Tailoring the trajectory of cell rolling with cytotactic surfaces

Cell separation technology is a key tool for biological studies and medical diagnostics that relies primarily on chemical labeling to identify particular phenotypes. An emergent method of sorting cells based on differential rolling on chemically patterned substrates holds potential benefits over existing technologies, but the underlying mechanisms being exploited are not well characterized. In order to better understand cell rolling on complex surfaces, a microfluidic device with chemically patterned stripes of the cell adhesion molecule P-selectin was designed. The behavior of HL-60 cells rolling under flow was analyzed using a high-resolution visual tracking system. This behavior was then correlated to a number of established predictive models. The combination of computational modeling and widely available fabrication techniques described herein represents a crucial step toward the successful development of continuous, label-free methods of cell separation based on rolling adhesion.

Separation and detection of VX and its methylphosphonic acid degradation products on a microchip using indirect laser-induced fluorescence

The application of indirect LIF (IDLIF) technique for on-chip electrophoretic separation and detection of the nerve agent O-ethyl S-[2-(diisopropylamino)ethyl] methylphosphonothiolate (VX) and its major phosphonic degradation products, ethyl methylphosphonic acid (EMPA) and methylphosphonic acid (MPA) was demonstrated. Separation and detection of MPA degradation products of VX and the nerve agent isopropyl methylphosphonofluoridate (GB) are presented. The negatively charged dye eosin was found to be a good fluorescent marker for both the negatively charged phosphonic acids and the positively charged VX, and was chosen as the IDLIF visualization fluorescent dye. Separation and detection of VX, EMPA, and MPA in a simple-cross microchip were completed within less than a minute, and consumed only a 50 pL sample volume. A characteristic system peak that appeared in all IDLIF electropherograms served as an internal standard that increased the reliability of peak identification. The negative peak of both VX and the MPAs is in agreement with indirect detection theory and with previous reports in the literature. The LOD of VX and EMPA by IDLIF was 30 and 37 microM, respectively. Despite the fact that the detection sensitivity is relatively low, the rapid simultaneous on-chip analysis of both VX and its degradation products as well as the separation and detection of the MPA degradation products of both VX and GB, increases detection reliability and may present a choice when sensitivity is not critical compared with speed and simplicity of the assay.

On-chip integrated hydrolysis, fluorescent labeling, and electrophoretic separation utilized for acetylcholinesterase assay

A sensitive on-chip acetylcholinesterase (AChE) assay that serves as a basis for the development of a fully integrated on-chip AChE-inhibitor detection assay is presented. The sequential steps required for the on-chip analysis process were integrated into a microchip. Transport and mixing of the reagents occurred by a combination of electroosmosis and electrophoresis using computer-controlled electrokinetic transport. AChE-catalyzed hydrolysis of acetylthiocholine to thiocholine was determined by on-chip reaction of thiocholine with eosinmaleimide, and the resulting thioether was electrophoretically separated and detected by laser-induced fluorescence (LIF). Enzyme-substrate mixing and reaction by confluent flow of reagents was compared with electrophoretically mediated microanalysis (EMMA), based on injection of an enzyme plug, and the utilization of differences in electrophoretic mobility as a driving force for efficient mixing and reaction. Both methods yielded similar results, however the EMMA-plug technique is preferable. The EMMA-plug technique was optimized for length and pushing time of enzyme plug, length of dyes mixture plug, acetylthiocholine concentration, and detector location. Detection of O-ethyl S-[2-(diisopropylamino)ethyl] methylphosphonothiolate (VX) and paraoxon, two AChE inhibitors, was demonstrated by off-chip mixing of the inhibitor and AChE, followed by the on-chip AChE assay. Limit of detection of VX for 5.5 min incubation and of paraoxon for 8 min incubation was 4x10(-10) and 4x10(-7) M, respectively. Utilization of the AChE microchip assay for inhibition kinetics was demonstrated also by evaluation of the inhibitor-enzyme bimolecular reaction constant (k(i)). The evaluated k(i) values for VX and paraoxon for AChE from the electric eel were 3.5 x 10(7) and 1.7 x 10(5) M(-1) min(-1), respectively, conforming well to reported values obtained by bulk methods.

Voltage-induced inhibition of antigen-antibody binding at conducting optical waveguides

Optical waveguides coated with electrically conducting indium-tin oxide (ITO) are demonstrated here as a new class of substrate for fluorescent immunosensors. These waveguides combine electrochemical control with evanescent excitation and image-based detection. Presented here are preliminary results utilizing these waveguides that demonstrate influence of waveguide voltage on antigen binding. Specifically, waveguide surfaces were bisected into electrically addressable halves, anti-ovalbumin immobilized in patterns on their surfaces, and a 1.3 V bias applied between waveguide halves in the presence of Cy5-labeled ovalbumin in 10 mM phosphate buffer (pH 7.4) containing 150 mM NaCl and 0.05% Tween-20. Fluorescence imaging indicated that binding of the antigen to positively biased waveguide halves was inhibited nearly 10-fold compared with negatively biased waveguide halves and unbiased controls. Furthermore, it is shown that ovalbumin binding to positively biased waveguide regions is regenerated after removal of applied voltage. These results suggest that electrochemical control of immunosensor substrates can be used as a possible strategy toward minimizing cross-reactive binding and/or nonspecific adsorption, immunosensor regeneration, and controlled binding.

Kinetics of antigen binding to arrays of antibodies in different sized spots

A fluorescence-based array biosensor has been developed which can measure the binding kinetics of an antigen to an immobilized antibody in real time. A patterned array of antibodies immobilized on the surface of a planar waveguide was used to capture a Cy5-labeled antigen present in a solution that was continuously flowed over the surface. The CCD image of the waveguide was monitored continuously for 25 min. The resulting exponential rise in fluorescence signal was determined by image analysis software and fitted to a reaction-limited kinetics model, giving a kf of 3.6 x 10(5) M(-1) s(-1). Different spot sizes were then patterned on the surface of the waveguide using either a PDMS flow cell or laser exposure, producing width sizes ranging from 80 to 1145 microm. It was demonstrated that under flow conditions, the reduction of spot size did not alter the association rate of the antigen with immobilized antibody; however, as the spot width decreased to < 200 nm, the signal intensity also decreased.

Acoustic and optical transduction of BuChE binding to procainamide modified surfaces

A novel polymer, poly(procainamide), PPA, containing numerous binding sites for cholinesterases was synthesized as a recognition layer for butyryl cholinesterase (BuChE) interaction with the ligand procainamide, utilizing TSM and SPR sensors. The polymer was synthesized by the reaction of methacryloyl chloride and procainamide followed by radical polymerization. Sensor surfaces (Au or SiO(2)) were spin-coated by the polymer solution to form thin layers. Binding of BuChE was found to be sensitive to the drying procedure of the polymer layer. The binding of BuChE to the polymer coated sensors was monitored on-line by following the response of thickness shear mode (TSM) and surface plasmon resonance (SPR) sensors. Binding of BuChE to PPA-coated TSM sensors were shown to follow a Langmuir isotherm giving association constant 3.4x10(6) M(-1).

Kinetics of water vapor sorption in porcine stratum corneum

The kinetics of water vapor absorption-desorption was studied in the porcine stratum corneum (sc) at 25 degrees C. Dry sc samples were exposed to several relative humidities followed by exposure to dry air, and the change in weight was monitored over time by use of a thermal gravimetric analysis technique. Diffusion coefficients were estimated by simulation and data fit optimization to modifications of the non-steady-state solution of Fick's law. Very good fits of experimental data to theoretical equations were obtained. The stratum corneum-water absorption isotherm was characterized by a linear increase from the origin, extending to about 55% relative humidity (10% water gain), followed by an upward curve where water gain increased further by increasing relative humidity. Water absorption and water desorption in the porcine sc were asymmetrical processes, desorption being the slower one. Water diffusivity for the absorption process was related to sc water content in two phases. In the first (water gain < 9%) diffusion coefficients increased as water concentration increased. In the second (water gain > 9%) diffusion coefficients were independent of water concentration, equal to 1.17 x 10(-10) cm2/s. Water diffusivity in the desorption phase was shown to decrease linearly as sc water content decreased. Analysis of the absorption isotherm and of the derived diffusion coefficients suggested at least two forms of water associated with the sc. The first, termed "firmly bound water" and characterized by low diffusion coefficients, was most apparent with dry sc and with a sc water gain of up to 10%.(ABSTRACT TRUNCATED AT 250 WORDS)

Water diffusivity in porcine stratum corneum measured by a thermal gravimetric analysis technique

Water is a natural constituent of the stratum corneum (sc) affecting its plasticity and modulating its barrier function. Diffusion coefficients (D) were calculated by measuring the desorption rates of water from porcine sc and dermis samples by a thermal gravimetric analysis (TGA) technique at isothermal conditions in the range 30-80 degrees C. Water-loaded samples were exposed to a flow of dry air, and the change of weight and of temperature were monitored with time. Distinct abrupt decreases in rate of desorption marked three different phases of water in the sc, designated as free, bulk, and bound water. Concomitant with the sharp change in evaporation rate, an increase in temperature was observed, in accordance with the absorption of heat accompanied with the water desorption process. Desorption curves were plotted against time and optimized. Values of D were estimated from the ratio of the evaporated water to the initial sc water content, as a function of the square root of time. The "initial slope" and the t1/2 (time for which Mt/Mo = 0.5, where Mo is the equilibrium amount of water absorbed in the membrane and Mt is the amount of water released by the membrane in a time t) methods gave similar results. The water D values of sc at 30 degrees C calculated by the two methods were 3.3 +/- 0.6 x 10(-10) and 2.7 +/- 0.8 x 10(-10) cm2/s, respectively. These values were about two orders of magnitude lower than the calculated D value for water in the dermis.(ABSTRACT TRUNCATED AT 250 WORDS)

Penetration of theophylline and adenosine into excised human skin from binary and ternary vehicles: effect of a nonionic surfactant

A nonionic surfactant, diethyleneglycol lauryl ether (PEG-2-L), increases the flux of either theophylline or adenosine by a factor of 2.2-2.7, when these are delivered from propionic acid solutions into human skin samples, with respect to propionic acid alone. At the same time, the flux of propionic acid from the same vehicles is decreased. Significant expansion of the partial molal volumes vi of both purines occurs following incorporation of PEG-2-L into their propionic acid solution. Hence, the enhancing effect of this surfactant arises mainly from an increase in the excess free energy of these solutes in the donor phase ("push" effect). Paraffin oil increases the flux of either drug from propionic acid by an entirely different mechanism. It enhances the flux of propionic acid into the skin, thus promoting the partitioning of the purine solute in the modified skin barrier ("pull" effect). Indeed, the magnitude of vi of either purine in propionic acid:paraffin oil solution gives no indication of a significant interaction between paraffin oil and the purine solute. Finally, the penetration enhancing effects of PEG-2-L and paraffin oil combined together in the same propionic acid vehicle are additive, resulting in a flux which is approximately the sum total of fluxes obtained separately with PEG-2-L or paraffin oil.

Penetration of adenosine into excised human skin from binary vehicles: the enhancement factor

Human skin samples are permeable to adenosine delivered from 0.5-3% solutions in vehicles consisting of propionic acid, hexanoic acid, or mixtures of these in various ratios. The observed optimal Kp, hence flux, of adenosine from the binary vehicles is 0.4 X 10(-3) cm/min, while that from propionic acid is 0.12 X 10(-3), and that from hexanoic acid is 0.16 X 10(-3) cm/min. The bell-shaped dependence of the observed Kp on donor vehicle composition may be resolved into two trends proceeding in opposite directions: an increase in the excess free energy of adenosine in the donor vehicle with an increase in volume fraction of hexanoic acid in that vehicle ("push" effect); and an increase in the flux of adenosine with an increase in the flux of propionic acid ("pull" effect) delivered from the binary vehicle. The Kp of adenosine anticipated from its excess free energy in the donor phase ("push" effect), independent of any enhancing effect due to the presence of propionic acid, is calculated from the partial molal volume of adenosine in a given vehicle; this volume correlates inversely with the solubility parameter of that vehicle. The ratio of the observed Kp to the calculated Kp gives an enhancement factor which is approximately 6 for a formulation consisting of four volumes of hexanoic acid and six volumes of propionic acid. A similar trend is found for binary vehicles of isopropyl myristate and propionic acid.

Studies on uptake of gamma-aminobutyric acid by mouse brain particles; toward the development of a model

Several substances were studied for their effect on enhancement and/or inhibition of uptake of GABA into a mouse brain microsomal fraction (P3) at pH 7.3 in the presence and absence of buffer. These were diverse: Na+, K+, NH4+, Hg2+, Cl-, and HCO3-; beta-guanidinopropionic and L-2,3-diaminopropionic acids and 1,2-diaminoethane; pyridine and several methylated pyridines; chlorpromazine and ketamine; and melittin. Kinetic experiments tested these substances for competition with GABA and Na+. Assuming the GABA transporter to consist of a GABA recognition entity and a Na+- and Cl-dependent protein required for its activity, a minimal provisional model for the GABA uptake process is proposed that is consistent with all current data and with relevant observations in the literature. It accounts for the activational effects of proton removal on GABA uptake, the stoichiometry of 2 Na+ and 1 Cl- associated with uptake of one GABA molecule, and the types of inhibition of uptake shown by the substances listed above. Factors are considered that may be necessary to maintain the transporter in a GABA-receptive configuration and that allow it the freedom of movement to undergo the structural variations necessary for the transport process to take place at rates that may be regulated by environmental factors.

Delivery of theophylline into excised human skin from alkanoic acid solutions: a "push-pull" mechanism

Human skin samples are permeable to theophylline delivered from 1.5% solutions in various alkanecarboxylic acids and their mixtures. The respective permeability coefficients of theophylline, calculated from steady-state flux, correlate negatively with the permeability coefficients of the donor carboxylic acids and positively with the excess free energy of theophylline in the donor phase, or "push" effect. An exception is propionic acid which enhances the penetration of theophylline by promoting its solubility in the skin-propionic acid medium through the "pull" effect. The two effects operate jointly in the delivery of theophylline from a mixture of propionic and a higher acid such as lauric, resulting in a much higher flux than expected from theory. The "push" effect can be estimated from the solubility parameters of theophylline and those of the holding phases even though regular solution behavior is not strictly obeyed. The increase in the permeability coefficient with partial molal volume of theophylline in the donor phase seems to reach an upper limit or decrease beyond 119 cm3.mol-1.

Theoretical derivation of solute-solvent interaction parameter in binary solution: case of the deviation from Raoult's law

In a binary mixture, partial vapor pressure may show either a positive or negative deviation from the predicted value of an ideal solution. In this report we derive the deviation from Raoult's law from the heat of mixing, delta H mix, and the molal volume, V, of each of the components of a binary solution. This derivation is then tested for seven sets of combinations of two different solvents, taken at random from the literature. Each set consists of several different ratios of solute-solvent. The correlation between the reported experimental values of the partial vapor pressure of a given component, P1, and the theoretically derived values is excellent. The same derivation is further applied to calculate the solute-solvent interaction parameter, beta 12, independently from the geometric mean assumption of regular solution theory. In a number of cases, especially in hydrocarbon-alcohol mixtures, beta 12 proves to be significantly different from the calculated geometric mean square root beta 11 beta 22 or from the Walker interaction parameter term, K.

Potentiation of Na+-dependent uptake of gamma-aminobutyric acid in mouse brain particles by buffer-mediated proton removal

A number of buffers showed a remarkable facilitatory effect on the uptake of GABA into a mouse brain microsomal subfraction (P3) at 0 degrees C and pH 7.3 in the presence of 80 mM NaCl. Complete dose-response curves were obtained for 21 buffers, ranging in pKa values from 6.2 to 9.9. The data are consistent with the interpretation that the unprotonated forms of the buffers are responsible for the enhancement of GABA uptake by P3 particles and that this is a result of the removal of protons from a membrane site (or sites) in such a manner as to allow the GABA transporter to function. However, the enhancing effects of the buffers could not solely be attributable to unhindered interaction of protonated membrane sites with unprotonated forms of the buffer. Additional factors related to structures of the buffers and membrane properties which might be importantly operative in the enhancement are discussed.

Dependence of solute solubility parameters on solvent polarity

In nonpolar solvents a solute may self-associate through polar interactions, exposing its nonpolar surface to a solvent with a low solubility parameter, delta 1. In polar solvents a solute is solvated, presumably, by the polar groups of the solvent. This "chameleonic" effect results in different solubility parameters for a solute, depending on the polarity of the solvent. This report presents data for solute solubility parameters in solvents of variable polarity and gives suggestions for dealing with the chameleonic effect associated with solute-solvent interaction.

Roles of proton removal and membrane fluidity in Na+- and Cl(-)-dependent uptake of gamma-aminobutyric acid by mouse brain particles

Proton removal is required for Na+-dependent uptake of GABA into mouse brain microsomal particles (P3) to take place at 0 degrees C and pH 7.3. No temporal coordination was demonstrated to exist between proton removal and GABA uptake processes, ruling out the possibilities that either a proton gradient or proton outflux from the particles is required. Observations on proton dissociation from the particles indicated that the protons are derived from a particulate compartment that is not in pH equilibrium with the bulk solution. Experiments on the effects of temperature on GABA uptake in the presence of 80 mM NaCl alone or with 10 mM triethanolamine (TREA) at pH 7.3 showed that the effects of TREA and temperature were interactive. The relative enhancing effects of TREA on GABA uptake diminished progressively with increasing temperature. The break points in Arrhenius plots obtained in presence and absence of TREA were the same, indicating that the effect of TREA was not on bulk viscosity of the membrane. Measurements made of fluorescence polarization as a function of temperature and of the absorbance-corrected fluorescence using TMA-DPH, a probe believed to be anchored at the lipid-water interface, showed clearly that the characteristic viscosity changes that take place with temperature in the membrane regions through which the probe is distributed were not correlated with the effects of temperature on GABA uptake. It is tentatively concluded that the protons may be attached to the membranes of the P3 particles by strong coulombic interactions in unstirred electrical double layers, possibly both on the inside and outside of particles. The importance of the existence of differences between pH at the surfaces of membranes, possibly both on the inside and outside of the particles, and that of the bulk solutions that bathe them was reiterated.

Verapamil is a competitive inhibitor of gamma-aminobutyric acid and calcium uptakes by mouse brain subcellular particles

We found that verapamil and its methoxy analogue, D600, were relatively potent (micromolar) inhibitors of Na+-dependent GABA uptake by a mouse brain microsomal subfraction (P3). Verapamil was competitive with GABA and uncompetitive with Na+ in the uptake assay with the P3 fraction. These substances were much less effective in inhibiting GABA binding in a receptor-related assay system with synaptosomal membranes. Inhibition by verapamil of Na+-dependent 45Ca++ uptake by the P3 particles was competitive with Ca++. A consideration of our results with those in the literature led to the suggestion that the interaction of verapamil and related substances with GABA and 45Ca uptake processes by the P3 fraction, as well as with many other membrane activities, may be allosteric in nature rather than directly competitive with specific ligands at their binding sites.

Percutaneous absorption of alkanoic acids I: A study of operational conditions

The rate of penetration of propionic and butyric acids through excised porcine skin was determined in vitro in specific apparatus allowing optimal control of operational conditions. In one technique, the rate was followed by continuous pH-stat titration of acid appearing in the perfusate, in another, by periodic monitoring of [14C]propionic acid in the perfusate. With the assumption that Fick's equation applies to the process of penetration, it was found that the permeability coefficient, Kp, increases with increasing mass of neat penetrant applied per unit area to the donor side, increases with increasing concentration of penetrant in n-heptane as vehicle, increases with increasing temperature, Ea = 11.4 kcal/mol, and decreases with decreasing perfusion rate of the acceptor side when this rate is smaller than 60 mL/h.

Percutaneous absorption of alkanoic acids II: Application of regular solution theory

The permeability coefficient, Kp, of pure unbranched alkanoic acids (C2-C7) applied to isolated porcine skin, reached a maximum in the solubility parameter (delta 2) range of 9.7-10 cal1 /2/cm3/2. When these and other penetrants were delivered from a solvent vehicle, the following linear relationships could be demonstrated: (a) between log Kp and the molar attraction constant of the penetrant [delta 2v2 or (-Ev)1/2] for six unbranched and six branched acids delivered from 1 M solution in n-heptane; (b) between Kp and the partial molal volume difference in n-heptane (-v2-v02) for the unbranched acids; and (c) between Kp and (-v2-v02) for propionic acid delivered from 1 M solutions in nine solvents having delta 1 values in the range 7.4-12.7 cal1 /2/cm3/2. Drug penetrability in a given series could be assessed from knowledge of the excess free energy of the penetrant in the delivery system used.

Densitometric determination of the solubility parameter and molal volume of compounds of medicinal relevance

A procedure is described for the simultaneous determination of molal volumes (v02) and solubility parameters (delta) of compounds of medicinal interest. These include alkanoic acids of various chain length and branching (some solid at room temperature), cholesterol, and cholesteryl esters. The procedure is based on the determination of partial molal volumes (v2) from high-precision density measurements of dilute solutions of these compounds in reference solvents, which range in polarity from carbon tetrachloride (delta = 8.6) to nitrobenzene (delta = 10.0). In some cases, the present results do not agree with values of delta published in the literature. Values calculated from group contributions proposed by other authors are prone to error particularly in the case of branched acids and cholesteryl esters.