Fabrication and in situ characterization of microcapsules in a microfluidic system

We have designed a microfluidic system that enables both the fabrication of calibrated capsules and the in situ characterization of their mechanical properties. The fabrication setup consists of a double flow-focusing system. A human serum albumin aqueous solution is introduced in the central channel of a first Y-junction. Intercepted by the lateral flows of a hydrophobic phase, it is dispersed into microdroplets. A cross-linking agent is then introduced at a second Y-junction allowing a membrane to form around the droplets. The time of cross-linking is controlled by the length of a wavy channel located downstream of the second junction. A cylindrical microchannel finally enables to deform and characterize the capsules thus formed. The mechanical properties of the capsule membrane are obtained by inverse analysis. The results show that the drop size increases with the flow rate ratio between the central and lateral channels. The mean shear modulus of the capsules fabricated after 23 s of cross-linking is of the order of the surface tension between the two phases indicating that a reaction time of 23 s is too short for an elastic membrane to form around the droplet. When the cross-linking time is increased to 60 s, the microcapsules surface is wrinkled, thus confirming that a solid membrane is formed around the drop. The mean shear modulus of the capsule membrane increases with the cross-linking time, which is in agreement with our previous chemical results and proves that a fine control of the mechanical properties is possible by choosing adequately the control parameters of the system.