Vesicular glutamate release from central axons contributes to myelin damage

Neuronal activity can lead to vesicular release of glutamate. Here the authors demonstrate that vesicular release of glutamate occurs in axons during ischemic conditions, and that an allosteric modulator of GluN2C/D is protective in models of ischemic injury

Anisotropic strength and fracture resistance of epoxy-ceramic composite materials produced by ultrasound freeze-casting

The anisotropic mechanical properties of ultrasound freeze cast epoxy-ceramic composite materials were studied by measuring flexural strength and fracture resistance curves (R-curves) using both unnotched and notched three-point beam bending experiments, respectively, cut in three different orientations relative to the directional freezing axis. Three ultrasound frequencies of 0.699, 1.39 and 2.097 MHz were used in order to introduce different length scales into the microstructure, with 0 MHz used as the control samples. For all cases, the composites showed higher strength and fracture resistance when the crack plane cut across the direction of ice growth (denoted as the YX orientation). The anisotropic properties were more evident for the material produced without ultrasound (0 MHz) where the flexural strength was approximately 160% higher for the YX orientation compared to two orthogonal orientations. Most of the fracture resistance increase was found to occur within a crack extension, Δa, of ∼0.5 mm. Comparing the fracture resistance at Δa = 0.5 mm for the highly anisotropic 0 MHz samples showed that the YX orientation was approximately 86% tougher than the two orthogonal orientations. When the ultrasound operation frequencies of 0.699, 1.39 and 2.097 MHz were applied, the amount of anisotropy in the strength and fracture resistance gradually decreased as the operating frequency increased. The high strength and fracture resistance for the YX orientation was attributed to the alignment of the ceramic particles along the freeze front direction creating a barrier for crack propagation. Ultrasound modifies the material microstructure, introducing relatively dense ceramic layers perpendicular to the freezing front direction that act as an additional, orthogonal barrier to crack propagation. The addition of the denser layers acts to improve the mechanical properties in the weaker orientations and reduce the overall anisotropy

Multiscale structure and damage tolerance of coconut shells

We investigated the endocarp of the fruit of Cocos nucifera (i.e., the inner coconut shell), examining the structure across multiple length scales through advanced characterization techniques and in situ testing of mechanical properties. Like many biological materials, the coconut shell possesses a hierarchical structure with distinct features at different length scales that depend on orientation and age. Aged coconut was found to have a significantly stronger (ultimate tensile strength, UTS = 48.5 MPa), stiffer (Young's modulus, E = 1.92 GPa), and tougher (fracture resistance (R-curve) peak of KJ = 3.2 MPa m1/2) endocarp than the younger fruit for loading in the latitudinal orientation. While the mechanical properties of coconut shell were observed to improve with age, they also become more anisotropic: the young coconut shell had the same strength (17 MPa) and modulus (0.64 GPa) values and similar R-curves for both longitudinal and latitudinal loading configurations, whereas the old coconut had 82% higher strength for loading in the latitudinal orientation, and >50% higher crack growth toughness for cracking on the latitudinal plane. Structural aspects affecting the mechanical properties across multiple length scales with aging were identified as improved load transfer to the cellulose crystalline nanostructure (identified by synchrotron x-ray diffraction) and sclerification of the endocarp, the latter of which included closing of the cell lumens and lignification of the cell walls. The structural changes gave a denser and mechanically superior micro and nanostructure to the old coconut shell. Additionally, the development of anisotropy was attributed to the formation of an anisotropic open channel structure throughout the shell of the old coconut that affected both crack initiation during uniaxial tensile tests and the toughening mechanisms of crack trapping and deflection during crack propagation

Ultrasound freeze-casting of a biomimetic layered microstructure in epoxy-ceramic composite materials to increase strength and hardness

Some natural materials, such as the dactyl club of the mantis shrimp, have impressive mechanical properties (e.g. strength) due to their microstructure that consists of periodic layers of high and low density material, which prevent crack propagation. Although such layered structures have the potential to increase the strength of engineered epoxy-ceramic composites relative to their constituents, synthetically replicating this class of layered structures in engineered materials has been challenging to date. To overcome this challenge, ultrasound freeze casting (UFC) was used to manufacture macroscale specimens of epoxy-ceramic composite materials with periodic layers of high and low density that mimic the structure of natural materials. The critical operating parameter of the UFC technique, the ultrasound operating frequency, was related to the resulting hardness, porosity, and flexural strength of the resultant epoxy-ceramic composite materials. Scanning electron microscopy and micro X-ray CT was used to visualize the microstructure of the specimens and connect it to the mechanical properties. The ultrasound operating frequency controlled the spacing of the layers as well as the local hardness of the epoxy-ceramic composite, which increased by up to 18%. The flexural strength of the epoxy-ceramic composite was also related to the ultrasound operating frequency, with a maximum increase of 52%

Helical and Bouligand Porous Scaffolds Fabricated by Dynamic Low Strength Magnetic Field Freeze Casting

Porous Fe3O4 scaffolds were fabricated while subject to a low (7.8 mT) magnetic field applied in helical and Bouligand motions using a custom-built tri-axial nested Helmholtz coils-based freeze-casting setup. This setup allowed for the control of a dynamic, uniform low-strength magnetic field to align particles during the freezing process, resulting in the majority of lamellar walls aligning within ± 30° of the magnetic field direction and a decrease in porosity by up to 42%. Similar to how helical and Bouligand structures in nature promote impact resistance, these magnetic field motions produced structures with improved high strain rate mechanical properties. Strain at failure was increased by up to 2 times as cracks deflected to match the applied angles of rotation of the magnetic field

Using graphene networks to build bioinspired self-monitoring ceramics

We acknowledge the EPSRC Grant Graphene 3D Networks (EP/K01658X/1), EPSRC founded Center for Advanced Structural Ceramics (CASC) at Imperial College London. O.T.P. thanks Nanoforce Technology Ltd. for their facilities and V.G.R. thank the European Commission (FP7—Marie Curie Intra-European Fellowship GRAPES). C.F. thanks the European Commission funding under the 7th Framework Programme (Marie Curie Initial Training Networks; grant number: 289958, Bioceramics for Bone Repair). N.N. acknowledges the funding from Imperial College Junior Research Fellowship. X-ray photoelectron spectra were obtained at the National EPSRC XPS Users’ Service (NEXUS) at Newcastle University, an EPSRC Mid-Range Facility. We acknowledge Dr. I.J.Villar-Garcia support on XPS data processing and analysis