Journal of Membrane Science & Technology

Infusion filters are of outstanding importance for the safety in intravenous drug therapy. They are commonly used as inline filters to minimize particle and microbiological burden, to reduce the incidence of phlebitis and sepsis associated with intravenous infusions [1-7]. Most of the membranes used in intensive care units are made of polyamide 6.6 or polyethersulfone. Materials such as cellulose and polycarbonate are not longer in use as membranes in inline filters, but they still can be found in the preparation of pharmaceutical solutions [8]. Polyethersulfone can be modified with different agents, to achieve different membrane properties [5,9]. For inline filtration, polyethersulfone is available in a positively and an uncharged form.

The measurement of the streaming potential is one of the numerous methods in membrane characterization. The streaming potential is an electokinetic effect, which occurs when a mobile charged phase– electrolyte solution–moves with respect to a solid charged membrane. The Zeta potential is related to the electrokinetic potential created at the boundary between the mobile and the solid phase. The Zeta potential depends on the pH and the electrolyte concentration of the feeding solution [10,11]. The membranes may develop a positive or negative Zeta potential, as a result of the interaction with functional groups present on the membrane surface. This aspect is systematically used to develop membranes for specific tasks in separation [12], and to demonstrate the importance of ions on protein transport through semipermeable membranes [11].

The electrokinetic double layer arises on the membrane surface, consisting of the inner Helmholtz layer and the outer Helmholtz layer, also known as Stern layer. The Stern layer is the first adsorbed ion layer, next to the membrane. The extent of ion adsorption is determined by electrical and other interactions between ions in the feeding solution, and the membrane surface. The ions outside the Stern layer form a diffuse layer. At the shear layer, which depends on the movement in the system, the Zeta potential may be measured. Its value is used to characterize the electrical properties of the membrane surface. A continuous measurement of the Zeta potential allows determining qualitative and quantitative changes at the membrane surface.

The aim of the present work is to analyze the changes in the Zeta potential of uncharged (PES⁰) and positively charged (PES⁺) polyethersulfone membranes, resulting from undesired furosemide sodium retention [13]. In the literature, numerous studies on the interactions between different drugs or pharmaceutical additives and filter materials, and/or syringe and tube materials can be found [14-25].

To analyze the chemical composition of the PES⁰ and PES⁺ membranes, X-Ray photoelectron spectroscopy (XPS) was used. XPS is a method, also known as electron spectroscopy for chemical analysis (ESCA). The membrane is irradiated with X-Rays, and photoelectrons are emitted from the surface. The kinetic energy of the emitted electrons is characteristic for the element to which the photoelectrons belong. The results of these experiments are required to gain information on the source of the positive charges at the PES⁺ membrane surface.

Commercial drug solutions

Lasix^® 20 mg solution for injection (Sanofi-Aventis, Frankfurt am Main, Germany): 1 ampoule containing 2 ml solution for injection with 21.3 mg furosemide sodium (corresponding to 20 mg furosemide); additives: sodium chloride, sodium hydroxide, water for injection. To measure the effect of furosemide sodium on the Zeta potential of the membrane, Furosemide was diluted to a concentration of 60 μmol/L.

Eluents

Ultrapure water was prepared with an Ultrapure Water Purification System (Super-AQUADEM^®, Wilhelm Werner GmbH, Leverkusen, Germany). Potassium chloride (analytical grade) and CertiPur^® buffer solution pH 4/7/10 for calibration (all from Merck, Darmstadt, Germany), were used.

Filters

Intrapur^® Paed (BBraun, Melsungen, Germany) is an endotoxinretentive filter with a 0.2 μm Supor^® (positively charged polyethersulfone, PES⁺) membrane and position-independent air removal. Filling volume is 1 ml; the maximum working pressure is 3 bars. Sterifix^® Paed (BBraun, Melsungen, Germany) is equivalent to Intrapur^® Paed. However, it contains an uncharged 0.2 μm polyethersulfone membrane filter (PES⁰).

Streaming potential measurements

The Zeta potential of isolated PES⁰ and PES⁺ filter membranes was determined by a streaming current electrokinetic analyzer (SurPass^®, Anton Paar, Graz, Austria), equipped with a conductivity electrode (Schott^® LF 5100, SI Analytics, Mainz, Germany) and a pH electrode (Schott^® N 61, SI Analytics, Mainz, Germany). VisioLab^® for electrokinetic analyzer (EKA) V1.03 was used for data acquisition. To minimize the background noise, 0.001% NaCl aqueous solution was taken.

To determine the Zeta potential over a period of 10 h, measurements were done every 30 min, during the first nine hours. After 24 h, one final measurement was done. During the same time interval of 30 min, conductivity of the samples and their pH values were measured. The pH value (5.07-6.54) remained constant during the time course of the experiment. All measurements were done in triplicate.

Surface characterization

The chemical characterization of the surface of both PES⁰ and PES⁺ membranes was done by X-ray photoelectron spectroscopy (XPS, Axis^® 165, Kratos Analytical, Manchester, Great Britain), with an Al Kα radiation as the excitation source. The spectra were recorded at a variable angle in the constant pass energy mode at 80 eV and 20 Ev, using an analysis area of 300 μm×700 μm, and a depth of 7 nm. The spectra were evaluated regarding to the C, N, O, and S peaks.

Zeta potential measurements

To detect changes in the Zeta potential over the time course of the experiment, temperature (Figures 1 and 2) and conductivity (Figure 3) were measured in a 0.001% aqueous NaCl solution. The measurements with PES⁺ were done on the apical side of the membrane, which is the side of the influx of the infusion solution, and the basolateral side, which is the side of the efflux of the infusion solution. Both sides seem to be different in their structure [13]. With PES⁰, the experiments were only done on the apical side, as in the SEM pictures taken; no difference between the influx side and efflux side of the PES⁰ membranes could be observed [13].

Figure 1: Zeta potential and temperature changes of PES⁺ membrane over a time period of 10 h (means, n=3). A: ____ Zeta potential of PES⁺ apical side, ••• Zeta potential of PES⁺ basolateral side B: ____ temperature of PES⁺ apical side, ••• temperature of PES⁺ basolateral side.

Figure 2: Zeta potential and temperature changes of PES° membrane over a time period of 10 h (means, n=3). A: Zeta potential of PES° apical side; B temperature of PES° apical side.

Figure 3: Conductivity changes of PES⁰ and PES⁺ membranes in 0.001% NaCl over a period of 10 h (means, n=3): ____ conductivity of PES⁺ apical side, ···· conductivity of PES⁺ basolateral side, - - - conductivity of PES⁰ apical side.

As shown in figures 1-3, all measured data is almost constant during the time course of the experiment.

With increasing furosemide sodium concentration in the feeding solution, the Zeta potential of the PES⁺ membrane changes from positive to negative values (Figure 4). In contrast, with the PES⁰ membrane, there are no remarkable changes of the Zeta potential.

Figure 4: Zeta potential changes of PES⁰ and PES⁺ membranes in dependence of the furosemide concentration over a period of 10 h (means, n=3): ____ Zeta potential of PES⁺ apical side, •••• Zeta potential of PES⁺ basolateral side, - - - Zeta potential of PES⁰ apical side.

To determine the furosemide sodium concentration in the measuring chamber during the time period of the experiment, conductivity was measured (Figure 5).

Figure 5: Conductivity changes of PES⁰ and PES⁺ membranes in dependence of the furosemide concentration over a period of 10 h (means, n=3): ____ conductivity of PES⁺ apical side, •••• conductivity of PES⁺ basolateral side, - - - conductivity of PES⁰ apical side.

Surface analysis (XPS)

The surface of PES⁰ and PES⁺ membranes consist basically of carbon and oxygen, with small amounts of nitrogen and sulfur (Table 1). In the PES⁰ membrane, the nitrogen content was found to be higher than in the PES⁺ membrane.

Table 1: Qualitative and quantitative composition of PES⁰ and PES⁺ membranes at a depth of 7 nm [8].

In figure 6, the XPS spectra of both membranes are displayed. The similarity is noticeable. It is obvious that both membranes have PES as the basic structure.

Figure 6: XPS spectra of PES⁺ (A) and PES⁰ (B) membranes.

In contrast to PES⁺, with PES⁰ a clear N1s-peak can be detected (Figure 7). From its binding energy (399.32 eV), an amine or amide nitrogen is expected. PES⁺ membranes show two structures: one with lower binding energy (398.92 eV) and another, which is positively charged ammonium nitrogen (401.85 eV).

Figure 7: N1s peaks of ____ PES⁰ and ••• PES⁺ membranes.

For both membranes, the O1s spectra are obviously different (Figure 8). In the PES⁺ membrane, the spectrum is characterized by three peaks (530.58 eV (weak), 531.

Figure 8: O1s peaks of ____ PES⁺ and •••• PES⁰ membranes.

The C1s spectra of the PES⁺ and PES⁰ membranes also show differences (Figure 9).

Figure 9: C1s peaks of ____ PES⁺ and •••• PES⁰ membranes.

The positive charge of the PES⁺ membrane, and the resulting positive Zeta potential, decreases with increasing furosemide sodium concentration. It even approximates the negative Zeta potential of the PES⁰ membrane. The membrane charge depends on its chemical structure. Unfortunately, most manufacturers do not give information on the cause of the positive charge. Because of this lack of information, inconsistent data is found in the literature. Even in the present study, a PES⁰ membrane declared as uncharged by the manufacture showed a negative Zeta potential. Usually, end-users do not have information on the exact composition of commercially available membranes [15].

With XPS, it could be shown that both membranes only contain oxygen, carbon, sulfur and nitrogen. The polymer matrix has two characteristic functional groups: the hydrophilic sulfonyl group, which can participate in hydrogen bonding, and the hydrophobic benzene, which interacts in the form of van der Waals forces. In the matrix itself, oxygen is likely to be the most reactive atom. In none of the membranes, ionic additives such as electrolytes could be detected. As a result of the surface analysis, the peaks of interest appear in the photoelectron spectrum at a kinetic energy, which indicates two different functional groups containing nitrogen. In combination with the binding energy of the oxygen, an amide structure in the PES⁰ membrane, and an ammonium structure in the PES⁺ membrane, is most likely.

In 1997, the BBraun Company conducted a research study on endotoxin-retaining PES⁺ membranes [26]. In this investigation, water for injection containing 5.63 EU/ml endotoxin was filtered with a 0.2 μm PES⁺ membrane. This membrane retained the endotoxin effectively. However, in these experiments, an ion-free aqueous endotoxin solution was used as test solution. In the daily routine, most of the infusibile drugs are administered in ionic solutions such as NaCl or Ringer solution. It is unusual to use water for injection as diluent for infusion stock solutions. Because of the ion-free test solution in the BBraun experiment, the Zeta potential hardly changes, and negatively charged endotoxins can be captured by the membrane with the positive Zeta potential. This experimental setup does not reflect the situation in the daily routine. Hence, conclusions for intravenous drug application in neonatology, where the drug concentrations and the infusion rates are very low, cannot be drawn. As a result, it has to be questioned, to what extent positively charged filter membranes do really have an advantage over “uncharged” membranes.