The chitosan coating and processing effect on the physiological corrosion behaviour of porous magnesium monoliths

Progress in Organic Coatings 99 (2016) 147–156

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The chitosan coating and processing effect on the physiological corrosion behaviour of porous magnesium monoliths Hanuma Reddy Tiyyagura a,b , Rebeka Rudolf b,c , Selestina Gorgieva b , Regina Fuchs-Godec d , Boyapati Venkatappa Rao e , Mantravadi Krishna Mohan a,∗ , Vanja Kokol b,∗∗ a

Department of Metallurgical and Material Engineering, National Institute of Technology, Warangal, Telangana, India Faculty of Mechanical Engineering, University of Maribor, Maribor, Slovenia c Zlatarna Celje d.d., Slovenia d Faculty of Chemistry and Chemical Engineering, University of Maribor, Maribor, Slovenia e Department of Chemistry, National Institute of Technology, Warangal, Telangana, India b

a r t i c l e

i n f o

Article history: Received 4 November 2015 Received in revised form 15 May 2016 Accepted 26 May 2016 Keywords: Mg monolith Porosity Surface coating Chitosan Mineralization Corrosion resistance

a b s t r a c t The present study evaluates the corrosion resistance effect of chitosan coating onto porous magnesium (Mg) monoliths with porosities of 14–40 vol.%, prepared by sintering of Mg powder with NH4 HCO3 used as spacer particles. The dip-coated chitosan was found to interact with corrosion products (Mg(OH)2 and MgO) via hydrogen bonding, providing the physiological stability and the corrosion resistance, at the same time affecting the mineralization process towards amorphous apatite with a small contribution of crystalline hydroxyapatite(HAP). The electrochemical studies reveal that porosity increases, and that the coating process affects the corrosion resistance positively. However, the values for compression strength (17–72 MPa) and elastic modulus (12–26 MPa) of chitosan-coated monolith indicate its applicability as a supporting, rather than self-standing implantation material. © 2016 Elsevier B.V. All rights reserved.

1. Introduction Biodegradable implants have already proved their clinical efficiency in cardiovascular and orthopaedic devices [1–7]. Among them, the magnesium (Mg)-based ones are the most promising, especially in hard tissue regeneration [8–10], due to the many advantages which they offer over the conventionally used metals (e.g. stainless steel, cobalt-based and titanium-based alloys [7]): greater fracture toughness, a good match of the elastic modulus and compressive yield strengths to cortical human bone [11], the reduced stress shielding [5], light weight due to low density (1.74 g cc−1 ), excellent biocompatibility, biodegradability and bioresorbability [12,13]. Acting as temporary implants they can

∗ Corresponding author at: National Institute of Technology, Department of Metallurgical and Material Engineering, 506004 Warangal, Telangana, India. ∗∗ Corresponding author at: University of Maribor, Faculty of Mechanical Engineering, Smetanova ul. 17, SI-2000 Maribor, Slovenia. E-mail addresses: [email protected] (K.M. Mantravadi), [email protected] (V. Kokol). http://dx.doi.org/10.1016/j.porgcoat.2016.05.019 0300-9440/© 2016 Elsevier B.V. All rights reserved.

dissolve within the body, without the need for a second surgical intervention for their removal. However, this behaviour (too rapid degradation [10] in the presence of human body fluids) brings the main limitations of Mg-based implants, which causes the reduction in the mechanical integrity before the tissue regeneration as well as simultaneous generation of H2 and OH− ions in the surrounding medium with consequent alkalinity increase, leading to delay of the healing process at the implantation site, as well as tissue necrosis [14,15]. There are different strategies adopted in developing magnesium based implants. Producing magnesium based new alloys, composites, developing surface coatings and microstructural modifications are a few methods reported in the literature [16–24]. Therefore, there is increasing demand for suitable Mgbased implants with an appropriate degrading rate and bio safe corrosion products. The choice of appropriate materials’ processing is one approach for degradation control, which explains the higher performance (by means of lower H2 evolution, as well as pH control) of porous-, comparing to compact (non-porous) Mg-based implants [25]. In this respect, different techniques for production of porous Mg materi-

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als have been applied, such as e.g. The Gasar process, infiltration process, laser perforating technique and powder metallurgy (P/M) [26]. The last one (P/M) has recently found its place in the biomedical area, advancing the production of near net shapes with narrow tolerances and controlled porosity, the use of different powders such as metallic and non-metallic (ceramics), achievement of high surface quality and lower cost than other conventional methods, like casting and extrusion. This process also allows the formation of a finer and uniform dispersion of second phases and grain refinement, grain size control, and consequent mechanical properties’ improvement [27]. The other approach for degradation delay or improved corrosion resistance, which affect the quality of the bone–implant integration simultaneously while preventing the occurrence of an intervening fibrous tissue layer, is surface modification of Mg-based implants, like alloying, micro arc oxidation, plasma electrolytic deposition, magnetron sputtering, electrophoretic deposition, solgel, physical and chemical vapour depositions, ion implantation, hydroxyapatite (HAP), and polymer coating [28–31]. In comparison with synthetic polymers (e.g. polylactic acid, poly lactic-co-glycolic acid, polycaprolactone),the biopolymer coatings using hyaluronic acid, alginate, poly-l-lysine, collagen, fibronectin etc. were found more biocompatible, mainly due to the lack of highly acidic degradation products related to the synthetics [32]. Their bioactivity and osteointegration support are additional advantages, being especially valuable in orthopaedic applications. The aim of the present study was thus to evaluate the effect of physiological stability and corrosion resistance of chitosan-coated porous Mg monoliths. Although the chitosan was already applied in the modification of Mg-containing implants for improving both anti-corrosion properties as well as biocompatibility [33–38], this effect onto pure and porous Mg-based materials has not been evaluated yet. In that respect, the Mg monoliths were prepared by P/M technology using ammonium bicarbonate(NH4 HCO3 ) as space holding particles to control the porosity profile, and being examined related to the microstructure by SEM and porosity analysis. The chitosan coating efficacy and stability were evaluated by FTIR and XRD spectroscopies, while the corrosion behaviour was followed by using an immersion test with gravimetrical and pH change evaluation and electrochemical analysis. Finally, the mechanical (compression) performance and the mineralization potential onto the monolith surface were examined. 2. Experimental 2.1. Materials Analytical grade Mg powder (99.9% purity; size <100 ␮m), NH4 HCO3 , and low molecular weight chitosan (deacetylation degree of 75–85%) were purchased from Sigma Aldrich. For the preparation of the Simulated Body Fluid (SBF) solution, the following reagents (being received from Sigma Aldrich, without further purification) were utilised: 7.996 g of NaCl, NaHCO3 0.350 g, KCl 0.224 g, K2 HPO4 × 3H2 O 0.228 g, MgCl2 × 6H2 O 0.305 g, CaCl2 0.278 g, Na2 SO4 0.071 g (CH2 OH)3 CNH2 6.057 g respectively.

pacts with 13 mm diameter and 16 mm thickness. The obtained monoliths were treated further by a two-step heat treatment: (I) T = 130 ◦ C for 4 h and (II) 550 ◦ C for 6 h, under an argon atmosphere in order to burn out the spacer particles and merge the Mg particles into larger grains. 2.3. Porosity analysis Pycnometry was used to evaluate the porosity as a function of the monoliths‘ starting composition. The percentage of porosity (P) in the sintered samples was determined according to the following equation [39]. P = (1 − /s ) × 100% where s is the density of Mg and  is the density of the porous Mg sample, being determined as volume/mass ratio. 2.4. Chitosan coating Mg monoliths were ground with SiC abrasive paper down to 1200 grid, rinsed ultrasonically in EtOH and dried in air. A chitosan (1% w/v) water solution was prepared being adjusted to pH of 5.8–6.0 by using HCl and NaOH. The chitosan solution was applied on the surface of porous Mg samples by the dip coating procedure at room temperature. 2.5. In vitro degradation study The prepared cylindrical-shape Mg monoliths were weighed (Wi ) and submerged into 25 ml of SBF solution of pH 7.4, and the immersion was carried out at 37 ± 0.5 ◦ C with constant shaking at 100 rpm for up to 120 h. At each time point (24, 48, 72, 96 and 120 h, respectively), the submerged samples were taken out from the solution, rinsed gently with deionized water, dried at room conditions for 24 h and weighed (Wt ). The weight loss was calculated according to the following equation: weight loss = (Wi − Wt )/Wi × 100%, where Wi is the weight of the Mg monolith before immersion in the SBF solution, Wt. is the weight of the Mg monolith after immersion in the SBF solution at certain time intervals. The pH changes of the immersing solutions were also measured for each respective degradation period. Three samples were tested for each group. 2.6. Attenuated total reflectance—Fourier transform infrared (ATR-FTIR) spectroscopy analysis An ATR-FTIR Spectroscopy analysis of the monoliths was performed to identify the chitosan presence and potential interactions with the Mg, the same as the potential mineralization process (after the incubation in SBF), by using a Perkin–Elmer IR Spectrophotometer with a Golden Gate Attenuated Total Reflection (ATR) attachment with a diamante crystal. The spectra were accumulated within 16 scans at a resolution of 4 cm−1 within a range of 4000 cm−1 –650 cm−1 . The background air spectrum was subtracted. The Spectrum 5.0.2 software programme was applied for the data acquisition analysis. The inspection of the second derivative and deconvolution of the FTIR spectra at specific spectral regions was performed by the Peak Fit v4.12 programme.

2.2. Processing of porous Mg monoliths 2.7. X-ray diffraction (XRD) analysis Porous Mg monoliths were prepared by the P/M process (sintering) using NH4 HCO3 powder as a space holder material. Liquid hexane was added to avoid the segregation of powders at a volume fraction of 30%. The Mg and NH4 HCO3 powders were mixed thoroughly according to the weight content of NH4 HCO3 (i.e. 0, 10 and 20 wt.%, respectively). The mixture powders were cold-pressed uniaxially under pressure of 265 MPa into cylindrical green com-

The phase component of the monoliths (after the incubation of SBF immersion) were analysed with XRD using Xpert-pro equipment with Cu K␣ radiation (␭ = 1.5406 A) in a continuous scan mode. The filament current of 30 mA and acceleration voltage of 45 kV were applied. The diffraction angles (2) range from 10◦ to 110◦ and a scanning speed of 10◦ /min was used. The XRD phase

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Fig. 1. SEM images of Mg monolith surfaces after first (at 130 ◦ C; a–c) and second (at 550 ◦ C; d–f) heat treatment, corresponding to different Mg/NH4 HCO3 ratio* (100/0, 90/10 and 80/20).

Fig. 2. ATR-FTIR spectra of chitosan and chitosan-coated Mg monoliths prepared with the lowest (14%) porosity, before and after the incubation in SBF solution at 37 ◦ C.

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Scheme 1. Anticipated surface and bulk phenomena of uncoated (above) and chitosan-coated (below) Mg-monolith, taking place before (a) and after (b) incubation in SBF media.

identification was performed using a JCPDS standard XRD card (09432) for identification of Mg and Mg corrosion products (MgO and Mg(OH)2 ), as well as the HAP. 2.8. Scanning electron microscopy (SEM) and energy dispersive X-ray (EDX) analysis The surface morphology of differently porous monoliths after each heat treatment, as well as after the SBF incubation, was examined by SEM imaging on the upper surface with the microscope FEI Quanta 200 3D using back-scattered modes and different magnifications. Images were analysed further by the Image J programme in order to obtain quantitative information regarding grain size, as well as semi-qualitative information about porosity and interconnectivity on the surface. SEM imaging coupled with an EDX detection system was also performed on the monoliths, before and after the coating process, the same as after SBF incubation, using the microscope Sirion NC 400, equipped with an EDX detector. EDX analysis was performed on the uppermost (coating-related) section to inspect the presence and type of deposited formations. 2.9. Electrochemical measurements A conventional three-electrode configuration was used for the potentiodynamic polarization studies. All the potentials were measured against a Saturated Calomel Electrode (SCE), and the counter electrode was made from Pt. The SCE was immersed in a Luggin capillary that was placed as close as possible to the working electrode. The potentiodynamic polarization curves were recorded by changing the electrode potential automatically, starting at 250 mV

vs. the open circuit potential (Eoc ), and continuing with increasing potential in the anodic direction with a potential scan rate of 1 mVs−1 . Impedance spectra were obtained at Eoc in the frequency range from 100 kHz to 1 mHz with 10 points per decade and a 10 mV (peak to peak) amplitude of the excitation signal. Nyquist and polarization plots were obtained from the results of these experiments one hour after the working electrode had been immersed in the solution, in order to allow stabilization of the stationary potential (three replicate measurements were performed). All the experiments were carried out at 37 ◦ C ± 1 ◦ C. The measurements were performed using the Solartron 1287 Electrochemical interface and with a Gamry 600TM potentiostat/galvanostat controlled by electrochemical programmes. The test specimens were fixed in a PTFE holder, where the geometric area of the electrode exposed to electrolyte was 0.785 cm2 .

2.10. Compression testing A uniaxial, unconfined compression test was performed on the mechanical testing machine Shimatzu AG-X plus with 10 kn load cell, according to the ASTM E9 Standard [40]. The system was used in displacement control and a rate of 1 mm/min was applied. Before testing, the dimensions of each sample were measured for calculation of stress (N/mm2 ) and strain (%) data, being normalised by the measured force (N) and stroke (mm) with surface area (mm2 ) and initiate thickness (mm), respectively. Compressive modulus was determined by dividing applied forces and dimensional changes into the initial cross-sectional area and dimension, with subsequent calculation of the slope of linear region in the stress vs. strain curve.

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Fig. 3. XRD spectra of i) uncoated and ii) chitosan-coated (14% porous) Mg monolith after the incubation in SBF solution at 37 ◦ C. The spectral lines in HAP-related region within both samples are inserted in higher magnification for visualization purpose.

3. Results and discussion 3.1. Monoliths’ processing and microstructure characterization Fig. 1 shows SEM micrographs of the upper surfaces of porous Mg monoliths after heat treatment (I) and (II). Porosities within the monoliths obtained by partial evaporation of NH4 HCO3 were mostly open type pores, which depended on the type of original powders used. The pore space structure after space holder removal displays irregular shaped macro pores inside the sintered material. According to the SEM images, two types of pores were observed after sintering—the first type with a diameter up to 100 ␮m and the second type pore with diameters ranging between 150 and 500 ␮m (indicated by the arrows in Fig. 1f). Observations indicated that the porosity increased from 14% via 30% to 40% with increasing the NH4 HCO3 content. 3.2. Coating identification and evaluation of monoliths’ stability The film-forming properties of chitosan [41] were utilised to advance the Mg monolith coating process, which results in good surface and inter-penetration coverage with a thickness of approximately 330 ␮m given in the supplementary information(graph1). Chitosan presence was confirmed by the inspected FTIR spectral lines (Fig. 2), where typical chitosan-related spectral bands were identified within the chitosan-coated Mg monolith: The broad absorption line in the region between 3200 and 3500 cm−1 attributed to OH and NH stretching, signals at about 1632 cm−1 , 1555 cm−1 and 1380 cm−1 attributed to the amide I, II, and III modes of the residual N-acetyl groups, respectively, as well as a band at about 1150 cm−1 being related to the anti-symmetric stretching of C O C bridge and at about 1080 cm−1 to skeletal vibration involving the C O stretching [42]. The small (3–8 cm−1 ) shifting in bands‘ position within the OH-related region in the case of chitosan-coated Mg monoliths from those in neat chitosan indicated possible hydrogen bonding with Mg(OH)2 or MgO segments being present on the monolith surface, as will be discussed later within the XRD data evaluation. According to the anticipated Mg-chitosan interaction (Scheme 1a), the presence of the chitosan layer is expected to improve the physiological stability of the monolith, affecting both the complex degradation/corrosion process and simultaneous mineralization potential by means of SBF ions‘ deposition (Scheme 1b). Indeed, the process of bio-corrosion occurred in vivo after Mg alloy implantation was approved [43] to proceed through simultaneous and complex mechanisms composed of Mg(OH)2

Fig. 4. (a) The weight-loss of uncoated and chitosan-coated Mg (coated Mg) monoliths of different porosity, and pH changes of SBF solution during 120 h of incubation at 37 ◦ C. For better visualization of relative changes, (b) the normalized weightloss and pH of uncoated (Mg) respective to chitosan-coated (Mg-Ch) monoliths are presented in the graph below.

formation (due to Mg dissolution and surface alkalization), further exchange with soluble MgCl2 (which progressed the degradation process readily), Ca2+ and PO4 3− deposition on non-dissolved Mg(OH)2 and, finally, the formation HAP, acting as a protective layer against further degradation. In order to evaluate the initial effect of chitosan coating on the Mg monolith‘s physiological stability, the incubation in SBF media was performed and traced by FTIR and XPS spectroscopies. As seen from the FTIR spectra in Fig. 2, the SBF incubation altered the spectra profile of chitosan-

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Fig. 5. SEM images with different magnifications (A–C) of chitosan-coated Mg monoliths of different porosity (14%, 30% and 40%) after the immersion in the SBF solution at 37 ◦ C for 24 h.

Fig. 6. SEM-EDX analysis of chitosan-coated Mg monoliths with different porosity (A 14%, B 30%, and C 40%) after the immersion in the SBF solution at 37 ◦ C for 24 h.

coated Mg monolith significantly, showing the dominant presence of a carbonate (CO3 2− )—related vibration as directly adsorbed (2000–2300 cm−1 region) or arising from the formed apatite (being approved by bands at about 1547 cm−1 , 1440 cm−1 , and 840 cm−1 ), being bonded ionically to the −PO4 3− groups (bands at about 1049 cm−1 , 1049 cm−1 , 1033 cm−1 ) [44]. A very low intensity of the later (PO4 3− —related) vibration band, which is normally followed by a less intensive carbonate-related band, may be a consequence of a thick carbonate layer [45], as well as an indication of the formation of an amorphous, non-stoichiometric type apatite, being already well described [46] as a pre-step in HAP formation. On the other hand, the XRD analysis of uncoated and chitosancoated Mg monoliths (of 14% porosity) after the incubation in SBF (Fig. 3), revealed the presence of typical crystal planes of HAP (visible in upright inserted spectral lines) [43], being however, (due to the relatively low intensity), not the dominant crystalline fraction found on the Mg-monolith surface. Indeed, beside typical Mg (2theta) angles at 32◦ , 48◦ , 57◦ and 67◦ degrees for (010) (012) (110) (020) planes, the major corrosion product was Mg(OH)2 as has been reported already for a similar experimental set-up [14,15]. The retention of chitosan after the incubation can anyhow not be identified with 100% accuracy due to the overlapping of Mg(OH)2

with the chitosan-related XRD peak, typically observed as a broad peak at ∼20◦ degrees [47]. 3.3. Anticorrosion resistance of monoliths Complementary to spectroscopically-examined surface-related changes, the bulk degradation rate (being related to the removal or dissolution of Mg ions and chitosan) were evaluated gravimetrically by tracking the weight loss of monoliths and pH changes of immersion solutions as a function of time for both uncoated and chitosan-coated monoliths of different porosities. As seen from Fig. 4a, the weight-loss phenomenon was most significant in the case of low (14%) porosity monoliths, which increased with the increased immersion time. This corresponds to the highest grain polydispersity, as well as the highest presence of grain-interfaces covered with MgO and Mg(OH)2 , being already elaborated by SEM image analysis for the same sample. On the other hand, the comparison betweenuncoated and chitosan-coated Mg monoliths (Fig. 4b) reveals significant variations in kinetic data between differently-porous samples, although the chitosan protective role in the degradation/corrosion process was also identified in all cases. Indeed, the weight-loss difference (between uncoated and chitosan-coated monoliths) increased twice within 120 h of incubation (from 16% to 32%) for the lowest (14%) porous monolith,

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Fig. 7. Nyquist plots (a) with respective electrochemical polarisation parameters (b) and Tafel plots (c) for differently porous (14%, 30% and 40%) uncoated (left) and chitosan-coated (right) Mg monoliths incubated in SBF at 37 ◦ C for 24 h.

leading to 6–9% of pH difference. On the other hand, the monoliths with 30% and 40% porosity gave an opposite trend during the same period, i.e. the reduction of weight-loss from about 40% to 24% and from about 50% to 22%, respectively, and significant pH change (from 8.2% to 12.7%) only in the case of the 40% porous sample. This result clearly implied a protective function of the chitosan layer, acting as a barrier between the porous Mg monolith surface and the SBF electrolytes. This led to a decreased degradation rate with the progression of incubation, being most effective in the lowest porous monolith while in the highly-porous ones the chitosan coating may also penetrate into the moonlight porous structure (as shown in graph 2 supplementary information), leaving the surface

irregularities. The elaborated pH change is a direct consequence of the corrosion process where the Cl− ion from the SBF media exchanged progressively with the −OH− ions within the Mg(OH)2, being present on the monolith‘s surface, leading to media alkalinity which was dominant in the lowest porosity samples. In the case of chitosan-coated monoliths, the surface oxides were not accessible to SBF ions to same extent, which decelerated the degradation and, consequently, diminished the alkalization. Due to chitosan insolubility in a high pH media, this behaviour is expected to prolong physiological stability further, which however needs to be confirmed, although the chitosan films processed separately (using

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Fig. 8. Stress-strain diagram of uncoated and chitosan-coated (14% and 40% porous) Mg monoliths (a) with extracted data for compression strength (MPa), maximum strain (%) and elastic modulus (MPa) for the same data set (b).

Table 1 EDX analysis of porous Mg monoliths after SBF incubation. Porosity

Element

Element (wt%)

Element (atomic%)

14%

O Mg Cl

60.54 36.31 3.15

70.51 27.83 1.66

30%

O Mg Cl

61.78 35.68 2.36

71.47 27.30 1.23

40%

O Mg

62.33 37.67

71.55 28.45

the same coating procedure) showed relatively high physiological stability (<13% weight-loss) in 15 days of SBF incubation. EDX analysis with respective SEM micrographs (Figs. 5 and 6) provided additional proof of the fastest degradation of the 14% porous sample. It was found that the presence of Mg, O and Cl elements (Table 1) in selected areas, and the calculations by the semi-quantitative ratio between these elements (inserted in Table 1) could confirm the presence of Mg-oxide products and MgCl2 being known to speed up the corrosion process due to its solubility [48]. Moreover, a large diversity of structures, from needleto globular- and flake-like minerals, were found as dominant on this sample (Fig. 5), indicated by the arrow. On the other hand, less diversity in the elements‘ release was identified in the 30% porous monolith, while Cl ions were absent in the case of the 40% porous sample, demonstrating the higher presence of MgO related precipitates (Fig. 6c). Interestingly, the mineralization products were not identified on any of the samples, which may be due to the presence of too densely covered corrosion products. Potentiodynamic polarization curves and Nyquist plots, obtained from the uncoated and chitosan-coated porous Mg monoliths in SBF were recorded (Fig. 7), and the electrochemical values of the polarization parameters presented (Fig. 7b). It was observed that the corrosion potential (Ecorr) of the 14% porous and uncoated monolith was quite negative (−1.54 V). In contrast, the corrosion potentials of the monoliths shifted to about −1.39 V and −0.71 V for 30% and 40% porosities, respectively. It was also noticed that the corrosion current densities (Icorr) of the same samples were around 736 ␮A cm−2 and 597 ␮A cm−2 for the same monoliths. On the other hand, the 14% porous Mg monolith showed a corrosion current density of around 806 ␮A cm−2 . It is obvious

that the corrosion current densities of the highly porous monoliths were lower than that of the least porous monolith, suggesting on a lower corrosion rate of the 40% porous monolith when compared to the other two samples. The potentiodynamic polarization test also revealed that the corrosion resistance of the monoliths was improved significantly when there was an increase in the porosity. The same trend was continued for the chitosan coated Mg monoliths where the corrosion potentials of about −1.81 V, −1.40 V and −0.88 V for 14%, 30%, and 40% porous monoliths, respectively, were shifted towards more positive values, confirming the protective behaviour of the chitosan coating. It was observed from the impedance spectra shown in Fig. 7a that the 40% Mg porous monolith samples with a polarization resistance of about 1052 cm2 became more corrosion resistant than the 14% and 30% porous ones which showed a polarization resistance of about 846 cm2 and 276 cm2 , respectively. In the case of the chitosan-coated 40% porous Mg monolith, the polarization resistance was further increased to about 1152 cm2 , which showed the highest corrosion resistant material. The same trend was observed for the other porous monoliths.

3.4. Mechanical properties of monoliths The effects of different porosity, as well as chitosan coating, on the mechanical properties of Mg monoliths were followed by samples’ compressions testing. The stress-strain curves being presented in Fig. 8 exhibit the typical linear elastic regime at low strain in all samples. However, the low porosity (14%) monolith differed significantly from the others due to its typical brittle failure profile, without plastic deformation, while all the others exhibited yield peak, followed by softening and strain hardening regimes, being especially pronounced in the chitosan-coated Mg monoliths. The increase of porosity affected the compression strength negatively, reducing it by about 70% while, at the same time, affected the elastic modulus positively by increasing it by the same (about 70%) except in the case of non-coated and chitosan-coated Mg monoliths with 40% of porosity. Moreover, the chitosan coating did not affect the elastic modulus significantly in the low porosity sample, while, in the case of the highly porous sample, a significant (about 40%) modulus reduction was measured, which indicated directly its porosity guided-elasticity, as well as the possibility for tuning of the same by controlling the Mg monoliths‘ processing. Importantly,

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the obtained mechanical properties of coated monoliths (elastic modulus of 12–26 MPa and compressive strength of 17–72 MPa) did not match those of cancellous bone having 50–100 MPa modulus and 5–10 MPa strength or even compact cortical bone tissue with 17–20 GPa modulus and 80–150 MPa strength [49], which indicate the monolith‘s possible applicability as supporting, rather than self-standing implantation material.

4. Conclusions The P/M process was used to produce different porous (14%–40%) Mg monoliths followed by dip-coating with natural biopolymer chitosan. The un-coated Mg monolith undergoes considerable degradation in SBF while the chitosan coating retards the degradation and also encourages the formation of an HA layer over the sample surface. Among the different porosities attempted, the 40% porous monolith was found to impart maximum corrosion resistance to the sample, being improved further by chitosan surface treatment. Such an Mg monolith was shown to be used potentially as a biomedical implant, however, as a supporting, rather than self-standing implantation material due to its compression strength limitations.

Acknowledgement This work was supported financially by the Erasmus Mundus grant no. EMA2-2013-2540/001-001-EM-EUPHRATES.

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