Since oscillations are a phenomena that require observation at a small scale (level of very few cells or even single cells), we designed microfluidic chambers in order to load our cells and observe oscillations.
We used standard soft lithography techniques to generate microfluidic channels. In brief, SU8 photoresist was spin coated on a silicon wafer to the height of 50μm. The desired pattern was generated using maskless lithography. A silicone elastomer was added with its curing agent in 10:1 volume ratio and poured over the micro-mold. After 4 hours of incubation at 65oC, PDMS was peeled off the silicon wafer and inlet and outlet holes were punched. The surface of PDMS and a cover slip were modified using a plasma cleaner and microfluidic channels were created by bonding the two together.
Maintaing Flow Rate
RIn order to visualize the oscillations, we needed to maintain a flow rate in the system. This is because as cells would grow, they would produce toxic compounds inside the chamber, and also reach stationary phase, which would cause a slowdown in protein production, which could kill the oscillations. Flow rate maintenance is something that commonly needs to be done in microfluidic chambers where oscillations are required to be seen, but such sophisticated systems for flow rate maintenance are extremely costly and require specialized equipment to handle.
We designed our own cost effective solution to the problem, by using commonly available material. From the required velocity of the fresh medium that would be needed, the flow rate was calculated. This was in turn used to calculate the pressure difference using the Hagen Poiseuille law. This pressure difference that would be required for the system was achieved by filling media inside a rubber pipe of small diameter (0.4 mm), and placing it up to a height h such that the hydrostatic pressure would be equal to the pressure difference required.
The height could be modulated to generate different flow rates and tune them specifically for our requirements. The following videos show the different flow rates that could be achieved –
