The same material that protects astronauts. This isn’t just a frozen gel pack in a fabric tube. The PCM (phase change material) inside was originally developed to protect astronauts from sudden temperature swings, absorbing heat once things warm past a set point and slowly releasing stored coolness as it does. That’s why it feels steady rather than icy-then-gone the way a simple ice pack does, and why it’s become a common sight around Tokyo necks every summer.
It’s genuinely reusable, not a one-summer item. There’s no battery, no gel pack to replace, and no ice required if the room is already under 28°C — it just cools on its own. One reviewer who bought two different sizes for her household found it kept working the same way trip after trip: freeze it, wear it, let it soften, freeze it again. It’s the kind of small, practical gadget that’s easy to pack and forget about between uses.
Measure your neck before picking a size. Sizes run S through LL, and the fit is meant to sit snugly rather than loosely. One reviewer found the standard L a little tight on a larger neck and needed the LL instead, so it’s worth checking the size chart’s measurements against your own neck size before ordering.
You’ve got options for a fast recharge. If you’re not near a freezer, a fridge gets it cold in about 20 minutes, ice water in about 15, and even a car’s air conditioning works in around 20 minutes. Keeping one in an office fridge or a hotel mini-fridge makes it easy to recharge between outings on a hot day.