Morphological induced improvements in the bulk mechanical properties of chemically etched additively manufactured Ti-6Al-4V micro-struts

Powder Bed Fusion (PBF) additive manufacturing techniques have enabled the fabrication of geometrically complex porous metallic lattice structures. Despite the many advantages of this approach, surface and near surface defects inherited from the fabrication process act to limit their structural and mechanical integrity. Consequently, surface cleaning techniques such as chemical etching have been increasingly adopted post process. Although the benefits of this technique have been captured across a range of lattice structures, the existing body of work is typically a characterisation of the engineering mechanical properties of lattices, rather than the bulk mechanical properties of the material. From a design perspective, characterisation of such properties is fundamental to prediction of mechanical performance and failure mechanisms. In this study, standardised micro-strut dog-bone geometries were adopted as a means of isolating geometrical variations between lattice designs; allowing us to study the bulk mechanical properties of the material. Incremental etching was conducted and changes to both the surface and morphological properties of the micro-structs was characterised. Improvements in these factors was associated with a corresponding increase in mechanical properties. Both strength and stiffness increased following removal of process inherited surface defects, which can be linked to a removal of non-load bearing material. Failure strain and fatigue resistance also improved following surface etching, although similar fracture surfaces were observed in both groups. Together these findings demonstrate the benefits of chemical etching for improving the mechanical properties of additively manufactured Ti-6Al-4V lattice structures.</p

Morphological induced improvements in the bulk mechanical properties of chemically etched additively manufactured Ti-6Al-4V micro-struts

Powder Bed Fusion (PBF) additive manufacturing techniques have enabled the fabrication of geometrically complex porous metallic lattice structures. Despite the many advantages of this approach, surface and near surface defects inherited from the fabrication process act to limit their structural and mechanical integrity. Consequently, surface cleaning techniques such as chemical etching have been increasingly adopted post process. Although the benefits of this technique have been captured across a range of lattice structures, the existing body of work is typically a characterisation of the engineering mechanical properties of lattices, rather than the bulk mechanical properties of the material. From a design perspective, characterisation of such properties is fundamental to prediction of mechanical performance and failure mechanisms. In this study, standardised micro-strut dog-bone geometries were adopted as a means of isolating geometrical variations between lattice designs; allowing us to study the bulk mechanical properties of the material. Incremental etching was conducted and changes to both the surface and morphological properties of the micro-structs was characterised. Improvements in these factors was associated with a corresponding increase in mechanical properties. Both strength and stiffness increased following removal of process inherited surface defects, which can be linked to a removal of non-load bearing material. Failure strain and fatigue resistance also improved following surface etching, although similar fracture surfaces were observed in both groups. Together these findings demonstrate the benefits of chemical etching for improving the mechanical properties of additively manufactured Ti-6Al-4V lattice structures.</p

Microstructural and mechanical insight into atherosclerotic plaques: an ex vivo DTI study to better assess plaque vulnerability

Non-invasive microstructural characterisation has the potential to determine the stability, or lack thereof, of atherosclerotic plaques and ultimately aid in better assessing plaques' risk to rupture. If linked with mechanical characterisation using a clinically relevant imaging technique, mechanically sensitive rupture risk indicators could be possible. This study aims to provide this link-between a clinically relevant imaging technique and mechanical characterisation within human atherosclerotic plaques. Ex vivo diffusion tensor imaging, mechanical testing, and histological analysis were carried out on human carotid atherosclerotic plaques. DTI-derived tractography was found to yield significant mechanical insight into the mechanical properties of more stable and more vulnerable microstructures. Coupled with insights from digital image correlation and histology, specific failure characteristics of different microstructural arrangements furthered this finding. More circumferentially uniform microstructures failed at higher stresses and strains when compared to samples which had multiple microstructures, like those seen in a plaque cap. The novel findings in this study motivate diagnostic measures which use non-invasive characterisation of the underlying microstructure of plaques to determine their vulnerability to rupture. </p

Phosphotungstic acid (PTA) preferentially binds to collagen- rich regions of porcine carotid arteries and human atherosclerotic plaques observed using contrast enhanced micro-computed tomography (CE-µCT)

Background and aims: Atherosclerotic plaque rupture in the carotid artery can cause small emboli to travel to cerebral arteries, causing blockages and preventing blood flow leading to stroke. Contrast enhanced micro computed tomography (CEμCT) using a novel stain, phosphotungstic acid (PTA) can provide insights into the microstructure of the vessel wall and atherosclerotic plaque, and hence their likelihood to rupture. Furthermore, it has been suggested that collagen content and orientation can be related to mechanical integrity. This study aims to build on existing literature and establish a robust and reproducible staining and imaging technique to non-destructively quantify the collagen content within arteries and plaques as an alternative to routine histology.  Methods: Porcine carotid arteries and human atherosclerotic plaques were stained with a concentration of 1% PTA staining solution and imaged using MicroCT to establish the in situ architecture of the tissue and measure collagen content. A histological assessment of the collagen content was also performed from picrosirius red (PSR) staining.  Results: PTA stained arterial samples highlight the reproducibility of the PTA staining and MicroCT imaging technique used with a quantitative analysis showing a positive correlation between the collagen content measured from CEμCT and histology. Furthermore, collagen-rich areas can be clearly visualised in both the vessel wall and atherosclerotic plaque. 3D reconstruction was also performed showing that different layers of the vessel wall and various atherosclerotic plaque components can be differentiated using Hounsfield Unit (HU) values.  Conclusion: The work presented here is unique as it offers a quantitative method of segmenting the vessel wall into its individual components and non-destructively quantifying the collagen content within these tissues, whilst also delivering a visual representation of the fibrous structure using a single contrast agent. </p

Development of in silico models to guide the experimental characterisation of penile tissue and inform surgical treatment of erectile dysfunction

This paper presents a computational study to investigate the mechanical properties of human penile tissues. Different experimental testing regimes, namely indentation and plate-compression tests, are compared to establish the most suitable testing regime for establishing the mechanical properties of the different penile tissues. An idealised MRI-based geometry of the penis, containing different tissue layers, is simulated using the finite element (FE) method to enable realistic predictions of the deformation of the penis. Unlike the linear elastic models used in the literature to-date, hyperelastic isotropic/anisotropic material models are used to capture material nonlinearity and anisotropy. The influence of material properties, morphological variations, material nonlinearity and anisotropy are investigated. Moreover, the implantation of an inflatable penile prosthesis (IPP) is simulated to assess the effects of the implantation procedure, material nonlinearity, and anisotropy on tissue stresses. The results indicate that the interior layers of the penis do not affect the overall stiffness of the penis in the indentation test, while the plate-compression test is able to capture the effects of these layers. Tunica Albuginea (TA) is found to have the most significant contribution to the total stiffness of the penis under load. It can also be observed that buckling occurs in the septum of the penis during the compression tests, and different morphologies dictate different compressive behaviours. There is a clear need for future experimental studies on penile tissues given the lack of relevant test data in the literature. Based on this study, plate-compression testing would offer the most insightful experimental data for such tissue characterisation. </p