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