Direct Pattern-to-Curve PDMS-Based Microstructures Fabrication via Thermal Air Expansion for Micro-Optics

Microlens arrays (MLAs) are widely used in imaging, sensing, and photonics, yet conventional fabrication strategies such as thermal reflow and grayscale lithography typically require fully equipped cleanrooms, expensive tools, and multi-step processing. Here, we present a rapid, cost-effective, benchtop-based fabrication method that employs isotropic heated air pressure to transform directly SU-8/silicon patterns into smooth, curved PDMS MLAs. The fabricated lenses exhibit optical-grade surfaces, with measured roughness values ranging from ∼0.608 to 4.47 nm (mean value between 1.18 ± 0.48 and 3.3 ± 0.9 nm). Optical characterization revealed tunable focal lengths spanning 127.21-683.05 µm and transmittance of 83.06% for high-curvature patterns, considering scattering and Fresnel losses. A simple double-casting step yielded convex counterparts, and proof-of-concept imaging with both concave-convex arrays and with fabricated compound eye confirmed the functionality of the MLAs. By reducing equipment complexity and processing time, this approach provides a reproducible and cost-effective alternative for scalable MLAs manufacturing.