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Anticorrosive Self-Assembled Alkylsilane Coatings for Resorbable Magnesium Metal Devices. Avinash J Patil, Olivia F Jackson, Laura Beth Fulton, Dandan Hong, Stephen A Kelleher, Palak A Desai, Da-Tren Chou, Susheng Tan, Prashant N Kumta, and Elia Beniash ACS Biomater. Sci. Eng., Just Accepted Manuscript • DOI: 10.1021/acsbiomaterials.6b00585 • Publication Date (Web): 24 Feb 2017 Downloaded from http://pubs.acs.org on February 26, 2017

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ACS Biomaterials Science & Engineering

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Anticorrosive Self-Assembled Hybrid Alkylsilane Coatings for Resorbable

2

Magnesium Metal Devices.

3 4

Avinash J. Patila,g,h, Olivia Jacksona, Laura B. Fultona, Dandan Honga,g,h, Palak A. Desaib,

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Stephen A. Kelleher c, Da-Tren Choua, Susheng Tand,e, Prashant N. Kumtaa,f,g,h,I,j, Elia

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Beniasha,f,g,h, *

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a

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Benedum Hall, 3700 O'Hara Street, Pittsburgh, PA 15261, USA

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b

Department of Bioengineering, Swanson School of Engineering, University of Pittsburgh, 302

Department of Biological Sciences, Dietrich School of Arts and Sciences, University of

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Pittsburgh, 4249 Fifth Avenue, Pittsburgh, PA 15260, USA

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c

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44074, USA

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d

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University of Pittsburgh, 1238 Benedum Hall, 3700 O'Hara Street, Pittsburgh, PA 15261, USA

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e

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f

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3501 Terrace Street, Pittsburgh, PA 15261, USA

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g

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Sutherland Drive, Pittsburgh, PA15261, USA

21

h

22

Suite 300, Pittsburgh, PA 15219, USA

23

i

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Hall, 3700 O'Hara Street, Pittsburgh, PA 15261, USA

Department of Biology, Oberlin College, Science Center K123, 119 Woodland St, Oberlin, OH

Department of Electrical and Computer Engineering, Swanson School of Engineering,

Petersen Institute for NanoScience and Engineering (PINSE), University of Pittsburgh,

Department of Oral Biology, School of Dental Medicine, University of Pittsburgh, 347 Salk Hall

Center for Craniofacial Regeneration, University of Pittsburgh, 501 Salk Pavilion, 335

McGowan Institute for Regenerative Medicine, University of Pittsburgh, 450 Technology Drive,

Department of Chemical and Petroleum Engineering, University of Pittsburgh, 940 Benedum

1 ACS Paragon Plus Environment


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j

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* Corresponding author at: Elia Beniash, PhD, Department of Oral Biology, School of Dental

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Medicine, University of Pittsburgh, 505 Salk Pavilion, 335 Sutherland Drive, Pittsburgh, 15213,

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PA, USA, Tel.: +1 412 6480108; fax: +1 412 624 6685; E-mail address: [email protected]

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Keywords: Magnesium; AZ31; Resorbable; Implant; Alkylsilane; Coating; Corrosion; Self-

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assembly; Cytocompatibility

Department of Mechanical Engineering and Materials Science, University of Pittsburgh, 636


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Abstract

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Magnesium (Mg) and its alloys are promising candidates for use as resorbable materials for

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biomedical devices which can degrade in situ following healing of the defect, eliminating need

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for second surgery to remove the device. Hydrogen gas is the main product of magnesium

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corrosion and one of the limitations for use of Mg devices in clinic is the formation of gas

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pockets around them. One potential solution to this problem is reducing the rate of corrosion to

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the levels at which H2 can diffuse through the body fluids. The study’s aim was to evaluate the

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potential of hybrid alkylsilane self-assembled multilayer coatings to reduce Mg corrosion and to

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modify physicochemical properties of the coatings using surface functionalization. The coating

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was made by co-polymerization of n-Decyltriethoxysilane and Tetramethoxysilane followed by

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dip coating of metal discs. This resulted in a formation of homogeneous micron thick and defect

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free coating. The coated surface was much more hydrophobic than bare Mg, however

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functionalization of the coating with 3-Aminopropyltriethoxysilane reduced the hydrophobicity of

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the coating. The coatings reduced several folds the rate of Mg corrosion based on the H2

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evolution and other assessment methods, and effectively prevented the initial corrosion burst

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over the first 24 hours. Our in vitro tissue culture studies demonstrated cytocompatibility of the

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coatings. These results reveal excellent anticorrosive properties and good cytocompatibility of

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the hybrid alkylsilane coatings and suggest a great potential for use of these coatings on

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resorbable Mg devices.



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1. Introduction

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Magnesium is a great candidate for use in resorbable devices because of its light weight,

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mechanical properties closely matching natural bone, low toxicity and high corrosion

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characteristics

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formation3-6. Attempts to use Mg as a biomaterial trace back to 19th century when several

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physicians and surgeons were experimenting with Mg for orthopedic, cardiovascular and other

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applications7. However these initial attempts faced limitations since the corrosion reaction of Mg

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produces H2 leading to formation of gas pockets around the device and increases the pH thus

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significantly limiting the applicability of Mg in the biomedical field7. Furthermore, fast,

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uncontrolled degradation can also lead to premature resorption of the devices. In the last couple

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of decades, a renewed interest in biomedical use of Mg has developed, with researchers around

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the world trying to overcome these obstacles. There are two major approaches for control of Mg

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degradation rate, namely developing new alloys with desirable corrosion properties8-9 and use of

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anticorrosive coatings10. In addition to corrosion regulation coatings can be used as controlled

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release systems

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improve tissue regeneration around coated devices13-14. A number of different chemistries are

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used for anticorrosive coatings, including organic polymers15-16, inorganic minerals

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peptides19 and polysiloxanes 20-23 .

1-2

. Moreover, Mg2+ can activate osteogenic differentiation and promote bone

11-12

and surface modifications of the coatings with bioactive molecules can

17-18

,

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Use of aliphatic alkylsilanes (AS) provides means to introduce structural organization

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into the coating through self-assembly allowing for greater control of the coating properties and

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increased functionality of the coatings

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with general formula CnH2n+1(SiOEt)3 can form self-assembled multilayered films on solid

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surfaces. CnTES molecules become amphiphilic upon hydrolysis and self-assemble into

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multilayered films comprising 3-4 nm thick lamellae, with alkyl tails of these amphiphilic

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molecules forming hydrophobic cores and silane heads undergo polycondensation, leading to

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formation of crosslinked polysiaxane planes sandwiching the hydrophobic cores

24-27

. ASs, and specifically alkyltriethoxysilanes (CnTES)


24-26

. The

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properties of the film can be modified by changing the length of the alkyl tails, by adding UV

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cross-linkable moieties

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(TEOS) or tetramethoxysilane (TMOS)

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can be achieved using well developed chemistries28-29. Another advantage of these coatings is

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that they comprise biocompatible and biodegradable compounds such as hydrocarbons (fat)

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and polysiloxanes, which are widely used as biomaterials30-33.

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or by co-polymerizing with other siloxanes, such as tetraethoxysilane 24

. Furthermore, surface functionalization of these films

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The aim of this study was to design anticorrosive hybrid self-assembled AS coatings for

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implantable Mg devices. We assessed compositional, structural and anticorrosive properties of

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the coatings, conducted surface functionalization of the coating and tested cytocompatibility of

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the coatings in vitro in tissue culture.

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2. Materials and Methods

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All the reagents were purchased from Sigma Aldrich (St. Louis, MO, USA) and used as

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received unless otherwise stated.

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2.1. Metal sample preparation.

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Six mm diameter discs were stamped from 1 mm thick Mg (99.9 % purity) and AZ31alloy

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(96% Mg, 3% Al, and 1% Zn ) sheets (Alfa Aesar, Ward Hill, MA, USA) and polished with 1200

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grit (5µm) MicroCut® SiC abrasive discs (Buehler Inc., Lake Bluff, IL, USA). The polished discs

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were then etched in etching solution comprising 20 ml 85% Glycerol, 5 ml 65% HNO3 and 5 ml

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glacial acetic acid for 60 seconds. Chemical etching was done to remove debris, impurities and

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the oxide layer from the surface of the metal and to smooth the scratches introduced during

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polishing. Etched discs were sonicated in acetone for 30 minutes and stored under vacuum until

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further use. Prior to the alkylsilane coating, a thin uniform hydroxide layer was formed on the

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discs by immersion in 3.0 M Sodium Hydroxide (NaOH) solution for 2 h. In addition to the

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passivating properties, MgOH2 provides means for covalent binding of the silanes to the metal

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surface as described elsewhere21, 34-35.



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2.2. Synthesis and deposition of self-assembled multilayer AS coating on Mg and Mg

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alloys

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Self-assembled hybrid alkylsilane (AS) films on Mg and AZ31 alloys were prepared by a

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dip-coating technique as described elsewhere24-26. The precursor solution was prepared by

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mixing 0.25 ml (0.73 mM) n-Decyltriethoxysilane (DTEOS) (Alfa Aesar, Ward Hill, MA, USA),

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0.43 ml (2.92 mM) Tetramethoxysilane (TMOS) (Alfa Aesar, Ward Hill, MA, USA), 2 ml (0.032

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mM) of Ethanol and 0.25 ml 0.010 mM HCl (aq.). The precursor solution was stirred for 24 h at

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room temperature to induce hydrolysis of DTEOS and TMOS. Mg or AZ31 alloy discs

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passivated with NaOH were dip coated in the solution for 1 minute and dried in air for 10

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minutes at room temperature. The discs were subsequently dried in an incubator at 37℃ for 24

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h for removal of any trace amount of organic solvents.

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2.3. Functionalization of the coatings.

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For functionalization with 99% pure 3-Aminopropyltriethoxysilane (APTES), a following

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solution was prepared: 0.25 mL of 0.01 M HCl was added to a mixture of 2.0 ml ethanol (EtOH)

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and 0.43 ml APTES solution. This precursor solution was stirred for 24 h at room temperature

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prior to functionalization. The AS coated samples were then dipped in the APTES solution at

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room temperature for 1 minute and dried in air for 10 minutes at room temperature. The disc

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was subsequently dried in an incubator at 37℃ for 24 h for removal of any trace amount of

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organic solvents.

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Four groups of samples were used in this study as follows:

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•

Polished and etched Mg and AZ31 discs (Mg, AZ31)

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•

NaOH treated Mg and AZ31 discs (Mg-OH; AZ31-OH),

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•

AS coated NaOH treated Mg and AZ31 discs (Mg-OH-AS; AZ31-OH-AS)

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•

APTES functionalized AS coated Mg and AZ31 discs (Mg-OH-AS-APTES; AZ31-OH-

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AS-APTES).


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2.4. Scanning Electron Microscope (SEM) characterization

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The surface morphology of the uncoated and AS coated Mg and AZ31 discs were

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studied using JSM-6330F (JEOL, Peabody, MA, USA) SEM at 3.0-20.0 kV operating voltage

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and the working distance of 10 mm. All samples were sputter-coated with gold prior to SEM

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using Cressington sputter coater 108 auto model (Cressington Scientific Instruments Ltd,

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Watford, United Kingdom). The elemental composition of the non-coated and coated samples

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were analyzed using energy dispersive X-ray spectroscopy (EDS) in SEM. EDS INCA analysis

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system (Oxford Instruments, Oxfordshire, UK) equipped with a beryllium-window protected Si

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(Li) detector was used for the elemental analysis at the operating voltage of 15 kV.

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To assess the thickness of the coating a several micron deep trough was created in

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some samples using the focused ion beam milling (FIB). The ion milling and imaging was done

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on a FEI SCIOS Focused Ion Beam/Scanning Electron Microscope dual beam system with

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gallium ion source (Thermo Fisher Scientific, Waltham, MA, USA). Au/Pt depositions were

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carried out before the ion milling to protect the coating from the gallium ion beam.

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2.5 Atomic Force Microscopy (AFM) characterization.

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High resolution AFM was used to assess the thickness of individual lamellae by

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scanning the surface of AS coatings deposited on AS coated Mg films deposited on glass

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substrates (generous gift from Dr. Sergey Yarmolenko, NCAT, Greensboro, NC). Use of Mg

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films instead of polished disks significantly reduced the roughness of the samples, allowing high

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resolution AFM imaging of the samples. The AFM analysis was carried out using a Veeco

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Dimension V scanning probe microscope (SPM) controlled by a Nanoscope V controller (Bruker

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Corporation, Billerica, MA, USA). Tapping mode was employed with a AFM probe of 58 – 80

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kHz nominal resonance frequency and ~3 N/m spring constant. The acquired images were

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flattened using the Nanoscope V software. Measurements were carried out on the flattened

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images.



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2.6. Attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy

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characterization

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The AS coatings on the Mg and AZ31 substrates were analyzed by Vertex-70 ATR-FTIR

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(Bruker Optik GmbH, Ettlingen, Germany) using a diamond Miracle ATR accessory (Pike

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Technology, WI, USA). Spectra were obtained at 4 cm-1 resolution averaging 120 scan in the

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400-4000 cm-1 frequency range.

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2.7. Contact angle measurement

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The wettability of the AS coated Mg and AZ31 discs were assessed by the sessile drop

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(5 µl, Milli-Q water) contact angle measurement using VCA 2000 Goniometer (AST Products,

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Billerica, MA, USA).

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2.8. In vitro corrosion analyses.

163 164

The corrosion resistance of AS coated Mg and AZ31 discs was measured using hydrogen evolution method, weight loss assessment and electrochemical corrosion tests.

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Hydrogen evolution test. The H2 evolution was measured in a simulated body fluid (SBF)

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pH 7.2, prepared according to a published formulation by Kokubo36 (Supplementary Table 1), at

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room temperature over 24 hrs and 7-day period. Each sample group was prepared in triplicate

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and the samples were immersed in SBF. A glass graduated measuring cylinder was placed over

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the samples in a glass beaker to collect the resealed gases (Supplementary Figure 1). The

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experiments were carried out for 1 week and the solution was changed on day 2, 4 and 6. The

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ratio of solution volume to the total surface area of the tested samples was kept above 6.7 to

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mimic in-vivo degradation behavior of Mg

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constant throughout the study to avoid evaporation effects affecting the validity of the

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measurements.

175

19, 37-38

. The SBF level in each beaker was maintained

Weight loss analysis. The mass loss analysis after 7 days of degradation in SBF at room

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temperature was performed according to ASTM G1 standards and works by Lorking

177

Bobe et al.

40

39

and

. Prior to corrosion experiments all the samples were weighted using a


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microbalance Mettler Toledo XPE26 (Mettler Toledo, Columbus, OH, USA). After experiments

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the corroded sample discs were removed from SBF and placed in a beaker containing an

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aqueous solution of CrO3 at a concentration of ~180.0 g/L. The beaker was sonicated for 10 min

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to remove the corroded products in the chromic acid solution. The chromic acid solution was

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changed between samples to ensure complete removal of the corroded products. After chromic

183

acid cleaning, the sample discs were rinsed with deionized water followed by absolute ethanol.

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The sample discs were completely dried in a vacuum desiccator overnight and weighed. The

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percentage weight change was calculated for all the samples.

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Electrochemical corrosion test. Electrochemical measurements of the non-coated Mg

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and AS coated Mg and Mg alloy samples were performed using a CH604A electrochemical

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workstation (CH Instruments Inc, Austin, TX, USA). All the samples were prepared as described

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above (sections 2.1. and 2.2.); one side of each sample disc was connected to an electric wire

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with conductive silver-containing epoxy and then electrically insulated with an epoxy resin, such

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that a working area of 0.283cm2 was exposed to the electrolyte for electrochemical testing. A

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platinum wire was used as the counter electrode and an Ag/AgCl electrode (4.0 M KCl,

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Accumet,

194

measurements were carried out in a three neck water circulated jacket flask (Ace Glassware,

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NJ, USA) filled with 120 ml of SBF and maintained at room temperature. Each sample was

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immersed in the SBF and then the stable open circuit potential (OCP) was monitored for 10

197

min., before starting the potentiostatic polarization test at a scan rate of 1.0mV s-1. The CHI

198

604A software program was used to perform the analysis and plot the data. The corrosion

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current density (Icorr) and corrosion potential (Ecorr) were determined by the Tafel extrapolation

200

method 41-42.

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2.9. Cell culture experiments and cytocompatibility tests.

Fisher

Scientific,

MA,

USA)

as

the reference electrode.

Electrochemical

202

Tissue culture experiments on AS coated Mg discs. MC3T3 cells were seeded on Mg-

203

OH-AS and Mg-OH-AS-APTES discs (n=3 per group) with the seeding density of 10,000 cells



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cm-2 and cultured for 15 days in phenol red free α-MEM medium (Catalog number: 41061-029,

205

Thermo

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penicillin/streptomycin at 37°C and 5% CO2. The medium was changed every 3 days. After 15

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days, cells were stained with Hoechet 33342 NucBlue Live Ready Probes nuclear stain

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(Thermo-Fisher Scientific, Waltham, MA, USA) and ActinGreen™ 488 ReadyProbes® f-actin

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stain (Life Technologies, Carlsbad, CA, USA) following the manufacturer’s protocols. After three

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washes in PBS, samples were studied using a Nikon TE2000 microscope (Nikon Instruments

211

Inc., Melville, NY, USA) in epifluorescence mode using a DAPI filter for nuclear DNA staining

212

and FITC filter for F-actin staining. Micrographs were captured and processed using Nikon NIS

213

Elements software.

Fisher

Scientific,

MA,

USA)

supplemented

with

10%

FBS

and

1%

214

Cell density on Mg-OH-AS, Mg-OH-AS-APTES discs after 15 days in tissue culture was

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calculated from integrated DAPI fluorescence intensity in 3 epifluorescence micrographs per

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group. Each micrograph contained 53.49×104 µm2 (731.42 µm×731.42 µm) field of view. A blue

217

channel was isolated from RGB images using ImageJ image processing package (ImageJ,

218

Bethesda, MD). The grayscale of each image was adjusted to minimize the background

219

intensity. In each micrograph 10 nuclei were randomly selected and their integrated intensity

220

values were measured. The average signal intensity per nucleus were calculated for each

221

image, and the integrated intensities of the images were divided by corresponding average

222

nucleus intensities, providing us with the total cell number per image. Cell density on the discs

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was presented as a number of nuclei per 1x104 µm2.

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Tissue culture experiments on AS coated glass coverslips. In a separate set of

225

experiments MC3T3 cells were seeded on bare glass coverslips (CS), AS coated coverslips

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(CS-AS) and AS coated coverslips functionalized with APTES (CS-AS-APTES). The cells were

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seeded at density of 10,000 cells cm-2 and allowed to grow for 2 weeks in phenol red free α-

228

MEM supplemented with 10% FBS and 1% penicillin/streptomycin at 37°C and 5% CO2.


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The cells were counted using the method described in the previous section. In addition,

230

the number of cells was calculated using a spectrophotometric method. The amount of DNA

231

per coverslip was assessed at 7th and 14th day in culture by spectrophotometric assay using

232

NanoDrop 1000 (Thermo-Fisher Scientific, Waltham, MA, USA) in each experimental group in

233

triplicates. The number of cells per coverslip was calculated using a conversion factor of 7.6 pg

234

of DNA per cell 43. Cell density was calculated as a number of nuclei per 1x104 µm2.

235

AlamarBlue fluorescence assay (Thermo-Fisher Scientific, Waltham, MA, USA) was

236

used to monitor cell viability44. AlamarBlue assays were conducted at 1st, 3rd, 7th and 14th day in

237

triplicates according to the manufacturer’s protocol. In brief, the culture medium was replaced

238

with 1 ml 10 % v/v almarBlue reagent in complete growth medium and the samples were

239

incubated for 4 hrs at 37ºC. After the incubation, the alamarBlue solution was collected and

240

replaced with regular medium and the tissue culture samples were allowed to grow till the next

241

time point. This design allowed us to monitor the metabolic activity in individual samples over 2-

242

week period.

243

separate readings measured at excitation/emission wavelengths of 530/590 nm using a Synergy

244

H1 microplate reader (BioTek instruments, Winooski, VT, USA). The wells containing only

245

alamarBlue solution in medium were used as controls for background fluorescence correction.

Fluorescence intensity of each sample was calculated as an average of 3

246

The cells grown for 1, 3, 5 and 7 days on all three experimental substrates were studied

247

using SEM. The samples were washed twice with phosphate buffered saline (PBS), fixed in

248

2.5% glutaraldehyde in 0.1 M PBS (pH 7.4) for 10 minutes and washed thoroughly with 0.1 M

249

PBS for 3x15 min. The samples were further fixed in 1% OsO4 in 0.1 M PBS for 60 minutes and

250

washed thoroughly with 0.1 M PBS for 3x15 min. Each sample was dehydrated in a graded

251

ethanol series (concentrations 30%, 70% and 100%) for 3x15 min. After dehydration, samples

252

were immersed in Hexamethyldisilizane (HMDS) solution for 15 min and subsequently air-dried

253

overnight and sputter coated with gold prior to SEM using Cressington sputter coater 108 Auto

254

(Cressington Scientific Instruments Ltd, Watford, UK). The SEM analysis was conducted using



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JSM-6330F (JEOL, Peabody, MA, USA) at 3.0-20.0 kV operating voltage and the working

256

distance of 10 mm.

257

2.10. Statistical analysis

258

All the experiments were performed in triplicates for each experimental group and time

259

point. The data were compared between groups using analysis of variance (ANOVA) followed

260

by Turkey test with 95% confidence level in Origin Pro 2015 software (OriginLabs,

261

Northhampton, MA). The data were presented as mean ± standard deviation (S.D.).

262

3. Results.

263

3.1. SEM characterization

264

SEM characterization of the coatings was conducted to assess the structural integrity of

265

the AS films (Figure 1). Analysis of the polished and etched Mg surface revealed individual

266

grains, grain boundary contours and the shallow scratch marks produced by polishing (Figure

267

1A). At the intermediate magnification, the alkali treated surface appeared very similar to the

Figure 1. SEM micrographs of A) uncoated Mg, B) Mg-OH, B’) a high magnification micrograph of a Mg-OH surface, C) Mg-OH-AS; D) Mg-OH-AS-APTES; E) low magnification Mg-OH-AS scratched surface; and F) a higher magnification image of a Mg-OH-AS scratched surface with pieces of AS coating peeling off the surface.


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268

bare Mg surface (Figure 1B). However, a nano-structured hydroxide layer on the Mg surface

269

was clearly recognizable at a higher magnification (Figure 1B’). Dip-coating procedure produces

270

a micron thick uniform AS film on the Mg-OH disc (Figure 1C). Interestingly, because the AS

271

films are extremely thin, the grain boundaries of the underlying metal were clearly visible.

272

APTES functionalized AS coated discs had similar appearance to AS coated discs (Figure 1D).

273

The grain boundaries and scratch marks were still visible on this sample although they were

274

less pronounced, suggesting that the coating followed the surface topography. Scratching of the

275

coated samples with a scalpel did not let to major defects of the coating away from the

276

scratched mark (Figure 1E). Analysis of the broken coating in the scratched area allowed us to

277

determine the thickness of the coating by measuring the pieces of the coating detached from

278

the metal surface (Figure 1F). Based on this analysis the thickness was approximately 1 µm. To

279

further assess the coating thickness a several µm deep trough was created in the coated

280

samples using FIB. The analysis of the sample cross-sections showed that the coating is ~1 µm

281

thick (Supplementary Figure 2).

282

3.2 AFM Analysis.

283

To assess the thickness of individual AS lamellae comprising the coating we have conducted

284

the AFM study of the AS coated Mg films. Edges of multiple lamellae are exposed on the

285

surface creating step like patterns (Supplementary Figure 3). By measuring the height of these

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steps an average thickness of the lamellae was determined to be 2.8 nm (SD=0.6; n=9) which is

287

in a good agreement with the lamellae thickness data reported elsewhere24, 26.

288

3.3 ATR-FTIR Analysis.

289

ATR-FTIR spectroscopy was used to analyze the chemical composition of the coatings

290

on the Mg surface. Figure 2A shows the spectra of non-coated and AS coated Mg discs.

291

Polished and etched Mg disc did not show any major absorbance peaks as expected (Figure

292

2A), while alkali treated discs presented a sharp peak around 3600 cm-1 consistent with the -O-

293

H stretching band. The spectra of Mg-OH-AS coating reveal peaks in the 1045 -1127 cm-1 that



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correspond to Si-O asymmetric stretching in –Si-O-Si- bond (Figure 2A). The C–H stretching in

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alkyl chain of DTEOS is evident around 3000–2850 cm-1. After amination with APTES, -NH2

296

absorption band appeared in 1500-1700 cm-1region indicating the presence of amines on the

297

surface of the coating. Results of the experiments with AZ31 samples were similar

298

(Supplementary figure 4A).

299

3.4 Contact angle measurement

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The surface weatability of non-coated Mg, alkali treated Mg and AS coated Mg were

301

examined by measuring the static contact angle using deionized water droplet. The contact

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angle of polished and etched Mg discs was approximately 58.98± 11.78º (Figure 2B). Alkali

303

treatment of Mg surface made it more hydrophilic and significantly reduced the contact angle

304

down to 21.62± 1.87º (p ≤ 0.05). The static contact angle of AS coated samples was110.60±

305

1.05º, indicating the hydrophobic nature of these samples. The contact angle of these samples

306

was significantly higher than both Mg (p ≤ 0.01) and Mg-OH samples (p ≤ 0.0001). Amination of

Figure 2. A) ATR-FTIR spectra of Mg (a), Mg-OH (b), Mg-OH-AS (c), Mg-OH-AS-APTES (d) and B) box graph of static water contact angles measured on Mg sample discs.


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the AS layer with APTES significantly reduced the contact angle of the surface to 67.44± 22.43º

308

(p ≤ 0.01), and made it similar to the bare Mg discs (P > 0.05). The results of contact angle

309

analysis on AZ31 samples showed similar trends (Supplementary Figure 4B.)

310

3.5 Determination of the corrosion rate by H2 evolution and weight loss methods.

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Cumulative H2 released from the uncoated Mg discs over 7 days was ~7 fold higher than

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that of AS coated Mg discs; this difference was highly significant (p0.9974) (Figure 3A, Supplementary Table 2). Experiments with

316

AZ31 samples yelded similarr results (Supplementary Figure 5A, Supplementary Table 2).

Figure 3. Cumulative hydrogen release profiles from Mg discs incubated in SBF over 7 days period (A), 24 hours period (B); C) weight loss of Mg discs incubated in SBF for 7 days; and D) Potentiodynamic polarization curves of Mg sample discs.



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To assess the ability of the AS coating to slow down the initial burst of corrosion we have

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followed H2 evolution over the initial 24 hours after immersion of the discs in SBF. The results of

319

these experiments indicate that the AS coating reduced the initial Mg corrosion rate 5 fold from

320

0.16 ml/h to 0.03 ml/h effectively preventing the initial burst of corrosion (Figure 3B,

321

Supplementary Table 2).

322

After the initial 24 hours, the corrosion rate of non-coated samples significantly

323

decreased and achieved steady state (Figure 3B, Supplementary Table 2). This decrease in the

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corrosion rate can be potentially attributed to depostion of corrosion products, which can have a

325

passivating effect. Nevertheless, the

326

corrosion rate of the AS coated Mg

327

over the following 6-days period was

(Icorr) values for non-coated and coated Mg and AZ31

328

4.5 times lower than that of the bare

substrate.

329

Mg, 0.013 ml/h vs. 0.06 ml/h,

Treatment

Ecorr(V)(SD)

330

(Figure 3A, Supplementary Table 2). Mg bare

-1.77 (0.014) 171.97 (26.40)

Mg-OH

-1.75 (0.027) 151.84 (31.78)

Mg-OH-AS

-1.78 (0.020) 10.85 (3.93)

Mg-OH-AS-APES

-1.80 (0.038) 3.66 (2.56)

AZ31 bare

-1.49 (0.009) 96.43 (65.90)

AZ31-OH

-1.54 (0.032) 100.84 (50.26)

AZ31-OH-AS

-1.62 (0.014) 6.06 (1.13)

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The results of the experiments with

332

AZ31 were very similar to those

333

obtained with Mg (Supplementary

334

Figure

335

Table 2).

5B,

and

Icorr (µA/cm-2) (SD)

Supplementary

336

The results of the weight loss

337

measurements were in an excellent

338

agreement

339

release experiments. Both Mg and

340

Mg-OH

341

significantly higher % weight loss

with

Table 1. Corrosion potential (Ecorr) and current density

the

samples

hydrogen

showed

AZ31-OH-AS-APES -1.59 (0.006) 4.07 (3.04)


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342

compared to Mg-OH-AS and Mg-OH-AS-APTES samples (Figure 3C). The weight loss of the

343

Mg-OH was slightly but significantly smaller than in Mg group. Weight loss measurements of

344

AZ31 samples also yelded similar results (Supplementary Figure 5C).

345

3.6 Determination of the corrosion rate by Potentiodynamic polarization and Impedance

346

spectroscopy technique. The average corrosion potential (Ecorr) measured for the untreated

347

Mg discs was -1.77 V and it was not significantly different from Mg-OH (p=0.81), Mg-OH-AS

348

(p=0.93) and Mg-OH-AS-APTES (p=0.55) samples (Figure 3D, Table 1). A slightly more

349

negative value of Ecorr was observed for both Mg and AZ31 coated samples suggesting a weak

350

dependence of the thermodynamic corrosion potential on the coating system and the nature of

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the coated films. However, the potentiodynamic polarization curves for the coated Mg samples

Figure 4. SEM images showing the surface morphologies of A) uncoated Mg, B) Mg-OH, C) Mg-OH-AS, D) Mg-OH-AS-APTES after 1-week incubation in SBF. All micrographs are taken at the same magnification.



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were shifted to lower Icorr values compared to pure Mg indicative of the changes in corrosion

353

kinetics. Passivation of the Mg discs with alkali did not seem to show а significant change

354

(p

Anticorrosive Self-Assembled Hybrid Alkylsilane ... - ACS Publications

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