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

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 (adfm.202211920). Furthermore, other new materials have been developed specifically to allow the fine tuning of bulk and surface properties of printed 3D microstructure by suitable post processing. Two papers, in particular, use a strategy based on refined nitroxide chemistry approaches. Spangenberger and co-workers developed a new photoresist suitable for two-photon lithography based on nitroxide mediated photopolymerization (NMP). This method enabled highly tuneable and precise surface modification of the 3D printed microstructures (adfm.202211971). Blasco´s team reported the inclusion of dynamic and living bonds (alkoxyamines) in two-photon printable formulations for the first time allowing for the preparation of “living” microstructures. The post-polymerization processing resulted in a dramatic increase of the volume (around 8 times) of the 3D printed microstructure, but also in an increase of the Young's Modulus (two orders of magnitude) (adfm.202207826R2). Worthy of note, for the generation of materials for biomedical applications, is included work which focussed on biocompatibility. Two papers proposed water soluble formulations able to produce completely biocompatible 3D microstructures. Zheng and co-workers reported a biocompatible anion ionic carbazole-based water-soluble two-photon initiator (TPI) 3,6-bis[2-(1-methylpyridinium)vinyl]-9-methylcarbazole ditosylate (BT) for the fabrication of 3D hydrogel structures in water (adfm.202210993). Magdassi´s team introduced a new resilin protein-based material that can be printed by two photon polymerization in water at a submicron length scale using a ruthenium-based photoinitiator. The printed microstructures are soft and resilient, similar to native resilin and importantly, they enabled cell growth and alignment (adfm.202300293). In addition, a review led by Van Vlierberghe discusses different types of biocompatible two-photon photoinitiators for their use in biomedical applications (adfm.202212641). Innovation in the two-photon lithography field also relates to technological advancements and the combination of the technique with others to achieve advanced functionalities. Two-step absorption lithography, or holographic approaches, represent examples of such advancement. Two original papers and two reviews report specifically on the advancement of two-photon lithography from a technological standpoint. In particular, Wegener's group recently demonstrated two-step absorption[5] as an emerging evolution of 2PP; here he and his co-workers systematically explored the possible alternatives to the few already know one-color and two-colour two-step absorption photoinitiators, a fundamental step toward the future implementation of inexpensive and compact setups based on continuous-wave lasers for micro 3D direct printing (adfm.202212482). The handling and manipulation of printed 3D micro-structures printed is not a trivial task; Mattoli and co-workers reported here on a novel technique to transfer 3D microstructures on arbitrary complex surfaces providing an extreme level of conformability. The method relies on the direct two-photon polymerization of micro structures on polymeric nano-membranes that can be efficiently transferred onto specific targets (adfm.202214409). One of the main limitations of the two-photon polymerization approach is the relatively low throughput, which has hampered its industrial application to date. Methods to improve this aspect have been proposed so far and here Balena, De Vittorio and co-workers, presented a review covering the fundamental concepts behind high-speed Two Photon Lithography and its combination with holography toward scalability of the approach (adfm.202211773). Finally, Sun's group reviewed the state-of-the-art techniques for 3D laser nano-printing of functional materials not directly printable by simple two-photon polymerization approach, such as polymer template-assisted printing, polymer-free printing or others, thus providing examples of cutting-edge applications and discussing existing technical bottleneck and potential strategy for future development (adfm.202211280). Concerning the applications of two-photon polymerization, it is interesting to underline that the actual trend seems to be driven by an increased interest in bio-related topics, including biomedical and bioinspired ones. Notably, in this special issue, five papers and one review report on bioinspired/biomimetic studies, spanning controlled hydrophobicity, micro actuation, or swimming capabilities. In particular, two different swimming micro-robots actuated with two different approaches have been presented. Pané Vidal and co-workers reported on biocompatible and degradable magnetic-driven hydrogel micro-robots based on Polyvinyl alcohol (PVA), implementing different morphologies and with tuneable stability. The fabrication of the micro-robots combines 2PP 3D printed template-assisted casting with a salting-out process for improving PVA stability in water and retaining biocompatibility (adfm.202212952). Di Leonardo's group propose a different approach to micro-robotic locomotion; integrating biological actuators, light-driven bacteria, as propellers in two photon printed 3D micro-structure. The resulting hybrid micro-roots can be controlled and steered by unbalancing light intensity over different micro-robot parts (adfm.202214801). Lewis Smith and co-workers reported on another interesting actuation principle inspired by nature, demonstrating a 3D-printed hydraulic actuator at the micro-scale. They printed 2PP structures with micron-scale thickness to create compliant membranes from rigid materials, using the encapsulated unpolymerized resist as the hydraulic media, thus implementing a micro-hydraulic system in analogy with spider's legs (adfm.202207435). Micro-devices built by 2PP can also be used for understanding fundamental mechanisms of nature. In this regard, Mazzolai's group investigated the transpiration and cavitation-driven dynamics observed in annulus cells by means of 2PP built synthetic structures. Specifically, they fabricated prismatic microcavities, assembled with a PDMS microfilm, to realize artificial micro-chambers mimicking the annulus cells, replicating for the first time their evaporation-driven collapse and their fast return triggered by the nucleation of bubbles (adfm.202214130). Moreover, Tricinci, Mattoli and co-worker exploited 2PP lithography and micro-contact printing to produce 3D micro-patterned surfaces inspired for the first time by both morphology and chemical coating of the hairs of Salvinia Molesta leaves, reproducing not trivial wettability behaviour, such as contemporary high hydrophobicity and high roll of angle (adfm.202206946). On a similar topic, Dong and Levkin reviewed applications of 3D micro-printing in design and fabrication of super-repellent microstructures, with a specific focus on properties and new functionalities offered by the possibility to have complex three dimensional topographies (adfm.202213916). Two original papers focus on advanced micro-optics, one review and one perspective on different aspects of 2 photon lithography enabled optic and plasmonic, complete this special issue. Giessen's group introduced a novel and simple way to create 3D 2PP printed opaque microscopic apertures used to increase the contrast of 3D-printed micro optics thus enabling new complex optical designs (adfm.202211159). With a specific focus on biological applications, Raimondi, Chirico and co-workers demonstrated the in situ fabrication of plano-convex micro-lenses for non-linear imaging of biological tissue. The micro-lenses tested on fibroblast cell culture, could open the way to the application of implanted micro-optics for optical in-vivo inspection of biological processes (adfm.202213926). Yang and co-worker extensively reviewed the field of two photon lithography application in optical and photonic fields, starting from fundamentals on light-matter interaction, reviewing relevant materials, reporting on fabrication technologies and post-processes, and analysing various optical applications (adfm.202214211). Finally, Juodkazis, Farsari and co-workers analysed and reported the recent advancement on the development of glass-ceramic 3D micro-optics obtained by two photon polymerization combined with calcination via high temperature annealing (adfm.202215230). This special issue has served to mark a time of great innovation and collaboration in a burgeoning field. It highlights a group of researchers at the forefront of a technology that spans several disciplines. With each innovation in materials and technologies, 3D Two Photon Polymerization reaches new and exciting frontiers. V.M. and L.F. acknowledge support from the European Union's Horizon 2020 research and innovation program under the FET Open grant agreement 5DNanoPrinting – no. 899349. E.B. acknowledges the funding from German Research Council (DFG) via the Excellence Cluster “3D Matter Made to Order” (EXC-2082/1-390761711) and the Carl Zeiss Foundation. The authors acknowledge Omar Tricinci and Matteo Archimi for providing pictures for Figure 1 composition. The authors declare no conflict of interest.