Ion-exchange fibers and drugs: a transient study.

The objective of this study was to theoretically model and experimentally measure the kinetics and extent of drug release from different ion-exchange materials using an in-house-designed flow-cell. Ion-exchange fibers (staple fibers and fiber cloth) were compared with commercially available ion-exchange materials (resins and gels). The functional ion-exchange groups in all the materials were weak -COOH or strong -SO3H groups. The rate and extent of drug release from the fibers (staple fiber>fiber cloth) was much higher than that from the resin or the gel. An increase in the hydrophilicity of the model drugs resulted in markedly higher rates of drug release from the fibers (nadolol>metoprolol>propranolol>tacrine). Theoretical modelling of the kinetics of ion exchange provided satisfactory explanations for the experimental observations: firstly, a change in the equilibrium constant of the ion-exchange reaction depending on the drug and the ion-exchange material and, secondly, a decrease in the Peclet number (Pe) with decreasing flow-rate of the drug-releasing salt solution.

Ion-exchange fibers and drugs: an equilibrium study.

The purpose of this study was to investigate the mechanisms of drug binding into and drug release from cation-exchange fibers in vitro under equilibrium conditions. Ion-exchange groups of the fibers were weakly drug binding carboxylic acid groups (-COOH), strongly drug binding sulphonic acid groups (-SO(3)H), or combinations thereof. Parameters determining the drug absorption and drug release properties of the fibers were: (i) the lipophilicity of the drug (tacrine and propranolol are lipophilic compounds, nadolol is a relatively hydrophilic molecule), (ii) the ion-exchange capacity of the fibers, which was increased by activating the cation-exchange groups with NaOH, (iii) the ionic strength of the extracting salt (NaCl), which was studied in a range of 1.5 mM to 1.5 M, and finally (iv) the effect of divalent calcium ions (CaCl(2)) on the release of the model drugs, which was tested and compared to monovalent sodium ions (NaCl), and combinations thereof. It was found that the lipophilic drugs, tacrine and propranolol, were retained in the fibers more strongly and for longer than the more hydrophilic nadolol. The more hydrophilic nadolol was released to a greater extent from the fibers containing strong ion-exchange groups (-SO(3)H), whereas the lipophilic drugs were attached more strongly to strong ion-exchange groups and released more easily from the weak (-COOH) ion-exchange groups. The salt concentration and the choice of the salt also had an effect: at lower NaCl concentrations more drug was released as a result of the influence of both electrostatic and volume effects (equimolar drug:salt ratio). Incorporation of CaCl(2) in the bathing solution increased drug release considerably as compared to NaCl alone. The equilibrium distribution of the drug species between the fiber and external solution phases was also simulated and it was found that the theoretical modelling proposed describes adequately the basic trends of the behavior of these systems.

Controlled transdermal iontophoresis by ion-exchange fiber.

The objective of this study was to assess the transdermal delivery of drugs using iontophoresis with cation- and anion-exchange fibers as controlled drug delivery vehicles. Complexation of charged model drugs with the ion-exchange fibers was studied as a method to achieve controlled transdermal drug delivery. Drug release from the cation-exchange fiber into a physiological saline was dependent on the lipophilicity of the drug. The release rates of lipophilic tacrine and propranolol were significantly slower than that of hydrophilic nadolol. Permeation of tacrine across the skin was directly related to the iontophoretic current density and drug concentration used. Anion-exchange fiber was tested with anionic sodium salicylate. The iontophoretic flux enhancement of sodium salicylate from the fiber was substantial. As the drug has to be released from the ion-exchange fiber before permeating across the skin, a clear reduction in the drug fluxes from the cationic and anionic fibers were observed compared to the respective fluxes of the drugs in solution. Overall, the ion-exchange fibers act as a drug reservoir, controlling the release and iontophoretic transdermal delivery of the drug.

Double layer potential and degree of dissociation in charged lipid monolayers.

One of the contributions to the surface potential in charged phospholipid monolayers at air-water interfaces is the double layer potential. In this note several misconceptions found in the literature concerning the relationship between the double layer potential and the degree of dissociation of the lipid polar headgroups are critically analyzed. The deviations of the double layer potential measurements from the Gouy-Chapman theory observed by several authors are explained by taking into account the dependence of the degree of dissociation with concentration, area per lipid molecule and pH.

True and apparent oxygen permeabilities of contact lenses.

We studied the passage of oxygen through some commercially available contact lenses. Oxygen diffusion coefficients were determined by the time-lag method and a 201T Redher permeometer was used to measure the oxygen permeability and transmissibility by the polarographic method. The measurements were carried out at room temperature with 0.09% sodium chloride physiologic solution. The following types of lenses were tested: (1) 12 lenses of cellulose acetate butyrate (CAB) of a mean thickness of 0.194 mm (observed Dk approximately 6.3 barrers) (1 barrier is equivalent to 10(-11) cm3 of O2 (STP).cm2/cm3.s.mm Hg). (2) 13 lenses of a cross-linked polyhydroxyethyl methacrylate (2-HEMA), manufactured by Lenticon and Bausch & Lomb, with 40 and 38.6% water content, respectively. The mean thicknesses were 0.160 and 0.148 mm, respectively (observed Dk approximately 12 to 13.5 barrers). (3) Finally 10 lenses of a copolymer of 2 HEMA with N-2-vinylpyrrolidone (2-HEMA/VP), manufactured by Bausch & Lomb under the name Hydrocurve II, with 55% water content and a mean thickness of 0.138 mm (observed Dk approximately 24.5 barrers). For a given lens thickness, the transmissibility and permeability of lenses whose main material is 2-HEMA are found to be equivalent. This fact suggests the use of such material as a standard in the study of diffusion processes in contact lenses of low oxygen permeability (Dk approximately 12 to 13.5 barrers). We studied the boundary layer effects and found significant discrepancies between true and apparent oxygen permeabilities. The apparent transmissibility decreased with increasing lens thickness, this effect being more apparent for lenses with low water content. Oxygen permeability is found to be exponentially dependent on water content rather than on the chemical composition of the hydrogel.