[1] He was an inventor of surgical devices, mostly artificial heart valves,[2] a scientist in the broad term and a medical educator.

[4] Marian Ionescu's entire medical career was dominated by scientific curiosity and a persistent desire and ability to make progress by creating change, by discovering the unknown, by improving the known and by inventing what he thought would be useful.

During 1961 and 1962 in Romania, he conducted, with his team, extensive experimental and clinical work on the physiology of organisms under deep hypothermia, with the aid of extracorporeal circulation, in animals and in man.

The body temperature was lowered to between 6 and 15 °C and complete circulatory arrest of up to 56 minutes was maintained without significant negative effects.

[6] At Leeds Ionescu designed, created and implanted in the mitral position in humans, for the first time ever, porcine aortic valves attached to a Teflon cloth collar, starting in February 1967.

Using autologous fascia lata tissue (the fibrous membrane which covers the outside aspect of the thigh muscles) taken from the patient and used to construct the valve during the operation by mounting the living fascia on a Dacron-covered titanium frame in the shape of a three cusp valve.

This procedure of valve construction and implantation had been used in many centres in the world for about three years until it was realised that fascia lata did not perform well over longer periods of time in the high pressure environment of the left heart.

[11] Freehand insertion of stentless fascia lata aortic valves was also used in a small number of cases with similar results.

[12] In his continuous search for the 'Holy Grail', Ionescu experimented with valves made from bovine pericardium (the membrane which surrounds the heart) treated with glutaraldehyde and mounted on a Dacron covered titanium frame.

[13] This original valve, designed and created by Ionescu, demonstrated excellent hydrodynamic performance with good durability during in-vitro fatigue testing.

In 1976 Shiley Laboratory in California began to manufacture and to distribute worldwide this valve under the name of 'Ionescu–Shiley Pericardial Xenograft'.

Its use by Ionescu and many other surgeons demonstrated very good performance in all aspects until 6 to 10 years post-operatively when signs of malfunction appeared progressively in younger patients and some of the valves had to be removed and replaced.

For the repair of congenital cyanotic cardiac diseases he created and built, initially in his hospital laboratory, two original devices: the 'mono-cusp patch' for the correct enlargement of the narrowed pulmonary artery and annulus and a valved conduit (a tube containing a three-cusped valve) for cases with discontinuity between the right ventricle and the pulmonary artery.