Introduction

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Device Applications

It is important to note that while these devices are just applications of already-established ideas from the microfluidics community, they are still very novel and not ready for practical use by either scientists or physicians. The primary obstacle is that the devices are by no means user-friendly: the prototypes currently appearing in various chemistry or applied physics journals show that certain methods of applying chemicals to cells in an organized manner will work, but realistically, only if done by the professor who invented it. Ease of use will need to be greatly improved before acceptance, and even then, the target audience will be the research community first and the clinical one second.

Nonetheless, it is possible to speculate on possible clinical applications for the years to come, as the microfluidics approach is currently the most promising plan towards cheap, fast, and accurate assays using a minimal amount of reagent. The current trend is to work towards the incorporation of microfluidic chips into first, the diagnosis of disease, and second, the design of patient-specific drug regimens based on the results of the diagnosis. Below is a general proof-of-concept for the use of microfluidic single-cell arrays in drug analysis.

Carboxylesterase Kinetics for Drug Uptake

Various types of cancer cells were stained with calcein AM, which fluoresces in response to contact with intracellular enzymes such as carboxylesterase, and then loaded into an array. NDGA is applied to the cells to inhibit carboxylesterase, and under a microscope, a CCD is used to collect fluorescence data over time from the trapped cells. Corresponding kinetics models are then generated, as well as distributions of enzyme concentrations in various cell types. Because carboxylesterase is involved in activation and deactivation of various drugs, this simple experiment, designed to validate a device’s function more than anything else, provides a very relevant example of the device’s use in clinical medicine.

Above: Device used in study. The fluid is perfused from top to bottom, and cells are isolated in the trapping area. The mechanism for trapping is the hydrodymaic method elaborated upon in Device Fabrication section.	Above: Data made possible by cell isolation array. In the top figure, enzyme concentration as measured by level of fluorescence is averaged across all cells, the result of a traditional assay. In the lower figure, knowledge of individual cell fluorescence allows for the generation of a histogram of enzyme concentration across the cells.

Cytotoxicity Assays Using Hepatocytes

A specialized application of the above assay is the cytotoxicity analysis, a common goal for microfluidics-based cell assays. This application was designed with the pharmaceutical industry in mind, though it very easily translates to a practical device for a pharmacist to deduce a personalized cocktail for a patient requiring aggressive treatment, such as chemotherapy. To perform the assay, the device is seeded with hepatocytes, obtained perhaps via a liver biopsy. Depending on the layout of the channels, various drug concentrations can be generated and multiple drugs can be mixed to form a large combination of possibilities. We can then perform a simple live-dead analysis on the cells, or go further and check for cytochrome p450 activity, the active toxin metabolizer in hepatocytes.

Above: This is one prototype device for cytoxicity testing. Hepatocytes can be perfused into the device from left to right, only instead of using wall as depicted in the first experiment, sieves are used to sort the cells, arranging them into tiny clusters. Drugs are then perfused vertically and in this particular experiment, a simple a live-dead assay is performed.	Results of the assay: The columns labeled H (for high toxin concentration) proved lethal to cells in several cases, without distinguishing between cell type. Potentially, a clinician can run various combinations of drugs over the array of cells using a more elaborate version of this proof-of-concept setup, optimizing a drug dosage on a patient by patient basis that will be aggressive yet safe.