Tunable microlens arrays actuated by various thermo-responsive hydrogel structures

We report on liquid-based tunable-focus microlens arrays made of a flexible polydimethylsiloxane (PDMS) polymer. Each microlens in the array is formed through an immiscible liquid–liquid interfacial meniscus. Here deionized water and silicone oil were used. The liquids were constrained in the PDMS structures fabricated through liquid-phase photopolymerization for molding and soft lithography. The microlenses were actuated by thermo-responsive N-isopropylacrylamide (NIPAAm) hydrogel microstructures and could be tuned individually by changing the local temperature. The NIPAAm hydrogels expanded and contracted, absorbing and releasing water, at different temperatures. Thus the pressure across the water–oil interface in the microlenses varied responding to the temperature, tuning their corresponding focal lengths. The microlens diameter was 2.4 mm. The typical microlens focal length was measured to be from 8 to 60 mm depending on the temperature. The microlens response time actuated by different structures and components of the NIPAAm hydrogels were compared. The normalized light intensities of the microlens focused spots were measured, matching well with a Zemax simulation, to study the microlens spherical aberrations. The NIPAAm hydrogel durability was also measured. (Some figures in this article are in colour only in the electronic version)