  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.
