In vitro degradation and biocompatibility of Fe–Pd and Fe–Pt
composites fabricated by spark plasma sinteringContents lists
available at ScienceDirect
Materials Science and Engineering C
j ourna l homepage: www.e lsev ie r .com/ locate /msec
In vitro degradation and biocompatibility of Fe–Pd and Fe–Pt
composites fabricated by spark plasma sintering
T. Huang a,b, J. Cheng c, Y.F. Zheng a,b,c, a State Key Laboratory
for Turbulence and Complex System, College of Engineering, Peking
University, Beijing 100871, China b Department of Materials Science
and Engineering, College of Engineering, Peking University, Beijing
100871, China c Center for Biomedical Materials and Tissue
Engineering, Academy for Advanced Interdisciplinary Studies, Peking
University, Beijing 100871, China
Corresponding author at: Department ofMaterials Sci Engineering,
Peking University, Beijing 100871, China. Tel
E-mail address:
[email protected] (Y.F. Zheng).
0928-4931/$ – see front matter © 2013 Elsevier B.V. All ri
http://dx.doi.org/10.1016/j.msec.2013.10.023
a b s t r a c t
a r t i c l e i n f o
Article history: Received 21 June 2013 Received in revised form 12
October 2013 Accepted 21 October 2013 Available online 31 October
2013
Keywords: Biodegradable metal Fe–Pt composite Fe–Pd composite
Corrosion Biocompatibility
In order to obtain biodegradable Fe-basedmaterialswith
similarmechanical properties as 316L stainless steel and faster
degradation rate than pure iron, Fe-5 wt.%Pd and Fe-5 wt.%Pt
composites were prepared by spark plasma sinteringwith powders of
pure Fe and Pd/Pt, respectively. The grain size of Fe-5 wt.%Pd and
Fe-5 wt.%Pt compos- ites wasmuch smaller than that of as-cast pure
iron. The metallic elements Pd and Pt were uniformly distributed in
the matrix and the mechanical properties of these materials were
improved. Uniform corrosion of Fe–Pd and Fe–Pt composites was
observed in both electrochemical tests and immersion tests, and the
degradation rates of Fe–Pd and Fe–Pt composites were much faster
than that of pure iron. It was found that viabilities of mouse
fibro- blast L-929 cells and human umbilical vein endothelial cells
(ECV304) cultured in extraction mediums of Fe–Pd and Fe–Pt
composites were close to that of pure iron. After 4 days' culture,
the viabilities of L-929 and ECV304 cells in extraction medium of
experimental materials were about 80%. The result of direct contact
cytotoxicity also indicated that experimental materials exhibited
no inhibition on vascular endothelial process. Meanwhile, iron ions
released from experimental materials could inhibit proliferation of
vascular smooth muscle cells (VSMC), which may be beneficial for
hindering vascular restenosis. Furthermore, compared with that of
as-cast pure iron, the hemolysis rates of Fe–Pd and Fe–Pt
composites were slightly higher, but still within the range of 5%,
which is the criteria for good blood compatibility. The numbers of
platelet adhered on the surface of Fe–Pd and Fe–Pt composites were
lower than that of pure iron, and the morphology of platelets kept
spherical. To sum up, the Fe-5wt.%Pd and Fe-5wt.%Pt composites
exhibited good mechanical properties and degradation be- havior,
closely approaching the requirements for biodegradable metallic
stents.
© 2013 Elsevier B.V. All rights reserved.
1. Introduction
Biodegradable stents are currently considered as ideal stents. They
are expected to effectively avoid the late stent thrombosis which
is often caused by permanent stents. Thematerial for biodegradable
stents is required to have good biocompatibility and it should be
kept intact for several months before completion of vascular
healing [1]. Two typical classes of metallic materials including
Mg-based [2–9] and Fe-based [10–18] alloys have been researched for
this application. Compared with Mg-based metals, Fe-based metals
possess more attractive me- chanical properties [10]. Iron is an
essential nutrient element in human body and plays an important
role in vital biochemical activities, such as oxygen sensing and
transport, electron transfer and catalysis [19]. Pure iron also has
goodmechanical property [10], biocompatibility and
hemocompatibility [20,21] close to 316L stainless steel, which
serves as the golden standard for stent material. Animal
experiments demonstrated that degradable iron stents can be safely
implanted
ence and Engineering, College of ./fax: +86 10 62767411.
ghts reserved.
without significant vascular restenosis caused by inflammation,
neointi- mal proliferation or thrombotic events, and there was no
evidence for local or systemic toxicity [20,22–24]. A clinical
outcome also showed that there was no association between iron
status and the incidence of major adverse cardiac events or
coronary restenosis in human body [25]. Therefore, iron-based
materials are considered as suitable mate- rials for further
development of novel biodegradable coronary artery stent. However,
a faster degradation rate of iron is desired to apply to stents
[20,22].
To obtain iron-based materials with appropriate properties for
clin- ical applications, researchers have focused on the
development of new kinds of Fe-based materials by modifying the
chemical composition and microstructure. Hermawan et al. [10,26]
were the first to study the effect of alloying elements on the
performance of biodegradable iron-based materials. They found that
alloying with Mn increased the strength and degradation rate of
pure iron. Liu et al. [27] discovered that the addition of 6 wt.%
of Si in Fe-30 wt.%Mn alloy established the shape memory effect. Xu
et al. [28] reported that Fe-30Mn-1C alloy showed lowermagnetic
susceptibility and bettermechanical properties than Fe-30Mn alloy.
Schinhammer et al. [29] added 1 wt.% of Pd into Fe- 10Mn alloy that
sped up the degradation rate ten times faster than that
44 T. Huang et al. / Materials Science and Engineering C 35 (2014)
43–53
of low carbon steel. They also did a research for Fe–Mn–C(–Pd)
alloys. The Fe–Mn–C–Pd alloys were characterized by an increased
degrada- tion rate compared to pure iron [16]. The effects of
alloying elements (Mn, Co, Al, W, Sn, B, C, and S) on the in vitro
degradation behavior and biocompatibility of pure iron were also
investigated by Liu and Zheng [30]. The results showed that the
addition of all alloying elements except for Sn improved the
mechanical properties of iron after rolling. New fabricating
methods were also tried by researchers, such as pow- der metallurgy
[31], electroforming [32–34], equal channel angular pressing (ECAP)
technique [35] and SPS sintering [36]. Although the predecessors
made a lot of efforts for the improvement of Fe-basedma- terials,
the corrosion rate of Fe-based materials still could not meet the
clinical application demand.
Palladium is a common alloying element in dental alloys. It has
rela- tively low price and density when compared to gold and
platinum. Pal- ladium possesses a good range of solubility in
several metals and could improve the mechanical properties of the
matrix metals, too [37]. There have also been reports of using
Pd-based organometallic com- pounds as antineoplastic agents [38].
Platinum has good mechanical property, low corrosivity and high
biocompatibility [39,40]. It is used as essential components for
many medical devices including implant- able defibrillators,
catheters, stents, pacemakers, neuromodulation device and upper-lid
implant [41,42]. So palladium and platinum them- selves are both
excellent biomedical materials with good biocompatibil- ity. In
addition, the standard electrode potential of palladium and
platinum (Pd is about −0.126 V, Pt is about 1.2 V) is higher than
that of iron (about −0.44 V) [43], and galvanic corrosion may occur
in the placewhere two kinds of metals contact, then accelerating
the degrada- tion of pure iron. Dispersion of platinum and
palladium into pure iron matrixmay alsomake the corrosion of pure
ironmore uniform. Further- more, the effect of dispersion
strengthening caused by the addition of platinum and palladiumwas
expected to improve themechanical prop- erties of pure iron. The
objective of the presentwork is to investigate the feasibility of
adding palladium and platinum to pure iron for the prepa- ration of
biodegradable iron-based composites.
2. Materials and methods
2.1. Material preparation
Pure iron powder (99.0%, −300 mesh), pure palladium powder (99.99%,
−200 mesh) and pure platinum powder (99.99%, −150 mesh), whichwere
provided by ChinaMetalMaterials Technology Com- pany, were used as
raw materials. Iron powder mixed with 5 wt.% of palladium and
platinum powder were prepared, respectively. Then the mixed powders
were put into a 20-mm diameter graphite die and sintered under
vacuum by SPS-1050 system (Sumitomo Coal Mining Company, Ltd.) with
sintering temperature of 1000 °C and holding time of 5 min. Pure
iron (99.9%) prepared by casting served as control.
The as-sintered Fe–Pd and Fe–Pt composite specimens (Φ20 × 7 mm)
and as-cast pure iron were cut into square pieces (10 × 10 × 1.5
mm3) and cylinders (Φ2 × 5 mm), respectively. Square pieces were
used for density measurements, microstructure characterization and
series tests of microhardness, electrochemical properties,
corrosion behavior, cytotoxicity, hemocompatibility and platelet
adhesiveness. Each sample was mechanically polished to 2000 grit,
then ultrasonically cleaned in anhydrous ethanol and dried in the
open air. Before cytotoxicity test, the samples were sterilized
with ultraviolet radiation for at least 2 h. Cylinder speci- mens
were used for compressive test.
2.2. Microstructure characterization
Specimens were polished by 0.15 μm diamond paste, further ultra-
sonically cleaned in anhydrous ethanol and dried in the open air.
Optical microscope (Olympus BX51M)was used to observe
themetallographic
structure after the specimens were etched with a 4% HNO3/alcohol
solution. X-ray diffractometer (XRD, Rigaku DMAX 2400) using CuKα
radiation was adopted to identify the constituent phases of Fe–Pd
and Fe–Pt composites with scanning range from 10° to 100° and scan
rate of 6°/min. Energy dispersive spectrometer (EDS) was used for
the analysis of chemical composition.
2.3. Mechanical test
Themechanical properties of experimental composites and pure iron
were determined by microhardness test and compressive test. Micro-
hardness tests were carried out by a microhardness tester (SHIMADZU
HMV-2t) measuring Vickers hardness with loading force of 0.2 kg and
dwell time of 10 s. Center point and other four points uniformly
distrib- uted around the center point on the surface of each sample
were select- ed for microhardness test. An average of five
measurements was taken for eachmaterial. Instron 5969 universal
testmachinewasused for com- pressive tests according to ASTME9-89a
[44]. The specimenswere in the form of cylinder 2.5 mm in diameter
and 5 mm in length. Compression strain rate was 2 × 10−4/s. An
average of at least three measurements was taken for each
group.
2.4. Electrochemical measurements
Electrochemical measurements were performed using three- electrode
system at an electrochemical work station (CHI660C, China). The
specimen, a platinum electrode and a saturated calomel electrode
(SCE) were set as the working electrode, auxiliary electrode and
the ref- erence electrode, respectively. All themeasurementswere
carried out at a temperature of 37 ± 0.5 °C in Hank's solution. The
area of working electrode exposed to the solution was 1 cm2. The
open circuit potential (OCP) measurement was set for 7200 s.
Electrochemical impedance spectroscopy (EIS) was measured from 100
kHz to 10 mHz at OCP value after 2 h immersion in Hank's solution.
The potentiodynamic po- larization curves were carried out from
−1000 mV (vs. SCE) to 0 mV (vs. SCE) at a scanning rate of 0.33
mV·s−1.
2.5. Static immersion test
In vitro static immersion test was performed in Hank's solution, 50
ml for each sample following ASTM-G31-72 [45] at 37 °C in water
bath. After 3, 10 and 30 days, the samples were removed from the
soaking solution, gently rinsed with distilled water and quickly
dried in case of oxidation. Changes on the surface morphologies of
the speci- mens after immersion were characterized by ESEM (Quanta
200FEG), equipped with an EDS attachment. Then, weighing after the
corrosion products on the surface of samples dissolved in the 10
mol/l NaOH solu- tion, an average of three measurements was taken
for each group. The degradation rate was calculated based on the
formula:
CR ¼ m St
where CR (mg·cm−2·day−1) is the corrosion rate, m (mg) is the mass
loss, S (cm2) is the surface area of the specimen and t (day) is
the time.
2.6. Dynamic immersion test
Dynamic immersion test was carried out using a dynamic corrosion
test bench which was built in our previous research work [11]. Dis-
solved oxygen is an important factor that influences degradation of
iron in a neutral medium, so the dissolved oxygen concentration in
Hank's solution was regulated to better simulate the blood environ-
ment. In this bench the wall shear stress (0.68 Pa), temperature
(36.0–37.1 °C), pH value (7.35–7.45), and dissolved oxygen (2.8–
3.2 mg−1) were controlled to approach the common values in
the
Fig. 1.Metallographs of experimentalmaterials: (a) pure iron and
(b) Fe–Pd and (c) Fe–Pt composites. The corresponding grain size
distribution estimated by themetallographs: (d) pure iron; (e)
Fe–Pd and (f) Fe–Pt composites.
45T. Huang et al. / Materials Science and Engineering C 35 (2014)
43–53
human coronary artery. Plate specimens were mounted in paraffin and
then placed in a specimen holder. All specimens were exposed to the
dynamic fluid flow (Hank's solution) for 30 days (considering the
gen- erally bare metallic coronary stents would be covered by
endothelial cells after 30 days in vivo implantation and no longer
contact with blood directly) [11]. After the test, the samples were
taken out, then the macroscopic surface morphology was
characterized using a 2.5 di- mensional precision image measuring
instrument (HLEO, VACD-1010, China). After that the specimens were
cleaned by deionized water and ethanol in turn, they were dried in
the air. Changes on the surface mor- phology were characterized by
ESEM (Quanta 200FEG), equipped with
Fig. 2. X-ray diffraction patterns of pure iron and Fe–Pd and Fe–Pt
composites.
an EDS attachment. The corrosion rate was calculated based on the
weight loss of specimens. Corrosion layers on the surface of
samples were peeled off at first. The residual corrosion products
were dissolved by 10 mol/l NaOH and rinsed by deionized water and
alcohol in turn, then samples were weighed after they were dried in
the open air. Cor- rosion rates were calculated by the same formula
in static immersion test.
2.7. Contact angle measurement
Water contact angle of iron-basedmaterials wasmeasured by a con-
tact angle analyzer (Kino SL200B); for each sample the measurements
were carried out three times.
2.8. Cytotoxicity test
The cytotoxicity test was performed by an indirect contact method.
Murine fibroblast cells (L-929), human vascular smooth muscle cells
(VSMC) and umbilical vein endothelial cells (ECV304) were used to
evaluate the cytotoxicity of experimental Fe–Pd and Fe–Pt
composites. At first, all the cell lineswere cultured in
theDulbecco'smodified Eagle's medium (DMEM) with 10% fetal bovine
serum (FBS), 100U·ml−1 pen- icillin and 100 μg·ml−1 streptomycin at
37 °C in a humidified atmo- sphere of 5% CO2. According to ISO
10993-12 [46], extraction medium was prepared using DMEM with a
surface area/extraction medium ratio of 1.25 cm2·ml−1 in a
humidified atmosphere with 5% CO2 at 37 °C for 72 h. After the
extracts were centrifuged, the supernatant fluid was withdrawn and
stored at 4 °C before cytotoxicity test. The control groups
involved DMEM medium as the negative control and DMEM including 10%
dimethyl sulfoxide (DMSO) as the positive con- trol. The metallic
concentrations of ions in the extraction medium were measured by
inductively coupled plasma atomic emission spectrometry (ICP-AES)
(Leeman, Profile). Cellswere incubated in the 96-well plates at the
density of approximately 5 × 103 cells per 100 μl medium in each
well and incubated for 24 h to allow attachment. DMEM was then
sucked out and replaced by extraction medium. Then 10 μl serum
was
Fig. 3. ESEM images of surface morphology and energy spectrum
analysis of (a) Fe–Pd and (b) Fe–Pt composites.
46 T. Huang et al. / Materials Science and Engineering C 35 (2014)
43–53
added to each well. After 1, 2 and 4 days' incubation in the
incubator re- spectively, the 96-well plates were observed under an
optical micro- scope. Thereafter, 10 μl of MTT was added to each
well. The cells were incubatedwithMTT for 4 h. Then 100 μl of
formazan solubilization solu- tion (10% sodium dodecyl sulfate in
0.01 MHCl) was added to each well and incubated for 10 h. The
absorbance of each well was tested by a mi- croplate reader
(Bio-RAD680) at 570 nmwith a referencewavelength of
Table 1 Density of Fe–Pd and Fe–Pt composites with as-cast pure
iron as the control at room temperature.
Materials Density (g/cm3) Relative density
Pure iron 7.83 (0.01)a \ Fe–Pd 7.15 (0.09)a 88.9% Fe–Pt 7.04
(0.03)a 86.3%
a Number in the brackets represents the standard deviation. Fig. 4.
Compressive stress–strain curves of pure iron and Fe–Pd and Fe–Pt
composites.
Fig. 5. Electrochemical test results of pure iron and Fe–Pd and
Fe–Pt composites in Hank's solution: (a) Potentiodynamic
polarization curve, (b) Nyquist plots.
47T. Huang et al. / Materials Science and Engineering C 35 (2014)
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630 nm. Viability of cells (X) was calculated using the following
formula according to ISO 19003-5 [47]:
X ¼ OD1
OD2 100%:
Here OD1 is themean absorbance of the experimental sample group and
positive control group. OD2 is the mean absorbance of the negative
con- trol group.
Direct contact cytotoxicity of ECV304 cells was also carried out
ac- cording to ISO 10993-5 [47]. 600 μl cell suspension was
respectively added to each sterilized sample at a cell density of 6
× 103/ml. After being cultured in the incubator for 24 h, the
medium was removed. Then the samples were gently washed by
phosphate buffered saline (PBS); cells on the samples were fixed
with 2.5% glutaraldehyde solu- tion at room temperature for 2 h,
then dewateredwith gradient alcohol solution (50%, 60%, 70%, 80%,
90%, 95% and 100%) for 10 min, and finally freeze-dried for 2 days.
The morphology of cells adhered on the speci- mens was observed by
ESEM (Quanta 200FEG).
2.9. Hemolysis test and platelet adhesion
Healthy human blood (anticoagulant was 3.8 wt.% citric acid sodi-
um) extracted from volunteers was diluted by physiological saline
according to volume ratio 4:5. Pure iron and Fe–Pd and Fe–Pt
compos- ites were installed in centrifugal tubes with 10 ml
physiological saline. Temperature was kept at 37 °C for 30 min.
Then 0.2 ml diluted blood was added to each tube and incubated at
37 °C for 60 min, 10 ml deion- ized water with 0.2 ml diluted blood
as the positive control and 10 ml physiological saline with 0.2 ml
diluted blood as the negative control. Upon completion of the above
operations, samples were removed, and then these tubes were
centrifuged at 800 g for 5 min. Supernatant was transferred to
96-well multiplates, the absorbance (OD) was
Table 2 Average electrochemical parameters of Fe–Pd and Fe–Pt
composites (as-cast pure iron as control).
Materials Vcorr (V) Icorr (μA/cm2)
Pure iron −0.324 (0.020) 0.871 (0.040) Fe–Pd −0.471 (0.010) 1.550
(0.120) Fe–Pt −0.545 (0.008) 6.698 (0.370)
Corrosion potential (Vcorr), corrosion current (Icorr).
determined by a microplate reader (Bio-RAD680) at a wavelength of
545 nm. Hemolysis of samples was calculated by the formula:
Hemolysis ¼ OD testð Þ−OD negative controlð Þ OD positive controlð
Þ−OD negative controlð Þ 100%:
For platelet adhesion, whole blood from healthy human body was
centrifuged at 1000 r/min for 10 min. Platelet rich plasma (PRP)
was obtained from the upper fluid. Samples after ultraviolet
disinfection were moved to 24-well multiplates and 0.2 ml PRP was
added to each well, incubated at 37 °C for 1 h. After gently
flushed by phosphate buff- ered saline (PBS), platelets on samples
were fixed with 2.5% glutaralde- hyde solution at room temperature
for 1 h, then dewatered with gradient alcohol solution (50%, 60%,
70%, 80%, 90%, 95% and 100%) for 10 min, and finally freeze-dried
for 2 days. The morphology of platelet adhered on the specimens was
observed by ESEM (Quanta 200FEG).
3. Results and discussion
3.1. Microstructures
Fig. 1 shows the metallographs of pure iron and Fe–Pd and Fe–Pt
composites and the corresponding grain size distribution measured
frommetallographs. The average grain sizes of these two kinds of
exper- imental composites are quite close, both about 20 ± 8 μm,
which are much smaller than that of as-cast pure iron (180 ± 90
μm). It may be caused by the different preparation methods. For
spark plasma sintering, lower temperature and shorter holding time
could sinter powders to crystal with little grain growth [48]. The
second phase par- ticles inhibiting grain growth should also be
taken into consideration. The decrease of the grain size could
contribute to the improvement of mechanical properties.
Fig. 2 displays XRD patterns of experimental composites, as-cast
pure iron as control. It can be found that the Fe-5wt.%Pd and Fe-
5wt.%Pt composites both consist of α-Fe phase and FeO phase, but
pure Pd or Pt phase was not found. PDF CARDS recorded that the
peaks of Fe9.7Pd0.3 and Fe9.7Pt0.3 phases almost overlap with that
of α-Fe phase in X-ray diffraction spectrum. As the contents of
both Pd and Pt in experimental composites are less than 3 at.%,
there might be quite a part of Pd and Pt solute in the surrounding
pure iron matrix forming Fe9.7Pd0.3 and Fe9.7Pt0.3 phases in the
sintering process, so the content of Pd or Pt phase was too low to
be detected by X-ray dif- fraction. Fig. 3 showed that Pd and Pt
were uniformly distributed in the iron matrix. The dark areas
mainly consisted of Fe and O, which
Fig. 6. Macrographs of surface morphology of experimental materials
static immersed in Hank's solution for 10 and 30 days.
Fig. 7. SEM images of samples' surfacemorphology after statically
immersed inHank's solution for different periods and the
corresponding X-ray diffraction patterns of corrosion products on
the surface of pure iron.
48 T. Huang et al. / Materials Science and Engineering C 35 (2014)
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Fig. 8. Corrosion rates calculated from the weight loss of samples
after static or dynamic immersion in Hank's solution for 30
days.
49T. Huang et al. / Materials Science and Engineering C 35 (2014)
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should be FeO phase, as revealed by the XRD results. Table 1 shows
the density of Fe–Pd and Fe–Pt composites with as-cast pure iron as
control at room temperature. Relative density of both Fe–Pd and
Fe–Pt compos- ites were less than 90%, which was probably due to
the relatively low sintering temperature and pressure, as well as
the existence of FeO in the composites.
3.2. Mechanical properties
Fig. 4 shows the compression stress–strain curves of Fe–Pd and Fe–
Pt composites, with pure iron as control. The yield strengths (YS)
of Fe–Pd and Fe–Pt composites were both about three times as that
of pure iron. The ultimate compressive strength (UCS) of Fe–Pt
composite was slightly higher than that of Fe–Pd composite. The
rigidity of Fe–Pd and Fe–Pt composites was also much larger than
that of pure iron. Ac- cording to the binary phase diagramof Fe–Pd
[49] and Fe–Pt [50] alloys, a small quantity of Pd and Pt could
dissolve into iron matrix. However, the atomic radii of Fe (1.40 ),
Pd (1.40 ) and Pt (1.35 ) are very close [51], so the lattice
distortion of α-Fe was negligible. Then the
Fig. 9. SEM images of samples' surfacemorphology after dynamically
immersed in Hank's soluti samples' surface morphology after removed
corrosion products: (d) pure iron and (e) Fe–Pd a
solution strengthening effect in these composites could not be
signifi- cant. Therefore, the strength enhancement of the
composites should be mainly attributed to the second phase
strengthening effects. More- over, the decrease of the grain size
could make considerable contribu- tions to the strength improvement
of these composites [52]. However, the Fe–Pd and Fe–Pt composites
exhibited low ductility, which can be explained mainly by the
relatively low density (Table 1) [53].
The microhardness of Fe–Pd (184 ± 10 HV0.2/10) and Fe–Pt (309 ± 8
HV0.2/10) composites was much higher than that of pure iron (119 ±
3 HV0.2/10), especially for Fe–Pt composite, which was nearly three
times as that of pure iron. Obviously, the addition of Pd or Pt
greatly enhanced the strength of iron matrix.
3.3. Corrosion properties of Fe–Pd and Fe–Pt Composites
3.3.1. Electrochemical measurements Fig. 5 shows the
potentiodynamic polarization curves (Fig. 5(a)) and
Nyquist plots (Fig. 5(b)) of Fe–Pd and Fe–Pt composites immersed in
Hank's solution, with as-cast pure iron as the control. The average
elec- trochemical parameters were listed in Table 2. Itwas found
that the cor- rosion potential was largely decreased and the
corrosion current densities were increased after adding Pd and Pt
into the iron matrix. As the Nyquist plots shown, plotted the
variation in the excitation frequency impedance imaginary part (z")
as a function of the imped- ance real part (z′) which obtained a
series of semicircles. The diameters of semicircles obtained from
Fe–Pd and Fe–Pt composites were smaller than that of pure iron,
revealing their worse corrosion resistance. Since the diameter of
high frequency capacitive loop can be considered as the charge
transfer resistance and smaller charge transfer resistance corre-
sponds to faster corrosion rate [54].
3.3.2. Static immersion corrosion behavior Samples after 3 days'
immersion in Hank's solution exhibited no
conspicuous change on the surface. After 10 days' immersion,
localized corrosion mainly happened at the edge of pure iron
sample, and the productswere reddish brown.However, uniform
corrosion could be ob- served on the surfaces of Fe–Pd and Fe–Pt
composites, with claybank corrosion products (Fig. 6).When the
sampleswere immersed inHank's solution for 30 days, most of
corrosion products fell off, as shown in Fig. 6. Different
orientations of the grains showed different colors.
on for 30 days: (a) pure iron and (b) Fe–Pd and (c) Fe–Pt
composites. SEM images of these nd (f) Fe–Pt composites.
Fig. 10.Water contact angles of (a) pure iron and (b) Fe–Pd and (c)
Fe–Pt composites.
50 T. Huang et al. / Materials Science and Engineering C 35 (2014)
43–53
Local distribution of black etch pits could also be found on the
surface of pure iron, indicating localized corrosion feature of
pure iron. However, a large number of corrosion pits were
distributed uniformly on the sur- face of Fe–Pd and Fe–Pt
composites, representing their macroscopically uniform corrosion
behavior. Fig. 7 shows the SEM images of surface morphology of
experimental samples, as can be seen from the figure:
(1) After 3 days' immersion, all the samples' surfaces almost kept
intact.
(2) After 10 days' immersion, most area on the surface of pure iron
remained intact, and only several corrosion pits were locally dis-
tributed on the surface. The surface of Fe–Pd composite was
Fig. 11. (a) Ion concentration in extraction mediums of
experimental materials and cell viabilit (c) ECV304 and (d)
VSMC.
covered by a thin corrosion layer, and this layer didn't closely
ad- here to thematrix. Fe–Pt composites corrodedmost severely and a
lot of deep corrosion pits were uniformly distributed on the
surface.
(3) After 30 days' immersion, because the most of the corrosion
products fell off from the surface of samples, grain boundary ob-
viously presented on the surface of pure iron. Fe–Pd and Fe–Pt
composites corroded much more seriously than pure iron. The iron
substrate surrounded Pd- and Pt-rich areas corroded more seriously.
This should be attributed to the galvanic corrosion oc- curred at
the contact place of Fe and Pd or Pt. Pd- and Pt-rich areas that
have relatively high corrosion potential as cathode
y after cultured in extraction mediums and positive control for 1,
2 and 4 days; (b) L-929,
Fig. 12. The morphologies of ECV 304 cells directly cultured on (a)
pure iron and (b) Fe–Pd and (c) Fe–Pt composites for 24 h.
51T. Huang et al. / Materials Science and Engineering C 35 (2014)
43–53
sites, iron as anode sites, then formed a lot of tiny galvanic
cells in the compositeswhen immersed inHank's solution, so as to
accel- erate the corrosion rate of iron.
(4) The XRD spectrum of corrosion products on the surface of
as-cast pure ironwhich had immersed inHank's solution for 10 days
indi- cated that the corrosion products were mainly the Fe + 3O(OH)
and Fe(OH)3. Corrosion products on the surface of Fe–Pd and Fe– Pt
composites were the same as that on pure iron.
Fig. 8 shows the corrosion rates thatwere calculated from theweight
loss after samples immersed in Hank's solution for 30 days. The
corro- sion rates of both Fe–Pd and Fe–Pt composites were faster
than that of as-cast pure iron. The corrosion rate of Fe–Pt
composite was the fastest, about 2.73 times as that of as-cast pure
iron. Compared to the corrosion rate of Fe-5wt.%W(about 1.15 times
as that of as-cast pure iron) and Fe- 0.5wt.%CNT composites (about
1.96 times as that of as-cast pure iron) [36], the addition of Pd
and Pt increased the corrosion rate of pure iron much more
significantly. In addition to the galvanic corrosion, FeO contained
in Fe–Pd and Fe–Pt composites which could be easily oxidized to
oxidation products was also an innegligible reason for increasing
corrosion rate.
3.3.3. Dynamic immersion corrosion behavior Fig. 9 panels (a)–(c)
were SEM images showing surfacemorphology
of experimental materials after corroding in dynamic environments.
As can be seen from these figures, the numbers of corrosion
products on the surface of Fe–Pd and Fe–Pt composites are much more
than that on the surface of pure iron.
Fig. 13. (a) Hemolysis rates of pure iron and Fe–Pd and Fe–Pt
composites. (b) The number of p
After the corrosion product layers were removed, surface morphol-
ogies of the substrates appeared, as shown in Fig. 9(d)–(f). Grain
boundaries could be observed obviously on the surface of pure iron.
The surface of Fe–Pd composite was much smoother. Fe–Pt composite
corrodedmost seriously, with a lot of deep corrosion pits on the
surface.
The corrosion rates calculated from weight loss of samples after
dynamic immersion tests were also shown in Fig. 8. Compared with
as- cast pure iron, both the Fe–Pd and Fe–Pt composites corroded
much faster, and the Fe–Pt composite performed the fastest
corrosion behavior. This result was in good consistencewith the
results from electrochemical tests and static immersion tests.
Furthermore, corrosion rate of samples in dynamic immersion tests
was much faster than that in static immersion tests. It was because
in the dynamic immersion tests, 2.8–3.2 mg·l−1 ox- ygen was
dissolved in Hank's solution to simulate blood environment, which
was much higher than that in static immersion tests. In addition,
continuous flow of Hank's solution could impede deposition of
ferrous ion and wash away part of the corrosion products, so the
contact time of materials and Hank's solution was prolonged.
3.4. Biocompatibility
3.4.1. Contact angle tests of Fe–Pd and Fe–Pt composites The
contact angles of Fe–Pd and Fe–Pt composites were shown
in Fig. 10, with pure iron as control. The contact angles of pure
iron and Fe–Pd and Fe–Pt compositeswere 51.73°, 64.74° and 51.83°,
respec- tively. Generally, the smaller the contact angle is, the
better the hydro- philicity will be and much easier the cell
adhesion becomes. So, it could be a preliminary judgment that the
biocompatibility of as-cast pure iron and Fe–Pt composite was
superior to that of Fe–Pd composite.
latelets per unit area adhered on the surface of pure iron and
Fe–Pd and Fe–Pt composites.
Fig. 14.Morphology of platelets adhered on the surface of pure iron
and Fe–Pd and Fe–Pt composites.
52 T. Huang et al. / Materials Science and Engineering C 35 (2014)
43–53
3.4.2. Cytotoxicity tests of Fe–Pd and Fe–Pt composites Fig. 11
illustrates the ion concentration in the extraction mediums
(Fig. 11(a)) and the cell viabilities of L929, ECV304 and VSMC
expressed as a percentage of the viability of cells cultured in the
negative control after 1, 2, and 4 days' incubation in pure iron
and Fe–Pd and Fe–Pt com- posite extraction mediums (Fig. 11(b) to
(d)). It can be seen that the order of iron ion concentration in
extraction mediums was: Fe–Pt composite N Fe–Pd composite N as-cast
pure iron, which matched well with the results of in vitro
corrosion tests. Pd and Pt ion concentra- tions were very low.
Firstly, the contents of these two kinds of metals added to the
composites were low. A more important reason was that these two
kinds ofmetals have high chemical stability andwere difficult to be
corroded. Based on Fig. 11(b) to (d), the cell viability slightly
de- creased after being cultured with Fe–Pd and Fe–Pt composite
extracts compared with pure iron group. This may be related to the
concentra- tion of leaching solution as higher iron ion
concentration led to lower cell viability value. As for palladium
and platinum ions, studies showed that palladium ion has a slight
toxicity to eukaryotic cells and it is con- sidered to be tolerated
by human body [37,55], but Pt ion has high cyto- toxicity [56].
However, the ion concentration of both palladium and platinum was
extremely low, so their effects on cell reproduction should be very
weak.
For L-929 and ECV304 cells, values of cell viability in
experimental materials' extracts were nearly 100% or more than 100%
after cultured for 1 day. However, viabilities decreased to around
80% compared with that of negative control after 2 or 4 days'
incubation. According to ISO 19003-5 [47], the cell viabilities of
L-929 and ECV304 cells for ex- perimental materials were both more
than 70%, indicating Fe–Pd and Fe–Pt composites' slight cell
toxicity to L929 and ECV304 cells. Zhu's work [57] also
demonstrated that iron ions almost have no effect onme- tabolism of
ECV304 cells. For VSMC, the cell viability in extracts of
experimental materials was just about 60% compared with that of
neg- ative control after 4 days' incubation. According to Mueller's
work [58], ferrous iron could reduce the proliferation rate of VSMC
by influencing growth-related gene expression. The cell toxicity
for VSMC was good for antagonizing restenosis in vivo [58].
Endothelial process is very important for stents. Thrombogenicity
would decrease as the stent surface is covered by regrowth of
endothe- lium [59]. So, direct contact cytotoxicity of ECV304 cells
was also per- formed. The morphology of ECV304 cells after 24 h
cultivation on the pure iron and Fe–Pd and Fe–Pt composites is
shown in Fig. 12. ECV304 cells spread out on the surface of
experimental materials in good condi- tion, indicating that the
experimental materials exhibited no inhibition on vascular
endothelial process.
3.4.3. Hemocompatibility tests of Fe–Pd and Fe–Pt composites The
hemolysis rates of experimental materials were shown in
Fig. 13(a). Hemolysis rates of Fe–Pd and Fe–Pt composites were
slightly higher than that of pure iron. Nevertheless, the hemolysis
rates of all
materials were still below 5%, the judging criterion for excellent
blood compatibility in ASTM F756-08 [60].
Fig. 13(b) illustrates the number of platelets per unit area
adhered on the surface of samples. The number of platelets adhered
to the Fe– Pd composite was the least, about 50% as that of pure
iron. Platelets ad- hered to the Fe–Pt composite was slightly less
than that to pure iron. Combined with the result of contact angle
test, platelet adhesion might be associated with materials'
hydrophilicity. Materials with poor hydrophilicity resist platelet
adhesion. The morphologies of ad- hered human platelets on the
experimental materials were shown in Fig. 14. Although a lot of
corrosion products can be observed on the sur- faces of Fe–Pd and
Fe–Pt composites, the platelets adhered on these sur- faces stayed
round. However, the platelets adhered on the as-cast pure iron
stretched a small amount of pseudopods. The generation of pseu-
dopods is an obvious sign of platelet activation [61]. Therefore,
com- pared to pure iron, the thrombosis of both Fe–Pd and Fe–Pt
composites would decrease.
4. Conclusions
In order to meet the clinical application demands of biodegradable
coronary stent, Fe-5wt.%Pd and Fe-5wt.%Pt composites were prepared
by spark plasma sintering. The mechanical properties, corrosion
behav- iors andbiocompatibility of these compositeswere
systematically inves- tigated by XRD, EDS, ESEM, ICP-AES and some
other characterization techniques. The main conclusions were listed
as follows:
(1) Pd and Pt were uniformly distributed in iron matrix. The grain
size of Fe-5wt.%Pd and Fe-5wt.%Pt composites wasmuch smaller than
that of as-cast pure iron. The addition of Pd or Pt improved the
yield strength, rigidity and microhardness of pure iron.
(2) Fe-5wt.%Pd and Fe-5wt.%Pt uniformly corrode inHank's solution.
The addition of Pd or Pt greatly accelerates the degradation rate
of pure iron, especially for the Fe–Pt composite.
(3) Fe-5wt.%Pd and Fe-5wt.%Pt composites exhibited slight cytotox-
icity to L-929 and ECV304 cells. At the same time, Fe–Pd and Fe– Pt
composites indicate an inhibitory potential to VSMC, which is
beneficial to prevent vascular restenosis.
(4) The hemolysis of both Fe-5wt.%Pd and Fe-5wt.%Pt composites is
slightly higher than that of as-cast pure iron, but still less than
5%. Furthermore, the number of platelets adhered to Fe–Pd and Fe–Pt
composites is less than that of as-cast pure iron, and the
platelets adhered on the specimenswere round, without apparent
induced thrombosis.
In summary, iron-based composites reinforced by Pd or Pt are prom-
ising biodegradable biomaterials for vascular stentswith good
combina- tion of mechanical properties and biocompatibility as well
as a proper degradation rate.
53T. Huang et al. / Materials Science and Engineering C 35 (2014)
43–53
Acknowledgments
This work was supported by the National Basic Research Program of
China (973 Program) (Grant Nos. 2012CB619102 and 2012CB619100),
National Science Fund for Distinguished Young Scholars (Grant No.
51225101), Research Fund for the Doctoral Program of Higher Educa-
tion under Grant No. 20100001110011, and National Natural Science
Foundation of China (No. 31170909).
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1. Introduction
3. Results and discussion
3.3. Corrosion properties of Fe–Pd and Fe–Pt Composites
3.3.1. Electrochemical measurements
3.4. Biocompatibility
3.4.1. Contact angle tests of Fe–Pd and Fe–Pt composites
3.4.2. Cytotoxicity tests of Fe–Pd and Fe–Pt composites
3.4.3. Hemocompatibility tests of Fe–Pd and Fe–Pt composites
4. Conclusions