Surface Modification of Polypropylene Membrane by Polyethylene Glycol Graft Polymerization Atiye Sadat Abednejad, Ghasem Amoabediny, Azadeh Ghaee PII: DOI: Reference:
S0928-4931(14)00343-9 doi: 10.1016/j.msec.2014.05.060 MSC 4684
To appear in:
Materials Science & Engineering C
Received date: Revised date: Accepted date:
4 January 2014 21 April 2014 29 May 2014
Please cite this article as: Atiye Sadat Abednejad, Ghasem Amoabediny, Azadeh Ghaee, Surface Modification of Polypropylene Membrane by Polyethylene Glycol Graft Polymerization, Materials Science & Engineering C (2014), doi: 10.1016/j.msec.2014.05.060
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Surface Modification of Polypropylene Membrane
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By Polyethylene Glycol Graft Polymerization
Department of Biomedical Engineering, Faculty of New Sciences and Technologies, University of
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Atiye Sadat Abednejada,*1, Ghasem Amoabedinyb,c and Azadeh Ghaeea.
b
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Tehran, P.O.Box 14395-1561, Tehran, Iran Department of Life Science Engineering, Faculty of New Sciences and Technologies, University of Tehran, P.O.Box 14395-1561, Tehran, Iran
c
Research Center for New Technologies in Life Science Engineering, University of Tehran, P.O. Box 63894-14179, Tehran, Iran
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*Corresponding author. Tel.: +98-21-88618430. E-mail address: [email protected].
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ACCEPTED MANUSCRIPT Abstract Polypropylene hollow fiber microporous membranes have been used in a wide range of
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applications, including blood oxygenator. The hydrophobic feature of the polypropylene surface
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causes membrane fouling. To minimize fouling, a modification consists of three steps: surface activation in H2 and O2 plasma, membrane immersion in polyethylene glycol (PEG) and plasma
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graft polymerization was performed. The membranes were characterized by contact angle
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measurement, Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), tensile test, Scanning electron microscopy (SEM) and Atomic force microscopy (AFM).
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Oxygen transfer of modified membranes was also tested. The stability of grafted PEG was measured in water and in phosphate buffer saline (PBS) at 37◦C. Blood compatibility of modified
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surfaces was evaluated by the platelet adhesion method. Water contact angel reduction from 110˚
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to 72˚ demonstrates the enhanced hydrophilicity, and XPS results verify the presence of
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oxygenated functional groups due to the peak existence in 286 eV as a result of PEG grafting. The results clearly indicate that plasma graft-polymerization of PEG is an effective way for antifouling improvement of polypropylene membranes. Also, the results show that oxygen transfer changes in PEG grafted membranes is not significant.
Key words: Polyethylene glycol; Polypropylene; hydrophilicity; plasma graft polymerization; blood compatibility.
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ACCEPTED MANUSCRIPT 1. Introduction There has been an increasing demand for membrane processes in the field of gas separation
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(GS), medicine, waste water treatment, and so on during the last few decades. A blood
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oxygenator is a medical device that is used for patients with acute respiratory problems, immature babies and, in open heart surgeries [1]. It was first designed in 1950s as a bubble type
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oxygenator [2]. Subsequently, film oxygenators were produced. In bubble and film oxygenators,
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blood came in contact with oxygen directly, causing damage to blood cells. The remarkable blood compatibility of membrane oxygenators, however, rather than bubble and film types, made
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them the most common type in extracorporeal oxygenating systems. Blood contact with the polypropylene membrane (PP) surface is followed by several bodily immune system responses,
to adhesion.
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as coagulation and complement activation, due to the PP membranes' hydrophobic surface, leads
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Polypropylene membranes with high and well-controlled porosity, as well as non-toxic and chemical inertness properties, are used in various applications [3-5]. Polypropylene hollow fiber microporous membranes have been commonly used in blood oxygenators, as well as in ultra filtration devices for waste water treatment [6]. However, the hydrophobic surface of PP membranes causes membrane fouling. Surface properties of polymers have significant importance in many branches of industrial applications, which also affect the performance of polymeric membranes. Thus, many studies focus on the membranes surface modification [7, 8]. In a number of attempts to improve the anti-fouling characteristics of the PP membranes, different methods such as UV irradiation, plasma treatment, gamma irradiation, and chemical reaction have been employed to modify the membrane surface [9]. Plasma surface modification of polymeric materials at a low temperature was developed several decades ago. Cold plasma
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ACCEPTED MANUSCRIPT processes are mainly used in both industrial applications and laboratory studies [10, 11]. H2O plasma treatment was used to enhance surface wettability of PP membranes by Yu et al. [9].
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Choi et al. [10] used O2 cold plasma to promote PP membrane surface hydrophilicity. Cheng et
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al. [12] tried to improve PP membrane hydrophilicity by argon plasma. Wei et al. [13-15] examined NH3, air and N2 plasma to improve PP membrane hydrophilicity, and Hei et al. [16]
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made an attempt to enhance PP membrane wettability by using CO2 plasma. Even though surface
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wettability was improved in all the aforementioned modifications, this improvement was not stable. In other studies, hydrophilic polymers were grafted to the surfaces after cold plasma
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activation. Chen et al. [17] grafted 2,3epoxypropylmethacylate (EPMA) to activated membrane for biomedical purposes. This modification improved both surface hydrophilicity and blood
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compatibility. Gycidyl methacrylate was used to modify the PP membrane by Paul et al. [18]; the membrane surface was also activated by UV radiation. PP membrane wettability enhancement
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and blood compatibility improvement were concluded in this modification. In this research, polyethylene glycol (PEG) graft polymerization was carried out to improve PP membrane surface hydrophilicity and blood compatibility. PEG-modified surfaces prevent protein and platelet adhesion due to the PEG's hydrophilicity characteristic. PEG is a gel type polymer that hydrates and swells when in contact with water. This polymer is grafted to numerous membranes, such as polyacrylonitrile and Polysulfone membranes, to prevent the adhesion of platelets [19]. Plasma-induced graft-polymerization is one of the most useful and effective methods of surface functionalization [20, 22]. Both hydrogen and oxygen plasma were used for covalent grafting of PEG in different experiments through three steps: PP membrane surface chemical activation by oxygen and hydrogen plasma, membrane immersion in PEG solution,and plasma induced graft polymerization [23]. The formation of oxygenated functional
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ACCEPTED MANUSCRIPT groups on the PP substrate was confirmed by FTIR and XPS analysis. AFM and SEM images were used to present membrane morphology before and after modification.
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2. Material and methods:
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2.1 Materials
Isotactic PP hollow fiber microporous membranes were commercially supplied by Membrana,
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Germany. The average inside and outside diameter of the fibers were 200 and 300 µm,
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respectively. The average pore diameter was 100 nm. Analytical grades of acetone, methanol, ethanol (98% v/v) NaOCl and glutaraldehyde (25%) were purchased from Merck. Phosphate
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buffer saline (PBS) was obtained from Medicago, Sweden and PEG (Mw: 1400) was supplied by Sigma Aldrich.
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2.2 Surface modification of polypropylene membrane H2 plasma treatments (both surface activation and graft-polymerization) were carried out in
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Reactive Ion Etching (RIE) system composed of a cubic vacuum chamber and made in Anatech CO, USA. An electrode was externally connected to a 13.56 MHz – RF power supplier to provide radio frequency (RF) voltage. PP samples were inserted in the chamber and kept at a distance of 10 cm from the RF electrode. In this research, the device was evacuated to 10-3 Pa by means of a turbo-molecular pump combined with a rotary pump before operation. Plasma power was set on 15 W (higher power may damage the membrane) and gas flow was 5 SCCS (standard cubic centimeter per second). 2.2.1 Modification of PP membrane in H2 plasma Before modifying the surfaces, membranes were washed with acetone to remove chemicals and wetting agents adsorbed from surfaces. Then, samples were dried at room temperature for 24 h. Modification started with activation of PP samples which were shaped manually into squares
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ACCEPTED MANUSCRIPT with a total area of 150 cm2. Surface activation of samples was performed in H2 plasma for 1.5, 2, 2.5, 3 and 3.5 min [24]. If exposed for a longer duration, H2 plasma would damage PP
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membrane surface [23]. Samples were then immersed in different concentration of PEG/ ethanol
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solution (10, 15, 20, 25 and 30 g/l). The physically adsorbed PEG was grafted on membrane using H2 Plasma for a longer period of time (3, 5, 7, 10 and 12 min) [24,25]. The samples finally
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were washed by methanol, NaOCl and deionized water,respectively, to remove any monomers or
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un-grafted PEG on the surface. The membrane surface modification was then followed by a stability test, which was conducted by immersion in phosphate buffer saline (PBS) for 24 h at
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37◦C. The samples were perfectly dried at ambient temperature for 24 h. 2.2.2 Modification of PP membrane in O2 plasma
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The aforementioned procedure with H2 plasma was repeated, this time, for O2 plasma, with only the activation times and grafting step differing. Surface activation by O2 cold plasma required
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less time than H2 plasma due to higher feasibility of oxygenated functional groups formation [24]. Surfaces were activated for 10, 30, 90, 120 and 150 sec. The grafting step was also performed for 1, 3, 5, 7 and 10 min. 2.3. Characterization
2.3.1 Adsorption Degree of PEG on PP membrane The PEG adsorption degree (AD) was obtained by the following equation [14]: AD = (Wa - W0)/W0
(1)
Where Wais the weight of the membrane after immersion in PEG solution and W0 is the weight of the unmodified membrane. 2.3.2 Grafting degree of PEG on PP membrane The grafting degree(GD) was calculated by equation 2 [14]:
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ACCEPTED MANUSCRIPT GD = (Wt -W0)/ W0
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Where Wt is the weight of the membrane after grafting, washing and drying and W0 is the weight
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of the unmodified membrane. All of the results were based on the average of three parallel
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experiments.
Chemical characterization of unmodified and modified membranes was determined by Fourier
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transform infrared spectroscopy (FT-IR, Tensor 27- Brucker). X-ray photoelectron spectra (XPS)
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were recorded using VG Macrotech, XR3E2 ESCA System apparatus, with an Al-Mg-anode source, 10-8 Torr pressure and an electron take-off angle of 45°. To examine the surface
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wettability, contact angles of the membrane surface were measured by the sessile drop method (Data physics OCA15+, Germany).
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To determine platelet adhesion to the PP membrane, the following procedure was carried out. Fresh platelet-rich plasma (PRP), obtained from 20ml of fresh human blood by centrifugation at
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1000 rpm for 10 min, was used in all experiments. All samples placed in tissue culture plates; 20 µl of fresh PRP was dropped on the center of the samples and incubated at 25°C for 30 min. The samples were rinsed gently with a phosphate buffered saline (PBS) solution, after which the adhered platelets were fixed with 2.5 wt% glutaraldehydesolutions in PBS for 30min. Finally, the samples were washed with PBS and dehydrated with a series of ethanol/water mixtures of increasing ethanol concentration (30, 40, 50, 60, 70, 80, 90, and 100% ethanol, 10 min in each mixture). The samples were coated with gold; platelet adhesion changes investigated by scanning electron microscopy (SEM). All results were the average of four parallel experiments. Surface and cross-section morphology of hollow fiber membranes in the unmodified and modified membranes were determined by Scanning electron microscopy (SEM) using a Hitachi S4160 scanning microscope. For cross-section analysis, membranes were fractured in liquid
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ACCEPTED MANUSCRIPT nitrogen before their surfaces (including the fractured cross-sections) were covered with a thin layer of gold using a sputter coater (SCDOOS – Baltec, Switzerland). Unless otherwise
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mentioned, a 5 mm/min test speed was used. AFM measurements were taken using a SolverNext
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(NT–MDT, Moscow, Russia) in tapping mode using high accuracy polysilicon tips (NT–MDT) with the typical spring constant of 10 N/m. A Zwick/Roell, Hct 400/25 model of dynamic testing
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machine, equipped with a data acquisition system, was utilized to carry out the tensile tests.
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Tensile tests were performed according to D882 ASTM standard. To measure oxygen transfer, a bundle of 200 fibers of unmodified and modified hollow fiber PP
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membranes were used in a cylindrical oxygenator. Fibers were assembled in a holding chamber in which oxygen and water came in contact indirectly, and in counter-current mode. The
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chamber consisted of oxygen and water inlets and outlets. Oxygen flows inside and water passes over the fibers. Diffused oxygen into water is measured by OXI3310- Germany.
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3. Results and discussion
3.1 Modification of PP membrane in H2 and O2 plasma The optimum activation time is chosen at the greatest amount of AD. Figure 1 shows adsorption degree versus time for H2 and O2 Plasma. In Figure 1, the greater AD shows higher activation of the PP membrane surface because surface activation leads to greater absorption of PEG, in other words H2 and O2 plasma cause active sites formation that results in PEG absorption. The ascending slope of the adsorption degree curve indicates the increase of membrane weight due to the interactions of surface and PEG functional groups, while the descending slope indicates membrane weight drop caused by membrane destruction in long-time plasma [16]. Figure 2 shows grafting degree versus time for H2 and O2 Plasma. As shown, the optimum plasma grafting time is obtained at maximum grafting degree (GD). In Figure 2, the ascending slope of
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ACCEPTED MANUSCRIPT the curve, which shows the growth of grafting degree, continues up to the limited time. The descending slope can be explained in terms of the destruction of grafted PEG PP membrane due
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to plasma exposure for a long duration. Plasma treatment would make bond break, which results
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in surface deterioration. As shown in Figure1, 1.5 min and 30 sec are the optimum times for activation in H2 and O2 plasma, respectively.
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According to Figure 2, 5 and 3 min, respectively, are the optimum times for PEG grafting in H2
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and O2 plasma. If plasma time increases greater than optimum times for activation and grafting, decreasing order of activation degree and grafting degree would be experienced. The same trend
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was observed for plasma treatment in Yu et al. research. They grafted PVP on PP membrane by means of plasma, grafting degree reduced when the plasma time increased [16, 21].
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Table 1 depicts the effect of PEG concentrations on grafting degree for H2 and O2 plasma. According to Table1, grafting degree improves as PEG concentration increases up to a specific
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value. GD is constant at specific PEG concentrations, contributing to the maximum capacity of the activated surface for PEG grafting. It is constant at 20 and 25 g/l of PEG concentration for H2 and O2 plasma, respectively. H2 and O2 plasma cause active sites formation on PP membrane which lead to PEG interaction with surfaces. In O2 plasma modification, the concentration at which GD becomes constant was greater rather than H2 plasma and membrane surface interacts with greater amount of PEG molecules per volume unit because of more active site formation. Kou et al. reported the same experience for grafting α-allyl Glucoside (AG) to modify PP membrane surface. Grafting degree became almost constant at specific polymer concentration [21].
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ACCEPTED MANUSCRIPT 3.2 FTIR Figure3 shows the FTIR spectra of the unmodified (a) and modified PP membranes in 20 g/l
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PEG concentration in H2 (b) and O2 (c) plasma. In particular, the typical bands of PP-PEG,
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namely O–H stretching 3417 cm-1 and 3422 cm-1 and C–O–C stretching vibration 1162 cm-1, are observed in spectra of PEG modified surfaces in H2 and O2 plasma ( figure 3 (b) and (c)) [24,26],
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which proves PEG grafting on membrane surface. O-H groups belong to PEG and C-O-C groups
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indicate the interaction between PP and PEG functional groups which end in named bonds formation [26]. The same peaks are sharper in the spectrum of the modified surface in O2
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plasma, which is due to higher concentration of oxygenated functional groups in this treatment. 3.3 XPS
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According to Table 2, PEG grafting introduced oxygenated functional groups in XPS analysis, which ascended O-C=O, C=O and C-OH to C fraction. The presence of oxygen and its
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functional groups is the most important evidence to prove PEG grafting, compared to the lack of oxygenated functional groups in PP samples. The percentage of oxygenated functional groups varies in H2 and O2 plasma as a result of higher formation of oxygenated functional groups in O2 plasma. Figure 4 displays different carbon bindings in modified PP membrane while figure 5 indicates carbon and oxygen in modified membranes elements. The spectra of the modified membrane shows a variety of components, which can be attributed to C–C (284.6 eV), C–O–C and C–OH (286.0 eV), C = O (287.3 eV) functional groups [23]. These peaks determine the formation of oxygenated functional groups as a result of PEG grafting on PP membranes. 3.4 SEM SEM images of unmodified and modified membranes' surface in H2 and O2 plasma with PEG concentrations of 10 g/l and 20 g/l are shown in Figure 6. As PEG concentration increases, more
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ACCEPTED MANUSCRIPT grafted PEG is seen in the SEM images, which leads to greater reduction in the pore size diameter or membrane pores blockage. According to AFM results, membrane average pore size
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is reduced from 100 to 87 nm in H2 plasma, and 105 to 86 nm in O2 plasma treatment. Surface
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destruction is also observed in O2 plasma at 10 g/l PEG concentration. The average pore diameter increased from 100 to 105 nm as a consequence of this destruction. Surface damage
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after O2 plasma is mentioned in Zanini et al. research. They tested O2 plasma for Poly Ethylene
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activation [23].
Figure 7 shows the SEM images of unmodified membranes and PEG modified membranes'
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cross- section in H2 and O2 plasma with a PEG concentration of 20 g/l. The number and diameter of cross-section pores decreases in both H2 and O2 plasma, which is due to the PEG diffusion
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inside the membranes. Because PEG grafting degree is high in O2 plasma, the pore size reduction is more obvious in O2 plasma [16].
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3.5 AFM
Pore diameter reduction, which was mentioned in SEM results, is proved by AFM software, Solver Next. According to Table 3, mean surface pore size decreases as PEG grafting degree increases, which can influence oxygen transfer efficiency. PEG grafting degree becomes constant in 20 g/l and 25 g/l for H2 and O2 plasma, respectively; therefore, pore diameter does not continue to change with increasing PEG concentration. Pore diameters increase at a PEG concentration of 10 g/l for O2 plasma modified membrane due to higher membrane destruction. In Table 4, it can be observed that PEG surface modification also alters surface properties such as average roughness, average ten-point height, density of summits of the surface and surface area ratio.
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ACCEPTED MANUSCRIPT Average surface roughness is an important factor, as it affects blood cells adhesion, and rises as the PEG grafting increases. Average surface roughness in unmodified PP membrane increases
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from 15.787 to 20.520 nm after H2 plasma modification with 10 g/l PEG concentration. It also
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increased to 31.974 nm after O2 plasma modification with 10 g/l PEG concentration. The greater average surface roughness in O2 plasma modification is attributed to more PEG grafting in this
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treatment. Average ten-point height enlargement incorporates blood cells in contact with
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hydrophobic PP membrane surface, which causes a reduction in body's immune response. The value of average ten points height is 281.05 nm for unmodified PP membranes. It rises to
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283.668 and 290.142 nm for H2 and O2 plasma modified membranes, respectively, both with 10 g/l PEG concentration. The increase in density of summits of the surface is a result of PEG
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grafting in H2 plasma. This parameter increases in O2 plasma due to an intensive surface destruction and formation. Generally, density of summits in unmodified PP membrane increased
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from 251.112 to 298.170 µm-1 for H2 plasma modification. This parameter reached to 381.582 µm-1 for O2 plasma modification. Surface area ratio (the increase in surface area in relation to a flat surface covering the same area in the X/Y plane [27]) increased from 8.238% to 9.658% in H2 plasma due to the grafted PEG. This parameter also rose in O2 plasma to 12.696%, since new surfaces would be formed after surface destruction and cause increase in GD, which confirmed the previous results. 3.6 Tensile test Figure 8 displays the stress-strain curves of unmodified and modified PP membranes. The stress/strain curve of H2 plasma modified membrane with 10 g/l PEG concentration indicates the same yield point as unmodified PP membrane. The higher ultimate point in H2 modified membranes occurred due to grafting PEG, which causes surface strengthening. On the other
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ACCEPTED MANUSCRIPT hand, the O2 plasma modified surface in the 10 g/l PEG concentration shows the least yield point of 10 Mpa. This parameter is 13 Mpa for unmodified and H2 plasma modified PP membrane
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surface. It is found out that surface damage in O2 plasma evolved in weakness in bearing tension
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[24].
On the other side, even though PEG grafting degree in O2 plasma modification is greater than H2
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plasma modified PP membranes, which considered strengthening PP membranes more, the
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membrane destruction overweighs strengthening and as a result the ultimate point for O2 modified membrane decreased.
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3.7 Hydrophilicity
Comparing the water contact angels of samples can provide a semi quantitative measure of the
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differences in the wettability for porous membranes. As a function of the age of the drop, the contact angle is plotted to determine a constant value. Figure 9 indicates a reduction of water
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contact angel after grafting degree increases in H2 and O2 plasma. The water contact angel dropped from 110° for unmodified PP membrane to 65° in the best condition of O2 plasma with 7.75% PEG grafting degree, while in H2 plasma, it dropped from 110° to 72°. The greater PEG grafting degree in O2 plasma coupled with the increased formation of oxygenated functional groups led to higher hydrophilicity improvement in O2 plasma [21]. It should be noted that surface wettability enhancement leads to adhesion reduction. Water contact reduction due to PEGA grafting was also experienced in Zanini et al. research. PEG and its derivates increase surface's wettability due to their hydrophilic characteristics [24]. 3.8 Hemocompatibility PEG as a hydrophilic polymer, influences blood platelet adhesion; increasing the PEG grafting degree causes a reduction of fouling and adhesion due to hydrophilicity enhancement. Figure 10
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ACCEPTED MANUSCRIPT shows SEM images of adhered platelets to unmodified and PEG modified membrane surfaces. By means of Nova PX, AFM software, adhered platelets were counted; remarkable adhesion
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diminishing is observed after PEG concentration changing from 10 g/l to 20 g/l (Table 5) [16,
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25]. The number of adhered platelets in 30 cm2 of PP membrane surface area was counted through AFM software - Nova PX. The numbers of adhered platelets reduced from 347 in
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unmodified membranes to 65 and 25 in H2 plasma treatment for 10 and 20 g/l PEG concentration
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respectively. In O2 plasma modified surfaces, the numbers of adhered platelets decreased to 49 and 16 for 10 and 20 g/l PEG concentration, respectively. PEG as a hydrophilic polymer
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minimizes the amount of adhered platelets. As the grafted PEG on membrane surface increases, the number of platelets, which adhered to hydrophobic PP surface, diminishes and fouling
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phenomenon would be prevented. That's why the numbers of adhered platelets on O2 plasma modified membranes are less than H2 plasma modified ones at the same PEG concentration. PVP
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as a hydrophilic and hemocompatible polymer was used to modify PP membrane surface by Liu et al.. By increasing PVP grafting degree on PP membrane the number of adhered platelets decreased [25].
3.9 Oxygen transfer
The concentration of dissolved oxygen was measured every 5 min during 2 h water oxygenation. Figure 11 shows the changes of O2 concentration in water versus time for unmodified and modified PP membranes. As mentioned, by grafting PEG the pores' diameter decreases, which leads to lower oxygen transfer in comparison to unmodified PP membranes. For all PEG modified surfaces, oxygen transfer is lower than that of the unmodified PP membrane. Concentration of dissolves oxygen after 2 hr was 26.5 mg/l for unmodified PP membrane. This parameter was 25.4 and 24.8 mg/l for H2 modified surface with 10 and 20 g/l PEG concentration,
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ACCEPTED MANUSCRIPT respectively. The dissolved oxygen after 2 hr was the least amount for O2 modified surface with 20 g/l PEG concentration, due to the greatest reduction in membrane pore. In contrast, in the O2
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plasma modified surface with 10 g/l of PEG concentration, the amount of transferred oxygen is
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higher compared to that of the other modified membranes, and even unmodified PP membrane, due to membrane surface destruction and pore diameter increase.The dissolved oxygen after 2 hr
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was the greatest amount, 26.8 mg/l.
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4. Conclusion
In this study, the surface modification of PP hollow fiber membrane for hydrophilicity
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enhancement and blood compatibility improvement was examined. The membrane was activated in both H2 and O2 plasma environments, followed by immersing in PEG solution. PEG grafting
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increased oxygenated functional groups, a finding which was confirmed by FTIR and XPS results. H2 plasma treatment demonstrated less oxygenated functional groups formation on PP
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surface compared to O2 plasma, with surface activation and PEG grafting degree being higher in O2 plasma treatment. As a result, enhanced wettability and improved blood compatibility, the main aims of the modification, were observed more so in O2 plasma. In contrast, surface damage appeared in the modified surface of O2 plasma in 10 g/l of PEG concentration. SEM images, tensile test and AFM results confirmed this finding, although the pore size diameter increase causes desirable oxygen transfer. PEG graft polymerization of surface seems to be one of the best ways to modify the PP membrane surface for hydrophilicity enhancement and reduction of platelet adhesion and fouling. 5. References [1] H.D. Parikh, D.C.G. Crabbe, A.W. Auldist , S.S. Rothenberg. Pediatric Thoracic Surgery, Springer-Verlag, London, 2009. [2] M.E. Voorhees, Oxygenator Technology: The Future ,Perfusion. 9 (1994) 229-232. 15
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[20] M. Nitschke. Polymer Surfaces and Interfaces chapter 10, Springer, Berlin- Heidelberg,
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[21] R. Q. Kou, Z. K. Xu, H. T. Deng, Z. M. Liu, P. Seta and Y. Xu, Surface Modification of Microporous Polypropylene Membranes by Plasma-Induced Graft Polymerization of α-Allyl Glucoside, Langmuir. 19 (2003) 6869-6875. [22] P.K Chu, J.K Chen, L.P. Wang and N. Huang, Surface Chemistry Influence Implant Biocompatibility, Mater. Sci. Eng. 36 (2002) 143- 206. [23] S. Zanini , M. Mu¨ller , C. Riccardi and M. Orlandi, Polyethylene Glycol Grafting on Polypropylene Membranes for Anti-fouling Properties, Plasma Chem Plasma Process. 27 (2007) 446- 457. [24] S. Zanini, M. Orlandi, C. Colombo, E.Grimoldi and C. Riccardi, Plasma-induced graftpolymerization of polyethylene glycolacrylate on polypropylene substrates, Eur. Phys. J. 54 (2009) 159- 164. [25] Z.M. Liu, Z.K. Xu, J.Q. Wang, J. Wu and J.J. Fu, Surface modification of Polypropylene Microfiltration Membranes by Graft Polymerization of Polyvinylpyrrolidone, Eur. Polym. J. 40 (2004) 2077-2087.
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ACCEPTED MANUSCRIPT [26] D.L. Pavia, G.M. Lampman, G.S. Kriz and J.R. Vyvyan, Introduction to Spectroscopy, 4th edition, Brooks/cole, Belmont, 2009.
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[27] A. Ghaee, M. Shariaty-Niassar, J. Barzin, T. Matsuura, Effects of chitosan membrane morphology on copper ion adsorption, Chem. Eng. J. 165 (2010) 46- 55.
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27 26.5 26 25.5 25 24.5 24 23.5 23 22.5 22 21.5 21 20.5 20 19.5 19 18.5 18 17.5 17 16.5 16 15.5 15 14.5 14 13.5 13 12.5 12 11.5 11 10.5 10 9.5 9 8.5 8 7.5 7
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Hydrogen modified PP membrane, C PEG= 20g/l" Oxygen modified PP membrane, C PEG=10g/l"
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Unmodified PP membrane
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Figures’ captions
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Figure 1: Adsorption degree versus plasma duration in Hydrogen and Oxygen plasma
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Figure 2: Grafting degree versus plasma duration in Hydrogen and Oxygen plasma
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Figure 3: FTIR spectra of a) unmodified PP membrane, b) modified PP membrane in Hydrogen plasma CPEG: 20 g/l and c) modified PP membrane in Oxygen plasma CPEG: 20 g/l
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Figure4: XPS spectra of PEG grafted PP membrane
Figure5: Element Composition of PEG modified PP surface
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Figure6: SEM images of a) unmodified PP membrane, b) modified PP membrane in H2 plasma
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PP membrane in Oxygen plasma and CPEG= 10 g/l, e) modified PP membrane in Oxygen plasma and CPEG= 20 g/l
Figure7: Cross section SEM images of a) unmodified PP membrane, b) modified membrane in Hydrogen plasma and CPEG= 20 g/l, c) modified PP membrane in Oxygen plasma and CPEG= 20 g/l
Figure 8: Stress versus strain of unmodified and modified PP membrane Figure 9: Water contact angel versus PEG grafting degree in Hydrogen and Oxygen plasma Figure 10: Platelet adhesion of a) Unmodified PP membrane, b) modified PP membrane in Hydrogen plasma and CPEG= 10 g/l, c) modified PP membrane in Hydrogen plasma and CPEG= 20 g/l. Figure 11: Oxygen concentration versus time
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CPEG(g/l) 10
15
20
In H2 plasma
1.48
3.78
5.98
In O2 plasma
2.96
4.5
6.8
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5.98
5.98
7.75
7.75
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Table 2 C=O
COH
Unmodified PP membrane
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-
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Modified PP membrane in H2 plasma, CPEG= 20 g/l Modified PP membrane in O2 plasma, CPEG= 20 g/l
3.4%
2.3%
12%
7.3%
5.3%
15,2%
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3.3%
58.5%
13.5%
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10
15
H2 plasma
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91
O2 plasma
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15.787
281.05
251.112
20.520
283.668
31.974
290.142
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Density of summits of the surface (1/μm2)
298.170
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Average ten points height(nm)
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Unmodified PP membrane
Average roughness(nm)
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381.582
Surface Area Ratio (%) 8.238
9.658
12.696
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Unmodified PP membrane
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Modified PP membrane in H2 plasma, CPEG= 10 g/l
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Modified PP membrane in H2 plasma, CPEG= 20 g/l
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Table 1: The effect of PEG concentrations, CPEG (g/l), on grafting degree, GD (wt%), for H2 and O2
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plasma Table 2: Oxygenated functional groups in unmodified and modified PP membrane (%)
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Table 3:Meansurface pore diameter (nm) versus PEG concentration (g/l) Table 4: Surface morphology features
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Table 5: Number of adhered platelets to unmodified and modified PP membrane of 30 cm2 surface area
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ACCEPTED MANUSCRIPT Highlights: H2 and O2 plasma graft polymerization of PEG on polypropylene membrane was carried.
Changes in surface properties were investigated by FTIR, XPS, SEM, AFM.
Surface wettability enhanced as a result of poly ethylene glycol grafting.
PEG grafting degree increase causes reduction of fouling and adhesion.
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S0928-4931(14)00343-9 doi: 10.1016/j.msec.2014.05.060 MSC 4684
To appear in:
Materials Science & Engineering C
Received date: Revised date: Accepted date:
4 January 2014 21 April 2014 29 May 2014
Please cite this article as: Atiye Sadat Abednejad, Ghasem Amoabediny, Azadeh Ghaee, Surface Modification of Polypropylene Membrane by Polyethylene Glycol Graft Polymerization, Materials Science & Engineering C (2014), doi: 10.1016/j.msec.2014.05.060
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Surface Modification of Polypropylene Membrane
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By Polyethylene Glycol Graft Polymerization
Department of Biomedical Engineering, Faculty of New Sciences and Technologies, University of
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Atiye Sadat Abednejada,*1, Ghasem Amoabedinyb,c and Azadeh Ghaeea.
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Tehran, P.O.Box 14395-1561, Tehran, Iran Department of Life Science Engineering, Faculty of New Sciences and Technologies, University of Tehran, P.O.Box 14395-1561, Tehran, Iran
c
Research Center for New Technologies in Life Science Engineering, University of Tehran, P.O. Box 63894-14179, Tehran, Iran
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*Corresponding author. Tel.: +98-21-88618430. E-mail address: [email protected].
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ACCEPTED MANUSCRIPT Abstract Polypropylene hollow fiber microporous membranes have been used in a wide range of
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applications, including blood oxygenator. The hydrophobic feature of the polypropylene surface
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causes membrane fouling. To minimize fouling, a modification consists of three steps: surface activation in H2 and O2 plasma, membrane immersion in polyethylene glycol (PEG) and plasma
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graft polymerization was performed. The membranes were characterized by contact angle
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measurement, Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), tensile test, Scanning electron microscopy (SEM) and Atomic force microscopy (AFM).
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Oxygen transfer of modified membranes was also tested. The stability of grafted PEG was measured in water and in phosphate buffer saline (PBS) at 37◦C. Blood compatibility of modified
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surfaces was evaluated by the platelet adhesion method. Water contact angel reduction from 110˚
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to 72˚ demonstrates the enhanced hydrophilicity, and XPS results verify the presence of
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oxygenated functional groups due to the peak existence in 286 eV as a result of PEG grafting. The results clearly indicate that plasma graft-polymerization of PEG is an effective way for antifouling improvement of polypropylene membranes. Also, the results show that oxygen transfer changes in PEG grafted membranes is not significant.
Key words: Polyethylene glycol; Polypropylene; hydrophilicity; plasma graft polymerization; blood compatibility.
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ACCEPTED MANUSCRIPT 1. Introduction There has been an increasing demand for membrane processes in the field of gas separation
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(GS), medicine, waste water treatment, and so on during the last few decades. A blood
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oxygenator is a medical device that is used for patients with acute respiratory problems, immature babies and, in open heart surgeries [1]. It was first designed in 1950s as a bubble type
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oxygenator [2]. Subsequently, film oxygenators were produced. In bubble and film oxygenators,
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blood came in contact with oxygen directly, causing damage to blood cells. The remarkable blood compatibility of membrane oxygenators, however, rather than bubble and film types, made
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them the most common type in extracorporeal oxygenating systems. Blood contact with the polypropylene membrane (PP) surface is followed by several bodily immune system responses,
to adhesion.
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as coagulation and complement activation, due to the PP membranes' hydrophobic surface, leads
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Polypropylene membranes with high and well-controlled porosity, as well as non-toxic and chemical inertness properties, are used in various applications [3-5]. Polypropylene hollow fiber microporous membranes have been commonly used in blood oxygenators, as well as in ultra filtration devices for waste water treatment [6]. However, the hydrophobic surface of PP membranes causes membrane fouling. Surface properties of polymers have significant importance in many branches of industrial applications, which also affect the performance of polymeric membranes. Thus, many studies focus on the membranes surface modification [7, 8]. In a number of attempts to improve the anti-fouling characteristics of the PP membranes, different methods such as UV irradiation, plasma treatment, gamma irradiation, and chemical reaction have been employed to modify the membrane surface [9]. Plasma surface modification of polymeric materials at a low temperature was developed several decades ago. Cold plasma
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ACCEPTED MANUSCRIPT processes are mainly used in both industrial applications and laboratory studies [10, 11]. H2O plasma treatment was used to enhance surface wettability of PP membranes by Yu et al. [9].
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Choi et al. [10] used O2 cold plasma to promote PP membrane surface hydrophilicity. Cheng et
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al. [12] tried to improve PP membrane hydrophilicity by argon plasma. Wei et al. [13-15] examined NH3, air and N2 plasma to improve PP membrane hydrophilicity, and Hei et al. [16]
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made an attempt to enhance PP membrane wettability by using CO2 plasma. Even though surface
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wettability was improved in all the aforementioned modifications, this improvement was not stable. In other studies, hydrophilic polymers were grafted to the surfaces after cold plasma
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activation. Chen et al. [17] grafted 2,3epoxypropylmethacylate (EPMA) to activated membrane for biomedical purposes. This modification improved both surface hydrophilicity and blood
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compatibility. Gycidyl methacrylate was used to modify the PP membrane by Paul et al. [18]; the membrane surface was also activated by UV radiation. PP membrane wettability enhancement
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and blood compatibility improvement were concluded in this modification. In this research, polyethylene glycol (PEG) graft polymerization was carried out to improve PP membrane surface hydrophilicity and blood compatibility. PEG-modified surfaces prevent protein and platelet adhesion due to the PEG's hydrophilicity characteristic. PEG is a gel type polymer that hydrates and swells when in contact with water. This polymer is grafted to numerous membranes, such as polyacrylonitrile and Polysulfone membranes, to prevent the adhesion of platelets [19]. Plasma-induced graft-polymerization is one of the most useful and effective methods of surface functionalization [20, 22]. Both hydrogen and oxygen plasma were used for covalent grafting of PEG in different experiments through three steps: PP membrane surface chemical activation by oxygen and hydrogen plasma, membrane immersion in PEG solution,and plasma induced graft polymerization [23]. The formation of oxygenated functional
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2. Material and methods:
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2.1 Materials
Isotactic PP hollow fiber microporous membranes were commercially supplied by Membrana,
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Germany. The average inside and outside diameter of the fibers were 200 and 300 µm,
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respectively. The average pore diameter was 100 nm. Analytical grades of acetone, methanol, ethanol (98% v/v) NaOCl and glutaraldehyde (25%) were purchased from Merck. Phosphate
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buffer saline (PBS) was obtained from Medicago, Sweden and PEG (Mw: 1400) was supplied by Sigma Aldrich.
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2.2 Surface modification of polypropylene membrane H2 plasma treatments (both surface activation and graft-polymerization) were carried out in
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Reactive Ion Etching (RIE) system composed of a cubic vacuum chamber and made in Anatech CO, USA. An electrode was externally connected to a 13.56 MHz – RF power supplier to provide radio frequency (RF) voltage. PP samples were inserted in the chamber and kept at a distance of 10 cm from the RF electrode. In this research, the device was evacuated to 10-3 Pa by means of a turbo-molecular pump combined with a rotary pump before operation. Plasma power was set on 15 W (higher power may damage the membrane) and gas flow was 5 SCCS (standard cubic centimeter per second). 2.2.1 Modification of PP membrane in H2 plasma Before modifying the surfaces, membranes were washed with acetone to remove chemicals and wetting agents adsorbed from surfaces. Then, samples were dried at room temperature for 24 h. Modification started with activation of PP samples which were shaped manually into squares
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ACCEPTED MANUSCRIPT with a total area of 150 cm2. Surface activation of samples was performed in H2 plasma for 1.5, 2, 2.5, 3 and 3.5 min [24]. If exposed for a longer duration, H2 plasma would damage PP
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membrane surface [23]. Samples were then immersed in different concentration of PEG/ ethanol
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solution (10, 15, 20, 25 and 30 g/l). The physically adsorbed PEG was grafted on membrane using H2 Plasma for a longer period of time (3, 5, 7, 10 and 12 min) [24,25]. The samples finally
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were washed by methanol, NaOCl and deionized water,respectively, to remove any monomers or
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un-grafted PEG on the surface. The membrane surface modification was then followed by a stability test, which was conducted by immersion in phosphate buffer saline (PBS) for 24 h at
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37◦C. The samples were perfectly dried at ambient temperature for 24 h. 2.2.2 Modification of PP membrane in O2 plasma
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The aforementioned procedure with H2 plasma was repeated, this time, for O2 plasma, with only the activation times and grafting step differing. Surface activation by O2 cold plasma required
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less time than H2 plasma due to higher feasibility of oxygenated functional groups formation [24]. Surfaces were activated for 10, 30, 90, 120 and 150 sec. The grafting step was also performed for 1, 3, 5, 7 and 10 min. 2.3. Characterization
2.3.1 Adsorption Degree of PEG on PP membrane The PEG adsorption degree (AD) was obtained by the following equation [14]: AD = (Wa - W0)/W0
(1)
Where Wais the weight of the membrane after immersion in PEG solution and W0 is the weight of the unmodified membrane. 2.3.2 Grafting degree of PEG on PP membrane The grafting degree(GD) was calculated by equation 2 [14]:
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Where Wt is the weight of the membrane after grafting, washing and drying and W0 is the weight
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of the unmodified membrane. All of the results were based on the average of three parallel
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Chemical characterization of unmodified and modified membranes was determined by Fourier
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transform infrared spectroscopy (FT-IR, Tensor 27- Brucker). X-ray photoelectron spectra (XPS)
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were recorded using VG Macrotech, XR3E2 ESCA System apparatus, with an Al-Mg-anode source, 10-8 Torr pressure and an electron take-off angle of 45°. To examine the surface
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wettability, contact angles of the membrane surface were measured by the sessile drop method (Data physics OCA15+, Germany).
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To determine platelet adhesion to the PP membrane, the following procedure was carried out. Fresh platelet-rich plasma (PRP), obtained from 20ml of fresh human blood by centrifugation at
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1000 rpm for 10 min, was used in all experiments. All samples placed in tissue culture plates; 20 µl of fresh PRP was dropped on the center of the samples and incubated at 25°C for 30 min. The samples were rinsed gently with a phosphate buffered saline (PBS) solution, after which the adhered platelets were fixed with 2.5 wt% glutaraldehydesolutions in PBS for 30min. Finally, the samples were washed with PBS and dehydrated with a series of ethanol/water mixtures of increasing ethanol concentration (30, 40, 50, 60, 70, 80, 90, and 100% ethanol, 10 min in each mixture). The samples were coated with gold; platelet adhesion changes investigated by scanning electron microscopy (SEM). All results were the average of four parallel experiments. Surface and cross-section morphology of hollow fiber membranes in the unmodified and modified membranes were determined by Scanning electron microscopy (SEM) using a Hitachi S4160 scanning microscope. For cross-section analysis, membranes were fractured in liquid
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mentioned, a 5 mm/min test speed was used. AFM measurements were taken using a SolverNext
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(NT–MDT, Moscow, Russia) in tapping mode using high accuracy polysilicon tips (NT–MDT) with the typical spring constant of 10 N/m. A Zwick/Roell, Hct 400/25 model of dynamic testing
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machine, equipped with a data acquisition system, was utilized to carry out the tensile tests.
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Tensile tests were performed according to D882 ASTM standard. To measure oxygen transfer, a bundle of 200 fibers of unmodified and modified hollow fiber PP
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membranes were used in a cylindrical oxygenator. Fibers were assembled in a holding chamber in which oxygen and water came in contact indirectly, and in counter-current mode. The
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chamber consisted of oxygen and water inlets and outlets. Oxygen flows inside and water passes over the fibers. Diffused oxygen into water is measured by OXI3310- Germany.
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3. Results and discussion
3.1 Modification of PP membrane in H2 and O2 plasma The optimum activation time is chosen at the greatest amount of AD. Figure 1 shows adsorption degree versus time for H2 and O2 Plasma. In Figure 1, the greater AD shows higher activation of the PP membrane surface because surface activation leads to greater absorption of PEG, in other words H2 and O2 plasma cause active sites formation that results in PEG absorption. The ascending slope of the adsorption degree curve indicates the increase of membrane weight due to the interactions of surface and PEG functional groups, while the descending slope indicates membrane weight drop caused by membrane destruction in long-time plasma [16]. Figure 2 shows grafting degree versus time for H2 and O2 Plasma. As shown, the optimum plasma grafting time is obtained at maximum grafting degree (GD). In Figure 2, the ascending slope of
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to plasma exposure for a long duration. Plasma treatment would make bond break, which results
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in surface deterioration. As shown in Figure1, 1.5 min and 30 sec are the optimum times for activation in H2 and O2 plasma, respectively.
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According to Figure 2, 5 and 3 min, respectively, are the optimum times for PEG grafting in H2
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and O2 plasma. If plasma time increases greater than optimum times for activation and grafting, decreasing order of activation degree and grafting degree would be experienced. The same trend
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was observed for plasma treatment in Yu et al. research. They grafted PVP on PP membrane by means of plasma, grafting degree reduced when the plasma time increased [16, 21].
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Table 1 depicts the effect of PEG concentrations on grafting degree for H2 and O2 plasma. According to Table1, grafting degree improves as PEG concentration increases up to a specific
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value. GD is constant at specific PEG concentrations, contributing to the maximum capacity of the activated surface for PEG grafting. It is constant at 20 and 25 g/l of PEG concentration for H2 and O2 plasma, respectively. H2 and O2 plasma cause active sites formation on PP membrane which lead to PEG interaction with surfaces. In O2 plasma modification, the concentration at which GD becomes constant was greater rather than H2 plasma and membrane surface interacts with greater amount of PEG molecules per volume unit because of more active site formation. Kou et al. reported the same experience for grafting α-allyl Glucoside (AG) to modify PP membrane surface. Grafting degree became almost constant at specific polymer concentration [21].
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ACCEPTED MANUSCRIPT 3.2 FTIR Figure3 shows the FTIR spectra of the unmodified (a) and modified PP membranes in 20 g/l
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PEG concentration in H2 (b) and O2 (c) plasma. In particular, the typical bands of PP-PEG,
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namely O–H stretching 3417 cm-1 and 3422 cm-1 and C–O–C stretching vibration 1162 cm-1, are observed in spectra of PEG modified surfaces in H2 and O2 plasma ( figure 3 (b) and (c)) [24,26],
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indicate the interaction between PP and PEG functional groups which end in named bonds formation [26]. The same peaks are sharper in the spectrum of the modified surface in O2
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plasma, which is due to higher concentration of oxygenated functional groups in this treatment. 3.3 XPS
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According to Table 2, PEG grafting introduced oxygenated functional groups in XPS analysis, which ascended O-C=O, C=O and C-OH to C fraction. The presence of oxygen and its
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functional groups is the most important evidence to prove PEG grafting, compared to the lack of oxygenated functional groups in PP samples. The percentage of oxygenated functional groups varies in H2 and O2 plasma as a result of higher formation of oxygenated functional groups in O2 plasma. Figure 4 displays different carbon bindings in modified PP membrane while figure 5 indicates carbon and oxygen in modified membranes elements. The spectra of the modified membrane shows a variety of components, which can be attributed to C–C (284.6 eV), C–O–C and C–OH (286.0 eV), C = O (287.3 eV) functional groups [23]. These peaks determine the formation of oxygenated functional groups as a result of PEG grafting on PP membranes. 3.4 SEM SEM images of unmodified and modified membranes' surface in H2 and O2 plasma with PEG concentrations of 10 g/l and 20 g/l are shown in Figure 6. As PEG concentration increases, more
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is reduced from 100 to 87 nm in H2 plasma, and 105 to 86 nm in O2 plasma treatment. Surface
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destruction is also observed in O2 plasma at 10 g/l PEG concentration. The average pore diameter increased from 100 to 105 nm as a consequence of this destruction. Surface damage
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after O2 plasma is mentioned in Zanini et al. research. They tested O2 plasma for Poly Ethylene
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activation [23].
Figure 7 shows the SEM images of unmodified membranes and PEG modified membranes'
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cross- section in H2 and O2 plasma with a PEG concentration of 20 g/l. The number and diameter of cross-section pores decreases in both H2 and O2 plasma, which is due to the PEG diffusion
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inside the membranes. Because PEG grafting degree is high in O2 plasma, the pore size reduction is more obvious in O2 plasma [16].
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3.5 AFM
Pore diameter reduction, which was mentioned in SEM results, is proved by AFM software, Solver Next. According to Table 3, mean surface pore size decreases as PEG grafting degree increases, which can influence oxygen transfer efficiency. PEG grafting degree becomes constant in 20 g/l and 25 g/l for H2 and O2 plasma, respectively; therefore, pore diameter does not continue to change with increasing PEG concentration. Pore diameters increase at a PEG concentration of 10 g/l for O2 plasma modified membrane due to higher membrane destruction. In Table 4, it can be observed that PEG surface modification also alters surface properties such as average roughness, average ten-point height, density of summits of the surface and surface area ratio.
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ACCEPTED MANUSCRIPT Average surface roughness is an important factor, as it affects blood cells adhesion, and rises as the PEG grafting increases. Average surface roughness in unmodified PP membrane increases
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from 15.787 to 20.520 nm after H2 plasma modification with 10 g/l PEG concentration. It also
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increased to 31.974 nm after O2 plasma modification with 10 g/l PEG concentration. The greater average surface roughness in O2 plasma modification is attributed to more PEG grafting in this
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treatment. Average ten-point height enlargement incorporates blood cells in contact with
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hydrophobic PP membrane surface, which causes a reduction in body's immune response. The value of average ten points height is 281.05 nm for unmodified PP membranes. It rises to
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283.668 and 290.142 nm for H2 and O2 plasma modified membranes, respectively, both with 10 g/l PEG concentration. The increase in density of summits of the surface is a result of PEG
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grafting in H2 plasma. This parameter increases in O2 plasma due to an intensive surface destruction and formation. Generally, density of summits in unmodified PP membrane increased
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from 251.112 to 298.170 µm-1 for H2 plasma modification. This parameter reached to 381.582 µm-1 for O2 plasma modification. Surface area ratio (the increase in surface area in relation to a flat surface covering the same area in the X/Y plane [27]) increased from 8.238% to 9.658% in H2 plasma due to the grafted PEG. This parameter also rose in O2 plasma to 12.696%, since new surfaces would be formed after surface destruction and cause increase in GD, which confirmed the previous results. 3.6 Tensile test Figure 8 displays the stress-strain curves of unmodified and modified PP membranes. The stress/strain curve of H2 plasma modified membrane with 10 g/l PEG concentration indicates the same yield point as unmodified PP membrane. The higher ultimate point in H2 modified membranes occurred due to grafting PEG, which causes surface strengthening. On the other
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surface. It is found out that surface damage in O2 plasma evolved in weakness in bearing tension
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[24].
On the other side, even though PEG grafting degree in O2 plasma modification is greater than H2
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plasma modified PP membranes, which considered strengthening PP membranes more, the
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membrane destruction overweighs strengthening and as a result the ultimate point for O2 modified membrane decreased.
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3.7 Hydrophilicity
Comparing the water contact angels of samples can provide a semi quantitative measure of the
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differences in the wettability for porous membranes. As a function of the age of the drop, the contact angle is plotted to determine a constant value. Figure 9 indicates a reduction of water
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contact angel after grafting degree increases in H2 and O2 plasma. The water contact angel dropped from 110° for unmodified PP membrane to 65° in the best condition of O2 plasma with 7.75% PEG grafting degree, while in H2 plasma, it dropped from 110° to 72°. The greater PEG grafting degree in O2 plasma coupled with the increased formation of oxygenated functional groups led to higher hydrophilicity improvement in O2 plasma [21]. It should be noted that surface wettability enhancement leads to adhesion reduction. Water contact reduction due to PEGA grafting was also experienced in Zanini et al. research. PEG and its derivates increase surface's wettability due to their hydrophilic characteristics [24]. 3.8 Hemocompatibility PEG as a hydrophilic polymer, influences blood platelet adhesion; increasing the PEG grafting degree causes a reduction of fouling and adhesion due to hydrophilicity enhancement. Figure 10
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ACCEPTED MANUSCRIPT shows SEM images of adhered platelets to unmodified and PEG modified membrane surfaces. By means of Nova PX, AFM software, adhered platelets were counted; remarkable adhesion
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diminishing is observed after PEG concentration changing from 10 g/l to 20 g/l (Table 5) [16,
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25]. The number of adhered platelets in 30 cm2 of PP membrane surface area was counted through AFM software - Nova PX. The numbers of adhered platelets reduced from 347 in
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unmodified membranes to 65 and 25 in H2 plasma treatment for 10 and 20 g/l PEG concentration
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respectively. In O2 plasma modified surfaces, the numbers of adhered platelets decreased to 49 and 16 for 10 and 20 g/l PEG concentration, respectively. PEG as a hydrophilic polymer
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minimizes the amount of adhered platelets. As the grafted PEG on membrane surface increases, the number of platelets, which adhered to hydrophobic PP surface, diminishes and fouling
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phenomenon would be prevented. That's why the numbers of adhered platelets on O2 plasma modified membranes are less than H2 plasma modified ones at the same PEG concentration. PVP
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as a hydrophilic and hemocompatible polymer was used to modify PP membrane surface by Liu et al.. By increasing PVP grafting degree on PP membrane the number of adhered platelets decreased [25].
3.9 Oxygen transfer
The concentration of dissolved oxygen was measured every 5 min during 2 h water oxygenation. Figure 11 shows the changes of O2 concentration in water versus time for unmodified and modified PP membranes. As mentioned, by grafting PEG the pores' diameter decreases, which leads to lower oxygen transfer in comparison to unmodified PP membranes. For all PEG modified surfaces, oxygen transfer is lower than that of the unmodified PP membrane. Concentration of dissolves oxygen after 2 hr was 26.5 mg/l for unmodified PP membrane. This parameter was 25.4 and 24.8 mg/l for H2 modified surface with 10 and 20 g/l PEG concentration,
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plasma modified surface with 10 g/l of PEG concentration, the amount of transferred oxygen is
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higher compared to that of the other modified membranes, and even unmodified PP membrane, due to membrane surface destruction and pore diameter increase.The dissolved oxygen after 2 hr
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4. Conclusion
In this study, the surface modification of PP hollow fiber membrane for hydrophilicity
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enhancement and blood compatibility improvement was examined. The membrane was activated in both H2 and O2 plasma environments, followed by immersing in PEG solution. PEG grafting
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increased oxygenated functional groups, a finding which was confirmed by FTIR and XPS results. H2 plasma treatment demonstrated less oxygenated functional groups formation on PP
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surface compared to O2 plasma, with surface activation and PEG grafting degree being higher in O2 plasma treatment. As a result, enhanced wettability and improved blood compatibility, the main aims of the modification, were observed more so in O2 plasma. In contrast, surface damage appeared in the modified surface of O2 plasma in 10 g/l of PEG concentration. SEM images, tensile test and AFM results confirmed this finding, although the pore size diameter increase causes desirable oxygen transfer. PEG graft polymerization of surface seems to be one of the best ways to modify the PP membrane surface for hydrophilicity enhancement and reduction of platelet adhesion and fouling. 5. References [1] H.D. Parikh, D.C.G. Crabbe, A.W. Auldist , S.S. Rothenberg. Pediatric Thoracic Surgery, Springer-Verlag, London, 2009. [2] M.E. Voorhees, Oxygenator Technology: The Future ,Perfusion. 9 (1994) 229-232. 15
ACCEPTED MANUSCRIPT [3] Z.K. Xu, J.L. Wang, L.Q. Shen, D.F. Meng and Y.Y. Xu, Microporous Polypropylene Hollow Fiber Membrane part1: Surface Modification By Graft Polymerization Of Acrylic Acid, J. Membrane. Sci. 196 (2002) 221–229.
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[4] S. Piletsky , H. Matuschewski , U. Schedler , A. Wilpert , E. Piletska and T.A. Thirle, Surface functionalization of porous polypropylene membranes with molecularly imprinted
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polymers by photo graft copolymerization in water, Macromolecules. 33 (2000) 3092- 3098.
Reduction, J. Membrane. Sci. 189 (2001) 255–270.
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[5] H. Ma, L.F. Kakim, C.N. Bowman and R.H.Davis.MainFactorsAffecting Membrane Fouling
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[6] Z.M. Liu, Z.K. Xu, J.Q. Wang, Q. Yang, J. Wu and P.Seta.Surface modification of microporous polypropylene membranes by the graftingof poly(γ-stearyl-l-glutamate), Eur.
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Polym. J. 39 (2003) 2291- 2299.
[7] K.C. Khuble, C. Feng and T. Matsuura, The Art of Surface Modification of Synthetic Polymeric Membranes. J. Appl. Polym. Sci. 115 (2010) 885-895.
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[8] K. Grundke. Polymer surface and interfaces, Characterization of Polymer Surfaces by
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Wetting and Electro-kinetic Measurements chapter 6, Springer, 2008, pp. 103-138. [9] H.Y. Yu, Z.Q. Tang, L. Huang, G. Cheng and W. Li, J. Zhou, M.G. Yan, J.S. Gu and X.W.
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Wei, Surface modification of polypropylene macro porous membrane to improve its antifouling characteristics in a submerged membrane-bioreactor: H2O plasma treatment, Water. Res. J. 42 (2008) 4341-4347.
[10] H.S. Choi, V.V. Rybkin, V.A. Titov, T.G. Shikova and T.A. Ageeva, Comparative actions of a low pressure oxygen plasma and an atmospheric pressure glow discharge on the surface modification of polypropylene,Surf.Coat. Tech. 200 (2006) 4479 - 4488. [11] V. Titov, T. Shikova, V. Rybkin, A.Kulentsan, T. Ageeva and H.S. Choi, Surface modification of polyethylene and polypropylene in low-pressureplasma and in atmospheric pressure plasma-solution system28th ICPIG. 15- 20 (2007) [12] C. Cheng , Z. Liye and
R.J. Zhan, Surface modification of polymer
fiber by new
atmospheric pressure cold plasma jet, Surf.Coat.Tech. 200 (2006) 6659- 6666. [13] H.Y. Yu, M.X. Hu, Z.K. Xu, J.L. Wang and S.Y. Wang, Surface modification of polypropylene microporous membranes to improve their antifouling property in MBR: NH3 plasma treatment, Sep. Purif. Technol. 45 (2005) 8-15.
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ACCEPTED MANUSCRIPT [14] H.Y. Yu, L.Q. Liu, Z.Q. Tang, M.G. Yan, J.S. Gu and X.W. Wei, Surface modification of polypropylene microporous membranes to improve its antifouling characterization in a SMBR:Air plasma treatment, J. Membrane.Sci. 311 (2008) 216- 224.
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[15] H.Y. Yu, X.C. He, L.Q. Liu, J.S. Gu and X.W. Wei, Surface modification of polypropylene microporous membranes to improve its antifouling characterization in a SMBR: N2 plasma
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treatment, Water.Res. J. 41(2007) 4703- 4709.
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[16] H.Y. Yu , Y.J. Xie , M.X. Hu , J.L. Wang, S.Y. Wang and Z.K. Xu. Flux Enhancement for Polypropylene Microporous Membrane in SMBR by the Immobilization Poly(N-Vinyl-2-
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Pyrrolidone), J. Membrane.Sci. 254 (2005) 219–227.
[17] H. Kwon, Y.C. Nho and J. Chen.Surface modification of polypropylene film by radiation-
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induced grafting and its blood compatibility, Appl. Polym. Sci. 88 (2003)1726-1736 [18] L.F. Macmanus. Thesis, department of chemistry (1998) [19] V.K Vendra, L. Wu and S. Krishna, NanomaterialforThe Life Science, chapter 5, Weily,
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Weinheim, 2010, pp. 1-51.
2008, pp. 203-214.
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[20] M. Nitschke. Polymer Surfaces and Interfaces chapter 10, Springer, Berlin- Heidelberg,
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[21] R. Q. Kou, Z. K. Xu, H. T. Deng, Z. M. Liu, P. Seta and Y. Xu, Surface Modification of Microporous Polypropylene Membranes by Plasma-Induced Graft Polymerization of α-Allyl Glucoside, Langmuir. 19 (2003) 6869-6875. [22] P.K Chu, J.K Chen, L.P. Wang and N. Huang, Surface Chemistry Influence Implant Biocompatibility, Mater. Sci. Eng. 36 (2002) 143- 206. [23] S. Zanini , M. Mu¨ller , C. Riccardi and M. Orlandi, Polyethylene Glycol Grafting on Polypropylene Membranes for Anti-fouling Properties, Plasma Chem Plasma Process. 27 (2007) 446- 457. [24] S. Zanini, M. Orlandi, C. Colombo, E.Grimoldi and C. Riccardi, Plasma-induced graftpolymerization of polyethylene glycolacrylate on polypropylene substrates, Eur. Phys. J. 54 (2009) 159- 164. [25] Z.M. Liu, Z.K. Xu, J.Q. Wang, J. Wu and J.J. Fu, Surface modification of Polypropylene Microfiltration Membranes by Graft Polymerization of Polyvinylpyrrolidone, Eur. Polym. J. 40 (2004) 2077-2087.
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ACCEPTED MANUSCRIPT [26] D.L. Pavia, G.M. Lampman, G.S. Kriz and J.R. Vyvyan, Introduction to Spectroscopy, 4th edition, Brooks/cole, Belmont, 2009.
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[27] A. Ghaee, M. Shariaty-Niassar, J. Barzin, T. Matsuura, Effects of chitosan membrane morphology on copper ion adsorption, Chem. Eng. J. 165 (2010) 46- 55.
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Unmodified PP membrane
Hydrogen modified PP membrane, C PEG= 10g/l"
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Figures’ captions
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Figure 1: Adsorption degree versus plasma duration in Hydrogen and Oxygen plasma
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Figure 2: Grafting degree versus plasma duration in Hydrogen and Oxygen plasma
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Figure 3: FTIR spectra of a) unmodified PP membrane, b) modified PP membrane in Hydrogen plasma CPEG: 20 g/l and c) modified PP membrane in Oxygen plasma CPEG: 20 g/l
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Figure4: XPS spectra of PEG grafted PP membrane
Figure5: Element Composition of PEG modified PP surface
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PP membrane in Oxygen plasma and CPEG= 10 g/l, e) modified PP membrane in Oxygen plasma and CPEG= 20 g/l
Figure7: Cross section SEM images of a) unmodified PP membrane, b) modified membrane in Hydrogen plasma and CPEG= 20 g/l, c) modified PP membrane in Oxygen plasma and CPEG= 20 g/l
Figure 8: Stress versus strain of unmodified and modified PP membrane Figure 9: Water contact angel versus PEG grafting degree in Hydrogen and Oxygen plasma Figure 10: Platelet adhesion of a) Unmodified PP membrane, b) modified PP membrane in Hydrogen plasma and CPEG= 10 g/l, c) modified PP membrane in Hydrogen plasma and CPEG= 20 g/l. Figure 11: Oxygen concentration versus time
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CPEG(g/l) 10
15
20
In H2 plasma
1.48
3.78
5.98
In O2 plasma
2.96
4.5
6.8
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GD(%Wt)
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5.98
5.98
7.75
7.75
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Table 2 C=O
COH
Unmodified PP membrane
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-
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Modified PP membrane in H2 plasma, CPEG= 20 g/l Modified PP membrane in O2 plasma, CPEG= 20 g/l
3.4%
2.3%
12%
7.3%
5.3%
15,2%
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3.3%
58.5%
13.5%
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10
15
H2 plasma
100
95
91
O2 plasma
100
105
98
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15.787
281.05
251.112
20.520
283.668
31.974
290.142
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Density of summits of the surface (1/μm2)
298.170
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Average ten points height(nm)
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Average roughness(nm)
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381.582
Surface Area Ratio (%) 8.238
9.658
12.696
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Unmodified PP membrane
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Modified PP membrane in H2 plasma, CPEG= 10 g/l
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Table 1: The effect of PEG concentrations, CPEG (g/l), on grafting degree, GD (wt%), for H2 and O2
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Table 3:Meansurface pore diameter (nm) versus PEG concentration (g/l) Table 4: Surface morphology features
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Table 5: Number of adhered platelets to unmodified and modified PP membrane of 30 cm2 surface area
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ACCEPTED MANUSCRIPT Highlights: H2 and O2 plasma graft polymerization of PEG on polypropylene membrane was carried.
Changes in surface properties were investigated by FTIR, XPS, SEM, AFM.
Surface wettability enhanced as a result of poly ethylene glycol grafting.
PEG grafting degree increase causes reduction of fouling and adhesion.
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