Biomedical Engineering Reference
In-Depth Information
2.2 Composition and microstructure characterization
X-ray diffraction (XRD, Philips-X'Pert) using Cu Ka radiation was employed for the
identification of the constituent phases in the as-cast Mg-Zn-Ca alloy and their corrosion
products after immersion. Microstructure observations of the alloys were conducted on
Olympus optical microscope. The specimens for optical microscopy were etched with a
solution of 8 vol. % acetic acid for 30 s, thoroughly flushed with water and alcohol, and then
dried by hot air.
2.3 Mechanical properties
Tensile tests were carried out using an Instron-5569 universal testing machine at a constant
crosshead speed of 1.0mm/min at room temperature. The tensile specimens with diameter
of 6mm and gauge length of 30mm were cut by electric-discharge machining from the ingot.
The Young's modulus was get from the tensile test. The fracture morphologies were
examined by SEM (SEM, Hitachi-3000N).
2.4 In vitro degradation tests
Electro-chemical measurements and immersion tests were performed in a Hank's simulated
body fluid to evaluate the in-vitro degradation properties. The chemical composite of
Hank's simulated body was listed in table 2.The pH value of Hank's solution was adjusted
with HCl and NaOH to 7.2~7.4, to avoid precipitation or formation of sediments in the
solution. Its temperature was controlled around 37 ± 0.5
0
C, which is equal to the human
body normal temperature. Pure Mg (>99.99%) was also tested as a contrast.
2.4.1 Electrochemical measurements
A typical three-electrode system consisting of graphite rod as counter electrode, saturated
calomel electrode (SCE) as a reference electrode and specimen (1cm
2
exposed areas) as a
working electrode was used. Potentiodynamic polarization experiments were carried out at
a scan rate of 0.5 mV/s. The electrochemical measurements of specimens with thickness of 4
mm and a gauge diameter of 15mm were machined from the ingot and ground with 2000
grit SiC paper, and they were rinsed with distilled water and dried by hot air.
2.4.2 Immersion tests
The immersion tests were carried out in Hank's solution according to ASTM-G31-72 [21].
Samples were removed after 30 days of immersion, rinsed with distilled water, and were
cleaned with chromic acid to remove the corrosion products. The degradation rates (in units
of mm
year
-
) were obtained according to ASTM-G31-72. An average of five measurements
was taken for each group. The pH value of the solution was also recorded in the immersion
tests at absolute group for 144 hours.
2.5 Cytotoxicity assessments
L-929 cells were adopted to evaluate the cytotoxicity of Mg-Zn-Ca alloys. The cells were
cultured in Dulbecco's modified Eagle's medium (DMEM), 10% fetal bovine serum (FBS),
100 Uml
-1
penicillin and 100 mg ml
-1
streptomycin at 37
o
C in a humidified atmosphere of 5%
CO2. The cytotoxicity tests were carried out by indirect contact. Extracts were prepared
using DMEM serum free medium as the extraction medium with the surface area of
extraction medium ratio 1.25 ml/cm
2
in a humidified atmosphere with 5% CO
2
at 37
o
C for
