Microfluidic tools for high-throughput screening.

Abstract

High-throughput screening (HTS) is an established technology in the pharmaceutical industry (1). Over the past decade, there has been a logarithmic increase in the industry’s ability to screen large combinatorial libraries of compounds against target molecules (2). The technology to achieve this has come in the form of robotics, high-density microplates, small volume (microliter) liquid handling, and sophisticated detection schemes. Signifi cant effort has gone into the design of HTS workstations capable of screening tens of thousands of compounds in a 24-h period. However, these workstations come with a large price tag, ranging from several hundred thousand to millions of dollars. A considerable amount of work is necessary to optimize nascent microfl uidic devices for HTS applications. Stateof-the-art commercial microfl uidic devices are principally made by the micromachining of silicon (3) and glass and rely on electroosmotic fl ow (4,5) to drive liquid through the channels, requiring high salt concentrations and a voltage source. This process generates gas bubbles, creating ionic conditions that are far from ideal for assays measuring enzymatic activity or protein-protein interactions. Other problems with hard polymer microfl uidic devices include the need to build up layers to effi ciently seal the channel networks, making layer-layer adhesion a serious concern during the fabrication process, and the lack of a good compartmentalization technology for the large-scale analysis of chemical or biological libraries. In thinking about a generic design for microfl uidic HTS devices for catalytic screening applications that confi ne both enzyme and substrate to picoliter volumes, both serial and parallel approaches can be explored. Using a serial strategy, each compound of interest is screened sequentially using a common microfl uidic channel with a single detection element. Mechanically, throughput depends on factors such as fl ow speed, sample concentration, and the acquisition time of the detector. In contrast, parallel screening functions like an ultra-high density microplate, in which thousands of compounds are arrayed into individual picoliter-scale compartments, with a detector element that probes the entire matrix. Throughput is principally limited by the number of compartments in the array. We have designed and developed microfl uidic chips employing both serial and parallel screening strategies. Unlike in state-ofthe-art microfl uidic devices relying on electroosmotic fl ow, fltraffi cking and compartmentalization in our chips are pressure-based, using integrated elastomeric valves whose function is independent of solvent composition.