Unexpected Behavior of Streaming Potential in Ion-Exchange Membranes.

Streaming potential is one of the numerous electrokinetic phenomena created when an electrolyte flows along a charged surface. In membranes, applying the charged cylindrical pore model, streaming potential can be used to estimate, e.g., the pore size and the charge density of such pores. In this study, we are extending streaming potential experiments to ion-exchange membranes (IEMs) and trying to verify the existing models with the measurements. According to the Donnan equilibrium between an electrolyte solution and an IEM, the solution concentration should not affect the streaming potential if the membrane charge is even moderately low. Yet, the streaming potential varied substantially with the solution concentration, as in the case of nearly neutral porous membranes. In addition, the existing theory does not include the membrane thickness, but we found that thinner membranes showed larger streaming potentials. These dilemmas are discussed in this paper.

Shape and Phase Transitions in a PEGylated Phospholipid System.

Poly(ethylene glycol) (PEG) polymers and PEG-conjugated lipids are widely used in bioengineering and drug transport applications. A PEG layer in a drug carrier increases hydrophilic repulsion, inhibits membrane fusion and serum opsonin interactions, and prolongs the storage and circulation time. It can also change the carrier shape and have an influence on many properties related to the content release of the carrier. In this paper, we focus on the physicochemical effects of PEGylation in the lipid bilayer. We introduce laurdanC as a fluorophore for shape recognition and phase transition detection. Together with laurdanC, cryogenic transmission electron microscopy, differential scanning calorimetry, molecular dynamics simulations, and small-angle X-ray scattering/wide-angle X-ray scattering, we acquire information of the particle/bilayer morphology and phase behavior in systems containing 1,2-dipalmitoyl- sn-glycero-3-phosphocholine:1,2-distearoyl- sn-glycero-3-phosphoethanolamine-PEG(2000) with different fractions. We find that PEGylation leads to two important and potentially usable features of the system. (1) Spherical vesicles present a window of elevated chain-melting temperatures and (2) lipid packing shape-controlled liposome-to-bicelle transition. The first finding is significant for targets requiring multiple release sequences and the second enables tuning the release by composition and the PEG polymer length. Besides drug delivery systems, the findings can be used in other smart soft materials with trigger-polymers as well.

Inverse Thermoreversible Mechanical Stiffening and Birefringence in a Methylcellulose/Cellulose Nanocrystal Hydrogel.

We show that composite hydrogels comprising methyl cellulose (MC) and cellulose nanocrystal (CNC) colloidal rods display a reversible and enhanced rheological storage modulus and optical birefringence upon heating, i.e., inverse thermoreversibility. Dynamic rheology, quantitative polarized optical microscopy, isothermal titration calorimetry (ITC), circular dichroism (CD), and scanning and transmission electron microscopy (SEM and TEM) were used for characterization. The concentration of CNCs in aqueous media was varied up to 3.5 wt % (i.e, keeping the concentration below the critical aq concentration) while maintaining the MC aq concentration at 1.0 wt %. At 20 °C, MC/CNC underwent gelation upon passing the CNC concentration of 1.5 wt %. At this point, the storage modulus ( G') reached a plateau, and the birefringence underwent a stepwise increase, thus suggesting a percolative phenomenon. The storage modulus ( G') of the composite gels was an order of magnitude higher at 60 °C compared to that at 20 °C. ITC results suggested that, at 60 °C, the CNC rods were entropically driven to interact with MC chains, which according to recent studies collapse at this temperature into ring-like, colloidal-scale persistent fibrils with hollow cross-sections. Consequently, the tendency of the MC to form more persistent aggregates promotes the interactions between the CNC chiral aggregates towards enhanced storage modulus and birefringence. At room temperature, ITC shows enthalpic binding between CNCs and MC with the latter comprising aqueous, molecularly dispersed polymer chains that lead to looser and less birefringent material. TEM, SEM, and CD indicate CNC chiral fragments within a MC/CNC composite gel. Thus, MC/CNC hybrid networks offer materials with tunable rheological properties and access to liquid crystalline properties at low CNC concentrations.

Extended Pharmacokinetic Model of the Rabbit Eye for Intravitreal and Intracameral Injections of Macromolecules: Quantitative Analysis of Anterior and Posterior Elimination Pathways.

PURPOSE: To extend the physiological features of the anatomically accurate model of the rabbit eye for intravitreal (IVT) and intracameral (IC) injections of macromolecules. METHODS: The computational fluid dynamic model of the rabbit eye by Missel (2012) was extended by enhancing the mixing in the anterior chamber with thermal gradient, heat transfer and gravity, and studying its effect on IC injections of hyaluronic acids. In IVT injections of FITC-dextrans (MW 10-157 kDa) the diffusion though retina was defined based on published in vitro data. Systematic changes in retinal permeability and convective transport were made, and the percentages of anterior and posterior elimination pathways were quantified. Simulations were compared with published in vivo data. RESULTS: With the enhanced mixing the elimination half-lives of hyaluronic acids after IC injection were 62-100 min that are similar to in vivo data and close to the theoretical value for the well-stirred anterior chamber (57 min). In IVT injections of FITC-dextrans a good match between simulations and in vivo data was obtained when the percentage of anterior elimination pathway was over 80%. CONCLUSIONS: The simulations with the extended model closely resemble in vivo pharmacokinetics, and the model is a valuable tool for data interpretation and predictions.

Indocyanine Green-Loaded Liposomes for Light-Triggered Drug Release.

Light-triggered drug delivery systems enable site-specific and time-controlled drug release. In previous work, we have achieved this with liposomes containing gold nanoparticles in the aqueous core. Gold nanoparticles absorb near-infrared light and release the energy as heat that increases the permeability of the liposomal bilayer, thus releasing the contents of the liposome. In this work, we replaced the gold nanoparticles with the clinically approved imaging agent indocyanine green (ICG). The ICG liposomes were stable at storage conditions (4-22 °C) and at body temperature, and fast near-infrared (IR) light-triggered drug release was achieved with optimized phospholipid composition and a 1:50 ICG-to-lipid molar ratio. Encapsulated small molecular calcein and FITC-dextran (up to 20 kDa) were completely released from the liposomes after light exposure for 15 s. Location of ICG in the PEG layer of the liposomes was simulated with molecular dynamics. ICG has important benefits as a light-triggering agent in liposomes: fast content release, improved stability, improved possibility of liposomal size control, regulatory approval to use in humans, and the possibility of imaging the in vivo location of the liposomes based on the fluorescence of ICG. Near-infrared light used as a triggering mechanism has good tissue penetration and safety. Thus, ICG liposomes are an attractive option for light-controlled and efficient delivery of small and large drug molecules.

Photothermally Triggered Lipid Bilayer Phase Transition and Drug Release from Gold Nanorod and Indocyanine Green Encapsulated Liposomes.

In light-activated liposomal drug delivery systems (DDSs), the light sensitivity can be obtained by a photothermal agent that converts light energy into heat. Excess heat increases the drug permeability of the lipid bilayer, and drug is released as a result. In this work, two near-IR responsive photothermal agents in a model drug delivery system are studied: either gold nanorods (GNRs) encapsulated inside the liposomes or indocyanine green (ICG) embedded into the lipid bilayer. The liposome system is exposed to light, and the heating effect is studied with fluorescent thermometers: laurdan and CdSe quantum dots (QDs). Both photothermal agents are shown to convert light into heat in an extent to cause a phase transition in the surrounding lipid bilayer. This phase transition is also proven with laurdan generalized polarization (GP). In addition to the heating results, we show that the model drug (calcein) is released from the liposomal cavity with both photothermal agents when the light power is sufficient to cause a phase transition in the lipid bilayer.

Electrochemically controlled proton-transfer-catalyzed reactions at liquid-liquid interfaces: nucleophilic substitution on ferrocene methanol.

The generation of α-ferrocenyl carbocations from ferrocenyl alcohols for S(N)1 substitution at the water-organic solvent interface is initiated by the transfer of protons into the organic phase. The proton flux, and hence the reaction rate, can be controlled by addition of a suitable "phase-transfer catalyst" anion or by external polarization with a potentiostat, providing a new method for the synthesis of ferrocene derivatives.

Biomimetic oxygen reduction by cofacial porphyrins at a liquid-liquid interface.

Oxygen reduction catalyzed by cofacial metalloporphyrins at the 1,2-dichlorobenzene-water interface was studied with two lipophilic electron donors of similar driving force, 1,1'-dimethylferrocene (DMFc) and tetrathiafulvalene (TTF). The reaction produces mainly water and some hydrogen peroxide, but the mediator has a significant effect on the selectivity, as DMFc and the porphyrins themselves catalyze the decomposition and the further reduction of hydrogen peroxide. Density functional theory calculations indicate that the biscobaltporphyrin, 4,5-bis[5-(2,8,13,17-tetraethyl-3,7,12,18-tetramethylporphyrinyl)]-9,9-dimethylxanthene, Co(2)(DPX), actually catalyzes oxygen reduction to hydrogen peroxide when oxygen is bound on the "exo" side ("dock-on") of the catalyst, while four-electron reduction takes place with oxygen bound on the "endo" side ("dock-in") of the molecule. These results can be explained by a "dock-on/dock-in" mechanism. The next step for improving bioinspired oxygen reduction catalysts would be blocking the "dock-on" path to achieve selective four-electron reduction of molecular oxygen.

Bipolar Membrane Electrodialysis for Sulfate Recycling in the Metallurgical Industries.

Demand for nickel and cobalt sulfate is expected to increase due to the rapidly growing Li-battery industry needed for the electrification of automobiles. This has led to an increase in the production of sodium sulfate as a waste effluent that needs to be processed to meet discharge guidelines. Using bipolar membrane electrodialysis (BPED), acids and bases can be effectively produced from corresponding salts found in these waste effluents. However, the efficiency and environmental sustainability of the overall BPED process depends upon several factors, including the properties of the ion exchange membranes employed, effluent type, and temperature which affects the viscosity and conductivity of feed effluent, and the overpotentials. This work focuses on the recycling of Na2SO4 rich waste effluent, through a feed and bleed BPED process. A high ion-exchange capacity and ionic conductivity with excellent stability up to 41 °C is observed during the proposed BPED process, with this temperature increase also leading to improved current efficiency. Five and ten repeating units were tested to determine the effect on BPED stack performance, as well as the effect of temperature and current density on the stack voltage and current efficiency. Furthermore, the concentration and maximum purity (>96.5%) of the products were determined. Using the experimental data, both the capital expense (CAPEX) and operating expense (OPEX) for a theoretical plant capacity of 100 m3 h-1 of Na2SO4 at 110 g L-1 was calculated, yielding CAPEX values of 20 M EUR, and OPEX at 14.2 M EUR/year with a payback time of 11 years, however, the payback time is sensitive to chemical and electricity prices.

Controlled complexation of plasmid DNA with cationic polymers: effect of surfactant on the complexation and stability of the complexes.

The aggregation of the cationic polymer-plasmid DNA complexes of two commonly used polymers, polyethyleneimine (PEI) and poly-l-lysine (PLL) were systematically compared. The complexation was studied in 5% glucose solution at 25 degrees C using dynamic light scattering and isothermal titration calorimetry. The aggregation of the complexes was controlled by addition of the surfactant polyoxyethylene stearate (POES). The stability of the complexes was evaluated using dextran sulphate (DS) as relaxing agent. The relaxation of the complexes in the presence of DS was studied using agarose gel electrophoresis. This study elucidates the role of surfactant in controlling the size of the PEI/pDNA complex and reveals the differences of the two polymers as complexing agents.

The effect of valence on the ion-exchange process: theoretical and experimental aspects on compound binding/release.

The effect of valence of mobile counter-ions (extracting electrolytes), mobile co-ions, and drug-like compounds was evaluated on drug binding/release in ion-exchange fibers. The experimental results support the Donnan theory and suggest that incorporation of monovalent salicylic acid (SA) and divalent 5-hydroxyisophthalic acid (di-COOH) into the anion-exchange fibers was attained mainly as a result of electrostatic (ionic) interaction, with additional contribution of non-electrostatic interactions. Increasing the capacity of ion-exchanger increased the molar amount of compound loading. More efficient release of model anions was observed at increasing valence or concentration of the extracting counter-ion. Potency to release the compounds decreased in the order of citrate (-3) > sulfate (-2) > chloride (-1). The valence of co-ions (sodium (+1) vs. calcium (+2)) in the external solution had only a slight effect on the release. Due to dual site binding (two ionized carboxylate groups), the amount of di-COOH bound into the fibers was half of that of monovalent SA. Also the release was significantly reduced, as the electrostatic interaction was stronger in the case of divalent compound. Simulations on the effect of valence on the Donnan potential and theoretical modeling of the release efficiencies by the external ions supported successfully the conclusions above.

Mechanistic evaluation of factors affecting compound loading into ion-exchange fibers.

Donnan theory was applied to gain mechanistic understanding on the factors affecting drug loading process, compound-fiber affinity and subsequent release from fibrous ion-exchangers. Impact of initial loading solution concentration on fiber occupancy and loading efficiency of compounds were assessed experimentally and theoretically. Relative affinity towards the anion-exchange fibers was studied by dual loading of monovalent salicylic acid and either more lipophilic 3-isopropylsalicylic acid or divalent 5-hydroxyisophthalic acid. The effect of fiber framework on compound binding was evaluated separately for weakly and strongly basic fibers of similar ion-exchange capacities. The results revealed that loading into the ion-exchange fibers can be efficiently adjusted by the concentration of loading solution, leading to improved controllability of drug release from the fiber and minimised drug loss during the loading procedure. Ion-exchange fibers can be utilised successfully in simultaneous delivery of two ionic drugs, which offers a potential drug delivery system for synergistically active drugs. However, physicochemical characteristics of the drug (lipophilicity, valence) and framework of fibrous ion-exchanger affect the relative affinity of the drug towards the fiber, and should not be neglected when selecting appropriate ion-exchange fiber or optimising the external conditions during loading/release. Application of Donnan theory in modelling calculations supported precisely the experimental observations of compound loading (fiber occupancy and loading efficiency).

Thermodynamic analysis of binding between drugs and glycosaminoglycans by isothermal titration calorimetry and fluorescence spectroscopy.

The thermodynamics of the interaction of positively charged drug molecules with negatively charged glycosaminoglycans (GAGs) is investigated by isothermal titration calorimetry (ITC) and fluorescence spectroscopy. The drugs considered are propranolol hydrochloride, tacrine, and aminacrine, and the polymers used as model GAGs are dextran sulfate, chondroitin sulfate, and hyaluronic acid. The ITC results show that the interaction between drugs and GAGs is via direct binding and that GAGs bind to drugs at one set of sites. Large negative values of heat capacity change (DeltaC(p)) are observed upon binding of GAGs to drugs. Such negative DeltaC(p) is not expected for purely electrostatic interactions and suggests that hydrophobic and other interactions may be also involved in the binding process. These results are corroborated by fluorescence spectroscopy measurements, which show that specific drug/GAG complex formation is accompanied by a clear enhancement of the fluorescence intensity. The results highlight the importance of the formation of drug/GAG complexes as a primary step for the drug delivery process into cell membranes. It is concluded that the interactions are dependent on the nature of both GAG and drug and this is a fact to be taken into account when new drugs are designed.

Rat epidermal keratinocyte organotypic culture (ROC) compared to human cadaver skin: the effect of skin permeation enhancers.

The objective of this study was to evaluate the response of the rat epidermal keratinocyte organotypic culture (ROC) to permeation enhancers, and to compare these responses to those in human cadaver skin. Different concentrations of two mixtures for enhancing permeation were investigated, sodium dodecyl sulfate:phenyl piperazine and methyl pyrrolidone:dodecyl pyridinium chloride, using skin impedance spectroscopy and two experimental compounds, the lipophilic corticosterone and the hydrophilic sucrose. The chemical irritation effects of the formulations were evaluated based on leakage of lactate dehydrogenase enzyme (LDH) and cellular morphological perturbation. This study provides evidence for direct correlations of permeation/permeation, impedance/impedance and permation/impedance between the culture model and human skin. The only exception was the enhancer induced permeation of sucrose which was 1-40-fold higher in ROC compared to human skin, reflecting the more disordered lipid organization in stratum corneum and consequently the greater number of polar pathways. LDH leakage and cellular morphology indicated that it was possible to differentiate between safe permeation enhancers from irritating agents. This is not only the first study to have compared the enhancer effects on a cultured skin model with human skin, but also it has demonstrated enhancer induced irritation using an artificial skin model.

Light activated liposomes: Functionality and prospects in ocular drug delivery.

Ocular drug delivery, especially to the retina and choroid, is a major challenge in drug development. Liposome technology may be useful in ophthalmology in enabling new routes of delivery, prolongation of drug action and intracellular drug delivery, but drug release from the liposomes should be controlled. For that purpose, light activation may be an approach to release drug at specified time and site in the eye. Technical advances have been made in the field of light activated drug release, particularly indocyanine green loaded liposomes are a promising approach with safe materials and effective light triggered release of small and large molecules. This review discusses the liposomal drug delivery with light activated systems in the context of ophthalmic drug delivery challenges.

Effect of ion-exchange fiber structure on the binding and release of model salicylates.

Salicylates were used as model anions to evaluate the effect of the structure (framework and ion-exchange groups) of fibrous anion-exchangers on the extent and mechanism(s) of compound binding and release. Binding was affected by the physicochemical properties of both the salicylates and the ion-exchange fibers. The highest molar amount of binding was obtained with the most lipophilic salicylate (5-chlorosalicylic acid) and the weak base (vinylpyridine) anion-exchange fibers. However, when the ion-exchange capacity was taken into account, higher binding was obtained in fibers of poly(ethylene) framework compared to the viscose-based fibers. The extent of salicylate release into NaCl solution(s) was dependent on the physicochemical characteristics of both the fiber and the bound model salicylate as well as on the amount of extracting ions. With strong base fibers (trimethylammonium), the viscose framework released the salicylates more efficiently than the poly(ethylene) framework. In the case of weak base fibers, the poly(ethylene) framework released the salicylates to a higher extent than the viscose framework. Calculated equilibrium constants (K) of the ion-exchange reactions illustrated that in addition to electrostatic interactions (pure ion-exchange mechanism), non-electrostatic interactions (hydrophobic interactions and/or hydrogen bonding) were also involved. However, the release of the salicylates was efficiently modified by the amount of extracting electrolyte, demonstrating that ion-exchange was the prevalent release mechanism.

Encapsulated cells for long-term secretion of soluble VEGF receptor 1: Material optimization and simulation of ocular drug response.

Anti-angiogenic therapies with vascular endothelial growth factor (VEGF) inhibiting factors are effective treatment options for neovascular diseases of the retina, but these proteins can only be delivered as intravitreal (IVT) injections. To sustain a therapeutic drug level in the retina, VEGF inhibitors have to be delivered frequently, every 4-8weeks, causing inconvenience for the patients and expenses for the healthcare system. The aim of this study was to investigate cell encapsulation as a delivery system for prolonged anti-angiogenic treatment of retinal neovascularization. Genetically engineered ARPE-19 cells secreting soluble vascular endothelial growth factor receptor 1 (sVEGFR1) were encapsulated in a hydrogel of cross-linked collagen and interpenetrating hyaluronic acid (HA). The system was optimized in terms of matrix composition and cell density, and long-term cell viability and protein secretion measurements were performed. sVEGFR1 ARPE-19 cells in the optimized hydrogel remained viable and secreted sVEGFR1 at a constant rate for at least 50days. Based on pharmacokinetic/pharmacodynamic (PK/PD) modeling, delivery of sVEGFR1 from this cell encapsulation system is expected to lead only to modest VEGF inhibition, but improvements of the protein structure and/or secretion rate should result in strong and prolonged therapeutic effect. In conclusion, the hydrogel matrix herein supported the survival and protein secretion from the encapsulated cells. The PK/PD simulation is a convenient approach to predict the efficiency of the cell encapsulation system before in vivo experiments.

Permeation of proteins, oligonucleotide and dextrans across ocular tissues: experimental studies and a literature update.

Proteins and oligonucleotides represent powerful tools for the treatment of several ocular diseases, affecting both anterior and posterior eye segments. Despite the potential of these compounds, their administration remains a challenge. The last years have seen a growing interest for the noninvasive administration of macromolecular drugs, but still there is only little information of their permeability across the different ocular barriers. The aim of this work was to evaluate the permeation of macromolecules of different size, shape and charge across porcine ocular tissues such as the isolated sclera, the choroid Bruch's membrane and the cornea, both intact and de-epitelialized. Permeants used were two proteins (albumin and cytochrome C), an oligonucleotide, two dextrans (4 and 40 kDa) and a monoclonal antibody (bevacizumab). Obtained data and its comparison with the literature highlight the difficulties in predicting the behavior of macromolecules based on their physicochemical properties, because the interplay between the charge, molecular radius and conformation prevent their analysis separately. However, the data can be of great help for a rough evaluation of the feasibility of a noninvasive administration and for building computational models to improve understanding of the interplay among static, dynamic and metabolic barriers in the delivery of macromolecules to the eye.

Light induced cytosolic drug delivery from liposomes with gold nanoparticles.

Externally triggered drug release at defined targets allows site- and time-controlled drug treatment regimens. We have developed liposomal drug carriers with encapsulated gold nanoparticles for triggered drug release. Light energy is converted to heat in the gold nanoparticles and released to the lipid bilayers. Localized temperature increase renders liposomal bilayers to be leaky and triggers drug release. The aim of this study was to develop a drug releasing system capable of releasing its cargo to cell cytosol upon triggering with visible and near infrared light signals. The liposomes were formulated using either heat-sensitive or heat- and pH-sensitive lipid compositions with star or rod shaped gold nanoparticles. Encapsulated fluorescent probe, calcein, was released from the liposomes after exposure to the light. In addition, the pH-sensitive formulations showed a faster drug release in acidic conditions than in neutral conditions. The liposomes were internalized into human retinal pigment epithelial cells (ARPE-19) and human umbilical vein endothelial cells (HUVECs) and did not show any cellular toxicity. The light induced cytosolic delivery of calcein from the gold nanoparticle containing liposomes was shown, whereas no cytosolic release was seen without light induction or without gold nanoparticles in the liposomes. The light activated liposome formulations showed a controlled content release to the cellular cytosol at a specific location and time. Triggering with visual and near infrared light allows good tissue penetration and safety, and the pH-sensitive liposomes may enable selective drug release in the intracellular acidic compartments (endosomes, lysosomes). Thus, light activated liposomes with gold nanoparticles are an attractive option for time- and site-specific drug delivery into the target cells.

Drug adsorption in human skin: a streaming potential study.

The objective of this study was to investigate the drug adsorption process in human skin using in vitro streaming potential measurements. Streaming potential is an electrokinetic phenomenon, which reflects both the charge density and the pore size of a membrane. Thus, the adsorption of charged solutes on the pore walls can be detected as a change of streaming potential, viz., as a change in the slope deltaE/deltaP. In these streaming potential measurements, hydrophilic nadolol and luteinizing hormone-releasing hormone, and lipophilic propranolol and Nafarelin were used as model drugs. As could be expected, the hydrophilic drugs did not change the slope. The more lipophilic propranolol and Nafarelin, instead, changed the slope. Propranolol changed the slope gradually from negative to positive when the concentration was increased from 1 to 10 mM. With Nafarelin, a straight line with a slope of about 0 was obtained at pH 7.3 and an ascending curve at pH 4.2. These results indicate that the negative charges on the pore walls of human skin are blocked by adsorption of the lipophilic cations. The adsorption of lipophilic cations in the skin alters the permselectivity of the skin, which, in turn, may lead to the inhibition of electroosmotic flow across the skin during iontophoresis and to the shut down of transdermal drug permeation of higher molecular weight drugs.