Electronic Supporting Information UV Polymerization of Hydrodynamically Shaped Fibers

The UV polymerization process dictates the materials that can be employed in the sheath-flow device downstream from the hydrodynamic focusing grooves. The primary requirement for UV polymerization is an integrated window through which the shaped core can be polymerized using a UV source. Possible options include the use of all PDMS parts, top and bottom, or a combination of PDMS and silicon/glass substrates. In both options, optical lithography can be utilized to make the mold for PDMS or to add the chevron features onto the substrate with SU-8 resist. However, both options have one major drawback; since the top and bottom are permanently bonded, clogging anywhere in the device would necessitate replacement. Thus, a modular device design with PMMA base and PDMS top was chosen to enable easy assembly and cleaning processes (see Figure 1S). The device was sandwiched between two aluminum clamps and held together by machine screws. The PMMA-PDMS device assembly technique provided for repeated cleaning without any problem. The sheath flow device used in this work was fabricated in two halves utilizing a combination of a computer numerical controlled (CNC) milling system (Mini Mill, Hass Automation Inc, Oxford, CA) and standard soft lithography. Figure 1S shows an overview of the sheath flow device assembly. The base piece was machined in transparent polymethylmethacrylate (PMMA) (Plexiglas®; Arkema Inc, Philadelphia, PA) with chevron-shaped grooves recessed below the surface of the channel. The top piece was made of polydimethysiloxane (PDMS; Dow Corning's Sylgard 184, Ellseworth Adhesives, Germantown, WI) to enable the UV transmission (350 nm) necessary for the polymerization. The mold for the PDMS, which has only the chevrons corresponding to those in the base, was machined in the PMMA using the same process as the base PMMA piece. The chevrons were recessed features on the PMMA mold and thus produced protruded chevron features on the molded PDMS piece. The top PDMS piece was aligned and assembled on the base PMMA part and screw holes were punched out using a coring tool. The assembled device was then sandwiched between two aluminum clamps with a thin PMMA interface on top of the PDMS. The PMMA interface layer had a small narrow slot, a little wider than the width of the channel) cut out to allow the UV to be transmitted through the PDMS. The slot is necessary as PMMA would absorb the UV and inhibit the polymerization. There were two primary functions …