Introduction: Even though titanium and its alloys are generally good materials for metallic implants, they lack per se in terms of biocompatibility, due to their intrinsic inertness. One of the strategies to improve their surface properties is to apply a coating, able to ensure a beneficial, pre-conditioning film of ions and proteins which will support the further cell adhesion and wound healing.
Thus, it is fundamental to study the electrokinetic behaviour of the implant surfaces, in connection with surface topography and wettability, in order to predict a priori the influence of the material properties on the events occurring at the bio-interface [1][2].
Materials and Methods: Electrochemical anodization (EA) was applied to grow TiO2-nanotubes (NTs), forming dense coatings on titanium foils [3].
The NTs characterisation was carried out in terms of morphology (scanning electron microscopy, SEM), wetting by sessile drop contact angle, and zeta potential (ZP) by streaming potential technique. The latter method allowed the quantification of surface charge in different electrolytic solutions and different ionic strengths, relevant for the biological assessment. Titration curves (ZP vs. pH) were systematically obtained in sodium chloride, phosphate buffer saline, artificial saliva, and Dulbecco’s Modified Eagle Medium. The experimental data were analysed and interpreted by a theoretical mean-field model of electric double layer which takes into account the orientational ordering of water dipoles and the asymmetric finite size of hydrated anions and cations in electrolyte solution [4]. Additionally, the effect of the ionic strength on ZP was quantitatively modelled by fitting the experimental data.
Results and Discussion: Tailoring the synthesis parameters applied during the EA, different NTs morphologies and porosity were obtained, which resulted in distinctive surface properties (i.e. high exposed surface area and wettability).
The ZP dependence on the pH revealed that, even though all the surfaces were negatively charged at physiological pH in the biologically-relevant electrolytes, ZP differed in magnitude. The isoelectric point shifted by mean of the present ions nature (valence, ionic and hydrated radius, affinity with the surface). The ZP dependence on the electrolytic concentration was also evaluated (Figure 1).
All these effects were computed by theoretical models, resulting in equations which represent the behaviour of the electrical double layer around the surface. Moreover, the effect of hydrophilicity, dipole interactions, and surface exposed area at the top-edge of NTs on ZP was also considered.
Figure 1. ZP dependence of the TiO2-NTs array (15 nm diameter) on the concentration of NaCl saline solution.
Conclusion: To our best knowledge, for the first time a systematic electrokinetic study of NTs-coated surfaces for body implants was carried out in terms of surface charge in biologically-relevant electrolytes. The results were supported by theoretical calculations, so that equations specific to our system, but applicable also to other nanoporous structures, were designed. The outcomes broad the scenario to the characterization of the bio-interface at the implant surface.
This work was supported by the Slovenian Research Agency (project grant numbers P2-0084 and P2-0232).
References:
[1] Lorenzetti, M.; Bernardini, G.; Luxbacher, T.; Santucci, A.; Kobe, S.; Novak, S., Surface properties of nanocrystalline TiO 2 coatings in relation to the in vitro plasma protein adsorption. Biomedical Materials 2015, 10 (4), 045012.
[2] Lorenzetti, M.; Luxbacher, T.; Kobe, S.; Novak, S., Electrokinetic behaviour of porous TiO2-coated implants. Journal of Materials Science: Materials in Medicine 2015, 26 (6), 1-4.
[3] Kulkarni, M.; Patil-Sen, Y.; Junkar, I.; Kulkarni, C. V.; Lorenzetti, M.; Iglič, A., Wettability studies of topologically distinct titanium surfaces. Colloids and Surfaces B: Biointerfaces 2015, 129 (0), 47-53.
[4] Gongadze, E.; Iglič, A., Asymmetric size of ions and orientational ordering of water dipoles in electric double layer model – an analytical mean-field approach. Electrochimica Acta 2015, 178), 541–545.