New Frontiers in Materials and Technologies for 3D Two Photon Polymerization

Abstract

Two-photon polymerization (2PP), also known as two photon lithograpy (2PL) or direct laser writing (DLW) is a microfabrication technique established in the last decade, which combines the advantages of 3D-printing with sub-micron resolution.[1] Micromachinery, photonics, surface design, microrobotics, and biomedical sciences are just some of the fields mostly impacted by the technology (see Figure 1). Recent developments in both materials[2, 3] and methods[4] have shown the potential to realize structures with unprecedented complexity, innovative functionality, and dynamic functions. Great efforts have been made by material scientists towards the development of new functional photoresists which encompasses nanomaterial inclusion, photoinitiators with enhanced two-photon absorption and biocompatibility, and soft responsive hydrogels, to highlight but a few noteworthy examples. Concurrently, greater understanding of reaction mechanisms, computational analysis, and development of optical systems, have made considerable inroads in extending the limits of resolution, fabrication speed, and application. This special issue brings together a host of scientists working in the broader field of two photon lithography and includes contributions at the cutting edge of this field, giving the occasion to document recent progress, and to critically analyze the opportunities and challenges for the future of this exciting field, through selected research papers, high-quality reviews, and perspective articles. We thank all the invited contributors that have made this issue possible with their wonderful contributions. Below, we identify papers by the name of the invited author. Lastly, we thank Dr. Maria Ronda Lloret for giving us the opportunity to be Guest Editors of this exciting Special Issue. It has been a real pleasure working with her. As introduced, the investigation of new functional materials, compatible with two-photon lithography approach, is one of most exciting and dynamic area of research in this specific topic. Enhanced functionalities, responsiveness to stimuli and biocompatibility are targeted proposing new monomers, initiators or composite formulations. Four papers assess this aspect, describing new materials able to respond to different stimuli (e.g., chemical, temperature or darkness), enabling the implementation of micro-sensor, micro-actuators, among other applications. Delaney and co-workers introduced a vapour responsive photoresist enabling the 4D microprinting of submicron 2D photonic structures with predictable uniform colour displays, which can be modulated on-demand. Also, the transmitted colour in the dry and hydrated states can be accurately modelled (adfm.202211735). Nocentini and co-workers presented tuneable photonic crystals relying on soft and elastic liquid crystalline networks that respond to their environment, in particular, with a physical deformation under temperature variation. This new approach enabled the fabrication of highly resolved, reproducible, and mechanically self-standing photonic crystals (adfm.202213162). Florea´s team showed the microfabrication of a novel sugar responsive, phenylboronic acid-based photoresist. This system permits the fabrication of 3D microstructures with programmable actuation (i.e., bending and opening) exhibiting a remarkable fast response (adfm.202213947). Barner-Kowollik´s team utilized light stabilized dynamic materials (LSDMs) to fabricate 3D microstructures that remain stable when continuously irradiated with green light and can be erased by exposure to darkness (adfm.202206303). Additionally, Turyanska and co-workers demonstrated the use of metal/polymer nanocomposites using two-photon polymerisation. In particular, three complementary strategies are detailed: (a) in-situ formation of metal nanoparticles (NPs) through a single step photoreduction process, (b) integration of pre-formed NPs into 2PP resin and (c) site-selective NPs decoration of 3D microstructures (