Article Cite This: ACS Biomater. Sci. Eng. XXXX, XXX, XXX-XXX
pubs.acs.org/journal/abseba
Turning Expanded Poly(tetrafluoroethylene) Membranes into Potential Skin Wound Dressings by Grafting a Bioinert Epoxylated PEGMA Copolymer Antoine Venault,*,† Cheng-Sian Liou,† Lu-Chen Yeh,† Jheng-Fong Jhong,† James Huang,‡ and Yung Chang*,† †
Department of Chemical Engineering and R&D Center for Membrane Technology, Chung Yuan Christian University, 200 Chung Pei Road, Chung-Li City 32023, Taiwan ‡ Yeu Ming Tai Chemical Industrial Co. Ltd, Taichung 407, Taiwan S Supporting Information *
ABSTRACT: Despite a set of properties ideal to the design of wound dressings, bioinert membranes are seldom applied as wound-healing systems. This work presents a unique series of random copolymers of glycidyl methacrylate (GMA) and poly(ethylene glycol) methacrylate (PEGMA), namely GMA-rPEGMA, used to surface-modify by grafting onto method polytetrafluorethylene membranes, with the aim of developing wound dressings for quick and efficient wound closure. It is shown that the membrane modified with G50P50 copolymer combines high surface hydrophilicity, high porosity, protein resistance, bacterial resistance, and hemocompatibility, essential properties to wound dressings. Fibrinogen adsorption was measured to be 11.4 ± 3.9% (compared with virgin membrane) correlating with a low water contact angle (14°), whereas the attachment of fluorescent Escherichia coli after 24 h, erythrocytes, leukocytes, thrombocytes, and cells from whole blood was reduced by 85−90%, compared with the virgin membrane. G50P50 membrane was tested as a wound dressing, which outperformed hydrophilic gels of PEGMA in terms of wound-closure kinetic and a commercial dressing in terms of homogeneity of the granulation layer. The facile surface-modification of ePTFE membrane using unique GMA-r-PEGMA copolymer leads to an antibiofouling porous material with improved hemocompatibility combining numerous essential properties of wound dressings and contributing toward the development of the ideal bandage. KEYWORDS: GMA-r-PEGMA, ePTFE membrane, antifouling, hemocompatibility, wound healing
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INTRODUCTION The development of efficient wound dressings has become a major preoccupation of biomaterials scientists to address in particular the concern of population aging. It is well-known that the world demographic shift is associated with constantly growing needs for better health services.1,2 For example, chronic diseases that damage the skin are more likely to be diagnosed in elderly people, and the tissue recovery time scale is increased as skin elasticity is reduced and collagen replacement is much slower. In addition, if continuous research on wound dressings is essential to the elderly, it is also important to the younger generations, particularly in developing countries that have a high birth rate but where the population suffers from limited access to health care.3 For these low- or middle-income countries, there is clearly a need for cheaper means to achieve the same healing results. Therefore, more efforts are needed on the design of more efficient, more cost-effective, and more readily prepared wound dressings. An efficient wound dressing should combine some key physical and chemical properties. Concerning the physical structure, a highly porous material is desired to avoid slowing © XXXX American Chemical Society
down gas exchanges, in particular oxygen transfer, essential to a prompt skin recovery.4 In addition, exudate drainage will be facilitated by porous dressings. The mechanical properties of the material chosen for designing the wound dressing are also essential and have concerned dedicated studies.5,6 The latter properties depend on the matrix composition and on its structure. Although there is not, to the best of our knowledge, an identified ideal elasticity, it is clear that the greater the Young’s modulus, the better the wound protection to environmental mechanical stress. In this respect, dry porous films are much better than hydrogels. As for the chemical properties, hydrophilicity is essential to constantly provide the wound with a moist environment, which benefits versus a dry environment have been thoroughly discussed several decades ago.7 An improvement of surface/matrix hydrophilicity would also prevent unwanted interactions and benefit wound healing for at least three reasons: (1) the interactions of proteins involved in wound repair with the Received: October 2, 2017 Accepted: October 24, 2017 Published: October 24, 2017 A
DOI: 10.1021/acsbiomaterials.7b00732 ACS Biomater. Sci. Eng. XXXX, XXX, XXX−XXX
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ACS Biomaterials Science & Engineering
Scheme 1. Schematic Illustration of the Surface Modification of ePTFE Membranes by Grafting onto GMA-r-PEGMA with the Aim of Preparing Antibiofouling Membranes Suitable for Wound Healing
presented nanofiber mats surface coated with poly(2-methacryloyloxyethyl phosphorylcholine).19 Given the excellent protein and bacterial resistance of the coatings, and also considering the high porosity of their electrospun mat, they suggested that their novel composite matrix could be tested in wound healing. Similarly, PEG-based protein and bacterial-resistant meltcoextruded nanofiber mat were introduced last year,20 and claimed to have potential as a wound-healing material. Another study in that direction is that of Lee et al., who reported on an integrated zwitterionic polymeric material, namely, poly(2-((2hydroxyethyl)(2- methacrylamidoethyl)ammonio)acetate), which depending on the pH of the environment showed either antifouling, antimicrobial, or buffering capacity, properties that are desired in the development of wound dressings for chronic wound healing.21 However, this promising material was not tested as a wound dressing. Similarly, Wang et al. presented a bactericidal and antifouling switchable chitosan materials.22 They covalently attached cationic carboxybetaine esters, hydrolysis of which yielded zwitterionic moieties, thus fulfilling switchable antibacterial and antifouling properties. The authors concluded that their material could present great potential in wound dressing applications. A recent study combined the antifouling properties of zwitterionic hydrogels (sulfobetaine-based) with the antimicrobial activity of silver nanoparticles,23 and the prepared composite hydrogel was tested as a wound dressing with a 100% skin-recovery (wound closure) observed after 15 days. This work, along with that of Lalani and Liu on the design of electrospun poly(sulfobetaine methacrylate) membranes, constitutes one rare example highlighting the suitability of antifouling materials as materials for wound-dressing designs.24 Our past studies on the development of wound dressings starred either zwitterionic or mixed charged groups grown on the surface matrices by a grafting from approach but the controlled growth of the brushes was hard to achieve, because unlike in grafting to, the grafting from approach often leads to partial disintegration of the monomers.25,26 Although fast wound closure was observed
dressing materials, via hydrophobic−hydrophobic interactions, will cause injury while removing the dressing; thus, nonfouling brushes grafted on the dressing would potentially prevent this by resisting protein adhesion;8,9 (2) direct interactions of human cells (fibroblasts, blood cells, etc..) with the dressing or their indirect interactions, mediated by earlier protein adsorption, will also cause a delay of wound healing while removing the matrix and (3) bacterial adhesion inducing further bacterial development can lead to infection. Concerning the third point, many studies are oriented toward the development of dressings incorporating antibacterial agent,10,11 but preventing the attachment of bacteria in the first place would likely be a better strategy as dead bacteria can still mediate the adhesion of live ones. It also appears, from these considerations, that the ideal wound dressing is extremely difficult to design, because compromises must be made. For instance, both dry porous membranes and hydrogels can benefit wound healing but if hydrogels provide a moist environment, some tend to prefer dry films, offering much better mechanical strength and oxygen transfer. An ideal compromise would be a material that combines the physical properties of dry films and the wetting properties of hydrogels. In this respect a hydrophobic membrane, surface-modified with nonfouling brushes, could lead to the desired results, as nonfouling is also associated with improved water entrapment. The development of nonfouling materials has been gaining momentum for the past 20 years as fouling is a major issue in numerous fields, in particular the biomedical one.12 Materials including PEG-based,13,14 zwitterionic-based,15,16 or mixedcharge-based17 have been introduced to reduce the propensity of biomaterials to interact with human fluids and tissues. Given their ability to prevent the interactions of proteins, cells, or bacteria with surfaces through the formation of a hydration layer,18 it could be interesting to incorporate them in wounddressing designs. Indeed, recent studies on antifouling surfaces often mention that the materials/processes developed hold potential in wound-healing applications. Kelowe et al. recently B
DOI: 10.1021/acsbiomaterials.7b00732 ACS Biomater. Sci. Eng. XXXX, XXX, XXX−XXX
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ACS Biomaterials Science & Engineering Table 1. Characterization of the Polymersa
(typically within 2 weeks), further efforts needed to be made to better control the surface modification and prevent the protein-, bacteria- and cell-membrane interactions. This could possibly be done by adopting a grafting onto approach, in which a fine-tuning of the antifouling brushes composition and size can be achieved. Therefore, it appears that the antifouling materials, although widely applied in water-treatment related applications or in the coating of biomedical devices, remains uncommon in the design of wound dressings. Difficult control of the synthesis process or of the surface modification at play may account for this. A readily achieved and controllable surface-modification process is clearly necessary to take advantage of the properties of antifouling materials in wound dressings. To address this need, and to contribute to shed light on an overlooked potential application of antifouling membranes, we designed a unique copolymer made of glycidyl methacrylate (GMA) and poly(ethylene glycol) methacrylate (PEGMA) units (Scheme 1). The polymer, that we will refer to as GMA-r-PEGMA, was grafted onto polytetrafluoroethylene (PTFE) membranes, chosen for their high porosity and outstanding bulk properties. PTFE is difficult to modify, but its bulk properties after functionalization will likely be maintained. As materials used as wound dressing constantly undergo mechanical and environmental stress that may damage their physical properties, PTFE appeared to be a reasonable choice as a starting material. After discussing some aspects of the polymer synthesis and of the surface modification and surface characterization, an in-depth study on the hydration and antibiofouling properties of the membranes was conducted, using fibrinogen, bacteria and blood cells as model biofoulants, in order to select the membrane presenting the best properties. The final aim was to study the applicability of these membranes as healing materials applied on rat wounds, and compare their efficiency with that of highly hydrated materials along with commercial dressing.
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atual theoretical composition of composition in the final polymer the reactive (mol %) solution (mol %) Polymer ID
GMA
PEGMA
GMA
G100P0 G90P10 G80P20 G70P30 G60P40 G50P50 G40P60 G30P70 G20P80 G10P90 G0P100
100 90 80 70 60 50 40 30 20 10 0
0 10 20 30 40 50 60 70 80 90 100
100 88 78 66 62 50 31 30 26 22 0
PEGMA
mol wt Mw (kg/mol)
polydispersity index
0 12 22 34 38 50 69 70 74 78 100
99 936 43 268 91 643 77 643 13 896 14 157 14 375 13 579 13 531 10 553 14 445
1.95 1.87 1.18 1.47 2.89 2.98 3.28 3.56 3.01 3.11 3.22
a
The actual composition was determined by 1H NMR, from the signals of the protons at δ = 4.11 ppm (GMA) and 3.24 ppm (PEGMA). The molecular weight was assessed by GPC.
index detector. For the determination of the molecular weight of copolymers soluble in methanol, the column was a A6000 M column (Malvern) and poly(ethylene glycol) was used as standard. The eluent was a 65% methanol aqueous solution and the elution rate was set to 0.5 mL/min. As for the copolymers soluble in THF, LT3000L, and LT4000L columns (Malvern) were combined, and polystyrene used as standard. The eluent was pure THF, and the elution rate was 1 mL/min. Surface Modification of ePTFE Membranes. ePTFE membranes were positioned in a plasma chamber and pretreated by a mixture of H2 and N2 gas in atmospheric conditions. During this pretreatment step that lasted 3 min, the power was fixed to 200 W and both H2 and N2 flow rate were 8 sccm. Notice that different gas or gas mixtures (H2, H2/N2 and H2/O2) along with different pretreatment durations (ranging from 30 s to 15 min) were previously tested to optimize the surfacemodification process (Figure 1) and the grafting density of the final membranes determined to decide on the best conditions. The pretreatment operation aimed at forming functional groups (−NH2 and − OH) at the surface of ePTFE membranes that would then react with the GMA groups of GMA-r-PEGMA. The optimization results of the pretreatment time reveal that if the grafting density tends to increase with the amount of GMA groups regardless of the pretreatment time, as
EXPERIMENTAL SECTION
Materials. ePTFE microporous matrices were provided by Yeu Ming Tai Chemical Industrial Co. Glycidyl methacrylate (GMA, Mw: 142.15 g/mol) and poly(ethylene glycol) methacrylate (PEGMA, Mw: 500 g/ mol) were obtained from Aldrich, respectively. Hexane (CAS: 110−54− 3, ≥ 95%), methanol (CAS: 67−56−1, ≥ 99%) and ethanol (CAS: 64− 17−5, ≥ 99%) were purchased from Echo Chemical Co. 2,2′-Azobis(2methylbutyronitrile) was bought from Sigma-Aldrich. Phosphate buffer saline (PBS) was obtained from Sigma-Aldrich Co. Tetrahydrofuran (THF) was bought from Tedia. Glutaraldehyde solution 25 wt % was obtained from Acros. Deionized (DI) water was produced in the lab using a Thermo Scientific Pure Lab water purification system. Synthesis and Characterization of GMA-r-PEGMA Random Copolymer. The random copolymer was synthesized by free radical polymerization. PEGMA homopolymer was mixed with GMA in THF, and the reaction was initiated with 2,2′-Azobis(2-methylbutyronitrile). The GMA:PEGMA molar ratio varied (Table 1), with the aim of optimizing the antifouling properties of the final material (grafted membrane). The total solid content in the reactive mixture was 30 wt %, while the total monomer (PEGMA + GMA) to initiator molar ratio was constant to 30:1. The reaction was performed at 55 °C for 24 h. Afterward, the polymer was purified using hexane as a nonsolvent. The obtained polymer was then dried using a vacuum pumping system for 24 h. Finally, a freeze-drying step at −50 °C was performed for 8h. The structure of the random copolymer was assessed by 1H NMR spectroscopy, whereas its molecular weight was determined by gel permeation chromatography (GPC). As for the NMR characterization, the copolymer was dissolved either in D2O (G0P100 to G60P40) or in D-chloroform (G70P30 to G100P0) (1 mg/mL) and the solution was analyzed with a 500 MHz Bruker instrument. GPC was performed on a Viscotek GPCmax chromatograph, associated to a Viscotec refractive-
Figure 1. Optimization of the plasma pretreatment time. C
DOI: 10.1021/acsbiomaterials.7b00732 ACS Biomater. Sci. Eng. XXXX, XXX, XXX−XXX
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ACS Biomaterials Science & Engineering
fluorescence of E. coli with GFP. For bacterial adhesion tests, the membrane samples (1 cm diameter) were positioned in a 24 well plate and thoroughly washed with PBS. Then, 1 mL of bacteria solution was added to each well, and the well plate was positioned in an incubator with the temperature set to 37 °C, for 24 h. To ensure that the membranes would constantly be in contact with live bacteria, we change the bacterial solutiond every 6 h. After the incubation, the bacterial solution was removed and the membranes were thoroughly washed with PBS, to remove loosely adhering bacteria. The extent of bacterial attachment was characterized by confocal microscopy coupled with a resonance scanner, using a Nikon microscope (CLSM A1R model). During the observations, the excitation wavelength (λex) and the emission wavelength (λem) were set to 488 and 520 nm, respectively. In addition, ImageJ software, an open source image processing software, was used to quantify bacterial attachment (presented in this work as bacterial density in cells/mm2) from confocal images. For each membrane, five independent tests were performed. Blood cell adhesion tests were performed using either erythrocyte concentrate, leukocyte concentratre, platelet-rich-plasma solution (PRP), or whole blood. Blood was obtained from healthy volunteers in a blood center. For individual cell tests, blood was fractionated, accordingly to an experimental protocol earlier reported.26 The membranes (5 independent samples) were placed in individual well of a 24-well plate, and equilibrated with PBS. The well plate was left for 24 h in an incubator with the temperature set to 37 °C. Afterward, 800 μL of either erythrocyte concentrate, leukocyte concentrate, PRP, or whole blood were dropped on the membranes. Incubation of the membrane samples with the blood fraction/whole blood was performed at 37 °C for 2 h. At the end of the incubation, the blood fraction/whole blood was removed and the membranes were washed with PBS, at least 5 times. The washing step was followed by a 4 h fixing step at 4 °C, achieved using 0.8 mL of a glutaraldehyde solution. Notice that the glutaraldehyde solution was previously prepared by mixing 1 mL of a commercial glutaraldehyde solution 25 wt % with 9 mL of PBS. Eventually, the membranes were washed with PBS, and observed with a confocal microscope (same instrument as that used in bacterial adhesion tests). The degree of cell adhesion was also quantified using ImageJ software. Wound Healing Properties of Virgin and Grafted ePTFE Membrane. GMA-r-PEGMA-grafted ePTFE membranes were used as wound dressings on rats, bought from BioLasco Taiwan Co. Each animal weighed approximatively 400 g. At first, tranquilizer (0.15 mL) was subcutaneously injected (Rompun 2% obtained from Bayer Healthcare A.G.). Subsequently, the animals were anesthetized using 0.2 mL of a general injectable anesthetic agent, Zoletil 50. The animals were then shaved using an electrical razor, and wounds created with a sterile disposable biopsy punch (Integra Miltex). The diameter of the wound created was 1.5 cm. Two wounds were patterned on each animal. The wounds were then immediately covered with the grafted membrane that provided the best combination of antibiofouling properties. Different controls were also used among with ePTFE membrane, PEGMA500 hydrogel, and a commercial DuoDerm dressing (composite material made of polyurethane, pectin, gelatin and sodium carboxymethyl cellulose). In addition, some wounds were left uncovered. Notice that PEGMA500 gel, ideal antifouling hydrogel, was selected as it offers a similar level of hydration to the selected membrane, according to a previous work;30 and were prepared following the same experimental protocol. The wound dressings were fixed to the wounded areas with a bandage. The extent of wound closure was monitored daily, in order to establish the kinetics of wound healing. In addition, histological analysis of the newly formed skin was performed after 2 weeks, using a Nikon Eclipse 80i microscope. For this purpose, the tissues were first dipped into formalin before undergoing a hematoxylin-eosin staining step.
also discussed later in the Results and discussion section, it is seen that above 3 min, the final grafting density of the membranes tends to be significantly higher than if 15 s, 30 s, or 1 min are chosen. On the other hand, further increasing the pretreatment duration to 15 min does not lead to a significant increase in the grafting density in the conditions tested, possibly because saturation of the membrane surface was reached at 3 min. Thus, 3 min was chosen as the pretreatment duration. Afterward, activated ePTFE membranes were immersed in a solution of GMA-r-PEGMA in THF solvent (G100P0 to G70P30) or methanol (G60P40 to G0P100) solvent at a concentration of 5 mg/mL and for 3 h at 60 °C. As a result, the epoxy group of GMA reacted with the hydroxyl and amine groups at the surface of the activated ePTFE membranes, resulting in a ring-opening reaction followed by the condensation of the copolymer. This operation eventually led to the surface modified membranes. Finally, membranes were immersed in an ultrasonic bath containing DI-water for 1 h to remove the loosely attached random copolymer brushes which may contain unreacted epoxides. Surface Characterization of Virgin and Grafted Membranes. The surface chemistry of grafted membranes was characterized by XPS, using a similar instrument and experimental protocol to those employed in our previous work.25 The structure of membranes surface was analyzed by SEM (Hitachi S-3000). Prior to observation, the virgin and grafted membranes were mounted on SEM holder, sputter-coated with gold for 150 s, and the holder positioned in the SEM chamber. The purpose of this analysis was to determine if the surface modification led to an apparent change of surface porosity. However, we also conducted porosity measurements. For this, membranes were dried in a vacuum oven and weighed. Subsequently, they were immersed in ethanol for 24 h. Afterward, wet membranes were weighed (after gently wiping out superficial liquid). The porosity of the membranes could then be evaluated using a formula reported in literature,27 from the knowledge of ePTFE and ethanol density (2.2 g/cm3 and 0.789 g/L at room temperature, respectively) and assuming that the surface modification did not lead to significant changes of the whole material density. The hydration properties of membranes were evaluated by carrying out water contact angle measurements in air and by measuring their hydration capacity. For water contact angle, we used a Kyowa Interface Science (Japan) automatic contact angle meter (CA-VP model), operated at ambient temperature. The 4 μL droplet is deposited by the automatic syringe onto the surface of the membrane sample, and the contact angle (spherical fit) automatically measured and recorded. Seven to ten independent measurements were performed on each membrane, and the average taken as the WCA of the membrane. Regarding the hydration capacity of membranes, membrane samples (disk of 1 cm diameter) were dried in a vacuum oven, weighed, and immersed in DI water for 24 h. Afterward, superficial water was gently wiped out with tissue, and wet weights were recorded. The hydration capacity is the difference between the wet and the dry weights per unit surface area (one could also have estimated it per unit volume, but as we performed a surface modification, most of the hydrophilic brushes enabling water trapping were at the surface of the grafted samples). Five independent measurements were done, and the average taken as the hydration capacity of the sample considered. Resistance to Biofouling and Hemocompatibility Tests. Biofouling and hemocomaptibility were studied in static conditions, by performing fibrinogen (FN) adsorption, bacterial (Escherichia coli) attachment tests and blood cell (erythrocytes, leukocytes, thrombocytes and whole blood) adhesion tests. FN adhesion was studied by enzyme linked immunosorbent assay (ELISA), and test well documented in literature.28,29 Bacterial attachment tests were performed with Escherichia coli modified with a Green Fluorescent Protein (E. coli with GFP). Bacteria were cultured in an erlenmeyer flask containing beef extract and peptone at a concentration of 3 and 5 mg/mL, respectively. The flask was shaken in an incubator which temperature was maintained constant to 37 °C. The stationary phase was reached after about 12 h, which corresponded to a bacterial concentration of 1 × 107 cells/mL. Then, 0.5 mL of the obtained solution was transferred into a 50 mL LB medium. After a 3-hincubation at 37 °C, isopropyl-β-d-1-thiogalactopyranoside (V = 200 μL, 0.1 M) was added to the mixture in order to further enhance the
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RESULTS AND DISCUSSION Some Aspects of the Random Copolymer Synthesis. The random copolymer was synthesized by free radical polymerization. To verify that the reaction between monomers was well-controlled, we performed 1H NMR tests. The NMR D
DOI: 10.1021/acsbiomaterials.7b00732 ACS Biomater. Sci. Eng. XXXX, XXX, XXX−XXX
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ACS Biomaterials Science & Engineering
Figure 2. NMR characterization of the different copolymers synthesized in view of modifying the ePTFE membranes. The signals at δ = 3.24 ppm (proton attached to the epoxy group of GMA) and δ = 4.11 ppm (proton in β-position of the ether group in PEGMA) were used to determine the actual composition of the copolymers.
Figure 3. Surface structure of the virgin and modified membranes. The porosity ε (%) is also indicated..
spectra of PEGMA, GMA and GMA-r-PEGMA are presented in Figure 2. The intensity of signal at δ = 3.24 ppm (proton attached to the epoxy group of GMA) and that at 4.11 ppm (CH2, PEGMA) were used to determine the actual composition of the polymers, as summarized in Table 1. It is seen that the actual ratios are close to the initial molar ratios in the reactive solution,
indicating that both types of monomer have similar reactivity, hence facilitating the control of the polymerization reaction. Additionally, it is worth mentioning that the composition of the copolymers strongly affects their solubility property as it was found that copolymers G100P0 to G70P30 were soluble in E
DOI: 10.1021/acsbiomaterials.7b00732 ACS Biomater. Sci. Eng. XXXX, XXX, XXX−XXX
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ACS Biomaterials Science & Engineering
Figure 4. Hydration properties (water contact angle and hydration capacity) and grafting density of surface-modified ePTFE membranes.
Figure 5. Effect of grafting on surface chemistry assessed by XPS. (a) C 1s spectra of all membranes; (b) magnification of C 1s spectra of virgin and G50P50 membrane.
tetrahydrofuran while copolymers G60P40 to G0P100 were soluble in methanol. Characterization of Grafted ePTFE Membranes. The structure of the membranes was observed by SEM, and related images are presented in Figure 3. It is seen that the surface modification did not affect in an important extent the surface of the membrane, as all surfaces look alike without any important decreasing of surface porosity. Measurements of surface porosity confirmed these observations as they were all found to be in the same range (62 to 69%). This suggests that surface modification is rather homogeneous and performed at the nanoscale, without the formation of aggregates that would lead to partial pore blockage accompanied by a significant reduction of surface porosity.
To evaluate the efficiency of the grafting, we conducted a number of physicochemical characterization tests. From the determination of the grafting density presented in Figure 4, it is seen that all polymers, except G0P100, could be efficiently anchored to the ePTFE matrix, with grafting density ranging between 0.10 mg/cm2 and 1.99 mg/cm2. In general, we observed that the grafting density was positively correlated with the amount of GMA repeat units in the polymer, because GMA is the anchoring block: membranes modified with G100P0, G90P10, G80P20 and G70P30 polymer presented a grafting density higher than 1 mg/cm2. On the other hand, membranes modified with random copolymers containing between 40% and 100% PEGMA (from G60P40 to G0P100) had a grafting density ranging between 0.05 and 0.35 mg/cm2: the low amount of GMA F
DOI: 10.1021/acsbiomaterials.7b00732 ACS Biomater. Sci. Eng. XXXX, XXX, XXX−XXX
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ACS Biomaterials Science & Engineering repeat units reflects on the final amount of grafted copolymer. However, these values may be still high enough to expect efficient surface modification and significant improvement of the antifouling properties of the membranes according to former studies reported on surface modification of membranes that showed that low coating/grafting density still led to improved fouling resistance,31−33 but this will be checked in the following sections. The efficiency of the grafting is also confirmed by a chemical analysis of the surface of the grafted membranes. In Figure 5, the characteristic peak of C−F bond in ePTFE (BE = 292 eV) can no longer be seen on the pretreated membrane, suggesting that the activation arose in the formation of numerous hydroxyl groups covering the entire membrane surface. Logically, this characteristic peak cannot be seen either on the spectra of all surface-modified membrane. The efficient and controlled surface grafting can be proven by analyzing in more detail the variation in the characteristic peak corresponding to C−O (BE = 286.2 eV) and O−C=O functional groups (BE = 288.7 eV).34−36 Larger intensities were measured for both G50P50 and G40P60, as these copolymers provide an ideal balance between the number of anchoring segment (GMA) in the copolymer and that of antifouling segments bearing the ether groups (PEGMA). Whether the GMA or the PEGMA content increased in the copolymer used to modify the membranes, the intensity of these peaks tended to decrease. In the first case, much more GMA in the copolymer was associated with many fewer ether groups, whereas in the second case, the increasing content of PEGMA was associated with a less efficient anchoring of the copolymer. Hydrophilic Properties of Grafted ePTFE Membranes. The water contact angle and the hydration capacity of membranes were determined to evaluate the hydrophilic properties of the membranes. Research on the development of wound dressings is currently oriented toward the design of superhydrophilic membranes or hydrogels.24,37 Indeed, to apply the grafted ePTFE membranes as wound dressings, they should first be able to maintain a high level of hydration above the wound, a first essential condition to healing. Our base material, ePTFE membrane, was obviously not chosen for its wetting properties and had to be modified to promote water penetration in the matrix. The results of these hydration tests are also displayed in Figure 4 along with those on grafting densities earlier discussed. They clearly indicate that the grafting with polymers containing between 80% (G80P30) and 100% GMA (G100P0) is not efficient to improve the surface hydrophilicity, as the water contact angle remained very high (>100°), whereas the hydration capacity was low (
