Introduction: Inclusions of certain inorganic elements in silica-based mesoporous bioactive glasses (MBGs) investigated for bone regeneration is a subject of interest because the positive biological effects that may occur when implanted[1]. Thus, gallium is reaching notoriety because is found at sites of rapid bone remodeling[2]. Moreover, Ga3+ has recently emerged as a new generation of antibacterial ions for treating and preventing localized infections associated with orthopaedic surgery[3]. In this communication, three mesoporous glasses SiO2–CaO–P2O5–Ga2O3, all containing 5 mol-% of Ga2O3 were synthesized, characterized and compared with an analogous Ga-free MBG (see Table). The aim of the study was relate the location of gallium ions in the glass network with the in vitro bioactive behavior and with the amount of Ga3+ ions released from glasses after soaking them in common cell culture media. The potential advantages of these glasses in bone regeneration applications and their antibacterial capacity were evaluated.
Experimental: MBGs were obtained by evaporation induced self-assembly by using a method described elsewhere[4]. Glasses were characterized by XRD, Nitrogen adsorption, TEM, SEM, EDX and solid-state NMR. In vitro bioactivity tests of MBGs were performed as both pellets and powders that were soaked in Simulated Body Fluid (SBF). After the in vitro tests, materials were characterized by FTIR and SEM-EDX and the variations in the chemical composition of solution were analysed by ICP. Ion release tests were performed in Todd Hewitt Broth (THB) and Dulbecco's Modified Eagle Medium (DMEM) in the conditions previously described[5].
Results and Discussion: Ga_1, Ga_2 and Ga_3 exhibited worm-like mesoporous structure determined by TEM and high SBET surface area and pore volume (see the Table). 29Si NMR analysis allowed to assess the location of Ga3+ ions in the glasses network (as network former or modifier) to explain their different in vitro behaviours. Ga_1 was coated by an apatite-like layer after only 1 d in SBF, as is observed by FTIR and XRD (see the Figure, top). This quick in vitro response was attributed to the higher amount of modifier ions (Ca2+ and Ga3+) in this MBG which consequently exhibited a higher depolymerized network. Furthermore, 31P NMR results suggested the presence of a Ca and P clustered zone in Ga_1 that would also favour its bioactivity. However, Ga_2 and Ga_3 show less favourable structural features. Accordingly, Ga_2 required 3 d and Ga_3 was not coated by apatite even after 7 d in SBF.
The cumulative release of Ga3+ ions in THB and DMEM was measured to assess the cytocompatibility and the antibacterial capacity of glasses. Ga_1 showed the maximum release of Ga3+ ions in both DMEM and THB (see the Figure, bottom). In DMEM, the maximum concentration reached was 2.5 ppm, i.e. below the toxicity limit of Ga3+ in blood plasma (14 ppm)[6]. In THB it was 9.8 ppm, that is, 140 times higher than the IC90 of P. aeruginosa and 2 times lower than that of S. aureus[7].
Conclusions: Ga_1 showed a very quick in vitro bioactive response in SBF due to the higher amount of modifier ions and the higher depolymerized network of this glass. Ga3+ amounts released from Ga_1 to DMEM and THB are inside non-cytotoxic levels, but in the effective range against P. aeruginosa and not far to the effective range against S. Aureus. Therefore, Ga_1 is a promising material for bone regeneration.
Ministerio de Ciencia e Innovación, Spain (project MAT2012-35556),; Agening Network of Excellence, Spain (CSO2010-11384-E); University of Modena and Reggio Emilia, Italy for the financial support by “Progetto Mobilità Internazionale”.
References:
[1] Wu, C.; Chang, J. Multifunctional mesoporous bioactive glasses for effective delivery of therapeutic ions and drug/growth factors. J. Control. Release. 2014, 193, 282-295.
[2] Verron, E.; Loubat, A.; Carle, G. F.; Vignes-Colombeix, C.; Strazic, I.; Guicheux, J.; Rochet, N.; Bouler, J. M.; Scimeca, J. C. Molecular effects of gallium on osteoclastic differentiation of mouse and human monocytes. Biochem. Pharmacol. 2012, 83, 671-679.
[3] J. R. Edwards, K. D. Peterson, Y. Mu, S. Banerjee, K. Allen- Bridson, G. Morrell, M. A. Dudeck, D. A. Pollock and T. C. Horan, National Healthcare Safety Network (NHSN) report: data summary for 2006 through 2008, issued December 2009. Am. J. Infect. Control, 2009, 37, 783.
[4] Shruti, S.; Salinas, A.J.; Malavasi, G.; Lusvardi, G.; Menabue, L.; Ferrara, C.; Mustarelli, P.; Vallet-Regí, M. Structural and in vitro study of cerium, gallium and zinc containing sol-gel bioactive glasses J. Mater. Chem. 2012, 22, 13698-13706.
[5] Sanchez-Salcedo, S.; Shruti, S.; Salinas, A.J.; Malavasi, G.; Menabue, L.; Vallet-Regì, M. In vitro antibacterial capacity and cytocompatibility of SiO2–CaO–P2O5 meso-macroporous glass scaffolds enriched with ZnO. J. Mater. Chem. B 2014, 2, 4836 -4847
[6] D. Campoccia, L.Montanaro, C.R. Arciola. The significance of infection related to orthopedic devices and issues of antibiotic resistance. Biomaterials 2006, 27, 233-2339.
[7] Minandri, F.; Bonchi, C.; Frangipani, E.; Imperi, F.; Visca, P. Promises and failures of gallium as an antibacterial agent. Future Microbiol. 2014, 9, 379-397.