Biodegradation Evaluation of Magnesium Alloys Corrosion Via Hydrogen Evolution Measurements

Recently, magnesium (Mg) based materials and its alloys are being explored for biomedical applications as a biodegradable metallic implant material (1, 2). However, the major disadvantage of magnesium alloy for biomedical applications is its low corrosion resistance especially in physiological environments (3). New magnesium based alloys and coating strategies are being developed to slow the corrosion of magnesium in physiological conditions (3). Surface modification strategies are also needed on magnesium to make it viable for the specific end application, for example drug delivery for stents and delivery of antibiotics for orthopedic devices. (4) Since these modifications are applications specific it is desired that these coatings do not significantly alter the corrosion behavior of the underlying magnesium material. Bio-corrosion of magnesium can be related to hydrogen gas evolution which would result as a byproduct of magnesium corrosion. This method (usually called hydrogen evolution method) is a reliable; easy to implement method for bio corrosion determination (5).

Self-assembled monolayers (SAMs) as a form of nanotechnology has being gaining a lot of attention for its potential medical applications due to the ease of modification of surface properties through the selection of the appropriate terminal functional group in the monolayer (6). SAMs have been also been reported for surface modification of metallic biomaterials (7). Our laboratory has recently reported the formation of (SAMs) of octadecylphosphonic acid (ODPA) on the native oxide layer of magnesium alloy (8).

In this manuscript we report the biodegradation evaluation of phosphonic SAMs coated magnesium alloys using hydrogen evolution measurements. 18-octadecylphosphonic acid (ODPA) and 16-Phosphonohexadecanoic acid (16-PA) SAMs were formed on magnesium alloy (1 x 1 cm) using protocols developed in our laboratory (6). After SAM formation the bio-corrosion behavior of the magnesium and SAM coated magnesium was evaluated using hydrogen evolution method.

Figure 1: Hydrogen Evolution Setup

Figure 1 schematically illustrates the hydrogen evolution setup. Test samples were soaked in phosphate buffer saline (PBS) test solution at 37^oC for a preset time (1, 3, 7, 14, 21 and 28 days. The emitted hydrogen volume was measured as a function of immersion time. Figure 2 shows the volume of hydrogen evolved over time for magnesium and magnesium coated samples. Figure 3 shows the magnesium hydroxide that was formed as a byproduct of the corrosion.

Figure 2:Hydrogen Evolution of ODPA, 16PA and Uncoated Mg alloy in PBS solution

Figure 3:  Magnesium hydroxide formation as result of biodegradation in PBS

 

Figure 2 showed that magnesium and magnesium coated substrates had similar hydrogen evolution profiles with the differences found to be statistically insignificant. The coatings on magnesium alloy did not significantly change the corrosion behavior of the underlying magnesium. Thus these SAM coatings could be used for surface modification of the magnesium for biomedical applications.

Reference:

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