Bio-compatible refractory metal alloys for patient-specific orthopaedic and dental implants


Although in principle, additive manufacturing (AM) technologies are perfect for ‘patient-specific’ applications, given 3D printing’s suitability in making complex one-off parts with no added cost, there are still some hurdles to overcome.

One hurdle that needs to be lept is that of the bio-compatibility of materials. With direct metal 3D printing, in particular to comply with constraints from the organisations like the FDA, concerns of the toxicology must be addressed.

Orthopaedic and dental implant materials are exposed to high mechanical loading. Even though many current materials used in the direct metal 3D printing processes, like stainless steel and cobalt-chrome alloys, can cope with the mechanical stresses, there are concerns surrounding the release of toxic or allergenic elements that could result in inflammation of tissue.

Metal alloys based on titanium (Ti) and niobium (Nb) represent higher biocompatibility with appropriate mechanical properties for avoiding stress-shielding and consecutive implant loosening. It is with this in mind that one of the forerunners of alloy materials development, H.C. Starck Tantalum and Niobium GmbH has designed AMPERTEC Spherical Ti-42Nb specifically for AM processes.

AMPERTEC Spherical Ti-42Nb powders are produced using electrode induction-melting gas atomisation (EIGA), the powders are fully spherical with a negligible amount of satellites. The spheroidal shape results in better processing properties in both powder bed fusion-based and laser-cladding processes.

Atomised alloys such as Ti-42Nb are compositionally entirely beta-phase (β-phase) alloys with a body-centred cubic (bcc) crystal structure, which is associated with a higher ductility to pure hexagonal close-packed (hcp) Ti or the commonly used Ti-6Al-4V alloy.

Thanks to unique processing properties, AMPERTEC Spherical Ti-42Nb powders can be printed to almost full density (99,95%) using the selective laser melting process. Internal stresses are usually low, accordingly, thermal post-processing such as diffusion annealing or HIP is not necessarily required. The phase composition is not affected by the laser melting process; similar to the atomised powders, as-printed Ti-42Nb is pure β-phase. Printed parts have a fine-grained microstructure with extremely homogeneous element distribution. Scanning electron microscopy with energy dispersive X-ray spectroscopy (SEM/EDX) investigations confirms that there is no segregation of Ti or Nb-rich phases.

Mechanical investigations by means of tensile and compression tests display a combination of high elasticity and strength. The Young’s modulus (tensile elasticity measure) of as-printed Ti-42Nb specimen measures ca. 60 GPa with strengths values of ca. 700 MPa. By comparison, standard Ti alloys such as Ti-6Al-4V of Ti-6Al-7Nb possess elastic moduli of 110 – 115 GPa, the elastic modulus of cortical bone is 16 – 22 GPa.

The closer match of Ti-42Nb in tensile elasticity to that of cortical bone means that stress shielding between bone and implant and associated inflammation or implant loosening due to mechanical mismatches is suppressed.

Finally, cell-biological investigations point to the fact that both osteoblasts (a cell that makes bones) and fibroblasts (the cell that synthesises collagen) exhibit higher metabolic activity on Ti-42Nb than on Ti-6Al-4V. This is of crucial importance since the implant adaption is mainly influenced by proper bone and tissue ingrowth, which are steered by osteoblasts and fibroblasts, respectively.

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15 May 2018

11:00





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