Scaling rules for flow focusing devices: From standard to large FFDs for cell encapsulation in alginates

Flow focusing devices (FFDs), with dimensions smaller than 100 mum have recently been used for cell encapsulation. However, encapsulation of larger clusters of cells, like islets of Langerhans, is of considerable interest. Producing larger capsules requires larger FFDs. We focus here on the geometrical scaling-up between a small FFD (50 mum) and a larger one (100 mum, 200 mum, 1 mm). We determine the driving pressure conditions to obtain the droplet regime at any size of the FFD. A similar behavior of the device is experimentally obtained by a scaling of the dimensions by a ratio lambda and a scaling of the driving pressures by the ratio 1/lambda.

INTRODUCTION

In the literature, microfluidic devices such as flow focusing devices (FFDs) are presented to perform encapsulation of biological objects with a typical size smaller than (cells for example) [1], [2]. The principle of FFDs is shown on Figure 1 [3]: pinching of a dispersed phase (alginate) by another immiscible continuous phase (oil) results in the continuous formation of droplets. Basically, droplet formation results from the balance between interfacial tension stresses and viscous stresses. Figure 1: Experimental setup: alginate droplets form at the nozzle due to the shear stress applied by the oil phase