The oxygenator was first conceptualized in the 17th century by Robert Hooke and developed into practical extracorporeal oxygenators by French and German experimental physiologists in the 19th century. Bubble oxygenators have no intervening barrier between blood and oxygen, these are called 'direct contact' oxygenators. Membrane oxygenators introduce a gas-permeable membrane between blood and oxygen that decreases the blood trauma of direct-contact oxygenators. Much work since the 1960s focused on overcoming the gas exchange handicap of the membrane barrier, leading to the development of high-performance microporous hollow-fibre oxygenators that eventually replaced direct-contact oxygenators in cardiac theatres.