What Are Underwires Made Of?

The wire itself

Most underwires are flattened spring steel, heat-treated to hold a curve under load and return to it, then coated, typically with nylon or a similar polymer, and tipped at both ends with rounded caps. The wire is a spring in the engineering sense: its job is to hold the cup’s shape against the cradle, transfer load into the band, and flex with breathing and movement without taking a new shape. Cheaper bras substitute plastic wires, which are lighter and can’t rust, but creep: plastic slowly adopts the shape it is stored or worn in, which is why a plastic-wired bra ages into a different bra.

Wire shapes are a spec, not a whim

Why wires poke, in materials terms

A wire almost never pokes because the steel failed; it pokes because the channel around it died first. The wire lives inside a fabric casing, and every wash cycle, twist and cram in a drawer abrades that casing from inside. When the casing wears through at the end cap, the wire’s tip, still perfectly healthy steel, exits into your underarm. That sequencing is why the poking fix guide treats escape as a retirement notice rather than a repair project: the casing that failed once has failed everywhere, and a patch buys weeks. Plastic wires fail differently: they fold at a stress point and stay folded, turning one cup subtly different from the other, a slow failure people usually blame on their own body first.

The nickel question

Steel alloys can contain nickel, and nickel is among the most common contact allergens. The coating exists partly to keep metal off skin, so a reaction usually appears only after the coating and casing both wear, another argument for retiring bras on the lifespan schedule. If your skin reliably reacts where wires sit even in newer bras, patch-tested nickel allergy is worth ruling in or out with a professional, and in the meantime the practical answers are plastic-wired, cushioned-wire, or wireless construction, all of which remove the metal question entirely.

Keeping a spring alive

Everything that kills wires is mechanical or thermal. Machine washing slams the wire against the drum and grinds the casing; the dryer’s heat fatigues the steel’s temper and cooks the casing’s elastic neighbors. Hand-wash or use a structured lingerie case on gentle, never the dryer, and store cups nested upright rather than folded through each other, which pre-bends the spring toward its eventual failure. The washing guide is the full protocol; the wire is its main beneficiary.