Advances in microfluidics: From state-of-the-art to two-photon polymerization 3D printing perspectives

Microfluidics is a promising technology for meeting different lab-scale or industrial-based needs. The fabrication of a plethora of devices, including droplet generators, reactors, organs-on-chip, mixers, and separators at the microscale, is rapidly advancing and aimed to overcome the actual limitations in chemical, physical and biomedical fields. This review provides a comprehensive analysis of the current methods and materials used in the fabrication of these systems, with a particular focus on the emerging two-photon polymerization (TPP) 3D printing. Although microfabrication techniques such as soft- and photolithography, micromachining and lamination, have been validated as milestones for the development of microfluidic devices, TPP 3D printing offers unprecedented precision, enabling the creation of complex microstructures with micrometric and nanometric resolution. Despite its potential, the application of TPP in microfluidic device production remains relatively underexplored. A discussion of the fundamental parameters guiding the TPP 3D printing process, the materials used as photoresist and the criteria for their formulations, and the current microfluidic applications is presented. Challenges, criticisms and opportunities associated to TPP are highlighted, showing the unexplored potential of this technology in microfluidics, with the aim of inspiring novel scenarios of applications looking at research and industrial innovation.