Catherine R. Cabrera

Lab-on-a-chip for drug development

Significant advances have been made in the development of micro-scale technologies for biomedical and drug discovery applications. The first generation of microfluidics-based analytical devices have been designed and are already functional. Microfluidic devices offer unique advantages in sample handling, reagent mixing, separation, and detection. We introduce and review microfluidic concepts, microconstruction techniques, and methods such as flow-injection analysis, electrokinesis, and cell manipulation. Advances in micro-device technology for proteomics, sample preconditioning, immunoassays, electrospray ionization mass spectrometry, and polymerase chain reaction are also reviewed.

Continuous concentration of bacteria in a microfluidic flow cell using electrokinetic techniques.

A novel method for the concentration of bacterial solutions is presented that implements electrokinetic techniques, zone electrophoresis (ZE) and isoelectric focusing (IEF), in a microfluidic device. The method requires low power (< 3e‐5 W) and can be performed continuously on a flowing stream. The device consists of two palladium electrodes held in a flow cell constructed from layers of polymeric film held together by a pressure‐sensitive adhesive. Both ZE and IEF are performed with carrier‐free solutions in devices in which the electrodes are in intimate contact with the sample fluid. IEF experiments were performed using natural pH gradients; no carrier ampholyte solution was required. Experiments performed in buffer alone resulted in significant electroosmotic flow. Pretreatment of the sample chamber with bleach followed by a concentrated solution of cationic detergent effectively suppressed electroosmotic flow.

Passive electrophoresis in microchannels using liquid junction potentials.

The formation of the liquid junction potential (LJP) is a well‐studied phenomenon that occurs in the presence of ionic concentration gradients. Although the LJP has been well characterized, its impact has generally been overlooked in microfluidic applications. The characteristics of flow in microfluidic channels cause this phenomenon to be particularly important, both as a source of deviation from anticipated results and as a tool capable of being harnessed to perform useful tasks. It is demonstrated that LJPs formed in microchannels can induce appreciable electrophoretic transport of charged species without the use of electrodes or an external power supply. This process is demonstrated in an H‐filter (an H‐shaped microfluidic channel used to bring two fluids into contact allowing extraction of diffusing species from one stream to another) by generating junction potentials between two flowing streams containing different concentrations of strong electrolytes and observing the mass transport of the charged dye fluorescein between those streams. It is shown that the LJP can be controlled to either accelerate or decelerate mass transport across a fluid interface in the absence of an interposed membrane. A preliminary mathematical description of the phenomena is offered to support the hypothesis that the observed mass transport is a result of the LJP. Possible practical microfluidic applications of electrophoretic transport without electrodes are discussed.

Applying Microfluidic Chemical Analytical Systems to Imperfect Samples

Over the last 4 years our group has been involved in developing a series of devices for chemical separation and analysis. These devices share a theme of utilizing the low Reynolds number properties of liquids flowing at slow speeds in small channels. These devices allow some types of function that are not possible in larger devices because of the possibility of bringing flows together without convective mixing. We have taken two somewhat different approaches to coping with samples that contain particles that are incompatible with one or more analytical methods to be used.

Analytical Devices Based on Transverse Transport in Microchannels

The manipulation of transport transverse to the flow direction has permitted development of microfluidic devices that allow continuous processing of samples. One of these is a rapid competition immunoassay that relies on apparent changes in the diffusivity of antigen due to binding of the antigen to relatively slowly diffusing antibody. It has also been possible to utilize isoelectric focusing to concentrate and separate different types of analytes into separate flowing streams.

Mapping of pH Gradients in Microfluidic Electrokinetic Devices

As part of a larger effort to perform isoelectric focusing and electrophoresis in a microfluidic device [1,2], image analysis techniques that permit non-invasive quantitative measurement of pH in a micro fluidic channel have been developed [3]. By using two independent color channels, the ratio of the two forms of the indicator dye can be measured; this ratio is then correlated to pH. Methods for accounting for illumination variation have also been developed and shown to reduce scatter in the data. Results of this technique are compared to model predictions of dye concentration and pH gradients and found to be in good agreement.

Use of isoelectric focusing for sample preconditioning in a microfluidic electrochemical flow cell

A novel free-flow isoelectric focusing (IEF) implementation method has been developed, called /spl mu/IEF, as part of a larger effort to design microfluidic sample preconditioning techniques. The /spl mu/IEF method consists of a novel device in which the electrodes are in intimate contact with the bulk fluid and pH gradient formation is driven by the products of electrolysis generated at the electrodes. The /spl mu/IEF flow cell is fabricated using rapid prototyping techniques. /spl mu/IEF has been used successfully to concentrate vegetative bacteria and proteins. The thickness and position of the focused band can be adjusted by varying the buffer composition and the initial pH.