Direct Laser Writing of Complex 3D Metal Nanoparticle Patterns within Polymer Microstructures for Photothermal Micro-Actuators

This work describes the fabrication of complex 3D structures comprising of metallic Ag nanoparticles (NPs) which were manufactured inside prefabricated polymer structures by direct laser writing (DLW). In recent years, DLW has been established as a powerful tool for the fabrication of 3D micro-objects with features below 300 nm. This additive manufacturing technology is an adaptable, high-resolution process, where structure fabrication can be achieved via multi-photon polymerisation or metal photo-reduction.^1,2 More recently, this technique has been applied to the fabrication of stimuli-responsive hydrogel microstructures. Due to their size, these structures offer dramatically improved response times, where the inherently slow diffusion-controlled hydrogel expansion is countervailed. In this context, stimuli-controlled hydrogel micro-actuators show improved performance compared to their macro-scale counterparts and find applications across many fields including micro-robotics, microfluidics and biomedical devices.³<br/>The micro-structures described herein were realised using a two-step approach. The first step comprised the fabrication of polymer structures via DLW by free-radical polymerisation. Following their fabrication, the polymer structures were immersed in a solution of Ag⁺ and a second DLW process was conducted to induce photoreduction of the Ag⁺ ions, thereby creating complex patterns of Ag particles inside the 3D polymer microstructures. TEM characterisation of microstructure cross-section was used to characterise the metal particle size and the distribution of particles. The size of the obtained particles was also compared to the voxel size obtained under the range of laser powers and scan speeds, in order to establish a protocol for the realisation of controlled metal particle size via DLW.<br/>The same two-step approach was used to create microstrucures showing photo-induced actuation. In this case, the microstructures were fabricated in the thermo-responsive polymer poly(<i>N</i>-isopropylacrylamide) (PNIPAAM). PNIPAAM is vastly employed for the realisation of thermo-actuators, owed to its thermo-responsive properties associated with a phase transition at the lower critical solution temperature (32 °C).⁴ Ag NP patterns were then written <i>in situ</i> inside the PNIPAAM structure, to act as photothermal converters. Laser irradiation of the Ag NPs patterns caused localised heating, inducing fast, controllable and reversible actuation of the microstructures. We further demonstrate how the actuation direction and speed can be tuned by controlling the Ag pattern and by the laser writing/scanning speed.<br/> <br/> <br/>References<br/>1. Nishiyama H., Umetsu K., Kimura K., Versatile Direct Laser Writing of Non-Photosensitive Materials using Multi-Photon Reduction-Based Assembly of Nanoparticles, Sci. Rep., 2016, 9<b>, </b>14310.<br/>2. Blasco E., Müller J., Müller P., Trouillet V., Schön M., Scherer T., Barner-Kowollik C., Wegener M., Fabrication of Conductive 3D Gold-Containing Microstructures via Direct Laser Writing, Adv. Mater. 2016, 28, 3592.<br/>3. Bogue R., Recent developments in MEMS sensors: a review of applications, markets and technologies, Sensor Rev. 2013 ,33, 300.<br/>4. Ashraf S., Park H., Park H., and Lee S., Snapshot of Phase Transition in Thermoresponsive Hydrogel PNIPAM: Role in Drug Delivery and Tissue Engineering, Macromol. Res., 2016, 24, 4, 297