Direction Of Electroosmotic Flow Research Articles

Bilaterally Aligned Electroosmotic Flow Generated by Porous Microneedle Device for Dual-Mode Delivery.

Here, a novel porous microneedle (PMN) device with bilaterally aligned electroosmotic flow (EOF) enabling controllable dual-mode delivery of molecules is developed. The PMNs placed at anode and cathode compartments are modified with anionic poly-2-acrylamido-2-methyl-1-propanesulfonic acid and cationic poly-(3-acrylamidopropyl) trimethylammonium, respectively. The direction of EOF generated by PMN at the cathode compartment is, therefore, reversed from cathode to anode, countering the unwanted cathodal suctioning of interstitial fluid caused by reverse iontophoresis. With the bilateral alignment of EOF, the versatility of the proposed device is evaluated by delivering molecules with different charges and sizes using Franz cell. In addition, a 3D printed probe device is developed to ease practical handling and minimize electrical stimulation by integrating two PMNs in closed proximity. Finally, the performance of the integrated probe device is demonstrated by dual delivery of a variety of molecules (methylene blue, rhodamine B, and fluorescein isothiocyanate-dextran) using pig skin and vaccination using mice with delivered ovalbumin.

Chiral Separation of Apremilast by Capillary Electrophoresis Using Succinyl-β-Cyclodextrin-Reversal of Enantiomer Elution Order by Cationic Capillary Coating.

A stereospecific capillary electrophoresis method was developed for the separation of the novel, antipsoriatic agent, apremilast (APR). Six anionic cyclodextrin (CD) derivatives were screened for their ability to discriminate between the uncharged enantiomers. Only succinyl-β-CD (Succ-β-CD) presented chiral interactions; however, the enantiomer migration order (EMO) was unfavorable, and the eutomer, S-APR, migrated faster. Despite the optimization of all possible parameters (pH, cyclodextrin concentration, temperature, and degree of substitution of CD), the method was unsuccessful for purity control due to the low resolution and the unfavorable enantiomer migration order. Changing the direction of electroosmotic flow (EOF) by the dynamic coating of the inner surface of the capillary with poly(diallyldimethylammonium) chloride or polybrene resulted in EMO reversal, and the developed method could be applied for the determination of R-APR as the enantiomeric purity. Thus, the application of the dynamic capillary coating offers a general opportunity for enantiomeric migration order reversal in particular cases when the chiral selector is a weak acid.

Electroosmotic flow in fused deposition modeling (FDM) 3D-printed microchannels.

Electroosmotic flow (EOF) was determined in tridimensional (3D)-printed microchannels with dimensions smaller than 100µm. Fused deposition modeling 3D printing using thermoplastic filaments of PETG (polyethylene terephthalate glycol), PLA (polylactic acid), and ABS (acrylonitrile butadiene styrene) were used to fabricate the microchannels. The current monitoring method and sodium phosphate solutions at different pH values (3-10) were used for the EOF mobility (µEOF ) measurements, which ranged from 2.00×10-4 to 12.52×10-4 cm2 V-1 s-1 . The highest and the smallest µEOF were obtained for the PLA and PETG microchannels, respectively. Adding the cationic surfactant cetyltrimethylammonium bromide to the sodium phosphate solution caused EOF direction reversion in all the studied microchannels. The obtained results can be interesting for developing 3D-printed microfluidic devices, in which EOF is relevant.

Reagent-free electrokinetic remediation coupled with anode oxidation for the treatment of phenanthrene polluted soil

Electrokinetic in-situ chemical oxidation (EK-ISCO) has attracted much attention during remediation of organic contaminated soil. Oxidants in EK-ISCO brings high cost and negative effects on soil physicochemical properties. In this study, a novel approach of combined electrokinetic treatment and anode oxidation was investigated to remediate phenanthrene polluted soils without adding oxidants. The fabricated Ti4O7 acted as anode, and could generate •OH at the rate of 9.31 × 10−7 mol h−1 at current 5.10 mA cm−2 through direct H2O electrolysis. Electro-osmotic flow (EOF) was used to transport phenanthrene to anode for the subsequent degradation. Sandy soil, fluvo-aquic soil and red soil were selected as typical soil samples, because pH and buffer capacity were two important factors affecting the direction of EOF. Strategies were developed to regulate the direction of EOF, including adding CEM membrane, maintaining soil pH at 3.5–4.0 and mixing solution from anode and cathode chambers. After treatment, more than 81.9% of phenanthrene was removed without adding any oxidants, and the remediated soil had low toxicity for Lolium perenne growth based on 3-d cultivation results. The results indicated that EK-AO had the advantage of less energy consumption and superior environmental friendliness than traditional EK-ISCO.

Electroosmosis Based Novel Treatment Approach for Cerebral Edema.

Cerebral edema characterized as an abnormal accumulation of interstitial fluid has not been treated effectively. We propose a novel edema treatment approach to drive edematous fluid out of the brain by direct current utilizing brain tissue's electroosmotic property. A finite element (FE) head model is developed and employed to assess the feasibility of the approach. First, the capacity of the model for electric field prediction is validated against human experiments. Second, two electrode configurations (S and D-montage) are designed to evaluate the distribution of the electric field, electroosmotic flow (EOF), current density, and temperature across the brain under an applied direct current. The S-montage is shown to induce an average EOF velocity of 7e-4 mm/s underneath the anode by a voltage of 15 V, and the D-montage induces a velocity of 9e-4 mm/s by a voltage of 5 V. Meanwhile, the brain temperature in both configurations is below 38 °C, which is within the safety range. Further, the magnitude of EOF is proportional to the electric field, and the EOF direction follows the current flow from anode to cathode. The EOF velocity in the white matter is significantly higher than that in the gray matter under the anode where the fluid is to be drawn out. The proposed electroosmosis based approach allows alleviating brain edema within the critical time window by direct current. The approach may be further developed as a new treatment solely or as a complement to existing conventional treatments of edema.

Analysis of metabolomics and proteomics based on capillary electrophoresis-mass spectrometry

Capillary electrophoresis-mass spectrometry (CE-MS) has the advantages of higher sensitivity, higher efficiency, and less sample consumption. Moreover, it possesses obvious advantages during the analysis of strongly charged and highly polar samples. CE-MS has been widely applied in life sciences, medicine, and pharmacology. In the past ten years, the main factors affecting its application were system stability, reproducibility, and data accuracy. In order to solve the existing problems of CE-MS, researchers have invested significant effort in technology innovation to further expand CE-MS application. In the fields of medicine and analytical chemistry, substantial research indicates that CE-MS is superior compared to other metabolomic and proteomic approaches. This study aims at reviewing the latest methods and applications developed in the fields of medicine and analytical chemistry since 2015. Furthermore, it also aims at enhancing the technology development-related application value of CE-MS and serving as a reference for future development. Further development of the CE-MS technology is discussed from the aspects of coating-sample interaction, interface types, and data processing methods. Concerning the coating types, neutral coatings had been applied extensively in CE-MS and there should be no limitation to the charge of the analyte. The coating decreased sample adsorption on the inner wall by covering the surface charge, greatly reducing the electroosmotic flow (EOF). A charged capillary coating could modify such an EOF direction. The cationic coating could reduce the hydrophobic interaction between the sample and the capillary column, resulting in higher EOF. If it is applied to the sheathless interface, the resolution could be improved by extending the capillary length. Anionic coatings are predominant among the anionic compounds, shortening the separation time by reducing the interaction between the anionic compounds and the capillary. The coating type should be chosen relative to the analyte characteristics. Concerning the interface technology, all interfaces should be simple, practical, and non-dependent on sheath liquid and background electrolytes. As far as data processing methods are concerned, it is necessary to design and develop a practical method for span space data comparison and processing. The optimized experimental conditions have effectively improved separation efficiency and data comparison analysis. Furthermore, they established a solid foundation for its application development. CE-MS analysis of complex samples in the fields of metabolomics and proteomics (e. g., of tissues, cells, body fluids, etc.) could provide a visualization method for future clinical analysis. It contributes to the development of cancer pathological analysis, drug development, disease surveillance, etc. The characteristic analysis of small molecule metabolites and protein biomarkers directly reflects on enzymatic activity in the biological systems. It could be associated with the development of various diseases/complications. Omics analysis also has an important directive to disease detection and surveillance with obvious advantages in disease diagnosis, staged treatment, drug development, and patient treatment progress. CE-MS is useful in detecting complications and promoting personalized medicine. It provides technical support for future clinical developments. In addition to a comprehensive review of the recent advances of CE-MS research, this paper also indicates the development directions of CE-MS. In order to avoid the problem of omics analysis and obtain the optimized analysis results, future analysis should be improved from the following three aspects:(i) The analysis conditions should be optimized concerning sample preparation methods and separation techniques. (ii) The analytic techniques should be supported to adjust to capillary coating and interface technology. (iii) New ideas should be developed in the fields of clinical research and statistical analysis.

Single-molecule electrophoresis: renewed understanding of nanopore electrochemistry

This study aims to understand nanopore technology from the standpoint of capillary electrophoresis separation. The nanopore electrochemical measurements could be regarded as "single molecule electrophoresis". Similar to the case of capillary electrophoresis, the single target molecules migrate inside a nanopore under an external electric field. The recognition ability of the nanopore mainly depends on the charge, shape, and size of the target molecules under the electric force. The confined space of an Aerolysin nanopore matches the size of single biomolecule, while the amino acid residues along the inner wall of the nanopore facilitate electrokinetic regulation inside the nanopore. Under the applied voltage, each molecule enters the nanopore, generating the characteristic migration velocity and trajectory. Therefore, statistical analysis of the current amplitude, duration, frequency, and shape of the electrochemical signals would help differentiate and identify a single analyte from the mixture. Herein, we used an Aerolysin nanopore for identifying the oligonucleotides of 5'-CAA-3' (CA2), 5'-CAAA-3' (CA3), and 5'-CAAAA-3' (CA4), which differ in length only by one nucleotide, as the model system to demonstrate single-molecule electrophoresis. The diameter of the Aerolysin nanopore is around 1 nm, and the pore length is approximately 10 nm. Under an applied voltage of 80 mV, the nanopore experiences a high electric field strength of about 80 kV/cm. The phosphate groups of the nucleotides carry negative charges in an electrolyte buffer solution of 1.0 mol/L KCl, at pH 8. Therefore, CA2, CA3, and CA4 carry 2, 3, and 4 negative charges, respectively. During nanopore sensing, CA2, CA3, and CA4 are subjected to electrophoretic forces and thus move inside the nanopore. Because the Aerolysin nanopore is anion selective, the direction of electroosmotic flow through the nanopore is consistent with the anion flow direction. Under the combined effects of the electrophoretic force and electroosmotic flow, CA2, CA3, CA4 will transverse through the Aerolysin nanopore at different migration velocities. Note that the oligonucleotide shows strong electrostatic interaction with the two sensitive regions of Aerolysin, which comprises polar amino acids around R220 and K238. The strong interaction between the sensitive region of Aerolysin and the analyte would further modulate the translocation of oligonucleotides. Therefore, each oligonucleotide follows a different migration trajectory as it individually transverses through the nanopore. The migration speed and migration trajectory are recorded as ionic blockages in nanopore electrochemistry. The scatter plots of the blockage current and blockage duration of the mixed sample of CA2, CA3, and CA4 show three characteristic distributions assigned to each type of oligonucleotide. Since the net charge increases with increasing length of the oligonucleotide, CA3 and CA4 experience a stronger electrophoretic force than does CA2 inside the nanopore, leading to higher migration velocity. Therefore, the blockage duration of CA3 and CA4 is 5 times longer than that of CA2. By Gaussian fitting, the fitted blockage currents of CA2, CA3, and CA4 are 20.7, 15.7, and 12.7 pA, respectively. Similar to our previous results, the blockage current increases with the chain length when the oligonucleotides comprise not more than 14 nucleotides. Therefore, nanopore-based single-molecule electrophoresis allows for the electrochemical identification of CA2, CA3, and CA4 that differ in a length by only one nucleotide. Understanding the "single-molecule electrophoresis" concept would promote the application of electrochemically confined effects in single-molecule electrophoresis separation. The combination of single-molecule electrophoresis with a microfluidic system and a nanopore array is expected to aid the separation and identification of single molecules.

Modulation of Electroosmotic Flow in Open Tubular Capillary Electrochromatography by Chitosan-Assisted Titanium Oxide Nanoparticles Liquid Phase Deposition

Electroosmotic flow is one of the most important driving forces in capillary electrophoresis, which can control the migration velocity and direction of the target substance, thus affecting the separation efficiency and reproducibility of capillary electrophoresis. In this study, a facile method was developed for the preparation of TiO2/chitosan open tubular capillary electrochromatography for modulating electroosmotic flow in capillary electrophoresis. The magnitude and direction of electroosmotic flow were modulated by changing a series of coating preparation parameters, such as deposition time of (NH4)2TiF6 and H3BO3, the concentration of chitosan, and the concentration ratio of (NH4)2TiF6 and chitosan. The results showed that when the concentration of (NH4)2TiF6 was below 2 mg/mL, the surface of the capillary was not modified completely by phosphate ion by Lewis acid–base interaction, resulting in an anodic electroosmotic flow. Furthermore, the feasibility and stability of the column were assessed by separation of nucleotides and alkaloid compounds. With the TiO2/chitosan liquid phase deposition columns with the concentration ratio of 3:7 [(NH4)2TiF6:chitosan], alkaloids were baseline separated within 11 min. The baseline separation of the four nucleotides was achieved using the TiO2/chitosan-coated columns with the concentration ratio of 1.8:7 [(NH4)2TiF6:chitosan]. Furthermore, the relative standard deviations of migration times for inter-day, intra-day, and column-to-column repeatability of the TiO2/chitosan-coated columns with the concentration ratio of 3:7 [(NH4)2TiF6:chitosan] were all less than 5.8%. The strategy of liquid phase deposition of TiO2 nanoparticles provides a promising approach for modulation electroosmotic flow.

Electrokinetic Phenomena in Organic Solvents.

Solid/liquid interfaces play a key role in separation processes, energy storage devices, and transport in nanoscale systems. Nanopores and mesopores with well-defined geometry and chemical characteristics have been a valuable tool to unravel electrochemical properties of interfaces, but the majority of studies have been focused on aqueous solutions. Here, we present experiments and numerical modeling aimed at characterizing effective surface charge of polymer pores in mixtures of water and alcohols as well as in propylene carbonate and acetone. The charge properties of pore walls are probed through analysis of current-voltage curves recorded in the presence of salt concentration gradients. The presence and direction of electro-osmotic flow lead to asymmetric current-voltage curves, with rectification characteristics determined by the polarity of surface charge. The results suggest that the effective surface charge of the pore walls depends not only on the type of solvent but also on the concentration of the electrolyte and voltage. We identified conditions at which polymer pores that are negatively charged in aqueous solutions become positively charged in propylene carbonate and acetone. The findings are of importance for nonaqueous separations, fundamental knowledge on solid/liquid interfaces in organic media, and preparation of porous devices with tunable surface charge characteristics.

Modulation of electroosmotic flow in capillary electrophoresis by plant polyphenol-inspired gallic acid/polyethyleneimine coatings: Analysis of small molecules

Plant polyphenols can form functional coatings on various materials through self-polymerization. In this paper, a series of modified capillary columns, which possess diversity of charge characteristics for modulating electroosmotic flow (EOF), were prepared by one-step co-deposition of gallic acid (GA), a plant-derived polyphenol monomer, and branched polyethyleneimine (PEI). The physicochemical properties of the prepared columns were characterized by Fourier transform infrared spectroscopy (FT-IR), UV–Vis spectroscopy and scanning electron microscopy (SEM). The magnitude and direction of EOF of GA/PEI co-deposited columns were modulated by changing a series of coating parameters, such as post-incubation of FeCl3, co-deposition time, and deposited amounts of GA and PEI with different relative molecular mass (PEI-600, PEI-1800, PEI-10000, and PEI-70000). Furthermore, the separation efficiencies of the prepared GA/PEI co-deposited columns were evaluated by separations of small molecules, including organic acids, polar nucleotides, phenols, nucleic acid bases and nucleosides. Results indicated that modulating of EOF plays an important role in enhancing the separation performance and reversing the elution order of the analytes. Finally, the developed method was successfully applied to quantitative analysis of acidic compounds in four real samples. The recoveries were in the range of 73.5%–85.8% for citric acid, benzoic acid, sorbic acid, salicylic acid and ascorbic acid in beverage and fruit samples, 101.6%–104.9% for cinnamic acid, vanillic acid, and ferulic acid in Angelica sinensis sample, while 84.6%–97.8% for guanosine-5′-monophosphate, uridine-5′-monophosphate, cytosine-5′- monophosphate and adenosine-5′-monophosphate in Cordyceps samples. These results indicated that the co-deposition of plant polyphenol-inspired GA/PEI coatings can provide new opportunities for EOF modulation of capillary electrophoresis.

A novel thread-based microfluidic device for capillary electrophoresis with capacitively coupled contactless conductivity detection

This work describes a novel thread-based microfluidic device to perform capillary electrophoresis separation and capacitively coupled contactless conductivity detection. Polyester threads with a diameter of about 315 μm were assembled (stretched) in a 3D-printed platform containing solution reservoirs and platinum electrodes. Separation of potassium, sodium, and lithium ions was achieved within 1 min with good peak resolutions and limits of detection close to 1 μmol L−1. Inversion of the electroosmotic flow direction by adding a cationic surfactant in the running electrolyte was demonstrated. The applicability of the thread-based microfluidic devices was successfully evaluated by the determination of sodium and potassium ions in commercial samples of diet soft drinks.

Citric Acid-Enhanced Electroremediation of Toxic Metal-Contaminated Dredged Sediments: Effect of Open/Closed Orifice Condition, Electric Potential and Surfactant

Citric acid (CA), a widely used eco-friendly electrolyte, can be employed as an agent for enhancing toxic metal (TM) removal from contaminated dredged sediment using electrokinetic (EK) technology. In this study, dredged harbor sediments co-contaminated by TMs were subjected to enhanced EK treatment using a mixture of chelating agent (CA) and surfactant as an additive in the processing fluids. Several control conditions that may influence the efficiency of TM removal were tested, including open/closed sediment chamber orifices, electric potential gradients (0.5, 1.0, and 1.5 V cm−1), and electrolyte surfactant. Tween 20 (4 mmol L−1) was used as a surfactant within the electrolyte to investigate the extent of TM removal in sediment with high organic matter content. The results showed that an open orifice led to a greater electro-osmotic flow (EOF) with moderate TM removal. In contrast, a closed orifice with a nonionic surfactant electrolyte allowed the highest removal of TMs from the matrix. Moreover, increasing the electric potential gradient led to a higher EOF under the open orifice condition, but no significant increase in TM removal was observed owing to a higher accumulation of TMs in the middle of the matrix, caused by the opposite direction of EOF and electro-migration of metal-citrate complexes.

Effect of Electroosmotic Flow on the Electrophoretic Deposition of Zeolite Powder on a Porous Alumina Support

Using a porous alumina support, seeding for the preparation of a zeolite separation membrane is conducted by placing the electrodes for electrophoretic deposition (EPD) on both sides of the support. In this case, two electrokinetic phenomena are observed: electrophoresis of the seed crystal particles and electroosmotic flow of the solvent within the porous alumina support. When water and acetone are used, the direction of electroosmotic flow is opposite to that of EPD, and the strength of electroosmotic flow with water is higher than that with acetone. In contrast, with methanol, the direction of the electroosmotic flow is the same as that for EPD, with a weak particle charge. The results show that both the support and the solvent contribute to the change in the strength and direction of the electroosmotic flow.

Enhanced cell trapping throughput using DC-biased AC electric field in a dielectrophoresis-based fluidic device with densely packed silica beads.

This paper presents the use of DC-biased AC electric field for enhancing cell trapping throughput in an insulator-based dielectrophoretic (iDEP) fluidic device with densely packed silica beads. Cell suspension is carried through the iDEP device by a pressure-driven flow. Under an applied DC-biased AC electric field, DEP trapping force is produced as a result of non-uniform electric field induced by the gap of electrically insulating silica beads packed between two mesh electrodes that allow both fluid and cells to pass through. While the AC component is mainly to control the magnitude of DEP trapping force, the DC component generates local electroosmotic (EO) flow in the cavity between the beads and the EO flow can be set to move along or against the main pressure-driven flow. Our experimental and simulation results show that desirable trapping is achieved when the EO flow direction is along (not against) the main flow direction. Using our proposed DC-biased AC field, the device can enhance the trapping throughput (in terms of the flowrate of cell suspension) up to five times while yielding almost the same cell capture rates as compared to the pure AC field case. Additionally, the device was demonstrated to selectively trap dead yeast cells from a mixture of flowing live and dead yeast cells.

Modulation of Electroosmotic Flow through Skin: Effect of Poly(Amidoamine) Dendrimers.

The objective of this work is to evaluate the effect of polyamidoamine (PAMAM) dendrimers on electroosmotic flow (EOF) through skin. The effect of size and concentration of dendrimer was studied, using generation 1, 4 and 7 dendrimer (G1, G4 and G7, respectively). As a marker molecule for the direction and magnitude of EOF, a neutral molecule, acetoaminophen (AAP) was used. The visualization of dendrimer permeation into the current conducting pore (CCP) of skin was made using G4–fluorescein isothiocyanate (FITC) conjugate and confocal microscopy. Without dendrimer, anodal flux of AAP was much higher than cathodal or passive flux. When G1 dendrimer was added, anodal flux decreased, presumably due to the decrease in EOF by the association of G1 dendrimer with net negative charge in CCP. As the generation increased, larger decrease in anodal flux was observed, and the direction of EOF was reversed. Small amount of methanol used for the preparation of dendrimer solution also contributed to the decrease in anodal flux of AAP. Cross-sectional view perpendicular to the skin surface by confocal laser scanning microscope (CLSM) study showed that G4 dendrimer-FITC conjugate (G4-FITC) can penetrate into the viable epidermis and dermis under anodal current. The permeation route seemed to be localized on hair follicle region. These results suggest that PAMAM dendrimers can permeate into CCP and change the magnitude and direction of EOF. Overall, we obtained a better understanding on the mechanistic insights into the electroosmosis phenomena and its role on flux during iontophoresis.

Alternating electric field-induced ion current rectification and electroosmotic pump in ultranarrow charged carbon nanocones.

Pumping fluid in ultranarrow (sub-2 nm) synthetic channels, analogous to protein channels, has widespread applications in nanofluidic devices, molecular separation, and related fields. In this work, molecular dynamics simulations were performed to study a symmetrical sinusoidal electric field-induced electroosmotic pump in ultranarrow charged carbon nanocone (CNC) channels. The results show that the CNC channels could rectify the ion current because of the different ion flow rates in the positive and negative half circles of the sinusoidal electric field. Electroosmotic flow (EOF) rectification yielded by the ion current rectification is also revealed, and net water flow from the base to the tip of the CNC channels is observed. The simulations also show that the preferential ion current conduction direction in the ultranarrow CNC channels (from base to tip) is opposite to that in conical nanochannels with tip diameters larger than 5 nm (from tip to base). However, the preferential EOF direction is the same as that of large conical nanochannels (from base to tip). We also investigated the influences of ion concentration and the amplitudes and periods of the sinusoidal electric field on the EOF pump. The results show that high ion concentration, large amplitudes, and long periods are desired for high EOF pumping efficiency. Finally, through comparison with a constant electric field and a pressure-induced water pump, we prove that the EOF pump under an alternating electric field has a higher pump efficiency. The approach outlined in this work provides a general scheme for pumping fluid in ultranarrow charged conical nanochannels.

A two-step method for rapid characterization of electroosmotic flows in capillary electrophoresis.

The measurement of electroosmotic flow (EOF) is important in a capillary electrophoresis (CE) experiment in terms of performance optimization and stability improvement. Although several methods exist, there are demanding needs to accurately characterize ultra-low electroosmotic flow rates (EOF rates), such as in coated capillaries used in protein separations. In this work, a new method, called the two-step method, was developed to accurately and rapidly measure EOF rates in a capillary, especially for measuring the ultra-low EOF rates in coated capillaries. In this two-step method, the EOF rates were calculated by measuring the migration time difference of a neutral marker in two consecutive experiments, in which a pressure driven was introduced to accelerate the migration and the DC voltage was reversed to switch the EOF direction. Uncoated capillaries were first characterized by both this two-step method and a conventional method to confirm the validity of this new method. Then this new method was applied in the study of coated capillaries. Results show that this new method is not only fast in speed, but also better in accuracy.

Polydopamine-functionalized poly(ether ether ketone) tube for capillary electrophoresis-mass spectrometry

Capillary electrophoresis-mass spectrometry (CE-MS) is a hyphenated technique that combines the advantages like low sample consumption, high separation efficiency, short analytical time in CE and high sensitivity, powerful molecular structure elucidation in MS. Polyimide-coated fused silica capillary has become the most dominant capillary for CE, but it suffers from swelling and aminolysis of polyimide coating when treated with organic solvents and alkaline buffer in the CE-MS interface in which the polyimide coating at the end of the capillary is exposed to the solution, and this phenomenon can result in current instability, irregular electrospray and clogging at outlet after prolonged use. In this work, poly(ether ether ketone) (PEEK) capillary was explored as separation capillary for CE-MS. The problems like swelling and aminolysis of polyimide coating were solved due to the high thermal and chemical stability of PEEK material. After modification with polydopamine, PEEK capillary (PD-PEEK) can generate adjustable electroosmotic flow and provide good separation selectivity. The zwitterion polymer of polydopamine can provide cathodic electroosmotic flow (EOF) at high pH value (pH ≥ 5) and anodic EOF at low pH value (pH ≤ 4), and the EOF mobility can also be adjusted by controlling the modification time of polydopamine. Good separation performance was obtained in the analysis for several classes of compounds including amino acids, phenols and plant hormones at rational EOF direction. Repeatability of the PD-PEEK capillary was studied, with relative standard deviations for intra-day, inter-day runs and between tubes less than 4.94%.

Electroosmotic flow through skin: effect of current duration and poly(ethylene imine)

The objective of this work is to investigate the effect of current duration and poly(ethylene imine) on the magnitude and direction of electroosmotic flow (EF), to have a better understanding on the design of iontophoretic transdermal formulation. Such factors as the length of current application time and the addition of poly(ethylene imine)750 K (PEI750K) in the formulation were studied. Experiments were performed in vitro at pH 7.4 using side-by-side diffusion cell and hairless mouse skin. The direction and magnitude of EF were evaluated simultaneously using two neutral marker molecules, acetaminophen (AAP) and caffeine (CAF). The flux data of AAP and CAF were very similar to each other, and demonstrated clearly that EF is playing an important role in their flux. As the current duration increased, not only the passive permeability but also the magnitude of EF increased. The magnitude of EF at 9 h of current application was estimated to be ~1.9 µl/cm2 hr. The flux of AAP and CAF substantially decreased by PEI750K during current application, and it increased after current off, indicating reversion in the direction of EF from the normal anode-to-cathode direction to cathode-to-anode direction. These results suggest that PEI750K molecules seem to adhere to the negatively charged interface of the current conducting pore (CCP), leading to the charge inversion to a positively charged interface. Overall, the results obtained in this work provide a better understanding on the effect of current duration and PEI750K, and the role of EF in transdermal flux during iontophoresis.

New design of hydrogels with tuned electro-osmosis: a potential model system to understand electro-kinetic transport in biological tissues.

Electro-osmosis is the bulk fluid flow of electrolyte solution generated by an electric field in contact with charged immobile objects, such as the surface of polymer gels. Electro-osmosis may be used to assist in analytical separation, and plays an important role in biological tissues. Despite its importance, the contribution of electro-osmosis to the overall solute transport has never been fully understood because of the complexity of the biological systems. To understand the electro-osmosis, a model system with controlled charge density, charge composition and stiffness is necessary. In this paper, we propose a new design of a gel-based system (tetra-PEG gel) with well controlled electro-osmotic properties. Tetra-PEG gels are formed by two four-armed poly(ethylene)glycol units with mutually reactive end groups, allowing the introduction of positive or negative charges by variation in the monomer stoichiometric ratio. Using this system, we for the first time succeeded in preparing a series of hydrogels with precisely and independently tuned charge densities and stiffness. Electro-osmotic flows of these gels were distinctly different from each other, and varied linearly with the total charge amount. Interestingly, a hydrogel with the same amount of positive and negative charges did not show any electro-osmotic flow. Our new hydrogel system provides an important basis for understanding the electro-osmotic flow as an additional mechanism in mass transport and will find use in biomaterials and analytical separations, and as a model system for biological tissues.