## 2 Microfabrication of Suspensions of Polymeric Fibers by Photo-Polymerization  
### Principle and Setup Description  
We adapted a microscope-based photolithography method initially developed by [PERSON] et al. [11] to produce confined fibers directly inside a microfluidic chip. The idea is to project UV light through a fiber-shaped lithography mask into a microfluidic channel filled with a reactive substance. The solution, a mixture of an oligomer and photo-initiator, crosslinks under UV light: the illumination of the target fluid region through the mask polymerizes it within hundreds of milliseconds. A schematic drawing of the method for fiber fabrication is presented in Figure 1. When using an all-PDMS (polydimethylsiloxane) channel, oxygen diffuses through the PDMS walls and inhibits the radical crosslinking reaction close to the channel walls. The two inhibition layers formed in this way at the upper and lower walls prevent the fibers from sticking to the latter. The light projection along the channel height therefore leads to free fibers with a rectangular cross-section. The inhibition layers can be taken advantage of to adapt the fiber geometry, for example to control the confinement, as we will see later in this article. Figure 1: **(a)** Schematic drawing of the in situ fiber fabrication technique, using a microscope-based projection lithography technique. UV light is projected into the channel through a fiber-shaped mask, polymerizing the polymer inside the channel, typically within 100 ms; **(b)** one or several fiber shapes can be used on the mask depending on the number of fibers to make in one step; **(c)** the polymerization is prevented close to the PDMS walls due to the presence of oxygen allowing free fibers to be fabricated. The thickness of the inhibition layer being constant, the confinement increases with the channel height. The choice of the oligomer and the concentration of photo-initiator should be made in regards to the targeted mechanical properties of the fabricated fiber [12; 13]; see Section 2.3.3. The photo-initiator excitation wavelength should match the light source available and the setup constraints (like the transmission of UV light). Here, we use Darocur 1173 photo-initiator (PI; Ciba Specialty Chemicals Inc., Basel, Switzerland) (2-hydroxy-2-methylpropiophenone, Sigma, Saint Louis, MO, USA), whose excitation peak is around 365 nm. This peak is twice more intense with an HBO lamp as compared to a Xcite120Q source, leading to faster curing with the HBO lamp. For an application requiring softer materials or the presence of water, one can benefit from the solubility of polyethylene glycol diacrylate (PEGDA) in water to fabricate soft hydrogel particles. As illustrated in Figure 1, we built a fabrication and flow study platform in a single setup. A transparent all-PDMS (Sylgard 184; Corning, Midland, MI, USA) microchannel is made with traditional soft-lithography techniques [12]. It is then placed on the stage of an inverted microscope (Zeiss Axio Observer, Oberkochen, Germany) equipped with a UV light source, a shutter and a fiber-shaped mask (Shutter Uniblitz V25, Lamp HBO 130W, Osram, Munchen, Germany). The UV light is filtered through a narrow-UV-excitation filter set (11004v2 Chroma, Burlington, VT, USA) with an emission peak at \(\lambda\sim 365\) nm. We work with rectangular-section microchannels where the width is up to ten-times larger than the height. The orders of magnitude of channel dimensions are: width 200-500 \(\upmu\)m, height 20-50 \(\upmu\)m and a length of several centimeters. The lithography mask determines specifically the aspect ratio in the plane (\(x,y\)) perpendicular to the optical axis. It consists of a transparent-fiber shape drawn in the center part of a 25 mm-diameter black disk. We place this mask in the field stop position of the microscope to get it focused into the objective image plane.