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Electrochemical investigations of magnesium in DMEM with biodegradable polycaprolactone coating as corrosion barrier
Applied Surface Science 282 (2013) 264–270
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Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Electrochemical investigations of magnesium in DMEM with biodegradable polycaprolactone coating as corrosion barrier Julia Degner a , Ferdinand Singer a , Luis Cordero b , Aldo R. Boccaccini b , Sannakaisa Virtanen a,∗ a b
Department of Materials Science, WW4-LKO, University of Erlangen-Nuremberg, Martenstr. 7, 91058 Erlangen, Germany Department of Materials Science, WW7-BioMat, University of Erlangen-Nuremberg, Cauerstr. 6, 91058 Erlangen, Germany
a r t i c l e
i n f o
Article history: Received 6 February 2013 Received in revised form 23 May 2013 Accepted 24 May 2013 Available online 30 May 2013 Keywords: Magnesium Biodegradable Coating Polycaprolactone DMEM Corrosion FTIR
a b s t r a c t Magnesium and its alloys are being increasingly investigated as biodegradable metallic implant materials. However, the high corrosion rate and accumulation of hydrogen gas upon degradation prevent the clinical application of many magnesium based materials. Applying polymer or ceramic coatings is a popular approach to improve the corrosion behaviour of magnesium and its alloys. In the current research, a biodegradable polymer film of polycaprolactone (PCL) is prepared in different concentrations by spin coating, in order to influence the corrosion behaviour of 99.9% pure magnesium. The resulting polymer coating was qualified by Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM) and tape-test according to ASTM D3359-09 to measure the adhesion strength of the coating on the substrate. Furthermore, coated and uncoated specimens were stored up to 30 days at 37 ◦ C in DMEM. The corrosion behaviour was investigated by polarization curves. The PCL-films were found to be uniform and without pores, but they show a low adhesion strength on the substrate. Nevertheless, remarkable improvement of the corrosion resistance of magnesium substrate can be obtained by the polymer films, depending on the film thickness and exposition time. In summary, coating magnesium with PCL is a promising method to tailor the degradation behaviour for biomedical applications. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Magnesium and its alloys are potential materials in the field of biodegradable materials such as bone implants and cardiovascular stents [1,2]. Magnesium exhibits excellent mechanical properties [1] such as high strength in comparison to polymers and high ductility in comparison to bio ceramics [3]. Moreover, it is one of the lightest metals and it has an elastic modulus close to that of bone [4], which is an important factor when it comes to load-bearing applications. The elastic moduli of most metallic biomaterials are much higher in comparison to bone which can cause the so called “stress shielding effect” that results in decreased stimulation of bone growth leading to reduced implant stability [5]. Another advantageous aspect is the degradability of magnesium in biological solutions which makes it an attractive candidate for resorbable implant materials [6,7]. In many applications, metal implants are removed after a certain time of implantation [8]. Therefore, further surgical intervention is necessary, accompanied by risks and costs [9]. A biomaterial with a tailored degradation rate adapted to new
∗ Corresponding author. Tel.: +49 91318527577. E-mail address: [email protected] (S. Virtanen). 0169-4332/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2013.05.115
bone formation would avoid additional surgery, reduce the risk of complications and shorten the healing phase. Beside this, magnesium is essential to human metabolism with approximately half of the total physiological magnesium stored in bone tissue [10,11]. This implies that a release of magnesium ions during implantation and degradation has no negative effect to the human body, which underlines its biocompatibility [12,13]. Using magnesium as biodegradable implant material is not a new approach. Already in the year 1907 Lambotte tried to fix a leg fracture with a magnesium plate [4,14]. Thereby he failed because magnesium dissolved too fast accompanied by the release of a high amount of hydrogen gas [15]. Even today, the fast degradation and the evolution of hydrogen gas limit the application of magnesium as implant material [16]. Different investigations exhibited that surface modifications show promising results regarding corrosion protection of magnesium and several methods have been reviewed recently to optimize the corrosion rate of magnesium [17,18]. One of the most popular approaches is to apply protective coatings. Hornberger et al. [19] summarized research in the field of degradable coatings on magnesium. In our current study, polycaprolactone (PCL) as deposited organic coating [19] is applied. Coating magnesium with polymers like PCL has previously been investigated by several authors. For instance, Chen et al. [2] investigated high purity
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magnesium (99.95%) coated with PCL and PLA (Poly-Lactic Acid). The electrochemical behaviour of the dip coated samples was studied for 10 days in buffered modified simulated body fluid (m-SBF). Furthermore, Wong et al. [16] and Conceicao et al. [20] examined the corrosion behaviour of dip-coated magnesium AZ91 with PCL and AZ31 with PEI, respectively. Xu et al. [1] applied Poly-Lactic Acid (PLLA) and PCL on 99.95% magnesium by spin coating. All these investigations demonstrate that polymer coatings improve the initial corrosion resistance of magnesium, as expected. Furthermore, cytocompatibility tests (SaOS-2 cells) showed that polymer coatings like PCL have a significantly better initial cytocompatibility than uncoated magnesium [1,16]. In our present study, magnesium (of 99.9% purity) was coated with PCL by spin coating, whereas spin coating is applied to achieve a homogenously distributed “model” PCL layer on the magnesium surface, according to Xu et al. [1]. The corrosion properties of this model surface coating are then investigated for the first time for up to 30 days in cell culture medium. The aim of the work was to investigate the tailoring of the degradation rate of magnesium and to gain information about the corrosion mechanism of PCL magnesium interface.
2. Materials and methods A commercially pure Magnesium rod (25.4 mm diameter, 99.9%purity, ChemPur Feinchemikalien und Forschungsbedarf GmbH) was employed. The samples were cut into 2 mm thick slices, ground on a microcut paper disc (1200 grit) and then ultrasonically cleaned in ethanol. Afterwards they were polished with diamond paste in four steps (6 m, 3 m, 1 m, 0.25 m) using an ethanol/glycerol (3:1) mixture as lubricant and again ultrasonically cleaned in ethanol and air dried. Before coating, the substrates were pre-heated over a heating plate at 120 ◦ C. The coating material was polycaprolactone (PCL) (Sigma–Aldrich) with a molecular weight of 70 000–90 000 g/mol. PCL was dissolved in chloroform in three different concentrations: 2.5 wt%, 5.0 wt% and 7.5 wt%. 200 l of each solution were applied with a micropipette on the surface of the samples and the polymer film was prepared by a spin coater (BLE Laboratory Equipment GmbH). Spin coating was carried out in a grey room at room temperature and a humidity of 30%. The conditions for spin coating were 5000 rpm for 30 s. Subsequently, the samples were dried at the room temperature. The film thickness was calculated by mass differences between coated and uncoated samples, assuming that the coating material is distributed homogenously on the substrate. To qualify the adhesion strength of the polymer film on the magnesium substrate, the standard test method ASTM D3359-09 (Tape Test) [21] was used. The adhesion of the coated film was assessed by applying and removing a pressure-sensitive tape over cuts scratched in the film [21]. Finally, the adhesion strength was categorized by examining the remaining percentage of polymer residues on the substrate. To characterize the films composition Fourier transform infrared spectroscopy was applied (FTIR, Nicolet 6700, Thermo Scientific) over the samples of PCL/Mg produced with different concentrations of PCL. The bands of the resulting spectra were compared to the spectrum of PCL as bulk material [22]. Furthermore, the PCL-layer on the magnesium substrate was examined by Scanning Electron Microscope (SEM) (Hitachi FE-SEM S-4800). The images were taken at an acceleration voltage of 10 kV using a cold field emitter. The secondary electrons were recorded with an upper and lower secondary electron detector. Prior to electrochemical testing, the samples were stored in 80 ml DMEM (Biochrome AG) at 37 ◦ C for 15 min, 60 min, 24 h, 7 days, 15 days and 30 days to study the long-term behaviour of the Magnesium-PCL-system at simulated body conditions. The composition of DMEM is shown in Table 1. For each coating
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Table 1 Composition of Dulbecco’s modified eagle medium. Substance
Concentration (mg/l)
Substance
Concentration (mg/l)
NaCl KCl CaCl2 MgSO4 ·7H2 O NaH2PO4 d-Glucose Fe(NO3 )3 ·9H2 O Na-Pyruvate Phenol red NaHCO3 l-Arginine·HCl l-Cysteine l-Glutamine l-Histidin·HCl H2 O l-Isoleucine l-Leucine
6400 400 200 200 124 1000 0.1 110 15 3700 84 48 580 42
l-Lysine HCl l-Methionine l-Phenylalanine l-Threonine l-Tryptophan l-Tyrosine l-Valine Glycine l-Serine Cholinchloride Folic acid Myo-Inositol Nicotinamide Da-Ca-Pantothenate Pyridoxal HCl Riboflavin Thiamine-HCL
146 30 66 95 16 72 94 30 42 4 4 7.2 4 4 4 0.4 4
106 106
thickness and exposure time a new sample was used for polarization measurement. An area of 1 cm2 was exposed to the electrolyte for testing. The same area was later used for electrochemical measurements. The polarization curve was measured with a potentiostat (Zahner Elektrik GmbH) using a three-electrode-system in 80 ml DMEM as corrosion medium. An Ag/AgCl electrode in 3-molar solution of KCl and a plate of platinum were used as reference electrode and counter electrode, respectively. The polarization scan started from −2 V to 1 V at a scan rate of 3 mV/s. The measurements were aborted when the current density exceeded 1 mA/cm2 . All results are shown with respect to the surface area. Every experimental setup was carried out at least three times and the average is shown. The corrosion current density icorr was determined using Tafel extrapolation. The Tafel lines were applied 50 mV away from Ecorr and proceeded over a decade of current density. After polarization measurements a picture of each surface was taken with a macroscope (Makroskop M420, Wild). 3. Results and discussion 3.1. Characterization of the coatings Fig. 1 illustrates the thicknesses of the resulting polymer films according to the different concentrations of PCL dissolved in chloroform. The solution of 2.5 wt% PCL resulted in a layer thickness
Fig. 1. Film thicknesses of different concentrations of PCL dissolved in chloroform.
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of 0.64 m ± 0.18 m. For 5.0 wt% and 7.5 wt% PCL the thicknesses were 2.29 m ± 0.82 m and 5.23 m ± 1.57 m; hence a straightforward way to control the achieved coating thickness is by adjusting the concentration of PCL in solution. The thickness of the layer increases with increasing PCL concentration due to density and viscosity of the solution. The resulting layer thicknesses achieved by the individual PCL concentrations vary widely from one coating to another. One possible reason could be the spin coating process [23] as the amount of PCL solution on the surface might slightly vary as well as the temperature and the humidity. On the other hand, similar divergences in layer thicknesses of PCL on magnesium have been observed by Xu et al. [1]. The adhesion strength of the PCL-film on the polished magnesium surface was categorized in class 0(B), according to ASTM D3359-09 [21]. That means that more than 65% of the layer got removed by detaching the tape, implying a low adhesion on the substrate. One possible reason could be the molecular structure of PCL. Polymers with higher ratio of O in its weight can provide more electrostatic interactions on the magnesium surface. Compared to PLLA, PCL has a lower ratio of oxygen and therefore exhibits lower adhesion to the surface [1,24]. When it comes to implantation of biomaterials into the human body, poor adhesion between substrate and coating could lead to delamination of the coating [25,26]. As a consequence, the success of the whole biomedical device can be affected [1]. However, as demonstrated below, in spite of this limited adhesion in the scratch test, failure of the coatings during corrosion testing by complete delamination and peeling-off of the coating was not observed. Fig. 2 shows the FTIR spectra for the coatings produced with different chloroform concentrations. The three samples present the same spectra with an asymmetric CH2 stretching at 2944 cm−1 and the symmetric CH2 stretching at 2865 cm−1 [27,28]. At 1719 cm−1 appears the carbonyl stretching (C O), 1295 cm−1 the C O and C C stretching in the crystalline phase [27,28]. The asymmetric COC stretching is visible at 1240 cm−1 and also the OC O stretching is present at 1190 cm−1 , finally the symmetric COC stretching is located at 1160 cm−1 [27,28]. According to the results the PCL is present in all the samples and no other type of signal is visible due to the chloroform [27,28]. FTIR results imply that during the coating process the solvent chloroform almost completely vanishes; as no bands corresponding to chloroform are observed. This is a crucial
Fig. 2. FTRI spectra for the PCL/Mg samples produced with different concentrations of PCL: 2.5 wt% (a), 5.0 wt% (b) and 7.5 wt% (c).
factor when it comes to implanting the device into human body, as chloroform is known as a potential health risk for human body and studies have revealed possible cancerogenic effects [29]. Fig. 3 shows the surface morphologies of magnesium coated with 2.5 wt%, 5.0 wt% and 7.5 wt% PCL. Compared to the SEM pictures of Xu et al. [1] the PCL coated surface looks very similar. It can be observed that all polymer films were smooth and homogeneous; the lack of any features in the images demonstrates that the coatings are free of pores or cracks. 3.2. Corrosion behavior Polarization curves were recorded in order to investigate the long term corrosion behaviour of uncoated and with PCL coated magnesium in DMEM. For this, the samples were immersed in DMEM for up to 30 days at 37 ◦ C and polarization curves were measured after different immersion times. The aim was to investigate the protective effect of the coating and to compare the long term stability of the different coating thicknesses. Fig. 4 shows the
Fig. 3. SEM observations of (a) Mg coated with 2.5 wt% PCL, (b) Mg coated with 5.0 wt% PCL, (c) Mg coated with 7.5 wt% PCL.
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Fig. 4. Polarization curves of (a) pure magnesium (pMg), (b) magnesium coated with 2.5 wt% PCL, (c) magnesium coated with 5.0 wt% PCL and (d) magnesium coated with 7.5 wt% PCL after 15 min, 60 min, 24 h, 7 days, 15 days, and 30 days storage in DMEM at 37 ◦ C. Polarization from −2 V to 1 V with a scan rate of 3 mV/s, stopping at 1 mA of current.
polarization curves obtained for (a) uncoated magnesium, (b) with 2.5 wt% PCL, (c) 5.0 wt% PCL and (d) 7.5 wt% PCL coated magnesium surfaces. Tables 1 and 2 show icorr and Ecorr values and the standard deviation determined from the polarization curves. The logarithmic icorr over the immersion time is plotted in Fig. 5. After polarization a picture of each sample surface was taken with a macroscope and is shown in Fig. 6. For pure magnesium polarization measurements were carried out for up to 30 days. In Fig. 4a and Table 2 a shift of the corrosion potential to more anodic directions can be observed with increasing immersion times in DMEM. The cathodic current decreases with increasing immersion time as can be seen in Fig. 4a. The anodic reactions are also slightly more blocked. The beginning of the unhindered fast dissolution of magnesium is also shifted to a more positive potential with longer exposition time. The rapid increase of the anodic current indicates a charge-transfer controlled dissolution mechanism for uncoated magnesium until
the measurement breakoff criteria of 1 mA/cm2 . In Fig. 5 and Table 1 a slight decrease of the corrosion current density can be observed with increasing immersion time. The improved corrosion resistance upon longer exposure to D-MEM is in line with previous observations by others [30]. This improvement might result from an increase of the pH-value in the DMEM solution caused by magnesium dissolution with elongated immersion time, leading to partial passivation of the Mg surface. An additional reason enhancing changes on the electrode surface is the static electrolyte used in the present experiments. As the DMEM is not changed for the immersion time nor is the system in motion, corrosion products that form on the interface are not transported away. Fig. 6 shows a black layer formed on pure magnesium, which may become more protective with increasing immersion time. Also for pure magnesium exposed to DMEM for 30 days white particles, probably magnesium hydroxide, can be observed on the surface. Therefore, dissolution products precipitated on the Mg surface
Table 2 Corrosion currents taken from polarization curves for different exposition times and uncoated and coated samples with 2.5 wt%, 5.0 wt% and 7.5 wt% PCL.
15 min 60 min 24 h 7d 15 d 30 d
pMg (A/cm2 )
Standard deviation (±)
2.5 wt% PCL (A/cm2 )
Standard deviation (±)
5.0 wt% PCL (A/cm2 )
Standard deviation (±)
7.5 wt% PCL (A/cm2 )
Standard deviation (±)
3 3 10 2 0.4 2
0.18 0.15 0.28 0.12 0.05 0.15
0.2 0.2 0.5 7
6 × 10−3 5 × 10−3 2 × 10−3 0.35
0.04 0.004 0.06 0.1 0.6
1.2 × 10−3 1.4 × 10−4 2.4 × 10−3 4 × 10−3 3.2 × 10−3
0.006 0.0004 0.002 0.02 0.1 0.7
1.8 × 10−4 1.2 × 10−5 8 × 10−5 7.6 × 10−4 5 × 10−3 4.2 × 10−2
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Fig. 5. Logarithmic illustration of corrosion current (icorr ) of pure magnesium (pMg) and coated samples with different concentrations (2.5 wt%, 5.0 wt%, 7.5 wt%) after different storage times (15 min, 60 min, 24 h, 7 days, 15 days, 30 days) in DMEM at 37 ◦ C.
may form an additional barrier layer to hinder further dissolution. The polarization measurements for magnesium coated with 2.5 wt% PCL show a different corrosion behaviour for up to 24 h compared to uncoated magnesium, as can be seen in Fig. 4b. The cathodic current density is lower than for the bare magnesium sample. The corrosion potential is in a similar range as for uncoated magnesium, see also Table 2. Fig. 5 and Table 1 shows that the corrosion current density is a magnitude of 10 lower than for uncoated magnesium. Comparing Fig. 4a and b the anodic current density for magnesium coated with 2.5 wt% PCL shows remarkably different behaviour compared to pure magnesium. Up to 1 V the current does not exhibit the measurement breakoff criteria of 1 mA/cm2 . Furthermore, the slow current increase indicates a change of dissolution mechanism from charge-transfer controlled to diffusion controlled. This change of dissolution mechanism indicates that PCL coatings hinder but do not completely block the dissolution of magnesium (Table 3). After 7 days of immersing the 2.5 wt% PCL coated magnesium samples in DMEM, the polarization curve shows similar behaviour as for the bare Mg sample. Fig. 5 and Table 1 indicate that the corrosion current is of the same magnitude as for pure magnesium. This implies that the PCL layer is completely dissolved and has no protective effect on the magnesium surface any longer. The shift of the corrosion potential to −1.84 V might be due to complete dissolution of the layer during the immersion time, and therefore the magnesium surface is freshly exposed to DMEM. Fig. 4(c) shows the polarization measurements for 5.0 wt% PCL coated magnesium. The measurements show a notable lowering of the cathodic and anodic current density compared to magnesium and the 2.5 wt% PCL coated magnesium for up to 7 days of immersion time in DMEM. The corrosion current density is up to a hundred times lower as for pure magnesium, which indicates a good protection of the magnesium surface. The anodic current density shows similar behaviour as for 2.5 wt% PCL coated magnesium. The dissolution mechanism seems to be also diffusion controlled, whereas the current density is significantly lower as for 2.5 wt% PCL coated magnesium. Magnesium coated with 5.0 wt% shows a slightly different corrosion behaviour for up to 7 days compared to 2.5 wt% magnesium. The measured currents and the corrosion current density are decreasing in the first 24 h, and afterwards slowly increasing until the current reaches a similar level as magnesium after 15 days;
see also Fig. 5 and Table 1. The corrosion potential is also slightly shifted in the anodic direction. The decrease of the corrosion current until 24 h might be due to water uptake of the PCL coating and the beginning bulk hydrolysis of the layer [31,32]. With this wateruptake the PCL-layer degrades from the inside out by hydrolysis of the ester-linkages, thus water is breaking the ester bond and reducing the molecular weight of the PCL chain [33,34], and the surface remains stable and nonporous, similar to the initial state shown in Fig. 3, for up to 24 h. The pictures of 5.0 wt% PCL magnesium in Fig. 6 for 15 and 60 min indicate that the surface is not visually changed during the storage time and the corresponding icorr drop. From 1 to 15 days immersion in DMEM, the progressive influence of the degradation of PCL and the corrosion of the sample can be seen. After 15 days the layer seems to have no longer a protective effect, and the surface shows an initial black layer similar to magnesium after 15 min of immersion in DMEM. Possibly this drop of current density is not present on 2.5 wt% PCL coated magnesium because the layer is too thin and the hydrolysis creates pores on the surface as soon as the samples are immersed in DMEM, what fastens the dissolution of magnesium. The polarization measurements for 7.5 wt% PCL coated magnesium show similar results as for 5.0 wt% PCL coated magnesium, as shown in Fig. 4d. Comparing the corrosion potential and the corrosion current density, Tables 1 and 2 show that the corrosion potential again is shifted slightly to more anodic values and compared to the coating with 5.0 wt% PCL, icorr of 7.5 wt% PCL is a factor of ten lower. Magnesium coated with 7.5 wt% PCL also shows the drop of corrosion current density after 60 min of immersion in DMEM. The pictures obtained of the surfaces after 15 and 60 min indicate that the coating is not influenced by the solution. After 24 h up to 15 days, the pictures show an increasing degradation of the coating with time and corrosion on the surface. Figs 5 and 6 indicate that after 30 days the coating has no longer a protective effect on magnesium. A factor influencing the corrosion behaviour could be the use of a static electrolyte. The DMEM is not changed during the experiments and therefore corrosion products may collect on the surface as well as the products of the degraded PCL. These remnants may form an additional barrier for dissolution of the surface. Also the pHvalue could be increased by corrosion of magnesium, slowing down further dissolution of the magnesium surface [35,36], whereas the increasing of the pH-value might also increase the degradation rate of PCL as described by Lam et al. [37]. For further experiments a dynamic system, more similar to human body conditions, should be considered to investigate the corrosion behaviour. In summary, pure magnesium in a static DMEM solution shows slightly improved corrosion resistance for immersion at least up to 1 day. A black layer is formed on pure magnesium after immersion in DMEM. For magnesium surfaces coated with PCL a significant improvement of the corrosion resistance can be achieved depending on coating thickness. The 2.5 wt% PCL coated magnesium, with a thickness of 0.64 m, shows a slightly improved corrosion resistance for up to 7 days. Afterwards the corrosion resistance is lowered to that of magnesium. For 5.0 wt% and 7.5 wt% PCL coated magnesium the corrosion resistance increases within the first 60 min. After 60 min the coating is degraded and corrosion starts to increase. 5.0 wt% PCL, with a thickness of 2.29 m, protected the magnesium at least for up to 7 days. Magnesium with 7.5 wt% PCL coating, with a thickness of 5.23 m, showed the best corrosion protection with icorr being 1000 times lower than for uncoated magnesium. The results suggest that 7.5 wt% PCL coated magnesium is well protected against dissolution in DMEM for at least up to 15 days. The obtained pictures in Fig. 6 furthermore indicate that the surface for 5.0 wt% and 7.5 wt% PCL are not changed during the first 60 min. Tables 1 and 2 also show the standard deviation for obtained Ecorr and icorr . The general deviation for
J. Degner et al. / Applied Surface Science 282 (2013) 264–270
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Fig. 6. Macroscopic images in 6.3 times magnification of pure magnesium (pMg) and magnesium coated with different concentrations of PCL (2.5 wt%, 5.0 wt%, 7.5 wt%) after different storage times in DMEM (15 min, 1 h, 24 h, 7 days, 15 days, 30 days).
magnesium is up to 6% whilst the deviation for PCL coated magnesium sample is increasing with increasing immersion time in DMEM. At first the deviation is at 3% going up to 6%, similar to magnesium, with increasing degradation of PCL. The anodic current densities measured for PCL coated magnesium indicate a
high stability of the layer up to 1 V with current densities below 1 mA/cm2 and a change from charge-transfer controlled to diffusion controlled dissolution. This suggests that the PCL coating acts as an ionic barrier layer, slowing the corrosion down by limiting the mass transport.
Table 3 Corrosion potentials taken from polarization curves for different exposition times and uncoated and coated samples with 2.5 wt%, 5.0 wt% and 7.5 wt% PCL.
15 min 60 min 24 h 7d 15 d 30 d
pMg (V)
Standard deviation (±)
2.5 wt% PCL (V)
Standard deviation (±)
5.0 wt% PCL (V)
Standard deviation (±)
7.5 wt% PCL (V)
Standard deviation (±)
−1.58 −1.54 −1.40 −1.50 −1.41 −1.45
0.10 0.09 0.07 0.09 0.08 0.06
−1.67 −1.55 −1.63 −1.84
0.01 0.02 0.02 0.03
−1.61 −1.45 −1.88 −1.54 −1.39
0.04 0.04 0.07 0.06 0.07
−1.61 −1.34 −1.53 −1.56 −1.37 −1.28
0.05 0.04 0.06 0.06 0.07 0.08
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4. Conclusion A biodegradable polymer film of PCL of different thickness was prepared by spin coating to improve the corrosion resistance of 99.9% pure magnesium. The PCL layer was characterized regarding microstructure, adhesive strength and molecular structure. The film was found to be nonporous and homogeneous. Furthermore, no residues of solvent could be found on the coated samples. Adhesion tests revealed low adhesion strength of all coatings. Measurement of polarization curves after different storage times in DMEM at 37 ◦ C showed that the coatings have a protective effect, correlating with film thickness, with respect to bare magnesium. The best coating achieved until now was able to efficiently protect Mg against corrosion at least for up to 15 days. Results suggest that the corrosion mechanism is altered from chargedcontrolled dissolution on bare magnesium surface to diffusion controlled dissolution on PCL coated surface. The initial current drop during the first 60 min for 5 wt% and 7.5 wt% PCL coated magnesium may be due to slow water uptake whereas the 2.5 wt% PCL coating seems to be penetrated by water instantaneously. Afterwards the dissolution follows a diffusion controlled bulk degradation of the PCL layer. The present PCL coated Mg substrates will be further characterized by cell culture tests to assess their application potential as bone substituting materials and for bone engineering. Acknowledgements The authors gratefully acknowledge financial support provided by German Research Foundation (GRF). We would like to thank Dr. Menti Goudouri and Anja Friedrich for their technical assistance. References [1] L. Xu, A. Yamamoto, Colloids and Surfaces B: Biointerfaces 93 (2012) 67–74. [2] Y. Chen, Y. Song, S.X. Zhang, J.N. Li, C.L. Zhao, X.N. Zhang, Biomedical Materials (2011) 6. [3] L.P. Xu, A. Yamamoto, Applied Surface Science 258 (2012) 6353–6358. [4] M.P. Staiger, A.M. Pietak, J. Huadmai, G. Dias, Biomaterials 27 (2006) 1728–1734. [5] J. Nagels, M. Stokdijk, P.M. Rozing, Journal of Shoulder and Elbow Surgery 12 (2003) 35–39. [6] B.E. Mc, Journal of the American Medical Association 111 (1938) 2464–2467.
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Electrochemical investigations of magnesium in DMEM with biodegradable polycaprolactone coating as corrosion barrier Julia Degner a , Ferdinand Singer a , Luis Cordero b , Aldo R. Boccaccini b , Sannakaisa Virtanen a,∗ a b
Department of Materials Science, WW4-LKO, University of Erlangen-Nuremberg, Martenstr. 7, 91058 Erlangen, Germany Department of Materials Science, WW7-BioMat, University of Erlangen-Nuremberg, Cauerstr. 6, 91058 Erlangen, Germany
a r t i c l e
i n f o
Article history: Received 6 February 2013 Received in revised form 23 May 2013 Accepted 24 May 2013 Available online 30 May 2013 Keywords: Magnesium Biodegradable Coating Polycaprolactone DMEM Corrosion FTIR
a b s t r a c t Magnesium and its alloys are being increasingly investigated as biodegradable metallic implant materials. However, the high corrosion rate and accumulation of hydrogen gas upon degradation prevent the clinical application of many magnesium based materials. Applying polymer or ceramic coatings is a popular approach to improve the corrosion behaviour of magnesium and its alloys. In the current research, a biodegradable polymer film of polycaprolactone (PCL) is prepared in different concentrations by spin coating, in order to influence the corrosion behaviour of 99.9% pure magnesium. The resulting polymer coating was qualified by Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM) and tape-test according to ASTM D3359-09 to measure the adhesion strength of the coating on the substrate. Furthermore, coated and uncoated specimens were stored up to 30 days at 37 ◦ C in DMEM. The corrosion behaviour was investigated by polarization curves. The PCL-films were found to be uniform and without pores, but they show a low adhesion strength on the substrate. Nevertheless, remarkable improvement of the corrosion resistance of magnesium substrate can be obtained by the polymer films, depending on the film thickness and exposition time. In summary, coating magnesium with PCL is a promising method to tailor the degradation behaviour for biomedical applications. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Magnesium and its alloys are potential materials in the field of biodegradable materials such as bone implants and cardiovascular stents [1,2]. Magnesium exhibits excellent mechanical properties [1] such as high strength in comparison to polymers and high ductility in comparison to bio ceramics [3]. Moreover, it is one of the lightest metals and it has an elastic modulus close to that of bone [4], which is an important factor when it comes to load-bearing applications. The elastic moduli of most metallic biomaterials are much higher in comparison to bone which can cause the so called “stress shielding effect” that results in decreased stimulation of bone growth leading to reduced implant stability [5]. Another advantageous aspect is the degradability of magnesium in biological solutions which makes it an attractive candidate for resorbable implant materials [6,7]. In many applications, metal implants are removed after a certain time of implantation [8]. Therefore, further surgical intervention is necessary, accompanied by risks and costs [9]. A biomaterial with a tailored degradation rate adapted to new
∗ Corresponding author. Tel.: +49 91318527577. E-mail address: [email protected] (S. Virtanen). 0169-4332/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2013.05.115
bone formation would avoid additional surgery, reduce the risk of complications and shorten the healing phase. Beside this, magnesium is essential to human metabolism with approximately half of the total physiological magnesium stored in bone tissue [10,11]. This implies that a release of magnesium ions during implantation and degradation has no negative effect to the human body, which underlines its biocompatibility [12,13]. Using magnesium as biodegradable implant material is not a new approach. Already in the year 1907 Lambotte tried to fix a leg fracture with a magnesium plate [4,14]. Thereby he failed because magnesium dissolved too fast accompanied by the release of a high amount of hydrogen gas [15]. Even today, the fast degradation and the evolution of hydrogen gas limit the application of magnesium as implant material [16]. Different investigations exhibited that surface modifications show promising results regarding corrosion protection of magnesium and several methods have been reviewed recently to optimize the corrosion rate of magnesium [17,18]. One of the most popular approaches is to apply protective coatings. Hornberger et al. [19] summarized research in the field of degradable coatings on magnesium. In our current study, polycaprolactone (PCL) as deposited organic coating [19] is applied. Coating magnesium with polymers like PCL has previously been investigated by several authors. For instance, Chen et al. [2] investigated high purity
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magnesium (99.95%) coated with PCL and PLA (Poly-Lactic Acid). The electrochemical behaviour of the dip coated samples was studied for 10 days in buffered modified simulated body fluid (m-SBF). Furthermore, Wong et al. [16] and Conceicao et al. [20] examined the corrosion behaviour of dip-coated magnesium AZ91 with PCL and AZ31 with PEI, respectively. Xu et al. [1] applied Poly-Lactic Acid (PLLA) and PCL on 99.95% magnesium by spin coating. All these investigations demonstrate that polymer coatings improve the initial corrosion resistance of magnesium, as expected. Furthermore, cytocompatibility tests (SaOS-2 cells) showed that polymer coatings like PCL have a significantly better initial cytocompatibility than uncoated magnesium [1,16]. In our present study, magnesium (of 99.9% purity) was coated with PCL by spin coating, whereas spin coating is applied to achieve a homogenously distributed “model” PCL layer on the magnesium surface, according to Xu et al. [1]. The corrosion properties of this model surface coating are then investigated for the first time for up to 30 days in cell culture medium. The aim of the work was to investigate the tailoring of the degradation rate of magnesium and to gain information about the corrosion mechanism of PCL magnesium interface.
2. Materials and methods A commercially pure Magnesium rod (25.4 mm diameter, 99.9%purity, ChemPur Feinchemikalien und Forschungsbedarf GmbH) was employed. The samples were cut into 2 mm thick slices, ground on a microcut paper disc (1200 grit) and then ultrasonically cleaned in ethanol. Afterwards they were polished with diamond paste in four steps (6 m, 3 m, 1 m, 0.25 m) using an ethanol/glycerol (3:1) mixture as lubricant and again ultrasonically cleaned in ethanol and air dried. Before coating, the substrates were pre-heated over a heating plate at 120 ◦ C. The coating material was polycaprolactone (PCL) (Sigma–Aldrich) with a molecular weight of 70 000–90 000 g/mol. PCL was dissolved in chloroform in three different concentrations: 2.5 wt%, 5.0 wt% and 7.5 wt%. 200 l of each solution were applied with a micropipette on the surface of the samples and the polymer film was prepared by a spin coater (BLE Laboratory Equipment GmbH). Spin coating was carried out in a grey room at room temperature and a humidity of 30%. The conditions for spin coating were 5000 rpm for 30 s. Subsequently, the samples were dried at the room temperature. The film thickness was calculated by mass differences between coated and uncoated samples, assuming that the coating material is distributed homogenously on the substrate. To qualify the adhesion strength of the polymer film on the magnesium substrate, the standard test method ASTM D3359-09 (Tape Test) [21] was used. The adhesion of the coated film was assessed by applying and removing a pressure-sensitive tape over cuts scratched in the film [21]. Finally, the adhesion strength was categorized by examining the remaining percentage of polymer residues on the substrate. To characterize the films composition Fourier transform infrared spectroscopy was applied (FTIR, Nicolet 6700, Thermo Scientific) over the samples of PCL/Mg produced with different concentrations of PCL. The bands of the resulting spectra were compared to the spectrum of PCL as bulk material [22]. Furthermore, the PCL-layer on the magnesium substrate was examined by Scanning Electron Microscope (SEM) (Hitachi FE-SEM S-4800). The images were taken at an acceleration voltage of 10 kV using a cold field emitter. The secondary electrons were recorded with an upper and lower secondary electron detector. Prior to electrochemical testing, the samples were stored in 80 ml DMEM (Biochrome AG) at 37 ◦ C for 15 min, 60 min, 24 h, 7 days, 15 days and 30 days to study the long-term behaviour of the Magnesium-PCL-system at simulated body conditions. The composition of DMEM is shown in Table 1. For each coating
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Table 1 Composition of Dulbecco’s modified eagle medium. Substance
Concentration (mg/l)
Substance
Concentration (mg/l)
NaCl KCl CaCl2 MgSO4 ·7H2 O NaH2PO4 d-Glucose Fe(NO3 )3 ·9H2 O Na-Pyruvate Phenol red NaHCO3 l-Arginine·HCl l-Cysteine l-Glutamine l-Histidin·HCl H2 O l-Isoleucine l-Leucine
6400 400 200 200 124 1000 0.1 110 15 3700 84 48 580 42
l-Lysine HCl l-Methionine l-Phenylalanine l-Threonine l-Tryptophan l-Tyrosine l-Valine Glycine l-Serine Cholinchloride Folic acid Myo-Inositol Nicotinamide Da-Ca-Pantothenate Pyridoxal HCl Riboflavin Thiamine-HCL
146 30 66 95 16 72 94 30 42 4 4 7.2 4 4 4 0.4 4
106 106
thickness and exposure time a new sample was used for polarization measurement. An area of 1 cm2 was exposed to the electrolyte for testing. The same area was later used for electrochemical measurements. The polarization curve was measured with a potentiostat (Zahner Elektrik GmbH) using a three-electrode-system in 80 ml DMEM as corrosion medium. An Ag/AgCl electrode in 3-molar solution of KCl and a plate of platinum were used as reference electrode and counter electrode, respectively. The polarization scan started from −2 V to 1 V at a scan rate of 3 mV/s. The measurements were aborted when the current density exceeded 1 mA/cm2 . All results are shown with respect to the surface area. Every experimental setup was carried out at least three times and the average is shown. The corrosion current density icorr was determined using Tafel extrapolation. The Tafel lines were applied 50 mV away from Ecorr and proceeded over a decade of current density. After polarization measurements a picture of each surface was taken with a macroscope (Makroskop M420, Wild). 3. Results and discussion 3.1. Characterization of the coatings Fig. 1 illustrates the thicknesses of the resulting polymer films according to the different concentrations of PCL dissolved in chloroform. The solution of 2.5 wt% PCL resulted in a layer thickness
Fig. 1. Film thicknesses of different concentrations of PCL dissolved in chloroform.
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of 0.64 m ± 0.18 m. For 5.0 wt% and 7.5 wt% PCL the thicknesses were 2.29 m ± 0.82 m and 5.23 m ± 1.57 m; hence a straightforward way to control the achieved coating thickness is by adjusting the concentration of PCL in solution. The thickness of the layer increases with increasing PCL concentration due to density and viscosity of the solution. The resulting layer thicknesses achieved by the individual PCL concentrations vary widely from one coating to another. One possible reason could be the spin coating process [23] as the amount of PCL solution on the surface might slightly vary as well as the temperature and the humidity. On the other hand, similar divergences in layer thicknesses of PCL on magnesium have been observed by Xu et al. [1]. The adhesion strength of the PCL-film on the polished magnesium surface was categorized in class 0(B), according to ASTM D3359-09 [21]. That means that more than 65% of the layer got removed by detaching the tape, implying a low adhesion on the substrate. One possible reason could be the molecular structure of PCL. Polymers with higher ratio of O in its weight can provide more electrostatic interactions on the magnesium surface. Compared to PLLA, PCL has a lower ratio of oxygen and therefore exhibits lower adhesion to the surface [1,24]. When it comes to implantation of biomaterials into the human body, poor adhesion between substrate and coating could lead to delamination of the coating [25,26]. As a consequence, the success of the whole biomedical device can be affected [1]. However, as demonstrated below, in spite of this limited adhesion in the scratch test, failure of the coatings during corrosion testing by complete delamination and peeling-off of the coating was not observed. Fig. 2 shows the FTIR spectra for the coatings produced with different chloroform concentrations. The three samples present the same spectra with an asymmetric CH2 stretching at 2944 cm−1 and the symmetric CH2 stretching at 2865 cm−1 [27,28]. At 1719 cm−1 appears the carbonyl stretching (C O), 1295 cm−1 the C O and C C stretching in the crystalline phase [27,28]. The asymmetric COC stretching is visible at 1240 cm−1 and also the OC O stretching is present at 1190 cm−1 , finally the symmetric COC stretching is located at 1160 cm−1 [27,28]. According to the results the PCL is present in all the samples and no other type of signal is visible due to the chloroform [27,28]. FTIR results imply that during the coating process the solvent chloroform almost completely vanishes; as no bands corresponding to chloroform are observed. This is a crucial
Fig. 2. FTRI spectra for the PCL/Mg samples produced with different concentrations of PCL: 2.5 wt% (a), 5.0 wt% (b) and 7.5 wt% (c).
factor when it comes to implanting the device into human body, as chloroform is known as a potential health risk for human body and studies have revealed possible cancerogenic effects [29]. Fig. 3 shows the surface morphologies of magnesium coated with 2.5 wt%, 5.0 wt% and 7.5 wt% PCL. Compared to the SEM pictures of Xu et al. [1] the PCL coated surface looks very similar. It can be observed that all polymer films were smooth and homogeneous; the lack of any features in the images demonstrates that the coatings are free of pores or cracks. 3.2. Corrosion behavior Polarization curves were recorded in order to investigate the long term corrosion behaviour of uncoated and with PCL coated magnesium in DMEM. For this, the samples were immersed in DMEM for up to 30 days at 37 ◦ C and polarization curves were measured after different immersion times. The aim was to investigate the protective effect of the coating and to compare the long term stability of the different coating thicknesses. Fig. 4 shows the
Fig. 3. SEM observations of (a) Mg coated with 2.5 wt% PCL, (b) Mg coated with 5.0 wt% PCL, (c) Mg coated with 7.5 wt% PCL.
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Fig. 4. Polarization curves of (a) pure magnesium (pMg), (b) magnesium coated with 2.5 wt% PCL, (c) magnesium coated with 5.0 wt% PCL and (d) magnesium coated with 7.5 wt% PCL after 15 min, 60 min, 24 h, 7 days, 15 days, and 30 days storage in DMEM at 37 ◦ C. Polarization from −2 V to 1 V with a scan rate of 3 mV/s, stopping at 1 mA of current.
polarization curves obtained for (a) uncoated magnesium, (b) with 2.5 wt% PCL, (c) 5.0 wt% PCL and (d) 7.5 wt% PCL coated magnesium surfaces. Tables 1 and 2 show icorr and Ecorr values and the standard deviation determined from the polarization curves. The logarithmic icorr over the immersion time is plotted in Fig. 5. After polarization a picture of each sample surface was taken with a macroscope and is shown in Fig. 6. For pure magnesium polarization measurements were carried out for up to 30 days. In Fig. 4a and Table 2 a shift of the corrosion potential to more anodic directions can be observed with increasing immersion times in DMEM. The cathodic current decreases with increasing immersion time as can be seen in Fig. 4a. The anodic reactions are also slightly more blocked. The beginning of the unhindered fast dissolution of magnesium is also shifted to a more positive potential with longer exposition time. The rapid increase of the anodic current indicates a charge-transfer controlled dissolution mechanism for uncoated magnesium until
the measurement breakoff criteria of 1 mA/cm2 . In Fig. 5 and Table 1 a slight decrease of the corrosion current density can be observed with increasing immersion time. The improved corrosion resistance upon longer exposure to D-MEM is in line with previous observations by others [30]. This improvement might result from an increase of the pH-value in the DMEM solution caused by magnesium dissolution with elongated immersion time, leading to partial passivation of the Mg surface. An additional reason enhancing changes on the electrode surface is the static electrolyte used in the present experiments. As the DMEM is not changed for the immersion time nor is the system in motion, corrosion products that form on the interface are not transported away. Fig. 6 shows a black layer formed on pure magnesium, which may become more protective with increasing immersion time. Also for pure magnesium exposed to DMEM for 30 days white particles, probably magnesium hydroxide, can be observed on the surface. Therefore, dissolution products precipitated on the Mg surface
Table 2 Corrosion currents taken from polarization curves for different exposition times and uncoated and coated samples with 2.5 wt%, 5.0 wt% and 7.5 wt% PCL.
15 min 60 min 24 h 7d 15 d 30 d
pMg (A/cm2 )
Standard deviation (±)
2.5 wt% PCL (A/cm2 )
Standard deviation (±)
5.0 wt% PCL (A/cm2 )
Standard deviation (±)
7.5 wt% PCL (A/cm2 )
Standard deviation (±)
3 3 10 2 0.4 2
0.18 0.15 0.28 0.12 0.05 0.15
0.2 0.2 0.5 7
6 × 10−3 5 × 10−3 2 × 10−3 0.35
0.04 0.004 0.06 0.1 0.6
1.2 × 10−3 1.4 × 10−4 2.4 × 10−3 4 × 10−3 3.2 × 10−3
0.006 0.0004 0.002 0.02 0.1 0.7
1.8 × 10−4 1.2 × 10−5 8 × 10−5 7.6 × 10−4 5 × 10−3 4.2 × 10−2
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Fig. 5. Logarithmic illustration of corrosion current (icorr ) of pure magnesium (pMg) and coated samples with different concentrations (2.5 wt%, 5.0 wt%, 7.5 wt%) after different storage times (15 min, 60 min, 24 h, 7 days, 15 days, 30 days) in DMEM at 37 ◦ C.
may form an additional barrier layer to hinder further dissolution. The polarization measurements for magnesium coated with 2.5 wt% PCL show a different corrosion behaviour for up to 24 h compared to uncoated magnesium, as can be seen in Fig. 4b. The cathodic current density is lower than for the bare magnesium sample. The corrosion potential is in a similar range as for uncoated magnesium, see also Table 2. Fig. 5 and Table 1 shows that the corrosion current density is a magnitude of 10 lower than for uncoated magnesium. Comparing Fig. 4a and b the anodic current density for magnesium coated with 2.5 wt% PCL shows remarkably different behaviour compared to pure magnesium. Up to 1 V the current does not exhibit the measurement breakoff criteria of 1 mA/cm2 . Furthermore, the slow current increase indicates a change of dissolution mechanism from charge-transfer controlled to diffusion controlled. This change of dissolution mechanism indicates that PCL coatings hinder but do not completely block the dissolution of magnesium (Table 3). After 7 days of immersing the 2.5 wt% PCL coated magnesium samples in DMEM, the polarization curve shows similar behaviour as for the bare Mg sample. Fig. 5 and Table 1 indicate that the corrosion current is of the same magnitude as for pure magnesium. This implies that the PCL layer is completely dissolved and has no protective effect on the magnesium surface any longer. The shift of the corrosion potential to −1.84 V might be due to complete dissolution of the layer during the immersion time, and therefore the magnesium surface is freshly exposed to DMEM. Fig. 4(c) shows the polarization measurements for 5.0 wt% PCL coated magnesium. The measurements show a notable lowering of the cathodic and anodic current density compared to magnesium and the 2.5 wt% PCL coated magnesium for up to 7 days of immersion time in DMEM. The corrosion current density is up to a hundred times lower as for pure magnesium, which indicates a good protection of the magnesium surface. The anodic current density shows similar behaviour as for 2.5 wt% PCL coated magnesium. The dissolution mechanism seems to be also diffusion controlled, whereas the current density is significantly lower as for 2.5 wt% PCL coated magnesium. Magnesium coated with 5.0 wt% shows a slightly different corrosion behaviour for up to 7 days compared to 2.5 wt% magnesium. The measured currents and the corrosion current density are decreasing in the first 24 h, and afterwards slowly increasing until the current reaches a similar level as magnesium after 15 days;
see also Fig. 5 and Table 1. The corrosion potential is also slightly shifted in the anodic direction. The decrease of the corrosion current until 24 h might be due to water uptake of the PCL coating and the beginning bulk hydrolysis of the layer [31,32]. With this wateruptake the PCL-layer degrades from the inside out by hydrolysis of the ester-linkages, thus water is breaking the ester bond and reducing the molecular weight of the PCL chain [33,34], and the surface remains stable and nonporous, similar to the initial state shown in Fig. 3, for up to 24 h. The pictures of 5.0 wt% PCL magnesium in Fig. 6 for 15 and 60 min indicate that the surface is not visually changed during the storage time and the corresponding icorr drop. From 1 to 15 days immersion in DMEM, the progressive influence of the degradation of PCL and the corrosion of the sample can be seen. After 15 days the layer seems to have no longer a protective effect, and the surface shows an initial black layer similar to magnesium after 15 min of immersion in DMEM. Possibly this drop of current density is not present on 2.5 wt% PCL coated magnesium because the layer is too thin and the hydrolysis creates pores on the surface as soon as the samples are immersed in DMEM, what fastens the dissolution of magnesium. The polarization measurements for 7.5 wt% PCL coated magnesium show similar results as for 5.0 wt% PCL coated magnesium, as shown in Fig. 4d. Comparing the corrosion potential and the corrosion current density, Tables 1 and 2 show that the corrosion potential again is shifted slightly to more anodic values and compared to the coating with 5.0 wt% PCL, icorr of 7.5 wt% PCL is a factor of ten lower. Magnesium coated with 7.5 wt% PCL also shows the drop of corrosion current density after 60 min of immersion in DMEM. The pictures obtained of the surfaces after 15 and 60 min indicate that the coating is not influenced by the solution. After 24 h up to 15 days, the pictures show an increasing degradation of the coating with time and corrosion on the surface. Figs 5 and 6 indicate that after 30 days the coating has no longer a protective effect on magnesium. A factor influencing the corrosion behaviour could be the use of a static electrolyte. The DMEM is not changed during the experiments and therefore corrosion products may collect on the surface as well as the products of the degraded PCL. These remnants may form an additional barrier for dissolution of the surface. Also the pHvalue could be increased by corrosion of magnesium, slowing down further dissolution of the magnesium surface [35,36], whereas the increasing of the pH-value might also increase the degradation rate of PCL as described by Lam et al. [37]. For further experiments a dynamic system, more similar to human body conditions, should be considered to investigate the corrosion behaviour. In summary, pure magnesium in a static DMEM solution shows slightly improved corrosion resistance for immersion at least up to 1 day. A black layer is formed on pure magnesium after immersion in DMEM. For magnesium surfaces coated with PCL a significant improvement of the corrosion resistance can be achieved depending on coating thickness. The 2.5 wt% PCL coated magnesium, with a thickness of 0.64 m, shows a slightly improved corrosion resistance for up to 7 days. Afterwards the corrosion resistance is lowered to that of magnesium. For 5.0 wt% and 7.5 wt% PCL coated magnesium the corrosion resistance increases within the first 60 min. After 60 min the coating is degraded and corrosion starts to increase. 5.0 wt% PCL, with a thickness of 2.29 m, protected the magnesium at least for up to 7 days. Magnesium with 7.5 wt% PCL coating, with a thickness of 5.23 m, showed the best corrosion protection with icorr being 1000 times lower than for uncoated magnesium. The results suggest that 7.5 wt% PCL coated magnesium is well protected against dissolution in DMEM for at least up to 15 days. The obtained pictures in Fig. 6 furthermore indicate that the surface for 5.0 wt% and 7.5 wt% PCL are not changed during the first 60 min. Tables 1 and 2 also show the standard deviation for obtained Ecorr and icorr . The general deviation for
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Fig. 6. Macroscopic images in 6.3 times magnification of pure magnesium (pMg) and magnesium coated with different concentrations of PCL (2.5 wt%, 5.0 wt%, 7.5 wt%) after different storage times in DMEM (15 min, 1 h, 24 h, 7 days, 15 days, 30 days).
magnesium is up to 6% whilst the deviation for PCL coated magnesium sample is increasing with increasing immersion time in DMEM. At first the deviation is at 3% going up to 6%, similar to magnesium, with increasing degradation of PCL. The anodic current densities measured for PCL coated magnesium indicate a
high stability of the layer up to 1 V with current densities below 1 mA/cm2 and a change from charge-transfer controlled to diffusion controlled dissolution. This suggests that the PCL coating acts as an ionic barrier layer, slowing the corrosion down by limiting the mass transport.
Table 3 Corrosion potentials taken from polarization curves for different exposition times and uncoated and coated samples with 2.5 wt%, 5.0 wt% and 7.5 wt% PCL.
15 min 60 min 24 h 7d 15 d 30 d
pMg (V)
Standard deviation (±)
2.5 wt% PCL (V)
Standard deviation (±)
5.0 wt% PCL (V)
Standard deviation (±)
7.5 wt% PCL (V)
Standard deviation (±)
−1.58 −1.54 −1.40 −1.50 −1.41 −1.45
0.10 0.09 0.07 0.09 0.08 0.06
−1.67 −1.55 −1.63 −1.84
0.01 0.02 0.02 0.03
−1.61 −1.45 −1.88 −1.54 −1.39
0.04 0.04 0.07 0.06 0.07
−1.61 −1.34 −1.53 −1.56 −1.37 −1.28
0.05 0.04 0.06 0.06 0.07 0.08
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4. Conclusion A biodegradable polymer film of PCL of different thickness was prepared by spin coating to improve the corrosion resistance of 99.9% pure magnesium. The PCL layer was characterized regarding microstructure, adhesive strength and molecular structure. The film was found to be nonporous and homogeneous. Furthermore, no residues of solvent could be found on the coated samples. Adhesion tests revealed low adhesion strength of all coatings. Measurement of polarization curves after different storage times in DMEM at 37 ◦ C showed that the coatings have a protective effect, correlating with film thickness, with respect to bare magnesium. The best coating achieved until now was able to efficiently protect Mg against corrosion at least for up to 15 days. Results suggest that the corrosion mechanism is altered from chargedcontrolled dissolution on bare magnesium surface to diffusion controlled dissolution on PCL coated surface. The initial current drop during the first 60 min for 5 wt% and 7.5 wt% PCL coated magnesium may be due to slow water uptake whereas the 2.5 wt% PCL coating seems to be penetrated by water instantaneously. Afterwards the dissolution follows a diffusion controlled bulk degradation of the PCL layer. The present PCL coated Mg substrates will be further characterized by cell culture tests to assess their application potential as bone substituting materials and for bone engineering. Acknowledgements The authors gratefully acknowledge financial support provided by German Research Foundation (GRF). We would like to thank Dr. Menti Goudouri and Anja Friedrich for their technical assistance. References [1] L. Xu, A. Yamamoto, Colloids and Surfaces B: Biointerfaces 93 (2012) 67–74. [2] Y. Chen, Y. Song, S.X. Zhang, J.N. Li, C.L. Zhao, X.N. Zhang, Biomedical Materials (2011) 6. [3] L.P. Xu, A. Yamamoto, Applied Surface Science 258 (2012) 6353–6358. [4] M.P. Staiger, A.M. Pietak, J. Huadmai, G. Dias, Biomaterials 27 (2006) 1728–1734. [5] J. Nagels, M. Stokdijk, P.M. Rozing, Journal of Shoulder and Elbow Surgery 12 (2003) 35–39. [6] B.E. Mc, Journal of the American Medical Association 111 (1938) 2464–2467.
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