microballoon
/ˈmʌɪkrəʊbəˌluːn/noun
1A man-made hollow microsphere, usually of glass or synthetic resin, especially one of a kind used in bulk to provide insulation or buoyancy.
2 Medicine. A very small balloon catheter; the inflatable part of such a catheter.
Origin
1950s; earliest use found in The Official Gazette. From micro- + balloon.