Patent Publication Number: US-2021186688-A1

Title: Heart valve

Description:
FIELD 
     The present disclosure relates to an artificial heart valve, a stent and an artificial heart valve system suitable for percutaneous delivery and methods for the percutaneous delivery of such an artificial heart valve, a stent and an artificial heart valve system. 
     BACKGROUND 
     The valves of the heart may be abnormal from birth, may become diseased, or may degenerate in old age. When their function becomes sufficiently impaired they may require to be replaced. There are many different types of artificial heart valves in established clinical use for the replacement of abnormal or aged natural heart valves. Mechanical replacement heart valves are typically constructed of rigid materials such as metallic alloys, pyrolytic carbon, or rigid polymers. They do not generally resemble natural heart valves. Biological replacement heart valves generally have leaflets constructed of flexible materials of human or animal origin such as human aortic or pulmonary valves, animal aortic or venous valves, or animal pericardium (the fibrous sheet surrounding the heart). Such animal tissues are commonly treated with agents such as glutaraldehyde to enhance their durability. Biological heart valves generally resemble the natural aortic or pulmonary valves. Artificial heart valves are also known, having flexible synthetic materials fashioned into flexible leaflets on a supporting frame. Some of these artificial heart valves have synthetic leaflets which differ appreciably in configuration from the leaflets of natural heart valves. 
     Valves have traditionally been implanted into the heart after removal of the abnormal valve by means of open-heart surgery. More recently, flexible valve leaflets have been attached within an expandable mesh-like cylinder or stent, for percutaneous delivery on a catheter. An antegrade approach may be used wherein the catheter is introduced into the apex of the heart. Or, a retrograde approach may be used wherein the catheter is introduced via a peripheral blood vessel. After manipulation into the correct location, the stent and artificial heart valve are expanded with a balloon to create a functional valve. Such known methods of percutaneously implanting a heart valve, although less invasive than an open-heart operation, have some drawbacks. The pressure applied to the flexible leaflets between the balloon and the stent during expansion can cause damage, such as tearing, to the flexible leaflets. If the flexible leaflets are damaged they may not function effectively. 
     During open-heart surgery, a peripheral portion of the heart valve is sewn into a region of natural valve leaflet attachment known as the valve annulus. The peripheral portion of the heart valve may be formed of a resilient material so that a seal is provided between the artificial heart valve and the artery wall when the peripheral portion of the heart valve is sewn into the valve annulus. This prevents a loss of efficiency in the functioning of the valve. Sewing the valve into the valve annulus is not possible percutaneously. 
     SUMMARY 
     An artificial heart valve is described herein which is suitable to be delivered percutaneously. The artificial heart valve comprises a plurality of support elements in the form of a plurality of base segments and a plurality of posts. The artificial heart valve further comprises a plurality of flexible leaflets. Each leaflet is attached to two of the posts. The artificial heart valve further comprises one or more strings passing through the plurality of support elements. The artificial heart valve is configurable between a collapsed delivery configuration in which the posts are located side-by-side and closer together and an expanded operational configuration in which the posts are located side-by-side and further apart. When the artificial heart valve is in the collapsed delivery configuration, application of tension to the one or more strings pulls each support element into engagement with adjacent support elements until the artificial heart valve adopts the expanded operational configuration in which the support elements together form a support structure defining an aperture for blood flow in which the leaflets are movable between an open configuration in which the leaflets permit blood flow through the artificial heart valve and a closed configuration in which the leaflets restrict blood flow through the artificial heart valve. 
     Such an artificial heart valve may be delivered percutaneously. Such an artificial heart valve does not require the leaflets to be compressed between a balloon and a stent during expansion. Consequently, such an artificial heart valve may be less prone to damage during expansion than known artificial heart valves which are delivered percutaneously and expanded using a balloon. 
     When the artificial heart valve is in the expanded operational configuration, the support elements may be in compression. When the artificial heart valve is in the collapsed delivery configuration the support elements may not be in compression. 
     The support structure may be generally annular or generally cylindrical. The support structure may be generally shaped to correspond to the shape of the aorta. 
     The number and/or distribution of base segments located between adjacent posts may be selected according to the size and/or shape of the valve annulus. The number of and/or distribution of base segments located between adjacent posts may be selected according to the size and/or shape of the leaflets attached to the adjacent posts. 
     The posts may be distributed circumferentially around the support structure with an equal number of base segments located between adjacent posts. 
     The posts may be distributed circumferentially around the support structure with an unequal number of base segments located between adjacent posts. For example, for a bicuspid valve such as a mitral valve having only first and second leaflets attached between first and second posts, the number of base segments extending along the base edge of the first leaflet may be different to the number of base segments extending along the base edge of the second leaflet, for example to accommodate different sizes and/or shapes of the first and second leaflets. 
     The artificial heart valve may comprise a plurality of base segments between each pair of adjacent posts. The artificial heart valve may comprise 10 to 16 base segments between each pair of adjacent posts. 
     Adjacent support elements may define complementary surfaces which are in engagement when the artificial heart valve is in the expanded operational configuration. 
     Each post may have a base portion. For each post, the base portion of the post and the base segment adjacent to the base portion of the post may define complementary surfaces which are in engagement when the artificial heart valve is in the expanded operational configuration. 
     When the artificial heart valve is in the expanded operational configuration, each leaflet may extend over one or more of the base segments between the two posts to which the leaflet is attached. 
     Each leaflet may define a base edge. The base edge may define a channel or recess. The channel or recess may have an internal profile. The one or more base segments between the two posts to which the leaflet is attached may define an external profile. The internal profile of the channel or recess may be complementary to the external profile defined by the one or more base segments between the two posts to which the leaflet is attached. 
     When the artificial heart valve is in the expanded operational configuration, the flexible leaflets may provide a seal between the support structure and the stent. When the artificial heart valve is in the expanded operational configuration, the flexible leaflets may provide a smooth surface for flow of blood through the aperture for blood flow. 
     Each of the flexible leaflets may comprise a polymer. The polymer may be polyurethane such as biostable polyurethane. Such polymer flexible leaflets may be resilient to folding. Furthermore, biostable polyurethane does not generally calcify in use, therefore the length of time for which the artificial heart valve can be used in the human body may be extended. 
     The support elements may be formed using an additive manufacturing technique such as direct laser sintering or  3 D printing to provide an accurate and/or complex shape. Sintering and/or  3 D printing may provide the support elements with a rough surface which can improve adhesion of the flexible leaflets to the support elements. The support elements may comprise a metal. The support elements may comprise titanium. The support elements may be formed using direct laser metal sintering. 
     Each flexible leaflet may be bonded or fused to at least one base segment located between the posts to which the flexible leaflet is attached. This may cause the base segments to preferentially locate in the channel or recess defined by the base edge of the flexible leaflet when the artificial heart valve is in the collapsed delivery configuration and tension is applied to the one or more strings. Furthermore, when the artificial heart valve is in the collapsed delivery configuration, this may result in a reduction in an axial distance over which the base segments extend, wherein the axial distance is defined relative to an axis defined by the aperture for blood flow. In effect, this may reduce the overall axial extent of the artificial heart valve in the collapsed delivery configuration. During delivery of the artificial heart valve through an artery, the heart may spontaneously eject blood through the artery and cause damage to the leaflets as the base segments are caught in the blood flow. Reducing the axial distance over which the base segments extend, may reduce the chance of damage to the leaflets because the base segments have less freedom of movement when disturbed by the blood flow. 
     When the artificial heart valve is in the expanded operational configuration, each leaflet may have a moveable free edge and a base edge opposite the free edge. 
     When the artificial heart valve is in the expanded operational configuration, the base segments between each pair of adjacent posts may be arranged along an outwardly convex path in a transverse plane relative to an axis defined by the aperture for blood flow. When the artificial heart valve is in the expanded operational configuration, the moveable free edge of each leaflet may be generally S-shaped and the base edge of each leaflet may be outwardly convex relative to the axis defined by the aperture for blood flow. Such a leaflet may fold in a predictable manner between the posts to which the leaflet is attached (rather than crumpling unpredictably) when the artificial heart valve is folded or manipulated into the collapsed delivery configuration and mounted on a catheter for percutaneous delivery. Such leaflets may, therefore, be particularly suitable for percutaneous delivery, as such leaflets may be subject to relatively low levels of stress when the artificial heart valve is folded or manipulated into the collapsed delivery configuration and mounted on a catheter for percutaneous delivery. 
     When the artificial heart valve is in the expanded operational configuration, the base edge of each leaflet may be arranged along an outwardly convex path having both circumferential and axial components relative to an axis defined by the aperture for blood flow. For example, the base edge of each leaflet may be “scallop shaped”. 
     When the artificial heart valve is in the expanded operational configuration, the moveable free edge of each leaflet may be generally outwardly convex when the leaflet is in the open configuration and the moveable free edge of each leaflet may be generally outwardly concave when the leaflet is in the closed configuration. 
     When the artificial heart valve is in the expanded operational configuration, the base segments between each pair of adjacent posts may be arranged along an outwardly convex path having both circumferential and axial components relative to an axis defined by the aperture for blood flow. 
     The artificial heart valve may comprise a sleeve. The sleeve may be located radially outwardly of the flexible leaflets. The sleeve may provide protection for the flexible leaflets during percutaneous delivery of the artificial heart valve. 
     An artificial heart valve system is described herein which is suitable to be delivered percutaneously. The artificial heart valve system comprises the previously described artificial heart valve, and a stent configurable between a collapsed delivery configuration and an expanded operational configuration. The one or more strings pass through the stent. When the stent is in its expanded operational configuration and the artificial heart valve is in its collapsed delivery configuration, application of tension to the one or more strings pulls each support element into engagement with the adjacent support elements and pulls the posts into engagement with the stent until the artificial heart valve adopts the expanded operational configuration and is in engagement with the stent. 
     Such an artificial heart valve system may be delivered percutaneously. Such an artificial heart valve system does not require the leaflets to be compressed between a balloon and a stent during expansion. Consequently, such an artificial heart valve system is likely to be less prone to damage during expansion than known artificial heart valve systems which are delivered percutaneously and expanded using a balloon. 
     Such an artificial heart valve system may provide a good seal with an artery wall. 
     One or more strings may pass alternatingly through the stent and the plurality of support elements of the artificial heart valve. 
     The artificial heart valve may be pulled into axial alignment with the stent when tension is applied to the one or more strings. 
     The stent may comprise at least one string guidance feature corresponding to each post. The one or more strings may extend through the at least one string guidance feature. When the stent is in its expanded operational configuration and the artificial heart valve is in its expanded operational configuration, each post may abut the stent at a location adjacent the corresponding at least one string guidance feature. 
     Each post may comprise a plurality of tunnels through which the one or more strings pass. Each base segment may comprise a tunnel through which the one or more strings pass. 
     For each post, the one or more strings may pass from at least one of the tunnels of the post through a corresponding string guidance feature of the stent. 
     Each post may comprise a string guidance feature. 
     For each post, the one or more strings may pass from at least one of the tunnels of the post through a first corresponding string guidance feature of the stent, through the corresponding string guidance feature of the post and through a second corresponding string guidance feature of the stent. 
     The one or more strings may comprise a single continuous string. On application of tension to the single continuous string, said tension is distributed more evenly to the plurality of support elements. 
     The one or more strings may comprise a plurality of strings. For each pair of adjacent posts, one of the strings may extend through each base segment located between the pair of adjacent posts, through the pair of adjacent posts, and through the stent. 
     The one or more strings comprise a plurality of strings, and, for each pair of adjacent posts, two or more of the strings extend through each base segment located between the pair of adjacent posts, through the pair of adjacent posts, and through the stent. 
     The one or more strings may comprise a plurality of strings, and, for each pair of adjacent posts, two or more of the strings may extend through each base segment located between the pair of adjacent posts, through the pair of adjacent posts, and through the stent 
     The artificial heart valve system may comprise a plurality of anchor strings. For each post, a corresponding anchor string may extend through a string guidance feature of the post and through a string guidance feature of the stent without extending through any of the base segments 
     When the stent is in its expanded operational configuration and the artificial heart valve is in its expanded operational configuration, the support structure may abut a surface of the stent which is radially inward relative to an axis defined by the aperture for blood flow. 
     The artificial heart valve system may comprise a plurality of string locking arrangements. Each string locking arrangement may be configured to allow the one or more strings to be pulled in a first direction relative to the stent but to prevent the one or more strings from being pulled relative to the stent in a second direction opposite to the first direction. Each string locking arrangement may comprise a male locking element attached to the one or more strings and a female locking element attached to the stent, wherein the male and female locking elements may be configured to allow the male locking element to be pulled with the one or more strings in a first direction through the female locking element but to prevent the male locking element from being pulled through the female locking element with the one or more strings in a second direction opposite to the first direction. The male and female locking elements may define complementary inter-engaging ratchet profiles. 
     The artificial heart valve system may further comprise a sealing arrangement configured to provide a seal between the artificial heart valve and the stent when the stent is in its expanded operational configuration and the artificial heart valve is in its expanded operational configuration. The sealing arrangement may extend around a base portion of the stent. The sealing arrangement may comprise a sealing member and a flexible sealing actuation string extending around a periphery of the sealing member. When the stent is in its expanded operational configuration and the artificial heart valve is in its expanded operational configuration, tension applied to the sealing actuation string may compress the sealing member and the plurality of flexible leaflets against the support structure. The sealing member may comprise a woven material such as woven polyester cloth. Sealing between the artificial heart valve and the stent may improve the efficiency of the artificial heart valve because flow of blood may pass through the aperture of the artificial heart valve, rather than between the artificial heart valve and the stent. 
     The support elements may be rigid. 
     A plurality of support elements for an artificial heart valve is described herein. The plurality of support elements comprises a plurality of base segments and a plurality of posts. The plurality of support elements is configurable between a collapsed delivery configuration in which the posts are located side-by-side and closer together and an expanded operational configuration in which the posts are located side-by-side and further apart. The plurality of support elements is configured such that, when the plurality of support elements is in the collapsed delivery configuration, application of tension to one or more strings passing through the plurality of support elements, pulls each support element into engagement with adjacent support elements until the plurality of support elements adopts the expanded operational configuration in which the support elements together form a support structure defining an aperture for blood flow. 
     A method of percutaneously implanting the artificial heart valve system into a human or animal subject is described herein. 
     The method may comprise mounting the artificial heart valve system on a catheter. The method may comprise mounting the artificial heart valve system on a catheter with the stent and the artificial heart valve in their respective collapsed delivery configurations. 
     The method may comprise mounting the stent and an inflatable member on the catheter at a first axial location of the catheter with the stent located radially outwardly of the inflatable member. The method may comprise mounting the artificial heart valve on the catheter at a second axial location of the catheter. 
     The second axial location of the artificial heart valve on the catheter may be proximal with respect to the first axial location of the stent and the inflatable member. 
     The second axial location of the artificial heart valve on the catheter may be distal with respect to the first axial location of the stent and the inflatable member. 
     The method may comprise inserting the catheter through an incision in the skin of the subject. The method may comprise inserting the catheter through an artery of the subject. The method may comprise aligning the stent and the inflatable member adjacent a valve annulus of the subject. 
     The method may comprise inflating the inflatable member to expand the stent from its collapsed delivery configuration to its expanded operational configuration. The method may comprise deflating the inflatable member after expansion of the stent. The method may then comprise moving the catheter relative to the subject until the artificial heart valve is generally aligned axially with the stent. 
     For example, if the second axial location of the artificial heart valve on the catheter is proximal with respect to the first axial location of the stent and the inflatable member, the method may comprise initially inserting the catheter into the subject until the stent and the inflatable member are generally aligned axially with the valve annulus and the artificial heart valve is located proximal of the valve annulus. The method may then comprise inflating the inflatable member to expand the stent from its collapsed delivery configuration to its expanded operational configuration. The method may comprise deflating the inflatable member after expansion of the stent. The method may comprise inserting the catheter further into the subject until the artificial heart valve is generally aligned axially with the stent and the inflatable member is inserted distally of the stent. Mounting the artificial heart valve on the catheter at a second axial location which is proximal with respect to the first axial location of the stent and the inflatable member, may avoid any requirement for the artificial heart valve to be inserted into, or through, the valve annulus before expansion of the stent. 
     Alternatively, if the second axial location of the artificial heart valve on the catheter is distal with respect to the first axial location of the stent and the inflatable member, the method may comprise inserting the catheter into the subject so as to insert the artificial heart valve through the valve annulus until the stent and the inflatable member are generally aligned axially with the valve annulus. The method may comprise inflating the inflatable member to expand the stent from its collapsed delivery configuration to its expanded operational configuration. The method may comprise deflating the inflatable member after expansion of the stent. The method may comprise retracting the catheter from the subject until the artificial heart valve is generally aligned axially with the stent and the inflatable member is proximal of the stent. 
     The method may comprise removing the sleeve. 
     The method may comprise applying tension to the one or more strings so as to expand the artificial heart valve from the collapsed delivery configuration to the expanded operational configuration and so as to pull the artificial heart valve radially outwardly into engagement with the stent. 
     The method may comprise applying the tension to the one or more the strings using one or more secondary strings, wherein each secondary string extends from the incision in the skin of the subject, through a corresponding loop in the one or more strings, and returns to the incision in the skin. 
     The method may comprise withdrawing the catheter and the deflated inflatable member from the subject. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Artificial heart valves, stents, artificial heart valve systems and methods will now be described by way of non-limiting example only with reference to the following drawings of which: 
         FIG. 1  is a schematic cut-away perspective view of a heart following percutaneous delivery of an artificial heart valve system to replace the natural aortic valve; 
         FIG. 2  is a detailed schematic section of the artificial heart valve system of  FIG. 1 ; 
         FIG. 3  is a perspective view of the artificial heart valve system of  FIG. 1 ; 
         FIG. 4A  is a perspective view of an artificial heart valve of the artificial heart valve system of  FIG. 1  in a closed configuration; 
         FIG. 4B  is a perspective view of an artificial heart valve of the artificial heart valve system of  FIG. 1  in an open configuration; 
         FIG. 5A  is a schematic cross section of a base portion of the artificial heart valve of  FIGS. 4A and 4B  when the artificial heart valve is in an expanded operational configuration; 
         FIG. 5B  is a schematic cross section of an alternative base portion of the artificial heart valve of  FIGS. 4A and 4B  when the artificial heart valve is in an expanded operational configuration; 
         FIG. 6A  is a perspective view of a plurality of support elements of the artificial heart valve of  FIGS. 4A and 4B  when the artificial heart valve is in an expanded operational configuration and each support element is pulled into engagement with the adjacent support elements so as to define a support structure in which the support elements are in compression; 
         FIG. 6B  is a perspective view of a plurality of support elements of the artificial heart valve of  FIGS. 4A and 4B  when the artificial heart valve is in a collapsed delivery configuration and the support elements are not in compression; 
         FIG. 7  is a plan view of the artificial heart valve system of  FIG. 1 ; 
         FIG. 8A  is a section on line A-A of  FIG. 7 , during expansion of the artificial heart valve system; 
         FIG. 8B  is a section on line A-A of  FIG. 7 , after expansion of the artificial heart valve system; 
         FIG. 8C  is a detailed view of the locking arrangement of  FIGS. 8A and 8B ; 
         FIG. 8D  is a detailed view of a sealing arrangement of  FIGS. 8A and 8B ; 
         FIG. 9A  is a schematic cross-section during percutaneous delivery of an artificial heart valve system to the aorta before expansion of a stent; 
         FIG. 9B  is a schematic cross-section during percutaneous delivery of an artificial heart valve system, after expansion of the stent but before expansion of a heart valve; 
         FIG. 9C  is a schematic cross-section during percutaneous delivery of an artificial heart valve system, after expansion of the stent and the heart valve, but before removal of secondary strings; 
         FIG. 10  is a detailed schematic cross-section during percutaneous delivery of an artificial heart valve system corresponding to  FIG. 9C ; 
         FIG. 11A  is a perspective view of a base segment of  FIG. 6 ; 
         FIG. 11B  is a perspective view of an alternative base segment; 
         FIG. 11C  is a perspective view of a further alternative base segment; 
         FIG. 12A  is a cross-section of a first alternative base segment; 
         FIG. 12B  is a cross-section of a second alternative base segment; 
         FIG. 12C  is a cross-section of a third alternative base segment; 
         FIG. 13  is a perspective view of an alternative artificial heart valve which is suitable to be delivered percutaneously; and 
         FIG. 14  is a perspective view of a further alternative artificial heart valve which is suitable to be delivered percutaneously. 
     
    
    
     DETAILED DESCRIPTION OF DRAWINGS 
     Referring initially to  FIG. 1 , there is shown a heart  14  following percutaneous delivery of an artificial heart valve system  10  to replace, or displace, the natural, abnormal, aortic valve. 
     Referring to  FIGS. 2 and 3  the artificial heart valve system  10  has a stent  22  and an artificial heart valve  24 . As will be described in more detail below, the artificial heart valve  24  has a plurality of support elements and a plurality of flexible leaflets  34  attached to the support elements. The support elements are rigid. The artificial heart valve  24  has a collapsed delivery configuration and an expanded operational configuration, as will be described in more detail below. The stent  22  also has a collapsed delivery configuration and an expanded operational configuration. 
       FIGS. 4A and 4B  show an artificial heart valve  24  in its expanded operational configuration when the flexible leaflets  34  are in closed and open positions respectively. The artificial heart valve  24  has three flexible leaflets  34 . When the artificial heart valve  24  in its expanded operational configuration, the support elements together form a support structure  32 . The support structure  32  has a base  36 , and three rigid posts  38 . When the artificial heart valve  24  is in the expanded operational configuration, the base  36  is generally annular. Each post  38  has a series of holes  40 . A continuous layer of polymer is formed around each post  38  and through the holes  40  in each post  38 . The polymer between two adjacent posts  38  forms one of the flexible leaflets  34 . 
     Each flexible leaflet  34  has an upper edge  42 . When the artificial heart valve  24  in its expanded operational configuration, the upper edge  42  is a moveable free edge. The free edge  42  of each flexible leaflet  34  has an approximate “S” shape. The flexible leaflets  34  are translucent. Consequently, the flexible leaflets  34  are indicated in  FIGS. 4A and 4B  by a solid line showing the free edge  42  of each flexible leaflet  34 . When the artificial heart valve is in the expanded operational configuration, the free edge  42  is movable between a closed position in which blood flow through the artificial heart valve is restricted (see  FIG. 4A ), and an open position in which blood flow through the artificial heart valve is permitted (see  FIG. 4B ). In the closed position, the upper portions or co-aptation regions of the flexible leaflets  34  engage one another so as to form a seal. In the open position, the co-aptation regions of the flexible leaflets  34  are spaced apart. 
     Each flexible leaflet  34  has a base edge or portion  41  opposite the free edge  42 . The base edge  41  is outwardly convex relative to an axis defined by the support structure. The base edge or portion  41  of each flexible leaflet  34  defines a channel or recess  43  defining an internal profile which is complementary to an external profile of the base  36 . Each channel  43  has a radially inner surface  43   a , a radially outer surface  43   b  and an opening for receiving a portion of the base  36  when the artificial heart valve  24  is in the expanded operational configuration. The radially inner surface  43   a  can extend the full depth of the base  36  (see  FIG. 5A ), or only a partial depth of the base  36  (see  FIG. 5B ). When the artificial heart valve is in the expanded operational configuration, the internal profile of the channel or recess  43  extends over the base  36  between the two posts to which the leaflet is attached. 
     As shown in  FIGS. 6A and 6B , the base  36  is formed from a plurality of base segments  50 . Each base segment  50  constitutes a support element of the artificial heart valve  24 . Similarly, each post  38  constitutes a support element the artificial heart valve  24 . As will be described in more detail below,  FIG. 6A  shows the plurality of support elements when the artificial heart valve is in an expanded operational configuration and each support element is pulled into engagement with the adjacent support elements so as to define the support structure  32  in which the support elements are in compression and  FIG. 6B  shows the plurality of support elements when the artificial heart valve is in a collapsed delivery configuration and the support elements are not in compression. 
     Each post  38  has a base portion  52 . The base portion  52  of post  38  is the portion of post  38  adjacent the base segments  50  of the support structure  32 . The base segments are grouped into three groups  62   a ,  62   b ,  62   c , with each group  62   a ,  62   b ,  62   c  of base segments  50  located between two adjacent posts  38 . In  FIGS. 6A and 6B  there are ten base segments  50  per group  62 . When the artificial heart valve  24  is in the expanded operational configuration, each group  62  of base segments  50  is arranged along an outwardly convex path in a transverse plane relative to an axis defined by the support structure  32 . The base edge  41  of the flexible leaflets  43  has a path corresponding to the path of the groups  62  of base segments  50 . As will be described in more detail below, a string  26  extends through the base portion  52  of each post  38  and all of the base segments  50 . 
     Each base segment  50 , and the base portion  52  of each post  38 , has a male engagement surface  54   a  at a first end or side and a female engaging surface  54   b  at a second end or side opposite the first end or side. Each male engagement surface  54   a  is configured for engagement with a complementary female engagement surface  54   b  of an adjacent base segment  50  or an adjacent base portion  52  of a post  38 . 
     Each of the base segments  50  has a tunnel  56  extending through it (see  FIG. 6B ) between the engagement surfaces  54   a ,  54   b  of the base segment  50 . The base portion  52  of each post  38  has two tunnels  58   a ,  58   b  extending through it. Each base portion tunnel  58   a ,  58   b  has a first end at respective engagement surfaces  54  on opposite sides of the post  38 . Each base portion tunnel  58   a ,  58   b  has a second end at a radially outer surface  60  of the post  38 . The string  26  (see  FIG. 6B ) extends through the base segment tunnels  56  and the base portion tunnels  58   a ,  58   b  and forms loops  26   a ,  26   b ,  26   c  between the base portion tunnel  58   a  and the base portion tunnel  58   b  at each post  38 . 
     The artificial heart valve  24  is expanded by applying tension to the loops  26   a ,  26   b ,  26   c  until adjacent engagement surfaces  54  are pulled into engagement so that the base segments  50  and the base portion  52  of each post  38  are engaged and the posts  38  are aligned side-by-side and evenly distributed around the annular base  36  as shown in  FIG. 6A . When the artificial heart valve is in the expanded operational configuration the support structure is rigid. 
     As tension in the string  26  pulls the engagement surfaces  54  of the base segments  50  and the base portion  52  of each post  38  into engagement, the base  36  of the support structure  32  forms within the channel formed by the flexible leaflets  34  such that the flexible leaflets  34  cover the radially outer surface  64  of each base segment  50 . 
     When the artificial heart valve  24  is in the collapsed delivery configuration (see  FIG. 6B ), the posts  38  are located side-by-side and spaced closer together for mounting on the catheter  16 . The flexible leaflets  34  fold up between the posts  38 . Each flexible leaflet  34  folds in a predictable manner consistent with the approximate “S” shape of the free edge  42  of the flexible leaflet  34 . To allow the posts  38  to be positioned closer together, the engagement surfaces  54  of the base segments  50  and the base portion  52  of each post  38  are disengaged and spaced apart so that each group  62  of base segments  50  hangs on the string  26  between two adjacent posts  38 . 
     Each post  38  has a string guidance feature in the form of a ring protruding from the post. The ring  66  is located on the radially outer surface  60  of the post  38 . Where the post  38  has a length measured between the bottom of the post  38  and the top of the post  38 , the ring  66  is located approximately three quarters of the length from the bottom of the post  38 . 
     As can be seen in  FIG. 7 , in the expanded operational configuration, the artificial heart valve  24  is circumferentially engaged with the stent  22 . 
     Referring to  FIGS. 8A and 8B , the stent  22  defines a plurality of string guidance features. Each string guidance feature may comprise a channel  70 . At least one channel  70  is associated with each post  38  of the artificial heart valve  24 . The string guidance features may further comprise apertures extending through the stent  22  from the radially inner surface  72  of the stent  22  to the channel  70 . A bottom aperture  74  is associated with the base portion tunnels  58   a ,  58   b . The channel  70  extends from the top of the stent  22  to the bottom aperture  74 . The top aperture  76  is associated with the ring  66  of the post  38 . Where the stent  22  has a length measured between the top of the stent  22  and the bottom of the stent, the top aperture  76  is located approximately three quarters of the length from the bottom of the stent  22 . A bottom portion of the channel  70  extends between the bottom aperture  74  and the top aperture  76 . A top portion of the channel  70  extends between the top aperture  76  and the top of the stent  22 . 
     The path of a loop  26   a ,  26   b ,  26   c  of the string  26  is shown in  FIGS. 8A and 8B  as a dashed string. As will be described in more detail below, the string  26  extends through the string guidance features alternatingly between the support structure  32  and the stent  22 . Specifically, a loop  26   a ,  26   b ,  26   c  of the string  26  extends from the second end of the base portion tunnels  58   a ,  58   b , through the bottom aperture  74 , through the bottom portion of the channel  70 , through the top aperture  76 , through the ring  66 , returns through the top aperture  76 , and extends through the top portion of the channel  70  and out of the stent  22 . When tension is applied to the loops  26   a ,  26   b ,  26   c , the artificial heart valve  24  is simultaneously moved from the collapsed configuration to the expanded configuration, and brought into axial alignment and circumferential engagement with the stent  22 .  FIGS. 7 and 8B  show the artificial heart valve  24  in engagement with the stent  22 . The portion of the flexible leaflets  34  that covers the base  36  of the support structure  32  is compressed between the artificial heart valve  24  and the stent  22 . 
     As shown in  FIGS. 8A and 8B , a sealing arrangement  78  is provided between the flexible leaflets  34  of artificial heart valve  24  and the stent  22  adjacent the base  36  of the support structure  32 . As shown in more detail in  FIG. 8D , the sealing arrangement  78  includes a sealing member  78   a  which has an annular channel  79 . The sealing arrangement  78  further includes a sealing actuation string  77  which passes through the annular channel  79 . Both ends of the sealing actuation string  77  pass through a point on a radially inner surface of the annular sealing member  78  such that tension can be applied to the sealing actuation string  77 . On application of tension to the sealing actuation string  77 , the sealing actuation string  77  compresses a portion of the annular sealing member  78   a  and a radially outer portion of each flexible leaflet  34  against the radially outer surface of the support structure  32  so as to form a circumferentially continuous seal between the stent  22  and the artificial heart valve  24  which serves to reduce or prevent leakage of blood between the stent  22  and the artificial heart valve  24 . 
     A plurality of string locking arrangements is provided. Each string locking arrangement may include a female locking element in the form of a cylindrical locking tube  80  located above each channel  70  in the stent  22 . Each locking tube  80  is a hollow cylinder. The radially inner surface  80   a  of the locking tube  80  is ridged. On exit from the stent channel  70 , the loop  26   a ,  26   b ,  26   c  extends through the locking tube  80 . Each string locking arrangement may include a male locking element in the form of a polymer bead  81  (see  FIG. 8C ) provided on the string  26 . The radially outer surface  81   a  of the polymer bead  81  is ridged. The polymer bead  81  enters the locking tube  80  such that the ridged surfaces  80   a  and  81   a  of the locking tube  80  and the polymer bead  81  engage one another when the artificial heart valve has been fully expanded. The engagement between the ridged surfaces  80   a ,  81   a  acts to prevent the loop  26   a ,  26   b ,  26   c  of the string  26  from moving back through the locking tube  80  towards the stent  22 . Tension in the loops  26   a ,  26   b ,  26   c  is maintained and the artificial heart valve  24  is locked in the expanded configuration in engagement with the stent  22 . 
     The process of percutaneously implanting the artificial heart valve system  10  into the aorta  12  of the heart  14  will now be described with reference to  FIGS. 9A, 9B, 9C and 10 . 
     With reference to  FIG. 9A , the artificial heart valve system  10  is mounted on a catheter  16  with the stent  22  and artificial heart valve  24  in their respective collapsed delivery configurations. The stent  22  and an inflatable member  30  are mounted on the catheter  16  at a first axial location  15  with the stent  22  located radially outwardly of the inflatable member  30 . The artificial heart valve  24  is mounted on the catheter  16  at a second axial location  17 . The second axial location  17  on the catheter  16  is proximal relative to the first axial location  15  on the catheter  16 , i.e. closer to the incision  18  in the skin  20 . The catheter  16  is inserted through an incision  18  in the skin  20  of the subject and through an artery to the aorta  12 . 
     Referring to  FIG. 9B , the inflatable member  30  is inflated to expand the stent  22  within the aorta  12  from its collapsed delivery configuration to its expanded operational configuration. 
     Referring to  FIGS. 9C and 10 , once the stent  22  is expanded, the inflatable member  30  is deflated. As one of ordinary skill in the art will understand, the stent  22  is configured such that, once expanded, the stent  22  remains in an expanded operational configuration. The catheter  16  is inserted into the subject further until the artificial heart valve  24  is generally aligned axially with the stent  22 . One of ordinary skill in the art will understand that a sleeve (not shown) may cover the artificial heart valve  24  whilst it is mounted on the catheter  16  for delivery. The sleeve (not shown) is removed using a secondary string (not shown) connected to the sleeve (not shown) and extending to the incision  18 . The artificial heart valve  24  is then expanded and pulled radially outwardly into engagement with the stent  22 . Specifically, the artificial heart valve  24  is expanded by applying tension to secondary strings  28   a ,  28   b ,  28   c . Each secondary string  28   a ,  28   b ,  28   c  extends from an incision  18  in the skin  20  of the subject, through a corresponding loop  26   a ,  26   b ,  26   c , of the string  26  and returns to the incision  18 . The tension applied to the secondary strings is translated to the loops  26   a ,  26   b ,  26   c  of the string  26 . As described above, on application of tension to the loops  26   a ,  26   b ,  26   c , the artificial heart valve  24  moves from the collapsed delivery configuration to the expanded operational configuration. 
     One skilled in the art will understand that various modifications may be made to the foregoing embodiments without departing from the scope of the present invention as defined by the appended claims. 
     For example, each of  FIGS. 11A, 11B and 11C , shows an alternative support element in the form of an alternative base segment  50 . Each alternative base segment defines a male engagement surface  54   a  at a first end or side and a female engagement surface  54   b  at a second end or side opposite the first end or side. In a first alternative base segment shown in  FIG. 11A , each of the engagement surfaces  54   a ,  54   b  is defined by two planar surfaces which meet along an edge which is aligned generally parallel to an axis of the artificial heart valve when the artificial heart valve is in the expanded operational configuration and the engagement surfaces  54   a ,  54   b  are inter-engaged such that the engagement surfaces  54   a ,  54   b  prevent relative radial movement between adjacent base segments  50 . One of skill in the art will also understand that each post  38  may define male and female engagement surfaces like the male and female engagement surfaces  54   a ,  54   b  of the first alternative base segments shown in  FIG. 11A  so that when the artificial heart valve is in the expanded operational configuration and the engagement surfaces are inter-engaged, the engagement surfaces prevent relative radial movement between a post  38  and an adjacent base segment  50 . 
     In a second alternative base segment shown in  FIG. 11B , each of the engagement surfaces  54   a ,  54   b  is defined by two planar surfaces which meet along an edge which is aligned generally perpendicular to an axis of the artificial heart valve when the artificial heart valve is in the expanded operational configuration and the engagement surfaces  54   a ,  54   b  are inter-engaged such that the engagement surfaces  54   a ,  54   b  prevent relative axial movement between adjacent base segments. One of skill in the art will also understand that each post  38  may define male and female engagement surfaces like the male and female engagement surfaces  54   a ,  54   b  of the second alternative base segments shown in  FIG. 11B  so that when the artificial heart valve is in the expanded operational configuration and the engagement surfaces are inter-engaged, the engagement surfaces prevent relative axial movement between a post  38  and an adjacent base segment  50 . 
     In a third alternative base segment shown in  FIG. 11C , each of the engagement surfaces  54   a ,  54   b  is defined by four planar surfaces which meet along four edges when the artificial heart valve is in the expanded operational configuration and the engagement surfaces  54   a ,  54   b  are inter-engaged such that the engagement surfaces  54   a ,  54   b  prevent relative radial and relative axial movement between adjacent base segments. One of skill in the art will also understand that each post  38  may define male and female engagement surfaces like the male and female engagement surfaces  54   a ,  54   b  of the third alternative base segments shown in  FIG. 11C  so that when the artificial heart valve is in the expanded operational configuration and the engagement surfaces are inter-engaged, the engagement surfaces prevent relative radial and relative axial movement between a post  38  and an adjacent base segment  50 . 
     In each of the alternative base segments shown in  FIGS. 11A-11C , the angle at which the planar surfaces meet may be between approximately 60 degrees and approximately 120 degrees. The planar surfaces may meet an angle of 90 degrees. Alternatively, the planar surfaces may meet at an angle of 120 degrees. 
     The base segments  50  may have a cross-sectional profile other than the rectangular cross-sectional profile shown in  FIGS. 5A and 5B . For example, each base segment  50  may have a rounded upper surface as shown in  FIG. 12A . In other embodiments, the base segments may have a circular cross-section as shown in  FIG. 12B . In yet other embodiments, the base segments may have a cross-section comprising a square or rectangle with a rounded corner as shown in  FIG. 12C . The rounded corner is the radially outer top corner of the base segment. Compared with the radially outer top corner of the base segments shown in shown in  FIGS. 5A and 5B , the rounded corner may serve to reduce localized pressure on the flexible leaflets  34  at the rounded corner. The rounded corner may be shaped so as to be commensurate with the shape of the flexible leaflets  34 . It should be understood that the base segments  50  may have one of the cross-sectional profiles shown in any of  FIGS. 12A-12C  in combination with one pair of male and female engagement surfaces  54   a ,  54   b  shown in any of  FIGS. 11A-11C . Alternatively, the base segments  50  may have one of the cross-sectional profiles shown in any of  FIGS. 12A-12C  in combination with a conical male engagement surface (not shown) and a conical female engagement surface (not shown). 
     An alternative embodiment of an artificial heart valve is shown in  FIG. 13 . Features of this embodiment that correspond to those of the previously described embodiment use corresponding reference numerals incremented by “100”. The artificial heart valve  124  has a plurality of support elements and flexible leaflets  134 . When the artificial heart valve  124  is in the expanded operational configuration, the plurality of support elements forms a generally cylindrical support structure  132 . An upper edge  133  of the support structure  132  defines a plurality of generally U-shaped or “scalloped” regions. The flexible leaflets  134  extend across the scalloped regions. A base edge of each flexible leaflet  134  has a generally U-shaped or “scalloped” shape defined by the corresponding U-shaped or “scalloped” region of the upper edge  133  of the support structure  132 . Each leaflet  134  has a movable free edge  142  located opposite the corresponding U-shaped or “scalloped” region of the upper edge  133  of the support structure  132 . The support structure  132  is segmented circumferentially into a plurality of base segments  150  and a plurality of posts  138 . Each flexible leaflet  134  is attached to two of the posts  138 . Each base segment  150  has upper and lower tunnels (not shown) for receipt of first and second strings (not shown). Each post  138  has two upper tunnels  158   aa ,  158   ab  and two lower tunnels  158   ba ,  158   bb  for receipt of first and second strings (not shown). The first string (not shown) forms a loop between the two upper tunnels  158   aa ,  158   ab  of each post  138 . The second string (not shown) forms a loop between the two lower tunnels  158   ba ,  158   bb  of each post  138 . In use, the loops formed in the first and second strings extend through a stent (not shown). Tension on the loops formed in the first and second strings (not shown) pulls the artificial heart valve  124  from a collapsed delivery configuration to an expanded operational configuration whilst also pulling the artificial heart valve  124  into engagement with the stent (not shown). 
     A further alternative embodiment of an artificial heart valve is shown in  FIG. 14 . Features of this embodiment that correspond to those of the first described embodiment use corresponding reference numerals incremented by “200”. The artificial heart valve  224  has a plurality of support elements and flexible leaflets  234 . When the artificial heart valve is in the expanded operational configuration the support elements together form a support structure  232  defining an aperture for blood flow. The plurality of support elements includes three posts  238  and a plurality of base segments  250 . Between each pair of adjacent posts  238  there is a group  262  of base segments. A string  226  extends through the posts  238  and the base segments  250 . When the artificial heart valve  224  is in the expanded operational configuration, each group  262  of base segments  250  is arranged along an outwardly convex path having both circumferential and axial components relative to an axis defined by the aperture for blood flow. Each of the base segments  250  has a tunnel (not shown) for receipt of a string. Each post  238  has two tunnels  258   a  and  258   b . Each tunnel  258   a  and  258   b  extends from an adjacent base segment  250  to a radially outer surface  260  of the post  238 . Loops are formed in the string  226  between the tunnels  258   a  and  258   b.    
     One skilled in the art will understand that other modifications may be made to the foregoing embodiments without departing from the scope of the present invention as defined by the appended claims. For example, each flexible leaflet may be attached, for example, fused, bonded or sewn to at least one base segment. For example, each flexible leaflet may be attached to a base segment in the middle of the group of base segments to which said base segment belongs. Attaching each flexible leaflet to at least one base segment may assist in guiding the base segments into the channel defined along the base edge of the flexible leaflet when the artificial heart valve is expanded from the collapsed delivery configuration to the expanded operational configuration. Furthermore, by having at least one base segment per group attached to the flexible leaflet, a distance by which the base segments extend below the base edges of the flexible leaflets when the artificial heart valve is in the collapsed delivery configuration is reduced in comparison to a distance by which the base segments extend below the base edges of the flexible leaflets when none of the base segments are fixed to the flexible leaflet. This may reduce the chance of damage to the artificial heart valve in the event of heart ejection during delivery of the artificial heart valve. 
     An artificial heart valve may be delivered such that it replaces the aortic valve, the mitral valve or any other valve within the heart. An artificial heart valve may comprise fewer or more than three posts and fewer or more than three flexible leaflets, provided that the number of leaflets corresponds to the number of posts. Similarly, the number of string guidance features in the stent may vary to correspond to the number of posts. 
     The flexible leaflets may be manufactured using any suitable method such as injection molding, dip molding or the like. A leaflet material may be formed around and/or through the posts so as to define the flexible leaflets. The leaflet material may extend continuously around and/or through the posts. Each post may define one or more apertures or slits through which the leaflet material extends. The leaflet material may be formed around and/or through one or more of the base segments so as to define the flexible leaflets. The leaflet material may extend continuously around and/or through one or more of the base segments. Each base segment may define one or more apertures or slits through which the leaflet material extends. 
     The flexible leaflets may comprise or be made of any suitable material, provided that the flexible leaflets can fold sufficiently to allow the artificial heart valve to assume the collapsed delivery configuration. For example, the flexible leaflets may comprise or be formed of a natural material such as human aortic or pulmonary valves, animal aortic or venous valves, or animal pericardium. The flexible leaflets may comprise or be formed of a synthetic polymer such as polyurethane. 
     The support elements may comprise, or be formed of, any suitable material. For example, the support elements may comprise, or be formed from, a metal. The support elements may comprise, or be formed of, stainless steel, titanium, or a polymer such as polyether ether ketone (PEEK). The support elements may have a rough or textured surface. A rough or textured surface may improve the adhesion of the leaflet material to the support elements. The support elements may comprise, or be formed of, a sintered material. Use of a sintered material may provide the support elements with a rough or textured surface. The support elements may be formed using an additive manufacturing technique. For example, the support elements may be formed by direct laser sintering such as direct laser metal sintering and/or by 3-D printing. 
     There may be more tunnels, strings and loops than described above. One, two or more tunnels may be provided in each base segment, wherein each tunnel receives a corresponding string. The tunnels may be substantially parallel, or may have dissimilar paths. The tunnels may be curved or straight. A string may be provided per group of base segments. A loop may be formed at each end of each string. 
     One or more of the strings may comprise or be formed from a cord, a line, a ligature, a suture, a thread or the like. One or more of the strings may be coated in polymer. The polymer may be configured so as to provide an interference fit between the string and the tunnel of each support element through which the string passes. Such an interference fit may be selected so that the string is not loosely fitted in the tunnel. Such an interference fit may be selected so that each support element through which the string passes does not slide along the string unless a predetermined force is applied between the support element and the string. This may prevent the support element from moving along the string without user manipulation. The polymer may comprise polyester, such as braided polyester. 
     The number of base segments per group of the base may be fewer or more than ten depending on the size of the artificial heart valve, the size of the base segments, the number of posts, etc. The engagement surfaces can have any suitable form such that each engagement surface abuts or inter-engages a complementary adjacent surface, whether of a base segment or a base portion of a post. 
     Although the string guidance feature of the post is described above as a ring or eyelet protruding from the post, the string guidance feature may be constructed in any other suitable manner such that the loop can pass through the string guidance feature. For example, the string guidance feature may be a generally “U” shaped tunnel on the radially outer face of the post. The location of the string guidance feature and the corresponding stent aperture can be closer to the top or closer to the bottom of the artificial heart valve system than the three quarter distance described previously, provided that the artificial heart valve can be suitably engaged with the stent on application of tension to the loops. 
     Separate strings may be provided to extend between the tunnels in the support elements and the stent, and between the post string guidance feature and the stent. For example, the previously described one or more strings may extend between the tunnels in the support elements and the stent, and one or more anchor strings may extend between the post string guidance features and the stent. The one or more anchor strings may not extend through any of the base segments. The system may comprise one or more secondary strings for applying tension to the one or more anchor strings. 
     The locking tubes may be integrally formed with the stent. The locking tubes may be located within the stent. 
     In the method of percutaneously implanting the artificial heart valve  24  described with reference to  FIGS. 9A to 9C and 10 , the stent  22  and the inflatable member  30  are mounted on the catheter  16  at a first axial location of the catheter  16  with the stent  22  located radially outwardly of the inflatable member  30  and the artificial heart valve  24  is mounted on the catheter at a second axial location on the catheter  16 , where the second axial location of the artificial heart valve  24  on the catheter  16  is proximal with respect to the first axial location of the stent  22  and the inflatable member  30 . Initially, the catheter  16  is inserted into the subject until the stent  22  and the inflatable member  30  are generally aligned axially with the valve annulus. The inflatable member  30  is inflated to expand the stent  22  from its collapsed delivery configuration to its expanded operational configuration. The inflatable member  30  is deflated after expansion of the stent  22 . The catheter  16  is inserted further into the subject until the artificial heart valve  24  is generally aligned axially with the stent  22  and the inflatable member  30  is inserted distally of the stent  22 . Once the artificial heart valve  24  is expanded, the inflatable member  30  is retracted back through the expanded artificial heart valve  24 . In effect, therefore, mounting the artificial heart valve  24  on the catheter  16  at a second axial location which is proximal with respect to the first axial location of the stent  22  and the inflatable member  30 , may avoid any requirement for the artificial heart valve  24  to be inserted into, or through, the valve annulus before expansion of the stent  22 . 
     Alternatively, the artificial heart valve may be mounted on the catheter at a second axial location which is distal with respect to the first axial location of the stent and the inflatable member. The method may then comprise inserting the catheter into the subject so as to insert the artificial heart valve through the valve annulus until the stent and the inflatable member are generally aligned axially with the valve annulus. The method may comprise inflating the inflatable member to expand the stent from its collapsed delivery configuration to its expanded operational configuration. The method may comprise deflating the inflatable member after expansion of the stent. The method may comprise retracting the catheter from the subject until the artificial heart valve is generally aligned axially with the stent for expansion of the artificial heart valve and the inflatable member is proximal of the stent.