Responsive polymer-based materials are capable of altering their chemical and/or physical properties upon exposure to external stimuli. These materials have diverse applications including: diagnostics, tissue engineering, 4D-printed soft devices. Interestingly, their physical properties, such as shapes, size, and hydrophobicity can be altered using external stimuli, including thermal, electrical, magnetic, and ionic interactions. Consequently, they have attracted much attention in recent years where so far, various applications have been demonstrated. For such materials, the responsiveness rate is vital. In fact, recent research has shown that combining nanomaterials and polymer matrices can be an effective approach to enhance responsiveness. Further, existing literature has shown that motion or shape control of microstructures containing magnetic nanoparticles is possible using modulation of external magnetic fields. This is however possible by dispersing nanomaterials uniformly in polymer matrices using chemical synthesis processes, which mitigates against the achievement of complex deformation controls. Unfortunately, the materials always exhibit the same deformation throughout; a limitation that causes difficulty in achieving different modes of motion from a single composite material. One approach to circumvent this limitation is by offering the ability of response only into the specific areas of polymer matrices.
Regardless, localized incorporation of nanomaterials into soft materials, particularly wet materials such as hydrogels, remains challenging because of their low compatibility with conventional microfabrication techniques, including vacuum deposition and plasma processes. To address this shortfall, researchers from the Yamagata University in Japan: Professor Hiroaki Nishiyama, Dr. Shun Odashima and Dr. Suguru Asoh applied a three-dimensional (3D) incorporation of plasmonic Ag nanoparticles inside temperature-responsive poly(N-isopropylacrylamide) microgels using near-infrared femtosecond laser multi-photon reduction. They focused on the water-content ratio of hydrogels: i.e. the latter being much higher than other solids. Their work is currently published in the research journal, Optics Express.
In their approach, the research team prepared PNIPAM gels from an aqueous solution containing N-isopropylacrylamide, N, N’-methylenebisacrylamide and a photo-initiator by UV exposure. Linearly polarized laser pulses were focused into PNIPAM gels on cover glasses through an oil immersion objective lens of NA 1.42. Laser translation was performed by moving the laser focus using a computer-controlled, three-axis, piezo stage system. Optical absorption spectra were measured using UV-VIS spectrophotometer. Elemental analysis was carried out using energy-dispersive X-ray spectroscopy
The authors found that the nanoparticles, formed by laser writing at lower doses, exhibited intense plasmonic absorption in the gels around 420 nm wavelength. Light-induced local shrinking of up to 86%, under assumption of isotropic shrinkage in volume, was achieved by the efficient photothermal conversion of Ag nanoparticles. Such shrinkages and deformation speeds strongly depended on the geometric design and 3D layout of the laser writing patterns of Ag nanoparticles inside the microgels. In particular, the team further reported that femtosecond laser incorporation enhanced the recovery speed by more than twice in comparison with the gels containing nanoparticles over the entire region.
In summary, the study by Hiroaki Nishiyama and colleagues demonstrated that plasmonic Ag nanoparticles can be successfully incorporated into PNIPAM microgels using femtosecond laser multi-photon reduction. Notably, the authors reported that the reported femtosecond laser 3D incorporation enabled them achieve both drastic shrinkage and rapid recovery of the microgels, compared to gels that had nanoparticles throughout their entirety. In a statement to Advances in Engineering, Professor Hiroaki Nishiyama mentioned that their flexible Ag nanoparticle incorporation process promises to be a powerful tool for developing of 4D-printed microdevices and highly controllable soft actuators. Further, he pointed out that the laser direct incorporation presented in their approach allowed for the control of the 3D position and extent and response speeds of gel deformation.
Hiroaki Nishiyama, Shun Odashima, Suguru Asoh. Femtosecond laser writing of plasmonic nanoparticles inside PNIPAM microgels for light-driven 3D soft actuators. Optics Express 26470; Volume 28, No. 18.