Growth Cone Pathfinding: a competition between deterministic and stochastic events

BACKGROUND: Growth cone migratory patterns show evidence of both deterministic and stochastic search modes. RESULTS: We quantitatively examine how these two different migration modes affect the growth cone's pathfinding response, by simulating growth cone contact with a repulsive cue and measuring the resultant turn angle. We develop a dimensionless number, we call the determinism ratio Ψ, to define the ratio of deterministic to stochastic influences driving the growth cone's migration in response to an external guidance cue. We find that the growth cone can exhibit three distinct types of turning behaviors depending on the magnitude of Ψ. CONCLUSIONS: We conclude, within the context of these in silico studies, that only when deterministic and stochastic migration factors are in balance (i.e. Ψ ~ 1) can the growth cone respond constructively to guidance cues

Blood–brain barrier water exchange measurements using contrast-enhanced ASL

A technique for quantifying regional blood–brain barrier (BBB) water exchange rates using contrast-enhanced arterial spin labelling (CE-ASL) is presented and evaluated in simulations and in vivo. The two-compartment ASL model describes the water exchange rate from blood to tissue, (Formula presented.), but to estimate (Formula presented.) in practice it is necessary to separate the intra- and extravascular signals. This is challenging in standard ASL data owing to the small difference in (Formula presented.) values. Here, a gadolinium-based contrast agent is used to increase this (Formula presented.) difference and enable the signal components to be disentangled. The optimal post-contrast blood (Formula presented.) ((Formula presented.)) at 3 T was determined in a sensitivity analysis, and the accuracy and precision of the method quantified using Monte Carlo simulations. Proof-of-concept data were acquired in six healthy volunteers (five female, age range 24–46 years). The sensitivity analysis identified the optimal (Formula presented.) at 3 T as 0.8 s. Simulations showed that (Formula presented.) could be estimated in individual cortical regions with a relative error (Formula presented.) % and coefficient of variation (Formula presented.) %; however, a high dependence on blood (Formula presented.) was also observed. In volunteer data, mean parameter values in grey matter were: arterial transit time (Formula presented.) s, cerebral blood flow (Formula presented.) mL blood/min/100 mL tissue and water exchange rate (Formula presented.) s−1. CE-ASL can provide regional BBB water exchange rate estimates; however, the clinical utility of the technique is dependent on the achievable accuracy of measured (Formula presented.) values

Selective laser melting of aluminium alloys

Metal additive manufacturing (AM) processes, such as selective laser melting, enable powdered metals to be formed into arbitrary 3D shapes. For aluminium alloys, which are desirable in many high-value applications for their low density and good mechanical performance, selective laser melting is regarded as challenging due to the difficulties in laser melting aluminium powders. However, a number of studies in recent years have demonstrated successful aluminium processing, and have gone on to explore its potential for use in advanced, AM componentry. In addition to enabling the fabrication of highly complex structures, selective laser melting produces parts with characteristically fine microstructures that yield distinct mechanical properties. Research is rapidly progressing in this field, with promising results opening up a range of possible applications across scientific and industrial sectors. This paper reports on recent developments in this area of research as well as highlighting some key topics that require further attention

Nanoindentation shows uniform local mechanical properties across melt pools and layers produced by selective laser melting of AlSi10Mg alloy

Single track and single layer AlSi10Mg has been produced by selective laser melting (SLM) of alloy powder on a AlSi12 cast substrate. The SLM technique produced a cellular-dendritic ultra-fined grained microstructure. Chemical composition mapping and nanoindentation showed higher hardness in the SLM material compared to its cast counterpart. Importantly, although there was some increase of grain size at the edge of melt pools, nanoindentation showed that the hardness (i.e. yield strength) of the material was uniform across overlapping tracks. This is attributed to the very fine grain size and homogeneous distribution of Si throughout the SLM material

Mechanical Properties of Selective Laser Melted AlSi10Mg: Nano, Micro, and Macro Properties

The selective laser melting (SLM) of aluminium alloys is of great current interest at both the industrial and research levels. Aluminium poses a challenge to SLM compared with other candidate materials, such as titanium alloys, stainless steels, and nickel-based alloys, because of its high thermal diffusivity and low infrared absorptivity and tendency to result in relatively porous parts. However, recent studies have reported the successful production of dense AlSi10Mg parts using SLM. In this study, we report on the nano, micro, and macroscopic mechanical properties of dense AlSi10Mg samples fabricated by SLM. Nanoindentation revealed the hardness profile across individual melt pools building up the parts to be uniform. This is due to the fine microstructure and uniform chemical elements distribution developed during the process due to rapid solidification. Micro-hardness testing showed anisotropy in properties according to the build orientation driven by the texture produced during solidification. Lastly, the tensile and compressive behaviours of the parts were examined showing high strength under both loading conditions as well as adequate amounts of strain. These superior mechanical properties compared to those achieved via conventional manufacturing promote SLM as promising for several applications.Mechanical Engineerin

The BCC Unit Cell for Latticed SLM Parts; Mechanical Properties as a Function of Cell Size

The existing framework describing the mechanical properties of lattices places strong emphasis on one important property, the relative density of the repeating cells. In this work, we explore the effects of cell size, attempting to construct more complete models for the performance of lattices. This was achieved by examining the elastic modulus and ultimate tensile strength of latticed parts with a range of unit cell sizes and fixed density. The parts were produced by selective laser melting (SLM). The examined cell type was body-centred-cubic (BCC), a cell of high relevance for SLM because of its self-supporting structure. We obtained power law relationships for the mechanical properties of our latticed specimens as a function of cell size, which are similar in form to the existing laws for the density dependence. These can be used to predict the properties of latticed column structures comprised of BCC cells, and may be easily amended for other situations. In addition, we propose a novel way to analyse the elastic modulus data, which may lead to more general models, applicable to parts of varying size. Lastly, our general methodology may be of use in future studies which explore the other parameters that determine lattice performance; the choice of cell type, the global shape of the lattice structure and the type of stress.Mechanical Engineerin

Is graphene on copper doped?

Angle-resolved photoemission spectroscopy (ARPES) and X-ray photoemission spectroscopy have been used to characterise epitaxially ordered graphene grown on copper foil by low-pressure chemical vapour deposition. A short vacuum anneal to 200 °C allows observation of ordered low energy electron diffraction patterns. High quality Dirac cones are measured in ARPES with the Dirac point at the Fermi level (undoped graphene). Annealing above 300 °C produces n-type doping in the graphene with up to 350 meV shift in Fermi level, and opens a band gap of around 100 meV. Dirac cone dispersion for graphene on Cu foil after vacuum anneals (left: 200 °C, undoped; right: 500 °C, n-doped). Centre: low energy electron diffraction from graphene on Cu foil after 200 °C anneal. Data from Antares (SOLEIL)

Design and characterisation of an additive manufacturing benchmarking artefact following a design-for-metrology approach

We present the design and characterisation of a high-speed sintering additive manufacturing benchmarking artefact following a design-for-metrology approach. In an important improvement over conventional approaches, the specifications and operating principles of the instruments that would be used to measure the manufactured artefact were taken into account during its design process. With the design-for-metrology methodology, we aim to improve and facilitate measurements on parts produced using additive manufacturing. The benchmarking artefact has a number of geometrical features, including sphericity, cylindricity, coaxiality and minimum feature size, all of which are measured using contact, optical and X-ray computed tomography coordinate measuring systems. The results highlight the differences between the measuring methods, and the need to establish a specification standards and guidance for the dimensional assessment of additive manufacturing parts

On the development of twinning-induced plasticity in additively manufactured 316L stainless steel

A report on twinning-induced plasticity in 316L stainless steel manufactured by metal additive manufacturing (AM) is presented. A tapered tensile test geometry was used which enabled the investigation of twin formation over a range of strain levels in a single specimen. Hardness and twinning concentration were observed to increase with strain up to peak values of 380 ± 10 HV and 28 ± 4%, respectively. Furthermore, twin formation was found to be regulated by grain size and crystal texture. This methodology can be applied to new AM materials development and will inform the design of energy-absorbing structures that maximise the benefits of AM design and strain-hardenable materials

Heteroepitaxial growth of ferromagnetic MnSb(0001) films on Ge/Si(111) virtual substrates

Molecular beam epitaxial growth of ferromagnetic MnSb(0001) has been achieved on high quality, fully relaxed Ge(111)/Si(111) virtual substrates grown by reduced pressure chemical vapor deposition. The epilayers were characterized using reflection high energy electron diffraction, synchrotron hard X-ray diffraction, X-ray photoemission spectroscopy, and magnetometry. The surface reconstructions, magnetic properties, crystalline quality, and strain relaxation behavior of the MnSb films are similar to those of MnSb grown on GaAs(111). In contrast to GaAs substrates, segregation of substrate atoms through the MnSb film does not occur, and alternative polymorphs of MnSb are absent