In {TAIL ~ these versions}, {HEAD ~ the required stiffness} of the leaflets is achieved by a mechanism activated hydraulically, electrically or with other similar means 
In { TAIL ~ these versions } , { HEAD ~ the required stiffness } of the leaflets is achieved by a mechanism activated hydraulically , electrically or with other similar means
REL TAIL TAIL TAIL TAIL TAIL TAIL OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH