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Investigation of a cyanate ester based formulation and stereolithography-based additive manufacturing via Hot Lithography to produce polycyanurates with a glass transition temperature of 336 °C without any additives.

Light-hardening dental composites can be used in a large number of applications in restorative dentistry. They are based on photopolymerizable resins, which are highly relevant also in other industries like 3D printing. Much effort is therefore being put into developing and optimizing photopolymers. Currently used photopolymers still have limitations regarding mechanical properties, shrinkage and leaching of uncured monomers. These issues are strongly linked to the network structure of the polymer and are usually addressed using trial and error methods. Therefore, it is of interest to have a model for the network structure of such materials and to have a tool to facilitate scientific progress and the development of high-performance photopolymers. This work presents a coarse grain model of Bis-GMA/TEGDMA formulations and their corresponding networks, following the Martini 3 guidelines and using a simulated polymerization algorithm. The model proved to reproduce the densities and volumetric shrinkage values found in the literature well. Furthermore, it was possible to estimate the final double bond conversion of the polymer material. Martini's building block-like design makes it easy to extend the model to other monomers in the future.

Additive manufacturing of phenolic resins as a proof-of-concept for the first photo-chemical induced polycondensation by Hot Lithography. Through the dual use of a photoacidgenerator, the first pure 3D printing of Bakelite© is investigated.

In lithography-based additive manufacturing, mostly crosslinking monomers with acrylate functionality are applied, which yield brittle materials with inhomogeneous network architectures. The toughening of state-of-the-art materials is an integral requirement for the advancement of photopolymer-based three-dimensional (3D) products. Here we show that the final material properties of acrylate networks can be adjusted through regulation of the radical curing mechanism using difunctional vinyl sulfonate esters to obtain toughened 3D structured materials. A substantial improvement of the thermomechanical behavior of resulting materials over materials regulated by state-of-the-art reagents (e.g. thiols) is presented and first 3D parts have successfully been printed. Resulting materials exhibit reduced shrinkage stress, reduced warpage, higher overall conversion and higher glass transition temperatures compared with the pure acrylate network.

Die Computerunterstutzte Modellierung und Fertigung (CAD/CAM = computer aided design/computer aided manufacturing) hat sich seit geraumer Zeit in der Welt der Zahnmedizin verbreitet. Ausgehend von dieser Neuerung hat sich der rein digitale Arbeitsablauf als angestrebtes Modell etabliert. Die Erfassung der Patientensituation per Oral- oder Tischscanner, das Modellieren per passender Software und das Fertigen uber CNC-Frasen sind heutzutage Stand der Technik. Neben dem Frasen hat die additive Fertigung als aufstrebende CAM-Technologie ihren Weg in Zahnarztpraxen und Zahntechnikerlabors gefunden. In den meisten Fallen werden lithografiebasierte Verfahren fur Modelle und Gussformen angewandt. Dieser Artikel soll einen Uberblick uber die additiven Fertigungsverfahren geben, die prinzipiell fur die Dentalbranche geeignet sind. Daruber hinaus werden Anwendungen beschrieben, und im letztem Abschnitt wagen wir einen kleinen Ausblick in zukunftige Entwicklungen.

Bottom-up stereolithography has become a common lithography-based additive manufacturing technology (L-AMT) to fabricate parts with high feature resolution for biomedical applications. Novel vinyl ester based photopolymers, with their good biocompatibility and biodegradation behavior, showed a promising capacity as bone replacement materials. Due to further tuning of the mechanical properties, those biophotopolymers exhibit reduced curing speed in comparison to highly crosslinked resins e.g. acrylates. The slow structuring of the polymer network results in difficulties at the printing process. The Jacobs working curve characterizes the cure- and penetration depth of resins, but gives no information about the mechanical properties of the cured layer. The information of cure depth and the mechanical properties of the cured layer (cure degree) is desired. In this work, the conditions at L‑AMT during the structuring process were simulated with a real-time near-infrared photorheometer to evaluate the cure degree of a cured layer at constant cure depth. Therefore, we investigated the curing behavior of mixtures with variable amount of photoinitiator (PI) and light absorber (LA) of vinyl ester based biophotopolymers. We found, that a high amount of LA is crucial for good mechanical properties at constant cure depth. Moreover, we present a technique how to optimize a resin formulation regarding the content of PI and LA.

Digital dentistry is increasingly replacing conventional methods of manually producing dental restorations. With regards to computer-aided manufacturing (CAM), milling is state of the art. Additive manufacturing (AM), as a complementary approach, has also found its way into dental practices and laboratories. Vat photo-polymerization is gaining increasing attention, because it enables the production of full ceramic restorations with high precision. One of the two predominantly used ceramic materials for these applications is lithium disilicate, Li2Si2O5. This glass ceramic exhibits a substantial fracture toughness, although possesses much lower bending strength, than the other predominantly used ceramic material, zirconia. Additionally, it shows a much more natural optical appearance, due to its inherent translucency, and therefore is considered for anterior tooth restorations. In this work, an optimized formulation for photo-reactive lithium disilicate suspensions, to be processed by vat photo-polymerization, is presented. Following the fundamental theoretical considerations regarding this processing technique, a variety of solvents was used to adjust the main properties of the suspension. It is shown that this solvent approach is a useful tool to effectively optimize a suspension with regards to refractive index, rheology, and debinding behavior. Additionally, by examining the effect of the absorber, the exposure time could be reduced by a factor of ten.

Light-based processing techniques triggering photopolymerization are among the most promising 3D printing technologies due to their benefits regarding resolution, surface quality and build speed. However, the main challenge remains the development of strong and tough materials, since most commercially available photopolymer resins are limited in terms of their thermomechanical performance. We therefore synthesized a heterotelechelic hybrid oligomer based on poly(propylene oxide) (PPO-H), bearing one methacrylic and one addition–fragmentation chain transfer group (AFCT) as end groups. This new compound was successfully implemented as a toughening agent by regulating the network structure via the AFCT mechanism and acted as reactive diluent for highly viscous resins. Formulations containing 10–25 db% (double bond percent) of PPO-H mixed into a commercially available resin (Bomar XR-741MS) were investigated and compared to the corresponding PPO dimethacrylate (PPO-D) mixtures, representing state of the art resins. Full double bond conversion could be reached for PPO-H containing formulations, while shrinkage stress was simultaneously reduced by up to 50% compared to the PPO-D mixtures. Glass transition temperatures decreased with increasing PPO contents. Toughness was enhanced by a factor of 2 (10 db% PPO-H) to 4 (15 db% PPO-H), measured by elongation at break, while decrease in tensile strength remained low (factor of 1.2 and 1.4 for 10 and 15 db% PPO-H, respectively). Impact strength increased by 55 and 92% for 10 and 15 db% PPO-H, respectively. By employing the recently developed Hot Lithography technique, toughened and migration-free 3D printed parts were obtained with PPO-H.

Lithography based additive manufacturing (AM) is one of the most established and widely used 3D-printing processes. It has enabled the processing of many different materials from thermoplast-like polymers to ceramics that have outstanding feature resolutions and surface quality, with comparable properties of traditional materials. This work focuses on the processing of glass ceramics, which have high optical demands, precision and mechanical properties specifically suitable for dental replacements, such as crowns. Lithography-based ceramic manufacturing (LCM) has been chosen as the optimal manufacturing process where a light source with a defined wavelength is used to cure and structure ceramic filled photosensitive resins. In the case of glass ceramic powders, plastic flow during thermal processing might reduce the precision, as well as the commonly observed sintering shrinkage associated with the utilized temperature program. To reduce this problem, particular sinter structures have been developed to optimize the precision of 3D-printed glass ceramic crowns. To evaluate the precision of the final part, testing using digitizing methods from optical to tactile systems were utilized with the best results were obtained from micro computed tomography (CT) scanning. These methods resulted in an optimized process allowing for possible production of high precision molar crowns with dimensional accuracy and high reproducibility.

Cyclopolymerizable monomers (CPMs) were discovered about 70 years ago but are rarely described in the field of photopolymerization up to now. Herein, we present a class of tertiary amine-based CPMs that undergo complete cyclopolymerization, forming linear polymer chains with cyclic structures in the backbone. Compared to similar monofunctional methacrylates, they show significantly improved reactivity and high double-bond conversions. Because of their low viscosity and low volatility, they are also suitable reactive diluents for highly viscous or even solid urethane methacrylates usually applied in the field of Hot Lithography.

Dental applications like crowns, veneers or bridges require high accuracy to be fitted on the patient’s stump. Stereolithography is an additive manufacturing method, which offers high precision by using light exposure as the layer generating mechanism. In the LCM process, this precision is combined with a thermal post-processing step to achieve full ceramic restorations. The overall production of such ceramic parts in a reproducible way is a highly complex procedure. The first requirement is to find a slurry formulation, which is stable against sedimentation and segregation, that is also processable in a stereolithographic system. Such a formulation has been found by us, which could be shown by rheology measurements. During experiments with this formulation, it could be observed that there is a correlation between wet film thickness and resolution. Several adjustments to the machine have been made, to fully control this parameter. Namely, changes to the vat, the doctor blade and the building platform have been made. The improvement of the process and the quality of the final parts are validated by fabricating Siemens stars and by biaxial bending tests.

Vat photopolymerization is used for printing very precise and accurate parts from photopolymer resins. Conventional 3D-printers based on vat photopolymerization are curing resins with low viscosity at or slightly above room temperature. The newly developed Hot Lithography provides vat photopolymerization where the resin is heated and cured at elevated temperatures. This study presents the influence of printing temperature (23 °C and 70 °C) on the properties of a printed dimethacrylate resin. The working curve was measured for 23 °C, 50 °C and 70 °C. Specimens were printed in XYZ and ZXY orientation. The resulting tensile properties were tested, dynamic mechanical analysis was carried out and the double-bond conversion was analyzed. It was found that the critical energy E0 was significantly reduced by a higher printing temperature. Therefore, the exposure time was reduced from 50 s to 30 s to reach similar curing depth. Higher printing temperature provided higher double-bond conversion, tensile strength and modulus of the green parts. However, printing temperature did not affect the properties after post-curing in XYZ orientation. Post-cured tensile specimens in ZXY orientation had higher tensile strength when printed at 23 °C, because higher over-polymerization led to a smoother surface of the specimens. Overall, higher printing temperatures lowered the viscosity of the resin, reduced the printing time and provided better mechanical properties of green parts while post-cured properties were mostly not affected.

Lithography-based Additive Manufacturing Technologies (L-AMT) exploit the curing of photosensitive materials upon light exposure. We developed a hybrid exposure concept. This system is able to overcome the challenge of providing good surface qualities and excellent feature resolution as well as a throughput similar to dynamic mask-based L-AMT systems by combining two light sources. A Digital Light Processing (DLP®) Light Engine (LE) with a building area of 144 x 90 mm² offers a pixelsize of 56 μm. In order to further improve the achievable resolution, a continuous laser-exposed contour line (spot size 20 μm) on the outside of the projected envelope can be written with an additional scanning laser-system. The matching of the DLP® projection mask and the laser-contour is crucial for accurate printing. Therefore a calibration tool was developed, which facilitates the alignment of the two light sources. A dichroic coated mirror enables a perpendicular alignment of the DLP® light beam and the laser beam.

A bis(acyl)phosphane oxide (BAPO) photoinitiator was conveniently synthesized in an efficient one-pot process. It shows excellent dispersibility in water, good storage stability, and high photo-reactivity in 3D printing of hydrogels under visible-light irradiation (460 nm).

Ceramic parts manufactured by lithography-based ceramic manufacturing (LCM) excel in resolution and surface quality. The material for LCM is a photosensitive ceramic particle-filled slurry which needs to have homogeneous properties over time and during each processing step. The goal of this study was to use “mechanical” stabilization for a tricalcium phosphate-filled slurry done by increasing slurry viscosity, solids loading, or inducing thixotropic behavior. The modified slurries were compared with a nonstable reference slurry. While all methods lead to increased storage stability, only the stabilized slurry with 0.5 wt% fumed silica is stable during the printing process.

The possibility of manufacturing textured materials and devices, with surface properties controlled from the design stage, instead of being the result of machining processes or chemical attacks, is a key factor for the incorporation of advanced functionalities to a wide set of micro- and nanosystems. High-precision additive manufacturing (AM) technologies based on photopolymerization, together with the use of fractal models linked to computer-aided design tools, allow for a precise definition of final surface properties. However, the polymeric master parts obtained with most commercial systems are usually inadequate for biomedical purposes and their limited strength and size prevents many potential applications. On the other hand, additive manufacturing technologies aimed at the production of final parts, normally based on layer-by-layer melting or sintering ceramic or metallic powders, do not always provide the required precision for obtaining controlled micro-structured surfaces with high-aspect-ratio details. Towards the desired degree of precision and performance, lithography-based ceramic manufacture is a remarkable option, as we discuss in the present study, which presents the development of two different micro-textured biodevices for cell culture. Results show a remarkable control of the surface topography of ceramic parts and the possibility of obtaining design-controlled micro-structured surfaces with high-aspect-ratio micro-metric details.

Recently, vinyl carbonates have been demonstrated to be a versatile alternative to acrylates and methacrylates in biomedical applications as they exhibit photoreactivity and mechanical properties on a level or even above (meth)acrylates. Furthermore, much lower cytotoxicity as well as degradation via a surface erosion mechanism qualify them for medical use. However, it is highly desirable to improve the mechanical properties of vinyl carbonates to reach the performance of PLA. Thus, the main focus of this study lies on designing vinyl carbonates with suitable functional groups that are capable of augmenting mechanical properties of vinyl carbonates, e.g. cyclic structures or urethane groups, and implementing them into the vinyl carbonate structures. Resulting monomers were tested regarding their photoreactivity and cytotoxicity. Furthermore, cured specimens were investigated concerning their mechanical properties. In addition, the thiol-ene reaction was utilized to further improve photoreactivity....

Photocurable vinyl esters have recently been introduced as suitable alternatives to (meth)acrylates in biomedical applications. While (meth)acrylates exhibit good mechanical properties, their cytotoxicity and degradation products principally disqualify them from medical use. Vinyl esters exhibit much lower cytotoxicity and give biocompatible degradation products, but their disadvantage are relatively low mechanical properties, particularly brittleness. This study focuses on the identification of suitable functional groups that are capable of introducing enhanced impact strength into the vinyl ester network, for example, cyclic structures or urethane groups. A new pathway for the synthesis of vinyl esters carrying these groups was established and resulting monomers were tested regarding their photoreactivity and cytotoxicity. Mechanical properties and degradation behavior of the new materials were investigated as well. In addition, the thiol-ene reaction was utilized to enhance photoreactivity and tune hydrolytical degradation. The new vinyl esters exhibit excellent biocompatibility and good photoreactivity that can be significantly enhanced with thiols on to the level of highly photoreactive acrylates. Ultimately, the impact strength was improved by a factor of more than ten compared to commercial vinyl esters. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2016, 54, 1987–1997

Multifunctional methacrylates are highly reactive monomers for radical photopolymerization, but yield brittle materials due to their inhomogeneous and highly crosslinked network architecture. Addition fragmentation chain transfer (AFCT) reagents serve as additives for the regulation of radical network formation and pave the way to photopolymer networks with high toughness. However, AFCT reagents (e.g. β-allyl sulfones) tend to have a negative influence on the reaction speed which limits them for lithography-based 3D fabrication. Vinyl sulfone esters are described as a new class of AFCT reagents for methacrylate-based photopolymerization without the drawback of retardation but good regulation of network architecture. The resulting materials show high network homogeneity, low shrinkage stress, and a significant increase in CC double bond conversion and toughness. This promises great potential for vinyl sulfone esters as AFCT reagents in photopolymer applications. First 3D parts have been successfully fabricated via digital light processing.

Photo-curable resins based on multifunctional acrylate monomers are commonly applied as thin films (e.g. protective coatings, printing inks, etc.) and in recent years are also used for the fabrication of bulk objects such as dental fillings and 3D-printed parts. While rapid curing and good spatial resolution are advantages to these systems, brittleness and poor impact resistance due to inhomogeneous polymer architecture and high crosslink density are serious drawbacks. By comparison, epoxy thermoset resins suffered many years ago from similar problems, but since then are found in ever demanding applications thanks to a variety of approaches to increase polymer toughness. Based on these successes, researchers have tried to translate strategies for toughening epoxy resins to photopolymer networks. Therefore, this review surveys relevant scientific papers and patents on the development of crosslinked epoxy-based polymers and also photo-curable polymers based on multifunctional acrylates with improved toughness. Strategies developed to reduce brittleness include working with monomers, which intrinsically give tougher polymers, particulate additives, and alternate forms of polymerization and polymer architecture (e.g., dual-cure networks, interpenetrating networks, thiol–ene chemistry). All of these strategies have advantages and yet application specific rigours must also be considered before and during formulation development.

Lithography-based Additive Manufacturing Technologies (AMT) exploit the curing of photosensitive materials upon light exposure. We developed a hybrid exposure concept, which is able to overcome the dilemma of providing high surfaces qualities and an enhanced throughput by combining two light sources. Digital Light Processing (DLP) Light Engine (LE) -exposure allows a rapid curing of extended areas. However, round envelopes are approximated by single pixels, which leads subsequently to a spatial approximation and thus to lower surface qualities, whereas laser-based AMT systems enable smooth structures. However, with increasing layer crosssection and smaller laser-spot size the exposure time increases, since single laser lines are applied next to each other in order to provide a fully cured layer. In our system, a laser focused to 20 μm shapes the outline of the item whereas a DLP LE, with a pixel-size of 56 μm, cures the inner area. A dichroic mirror, which reflects the wavelength of the laser and transmits the light of the DLP LE, facilitates the beam-alignment. An online-monitoring camera array guaranties the control of both light sources. Our new technology enables the production of parts which previously could not be produced with traditional manufacturing methods. The layer-by-layer approach enables highly complex structures, leading to a design oriented engineering of items. A system with these specifications provides an alternative to many microinjection molding or injection molding processes for complex structures and small lot sizes.

Additive manufacturing techniques (AMTs) directly benefit from their fast and flexible way of realizing a small or medium number of parts in a cost-effective way. By using automated manufacturing processes, three-dimensional parts with a high degree of geometrical complexity can be directly built from the user-defined computer-aided design data. The complete part is assembled in a layer-based production fashion from the bottom of the structure to its top. The rapidly expanding market for microsystem technologies is also constantly raising the requirements for fast and flexible production of functional micro parts. Stereolithography and laser sintering offer a huge potential for further miniaturization by systematically adapting the right parameters and tailoring the materials. From micro components to functional micro systems with detailed features on the micron level, optimized AMTs enable fabrication on either a polymer or metal basis.

Lithography-based additive manufacturing was introduced in the 1980s, and is still the method of choice for printing accurate plastic parts with high surface quality. Recent progress in this field has made tough photopolymer resins and cheap LED light engines available. This study presents the influence of photoinitiator selection and post-processing on the thermomechanical properties of various tough photopolymers. The influence of three photoinitiators (Ivocerin, BAPO, and TPO-L) on the double-bond conversion and mechanical properties was investigated by mid infrared spectroscopy, dynamic mechanical analysis and tensile tests. It was found that 1.18 wt % TPO-L would provide the best overall results in terms of double-bond conversion and mechanical properties. A correlation between double-bond conversion, yield strength, and glass transition temperature was found. Elongation at break remained high after post-curing at about 80-100%, and was not influenced by higher photoinitiator concentration. Finally, functional parts with 41 MPa tensile strength, 82% elongation at break, and 112 °C glass transition temperature were printed on a 405 nm DLP (digital light processing) printer.

The two-photon absorption (2PA) phenomenon is the basis of many unique applications involving suitable chromophores as photoinitiators. Ideally the 2PA cross section should, therefore, be a unique parameter, allowing quantification and comparing 2PA capabilities of different substances. In this report, the most straightforward and widespread method, the Z-scan technique, was used for determining the 2PA cross-section values of three different synthesized photoinitiators and one laser dye as a standard. It is demonstrated that the experimentally obtained values strongly depend on the molar concentration of a measured solution. A tenfold decrease in substance concentration can lead to the doubling of the 2PA cross-section. A similar concentration dependence was confirmed for all three investigated substances. Among the crucial implications of this observed behavior is the questionable possibility to compare the 2PA characteristics of different compounds based on the values reported in the literature. An example of another important consequence of this effect extends i.e. to the calculation of the dose necessary for killing the tumor cells in 2PA-based photodynamic therapy applications. The possible factors responsible for this contra-intuitive behavior are discussed and investigated. Finally, a reliable measurement protocol for comprehensive characterization of 2PA capability of different substances is proposed. Herewith an attempt to establish a standard method, which takes into account the concentration dependence, is made. This method provides means for faultless comparison of different compounds.

Additive Manufacturing (AM) received a lot of attention in the last years. Organizations are using AM systems for a range of applications such as prototypes for fitting an assembly, tooling components, patterns for prototype tooling, functional parts and many more. Nearly a third is applied for functional parts [1][2]. Hence, the SL method provides a smoother surface finish than other AMT[3]. Not only is the smoother surface a benefit but the good precision is also a positive feature. The ongoing development of new material systems and applications make them suitable alternatives for conventional series production like injection molding or machined-core fabrication for foundry use. Small to middle series cores for faucets with quantities from around 50,000 pieces produced using AM methods are already a reality [1]. From the economical point of view, the SL is a cheap and fast process in comparison to AM systems. The SL technology used in this work is based on an active mask exposure, the digital light processing (DLP). The term DLP refers to the digital mirror devices which are used for selectively tuning individual mirrors on and off in order to selectively expose a photosensitive resin with visible or ultraviolet light. The resin contains a photoinitiator which triggers radical polymerization when irradiated with light. The polymerization process leads to a solidification of the resin, leading finally to a solid polymer part [4]. A digital Mirror Device (DMD) chip acts as a dynamic mask to expose a defined area on the bottom of a transparent material vat above the optical system. The generated picture enables layer-wise polymerization of the photosensitive resin resulting in a 3-dimensional object. The light source radiates light with a wavelength of 460 nm which means blue visible light. At this wavelength the curing takes place. At the Institute of Materials Science and Technology at the Vienna University of Technology six generations of these Blueprinter machines have been developed and built to date. The largest parts that a Blueprinter can currently generate are 110 x 110 x 80 mm3 with a resolution of 25 x 25 x 25 μm3. The wall thickness can go down to four pixels which means one tenth of a millimetre.

Within the large variety of different additive manufacturing technologies stereolithography excels in high precision and surface quality. Using the Digital Light Processing (DLP) Technology a stereolithography-based system was developed, which is specifically designed for the processing of highly filled photopolymers.The powder-filled suspension enables the 3D-fabrication of a so called ceramic green part. In order to get a dense ceramic structure, subsequent thermal processing steps after the 3D-printing process are necessary. First, the polymer-ceramic composites heated up to 400°C. During this processing step, called debinding, the organic components are burned out. The resulting part, consisting of powder particles stabilized by physical interactions, is further heated to sinter the particles together, and the final, fully dense ceramic part is obtained.The debinding step is the most critical process. The used components have different evaporation or decomposition temperatures and behaviors. Thereby a reduction in weight and also in dimension occurs, which depends on the portion and composition of the organic components and especially on the temperature cycle. Furthermore, the physical characteristics of the ceramic powder, such as the particle size and the size distribution influence the debinding behavior. To measure the changes in weight and dimension a thermo-gravimetric (TGA) and a thermo-mechanical analysis (TMA) can be used. To avoid too high internal gas pressures inside the green parts a preferably constant gas evolution rate is seeked. Also the ‘surface-to-volume ratio’ affects the debinding characteristics. Therefore, optimized debinding cycles for specific geometries allow the crack-free debinding of parts with a wall thickness up to 20 mm.

Additive manufacturing (AM) alias 3D printing translates computer-aided design (CAD) virtual 3D models into physical objects. By digital slicing of CAD, 3D scan, or tomography data, AM builds objects layer by layer without the need for molds or machining. AM enables decentralized fabrication of customized objects on demand by exploiting digital information storage and retrieval via the Internet. The ongoing transition from rapid prototyping to rapid manufacturing prompts new challenges for mechanical engineers and materials scientists alike. Because polymers are by far the most utilized class of materials for AM, this Review focuses on polymer processing and the development of polymers and advanced polymer systems specifically for AM. AM techniques covered include vat photopolymerization (stereolithography), powder bed fusion (SLS), material and binder jetting (inkjet and aerosol 3D printing), sheet lamination (LOM), extrusion (FDM, 3D dispensing, 3D fiber deposition, and 3D plotting), and 3D bioprinting. The range of polymers used in AM encompasses thermoplastics, thermosets, elastomers, hydrogels, functional polymers, polymer blends, composites, and biological systems. Aspects of polymer design, additives, and processing parameters as they relate to enhancing build speed and improving accuracy, functionality, surface finish, stability, mechanical properties, and porosity are addressed. Selected applications demonstrate how polymer-based AM is being exploited in lightweight engineering, architecture, food processing, optics, energy technology, dentistry, drug delivery, and personalized medicine. Unparalleled by metals and ceramics, polymer-based AM plays a key role in the emerging AM of advanced multifunctional and multimaterial systems including living biological systems as well as life-like synthetic systems.

Lithography-based additive manufacturing (AM) is increasingly becoming the technology of choice for the small series or single unit production. At the TU Vienna a digital light processing (DLP) system was developed for the fabrication of complex technical ceramics, requiring high levels of detail and accuracy. The DLP-system used in this study creates a ceramic green part by stacking up layers of a photo-curable resin with a solid loading of around 45 vol.% zirconia. After a thermal debinding and sintering step the part turns into a dense ceramic and gains its final properties. The native resolution of the DLP process depends on the light engine's DMD (digital mirror device) chip and the optics employed. Currently it is possible to print 3D-structures with a spatial resolution down to 40 μm. A modification of the light source allows for the customization of the light curing strategy for each pixel of the exposed layers. This work presents methods to improve the geometrical accuracy as well as the structural properties of the final 3D-printed ceramic part by using the full capabilities of the light source. On the one hand, the feasibility to control the dimensional overgrowth to gain resolution below the native resolution of the light engine—a sub-pixel resolution—was evaluated. Overgrowth occurs due to light scattering and was found to be sensitive to both exposure time and exposed area. On the other hand, different light curing strategies (LCSs) and depths of cure (Cd) were used for the 3D-printing of ceramic green parts and their influence on cracks in the final ceramic was evaluated. It was concluded that softstart LCSs, as well as higher values for Cd, reduce cracks in the final ceramic. Applying these findings within the 3D-printing process may be another step toward flawless and highly accurate ceramic parts.

Bioactive glasses and glass ceramics like the 45S5 formulation have been studied toward biocompatibility and biodegradability for years. Nevertheless, clinical applications as bone substitute or scaffold material are highly limited because of the often poor mechanical behavior of bioactive glasses. In this study, we are able to provide a new production alternative for 45S5 bioactive glass structures resulting in parts with high density and strength. Using the stereolithographic ceramic manufacturing (SLCM) process, it is possible to additively produce solid bulk glass ceramics as well as delicate scaffold structures. Recent developments in SLCM slurry preparation together with an appropriate selection of raw materials led to 3D parts with a very homogeneous microstructure and a density of about 2.7 g/cm³. Due to the low number and small size of defects, a high biaxial bending strength of 124 MPa was achieved. Weibull distribution also underlines good process control showing a Weibull modulus of 8.6 and a characteristic strength of 131 MPa for the samples tested here. By reaching bending strength values of natural cortical bone, bioactive glasses processed with SLCM could eventually advance to be an interesting bone substitute material even in load-bearing applications, valuing the huge efforts undertaken to understand their bioactive behavior.

Engineering three-dimensional (3D) hydrogels with well-defined architectures has become increasingly important for tissue engineering and basic research in biomaterials sci- ence. To fabricate 3D hydrogels with (sub)cellular-scale fea- tures, two-photon polymerization (2PP) shows great promise although the technique is limited by the selection of appropri- ate hydrogel precursors. In this study, we report the synthesis of gelatin hydrolysate vinyl esters (GH-VE) and its copolymer- ization with reduced derivatives of bovine serum albumin (acting as macrothiols). Photorheology of the thiol-ene copoly- merization shows a much more rapid onset of polymerization and a higher end modulus in reference to neat GH-VE. This allowed 2PP to provide well-defined and stable hydrogel microstructures. Efficiency of the radical-mediated thiol-vinyl ester photopolymerization allows high 2PP writing speed (as high as 50 mm s 21 ) with low laser power (as low as 20 mW). MTT assays indicate negligible cytotoxicities of the GH-VE macromers and of the thiol-ene hydrogel pellets. Osteosar- coma cells seeded onto GH-VE/BSA hydrogels with different macromer relative ratios showed a preference for hydrogels with higher percentage of GH-VE. This can be attributed both to a favorable modulus and preferable protein environment since gelatin favors cell adhesion and albumin incurs nonspe- cific binding. V C 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2013, 51, 4799-4810

A combination of high internal phase emulsion (HIPE) templating and additive manufacturing technology (AMT) is applied for creating hierarchical porosity within an acrylate and acrylate/ thiol-based polymer network. The photopolymerizable formulation is optimized to produce emulsions with a volume fraction of droplet phase greater than 80 vol%. Kinetic stability of the emulsions is suffi cient enough to withstand in-mold curing or computer-controlled layer-by-layer stereolithography without phase separation. By including macroscale cellular cavities within the build fi le, a level of controlled porosity is created simultaneous to the formation of the porous microstructure of the polyHIPE. The hybrid HIPE–AMT technique thus provides hierarchically porous materials with mechanical properties tailored by the addition of thiol chain transfer agent.

The development of practical two-photon absorption photoinitiators (TPA PIs) has been slow due to their complicated syntheses often reliant on expensive catalysts. These shortcomings have been a critical obstruction for further advances in the promising field of two-photon-induced photopolymerization (TPIP) technology. This paper describes a series of linear and cyclic benzylidene ketone-based two-photon initiators containing double bonds and dialkylamino groups synthesized in one step via classical aldol condensation reactions. Systematic investigations of structure–activity relationships were conducted via quantum-chemical calculations and experimental tests. These results showed that the size of the central ring significantly affected the excited state energetics and emission quantum yields as well as the two-photon initiation efficiency. In the TPIP tests the 4-methylcyclohexanone-based initiator displayed much broader ideal processing windows than its counterparts with a central five-membered ring and previously described highly active TPA PIs. Surprisingly, a writing speed as high as 80 mm/s was obtained for the microfabrication of complex 3D structures employing acrylate-based formulations. These highly active TPA PIs also exhibit excellent thermal stability and remain inert to one-photon excitation. Straightforward synthesis combined with high TPA initiation efficiency makes these novel initiators promising candidates for commercialization.

The natural extracellular matrix (ECM) represents a complex and dynamic environment. It provides numerous spatio-temporal signals mediating many cellular functions including morphogenesis, adhesion, proliferation and differentiation. The cell–ECM interaction is bidirectional. Cells dynamically receive and process information from the ECM and remodel it at the same time. Theses complex interactions are still not fully understood. For better understanding, it is indispensable to deconstruct the ECM up to the point of investigating isolated characteristics and cell responses to physical, chemical and topographical cues. Two-photon polymerization (2PP) allows the exact reconstruction of cell specific sites in 3D at micro- and nanometer precision. Processing biocompatible synthetic and naturally-derived hydrogels, the microenvironment of cells can be designed to specifically investigate their behavior in respect to key chemical, mechanical and topographical attributes. Moreover, 3D manipulation can be performed in the presence of cells, guiding biological tissue formation in all stages of its development. Here, advances in 2PP microfabrication of synthetic and naturally based hydrogels are reviewed. Key components of photopolymerizable hydrogel precursors, their structure–property relationships and their polymerization mechanisms are presented. Furthermore, it is shown how biocompatible 2PP fabricated constructs can act as biologically relevant matrices to study cell functions and tissue development.