Magnesium-Based Stents: Testing & Controlling Corrosion Rates

[jared530]

Hi everyone,

Noob to the forums, but forum junkie in different hobbies. Anyways, my senior project is to identify appropriate bench test to characterize degradation behaviors of magnesium-based alloys (stents). Form of final device and complexities of an in vivo environment can impact results, so made assumptions whenever possible.

Also to identify and test options to slow/or control corrosion rates of mg-based alloys.

Currently were modeling different stent designs in COMSOL, and modeling the degradation rate(erosion corrosion) as a function of ion concentration, pH and flow velocity.

Our team is having some trouble formulating equations to plug into COMSOL.

The question is, does anyone have any suggestions on how I can arrive at a time dependent relationship for erosion corrosion of a metal as a functions of ion concentration, pH and flow velocity?

Please feel free to ask any questions.

 

[Enthalpy]

Sorry it's not exactly your question, but...

I don't expect a software to make any decent prediction of corrosion rate.
The best human experts don't make accurate predictions. They experiment.

Corrosion rate does not relate to a limited set of parameters like pH, ion concentration and flow velocity!
If your software proposes this to deduce a corrosion rate, be sure the prediction will be meaningless.

Corrosion rate does heavily depend on the alloy composition, on its thermal history... For stainless steel, if welds are to resist corrosion, you take a variant with <0.02% C instead of <0.06% C - just as an example. But a bit of chlorine ions in water would let you choose an other alloy, with Mo.

More: in a medical implant, the release of toxic ions is more important than the corrosion rate. Do you enjoy zirconium in your patient's body? Chromium? Usual in magnesium.

I'm also surprised that people consider magnesium for an implant. Though the Mg ingots I had did not corrode quickly, you should consider that implants use very special alloys, because stainless isn't good enough for them. They have specially-designed cobalt and titanium alloys, produced only for implants.

For such a demanding use (vital, in fact) I'd never rely on a software.

 

[jared530]

Sorry it's not exactly your question, but...

I don't expect a software to make any decent prediction of corrosion rate.
The best human experts don't make accurate predictions. They experiment.

Corrosion rate does not relate to a limited set of parameters like pH, ion concentration and flow velocity!
If your software proposes this to deduce a corrosion rate, be sure the prediction will be meaningless.

Corrosion rate does heavily depend on the alloy composition, on its thermal history... For stainless steel, if welds are to resist corrosion, you take a variant with <0.02% C instead of <0.06% C - just as an example. But a bit of chlorine ions in water would let you choose an other alloy, with Mo.

More: in a medical implant, the release of toxic ions is more important than the corrosion rate. Do you enjoy zirconium in your patient's body? Chromium? Usual in magnesium.

I'm also surprised that people consider magnesium for an implant. Though the Mg ingots I had did not corrode quickly, you should consider that implants use very special alloys, because stainless isn't good enough for them. They have specially-designed cobalt and titanium alloys, produced only for implants.

For such a demanding use (vital, in fact) I'd never rely on a software.

Thank you so much for your reply. Your actually very right, variables that effect corrosion are essentially unlimited and it would be impossible to model the rate in any software. I guess we can use the model only as a stress-point indicator to formulate an appropriate geometry for the stent.

I realize the metal ions absorbed into the blood stream should be biocompatible. The composition were supposed to be testing wasn’t specified by the company who’s hosting this project, so I’m guessing we can alloy with what we feel appropriate. But magnesium should be the main component because the scope of the project is to control the degradation rate of Mg-based stents to 6-8 months, and after this period, should start degrading and absorbing into the blood stream. This is mainly to avoid a second surgery, which would happen if you used SS or NiTi.

Do you have any ideas on how we can control this degradation rate? Currently bare-metal magnesium degrades in a period of 1 month, and we would like to extend this to 6-8 months. Currently we have come up with the options: heat treatments of the Mg-alloy; and coating the metal with a biodegradable polymer.

 

[Enthalpy]

Again an offroad question, sorry for that, but...

Do you have an absolutely safe means to guarantee that only ions from your stent will move in the blood? Metal chips are not desired in a patient!

Slower degradation: maybe with purer magnesium, but it should get even softer then.
Could you try to anodize your alloy? (I'm not very sure it helps with magnesium...)

 

[jared530]

Again an offroad question, sorry for that, but...

Do you have an absolutely safe means to guarantee that only ions from your stent will move in the blood? Metal chips are not desired in a patient!

Slower degradation: maybe with purer magnesium, but it should get even softer then.
Could you try to anodize your alloy? (I'm not very sure it helps with magnesium...)

The patient will experience "chunking" of metal especially w/ bare Mg because of the super fast rate.

Would anodizing result in a extremely slow degradation rate(over 10 months)? Or would the oxide even break down in the body?