Describes items that must be inflated for normal use, such as life rafts, air mattresses, or emergency slides. Often used attributively before a noun.
飛行機には膨張式の脱出スライドが装備されている。
The airplane is equipped with inflatable evacuation slides.
キャンプ用に膨張式マットレスを買った。
I bought an inflatable mattress for camping.
Compound of 膨張 (expansion, inflation) and 式 (style, type). The exact historical derivation is uncertain, but the word follows a common pattern for describing functional types.