Microfabrication on low-refractive-index hydrogels using femtosecond laser direct writing

Polyacrylamide (PAA) hydrogels have applications as biochip materials due to their unique optical properties, including high transparency and low refractive indices similar to that of water, as well as their low cost. However, it is difficult to carry out microfabrication using these materials because they undergo nonuniform shrinkage when exposed to the air as water is lost via evaporation. The present study demonstrates the surface microfabrication of PAA hydrogels using a femtosecond laser under two different processing conditions. In some trials, the PAA hydrogel was swollen completely in water and then irradiated with the laser to fabricate a microgroove. In others, a dried hydrogel containing glycerol and having undergone uniform shrinkage was irradiated in air to fabricate the microgroove. Following the fabrication process, the dried gel was again swollen in water to reduce its refractive index. In both scenarios, microgrooves were fabricated by both single and multiple laser scanning. The results show that, for both processing conditions, single laser scanning generated narrow grooves with minimal depths as a consequence of the flexibility and swelling behavior of the hydrogel. In the case of multiple laser scanning, the shape of a microgroove having an aspect ratio of approximately 1.0 was retained on the surface of a PAA hydrogel with a refractive index of 1.34.