This paper reviews current physicochemical and biochemical coating techniques that are investigated to enhance bone regeneration at the interface of titanium implant materials. By applying coatings onto titanium surfaces that mimic the organic and inorganic components of living bone tissue, a physiological transition between the non-physiological titanium surface and surrounding bone tissue can be established. In this way, the coated titanium implants stimulate bone formation from the implant surface, thereby enhancing early and strong fixation of bone-substituting implants. As such, a continuous transition from bone tissue to implant surface is induced. This review presents an overview of various techniques that can be used to this end, and that are inspired by either inorganic (calcium phosphate) or organic (extracellular matrix components, growth factors, enzymes, etc.) components of natural bone tissue. The combination, however, of both organic and inorganic constituents is expected to result into truly bone-resembling coatings, and as such to a new generation of surface-modified titanium implants with improved functionality and biological efficacy.

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Organic–Inorganic Surface Modifications for Titanium Implant Surfaces

Additive manufacturing (AM) technology from engineering has helped to achieve several advances in the medical field, particularly as far as fabrication of implants is concerned. The use of AM has made it possible to carry out surgical planning and simulation using a three-dimensional physical model which accurately represents the patient's anatomy. AM technology enables the production of models and implants directly from a 3D virtual model, facilitating surgical procedures and reducing risks. Furthermore, AM has been used to produce implants designed for individual patients in areas of medicine such as craniomaxillofacial surgery, with optimal size, shape and mechanical properties. This work presents AM technologies which were applied to design and fabricate a biomodel and customized implant for the surgical reconstruction of a large cranial defect. A series of computed tomography data was obtained and software was used to extract the cranial geometry. The protocol presented was used to create an anatomic biomodel of the bone defect for surgical planning and, finally, the design and manufacture of the patient-specific implant.

Cranial reconstruction: 3D biomodel and custom-built implant created using additive manufacturing

Zirconium alloys for biomedical applications are receiving increasing attention due to their two unique properties: 1) the formation of an intrinsic bone-like apatite layer on their surfaces in body environments, and 2) better compatibility with magnetic resonance imaging (MRI) diagnostics due to their low magnetic susceptibility, as well as their overall excellent biocompatibility, mechanical properties, and bio-corrosion resistance. In particular, since both of the MRI quality and speed depend on magnetic field strength, there is a compelling drive for use of high magnetic field strength (>3 Tesla) MRI systems. This paper provides a comprehensive review of the characteristics of commercially pure (CP) Zr and Zr-based alloys as orthopaedic and dental implant materials. These include their 1) phase transformations; 2) unique properties including corrosion resistance, biocompatibility, magnetic susceptibility, shape memory effect, and super-elasticity; 3) mechanical properties; 4) current orthopaedic and dental applications; and 5) the d-electron theory for Zr alloy design and novel Zr-alloys. The mechanical properties of Zr-based bulk metallic glasses (BMGs) and their application as implant materials are also assessed. Future directions for extending the use of Zr-alloys as orthopaedic and dental implants are discussed.

Zirconium Alloys for Orthopaedic and Dental Applications

https://hal.archives-ouvertes.fr/hal-00857502/document

Progress on the preparation of nanocrystalline apatites and surface characterization: Overview of fundamental and applied aspects

Considering that the densification level and the attendant quality of selective laser melted Ti6Al4V parts depend strongly on the operating temperature of the melting system, which is mainly controlled by the processing parameters. The processing parameters of selective laser melting were thus investigated in this study to fabricate denser Ti6Al4V parts without post-processes. Temperature distribution calculation was firstly carried out based on a three-dimensional model. It was found that there exists a great temperature gradient from the surface of powder bed to the experimental platform, and the maximum depth of molten powder layer is about 45 μm, very close to the total thickness of powder bed (50 μm) under the condition of laser power of 110 W and scan rate of 0.2 m/s. The Ti6Al4V parts with lower porosity and higher density were then well fabricated by experimental method under the condition of laser power of 110 W and scan rate of 0.2 m/s. The experimental results also indicate that the microstructures exhibit more and more pores and the layer structures are more and more obvious with the increase in the scan rate. Moreover, the microhardness measurement yields different values with increasing scan rate, owing to the increase of α phase and porosity.

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Process parameter selection for selective laser melting of Ti6Al4V based on temperature distribution simulation and experimental sintering

Titanium may be considered a relatively new engineering material. It was discovered much later than the other commonly used metals, its commercial application starting in the late 1940s. Its usage as an implant material began in the 1960s, despite the fact that titanium exhibits superior corrosion resistance and tissue acceptance when compared with stainless steels and Cr-Co-based alloys. This paper reviews the use of titanium and titanium alloys for use in biomedical applications.

Review: titanium and titanium alloy applications in medicine

A single component can often benefit from being built using more than a single processing technique. Here, we investigated the hybrid additive manufacturing (HAM) of Ti6Al4V using a combination of powder-bed fusion (PBF) and direct-energy deposition (DED). The aim was to identify critical areas and assess the performance of the hybrid process relative to the individual processes. The PBF sub-parts were built first, and then completed by DED. The builds were in the horizontal and vertical directions, so we could observe the mechanical anisotropy relative to the build direction. X-ray computed tomography, microstructural examinations, and tensile testing coupled with digital image correlation were employed to assess the parts. The as-built PBF surface can be used to build HAM Ti6Al4V samples with DED, thus eliminating steps like machining. The HAM samples built in horizontally had intermediate tensile strengths of about 1050 MPa, and in the vertical direction, about 860 MPa, i.e., lower than the DED samples. Strength-wise, horizontally built parts exceeded the requirements. However, a reduction in deposition size (especially <102 mm2) promoted a different temperature evolution and, in the worst-case scenario, heat accumulation, which led to the formation of an undesirable microstructure and local plastic deformation in the DED part. 

Hybrid additive manufacturing of Ti6Al4V with powder-bed fusion and direct-energy deposition

Additive manufacturing is a complicated process, including powder preparation and post-processing. The atmosphere during the processing of sensitive alloys like Ti6Al4V is among the most important factors. The N content influences the mechanical properties in a significant way. Therefore, it is essential to understand, recognise, and identify the problems in case of machine malfunction or human error. The influence of a low-quality atmosphere on the microstructure evolution and mechanical properties was investigated during the direct energy deposition of hybrid additive-manufactured Ti6Al4V. The properties of the built parts were characterised by chemical analyses, light and electron microscopy, and mechanical testing. High concentrations of N, which absorbed and dissolved with every deposition layer of the direct-energy-deposition process, promoted the formation of a layer of α-scale on the part’s surface. The presence of α-scale was confirmed by observing the sample surfaces and the lack of fusion pores where a layer of α-scale remained. The α-scale partially or entirely dissolved during the next deposition layer. The morphology of the partially melted, α-scale depended on the size of the deposition and, thus, on the temperature evolution during the direct energy deposition.

Matej Balazic

Papers

Review: titanium and titanium alloy applications in medicine

Hybrid additive manufacturing of Ti6Al4V with powder-bed fusion and direct-energy deposition

CHARACTERISATION OF THE MICROSTRUCTURE AND DEFECTS DURING THE DIRECT ENERGY DEPOSITION OF HYBRID Ti6Al4V IN A LOW-QUALITY ATMOSPHERE