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Surface and interface properties of functionalized polysulfones: cell-material interaction and antimicrobial activity.

INTRODUCTION

The development of new polymers, that are both bactericidal and biocompatible, represents a substantial research interest with many applications. Literature highlights that polysulfones (PSFs) are widely used as biomaterials due to several outstanding properties, such as high mechanical strength, resistance to acids and alkalis, thermal and chemical stability, and most importantly, film-forming properties and excellent biocompatibility [1-4]. Therefore, they appear as some of the few biomaterials that can withstand all sterilization techniques, namely steam, ethylene oxide, gamma radiation. Polysulfones are noncrystalline polymers; their chain rigidity is derived from the relatively inflexible and immobile phenyl and S[O.sub.2] groups, whereas their toughness is derived from the connecting ether oxygen [5].

Currently, the application of PSFs is focused on bloodcontact devices (e.g., hemodialysis, hemodiafiltration, and hemofiltration in the form of membranes) [6, 7], cell- and tissue-contact devices (e.g., bioreactors made of hollow-fiber membranes) [8-10], and nerve generation (through semipermeable, hollow PSF membranes) [11, 12]. It is well-known that surface-induced blood coagulation is one of the main problems in the development of blood-contacting materials. From a clinical point of view, literature shows that a biomaterial can be considered as hemocompatible only when its interaction with blood does not cause damage of blood cells or modification in the structure of plasma proteins [13, 14], The surface free energy of biomaterials and the corresponding values of the work of spreading can be used as characterization parameters for predicting cell spreading onto their surfaces and hence, for establishing their blood compatibility.

On the other hand, biomedical polysulfones have been widely used in extracorporeal and biomedical devices. In biomedical polymer research, it is believed that the adsorption of blood components onto polymer surface has a great influence on several biological responses, such as homeostasis, complement activation, and platelet adhesion [15]. Therefore, a thorough understanding of the relationships between polymer surface and blood components is very important for the development of novel biomedical polymers.

Although these materials have excellent overall properties, their intrinsic hydrophobic nature often restricts their utility in membrane applications, such as saline solution perfusion, artificial kidney membranes, and cell culture bioreactors, which require hydrophilic characteristics. Hydrophobic surfaces possess high interfacial free energies in aqueous solutions, which is disadvantageous for such blood-contacting applications [16, 17]. In order to meet these demands, structural modification of conventional materials is often essential. On the basis of the knowledge acquired from the structure-property relationships, a variety of PSFs have been molecularly designed [18-21], however, for an enhanced performance, biologically inert and nontoxic compounds, which exhibit stable mechanical, morphological, and adhesion characteristics during exploitation are preferred. The combination of these factors assures the biocompatibility of PSFs and, implicitly, the possibility of their action in a living organism or in contact with living system without negative consequences. Therefore, chemical modification of PSFs, especially the chloromethylation reaction, is a topic of considerable interest from both theoretical and practical points of view; special efforts are taken for obtaining precursors for functional membranes, coatings, ion exchange resins, ion exchange fibers, and selectively permeable membranes [3, 22]. Also, quaternization with ammonium groups is an efficient method to obtain some properties recommended in multiple applications, for example, as biomaterials and semipermeable membranes. It was shown that these groups can modify hydrophilicity and increase water permeability (of special interest for biomedical applications) [23, 24], antimicrobial action [25, 26], solubility characteristics [27-30], and improve separation [31, 32],

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Previously, the synthesis [33-35] and some solution properties [27-30] of modified polysulfones were presented. Studies have been devoted to the quatemization reaction of chloromethylated polysulfones (CMPSF) for obtaining water-soluble polymers with different ionic chlorine content. The conformational behavior in solution and the experimental and theoretical results of the preferential adsorption coefficients versus mixed solvent composition have been discussed in correlation with the thermodynamic interaction parameters [28, 29, 36, 37]. Also, the structural characteristics of modified polysulfone films were discussed in correlation with their different properties [25, 38], for obtaining membranes with potential applications.

The aim of this work is to provide information concerning improving of some properties of polysulfones by chemical modifications, to obtain advanced membrane materials used in biomedical fields. For establishing the biocompatibility of functionalized polysulfones, the relations between the physicochemical properties of material surface and the adhesion of blood components should be known. Thus, the biocompatibility of these polymers was evaluated on the basis of interfacial tension and balance between adhesion and cohesion of blood cells and sanguine plasma proteins on polysulfones surface. The results were discussed in correlation with the conformational, structural, and morphological aspects of the synthesized polysulfones, in order to identify the optimal molecular aspects, which make them suitable for biomedical applications. Additionally, bacterial adhesion of Pseudomonas aeruginosa ATCC 27853 and Staphylococcus aureus ATCC 25923 microorganisms to the surface will allow extended applications of quatemized polysulfones, as membranes in biomedical domains.

EXPERIMENTAL

Polymer Synthesis

UDEL-1700 polysulfone (Union Carbide, Texas City), designated as PSF in the following text (Scheme 1), a commercial product, was purified by repeated reprecipitations from chloroform and dried for 24 h in vacuum, at 40[degrees]C, before being used in the synthesis of chloromethylated polysulfone. Functionalized polysulfones (Scheme 1) were prepared by a conventional twostage substitution process [33-35], In the first stage, a mixture of commercial paraformaldehyde with an equimolar amount of chlorotrimethylsilane ([Me.sub.3]SiCl, 99% pure, Merck) as a chloromethylation agent, and stannic tetrachloride (Sn[Cl.sub.4], Fluka) as a catalyst, was used for the chloromethylation reaction of polysulfone, at 50[degrees]C. The reaction time was varied from 24 to 140 h, to obtain different substitution degrees (DS) of the chloromethylated polysulfones (CMPSF) [33], In a subsequent stage, CMPSF was dissolved in N,N-dimethylformamide (DMF, 99.8% pure, Fluka), after which the N,N-dimethylethanolamine (DMEA, 99.5% pure, Fluka) quatemized derivative was poured into the reactor. The reaction time was of 48 h, at 60[degrees]C [35].

The characteristics of the synthesized polysulfones are presented in Table 1. The ionic chlorine, [Cl.sub.i] and total chlorine contents were determined by potentiometric titration with a 0.02N AgN[O.sub.3] aqueous solutions (TitraLab Radimeter 840, Copenhagen, Denmark), using the Schoninger method [39]. The ratios between the ionic chlorine and total chlorine contents show that the quatemization reaction of chloromethylated polysulfones occurs at a transformation degree close to 98%. Thus, one may consider that almost all chloromethylenic groups were quatemized.

Contact Angle Measurements

For obtaining thin polysulfone films, polymer solutions of 20 g/dL in DMF for PSF, CMPSF1, and CMPSF2, DMF/methanol (MeOH) and DMF/water for PSFQ1, and in DMF/MeOH and MeOH/water for PSFQ2, respectively, were mixed at different composition ratios of these solvent mixtures, stirred in capped reagent bottles for 2 h at room temperature, and subsequently degassed. The polymer solutions were cast on flat glass and gradually oven-dried at different temperatures, to control the solvent evaporation rate. Finally, the films were placed in a vacuum oven, for 2 days at 50[degrees]C, to remove the residual solvent. Thickness of the resulting films was found to be around 40 [micro]m.

The static contact angles were measured by the sessile-drop method, with a CAM-101 (KSV Instruments, Helsinki, Finland) contact angle measurement system equipped with a liquid dispenser, video camera, and drop-shape analysis software at room temperature. Water (W), glycerol (G), ethylene glycol (EG), and 1-brom-naphthalene (1-Br) were used as test liquids. For each kind of liquid, three different regions of the surface were selected to obtain a statistical result, on taking into consideration the contact angle values of three measurements with an error of [+ or -] 1[degrees]. The relative error on the calculated parameters was in the range of 0.02-0.06%.

Atomic Force Microscopy

Atomic force microscopy (AFM) measurements were made in air, at room temperature, using a Solver PRO-M scanning probe microscope (NT-MDT, Zelenograd, Russia). The surface morphology of films was studied in semi-contact mode, with a commercially available NSG03 rectangularly shaped silicon cantilever (length =135 [+ or -] 5 [micro]m, width =30 [+ or -] 5 [micro]m, thickness = 1-2 [micro]m, probe tip radius = 10 nm). The scanning area was 60 [micro]m x 60 [micro]m.

Antimicrobial Activity Tests

The in vitro antimicrobial activity of PSFQ1 and PSFQ2, assessed on two bacteria strains, by the diffusion method (Kirby-Bauer) certificated by the National Committee on Clinical Laboratory Standards (NCCLS), was evidenced by the occurrence of an inhibition zone. The Gram-positive Staphylococcus aureus ATCC 25923 (S. aureus) and the Gram-negative Pseudomonas aeruginosa ATCC 27853 (P. aeruginosa) were used as test microorganisms, because they are the major cause of cross-infection in hospitals. The preincubation conditions of the test bacteria were 37[degrees]C for 18 h.

Plates with Miieller-Hinton-Broth medium (pH 7.3 at 25[degrees]C) were uniformly inoculated with the test microorganism using a sterile cotton swab. Circular polysulfones films, 10 mm in diameter and 40 [micro]n thick, were applied on the surface of the medium. The plates were incubated at 37[degrees]C for 24 h. The diameter of the inhibition zone (mm) depends both on the polymer present on the medium and on microorganism susceptibility. Finally, the diameter of the inhibition clear zone was measured with a ruler. During antibacterial testing, all measurements were completed in a microbiology laboratory environment of about 24[degrees]C and 55% relative humidity, and repeated four times.

RESULTS AND DISCUSSION

Structure-Wettability-Morphology Relationship in Complex Systems of Polysulfones

The surface energy of a polymer can be evaluated from contact angle measurements using equations of the wetting molecular theory. As known, the water contact angle is an indicator of the wettability of the film surface, and the approach of polymer surface energy permits analysis of sample surface tension according to the surface tension components given by a combination of dispersion (van der Waals forces) and polar (hydrogen bonding) forces. In this context, the acid-base theory (LW/AB) developed by van Oss et al. [40-42] (Eqs. 1-3) was used to calculate the surface tension parameters of some polysulfones. This theory attempts to correlate the surface tension components with their chemical nature.

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[[gamma].sup.AB.sub.sv] = 2[square root of ([[gamma].sup.+.sub.sv][[gamma].sup.-.sub.sv])] (2)

[[gamma].sup.LW/AB.sub.sv] = [[gamma].sup.LW.sub.sv] + [[gamma].sup.AB.sub.sv] (3)

where [theta] is the contact angle determined for test liquids, subscripts "lv" and "vv" denote the liquid-vapor and surface-vapor interfacial tension, respectively, while superscripts "LW" or "d" and "AB" indicate the disperse and polar components obtained from the [[gamma].sup.-.sub.sv] electron-donor and [[gamma].sup.+.sub.sv] electron-acceptor interactions of total surface tension, [[gamma].sup.LW/AB.sub.sv].

As to the acid-base method, the literature [42, 43] indicates that three polar liquids and a set of three equations may be used, if the values of the liquids parameters are not too close. Instead, Della Volpe et al. [44] showed that an improper utilization of the three liquids, without dispersive liquids, or with two prevalently basic or prevalently acidic liquids, strongly increases the ill-conditioning of the system. The different estimates of the acid-base theory, obtainable for the same surface by different triplets of liquids, do not necessarily imply that this method is inconsistent, but may simply reflect the large inaccuracies affecting the results, as due to ill-conditioning. Also, literature [42] shows that contact angles should be measured with a liquid which possesses a surface tension higher than the anticipated solid surface tension, i.e., [[gamma].sub.lv] > [[gamma].sub.sv]. According to these remarks, to determine the disperse and polar components of surface tension of the studied polysulfones, knowledge on the surface tension parameters of liquids test, taken over from literature [45] (Table 2), and on the values of contact angles listed in Table 3, is necessary.

It is known that the values of contact angles are affected by the chemical structure and morphology of the polymer surface, as evidenced in Table 3, which presents the results for relatively hydrophobic surfaces (with high contact angles) converted into hydrophilic surfaces (with low contact angles), by chemical modification of the studied samples. Functionalization of polysulfones--through chloromethylation and quatemization--determines a decline of the contact angles and a corresponding increase of polymer wettability. The increase of the latter parameter is caused by the addition of polar groups to the polysulfone backbone, which exhibit high hydrophilicity. Table 4 lists the results obtained for surface tension components, as well as the electron-donor and electron-acceptor parameters obtained by the LW/AB method.

For these systems, the results obtained indicate that the surface tension parameters were influenced by the charged groups attached to the polymer backbone, as well as by the composition of the solvent/nonsolvent mixtures. Thus, the PSF formed from the aromatic rings connected by one carbon and two methyl groups, oxygen elements, and sulfonic groups, possesses the lowest hydrophilicity ([[gamma].sup.LW.sub.sv] > [[gamma].sup.AB.sub.sv]), whereas the chloromethylated polysulfones with different chlorine content (CMPSF1 and CMPSF2) evidence an increased hydrophilicity, generated by the functional group -C[H.sub.2]Cl (see the values of the disperse and polar surface parameters for PSF, comparatively with CMPSF, in Table 4). Moreover, the results indicate that the PSFQ films are the most hydrophilic ones of all studied samples (lowest water contact angle, Table 3), due to the charged groups--N,N-dimethylethanolamine--attached to the PSF backbone. Hence, it is observed that total surface tension, [[gamma].sup.LW/AB.sub.sv], and the polar component, [[gamma].sup.AB.sub.sv] increase with the substitution degree of CMPSF and with the ionic chlorine content for PSFQ samples, while the disperse component, [[gamma].sup.LW.sub.sv], decreases from PSF to CMPSF and PSFQ. Therefore, the total surface tensions of PSF and CMPSF1 (DS < 1) are dominated by the apolar component, while the total surface tension values of CMPSF2 (DS > 1) and PSFQ are dominated by the polar term, with the electron-donor interactions, [[gamma].sup.-.sub.sv] smaller than the electron-acceptor ones, [[gamma].sup.+.sub.sv] Consequently, the functional -C[H.sub.2]Cl groups introduced by chloromethylation of PSF increase polarity for CMPSF2 with a substitution degree DS > 1; also, the N,N-dimethylethanolamine charged groups introduced by quatemization of CMPSF, increase polarity for PSFQ1 and PSFQ2.

Apart from the nature of the functional groups spread all along the chain and the charge density of the quatemized polysulfones, the history of the films formed from solutions in solvent/nonsolvent mixtures, as well as their surface morphology can also influence the surface tension parameters. Therefore, the apolar and polar surface tension parameters are influenced by the nature and composition of solvent/nonsolvent mixtures from which the films had been prepared. As mentioned in previous studies [25, 28, 29], the solvent systems were selected as a function of the ionic chlorine content of PSFQ, which dictated solubility.

Additionally, literature [25, 49, 50] reported that the chain shape of a polymer in solution could affect the polymer morphology in bulk. Consequently, conformational modifications of quatemized polysulfones chains in mixed solvents, generated by specific interactions, are reflected in the topographic reorganization of the polysulfonic films induced by the nature of solvent/ nonsolvent mixtures.

As known, mixing of DMF with MeOH will induce changes in the formation of hydrogen bonding and dipolar interactions [51]. In addition, in the presence of the polymer, addition of DMF to MeOH leads to self-association and to increased hydrogen bonding between the two components. In particular, it was found out that increasing of the nonsolvent content in the casting solutions favored modification of the ordered domains; increasing of the MeOH content favored the increase of pore number and of their characteristics--area, volume, diameter, while their depth decreases (Table 5, Fig. 1b). This changing trend in morphology is due to the modification of chain conformation of PSFQs, influenced by the quality of the mixed solvents [25]. Moreover, the strong electrostatic interactions in the PSFQ2/DMF/MeOH system are generated by the pronounced electron-donor character of PSFQ2, comparatively with PSFQ1. On the other hand, a higher charge density in PSFQ2 determined the occurrence of nodules, as illustrated in Fig. 2a and b.

Instead, the presence of water (higher content of water) as a nonsolvent in the solutions used for casting films led to lower area pores for both polymer films under study (Table 5, Fig. Id) for PSFQ1 films and Fig. 2d for PSFQ2 films), and to fewer nodules in the PSFQ2 films (Fig. 2d). Also, it may be assumed that the association phenomena of MeOH with water over different composition domains of their mixtures might have been one of the factors affecting the morphology of the PSFQ2 film surface. Therefore, these phenomena changed PSFQ2 solubility, determining modification of solution properties [28, 29], as well as of surface morphology.

As a consequence, knowledge on their hydrophilic/hydrophobic balance in correlation with morphological aspects lays at the basis of their future applications, namely, obtaining of semipermeable membranes with biomedical applications.

Hydrophilic/Hydrophobic Balance of Polysulfone Surfaces

Besides the bulk characteristics, biomedical applications of polysulfones are affected by their low surface energy, determined by the absence of functional groups, which generates weak wettability [17]. Functionalization of PSF with hydrophilic groups (i.e., chloromethylenic and ammonium groups), has been used to improve the wettability, biocompatibility, and other related surface properties, which are responsible for the resulting biological reactions [4, 17]. In most physiological situations, the biomaterial is in contact with aqueous environments, therefore a better understanding of the biological materials-biomaterial interactions can be obtained through interfacial water-polymer tension, which provides information on the intermolecular forces and interfacial structure between two phases. Therefore, in order to design polysulfones as biomaterials, interpretation of their surface characteristics and control of their physical or/and chemical modifications should be especially had in view. Generally, their biocompatibility is mainly affected by surface characteristics, including surface smoothness, distribution and functionality, and hydrophilic/hydrophobic balance of surface [52], In particular, the effect of different functionalized groups and of the history of the films formed from solutions on the surface properties was evaluated by surface free energy, [DELTA][G.sub.w]. (Eq. 4), which expresses the balance between surface hydrophobicity and hydrophilicity [53], Also, the balance hydrophobic/hydrophilic can be described by the interfacial tension, [[gamma].sub.sl] - Eq. 5, and by the work of spreading, [W.sub.s], which represents the difference between the adhesion work, [W.sub.a], and the cohesion work, [W.sub.c] Eq. 6. The values of [DELTA][G.sub.w], presented in Table 4 were obtained from the total surface tension of water, [[gamma].sub.lv] (Table 2), and contact angle of water for the studied samples, [[theta].sub.water] (Table 3).

[DELTA][G.sub.w] = - [[gamma].sub.lv] (1 + COS [[theta].sub.water]) (4)

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Literature [53] mentions that the surface polymer can be considered more hydrophilic for [DELTA][G.sub.w] < -113 mJ/[m.sup.2], whereas, when [DELTA][G.sub.w], > -113 mJ/[m.sup.2], it should be considered more hydro phobic. Thus, the negative values of [DELTA][G.sub.w], (Table 4) and the positive values of the interfacial polymer-water tension (Fig. 3a) reveal an increased wettability, implicitly a high hydrophilicity, a property useful for biomedical applications, from polysulfone to chloromethylated polysulfones and quatemizated polysulfones films.

Also, the hydrophilic/hydrophobic balance of these polymers was described by the work of spreading of water, [W.sub.s,W], over the surface, expressed by Eq. 6. Thus, Fig. 4a plots graphically the negative values of [W.sub.s,W], indicating a lower work of water adhesion, comparatively with the work of water cohesion. Moreover, a simultaneous increase of the work of spreading of water, i.e., [W.sup.PSF.sub.s,W] < [W.sup.CMPSF.sub.s,W] < [W.sup.PSFQ.sub.s,W] is observed, and consequently, an increase of films surface hydrophilicity.

Blood Components--Functionalized Polysulfones Interactions

The interaction between a biomaterial and blood is a key factor in the design of biomedical polymers. It is well-known that the response of a cell in contact with the surface, and the adhesion of cells to the material play an important role in the biocompatibility of materials. Biocompatibility of polymers can be enhanced through changes in polarity and wettability, addition of functional groups that help prevent thrombogenicity, and surface coatings with biologically compatible species, e.g., proteins and antibiotics [54], In addition, blood compatibility is dictated by the manner in which their surfaces interact with blood constituents, such as red blood cells and platelets. In this context, another objective of surface analysis of functionalized polysulfones aimed at a better understanding of the interfacial chemistry of adhesion not only with water, but also with some blood components and plasma proteins. Interactions between blood and a polymer surface depend on blood composition, blood flow, and physico-chemical properties of the polymer surface, such as hydrophobicity/hydrophilicity, roughness, and flexibility, or on the toxicological and electrical properties [55]. Consequently, for assuring a direct contact of the biomaterial with blood, a clear understanding of their interactions is a prerequisite.

For analyzing the possibilities of using quatemized polysulfones in biomedical applications and for establishing their compatibility with some blood compounds--such as red blood cells (RBC) and platelets-- and also with plasma proteins--such as albumin, immunoglobulin G (IgG), and fibrinogen--their interfacial tension parameters (Eq. 5) and work of spreading (Eq. 6) were analyzed. When blood is brought into contact with a polymer surface, adsorption of proteins from sanguine plasma and/or cell adhesion occurs. Therefore, Fig. 3a and b show that the interfacial tensions between polysulfone films and some blood constituents and plasma proteins are influenced by the nature of the functional groups and of the solvent/nonsolvent mixtures (DMF/MeOH, DMF/water, and MeOH/water). The obtained values for red blood cells, platelets, fibrinogen, and IgG increase in the following order: polysulfone, chloromethylated polysulfones, and quatemizated polysulfones, becoming higher with increasing the ionic chlorine content (PSFQ2). Also, the interfacial tensions between quatemized polysulfones films and the above-mentioned components increase in organic solvent-water binary mixtures, namely PSFQ1 in DMF/water and PSFQ2 in MeOH/water. In addition, the values obtained for blood and albumin decrease with increasing the ionic chlorine content, being higher for blood than for albumin. Accordingly, the positive values for the work of spreading of red blood cells, [W.sub.s,RBC]. and the negative values for the work of spreading of platelets, [W.sub.s,p], (Fig. 4a) were used for characterizing biomaterials versus cell adhesion. As a premise, it is known that materials which exhibit a lower work of adhesion would lead to a lower extent of adherent cells than those with a higher work of adhesion [47], Polymer interaction with red blood cells is mediated mostly by the hydrophobic interaction with the lipid bilayer (the red blood cell hydrophobic layer containing transmembrane proteins), the electrostatic interaction with the surface charges or/and the direct interaction with membrane proteins, depending on polymer characteristics. Moreover, electrostatic interactions can appear between the modified polysulfones surface and the red blood cells, due to the positive charge of the quaternized side groups and negative charges of the surface of red blood cells. This type of interaction becomes decisive, along with increasing the contribution of these polysulfones, especially as gene carriers, to biomedical applications.

Blood platelets are essential in maintaining hemostasis, platelets being very sensitive to changes in blood microenvironment. Platelet aggregation is used as a marker for materials' thrombogenic properties, the polymer-platelet interaction being an important step towards understanding their hematocompatibility [56]. Considering the exposure to blood platelets, the negative values of the work of spreading indicate that all samples present a pronounced cohesion (Fig. 4a) which suggests that polymers do not interact with platelets, thus preventing activation of coagulation at the blood-biomaterial interface.

An important aspect in the evaluation of biocompatibility refers to the analysis of the competitive or selective adsorption of blood proteins on biomaterial surface; predictions about these interactions can only be formulated if knowing exactly the structure of the biomaterial. Initially, the surface of an implanted material is mainly coated with albumin, immunoglobulins [especially immunoglobulin G (IgG)] and fibrinogen from plasma, namely the sanguine plasma proteins selected for investigating the affinity of polymer towards physiological fluid media, due to their presence in the biological events from blood. Hence, Fig. 4b exhibits negative values of the work of spreading for albumin, revealing that cohesion prevails, thus favoring a nonadsorbent behavior at the interface. Also, all samples exhibit lower values of the work of spreading for albumin that, along with the rejection of platelets, emphasizes the important role played in material-host interactions. On the other hand, quaternized polysulfones may be considered as compatible with certain elements from the physiological environment (i.e., tissue, cells), since their interaction with the studied biological materials would cause neither damage of the blood cells nor change in the structure of plasma proteins. All these properties, correlated with the microarchitecture of films, recommend them as proper candidates for applications in cellular and tissue engineering. In addition, positive values of the work of spreading for fibrinogen and IgG (Fig. 4b) indicate a higher work of adhesion, comparatively with that of cohesion. These results show them as promising materials for blood-contacting devices (including vascular grafts, stents, pacemakers, extracorporeal circuits, etc.).

Consequently, blood compatibility implies prevention of platelet adhesion and deactivation of the intrinsic coagulation system, generated by the competitive blood protein adsorption on the polymer surface [57], Moreover, literature shows that adhesion of red blood cells onto a surface requires knowledge on their interactions with the vascular components [58]. Thus, endothelial glycocalyx, along with the mucopolysaccharides adsorbed onto the endothelial surface of the vascular endothelium, reject the clotting factors and platelets, known as playing a significant role in thrombus formation. Finally, these results seem to be applicable for evaluating bacterial adhesion on polymer surfaces, and could be subsequently employed for studying possible implanted induced infections, or for obtaining biomembranes.

Structure-Bioactivity Relationship of Polysulfones Containing Quaternary Ammonium Groups

Besides the cell-material interaction, the antibacterial properties also play an important role in medical implants. When an acellular scaffold is implanted, both cells and bacteria compete to adhere and grow onto the surface; this process is called "race for the surface" [59], For a correct functioning of an implant, it is thus critical that the attachment of bacteria be prevented. This can be achieved by making the surface of the implant antibacterial. Literature [60] discusses different possible approaches that prevent the adhesion of bacteria onto the surface. One way is to deposit on the implant surface a coating that offers resistance against bacterial colonization. However, it should be kept in mind that the antibacterial properties of the surface should not compromise the attachment of cells from the surrounding tissue. Some antibacterial polymers, that kill bacteria or prevent their attachment, may be used for such a coating [26]. It is well-known that most bacterial cell walls are negatively charged, containing phosphatidylethanolamine as the major component; hence, most antimicrobial polymers are positively charged. In this regard, polymers with quaternary ammonium groups are the most explored kind of polymeric biocides. It is generally accepted that the mechanism of bactericidal action of polycationic biocides involves destructive interactions with the cell wall and/or cytoplasmic membranes [61]. In this context, the performance of a bactericidal polymer is measured by its ability to strike the balance between microbicidal and biocompatible properties, maximizing the selectivity of the material [62], Therefore, the bioactivity dependence on the pendant chain size of quatemamized polysulfones (PSFQ), allowing establishment of a structure-bioactivity relationship, was investigated against Pseudomonas aeruginosa (P. aeruginosa) and Staphylococcus aureus (5. aureus). As shown in Table 6, these polymers inhibit the growth of microorganisms, the inhibition becoming stronger with increasing the polycationic nature of the modified polysulfone. In addition, the type of nonsolvents--organic (MeOH for PSFQ1 and DMF for PSFQ2) or water--influences the growth and development of microorganisms. Also, for all samples, inhibition is compared to solvent/nonsolvent mixture corresponding to each polymer, which was used as a control sample. These conclusions are evaluated in terms of the diameter of the inhibition zone presented in Table 6.

Worth mentioning here is that the cationic polysulfones modified with quaternary ammonium groups interfere with the bacterial metabolism by electrostatic stacking at the cell surface of bacteria [63], In this context, the specific compositions of the cell wall of Gram-negative (P. aeruginosa) and Gram-positive (S. aureus) bacteria induce different antimicrobial activity. It is known that the component of Gram-positive bacteria cell walls is peptidoglycan, which confers the hydrophobic character of S. aureus, while the major constituent of Gram-negative bacteria cell walls is peptidoglycan, together with other membranes, such as lipopolysaccharides and proteins, assuring the hydrophilic character of P. aeruginosa [63]. Thus, it was found out that inhibition of the hydrophilic P. aeruginosa to the hydrophilic quatemized polysulfones is lower than the inhibition of hydrophobic S. aureus cells. Therefore, all these aspects indicate that the antimicrobial activity depends not only on the substituent groups of the quatemized polysulfones, but also on the hydrophilic or hydrophobic character of bacteria, generating different interactions of the quaternary ammonium salt groups with the bacterial cell membrane. It can be concluded that the exact mechanism of the inhibiting effect of these microorganisms is quite complex, considering that, besides the wall compositions of these bacteria and the surface properties of polysulfones, other types of interactions also occur, e.g., van der Waals and electrostatic interactions.

CONCLUSIONS

New quatemized polysulfones, prepared by quatemization of chloromethylated polysulfones with N,N-dimethylethanolamine, were investigated for establishing the structural characteristics with impact on the surface and morphological properties, as well as their interactions with some blood compounds and plasma proteins.

The polyelectrolyte effect and type of nonsolvents in casting solutions of polymer, identified by viscometric investigations, significantly influenced membrane morphology, the surface tension parameters, surface and interfacial free energy, and the work of spreading of water, and of some blood compounds and plasma proteins. Additionally, the effect of the chemical structure on the surface and interfacial properties evidences a higher hydrophilicity of quatemized polysulfones, characterized by a surface free energy lower than -113 mJ/[m.sup.2], and also positive values of interfacial tensions. These results reflect the capacity of the N-dimethylethanolammonium chloride pendant group to determine the acceptor or donor character of the polar terms, generated by the inductive phenomena of the pendant group.

Surface hydrophobicity and roughness are the parameters controlling the compatibility with blood components and plasma proteins. Therefore, the compatibility between biomaterials and cell adhesion, expressed by the work of adhesion of the red blood cells and platelets, indicates in all cases a pronounced adhesion for red blood cells, and a pronounced cohesion for platelets; the results show that the polysulfones films do not interact with platelets and thus prevent activation of coagulation at the blood/biomaterial interface. As to the compatibility with plasma proteins, such as albumin, immunoglobulin G and fibrinogen--that depends on the characteristics of the polymers themselves--all samples exhibit negative values of spreading work for albumin, revealing that cohesion prevails, along with the rejection of platelets, has an important role in material-host interactions. In addition, positive values of the work of spreading for fibrinogen and IgG indicate a higher work of adhesion comparatively with that of cohesion. As the polysulfone films would cause no damage of blood cells or changes in the structure of plasma proteins, they may be suitable for applications in medical fields.

The antimicrobial activity of polysulfones with quaternary ammonium groups is considered to be one of the most important properties, directly related to new possible applications. Apart from this, certain inhibitory effects of quatemized polysulfones on the growth of P. aeruginosa and S. aureus bacteria have been observed. These properties are useful in investigations on specific biomedical applications and in the utilization of modified polysulfones as semipermeable membranes.

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Anca Filimon, (1) Ecaterina Avram, (1) Simona Dunca (2)

(1) Department of Physical Chemistry of Polymers, "Petru Poni" Institute of Macromolecular Chemistry, 41A Gr. Ghica Voda Alley, 700487 Iasi, Romania

(2) Department of Microbiology, Biology Faculty, "Alexandru loan Cuza" University of Iasi, 11 Carol I Bvd., 700506 Iasi, Romania

Correspondence to: A. Filimon; e-mail: capataanca@yahoo.com

Contract grant sponsor: Romanian National Authority for Scientific Research, CNCS--UEF1SCDI; contract grant number: PN-II-RU-TE-20123-143.

DOI 10.1002/pen.24103

TABLE 1. Characteristics8 of the polysulfone and synthetized
polysulfones.

Properties                            Samples

                     PSF     CMPSF1   CMPSF2   PSFQ1    PSFQ2

DS                    0      0.437    1.541     1.00     1.00
Cl (%)                0       3.34    10.53      0        0
[Cl.sub.i] (%)        0        0        0       2.15     5.71
[m.sub.0]           442.51   463.68   517.17   479.47   647.31
[M.sub.g] (g/mol)   39,000   41,000   46,000   42,000   57,000

(a) Characteristics include substitution degree, DS, chlorine content,
ionic chlorine content, molecular weights of structural units,
[m.sub.0], and number-average molecular weights, [M.sub.g].

TABLE 2. Disperse, [[gamma].sup.d.sub.lv], Polar,
[[gamma].sup.p.sub.lv] electron-donor, [[gamma].sup.-.sub.lv], and
electron-acceptor, [[gamma].sup.+.sub.lv], components to total surface
tension, [[gamma].sub.lv], (mN-m) of the different test liquids used
for contact angle measurements, and some biological materials.

                               [[gamma].sup.d.sub.lv]

Test liquids [45]
Water (W)                              21.80
Ethylene glycol (EG)                   29.00
Glycerol (G)                           34.00
1-Brom-naphtalene (1-Bn)               44.40
Biological materials [46-48]
Blood                                  11.20
Platelet (p)                           99.14
Red blood cell (RBC)                   35.20
Fibrinogen                             37.60
Albumin                                26.80
Immunoglobulin G (IgG)                 34.00

                               [[gamma].sup.p.sub.lv]

Test liquids [45]
Water (W)                              51.00
Ethylene glycol (EG)                   19.00
Glycerol (G)                           30.00
1-Brom-naphtalene (1-Bn)                 0
Biological materials [46-48]
Blood                                  36.30
Platelet (p)                           19.10
Red blood cell (RBC)                    1.36
Fibrinogen                              3.89
Albumin                                35.70
Immunoglobulin G (IgG)                 17.30

                               [[gamma].sup.+.sub.lv]

Test liquids [45]
Water (W)                              25.50
Ethylene glycol (EG)                    1.92
Glycerol (G)                            3.92
1-Brom-naphtalene (1-Bn)                 0
Biological materials [46-48]
Blood                                    --
Platelet (p)                           12.26
Red blood cell (RBC)                    0.01
Fibrinogen                              0.10
Albumin                                 6.30
Immunoglobulin G (IgG)                  1.50

                               [[gamma].sup.-.sub.lv]

Test liquids [45]
Water (W)                              25.50
Ethylene glycol (EG)                   47.00
Glycerol (G)                           57.40
1-Brom-naphtalene (1-Bn)                 0
Biological materials [46-48]
Blood                                    --
Platelet (p)                            7.44
Red blood cell (RBC)                   46.20
Fibrinogen                             38.00
Albumin                                50.60
Immunoglobulin G (IgG)                 49.60

                               [[gamma].sub.lv]

Test liquids [45]
Water (W)                           72.80
Ethylene glycol (EG)                48.00
Glycerol (G)                        64.00
1-Brom-naphtalene (1-Bn)            44.40
Biological materials [46-48]
Blood                               47.50
Platelet (p)                        118.24
Red blood cell (RBC)                36.56
Fibrinogen                          41.50
Albumin                             62.50
Immunoglobulin G (IgG)              51.30

TABLE 3. Contact angle (degree) of different test liquids on
polysulfones films preparation from the solutions in different
solvent/nonsolvent mixtures.

                                          Contact angle

Code     Polymer/solvent mixtures    W     G    EG    1-Bn

1        PSF in DMF                 79    70    60     34
2        CMPSF 1 in DMF             73    72    57     30
3        CMPSF2 in DMF              71    68    52     29
PSFQI/DMF/MeOH
4        100/0 DMF/MeOH             70    67    58     29
5        60/40 DMF/MeOH             68    64    56     27
6        20/80 DMF/MeOH             62    61    50     25
PSFQ 1 /DMF/water
7        60/40 DMF/water            61    58    51     27
8        40/60 DMF/water            57    55    49     21
PSFQ2/DMF/MeOH
9        0/100 DMF/MeOH             62    60    51     26
10       40/60 DMF/MeOH             58    56    49     24
11       80/20 DMF/MeOH             54    52    43     22
PSFQ2/MeOH/water
12       60/40 MeOH/water           57    55    48     23
13       20/80 MeOH/water           54    51    44     19

TABLE 4. Surface tension parameters (mN/m): disperse,
[[gamma].sup.LW.sub.sv], and polar, [[gamma].sup.AB.sub.sv] components
of total surface tension, [[gamma].sup.LW/AB.sub.sv]   and the
electron/acceptor, [[gamma].sup.+.sub.sv], and electron/donor,
[[gamma].sup./.sub.sv], parameters and surface free energy,
[DELTA][G.sub.w] (mJ/[m.sup.2]), for films of studied polysulfones.

Code    Polymer/solvent         LW/AB method
        mixtures
                           [[gamma].sup.LW.sub.sv]

1       PSF in DMF                  18.77
2       CMPSF1 in DMF               17.85
3       CMPSF2 in DMF               13.12
PSFQ 1/DMF/MeOH
4       100/0 DMF/MeOH              9.51
5       60/40 DMF/MeOH              10.02
6       20/80 DMF/MeOH              9.82
PSFQ 1 /DMF/water
7       60/40 DMF/water             9.20
8       40/60 DMF/water             8.52
PSFQ2/DMF/MeOH
9       0/100 DMF/MeOH              8.51
10      40/60 DMF/MeOH              8.20
11      80/20 DMF/MeOH              9.20
PSFQ2/DMF/water
12      60/40 MeOH/water            9.80
13      20/80 MeOH/water            8.80

Code    Polymer/solvent         LW/AB method
        mixtures
                           [[gamma].sup.AB.sub.sv]

1       PSF in DMF                  9.57
2       CMPSF1 in DMF               10.56
3       CMPSF2 in DMF               16.91
PSFQ 1/DMF/MeOH
4       100/0 DMF/MeOH              28.02
5       60/40 DMF/MeOH              31.15
6       20/80 DMF/MeOH              29.41
PSFQ 1 /DMF/water
7       60/40 DMF/water             31.74
8       40/60 DMF/water             30.14
PSFQ2/DMF/MeOH
9       0/100 DMF/MeOH              31.05
10      40/60 DMF/MeOH              31.51
11      80/20 DMF/MeOH              33.32
PSFQ2/DMF/water
12      60/40 MeOH/water            33.13
13      20/80 MeOH/water            31.94

Code    Polymer/solvent         LW/AB method
        mixtures
                           [[gamma].sup.+.sub.sv]

1       PSF in DMF                  9.96
2       CMPSF1 in DMF              11.01
3       CMPSF2 in DMF              17.02
PSFQ 1/DMF/MeOH
4       100/0 DMF/MeOH             22.08
5       60/40 DMF/MeOH             25.01
6       20/80 DMF/MeOH             23.50
PSFQ 1 /DMF/water
7       60/40 DMF/water            26.80
8       40/60 DMF/water            26.10
PSFQ2/DMF/MeOH
9       0/100 DMF/MeOH             26.20
10      40/60 DMF/MeOH             26.40
11      80/20 DMF/MeOH             26.70
PSFQ2/DMF/water
12      60/40 MeOH/water           26.90
13      20/80 MeOH/water           26.30

Code    Polymer/solvent         LW/AB method
        mixtures
                           [[gamma].sup.-.sub.sv]

1       PSF in DMF                  2.30
2       CMPSF1 in DMF               2.53
3       CMPSF2 in DMF               4.20
PSFQ 1/DMF/MeOH
4       100/0 DMF/MeOH              8.89
5       60/40 DMF/MeOH              9.70
6       20/80 DMF/MeOH              9.20
PSFQ 1 /DMF/water
7       60/40 DMF/water             9.40
8       40/60 DMF/water             8.70
PSFQ2/DMF/MeOH
9       0/100 DMF/MeOH              9.20
10      40/60 DMF/MeOH              9.40
11      80/20 DMF/MeOH             10.40
PSFQ2/DMF/water
12      60/40 MeOH/water           10.20
13      20/80 MeOH/water            9.70

Code    Polymer/solvent           LW/AB method
        mixtures
                           [[gamma].sup.LW/AB.sub.sv]

1       PSF in DMF                   28.34
2       CMPSF1 in DMF                28.41
3       CMPSF2 in DMF                30.03
PSFQ 1/DMF/MeOH
4       100/0 DMF/MeOH               37.53
5       60/40 DMF/MeOH               41.17
6       20/80 DMF/MeOH               39.23
PSFQ 1 /DMF/water
7       60/40 DMF/water              40.94
8       40/60 DMF/water              38.66
PSFQ2/DMF/MeOH
9       0/100 DMF/MeOH               39.56
10      40/60 DMF/MeOH               39.71
11      80/20 DMF/MeOH               42.52
PSFQ2/DMF/water
12      60/40 MeOH/water             42.93
13      20/80 MeOH/water             40.74

Code    Polymer/solvent
        mixtures            [DELTA][G.sub.w]

1       PSF in DMF              -86.69
2       CMPSF1 in DMF           -94.08
3       CMPSF2 in DMF           -96.50
PSFQ 1/DMF/MeOH
4       100/0 DMF/MeOH          -97.70
5       60/40 DMF/MeOH         -100.07
6       20/80 DMF/MeOH         -106.98
PSFQ 1 /DMF/water
7       60/40 DMF/water        -108.09
8       40/60 DMF/water        -112.45
PSFQ2/DMF/MeOH
9       0/100 DMF/MeOH         -106.98
10      40/60 DMF/MeOH         -111.98
11      80/20 DMF/MeOH         -115.59
PSFQ2/DMF/water
12      60/40 MeOH/water       -112.45
13      20/80 MeOH/water       -115.59

TABLE 5. Pore characteristics8 of films prepared from quatemized
polysulfones with different solvent/nonsolvent mixtures.

Sample   Cast solvents                   Pore characteristics

                            No. p   Area    Volume   Depth    Diameter

PSFQ1    60/40 DMF/MeOH      36     0.499   74.72    237.62     0.793
         20/80 DMF/MeOH      57     1.441   83.47     88.54     1.350
         60/40 DMF/water     68     0.342    8.85     46.21     0.665
         40/60 DMF/water     212    0.165    4.60     45.74     0.460
PSFQ2    40/60 DMF/MeOH       7     0.186    1.00      6.82     0.489
         80/20 DMF/MeOH       9     0.216    0.88      6.99     0.528
         60/40 MeOH/water    14     0.485    5.84     20.07     0.783
         20/80 MeOH/water     6     0.145    0.25      3.01     0.431

(a) Pore characteristics including number of collected pores (No. p),
area ([micro]m X [micro]m), volume ([micro]m X [micro]m X nm), depth
(nm), and diameter ([micro]m).

TABLE 6. Antimicrobial activity expressed by the diameter of the
inhibition zone (mm) for PSFQ1 and PSFQ2 films prepared from
solutions in different solvent/nonsolvent mixtures, and for solvent/
nonsolvent mixture corresponding to each polymer used as a control
sample against S. aureus and P. aeruginosa microorganisms.

                                    Microorganism test
                                                         Control
Code   Polymeric films              S.          P.       sample
                                   aureus   aeruginosa

5      PSFQ1 in 60/40 DMF/MeOH       11         7           6
8      PSFQ1 in 40/60 DMF/water      7          5           4
10     PSFQ2 in 40/60 DMF/MeOH       14         10          5
13     PSFQ2 in 20/80 MeOH/water     9          7           6
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