The formulation of novel calcium phosphate containing culture beads for cell therapy

Cell therapy has recently gained much attention as a novel treatment method for a range of diseases. Many early examples of mesenchymal stem cell use have focussed on infusion of a cell population and rely on the cells locating to the area of tissue damage. The development of implantable materials in the form of microcarriers to deliver cell populations to the site of injury could help enhance efficacy of treatment. Such carriers may also be used to expand cell populations $in$ $vitro$. In this thesis, a range of cell culture beads have been formulated using calcium phosphate ceramics, with and without the addition of a hydrogel such as gellan gum. The processing of the calcium phosphate (brushite) cement beads was shown to be critical to cell attachment, with the use of a crystallisation inhibitor in the formulation causing cell detachment. By conditioning the beads post manufacture or by using a process of granulation of brushite crystals, it was possible to generate beads that enable attachment and proliferation of the cells: a ~34 fold increase in the case of post treated beads and ~6 fold in the case of granulated beads. It was also shown that modification of gellan gum with nanoscale HA (nHA) at a concentration of 50 wt% allowed the control of mechanical properties by increasing yield strength and bulk modulus by four- and nine fold, respectively. Finally, it was shown that both ceramic and ceramic-hydrogel beads were conducive to bone formation when culturing MC3T3 pre-osteoblast cells in static and dynamic conditions, respectively

An investigation into patient-specific 3D printed titanium stents and the use of etching to overcome Selective Laser Melting design constraints

Due to limitations in available paediatric stents for treatment of aortic coarctation, adult stents are often used off-label resulting in less than optimal outcomes. The increasingly widespread use of CT and/or MR imaging for pre-surgical assessment, and the emergence of additive manufacturing processes such as 3D printing, could enable bespoke devices to be produced efficiently and cost-effectively. However, 3D printed metallic stents need to be self-supporting leading to limitations in their design. In this study, we investigate the use of etching to overcome these design constraints and improve stent surface finish. Furthermore, using a combination of experimental bench testing and finite element (FE) methods we investigate how etching influences stent performance. Then using an inverse finite element approach the material properties of the printed and etched stents were calibrated and compared. We show that without etching the titanium stents, the inverse FE approach underestimates the stiffness of the as-built stent (E = 33.89 GPa) when compared to an average of 76.84 GPa for the etched stent designs. Finally, using patient-specific finite element models the different stents’ performance were tested to assess patient outcomes and lumen gain and vessel stresses were found to be strongly influenced by the stent design and postprocessing. Within this study, etching is confirmed as a means to create open-cell stent designs whilst still conforming to additive manufacturing ‘rules’ and concomitantly improving stent surface finish. Additionally, the feasibility of using an in-vivo imaging-to-product development pipeline is demonstrated that enables patient-specific stents to be produced for varying anatomies to achieve optimum device performance.</p

Microscale stress-geometry interactions in an additively manufactured NiTi cardiovascular stent: A synchrotron dual imaging tomography and diffraction study

This study explores cardiovascular stents fabricated using laser powder bed fusion (LPBF); an emerging method to offer patient-specific customisable parts. Here, the shape memory alloy NiTi, in a near equiatomic composition, was investigated to deconvolve the material response from macroscopic component effects. Specifically, stress-geometry interactions were revealed, in-situ, for a minaturised cardiovascular stent subjected to an externally applied cylindrical stress whilst acquiring synchrotron X-ray imaging and diffraction data. The approach enabled the collection of spatially resolved micromechanical deformation data; the formation of stress-induced martensite and R-phase was evident, occurring in locations near junctions between stent ligaments where stress concentrations exist. In the as-fabricated condition, hardness maps were obtained through nanoindentation, demonstrating that the localised deformation and deformation patterning is further controlled by porosity and microstructural heterogeneity. Electron backscatter diffraction (EBSD) supported these observations, showing a finer grain structure near stent junctions with higher associated lattice curvature. These features, combined with stress concentrations when loaded will initiate localised phase transformations. If the stent was subjected to repeated loading, representing in-vivo conditions, these regions would be susceptible to cyclic damage through transformation memory loss, leading to premature component failure. This study highlights the challenges that must be addressed for the post-processing treatment of LABF-processed stents for healthcare-related applications

Tailoring selective laser melting process for titanium drug-delivering implants with releasing micro-channels

Ganoderma is a large, diverse and globally-distributed genus in the Basidiomycota that includes species causing a white rot form of wood decay on a variety of tree species. For the past century, many studies of Ganoderma in North America and other regions of the world have used the name G. lucidum sensu lato for any laccate (shiny or varnished) Ganoderma species growing on hardwood trees or substrates. Molecular studies have established that G. lucidum sensu stricto (Curtis) Karst is native to Europe and some parts of China. To determine the species of the laccate Ganoderma that are present in the United States, we studied over 500 collections from recently collected samples and herbarium specimens from hardwoods, conifers, and monocots. A multilocus phylogeny using ITS, tef1α, rpb1 and rpb2 revealed three well-supported clades, similar to previously reported findings. From the U.S. collections, thirteen taxa representing twelve species were identified, including: G. curtisii, G. lucidum sensu stricto, G. martinicense, G. oregonense, G. polychromum, G. ravenelii, G. sessile, G. tsugae, G. tuberculosum, G. cf. weberianum, G. zonatum, and Tomophagus colossus (syn. G. colossus). The species G. meredithiae is synonymized with G. curtisii, and considered a physiological variant that specializes in decay of pines. The designation G. curtisii f.sp. meredithiae forma specialis nov. is proposed. Species such as G. curtisii and G. sessile, once considered as G. lucidum sensu lato, were found to be divergent from one another, and highly divergent from G. lucidum sensu stricto. Morphological characteristics such as context tissue color and features (e.g. melanoid bands), basidiospore shape and size, geographic location, and host preference were found to aid in species identification. Surprisingly, G. lucidum sensu stricto was found in the U.S., but only in geographically restricted areas of northern Utah and California. These collections appear to have resulted from the introduction of this species into the United States possibly from mushroom growers producing G. lucidum outdoors. Overall, this study clarifies the chaotic taxonomy of the laccate Ganoderma in the United States, and will help to remove ambiguities from future studies focusing on the North American species of laccate Ganoderma

The design of additively manufactured lattices to increase the functionality of medical implants

The rise of antibiotic resistant bacterial species is driving the requirement for medical devices that minimise infection risks. Antimicrobial functionality may be achieved by modifying the implant design to incorporate a reservoir that locally releases a therapeutic. For this approach to be successful it is critical that mechanical functionality of the implant is maintained. This study explores the opportunity to exploit the design flexibilities possible using additive manufacturing to develop porous lattices that maximise the volume available for drug loading while maintaining load-bearing capacity of a hip implant. Eight unit cell types were initially investigated and a volume fraction of 30% was identified as the lowest level at which all lattices met the design criteria in ISO 13314. Finite element analysis (FEA) identified three lattice types that exhibited significantly lower displacement (10-fold) compared with other designs; Schwartz primitive, Schwartz primitive pinched and cylinder grid. These lattices were additively manufactured in Ti-6Al-4V using selective laser melting. Each design exceeded the minimum strength requirements for orthopaedic hip implants according to ISO 7206-4. The Schwartz primitive (Pinched) lattice geometry, with 10% volume fill and a cubic unit cell period of 10, allowed the greatest void volume of all lattice designs whilst meeting the fatigue requirements for use in an orthopaedic implant (ISO 7206-4). This paper demonstrates an example of how additive manufacture may be exploited to add additional functionality to medical implants

Adding functionality with additive manufacturing : fabrication of titanium-based antibiotic eluting implants

Additive manufacturing technologies have been utilised in healthcare to create patient-specific implants. This study demonstrates the potential to add new implant functionality by further exploiting the design flexibility of these technologies. Selective laser melting was used to manufacture titanium-based (Ti-6Al-4V) implants containing a reservoir. Pore channels, connecting the implant surface to the reservoir, were incorporated to facilitate antibiotic delivery. An injectable brushite, calcium phosphate cement, was formulated as a carrier vehicle for gentamicin. Incorporation of the antibiotic significantly (p=0.01) improved the compressive strength (5.8±0.7MPa) of the cement compared to non-antibiotic samples. The controlled release of gentamicin sulphate from the calcium phosphate cement injected into the implant reservoir was demonstrated in short term elution studies using ultraviolet-visible spectroscopy. Orientation of the implant pore channels were shown, using micro-computed tomography, to impact design reproducibility and the back-pressure generated during cement injection which ultimately altered porosity. The amount of antibiotic released from all implant designs over a 6hour period (<28% of the total amount) were found to exceed the minimum inhibitory concentrations of Staphylococcus aureus (16μg/mL) and Staphylococcus epidermidis (1μg/mL); two bacterial species commonly associated with periprosthetic infections. Antibacterial efficacy was confirmed against both bacterial cultures using an agar diffusion assay. Interestingly, pore channel orientation was shown to influence the directionality of inhibition zones. Promisingly, this work demonstrates the potential to additively manufacture a titanium-based antibiotic eluting implant, which is an attractive alternative to current treatment strategies of periprosthetic infections

Selective Laser Melting of Ti-6Al-4V: The Impact of Post-processing on the Tensile, Fatigue and Biological Properties for Medical Implant Applications

One of the main challenges in additive manufacturing (AM) of medical implants for the treatment of bone tissue defects is to optimise the mechanical and biological performance. The use of post-processing can be a necessity to improve the physical properties of customised AM processed implants. In this study, Ti-6Al-4V coupons were manufactured using selective laser melting (SLM) in two build orientations (vertical and horizontal) and subsequently post-processed using combinations of hot isostatic pressing (HIP), sandblasting (SB), polishing (PL) and chemical etching (CE). The effect of the different post-manufacturing strategies on the tensile and fatigue performance of the SLMed parts was investigated and rationalised by observing the surface topography. Vertically built samples showed higher yield strength (YS) and ultimate tensile strength (UTS) than the horizontal samples, increasing from 760.9 &plusmn; 22.3 MPa and 961.3 &plusmn; 50.2 MPa in the horizontal condition to 820.09 &plusmn; 16.5 MPa and 1006.7 &plusmn; 6.3 MPa in the vertical condition, respectively. After the HIP treatment, the ductility was substantially improved in both orientations; by 2.1 and 2.9 folds in the vertical and horizontal orientations, respectively. The vertically built samples demonstrated a superior ductility of 22% following HIP and polishing. Furthermore, chemical etching was found to be the most effective surface post-processing treatment to improve the fatigue performance after HIP, achieving the highest run-out strength of 450 MPa. Most importantly, chemical etching after HIP enhanced the cellular affinity of the surface, in addition to its good fatigue performance, making it a promising post-processing approach for bone implants where tissue integration is needed