Chips capable of microfluidic hydrodynamic focusing were designed using AutoDesk360 (See Supplementary Information Figure ). Initial designs were loosely based on commercial Dolomite chips used in MHF liposome synthesis. They featured a flow focusing junction made up of three channels: two with a shared inlet and one central channel with a separate inlet. Channels adjacent to the central channel were kept at 45°. Pre-junction dimensions were equalised by adding a serpentine section in the central channel, and the total length roughly covered that of a microscope slide (70 mm). With a view toward introducing secondary flow regimes, serpentine channels were added near the outlet in some later designs but are not shown here. CAD designs were sent to Micro Lithography Services Ltd. (Chelmsford CM3 5ZF, UK) for laser cut photolithography masks. In a cleanroom, under UV filtered light, SU-8 2075 (Kayaku Advanced Materials Inc., Westborough, MA 01581 USA) was deposited onto a silicon wafer by spin coating at a specified rpm using a spin coater (WS-650-8 model, Laurell Technologies Corporation, North Wales, PA 19454-4150 USA). The wafer was subjected to a pre-exposure "soft bake" on a heating mantle. The lasercut mask was placed on the wafer and exposed to UV light (UV-OAI 150), followed by a pre-development heating cycle. The master was carefully submerged in a propylene glycol methyl ether acetate (PGMEA) (Sigma Aldrich) bath to remove any excess photoresist polymer. The wafer was then treated with acetone (Sigma Aldrich). The PGMEA wash was repeated until it was evident all excess SU-8 had been removed (waste acetone ran clear rather than white). The wafer was dried under a steady stream of nitrogen, then heated at 65 °C for 10 mins. Details of photoresist deposition and curing conditions required for a feature height of ~150 µm are shown in Table Features were inspected using a microscope (MZ12 module, Leica Camera AG; 2.5 x objective) for any defects. If judged to be of a satisfactory quality, the master was plasma treated for 1 minute in a plasma oven (Harick Plasma oven; model: PDC-002), after which it was placed under vacuum in a desiccator for 1 hour, along with a 1 ml glass vial containing trichloro(1H,1H,2H,2H-perfluorooctyl)silane (0.1 ml; Sigma Aldrich). The wafer was placed into a petri dish to await further use in PDMS chip fabrication. Vinyl-terminated polydimethylsulfoxane (PDMS) (50 g; Sylgard 184, Sigma Aldrich) was poured into a 100 ml weigh boat. The cross-linking agent (5 g) was incorporated by mixing with a plastic spatula. Some of the mixture (40 g) was poured onto the featured wafer, and the remainder into a separate petri dish. Both dishes were degassed in a desiccator under vacuum for 1 hour, then placed onto a hotplate at 65 °C to cure overnight. The featured PDMS block was cut out using a scalpel, and the inlets and outlet punched using a 2 mm biopsy punch. The featured PDMS block (feature side up) and the PDMS blank were both plasma-treated for 1 minute then pressed firmly together, taking care to remove any air bubbles. RainEx (hydrophobic glass treatment solution made up of a silane mixture; 0.2 ml) was injected into the channels at this stage, then immediately blown out using a nitrogen stream. Excess surface treatment was evaporated, and the bonding step completed by heating the sandwiched blocks to 65 °C for 15 minutes. Chips were cut to the required shape, then plasma bonded onto a glass slide. Magic tape was used placed over the surface of the chips to prevent any channel contamination before use.