In silico evaluation of additively manufactured 316L stainless steel stent in a patient-specific coronary artery.

Additive manufacturing (AM) is an emerging method for the fabrication of stents, which is cost-saving and capable of producing personalised stent designs. However, poor surface finish and dimension discrepancy in the manufactured stents can significantly affect not only their own mechanical behavior but also mechanical response of arteries. This study investigates the effects of surface irregularities and dimension discrepancy of a 316L stainless steel stent, manufactured using laser powder bed fusion (LPBF), on its biomechanical performance, in comparison with the original design and a commercial stent. In silico simulations of stent deployment in a patient-specific coronary artery, based on intravital optical coherency tomography imaging, are conducted to assess the stent deformation as well as arterial stress and damage. Severe plastic strain concentrations (with a maximum value of 1.93) occur in the LPBF stent after deployment due to surface irregularities, suggesting a high risk of stent fracture. The LPBF stent is harder to expand due to its thicker struts and closed-cell design (diameter of 4.14 mm at the peak inflating pressure during deployment, compared to 4.58 mm and 4.65 mm for the designed and MULTI-LINK RX ULTRA stents, respectively). Also, the LPBF stent induces a higher level of stress concentration (with a maximum value of 23.04 MPa) to the arterial layers, suggesting a higher risk of tissue damage and in-stent restenosis. This study demonstrates a clear need for further development of the AM process for manufacturing medical implants, especially the surface finish and dimension accuracy.

Surface Free Energy Dominates the Biological Interactions of Postprocessed Additively Manufactured Ti-6Al-4V.

Additive manufacturing (AM) has emerged as a disruptive technique within healthcare because of its ability to provide personalized devices; however, printed metal parts still present surface and microstructural defects, which may compromise mechanical and biological interactions. This has made physical and/or chemical postprocessing techniques essential for metal AM devices, although limited fundamental knowledge is available on how alterations in physicochemical properties influence AM biological outcomes. For this purpose, herein, powder bed fusion Ti-6Al-4V samples were postprocessed with three industrially relevant techniques: polishing, passivation, and vibratory finishing. These surfaces were thoroughly characterized in terms of roughness, chemistry, wettability, surface free energy, and surface ζ-potential. A significant increase in Staphylococcus epidermidis colonization was observed on both polished and passivated samples, which was linked to high surface free energy donor γ- values in the acid-base, γAB component. Early osteoblast attachment and proliferation (24 h) were not influenced by these properties, although increased mineralization was observed for both these samples. In contrast, osteoblast differentiation on stainless steel was driven by a combination of roughness and chemistry. Collectively, this study highlights that surface free energy is a key driver between AM surfaces and cell interactions. In particular, while low acid-base components resulted in a desired reduction in S. epidermidis colonization, this was followed by reduced mineralization. Thus, while surface free energy can be used as a guide to AM device development, optimization of bacterial and mammalian cell interactions should be attained through a combination of different postprocessing techniques.

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.

Microstructural Evolution, Mechanical Properties, and Preosteoblast Cell Response of a Post-Processing-Treated TNT5Zr ß Ti Alloy Manufactured via Selective Laser Melting.

A Ti-34Nb-13Ta-5Zr (TNT5Zr) ß Ti alloy with a high strength-to-modulus ratio has been developed, showing its potential to become another candidate material in load-bearing implant applications. This work mainly investigates the microstructural evolution, mechanical properties, and biocompatibility of a post-processing-treated TNT5Zr alloy manufactured via selective laser melting (SLM). Transmission electron microscopy observation shows the existence of the single beta grain matrix and alpha precipitates along the grain boundary in the SLM + HIP manufactured TNT5Zr alloy (TNT5Zr-AF + HIP), and ellipsoidal nano-sized intragranular α″ precipitates (approx. 5-10 nm) were introduced after the subsequent low-temperature aging treatment. The precipitation strengthening enables the SLM + HIP + aging manufactured TNT5Zr (TNT5Zr-AF + HIPA) alloy to show a comparable ultimate tensile strength (853 ± 9 MPa) to that of the reference material (Ti64-AF + HIP, 926 ± 23 MPa). Including the inferior notch-like surface of the test pieces, the slip-band cracking that occurs in this ductile TNT5Zr-AF + HIPA alloy is regarded as the main factor in determining its fatigue strength (170 MPa). In vitro short-term biocompatibility evaluation reveals almost no significant difference in the preosteoblast viability, differentiation, and mineralization between TNT5Zr-AF + HIPA and the reference biomaterial (Ti64-AF + HIP).

The influence of zirconium content on the microstructure, mechanical properties, and biocompatibility of in-situ alloying Ti-Nb-Ta based ß alloys processed by selective laser melting.

This study investigates Ti-Nb-Ta based ß alloys with different zirconium additions (0, 5, 9 wt%) manufactured by SLM. A low level of as-fabricated defects is obtained: the relative density of TNT (Z) alloys is >99.97% with the keyhole size in a range of 3-20 µm. BF TEM images combining SAD patterns of TNT(Z) alloys show single ß phase obtained inside the beta matrix; BF-STEM images reveal potential nano-scale grain boundary alpha phase precipitation. Zirconium functions as a neutral element in these high ß-stabilized Ti-Nb-Ta based alloys. An increase in Vickers hardness and UTS caused by zirconium additions is observed, which is explained by beta grain refinement because higher degree of undercooling occurs. Corrosion ions of TNT(Z) alloys released from immersion testing at each time intervals show extremely small concentrations (<10 µg/L). It indicated that good biocompatibility during culture with the negligible corrosion ions. High strength-to-modulus ratio ß Ti alloys together with excellent biological response show their prospect for biomedical applications.

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 ± 22.3 MPa and 961.3 ± 50.2 MPa in the horizontal condition to 820.09 ± 16.5 MPa and 1006.7 ± 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.

Novel Hybrid Manufacturing Process of CM247LC and Multi-Material Blisks.

The study on CM247LC used the traditional approach for Near-Netshape Hot Isostatic Pressing (NNSHIP) with sacrificial low carbon steel tooling, which was built using Selective Laser Melting (SLM), to produce a shaped CM247LC blisk. The assessment of the microstructure focused on both the exterior components in order to determine the depth of the Fe-diffusion layer and on the interior microstructure. Samples were extracted from the Hot Isostatic Pressed (HIPped) components for tensile testing at both room and elevated temperatures. The components were scanned to assess the geometrical shrinkages due to Hot Isostatic Pressing (HIPping). An oversized blisk was also produced based on the measurements as a demonstrator component. In addition, a further study was carried out on a novel idea that used a solid IN718 disk in the centre of the blisk to create a multi-material component.

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.

Additive manufacturing of magnetic shielding and ultra-high vacuum flange for cold atom sensors.

Recent advances in the understanding and control of quantum technologies, such as those based on cold atoms, have resulted in devices with extraordinary metrological performance. To realise this potential outside of a lab environment the size, weight and power consumption need to be reduced. Here we demonstrate the use of laser powder bed fusion, an additive manufacturing technique, as a production technique relevant to the manufacture of quantum sensors. As a demonstration we have constructed two key components using additive manufacturing, namely magnetic shielding and vacuum chambers. The initial prototypes for magnetic shields show shielding factors within a factor of 3 of conventional approaches. The vacuum demonstrator device shows that 3D-printed titanium structures are suitable for use as vacuum chambers, with the test system reaching base pressures of 5 ± 0.5 × 10-10 mbar. These demonstrations show considerable promise for the use of additive manufacturing for cold atom based quantum technologies, in future enabling improved integrated structures, allowing for the reduction in size, weight and assembly complexity.

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.