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.

This review surveys relevant scientific papers and patents on the development of crosslinked epoxies and also photo-curable polymers based on multifunctional acrylates with improved toughness.

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.

Stereolithography (SLA) is a widely used technique for the fabrication of prototypes and small series products. The main advantage of SLA and related solid freeform fabrication (SFF) techniques is their capability to fabricate parts with complex shapes with high resolution. Although the spectrum of available materials has been widened in recent years, there is still a lack of materials which can be processed with SLA on a routine basis. In this work, a micro-SLA (μSLA) system is presented which can shape a number of different photopolymers with resolutions down to 5 μm in the xy-plane and 10 μm in the z-direction. The system is capable of processing various specifically tailored photopolymers which are based on acrylate chemistry. The materials processed for this work range from hybrid sol-gel materials (ORMOCER) to photo-crosslinked elastomers and hydrogels. The elastic moduli of these materials can be tuned over several orders of magnitude and range from 0.1 MPa to 8000 MPa. The reactivity of these monomers is sufficient for achieving writing speeds up to 500 mm s −1 which is comparable to commercial SLA resins. Various test structures are presented which show the suitability of the process for fabricating parts required for applications in micro-mechanical systems as well as for applications in biomedical engineering. Using the presented system, internal channels with a diameter of 50 μm and a length of 1500 μm could be fabricated. It was also possible to manufacture a micro-mechanical system consisting of a fixed axe and a free spinning turbine wheel.

The two-photon polymerization (2PP) of photosensitive gelatin in the presence of living cells is reported. The 2PP technique is based on the localized cross-linking of photopolymers induced by femtosecond laser pulses. The availability of water-soluble photoinitiators (PI) suitable for 2PP is crucial for applying this method to cell-containing materials. Novel PIs developed by our group allow 2PP of formulations with up to 80% cell culture medium. The cytocompatibility of these PIs was evaluated by an MTT assay. The results of cell encapsulation by 2PP show the occurrence of cell damage within the laser-exposed regions. However, some cells located in the immediate vicinity and even within the 2PP-produced structures remain viable and can further proliferate. The control experiments demonstrate that the laser radiation itself does not damage the cells at the parameters used for 2PP. On the basis of these findings and the reports by other groups, we conclude that such localized cell damage is of a chemical origin and can be attributed to reactive species generated during 2PP. The viable cells trapped within the 2PP structures but not exposed to laser radiation continued to proliferate. The live/dead staining after 3 weeks revealed viable cells occupying most of the space available within the 3D hydrogel constructs. While some of the questions raised by this study remain open, the presented results indicate the general practicability of 2PP for 3D processing of cell-containing materials. The potential applications of this highly versatile approach span from precise engineering of 3D tissue models to the fabrication of cellular microarrays.

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

Hydrogels are polymeric materials with water contents similar to that of soft tissues. Due to their biomimetic properties, they have been extensively used in various biomedical applications including cell encapsulation for tissue engineering. The utilization of photopolymers provides a possibility for the temporal and spatial controlling of hydrogel cross-links. We produced three-dimensional (3-D) hydrogel scaffolds by means of the two-photon polymerization (2PP) technique. Using a highly efficient water-soluble initiator, photopolymers with up to 80 wt.% water were processed with high precision and reproducibility at a writing speed of 10  mm/s. The biocompatibility of the applied materials was verified using Caenorhabditis elegans as living test organisms. Furthermore, these living organisms were successfully embedded within a 200×200×35 μm³ hydrogel scaffold. As most biologic tissues exhibit a window of transparency at the wavelength of the applied femtosecond laser, it is suggested that 2PP is promising for an in situ approach. Our results demonstrate the feasibility of and potential for bio-fabricating 3-D tissue constructs in the micrometre-range via near-infrared lasers in direct contact with a living organism.

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