Effect of alkali-acid-heat chemical surface treatment on electron beam melted porous titanium and its apatite forming ability

Suzan Bsat (Carleton University)
Saber Yavari (Delft University of Technology)
Maximilian Munsch (implantcast GmbH)
Edward Valstar (Delft University of Technology)
Amir Zadpoor (Delft University of Technology)

Introduction

Advanced additive manufacturing techniques such as electron beam melting (EBM), can produce highly porous structures that resemble the mechanical properties and structure of native bone. However, for orthopaedic applications, such as joint prostheses surface coatings or bone substitution, the surface must also be biofunctionalized to promote bone growth. Alkali acid heat (AlAcH) treatment has been shown to effectively biofunctionalize the surface of plasma sprayed[1] and selective laser melted[2] porous titanium by creating nanotopographical features and modifying the surface chemistry, while maintaining adequate mechanical properties The current work evaluates the use of AlAcH treatment to biofunctionalize the surface of porous Ti-6Al-4V fabricated by EBM by examining its apatite forming ability through simulated body fluid (SBF) testing. Various molar concentrations and immersion times of the alkali treatment were used to determine optimal parameters.

Materials and Methods

Porous Ti-6Al-4V samples (EPORE®), designed and manufactured by implantcast were prepared by EBM. The samples were exposed to AlAcH treatment by first immersing the samples in either 3, 5 or 10M NaOH at 60°C for either 6 or 24 hours. After the alkali treatment, the samples were immersed in ultrapure water at 40°C for 24 hours then in 0.5mM HCl at 40°C for 24 hours. After the acid treatment, the samples were washed with ultrapure water then dried in an oven at 40°C for 24 hours before they were placed in the furnace at 600°C for 1 hour dwelling time. All surface treated and as manufactured (AsM) samples were immersed in SBF as per ISO 23317 for 3 weeks to evaluate their apatite forming ability. The surface topography and chemistry were examined before and after immersion in SBF using SEM and EDS.

Results

The AlAcH treatment successfully modified the topographical and chemical characteristics of EBM porous titanium surface creating nanotopographical features ranging from 200-300 nm in size (Fig. 1 vs. Fig. 2) with a crystalline titania layer ideal for apatite formation. An increase in molar concentration and/or immersion time of alkali treatment resulted in an increase in the number of nanotopographical features per unit area as well as the amount of crystalline titania on the surface suggesting that stronger molar concentration/longer immersion time alkali treatment may produce more promising results for apatite forming ability. After 3 weeks immersion in SBF there was no Ca or P present on the surface of AsM porous titanium while both elements were present on all AlAcH treated samples except those exposed to 3M, 6 hr alkali treatment (Table 1). Samples exposed to the strongest and longest alkali treatment had the greatest wt.% of Ca and P.

Discussion and Conclusion

Studies have shown that nanotopographical features help promote the initial formation of Ca and P, and later apatite or bone [3]. Based on this understanding of the significance of nanofeatures, the current results suggest that an AlAcH treatment with higher NaOH molar concentration is ideal as stronger molar concentrations of NaOH resulted in increased nanotopographical features per unit area potentially providing more opportunity for interaction between the implant surface and cells. Stronger molar concentrations and longer immersion times of NaOH are also favorable as they resulted in a more prominent titania layer, ideal for apatite formation. After SBF immersion no Ca or P was present on AsM surfaces, however, both were present on all AlAcH treated samples except those exposed to 3M, 6 hr alkali treatment. It is hypothesized that the low concentration and limited time may have contributed to the lack of Ca formation. The greatest amount of Ca and P was observed for samples exposed to 10M, 24 hr alkali treatment which coincides with the greatest density of nanotopographical features and the most prominent titania layer. Although no apatite was visually observed, the presence of Ca and P does indicate that AlAcH treatment does encourage the formation of both elements as AsM surfaces contained neither. It is important to consider that SBF tests only stimulate chemical bioactivity pathways, while cell culture tests stimulate chemical and biological bioactivity pathways. In some cases the results of SBF and cell culture tests disagree [4], presumably due to differing bioactivity pathways.The lack of apatite may therefore be explained by the absence of biological bioactivity pathways. The study is limited in that parameters of the alkali treatment were varied, however, various other parameters should be considered to find ideal conditions for apatite formation as apatite forming ability greatly depends on the parameters chosen. Also, SEM/EDS elemental analysis is limited in characterizing the surface of porous titanium and should only be used for preliminary detection of apatite by visual observation and Ca and P formation; following studies should use x-ray diffraction to detect apatite phase.

Figure 1. (a) Macrograph of test sample and (b) SEM micrograph of as manufactured

Figure 2. SEM micrographs of samples after AlAcH treatment for various alkali parameters (a) 3M, 6 hr (b) 3M, 24 hr (c) 5M, 6 hr (d) 5M, 24 hr (e) 10M, 6 hr and (f) 10M, 24 hr

Table 1. EDS elemental analysis (wt. %) after 3 week SBF immersion for AsM and AlAcH treated samples with various alkali concentrations and immersion times

References

[1] Takemoto, M. et al. Osteoinductive porous titanium implants: effect of sodium removal by dilute HCl treatment, Biomat, 2006, 27, 2682-91. [2] Yavari, S. et al. Effects of bio-functionalizing surface treatments on the mechanical behavior of open porous titanium biomaterials, J Mech Behavior Biomed Mat, 2014, 36, 109-19. [3] Brammer, K. et al. TiO2 nanotubes for bone regeneration, Trends Biotech, 2012, 30, 315-22. [4] Yavari, S. et al. Bone regeneration performance of surface-treated porous titanium, Biomat, 2014, 35, 6172-81.

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