3D Printing Mechanically Robust and Transparent Polyurethane Elastomers for Stretchable Electronic Sensors

Interfacial Charge Transport Enhancement of Liquid-Crystalline Polymer Transistors Enabled by Ionic Polyurethane Dielectric

In organic field-effect transistors (OFETs) using disordered organic semiconductors, interface traps that hinder efficient charge transport, stability, and device performance are inevitable. Benchmark poly(9,9-dioctylfuorene-co-bithiophene) (F8T2) liquid-crystalline polymer semiconductor has been extensively investigated for organic electronic devices due to its promising combination of charge transport and light emission properties. This study demonstrates that high-capacitance single-layered ionic polyurethane (PU) dielectrics enable enhanced charge transport in F8T2 OFETs. The ionic PU dielectrics are composed of a mild blending of PU ionogel and PU solution, thereby forming a solid-state film with robust interfacial characteristics with semiconductor layer and gate electrode in OFETs and measuring high capacitance values above 10 µF cm-2 owing to the combined dipole polarization and electric double layer formation. The optimized fabricated ionic PU-gated OFETs exhibit a low-voltage operation at -3 V with a remarkable hole mobility of over 5 cm2 V-1 s-1 (average = 2.50 ± 1.18 cm2 V-1 s-1), which is the highest mobility achieved so far for liquid-crystalline F8T2 OFETs. This device also provides excellent bias-stable characteristics in ambient air, exhibiting a negligible threshold voltage shift of -0.03 V in the transfer curves after extended bias stress, with a reduced trap density.

3D-Printed Eosin Y-Based Heterogeneous Photocatalyst for Organic Reactions

Heterogenization of Eosin Y by 3D-printing and its application in photocatalysis are reported. The approach allows a fine tuning of the photocatalyst morphology and its rapid preparation. Photocatalytic activity was evaluated through model organic reactions involving oxidation, reduction, and photosensitization pathways. The efficiency, recyclability and stability of 3D printed EY is remarkable paving the way to new generation of heterogeneous photocatalysts with a perfect control of their shape and adaptable to any photoreactors.

Continuous Polymer Synthesis and Manufacturing of Polyurethane Elastomers Enabled by Automation

Connecting polymer synthesis and processing is an important challenge for streamlining the manufacturing of polymeric materials. In this work, the automated synthesis of acrylate-capped polyurethane oligomers is integrated with vat photopolymerization 3D printing. This strategy enabled the rapid manufacturing of a library of polyurethane-based elastomeric materials with differentiated thermal and mechanical properties. The automated semicontinuous batch synthesis approach proved enabling for resins with otherwise short shelf lives because of the intimate connection between synthesis, formulation, and processing. Structure-property studies demonstrated the ability to tune properties through systematic alteration of cross-link density and chemical composition.

Three-Dimensional Printing of Ultrasoft Silicone with a Functional Stiffness Gradient

A methodology for three-dimensionally printing ultrasoft silicone with a functional stiffness gradient is presented. Ultraviolet-cure silicone was deposited via two independently controlled extruders into a thixotropic, gel-like, silicone oil-based support matrix. Each extruder contained a different liquid silicone formulation. The extrusion rates were independently varied during printing such that the combined selectively deposited material contained different ratios of the two silicones, resulting in localized control of material stiffness. Tests to validate the process are reported, including tensile testing of homogeneous cubic specimens to quantify the range of material stiffness that could be printed, indentation testing of cuboid specimens to characterize printed stiffness gradients, and vibratory testing of synthetic multilayer vocal fold (VF) models to demonstrate that the method may be applied to the fabrication of biomechanical models for voice production research. The cubic specimens exhibited linear stress-strain data with tensile elasticity modulus values between 1.11 and 27.1 kPa, more than a factor of 20 in stiffness variation. The cuboid specimens exhibited material variations that were visually recognizable and quantifiable via indentation testing. The VF models withstood rigorous phonatory flow-induced vibration and exhibited vibratory characteristics comparable to those of previous models. Overall, while process refinements are needed, the results of these tests demonstrate the ability to print ultrasoft silicone with stiffness gradients.

High-Resolution Additive Manufacturing of a Biodegradable Elastomer with A Low-Cost LCD 3D Printer

Artificial organs and organs-on-a-chip (OoC) are of great clinical and scientific interest and have recently been made by additive manufacturing, but depend on, and benefit from, biocompatible, biodegradable, and soft materials. Poly(octamethylene maleate (anhydride) citrate (POMaC) meets these criteria and has gained popularity, and as in principle, it can be photocured and is amenable to vat-photopolymerization (VP) 3D printing, but only low-resolution structures have been produced so far. Here, a VP-POMaC ink is introduced and 3D printing of 80 µm positive features and complex 3D structures is demonstrated using low-cost (≈US$300) liquid-crystal display (LCD) printers. The ink includes POMaC, a diluent and porogen additive to reduce viscosity within the range of VP, and a crosslinker to speed up reaction kinetics. The mechanical properties of the cured ink are tuned to match the elastic moduli of different tissues simply by varying the porogen concentration. The biocompatibility is assessed by cell culture which yielded 80% viability and the potential for tissue engineering illustrated with a 3D-printed gyroid seeded with cells. VP-POMaC and low-cost LCD printers make the additive manufacturing of high resolution, elastomeric, and biodegradable constructs widely accessible, paving the way for a myriad of applications in tissue engineering and 3D cell culture as demonstrated here, and possibly in OoC, implants, wearables, and soft robotics.

Fabrication of Soft Transparent Patient-Specific Vascular Models with Stereolithographic 3D printing and Thiol-Based Photopolymerizable Coatings

An ideal vascular phantom should be anatomically accurate, have mechanical properties as close as possible to the tissue, and be sufficiently transparent for ease of visualization. However, materials that enable the convergence of these characteristics have remained elusive. The fabrication of patient-specific vascular phantoms with high anatomical fidelity, optical transparency, and mechanical properties close to those of vascular tissue is reported. These final properties are achieved by 3D printing patient-specific vascular models with commercial elastomeric acrylic-based resins before coating them with thiol-based photopolymerizable resins. Ternary thiol-ene-acrylate chemistry is found optimal. A PETMP/allyl glycerol ether (AGE)/polyethylene glycol diacrylate (PEGDA) coating with a 30/70% AGE/PEGDA ratio applied on a flexible resin yielded elastic modulus, UTS, and elongation of 3.41 MPa, 1.76 MPa, and 63.2%, respectively, in range with the human aortic wall. The PETMP/AGE/PEGDA coating doubled the optical transmission from 40% to 80%, approaching 88% of the benchmark silicone-based elastomer. Higher transparency correlates with a decrease in surface roughness from 2000 to 90 nm after coating. Coated 3D-printed anatomical replicas are showcased for pre-procedural planning and medical training with good radio-opacity and echogenicity. Thiol-click chemistry coatings, as a surface treatment for elastomeric stereolithographic 3D-printed objects, address inherent limitations of photopolymer-based additive manufacturing.

Thermal and UV Curable Formulations of Poly(propylene glycol)-Poly(hydroxyurethane) Elastomers toward Nozzle-Based 3D Photoprinting

In this work, isocyanate-free formulations for poly(propylene glycol) polyurethane elastomers are studied. These formulations are based on poly(propylene glycol) end-capped by CO2-sourced cyclic carbonate (bisCC PPG) macromonomers able to react with amines leading to poly(hydroxyurethane)s. In order to obtain covalent networks, two curing approaches are studied. First, the direct thermally activated cross-linking of bisCC PPG with a mixture of various aliphatic or aromatic diamines and a triamine is investigated, and in particular the nature of the diamine on the mechanical properties. In the second approach, UV-activated formulations are developed by reacting bisCC PPG with allylamine followed by the addition of a trithiol by photoactivated thiol-ene reaction. The swelling tests show that both systems provide highly cross-linked polymer networks and complementary characterizations highlighted excellent mechanical properties. Thanks to the fast curing and adapted viscosity of the developed photoactive formulation, the latter was found suitable for use as a photoresin for 3D printing as demonstrated by printing a vaginal ring by a nozzle-based photoprinter.

Progress in Photocurable 3D Printing of Photosensitive Polyurethane: A Review

In recent years, as a class of advanced additive manufacturing (AM) technology, photocurable 3D printing has gained increasing attention. Based on its outstanding printing efficiency and molding accuracy, it is employed in various fields, such as industrial manufacturing, biomedical, soft robotics, electronic sensors. Photocurable 3D printing is a molding technology based on the principle of area-selective curing of photopolymerization reaction. At present, the main printing material suitable for this technology is the photosensitive resin, a composite mixture consisting of a photosensitive prepolymer, reactive monomer, photoinitiator, and other additives. As the technique research deepens and its application gets more developed, the design of printing materials suitable for different applications is becoming the hotspot. Specifically, these materials not only can be photocured but also have excellent properties, such as elasticity, tear resistance, fatigue resistance. Photosensitive polyurethanes can endow photocured resin with desirable performance due to their unique molecular structure including the inherent alternating soft and hard segments, and microphase separation. For this reason, this review summarizes and comments on the research and application progress of photocurable 3D printing of photosensitive polyurethanes, analyzing the advantages and shortcomings of this technology, also offering an outlook on this rapid development direction.

3D Printing of High Viscosity UV-Curable Resin for Highly Stretchable and Resilient Elastomer

Elastomers prepared via vat photopolymerizationus ually exhibit unsatisfied mechanical properties owing to their insufficient growth of molecular weight upon UV exposure. Increasing the weight ratio of oligomer in the resin system is an effective approach to enhance the mechanical properties, yet the viscosity of the UV-curable resin increases dramatically; this hinders its printing. In this study, a linear scan-based vat photopolymerization (LSVP) system which can print high-viscosity resins is implemented to 3D print the oligomer-dominated UV-curable resin via a dual-curing mechanism. A polyurethane methacrylate blocking oligomer is first synthesized and then mixed with a commercialized bifunctional oligomer, photoinitiator, and primary amine as a chain extender to prepare high-viscosity UV-curable resin for the LSVP system. The deblocked isocyanate is further crosslinked with a chain extender via thermal treatment to construct a highly entangled polymer chain network. The optimal thermal treatment parameters are investigated, and the resilience of the 3D-printed elastomer is evaluated through continuous tensile loading and unloading tests. Subsequently, complex structured elastomers are printed, exhibiting favorable mechanical durability without defects. The results obtained from this work will provide a reference for preparing elastomeric devices with excellent physical properties and expand the application scope of vat photopolymerization to new fields.

3D printing of ultra-high viscosity resin by a linear scan-based vat photopolymerization system

The current printing mechanism of the bottom-up vat photopolymerization 3D printing technique places a high demand on the fluidity of the UV-curable resin. Viscous high-performance acrylate oligomers are compounded with reactive diluents accordingly to prepare 3D printable UV-curable resins (up to 5000 cps of viscosity), yet original mechanical properties of the oligomers are sacrificed. In this work, an elaborated designed linear scan-based vat photopolymerization system is developed, allowing the adoption of printable UV-curable resins with high viscosity (> 600,000 cps). Briefly, this is realized by the employment of four rollers to create an isolated printing area on the resin tank, which enables the simultaneous curing of the resin and the detachment of cured part from the resin tank. To verify the applicability of this strategy, oligomer dominated UV-curable resin with great mechanical properties, but high viscosity is prepared and applied to the developed system. It is inspiring to find that high stress and strain elastomers and toughened materials could be facilely obtained. This developed vat photopolymerization system is expected to unblock the bottleneck of 3D printed material properties, and to build a better platform for researchers to prepare various materials with diversiform properties developed with 3D printing.

Single-vat single-cure grayscale digital light processing 3D printing of materials with large property difference and high stretchability

Multimaterial additive manufacturing has important applications in various emerging fields. However, it is very challenging due to material and printing technology limitations. Here, we present a resin design strategy that can be used for single-vat single-cure grayscale digital light processing (g-DLP) 3D printing where light intensity can locally control the conversion of monomers to form from a highly stretchable soft organogel to a stiff thermoset within in a single layer of printing. The high modulus contrast and high stretchability can be realized simultaneously in a monolithic structure at a high printing speed (z-direction height 1 mm/min). We further demonstrate that the capability can enable previously unachievable or hard-to-achieve 3D printed structures for biomimetic designs, inflatable soft robots and actuators, and soft stretchable electronics. This resin design strategy thus provides a material solution in multimaterial additive manufacture for a variety of emerging applications.

3D printed superparamagnetic stimuli-responsive starfish-shaped hydrogels

Magnetic-stimuli responsive hydrogels are quickly becoming a promising class of materials across numerous fields, including biomedical devices, soft robotic actuators, and wearable electronics. Hydrogels are commonly fabricated by conventional methods that limit the potential for complex architectures normally required for rapidly changing custom configurations. Rapid prototyping using 3D printing provides a solution for this. Previous work has shown successful extrusion 3D printing of magnetic hydrogels; however, extrusion-based printing is limited by nozzle resolution and ink viscosity. VAT photopolymerization offers a higher control over resolution and build-architecture. Liquid photo-resins with magnetic nanocomposites normally suffer from nanoparticle agglomeration due to local magnetic fields. In this work, we develop an optimised method for homogenously infusing up to 2 wt % superparamagnetic iron oxide nanoparticles (SPIONs) with a 10 nm diameter into a photo-resin composed of water, acrylamide and PEGDA, with improved nanoparticle homogeneity and reduced agglomeration during printing. The 3D printed starfish hydrogels exhibited high mechanical stability and robust mechanical properties with a maximum Youngs modulus of 1.8 MPa and limited shape deformation of 10% when swollen. Each individual arm of the starfish could be magnetically actuated when a remote magnetic field is applied. The starfish could grab onto a magnet with all arms when a central magnetic field was applied. Ultimately, these hydrogels retained their shape post-printing and returned to their original formation once the magnetic field had been removed. These hydrogels can be used across a wide range of applications, including soft robotics and magnetically stimulated actuators.

Lignin-Based Materials for Additive Manufacturing: Chemistry, Processing, Structures, Properties, and Applications

The utilization of lignin, the most abundant aromatic biomass component, is at the forefront of sustainable engineering, energy, and environment research, where its abundance and low-cost features enable widespread application. Constructing lignin into material parts with controlled and desired macro- and microstructures and properties via additive manufacturing has been recognized as a promising technology and paves the way to the practical application of lignin. Considering the rapid development and significant progress recently achieved in this field, a comprehensive and critical review and outlook on three-dimensional (3D) printing of lignin is highly desirable. This article fulfils this demand with an overview on the structure of lignin and presents the state-of-the-art of 3D printing of pristine lignin and lignin-based composites, and highlights the key challenges. It is attempted to deliver better fundamental understanding of the impacts of morphology, microstructure, physical, chemical, and biological modifications, and composition/hybrids on the rheological behavior of lignin/polymer blends, as well as, on the mechanical, physical, and chemical performance of the 3D printed lignin-based materials. The main points toward future developments involve hybrid manufacturing, in situ polymerization, and surface tension or energy driven molecular segregation are also elaborated and discussed to promote the high-value utilization of lignin.

Photocurable and elastic polyurethane based on polyether glycol with adjustable hardness for 3D printing customized flatfoot orthosis

Orthopedic insoles is the most commonly used nonsurgical treatment method for the flatfoot. Polyurethane (PU) plays a crucial role in the manufacturing of orthopedic insoles due to its high wear resistance and elastic recovery. However, preparing orthopedic insoles with adjustable hardness, high-accuracy, and matches the plantar morphology is challenging. Herein, a liquid crystal display (LCD) three-dimensional (3D) printer was used to prepare the customized arch-support insoles based on photo-curable and elastic polyurethane acrylate (PUA) composite resins. Two kinds of photo-curable polyurethanes (DL1000-PUA and DL2000-PUA) were successfully synthesized, and a series of fast-photocuring polyurethane acrylate (PUA) composite resins for photo-polymerization 3D printing were developed. The effects of different acrylate monomers on the Shore hardness, viscosity, and mechanical properties of the PUA composite resins were evaluated. The PUA-3-1 composite resin exhibited low viscosity, optimal hardness, and mechanical properties. A deviation analysis was conducted to assess the accuracy of printed insole. Furthermore, the stress conditions of the PUA composite resin and ethylene vinyl acetate (EVA) under the weight load of healthy adults were compared by finite element analysis (FEA) simulation. The results demonstrated that the stress of the PUA composite resin and EVA were 0.152 MPa and 0.285 MPa, and displacement were 0.051 mm and 3.449 mm, respectively. These results indicate that 3D-printed arch-support insole based on photocurable PUA composite resin are high-accuracy, and can reduce plantar pressure and prevent insoles premature deformation, which show great potential in the physiotherapeutic intervention for foot disorders.

Mechanically tunable resins based on acrylate-based resin for digital light processing (DLP) 3D printing

Until now, only a few materials are available for additive manufacturing technologies that employ photopolymerization, such as stereolithography (SLA) and digital light processing (DLP) 3D printing systems. This study investigates a newly formulated resins as an alternative 3D printing materials with tunable mechanical properties to expand the potential applications of advanced engineering products such as wearable devices and small reactors. A commercial acrylate-based resin was selected as a standard resin (STD). The resin was formulated by combining various volume ratios of a low-cost polypropylene glycol (PPG) having various molecular weights (400, 1000, and 2000 g/mol) with the STD resin. The printability of the formulated resins was optimized using the digital light processing (DLP) 3D printing technique. The effects of the PPG contents on the properties of the printed parts were studied, including printability, thermal properties, mechanical properties, and thermo-mechanical properties. As a result, the formulated resins with 5–30%vol of PPG could be printed while higher PPG content led to print failure. Results suggest that increasing the PPG contents reduced the dimensional accuracy of the printed parts and decreased the mechanical properties, including the flexural strength, flexural modulus, impact strength, hardness, and elastic modulus. interestingly, at small loading, 5%vol, the mechanical performance of the printed specimens was successfully enhanced. These results are intriguing to use a tunable mechanical acrylate-based resin for a specific application such as a microreactor.

Photo-Curing Kinetics of 3D-Printing Photo-Inks Based on Urethane-Acrylates

In this study, photo-curing kinetics for urethane-acrylate-based photo-inks for 3D printing were evaluated using a photo-differential scanning calorimetry analysis. Initially, the photopolymerization kinetics of di- and monofunctional monomers were separately studied at different temperatures (5–85 °C). Later, the photo-curing kinetics and mechanical properties of photo-inks based on different monomer mixtures (40/60–20/80) were evaluated. The results showed that urethane-dimethacrylate (UrDMA) and urethane-acrylate (UrA) had no light absorption in the region of 280–700 nm, making them a proper crosslinker and a reactive diluent, respectively, for the formulation of 3D-printing photo-inks. The kinetics investigations showed a temperature dependency for the photo-curing of UrDMA, where a higher photopolymerization rate (Rp,max: from 5.25 × 10−2 to 8.42 × 10−2 1/s) and double-bound conversion (DBCtotal: from 63.8% to 92.2%) were observed at elevated temperatures (5–85 °C), while the photo-curing of UrA was independent of the temperature (25–85 °C). Enhancing the UrA content from 60% to 80% in the UrDMA/UrA mixtures initially increased and later decreased the photopolymerization rate and conversion, where the mixtures of 30/70 and 25/75 presented the highest values. Meanwhile, increasing the UrA content led to lower glass transition temperatures (Tg) and mechanical strength for the photo-cured samples, where the mixture of 30/70 presented the highest maximum elongation (εmax: 73%).

Facile Fabrication of Highly Sensitive Thermoplastic Polyurethane Sensors with Surface- and Interface-Impregnated 3D Conductive Networks

This research aims to develop a practical, scalable, and highly conductive flexible 3D printed piezoresistive sensor with low filler content. Here, we introduced a fused deposition modeling 3D printing combined in situ spray-coating technique to develop a conductive sensor in a single shot. The graphene suspension is sprayed over each layer during the 3D printing of the sensor, which helps develop a conductive network on the surface and at the interface of the printed system. Graphene deposited on the overall surface is often affected by nanoparticle delamination and loses its function over time. To avoid this, the prepared samples are subjected to foaming. The foaming process created a low-mass-density sensor by forming a microcellular structure, and the surface-deposited graphene is embedded well on the TPU surface. The method followed in this work reveals a stable and connected conduction path with excellent electrical resistance and resistance against harsh conditions (exposure to organic solvents). Besides, the compression sensor withstood its sensitivity over a severe compressive strain of 80% and showed a GF of 1.82 and a sensitivity of 2.316 kPa^-1. The conductive network path varied based on the infill pattern, affecting its electrical sensitivity. The wiggle pattern shows good resistance; under stretching, the pattern generated a higher current and showed a delayed conductive path disconnection than other patterns. Thus, the embedded graphene/TPU conductive sensors show good stability and promising sensitivity. Furthermore, the developed sensor is used to monitor human motion and actions.

Waterborne Polyurethane Acrylates Preparation towards 3D Printing for Sewage Treatment

Conventional immobilized nitrifying bacteria technologies are limited to fixed beds with regular shapes such as spheres and cubes. To achieve a higher mass transfer capacity, a complex-structured cultivate bed with larger specific surface areas is usually expected. Direct ink writing (DIW) 3D printing technology is capable of preparing fixed beds where nitrifying bacteria are embedded in without geometry limitations. Nevertheless, conventional bacterial carrier materials for sewage treatment tend to easily collapse during printing procedures. Here, we developed a novel biocompatible waterborne polyurethane acrylate (WPUA) with favorable mechanical properties synthesized by introducing amino acids. End-capped by hydroxyethyl acrylate and mixed with sodium alginate (SA), a dual stimuli-responsive ink for DIW 3D printers was prepared. A robust and insoluble crosslinking network was formed by UV-curing and ion-exchange curing. This dual-cured network with a higher crosslinking density provides better recyclability and protection for cryogenic preservation. The corresponding results show that the nitrification efficiency for printed bioreactors reached 99.9% in 72 h, which is faster than unprinted samples and unmodified WPUA samples. This work provides an innovative immobilization method for 3D printing bacterial active structures and has high potential for future sewage treatment.

Mechanical and wear behaviour of PEEK, PTFE and PU: review and experimental study

Soft polymers such as polyether ether ketone (PEEK), polyurethane (PU) and polytetrafluoroethylene (PTFE) have gained significant research interest in the last few decades owing to their excellent material properties which can be harnessed to meet the demands of various applications such as biomedical implants and accessories, insulation panels to cooking utensils, inner coating material for non-stick cookware etc. In the present study, we provide a comprehensive review on the mechanical and tribological behaviour of PEEK, PU and PTFE polymers. Samples of these materials were also fabricated and the experimentally obtained tensile strength, flexural strength, wear rate and coefficient of frictions were ascertained with values reported in literature. It is highlighted that coefficient of friction of polymers were highly dependent on the surface texture of the polymer’s surface; where an uneven surface exhibited higher coefficient of friction. Perspectives for future progress are also highlighted in this paper.

High-Resolution 3D Printing for Electronics

The ability to form arbitrary 3D structures provides the next level of complexity and a greater degree of freedom in the design of electronic devices. Since recent progress in electronics has expanded their applicability in various fields in which structural conformability and dynamic configuration are required, high-resolution 3D printing technologies can offer significant potential for freeform electronics. Here, the recent progress in novel 3D printing methods for freeform electronics is reviewed, with providing a comprehensive study on 3D-printable functional materials and processes for various device components. The latest advances in 3D-printed electronics are also reviewed to explain representative device components, including interconnects, batteries, antennas, and sensors. Furthermore, the key challenges and prospects for next-generation printed electronics are considered, and the future directions are explored based on research that has emerged recently.

Emerging Roles of Microfluidics in Brain Research: From Cerebral Fluids Manipulation to Brain-on-a-Chip and Neuroelectronic Devices Engineering

Remarkable progress made in the past few decades in brain research enables the manipulation of neuronal activity in single neurons and neural circuits and thus allows the decipherment of relations between nervous systems and behavior. The discovery of glymphatic and lymphatic systems in the brain and the recently unveiled tight relations between the gastrointestinal (GI) tract and the central nervous system (CNS) further revolutionize our understanding of brain structures and functions. Fundamental questions about how neurons conduct two-way communications with the gut to establish the gut-brain axis (GBA) and interact with essential brain components such as glial cells and blood vessels to regulate cerebral blood flow (CBF) and cerebrospinal fluid (CSF) in health and disease, however, remain. Microfluidics with unparalleled advantages in the control of fluids at microscale has emerged recently as an effective approach to address these critical questions in brain research. The dynamics of cerebral fluids (i.e., blood and CSF) and novel in vitro brain-on-a-chip models and microfluidic-integrated multifunctional neuroelectronic devices, for example, have been investigated. This review starts with a critical discussion of the current understanding of several key topics in brain research such as neurovascular coupling (NVC), glymphatic pathway, and GBA and then interrogates a wide range of microfluidic-based approaches that have been developed or can be improved to advance our fundamental understanding of brain functions. Last, emerging technologies for structuring microfluidic devices and their implications and future directions in brain research are discussed.

A colorless, transparent and mechanically robust polyurethane elastomer: synthesis, chemical resistance and adhesive properties

In this work, a transparent, mechanically strong and chemically resistant XDI-PUE adhesive was fabricated, which exhibited a remarkable tensile stress of 21.0 MPa with a break strain of 1608%. XDI-PUE also showed good chemical resistance towards toluene and NaOH aqueous solution.

3D Printed, Solid-State Conductive Ionoelastomer as a Generic Building Block for Tactile Applications

Shaping soft and conductive materials into preferential architectures via 3D printing is highly attractive for numerous applications ranging from tactile devices to bioelectronics. A landmark type of soft and conductive materials is hydrogels/ionogels. However, 3D-printed hydrogels/ionogels still suffer from a fundamental bottleneck: limited stability in their electrical-mechanical properties caused by the evaporation and leakage of liquid within hydrogels/ionogels. Although photocurable liquid-free ion-conducting elastomers can circumvent these limitations, the associated photocurable process is cumbersome and hence the printing quality is relatively poor. Herein, a fast photocurable, solid-state conductive ionoelastomer (SCIE) is developed that enables high-resolution 3D printing of arbitrary architectures. The printed building blocks possess many promising features over the conventional ion-conducting materials, including high resolution architectures (even ≈50 µm overhanging lattices), good Young's modulus (up to ≈6.2 MPa), and stretchability (fracture strain of ≈292%), excellent conductivity tolerance in a wide range of temperatures (from -30 to 80 °C), as well as fine elasticity and antifatigue ability even after 10 000 loading-unloading cycles. It is further demonstrated that the printed building blocks can be programmed into 3D flexible tactile sensors such as gyroid-based piezoresistive sensor and gap-based capacitive sensor, both of which exhibit several times higher in sensitivity than their bulky counterparts.

Rubber Seed Oil-Based UV-Curable Polyurethane Acrylate Resins for Digital Light Processing (DLP) 3D Printing

Novel UV-curable polyurethane acrylate (PUA) resins were developed from rubber seed oil (RSO). Firstly, hydroxylated rubber seed oil (HRSO) was prepared via an alcoholysis reaction of RSO with glycerol, and then HRSO was reacted with isophorone diisocyanate (IPDI) and hydroxyethyl acrylate (HEA) to produce the RSO-based PUA (RSO-PUA) oligomer. FT-IR and 1H NMR spectra collectively revealed that the obtained RSO-PUA was successfully synthesized, and the calculated C=C functionality of oligomer was 2.27 per fatty acid. Subsequently, a series of UV-curable resins were prepared and their ultimate properties, as well as UV-curing kinetics, were investigated. Notably, the UV-cured materials with 40% trimethylolpropane triacrylate (TMPTA) displayed a tensile strength of 11.7 MPa, an adhesion of 2 grade, a pencil hardness of 3H, a flexibility of 2 mm, and a glass transition temperature up to 109.4 °C. Finally, the optimal resin was used for digital light processing (DLP) 3D printing. The critical exposure energy of RSO-PUA (15.20 mJ/cm2) was lower than a commercial resin. In general, this work offered a simple method to prepare woody plant oil-based high-performance PUA resins that could be applied in the 3D printing industry.

Superelastic, Hygroscopic, and Ionic Conducting Cellulose Nanofibril Monoliths by 3D Printing

Compressible and superelastic 3D printed monoliths have shown great promise in various applications including energy storage, soft electronics, and sensors. Although such elastic monoliths have been constructed using some limited materials, most notably graphene, it has not yet been achieved in nature's most abundant material, cellulose, partly due to the strong hydrogen-bonding network within cellulose. Here, we report a 3D-printed cellulose nanofibril monolith that demonstrates superb elasticity (over 91% strain recovery after 500 cycles of compressive test), compressibility (up to 90% compressive strain), and pressure sensitivity (0.337 kPa^-1) at 43% relative humidity. Such a high-performance CNF monolith is achieved through both hierarchical architecture design by 3D printing and freeze-drying and incorporation of hygroscopic salt for water absorption. The facile and efficient design strategy for a highly flexible CNF monolith is expected to expand to materials beyond cellulose and can realize much broader applications in flexible sensors, thermal insulation, and many other fields.

Building biobased, degradable, flexible polymer networks from vanillin via thiol–ene “click” photopolymerization

A new biobased allyl ether monomer with acetal groups is synthesized from renewable vanillin for building flexible transparent thiol–ene networks with good degradability under mild acidic conditions.