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Effect of extreme wettability on platelet adhesion on metallic implants: from superhydrophilicity to superhydrophobicity Sona Moradi, Narges Hadjesfandiari, Salma Fallah Toosi, Jayachandran N Kizhakkedathu, and Savvas G. Hatzikiriakos ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b03644 • Publication Date (Web): 20 Jun 2016 Downloaded from http://pubs.acs.org on June 26, 2016

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ACS Applied Materials & Interfaces

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Effect of Extreme Wettability on Platelet Adhesion on Metallic Implants: from Superhydrophilicity to Superhydrophobicity Sona Moradi1, Narges Hadjesfandiari2,3, Salma Fallah Toosi1, Jayachandran N. Kizhakkedathu2,3,*, and Savvas G. Hatzikiriakos1, * 1

Department of Chemical and Biological Engineering, University of British Columbia, Vancouver, BC, V6T-1Z3, Canada.

2

Centre for Blood Research, Pathology and Laboratory Medicine, Life Science Centre, University of British Columbia, Vancouver, BC, V6T-1Z3, Canada. 3

Department of Chemistry, University of British Columbia, Vancouver, BC, V6T-1Z1,Canada.

Abstract In order to design anticoagulant implants, the effect of extreme wettability (superhydrophilicity to superhydrophobicity) on the biocompatibility of the metallic substrates (stainless steel and titanium) was investigated. The wettability of the surface was altered by chemical treatments and laser ablation methods. The chemical treatments generated different functionality and chemical composition as evident from XPS analysis. The micro/nano patterning by laser ablation resulted in three different pattern geometry and different surface roughness and consequently wettability. The patterned surface was further modified with chemical treatments to generate a wide range of surface wettability. The influence of chemical functional groups, pattern geometry and surface wettability on protein adsorption and platelet adhesion was studied.

On chemically treated flat surfaces, the type of hydrophilic

treatment was shown to be a contributing factor that determines the platelet adhesion, since the hydrophilic oxidized substrates exhibit less platelet adhesion and protein adsorption in comparison to the control untreated or fluorinated surfaces. Also, the surface morphology, surface roughness and superhydrophobic character of the surfaces are contributing factors to platelet adhesion on the surface. Our results show that superhydrophobic cauliflower-like patterns are highly resistant to platelet adhesion possibly due to the stability of Cassie-Baxter state for this pattern compared to others. Our results also show that simple surface treatments on metals offer a novel way to improve the hemocompatibility of metallic substrates. Keywords: Superhydrophobicity, Metallic implants, Chemical treatment, Surface modification, Platelet adhesion, Blood compatibility *

Corresponding authors: [email protected]; [email protected] ACS Paragon Plus Environment


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2 1. Introduction Stainless steel (SS) and titanium (Ti) both possess a number of advantages compared to other metals, ceramics, and polymers as materials for manufacturing biomedical implants; namely satisfactory corrosion resistance, wear resistance, and high strength and toughness1–5. Metallic implants for long-term intravascular use, such as heart valves, blood pumps, and pacemaker leads, as well as for temporary intravascular use, such as catheters and guide wires are a few examples of biomaterials made from SS and Ti that are brought into contact with blood. SS and Ti are also the most common metals being used for balloon–extendable and self-extendable stents respectively6. Despite the extensive use of SS and Ti as blood-contacting biomaterials, the surface of implants made of these two metals suffers from the adsorption of blood proteins, adhesion and activation of platelets resulting in thrombus formation, and inflammatory responses upon contact with blood 2–4. The interaction of proteins occurs immediately after the implant comes in contacts with the blood, and then platelets attach to the surface resulting in the activation of blood coagulation system further contributing to the thrombus generation. In order to minimize these drawbacks and render the SS and Ti implants surfaces more biocompatible for blood-contacting applications, a number of surface modification strategies were designed and applied

7,8

as biocompatibility is mainly determined by the

surface properties of the implants. The chemistry of the implant surface as well as its physicochemical properties such as wettability or surface energy, and surface topography were found to be important factors that dictate protein adsorption, and platelet adhesion and activation

9–12.

Regulation of platelet

adhesion onto the surface of a blood contacting implant is a key aspect in the field of biomedicine and tissue engineering13–15. Surface chemistry is an important characteristic in designing implants as it determines the available functional groups on the surface that contribute to the interaction of biomolecules that are exposed to. The most direct way to affect protein and cell interaction behavior is to modify the surface

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3 chemistry with functional groups (e.g. -CH3, -NH2, -OH, -COOH), peptide motifs or proteins that are known to interact with certain cell-surface structures, and accordingly trigger specific behaviors

15,16

.

By tailoring the available functional groups on the implant surface, the surface free energy and consequently wettability, and surface electrical charges also can be modified 9,12,17. In addition to the chemistry, the topography of the surface is also a key factor affecting protein and cell interactions8,18,19. Most of the implant surfaces intended for biomedical applications typically contain some complex topographical patterns8. For instance, in some biomedical applications such as bone remodeling, drug release, bio-separation and bio-detection applications, a rough surface is desired as a substrate with more topographical features offer more surface area for possible cell and protein interactions

20,21

. Topographical and chemical modifications of the surfaces change the surface

wettability. Introducing roughness makes a hydrophobic surface more hydrophobic (superhydrophobic), and a hydrophilic surface more hydrophilic (superhydrophilic)22. Superhydrophobic surfaces exhibit water contact angle higher than 150° and contact angle hysteresis less than 5° (the difference between the advancing and receding contact angles). The poor wettability of such surfaces is mainly attributed to the combined effect of low surface energy materials and dual-scaled micro/nano-asperities. Studies on blood–material interactions on surfaces with varied wettable properties have largely been focused within the hydrophilic–hydrophobic range (20° < CAs < 110°)

23

. However, curiosity

regarding the biological response beyond such contact angle intervals has been encouraging the development of new approaches in order to obtain further insights into the interaction of blood with surfaces possessing extreme wettability properties especially superhydrophobic ones. The studies related to the biological integration of superhydrophobic surfaces are still in the early stages and few studies have been published reporting the interactions of proteins or blood/cells with such micro/nanostructured highly repellent surfaces particularly for metallic surfaces24–30. The superhydrophobicity can



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4 be a potential solution to increase the biocompatibility of the temporary metallic (and polymeric surfaces) medical tools such as guide wires, catheters and surgery tools. In this research we report on the effect of extreme surface wettability (from superhydrophilicity to superhydrophobicity) on platelet adhesion and activation in SS and Ti surfaces. By tailoring the surface chemical composition, surface roughness and, consequently, the wettability constitutes a possible strategy to achieve adequate properties for this purpose. More specifically, different surface chemistries (coatings) and surface micro/nano structures (superhydrophilic and superhydrophobic) have been applied to investigate their effects on protein adsorption and platelet adhesion on metallic SS and Ti substrates.

2. Materials and Methods Stainless steel 316L and pure Ti (99.6 %) samples 1 mm in thickness were used as substrates in this work. They were polished using sandpaper to an average roughness value ( Ra ) of 500 nm. 2.1 Surface Modification 2.1.1 Surface patterning via laser ablation One of the promising methods to create durable micro-nano structures on the metallic surface is laser ablation

31–33

. In order to create various surface morphologies on the surface, ultrashort laser pulses

were generated by an amplified all solid-state Ti:Sapphire laser. Outcome of amplifier was femtosecond laser pulses with center wavelength of 800 nm and Gaussian distribution. The repetition rate of laser pulses was 1 kHz with pulse duration of 120 fs and the maximum output power of about 2W. A set of neutral density (ND) filters were used to attenuate and adjust the energy of the laser beam with spot size of 30 µm at the focal point. In order to translate the samples under the laser beam, the samples were mounted on a precise, computer-controlled ZABER T-LS80 X-Y translation stage. The power of the incident laser beam was adjusted in the range of 5 to 1700 mW (peak fluence: 1.5 to 480 J .cm −2 ) and the scanning


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5 speed range varied from 250 to 1850 µ m.s −1 . The samples were irradiated at normal incidence in air and then subjected to an ultrasonic bath for 2 min in acetone to remove all the debris off the patterned surface. More details can be found elsewhere 31. 2.1.2 Modification of surface chemistry-acid treatment To modify the surface functional groups, a mixture of hydrogen peroxide (H2O2, 30%, SigmaAldrich) and sulfuric acid (H2SO4, 98%, Sigma-Aldrich), Piranha solution ( H2 SO4 / H2O2 , 4:1 v/v), which is a strong oxidizing agent, was utilized on the flat and patterned Ti and SS surfaces for 1 hour at room temperature. It should be noted that piranha solution is extremely reactive and as such should be handled with great care. This treatment removes any organic contaminations and generates hydroxyl groups on the metallic substrates to make it superhydrophilic (CA < 10°). After acid treatment, the surfaces were rinsed with ethanol, water and acetone respectively for 10 minutes in ultrasonic bath. 2.1.3 Heat treatment To investigate the effect oxidation of metallic substrates on platelet adhesion, Ti and SS sheets, both flat and patterned ones, were heated at 700°C in a preheated furnace for 30 min. 2.1.4 Hydrophobic coatings In order to render the surfaces hydrophobic and superhydrophobic, two types of chemical treatments have been performed on the laser-irradiated structures. The first and common method is coating the substrate with low surface energy materials such as alkyl fluorosilane. Here, trichloro (1H,1H,2H,2H-perfluorooctyl) silane, FTS (97%, Sigma-Aldrich, USA) was deposited on the sample surfaces using a dip coating method. The silanization was performed following acid-treatment by immersing the samples in 0.5% wt FTS in n-hexane (0.075 gr per 10 mL of n-hexane) for 1 h at 60°C. Finally, the samples were cured to 120° C for 1 hr to dry the samples and stabilize the coating 34. The second method used was exposing the laser-ablated patterns in CO2 gas for two days right after laser irradiation. Due to existence of some activated sites on the laser-induced metallic substrates, ACS Paragon Plus Environment


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6 and by absorbing and reacting carbon in air or CO2 gas, the surface energy decreases and the dualscaled patterns evolve to slowly become superhydrophobic ones 35. 2.2 Evaluation of Biocompatibility 2.2.1 Protein Adsorption Test To evaluate the protein adsorption, Bovine Serum Albumin conjugated to fluorescent tags (BSA, Sigma Aldrich) and Fibrinogen from human plasma conjugated to fluorescent tags ((Alexa-fluor 594 conjugate (Invitrogen, F13193)) were chosen as model protein. All of samples were placed into 6-well tissue culture plates (Corning Inc.). To equilibrate the surface, 2 ml of phosphate-buffered solution (PBS, pH 7.4) was added into each well, which was then allowed to stand for 30 min. After removing the PBS, the samples were incubated in 1mg ml-1 BSA in 200 mM PBS for 4 hrs at 80 rpm and 37°C in a humidified 5% CO2 atmosphere. Then the samples were gently washed with PBS three times and dried in the air. The same protocol has been used for Fibrinogen adsorption test, simply instead of BSA, 0.25 mg ml-1 Fibrinogen in 0.1M sodium bicarbonate buffer (pH=8.4) was utilized.

2.2.2 Platelet Adhesion The interaction of human platelets on various surfaces in vitro was performed using platelet-rich plasma (PRP) prepared from fresh human whole blood. The fresh venous blood was collected from two unmedicated healthy adult volunteers after obtaining their informed consent. The protocol was approved by the University of British Columbia clinical ethical committee. PRP was prepared by centrifuging citrate anticoagulated (using sodium citrate:blood 1:9) blood at 150 g (Allegra X-22R Cengtrifuge, Beckman Coulter) for 15 min at room temperature. All modified and unmodified metallic samples were placed into 6-well tissue culture plates. Prior to the addition, the samples were equilibrated with 2 mL of PBS (pH 7.4) for 30 min. After removing the PBS, the samples were immersed in PRP with continuous shaking (80 rpm) for 4 hrs at 37 °C. Then the ACS Paragon Plus Environment

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7 samples were taken out, and gently rinsed by PBS for three times to remove the loosely attached and nonadherent platelets. To fix the surface-adhered platelets, samples were immersed in glutaraldehyde solution (2.5 v/v% solution in PBS, prepared from glutaraldehyde solution 25%, Sigma-Aldrich) for 1 h and washed by PBS. Then, the samples were incubated with fluorescently labeled antibody, monoclonal antiCD42a-FITC (1% v/v solution in PBS, prepared from IOSTest CD42a-FITC conjugated antibody, Beckman Coulter Inc.), and incubated at 37°C for 1 hr. At the end of the incubation, the samples were rinsed with deionized water three times and were dried for the scanning electron microscopy (SEM) and fluorescence microscope observations.

2.3 Surface Characterization The morphology of the surface structures was studied with a variable-pressure Scanning Electron Microscope (Hitachi S-3000N SEM). Samples with adhered platelets were sputter coated with gold prior to observation by SEM. The physicochemical properties of the various surfaces were studied by X-ray photoelectron spectroscopy (XPS) using monochromated AlKR radiation. The X-ray source was operated at 12 mA and 15 kV. The binding energy (BE) scales were referenced to 285.0 eV as determined by the locations of the maximum peaks on the C1s spectra of hydrocarbon (CHx), associated with adventitious contamination. The wetting behavior of the samples was evaluated by measuring water contact angle through sessile drop method on the surface; dispensing a water droplet (with 5µL volume) on the respective surfaces and analyzing the droplet images with image processing methods (here: using MATLAB). The contact angle measurements have been done 5 times for each surface and the standard deviation errors of the reported contact angle values have less than 2°. In order to quantify the platelet adhesion and protein adsorption on the surface, three random images were taken from each sample using a Fluorescent light microscope with FITC filter (Zeiss



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8 Axioskop 2 plus, Carl Zeiss Microimaging Inc.). The adherent platelets and adsorbed protein were quantified based on the surface fluorescent light intensity of images using ImageJ software. Results were normalized to the flat untreated metallic (Ti and SS) control substrates for comparison.

3. Results and Discussion 3.1 Surface Chemistry and Morphology Analysis Laser ablation and chemical modification have resulted in samples with different wettability and morphologies. As shown in Figure 1, depending on the laser ablation parameters, three types of micro/nano structures have been produced on metallic surfaces, namely paraboloidal, triple and cauliflower-like structures32. The first structure is a pillared morphology with paraboloidal shapes shown in Figure 1(a) created with laser beam peak fluence, Φ o , of 20 J/cm2 and scanning speed V, of 465 µm/s. A higher magnification of the sample depicted as inset in Figure 1(a) reveals that the laser induced surface structures are consisted of micro-paraboloids covered with approximately hemicylindrical nano-scaled ripples. Increasing the ablation energy results in tripled pattern depicted in Figure 1(b) produced by laser fluence of 38 J/cm2 and scanning speed of 465 µm/s. As shown in the inset of Figure 1(b), the triple pattern consists of coarse micro asperities of irregular shape covered by small spheres with characteristic diameter of 2-3 µm as second degree of roughness. Furthermore on top of these microspheres, there seem to exist smaller nano-scaled ripples (inset of image in Figure 1(b)). The pattern depicted in Figure 1(c), resembles the surface of cauliflower with fractal and reentrance structure, here referred to as cauliflower-like pattern. It has been produced with laser fluence of 240 J/cm2 and scanning speed of 370 µm/s. The cauliflower-like structure is an essentially triple roughness structure with re-entrant micro pillar size in the range of 50 to 100 µm.


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a

4μm

b

20μm

c

10μm

50μm

20μm

100μm

f

e

d

10μm

50μm

100μm

Figure 1: Scanning electron microscope (SEM) images of laser ablated SS surfaces with: a. Paraboloidal, b. Triple, and c. Cauliflower-like micro/nano structures, insets: higher magnification images. SEM images of laser ablated Ti surfaces with: d. Paraboloidal, e. Triple, and f. Cauliflower-like structures.

Figure 1(d-f) shows the morphology of Ti samples fabricated with laser ablation with the laser parameters (fluence and scanning speed) similar to those applied to fabricate the SS samples in Figure 1(a-c). Due to nearly the same melting point of Ti and SS, the morphology of produced structures on both of SS and Ti samples are similar. Atomic compositions of the laser ablated samples and flat samples were determined using elemental XPS measurements. A comparison of different surface chemical treatments and untreated surfaces were also made. The results are summarized in Figure 2 (more details are presented in Tables S1 and S2 of supplementary materials). The data given in Figure 2 (and Table S1) shows that the dominant signals for a flat untreated SS316 surface are Fe, O, and C, with weaker contributions from Cr and N. Likewise, flat control Ti surface is mainly composed of Ti, O and C. The carbon signal on the flat control, the acid-treated SS and Ti substrates arose from the intrinsic carbon content and organic contaminations. The origin of C is probably from hydrocarbons from air exposure and cleaning ACS Paragon Plus Environment


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10 solvents. Nitrogen, also from organic surface contamination was detected on all surfaces, regardless of the treatment. The level of metal on surface of the samples varies in accordance with the amounts of surface organics. Higher surface organic levels translate into less dominate metal signal. The dominant metal and O percentage of the XPS data of Figure 2 (Table S1 and S2) show that the surface film mainly consists of a metal oxide, which is iron-oxide and chromium-oxide in the case of SS, and titanium oxide in the case of Ti.

Figure 2: XPS chemical analysis of SS and Ti samples with different chemical treatments; C: Carbon, O: Oxygen, Ti: Titanium, and SS: Stainless steel.

XPS data of SS indicates that the Fe peek is in the Fe2p (3/2 and ½) region (binding energy: 713,727eV), which is attributed to Fe+3. Likewise, Cr is in Cr+3 state (binding energy: 576 eV). This type of iron oxide and chromium oxide are mostly Fe2O3 and Cr2O3. Ti peak in XPS data of untreated


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11 flat control surface is mainly in the Ti 2p region (binding energy: 460eV), which presents Ti as Ti+4. Thus, the dominant surface oxide is TiO2 36. In the case of acid-treated surfaces, piranha solution is used to generate hydroxyl groups on the surface. The chemical reaction between concentrated sulfuric acid with hydrogen peroxide generates reactive oxygen species, leading to attack the metal surface to remove the native oxides and form a fresh OH-rich oxide on the SS and Ti surfaces37. Therefore, the strong oxygen signal was attributed to the hydroxyl group generated on the SS and Ti. In the case of silanized surfaces, the process starts with an acid treatment step to generate hydroxyl groups on the surface followed by a reaction with fluoroalkylsilane compound. This was confirmed by an increase carbon and fluorine content on surface with a consequent decrease in metal content in the XPS data of the silanized surfaces (Table S1 & S2). The formation of a silane coating was also confirmed by the enhancement in the Si signal. Comparison of XPS data for control SS and Ti surfaces (Flat, untreated) with heat-treated samples reveal that a progressive thickening of the surface oxide is happening with an increase in temperature. Therefore heat treatment causes an enrichment of metal oxides on the surface. For heattreated titanium sheet, the Ti 2p binding energy (460 eV) indicates the presence of TiO2 in the oxide layer. For SS heat-treated, the surface layer composed of mostly iron oxides (Fe binding energy: 713727eV) such as Fe3O4 and Fe2O3. For fresh laser irradiated surfaces, due to high-energy laser ablation on the SS surface, the dominant iron oxide on the SS surface is Fe3O4. It has been previously reported

35

that the metallic

surface after patterning by laser ablation is completely hydrophilic, although by exposing the samples to air or CO2, it gradually becomes hydrophobic or even superhydrophobic depending on the pattern morphology. XPS data of the freshly laser patterned and the air-exposed flat surfaces for both SS reveal that surface chemistry changes over time by enhancing the carbon content, leading to less



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12 oxygen content on the patterned substrate 35. As a hypothesis, decomposition of environmental carbon dioxide into carbon with active magnetite is a potential explanation assuming that laser energy is accountable for the creation of an active magnetite scale. Active magnetite Fe3O4-δ (0 < δ < 1) is a nonstoichiometric oxygen deficient iron oxide scale, which was found to catalyze the dissociative adsorption of carbon dioxide 35. Carbon dioxide converts to carbon monoxide and zero valence carbon, and oxygen anions are transferred into lattice vacancies of the metal to form stoichiometric Fe3O4. Deconvolution of XPS data of fresh laser ablated Ti surface demonstrates that the ablated surface is covered mostly with TiO2 and a trace amount of Ti2O3. Similar to the SS, the surface chemistry of ablated Ti is changing over time, and the change is probably attributed to the existence of activated titanium oxide (TiO2-δ) after laser irradiation, which reacts with carbon species in air or surrounding atmosphere to convert to TiO2.

3.2 Wetting Properties Figure 3 (Table S3) illustrates how surface patterning and coating affect the wettability of various stainless steel substrates. Since the coated and patterned structures of both SS and Ti substrates are almost same, resulting similar contact angles (CAs), the values of CAs of Ti surfaces have been reported in Supporting information as Table S3 and Figure S1. The bare flat SS and Ti surfaces are hydrophilic. Oxidation of surface during heat and acid treatment results in more hydrophilic surfaces when compared to controls. Polar hydroxyl (-OH) groups make the surface polar and facilitate formation of hydrogen bonding between solid surface and water molecules causing superhydrophilicity. According to the Wenzel’s theory, the roughness magnifies the surface intrinsic wettability that the flat hydrophilic and hydrophobic surface becomes more hydrophilic and hydrophobic, respectively. Therefore the acid-treated patterned Ti and SS samples are superhydrophilic.


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13 Contact angle measurements reveal higher hydrophobicity for flat heat-treated surfaces in comparison to the flat acid-treated ones. The intrinsic hydrophilicity of the polar iron oxides is amplified by the surface roughness according to Wenzel’s theory and water droplet completely spreads on the surface with higher roughness with patterns such as cauliflower-like on SS and Ti. Stainless Steel 180 160

No Coa ng

Carbonized

Silanized

Acid Treated

Heat Treated

140

Contact Angle ( )

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°

120 100 80 60 40 20 0 Flat Surface

Paraboloidal

Triple Pa ern

Cauliflowered

Surface Chemical Treatment Figure 3: Water contact angles of flat and laser ablated SS surfaces

Due to oxide layers and activated sites on the fresh laser ablated surfaces, they became superhydrophilic. By exposing to air, the CO2 decomposition reaction continues slowly and nonpolar carbon accumulates on the rough surface35. The deposited nonpolar carbon layer on high surface energy iron oxide and titanium oxide shields the surface hydrophilicity of polar metal oxide and results in superhydrophobicity of patterned substrates. Therefore, after CO2 exposure the laser ablated Ti and



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14 SS are highly hydrophobic on paraboloidal pattern and superhydrophobic on triple and cauliflower-like patterns with CA ranging from 150° to 165°. The level of surface hydrophobicity increases with surface silanization. The contact angle of flat surface after silanization is increased to 105° and the contact angle on cauliflower-like pattern to 172°. Moreover as presented in Table 1 the contact angle hysteresis (CAH) was also assessed for the substrates with CA>150o to confirm that these are truly superhydrophobic surfaces with CAH

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