Biomedical subjects

Dongqing Li

Publications and source records attributed to Dongqing Li.

52 records · Page 3Linked to original sources

PubMed2003

Electrokinetic transport through rough microchannels.

Surface roughness is present in most microfluidic devices as a result of the microfabrication techniques or particle adhesion. It is highly desirable to understand the roughness effect on microscale transport processes. In this study, we developed a 3-D, finite-volume-based numerical model to simulate electroosmotic transport in microchannels with rectangular prism rough elements on the surfaces. Various configurations of roughness were investigated, and the results show different degrees of an even-out effect on liquid transport due to the roughness-induced local pressure field and the variation of the electroosmotic slip boundary velocities. 3D-sample transport through rough microchannels was analyzed. The results demonstrate that the sample's transport under the electrical field is much faster in the pathway between the rough elements; the concentration field in the height and width direction is not uniform. The influence of the electrokinetic properties on liquid flow and sample transport was studied. It was found that the increase of the electroosmotic mobility or the decrease of the electrophoretic mobility can dramatically enhance the uniformity of the concentration field.

Journal Article↗

PubMed2003

A dynamic loading method for controlling on-chip microfluidic sample injection.

A new technique for controlling discrete sample injection in straight-cross microfluidic chips is presented here. This technique involves a three-part process with a dynamic loading step in between the steady-state loading step and the dispensing step. During the intermediate step, sample is pumped into the intersection and into the three connecting channels. The key features of this technique are the ability to dynamically control the sample size and the ability to inject well-defined samples at the original sample concentration. Injections of these samples with lengths varying from 2 channel widths (100 microm) to 20 channel widths (millimeter-sized) are demonstrated. The sample concentration profiles obtained are compared with those of focused and less-focused pinched-valve injections. In applications such as high-speed capillary zone electrophoresis, this technique can provide an increase in signal with a small increase in sample length. This technique is especially applicable to many large-sample applications in which the offset twin-T microchip has been previously employed.

Journal Article↗

PubMed2003

Joule heating and heat transfer in poly(dimethylsiloxane) microfluidic systems.

Joule heating is a significant problem in electrokinetically driven microfluidic chips, particularly polymeric systems where low thermal conductivities amplify the difficulty in rejecting this internally generated heat. In this work, a combined experimental (using a microscale thermometry technique) and numerical (using a 3D "whole-chip" finite element model) approach is used to examine Joule heating and heat transfer at a microchannel intersection in poly(dimethylsiloxane)(PDMS), and hybrid PDMS/Glass microfluidic systems. In general the numerical predictions and the experimental results agree quite well (typically within +/- 3 degree C), both showing dramatic temperature gradients at the intersection. At high potential field strengths a nearly five fold increase in the maximum buffer temperature was observed in the PDMS/PDMS chips over the PDMS/Glass systems. The detailed numerical analysis revealed that the vast majority of steady state heat rejection is through lower substrate of the chip, which was significantly impeded in the former case by the lower thermal conductivity PDMS substrate. The observed higher buffer temperature also lead to a number of significant secondary effects including a near doubling of the volume flow rate. Simple guidelines are proposed for improving polymeric chip design and thereby extend the capabilities of these microfluidic systems.

Computer Simulation↗

PubMed2003

Effects of liquid conductivity differences on multi-component sample injection, pumping and stacking in microfluidic chips.

As an increasing number of processes are being integrated into Lab-on-a-chip devices, there is an increasing need for flexible and accurate sample manipulation techniques for effective transport and separation. Conductivity differences between running buffer and analyte samples can arise as a product of on-chip processing, or by design. The two situations studied here are sample pumping (where bulk transport is increased and separation of charged analytes is delayed using a relatively high conductivity sample), and sample stacking (where bulk transport is decreased and separation of charged analytes is expedited using a relatively low conductivity sample). A recently developed dynamic loading method for on-chip sample injection in a straight-cross channel configuration is applied here to both pumping and stacking cases. A key characteristic of the dynamic loading method is the ability to inject samples of high concentration density and uniformity of any length. By employing the conductivity differences alone, the effectiveness of either sample transport or sample separation are shown to improve over the uniform conductivity case. Then it is demonstrated that increasing the sample length, through dynamic loading, greatly increases the effectiveness of sample pumping, evidenced in an eight-fold increase in peak height as well as a decrease in total sample length at a downstream detector. Dynamic loading in the sample stacking case was shown to also increase peak intensity height (three-fold) in rapid separations. These results demonstrate that the dynamic loading technique, used in conjunction with strategic conductivity differences, significantly extends the capabilities of microfluidic chips.

Journal Article↗

PubMed2003

Modeling of DNA hybridization kinetics for spatially resolved biochips.

The marriage of microfluidics with detection technologies that rely on highly selective nucleic acid hybridization will provide improvements in bioanalytical methods for purposes such as detection of pathogens or mutations and drug screening. The capability to deliver samples in a controlled manner across a two-dimensional hybridization detection platform represents a substantial technical challenge in the development of quantitative and reusable biochips. General theoretical and numerical models of heterogeneous hybridization kinetics are required in order to design and optimize such biochips and to develop a quantitative method for online interpretation of experimental results. In this work we propose a general kinetic model of heterogeneous hybridization and develop a technique for estimating the kinetic coefficients for the case of well-spaced, noninteracting surface-bound probes. The experimentally verified model is then incorporated into the BLOCS (biolab-on-a-chip simulation) 3D microfluidics finite element code and used to model the dynamic hybridization on a biochip surface in the presence of a temperature gradient. These simulations demonstrate how such a device can be used to discriminate between fully complementary and single-base-pair mismatched hybridization using fluorescence detection by interpretation of the unique spatially resolved intensity pattern. It is also shown how the dynamic transport of the targets is likely to affect the rate and location of hybridization as well as that, although nonspecific hybridization is present, the change in the concentration of hybridized targets over the sensor platform is sufficiently high to determine if a fully complementary match is present. Practical design information such as the optimum transport speed, target concentration, and channel height is presented. The results presented here will aid in the interpretation of results obtained with such a temperature-gradient biochip.

Biosensing Techniques↗

PubMed2003

Zeta-potential measurement using the Smoluchowski equation and the slope of the current-time relationship in electroosmotic flow.

The zeta -potential of a solid-liquid interface is an important surface characterization quantity for applications ranging from the development of biomedical polymers to the design of microfluidic devices. This study presents a novel experimental technique to measure the zeta -potentials of flat surfaces. This method combines the Smoluchowski equation with the measured slope of current-time relationship in electroosmotic flow. This method is simple and accurate in comparison with the traditional streaming potential and electrophoresis techniques. Using this method the zeta -potentials of glass and poly(dimethylsiloxane) (PDMS) coated surfaces in KCl and LaCl3 aqueous solutions were measured using several flow channels ranging from 200 to 300 microm in height. The zeta -potential was found to vary from -88 to -66 mV for glass surface and -110 to -68 mV for PDMS surfaces depending on the electrolyte and the ionic concentration. The measured values of the zeta -potential are found to be independent of the channel size and the applied driving voltage and generally are repeatable within +/-6%.

Journal Article↗

PubMed2003

Visualization and numerical modelling of microfluidic on-chip injection processes.

Sample injection processes accomplished using a microfluidic-cross chip are investigated experimentally and numerically. Fluorescent dye is employed to differentiate the sample solution from the pure buffer. Different sample geometries are achieved using different applied electric fields and dyes with different electrophoretic mobilities. Of particular interest here are concentration-dense samples with large axial extent (extending beyond the intersection). The ability to load and subsequently dispense these large axial extent samples is predicted numerically and verified experimentally by direct visualization. Containing more mass, larger samples exhibited lower concentration gradients, making them less sensitive to diffusion and well-suited to transport once dispensed. In the loading process, however, larger samples were found to be more sensitive to pressure effects than more focused samples. This was investigated by imaging sample geometries under various applied fields in the presence of a constant pressure gradient. Laplace pressure originating from differential meniscus curvatures in the reservoirs was found to be the most significant source of such pressure disturbances in these geometries.

Journal Article↗

PubMed2003

Experimental and theoretical study of the displacement process between two electrolyte solutions in a microchannel.

Displacement of one electrolyte solution by another in a microchannel is required in many biolab chip devices. The objective of this paper is to develop a better understanding of the displacement process between two electrolyte solutions under an applied electric field in a cylindrical microchannel in terms of the traveling distance of the interface between these two electrolyte solutions. In order to develop a general model to predict the location of the interface, two different situations are considered; one model assumes the presence of a sharp interface between the two solutions and the other model considers a mixing zone between the two solutions. Carefully conducted experiments were carried out to obtain the current-time relationship, which is used in the model to predict the location of the interface. In these experiments, deionized ultrafiltered water (DIUF water), 10 mM KCl, 0.1 mM KCl, and 0.1 mM LaCl3 solutions were used as the testing liquids. Polyamide-coated silica capillary tubes of internal diameter 100 mum and length 10 cm were employed in this study. The relationship between traveled distance of the interface and time was predicted by a developed model based on the measured current-time relationship for such a displacement process under a constant applied electric field. The characteristics of the nonlinear change of the traveling distance with the time were also discussed in this paper.

Hypertonic Solutions↗

PubMed2002

Theoretical studies of microfluidic dispensing processes.

The understanding of electrokinetic transport phenomena in microfluidic dispensers, an important component of biochips, is very important for designing and controlling biochips. A theoretical model to study the electrokinetic transport processes in microfluidic dispensers was developed in the work to study the controlling parameters for the dispensing process. The electrical field, the flow field, and the concentration field during dispensing processes were obtained by solving this theoretical model numerically. The effects of the electroosmotic mobility of the buffer solution, the diffusion coefficient and the electrophoretic mobility of the sample, the applied electrical field strength, and the channel size on the dispensing process are examined in this paper. The investigations show that optimal controlling parameter values can be found by using this model for dispensing any desired amount of the sample.

Journal Article↗

PubMed2002

Direct and indirect electroosmotic flow velocity measurements in microchannels.

As microfluidic technologies mature, increasingly complex solutions are employed, and accurate methods for the measurement of electroosmotic flow rates are becoming increasingly important. The methodologies of both a direct method and an indirect method of flow rate measurement are presented here. The direct method involves flow visualization using trace amounts of a caged fluorescent dye. The indirect method is based on the change in current that occurs when one solution in the microchannel is replaced by another. The results of concurrent and independent measurements of electroosmotic velocities of Tris-acetate with EDTA (TAE) and Tris-borate with EDTA (TBE) at 1x concentration in fused silica capillaries are presented. Although these buffers are commonly used in biological chemistry, these mobilities have not previously been reported. Strong agreement among data collected with both methods establishes confidence in the electroosmotic mobility values obtained and indicates that the current-based method, which requires less infrastructure than the direct method, can provide accurate flow rate measurements under these conditions. Constant electroosmotic mobilities of 4.90 x 10(-8) m(2) V(-1) s(-1) for TAE and 3.10 x 10(-8) m(2) V(-1) s(-1) for TBE were determined by tests in a range of electrical field strengths from 5 to 20 kV/m. A linear flow rate increase with applied field strength indicated that constant mobility and negligible Joule heating effects were present. Applicability and limitations of both the measurement methods and these buffers are discussed in the context of microfluidic applications.

Journal Article↗

PubMed2002

Electrophoretic motion of a sphere in a microchannel under the gravitational field.

This paper considered electrophoretic motion of a sphere in an aqueous electrolyte solution in a microchannel under the gravitational field. In an externally applied electric field, the negatively charged sphere will move toward the anode. At the same time, the sphere will move toward the lower channel wall due to the density difference and the gravity. When the sphere moves very close to the lower wall, the buoyancy, the electric double layer interaction force, and the van der Waals force balance the gravity force, so the sphere moves parallel to the lower wall. A theoretical model for the electrophoretic motion of a sphere in a microchannel, with the consideration of the electrophoretic retardation effect, is presented in this paper. It was found that the sphere's motion in the microchannel is affected by its size, the density difference, the zeta potentials of the sphere and the channel wall, and the applied electric strength. The effects of these factors on the sphere's transport distance in the microchannel are discussed. It was found that the spheres with the same surface charge could be separated by their size within a certain range of ka in aqueous solutions in the microchannel.

Electrophoresis↗

PubMed2002

A new method of evaluating the average electro-osmotic velocity in microchannels.

A new and simple method to evaluate the average electro-osmotic flow velocity in microchannels is presented in this paper. In this method, the average electro-osmotic flow velocity is determined by using the slope of the measured current-time relationship during the electro-osmotic flow of one solution replacing another similar solution. The two solutions have the same electrolyte and a small difference in ionic concentration. Careful experiments were conducted to measure the electrical current change with time during such a displacing process under a constant applied electrical field. KCl and LaCl3 electrolyte solutions and 10-cm-long polyamide-coated silica capillary tubes of 100 and 200 microm in internal diameter were used in this study. The average velocities were determined by using the slope method. A numerical model was also developed to predict the average velocity of such an electro-osmotic flow. An excellent agreement in the average velocities between the slope method and the model predictions was found.

Journal Article↗

PubMed2002

Deposition of spherical particles onto cylindrical solid surfaces. I. Numerical simulations.

In this article, analytical and numerical solutions are given for the deposition of spherical particles onto cylindrical solid surfaces. If only the retarded van der Waals (vdW) force and the electrical double-layer (EDL) force between the spherical particle and the cylindrical surface are considered, such a deposition process is governed by a one-dimensional (1D) mass transfer equation for which both analytical and numerical solutions are obtained. A parametric study was conducted to examine the effects of these two colloidal forces on the deposition process. The numerical results show that the attractive vdW force, as represented by the dimensionless adhesion number A(d), is a necessary condition for deposition to occur. However, it affects only the drop number concentration distribution and, to a limited extent, the mass transfer rate. In general, the 1D deposition process is dominated by the EDL force, as represented by the dimensionless EDL parameter Dl and the reduced radius tau=kappaa of the spherical drop. It is noted that tau plays a key role in determining the effect of the EDL force on the mass transfer rate since an appreciable EDL force exists only within small separation distances (</=3tau(-1)). In addition to the two colloidal forces, the deposition process was also investigated in the presence of gravity and with a vertically applied electrical force. With these two external forces, the deposition becomes a two-dimensional (2D) boundary value problem for which a numerical solution was achieved using the implicit Crank-Nicolson method. Their effects on the 2D deposition process were studied.

Journal Article↗

PubMed2002

Deposition of spherical particles onto cylindrical solid surfaces. II. Experimental studies.

This paper presents experimental studies of the deposition of silicone oil drops onto two different solid surfaces in an aqueous solution. A series of deposition tests were conducted to measure the dimensionless mass transfer rate (Sherwood number). The effects of three kinds of aqueous solutions and two solid surfaces on the deposition process were studied and compared with the numerical predictions based on the well-known DLVO theory. More specifically, both the experimentally measured and the numerically predicted Sherwood numbers monotonically decrease as the pH value of the aqueous solution increases. It was also found that two ionic surfactant solutions have similar influences while the electrolyte solutions have opposite effects on the deposition rate on different solid surfaces. Finally, comparison of all the experimental results for the bare glass surface with the numerical simulations shows that the deposition process of the silicone oil drops onto the hydrophilic solid surface can be satisfactorily described by the classical DLVO theory. However, the deposition data for the FC725 precoated surface are significantly larger than the numerical predictions. This fact suggests that the so-called non-DLVO attractive interaction is involved in the deposition process with the hydrophobic solid surface. This additional non-DLVO attractive interaction, which is generally called the hydrophobic interaction, still remains to be incorporated into the existing DLVO theory, if this is possible.

Journal Article↗

PubMed2001

Streaming Potential and Streaming Current Methods for Characterizing Heterogeneous Solid Surfaces.

By monitoring changes in electrokinetic parameters, the streaming potential technique has been used as a method of characterizing heterogeneous surfaces, for example, due to protein adsorption. In general it is assumed that the change in the streaming potential is proportional to the degree of heterogeneity. In this study a simple model of the electrokinetic flow through heterogeneous slit channels has been developed with the goal of comparing the streaming potential and streaming current techniques and determining under what conditions the aforementioned proportionality assumption will produce erroneous results. The flow simulations have shown that, when the streaming potential induces significant flow impedance, a severe deviation from the linear assumption is observed. Since streaming current measurements are unaffected by electrokinetic flow effects, more consistent results are predicted and they are preferred for measurements made in small channels. Additionally it has been shown that the distribution of the heterogeneous coverage has a negligible effect on both the streaming potential and the streaming current in cases where the double-layer overlap is not significant. Copyright 2001 Academic Press.

Journal Article↗

PubMed2000

Determining zeta Potential and Surface Conductance by Monitoring the Current in Electro-osmotic Flow.

In this paper we have outlined a simple procedure for determining zeta potential, zeta(0), and surface conductance, lambda(s), based on current monitoring in electro-osmosis. In these experiments, the average velocity was determined by measuring the amount of time required to completely displace a solution by another solution in the capillary tube. The average velocity during electro-osmosis was found to be independent of capillary size, although it was dependent on the electrolyte concentration and applied electrical field. The measured values of the zeta potential, zeta(0), were found to be independent of capillary size and the applied field, while zeta(0) is strongly dependent on the electrolyte concentration. Calculations of the specific surface conductivity lambda(s) based on current measurements reveal a relationship between lambda(s) and capillary size, in agreement with the results reported in the literature. Copyright 2000 Academic Press.

Journal Article↗