John Albert Thornby

3D printing of porous hydroxyapatite scaffolds intended for use in bone tissue engineering applications.

A systematic characterisation of bone tissue scaffolds fabricated via 3D printing from hydroxyapatite (HA) and poly(vinyl)alcohol (PVOH) composite powders is presented. Flowability of HA:PVOH precursor materials was observed to affect mechanical stability, microstructure and porosity of 3D printed scaffolds. Anisotropic behaviour of constructs and part failure at the boundaries of interlayer bonds was highlighted by compressive strength testing. A trade-off between the ability to facilitate removal of PVOH thermal degradation products during sintering and the compressive strength of green parts was revealed. The ultimate compressive strength of 55% porous green scaffolds printed along the Y-axis and dried in a vacuum oven for 6h was 0.88 ± 0.02 MPa. Critically, the pores of 3D printed constructs could be user designed, ensuring bulk interconnectivity, and the imperfect packing of powder particles created an inherent surface roughness and non-designed porosity within the scaffold. These features are considered promising since they are known to facilitate osteoconduction and osteointegration in-vivo. Characterisation techniques utilised in this study include two funnel flow tests, scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR), compressive strength testing and computed tomography (CT).

Inconsistency in 9 mm bullets: Correlation of jacket thickness to post-impact geometry measured with non-destructive X-ray computed tomography

Fundamental to any ballistic armour standard is the reference projectile to be defeated. Typically, for certification purposes, a consistent and symmetrical bullet geometry is assumed, however variations in bullet jacket dimensions can have far reaching consequences. Traditionally, characteristics and internal dimensions have been analysed by physically sectioning bullets--an approach which is of restricted scope and which precludes subsequent ballistic assessment. The use of a non-destructive X-ray computed tomography (CT) method has been demonstrated and validated (Kumar et al., 2011 [15]); the authors now apply this technique to correlate bullet impact response with jacket thickness variations. A set of 20 bullets (9 mm DM11) were selected for comparison and an image-based analysis method was employed to map jacket thickness and determine the centre of gravity of each specimen. Both intra- and inter-bullet variations were investigated, with thickness variations of the order of 200 μm commonly found along the length of all bullets and angular variations of up to 50 μm in some. The bullets were subsequently impacted against a rigid flat plate under controlled conditions (observed on a high-speed video camera) and the resulting deformed projectiles were re-analysed. The results of the experiments demonstrate a marked difference in ballistic performance between bullets from different manufacturers and an asymmetric thinning of the jacket is observed in regions of pre-impact weakness. The conclusions are relevant for future soft armour standards and provide important quantitative data for numerical model correlation and development. The implications of the findings of the work on the reliability and repeatability of the industry standard V50 ballistic test are also discussed.

Additive manufacture of impedance matching layers for air-coupled ultrasonic transducers

A key problem in designing an efficient ultrasonic transducer for operating in a low acoustic impedance medium such as air is the large impedance mismatch between the active piezoceramic material and the load medium. While acoustic matching layers can be added to the face of the piezoceramic, the associated manufacturing difficulties and reliability can impact upon the cost and longevity of the resultant transducer. This paper presents some preliminary investigations conducted using an additive manufacturing technique, to develop a new material system for matching layer fabrication for air-coupled ultrasonic transducers. Results to date are very encouraging, and could result in a robust, reproducible, economical and improved fabrication method for air-coupled transducers.

Correction to: Quantitative topographic anatomy of the femoral ACL footprint: a micro-CT analysis

An author has corrected their first name and updated their email address - see the affiliation section below. Daniel G. Norman should now be Danielle G. Norman as shown in the authorgroup section above.

Quantitative topographic anatomy of the anterior cruciate ligament femoral footprint - do the ridges exist

Objectives: Anatomic ACL reconstruction has become an accepted technique to restore functional stability to the knee. Bony landmarks have been described which aid placement of the graft in mid bundle or specific anteromedial/posterolateral bundle positions. The objective of this study was to produce objective quantitative validation of the lateral intercondylar ridge and bifurcate ridge using micro CT technology, so a to better understand these important anatomical landmarks. Methods: Cadaveric human knees were imaged using a XT 320 H LC subjects (mean length 9.3 mm; mean height 1.5 mm) with the bifurcate ridge present in 14% (mean length 4.3 mm, height 0.9 mm). A relief map of the ACL footprint was produced which showed the individual variability of the anatomical ridges. Conclusions: This study demonstrates that the bony anatomical landmarks of the ACL footprint are a variable entity in individuals. When present, they can be used as a guide for femoral tunnel placement in ACL reconstruction. However, if absent, other techniques need to be adopted such as the ‘ruler technique’ or intraoperative fluoroscopy, so as to be able to reproducibly replicate anatomic tunnel positioning.