RESOURCES

Abstract
Neuromuscular interfaces are required to translate bioelectronic technologies for application in clinical medicine. Here, by leveraging the robotically controlled ink-jet deposition of low-viscosity conductive inks, extrusion of insulating silicone pastes and in situ activation of electrode surfaces via cold-air plasma, we show that soft biocompatible materials can be rapidly printed for the on-demand prototyping of customized electrode arrays well adjusted to specific anatomical environments, functions and experimental models. We also show, with the monitoring and activation of neuronal pathways in the brain, spinal cord and neuromuscular system of cats, rats and zebrafish, that the printed bioelectronic interfaces allow for long-term integration and functional stability. This technology might enable personalized bioelectronics for neuroprosthetic applications.

Abstract
Abstract Porosity is an essential feature in a wide range of applications that combine light weight with high surface area and tunable density. Porous materials can be easily prepared with a vast variety of chemistries using the salt-leaching technique. However, this templating approach has so far been limited to the fabrication of structures with random porosity and relatively simple macroscopic shapes. Here, a technique is reported that combines the ease of salt leaching with the complex shaping possibilities given by additive manufacturing (AM). By tuning the composition of surfactant and solvent, the salt-based paste is rheologically engineered and printed via direct ink writing into grid-like structures displaying structured pores that span from the sub-millimeter to the macroscopic scale. As a proof of concept, dried and sintered NaCl templates are infiltrated with magnesium (Mg), which is typically highly challenging to process by conventional AM techniques due to its highly oxidative nature and high vapor pressure. Mg scaffolds with well-controlled, ordered porosity are obtained after salt removal. The tunable mechanical properties and the potential to be predictably bioresorbed by the human body make these Mg scaffolds attractive for biomedical implants and demonstrate the great potential of this additive technique.

Abstract
Ceramics and metals are important materials that modern technologies are constructed from. The capability to produce such materials in a complex geometry with good mechanical properties can revolutionize the way we engineer our devices. Current curing techniques pose challenges such as high energy requirements{,} limitations of materials with high refractive index{,} tedious post-processing heat treatment processes{,} uneven drying shrinkages{,} and brittleness of green bodies. In this paper{,} a novel modified self-curable epoxide–amine 3D printing system is proposed to print a wide range of ceramics (metal oxides{,} nitrides{,} and carbides) and metals without the need for an external curing source. Through this technique{,} complex multi-material structures (with metal–ceramic and ceramic–ceramic combinations) can also be realized. Tailoring and matching the sintering temperatures of different materials through sintering additives and dopants{,} combined with a structural design providing maximum adhesion between interfaces{,} allow us to successfully obtain superior quality sintered multi-material structures. High-quality ceramic and metallic materials have been achieved (e.g.{,} zirconia with >98% theoretical density). Also{,} highly conductive metals and magnetic ceramics were printed and shaped uniquely without the need for a sacrificial support. With the addition of low molecular weight plasticizers and a multi-stage heat treatment process{,} crack-free and dense high-quality integrated multi-material structures fabricated by 3D printing can thus be a reality in the near future.

Abstract
Electroactive biomaterials are fascinating for tissue engineering applications because of their ability to deliver electrical stimulation directly to cells, tissue, and organs. One particularly attractive conductive filler for electroactive biomaterials is silver nanoparticles (AgNPs) because of their high conductivity, antibacterial activity, and ability to promote bone healing. However, production of AgNPs involves a toxic reducing agent which would inhibit biological scaffold performance. This work explores facile and green synthesis of AgNPs using extract of Cilembu sweet potato and studies the effect of baking and precursor concentrations (1, 10 and 100 mM) on AgNPs’ properties. Transmission electron microscope (TEM) results revealed that the smallest particle size of AgNPs (9.95 ± 3.69 nm) with nodular morphology was obtained by utilization of baked extract and ten mM AgNO3. Polycaprolactone (PCL)/AgNPs scaffolds exhibited several enhancements compared to PCL scaffolds. Compressive strength was six times greater (3.88 ± 0.42 MPa), more hydrophilic (contact angle of 76.8 ± 1.7°), conductive (2.3 ± 0.5 × 10−3 S/cm) and exhibited anti-bacterial properties against Staphylococcus aureus ATCC3658 (99.5% reduction of surviving bacteria). Despite the promising results, further investigation on biological assessment is required to obtain comprehensive study of this scaffold. This green synthesis approach together with the use of 3D printing opens a new route to manufacture AgNPs-based electroactive with improved anti-bacterial properties without utilization of any toxic organic solvents.

Abstract
Abstract 3D printing strategies have acquired great relevance toward the design of 3D scaffolds with precise macroporous structures, for supported mammalian cell growth. Despite advances in 3D model designs, there is still a shortage of detection tools to precisely monitor in situ cell behavior in 3D, thereby allowing a better understanding of the progression of diseases or to test the efficacy of drugs in a more realistic microenvironment. Even if the number of available inks has exponentially increased, they do not necessarily offer the required functionalities to be used as internal sensors. Herein the potential of surface-enhanced Raman scattering (SERS) spectroscopy for the detection of biorelevant analytes within a plasmonic hydrogel-based, 3D-printed scaffold is demonstrated. Such SERS-active scaffolds allow for the 3D detection of model molecules, such as 4-mercaptobenzoic acid. Flexibility in the choice of plasmonic nanoparticles is demonstrated through the use of gold nanoparticles with different morphologies, gold nanorods showing the best balance between SERS enhancement and scaffold transparency. Detection of the biomarker adenosine is also demonstrated as a proof-of-concept toward the use of these plasmonic scaffolds for SERS sensing of cell-secreted molecules over extended periods of time.

Abstract
Bioelectronic interfaces employing arrays of sensors and bioactuators are promising tools for the study, repair and engineering of cardiac tissues. They are typically constructed from rigid and brittle materials processed in a cleanroom environment. An outstanding technological challenge is the integration of soft materials enabling a closer match to the mechanical properties of biological cells and tissues. Here we present an algorithm for direct writing of elastic membranes with embedded electrodes, optical waveguides and microfluidics using a commercial 3D printing system and a palette of silicone elastomers. As proof of principle, we demonstrate interfacing of cardiomyocytes derived from human induced pluripotent stem cells (hiPSCs), which are engineered to express Channelrhodopsin-2. We demonstrate electrical recording of cardiomyocyte field potentials and their concomitant modulation by optical and pharmacological stimulation delivered via the membrane. Our work contributes a simple prototyping strategy with potential applications in organ-on-chip or implantable systems that are multi-modal and mechanically soft.

Abstract
One of the important challenges in bone tissue engineering is the development of biodegradable bone substitutes with appropriate mechanical and biological properties for the treatment of larger defects and those with complex shapes. Recently, magnesium phosphate (MgP) doped with biologically active ions like strontium (Sr2+) have shown to significantly enhance bone formation when compared with the standard calcium phosphate-based ceramics. However, such materials can hardly be shaped into large and complex geometries and more importantly lack the adequate mechanical properties for the treatment of load-bearing bone defects. In this study, we have fabricated bone implants through extrusion assisted three-dimensional (3D) printing of MgP ceramics modified with Sr2+ ions (MgPSr) and a medical grade polycaprolactone (PCL) polymer phase. MgPSr with 30 wt% PCL (MgPSr-PCL30) allowed the printability of relevant size structures (>780 mm3) at room temperature with an interconnected macroporosity of approximately 40%. The printing resulted in implants with a compressive strength of 4.3 MPa, which were able to support up to 50 cycles of loading without plastic deformation. Notably, MgPSr-PCL30 scaffolds were able to promote in vitro bone formation in medium without the supplementation with osteo-inducing components. In addition, long-term in vivo performance of the 3D printed scaffolds was investigated in an equine tuber coxae model over 6 months. The micro-CT and histological analysis showed that implantation of MgPSr-PCL30 induced bone regeneration, while no bone formation was observed in the empty defects. Overall, the novel polymer modified MgP ceramic material and extrusion-based 3D printing process presented here greatly improved the shape ability and load bearing properties of MgP-based ceramics with simultaneously induction of new bone formation.

Abstract
Local tissue microenvironment is able to regulate cell-to-cell interaction that leads to effective tissue repair. This study aims to demonstrate a tunable magnesium ionic (Mg2+) microenvironment in bony tissue that can significantly induce bone defect repair. The concept can be realized by using a newly fabricated nanocomposite comprising of custom-made copolymer polycaprolactone-co-poly(ethylene glycol)-co-polycaprolactone (PCL-PEG-PCL) and surface-modified magnesium oxide (MgO) nanoparticles. In this study, additive manufacturing (AM) technology had been adopted to help design the porous three-dimensional (3D) scaffolds with tunable Mg2+ microenvironment. We found that the wettability and printability of new copolymer had been improved as compared with that of PCL polymer. Additionally, when MgO nanoparticles incorporated into the newly synthesized hydrophilic copolymer matrix, it could lead to increased compressive moduli significantly. In the in vitro studies, the fabricated nanocomposite scaffold with low concentration of Mg2+ microenvironment not only demonstrated better cytocompatibility, but also remarkably enhanced osteogenic differentiation in vitro as compared with the pure PCL and PCL-PEG-PCL co-polymer controls. In the animal studies, we also found that superior and early bone formation and tissue mineralization could be observed in the same 3D printed scaffold. However, the nanocomposite scaffold with high concentration of Mg2+ jeopardized the in situ bony tissue regeneration capability due to excessive magnesium ions in bone tissue microenvironment. Lastly, this study demonstrates that the nanocomposite 3D scaffold with controlled magnesium concentration in bone tissue microenvironment can effectively promote bone defect repair.

Abstract
In this work, the fabrication of multi-responsive and hierarchically organized nanomaterial by using core-shell SrF2 upconverting nanoparticles, doped with Yb3+, Tm3+, Nd3+ incorporated into gelatin methacryloyl matrix, is reported. Upon 800 nm excitation, deep monitoring of 3D printed constructs is demonstrated. Addition of magnetic self-assembly of iron oxide nanoparticles within the hydrogel provides anisotropic structuration from the nano- to the macro-scale and magnetic responsiveness permitting remote manipulation. The present study provides a new strategy for the fabrication of a novel highly organized multi-responsive material using additive manufacturing, which could have important implications in biomedicine. This article is protected by copyright. All rights reserved.

Abstract
Abstract Bioelectronics platforms are gaining widespread attention as they provide a template to study the interactions between biological species and electronics. Decoding the effect of the electrical signals on the cells and tissues holds the promise for treating the malignant tissue growth, regenerating organs and engineering new-age medical devices. This work is a step forward in this direction, where bio- and electronic materials co-exist on one platform without any need for post processing. We fabricate a freestanding and flexible hydrogel based platform using 3D bioprinting. The fabrication process is simple, easy and provides a flexible route to print materials with preferred shapes, size and spatial orientation. Through the design of interdigitated electrodes and heating coil, the platform can be tailored to print various circuits for different functionalities. The biocompatibility of the printed platform is tested using C2C12 murine myoblasts cell line. Furthermore, normal human dermal fibroblasts (primary cells) are also seeded on the platform to ascertain the compatibility.

Abstract
Abstract Stimuli-responsive materials have the potential to enable the generation of new bioinspired devices with unique physicochemical properties and cell-instructive ability. Enhancing biocompatibility while simplifying the production methodologies, as well as enabling the creation of complex constructs, i.e., via 3D (bio)printing technologies, remains key challenge in the field. Here, a novel method is presented to biofabricate cellularized anisotropic hybrid hydrogel through a mild and biocompatible process driven by multiple external stimuli: magnetic field, temperature, and light. A low-intensity magnetic field is used to align mosaic iron oxide nanoparticles (IOPs) into filaments with tunable size within a gelatin methacryloyl matrix. Cells seeded on top or embedded within the hydrogel align to the same axes of the IOPs filaments. Furthermore, in 3D, C2C12 skeletal myoblasts differentiate toward myotubes even in the absence of differentiation media. 3D printing of the nanocomposite hydrogel is achieved and creation of complex heterogeneous structures that respond to magnetic field is demonstrated. By combining the advanced, stimuli-responsive hydrogel with the architectural control provided by bioprinting technologies, 3D constructs can also be created that, although inspired by nature, express functionalities beyond those of native tissue, which have important application in soft robotics, bioactuators, and bionic devices.

Abstract
Intelligent or smart materials have one or more properties that can be significantly changed in a controlled fashion by external stimuli, such as temperature, pH, electric or magnetic fields, etc. Magnetorheological (MR) materials are a class of smart materials whose properties can be varied by applying an external magnetic field. In this work, the possibility of employing a suitable 3D printing technology for the development of one of the smart MR materials, the magnetorheological elastomer (MRE) has been explored. In order to achieve such 3D printing, a multi-material printing is implemented, where a controlled volume of MR fluid is encapsulated within an elastomer matrix in the layer-by-layer fashion. The choice of printing materials determines the final structure of the 3D printed hybrid MR elastomer. Printing with a vulcanizing MR suspension produces the solid MR structure inside the elastomer matrix while printing with a non-vulcanizing MR suspension (MR fluid) results in the structures that the MR fluid is encapsulated inside the elastomer matrix. The 3D printability of different materials has been studied by measuring their rheological properties and we found that the highly shear thinning and thixotropic properties are important for 3D printability. The quality of the printed filaments strongly depends on the key printing parameters such as extrusion pressure, initial height and feed rate. The experimental results from the forced vibration testing show that the 3D printed MR elastomers could change their elastic and damping properties when exposed to the external magnetic field. Furthermore, the 3D printed MR elastomer also exhibits the anisotropic behavior when the direction of the magnetic field is changed with respect to the orientation of the printed filaments. This study has demonstrated that the 3D printing is viable for fabrication of hybrid MR elastomers with controlled structures of magnetic particles or MR fluids.

Abstract
Abstract In this study, a novel magnetorheological (MR) hybrid elastomer has been developed using a 3D printing method. In such an MR hybrid elastomer, a controlled volume of an MR fluid was encapsulated layer by layer into an elastomer matrix by means of a 3D printer and each layer was a composite structure consisting of an MR fluid and an elastomer. Similar to current MR fluids and MR elastomers, mechanical properties of 3D printed MR hybrid elastomers could be controlled via an externally applied magnetic field. The experimental results showed that the relative change in the damping capability of the new MR elastomer was more pronounced than the change in its stiffness when exposed to an external magnetic field. The study demonstrated that the 3D printing technique is feasible for fabrication of MR elastomers with controlled microstructures including magnetic particles or MR fluids. The 3D printed MR hybrid elastomer is also a potential material as a tunable spring-damper element.