Expo 2025 Osaka had a working sauna. Taiyo no Tsubomi ("Bud of the Sun") stood in the Future Life zone as a sponsored structure in its own right: built by Taiyo Kogyo Corporation, one of Japan's major membrane-architecture manufacturers, produced by TTNE, the Tokyo sauna creative collective, and open to the public by lottery for the full six-month run.
Japan's sauna boom has moved through manga, television, and bathhouse renovations. This was its first stop at a World Expo.
The bud of the sun
Strip away the theming and the building is membrane-technology theater as much as a bathing facility. Three connected units — sauna, lounge, cold plunge — are built from an aluminum frame and 0.25-millimetre ETFE film, the lightweight plastic used in stadium roofs, inflated into a cluster of tetrahedral "petals" that let daylight flood the interior. The structure was designed from the start to be deflated, packed down, and rebuilt elsewhere.
It holds one third-party-verified record. On October 8, 2025, five days before the Expo closed, Guinness World Records certified it as the tallest membrane-structure sauna building: 8.4 meters. (The "world's first membrane sauna" line in the project's marketing is the company's own claim, which this journal hasn't verified.)
Getting in was hard. The sauna held fourteen people per session, with a handful of sessions a day, allocated through the Expo's own lottery system. Visitors who wrote about the experience describe reservations as hard to get for most of the run.
What the Expo slot means
Japan's current sauna boom traces to 2019: a manga-turned-TV-drama, a wave of self-service löyly, and several years of being described, fairly, as a youth trend. The trend hardened fast. Self-service löyly listings on Sauna Ikitai went from 37 facilities in early 2019 to 1,347 by November 2022.
The Expo slot deserves precise framing. Taiyo no Tsubomi was an organizer event — sponsored programming funded by corporate partners, here a Silver Partner deal — and not a government statement of national culture. What happened is narrower, and still notable: a major industrial manufacturer decided that a working sauna, not a scale model or a video wall, was the best showcase for its membrane technology in front of an Expo audience. Whether sauna keeps that level of visibility is a question one Expo season can't answer.
The question nobody has answered
Expo 2025 Osaka closed on October 13, 2025. At the Guinness ceremony five days earlier, company president Yumi Nomura told press that Taiyo Kogyo was "considering relocation" of the structure and had already received multiple inquiries about where it might go next.
That is still the last public statement. As of July 17, 2026 — nine months after the Expo closed — there is no announced site, no reopening date, and no decision to retire the structure. A building engineered to be rebuilt is only a portable building if it gets a second address. Right now it doesn't have one.
If Taiyo Kogyo announces a next site, this journal will report it.
— Takayama Sauna, Editor-in-Chief
Lead photography: Taiyo no Tsubomi's membrane petals at Expo 2025 Osaka. Photo: Taiyo no Tsubomi official website (Taiyo Kogyo), taiyo-tsubomi.jp.