Nonlinear Concentration Gradients Research Articles

Micro-fluidic covalent immobilization of multi-gradient RGD peptides on a gelatin surface for studying endothelial cell migration.

Collective endothelial migration is a hallmark of wound healing, which is regulated by spatial concentration gradients of extracellular biochemical factors. Arginine-glycine-aspartate (RGD) peptides play a vital role in regulating cell migration through specific binding to integrins. In this study, a micro-fluidic technology combined with a photopolymerization technique is developed to create gelatin methacryloyl (GelMA)-based substrates with various concentration gradients of RGD peptides. The capability of generating linear and nonlinear RGD concentration gradients was quantitatively verified through numerical simulation and immunohistochemical quantitative experiments. The results of the concentration gradients show a strong concurrence between the immunohistochemical quantification experiments and numerical simulations. Furthermore, endothelial migration experiments were conducted with various concentration gradients of RGD peptides. We have observed that endothelial cells on the surface of gels with a linear concentration gradient exhibit a larger cell area, a longer cell perimeter, and more stress fiber density. Furthermore, the cells demonstrate directional alignment and migration towards regions with a higher RGD concentration. High concentration gradients significantly enhance endothelial cell migration, consistent with observations on surfaces of gels with nonlinear concentration gradients. In brief, we proposed a simple and effective micro-fluidic photopolymerization technique capable of generating diverse concentration gradients of RGD and probing their effects on cell migration. The results suggest that regulating the RGD peptide concentration gradients can alter the migration of endothelial cells, showing potential for promoting wound healing.

Complete experimental and theoretical characterization of nonlinear concentration gradient generator microfluidic device for analytical purposes.

Microfluidic devices that generate stable concentration gradients are efficient instruments for automated calibration for analytical and bioanalytical systems. However, little attention has been paid tothe development of reusable microfluidic concentration gradient generators, which can be useful for a range of species through mathematical characterization. In this work, we develop a microfluidic device based on three steps of serial dilution that were able to generate nonlinear concentration gradient for dyes and biomolecules. The microfluidic device was described mathematically, statistically and was suitable for reusable analytical and bioanalytical analysis. The device reproducibility was assessed by experimental tests, which have shown the same gradient concentration profile for different dyes and statistical reproducibility with 95% confidence interval for bovine serum albumin (BSA). Moreover, the experimental data converged well withthose obtained by computational fluid dynamics simulation. Applicability was verified by coupling the microfluidic device to a surface plasmon resonance (SPR) biosensor, based on nanohole arrays with sensitivity of 358.7 nm RIU-1 determined by white-light SPR excitation exposed to different D-(+)-glucose aqueous solutions with 1.3361-1.4035 refractive index interval. The transmission light intensities obtained by the array of images allowed to quantify a pseudo-unknown BSA sample (160 µg mL-1) at 138 µg mL-1. The SPR analysis has been validated in parallel by fluorescence emissions, which showed a concentration of 154.8 ± 16.6 µg mL-1.

Properties of Mosquito Repellent-Plasticized Poly(lactic acid) Strands.

Poly(lactic acid) (PLA) is an attractive candidate for replacing petrochemical polymers because it is fully biodegradable. This study investigated the potential of PLA as a sustainable and environmentally friendly alternative material that can be developed into commercially viable wearable mosquito repellent devices with desirable characteristics. PLA strands containing DEET and IR3535 were prepared by twin screw extrusion compounding and simultaneously functioned as plasticizers for the polymer. The plasticizing effect was investigated by thermal and rheological studies. DSC studies showed that the addition of DEET and IR3535 into PLA strands reduced the glass transition temperature consistent with predictions of the Fox equation, thus proving their efficiency as plasticizers. The rheology of molten samples of neat PLA and PLA/repellents blends, evaluated at 200 °C, was consistent with shear-thinning pseudoplastic behaviour. Raman studies revealed a nonlinear concentration gradient for DEET in the PLA strand, indicating non-Fickian Type II transport controlling the desorption process. Release data obtained at 50 °C showed initial rapid release followed by a slower, near constant rate at longer times. The release rate data were fitted to a novel modification of the Peppas-Sahlin desorption model.

Dendritic cell subset positioning governs clonal diversity and magnitude of the CD8 T cell response

Abstract Although the molecular requirements for CD8 T cell responses have been well characterized, subunit vaccine formulations struggle to generate robust CD8 T cell immunity. One possible basis for this may be the relationship of dendritic cell (DC) subset topography to micro-anatomically dictated antigen dispersal and subsequent down-stream innate and adaptive cellular engagement. To address this issue directly in situ we utilized advanced quantitative imaging approaches (histo-cytometry) and found that protein immunization led to non-linear concentration gradients across draining lymph nodes. This graded distribution in turn affected which DC subset had access to high vs. low concentrations of antigen. Because DC subsets are specialized in MHC-I vs. MHC-II antigen processing and presentation, the result of this intersection of antigen dispersal and cell positioning was the predominant residence of MHC-I presenting DC in regions with lower antigen bioavailability. Modulating the antigen dose directly influenced the fraction of MHC-I presenting DC with access to sufficient antigen to activate T cells, allowing for direct control over the clonal diversity and magnitude of the response. These findings indicate that tissue microanatomy related to DC subset distribution and antigen dissemination within the lymph node is a critical regulator of CD8 T cell immunity. This research was supported by the Intramural Research Program of the NIH, NIAID.

Nonlinear concentration gradients regulated by the width of channels for observation of half maximal inhibitory concentration (IC50) of transporter proteins.

This paper describes a simple microfluidic device that can generate nonlinear concentration gradients. We changed the "width" of channels that can drastically shorten the total microfluidic channel length and simplify the microfluidic network design rather than the "length" of channels. The logarithmic concentration gradients generated by the device were in good agreement with those obtained by simulation. Using this device, we evaluated a probable IC50 value of the ABC transporter proteins by the competitive transport assays at five different logarithmic concentrations. This probable IC50 value was in good agreement with an IC50 value (0.92 μM) obtained at the diluted concentrations of seven points.

Rapid and accurate generation of various concentration gradients using polydimethylsiloxane-sealed hydrogel device

We report a microfluidic device that can rapidly and accurately generate various concentration gradients in a controllable manner for the chemotaxis study of motile bacterial cells by integrating hydrogel into polydimethylsiloxane (PDMS) microchannels. We performed numerical simulations for both the PDMS-sealed hydrogel hybrid device and a representative conventional hydrogel-based device to theoretically compare their characteristics. In addition, we experimentally demonstrated that the PDMS-sealed hydrogel device not only produces various linear and nonlinear concentration gradients without flow-induced shear stresses on motile bacterial cells but also exhibits remarkable advantages over conventional hydrogel-based devices. For example, the PDMS-sealed hydrogel device can be used for fast and accurate generation of various concentration gradients, prevents dehydration of hydrogel and evaporation of solutions, directs diffusion of chemicals such as chemoattractants, exhibits long-term durability, and is easy to handle. Because the hydrogel used is biocompatible and arbitrary concentration profiles can be easily designed and produced on a chip, we believe that not only the PDMS-sealed hydrogel fabrication method but also the versatile concentration gradient generation device can be used for various studies on interaction between chemicals and cells including bacterial chemotaxis assays.

A microfluidic device mimicking acinar concentration gradients across the liver acinus

The acinus-mimicking microfluidic chip, which simulates the in vivo condition of the liver, was developed and reported in this paper. The gradient microenvironment of the liver acinus is replicated within this proposed microfluidic chip. The advantage of this acinus-mimicking chip is capable of adjusting the concentration gradient in a relatively short period of time at around 10 s. At the same instance the non-linear concentration gradient can be presented in the various zones within this microfluidic chip. The other advantage of this proposed design is in the convenience of allowing the direct injection of the cells into the chip. The environment within the chip is multi-welled and gel-free with high cell density. The multi-row pillar microstructure located at the entrance of the top and bottom flow channels is designed to be able to balance the pressure of the perfusion medium. Through this mechanism the shear stress experienced by the cultured cells can be minimized to reduce the potential damage flow from the perfusion process. The fluorescence staining and the observations of the cell morphology verify the life and death of the cells. The shear stress experienced by the cells in the various zones within the chip can be effectively mapped. The serum glutamic oxaloacetic transaminase (SGOT) collected from the supernatants was used to determine the effects of the degassing process and the shear stress of the medium flow on the cultured cells.

Polyester μ-assay chip for stem cell studies.

The application of microfluidic technologies to stem cell research is of great interest to biologists and bioengineers. This is chiefly due to the intricate ability to control the cellular environment, the reduction of reagent volume, experimentation time and cost, and the high-throughput screening capabilities of microscale devices. Despite this importance, a simple-to-use microfluidic platform for studying the effects of growth factors on stem cell differentiation has not yet emerged. With this consideration, we have designed and characterized a microfluidic device that is easy to fabricate and operate, yet contains several functional elements. Our device is a simple polyester-based microfluidic chip capable of simultaneously screening multiple independent stem cell culture conditions. Generated by laser ablation and stacking of multiple layers of polyester film, this device integrates a 10 × 10 microwell array for cell culture with a continuous perfusion system and a non-linear concentration gradient generator. We performed numerical calculations to predict the gradient formation and calculate the shear stress acting on the cells inside the device. The device operation was validated by culturing murine embryonic stem cells inside the microwells for 5 days. Furthermore, we showed the ability to maintain the pluripotency of stem cell aggregates in response to concentrations of leukemia inhibitory factor ranging from 0 to ∼1000 U/ml. Given its simplicity, fast manufacturing method, scalability, and the cell-compatible nature of the device, it may be a useful platform for long-term stem cell culture and studies.

Induced charge electro-osmotic concentration gradient generator

Biomolecule gradients play an important role in the understanding of various biological processes. Typically, biological cells are exposed to linear and nonlinear concentration gradients and their response is studied for understanding cell growth, cell migration, and cell differentiation mechanisms. Recent studies have demonstrated the use of microfluidic devices for precise and stable concentration gradient generation. However, most of the reported devices are geometrically complex and lack dynamic controllability. In this work, a novel microfluidic gradient generator is presented which utilizes the induced charge electro-osmosis (ICEO) by introducing conducting obstacle in the microchannel. With the ICEO flow component, significant transverse convection can be generated within the microchannel, which can, in turn, be used to create nonlinear as well as asymmetric gradients. The characteristics of the developed concentration gradient are dependent on the interplay between fixed charge electro-osmotic and ICEO flows. It is shown that the proposed device can switch between linear and nonlinear gradients by just altering the applied electric field. Finally, the formation of user-defined concentration profiles (linear, convex, and concave) is demonstrated by varying the conducting obstacle size.

Nanoscale yttrium distribution in yttrium-doped ceria powder

The microstructure and geometry of yttrium-doped ceria particles, synthesized by spray pyrolysis (SP), were characterized by transmission electron microscopy, and the distribution of yttrium was investigated by energy-dispersive X-ray spectroscopy (EDX). The yttrium distribution was investigated by considering an electron interaction volume and geometry of the particle in an EDX profile analysis. Two models, linear and non-linear, for the yttrium concentration gradient were tested against the experimental EDX data for hollow particles. The results showed that the linear concentration distribution of yttrium fits the EDX profiles appreciably better than the non-linear concentration gradient model. This suggests that yttrium distribution is mainly controlled by the different diffusion rates of the precursors from the centre to the surface of the particle, which could be related to their different solubilities, during the heating process in SP.

Generation of linear and non-linear concentration gradients along microfluidic channel by microtunnel controlled stepwise addition of sample solution

The ability to generate stable chemical gradients in microfluidics has important applications, since such gradients are useful in both chemical and biological studies. Growing evidence reveals that many cellular responses are specific to non-linear spatial gradients, hence a need to control complex concentration gradient profiles with and within microfluidics. In this paper, we present a structure-based approach to generate linear and non-linear chemical gradients, with profiles controlled by microtunnels fabricated alongside two main channels. Using single-step photolithography, microtunnels and main channels were fabricated at different heights thus having different fluidic resistance. Through these microtunnels, sample solutions were stepwise dispensed into the buffer stream to generate a chemical gradient profile. By varying the lengths of microtunnels that dictated the volume of sample solutions being dispensed, complex gradient profiles were generated. We have successfully demonstrated the formation of linear, convex and concave gradient profiles and a simple mathematical expression was established to approximate the profiles produced in our microfluidic gradient-generators.

Simulation of Non-Linear Flow Density in Transition State from the Mathematical Model of the Diffusion of Metal Anions Through a Flat Sheet Supported Liquid Membrane

A mathematical model is developed for the carrier facilitated transport of metal ions through a flat sheet support liquid membrane (FSSLM) in transition state from Fick's second law. From this model, and from Fick's first law, the flow density is derived as a non-linear concentration gradient. Both expressions, concentration and flow density, depend on the thickness of the membrane and on time. Since the rate constant plays an important role in the model, it is considered as the parameter that controls the system and an equation for it is obtained. This equation explains the velocity of the co-transport process. The proposed model takes into account the species co-transported together with the metal ions. An equation for the number of moles of this species is obtained as a function of the metal species. The concentration gradient of this species explains the behaviour of pH in the feed phase during the process. The model is tested against experimental data corresponding to the transport of metal anions in acidic solution and it is shown that the co-transport process is reproduced with high accuracy.

Unified treatment of structural effects in fluid‐solid reactions

AbstractBy considering reaction and diffusion through a product layer and concomitant movement of pore and reaction surfaces, a general rate equation is derived for fluid‐solid reactions. Application to the random pore model extends the prior results (Bhatia and Perlmutter, 1980, 1981a) to account for nonlinear concentration gradients in the product layer, by assuming that the product is deposited as overlapping cylindrical annulae. For the Petersen (1957) model, new results are derived which account for product layer diffusion. A comparison of numerical conversion‐time predictions from the grain model (Szekely et al., 1976), the Petersen model, and the random pore models suggests that they are more strongly affected by the representation of the reaction surface than by that of the pore surface. The model is applied to the data of Borgwardt (1970) on the SO2‐lime reaction, and the results are compared with a previous interpretation using a linear concentration gradient approximation.

The permeability coefficient of the wall of a villous membrane.

The equations hitherto used to correct the permeability coefficient for the unstirred layer influence are valid only for flat membranes. Therefore, appropriate equations for membranes with a villous surface (e.g., small intestine) have been derived. They take into account the non-linear concentration gradient in the intervillous part of the unstirred layer. Quantitative information about the geometry of the villous surface and the unstirred layer thickness are needed to calculate the permeability coefficient of the membrane wall (e.g., intestinal epithelium). The concentration of highly permeable substances drops sharply already in the upper part of intervillous space, so that the tips of the villi function as effective absorbing area. The intervillous concentration gradient of a substance with a low permeability coefficient is so small, that such a substance is absorbed by the total surface area of the villous membrane. The effective absorbing area of substances with intermediate permeability coefficient lies between the described limits.