Induction Heating Vs Conventional Heating for the Hydrothermal

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Induction Heating Vs Conventional Heating for the Hydrothermal Treatment of Nitinol and Its Subsequent 2-(Methacryloyloxy)ethyl 2-(trimethylammonio)ethyl Phosphate Coating by Surface-Initiated Atom Transfer Radical Polymerization S. Devillers,† B. Barthelemy,† J. Delhalle, and Z. Mekhalif* Laboratory of Chemistry and Electrochemistry of Surfaces (CES), University of Namur (FUNDP), rue de Bruxelles, 61, B-5000 Namur, Belgium

bS Supporting Information ABSTRACT: Nitinol is an alloy of great interest in general and especially in the biomedical field where many researches are aimed to improve both its corrosion resistance and its biocompatibility. In this work, we report on the advantage of an induction heating treatment in pure water compared to a conventional hydrothermal procedure. Both treatments lead to a hydroxylation of the surface, a decrease of the nickel amount in the outer part of the oxide layer, and a drastically decreased corrosion current density. However, the amount of surface hydroxyl groups is higher in the case of the induction heating treatment, which in turn leads to a denser grafting of atom transfer radical polymerization initiators and ultimately to a thicker 2-(methacryloyloxy)ethyl 2-(trimethylammonio)ethyl phosphate (MPC) polymer layer than in the case of conventional heating treatments. X-ray photoelectron spectroscopy (XPS), static contact angle, and polarization curves measurements as well as scanning electron microscopy (SEM) have been used to characterize the obtained modified surfaces. KEYWORDS: nitinol, induction heating, hydrothermal treatment, 11-(2-bromoisobutyrate)-undecyl-1-phosphonic acid, ATRP, 2-(methacryloyloxy)ethyl 2-(trimethylammonio)ethyl phosphate

’ INTRODUCTION Nitinol alloys are widely studied as biomaterials for medical implants because of their unique mechanical properties, such as shape memory, superelasticity and a relatively good biocompatibility.1,2 The biocompatibility of Nitinol implants obviously depends on their corrosion resistance but also on a low release of Ni ions after implantation as this chemical element is known to be allergenic and toxic, though essential for the human body.3 Numerous works have been published on the Ni release from Nitinol4 6 and its corrosion resistance compared to other commonly used alloys.6 13 Several ways to improve Nitinol surface properties have been assessed by modifying the oxide layer itself with methods such as electropolishing and heat treatment,14 16 mechanical polishing,15,17 passivation in nitric acid solutions,14,18 surface nitridation,19 21 selective surface oxidation,21,22 laser surface melting,23,24 tantalum implantation25 or electrodeposition,26 implantation of Ar+ and N+,27 oxygen28 or carbon,29 deposition of calcium phosphate coatings.30 Many works have also been published on the modification of Nitinol surface with organic coatings. A large part of these studies report on the Nitinol coating with a polymeric layer such as polyurethane,31,32 poly(p-xylene) derivatives,33 phosphorylcholine,34 polyallylamine bisphosphonate,35 hyaluronan derivatives,36 polytetrafluoroethylene37 or polyethylene glycol derivatives38,39 on which an antithrombogene r 2011 American Chemical Society

agent such as heparin is often immobilized. Since the past few years, several publications report also on the formation of layer-by-layer assembly of polyanions and polycations into multilayers on Nitinol.40 44 Another way commonly used to create a protective organic coating on oxides surfaces such as Nitinol is the formation of a self-assembled monolayer. Several publications report on the modification of the Nitinol oxide layer with organosilanes,45 organophosphonic acids,19,46,47 and the use of such covalently grafted organic monolayers as adhesion promoters or polymerization initiators for the formation of a polymeric layer.48 The boiling procedure (also called hydrothermal treatment, i.e., the immersion of Nitinol in boiling water) was introduced for Nitinol many years ago49 and proved to be very effective in blocking the Ni release.50,51 Shabalovskaya et al. also showed that boiling in water of chemically etched samples results in a slight extension of the passivity region and a reduction of the current density by 1 order of magnitude at the potentials above the corrosion potential, indicating a significant drop in the anodic dissolution of material.15 Received: July 12, 2011 Accepted: September 12, 2011 Published: September 12, 2011 4059

dx.doi.org/10.1021/am200912k | ACS Appl. Mater. Interfaces 2011, 3, 4059–4066

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Figure 1. 11-(2-Bromoisobutyrate)-undecyl-1-phosphonic acid) (BUPA) (A) and 2-(methacryloyloxy)ethyl 2-(trimethylammonio)ethyl phosphate (MPC) (B).

Induction heating is a direct and contactless heating method based on Faraday’s law of induction and Joule’s law. In practice, a piece of metal is placed in a solenoid in which a high frequency alternating current flows. The piece of metal is thus exposed to rapid magnetic field changes that induce eddy currents in it and therefore a temperature increase according the Joule’s law. In addition to being contactless and localized, this heating method has the main advantages to be more rapid and energetically economic than other conventional heating methods. The aim of the present work is to carry out induction heating (IH) of Nitinol in water and compare its effects to those obtained with a conventional hydrothermal (CH) treatment on the elemental and chemical composition of the resulting surface oxide layer. The grafting of 11-(2-bromoisobutyrate)-undecyl-1-phosphonic acid (BUPA; Figure 1) on the Nitinol surface oxide layers after IH and CH hydrothermal treatments is also compared. Such modified substrates are expected to constitute convenient platforms for surface-initiated atom transfer radical polymerization (ATRP) of biocompatible monomers. In recent works, we have successfully modified stainless steel and titanium oxide surfaces with BUPA to perform ATRP of acrylonitrile, methylmethacrylate and polystyrene.52,53 In the frame of this work, we carried out the ATRP of 2-(methacryloyloxy)ethyl 2-(trimethylammonio)ethyl phosphate (MPC; Figure 1) on the modified Nitinol surfaces. Polymerized MPC is highly hydrophilic and well-known for its resistance to protein adsorption, cell adhesion, prevention of biofouling, and nonthrombogenic nature.54,55 This makes polymerized MPC a candidate of choice for several biomedical applications. Surface-initiated ATRP is by now an established effective method to graft the MPC onto a substrate via a covalent bond.56 Therefore surface-initiated ATRP appears to be a suitable method in order to obtain an MPC polymer layer on Nitinol surfaces. XPS, static contact angle and polarization curves measurements as well as SEM have been used to characterize the obtained modified surfaces.

’ EXPERIMENTAL SECTION Chemicals. Nitinol substrates (20 10 0.3 mm3) were purchased from AMF, France. This alloy is mainly composed of Ni (56 wt %) and Ti (balance). The surface composition of mechanically polished Nitinol is somewhat different from the bulk composition: the main metallic element constitutive of the protective oxide layer is titanium. All reagents used for the synthesis of BUPA were purchased from Aldrich Chemical Co.: 10-bromo-1-undecanol (98%), 3,4-dihydro-2Hpyrane (97%), p-toluenesulfonic acid (98%), triethyl phosphate (98%), pyridinium-p-toluene-sulfonate (98%), 2-bromoisobutyryl-bromide (98%), pyridine (97%) dried on CaH2, bromotrimethylsilane (97%), anhydrous anisole (99.7%), dichloromethane, petroleum ether,

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diethylether, ethyl acetate, methanol, acetonitrile, acetone, ethanol, absolute ethanol (>99.8%), and anhydrous tetrahydrofuran. All these chemicals were used without any further purification. Reagents for the ATR polymerization were also purchased from Aldrich Chemical Co. and purified before use: Cu(I)Br was purified according to a method described elsewhere.57 N, N, N, N, NPentamethyldiethylenetriamine (PMDETA, 99%) was distilled. 2-(Methacryloyloxy)ethyl 2-(trimethylammonio)ethyl phosphate (MPC, > 96.0%) was purchased from Wonda Science. Sodium chloride (99+%) was purchased from Acros Organics. Absolute ethanol was purchased from Merck. These chemicals were also used without any further purification. Milli-Q water was used for preparation of all aqueous solutions.

Synthesis of the 11-(2-Bromoisobutyrate)-undecyl-1phosphonic acid. The synthesis of BUPA has already been reported elsewhere.52 The same conditions were used in this work to synthesize this molecule with an overall yield of 30%. Substrates Preparations. The Nitinol substrates were mechanically polished down to 1 μm on a Buehler Phoenix 4000 instrument using various grit silicon carbide papers and diamond pastes. At the end of the polishing steps, the metal coupons were cleaned by sonication 15 min in ethanol and blown dry under a nitrogen flow before being submitted to a heat treatment for 15 min at 500 °C in air atmosphere in a resistance furnace (a treatment similar to the common shape setting treatment applied to Nitinol). The substrates were then either immersed in boiling milli-Q water for 1 h (named “CH” for “conventional heating”) or immersed in 10 mL room temperature milli-Q water and submitted to the selected induction heating profile (named “IH” for “induction heating”). After this treatment, the substrates were copiously rinsed with ethanol, cleaned by sonication 15 min in the same solvent, blown dry under a nitrogen flow and characterized directly or stored until their modification. The BUPA layer was grafted by immersing the hydrothermally treated Nitinol substrates in a 1 mM solution of the molecule in absolute ethanol during 24 h at room temperature in the dark. These substrates were then rinsed copiously with ethanol, cleaned by sonication 15 min in the same solvent, blown dry under a nitrogen flow, and characterized directly. ATRP was carried out according the following procedure: 48 mg of Cu(I)Br, 10 mL of milli-Q water and 0.5 g of MPC are added to a reaction flask sealed with a rubber septum. The reaction mixture is deaerated five times by freeze-pump-backfilling with argon. 113 mg of PMDETA is added to the mixture with a syringe. The mixture is then stirred during 1 h at 90 °C. After the polymerization, the coated substrates were cleaned by sonication 15 min in methanol and rinsed copiously with methanol in order to remove the untethered polymer. Substrates Characterizations. The Nitinol substrates were characterized by X-ray photoelectron spectroscopy (XPS), static contact angle measurements, polarization curves measurements (PC), and scanning electron spectroscopy (SEM). XPS is used to evaluate the elemental composition of the oxide layer and to confirm the presence of the grafted initiator and of the polymer. The photoelectron spectra have been obtained with a SSX-100 spectrometer using a monochromatized X-ray Al Kα radiation (1486.6 eV), the photoemitted electrons being collected at 35° takeoff angle. Nominal resolution was measured as full width at half-maximum of 1.0 1.5 eV for core levels and survey spectra, respectively. The binding energy of core levels was calibrated against the C1s binding energy set at 285.0 eV, an energy characteristic of alkyl moieties. The peaks were analyzed using mixed Gaussian Lorentzian curves (80% of Gaussian character). Static contact angle measurements were carried out using a DIGIDROP (GBX Surface Science Technology) contact angle goniometer at room temperature. A syringe was used to dispense 2 μL of probe droplets of milli-Q water to the sample surface. 4060

dx.doi.org/10.1021/am200912k |ACS Appl. Mater. Interfaces 2011, 3, 4059–4066


Figure 2. Evolution of the temperature at the surface of a thermally treated (15 min at 500 °C) NiTi substrate immersed into 10 mL of milliQ water during the induction heating sequence. Polarization curves characterizations were carried out with an EG&G Instruments potensiostat, model 263A, monitored by computer and M270 electrochemistry software. A three-electrode electrochemical cell was used with SCE as reference electrode and a platinum foil as counter electrode. The cell used enables analysis of a well-defined and reproducible spot (0.28 cm2) on the sample. These experiments were carried out in a 0.5 M sodium chloride solution by sweeping a range of potential from 1 to 1 V vs SCE at 1 mV/s. SEM studies were performed on a JEOL 7500 F microscope with an acceleration voltage of 15 kV.

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Figure 3. O1s core level photoelectrons high-resolution XPS spectra of thermally treated Nitinol samples (A) without any further treatment, (B) immersed in boiling water for 1 h, and (C) submitted to induction heating in milli-Q water.

’ RESULTS AND DISCUSSIONS Temperature Measurements. First, we assessed the efficiency of the induction heating (IH) of Nitinol substrates. Induction heating was performed with an Ambrell EasyHeat induction heating system with a power output of 725 W and a frequency of 198 kHz. The used solenoid is composed of 7 loops with an internal diameter of 9 cm. Temperature measurements (FLUKE thermometer 54-II) were carried out with K-type thermocouples soldered on the substrate surface. Note that with this method, it was impossible to measure the temperature of the substrate during the induction phase because of the creation of eddy currents in the thermocouple wires themselves. The heating experiments were thus realized by applying sequences of induction heating followed by cooling periods allowing temperature measurements. Note also that we ensured that the eddy currents induced in the thermocouple wires did not induce any significant heating of these wires at 198 kHz and thus that the temperature curves measured during the cooling down periods are meaningful. The substrates were immersed into a controlled volume of milli-Q water (10 mL) and centered into the solenoid with their plane perpendicular to the solenoid axis. Several heating sequences were tested (not shown here) and we selected one of them for the rest of this study as it allows the surface to quickly reach a “stationary” temperature range comprised between 75 and 100 °C. This sequence consists in an alternation of a 120 s IH pulse and a 180 s cooling period. This 5 min sequence is repeated during 1 h (thus 12 times). The temperature curve of this profile is presented in Figure 2. Note that after the third 120 s IH pulse, the water starts to boil at the surface. After this point, boiling is

Figure 4. Left, OH /O2 ratio; right, Ni/Ti ratio of thermally treated Nitinol samples (A) without any further treatment, (B) immersed in boiling water for 1 h, and (C) submitted to induction heating in milli-Q water calculated on the basis of the relative XPS analysis.

observed at the surface during each induction heating pulse and stops during each cooling period. Impact of the Induction Heating/Water Treatment of NiTi on Its Oxide Layer. To compare the impact of induction heating and conventional heating boiling treatment on the Nitinol oxide layer, we characterized substrates by XPS and polarization curves techniques. XPS characterizations has been carried out on reference substrates (Nitinol heat-treated 15 min at 500 °C), samples treated in boiling milli-Q water for 1 h with CH (called “CH-treated”) and samples submitted to IH in 10 mL of milliQ water (called “IH treated”). The experimental O1s core level photoelectrons XPS spectra of these samples are analyzed with three to four different peaks (Figure 3). The first one, centered at 530.0 eV, is attributed to the photoelectrons coming from the metal oxides (mainly TiO2 and NiO) and is noted “O2-”. The second peak, 4061



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Table 1. Water Static Contact Angle for a Mechanically Polished and Thermally Treated (500°C 15 min) Nitinol without Any Further Treatment (reference), after a 1 h Immersion in Boiling Water (CH treated), after Being Submitted to Induction Heating in 10 mL of Milli-Q Water (IH treated), after the Grafting of 11-(2-Bromoisobutyrate)-undecyl-1-phosphonic Acid (initiator) on CH and IH-Treated Nitinol Samples, and after the ATRP Polymerization of MPC on CH and IH-Treated Modified Nitinol Samples substrates

contact angle (deg)

Table 2. Corrosion Potentials and Corrosion Current Densities Measured for a Mechanically Polished and Thermally Treated (500°C 15 min) Nitinol without Any Further Treatment (reference), after a 1 h Immersion in Boiling Water (CH treated), after Being Submitted to Induction Heating in 10 mL of Milli-Q Water (IH treated), after the Grafting of 11-(2-Bromoisobutyrate)-undecyl-1-phosphonic Acid (initiator) on CH and IH-Treated Nitinol Samples, and after the ATRP Polymerization of MPC on CH and IH-Treated Modified Nitinol Samplesa Ecor (mV vs SCE)

icor (A cm 2)

reference

68 ( 2

CH treated

59 ( 3

reference

220

7.8 10

7

IH treated CH treated initiator

55 ( 1 72 ( 1

CH treated

275

7.5 10

9

IH treated

355

2.2 10

8

IH treated initiator

88 ( 1

CH treated initiator

294

4.0 10

9

CH treated initiator PMPC

35 ( 1

IH treated initiator

280

9.7 10

9

IH treated initiator PMPC

36 ( 1

CH treated initiator PMPC IH treated initiator PMPC

263 265

1.4 10 2.6 10

9 8

a

The polarization curves were acquired by scanning potentials from 1 to +1 V vs. SCE at 1 mV/s in a 0.5 M NaCl aqueous solution.

Figure 5. Polarization curves of a mechanically polished and thermally treated (500 °C 15 min) Nitinol sample without any further treatment (thick plain line); after the CH treatment (immersion of 1 h in boiling water) (dotted line) and after the IH treatment (induction heating in 10 mL milli-Q water) (dashed line). Curves acquired by scanning potentials from 1 to +1 V vs SCE at 1 mV/s in a 0.5 M NaCl aqueous solution.

centered at 531.5 eV, corresponds to the different metal hydroxides present at the surface (mainly Ti(OH)4 and Ni(OH)2) and is noted “OH ”. The third peak, centered at 532.8 eV, is related to oxygen atoms from atmospheric contaminations. In the case of the IH-treated samples, a fourth peak centered at 533.7 eV is observed, which corresponds to adsorbed water molecules on the surface. From this analysis, the OH /O2 ratio (i.e., the peaks area ratio) is calculated; it is representative of the hydroxylation level of the outer part of the oxide layer (Figure 4). The OH /O2 ratio is around 0.25 for the reference samples, it increases to 0.38 after the CH treatment (1 h immersion in boiling water) and increases drastically more (0.56) with the IH treatment (induction heating for 1 h in 10 mL of milli-Q water). In the case of the IH treated samples, the presence of strongly adsorbed water on the Nitinol surface is noticeable and indicative of an improved hydroxylation of these oxides. This OH /O2 ratio is directly related to the surface hydrophilic character as seen from the water contact angle (θw)

values: 68, 59, and 55° for the reference, CH-treated, and IH-treated samples, respectively (Table 1). Besides the fact that this improved hydrophilicity could be attractive for the hemocompatibility properties seeked for Nitinol based devices,2,58 it is of particular interest in the context of the present work (section 3.3) because it is expected to lead to a more effective grafting of phosphonic acids known to bind to metal oxides substrates via hydroxyl groups.48,59 The use of Nitinol in biological applications is crucially concerned with the amount of nickel in the outer part of the oxide layer: a smallest quantity of nickel at the surface leads to a better biocompatibility of the resulting Nitinol device. Accordingly, the Ni/Ti ratio has been calculated from XPS data for each of the studied samples (where “Ni” is the normalized area of the Ni2p peak and “Ti” is the normalized area of the Ti2p peak). The corresponding values are reported in Figure 4. This ratio is much more important for the reference samples (0.51) than for the CH- and IH-treated samples (0.37 and 0.31, respectively). These results indicate that the IH treatment leads to similar results than a classical boiling procedure with conventional heating, i.e., a hydroxylated Nitinol surface with a lower amount of nickel in the outer part of the oxide layer (and thus a bigger proportion of TiO2). However, it appears that the surface hydroxylation as well as the Ni/Ti ratio decrease are more important when induction heating is used than when conventional heating is used. Polarization curves experiments provide information on the corrosion resistance of CH and IH treated Nitinol samples. The resulting curves and corresponding numerical values are presented in Figure 5 and Table 2, respectively. For each of the treated samples, an important and comparable decrease in the corrosion current density (from 7.8 10 7 A cm 2 for the reference sample to 7.5 10 9 and 2.2 10 8 A cm 2 for CH and IH, respectively) is observed. Similarly, a shift of the corrosion potential toward more cathodic values occurs. The observed cathodic shift of the corrosion potentials is indicative of a mixed yet slightly more cathodic corrosion inhibition, which can be correlated with the XPS results. It is found that both CH and IH treatments lead, on the one hand, to a larger proportion of TiO2 in the outer part of the oxide layer (which is more stable than nickel oxides) and to more OH , on the other hand. 4062



Figure 6. XPS survey spectra of Nitinol CH-treated samples (A) before and (C) after the grafting of 11-(2-bromoisobutyrate)-undecyl-1-phosphonic acid and of Nitinol IH-treated samples (B) before and (D) after the grafting of 11-(2-bromoisobutyrate)-undecyl-1-phosphonic acid.

Figure 7. P 2p core levels photoelectrons high resolution XPS spectra of (C) CH-treated and (A) IH-treated Nitinol samples and Br 3d core levels photoelectrons high-resolution XPS spectra of (D) CHtreated and (B) IH-treated Nitinol samples after the grafting of the 11-(2-bromoisobutyrate)-undecyl-1-phosphonic acid.

Grafting of 11-(2-Bromoisobutyrate)-undecyl-1-phosphonic Acid (BUPA). In the previous section, it was shown that

the IH treatment induces a higher proportion of OH groups on the Nitinol surface than the CH treatment does. In the present section, we assess the consequence of this higher OH groups proportion on the grafting of BUPA on the Nitinol surface. First, the CH- and IH-treated Nitinol substrates were characterized by XPS before and after their modification with BUPA (Figure 6). The presence of P2s, P2p, Br3p, and Br3d core levels photoelectrons peaks can be pointed out after the grafting on both CH and IH treated samples. XPS high-resolution spectra of the P2p and Br3d core levels photoelectrons are presented in Figure 7. The P2p and Br3d core level peaks are centered at a binding energy of 133.3 and 70.6 eV respectively. The peak centered at a binding energy of 65.0 eV can be attributed to the

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Figure 8. Polarization curves of a mechanically polished and thermally treated (500 °C 15 min) Nitinol sample without any further treatment (thick plain line); after the grafting of 11-(2-bromoisobutyrate)-undecyl-1-phosphonic acid on CH-treated Nitinol sample (dotted line) and IH treated Nitinol sample (dashed line). Curves acquired by scanning potentials from 1 to +1 V vs SCE at 1 mV/s in a 0.5 M NaCl aqueous solution.

Figure 9. XPS survey spectra of modified Nitinol CH-treated samples (A) before and (C) after ATRP polymerization of MPC and of modified Nitinol IH-treated samples (B) before and (D) after ATRP polymerization of MPC.

Ni3p core level. These binding energies are characteristic of the phosphorus and the bromine atoms constitutive of the grafted molecule as shown in previous studies.52,53 This confirms the presence of the grafted initiator both on CH and IH treated Nitinol substrates. The P/(Ni+Ti) ratios were also calculated and compared (where “P” is the normalized area of the P2p peak, “Ni” is the normalized area of the Ni2p peak and “Ti” is the normalized area of the Ti2p peak). It appears that this ratio is higher after the modification of IH treated samples than after the modification of CH treated samples (0.09 and 0.06, respectively). This confirms our hypothesis that a higher proportion of OH groups on the surface leads to a higher amount of grafted phosphonic acid. Water static contact angles measurements corroborate the results obtained by XPS. Indeed the presence of a BUPA layer 4063


ACS Applied Materials & Interfaces induces an increase of the contact angle (and thus of the surface hydrophobicity) from 61 to 72° when grafted on CH treated samples and from 57 to 88° when grafted on IH treated samples. The higher hydrophobicity obtained in the case of IH-treated Nitinol substrates modified by grafted initiator correlates with a higher amount of grafted molecules. The value of 88° is quite close to the values reported in studies wherein the initiator has been grafted on steel52 and titanium.53 The polarization curves analyses of these samples have also been systematically carried out (see Figure 8 and Table 2). The BUPA layers formed on CH treated Nitinol samples and on IH treated Nitinol samples lead to similar corrosion current densities (4.0 10 9 and 9.7 10 9 A cm 2, respectively) and corrosion potential ( 294 and 290 mV/SCE, respectively). Note that BUPA layers formed on the CH and IH treated Nitinol samples lead to corrosion current densities slightly lower than those measured after the hydrothermal treatments. Moreover, an anodic shift of the corrosion potential is observed after the formation of the BUPA layer on IH-treated Nitinol samples, whereas a cathodic shift can be pointed out after the formation of a BUPA layer on CH-treated Nitinol samples.

Figure 10. P 2p core levels photoelectrons high-resolution XPS spectra of (C) CH-treated and (A) IH-treated modified Nitinol samples and N 1s core levels photoelectrons high-resolution XPS spectra of (D) CHtreated and (B) IH-treated modified Nitinol samples after ATRP polymerization of MPC.

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Regarding these results, it can be concluded that an IH treatment of Nitinol is favorable to the grafting of the ATRP initiator (i.e., BUPA) compared to a CH treatment, most likely because of the higher quantity of OH groups present on the surface of the formed oxide layer after an IH treatment. ATRP of MPC on Modified Nitinol Substrates. In this last section, we compare the efficiency of the MPC surface-initiated ATRP on modified IH and CH treated Nitinol substrates. Figure 9 shows the XPS survey spectra of CH and IH treated substrates, modified with the ATRP initiator layer before and after the ATR polymerization of MPC. An increase of the C1s peak intensity as well as an attenuation of Ti2p and Ni2p peaks intensity can be pointed after the polymerization in both cases. However this trend is more pronounced for the IH treated Nitinol samples as the Ni2p peak is not visible anymore (suggesting the presence of a thicker polymer layer than the one obtained on CH treated samples). The two peaks centered at 100.0 and 150.0 eV appearing after ATRP polymerization are attributed to Si2p and Si2s core levels photoelectrons, respectively. This Si contamination is most likely due to the silicon grease used in the experimental setup to seal the glassware pieces. The corresponding XPS high-resolution spectra of the P2p and N1s core levels are presented in Figure 10. On these spectra, the photoelectrons peaks centered at 135.1 and 404.1 eV are attributed to the phosphorus and nitrogen atoms, respectively, present in MPC, substantiating its polymerization. The P2p peak centered at 133.9 eV corresponds to the phosphorus atom in BUPA. Because this component is not visible in the case of the ATRP carried out on IH-treated Nitinol samples, it can be inferred that a thicker polymer layer has been formed on the IH treated substrates than on the CH treated ones. The presence of a N1s peak centered at 401.1 eV could be due to the adsorption of some N2 used to dry the samples and/or the entrapment of some PMDETA in the formed polymer layer. The calculated P/(Ni+Ti) ratio appears to be three times higher when the MPC polymerization is carried out on IH treated samples than when it is carried out on the CH-treated samples (0.22 and 0.68 respectively). This confirms our assumption that a higher quantity of grafted phosphonic acid ATRP initiator leads to the formation of a more important polymer layer. Water static contact angles measurements show a drastic increase in the hydrophilicity after ATRP for both types of samples. Indeed, the contact angle value decreases with the polymerization from 72 to 35° for CH treated samples and from 88 to 36° for IH treated ones (thus the obtained polymer layer have similar hydrophilic properties in both cases). This observation is in line with the hydrophilic character of MPC.54

Figure 11. SEM pictures of a bare Nitinol substrate (a) and the polyMPC layer formed by ATRP on (b) CH-treated and (c) IH-treated modified Nitinol substrates. 4064



Figure 12. Polarization curves of a mechanically polished and thermally treated (500 °C 15 min) Nitinol sample without any further treatment (thick plain line); after the ATRP polymerization of MPC on CHtreated modified Nitinol sample (dotted line) and IH=treated modified Nitinol sample (dashed line). Curves acquired by scanning potentials from 1 to +1 V vs SCE at 1 mV/s in a 0.5 M NaCl aqueous solution.

SEM observations were also carried out in order to characterize the morphology of the obtained polyMPC layers. Representative pictures are presented in Figure 11. It appears that the presence of a polymer layer on Nitinol induces an increase of the surface roughness compared to a bare Nitinol surface. It can also be noticed that the obtained polymer layers are homogeneous and that no significant morphological difference can be pointed out between CH- and IH-treated samples. Polarization curves measurements were also performed on these substrates after the ATRP of MPC (see Figure 12 and Table 2). A mixed inhibition can be observed after the polymerization compared to the reference with a decrease of the corrosion current density by at least 1 order of magnitude, the highest decrease being observed in the case of the polymer layer formed on CH treated Nitinol substrates. These observations are consistent with the hydrophilic nature of the polymer.

’ CONCLUSIONS We investigated for the first time the impact of an induction heating treatment in pure water (IH treatment) on thermally treated (15 min at 500 °C) Nitinol surface composition and corrosion resistance and compared the efficiency of this treatment with a conventional boiling water treatment (CH treatment). Both CH and IH treatments lead to qualitatively similar results. Both CH and IH treatments lead to an important decrease in the Ni/Ti ratio and a significant increase in the amount of hydroxyl groups present at the Nitinol surface. However, this behavior is slightly more important with IH treatment than with CH treatment. Regarding corrosion resistance, both CH and IH treatments lead to an important decrease of the measured corrosion current densities (at least ten times lower than the one measured for a reference untreated Nitinol sample). These CH- and IH-treated Nitinol samples have been successfully modified with 11-(2-bromoisobutyrate)-undecyl-1-phosphonic (our ATRP initiator). It has been shown that the higher amount of hydroxyl groups obtained with IH treatment leads to the grafting of a larger quantity of initiator molecules, which in turn leads to the

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formation of a thicker polyMPC layer. Both treatments lead to a hydrophilic layer combined with a beneficial effect in terms of corrosion resistance: the corrosion current densities measured on polymerized samples is decreased by at least 1 order of magnitude compared to bare thermally treated samples. It thus appears that induction heating can be regarded as a very promising alternative to other conventional heating methods for surface treatments that require thermal activation. Beyond these results one should also consider the potential advantages of the induction heating method such as its high versatility resulting from its nature (direct heating from the surface itself) and the possibility of post- and/or pretreatment of the surface combined with the one studied in the frame of this work. For instance, IH could be used in the full sequence of operations leading ultimately to the polymerization of MPC. To illustrate this, it has been shown that IH can be used to control the grafting of phosphonic acids derivatives on Phynox60 and to trigger ATRP polymerization.53 Another aspect that also has to be studied is a direct application of IH to this sequence of operations onto Nitinol wires of diameters typical of those found in actual stents and, ultimately, to the stents themselves. Works are in progress along these lines.

’ ASSOCIATED CONTENT

bS

Supporting Information. The temperature measurements carried out in order to ensure that the eddy currents induced in the thermocouple wires did not induce any significant heating of the thermocouple wires at 198 kHz and thus that the temperature curves measured during the cooling down periods are meaningful. This material is available free of charge via the Internet at http:// pubs.acs.org/.

’ AUTHOR INFORMATION Corresponding Author

*Tel: +32-(0)81-72 52 30. Fax: +32-(0)81-72 46 00. E-mail: [email protected]. Author Contributions †

These authors contributed equally to this work.

’ REFERENCES (1) Duerig, T.; Pelton, A.; St€ockel, D. Mater. Sci. Eng., A 1999, 273 275, 149–160. (2) Mani, G.; Feldman, M. D.; Patel, D.; Mauli Agrawal, C. Biomaterials 2007, 28, 1689–1710. (3) Lu, X.; Bao, X.; Huang, Y.; Qu, Y.; Lu, H.; Lu, Z. Biomaterials 2009, 30, 141–148. (4) Gil, F. X.; Manero, J. M.; Planell, J. A. J. Mater. Sci.: Mater. Med. 1996, 7, 403–406. (5) Esenwein, S. A.; Bogdanski, D.; Habijan, T.; Pohl, M.; Epple, M.; Muhr, G.; K€oller, M. Mater. Sci. Eng., A 2008, 481 482, 612–615. (6) Shabalovskaya, S. A.; Tian, H.; Anderegg, J. W.; Schryvers, D. U.; Carroll, W. U.; Van Humbeeck, J. Biomaterials 2009, 30, 468–477. (7) Es-Souni, M.; Es-Souni, M.; Fischer-Brandies, H. Biomaterials 2002, 23, 2887–2894. (8) Rocher, P.; El Medawar, L.; Hornez, J. C.; Traisnel, M.; Breme, J.; Hildebrand, H. F. Scr. Mater. 2004, 50, 255–260. (9) Schulte, A.; Belger, S.; Etienne, M.; Schuhmann, W. Mater. Sci. Eng., A 2004, 378, 523–526. (10) Clarke, B.; Carroll, W.; Rochev, Y.; Hynes, M.; Bradley, D.; Plumpley, D. J. Biomed. Mater. Res.,Part A 2006, 79, 61–70. (11) Robertson, S. W.; Ritchie, R. O. Biomaterials 2007, 28, 700–709. 4065


ACS Applied Materials & Interfaces (12) Figueira, N.; Silva, T. M.; Carmezim, M. J.; Fernandes, J. C. S. Electrochim. Acta 2009, 54, 921–926. (13) Schiff, N.; Grosgogeat, B.; Lissac, M.; Dalard, F. Biomaterials 2002, 23, 1995–2002. (14) Trepanier, C.; Tabrizian, M.; Yahia, L. H.; Bilodeau, L.; Piron, D. L. J. Biomed. Mater. Res., Part A 1998, 43, 433–440. (15) Shabalovskaya, S. A.; Rondelli, G. C.; Undisz, A. L.; Anderegg, J. W.; Burleigh, T. D.; Rettenmayr, M. E. Biomaterials 2009, 30, 3662–3671. (16) Plant, S. D.; Grant, D. M.; Leach, L. Biomaterials 2005, 26, 5359–5367. (17) El Medawar, L.; Rocher, P.; Hornez, J. C.; Traisnel, M.; Breme, J.; Hildebrand, H. F. Biomol. Eng. 2002, 19, 153–160. (18) O’Brien, B.; Carroll, W. M.; Kelly, M. J. Biomaterials 2002, 23, 1739–1748. (19) Zorn, G.; Adadi, R.; Brener, R.; Yakovlev, V. A.; Gotman, I.; Gutmanas, E. Y.; Sukenik, C. N. Chem. Mater. 2008, 20, 5368–5374. (20) Zhang, D.; Zeng, W.; Zi, Z.; Chu, P. K. Mater. Sci. Eng., C. 2009, 29, 1599–1603. (21) Neelakantan, L.; Swaminathan, S.; Spiegel, M.; Eggeler, G.; Hassel, A. W. Corros. Sci. 2009, 51, 635–641. (22) Pohl, M.; Glogowski, T.; K€uhn, S.; Hessing, C.; Unterumsberger, F. Mater. Sci. Eng., A 2008, 481 482, 123–126. (23) Cui, Z. D.; Man, H. C.; Yang, X. J. Surf. Coat. Technol. 2005, 192, 347–353. (24) Yan, X. J.; Yang, D. Z.; Liu, X. P. Mater. Charact. 2007, 58, 623–628. (25) Li, Y.; Wei, S.; Cheng, X.; Zhang, T.; Cheng, G. Surf. Coat. Technol. 2008, 202, 3017–3022. (26) Zein El Abedin, S.; Welz-Biermann, U; Endres, F. Electrochem. Commun. 2005, 7, 941–946. (27) Shevchenko, N.; Pham, M. T.; Maitz, M. F. Appl. Surf. Sci. 2004, 235, 126–131. (28) Tan, L.; Dodd, R. A.; Crone, W. C. Biomaterials 2003, 24, 3931–3939. (29) Poon, R. W. Y.; Yeung, K. W. K.; Liu, X. Y.; Chu, P. K.; Chung, C. Y.; Lu, W. W.; Cheung, K. M. C.; Chan, D. Biomaterials 2005, 26, 2265–2272. (30) Bogdanski, D.; Esenwein, S. A.; Prymak, O.; Epple, M.; Muhr, G.; K€oller, M. Biomaterials 2004, 25, 4627–4632. (31) Han, Y. M.; Hwang, S. B.; Lee, S. T.; Lee, J. M.; Chung, G. H. Cardiovasc. Intervent. Radiol. 2002, 25, 381–387. (32) Tepe, G.; Schmehl, J.; Wendel, H. P.; Schaffner, S.; Heller, S.; Gianotti, M.; Claussen, C. D.; Duda, S. H. Biomaterials 2006, 27, 643–650. (33) Kong, X.; Grabitz, R. G.; Van Oeveren, W.; Klee, D.; Van Kooten, T. G.; Freudenthal, F.; Qing, M.; Von Bernuth, G.; Seghaye, M. C. Biomaterials 2002, 23, 1775–1783. (34) Babapulle, M. N.; Eisenberg, M. J. Circulation 2002, 106, 2734–2740. (35) Fishbein, I.; Alferiev, I. S.; Nyanguile, O.; Gaster, R.; Vohs, J. M.; Wong, G. S.; Felderman, H.; Chen, I. W.; Choi, H.; Wilensky, R. L.; Levy, R. J. Proc. Natl. Acad. Sci. U. S. A. 2006, 103, 159–164. (36) Thierry, B.; Winnik, F. M.; Merhi, Y.; Griesser, H. J.; Tabrizian, M. Langmuir 2008, 24, 11834–11841. (37) Chung, H. H.; Lee, S. H.; Cho, S. B.; Park, H. S.; Kim, Y. S.; Kang, B. C.; Frisoli, J. K.; Razavi, M. K. Cardiovasc. Intervent. Radiol. 2008, 31, 619–628. (38) Yang, J.; Wang, J. J. Mater. Sci. Technol. 2004, 20, 769–771. (39) Shin, H. S.; Park, K.; Kim, J. J.; Kim, H. K.; Han, D. K. Polymer (Korea) 2009, 33, 84–90. (40) Brunot, C.; Ponsonnet, L.; Lagneau, C.; Farge, P.; Picart, C.; Grosgogeat, B. Biomaterials 2007, 28, 632–640. (41) Schweizer, S.; Schuster, T.; Junginger, M.; Siekmeyer, G.; Taubert, A. Macromol. Mater. Eng. 2010, 295, 535–542. (42) Lackmann, J.; Regenspurger, R.; Maxisch, M.; Grundmeier, G.; Maier, H. J. J. Mech. Behavior Biomed. Mater. 2010, 3, 436–445. (43) Ma, Y.; Liu, M.; Yue, X.; Zha, Z.; Dai, Z. Int. J. Biol. Macromol. 2010, 46, 109–114. (44) Liu, M.; Yue, X.; Dai, Z.; Ma, Y.; Xing, L.; Zha, Z.; Liu, S.; Li, Y. ACS Appl. Mater. Interfaces 2009, 1, 113–123.

RESEARCH ARTICLE

(45) Sargeant, T. D.; Rao, M. S.; Koh, C. Y.; Stupp, S. I. Biomaterials 2008, 29, 1085–1098. (46) Raman, A.; Qui~ nones, R.; Barriger, L.; Eastman, R.; Parsi, A.; Gawalt, E. S. Langmuir 2010, 26, 1747–1754. (47) Maxisch, M.; Ebbert, C.; Torun, B.; Fink, N.; de los Arcos, T.; Lackmann, J.; Maier, H. J.; Grundmeier, G. Appl. Surf. Sci. 2011, 257, 2011–2018. (48) Qui~ nones, R.; Gawalt, E. S. Langmuir 2008, 24, 10858–10864. (49) Shabalovskaya, S.; Anderegg, J. J. Vac. Sci. Technol., A 1995, 13, 2624–2632. (50) Shabalovskaya, S. Biomed. Mater. Eng. 1996, 6, 267–289. (51) Perez, L. M.; Gracia-Villa, L.; Puertolas, J. A.; Arruebo, M.; Irusta, S.; Santamaria, J. J. Biomed. Mater. Res., Part B 2009, 91B, 337–347. (52) Minet, I.; Delhalle, J.; Hevesi, L.; Mekhalif, Z. J. Colloid Interface Sci. 2009, 332, 317–326. (53) Barthelemy, B.; Devillers, S.; Minet, I.; Delhalle, J.; Mekhalif, Z. J. Colloid Interface Sci. 2011, 354, 873–879. (54) Ishihara, K.; Nomura, H.; Mihara, T.; Kurita, K.; Iwasaki, Y.; Nakabayashi, N. J. Biomed. Mater. Res. 1998, 39, 323–330. (55) Lewis, A. L. Colloids Surf., B 2000, 18, 261–275. (56) Kyomoto, M.; Ishihara, K. ACS Appl. Mater. Interfaces 2009, 1, 537–542. (57) Keller, R. N.; Wycoff, H. D. Inorg. Synth. 1946, 2, 1–4. (58) Tulloch, A. W.; Chun, Y.; Levi, D. S.; Mohanchandra, K. P.; Carman, G. P.; Lawrence, P. F.; Rigberg, D. A. J. Surg. Res. 2010, doi:10.1016/j.jss.2010.01.014. (59) Guerrero, G.; Mutin, P. H.; Vioux, A. Chem. Mater. 2001, 13, 4367–4373. (60) Devillers, S.; Lanners, L.; Delhalle, J.; Mekhalif, Z. Appl. Surf. Sci. 2011, 257, 6152–6162.

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Induction Heating Vs Conventional Heating for the Hydrothermal

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