Biomedical Materials and Devices Overview

Magnesium as a Biodegradable and

Bioabsorbable Material for Medical
Implants
Harpreet S. Brar, Manu O. Platt, Malisa Sarntinoranont, Peter I. Martin, and Michele V. Manuel

For many years, stainless steel, co- are able to be metabolized by the body,
balt-chromium, and titanium alloys How would you… and thus are bioabsorbable.2,3
have been the primary biomaterials …describe the overall significance Polymers were the first materials to
used for load-bearing applications. of this paper? be used as commercial biodegradable
However, as the need for structural ma- Medical advances have significantly and bioabsorbable implant materials.
increased the average life
terials in temporary implant applica- expectancy and have resulted The earliest and most commonly used
tions has grown, materials that provide in an ever increasing aging absorbable materials include poly-
short-term structural support and can population. Billions of dollars are glycolic acid (PGA), poly-lactic acid
spent in cardiac and orthopedic (PLA), and poly-dioxanone (PDS).4,5
be reabsorbed into the body after heal- implants alone. However, most
ing are being sought. Since traditional of the implants being used today However, these materials are limited
metallic biomaterials are typically bio- are only biocompatible and by their low mechanical properties and
compatible but not biodegradable, the not bioabsorbable. Magnesium radiolucency.6 Low strength severely
combines bioabsorbability with
potential for magnesium-based alloys high specific strength, allowing for restricts the applications of polymeric
in biomedical applications has gained the possibility to combine optimal materials in load-bearing and tissue-
more interest. This paper summarizes mechanical performance with supporting applications, as a greater
biodegradable and bioabsorbable amount of material is required to meet
the history and current status of mag- properties.
nesium as a bioabsorbable implant the mechanical needs of the body. It
material. Also discussed is the devel- …describe this work to a has also been observed that radiolucent
materials science and engineering
opment of a magnesium-zinc-calcium professional with no experience in polymer stents can decrease the accu-
alloy that demonstrates promising deg- your technical specialty? racy during the positioning of coronary
radation behavior. Recently, there has been an stents.6 Metals have desirable mechani-
increased interest in developing cal properties due to their relatively
INTRODUCTION biocompatible and bioabsorbable high strength and fracture toughness;
materials for numerous medical
Biomaterial implants can either be applications ranging from cardiac however, the majority of metals are
used to replace a diseased part or to as- stents to orthopedic devices. This biologically non-absorbable or toxic.
sist in the healing process. While the paper provides an overview of the Studies have shown that conventional
history of the use of magnesium
former application requires implants to as a biomaterial, biological surgical alloys, like stainless steel, co-
stay in the body permanently, the latter considerations in designing balt, chromium, and nickel-based al-
only requires that the implant remain bioabsorbable magnesium-based loys produce corrosion products, which
materials, and compares the in vitro are harmful to the human body.7–10 On
in the body temporarily. Thus, in situ- performance of a new magnesium-
ations where a permanent implant is zinc-calcium alloy against pure the other hand, magnesium (Mg) and
used for a short-term application, addi- magnesium and commercially its corrosion products have excellent
tional surgeries are required to remove available AZ91. biocompatibility and are considered to
these devices once the healing process …describe this work to a be a promising technology for tempo-
is complete. This removal process in- layperson? rary medical implants.8,11 As a result,
creases the cost of health care and Biomedical implants are required the use of Mg as a biodegradable and
to aid in the repair or replacement bioabsorbable medical material has
patient morbidity.1 In contrast, biode-
of damaged body parts. Although
gradable materials dissolve after the there is a specific need for temporary gained significant attention in the area
healing process is complete and thus, implants, the majority of traditional of biomaterials.
no additional surgeries are required for metallic biomaterials are designed
for long-term use. Bioabsorbable MAGNESIUM AS A
removal of these implants. This also implants provide short-term BIOMATERIAL
eliminates the complications associ- support and can be easily absorbed
ated with the long-term presence of biologically. Because of its ability Magnesium shows great promise as a
to be metabolized by the body, potential biocompatible and biodegrad-
implants in the body. Lastly, once these magnesium shows great promise
materials degrade within the body, it is as a bioabsorbable implant material. able material. Some of the attractive
important that the degradation products physical characteristics of Mg include

Vol. 61 No. 9 • JOM www.tms.org/jom.html 31

 substrate on the implant comprised of
Table I. Summary of Mechanical Properties of Natural and Implant Materials
the proteins necessary for their function
Tensile Elastic and survival. For example, osteoblasts
Strength Modulus
Material (MPa) (GPa) Bio-degradable are bone-forming cells that lay down
the tough protein collagen and then
Natural Materials
mineralize it to make new bone. Grow-
Collagen 60 b 1b Yes
Cortical bone 100–200 b 10–20 b Yes ing these cells on collagen increases
Inorganic Materials their bone mineralization activity.27
Magnesium 185–232c 41–45d Yes Alternatively, an elastic protein, aptly
Stainless steels 480–834 b 193 b No named “elastin,” is abundant in arter-
Cobalt alloys 655–1400 b 195–210 b No ies to expand and recoil when the large
Titanium alloys 550–985 b 100–105 b No volume of blood comes from heart con-
Platinum alloys 152–485 b 147 b No
Synthetic hydroxyapatite 600 d,* 73–117d Variable traction.28 After the cells are supported,
absorption of the implant would leave
Organic Materials
L-PLA 28–50 a 1.2–3 a Yes behind a naturally synthesized protein
D,L-PLA 29–35 a 1.9–2.4 a Yes structure appropriate for those cells at
UHMWPE 39–40 b 0.94–1.05 b No that specific site. For these reasons, a
*Indicates compressive strength (MPa) significant, uncontrolled, local change
(a) Reference 36; (b) Reference 37; (c) Reference 30; (d) Reference 15 in Mg concentration due to implant
degradation can have a deleterious ef-
high specific strength and an elastic growth and healing. Later experiments fect on human physiology and must be
modulus that is closest to the human have confirmed that the presence of Mg managed through proper engineering
bone when compared to traditional me- enhances the bone cell adhesion on alu- design. Hence for extensive use of Mg
tallic implant materials (Table I). These mina19 and has no inhibitory effect on and its alloys in biomedical implant ap-
properties are of great importance as cell growth.20 Furthermore, corrosion plications, understanding and control
high mechanical strength reduces the and degradation of Mg leads to the for- over the degradation rate are required.
amount of implant material needed for mation of harmless corrosion products,
CONTROLLED
a given applied load and reducing the which are excreted through urine.21
DEGRADATION AND
elastic modulus mismatch alleviates However, the major limitation of Mg
ALLOY DESIGN
stress-shielding effects between bone is its low corrosion resistance. High
and the implant material. corrosion rates result in the rapid re- Although the general biocompat-
Magnesium is an essential mineral lease of degradation products. A high ibility of Mg is high, increased deg-
for human metabolism and its deficien- rate of degradation under physiological radation rates under physiological pH
cy has been linked to various patholog- conditions can cause a reduction in the conditions can locally reduce the bio-
ical conditions.7 The human body con- mechanical integrity of the implant be- compatibility near the implant surface.
tains about 1 mole (24 g) of Mg. It is fore the bone or tissue has sufficiently Under typical atmospheric conditions,
the second most common intracellular healed.15 Magnesium’s low corrosion Mg reacts with water to produce a
ion and serves as a cofactor for more resistance also leads to the rapid pro- mildly protective film of magnesium
than 300 enzymatic reactions ranging duction of hydrogen gas and the for- hydroxide (Mg(OH)2).29 Although this
from muscle contraction to neuronal mation of gas bubbles. These bubbles film slows corrosion under aqueous
control.12 Most of the Mg in the body can accumulate around the implant and conditions, it reacts with chlorine ions
(53%) is stored in bone, in an apatite delay the healing of the tissue.15 The lo- present in physiological conditions to
inorganic matrix subject to regulated calized formation of hydrogen gas can produce MgCl2 and hydrogen gas.15
release.13 Magnesium-based materials also result in a pH increase around the Efforts to control the corrosion rate
were first introduced for orthopedic ap- implant.22 This can cause local alkali- of Mg have utilized various processing
plications in the beginning of the 20th zation and severely affect the pH-de- methods such as purification, alloying,
century. Lambotte14,15 first reported the pendent physiological processes in the anodizing, and surface coating. Studies
use of a pure Mg plate along with gold- vicinity of the implant.23 have shown that purification of Mg re-
plated steel nails to secure a lower leg To successfully employ bioabsorb- duces the corrosion rate considerably;
bone fracture. However, the in vivo able metal implants, the time frame however, due to the low yield strength
corrosion of the implant was too rap- of degradation must be sufficient such of pure Mg,30 its application in orthope-
id as it degraded in just 8 days. Since that the cells can synthesize and depos- dics and other load bearing applications
then, several attempts have been made it an extracellular matrix for their own is limited.23 Alloying elements can be
to increase the corrosion resistance of support and function before the struc- added to increase the strength of pure
Mg implants and a variety of new al- tural integrity of the implant is com- Mg31 but alloying elements should be
loys have been tested in vitro and in promised. Surfaces have been treated selected carefully to maintain the Mg’s
vivo.16–18 These studies highlighted or coated with a variety of chemistries biocompatibility. Elements like Fe, Ni,
Mg’s additional beneficial attributes, and polymers to encourage cell attach- Cu, and Co have extremely deleterious
namely its ability to stimulate bone ment.24–26 Once adhered, cells create a effects on the corrosion properties of

32 www.tms.org/jom.html JOM • September 2009