Bioactive and antimicrobial macro-/micro-nanoporous selective laser melted Ti-6Al-4V alloy for biomedical applications

Abstract

Metal Additive Manufacturing (AM) technology is an emerging technology in biomedical field due to its unique ability to manufacture customized implants [Patients-specific Implants (PSIs)] replicating the complex bone structure from the relevant metal powders. PSIs could be developed through any AM technology, but the ultimate challenge lies in integrating the metallic implant with the living bone. Considering this aspect, in the present study, Ti alloy (Ti-6Al-4V) powder has been used to fabricate scaffolds of channel type macropores with 0-60% porosity using selective laser melting (SLM) and subsequent post-treatments paving way for surface microporosities. Surface chemical and subsequent heat treatments were carried out on thus developed Ti alloy scaffolds to improve its bioactivity, antibacterial activity and osteoblastic cell compatibility. NaOH and subsequent Ca(NO₃)₂/AgNO₃ treatment induced the formation of a nanoporous network structure decorated with Ca-Ag ions. Ag nanoparticles covering the entire scaffold provided antibacterial activity and the presence of Ca²⁺ ions with anatase TiO₂ layer further improved the bioactivity and osteoblastic cell compatibility of the scaffold. Therefore, SLM technology combined with heat treatment and surface modification could be effectively utilized to create macro-micro-nano structure scaffolds of Ti alloy that are bioactive, antibacterial, and cytocompatible.

Keywords: Antibacterial activity; Bioactivity; Osteoblastic cell compatibility; Patient-specific implants; Selective laser melting; Ti–6Al–4V alloy powder.

Figure 1

(a) SEM image of as-purchased Ti–6Al–4V powder used for the study. Powder morphology appeared to be spherical with a range of particle size. (b) XRD spectra of Ti–6Al–4V powder shows peaks corresponding to pure Ti metal.

Figure 2

(a) Photographs of SLM Ti–6Al–4V cubical samples with varying pore dimensions from 200 to 1000 μm. For comparison dense sample image is also shown. (b) Graphical representation of density/porosity Vs pore size as measured by Archimedes principle.

Figure 3

(a) SEM images of a typical SLM built Ti–6Al–4V samples with square shaped pores. (b) SEM images of inner walls of as-printed porous Ti–6Al–4V sample and heat treated at 1200 and 1300 °C. As-printed samples showed weakly bonded Ti–6Al–4V particles even up to 1200 °C and these particles became soften when sintered above 1300 °C and finally bonded to the walls of the components leading to micro-pockets.

Figure 4

SEM images of porous Ti–6Al–4V samples (a) heat treated at 1300 °C, (b) & (c) subjected to subsequent soaking in NaOH solution and further heat treatment at 600 °C. NaOH treatment showed the formation of fine porous network structure both walls as well as melted particles inside the pores confirming the uniform chemical treatment.(d) & (e) SEM image & EDS result of porous Ti–6Al–4V samples, heat treated at 1300 °C, subjected to NaOH and subsequently treated with a mixture of Ca and Ag nitrate solutions and heat treatment at 600 °C.

Figure 5

Raman spectra of porous Ti–6Al–4V samples heat treated at 1300 °C, subjected to various chemical and heat treatments.

Figure 6

(a) & (b) SEM images of porous Ti–6Al–4V samples, subjected to various chemical and heat treatments and soaked in SBF for 3 days, and observed at outer surface as well as inside the pores. Bone-like apatite particles could be observed even in the inner walls of chemical and heat treated samples.

Figure 7

(a) Antibacterial study carried out for porous Ti–6Al–4V samples, subjected to various chemical and heat treatments. Untreated Ti–6Al–4V as well as NaOH and heat-treated sample did not show any antibacterial activity (presence of bacterial colonies) while Ca–Ag decorated Ti–6Al–4V showed good antibacterial activity (no bacterial colonies). (b) Quantitative analysis of antibacterial activity for the samples measured as % bacterial colony formation.

Figure 8

(a) Cell viability of SLM built porous Ti–6Al–4V samples subjected to various chemical and thermal treatments as measured by MTT assay. (b) CLSM images of MG 63 cells adhered on as-printed Ti–6Al–4V samples, SLM built porous Ti–6Al–4V samples sintered at 1300 °C subjected to various optimized chemical and heat treatments, after acridine orange staining. Significant amount of MG 63 cells adherence on Ca–Ag sample indicates the non-toxicity of the Ag concentration present on the Ti–6Al–4V sample surface.