MATSE 403 Paper

8/8/2019 MATSE 403 Paper

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MATSE 403 ‐ Final Paper

Part III: State ‐of ‐the ‐art Advances/ Production/ Synthesis

Daniel Cook

Introduction

As mentioned in previous portions of this paper, Magnesium is an ideal candidate as a biodegradable

material. It is highly biocompatible (approximately three ‐hundred milligrams per day is the

recommended daily intake), has a high primary stability, and has a superior strength to weight ratio.

Even with such promising characteristics Magnesium has yet to make a significant contribution in the field biomaterials. A major factor preventing its application is its high rate of corrosion in‐vivo. High rates

of corrosion are the source of two problems. First, magnesium corrodes in‐vivo through the following

equation:

2 ↔

As can be seen from this one mole of magnesium produces one mole of hydrogen gas, causing a large accumulation of subcutaneous gas bubbles. The second issue with high corrosion rates is the tissue the

implant is supporting will not have sufficient time to heal before the implant completely corrodes. This

is most evident in bone fracture and stent use. Both bone and arteries require approximately six weeks to fully heal, meaning the implant must last this amount of time. This section of the paper will look at what methods are being investigated to control the problem of rapid corrosion.

Alloying

Alloying magnesium with more corrosion resistant materials is the first method being investigated. Two of the main alloys currently being researched are WE43 and AZ91D. The composition of these alloys are as follows:

WE43Element Weight Percent Yttrium 4.11 Neodymium 2.28 Rhenium 0.98 Zirconium 0.45

Dysprosium 0.27 Magnesium 91.91

AZ91DElement Weight Percent Aluminum 8.1 Zinc 1 Silicon 0.3



Manganese 0.13 Magnesium 90.47

There are two methods used to produce these alloys, depending on the intended application. The first method is die casting. This method is used when the implant needs to be a bulk metal, such as when

the implant is used for bone repair. Die casting involves heating the metals until they reach a molten state. Pressure is then used to force this liquid into a mold where it cools and hardens, forming the

alloy.

The second method for production is electron beam physical vapor deposition. This method is preferred when the alloy must be very thin (micrometers range), or when the implant must have a very specific surface morphology. These requirements are common to stent use. In this method electrons ejected

from a charged tungsten filament bombard the metals, causing them to vaporize. The vapor containing

the metals then precipitates, forming a thin solid alloy.

The purpose of adding these metals to pure magnesium is that they provide electrochemical protection, as well as acting as hardeners. From an electrochemical perspective, all of the listed alloying metals are

more noble than magnesium thus providing cathodic protection. From a physical point ‐of ‐view many of the metals, specifically aluminum, zinc, and the rare earth metals cause solid solution strengthening.

Solid solution strengthening helps prevent dislocations from easily sliding, thus increasing the yield strength of the material.

The benefit of alloying can be seen by looking at table x which displays the experimental corrosion rate

of both WE43 and AZ91D tested in simulated body fluid at thirty seven degrees Celsius.

Alloy Corrosion Rate (mm/year) AZ91D 2.8 WE43 6.9 Pure Magnesium 105

Coatings It is immediately apparent that the addition of alloying elements significantly decreases the rate of corrosion. However even with such drastic drops in corrosion rate, some issues still remain. While bulk

metal applications can typically be made thicker to increase the amount of time before total

degradation, applications of thin films typically have very tight space requirements meaning increasing the thickness of the implant is not viable. This is where another, more recent, research topic in magnesium biomaterials has taken hold. This new topic involves the formation of a very thin coating on top of the magnesium implant.



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m can signifi

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ll as testing

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nd

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