Russell A. Harris

Selective laser sintering of hydroxyapatite reinforced polyethylene composites for bioactive implants and tissue scaffold development

Abstract Selective laser sintering (SLS) has been investigated for the production of bioactive implants and tissue scaffolds using composites of high-density polyethylene (HDPE) reinforced with hydroxyapatite (HA) with the aim of achieving the rapid manufacturing of customized implants. Single-layer and multilayer block specimens made of HA-HDPE composites with 30 and 40 vol % HA were sintered successfully using a CO2 laser sintering system. Laser power and scanning speed had a significant effect on the sintering behaviour. The degree of particle fusion and porosity were influenced by the laser processing parameters, hence control can be attained by varying these parameters. Moreover, the SLS processing allowed exposure of HA particles on the surface of the composites and thereby should provide bioactive products. Pores existed in the SLS-fabricated composite parts and at certain processing parameters a significant fraction of the pores were within the optimal sizes for tissue regeneration. The results indicate that the SLS technique has the potential not only to fabricate HA-HDPE composite products but also to produce appropriate features for their application as bioactive implants and tissue scaffolds.

In vitro biocompatibility of hydroxyapatite-reinforced polymeric composites manufactured by selective laser sintering

The selective laser sintering (SLS) technique was used to manufacture hydroxyapatite-reinforced polyethylene and polyamide composites as potential customized maxillofacial implants. In vitro tests were carried out to assess cellular responses, in terms of cell attachment, morphology, proliferation, differentiation, and mineralized nodule formation, using primary human osteoblast cells. This study showed that the SLS composite processed was biocompatible, with no adverse effects observed on cell viability and metabolic activity, supporting a normal metabolism and growth pattern for osteoblasts. Positive von Kossa staining demonstrated the presence of bone-like mineral on the SLS materials. Higher hydroxyapatite content composites enhanced cell proliferation, increased alkaline phosphatase activity, and produced more osteocalcin. The present findings showed that SLS materials have good in vitro biocompatibility and hence demonstrated biologically the potential of SLS for medical applications.

Fabrication of porous bioactive structures using the selective laser sintering technique.

Abstract Hydroxyapatite, a ceramic with which natural bone inherently bonds, has been incorporated into a polymer matrix to enhance the bioactivity of implant materials. In order to manufacture custom-made bioactive implants rapidly, selective laser sintering has been investigated to fabricate hydroxyapatite and polyamide composites and their properties investigated. One objective of this research was to identify the maximum hydroxyapatite content that could be incorporated into the matrix, which was sintered at various parameters. The study focused on investigating the control of porosity and pore size of the matrix by manipulating the selective laser sintering parameters of the laser power and laser scan speed. The interception method was used to analyse the internal porous morphology of the matrices which were cross-sectioned through the vertical plane. Most notably, all structures built demonstrated interconnection and penetration throughout the matrix. Liquid displacement was also used to analyse the porosity of the matrices. The laser power showed a negative relationship between porosity and variation in parameter values until a critical power value was reached. However, the same relationship for laser scan speed matrices was inconsistent. The effects of the laser power and laser scanning speed on the features of porous structures that could influence cell spreading, proliferation, and bone regeneration are presented.

Evaluation of CO2 and Nd:YAG Lasers for the Selective Laser Sintering of HAPEX®

Abstract This paper evaluates and compares the performance of a CO2 and Nd:YAG laser for the selective laser sintering (SLS) of a commercial hydroxyapatite reinforced polyethylene (HA-HDPE) bioactive ceramic polymer composite material. Single-line and layer specimens were produced to compare the effects of different lasers on the material sintering. It was found that the processing window was much larger for the CO2 laser as compared to the Nd:YAG laser. Furthermore, the Nd:YAG processing window was highly dependent on the pulse width and pulse repetition rate parameter settings. Furthermore, the processing windows for both the laser systems were affected by the particle size of the HA-HDPE powders. The degree and mechanism of particle fusion existing in the composites layers were greatly influenced by the laser source and particle size. The results presented in this work clearly indicate that the CO2 laser would present a better performance than the Nd:YAG laser for the SLS of HAPEX® in terms of operation range, speed, processing efficiency, and, subsequently, greater potential as an SLS processing method for bioactive implant products.

The effects and interactions of fabrication parameters on the properties of selective laser sintered hydroxyapatite polyamide composite biomaterials

Purpose – Hydroxyapatite‐polymer composite materials are being researched for the development of low‐load bearing implants because of their bioactive and osteoconductive properties, while avoiding modulus mismatch found in homogenous materials. For the direct production of hydroxyapatite‐polymer composite implants, selective laser sintering (SLS) has been used and various parameters and their effects on the physical properties (micro and macro morphologies) have been investigated. The purpose of this paper is to identify the most influential parameters on the micro and macro pore morphologies of sintered hydroxyapatite‐polymer composites.Design/methodology/approach – A two‐level full factorial experiment was designed to evaluate the effects of the various processing parameters and their effects on the physical properties, including open porosity, average pore width and the percentage of pores which could enable potential bone regeneration and ingrowth of the sintered parts. The density of the sintered par...

Application of rapid manufacturing techniques in support of maxillofacial treatment: evidence of the requirements of clinical applications

Abstract The concept of applying rapid manufacturing technology to maxillofacial treatment has been described previously; however, these reports did not take into account the practicality of its actual incorporation into clinical practice. Patents in the field are based on imaging techniques combined with rapid manufacturing, which theoretically lead to reconstruction of faces. Some cases studies reported have dealt with the manufacture of prostheses on the laboratory scale. Here two case studies are reported that used imaging and rapid manufacturing techniques for making an ear prosthesis and a burns mask for two patients. Laser scanning was chosen for imaging and Thermojet printing and fused deposition modelling for rapid manufacturing. Outcomes of the study were threefold: improvement in the process, improvement in patient care, and clinical application of existing technology to healthcare. With further research this technology may aid maxillofacial prosthetists in busy facial clinics, reduce patient clinic time, and improve the final product.

Effects of material morphology and processing conditions on the characteristics of hydroxyapatite and high-density polyethylene biocomposites by selective laser sintering

Abstract Hydroxyapatite (HA), a ceramic to which bone inherently bonds, incorporated into a polymer matrix enhances the bioactivity of implants. In order to rapid-manufacture bioactive implants, selective laser sintering (SLS) has been used to fabricate HA and high-density polyethylene (HDPE) composite (HA-HDPE). The properties of SLS-fabricated specimens have been investigated. The main aspects to be considered in the SLS technology are the properties of the materials used in the process and processing parameters (PPs). HA-HDPE composite specimens have been fabricated using five different powders with variations in particle size (PS), PS distribution, and five different laser PPs. The sintering height, the width, and the shrinkage of the specimens were determined and the effects of the particles and PPs on the physical properties were investigated. The HA-HDPE specimens were found to be highly porous and the sintered density and porosity of the specimens were influenced by the PS and PPs. The interparticle connectivity and the pore size range of the specimens were found to be predominantly determined by the PS and to a lesser extent also influenced by the PPs. The strength of these specimens and the relationship with porosity are discussed.

The scoliosis of Richard III, last Plantagenet King of England: diagnosis and clinical significance

Richard III was king of England from 1483 to 1485, after declaring his nephew, Edward V, illegitimate. On Aug 20, 1485, Richard was killed in battle with the rebel Lancastrian claimant Henry Tudor at Bosworth. His body was carried back to Leicester and buried in the Greyfriars Minor Friary, where it remained until its excavation in 2012, when it was seen to have a severe scoliosis. Famously, Shakespeare described Richard III as “hunchbacked” in his eponymous play of 1593. There has been considerable disagreement whether this “hunchback” was real or an invention of his enemies after death, with political motivations. However, the chronicler John Rous wrote around 1490 that Richard “was small of stature, with a short face and unequal shoulders, the right higher and the left lower.” This description is compatible with the presence of a rightsided scoliosis. We analysed the skeleton macroscopically for evidence of spinal curvature and related lesions. From CT 3D reconstructions of each bone, we created polymer replicas and built a model of the spine to recreate its alignment in life (fi gure, appendix). The apex of the right-sided thoracic curve noted at excavation was at vertebrae T8–T9. The Cobb angle, determined from vertical excavation photos, was 75° from the upper border of T6 to the lower border of T11. Since this was measured supine, whereas clinical angles are taken standing, we estimate the Cobb angle to have been in the range 70–90° during life. The curve was well balanced, with cervical and lumbar spines reasonably well aligned (King Moe type 3). Abnormalities of individual vertebrae (eg, wedging of vertebral end plates, lateral angulation of spinous processes, asymmetry of facet joints) were restricted to the thoracic region (appendix). The foramen magnum was normal in size and shape. The 3D reconstruction closely matches the 2D images recorded at excavation, and shows the spiral nature of the scoliosis (appendix, video). Determining the cause of Richard’s scoliosis allows us to estimate the age at which it developed, and how it may have aff ected him. Since the spinal ligaments are some of the last to decompose after death, and in this case had partly ossifi ed, the position of the vertebrae should show only minimum change from the time of burial, having been surrounded by soil. Such small change is supported by the similarity to the reconstructed model, which relied on joint morphology to determine each joint position. The absence of structural spinal abnormalities, such as hemivertebrae and unilateral bars, makes congenital scoliosis improbable. Neuromuscular causes, such as cerebral palsy, are unlikely because of the normal structure, muscle markings, and cortical thickness of the legs and hips, compatible with a normal weight-bearing gait. Skeletal changes associated with syndromes such as Marfan’s (eg, high arched palate and tall stature) were not present, and a normal foramen magnum makes a Chiari malformation unlikely. The subtle nature of the changes in vertebral anatomy suggest onset in the last few years of growth, which is compatible with adolescent onset idiopathic scoliosis, probably starting after 10 years of age. The physical disfi gurement from Richard’s scoliosis was probably slight since he had a well balanced curve. His trunk would have been short relative to the length of his limbs, and his right shoulder a little higher than the left. However, a good tailor and custom-made armour could have minimised the visual impact of this. A curve of 70–90° would not have caused impaired exercise tolerance from reduced lung capacity, and we identifi ed no evidence that Richard would have walked with an overt limp, because the leg bones are symmetric and well formed.

Part shrinkage anomilies from stereolithography injection mould tooling

Abstract The use of stereolithography (SL) tooling allows plastic parts to be produced by injection moulding in a very short time due to the speed of mould production. One of the supposed advantages of the process is that it provides a low volume of parts that are the same as parts that would be produced by the conventional hard tooling in a fraction of the time and cost. However, this work demonstrates different rates of polymer shrinkage are developed by parts produced by SL and conventional tooling methods. These revelations may counter the greatest advantages of the SL injection moulding tooling process as the parts do not replicate those that would be produced by conventional hard tooling. This work identifies the different shrinkage that occurs in mouldings produced by an SL mould as compared to those produced from an aluminium mould. The experiments utilise two very different types of polymers and two mould geometries, which are processed in the same manner so that the heat transfer characteristics of the moulds are isolated as the only experimental variable. The work demonstrates how the two mould materials exhibit very different rates of expansion due to the temperature profiles experienced during moulding. This expansion must be compensated for to establish the total amount of shrinkage incurred by moulded parts. The compensation is derived by a mathematical approach and by modelling using finite element analysis. Both techniques depend upon knowledge of the thermal conditions during moulding. Knowledge of these thermal conditions are obtained by real-time data acquisition and simulated by FEA modeling. The application of the findings provide knowledge of the complete shrinkage values relating to the mould material and polymer used which would enable the production of geometrically accurate parts.

Characterization of selective laser sintered hydroxyapatite based biocomposite structures for bone replacement

Integration of the bone into the implant is highly desirable for the long-term performance of the implant. The development of a bone–implant interface is influenced by the surface morphology and roughness, surface wettability and porosity of the implants. This study characterizes these important properties of a hydroxyapatite-based biocomposite structure fabricated by selective laser sintering (SLS) with a comparison to a moulded specimen. The sintered specimens exhibited a rougher surface with open surface pores and a highly interconnected internal porous structure. It was shown that the characteristics of the powder particles used in the SLS provided a more influential means to modify the surface morphology and the features of the internal pores than laser parameter variation. The correlation of wettability and porous structure shows that although surface open pores could help cell ingrowth and bone regeneration, they resulted in a poorer wettability of the materials, which may not encourage initial cell attachment and adhesion. The potential solution to improve the wettability and cell anchorage is discussed.