- Email: [email protected]
Peptide-based biocoatings for corrosion protection of stainless steel biomaterial in a chloride solution
Materials Science and Engineering C 68 (2016) 695–700
Contents lists available at ScienceDirect
Materials Science and Engineering C journal homepage: www.elsevier.com/locate/msec
Peptide-based biocoatings for corrosion protection of stainless steel biomaterial in a chloride solution Noah G.G. Muruve a,b, Y. Frank Cheng a,⁎, Yuanchao Feng a, Tao Liu a, Daniel A. Muruve b, Daniel J. Hassett c, Randall T. Irvin d a
Department of Mechanical & Manufacturing Engineering, University of Calgary, Calgary, AB T2N 1N4, Canada Department of Medicine, University of Calgary, Calgary, AB T2N 1N4, Canada Department of Molecular Genetics, Biochemistry and Microbiology, University of Cincinnati, Cincinnati, OH 45267, USA d Department of Medical Microbiology & Immunology, University of Alberta, Edmonton, AB T6G 2G6, Canada b c
a r t i c l e
i n f o
Article history: Received 11 February 2016 Received in revised form 20 May 2016 Accepted 14 June 2016 Available online 16 June 2016 Keywords: Peptides Biocoatings Corrosion protection Stainless steel Electrochemical measurements Molecular dynamics simulation
a b s t r a c t In this work, PEGylated D-amino acid K122-4 peptide (D-K122-4-PEG), derived from the type IV pilin of Pseudomonas aeruginosa, coated on 304 stainless steel was investigated for its corrosion resistant properties in a sodium chloride solution by various electrochemical measurements, surface characterization and molecular dynamics simulation. As a comparison, stainless steel electrodes coated with non-PEGylated D-amino acid retroinverso peptide (RI-K122-4) and D-amino acid K122-4 peptide (D-K122-4) were used as control variables during electrochemical tests. It was found that the D-K122-4-PEG coating is able to protect the stainless steel from corrosion in the solution. The RI-K122-4 coating shows corrosion resistant property and should be investigated further, while the D-K122-4 peptide coating, in contrast, shows little to no effect on corrosion. The morphological characterizations support the corrosion resistance of D-K122-4-PEG on stainless steel. The adsorption of D-K122-4 molecules occurs preferentially on Fe2O3, rather than Cr2O3, present on the stainless steel surface. © 2016 Elsevier B.V. All rights reserved.
1. Introduction Metallic alloys, such as stainless steels, cobalt, titanium-based alloys, etc., have been commonly used as biomaterials for medical applications, including cranial plates, orthopedic fracture plates, spinal rods, endovascular stents, prosthetic joints and dental implants [1]. The metals are also used in integrated circuits for implantable electronic devices such as cardiac pacemakers and neuroprostheses such as cochlear stimulators [2]. Statistics show that, between 2012 and 2013, there were more than 30,000 primary hip replacements in Canada, the majority of which had complete or partial metallic bearing surfaces [3]. Given the huge burden of metallic implants used in medicine today, significant focus has been placed on developing materials that provide maximal biocompatibility and durability while maintaining the desired properties. Metallic biomedical implants are under constant corrosion attack due to the chemistry and pH of physiologic fluids [1,4–6]. Cations and anions, as well as negatively charged proteins are abundant in extracellular fluids and cause corrosion and degradation of the biomedical device [1,6–9]. Corrosion of metallic biomaterials results in not only the device failure, but also undesirable host responses. This problem is ⁎ Corresponding author. E-mail address: [email protected] (Y.F. Cheng).
http://dx.doi.org/10.1016/j.msec.2016.06.053 0928-4931/© 2016 Elsevier B.V. All rights reserved.
especially serious given the long lifespan expected for devices such as joint prostheses, endovascular stents or cardiac pacemakers. Corrosion releases metallic ions into local and systemic environments, causing adverse effects in patients. Metallic ions or debris can activate macrophages or osteoblasts, resulting in localized tissue reactions, chronic inflammation and granuloma formation [7,10]. Moreover, the release of metal ions from prosthetic devices into circulation is associated with multiple systemic toxicities and allergy in humans [11,12]. Thus, strategies to inhibit corrosion of metallic biomaterials used in medical devices are critical to improve the longevity and safety of implants in humans. In previous work, a series of peptides (K122-4) derived from the receptor-binding domain of the type IV pilin of Pseudomonas aeruginosa bacteria were identified, and the peptides effectively bind via a semi-covalent interaction to a variety of metallic materials including stainless steels [13,14]. It was found that these peptides demonstrate highly adhesive properties on the steels, and alter their surface characteristics by increasing the electron work function [13]. Furthermore, peptides synthesized using D-amino acids to create enantiomeric and retroinverso forms were found to be extremely durable and proteaseresistant when bound to surfaces such as stainless steels [15,16]. The physical characteristics of the K122-4 peptides suggest that they may serve as effective coatings to improve the corrosion resistance of stainless steels and other metals in biological fluids.
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In this work, the corrosion resistance of a D-amino acid K122-4 peptide conjugated to a hydrophobic polyethylene glycol moiety (D-K1224-PEG) coated on 304 stainless steel was investigated by various electrochemical measurements, surface characterization and molecular dynamics simulations. For comparison, the steel specimens were also coated with D-K122-4 peptide and non-PEGylated D-amino acid retroinverso peptide (RI-K122-4), respectively. The corrosion current density and corrosion potential of the coated steel, as well as the efficiencies of enhanced corrosion resistance by various biocoatings were determined from potentiodynamic polarization curve and electrochemical impedance measurements. The morphological observations were conducted to confirm the corrosion testing results obtained on the steel electrodes coated with D-K122-4-PEG. Moreover, the molecular dynamics simulation was conducted to model the preferential adsorption of peptide molecules and their orientation on the steel surface. 2. Materials and methods 2.1. Peptide synthesis D-amino acid enantiomeric peptides ACTSNADNKYLPKTCQT-amide corresponding to AA128-144 of the PilA receptor binding domain of Pseudomonas aeruginosa strain K122-4 were synthesized with a N-terminal tetra-glycine linker by solid phase peptides, and purified by a reversed-phase high-performance liquid chromatography (HPLC). A disulfide bridge was generated by air oxidation, and the peptides were biotinylated at the N-terminus. These peptides were referred to as DK122-4. Peptides were reconstituted in phosphate buffered saline (PBS) with a pH 7.4. The oxidized D-K122-4 peptide was coupled to polydisperse polyethylene glycol (PEG, molecular weight of ~ 1000) via a tri-glycine linker at the N-terminus to generate D-K122-4-PEG, which was then purified by a reversed phase HPLC to yield a product with over 95% purity determined by HPLC analysis. The polydispersed D-K122-4-PEG was synthesized by Ambiopharm Inc. (North Augusta, SC). PEGylated peptides were reconstituted in dimethyl sulfoxide (DMSO). The retroinverso (RI) D-amino acid peptide was synthesized as TQCTKPLYKNDANSTCA-amide with an N-terminal tetra-glycine linker and biotin, and was referred to as RI-K122-4. 2.2. Preparation of steel specimens
environments present in bodily fluids, the testing was conducted in an environment which was more aggressive (or more corrosive) than physiologic fluids in order to accelerate the experimental testing. Although the corrosion process was accelerated, the results remained directly applicable to the expected corrosion dynamics and the coatings' behavior in physiologic chloride solutions. After a steady value of OCP was reached, electrochemical impedance spectroscopy (EIS) was measured with a frequency range of 0.05 Hz to 2 × 104 Hz and an AC disturbance amplitude of 10 mV. The obtained Nyquist diagram and Bode plots were fitted with appropriate electrochemical equivalent circuits to determine the electrochemical impedance parameters that are directly relevant to the corrosion and coating properties of the WE. Immediately after the EIS measurement, the potentiodynamic polarization curve was measured on the WE at a potential scanning rate of 0.1 mV/s. Corrosion parameters, including corrosion potential (Ecorr), corrosion current density (icorr), and anodic and cathodic Tafel slopes (βa and βc), were determined by fitting the anodic and cathodic polarization curves with a CView software. The efficiency of the enhanced resistance by the biocoatings for steel corrosion, E%, was determined by: E% ¼
i0corr −icorr i0corr
100%
ð1Þ
where i0corr and icorr were corrosion current densities of non-coated and coated steel electrodes, respectively. To ensure the reproducibility of the testing results, each test was repeated at least three times. 2.4. Surface characterization To characterize the protective performance of the biocoatings on corrosion of the steel electrode in chloride solution, an accelerating corrosion testing was conducted in 3.5 wt.% NaCl solution with pH 1.0 (adjusted by 0.1 M HCl solution) for 24 h. The morphologies of the coated steel electrode before and after corrosion testing were characterized by a Keysight 5500 atomic force microscope (AFM). Silicon nitride probes with average spring constants of 0.02–0.77 N/m were used while the AFM was set as “contact mode”. The scanning rate was 1 Hz and the imaging was conducted over frames of 50 μm × 50 μm and 10 μm × 10 μm. Further tests were conducted with a scanning electron microscope (SEM) in order to image the surface morphology of the steel electrode at a larger scale after corrosion testing.
Specimens used in this work were cut from a plate of 304 stainless steel, with a surface area of 1.0 cm2. The specimens were ground up to grit 1200 SiC emery paper, polished with 1 μm diamond paste, and cleaned using deionized water and ethanol. The specimens were then coated with one of the following peptides, i.e., D-K122-4-PEG (1 μg/ mL, 5 μg/mL, 10 μg/mL, 40 μg/mL and 100 μg/mL), D-K122-4 (10 μg/ mL), or RI-K122-4 (10 μg/mL), in coating solutions for 2 h. The DK122-4-PEG was diluted in 100% methanol, and the D-K122-4 and RIK122-4 were diluted in deionized water.
The molecular dynamics simulation was conducted to model the adsorption of peptide molecules on the surface of Fe2O3 and Cr2O3, the key oxides formed on the surface of a stainless steel to resist corrosion. The atomic and molecular orientations and the adsorption energy were determined.
2.3. Electrochemical corrosion measurements
3. Results
Electrochemical corrosion measurements were performed using a Solartron 1280C electrochemical system on a three-electrode system, where the coated stainless steel specimen was used as working electrode (WE), a saturated calomel electrode (SCE) as reference electrode (RE), and a carbon rod as counter electrode (CE). The test solution was 3.5 wt.% NaCl solution. The WE was immersed in the solution for at least 1 h prior to electrochemical testing in order to reach a quasi-stationary open-circuit potential (OCP) value of the electrode. It was noted that corrosion of stainless steel in 0.9 wt.% NaCl solution (which resembles physiologic fluid chemistry) proceeded at a slow rate creating impractical experimental conditions. Since this work attempted to investigate the prepared biocoatings' resistance to corrosion in chloride
3.1. Polarization curve measurements
2.5. Molecular dynamics simulation
Fig. 1 shows the potentiodynamic polarization curves measured on steel electrodes uncoated and coated with D-K122-4-PEG at various concentrations in 3.5 wt.% NaCl solution. It is seen that, as the D-K1224-PEG concentration increases, the corrosion potential is shifted less negatively, with the exception of 1 μg/mL of D-K122-4-PEG. At individual potential, the anodic current density decreases with the increasing D-K122-4-PEG concentration, indicating the increasing corrosion resistance of the coated steel. Moreover, passivity is obvious on both uncoated and coating stainless steel electrodes, and the passive current density becomes smaller with the increase of the D-K122-4-PEG concentration.
N.G.G. Muruve et al. / Materials Science and Engineering C 68 (2016) 695–700
Fig. 1. Potentiodynamic polarization curves measured on steel electrodes uncoated and coated with D-K122-4-PEG at various concentrations in 3.5 wt.% NaCl solution.
Thus, the D-K122-4-PEG is able to improve the stability of the steel passivity. It is also noted that the cathodic polarization curve is approximately independent of the D-K122-4-PEG concentration. The exception to the observation for the steel electrode coated with 1 μg/ mL of D-K122-4-PEG is possibly due to an incomplete coverage of the steel surface by the biocoating at a low concentration, resulting in generation of a potential gradient between the coated and uncoated segments on the electrode surface which would increase the corrosion rate of the steel. Table 1 shows the electrochemical corrosion parameters derived from the polarization curves in Fig. 1. It is seen that, generally, the corrosion current density decreases with the increasing D-K122-4-PEG concentration. Moreover, the biocoating application is able to reduce the corrosion rate of the steel. When the D-K122-4-PEG is 100 μg/mL, the enhanced corrosion resistance is up to 85.3%. Fig. 2 shows the potentiodynamic polarization curves measured on uncoated and coated steel electrodes with 10 μg/mL of D-K122-4-PEG, D-K122-4 and RI-K122-4 coatings, respectively, in 3.5 wt.% NaCl solution. It is seen that the D-K122-4 coating is not able to inhibit corrosion of the steel. Instead, there is a larger anodic current density than the uncoated steel in the solution, indicating that the steel coated with unPEGylated D-K122-4 has a larger corrosion activity. Both D-K122-4PEG and RI-K122-4 coatings can reduce the steel corrosion, and display similar polarization behavior. At the concentration of 10 μg/mL, the efficiencies of the D-K122-4-PEG and RI-K122-4 coatings are 54.7% and 34.5%, respectively. Thus, the D-K122-4-PEG biocoating is more effective to resist corrosion of stainless steel in the solution. 3.2. Electrochemical impedance measurements Fig. 3 shows the Nyquist diagrams measured on the stainless steel electrode uncoated and coated with D-K122-4-PEG at various concentrations in 3.5 wt.% NaCl solution. It is seen that all plots are featured with an incomplete semicircle, indicating that there is one time constant associated with the electrochemical impedance behavior over the measuring frequency range. Moreover, the size of the semicircle, which is proportional to the charge-transfer resistance at the steel/solution
697
Fig. 2. Potentiodynamic polarization curves measured on uncoated and coated steel electrodes with 10 μg/mL of D-K122-4-PEG, D-K122-4 and RI-K122-4 coatings, respectively, in 3.5 wt.% NaCl solution.
interface, increases with the increasing concentration. Thus, the steel's corrosion rate is reduced as the coating concentration is increased. The result is well consistent with the polarization curve measurements in Fig. 1. Fig. 4 shows the Nyquist diagrams measured on uncoated and coated steel electrodes with 10 μg/mL of D-K122-4-PEG, D-K122-4 and RIK122-4 coatings, respectively, in 3.5 wt.% NaCl solution. The largest semicircle corresponds to the electrode coated with D-K122-4-PEG, indicating that the PEGylated peptide has the largest charge-transfer resistance and thus is the best corrosion-resistant coating. The nonPEGylated D-K122-4 coating showed the smallest charge-transfer resistance and proved to be the least effective corrosion inhibitor of the three measured. 3.3. Morphological characterization Fig. 5 shows the AFM topographic views of uncoated and coated steel electrode with 10 μg/mL D-K122-4-PEG before and after 24 h of corrosion testing in 3.5 wt.% NaCl solution (pH 1.0), respectively. The AFM is able to provide information on the surface roughness of steels at a resolution smaller than a nano-meter. It is seen that, prior to corrosion, there is a similar surface roughness for the uncoated and coated steels. After 24 h of immersion in the solution, the uncoated steel shows extensive corrosion products and isolated corrosion pits on the steel surface. However, the D-K122-4-PEG coated steel is quite smooth and does not show sign of corrosion. It is noted that, on the surface of the coated steel, sporadic bright dots are observed, which represent residue salt from the solution. After 24 h of testing, the D-K122-4-PEG coated steel is only 27.5 times rougher than its original state, having a maximum fluctuation of 115.2 nm from the norm. This is significantly smaller than the bare steel that is more than 300 times rougher than its original state which has a maximum fluctuation of 747.5 nm from the norm. Fig. 6 shows the SEM morphological views of the surface of uncoated and coated steels before and after 24 h of testing in 3.5 wt.% NaCl solution (pH 1.0). Consistent with the results obtained from the AFM
Table 1 Electrochemical corrosion parameters and inhibiting efficiencies derived from the polarization curves in Fig. 1. Concentration 0 5 μg/mL 10 μg/mL 40 μg/mL 100 μg/mL
icorr (A/cm2) −8
7.39 × 10 4.93 × 10−8 3.35 × 10−8 1.40 × 10−8 1.09 × 10−8
Ecorr (V, SCE)
βa (mV/dec)
βc (mV/dec)
E (%)
−0.263 −0.200 −0.243 −0.182 −0.162
72.421 142.12 73.773 58.052 55.866
44.426 77.774 50.203 37.065 40.626
0 49.1 54.7 81.0 85.3
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the peptide, and become shared through multiple sites with the steel. The peptide-based biocoatings possess the hydrophobic properties due to the long-chain fatty acid and benzene ring included in the molecular structure. When the D-K122-4-PEG concentration is 1 μg/mL only, the steel surface cannot be completely covered by the biocoating. A potential gradient is generated between the coated and uncoated segments. Chloride ions present in the solution can easily reach the uncoated area on the steel surface and induce pitting corrosion. With the increase of the coating concentrations, the peptide can form fibrous gels, which resist the penetration of chloride ions and improve the hydrophobic property of the coated surface. Hence, the efficiency for corrosion resistance increases. The interaction energy, EM2O3 − peptide, between the D-K122-4-PEG molecule and the metal oxide is calculated by: EM2 O3 −peptide ¼ EM2 O3 þpeptide −EM2 O3 −Epeptide
Fig. 3. Nyquist diagrams measured on the stainless steel electrode uncoated and coated with D-K122-4-PEG of various concentrations in 3.5 wt.% NaCl solution.
imaging, the SEM views demonstrate that the D-K122-4-PEG coating protect the steel from corrosion and pitting corrosion compared to the uncoated steel electrode. The uncoated steel shows heavy signs of corrosion and pitting corrosion, whereas the surface of the coated steel is quite smooth and free of corrosion. It is evident that the D-K122-4PEG provides a promising biological coating that effectively improves the corrosion resistance of stainless steel in corrosive, chloride environments.
ð2Þ
where EM2O3 +peptide is the total potential energy of the metal oxide crystal combined with the adsorbed D-K122-4-PEG molecule, EM2O3 is the total potential energy of the metal oxide crystal, and Epeptide is the total potential energy of D-K122-4-PEG molecule. The energy for the adsorption of D-K122-4-PEG on Fe2O3 and Cr2O3 is equal to 3451 kJ/mol and 1230 kJ/mol, respectively, implying that the interaction between D-K122-4-PEG molecules and Fe2O3 is much stronger than that with Cr2O3 surface. Thus, the D-K122-4-PEG molecules adsorb preferentially on Fe2O3, rather than Cr2O3, on the stainless steel surface. The analysis indicates that the adsorption of peptide biocoating does not damage the Cr2O3 passive film on stainless steels. The interaction that generates a new bond would result in changes of the electronic state of the steel surface. It is consistent with the XPS analysis results previously [13].
3.4. Molecular dynamics simulation of the coated steel in chloride solution 4. Discussion The present work demonstrates that the peptide-based biocoating D-K122-4-PEG is effective to protect stainless steel from corrosion in chloride solution. It was demonstrated previously [13] that there is a chemical interaction of the peptide coatings with stainless steels, and the interaction can change the electronic state of the steel surface and produce a strong adhesion of the peptide to the steel. Fig. 7 shows the schematic configuration of a D-K122-4-PEG molecule adsorbed on Fe2O3 (0 0 1) and Cr2O3 (0 0 1), the primary components contained on the surface of a stainless steel, by molecular dynamics simulation. The protein contains two sulfur atoms within its disulfide loop, with two β-turns and a hydrophobic core. From the simulation results, it is likely that electrons are delocalized throughout the disulfide loop of
Fig. 4. Nyquist diagrams measured on uncoated and coated steel electrodes with 10 μg/mL of D-K122-4-PEG, D-K122-4 and RI-K122-4 coatings, respectively, in 3.5 wt.% NaCl solution.
The present work demonstrates the anti-corrosion properties of a peptide-based coating on stainless steel exposed to chloride solutions. The results are confirmed by electrochemical corrosion measurements, morphological observations and molecular dynamics simulation. Through analysis of the potentiodynamic polarization curves, it was found that the D-K122-4-PEG coating can enhance the corrosion resistance of the steel by up to 85.3%, decrease the anodic current density and increase the stability of the steel's passivity. The Nyquist plots indicate that the D-K122-4-PEG coating increases the charge-transfer resistance. Moreover, the corrosion resistance is increased with the increasing concentration of the coating. The AFM and SEM images support the electrochemical results. From the AFM images, the non-coated steel samples are 6.5 times rougher than the coated ones after 24 h of testing in the solution, indicating that the samples coated with DK122-4-PEG are protected against the corrosion attack. In the SEM images pitting corrosion can be observed on the non-coated sample surface whereas the coated sample remained nearly pristine. Finally, upon the molecular dynamics simulation conducted on D-K122-4PEG, the coating has no impact on the passive film formed on the stainless steel, as it bonds preferably with Fe2O3 rather than Cr2O3. The anti-corrosion properties of D-K122-4-PEG are dose dependent. At low peptide concentrations such as 1 μg/mL, a complete biocoating cannot be formed to cover the steel surface. Both corrosion and pitting corrosion occur on the steel surface. In comparison to the RI-K122-4 coating, the D-K122-4-PEG coating is a more effective corrosion-resistant coating. The RI-K122-4 also shows corrosion protection ability, and thus should be examined further in later studies. The unPEGylated D-K122-4 biocoating, however, does not show any significant corrosion resistance properties. The D-K122-4-PEG molecule warrants further study as a potentially useful anti-corrosion coating for metal-containing medical devices. In this regard, studies examining anti-corrosion properties of the DK122-4-PEG coating on other alloys including titanium will expand
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Fig. 5. AFM topographic views of uncoated and coated steel electrode with 10 μg/mL D-K122-4-PEG before and after 24 h of corrosion testing in 3.5 wt.% NaCl solution (pH 1.0), respectively.
the potential usefulness of this material. Furthermore, evaluation of the biocompatibility properties of D-K122-4-PEG that include effects on immune cell activation or cellular adhesion will be needed. In authors'
previous work, it was demonstrated that the D-K122-4-PEG improves the biocompatibility of polysulfone, a polymer commonly used to manufacture dialysis membranes [16]. Whether these properties extend to
Fig. 6. SEM morphological views of uncoated and coated steel electrode with 10 μg/mL D-K122-4-PEG before and after 24 h of corrosion testing in 3.5 wt.% NaCl solution (pH 1.0), respectively.
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rather than Cr2O3, on the steel surface, with a stronger bonding strength. The D-K122-4-PEG demonstrates promising as an anti-corrosion coating for metal-containing medical devices, and further study is required for the purpose.
Acknowledgements This work was supported partially by Arch Biopartners Inc. (CIS2015-T-06).
References
Fig. 7. Ball and stick model of a D-K122-4-PEG molecule adsorbed on (a) Fe2O3 (0 0 1), and (b) Cr2O3 (0 0 1), the primary components contained on the surface of a stainless steel, given by molecular dynamics simulation. The different colors indicate different atom types, with gray, red, blue, violet and green symbolize carbon, oxygen, nitrogen, iron and chromium, respectively. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
stainless steels and other alloys used in the medical devices will be the focus of future studies. 5. Conclusions The D-K122-4-PEG peptide-based biocoating is able to protect stainless steel from corrosion in a chloride solution. This conclusion is confirmed by potentiodynamic polarization curve and electrochemical impedance measurements, as well as morphological characterizations. Moreover, corrosion resistance is enhanced by increasing the biocoating concentration, as indicated by the decreasing anodic current density and increasing charge-transfer resistance. The adsorption of D-K122-4-PEG molecules on the surface of stainless steel is selective. The adsorption occurs preferentially on Fe2O3,
[1] R. Singh, N.B. Dahotre, Corrosion degradation and prevention by surface modification of biometallic materials, J. Mater. Sci. Mater. Med. 18 (2007) 725–751. [2] A. Vanhoestenberghe, N. Donaldson, Corrosion of silicon integrated circuits and lifetime predictions in implantable electronic devices, J. Neural Eng. 10 (2013) 031002. [3] Canadian Institute for Health Information, Hip and Knee Replacements in Canada: Canadian Joint Replacement Registry 2014 Annual Report, CIHI, Ottawa, ON, 2014. [4] A. Igual Munoz, J. Schwiesau, B.M. Jolles, S. Mischler, In vivo electrochemical corrosion study of a CoCrMo biomedical alloy in human synovial fluids, Acta Biomater. 21 (2015) 228–236. [5] S. Virtanen, I. Milosev, E. Gomez-Barrena, R. Trebse, J. Salo, Y.T. Konttinen, Special modes of corrosion under physiological and simulated physiological conditions, Acta Biomater. 4 (2008) 468–476. [6] B. Thierry, M. Tabrizian, Biocompatibility and biostability of metallic endovascular implants: state of the art and perspectives, J. Endovasc. Ther. 10 (2003) 807–824. [7] H.J. Cooper, The local effects of metal corrosion in total hip arthroplasty, Orthop. Clin. N. Am. 45 (2014) 9–18. [8] V.G. Pina, A. Dalmau, F. Devesa, V. Amigo, A.I. Munoz, Tribocorrosion behavior of beta titanium biomedical alloys in phosphate buffer saline solution, J. Mech. Behav. Biomed. Mater. 46 (2015) 59–68. [9] Y. Yan, A. Neville, D. Dowson, Understanding the role of corrosion in the degradation of metal-on-metal implants, Proc. Inst. Mech. Eng. H 220 (2006) 173–181. [10] R.A. Antunes, M.C. de Oliveira, Corrosion fatigue of biomedical metallic alloys: mechanisms and mitigation, Acta Biomater. 8 (2012) 937–962. [11] B.D. Gessner, T. Steck, E. Woelber, S.S. Tower, A systematic review of systemic cobaltism after wear or corrosion of chrome-cobalt hip implants, J. Patient Saf. 12 (June 2015) 1549–8417. [12] B. Krecisz, M. Kiec-Swierczynska, D. Chomiczewska-Skora, Allergy to orthopedic metal implants — a prospective study, Int. J. Occup. Med. Environ. Health 25 (2012) 463–469. [13] E.M. Davis, D.Y. Li, R.T. Irvin, A peptide-stainless steel reaction that yields a new bioorganic-metal state of matter, Biomaterials 32 (2011) 5311–5319. [14] C.L. Giltner, E.J. van Schaik, G.F. Audette, D. Kao, R.S. Hodges, D.J. Hassett, The Pseudomonas aeruginosa type IV pilin receptor binding domain functions as an adhesin for both biotic and abiotic surfaces, Mol. Microbiol. 59 (2006) 1083–1096. [15] E.M. Davis, D. Li, M. Shahrooei, B. Yu, D. Muruve, R.T. Irvin, Evidence of extensive diversity in bacterial adherence mechanisms that exploit unanticipated stainless steel surface structural complexity for biofilm formation, Acta Biomater. 9 (2013) 6236–6244. [16] E.M. Davis, J.M. Platnich, R.T. Irvin, D.A. Muruve, Peptide-mediated PEGylation of polysulfone reduces protein adsorption and leukocyte activation, ASAIO J (2015).
Contents lists available at ScienceDirect
Materials Science and Engineering C journal homepage: www.elsevier.com/locate/msec
Peptide-based biocoatings for corrosion protection of stainless steel biomaterial in a chloride solution Noah G.G. Muruve a,b, Y. Frank Cheng a,⁎, Yuanchao Feng a, Tao Liu a, Daniel A. Muruve b, Daniel J. Hassett c, Randall T. Irvin d a
Department of Mechanical & Manufacturing Engineering, University of Calgary, Calgary, AB T2N 1N4, Canada Department of Medicine, University of Calgary, Calgary, AB T2N 1N4, Canada Department of Molecular Genetics, Biochemistry and Microbiology, University of Cincinnati, Cincinnati, OH 45267, USA d Department of Medical Microbiology & Immunology, University of Alberta, Edmonton, AB T6G 2G6, Canada b c
a r t i c l e
i n f o
Article history: Received 11 February 2016 Received in revised form 20 May 2016 Accepted 14 June 2016 Available online 16 June 2016 Keywords: Peptides Biocoatings Corrosion protection Stainless steel Electrochemical measurements Molecular dynamics simulation
a b s t r a c t In this work, PEGylated D-amino acid K122-4 peptide (D-K122-4-PEG), derived from the type IV pilin of Pseudomonas aeruginosa, coated on 304 stainless steel was investigated for its corrosion resistant properties in a sodium chloride solution by various electrochemical measurements, surface characterization and molecular dynamics simulation. As a comparison, stainless steel electrodes coated with non-PEGylated D-amino acid retroinverso peptide (RI-K122-4) and D-amino acid K122-4 peptide (D-K122-4) were used as control variables during electrochemical tests. It was found that the D-K122-4-PEG coating is able to protect the stainless steel from corrosion in the solution. The RI-K122-4 coating shows corrosion resistant property and should be investigated further, while the D-K122-4 peptide coating, in contrast, shows little to no effect on corrosion. The morphological characterizations support the corrosion resistance of D-K122-4-PEG on stainless steel. The adsorption of D-K122-4 molecules occurs preferentially on Fe2O3, rather than Cr2O3, present on the stainless steel surface. © 2016 Elsevier B.V. All rights reserved.
1. Introduction Metallic alloys, such as stainless steels, cobalt, titanium-based alloys, etc., have been commonly used as biomaterials for medical applications, including cranial plates, orthopedic fracture plates, spinal rods, endovascular stents, prosthetic joints and dental implants [1]. The metals are also used in integrated circuits for implantable electronic devices such as cardiac pacemakers and neuroprostheses such as cochlear stimulators [2]. Statistics show that, between 2012 and 2013, there were more than 30,000 primary hip replacements in Canada, the majority of which had complete or partial metallic bearing surfaces [3]. Given the huge burden of metallic implants used in medicine today, significant focus has been placed on developing materials that provide maximal biocompatibility and durability while maintaining the desired properties. Metallic biomedical implants are under constant corrosion attack due to the chemistry and pH of physiologic fluids [1,4–6]. Cations and anions, as well as negatively charged proteins are abundant in extracellular fluids and cause corrosion and degradation of the biomedical device [1,6–9]. Corrosion of metallic biomaterials results in not only the device failure, but also undesirable host responses. This problem is ⁎ Corresponding author. E-mail address: [email protected] (Y.F. Cheng).
http://dx.doi.org/10.1016/j.msec.2016.06.053 0928-4931/© 2016 Elsevier B.V. All rights reserved.
especially serious given the long lifespan expected for devices such as joint prostheses, endovascular stents or cardiac pacemakers. Corrosion releases metallic ions into local and systemic environments, causing adverse effects in patients. Metallic ions or debris can activate macrophages or osteoblasts, resulting in localized tissue reactions, chronic inflammation and granuloma formation [7,10]. Moreover, the release of metal ions from prosthetic devices into circulation is associated with multiple systemic toxicities and allergy in humans [11,12]. Thus, strategies to inhibit corrosion of metallic biomaterials used in medical devices are critical to improve the longevity and safety of implants in humans. In previous work, a series of peptides (K122-4) derived from the receptor-binding domain of the type IV pilin of Pseudomonas aeruginosa bacteria were identified, and the peptides effectively bind via a semi-covalent interaction to a variety of metallic materials including stainless steels [13,14]. It was found that these peptides demonstrate highly adhesive properties on the steels, and alter their surface characteristics by increasing the electron work function [13]. Furthermore, peptides synthesized using D-amino acids to create enantiomeric and retroinverso forms were found to be extremely durable and proteaseresistant when bound to surfaces such as stainless steels [15,16]. The physical characteristics of the K122-4 peptides suggest that they may serve as effective coatings to improve the corrosion resistance of stainless steels and other metals in biological fluids.
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In this work, the corrosion resistance of a D-amino acid K122-4 peptide conjugated to a hydrophobic polyethylene glycol moiety (D-K1224-PEG) coated on 304 stainless steel was investigated by various electrochemical measurements, surface characterization and molecular dynamics simulations. For comparison, the steel specimens were also coated with D-K122-4 peptide and non-PEGylated D-amino acid retroinverso peptide (RI-K122-4), respectively. The corrosion current density and corrosion potential of the coated steel, as well as the efficiencies of enhanced corrosion resistance by various biocoatings were determined from potentiodynamic polarization curve and electrochemical impedance measurements. The morphological observations were conducted to confirm the corrosion testing results obtained on the steel electrodes coated with D-K122-4-PEG. Moreover, the molecular dynamics simulation was conducted to model the preferential adsorption of peptide molecules and their orientation on the steel surface. 2. Materials and methods 2.1. Peptide synthesis D-amino acid enantiomeric peptides ACTSNADNKYLPKTCQT-amide corresponding to AA128-144 of the PilA receptor binding domain of Pseudomonas aeruginosa strain K122-4 were synthesized with a N-terminal tetra-glycine linker by solid phase peptides, and purified by a reversed-phase high-performance liquid chromatography (HPLC). A disulfide bridge was generated by air oxidation, and the peptides were biotinylated at the N-terminus. These peptides were referred to as DK122-4. Peptides were reconstituted in phosphate buffered saline (PBS) with a pH 7.4. The oxidized D-K122-4 peptide was coupled to polydisperse polyethylene glycol (PEG, molecular weight of ~ 1000) via a tri-glycine linker at the N-terminus to generate D-K122-4-PEG, which was then purified by a reversed phase HPLC to yield a product with over 95% purity determined by HPLC analysis. The polydispersed D-K122-4-PEG was synthesized by Ambiopharm Inc. (North Augusta, SC). PEGylated peptides were reconstituted in dimethyl sulfoxide (DMSO). The retroinverso (RI) D-amino acid peptide was synthesized as TQCTKPLYKNDANSTCA-amide with an N-terminal tetra-glycine linker and biotin, and was referred to as RI-K122-4. 2.2. Preparation of steel specimens
environments present in bodily fluids, the testing was conducted in an environment which was more aggressive (or more corrosive) than physiologic fluids in order to accelerate the experimental testing. Although the corrosion process was accelerated, the results remained directly applicable to the expected corrosion dynamics and the coatings' behavior in physiologic chloride solutions. After a steady value of OCP was reached, electrochemical impedance spectroscopy (EIS) was measured with a frequency range of 0.05 Hz to 2 × 104 Hz and an AC disturbance amplitude of 10 mV. The obtained Nyquist diagram and Bode plots were fitted with appropriate electrochemical equivalent circuits to determine the electrochemical impedance parameters that are directly relevant to the corrosion and coating properties of the WE. Immediately after the EIS measurement, the potentiodynamic polarization curve was measured on the WE at a potential scanning rate of 0.1 mV/s. Corrosion parameters, including corrosion potential (Ecorr), corrosion current density (icorr), and anodic and cathodic Tafel slopes (βa and βc), were determined by fitting the anodic and cathodic polarization curves with a CView software. The efficiency of the enhanced resistance by the biocoatings for steel corrosion, E%, was determined by: E% ¼
i0corr −icorr i0corr
100%
ð1Þ
where i0corr and icorr were corrosion current densities of non-coated and coated steel electrodes, respectively. To ensure the reproducibility of the testing results, each test was repeated at least three times. 2.4. Surface characterization To characterize the protective performance of the biocoatings on corrosion of the steel electrode in chloride solution, an accelerating corrosion testing was conducted in 3.5 wt.% NaCl solution with pH 1.0 (adjusted by 0.1 M HCl solution) for 24 h. The morphologies of the coated steel electrode before and after corrosion testing were characterized by a Keysight 5500 atomic force microscope (AFM). Silicon nitride probes with average spring constants of 0.02–0.77 N/m were used while the AFM was set as “contact mode”. The scanning rate was 1 Hz and the imaging was conducted over frames of 50 μm × 50 μm and 10 μm × 10 μm. Further tests were conducted with a scanning electron microscope (SEM) in order to image the surface morphology of the steel electrode at a larger scale after corrosion testing.
Specimens used in this work were cut from a plate of 304 stainless steel, with a surface area of 1.0 cm2. The specimens were ground up to grit 1200 SiC emery paper, polished with 1 μm diamond paste, and cleaned using deionized water and ethanol. The specimens were then coated with one of the following peptides, i.e., D-K122-4-PEG (1 μg/ mL, 5 μg/mL, 10 μg/mL, 40 μg/mL and 100 μg/mL), D-K122-4 (10 μg/ mL), or RI-K122-4 (10 μg/mL), in coating solutions for 2 h. The DK122-4-PEG was diluted in 100% methanol, and the D-K122-4 and RIK122-4 were diluted in deionized water.
The molecular dynamics simulation was conducted to model the adsorption of peptide molecules on the surface of Fe2O3 and Cr2O3, the key oxides formed on the surface of a stainless steel to resist corrosion. The atomic and molecular orientations and the adsorption energy were determined.
2.3. Electrochemical corrosion measurements
3. Results
Electrochemical corrosion measurements were performed using a Solartron 1280C electrochemical system on a three-electrode system, where the coated stainless steel specimen was used as working electrode (WE), a saturated calomel electrode (SCE) as reference electrode (RE), and a carbon rod as counter electrode (CE). The test solution was 3.5 wt.% NaCl solution. The WE was immersed in the solution for at least 1 h prior to electrochemical testing in order to reach a quasi-stationary open-circuit potential (OCP) value of the electrode. It was noted that corrosion of stainless steel in 0.9 wt.% NaCl solution (which resembles physiologic fluid chemistry) proceeded at a slow rate creating impractical experimental conditions. Since this work attempted to investigate the prepared biocoatings' resistance to corrosion in chloride
3.1. Polarization curve measurements
2.5. Molecular dynamics simulation
Fig. 1 shows the potentiodynamic polarization curves measured on steel electrodes uncoated and coated with D-K122-4-PEG at various concentrations in 3.5 wt.% NaCl solution. It is seen that, as the D-K1224-PEG concentration increases, the corrosion potential is shifted less negatively, with the exception of 1 μg/mL of D-K122-4-PEG. At individual potential, the anodic current density decreases with the increasing D-K122-4-PEG concentration, indicating the increasing corrosion resistance of the coated steel. Moreover, passivity is obvious on both uncoated and coating stainless steel electrodes, and the passive current density becomes smaller with the increase of the D-K122-4-PEG concentration.
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Fig. 1. Potentiodynamic polarization curves measured on steel electrodes uncoated and coated with D-K122-4-PEG at various concentrations in 3.5 wt.% NaCl solution.
Thus, the D-K122-4-PEG is able to improve the stability of the steel passivity. It is also noted that the cathodic polarization curve is approximately independent of the D-K122-4-PEG concentration. The exception to the observation for the steel electrode coated with 1 μg/ mL of D-K122-4-PEG is possibly due to an incomplete coverage of the steel surface by the biocoating at a low concentration, resulting in generation of a potential gradient between the coated and uncoated segments on the electrode surface which would increase the corrosion rate of the steel. Table 1 shows the electrochemical corrosion parameters derived from the polarization curves in Fig. 1. It is seen that, generally, the corrosion current density decreases with the increasing D-K122-4-PEG concentration. Moreover, the biocoating application is able to reduce the corrosion rate of the steel. When the D-K122-4-PEG is 100 μg/mL, the enhanced corrosion resistance is up to 85.3%. Fig. 2 shows the potentiodynamic polarization curves measured on uncoated and coated steel electrodes with 10 μg/mL of D-K122-4-PEG, D-K122-4 and RI-K122-4 coatings, respectively, in 3.5 wt.% NaCl solution. It is seen that the D-K122-4 coating is not able to inhibit corrosion of the steel. Instead, there is a larger anodic current density than the uncoated steel in the solution, indicating that the steel coated with unPEGylated D-K122-4 has a larger corrosion activity. Both D-K122-4PEG and RI-K122-4 coatings can reduce the steel corrosion, and display similar polarization behavior. At the concentration of 10 μg/mL, the efficiencies of the D-K122-4-PEG and RI-K122-4 coatings are 54.7% and 34.5%, respectively. Thus, the D-K122-4-PEG biocoating is more effective to resist corrosion of stainless steel in the solution. 3.2. Electrochemical impedance measurements Fig. 3 shows the Nyquist diagrams measured on the stainless steel electrode uncoated and coated with D-K122-4-PEG at various concentrations in 3.5 wt.% NaCl solution. It is seen that all plots are featured with an incomplete semicircle, indicating that there is one time constant associated with the electrochemical impedance behavior over the measuring frequency range. Moreover, the size of the semicircle, which is proportional to the charge-transfer resistance at the steel/solution
697
Fig. 2. Potentiodynamic polarization curves measured on uncoated and coated steel electrodes with 10 μg/mL of D-K122-4-PEG, D-K122-4 and RI-K122-4 coatings, respectively, in 3.5 wt.% NaCl solution.
interface, increases with the increasing concentration. Thus, the steel's corrosion rate is reduced as the coating concentration is increased. The result is well consistent with the polarization curve measurements in Fig. 1. Fig. 4 shows the Nyquist diagrams measured on uncoated and coated steel electrodes with 10 μg/mL of D-K122-4-PEG, D-K122-4 and RIK122-4 coatings, respectively, in 3.5 wt.% NaCl solution. The largest semicircle corresponds to the electrode coated with D-K122-4-PEG, indicating that the PEGylated peptide has the largest charge-transfer resistance and thus is the best corrosion-resistant coating. The nonPEGylated D-K122-4 coating showed the smallest charge-transfer resistance and proved to be the least effective corrosion inhibitor of the three measured. 3.3. Morphological characterization Fig. 5 shows the AFM topographic views of uncoated and coated steel electrode with 10 μg/mL D-K122-4-PEG before and after 24 h of corrosion testing in 3.5 wt.% NaCl solution (pH 1.0), respectively. The AFM is able to provide information on the surface roughness of steels at a resolution smaller than a nano-meter. It is seen that, prior to corrosion, there is a similar surface roughness for the uncoated and coated steels. After 24 h of immersion in the solution, the uncoated steel shows extensive corrosion products and isolated corrosion pits on the steel surface. However, the D-K122-4-PEG coated steel is quite smooth and does not show sign of corrosion. It is noted that, on the surface of the coated steel, sporadic bright dots are observed, which represent residue salt from the solution. After 24 h of testing, the D-K122-4-PEG coated steel is only 27.5 times rougher than its original state, having a maximum fluctuation of 115.2 nm from the norm. This is significantly smaller than the bare steel that is more than 300 times rougher than its original state which has a maximum fluctuation of 747.5 nm from the norm. Fig. 6 shows the SEM morphological views of the surface of uncoated and coated steels before and after 24 h of testing in 3.5 wt.% NaCl solution (pH 1.0). Consistent with the results obtained from the AFM
Table 1 Electrochemical corrosion parameters and inhibiting efficiencies derived from the polarization curves in Fig. 1. Concentration 0 5 μg/mL 10 μg/mL 40 μg/mL 100 μg/mL
icorr (A/cm2) −8
7.39 × 10 4.93 × 10−8 3.35 × 10−8 1.40 × 10−8 1.09 × 10−8
Ecorr (V, SCE)
βa (mV/dec)
βc (mV/dec)
E (%)
−0.263 −0.200 −0.243 −0.182 −0.162
72.421 142.12 73.773 58.052 55.866
44.426 77.774 50.203 37.065 40.626
0 49.1 54.7 81.0 85.3
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the peptide, and become shared through multiple sites with the steel. The peptide-based biocoatings possess the hydrophobic properties due to the long-chain fatty acid and benzene ring included in the molecular structure. When the D-K122-4-PEG concentration is 1 μg/mL only, the steel surface cannot be completely covered by the biocoating. A potential gradient is generated between the coated and uncoated segments. Chloride ions present in the solution can easily reach the uncoated area on the steel surface and induce pitting corrosion. With the increase of the coating concentrations, the peptide can form fibrous gels, which resist the penetration of chloride ions and improve the hydrophobic property of the coated surface. Hence, the efficiency for corrosion resistance increases. The interaction energy, EM2O3 − peptide, between the D-K122-4-PEG molecule and the metal oxide is calculated by: EM2 O3 −peptide ¼ EM2 O3 þpeptide −EM2 O3 −Epeptide
Fig. 3. Nyquist diagrams measured on the stainless steel electrode uncoated and coated with D-K122-4-PEG of various concentrations in 3.5 wt.% NaCl solution.
imaging, the SEM views demonstrate that the D-K122-4-PEG coating protect the steel from corrosion and pitting corrosion compared to the uncoated steel electrode. The uncoated steel shows heavy signs of corrosion and pitting corrosion, whereas the surface of the coated steel is quite smooth and free of corrosion. It is evident that the D-K122-4PEG provides a promising biological coating that effectively improves the corrosion resistance of stainless steel in corrosive, chloride environments.
ð2Þ
where EM2O3 +peptide is the total potential energy of the metal oxide crystal combined with the adsorbed D-K122-4-PEG molecule, EM2O3 is the total potential energy of the metal oxide crystal, and Epeptide is the total potential energy of D-K122-4-PEG molecule. The energy for the adsorption of D-K122-4-PEG on Fe2O3 and Cr2O3 is equal to 3451 kJ/mol and 1230 kJ/mol, respectively, implying that the interaction between D-K122-4-PEG molecules and Fe2O3 is much stronger than that with Cr2O3 surface. Thus, the D-K122-4-PEG molecules adsorb preferentially on Fe2O3, rather than Cr2O3, on the stainless steel surface. The analysis indicates that the adsorption of peptide biocoating does not damage the Cr2O3 passive film on stainless steels. The interaction that generates a new bond would result in changes of the electronic state of the steel surface. It is consistent with the XPS analysis results previously [13].
3.4. Molecular dynamics simulation of the coated steel in chloride solution 4. Discussion The present work demonstrates that the peptide-based biocoating D-K122-4-PEG is effective to protect stainless steel from corrosion in chloride solution. It was demonstrated previously [13] that there is a chemical interaction of the peptide coatings with stainless steels, and the interaction can change the electronic state of the steel surface and produce a strong adhesion of the peptide to the steel. Fig. 7 shows the schematic configuration of a D-K122-4-PEG molecule adsorbed on Fe2O3 (0 0 1) and Cr2O3 (0 0 1), the primary components contained on the surface of a stainless steel, by molecular dynamics simulation. The protein contains two sulfur atoms within its disulfide loop, with two β-turns and a hydrophobic core. From the simulation results, it is likely that electrons are delocalized throughout the disulfide loop of
Fig. 4. Nyquist diagrams measured on uncoated and coated steel electrodes with 10 μg/mL of D-K122-4-PEG, D-K122-4 and RI-K122-4 coatings, respectively, in 3.5 wt.% NaCl solution.
The present work demonstrates the anti-corrosion properties of a peptide-based coating on stainless steel exposed to chloride solutions. The results are confirmed by electrochemical corrosion measurements, morphological observations and molecular dynamics simulation. Through analysis of the potentiodynamic polarization curves, it was found that the D-K122-4-PEG coating can enhance the corrosion resistance of the steel by up to 85.3%, decrease the anodic current density and increase the stability of the steel's passivity. The Nyquist plots indicate that the D-K122-4-PEG coating increases the charge-transfer resistance. Moreover, the corrosion resistance is increased with the increasing concentration of the coating. The AFM and SEM images support the electrochemical results. From the AFM images, the non-coated steel samples are 6.5 times rougher than the coated ones after 24 h of testing in the solution, indicating that the samples coated with DK122-4-PEG are protected against the corrosion attack. In the SEM images pitting corrosion can be observed on the non-coated sample surface whereas the coated sample remained nearly pristine. Finally, upon the molecular dynamics simulation conducted on D-K122-4PEG, the coating has no impact on the passive film formed on the stainless steel, as it bonds preferably with Fe2O3 rather than Cr2O3. The anti-corrosion properties of D-K122-4-PEG are dose dependent. At low peptide concentrations such as 1 μg/mL, a complete biocoating cannot be formed to cover the steel surface. Both corrosion and pitting corrosion occur on the steel surface. In comparison to the RI-K122-4 coating, the D-K122-4-PEG coating is a more effective corrosion-resistant coating. The RI-K122-4 also shows corrosion protection ability, and thus should be examined further in later studies. The unPEGylated D-K122-4 biocoating, however, does not show any significant corrosion resistance properties. The D-K122-4-PEG molecule warrants further study as a potentially useful anti-corrosion coating for metal-containing medical devices. In this regard, studies examining anti-corrosion properties of the DK122-4-PEG coating on other alloys including titanium will expand
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Fig. 5. AFM topographic views of uncoated and coated steel electrode with 10 μg/mL D-K122-4-PEG before and after 24 h of corrosion testing in 3.5 wt.% NaCl solution (pH 1.0), respectively.
the potential usefulness of this material. Furthermore, evaluation of the biocompatibility properties of D-K122-4-PEG that include effects on immune cell activation or cellular adhesion will be needed. In authors'
previous work, it was demonstrated that the D-K122-4-PEG improves the biocompatibility of polysulfone, a polymer commonly used to manufacture dialysis membranes [16]. Whether these properties extend to
Fig. 6. SEM morphological views of uncoated and coated steel electrode with 10 μg/mL D-K122-4-PEG before and after 24 h of corrosion testing in 3.5 wt.% NaCl solution (pH 1.0), respectively.
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rather than Cr2O3, on the steel surface, with a stronger bonding strength. The D-K122-4-PEG demonstrates promising as an anti-corrosion coating for metal-containing medical devices, and further study is required for the purpose.
Acknowledgements This work was supported partially by Arch Biopartners Inc. (CIS2015-T-06).
References
Fig. 7. Ball and stick model of a D-K122-4-PEG molecule adsorbed on (a) Fe2O3 (0 0 1), and (b) Cr2O3 (0 0 1), the primary components contained on the surface of a stainless steel, given by molecular dynamics simulation. The different colors indicate different atom types, with gray, red, blue, violet and green symbolize carbon, oxygen, nitrogen, iron and chromium, respectively. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
stainless steels and other alloys used in the medical devices will be the focus of future studies. 5. Conclusions The D-K122-4-PEG peptide-based biocoating is able to protect stainless steel from corrosion in a chloride solution. This conclusion is confirmed by potentiodynamic polarization curve and electrochemical impedance measurements, as well as morphological characterizations. Moreover, corrosion resistance is enhanced by increasing the biocoating concentration, as indicated by the decreasing anodic current density and increasing charge-transfer resistance. The adsorption of D-K122-4-PEG molecules on the surface of stainless steel is selective. The adsorption occurs preferentially on Fe2O3,
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