A Maskless, Equipment-Free Approach for Fabricating Tunable Micro-Convex/Concave Structures Using Self-Assembled Microspheres and PNIPAM Hydrogel Swelling

We present a simple, cost-effective, and equipment-free method for fabricating soft molds with tunable three-dimensional micro-convex and concave structures. This technique leverages the self-assembly of polystyrene (PS) microspheres patterned onto a PNIPAM hydrogel layer, where particle size, hydrogel swelling, and solution properties collectively control the final microstructure geometry. The lateral spacing is adjusted through hydrogel swelling behavior, while microstructure height is controlled by the vertical position of microspheres at the liquid-air interface. Convex microdomes with diameters of 460 <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\mu \mathrm{m}$</tex> and 210 <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\mu \mathrm{m}$</tex> were reliably fabricated on polymeric substrates, demonstrating high uniformity, with over 80% of structures falling within narrow size ranges. Interparticle spacing was tuned from <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$4.2 \mu \mathrm{m}$</tex> to over <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$300 \mu \mathrm{m}$</tex> by modulating the PNIPAM swelling. The dome height was adjusted by changing the ethanol concentration in the pre-polymer solution, which alters both fluid density and surface tension. This method enables dynamic and precise control over both lateral and vertical microstructure features without the need for photolithography, etching, or molding equipment.