Synthesis of Photopolymerizable Hydrophilic Macromers and Evaluation of Their Applicability as Reactive Resin Components for the Fabrication of Three‐Dimensionally Structured Hydrogel Matrices by 2‐Photon‐Polymerization

Two‐photon polymerization (2‐PP) is a promising new photolithographic technique to fabricate three‐dimensional (3D), micro‐ and nano‐structured tissue engineering scaffolds from photopolymerizable monomers. Although various photo resins are known for the use in 2‐PP, there is currently a need for photo‐curable monomers processable by 2‐PP to generate biocompatible 3D‐structured hydrogel materials for soft or cartilage tissue regeneration. In the present work hydrophilic methacrylate monomers and macromers based on synthetic poly(glycerine) and poly(ethylene glycol) urethanes as well as on the biopolymers dextran and hyaluronan is prepared. The photopolymerization behavior of these substances are investigated and formed hydrogel networks are studied with regard to their mechanical properties, cytocompatibility, and hydrolytic degradation. Based on these examinations simple 3D model structures are fabricated from these photo‐curable monomers and macromers by 2‐PP. It is shown that both the synthetic monomers and the dextran methacrylate macromer are efficient 2‐PP starting materials whereas the hyaluronan methacrylate can be used for 2‐PP only in combination with suitable water‐soluble co‐monomers. No cytotoxic effects are found in preliminary chondrocyte cultivation experiments on 2‐PP‐fabricated scaffolds but initial cell adhesion on the hydrophilic scaffold surfaces is rather low and has to be further improved to apply these structures in tissue engineering.     

Three‐Dimensional Printing of Multifunctional Nanocomposites: Manufacturing Techniques and Applications

The integration of nanotechnology into three‐dimensional printing (3DP) offers huge potential and opportunities for the manufacturing of 3D engineered materials exhibiting optimized properties and multifunctionality. The literature relating to different 3DP techniques used to fabricate 3D structures at the macro‐ and microscale made of nanocomposite materials is reviewed here. The current state‐of‐the‐art fabrication methods, their main characteristics (e.g., resolutions, advantages, limitations), the process parameters, and materials requirements are discussed. A comprehensive review is carried out on the use of metal‐ and carbon‐based nanomaterials incorporated into polymers or hydrogels for the manufacturing of 3D structures, mostly at the microscale, using different 3D‐printing techniques. Several methods, including but not limited to micro‐stereolithography, extrusion‐based direct‐write technologies, inkjet‐printing techniques, and popular powder‐bed technology, are discussed. Various examples of 3D nanocomposite macro‐ and microstructures manufactured using different 3D‐printing technologies for a wide range of domains such as microelectromechanical systems (MEMS), lab‐on‐a‐chip, microfluidics, engineered materials and composites, microelectronics, tissue engineering, and biosystems are reviewed. Parallel advances on materials and techniques are still required in order to employ the full potential of 3D printing of multifunctional nanocomposites.     

Influence of the Thermodynamic and Kinetic Control of Self‐Assembly on the Microstructure Evolution of Silk‐Elastin‐Like Recombinamer Hydrogels

Complex recombinant biomaterials that merge the self‐assembling properties of different (poly)peptides provide a powerful tool for the achievement of specific structures, such as hydrogel networks, by tuning the thermodynamics and kinetics of the system through a tailored molecular design. In this work, elastin‐like (EL) and silk‐like (SL) polypeptides are combined to obtain a silk‐elastin‐like recombinamer (SELR) with dual self‐assembly. First, EL domains force the molecule to undergo a phase transition above a precise temperature, which is driven by entropy and occurs very fast. Then, SL motifs interact through the slow formation of β‐sheets, stabilized by H‐bonds, creating an energy barrier that opposes phase separation. Both events lead to the development of a dynamic microstructure that evolves over time (until a pore size of 49.9 ± 12.7 µm) and to a delayed hydrogel formation (obtained after 2.6 h). Eventually, the network is arrested due to an increase in β‐sheet secondary structures (up to 71.8 ± 0.8%) within SL motifs. This gives a high bond strength that prevents the complete segregation of the SELR from water, which results in a fixed metastable microarchitecture. These porous hydrogels are preliminarily tested as biomimetic niches for the isolation of cells in 3D cultures.     

Biotinylated Photopolymers for 3D‐Printed Unibody Lab‐on‐a‐Chip Optical Platforms

The present work reports the first demonstration of straightforward fabrication of monolithic unibody lab‐on‐a‐chip (ULOCs) integrating bioactive micrometric 3D scaffolds by means of multimaterial stereolithography (SL). To this end, a novel biotin‐conjugated photopolymer is successfully synthesized and optimally formulated to achieve high‐performance SL‐printing resolution, as demonstrated by the SL‐fabrication of biotinylated structures smaller than 100 µm. By optimizing a multimaterial single‐run SL‐based 3D‐printing process, such biotinylated microstructures are incorporated within perfusion microchambers whose excellent optical transparency enables real‐time optical microscopy analyses. Standard biotin‐binding assays confirm the existence of biotin‐heads on the surfaces of the embedded 3D microstructures and allow to demonstrate that the biofunctionality of biotin is not altered during the SL‐printing, thus making it exploitable for further conjugation with other biomolecules. As a step forward, an in‐line optical detection system is designed, prototyped via SL‐printing and serially connected to the perfusion microchambers through customized world‐to‐chip connectors. Such detection system is successfully employed to optically analyze the solution flowing out of the microchambers, thus enabling indirect quantification of the concentration of target interacting biomolecules. The successful application of this novel biofunctional photopolymer as SL‐material enables to greatly extend the versatility of SL to directly fabricate ULOCs with intrinsic biofunctionality.     

Graphoepitaxial Directed Self‐Assembly of Polystyrene‐Block‐Polydimethylsiloxane Block Copolymer on Substrates Functionalized with Hexamethyldisilazane to Fabricate Nanoscale Silicon Patterns

In block copolymer (BCP) nanolithography, microphase separated polystyrene‐block‐polydimethylsiloxane (PS‐b‐PDMS) thin films are particularly attractive as they can form small features and the two blocks can be readily differentiated during pattern transfer. However, PS‐b‐PDMS is challenging because the chemical differences in the blocks can result in poor surface‐wetting, poor pattern orientation control and structural instabilities. Usually the interfacial energies at substrate surface are engineered with the use of a hydroxyl‐terminated polydimethylsiloxane (PDMS‐OH) homopolymer brush. Herein, we report a facile, rapid and tuneable molecular functionalization approach using hexamethyldisilazane (HMDS). The work is applied to both planar and topographically patterned substrates and investigation of graphoepitaxial methods for directed self‐assembly and long‐range translational alignment of BCP domains is reported. The hexagonally arranged in‐plane and out‐of‐plane PDMS cylinders structures formed by microphase separation were successfully used as on‐chip etch masks for pattern transfer to the underlying silicon substrate. The molecular approach developed here affords significant advantages when compared to the more usual PDMS‐OH brushes used.     

Self‐Assembly of Shaped Nanoparticles into Free‐Standing 2D and 3D Superlattices

This article describes a novel supramolecular assembly‐mediated strategy for the organization of Au nanoparticles (NPs) with different shapes (e.g., spheres, rods, and cubes) into large‐area, free‐standing 2D and 3D superlattices. This robust approach involves two major steps: (i) the organization of polymer‐tethered NPs within the assemblies of supramolecular comblike block copolymers (CBCPs), and (ii) the disassembly of the assembled CBCP structures to produce free‐standing NP superlattices. It is demonstrated that the crystal structures and lattice constants of the superlattices can be readily tailored by varying the molecular weight of tethered polymers, the volume fraction of NPs, and the matrix of CBCPs. This template‐free approach may open a new avenue for the assembly of NPs into 2D and 3D structures with a wide range of potential applications.     

3D Printing of Textiles: Potential Roadmap to Printing with Fibers

3D printing (3DP) has transformed engineering, manufacturing, and the use of advanced materials due to its ability to produce objects from a variety of materials, ranging from soft polymers to rigid ceramics. 3DP offers the advantage of being able to print at a variety of lengths scales; from a few micrometers to many meters. 3DP has the unique ability to produce customized small lots, efficiently. Yet, one crucial industry that has not been able to adequately explore its potential is textile manufacturing. The research in 3DP of textiles has lagged behind other areas primarily due to the difficulty in obtaining some of the unique characteristics of strength, flexibility, etc., of textiles, utilizing a fundamentally different manufacturing technology. Textiles are their own class of materials due to the specific structural developments that occur during the various stages of textile manufacturing: from fiber extrusion to assembly of the fibers to fabrics. Here, the current 3DP technologies are reviewed with emphasis on soft and anisotropic structures, as well as the efforts toward 3DP of textiles. Finally, a potential pathway to 3DP of textiles, dubbed as printing with fibers to create textile structures is proposed for further exploration.     

Automated Optical‐Tweezers Assembly of Engineered Microgranular Crystals

In this work, a scalable automated approach for fabricating 3D microgranular crystals consisting of desired arrangements of microspheres using holographic optical tweezers and two‐photon polymerization is introduced. The ability to position microspheres as desired within lattices of any configuration allows designers to engineer the behavior of new metamaterials that enable advanced applications (e.g., armor that mitigates or redirects shock waves, acoustic lens for underwater imaging, damage detection, and noninvasive surgery, acoustic cloaking, and photonic crystals). Currently, no self‐assembly or automated approaches exist with the flexibility necessary to place specific microspheres at specific locations within a crystal. Moreover, most pick‐and‐place approaches require the manual assembly of spheres one by one and thus do not achieve the speed and precision required to repeatably fabricate practical volumes of engineered crystals. In this paper, the rapid assembly of 4.86 µm diameter silica spheres within differently packed 3D crystal‐lattice examples of unprecedented size using fully automated optical tweezers is demonstrated. The optical tweezers independently and simultaneously assemble batches of spheres that are dispensed to the build site via an automated syringe pump where the spheres are then joined together within previously unattainable patterns by curing regions of photocurable prepolymer between each sphere using two‐photon polymerization.     

Virus‐Templated Plasmonic Nanoclusters with Icosahedral Symmetry via Directed Self‐Assembly

The assembly of plasmonic nanoparticles with precise spatial and orientational order may lead to structures with new electromagnetic properties at optical frequencies. The directed self‐assembly method presented controls the interparticle‐spacing and symmetry of the resulting nanometer‐sized elements in solution. The self‐assembly of three‐dimensional (3D), icosahedral plasmonic nanosclusters (NCs) with resonances at visible wavelengths is demonstrated experimentally. The ideal NCs consist of twelve gold (Au) nanospheres (NSs) attached to thiol groups at predefined locations on the surface of a genetically engineered cowpea mosaic virus with icosahedral symmetry. In situ dynamic light scattering (DLS) measurements confirm the NSs assembly on the virus. Transmission electron micrographs (TEM) demonstrate the ability of the self‐assembly method to control the nanoscopic symmetry of the bound NSs, which reflects the icosahedral symmetry of the virus. Both, TEM and DLS show that the NCs comprise of a distribution of capsids mostly covered (i.e., 6–12 NS/capsid) with NSs. 3D finite‐element simulations of aqueous suspensions of NCs reproduce the experimental bulk absorbance measurements and major features of the spectra. Simulations results show that the fully assembled NCs give rise to a 10‐fold surface‐averaged enhancement of the local electromagnetic field.     

Remotely Triggered Assembly of 3D Mesostructures Through Shape‐Memory Effects

3D structures that incorporate high‐performance electronic materials and allow for remote, on‐demand 3D shape reconfiguration are of interest for applications that range from ingestible medical devices and microrobotics to tunable optoelectronics. Here, materials and design approaches are introduced for assembly of such systems via controlled mechanical buckling of 2D precursors built on shape‐memory polymer (SMP) substrates. The temporary shape fixing and recovery of SMPs, governed by thermomechanical loading, provide deterministic control over the assembly and reconfiguration processes, including a range of mechanical manipulations facilitated by the elastic and highly stretchable properties of the materials. Experimental demonstrations include 3D mesostructures of various geometries and length scales, as well as 3D aquatic platforms that can change trajectories and release small objects on demand. The results create many opportunities for advanced, programmable 3D microsystem technologies.     

Patterning of Nanoparticle‐Based Aerogels and Xerogels by Inkjet Printing

Nanoparticle‐based voluminous 3D networks with low densities are a unique class of materials and are commonly known as aerogels. Due to the high surface‐to‐volume ratio, aerogels and xerogels might be suitable materials for applications in different fields, e.g. photocatalysis, catalysis, or sensing. One major difficulty in the handling of nanoparticle‐based aerogels and xerogels is the defined patterning of these structures on different substrates and surfaces. The automated manufacturing of nanoparticle‐based aerogel‐ or xerogel‐coated electrodes can easily be realized via inkjet printing. The main focus of this work is the implementation of the standard nanoparticle‐based gelation process in a commercial inkjet printing system. By simultaneously printing semiconductor nanoparticles and a destabilization agent, a 3D network on a conducting and transparent surface is obtained. First spectro‐electrochemical measurements are recorded to investigate the charge–carrier mobility within these 3D semiconductor‐based xerogel networks.     

A novel quasicrystal-resin composite for stereolithography

Laser stereolithography (SL) is an additive manufacturing technology which is increasingly being used to produce customized end-user parts of any complex shape. It requires the use of a photo-curable resin which can be loaded with ceramic powders or carbon fibers to produce composite parts. However, the range of available materials compatible with the SL process is rather limited. In particular, photo-curable resins reinforced by metal particles are difficult to process, because of fundamental limitations related to the high reflectivity of intermetallics in the UV–visible range. In this work, the unique properties of Al-based quasicrystalline alloys are being used to develop a new UV-curable resin reinforced by such metal particles. The optical properties of the quasicrystalline particles and of the filled resin are studied and they are found to be compatible with the SL process. The volume fraction of the filler particles in the liquid resin is optimized to increase the polymerization depth while preserving suitable rheological behaviour. Finally, 3D composite parts are being built by SL. The composite parts have improved mechanical properties compared to the unfilled resin (higher hardness, reduced wear losses and lower friction coefficient) and compete favourably with the other commercial photo-curable resins.     

Amino Acids and Peptide‐Based Supramolecular Hydrogels for Three‐Dimensional Cell Culture

Supramolecular hydrogels assembled from amino acids and peptide‐derived hydrogelators have shown great potential as biomimetic three‐dimensional (3D) extracellular matrices because of their merits over conventional polymeric hydrogels, such as non‐covalent or physical interactions, controllable self‐assembly, and biocompatibility. These merits enable hydrogels to be made not only by using external stimuli, but also under physiological conditions by rationally designing gelator structures, as well as in situ encapsulation of cells into hydrogels for 3D culture. This review will assess current progress in the preparation of amino acids and peptide‐based hydrogels under various kinds of external stimuli, and in situ encapsulation of cells into the hydrogels, with a focus on understanding the associations between their structures, properties, and functions during cell culture, and the remaining challenges in this field. The amino acids and peptide‐based hydrogelators with rationally designed structures have promising applications in the fields of regenerative medicine, tissue engineering, and pre‐clinical evaluation.     

Laminated Object Manufacturing of 3D‐Printed Laser‐Induced Graphene Foams

Laser‐induced graphene (LIG), a graphene structure synthesized by a one‐step process through laser treatment of commercial polyimide (PI) film in an ambient atmosphere, has been shown to be a versatile material in applications ranging from energy storage to water treatment. However, the process as developed produces only a 2D product on the PI substrate. Here, a 3D LIG foam printing process is developed on the basis of laminated object manufacturing, a widely used additive‐manufacturing technique. A subtractive laser‐milling process to yield further refinements to the 3D structures is also developed and shown here. By combining both techniques, various 3D graphene objects are printed. The LIG foams show good electrical conductivity and mechanical strength, as well as viability in various energy storage and flexible electronic sensor applications.     

Molecular Modelling of Bone Morphogenetic Protein‐2 (BMP‐2) by 3D‐Rapid Prototyping

Bone morphogenetic proteins (BMP) play a decisive role in bone development and osteogenesis. In the past they have been the subject of widespread research and clinical trials as stimulants of bone growth. Recently BMP‐2 has been chemically immobilized on implant surfaces leading to enhanced bone growth and accelerated integration in sheep. Although the 3D‐structure of BMP‐2 is known the surface topography has not been the subject of a detailed analysis. Therefore we have begun implementing the technique of 3D‐rapid prototyping as a novel method for gaining topographical information on the structure‐function relationship of proteins (Laub et al., 2001, FASEB J. 15, A543). 3D‐rapid prototyping allows the construction of accurate three‐dimensional models of proteins based on their x‐ray crystallographic data. In this way we constructed a 3D scale image of BMP‐2 of the size 140 mm × 70 mm × 50 mm corresponding to a ca. 20 × 10⁶ fold magnification (scale 1 nm = 2 cm). BMP‐2 is a twisted banana‐shaped molecule consisting of a convex and a concave face and has a horn‐like protuberance cross‐turned at 180° (long axis) at each end. In the center of the convex face there is a ca. 1 nm deep crater like pit ca. 1.8 nm in diameter. The concave face is characterized by a 6–7 nm long helical groove 0.8–1.6 nm wide and ca 0.8 nm deep, into which a left‐handed helix with a pitch of 8–9 nm and a helical radius of 0.35–0.45 nm can be fitted. The concave face of BMP‐2 therefore corresponds to an imprint (groove) of a left‐handed helix i. e. to an anti‐helix or anthelix. The possible endogenous ligands and functions of these structures are unknown. These results demonstrate that full scale 3D molecular models of proteins can lead to new perceptions in understanding the interactions between ligands and proteins by macroscopic viewing and in‐hand fitting of the molecules without the aid of a computer.     

Unraveling the Solution‐State Supramolecular Structures of Donor–Acceptor Polymers and their Influence on Solid‐State Morphology and Charge‐Transport Properties

Polymer self‐assembly in solution prior to film fabrication makes solution‐state structures critical for their solid‐state packing and optoelectronic properties. However, unraveling the solution‐state supramolecular structures is challenging, not to mention establishing a clear relationship between the solution‐state structure and the charge‐transport properties in field‐effect transistors. Here, for the first time, it is revealed that the thin‐film morphology of a conjugated polymer inherits the features of its solution‐state supramolecular structures. A “solution‐state supramolecular structure control” strategy is proposed to increase the electron mobility of a benzodifurandione‐based oligo(p‐ phenylene vinylene) (BDOPV)‐based polymer. It is shown that the solution‐state structures of the BDOPV‐based conjugated polymer can be tuned such that it forms a 1D rod‐like structure in good solvent and a 2D lamellar structure in poor solvent. By tuning the solution‐state structure, films with high crystallinity and good interdomain connectivity are obtained. The electron mobility significantly increases from the original value of 1.8 to 3.2 cm² V^?1 s^?1. This work demonstrates that “solution‐state supramolecular structure” control is critical for understanding and optimization of the thin‐film morphology and charge‐transport properties of conjugated polymers.     

Biomaterial‐Based “Structured Opals” with Programmable Combination of Diffractive Optical Elements and Photonic Bandgap Effects

Naturally occurring iridescent systems produce brilliant color displays through multiscale, hierarchical assembly of structures that combine reflective, diffractive, diffusive, or absorbing domains. The fabrication of biopolymer‐based, hierarchical 3D photonic crystals through the use of a topographical templating strategy that allows combined optical effects derived from the interplay of predesigned 2D and 3D geometries is reported here. This biomaterials‐based approach generates 2D diffractive optics composed of 3D nanophotonic lattices that allow simultaneous control over the reflection (through the 3D photonic bandgap) and the transmission (through 2D diffractive structuring) of light with the additional utility of being constituted by a biocompatible, implantable, edible commodity textile material. The use of biopolymers allows additional degrees of freedom in photonic bandgap design through directed protein conformation modulation. Demonstrator structures are presented to illustrate the lattice multifunctionality, including tunable diffractive properties, increased angle of view of photonic crystals, color‐mixing, and sensing applications.     

3D Printed High‐Performance Lithium Metal Microbatteries Enabled by Nanocellulose

Batteries constructed via 3D printing techniques have inherent advantages including opportunities for miniaturization, autonomous shaping, and controllable structural prototyping. However, 3D‐printed lithium metal batteries (LMBs) have not yet been reported due to the difficulties of printing lithium (Li) metal. Here, for the first time, high‐performance LMBs are fabricated through a 3D printing technique using cellulose nanofiber (CNF), which is one of the most earth‐abundant biopolymers. The unique shear thinning properties of CNF gel enables the printing of a LiFePO₄ electrode and stable scaffold for Li. The printability of the CNF gel is also investigated theoretically. Moreover, the porous structure of the CNF scaffold also helps to improve ion accessibility and decreases the local current density of Li anode. Thus, dendrite formation due to uneven Li plating/stripping is suppressed. A multiscale computational approach integrating first‐principle density function theory and a phase‐field model is performed and reveals that the porous structures have more uniform Li deposition. Consequently, a full cell built with a 3D‐printed Li anode and a LiFePO₄ cathode exhibits a high capacity of 80 mA h g^?1 at a charge/discharge rate of 10 C with capacity retention of 85% even after 3000 cycles.     

Hydrophobic,Electrostatic, and Dynamic Polymer Forces at Silicone Surfaces Modified with Long‐Chain Bolaform Surfactants

Surfactant self‐assembly on surfaces is an effective way to tailor the complex forces at and between hydrophobic‐water interfaces. Here, the range of structures and forces that are possible at surfactant‐adsorbed hydrophobic surfaces are demonstrated: certain long‐chain bolaform surfactants—containing a polydimethylsiloxane (PDMS) mid‐block domain and two cationic α, ω‐quarternary ammonium end‐groups—readily adsorb onto thin PDMS films and form dynamically fluctuating nanostructures. Through measurements with the surface forces apparatus (SFA), it is found that these soft protruding nanostructures display polymer‐like exploration behavior at the PDMS surface and give rise to a long‐ranged, temperature‐ and rate‐dependent attractive bridging force (not due to viscous forces) on approach to a hydrophilic bare mica surface. Coulombic interactions between the cationic surfactant end‐groups and negatively‐charged mica result in a rate‐dependent polymer bridging force during separation as the hydrophobic surfactant mid‐blocks are pulled out from the PDMS interface, yielding strong adhesion energies. Thus, (i) the versatile array of surfactant structures that may form at hydrophobic surfaces is highlighted, (ii) the need to consider the interaction dynamics of such self‐assembled polymer layers is emphasized, and (iii) it is shown that long‐chain surfactants can promote robust adhesion in aqueous solutions.