmable cell manipulation with calibrated force control.