Design and Characterization of Phosphatizing Coatings for Magnesium Implants

TL;DR

Researchers developed a coating for magnesium implants that slows down their corrosion in the body, making them more suitable for medical use.

Contribution

A phosphatizing coating strategy is introduced to control magnesium implant degradation in physiological environments.

Findings

Coatings induced magnesium phosphate formation, effectively limiting surface degradation.

Thermally cross-linked HEC improved coating stability and slightly retarded degradation.

Coated AZ31 implants showed biocompatibility and potential bioactivity in vitro.

Abstract

Magnesium alloys
are promising biodegradable implant materials,
but their rapid corrosion in physiological environments limits their
clinical applications. This work is focused on the development of
cementitious coatings inducing magnesium phosphate formation on magnesium
AZ31 alloys. First, the alloy surfaces immersed in orthophosphoric
acid (OPA) solutions with six additives of various functions (sodium
chloride, magnesium chloride, calcium nitrate, magnesium nitrate,
trisodium citrate, and hydroxyethyl cellulose (HEC)) were comparatively
analyzed to understand the effect of solution chemistry on surface
evolution. OPA solutions were also saturated with respect to magnesium
ions, which effectively limited surface degradation. Sample mass and
solution pH were monitored for 21 days, and depositions were characterized
using SEM, EDX, and electrochemical methods to identify the surface
composition and investigate its effectiveness against Mg degradation.
In the next stage, alloy plates were dip-coated with the multicomponent
suspension of the most effective composition (OPA, MgCl2, HEC, and Mg-saturated deionized water). The phase evolution of
the dried samples in 3.5 wt % NaCl solution was monitored with regular
gravimetric, pH, quantitative XRD, SEM, EDX, and electrochemical Tafel
analyses. Samples passivated despite the high chlorine concentration,
as initially formed newberyite crystals, were replaced by Mg oxychlorides,
Mg phosphates, and Mg hydroxide in order, in response to the shift
in solution pH from acidic to alkaline values that is driven by the
dissolution and transformation of the alloy and coating phases. Thermally
cross-linking HEC improved the stability of the coatings, which slightly
retarded the degradation kinetics. In vitro cell culture tests validated
the coated AZ31 as both being biocompatible and potentially bioactive.
Thus, the phosphatizing coating approach offers a promising strategy
for controlled biodegradation of magnesium implants in physiological
environments.

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