Tuna-step: Tunable Parallelized Step Emulsification for the Generation of Droplets with Dynamic Volume Control to 3D Print Functionally Graded Porous Materials

We present Tuna-step, a novel microfluidic module based on step emulsification that allows for reliable generation of droplets of different sizes. Until now, sizes of droplets generated with step emulsification were hard-wired into the geometry of the step emulsification nozzle. To overcome this, we incorporate a thin membrane underneath the step nozzle that can be actuated by pressure, enabling the tuning of the nozzle size on-demand. By controllably reducing the height of the nozzle, we successfully achieved a three-order-of-magnitude variation in droplet volume without any adjustment of the flow rates of the two phases. We further enhanced our system by developing and applying a new hydrophilic surface modification, ensuring long-term stability and preventing swelling of the device when generating Oil-in-Water droplets. We used our system in the manufacturing of functionally graded soft materials with adjustable porosity and material content. By combining our microfluidic device with a custom 3D printer, we generated and extruded Oil-in-Water emulsions in an agarose gel bath, allowing for the creation of unique self-standing 3D hydrogel structures with porosity decoupled from flow rate and with composition gradients of external phases. We upscaled Tuna-step by setting 14 actuatable nozzles in parallel, offering a step-emulsification-based single chip solution that can accommodate various requirements in terms of throughput, droplet volumes, flow rates, and surface chemistry.