Binders used in binder jetting often pose health and environmental risks during processing and post processing operations. The print-heads which are used to deposit binder selectively on the feedstock are prone to clogging, despite the trend of print-heads being highly customised to suit different kinds of binders. These factors often hide the advantages of binder jetting as an additive manufacturing process, especially its scalability and its faster printing rates in comparison to powder bed fusion methods. The work presented here takes a step back and focuses on the development of an aqueous, polyvinyl alcohol (PVA)-based liquid binder that is easy to manufacture and store, safe to handle, and can be reliably jetted to print parts. The feedstock considered was Inconel 718, a nickel-based super alloy which can be effectively processed by binder jetting without niobium segregation. PVA was added to the Inconel 718 powder in dry, granular form to manufacture a modified feedstock. The study also investigated the role of molecular weight of the PVA used, sintering environments and post-processing methods like hot isostatic pressing (HIP) on process responses like part densification, tensile strength, and hardness. Three different types of PVA were chosen which had molecular weights 10,000 g/mol (low molecular weight or LMW), 26,000 g/mol (medium molecular weight or MMW), and 84,000 g/mol (high molecular weight or HMW). The compatibility of the liquid, aqueous PVA-based binders with virgin Inconel 718 was examined by measuring the contact angle. The liquid, aqueous binder having MMW PVA reported better wetting with the Inconel 718 powder with a wetting angle of 26.6 which was lower than the wetting angle of 42.4°, seen in case of a commercial resin-based binder. The green strength reported by the MMW PVA liquid binder was 220 kPa which was higher than the other two PVA-based liquid binders. Green parts, upon successful printing, were sintered at 1260 °C. It was observed that a part printed using MMW PVA had a densification of 96.16

Inkjet printing of magnetic materials has increased in recent years, as it has the potential to improve research in smart, functional materials. Magnetostriction is an inherent property of magnetic materials which allows strain or magnetic fields to be detected. This makes it very attractive for sensors in the area of structural health monitoring by detecting internal strains in carbon fibre-reinforced polymer (CFRP) composite. Inkjet printing offers design flexibility for these sensors to influence the magnetic response to the strain. This allows the sensor to be tailored to suit the location of defects in the CFRP. This research has looked into the viability of printable soft magnetic materials for structural health monitoring (SHM) of CFRP. Magnetite and nickel ink dispersions were selected to print using the JetLab 4 drop-on-demand technique. The printability of both inks was tested by selecting substrate, viscosity and solvent evaporation. Clogging was found to be an issue for both ink dispersions. Sonicating and adjusting the jetting parameters helped in distributing the nanoparticles. We found that magnetite nanoparticles were ideal as a sensor as there is more than double increase in saturation magnetisation by 49 Am2/kg and more than quadruple reduction of coercive field of 5.34 kA/m than nickel. The coil design was found to be the most sensitive to the field as a function of strain, where the gradient was around 80% higher than other sensor designs. Additive layering of 10, 20 and 30 layers of a magnetite square patch was investigated, and it was found that the 20-layered magnetite print had an improved field response to strain while maintaining excellent print resolution. SHM of CFRP was performed by inducing a strain via bending and it was found that the magnetite coil detected a change in field as the strain was applied.

Reactive inkjet printing (RIJ) is demonstrated as a new approach to the patterning of surfaces with metal–organic frameworks (MOFs). RIJ is an emerging manufacturing technique that jets solutions of reagents onto a substrate allowing them to react in situ to form the desired material. MOFs have the potential to perform a variety of useful sensing, catalytic, separation and storage applications within sophisticated devices, however, their insolubility makes them challenging to process into complex shapes and patterns. The RIJ approach offers advantages over conventional inkjet printing in that it allows stable solutions of different ligand and metal ions to be combined in a “mix‐and‐match” way. Here, the benefits of the RIJ approach are demonstrated to optimize the stoichiometry of the printed MOF, print a variety of different frameworks using common inks, and create gradients where the composition of the printed MOFs gradually varies between one isoreticular structure and another. Proof of principle is also demonstrated for the approach by demonstrating size selective encapsulation of a dye within a RIJ printed MOF. It is anticipated that this approach will be broadly applicable to the printing of MOFs and related materials enhancing their use across a variety of different applications.

Nature uses proteins as building blocks to create three-dimensional (3D) structural components (like spiderwebs and tissue) that are recycled within a closed loop. Furthermore, it is difficult to replicate the mechanical properties of these 3D architectures within synthetic systems. In the absence of biological machinery, protein-based materials can be difficult to process and can have a limited range of mechanical properties. Herein, we present an additive manufacturing workflow to fabricate tough, protein-based composite hydrogels and bioplastics with a range of mechanical properties. Briefly, methacrylated bovine-serum-albumin-based aqueous resins were 3D-printed using a commercial vat photopolymerization system. The printed structures were then treated with tannic acid to introduce additional non-covalent interactions and form tough hydrogels. The hydrogel material could be sutured and withstand mechanical load, even after immersion in water for 24 h. Additionally, a denaturing thermal cure could be used to virtually eliminate rehydration of the material and form a bioplastic. To highlight the functionality of this material, a bioplastic screw was 3D-printed and driven into wood without damage to the screw. Moreover, the 3D-printed constructs enzymatically degraded up to 85% after 30 days in pepsin solution. Thus, these protein-based 3D-printed constructs show great potential for biomedical devices that degrade in situ.

Additive manufacturing (AM) of carbon fibre reinforced thermoplastic composites can offer advantages over traditional carbon fibre manufacturing through improved design freedom and reduction in production time and cost. However, the carbon fibre composites produced using current state-of-the-art AM approaches generally possess high porosity (18-25 %) compared to those produced by conventional manufacturing (1 %). An approach known as composite fibre additive manufacturing (CFAM) is presented, involving selectively printing a binder and polymer powder onto discontinuous carbon fibre sheets, which are then compressed, heated and post -processed to form net shape components. The results demonstrate a correlation between compaction pressure applied and porosity/fibre volume fraction within components. Composite components were produced con-taining porosity of 1.5 % and fibre volume content of 15 % with 97 MPa tensile strength and 8.9 GPa elastic modulus, presenting a new approach for production of discontinuous carbon fibre reinforced polymer parts with mechanical properties exceeding those of state-of-the-art AM.

The salting effect, of kosmotropic and chaotropic salts, on nanoscale and giant-poly(ethylene oxide) 16 -block-poly(butylene oxide) 22 polymersomes was studied. Nanovesicles were studied by dynamic light scattering and transmission electron microscopy and their aggregates ( i.e. , gel-like structures) by optical microscopy. Giant polymersomes were formed using drop-on-demand inkjet printing and subsequently observed using an optical microscope. The kosmotropic salts were found to strongly influence both vesicle aggregation and giant vesicle (above 3 µm) formation at high salt concentrations ( i.e. , ∼0.5 M for nanovesicle aggregation and 0.23 M for giant polymersomes) where the salting effect dominates. Chaotropic salts have very little effect in both systems. The ionic specificity effect essentially determined the outcome of experiments since the type of anion provides different ion/water/poly(ethylene oxide) interactions and different osmotic stress values, which can be considered the main driving forces and lead the effects observed in experiments (nanovesicle aggregate experiments and abundant giant vesicle formation experiments). The applications of the present work may be lead to better methodologies to produce gel scaffolds for biomaterials or giant unilamellar polymersomes.

Stimuli responsive hydrogels that can change shape in response to applied external stimuli are appealing for soft robotics, biomedical devices, drug delivery, and actuators. However, existing 3D printed shape morphing materials are non‐biodegradable, which limits their use in biomedical applications. Here, 3D printed protein‐based hydrogels are developed and applied for programmable structural changes under the action of temperature, pH, or an enzyme. Key to the success of this strategy is the use of methacrylated bovine serum albumin (MA–BSA) as a biodegradable building block to Pickering emulsion gels in the presence of N ‐isopropylacrylamide or 2‐dimethylaminoethyl methacrylate. These shear‐thinning gels are ideal for direct ink write (DIW) 3D printing of multi‐layered stimuli‐responsive hydrogels. While poly( N ‐isopropylacrylamide) and poly(dimethylaminoethyl methacrylate) introduce temperature and pH‐responsive properties into the printed objects, a unique feature of this strategy is an enzyme‐triggered shape transformation based on the degradation of the bovine serum albumin network. To highlight this technique, protein‐based hydrogels that reversibly change shape based on environmental temperature and pH are fabricated, and irreversibly altered by enzymatic degradation, which demonstrates the complexity that can be introduced into 4D printed systems.

Structural health monitoring (SHM) represents the next generation of carbon fiber‐reinforced composite nondestructive testing. One challenge facing the application of magnetostrictive SHM is the lightweighting and ease of installation of actuators and sensors. Inkjet printing (IJP) technology is well suited to produce miniaturized electronic induction sensors that can be paired with magnetostrictive actuators to detect strain. These sensors have several advantages: their thicknesses can be minimized, the surface area can be maximized to increase sensitivity, and complex multifilar coil configurations can be fabricated. A parametric study of the efficacy of IJP induction coils with different parameters (number of coils, monofilar/bifilar, size) tested on a number of actuator‐functionalized composite coupons (FeSiB ribbon and impregnated epoxy sensors) is conducted. The samples are characterized by measuring their inductance response through induced strains. Increased sensitivity and accuracy of the 10‐turn monofilar IJP sensor are shown with respect to 1) 70‐turn hand‐wound coils, 2) a three‐axis AMR sensor, and 3) other IJP actuators with <10 turns. This is attributed to increased contact area to the composite surface and the requirement of minimum sensitivity (i.e., the number of turns and surface area) for strain detection.

High Speed Sintering (HSS) is a novel polymer additive manufacturing process which utilises inkjet printing of an infrared-absorbing pigment onto a heated polymer powder bed to create 2D cross-sections which can be selectively sintered using an infrared lamp. Understanding and improving the accuracy and repeatability of part manufacture by HSS are important, ongoing areas of research. In particular, the role of the ink is poorly understood; the inks typically used in HSS have not been optimised for it, and it is unknown whether they perform in a consistent manner in the process. Notably, the ambient temperature inside a HSS machine increases as a side effect of the sintering process, and the unintentional heating to which the ink is exposed is expected to cause changes in its fluid properties. However, neither the extent of ink heating during the HSS process nor the subsequent changes in its fluid properties have ever been investigated. Such investigation is important, since significant changes in ink properties at different temperatures would be expected to lead to inconsistent printing and subsequently variations in part accuracy and even the degree of sintering during a single build. For the first time, we have quantified the ink temperature rise caused by unintentional, ambient heating during the HSS process, and subsequently measured several of the ink’s fluid properties across the ink temperature range which is expected to be encountered in normal machine operation (25 to 45 ∘ C). We observed only small changes in the ink’s density and surface tension due to this heating, but a significant drop (36%) in its viscosity was seen. By inspection of the ink’s Z number throughout printing, it is concluded that these changes would not be expected to change the manner in which droplets are delivered to the powder bed surface. In contrast, the viscosity decrease during printing is such that it is expected that the printed droplet sizes do change in a single build, which may indeed be a cause for concern with regard to the accuracy and repeatability of the inkjet printing used in HSS, and subsequently to the properties of the polymer parts obtained from the process.

Guiding neuronal cell growth is desirable for neural tissue engineering but is very challenging. In this work, a self-assembling ultra-short surfactant-like peptide I3K which possesses positively charged lysine head groups, and hydrophobic isoleucine tails, was chosen to investigate its potential for guiding neuronal cell growth. The peptides were able to self-assemble into nanofibrous structures and interact strongly with silk fibroin (SF) scaffolds, providing a niche for neural cell attachment and proliferation. SF is an excellent biomaterial for tissue engineering. However neuronal cells, such as rat PC12 cells, showed poor attachment on pure regenerated SF (RSF) scaffold surfaces. Patterning of I3K peptide nanofibers on RSF surfaces significantly improved cellular attachment, cellular density, as well as morphology of PC12 cells. The live / dead assay confirmed that RSF and I3K have negligible cytotoxicity against PC12 cells. Atomic force microscopy (AFM) was used to image the topography and neurite formation of PC12 cells, where results revealed that self-assembled I3K nanofibers can support the formation of PC12 cell neurites. Immunolabelling also demonstrated that coating of I3K nanofibers onto the RSF surfaces not only increased the percentage of cells bearing neurites but also increased the average maximum neurite length. Therefore, the peptide I3K could be used as an alternative to poly-l-lysine for cell culture and tissue engineering applications. As micro-patterning of neural cells to guide neurite growth is important for developing nerve tissue engineering scaffolds, inkjet printing was used to pattern self-assembled I3K peptide nanofibers on RSF surfaces for directional control of PC12 cell growth. The results demonstrated that inkjet-printed peptide micro-patterns can effectively guide the cell alignment and organization on RSF scaffold surfaces, providing great potential for nerve regeneration applications.

Inkjet-printing technology enables the contactless deposition of functional materials such as conductive inks on surfaces, hence reducing contamination and the risk of substrate damage. In printed electronics, inkjet technology offers the significant advantage of controlling the volume of material deposited, and therefore the fine-tuning of the printed geometry, which is crucial for the performance of the final printed electronics. Inkjet printing of functional inks can be used to produce sensors to detect failure of mechanical structures such as carbon fiber reinforced composite (CFRC) components, instead of using attached sensors, which are subject to delamination. Here, silver nanoparticle-based strain sensors were embedded directly in an insulated carbon-fiber laminate by using inkjet printing to achieve an optimized conductive and adhesive geometry, forming a piezoresistive strain sensor. Following the inkjet-printing optimization process, the sensor conductivity and adhesion performance were evaluated. Finally, the sensor was quantified by using a bending rig which applied a pre-determined strain, with the response indicating an accurate sensitivity as the resistance increased with an increased strain. The ability to embed the sensor directly on the CFRC prevents the use of interfacial adhesives which is the main source of failure due to delamination.

Bio-based plastics that can supplant petroleum-derived materials are necessary to meet the future demands of sustainability in the life cycle of plastic materials. While there are significant efforts to develop protein-based plastic materials for commercial use, their application is limited by poor processability and limitations in mechanical performance. Here, we present a bovine serum albumin (BSA)-based resin for stereolithographic apparatus (SLA) 3D printing that affords bioplastic objects with shape-memory behavior. We demonstrate that the native conformation of these globular proteins is largely retained in the 3D-printed constructs and that each protein molecule possesses a "stored length" that could be revealed during mechanical deformation (extension or compression) of the 3D bioplastic objects. While the plastically deformed objects could retain this state for an indefinite period of time, heating the object or submerging in water allowed it to return to its original 3D-printed shape.

A novel facile approach to creating many localized giant polymersomes (GPs) (vesicles) is developed. This method allows for the easy capture and testing of the bulk properties of the GPs. Inkjet printing is shown to produce vesicles of a non-ionic block copolymer, poly(ethylene oxide)(16)-poly(butylene oxide)(22)(E16B22). The printer deposits a polymer solution (i.e., water/glucose/polymer) on a hydrophobic glass slide. Upon drying, glucose is left between polymer lamellar. Subsequent hydration of the sample dissolves glucose, and as a result, a concentration gradient is generated, which provides the energy necessary for separating the neutral polymer layers. After about 2 h in water, GPs are observed and are free of organic solvents. This novel methodology provides for the efficient formation of localized GPs. The distribution of the solution allows for a metered approach to releasing the GPs. This combination facilitates the collection of vesicles for micropipette aspiration experiments.

Inkjet printing has been used to produce a range of printed electronic devices, such as solar panels, sensors, and transistors. This article discusses inkjet printing and its employment in the field of printed electronics. First, printing as a field is introduced before focusing on inkjet printing. The materials that can be employed as inks are then introduced, leading to an overview of wetting, which explains the influences that determine print morphology. The article considers how the printing parameters can affect device performance and how one can account for these influences. The article concludes with a discussion on adhesion. The aim is to illustrate that the factors chosen in the fabrication process, such as dot spacing and sintering conditions, will influence the performance of the device.

Bio-sourced and biodegradable polymers for additive manufacturing could enable the rapid fabrication of parts for a broad spectrum of applications ranging from healthcare to aerospace. Herein, we report a resin for vat photopolymerization based on the protein, bovine serum albumin, using a commercially available laser-scanning stereolithographic apparatus (SLA) printer. Methacrylated bovine serum albumin (MA-BSA) was synthesized and formulated into aqueous resins that were used to print complex 3D objects with a resolution comparable to a commercially available resin. The MA-BSA resins were characterized by rheometry to determine the viscosity and the cure rate, as both parameters can ultimately be used to predict the printability of the resin. In the first step of patterning these materials, the MA-BSA resin was 3D printed, and in the second step, the printed construct was thermally cured to denature the globular protein and increase the intermolecular noncovalent interactions. These resins are expected to be compatible with other vat photopolymerization processes, including digital light projection (DLP) and continuous liquid interface production (CLIP).

Additive manufacturing (AM) technologies are expanding the boundaries of materials science and providing an exciting forum for interdisciplinary research. The ability to fabricate arbitrarily complex objects has made AM technologies indispensable in personalized healthcare, soft electronics, and renewable energy. At the intersection of AM technologies and materials chemistry are stimuli-responsive polymers, which change their chemical and physical properties in response to specific environmental cues. The responsiveness of these "smart" polymers makes them suitable for AM and provides functionality to the additively manufactured objects. Furthermore, the type and degree of stimulus response of smart polymers can be regulated through precise synthetic design or via incorporation of additives. Herein, we review recently reported stimuli-responsive polymers used in AM, with a focus on the design and chemistry of the polymers. The materials are broadly classified by type of printing, and more specifically classified by type of stimulus response. Finally, we briefly consider existing challenges that stimuli-responsive materials in AM can address in the future. (C) 2019 Elsevier B.V. All rights reserved.

Engineered tubular constructs made from soft biomaterials are employed in a myriad of applications in biomedical science. Potential uses of these constructs range from vascular grafts to conduits for enabling perfusion of engineered tissues and organs. The fabrication of standalone tubes or complex perfusable constructs from biofunctional materials, including hydrogels, via rapid and readily accessible routes is desirable. Here we report a methodology in which customized coaxial nozzles are 3D printed using commercially available stereolithography (SLA) 3D printers. These nozzles can be used for the fabrication of hydrogel tubes via coextrusion of two shear-thinning hydrogels: an unmodified Pluronic® F-127 (F127) hydrogel and an F127-bisurethane methacrylate (F127-BUM) hydrogel. We demonstrate that different nozzle geometries can be modeled via computer-aided design and 3D printed in order to generate tubes or coaxial filaments with different cross-sectional geometries. We were able to fabricate tubes with luminal diameters or wall thicknesses as small as ∼150 μm. Finally, we show that these tubes can be functionalized with collagen I to enable cell adhesion, and human umbilical vein endothelial cells can be cultured on the luminal surfaces of these tubes to yield tubular endothelial monolayers. Our approach could enable the rapid fabrication of biofunctional hydrogel conduits which can ultimately be utilized for engineering in vitro models of tubular biological structures.

Bio-sourced and biodegradable polymers for additive manufacturing couldenable the rapid fabrication of parts for a broad spectrum of applicationsranging from healthcare to aerospace. However, a limited number of thesematerials are suitable for vat photopolymerization processes. Herein, we report a two-step additivemanufacturing process to fabricate robust protein-based constructs using acommercially available laser-based SLA printer. Methacrylated bovine serumalbumin (MA-BSA) was synthesized and formulated into aqueous resins that were usedto print complex 3D objects with a resolution comparable to a commerciallyavailable resin. The MA-BSA resins were characterized by rheometry to determinethe viscosity and the cure rate, as both of these parameters can ultimately beused to predict the printability of the resin. In the first step of patterningthese materials, the MA-BSA resin was 3D printed, and in the second step, theprinted construct was thermally cured to denature the globular protein andincrease the intermolecular noncovalent interactions. Thus, the final 3Dprinted part was comprised of both chemical and physical cross-links. Compressionstudies of hydrated and dehydrated constructs demonstrated a broad range ofcompressive strengths and Young’s moduli that could be further modulated byadjusting the type and amount of co-monomer. The printed hydrogel constructsdemonstrated good cell viability (> 95%) after a 21-day culture period. TheseMA-BSA resins are expected to be compatible with other vat photopolymerizationtechniques including digital light projection (DLP) and continuous liquidinterface production (CLIP).

Shear-thinning hydrogels are often employed in direct-write 3D printing, however, the viscoelastic behaviors that define a printable hydrogel have not been fully established. Herein, we demonstrate a library of hydrogel inks based on the incorporation of water-soluble reactive meth(acrylate) monomers into F127-dimethacrylate hydrogels. This strategy afforded printed hydrogels with a broad range of chemical functionalities and mechanical properties. A systematic investigation was also performed to correlate the printability and mechanical properties to the viscoelastic properties of the hydrogel ink formulations. The materials with a high dynamic yield stress afforded extruded filaments that correlated well with the inner diameter of the printing nozzle. The static yield stress of the material was correlated to the extrusion pressure and print speed required for optimal printing. Thus, this study provides a guide for the future development of hydrogel inks for direct-write 3D printing along with a new set of functional hydrogel inks.