Proximal sound printing: direct 3D printing of microstructures on polymers.

Proximal Sound Printing (PSP) is a new class of additive manufacturing (AM) processes where on-demand polymerization occurs through ultrasound waves interacting with printing material right at the proximity of the acoustic aperture by inducing cavitation. Despite recent developments in sound-based AM techniques, inherent practical limitations still remain, such as low resolution and repeatability, as well as the inability to print multi-material structures. PSP overcomes these limitations, enhancing resolution tenfold, reducing printing power fourfold, and decreasing maximum acoustic streaming velocity 1600 times compared to common sound-based printing methods, enhancing repeatability and resolution. PSP offers greater versatility than existing methods in modulating feature size through printing aperture tuning. This capability is particularly valuable for fabricating microsystems, where high-resolution patterning and material integrity are essential. Furthermore, PSP enables the direct printing of heat-curing materials such as polydimethylsiloxane (PDMS), a widely used thermoset in microfluidics and soft lithography, without altering its native formulation. The PSP process is explored through sonochemiluminescence experiments and high-speed imaging and demonstrated by the successful printing of multi-material composite structures and functional microfluidic devices. Overall, PSP establishes a practical, high-resolution approach for sound-driven additive manufacturing.