Real-Time Holography Guided 3D Printing for Photocurable Hydrogel Microstructures With Tailored Morphology and Stiffness

Digital light processing (DLP) enables rapid fabrication of photocurable hydrogel microstructures, which serve as critical functional components in micro-optical systems and microrobotics, and whose performance depends on micron-scale morphological features and local stiffness. Traditional visual feedback methods provide horizontal data but struggle to quantify axial topography and local stiffness in real-time. Although digital holographic microscopy (DHM) offers advantages for sample characterization, its real-time capabilities are limited by the challenge of dynamically correcting for optical distortions. To address this, we present a real-time feedback control algorithm featuring a novel partial matrix Zernike fitting (PMZF) method. PMZF is analytically derived to estimate background phase distortion from partially occluded fields of view, enabling accurate phase reconstruction at 5 fps. With PMZF-based feedback, the system achieves 4.88 μm axial precision and 4.29 kPa stiffness precision, improving accuracy by 71.9% over open-loop DLP. Moreover, the spatially resolved control of stiffness within single microgels leads to region-specific fluorescent release, demonstrating a functional behavior not attainable with conventional printing. This work provides an effective closed-loop strategy for controlling both geometry and stiffness, paving the way for advanced functional devices in tissue engineering, MEMS, and beyond.