Scalable submicrometer additive manufacturing

Universal point spread function engineering for 3D optical information processing

Point spread function (PSF) engineering has been pivotal in the remarkable progress made in high-resolution imaging in the last decades. However, the diversity in PSF structures attainable through existing engineering methods is limited. Here, we report universal PSF engineering, demonstrating a method to synthesize an arbitrary set of spatially varying 3D PSFs between the input and output volumes of a spatially incoherent diffractive processor composed of cascaded transmissive surfaces. We rigorously analyze the PSF engineering capabilities of such diffractive processors within the diffraction limit of light and provide numerical demonstrations of unique imaging capabilities, such as snapshot 3D multispectral imaging without involving any spectral filters, axial scanning or digital reconstruction steps, which is enabled by the spatial and spectral engineering of 3D PSFs. Our framework and analysis would be important for future advancements in computational imaging, sensing, and diffractive processing of 3D optical information.

Micrometer-scale poly(ethylene glycol) with enhanced mechanical performance

Strong and lightweight materials are highly desired. Here we report the emergence of a compressive strength exceeding 2 GPa in a directly printed poly(ethylene glycol) micropillar. This strong and highly crosslinked micropillar is not brittle, instead, it behaves like rubber under compression. Experimental results show that the micropillar sustains a strain approaching 70%, absorbs energy up to 310 MJ/m3, and displays an almost 100% recovery after cyclic loading. Simple micro-lattices (e.g., honeycombs) of poly(ethylene glycol) also display high strength at low structural densities. By combining a series of control experiments, computational simulations and in situ characterization, we find that the key to achieving such mechanical performance lies in the fabrication of a highly homogeneous structure with suppressed defect formation. Our discovery unveils a generalizable approach for achieving a performance leap in polymeric materials and provides a complementary approach to enhance the mechanical performance of low-density latticed structures.

Emerging Technologies for Multiphoton Writing and Reading of Polymeric Architectures for Biomedical Applications

The rise in popularity of two-photon polymerization (TPP) as an additive manufacturing technique has impacted many areas of science and engineering, particularly those related to biomedical applications. Compared with other fabrication methods used for biomedical applications, TPP offers 3D, nanometer-scale fabrication dexterity (free-form). Moreover, the existence of turnkey commercial systems has increased accessibility. In this review, we discuss the diversity of biomedical applications that have benefited from the unique features of TPP. We also present the state of the art in approaches for patterning and reading 3D TPP-fabricated structures. The reading process influences the fidelity for both in situ and ex situ characterization methods. We also review efforts to leverage machine learning to facilitate process control for TPP. Finally, we conclude with a discussion of both the current challenges and exciting opportunities for biomedical applications that lie ahead for this intriguing and emerging technology.

Nanophotonic Zero-Index-Material-Enabled Optical Coherent Interferometry with High Signal-to-Noise Ratio

Optical interferometry enables distance measurement with record-high precision. However, the conventional interference intensity shows fast spatial variations, limiting photodetection to a single spot. The light intensity at this spot is affected by various noises, restricting the signal-to-noise ratio (SNR) of a coherent system. Here, we generate uniform interference patterns based on zero-index metawaveguides, enabling the photodetection of spatially extended intensity through multiple-pixel measurements of a charge-coupled device. Averaging the intensities measured across multiple pixels cancels uncorrelated noise, significantly improving the SNR and allowing the detection of weakly reflected optical power. Experiments show that nanoscale displacements down to 26 nm (λ0/59) can be converted into high-contrast intensity changes over a macroscopic area, leading to an SNR 10+ times higher (11.68 dB) than that of conventional single-spot photodetection. Our nanophotonic zero-index platform can be implemented in any coherent system, possibly leading to a transformative impact in precise measurement of the distance, profile, and spectrum.

3D printed edible electronics: Components, fabrication approaches and applications

A recently minted field of 3D-printed edible electronics (EEs) represents a cutting-edge convergence of edible electronic devices and 3D printing technology. This review presents a comprehensive view of this emerging discipline, which has gathered significant scientific attention for its potential to create a safe, environmentally friendly, economical, and naturally degraded inside the human body. EEs have the potential to be used as medical and health devices to monitor physiological conditions and possibly treat diseases. These edible devices include different components, such as sensors, actuators, and other electronic elements, all made from edible ingredients such as sugars, proteins, polysaccharides, polymers, and others. Among the different fabrication approaches, 3D printing can provide reliable solutions to specific requirements. The concept of EEs has the potential to transform healthcare, providing more convenient, less invasive alternatives and personalized, customizable products for patients that beat traditional manufacturing methods. While the potential is enormous, there are critical challenges, notably ensuring the long-term stability, and regulatory and safety of these devices within the human body. Accordingly, a detailed understanding of the underlying concepts, fabrication approaches, design considerations, and action in the body/application range has been presented. As an evolving field, there is ample scope for research and multiple challenges must be addressed; these are elaborated towards the concluding sections of this article.

Additive manufacturing in spatial patterning for spinal cord injury treatment

Combinatorial treatments integrating cells and biomolecules within scaffolds have been investigated to address the multifactorial nature of spinal cord injury (SCI). Current regenerative treatments have been ineffective as they do not consider the spatial positions of various cell types to effectively form functional neural pathways. Emulating the complex heterogeneity of cells in the native spinal cord requires translating the existing biological understanding of spatial patterning in neural development, as well as the influence of biomolecule and mechanical patterning on regional specification and axonal regeneration, to engineer a scaffold for spinal cord regeneration. This review explores the potential of 3D bioprinting to precisely control material, cell and drug patterns in scaffolds, achieving spatial phenotype specification and providing axonal guidance to form appropriate connections. We also discuss the application of extrusion-based and digital light processing bioprinting in integrating mechanical, chemical and biological cues within a scaffold to advance spatially patterned 3D bioprinted scaffold, as well as current challenges and future perspectives in these bioengineering strategies.

Two-photon absorption under few-photon irradiation for optical nanoprinting

Two-photon absorption (TPA) has been widely applied for three-dimensional imaging and nanoprinting; however, the efficiency of TPA imaging and nanoprinting using laser scanning techniques is limited by its trade-off to reach high resolution. Here, we unveil a concept, few-photon irradiated TPA, supported by a spatiotemporal model based on the principle of wave-particle duality of light. This model describes the precise time-dependent mechanism of TPA under ultralow photon irradiance with a single tightly focused femtosecond laser pulse. We demonstrate that a feature size of 26 nm (1/20 λ) and a pattern period of 0.41 λ with a laser wavelength of 517 nm can be achieved by performing digital optical projection nanolithography under few-photon irradiation using the in-situ multiple exposure technique, improving printing efficiency by 5 orders of magnitude. We show deeper insights into the TPA mechanism and encourage the exploration of potential applications for TPA in nanoprinting and nanoimaging.

DLP Printing High-Performance Acryloyl-Functional Benzoxazine Based on Dual-Cure System

Light-based three-dimensional (3D) printing technologies are regarded as one of the most promising manufacturing methods. However, 3D printed objects based on free radical polymerization have disadvantages such as anisotropy, poor mechanical strength, and heat resistance and cannot meet the requirements of high-performance applications. This study reports a class of acryloyl-functional benzoxazine exhibiting excellent mechanical properties, heat resistance, and flame retardancy. 3-Acryloyl-3,4-dihydro-2H-1,3-benzoxazine (Ph-am) and bis(3-phenyl-(3-acryloyl-3,4-dihydro-2H-1,3)-benzoxazine) (Ba-am) have been synthesized by the Mannich reaction using acrylamide as the amine source. The acryloyl-functional benzoxazine was printed by a DLP printer. Then, the oxazine ring undergoes ring-opening polymerization under a high temperature to form a dense polymer, enabling the printed object to exhibit isotropy. The Mannich bridge and hydrogen-bond structure in polybenzoxazine enhance the mechanical properties and thermal resistance of printed objects. The glass-transition temperature (Tg) and tensile strength of dual-cured Ph-am reach 270 °C and 116.55 MPa, respectively. Meanwhile, after blending modification with tris(2-hydroxyethyl)-isocyanurate triacrylate, hybrid polymer networks have been formed. The tensile strength and Young's modulus of dual-cured P8B2T1 reaches 130.89 MPa and 6.45 GPa, respectively. Moreover, the dual-cured P8B2T1 shows good flame retardancy. This high-performance photosensitive resin has great potential in fields such as aerospace, automotive, and electronics.

From Single to Multi-Material 3D Printing of Glass-Ceramics for Micro-Optics

Feynman's statement, "There is plenty of room at the bottom", underscores vast potential at the atomic scale, envisioning microscopic machines. Today, this vision extends into 3D space, where thousands of atoms and molecules are volumetrically patterned to create light-driven technologies. To fully harness their potential, 3D designs must incorporate high-refractive-index elements with exceptional mechanical and chemical resilience. The frontier, however, lies in creating spatially patterned micro-optical architectures in glass and ceramic materials of dissimilar compositions. This multi-material capability enables novel ways of shaping light, leveraging the interaction between diverse interfaced chemical compositions to push optical boundaries. Specifically, it encompasses both multi-material integration within the same architectures and the use of different materials for distinct architectural features in an optical system. Integrating fluid handling systems with two-photon lithography (TPL) provides a promising approach for rapidly prototyping such complex components. This review examines single and multi-material TPL processes, discussing photoresin customization, essential physico-chemical conditions, and the need for cross-scale characterization to assess optical quality. It reflects on challenges in characterizing multi-scale architectures and outlines advancements in TPL for both single and spatially patterned multi-material structures. The roadmap provides a bridge between research and industry, emphasizing collaboration and contributions to advancing micro-optics.

Fabrication of 3D Functional Nanocomposites Through Post-Doping of Two-Photon Microprinted Nanoporous Architectures

Two-photon lithography (TPL) enables the fabrication of complex 3D structures with sub-micrometer precision. Incorporation of new functionalities into TPL-printed structures is key to advance their applications. A prevalent approach to achieve this is by directly adding functional nanomaterials into the photoresist (called "pre-doping"), which has several inherent challenges including material compatibility, light scattering, and nanoparticle agglomeration. Here, a conceptually different "post-doping" strategy is proposed, where the functionality of the TPL-printed architectures is achieved by impregnating functional materials into their nanoporous 3D mimics. Using the principle of polymerization-induced phase separation, TPL printing of complex microarchitectures with well-defined nanoporous structures having pores of ≈420 nm is realized, which allows spontaneous impregnation of functional liquids via capillary effect. Importantly, unlike the "pre-doping" approach that requires printing optimization for each photoresist, this strategy is highly versatile in terms of functionalities possible. As a proof-of-concept, the impregnation of several functional liquids into TPL-printed porous microstructures is demonstrated: a fluorinated-lubricant, an ionic liquid, and three types of fluorescent liquids, conferring the microstructures with slippery, conductive, and localized fluorescence properties, respectively. Such versatility to fabricate complex microstructures with tailorable and localized functionalities is expected to open new possibilities in wide fields including bionics, electronics, and cell biology.

Bio-Informed Porous Mineral-Based Composites

Certain biominerals, such as sea sponges and echinoderm skeletons, display a fascinating combination of mechanical properties and adaptability due to the well-defined structures spanning various length scales. These materials often possess high density normalized mechanical properties because they contain well-defined pores. The density-normalized mechanical properties of synthetic minerals are often inferior because the pores are stochastically distributed, resulting in an inhomogeneous stress distribution. The mechanical properties of synthetic materials are limited by the degree of structural and compositional control currently available fabrication methods offer. In the first part of this review, examples of structural elements nature uses to impart exceptional density normalized Young's moduli to its porous biominerals are showcased. The second part highlights recent advancements in the fabrication of bio-informed mineral-based composites possessing pores with diameters that span a wide range of length scales. The influence of the processing of mineral-based composites on their structures and mechanical properties is summarized. Thereby, it is aimed at encouraging further research directed to the sustainable, energy-efficient fabrication of synthetic lightweight yet stiff mineral-based composites.

Medium-assisted scalable printing of LIPSS via femtosecond laser projection manufacturing

Femtosecond laser has been widely applied for manufacturing micro-/nanostructures in micro-optics, microelectronics, and biomedical engineering. However, tight focusing remains restricted by efficiency, and large-field scanning with a scanning mirror sacrifices precision. In this paper, we proposed a medium-assisted femtosecond laser projection manufacturing (MA-FPM) approach, which enables the fabrication of controllable patterns that incorporate 115 nm periodic laser-induced periodic surface structures (LIPSS). We demonstrated the capability of this technology in fabricating different patterns and pattern arrays. Additionally, we illustrated the application of the micro-nanostructures in plasmon enhancement through Raman spectroscopy. The MA-FPM technique holds promise for metasurface, biosensing, label-free microscopy, and tissue engineering.

Bayesian optimization with Gaussian-process-based active machine learning for improvement of geometric accuracy in projection multi-photon 3D printing

Multi-photon polymerization is a well-established, yet actively developing, additive manufacturing technique for 3D printing on the micro/nanoscale. Like all additive manufacturing techniques, determining the process parameters necessary to achieve dimensional accuracy for a structure 3D printed using this method is not always straightforward and can require time-consuming experimentation. In this work, an active machine learning based framework is presented for determining optimal process parameters for the recently developed, high-speed, layer-by-layer continuous projection 3D printing process. The proposed active learning framework uses Bayesian optimization to inform optimal experimentation in order to adaptively collect the most informative data for effective training of a Gaussian-process-regression-based machine learning model. This model then serves as a surrogate for the manufacturing process: predicting optimal process parameters for achieving a target geometry, e.g., the 2D geometry of each printed layer. Three representative 2D shapes at three different scales are used as test cases. In each case, the active learning framework improves the geometric accuracy, with drastic reductions of the errors to within the measurement accuracy in just four iterations of the Bayesian optimization using only a few hundred of total training data. The case studies indicate that the active learning framework developed in this work can be broadly applied to other additive manufacturing processes to increase accuracy with significantly reduced experimental data collection effort for optimization.

Improving the Lithography Sensitivity of Atomically Precise Tin-Oxo Nanoclusters via Heterometal Strategy

Tin-oxo clusters are increasingly recognized as promising materials for nanolithography technology due to their unique properties, yet their structural impacts on lithography performance remain underexplored. This work explores the structural impacts of heterometal strategies on the performance of tin-oxo clusters in nanolithography, focusing on various metal dopants and their coordination geometries. Specifically, SnOC-1(In), SnOC-1(Al), SnOC-1(Fe), and SnOC-2 were synthesized and characterized. These clusters demonstrate excellent solubility, dispersibility, and stability, facilitating the preparation of high-quality films via spin-coating for lithographic applications. Notably, this work innovatively employs Atomic Force Microscopy-based Infrared Spectroscopy (AFM-IR), neutron reflectivity (NR), and X-ray reflectivity (XRR) measurements to confirm film homogeneity. Upon electron beam lithography (EBL), all four materials achieve 50 nm line patterns, with SnOC-1(In) demonstrating the highest lithography sensitivity. This enhanced sensitivity is attributed to indium dopants, which possess superior EUV absorption capabilities and unsaturated coordination environments. Further studies on exposure mechanisms indicated that Sn-C bond cleavage generates butyl free radicals, promoting network formations that induce solubility-switching behaviors for lithography. These findings underscore the efficacy of tailored structural design and modulation of cluster materials through heterometal strategies in enhancing lithography performance, offering valuable insights for future material design and applications.

3D Nanofabrication via Directed Material Assembly: Mechanism, Method, and Future

Fabrication of complex three-dimensional (3D) structures at nanoscale is the core of nanotechnology, as it enables the creation of various micro-/nano-devices such as micro-robots, metamaterials, sensors, photonic devices, etc. Among most 3D nanofabrication strategies, the guided material assembly, an efficient bottom-up approach capable of directly constructing designed structures from precise integration of material building blocks, possesses compelling advantages in diverse material compatibility, sufficient driving forces, facile processing steps, and nanoscale resolution. In this review, we focus on assembly-based fabrication methods capable of creating complex 3D nanostructures (instead of periodic or 2.5D-only structures). Recent advances are classified based on the different assembly mechanisms, i.e., assembly driven by chemical reactions, physical interactions, and the synergy of multiple microscopic interactions. The design principles of representative fabrication strategies with an emphasis on their respective advantages, e.g., structural design flexibility, material compatibility, resolution, or applications are analyzed. In the summary and outlook, existing challenges, as well as possible paths to solutions for future development are reviewed. We believe that with recent advances in assembly-based nanofabrication strategies, 3D nanofabrication has achieved tremendous progress in resolution, material generality, and manufacturing cost, for it to make a greater impact in the near future.

Single-step and parallel high-speed fabrication for Au micro-/nanostructures

Patterns fabricated with Au nanoparticles (Au NPs) exhibit significant potential in fields such as micro-optics and bioelectronics. However, the current manufacturing methods still suffer from complex processes and low efficiency. In this work, we propose a high-speed manufacturing method for Au micro-/nanostructures based on spatiotemporal focusing and maskless lithography, achieving a throughput of 0.5 mm2/min. The surface electric field enhancement of the structure was confirmed using Raman spectroscopy, indicating that this method holds significant promise for metasurface, biosensing, and plasmonic chemistry.

Toward Integrated Multifunctional Laser-Induced Graphene-Based Skin-Like Flexible Sensor Systems

The burgeoning demands for health care and human-machine interfaces call for the next generation of multifunctional integrated sensor systems with facile fabrication processes and reliable performances. Laser-induced graphene (LIG) with highly tunable physical and chemical characteristics plays vital roles in developing versatile skin-like flexible or stretchable sensor systems. This Progress Report presents an in-depth overview of the latest advances in LIG-based techniques in the applications of flexible sensors. First, the merits of the LIG technique are highlighted especially as the building blocks for flexible sensors, followed by the description of various fabrication methods of LIG and its variants. Then, the focus is moved to diverse LIG-based flexible sensors, including physical sensors, chemical sensors, and electrophysiological sensors. Mechanisms and advantages of LIG in these scenarios are described in detail. Furthermore, various representative paradigms of integrated LIG-based sensor systems are presented to show the capabilities of LIG technique for multipurpose applications. The signal cross-talk issues are discussed with possible strategies. The LIG technology with versatile functionalities coupled with other fabrication strategies will enable high-performance integrated sensor systems for next-generation skin electronics.

Exploring the Potentials of Chitin and Chitosan-Based Bioinks for 3D-Printing of Flexible Electronics: The Future of Sustainable Bioelectronics

Chitin and chitosan-based bioink for 3D-printed flexible electronics have tremendous potential for innovation in healthcare, agriculture, the environment, and industry. This biomaterial is suitable for 3D printing because it is highly stretchable, super-flexible, affordable, ultrathin, and lightweight. Owing to its ease of use, on-demand manufacturing, accurate and regulated deposition, and versatility with flexible and soft functional materials, 3D printing has revolutionized free-form construction and end-user customization. This study examined the potential of employing chitin and chitosan-based bioinks to build 3D-printed flexible electronic devices and optimize bioink formulation, printing parameters, and postprocessing processes to improve mechanical and electrical properties. The exploration of 3D-printed chitin and chitosan-based flexible bioelectronics will open new avenues for new flexible materials for numerous industrial applications.

Aqueous Developable and CO2-Sourced Chemical Amplification Photoresist with High Performance

Seeking high-performance photoresists is an important item for semiconductor industry due to the continuous miniaturization and intelligentization of integrated circuits. Polymer resin containing carbonate group has many desirable properties, such as high transmittance, acid sensitivity and chemical formulation, thus serving as promising photoresist material. In this work, a series of aqueous developable CO2-sourced polycarbonates (CO2-PCs) were produced via alternating copolymerization of CO2 and epoxides bearing acid-cleavable cyclic acetal groups in the presence of tetranuclear organoborane catalyst. The produced CO2-PCs were investigated as chemical amplification resists in deep ultraviolet (DUV) lithography. Under the catalysis of photogenerated acid, the acetal (ketal) groups in CO2-PC were hydrolysed into two equivalents of hydroxyl groups, which change the exposed area from hydrophobicity to hydrophilicity, thus enabling the exposed area to be developed with water. Through normalized remaining thickness analysis, the optimal CO2-derived resist achieved a remarkable sensitivity of 1.9 mJ/cm2, a contrast of 7.9, a favorable resolution (750 nm, half pitch), and a good etch resistance (38 % higher than poly(tert-butyl acrylate)). Such performances outperform commercial KrF and ArF chemical amplification resists (i.e., polyhydroxystyrene-derived and polymethacrylate-based resists), which endows broad application prospects in the field of DUV (KrF and ArF) and extreme ultraviolet (EUV) lithography for nanomanufacturing.

Toward Multiscale, Multimaterial 3D Printing

Biological materials and organisms possess the fundamental ability to self-organize, through which different components are assembled from the molecular level up to hierarchical structures with superior mechanical properties and multifunctionalities. These complex composites inspire material scientists to design new engineered materials by integrating multiple ingredients and structures over a wide range. Additive manufacturing, also known as 3D printing, has advantages with respect to fabricating multiscale and multi-material structures. The need for multifunctional materials is driving 3D printing techniques toward arbitrary 3D architectures with the next level of complexity. In this paper, the aim is to highlight key features of those 3D printing techniques that can produce either multiscale or multimaterial structures, including innovations in printing methods, materials processing approaches, and hardware improvements. Several issues and challenges related to current methods are discussed. Ultimately, the authors also provide their perspective on how to realize the combination of multiscale and multimaterial capabilities in 3D printing processes and future directions based on emerging research.

Battery-less long-range wireless fluidic sensing system using flexible additive manufacturing ambient energy harvester and microfluidics

Fluid sensing has been an important but missing part of the massive Internet-of-Things sensor networks due to challenges including excessive manufacturing time/cost, finite wireless interrogation range, limited immunity to ambient clutter, and excessive required power for autonomous microfluidics operability. Here, we proposed an additive manufacturing flexible system as a solution to those challenges while enabling fluid analysis from controlled labs to virtually everywhere. Energy harvesting provides all required power for the actuation of the micro-pump enabling battery-less liquid sample acquisition. Energy sources including ultra-high-frequency radio frequency identification and hand-held devices like two-way talk radio are harvested simultaneously to support energy requirements for periodic monitoring every 6.6 min and on-demand monitoring within 4.63 s. Backscattering topologies are used to significantly extend the reading range while increasing the immunity to interferences and reducing the cost to the reader. A new additive manufacturing process is proposed to reduce fabrication time and cost while enabling massive scalability of flexible microfluidics. The good flexibility makes the system suitable for working toward future wearable applications. Prototypes of a sweat sensing system are demonstrated and successfully interrogated at 3 m with more than 15 dB signal-to-noise ratio using only a 14 dBm transmitter equivalent isotropic radiated power.

Approaching Standardization: Mechanical Material Testing of Macroscopic Two-Photon Polymerized Specimens

Two-photon polymerization (2PP) is becoming increasingly established as additive manufacturing technology for microfabrication due to its high-resolution and the feasibility of generating complex parts. Until now, the high resolution of 2PP is also its bottleneck, as it limited throughput and therefore restricted the application to the production of microparts. Thus, mechanical properties of 2PP materials can only be characterized using nonstandardized specialized microtesting methods. Due to recent advances in 2PP technology, it is now possible to produce parts in the size of several millimeters to even centimeters, finally permitting the fabrication of macrosized testing specimens. Besides suitable hardware systems, 2PP materials exhibiting favorable mechanical properties that allow printing of up-scaled parts are strongly demanded. In this work, the up-scalability of three different photopolymers is investigated using a high-throughput 2PP system and low numerical aperture optics. Testing specimens in the cm-range are produced and tested with common or even standardized material testing methods available in conventionally equipped polymer testing labs. Examples of the characterization of mechanical, thermo-mechanical, and fracture properties of 2PP processed materials are shown. Additionally, aspects such as postprocessing and aging are investigated. This lays a foundation for future expansion of the 2PP technology to broader industrial application.

Fast and Facile Liquid Metal Printing via Projection Lithography for Highly Stretchable Electronic Circuits

Soft electronic circuits are crucial for wearable electronics, biomedical technologies, and soft robotics, requiring soft conductive materials with high conductivity, high strain limit, and stable electrical performance under deformation. Liquid metals (LMs) have become attractive candidates with high conductivity and fluidic compliance, while effective manufacturing methods are demanded. Digital light processing (DLP)-based projection lithography is a high-resolution and high-throughput printing technique for primarily polymers and some metals. If LMs can be printed with DLP as well, the entire soft devices can be fabricated by one printer in a streamlined and highly efficient process. Herein, fast and facile DLP-based LM printing is achieved. Simply with 5-10 s of patterned ultraviolet (UV)-light exposure, a highly conductive and stretchable pattern can be printed using a photo-crosslinkable LM particle ink. The printed eutectic gallium indium traces feature high resolution (≈20 µm), conductivity (3 × 106 S m-1), stretchability (≈2500%), and excellent stability (consistent performance at different deformation). Various patterns are printed in diverse material systems for broad applications including stretchable displays, epidermal strain sensors, heaters, humidity sensors, conformal electrodes for electrography, and multi-layer actuators. The facile and scalable process, excellent performance, and diverse applications ensure its broad impact on soft electronic manufacturing.

Two-color 3D printing for reduction in femtosecond laser printing power

Two-photon polymerization (TPP) has emerged as a favored advanced manufacturing tool for creating complex 3D structures in the sub-micron regime. However, the widescale implementation of this technique is limited partly due to the cost of a high-power femtosecond laser. In this work, a method is proposed to reduce the femtosecond laser 3D printing power by as much as 50% using a combination of two-photon absorption from an 800 nm femtosecond laser and single photon absorption from a 532 nm nanosecond laser. The underlying photochemical process is explained with modeling of the photopolymerization reaction. The results show that incorporating single-photon absorption from a visible wavelength laser efficiently reduces inhibitor concentration, resulting in a decreased requirement for femtosecond laser power. The radical to macroradical conversion is dominated by the reduction in oxygen concentration, while the reduction in photoinitiator concentration limits the threshold power reduction of the femtosecond laser.

Growing three-dimensional objects with light

Vat photopolymerization (VP) additive manufacturing enables fabrication of complex 3D objects by using light to selectively cure a liquid resin. Developed in the 1980s, this technique initially had few practical applications due to limitations in print speed and final part material properties. In the four decades since the inception of VP, the field has matured substantially due to simultaneous advances in light delivery, interface design, and materials chemistry. Today, VP materials are used in a variety of practical applications and are produced at industrial scale. In this perspective, we trace the developments that enabled this printing revolution by focusing on the enabling themes of light, interfaces, and materials. We focus on these fundamentals as they relate to continuous liquid interface production (CLIP), but provide context for the broader VP field. We identify the fundamental physics of the printing process and the key breakthroughs that have enabled faster and higher-resolution printing, as well as production of better materials. We show examples of how in situ print process monitoring methods such as optical coherence tomography can drastically improve our understanding of the print process. Finally, we highlight areas of recent development such as multimaterial printing and inorganic material printing that represent the next frontiers in VP methods.

The importance of 3D fibre architecture in cancer and implications for biomaterial model design

The need for improved prediction of clinical response is driving the development of cancer models with enhanced physiological relevance. A new concept of 'precision biomaterials' is emerging, encompassing patient-mimetic biomaterial models that seek to accurately detect, treat and model cancer by faithfully recapitulating key microenvironmental characteristics. Despite recent advances allowing tissue-mimetic stiffness and molecular composition to be replicated in vitro, approaches for reproducing the 3D fibre architectures found in tumour extracellular matrix (ECM) remain relatively unexplored. Although the precise influences of patient-specific fibre architecture are unclear, we summarize the known roles of tumour fibre architecture, underlining their implications in cell-matrix interactions and ultimately clinical outcome. We then explore the challenges in reproducing tissue-specific 3D fibre architecture(s) in vitro, highlighting relevant biomaterial fabrication techniques and their benefits and limitations. Finally, we discuss imaging and image analysis techniques (focussing on collagen I-optimized approaches) that could hold the key to mapping tumour-specific ECM into high-fidelity biomaterial models. We anticipate that an interdisciplinary approach, combining materials science, cancer research and image analysis, will elucidate the role of 3D fibre architecture in tumour development, leading to the next generation of patient-mimetic models for mechanistic studies and drug discovery.

Intelligent Biomaterialomics: Molecular Design, Manufacturing, and Biomedical Applications

Materialomics integrates experiment, theory, and computation in a high-throughput manner, and has changed the paradigm for the research and development of new functional materials. Recently, with the rapid development of high-throughput characterization and machine-learning technologies, the establishment of biomaterialomics that tackles complex physiological behaviors has become accessible. Breakthroughs in the clinical translation of nanoparticle-based therapeutics and vaccines have been observed. Herein, recent advances in biomaterials, including polymers, lipid-like materials, and peptides/proteins, discovered through high-throughput screening or machine learning-assisted methods, are summarized. The molecular design of structure-diversified libraries; high-throughput characterization, screening, and preparation; and, their applications in drug delivery and clinical translation are discussed in detail. Furthermore, the prospects and main challenges in future biomaterialomics and high-throughput screening development are highlighted.

Upconversion 3D Bioprinting for Noninvasive In Vivo Molding

Tissue engineered bracket materials provide essential support for the physiological protection and therapeutics of patients. Unfortunately, the implantation process of such devices poses the risk of surgical complications and infection. In this study, an upconversion nanoparticles (UCNPs)-assisted 3D bioprinting approach is developed to realize in vivo molding that is free from invasive surgery. Reasonably designed UCNPs, which convert near-infrared (NIR) photons that penetrate skin tissues into blue-violet emission (300-500 nm), induce a monomer polymerization curing procedure in vivo. Using a fused deposition modeling coordination framework, a precisely predetermined trajectory of the NIR laser enables the manufacture of implantable medical devices with tailored shapes. A proof of the 3D bioprinting of a noninvasive fracture fixation scaffold is achieved successfully, thus demonstrating an entirely new method of in vivo molding for biomedical treatment.

Gelatine-collagen photo-crosslinkable 3D matrixes for skin regeneration

Immediate care of skin wounds and burns is essential to repair this mechanical and chemical barrier to infections. Hydrogels have become one of the standard methods for wound care. Here, gelatine-collagen photo-crosslinkable matrixes or hydrogels were manufactured by two-photon polymerization (TPP) or one-photon UV exposure using a Digital Light Processing (DLP) setup. Both techniques are able to construct matrixes from computer-aided design models, which is important for future clinical applications in which wound dressings should be customized. Although TPP can mimic the 3D dermo-epidermal junction with a high spatial resolution (i.e., ∼6 μm3), the manufacturing time was too slow to produce large wound dressings. Therefore, a DLP setup was explored in this study to fabricate large 2D matrixes of several cm2 using the same photo-resist as for TPP, except for the photoinitiator. The fibroblast viability, adherence, and proliferation were analysed in time on both 3D and 2D matrixes in vitro using two-photon microscopy. For both types of matrixes, the adherence and proliferation of fibroblasts (3T3-NIH) were optimal for stiff structures with a Young's modulus of 191 ± 35 kPa compared to softer matrixes of 37 ± 12 kPa. Fibroblast showed complete confluence on Day 14 after seeding on these matrixes, which may create the granulation tissue composed of fibronectin, collagen, and various proteoglycans in the future dermis before repair of the epidermis and disintegrating of their host matrix. For the monitoring of this repair, gelatine-collagen matrixes can easily incorporate bio-optical sensors for the simultaneous monitoring of inflammation processes and wound healing in time.

High-Throughput Two-Photon 3D Printing Enabled by Holographic Multi-Foci High-Speed Scanning

The emerging two-photon polymerization (TPP) technique enables high-resolution printing of complex 3D structures, revolutionizing micro/nano additive manufacturing. Various fast scanning and parallel processing strategies have been proposed to promote its efficiency. However, obtaining large numbers of uniform focal spots for parallel high-speed scanning remains challenging, which hampers the realization of higher throughput. We report a TPP printing platform that combines galvanometric mirrors and liquid crystal on silicon spatial light modulator (LCoS-SLM). By setting the target light field at LCoS-SLM's diffraction center, sufficient energy is acquired to support simultaneous polymerization of over 400 foci. With fast scanning, the maximum printing speed achieves 1.49 × 108 voxels s-1, surpassing the existing scanning-based TPP methods while maintaining high printing resolution and flexibility. To demonstrate the processing capability, functional 3D microstructure arrays are rapidly fabricated and applied in micro-optics and micro-object manipulation. Our method may expand the prospects of TPP in large-scale micro/nanomanufacturing.

Digital inverse patterning solutions for fabrication of high-fidelity microstructures in spatial light modulator (SLM)-based projection lithography

Digital mask projection lithography (DMPL) technology is gaining significant attention due to its characteristics of free-mask, flexibility, and low cost. However, when dealing with target layouts featuring sizes smaller than the wavelength scale, accurately producing resist patterns that closely match the target layout using conventional methods to design the modulation coefficients of digital masks produced by spatial light modulators (SLM) becomes challenging. Here, we present digital inversion lithography technology (DILT), which offers what we believe to be a novel approach to reverse engineer the modulation coefficients of digital masks. In the case of binary amplitude modulation, DILT achieves a remarkable reduction in pattern errors (PE), reaching the original 0.26. At the same time, in the case of the gray amplitude modulation, the PE can be reduced to the original 0.05, which greatly improves the high-fidelity transfer of the target layout. This significant improvement enhances the accuracy of target design transfer. By leveraging the capabilities of DILT, DMPL can now attain higher precision and reliability, paving the way for more advanced applications in the field of micro-nano device manufacturing.

Photophysics and its application in photon upconversion

Photoluminescence (PL) upconversion is a phenomenon involving light-matter interaction, where the energy of the emitted photons is higher than that of the incident photons. PL upconversion has promising applications in optoelectronic devices, displays, photovoltaics, imaging, diagnosis and treatment. In this review, we summarize the mechanism of PL upconversion and ultrafast PL physical processes. In particular, we highlight the advances in laser cooling, biological imaging, volumetric displays and photonics.

Femtosecond Laser Direct Writing of Flexible Electronic Devices: A Mini Review

By virtue of its narrow pulse width and high peak power, the femtosecond pulsed laser can achieve high-precision material modification, material additive or subtractive, and other forms of processing. With additional good material adaptability and process compatibility, femtosecond laser-induced application has achieved significant progress in flexible electronics in recent years. These advancements in the femtosecond laser fabrication of flexible electronic devices are comprehensively summarized here. This review first briefly introduces the physical mechanism and characteristics of the femtosecond laser fabrication of various electronic microdevices. It then focuses on effective methods of improving processing efficiency, resolution, and size. It further highlights the typical progress of applications, including flexible energy storage devices, nanogenerators, flexible sensors, and detectors, etc. Finally, it discusses the development tendency of ultrashort pulse laser processing. This review should facilitate the precision manufacturing of flexible electronics using a femtosecond laser.

A Computational Evaluation of Minimum Feature Size in Projection Two-Photon Lithography for Rapid Sub-100 nm Additive Manufacturing

Two-photon lithography (TPL) is a laser-based additive manufacturing technique that enables the printing of arbitrarily complex cm-scale polymeric 3D structures with sub-micron features. Although various approaches have been investigated to enable the printing of fine features in TPL, it is still challenging to achieve rapid sub-100 nm 3D printing. A key limitation is that the physical phenomena that govern the theoretical and practical limits of the minimum feature size are not well known. Here, we investigate these limits in the projection TPL (P-PTL) process, which is a high-throughput variant of TPL, wherein entire 2D layers are printed at once. We quantify the effects of the projected feature size, optical power, exposure time, and photoinitiator concentration on the printed feature size through finite element modeling of photopolymerization. Simulations are performed rapidly over a vast parameter set exceeding 10,000 combinations through a dynamic programming scheme, which is implemented on high-performance computing resources. We demonstrate that there is no physics-based limit to the minimum feature sizes achievable with a precise and well-calibrated P-TPL system, despite the discrete nature of illumination. However, the practically achievable minimum feature size is limited by the increased sensitivity of the degree of polymer conversion to the processing parameters in the sub-100 nm regime. The insights generated here can serve as a roadmap towards fast, precise, and predictable sub-100 nm 3D printing.

Polymer Composites in 3D/4D Printing: Materials, Advances, and Prospects

Additive manufacturing (AM), commonly referred to as 3D printing, has revolutionized the manufacturing landscape by enabling the intricate layer-by-layer construction of three-dimensional objects. In contrast to traditional methods relying on molds and tools, AM provides the flexibility to fabricate diverse components directly from digital models without the need for physical alterations to machinery. Four-dimensional printing is a revolutionary extension of 3D printing that introduces the dimension of time, enabling dynamic transformations in printed structures over predetermined periods. This comprehensive review focuses on polymeric materials in 3D printing, exploring their versatile processing capabilities, environmental adaptability, and applications across thermoplastics, thermosetting materials, elastomers, polymer composites, shape memory polymers (SMPs), including liquid crystal elastomer (LCE), and self-healing polymers for 4D printing. This review also examines recent advancements in microvascular and encapsulation self-healing mechanisms, explores the potential of supramolecular polymers, and highlights the latest progress in hybrid printing using polymer-metal and polymer-ceramic composites. Finally, this paper offers insights into potential challenges faced in the additive manufacturing of polymer composites and suggests avenues for future research in this dynamic and rapidly evolving field.

Scalable Printing of Metal Nanostructures through Superluminescent Light Projection

Direct printing of metallic nanostructures is highly desirable but current techniques cannot achieve nanoscale resolutions or are too expensive and slow. Photoreduction of solvated metal ions into metallic nanoparticles is an attractive strategy because it is faster than deposition-based techniques. However, it is still limited by the resolution versus cost tradeoff because sub-diffraction printing of nanostructures requires high-intensity light from expensive femtosecond lasers. Here, this tradeoff is overcome by leveraging the spatial and temporal coherence properties of low-intensity diode-based superluminescent light. The superluminescent light projection (SLP) technique is presented to rapidly print sub-diffraction nanostructures, as small as 210 nm and at periods as small as 300 nm, with light that is a billion times less intense than femtosecond lasers. Printing of arbitrarily complex 2D nanostructured silver patterns over 30 µm × 80 µm areas in 500 ms time scales is demonstrated. The post-annealed nanostructures exhibit an electrical conductivity up to 1/12th that of bulk silver. SLP is up to 480 times faster and 35 times less expensive than printing with femtosecond lasers. Therefore, it transforms nanoscale metal printing into a scalable format, thereby significantly enhancing the transition of nano-enabled devices from research laboratories into real-world applications.

Continuous Volumetric 3D Printing: Xolography in Flow

Additive manufacturing techniques continue to improve in resolution, geometrical freedom, and production rates, expanding their application range in research and industry. Most established techniques, however, are based on layer-by-layer polymerization processes, leading to an inherent trade-off between resolution and printing speed. Volumetric 3D printing enables the polymerization of freely defined volumes allowing the fabrication of complex geometries at drastically increased production rates and high resolutions, marking the next chapter in light-based additive manufacturing. This work advances the volumetric 3D printing technique xolography to a continuous process. Dual-color photopolymerization is performed in a continuously flowing resin, inside a tailored flow cell. Supported by simulations, the flow profile in the printing area is flattened, and resin velocities at the flow cell walls are increased to minimize unwanted polymerization via laser sheet-induced curing. Various objects are printed continuously and true to shape with smooth surfaces. Parallel object printing paves the way for up-scaling the continuous production, currently reaching production rates up to 1.75 mm3  s-1 for the presented flow cell. Xolography in flow provides a new opportunity for scaling up volumetric 3D printing with the potential to resolve the trade-off between high production rates and high resolution in light-based additive manufacturing.

Bioprintedin vitrotissue models: an emerging platform for developing therapeutic interventions and disease modelling

In the past decade, the use of three-dimensional (3D) bioprinting technology for the development ofin vitrotissue models has attracted a great deal of attention. This is due to its remarkable precision in constructing different functional tissues and organs, enabling studies of their biology. In addition, this high-throughput technology has been extended to therapeutics, as it provides an alternative functional platform for rapid drug screening and disease modelling. Functional tissue models fabricated using 3D bioprinting mimic native tissues and help in the development of platforms for personalized drug screening and disease modelling due to their high throughput and ease of customization. Moreover, bioprinted 3D tissue models mimic native tissues more closely and provide added advantages over earlier conventional tissue models, such as monoculture, co-culture, explants, etc. In this context, this review article provides an overview of different bioprintedin vitrotissue models of skin, bone, neural tissue, vascular tissue, cartilage, liver and cardiac tissue. This article explores advancements and innovations in these models in terms of developing improved therapeutic interventions. Herein, we provide an insight into the development of different bioprinted tissue models for applications in drug screening and disease modelling. The needs and advantages of bioprinted tissue models as compared with conventionalin vitromodels are discussed. Furthermore, the different biomaterials, cell sources and bioprinting techniques used to develop tissue models are briefly reviewed. Thereafter, different bioprinted tissue models, namely skin, liver, vascular, cardiac, cartilage, bone and neural tissue, are discussed in detail with a special emphasis on drug screening and disease modelling. Finally, challenges and future prospects are highlighted and discussed. Taken together, this review highlights the different approaches and strategies used for the development of different 3D bioprintedin vitrotissue models for improved therapeutic interventions.

Ultrahigh-printing-speed photoresists for additive manufacturing

Printing technology for precise additive manufacturing at the nanoscale currently relies on two-photon lithography. Although this methodology can overcome the Rayleigh limit to achieve nanoscale structures, it still operates at too slow of a speed for large-scale practical applications. Here we show an extremely sensitive zirconium oxide hybrid-(2,4-bis(trichloromethyl)-6-(4-methoxystyryl)-1,3,5-triazine) (ZrO2-BTMST) photoresist system that can achieve a printing speed of 7.77 m s-1, which is between three and five orders of magnitude faster than conventional polymer-based photoresists. We build a polygon laser scanner-based two-photon lithography machine with a linear stepping speed approaching 10 m s-1. Using the ZrO2-BTMST photoresist, we fabricate a square raster with an area of 1 cm2 in ~33 min. Furthermore, the extremely small chemical components of the ZrO2-BTMST photoresist enable high-precision patterning, leading to a line width as small as 38 nm. Calculations assisted by characterizations reveal that the unusual sensitivity arises from an efficient light-induced polarity change of the ZrO2 hybrid. We envisage that the exceptional sensitivity of our organic-inorganic hybrid photoresist may lead to a viable large-scale additive manufacturing nanofabrication technology.

3D printing of inorganic nanomaterials by photochemically bonding colloidal nanocrystals

3D printing of inorganic materials with nanoscale resolution offers a different materials processing pathway to explore devices with emergent functionalities. However, existing technologies typically involve photocurable resins that reduce material purity and degrade properties. We develop a general strategy for laser direct printing of inorganic nanomaterials, as exemplified by more than 10 semiconductors, metal oxides, metals, and their mixtures. Colloidal nanocrystals are used as building blocks and photochemically bonded through their native ligands. Without resins, this bonding process produces arbitrary three-dimensional (3D) structures with a large inorganic mass fraction (~90%) and high mechanical strength. The printed materials preserve the intrinsic properties of constituent nanocrystals and create structure-dictated functionalities, such as the broadband chiroptical responses with an anisotropic factor of ~0.24 for semiconducting cadmium chalcogenide nanohelical arrays.

Review of Additively Manufactured Polymeric Metamaterials: Design, Fabrication, Testing and Modeling

Metamaterials are architected cellular materials, also known as lattice materials, that are inspired by nature or human engineering intuition, and provide multifunctional attributes that cannot be achieved by conventional polymeric materials and composites. There has been an increasing interest in the design, fabrication, and testing of polymeric metamaterials due to the recent advances in digital design methods, additive manufacturing techniques, and machine learning algorithms. To this end, the present review assembles a collection of recent research on the design, fabrication and testing of polymeric metamaterials, and it can act as a reference for future engineering applications as it categorizes the mechanical properties of existing polymeric metamaterials from literature. The research within this study reveals there is a need to develop more expedient and straightforward methods for designing metamaterials, similar to the implicitly created TPMS lattices. Additionally, more research on polymeric metamaterials under more complex loading scenarios is required to better understand their behavior. Using the right machine learning algorithms in the additive manufacturing process of metamaterials can alleviate many of the current difficulties, enabling more precise and effective production with product quality.

4D Additive-Subtractive Manufacturing of Shape Memory Ceramics

The development of high-temperature structural materials, such as ceramics, is limited by their extremely high melting points and the difficulty in building complicated architectures. Four-dimensional (4D) printing helps enhance the geometrical flexibility of ceramics. However, ceramic 4D printing systems are limited by the separate processes for shape and material transformations, low accuracy of morphing systems, low resolution of ceramic structures, and their time-intensive nature. Here, a paradigm for a one-step shape/material transformation, high-2D/3D/4D-precision, high-efficiency, and scalable 4D additive-subtractive manufacturing of shape memory ceramics is developed. Original/reverse and global/local multimode shape memory capabilities are achieved using macroscale SiOC-based ceramic materials. The uniformly deposited Al2 O3 -rich layer on the printed SiOC-based ceramic lattice structures results in an unusually high flame ablation performance of the complex-shaped ceramics. The proposed framework is expected to broaden the applications of high-temperature structural materials in the aerospace, electronics, biomedical, and art fields.

Universal linear intensity transformations using spatially incoherent diffractive processors

Under spatially coherent light, a diffractive optical network composed of structured surfaces can be designed to perform any arbitrary complex-valued linear transformation between its input and output fields-of-view (FOVs) if the total number (N) of optimizable phase-only diffractive features is ≥~2NiNo, where Ni and No refer to the number of useful pixels at the input and the output FOVs, respectively. Here we report the design of a spatially incoherent diffractive optical processor that can approximate any arbitrary linear transformation in time-averaged intensity between its input and output FOVs. Under spatially incoherent monochromatic light, the spatially varying intensity point spread function (H) of a diffractive network, corresponding to a given, arbitrarily-selected linear intensity transformation, can be written as H(m, n; m', n') = |h(m, n; m', n')|2, where h is the spatially coherent point spread function of the same diffractive network, and (m, n) and (m', n') define the coordinates of the output and input FOVs, respectively. Using numerical simulations and deep learning, supervised through examples of input-output profiles, we demonstrate that a spatially incoherent diffractive network can be trained to all-optically perform any arbitrary linear intensity transformation between its input and output if N ≥ ~2NiNo. We also report the design of spatially incoherent diffractive networks for linear processing of intensity information at multiple illumination wavelengths, operating simultaneously. Finally, we numerically demonstrate a diffractive network design that performs all-optical classification of handwritten digits under spatially incoherent illumination, achieving a test accuracy of >95%. Spatially incoherent diffractive networks will be broadly useful for designing all-optical visual processors that can work under natural light.

Very High-Aspect-Ratio Polymeric Micropillars Made by Two-Photon Polymerization

Polymeric micropillars with a high-aspect-ratio (HAR) are of interest for a wide range of applications, including drug delivery and the micro-electro-mechanical field. While molding is the most common method for fabricating HAR microstructures, it is affected by challenges related to demolding the final structure. In this study, we present very HAR micropillars using two-photon polymerization (TPP), an established technique for creating complex 3D microstructures. Polymeric micropillars with HARs fabricated by TPP often shrink and collapse during the development process. This is due to the lack of mechanical stability of micropillars against capillary forces primarily acting during the fabrication process when the solvent evaporates. Here, we report different parameters that have been optimized to overcome the capillary force. These include surface modification of the substrate, fabrication parameters such as laser power, exposure time, the pitch distance between the pillars, and the length of the pillars. On account of adopting these techniques, we were able to fabricate micropillars with a very HAR up to 80.

A Review of Critical Issues in High-Speed Vat Photopolymerization

Vat photopolymerization (VPP) is an effective additive manufacturing (AM) process known for its high dimensional accuracy and excellent surface finish. It employs vector scanning and mask projection techniques to cure photopolymer resin at a specific wavelength. Among the mask projection methods, digital light processing (DLP) and liquid crystal display (LCD) VPP have gained significant popularity in various industries. To upgrade DLP and LCC VPP into a high-speed process, increasing both the printing speed and projection area in terms of the volumetric print rate is crucial. However, challenges arise, such as the high separation force between the cured part and the interface and a longer resin refilling time. Additionally, the divergence of the light-emitting diode (LED) makes controlling the irradiance homogeneity of large-sized LCD panels difficult, while low transmission rates of near ultraviolet (NUV) impact the processing time of LCD VPP. Furthermore, limitations in light intensity and fixed pixel ratios of digital micromirror devices (DMDs) constrain the increase in the projection area of DLP VPP. This paper identifies these critical issues and provides detailed reviews of available solutions, aiming to guide future research towards developing a more productive and cost-effective high-speed VPP in terms of the high volumetric print rate.

Laser-Induced Cavitation-Assisted True 3D Nano-Sculpturing of Hard Materials

Femtosecond lasers enable flexible and thermal-damage-free ablation of solid materials and are expected to play a critical role in high-precision cutting, drilling, and shaping of electronic chips, display panels, and industrial parts. Although the potential applications are theoretically predicted, true 3D nano-sculpturing of solids such as glasses and crystals, has not yet been demonstrated, owing to the technical challenge of negative cumulative effects of surface changes and debris accumulation on the delivery of laser pulses and subsequent material removal during direct-write ablation. Here, a femtosecond laser-induced cavitation-assisted true 3D nano-sculpturing technique based on the ingenious combination of cavitation dynamics and backside ablation is proposed to achieve stable clear-field point-by-point material removal in real time for precise 3D subtractive fabrication on various difficult-to-process materials. As a result, 3D devices including free-form silica lenses, micro-statue with vivid facial features, and rotatable sapphire micro-mechanical turbine, all with surface roughness less than 10 nm are readily produced. The true 3D processing capability can immediately enable novel structural and functional micro-nano optics and non-silicon micro-electro-mechanical systems based on various hard solids.

Additive Manufacturing of Polymer/Bioactive Glass Scaffolds for Regenerative Medicine: A Review

Tissue engineering (TE) is a branch of regenerative medicine with enormous potential to regenerate damaged tissues using synthetic grafts such as scaffolds. Polymers and bioactive glasses (BGs) are popular materials for scaffold production because of their tunable properties and ability to interact with the body for effective tissue regeneration. Due to their composition and amorphous structure, BGs possess a significant affinity with the recipient's tissue. Additive manufacturing (AM), a method that allows the creation of complex shapes and internal structures, is a promising approach for scaffold production. However, despite the promising results obtained so far, several challenges remain in the field of TE. One critical area for improvement is tailoring the mechanical properties of scaffolds to meet specific tissue requirements. In addition, achieving improved cell viability and controlled degradation of scaffolds is necessary to ensure successful tissue regeneration. This review provides a critical summary of the potential and limitations of polymer/BG scaffold production via AM covering extrusion-, lithography-, and laser-based 3D-printing techniques. The review highlights the importance of addressing the current challenges in TE to develop effective and reliable strategies for tissue regeneration.

Multi-photon polymerization using upconversion nanoparticles for tunable feature-size printing

The recent development of light-based 3D printing technologies has marked a turning point in additive manufacturing. Through photopolymerization, liquid resins can be solidified into complex objects. Usually, the polymerization is triggered by exciting a photoinitiator with ultraviolet (UV) or blue light. In two-photon printing (TPP), the excitation is done through the non-linear absorption of two photons; it enables printing 100-nm voxels but requires expensive femtosecond lasers which strongly limit their broad dissemination. Upconversion nanoparticles (UCNPs) have recently been proposed as an alternative to TPP for photopolymerization but using continuous-wave lasers. UCNPs convert near-infrared (NIR) into visible/UV light to initiate the polymerization locally as in TPP. Here we provide a study of this multi-photon mechanism and demonstrate how the non-linearity impacts the printing process. In particular, we report on the possibility of fine-tuning the size of the printed voxel by adjusting the NIR excitation intensity. Using gelatin-based hydrogel, we are able to vary the transverse voxel size from 1.3 to 2.8 μm and the axial size from 7.7 to 59 μm by adjusting the NIR power without changing the degree of polymerization. This work opens up new opportunities to construct 3D structures with micrometer feature size by direct laser writing with continuous wave inexpensive light sources.

Photo-activated polymerization inhibition process in photoinitiator systems for high-throughput 3D nanoprinting

The systems for multiphoton 3D nanoprinting are rapidly increasing in print speed for larger throughput and scale, unfortunately without also improvement in resolution. Separately, the process of photoinhibition lithography has been demonstrated to enhance the resolution of multiphoton printing through the introduction of a secondary laser source. The photo-chemical dynamics and interactions for achieving photoinhibition in the various multiphoton photoinitiator systems are complex and still not well understood. Here, we examine the photoinhibition process of the common photoinitiator 7-diethylamino 3-thenoylcoumarin (DETC) with inhibition lasers near or at the multiphoton printing laser wavelength in typical low peak intensity, high repetition rate 3D nanoprinting processes. We demonstrate the clear inhibition of the polymerization process consistent with a triplet absorption deactivation mechanism for a DETC photoresist as well as show inhibition for several other photoresist systems. Additionally, we explore options to recover the photoinhibition process when printing with high intensity, low repetition rate lasers. Finally, we demonstrate photoinhibition in a projection multiphoton printing system. This investigation of photoinhibition lithography with common photoinitiators elucidates the possibility for photoinhibition occurring in many resist systems with typical high repetition rate multiphoton printing lasers as well as for high-speed projection multiphoton printing.