What kind of material is 'Teijin Octa' mentioned in the beanie description?

The name Octa originates from the word referring to the number eight. Teijin Octa is a special polyester fiber with a hollow center in the yarn and eight radiating projections along the surface. It is light and warm. This is because warmed air stays trapped in the hollow core, generating superior warmth relative to its thickness.

SIERRA DESIGNS's Octa 2WAY Beanie uses 100 percent Teijin Octa polyester yarn for the outer fabric^[1]. General knit wool or fleece hats are cozy when staying still, but with just a little movement, heat and sweat easily build up, making them damp. This hat maintains warmth while simultaneously providing breathability that quickly dries sweat and vents heat outside^[2]. This is thanks to the yarn's characteristics, where moisture quickly spreads along the gaps on the fiber surface to evaporate, and air flows through the empty spaces.

hmmm, going beyond simply blocking the cold, this foundation supports you so you can keep running without taking off the hat, even during moments when you sweat profusely, such as hiking or running.