Fig. 1: Characterization of human MSU in synovial fluids.

a Representative XRD pattern of MSU (with small amounts of NaCl crystallized during drying of the samples: ICDD card number for the MSU and halite phases are indicated, as well as the position of their Bragg peaks); b FTIR spectra of biotic MSU before and after synovial fluid protein elimination (proteinase K treatment for the proper visualization of the main IR bands of MSU crystals). Main bands and corresponding molecular vibration are indicated (shaded areas); c Polarized light microcopy image (with 550 nm compensator) showing MSU crystals with their distinctive negative elongation; d FESEM image of MSU fibers, and e detail of the fiber in the squared area of (d) showing the nanogranular surface structure. f Representative TEM image of MSU crystals. g SAED pattern of the larger fiber in (f) demonstrating it is crystalline MSU. h Detail of a MSU fiber showing a nanogranular topography indicative of a non-classical growth via aggregation of nanoparticles. Nanogranular features are not caused by protein adsorption because the MSU crystals in this image were treated with proteinase K to eliminate any absorbed protein.