Meta-biomaterials are part of the emerging concept of metamaterials that possess a desired combination of mechanical (i.e. negative Poisson’s ratio), mass transport (e.g. permeability and diffusivity) and biological properties (e.g. tissue regeneration performance).
Total hip replacement (THR) implants often encounter mechanical failure at the implant-bone interface (aseptic loosening), which limits their lifetime. The femoral part of THR is repeatedly loaded under bending for ~2 million cycles per year, which creates tensile loading and compression on either side of the neutral axis of the implant. The implant–bone interface is more susceptible to failure when subjected to tension as compared to compression. Since bone exhibits higher mechanical strength in compression than in tension, the side of the THR that experiences tension (i.e. retracts from the bone) is more susceptible to interface failure. Hence, it is necessary to design THR implants in such as way to create compression on both sides of its neutral axis.
Researchers at Delft University of Technology, The Netherlands, 3D Systems, Leuven, Belgium and University Medical Centre Utrecht, The Netherlands have demonstrated a proof-of-concept of applying a combination of rational design and additive manufacturing in the design of meta-biomaterials to improve longevity of implants. (Reference: Helena M. A. Kolken et al., Rationally designed meta-implants: a combination of auxetic and conventional meta-biomaterials, Mater. Horiz., 2017, DOI: 10.1039/C7MH00699C)
Two types of meta-biomaterials, one with a negative Poisson’s ratio (i.e. auxetic) (‘A’ in Fig. 1) while the other one with a positive Poisson’s ratio (i.e. conventional) (‘B’ in Fig. 1) were designed. Subsequently, both types of meta-biomaterials were combined to create a hybrid meta-biomaterial with different values of the Poisson’s ratio (‘C’ in Fig. 1). The meta-implants were then designed using these combined meta-biomaterials, in which the Poisson’s ratio of the meta-biomaterials changed around the neutral axis to compress the implant against the bone on both sides. Totally, six different combinations were designed and they were manufactured by selective laser melting (SLM) using biomedical-grade titanium alloy Ti6Al4V-ELI powders.
Fig. 1 Schematic drawings showing the topological designs of (A) auxetic and (B) conventional meta-biomaterials, (C) hybrid meta-biomaterials (left); and design of meta-implants (right): (C1) control type 1 with conventional hexagonal honeycombs. (H1) Hybrid type 1 with a 50/50 cell ratio. (C2) Control type 2 with re-entrant hexagonal honeycombs, showing the different parts of the implant: (1) top, (2) porous region and (3) bottom. (H2) Hybrid type 2 with a 50/50 cell ratio and a solid core. (H1) Hybrid type 1 showing the different parts of the implant: (1) top-middle-bottom and (2) porous region. (H3) Hybrid type 3 with a 70/30 cell ratio
Fig. 2 shows the photographs of the selective laser melted Ti6Al4V-ELI THR meta-implants (Fig. 2(a)); the test set-up in which the THR implant was loaded including bone-mimicking materials (Fig. 2(b)); and the horizontal strains in the bone-mimicking materials surrounding the meta-implants at t = 0 and t = 180 s at 1.5 mm displacement for C1, C2, H1, H2 and H3 (Fig. 2(c)).
Fig. 2 (a) Additively manufactured (selective laser melting) Ti6Al4V-ELI THR meta-implants; (b) test set-up in which the THR implant was loaded including bone-mimicking materials; and (c) Horizontal strains in the bone-mimicking materials surrounding the meta-implants at t = 0 and t = 180 s at 1.5 mm displacement for C1, C2, H1, H2 and H3.
The findings of the study clearly reveal that meta-implant with design H2 compress against the bone under repetitive loads that are applied during gait and other daily activities. According to the Hoffman’s failure criterion, this combination of compression and shear is less deleterious than tension and shear.
The current proof-of-concept study demonstrated the feasibility of applying rational design and metamaterials for the development of the next generation of medical devices. Nevertheless, the performance of these materials has to be evaluated using animal models and clinical trials.
T.S.N. Sankara Narayanan
For further information, the reader may kindly refer: Helena M. A. Kolken et al., Mater. Horiz., 2017, DOI: 10.1039/C7MH00699C)
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