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Mural cells in the vasculature provide mechanical support to the vessels and regulate blood flow, while participating in a wealth of biological processes which often involve physical cues. Here, we introduce the Microfiber Force Probe, a novel in vitro tool that replicates key geometrical and mechanical vessel features to measure compressive forces exerted by mural cells, in an effort to unravel underlying laws that drive mural cell behavior. Soft micrometric fibers of various sizes, with embedded fluorescent microbeads to track their deformation, are used as a model for blood microvessels. In contrast to Traction Force Microscopy, which is restricted to the measurement of shear forces over flat surfaces, the high convex curvature of these microfibers is much closer to the conditions mural cells experience in vivo and allows to measure normal forces in addition to shear forces. We fabricate fibers with a UV sensitive mixture of poly(ethylene glycol) diacrylate (PEG-DA) and photoinitiator, flowed in a flow-focusing microfluidic chip. A computer-controlled short pulse of UV, sent through the microscope's objective, crosslinks the polymer. The polymerized fibers are retrieved at the outlet of the chip and purified before characterization and cell culture. To fabricate fibers that replicate the diameter (5-20 µm) and stiffness (0.1-10kPa) of small blood vessels, four parameters are tuned: the composition of sheath and core fluids, the dimensions of the chip, the length and intensity of the UV pulse and the flow rates. Fibers of 30 µm in diameter loaded with homogeneously dispersed microparticles are successfully produced. Biocompatibility is tested for endothelial and mural cells using RGD (arginine-glycine-aspartic acid) peptide to promote cell adhesion. AFM measurements show that fibers with a lower content of PEG-DA have smaller Young's modulus; E-10 kPa for fibers with 30% of PEG-DA compared to E-300 kPa for those with 50% of PEG-DA. Current efforts are focused on the assessment of the gel Young's modulus with macro-scale experiments to complement the local AFM measurements. Tests to measure compressive forces exerted by smooth muscle cells on fibers of E-10 kPa are being performed. [ABSTRACT FROM AUTHOR]