Controlling Optical Response of Hydrogel using Metallic Microstructures Fabricated with High-Speed Scanning of Femtosecond Laser Pulses

A temperature-responsive hydrogel shows reversible volume-phase transitions in response to changes in their surrounding temperature. By containing metal nanoparticles as light absorbers inside the temperature-responsive hydrogel, the hydrogel`s transmittance can be changed by photo-thermal conversion of the metal nanoparticles stimulated by light irradiation. The multi-photon photoreduction (MPR) method can spatially and selectively fabricate metal microstructures consisting of metal nanoparticles inside the hydrogel. In the MPR method, metal microstructures can be fabricated by femtosecond laser pulses inside the hydrogel containing metal ions. However, temperature-responsive hydrogels are thermally affected by the irradiation of femtosecond laser pulses at high repetition rates during the structure fabrication process which induce an undesirable volume phase transition, resulting in the hindrance of fabricating the arbitrarily designed structures. In this study, we fabricated microstructures consisting of multiple gold thin lines inside a poly (N-isopropylacrylamide) (PNIPAM) hydrogel by laser scanning at high scanning speed and multiple times (hereinafter called “multiple scans”) by using a galvanometer scanner system. Under the experimental conditions of low scanning speed at a single time (hereinafter called “single scan”), the density of gold nanoparticles in the gold microstructures was low and the lines formed curls, curves, and/or kinks. On the other hand, under the experimental conditions of high scanning speed and multiple scans, the density of gold nanoparticles in the gold microstructure was high and straight lines without curls and curves were fabricated. The temperature around the irradiated area during laser pulse irradiation was measured by a thermography camera (T865, FLIR), and it was confirmed that the maximum temperature at high scanning speed with multiple scans was lower than that at low scanning speed with a single scan. Next, transmittance control by light stimulation was performed using the PNIPAM hydrogel with the fabricated gold microstructure. The two laser diodes with wavelengths of 520 nm and 405 nm were used as a stimulating light and a signal light, respectively. When the stimulating light was illuminated to the fabricated gold microstructure, the transmittance of the signal light in the PNIPAM hydrogel significantly decreased. Furthermore, the rate of the decline in the hydrogel`s transmittance of the signal light and the response time of the hydrogel around the gold microstructure during irradiation of the stimulating light depended on the laser parameters during the structure fabrication process. The gold microstructures fabricated with high scanning speed and multiple scans showed the lowest transmittance and the fastest response time during the stimulated light irradiation in this experiment. This result is attributable to the higher density of gold nanoparticles in the gold microstructures fabricated with high scanning speed and multiple scans than in the microstructures fabricated with low scanning speed with a single scan. In the case of high scanning speed and multiple scans, the hydrogel around the fabricated gold microstructure is easier to reach the phase transition temperature. In conclusion, we experimentally demonstrated that gold microstructures can be simply fabricated into the temperature-responsive hydrogel by scanning femtosecond laser pulses with a high scanning speed and multiple scans. The fabricated gold microstructure can increase the degree of the hydrogel’s transmittance change and shorten the response time of the hydrogel.