Abstract

Probing the behaviour of single cells in time-course is very important to understand the heterogeneity of cells at given chemical conditions, provided the environment of experiments keeps constant. We have developed a microfluidic system to investigate cell to cell variations in the expression level of a protein, Red Fluorescence Protein (mRFP1), and in the activity of co-expressed enzyme, alkaline phosphatase (AP), while cells have been encapsulated in microdroplets. The microfluidic device was constructed by multi layer soft lithography. Pico-liter sized aqueous droplets were formed in oil, stored in wells, and monitored. The time-course measurement onto significant number of individual cell requires indexing them to identify where cells are located. The storage well built in the device provides a physical address as droplets are securely docked after storage. Also, the droplet shrinking due to water diffusion into the PDMS matrix that jeopardizes quantitative measurements of droplet contents was resolved by integrating a reservoir underneath the wells. The reservoir continuously supply water to the stored droplets to keep the water content constant over long periods. With long term experiments based on the microfluidic system that is able to specifically monitor single cells, we can study how cells differently behave with respect to time in an identical environment. These variations can not be measured in ensembles even though the current technology is not matured enough to describe the pathway of heterogeneity from the source to the individual phenotype. Also, a novel way to deliver small molecules to cells after encapsulations in droplets will be discussed.