3D-Printed Biomimetic Structural Colors.

Resolution control and expansibility have always been challenges to the fabrication of structural color materials. Here, a facile strategy to print cholesteric liquid crystal elastomers (CLCEs) into complex structural color patterns with variable resolution and enhanced expansibility is reported. A volatile solvent is introduced into the synthesized CLC oligomers, modifying its rheological properties and allowing direct-ink-writing (DIW) under mild conditions. The combination of printing shear flow and anisotropic deswelling of ink drives the CLC molecules into an ordered cholesteric arrangement. The authors meticulously investigate the influence of printing parameters to achieve resolution control over a wide range, allowing for the printing of multi-sized 1D or 2D patterns with constant quality. Furthermore, such solvent-cast direct-ink-writing (DIW) strategy is highly expandable and can be integrated easily into the DIW of bionic robots. Multi-responsive bionic butterfly and flower are printed with biomimetic in both locomotion and coloration. Such designs dramatically reduced the processing difficulty of precise full-color printing and expanded the capability of structural color materials to collaborate with other systems.

Versatile Patterning of Liquid Metal via Multiphase 3D Printing.

This paper presents a scalable and straightforward technique for the immediate patterning of liquid metal/polymer composites via multiphase 3D printing. Capitalizing on the polymer's capacity to confine liquid metal (LM) into diverse patterns. The interplay between distinctive fluidic properties of liquid metal and its self-passivating oxide layer within an oxidative environment ensures a resilient interface with the polymer matrix. This study introduces an inventive approach for achieving versatile patterns in eutectic gallium indium (EGaIn), a gallium alloy. The efficacy of pattern formation hinges on nozzle's design and internal geometry, which govern multiphase interaction. The interplay between EGaIn and polymer within the nozzle channels, regulated by variables such as traverse speed and material flow pressure, leads to periodic patterns. These patterns, when encapsulated within a dielectric polymer polyvinyl alcohol (PVA), exhibit an augmented inherent capacitance in capacitor assemblies. This discovery not only unveils the potential for cost-effective and highly sensitive capacitive pressure sensors but also underscores prospective applications of these novel patterns in precise motion detection, including heart rate monitoring, and comprehensive analysis of gait profiles. The amalgamation of advanced materials and intricate patterning techniques presents a transformative prospect in the domains of wearable sensing and comprehensive human motion analysis.

Embedded Sensors with 3D Printing Technology: Review.

Embedded sensors (ESs) are used in smart materials to enable continuous and permanent measurements of their structural integrity, while sensing technology involves developing sensors, sensory systems, or smart materials that monitor a wide range of properties of materials. Incorporating 3D-printed sensors into hosting structures has grown in popularity because of improved assembly processes, reduced system complexity, and lower fabrication costs. 3D-printed sensors can be embedded into structures and attached to surfaces through two methods: attaching to surfaces or embedding in 3D-printed sensors. We discussed various additive manufacturing techniques for fabricating sensors in this review. We also discussed the many strategies for manufacturing sensors using additive manufacturing, as well as how sensors are integrated into the manufacturing process. The review also explained the fundamental mechanisms used in sensors and their applications. The study demonstrated that embedded 3D printing sensors facilitate the development of additive sensor materials for smart goods and the Internet of Things.

Flow-through Gas Phase Photocatalysis Using TiO2 Nanotubes on Wirelessly Anodized 3D-Printed TiNb Meshes.

In this work, for the first time 3D Ti-Nb meshes of different composition, i.e., Ti, Ti-1Nb, Ti-5Nb, and Ti-10 Nb, were produced by direct ink writing. This additive manufacturing method allows tuning of the mesh composition by simple blending of pure Ti and Nb powders. The 3D meshes are extremely robust with a high compressive strength, giving potential use in photocatalytic flow-through systems. After successful wireless anodization of the 3D meshes toward Nb-doped TiO2 nanotube (TNT) layers using bipolar electrochemistry, they were employed for the first time for photocatalytic degradation of acetaldehyde in a flow-through reactor built based on ISO standards. Nb-doped TNT layers with low concentrations of Nb show superior photocatalytic performance compared with nondoped TNT layers due to the lower amount of recombination surface centers. High concentrations of Nb lead to an increased number of recombination centers within the TNT layers and reduce the photocatalytic degradation rates.

Tailoring Writability and Performance of Star Block Copolypeptides Hydrogels through Side-Chain Design.

Shear-recoverable hydrogels based on block copolypeptides with rapid self-recovery hold potential in extrudable and injectable 3D-printing applications. In this work, a series of 3-arm star-shaped block copolypeptides composed of an inner hydrophilic poly(l-glutamate) domain and an outer ß-sheet forming domain is synthesized with varying side chains and block lengths. By changing the ß-sheet forming domains, hydrogels with diverse microstructures and mechanical properties are prepared and structure-function relationships are determined using scattering and rheological techniques. Differences in the properties of these materials are amplified during direct-ink writing with a strong correlation observed between printability and material chemistry. Significantly, it is observed that non-canonical ß-sheet blocks based on phenyl glycine form more stable networks with superior mechanical properties and writability compared to widely used natural amino acid counterparts. The versatile design available through block copolypeptide materials provides a robust platform to access tunable material properties based solely on molecular design. These systems can be exploited in extrusion-based applications such as 3D-printing without the need for additives.

Hybrid Multimaterial 3D Printing Using Photocuring-While-Dispensing.

Three-dimensional (3D) printing methods, such as vat photopolymerization (VPP) and direct-ink-writing (DIW) processes, are known for their high-resolution and multimaterial capabilities, respectively. Here a novel hybrid 3D printing technique that combines the strengths of VPP and DIW processes to achieve multimaterial and high-resolution printing of functional structures and devices, is presented. The method involves dispensing liquid-like materials via syringes into a photocurable matrix material and subsequently using a Galvano mirror-controlled laser beam to selectively photocure the dispensed material trace or the matrix material surrounding the trace. The laser beam scanning and syringe dispensing are synchronized with a set delay to control liquid diffusion and in situ fixture. The versatility of the method is demonstrated by fabricating intricate 3D ant and wheel prototypes using various materials available for VPP and DIW technologies. The proposed photocuring-while-dispensing strategy offers advantages over conventional multimaterial 3D printing methods, such as integrating materials regardless of photocurability and viscosity, and fabricating heterogeneous structures with complex geometries and high resolution. With its principle demonstrated, this multimaterial 3D printing process will open up a wide range of potential applications with diverse functionalities and materials.

3D Bioprinting of Diatom-Laden Living Materials for Water Quality Assessment.

Diatoms have long been used as living biological indicators for the assessment of water quality in lakes and rivers worldwide. While this approach benefits from the great diversity of these unicellular algae, established protocols are time-consuming and require specialized equipment. Here, this work 3D prints diatom-laden hydrogels that can be used as a simple multiplex bio-indicator for water assessment. The hydrogel-based living materials are created with the help of a desktop extrusion-based printer using a suspension of diatoms, cellulose nanocrystals (CNC) and alginate as bio-ink constituents. Rheology and mechanical tests are employed to establish optimum bio-ink formulations, whereas cell culture experiments are utilized to evaluate the proliferation of the entrapped diatoms in the presence of selected water contaminants. Bioprinting of diatom-laden hydrogels is shown to be an enticing approach to generate living materials that can serve as low-cost bio-indicators for water quality assessment.

In Situ Self-Assembly of Nanoscale Particles into Macroscale Ordered Monolayers with Enhanced Memory Performance.

In situ fabrication of macroscale ordered monolayers of nanoparticles (NPs) on targeted substrates is highly desirable for precision electronic and optical devices, while it remains a great challenge. In this study, a solution is provided to address this challenge by developing a colloidal ink formulation and employing the direct-ink-writing (DIW) technique, where on-demand delivery of ink at a targeted location and directional evaporation with controllable rate are leveraged to precisely guide the deposition of polystyrene-grafted gold NPs (Au@PS NPs) into a macroscale monolayer with an ordered Au NP array embedded in a PS thin film. A 2D steady-state diffusion-controlled evaporation model, which explains the parameter dependence of the experimental results and gives semiquantitative agreement with the experimental evaporation kinetics is proposed. The ordered monolayer is used as both nanocrystal floating gates and the tunneling layer for nonvolatile memory devices. It shows significantly enhanced performance compared with a disordered NP film prepared by spin coating. This approach allows for fine control of NP self-assembly to print macroscaleordered monolayers directly onto substrates, which has great promise for application in broad fields, including microelectronic and photoelectronic devices, sensors, and functional coatings.

Direct Ink Writing of Pickering Emulsions Generates Ultralight Conducting Polymer Foams with Hierarchical Structure and Multifunctionality.

Porous materials with multiple hierarchy levels can be useful as lightweight engineering structures, biomedical implants, flexible functional devices, and thermal insulators. Numerous routes have integrated bottom-up and top-down approaches for the generation of engineering materials with lightweight nature, complex structures, and excellent mechanical properties. It nonetheless remains challenging to generate ultralight porous materials with hierarchical architectures and multi-functionality. Here, the combined strategy based on Pickering emulsions and additive manufacturing leads to the development of ultralight conducting polymer foams with hierarchical pores and multifunctional performance. Direct writing of the emulsified inks consisting of the nano-oxidant-hydrated vanadium pentoxide nanowires-generated free-standing scaffolds, which are stabilized by the interfacial organization of the nanowires into network structures. The following in situ oxidative polymerization transforms the nano-oxidant scaffolds into foams consisting of a typical conducting polymer-polyaniline. The lightweight polyaniline foams featured by hierarchical pores and high surface areas show excellent performances in the applications of supercapacitor electrodes, planar micro-supercapacitors, and gas sensors. This emerging technology demonstrates the great potential of a combination of additive manufacturing with complex fluids for the generation of functional solids with lightweight nature and adjustable structure-function relationships.

Microribbons composed of directionally self-assembled nanoflakes as highly stretchable ionic neural electrodes.

Many natural materials possess built-in structural variation, endowing them with superior performance. However, it is challenging to realize programmable structural variation in self-assembled synthetic materials since self-assembly processes usually generate uniform and ordered structures. Here, we report the formation of asymmetric microribbons composed of directionally self-assembled two-dimensional nanoflakes in a polymeric matrix during three-dimensional direct-ink printing. The printed ribbons with embedded structural variations show site-specific variance in their mechanical properties. Remarkably, the ribbons can spontaneously transform into ultrastretchable springs with controllable helical architecture upon stimulation. Such springs also exhibit superior nanoscale transport behavior as nanofluidic ionic conductors under even ultralarge tensile strains (>1,000%). Furthermore, to show possible real-world uses of such materials, we demonstrate in vivo neural recording and stimulation using such springs in a bullfrog animal model. Thus, such springs can be used as neural electrodes compatible with soft and dynamic biological tissues.

Dual-Channel Stretchable, Self-Tuning, Liquid Metal Coils and Their Fabrication Techniques.

Flexible and stretchable radiofrequency coils for magnetic resonance imaging represent an emerging and rapidly growing field. The main advantage of such coil designs is their conformal nature, enabling a closer anatomical fit, patient comfort, and freedom of movement. Previously, we demonstrated a proof-of-concept single element stretchable coil design with a self-tuning smart geometry. In this work, we evaluate the feasibility of scaling this coil concept to a multi-element coil array and the associated engineering and manufacturing challenges. To this goal, we study a dual-channel coil array using full-wave simulations, bench testing, in vitro, and in vivo imaging in a 3 T scanner. We use three fabrication techniques to manufacture dual-channel receive coil arrays: (1) single-layer casting, (2) double-layer casting, and (3) direct-ink-writing. All fabricated arrays perform equally well on the bench and produce similar sensitivity maps. The direct-ink-writing method is found to be the most advantageous fabrication technique for fabrication speed, accuracy, repeatability, and total coil array thickness (0.6 mm). Bench tests show excellent frequency stability of 128 ± 0.6 MHz (0% to 30% stretch). Compared to a commercial knee coil array, the stretchable coil array is more conformal to anatomy and provides 50% improved signal-to-noise ratio in the region of interest.

Physically Entangled Antiswelling Hydrogels with High Stiffness.

Physically cross-linked hydrogels have great potential for tissue engineering because of their excellent biocompatibility and easy fabrication. However, physical cross-linking points are typically weaker compared to chemical ones and therefore cannot form robust hydrogels with excellent water stability, which greatly hinder their further applications. In this work, a novel hydrogel with high stiffness and outstanding antiswelling performance cross-linked by hydrophobic polymer chains entanglements is reported. The hydrophobic polymer polyimide (PI) is mixed with the hydrophilic polymer poly-(vinyl pyrrolidone) (PVP) to form cross-linking points between the chains. At the equilibrium swelling state, tensile modulus of the hydrogel can be up to 22.57 MPa (higher than most existing hydrogels) and the equilibrium water swelling ratio (ESR) can be as low as 125.0%. By decreasing the PI mass ratio, tensile modulus and ESR of the hydrogel can be tuned in a wide range from 22.57 to 0.005 MPa and 125.0% to 765.6%, respectively. Using PVP/PI solutions as inks, uniform structures and multi-material structures are fabricated having mechanical properties close to cartilage through a direct ink writing 3D printing platform. This current work demonstrates that entangled PVP/PI hydrogels have excellent tailoring capabilities and are promising candidates for tissue engineering applications.

High-Resolution 3D Printing of Mechanically Tough Hydrogels Prepared by Thermo-Responsive Poloxamer Ink Platform.

High-resolution 3D-printable hydrogels with high mechanical strength and biocompatibility are in great demand because of their potential applications in numerous fields. In this study, a material system comprising Pluronic F-127 dimethacrylate (FDMA) is developed to function as a direct ink writing (DIW) hydrogel for 3D printing. FDMA is a triblock copolymer that transforms into micelles at elevated temperatures. The transformation increases the viscosity of FDMA and preserves its structure during DIW 3D printing, whereupon the printed structure is solidified through photopolymerization. Because of this viscosity shift, various functionalities can be incorporated through the addition of other materials in the solution state. Acrylic acid is incorporated into the pregel solution to enhance the mechanical strength, because the carboxylate group of poly(acrylic acid) ionically crosslinks with Fe3+ , increasing the toughness of the DIW hydrogel 37 times to 2.46 MJ m-3 . Tough conductive hydrogels are also 3D printed by homogenizing poly(3,4-ethylenedioxythiophene) polystyrene sulfonate into the pregel solution. Furthermore, the FDMA platform developed herein uses DIW, which facilitates multicartridges 3D printing, and because all the materials included are biocompatible, the platform may be used to fabricate complex structures for biological applications.

Anodic TiO2 Nanotubes on 3D-Printed Titanium Meshes for Photocatalytic Applications.

In this work, large 3D Ti meshes fabricated by direct ink writing were wirelessly anodized for the first time to prepare highly photocatalytically active TiO2 nanotube (TNT) layers. The use of bipolar electrochemistry enabled the fabrication of TNT layers within the 3D Ti meshes without the establishment of an electrical contact between Ti meshes and the potentiostat, confirming its unique ability and advantage for the synthesis of anodic structures on metallic substrates with a complex geometry. TNT layers with nanotube diameters of up to 110 nm and thicknesses of up to 3.3 µm were formed. The TNT-layer-modified 3D Ti meshes showed a superior performance for the photocatalytic degradation of methylene blue in comparison to TiO2-nanoparticle-decorated and nonanodized Ti meshes (with a thermal oxide layer), resulting in multiple increases in the dye degradation rate. The results presented here open new horizons for the employment of anodized 3D Ti meshes in various flow-through (photo)catalytic reactors.

3D Printing in Regenerative Medicine: Technologies and Resources Utilized.

Over the past ten years, the use of additive manufacturing techniques, also known as "3D printing", has steadily increased in a variety of scientific fields. There are a number of inherent advantages to these fabrication methods over conventional manufacturing due to the way that they work, which is based on the layer-by-layer material-deposition principle. These benefits include the accurate attribution of complex, pre-designed shapes, as well as the use of a variety of innovative raw materials. Its main advantage is the ability to fabricate custom shapes with an interior lattice network connecting them and a porous surface that traditional manufacturing techniques cannot adequately attribute. Such structures are being used for direct implantation into the human body in the biomedical field in areas such as bio-printing, where this potential is being heavily utilized. The fabricated items must be made of biomaterials with the proper mechanical properties, as well as biomaterials that exhibit characteristics such as biocompatibility, bioresorbability, and biodegradability, in order to meet the strict requirements that such procedures impose. The most significant biomaterials used in these techniques are listed in this work, but their advantages and disadvantages are also discussed in relation to the aforementioned properties that are crucial to their use.

Tailoring Pore Structures of 3D Printed Cellular High-Loading Cathodes for Advanced Rechargeable Zinc-Ion Batteries.

Developing high-loading cathodes with superior electrochemical performance is desirable but challenging in aqueous zinc-ion batteries (ZIBs) for commercialization. Advanced 3D printing of cellular and hierarchical porous cathodes with high mass loading for superior ZIBs is explored here. To obtain a high-performance 3D printable ink, a composite material of iron vanadate and reduced holey graphene oxide is synthesized as the ink component. A cellular cathode with hierarchical porous architecture for aqueous ZIBs is then designed and fabricated by 3D printing for the first time. The unique structures of 3D printed composite cathode provide interpenetrating transmission paths as well as channels for electrons and ions. 3D printed cathodes with high mass loading over 10 mg cm-2 exhibit a high specific capacity of 344.8 mAh g-1 at 0.1 A g-1 and deliver outstanding cycling stability over 650 cycles at 2 A g-1 . In addition, the printing strategy enables the ease increase in mass loading up to 24.4 mg cm-2 , where a remarkably high areal capacity of 7.04 mAh cm-2 is reached. The superior electrochemical performance paves the new way to design the state-of-the-art cathodes for ZIBs.

Direct Ink Writing of Li1.3 Al0.3 Ti1.7 (PO4 )3 -Based Solid-State Electrolytes with Customized Shapes and Remarkable Electrochemical Behaviors.

All-solid-state lithium batteries have received extensive attention due to their high safety and promising energy density and are considered as the next-generation electrochemical energy storage system. However, exploring solid-state electrolytes in customized geometries without sacrificing the ionic transport is significant yet challenging. Herein, various 3D printable Li1.3 Al0.3 Ti1.7 (PO4 )3 (LATP)-based inks are developed to construct ceramic and hybrid solid-state electrolytes with arbitrary shapes as well as high conductivities. The obtained inks show suitable rheological behaviors and can be successfully extruded into solid-state electrolytes using the direct ink writing (DIW) method. As-printed free-standing LATP ceramic solid-state electrolytes deliver high ionic conductivity up to 4.24 × 10-4  S cm-1 and different shapes such as "L", "T," and "+" can be easily realized without sacrificing high ionic transport properties. Moreover, using this printing method, LATP-based hybrid solid-state electrolytes can be directly printed on LiFePO4 cathodes for solid-state lithium batteries, where a high discharge capacity of 150 mAh g-1 at 0.5 C is obtained. The DIW strategy for solid-state electrolytes demonstrates a new way toward advanced solid-state energy storage with the high ionic transport and customized manufacturing ability.

Facile Photo and Thermal Two-Stage Curing for High-Performance 3D Printing of Poly(Dimethylsiloxane).

Three-dimensional (3D) printing of poly(dimethylsiloxane) (PDMS) is realized with a two-state curing strategy, i.e., photocuring for additively manufacturing high-precision architectures followed by thermal cross-linking for high-performance objects, taking Sylgard-184 as an example. In the mixture of base and curing agent of Sylgard-184, the photocuring ingredient methacrylated PDMS is incorporated to form hybrid inks with not only high-efficiency UV curing ability but also moderate rheological properties for 3D printing. The inks are then used to additively manufacture high-precision architectures by UV-assisted direct ink writing. Various architectures such as lattices and honeycombs, channels that can be used as microfluidics, and pressure-proof pipes with a feature size of ≈100 µm, can be readily printed. Thereafter, thermal cross-linking at elevated temperature is conducted to obtain the 3D PDMS objects with comparable properties to Sylgard-184. The facile, universal two-stage approach to 3D printing of PDMS can facilitate the development of microfluidics, flexible electronics, soft robots, and so on.

Design and Development of a Fully Printed Accelerometer with a Carbon Paste-Based Strain Gauge.

In this paper, we present a fully printed accelerometer with a piezoresistive carbon paste-based strain gauge printed on its surface, which can be manufactured at low cost and with high efficiency. This accelerometer is composed of two parts: a sensor substrate made from high-temperature resin, which is printed by a 3D printer based on stereolithography apparatus (SLA), and a carbon paste-based strain gauge fabricated by screen-printing technology and by direct ink writing (DIW) technology for the purposes of comparison and optimization. First, the structural design, theoretical analysis, simulation analysis of the accelerometer, and analyses of the conductive mechanism and the piezoresistive mechanism of the carbon paste-based strain gauge were carried out. Then the proposed accelerometer was fabricated by a combination of different printing technologies and the curing conditions of the carbon paste were investigated. After that, the accelerometers with the screen-printed strain gauge and DIW strain gauge were characterized. The results show that the printing precision of the screen-printing process on the sensor substrate is higher than the DIW process, and both accelerometers can perform acceleration measurement. Also, this kind of accelerometer can be used in the field of measuring body motion. All these findings prove that 3D printing technology is a significant method for sensor fabrication and verification.

Advanced Polymer Designs for Direct-Ink-Write 3D Printing.

The rapid development of additive manufacturing techniques, also known as three-dimensional (3D) printing, is driving innovations in polymer chemistry, materials science, and engineering. Among current 3D printing techniques, direct ink writing (DIW) employs viscoelastic materials as inks, which are capable of constructing sophisticated 3D architectures at ambient conditions. In this perspective, polymer designs that meet the rheological requirements for direct ink writing are outlined and successful examples are summarized, which include the development of polymer micelles, co-assembled hydrogels, supramolecularly cross-linked systems, polymer liquids with microcrystalline domains, and hydrogels with dynamic covalent cross-links. Furthermore, advanced polymer designs that reinforce the mechanical properties of these 3D printing materials, as well as the integration of functional moieties to these materials are discussed to inspire new polymer designs for direct ink writing and broadly 3D printing.