Harnessing chemistry for plant-like machines: from soft robotics to energy harvesting in the phytosphere

Nature, especially plants, can inspire scientists and engineers in the development of bioinspired machines able to adapt and interact with complex unstructured environments. Advances in manufacturing techniques, such as 3D printing, have expanded the range of materials and structures that can be fabricated, enabling better adaptation to specific applications and closer mimicking of natural systems. Furthermore, biohybrid systems-integrating plant-based or living materials-are getting attention for their ability to introduce functionalities not possible with purely synthetic materials. This joint feature article reviews and highlights recent works of two groups in microfabrication and plant-inspired robotics as well as plant-hybrid systems for energy conversion with applications in soft robotics to environmental sensing, reforestation, and autonomous drug-delivery in plant tissue.

A Comprehensive Review on Bioprinted Graphene-Based Material (GBM)-Enhanced Scaffolds for Nerve Guidance Conduits

Peripheral nerve injuries (PNIs) pose significant challenges to recovery, often resulting in impaired function and quality of life. To address these challenges, nerve guidance conduits (NGCs) are being developed as effective strategies to promote nerve regeneration by providing a supportive framework that guides axonal growth and facilitates reconnection of severed nerves. Among the materials being explored, graphene-based materials (GBMs) have emerged as promising candidates due to their unique properties. Their unique properties-such as high mechanical strength, excellent electrical conductivity, and favorable biocompatibility-make them ideal for applications in nerve repair. The integration of 3D printing technologies further enhances the development of GBM-based NGCs, enabling the creation of scaffolds with complex architectures and precise topographical cues that closely mimic the natural neural environment. This customization significantly increases the potential for successful nerve repair. This review offers a comprehensive overview of properties of GBMs, the principles of 3D printing, and key design strategies for 3D-printed NGCs. Additionally, it discusses future perspectives and research directions that could advance the application of 3D-printed GBMs in nerve regeneration therapies.

Holographic tomographic volumetric additive manufacturing

Several 3D light-based printing technologies have been developed that rely on the photopolymerization of liquid resins. A recent method, so-called Tomographic Volumetric Additive Manufacturing, allows the fabrication of microscale objects within tens of seconds without the need for support structures. This method works by projecting intensity patterns, computed via a reverse tomography algorithm, into a photocurable resin from different angles to produce a desired 3D shape when the resin reaches the polymerization threshold. Printing using incoherent light patterning has been previously demonstrated. In this work, we show that a light engine with holographic phase modulation unlocks new potential for volumetric printing. The light projection efficiency is improved by at least a factor 20 over amplitude coding with diffraction-limited resolution and its flexibility allows precise light control across the entire printing volume. We show that computer-generated holograms implemented with tiled holograms and point-spread-function shaping mitigates the speckle noise which enables the fabrication of millimetric 3D objects exhibiting negative features of 31 μm in less than a minute with a 40 mW light source in acrylates and scattering materials, such as soft cell-laden hydrogels, with a concentration of 0.5 million cells per mL.

Engineered Living Systems Based on Gelatin: Design, Manufacturing, and Applications

Engineered living systems (ELSs) represent purpose-driven assemblies of living components, encompassing cells, biomaterials, and active agents, intricately designed to fulfill diverse biomedical applications. Gelatin and its derivatives have been used extensively in ELSs owing to their mature translational pathways, favorable biological properties, and adjustable physicochemical characteristics. This review explores the intersection of gelatin and its derivatives with fabrication techniques, offering a comprehensive examination of their synergistic potential in creating ELSs for various applications in biomedicine. It offers a deep dive into gelatin, including its structures and production, sources, processing, and properties. Additionally, the review explores various fabrication techniques employing gelatin and its derivatives, including generic fabrication techniques, microfluidics, and various 3D printing methods. Furthermore, it discusses the applications of ELSs based on gelatin in regenerative engineering as well as in cell therapies, bioadhesives, biorobots, and biosensors. Future directions and challenges in gelatin fabrication are also examined, highlighting emerging trends and potential areas for improvements and innovations. In summary, this comprehensive review underscores the significance of gelatin-based ELSs in advancing biomedical engineering and lays the groundwork for guiding future research and developments within the field.

Helical Surface Relief Formation by Two-Photon Polymerization Reaction Using a Femtosecond Optical Vortex Beam

Optical vortices possess a helical phase wavefront with central phase dislocation and orbital angular momentum. We demonstrated three-dimensional microstructure formation using a femtosecond optical vortex beam. Two-photon polymerization of photocurable resin was induced by long-term exposure, resulting in the fabrication of cylindrical structures. The ring shape represents the intensity profile of optical vortex beam, and the diameter and height of the structures are related to the laser power. Periodic helical surface relief was observed on the inner surface. Significantly, the helical direction of the surface relief is consistent with the direction in which the orbital angular momentum acts and changes depending on the sign of the topological charge. Our proposed method can form three-dimensional microstructures with helical periodic surface relief, and the pitch is smaller than the diffraction limit without laser scanning. This method paves the way for further applications in optical devices such as three-dimensional chiral metamaterials.

Dissolving microneedles: standing out in melanoma treatment

Melanoma is one of the most significant and dangerous superficial skin tumors with a high fatality rate, thanks to its high invasion rate, drug resistance and frequent metastasis properties. Unfortunately, researchers for decades have demonstrated that the outcome of using conventional therapies like chemotherapy and immunotherapy with normal drug delivery routes, such as an oral route to treat melanoma was not satisfactory. The severe adverse effects, slow drug delivery efficiency and low drug accumulation at targeted malignancy sites all lead to poor anti-cancer efficacy and terrible treatment experience. As a novel transdermal drug delivery system, microneedles (MNs) have emerged as an effective solution to help improve the low cure rate of melanoma. The excellent characteristics of MNs make it easy to penetrate the stratum corneum (SC) and then locally deliver the drug towards the lesion without drug leakage to mitigate the occurrence of side effects and increase the drug accumulation. Therefore, loading chemotherapeutic drugs or immunotherapy drugs in MNs can address the problems mentioned above, and MNs play a crucial role in improving the curative effect of conventional treatment methods. Notably, novel tumor therapies like photothermal therapy (PTT), photodynamic therapy (PDT) and chemodynamic therapy (CDT) have shown good application prospects in the treatment of melanoma, and MNs provide a valid platform for the combination of conventional therapies and novel therapies by encompassing different therapeutic materials in the matrix of MNs. The synergistic effect of multiple therapies can enhance the therapeutic efficacy compared to single therapies, showing great potential in melanoma treatment. Dissolving MNs have been the most commonly used microneedles in the treatment of melanoma in recent years, mainly because of their simple fabrication procedure and enough drug loading. So, considering the increasing use of dissolving MNs, this review collects research studies published in the last four years (2020-2024) that have rarely been included in other reviews to update the progress of applications of dissolving MNs in anti-melanoma treatment, especially in synergistic therapies. This review also presents current design and fabrication methods of dissolving MNs; the limitations of microneedle technology in the treatment of melanoma are comprehensively discussed. This review can provide valuable guidance for their future development.

Machine Learning Assists in the Design and Application of Microneedles

Microneedles (MNs), characterized by their micron-sized sharp tips, can painlessly penetrate the skin and have shown significant potential in disease treatment and biosensing. With the development of artificial intelligence (AI), the design and application of MNs have experienced substantial innovation aided by machine learning (ML). This review begins with a brief introduction to the concept of ML and its current stage of development. Subsequently, the design principles and fabrication methods of MNs are explored, demonstrating the critical role of ML in optimizing their design and preparation. Integration between ML and the applications of MNs in therapy and sensing were further discussed. Finally, we outline the challenges and prospects of machine learning-assisted MN technology, aiming to advance its practical application and development in the field of smart diagnosis and treatment.

Laser-Based Selective Material Processing for Next-Generation Additive Manufacturing

The connection between laser-based material processing and additive manufacturing is quite deeply rooted. In fact, the spark that started the field of additive manufacturing is the idea that two intersecting laser beams can selectively solidify a vat of resin. Ever since, laser has been accompanying the field of additive manufacturing, with its repertoire expanded from processing only photopolymer resin to virtually any material, allowing liberating customizability. As a result, additive manufacturing is expected to take an even more prominent role in the global supply chain in years to come. Herein, an overview of laser-based selective material processing is presented from various aspects: the physics of laser-material interactions, the materials currently used in additive manufacturing processes, the system configurations that enable laser-based additive manufacturing, and various functional applications of next-generation additive manufacturing. Additionally, current challenges and prospects of laser-based additive manufacturing are discussed.

Micro- and nanofabrication of dynamic hydrogels with multichannel information

Creating micro/nanostructures containing multi-channel information within responsive hydrogels presents exciting opportunities for dynamically changing functionalities. However, fabricating these structures is immensely challenging due to the soft and dynamic nature of hydrogels, often resulting in unintended structural deformations or destruction. Here, we demonstrate that dehydrated hydrogels, treated by a programmable femtosecond laser, can allow for a robust fabrication of micro/nanostructures. The dehydration enhances the rigidity of the hydrogels and temporarily locks the dynamic behaviours, significantly promoting their structural integrity during the fabrication process. By utilizing versatile dosage domains of the femtosecond laser, we create micro-grooves on the hydrogel surface through the use of a high-dosage mode, while also altering the fluorescent intensity within the rest of the non-ablated areas via a low-dosage laser. In this way, we rationally design a pixel unit containing three-channel information: structural color, polarization state, and fluorescent intensity, and encode three complex image information sets into these channels. Distinct images at the same location were simultaneously printed onto the hydrogel, which can be observed individually under different imaging modes without cross-talk. Notably, the recovered dynamic responsiveness of the hydrogel enables a multi-information-encoded surface that can sequentially display different information as the temperature changes.

Near-Infrared Light-Induced Reversible Deactivation Radical Polymerization: Expanding Frontiers in Photopolymerization

Photoinduced reversible deactivation radical polymerization (photo-RDRP) or photoinduced controlled/living radical polymerization has emerged as a versatile and powerful technique for preparing functional and advanced polymer materials under mild conditions by harnessing light energy. While UV and visible light (λ = 400-700 nm) are extensively employed in photo-RDRP, the utilization of near-infrared (NIR) wavelengths (λ = 700-2500 nm) beyond the visible region remains relatively unexplored. NIR light possesses unique properties, including enhanced light penetration, reduced light scattering, and low biomolecule absorption, thereby providing opportunities for applying photo-RDRP in the fields of manufacturing and medicine. This comprehensive review categorizes all known NIR light-induced RDRP (NIR-RDRP) systems into four mechanism-based types: mediation by upconversion nanoparticles, mediation by photocatalysts, photothermal conversion, and two-photon absorption. The distinct photoinitiation pathways associated with each mechanism are discussed. Furthermore, this review highlights the diverse applications of NIR-RDRP reported to date, including 3D printing, polymer brush fabrication, drug delivery, nanoparticle synthesis, and hydrogel formation. By presenting these applications, the review underscores the exceptional capabilities of NIR-RDRP and offers guidance for developing high-performance and versatile photopolymerization systems. Exploiting the unique properties of NIR light unlocks new opportunities for synthesizing functional and advanced polymer materials.

Going below and beyond the surface: Microneedle structure, materials, drugs, fabrication, and applications for wound healing and tissue regeneration

Microneedle, as a novel drug delivery system, has attracted widespread attention due to its non-invasiveness, painless and simple administration, controllable drug delivery, and diverse cargo loading capacity. Although microneedles are initially designed to penetrate stratum corneum of skin for transdermal drug delivery, they, recently, have been used to promote wound healing and regeneration of diverse tissues and organs and the results are promising. Despite there are reviews about microneedles, few of them focus on wound healing and tissue regeneration. Here, we review the recent advances of microneedles in this field. We first give an overview of microneedle system in terms of its potential cargos (e.g., small molecules, macromolecules, nucleic acids, nanoparticles, extracellular vesicle, cells), structural designs (e.g., multidrug structures, adhesive structures), material selection, and drug release mechanisms. Then we briefly summarize different microneedle fabrication methods, including their advantages and limitations. We finally summarize the recent progress of microneedle-assisted wound healing and tissue regeneration (e.g., skin, cardiac, bone, tendon, ocular, vascular, oral, hair, spinal cord, and uterine tissues). We expect that our article would serve as a guideline for readers to design their microneedle systems according to different applications, including material selection, drug selection, and structure design, for achieving better healing and regeneration efficacy.

Progress in Photocurable 3D Printing of Photosensitive Polyurethane: A Review

In recent years, as a class of advanced additive manufacturing (AM) technology, photocurable 3D printing has gained increasing attention. Based on its outstanding printing efficiency and molding accuracy, it is employed in various fields, such as industrial manufacturing, biomedical, soft robotics, electronic sensors. Photocurable 3D printing is a molding technology based on the principle of area-selective curing of photopolymerization reaction. At present, the main printing material suitable for this technology is the photosensitive resin, a composite mixture consisting of a photosensitive prepolymer, reactive monomer, photoinitiator, and other additives. As the technique research deepens and its application gets more developed, the design of printing materials suitable for different applications is becoming the hotspot. Specifically, these materials not only can be photocured but also have excellent properties, such as elasticity, tear resistance, fatigue resistance. Photosensitive polyurethanes can endow photocured resin with desirable performance due to their unique molecular structure including the inherent alternating soft and hard segments, and microphase separation. For this reason, this review summarizes and comments on the research and application progress of photocurable 3D printing of photosensitive polyurethanes, analyzing the advantages and shortcomings of this technology, also offering an outlook on this rapid development direction.

High-aspect-ratio three-dimensional polymer and metallic microstructure microfabrication using two-photon polymerization

Creating micrometer-resolution high-aspect-ratio three-dimensional (3D) structures remain very challenging despite significant microfabrication methods developed for microelectromechanical systems (MEMS). This is especially the case when such structures are desired to be metallic to support electronic applications. Here, we present a microfabrication process that combines two-photon-polymerization (2PP) printing to create a polymeric high-aspect-ratio three-dimensional structure and electroless metal plating that selectively electroplates only the polymeric structure to create high-aspect-ratio 3D metallic structures having micrometer-resolution. To enable this, the effect of various 2PP processing parameters on SU-8 photoresist microstructures were first systematically studied. These parameters include laser power, slicing/hatching distances, and pre-/post-baking temperature. This optimization resulted in a maximum aspect ratio (height to width) of ~ 12. Following this polymeric structure printing, electroless plating using Tollens' Reagent were utilized to selectively coat silver particles only on the polymeric structure, but not on the silicon substrate. The final 3D metallic structures were evaluated in terms of their resistivity, reproducibly showing resistivity of ~ 10-6 [Ω·m]. The developed 3D metallic structure microfabrication process can be further integrated with conventional 2D lithography to achieve even more complex structures. The developed method overcomes the limitations of current MEMS fabrication processes, allowing a variety of previously impossible metallic microstructures to be created.

Machine learning to optimize additive manufacturing for visible photonics

Additive manufacturing has become an important tool for fabricating advanced systems and devices for visible nanophotonics. However, the lack of simulation and optimization methods taking into account the essential physics of the optimization process leads to barriers for greater adoption. This issue can often result in sub-optimal optical responses in fabricated devices on both local and global scales. We propose that physics-informed design and optimization methods, and in particular physics-informed machine learning, are particularly well-suited to overcome these challenges by incorporating known physics, constraints, and fabrication knowledge directly into the design framework.

Two Photon Induced Pulsed Laser Polymerization with Near Infrared Light

We introduce two-photon (2P) pulsed laser polymerization (PLP) at 800 nm, demonstrating its working principle even through biological tissue. We show that 2P PLP is reliable in determining propagation rate coefficients on the example of the free radical polymerization of methyl methacrylate (MMA) at frequencies ranging from 10 to 100 Hz.

Mechanical Control of the Optical Bandgap in One-Dimensional Photonic Crystals

Over the last several years, two-photon polymerization has been a popular fabrication approach for photonic crystals due to its high spatial resolution. One-dimensional photonic crystals with photonic bandgap reflectivities over 90% have been demonstrated for the infrared spectral range. With the success of these structures, methods which can provide tunability of the photonic bandgap are being explored. In this study, we demonstrate the use of mechanical flexures in the design of one-dimensional photonic crystals fabricated by two-photon polymerization for the first time. Experimental results show that these photonic crystals provide active mechanically induced spectral control of the photonic bandgap. An analysis of the mechanical behavior of the photonic crystal is presented and elastic behavior is observed. These results suggest that one-dimensional photonic crystals with mechanical flexures can successfully function as opto-mechanical structures.

Automation of cell culture assays using a 3D-printed servomotor-controlled microfluidic valve system

Microfluidic valve systems show great potential to automate mixing, dilution, and time-resolved reagent supply within biochemical assays and novel on-chip cell culture systems. However, most of these systems require a complex and cost-intensive fabrication in clean room facilities, and the valve control element itself also requires vacuum or pressure sources (including external valves, tubing, ports and pneumatic control channels). Addressing these bottlenecks, the herein presented biocompatible and heat steam sterilizable microfluidic valve system was fabricated via high-resolution 3D printing in a one-step process - including inlets, micromixer, microvalves, and outlets. The 3D-printed valve membrane is deflected via miniature on-chip servomotors that are controlled using a Raspberry Pi and a customized Python script (resulting in a device that is comparatively low-cost, portable, and fully automated). While a high mixing accuracy and long-term robustness is established, as described herein the system is further applied in a proof-of-concept assay for automated IC₅₀ determination of camptothecin with mouse fibroblasts (L929) monitored by a live-cell-imaging system. Measurements of cell growth and IC₅₀ values revealed no difference in performance between the microfluidic valve system and traditional pipetting. This novel design and the accompanying automatization scripts provide the scientific community with direct access to customizable full-time reagent control of 2D cell culture, or even novel organ-on-a-chip systems.

Micropatterning of electrochemiluminescent polymers based on multipolar Ru-complex two-photon initiators

In this work, we present an original stereolithography strategy based on multibranched Ru-complexes with a high two-photon initiating ability allowing the 'one-pot' direct laser writing of ECL-active materials deposited onto electro-active surfaces at the μm scale.

Controlling Light in Scattering Materials for Volumetric Additive Manufacturing

3D printing has revolutionized the manufacturing of volumetric components and structures in many areas. Several fully volumetric light-based techniques have been recently developed thanks to the advent of photocurable resins, promising to reach unprecedented short print time (down to a few tens of seconds) while keeping a good resolution (around 100 μm). However, these new approaches only work with homogeneous and relatively transparent resins so that the light patterns used for photo-polymerization are not scrambled along their propagation. Herein, a method that takes into account light scattering in the resin prior to computing projection patterns is proposed. Using a tomographic volumetric printer, it is experimentally demonstrated that implementation of this correction is critical when printing objects whose size exceeds the scattering mean free path. To show the broad applicability of the technique, functional objects of high print fidelity are fabricated in hard organic scattering acrylates and soft cell-laden hydrogels (at 4 million cells mL-1 ). This opens up promising perspectives in printing inside turbid materials with particular interesting applications for bioprinting cell-laden constructs.

In-Situ Imaging of a Light-Induced Modification Process in Organo-Silica Films via Time-Domain Brillouin Scattering

We applied time-domain Brillouin scattering (TDBS) for the characterization of porogen-based organosilicate glass (OGS) films deposited by spin-on-glass technology and cured under different conditions. Although the chemical composition and porosity measured by Fourier-transform infrared (FTIR) spectroscopy and ellipsometric porosimetry (EP) did not show significant differences between the films, remarkable differences between them were revealed by the temporal evolution of the Brillouin frequency (BF) shift of the probe light in the TDBS. The observed modification of the BF was a signature of the light-induced modification of the films in the process of the TDBS experiments. It correlated to the different amount of carbon residue in the samples, the use of ultraviolet (UV) femtosecond probe laser pulses in our optical setup, and their intensity. In fact, probe radiation with an optical wavelength of 356 nm appeared to be effective in removing carbon residue through single-photon absorption processes, while its two-photon absorption might have led to the breaking of Si-CH3 bonds in the OSG matrix. The quantum chemical calculations confirmed the latter possibility. This discovery demonstrates the possibility of local modifications of OSG films with a nanometric resolution via nonlinear optical processes, which could be important, among other applications, for the creation of active surface sites in the area-selective deposition of atomic layers.

Two-Photon 3D Printing in Metal-Organic Framework Single Crystals

Two-photon polymerization (TPP) is a micro/nano-fabrication technology for additive manufacturing, enabling 3D printing of polymeric materials using ultrafast laser pulses. In this work, two-photon polymerization is realized inside a metal-organic framework (MOF) crystal. Intricate structures are built in the porous crystal to create a microstructure-in-crystal hybrid. Furthermore, the MOF can be removed by acid treatment to release the printed structure. The two-photon polymerization inside the crystal has the potential for MOF sensing device fabrication and data storage applications. In the future development, printing different materials in the same MOF crystal for creating functional 3D devices is hoped.

Femtosecond Spectroscopy and Nonlinear Optical Properties of aza-BODIPY Derivatives in Solution

The fast relaxation processes in the excited electronic states of functionalized aza-boron-dipyrromethene (aza-BODIPY) derivatives (1-4) were investigated in liquid media at room temperature, including the linear photophysical, photochemical, and nonlinear optical (NLO) properties. Optical gain was revealed for nonfluorescent derivatives 3 and 4 in the near infrared (NIR) spectral range under femtosecond excitation. The values of two-photon absorption (2PA) and excited-state absorption (ESA) cross-sections were obtained for 1-4 in dichloromethane using femtosecond Z-scans, and the role of bromine substituents in the molecular structures of 2 and 4 is discussed. The nature of the excited states involved in electronic transitions of these dyes was investigated using quantum-chemical TD-DFT calculations, and the obtained spectral parameters are in reasonable agreement with the experimental data. Significant 2PA (maxima cross-sections ∼2000 GM), and large ESA cross-sections ∼10^-20  m² of these new aza-BODIPY derivatives 1-4 along with their measured high photostability reveal their potential for photonic applications in general and optical limiting in particular.

Advanced Strategies for 3D Bioprinting of Tissue and Organ Analogs Using Alginate Hydrogel Bioinks

Alginate is a natural polysaccharide that typically originates from various species of algae. Due to its low cost, good biocompatibility, and rapid ionic gelation, the alginate hydrogel has become a good option of bioink source for 3D bioprinting. However, the lack of cell adhesive moieties, erratic biodegradability, and poor printability are the critical limitations of alginate hydrogel bioink. This review discusses the pivotal properties of alginate hydrogel as a bioink for 3D bioprinting technologies. Afterward, a variety of advanced material formulations and biofabrication strategies that have recently been developed to overcome the drawbacks of alginate hydrogel bioink will be focused on. In addition, the applications of these advanced solutions for 3D bioprinting of tissue/organ mimicries such as regenerative implants and in vitro tissue models using alginate-based bioink will be systematically summarized.

Shaping collagen for engineering hard tissues: Towards a printomics approach

Hard tissue engineering has evolved over the past decades, with multiple approaches being explored and developed. Despite the rapid development and success of advanced 3D cell culture, 3D printing technologies and material developments, a gold standard approach to engineering and regenerating hard tissue substitutes such as bone, dentin and cementum, has not yet been realised. One such strategy that differs from conventional regenerative medicine approach of other tissues, is the in vitro mineralisation of collagen templates in the absence of cells. Collagen is the most abundant protein within the human body and forms the basis of all hard tissues. Once mineralised, collagen provides important support and protection to humans, for example in the case of bone tissue. Multiple in vitro fabrication strategies and mineralisation approaches have been developed and their success in facilitating mineral deposition on collagen to achieve bone-like scaffolds evaluated. Critical to the success of such fabrication and biomineralisation approaches is the collagen template, and its chemical composition, organisation, and density. The key factors that influence such properties are the collagen processing and fabrication techniques utilised to create the template, and the mineralisation strategy employed to deposit mineral on and throughout the templates. However, despite its importance, relatively little attention has been placed on these two critical factors. Here, we critically examine the processing, fabrication and mineralisation strategies that have been used to mineralise collagen templates, and offer insights and perspectives on the most promising strategies for creating mineralised collagen scaffolds. STATEMENT OF SIGNIFICANCE: In this review, we highlight the critical need to fabricate collagen templates with advanced processing techniques, in a manner that achieves biomimicry of the hierarchical collagen structure, prior to utilising in vitro mineralisation strategies. To this end, we focus on the initial collagen that is selected, the extraction techniques used and the native fibril forming potential retained to create reconstituted collagen scaffolds. This review synthesises current best practises in material sourcing, processing, mineralisation strategies and fabrication techniques, and offers insights into how these can best be exploited in future studies to successfully mineralise collagen templates.

Additive Manufacturing of Gold Nanostructures Using Nonlinear Photoreduction under Controlled Ionic Diffusion

Multiphoton photoreduction of photosensitive metallic precursors via direct laser writing (DLW) is a promising technique for the synthesis of metallic structures onto solid substrates at the sub-micron scale. DLW triggered by a two photon absorption process is done using a femtosecond NIR laser (λ = 780 nm), tetrachloroauric acid (HAuCl₄) as a gold precursor, and isinglass as a natural hydrogel matrix. The presence of a polymeric, transparent matrix avoids unwanted diffusive processes acting as a network for the metallic nanoparticles. After the writing process, a bath in deionized water removes the gold precursor ions and eliminates the polymer matrix. Different aspects underlying the growth of the gold nanostructures (AuNSs) are here investigated to achieve full control on the size and density of the AuNSs. Writing parameters (laser power, exposure time, and scanning speed) are optimized to control the patterns and the AuNSs size. The influence of a second bath containing Au³⁺ to further control the size and density of the AuNSs is also investigated, observing that these AuNSs are composed of individual gold nanoparticles (AuNPs) that grow individually. A fine-tuning of these parameters leads to an important improvement of the created structures’ quality, with a fine control on size and density of AuNSs.

In situ photografting during direct laser writing in thermoplastic microchannels

A method for in situ photografting during direct laser writing by two-photon polymerization is presented. The technique serves as a powerful approach to the formation of covalent bonds between 3D photoresist structures and thermoplastic surfaces. By leveraging the same laser for both pattern generation and localized surface reactions, crosslinking between the bulk photoresist and thermoplastic surface is achieved during polymerization. When applied to in-channel direct laser writing for microfluidic device fabrication, the process yields exceptionally strong adhesion and robust bond interfaces that can withstand pressure gradients as high as 7 MPa through proper channel design, photoinitiator selection, and processing conditions.

Rapid Prototyping of 3D Biochips for Cell Motility Studies Using Two-Photon Polymerization

The study of cellular migration dynamics and strategies plays a relevant role in the understanding of both physiological and pathological processes. An important example could be the link between cancer cell motility and tumor evolution into metastatic stage. These strategies can be strongly influenced by the extracellular environment and the consequent mechanical constrains. In this framework, the possibility to study the behavior of single cells when subject to specific topological constraints could be an important tool in the hands of biologists. Two-photon polymerization is a sub-micrometric additive manufacturing technique that allows the fabrication of 3D structures in biocompatible resins, enabling the realization of ad hoc biochips for cell motility analyses, providing different types of mechanical stimuli. In our work, we present a new strategy for the realization of multilayer microfluidic lab-on-a-chip constructs for the study of cell motility which guarantees complete optical accessibility and the possibility to freely shape the migration area, to tailor it to the requirements of the specific cell type or experiment. The device includes a series of micro-constrictions that induce different types of mechanical stress on the cells during their migration. We show the realization of different possible geometries, in order to prove the versatility of the technique. As a proof of concept, we present the use of one of these devices for the study of the motility of murine neuronal cancer cells under high physical confinement, highlighting their peculiar migration mechanisms.

Inverse Design and 3D Printing of a Metalens on an Optical Fiber Tip for Direct Laser Lithography

An inverse-designed metalens is proposed, designed, and fabricated on an optical fiber tip via a 3D direct laser-writing technique through two-photon polymerization. A computational inverse-design method based on an objective-first algorithm was used to design a thin circular grating-like structure to transform the parallel wavefront into a spherical wavefront at the near-infrared range. With a focal length about 8 μm at an operating wavelength of 980 nm and an optimized focal spot at the scale of 100 nm, our proposed metalens platform is suitable for two-photon direct laser lithography. We demonstrate the use of the fabricated metalens in a direct laser lithography system. The proposed platform, which combines the 3D printing technique and the computational inverse-design method, shows great promise for the fabrication and integration of multiscale and multiple photonic devices with complex functionalities.

Two-photon induced polymerization in a porous polymer film to create multi-layer structures

Two-photon induced polymerization for three-dimensional (3D) printing has attracted increasing attention. Here, we report the two-photon induced polymerization of triethylene glycol divinyl ether (TEGDVE) in a porous polymer film using 4,4',4''-nitrilotribenzoic acid (NTB) as the photosensitizer and diphenyliodonium hexafluorophosphate (HIP) as the initiator, enabling the printing of multi-layer structures in the porous support.

A Review of Stereolithography: Processes and Systems

Being the earliest form of additive manufacturing, stereolithography (SLA) fabricates 3D objects by selectively solidifying the liquid resin through a photopolymerization reaction. The ability to fabricate objects with high accuracy as well as a wide variety of materials brings much attention to stereolithography. Since its invention in the 1980s, SLA underwent four generations of major technological innovation over the past 40 years. These innovations have thus resulted in a diversified range of stereolithography systems with dramatically improved resolution, throughput, and materials selection for creating complex 3D objects and devices. In this paper, we review the four generations of stereolithography processes, which are scanning, projection, continuous and volumetric stereolithography. For each generation, representative stereolithography system configurations are also discussed in detail. In addition, other derivative technologies, such as scanning–projection, multi-material, and magnetically assisted stereolithography processes, are also included in this review.

Voxels Optimization in 3D Laser Nanoprinting

Voxels, the 3D equivalent of 2D pixels, are obtained by individual point exposures in 3D laser nanoprinting, and are the building blocks of laser printed 3D micro/nano-structures, and their optimization is important in determining the resolution of printed 3D objects. Here, we report what is believed the first detailed study of the voxel size dependence on the z-potion of the laser spot in 3D nano-printing. That is, we study the evolution and the low-limit size (diameter and length) of voxels fabricated in the vicinity of the substrate/resin interface. We use two-photon absorption in a photopolymerizable resin, and we vary the position of the laser’s focal spot, with respect to the cover glass/resin interface; i.e. in the longitudinal direction (z-direction). We found that the minimum lateral and the longitudinal sizes of complete voxels depend on the extent of penetration of the laser focal spot inside the resin. Truncated voxels, which are fabricated by partial overlap of the resin and the laser spot, allow for the fabrication of nano-features that are not diffraction limited, and we achieved near 100 nm feature sizes in our 3D fabricated objects. Our work is of central interest to 3D nanoprinting, since it addresses the spatial resolution of 3D printing technology, and might have potential impact for industry.

Meta-biomaterials

Meta-biomaterials are designer biomaterials with unusual and even unprecedented properties that primarily originate from their geometrical designs at different (usually smaller) length scales.

Shape anisotropic colloidal particle fabrication using 2-photon polymerization

HYPOTHESIS

Our ability to dictate the colloid geometry is intimately related to self-assembly. The synthesis of anisotropic colloidal particles is currently dominated by wet chemistry and lithographic techniques. The wet chemical synthesis offers limited particle geometries at bulk quantities. Lithographic techniques, on the other hand, provide precise control over the particle shape, although at lower yields. In this respect, two-photon polymerization (2PP)¹ has attracted growing attention due to its ability to automatically fabricate complex micro/nano structures with high resolution.

EXPERIMENTS

We manufacture precisely designed colloids with sizes ranging from 1 µm to 10 µm with 2PP and optimize the process parameters for each dimension. Moreover, we study the shape dependent Brownian motion of these particles with video microscopy and estimate their diffusion coefficients.

FINDINGS

We observe that increasing the geometrical anisotropy leads to a pronounced deviation from the analytically predicted diffusion coefficient for disks with a given aspect ratio. The deviation is attributed to stronger hydrodynamic coupling with increasing anisotropy. We demonstrate, for the first time, 2PP manufacturing of colloids with tailored geometry. This study opens synthesis of colloidal building blocks to a broader audience with limited access to cleanrooms or wet-chemistry know-how.

Tailoring the Mechanical Properties of 3D Microstructures Using Visible Light Post-Manufacturing

The photochemistry of anthracene, a new class of photoresist for direct laser writing, is used to enable visible-light-gated control over the mechanical properties of 3D microstructures post-manufacturing. The mechanical and viscoelastic properties (hardness, complex elastic modulus, and loss factor) of the microstructures are measured over the course of irradiation via dynamic mechanical analysis on the nanoscale. Irradiation of the microstructures leads to a strong hardening and stiffening effect due to the generation of additional crosslinks through the photodimerization of the anthracene functionalities. A relationship between the loss of fluorescence-a consequence of the photodimerization-and changes in the mechanical properties is established. The fluorescence thus serves as a proxy read-out for the mechanical properties. These photoresponsive microstructures can potentially be used as "mechanical blank slates": their mechanical properties can be readily adjusted using visible light to serve the demands of different applications and read out using their fluorescence.

3D arrays of microcages by two-photon lithography for spatial organization of living cells

This paper addresses a nanoengineering approach to create a fully three-dimensional (3D) network of living cells, providing an advanced solution to in vitro studies on either neuronal networks or artificial organs. The concept of our work relies on stackable scaffolds composed of microcontainers designed and dimensioned to favor the geometrically constrained growth of cells. The container geometry allows cells to communicate in the culture medium and freely grow their projections to form a 3D arrangement of living cells. Scaffolds are fabricated using two-photon polymerization of IP-L 780 photoresist and are coated with collagen. They are stacked by mechanical micromanipulation. Technical details of the proposed nanofabrication scheme and assembly of the modular culture environment are explained. Preliminary in vitro results using PC12 cells have shown that this structure provides a good basis for healthy cell growth for at least 16 days. Our approach is envisioned to provide tailor-made solutions of future 3D cell assemblies for potential applications in drug screening or creating artificial organs.

A two-photon active chevron-shaped type I photoinitiator designed for 3D stereolithography

A highly reactive two-photon cleavable initiator with a chevron-shaped architecture has been designed which presents very promising performances for three-dimensional multiphoton fabrication.

Deformation Behavior of Foam Laser Targets Fabricated by Two-Photon Polymerization

Two-photon polymerization (2PP), which is a three-dimensional micro/nano-scale additive manufacturing process, is used to fabricate component for small custom experimental packages (“targets”) to support laser-driven, high-energy-density physics research. Of particular interest is the use of 2PP to deterministically print millimeter-scale, low-density, and low atomic number (CHO) polymer matrices (“foams”). Deformation during development and drying of the foam structures remains a challenge when using certain commercial acrylic photo-resins. Acrylic resins were chosen in order to meet the low atomic number requirement for the foam; that requirement precludes the use of low-shrinkage organic/inorganic hybrid resins. Here, we compare the use of acrylic resins IP-S and IP-Dip. Infrared and Raman spectroscopy are used to quantify the extent of the polymerization during 2PP vs. UV curing. The mechanical strength of beam and foam structures is examined, particularly the degree of deformation that occurs during the development and drying processes. The magnitude of the shrinkage is quantified, and finite element analysis is used in order to simulate the resulting deformation. Capillary drying forces during development are shown to be small and are likely below the elastic limit of the foam log-pile structures. In contrast, the substantial shrinkage in IP-Dip (~5⁻10%) causes large shear stresses and associated plastic deformation, particularly near constrained boundaries and locations with sharp density transitions. Use of IP-S with an improved writing procedure results in a marked reduction in deformation with a minor loss of resolution.

Effects of intramolecular hydrogen bonding and sterically forced non-coplanarity on organic donor/acceptor two-photon-absorbing molecules

Excited-state decay rate constant is controlled by hydrogen bonding, which is switched from intramolecular to intermolecular by choice of solvent.

Additive manufacture of complex 3D Au-containing nanocomposites by simultaneous two-photon polymerisation and photoreduction

The fabrication of complex three-dimensional gold-containing nanocomposite structures by simultaneous two-photon polymerisation and photoreduction is demonstrated. Increased salt delivers reduced feature sizes down to line widths as small as 78 nm, a level of structural intricacy that represents a significant advance in fabrication complexity. The development of a general methodology to efficiently mix pentaerythritol triacrylate (PETA) with gold chloride hydrate (HAuCl4∙3H2O) is reported, where the gold salt concentration is adjustable on demand from zero to 20 wt%. For the first-time 7-Diethylamino-3-thenoylcoumarin (DETC) is used as the photoinitiator. Only 0.5 wt% of DETC was required to promote both polymerisation and photoreduction of up to 20 wt% of gold salt. This efficiency is the highest reported for Au-containing composite fabrication by two-photon lithography. Transmission Electron Microscopy (TEM) analysis confirmed the presence of small metallic nanoparticles (5.4 ± 1.4 nm for long axis / 3.7 ± 0.9 nm for short axis) embedded within the polymer matrix, whilst X-ray Photoelectron Spectroscopy (XPS) confirmed that they exist in the zero valent oxidation state. UV-vis spectroscopy defined that they exhibit the property of localised surface plasmon resonance (LSPR). The capability demonstrated in this study opens up new avenues for a range of applications, including plasmonics, metamaterials, flexible electronics and biosensors.

Specific Monitoring of Excited-State Symmetry Breaking by Femtosecond Broadband Fluorescence Upconversion Spectroscopy

Most quadrupolar molecules designed for large two-photon absorption cross section have been shown to undergo symmetry breaking upon excitation to the S₁ state. This was originally deduced from their strong fluorescence solvatochromism and later visualized in real time using transient infrared spectroscopy. For molecules not containing clear IR marker modes, however, a specific real-time observation of the symmetry breaking process remains lacking. Here we show that this process can be resolved using broadband fluorescence upconversion spectroscopy by monitoring the instantaneous emission transition dipole moment. This approach is illustrated with measurements performed on two quadrupolar molecules, with only one of them undergoing excited-state symmetry breaking in polar solvents.

Solute-Solvent Interactions and Excited-State Symmetry Breaking: Beyond the Dipole-Dipole and the Hydrogen-Bond Interactions

Symmetry breaking of the excited state of a linear quadrupolar acceptor-donor-acceptor molecule was investigated using time-resolved infrared spectroscopy in 55 solvents allowing the influence of several solute-solvent interactions to be examined separately. No symmetry breaking was found in nonpolar solvents irrespective of their refractive index, indicating that differences in dispersion interactions between the two arms of the molecule do not suffice to induce an asymmetric distribution of the excitation. However, symmetry breaking was observed in nondipolar but quadrupolar solvents like benzene to an extent that can be as large as that found in medium dipolar solvents like THF. Whereas larger symmetry breaking occurs in the most dipolar solvents, the strongest are observed in protic solvents due to hydrogen bonding. Strong evidence of the formation of halogen bonds in the excited state is also presented, confirming the idea of symmetry-breaking-induced asymmetrical photochemistry.

Scaling-Up Techniques for the Nanofabrication of Cell Culture Substrates via Two-Photon Polymerization for Industrial-Scale Expansion of Stem Cells

Stem-cell-based therapies require a high number (10⁶–10⁹) of cells, therefore in vitro expansion is needed because of the initially low amount of stem cells obtainable from human tissues. Standard protocols for stem cell expansion are currently based on chemically-defined culture media and animal-derived feeder-cell layers, which expose cells to additives and to xenogeneic compounds, resulting in potential issues when used in clinics. The two-photon laser polymerization technique enables three-dimensional micro-structures to be fabricated, which we named synthetic nichoids. Here we review our activity on the technological improvements in manufacturing biomimetic synthetic nichoids and, in particular on the optimization of the laser-material interaction to increase the patterned area and the percentage of cell culture surface covered by such synthetic nichoids, from a low initial value of 10% up to 88% with an optimized micromachining time. These results establish two-photon laser polymerization as a promising tool to fabricate substrates for stem cell expansion, without any chemical supplement and in feeder-free conditions for potential therapeutic uses.

Optically Clear and Resilient Free-Form µ-Optics 3D-Printed via Ultrafast Laser Lithography

We introduce optically clear and resilient free-form micro-optical components of pure (non-photosensitized) organic-inorganic SZ2080 material made by femtosecond 3D laser lithography (3DLL). This is advantageous for rapid printing of 3D micro-/nano-optics, including their integration directly onto optical fibers. A systematic study of the fabrication peculiarities and quality of resultant structures is performed. Comparison of microlens resiliency to continuous wave (CW) and femtosecond pulsed exposure is determined. Experimental results prove that pure SZ2080 is ∼20 fold more resistant to high irradiance as compared with standard lithographic material (SU8) and can sustain up to 1.91 GW/cm² intensity. 3DLL is a promising manufacturing approach for high-intensity micro-optics for emerging fields in astro-photonics and atto-second pulse generation. Additionally, pyrolysis is employed to homogeneously shrink structures up to 40% by removing organic SZ2080 constituents. This opens a promising route towards downscaling photonic lattices and the creation of mechanically robust glass-ceramic microstructures.

Direct Visualization of Excited-State Symmetry Breaking Using Ultrafast Time-Resolved Infrared Spectroscopy

Most symmetric quadrupolar molecules designed for two-photon absorption behave as dipolar molecules in the S1 electronic excited state. This is usually explained by a breakup of the symmetry in the excited state. However, the origin of this process and its dynamics are still not fully understood. Here, excited-state symmetry breaking in a quadrupolar molecule with a D-π-A-π-D motif, where D and A are electron donating and accepting units, is observed in real time using ultrafast transient infrared absorption spectroscopy. The nature of the relaxed S1 state was found to strongly depend on the solvent polarity: (1) in nonpolar solvents, it is symmetric and quadrupolar; (2) in weakly polar media, the quadrupolar state observed directly after excitation transforms to a symmetry broken S1 state with one arm bearing more excitation than the other; and (3) in highly polar solvents, the excited state evolves further to a purely dipolar S1 state with the excitation localized entirely on one arm. The time scales associated with the transitions between these states coincide with those of solvation dynamics, indicating that symmetry breaking is governed by solvent fluctuations.

Stereolithography of SiOC Ceramic Microcomponents

The first example of the fabrication of complex 3D polymer-derived-ceramic structures is presented with micrometer-scale features by a 3D additive manufacturing (AM) technology, starting with a photosensitive preceramic precursor. Dense and crack-free silicon-oxycarbide-based microparts with features down to 200 μm are obtained after pyrolysis at 1000 °C in a nitrogen atmosphere.