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