A pipette-integrated nanoliter-volume well plate for cell-based drug assays
Swastika S. Bithi and Siva A. Vanapalli
Department of Chemical Engineering, Texas Tech University, Lubbock, TX 79409
There is a growing interest in conducting drug screens with primary cells derived from human tissues and biofluids to predict patient outcomes. In contrast to immortalized cell lines, primary cells are a scarce resource and yet preclinical studies demand diverse assays probing specific targets, off-targets and cytotoxicity. Therefore there is a need for a technology that can screen drugs with minimal requirements on cell sample and compound volumes. Currently, multiwell plates and pipetting systems are the established methods for drug assays. However, this technology is difficult to scale down to nanoliter volumes due to fluid evaporation and pipetting errors. In recent years, drop-based microfluidics has emerged as a powerful technology to compartmentalize cells in volumes down to picoliters. Despite its great potential, drop-based microfluidics has not been configured to conduct cell based drug assays with the same ease and parallelized fluid handling capability as well plates and pipetting systems. In this study, we develop a new method to integrate pipettes into a uniquely designed microfluidic well plate for storing arrays of nanoliter droplets and demonstrate cell-based drug assays. The method involves a microchannel network that contains traps to store nanoliter-scale fluid volumes in a grid format similar to that of a multiwell plate. Both the carrier fluids as well as the assay fluids are delivered into the device using multi-channel pipettes. We find that the nanoliter volumes containing either soluble reagents or cells can be stored in a reproducible manner despite pipetting errors as the trapped droplet volume is preset by the geometry of the trap. We investigate potential loss of reagent fluid due to evaporation and find that there is negligible change in droplet volumes during 24 hours. We instrument our microfluidic well plate to an automated microscopy system and pursue cell-based assays using human leukemia cancer cell lines. Each nanoliter-scale droplet is designed to store 1 – 50 cells. Tests of cell viability show no adverse effects of the carrier fluid and system components. Cytotoxicity assays conducted with this method and the results compared to a conventional well plate assay. In summary, our method has significant potential for conducting drug screens using only as little as 200 cells and a microliter of compound volumes without any need for sophisticated flow-control interface. Such a technology is expected to find applications in testing of drugs with primary cells and patient-derived samples.