Biomedical subjects

Liqing Ren

Publications and source records attributed to Liqing Ren.

8 recordsLinked to original sources

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.

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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.

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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%.

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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.

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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.

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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.

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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.

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