Collagen, the predominant structural protein in animals, is a common biomaterial for clinical applications. Natural collagen is a triple-helix protein made of repeating proline-rich tripeptides. Posttranslational modifications make human collagen difficult to produce. And nonhuman collagens can trigger allergic reactions, complicating clinical use. So researchers have tried to make synthetic collagen-mimetic peptides instead. So far, these have been restricted to short pieces that aren’t particularly useful. Now, Ronald T. Raines and coworkers at the University of Wisconsin, Madison, have designed collagen-mimetic peptides that can self-assemble into symmetric triple-helix fibers that are almost 1 μm long, which is on the length scale of human collagen fibers (Nat. Chem. 2016, DOI: 10.1038/nchem.2556). Raines and coworkers achieve such long fibers by using peptides that have regularly spaced lysine and aspartic acid residues along with the proline, 4-hydroxyproline, and glycine that are found in natural collagen. The lysine and aspartic acid residues form salt bridges between neighboring strands and hold the peptides in place like tiles. They find that peptides must have 3n ± 1 tripeptide repeat units to assemble symmetrically.