Patent Publication Number: US-9408687-B2

Title: Tissue modification

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. application Ser. No. 14/481,477 file Sep. 9, 2014, which is a continuation of U.S. application Ser. No. 13/626,578 filed Sep. 25, 2012, which claims the benefit of U.S. Provisional Application Ser. No. 61/539,675, filed on Sep. 27, 2011, which is incorporated by reference herein. 
    
    
     TECHNICAL FIELD 
     The following disclosure relates to tissue modification and, more particularly, to modification of biological tissue for implantation in a mammal. 
     BACKGROUND 
     Heart valve surgery can be used to repair or replace diseased heart valves. For example, heart valve replacement may be indicated when there is a narrowing of the native heart valve, commonly referred to as stenosis, or when the native valve leaks or regurgitates. The repair or replacement of diseased heart valves can include, for example, the introduction of a prosthetic heart valve that includes biological tissue heterologous to the patient (e.g., a heterograft or xenograft). 
     Biological tissue can have mechanical properties that vary within a single donor and/or from among several donors of the same species. For example, biological tissue from a single donor can have non-uniform thickness, and the average thickness of biological tissue can vary from one donor to another. The variation in mechanical properties of biological tissue used in replacement heart valves can impact the performance and/or durability of a replacement heart valve implanted in a patient. 
     SUMMARY 
     Tissue modification changes one or more mechanical properties of biological tissue used for implantation in a mammal. 
     In one aspect, a tissue modification apparatus includes a first and a second guide, a first and a second set of mounts, and a first and a second actuator. The first guide defines a first axis, and the second guide defines a second axis intersecting the first axis. The mounts of the first set of mounts are movable relative to one another along the first axis, and the mounts of the second set of mounts are movable relative to one another along the second axis. The first actuator and the second actuator are each settable to a stress load, with the first actuator and the second actuator movable, respectively, along the first axis and the second axis to transmit each respective stress load to a piece of tissue mechanically coupled to the first and second set of mounts. 
     In some embodiments, the second axis is substantially perpendicular to the first axis. In certain embodiments, the second axis intersects the first axis in a plane substantially parallel to a substantially planar surface of the piece of tissue mechanically coupled to the first and second set of mounts. 
     In some embodiments, at least a portion of the first guide is mechanically coupled to at least a portion of the second guide. For example, the first guide can be mechanically coupled to the second guide at the intersection of the first axis and the second axis. 
     In certain embodiments, a plurality of first guides is substantially parallel to or coaxial with the first axis and a plurality of second guides is substantially parallel to or coaxial with the second axis. For example, the plurality of first guides can include a first pair of rods substantially parallel to one another and the plurality of second guide can include a second pair of rods substantially parallel to one another. Additionally or alternatively, at least one of the first pair of rods is disposed along the first axis and at least one of the second pair of rods is disposed along the second axis. Each rod of the first and second pair of rods can have an outer diameter great than or equal to about 0.75 mm and less than or equal to about 13 mm. For example, each rod of the first and second pair of rods is stainless steel with an outer diameter of about 3.2 mm. 
     In some embodiments, the first pair of rods are substantially perpendicular to the second pair of rods to form a substantially cruciform frame with an aperture defined by the first pair of rods and the second pair of rods and portions of each of the first and second pair of rods extending away from the aperture. For example, the first actuator and the second actuator are disposed on the respective portions of the first and second pair of rods extending away from the aperture. Additionally or alternatively, the first set of mounts and the second set of mounts are disposed on the respective portions of the first and second pair of rods extending away from the aperture. 
     In certain embodiments, the first guide and the second guide each include at least one passivated surface. Additionally or alternatively, the first guide and the second guide can each include at least one polished surface. 
     In some embodiments, the first guide and the second guide each include at least one collar stationary relative to the respective first and second guide. The at least one collar can be disposed along the respective first and second axes, and the at least one collar can limit movement of the respective first and second set of mounts along the respective first and second axes. 
     In certain embodiments, each mount of the first set of mounts is larger than each of the second set of mounts along axes perpendicular to the respective first and second axes. For example, each mount of the first set of mounts can extend about 100 mm to about 150 mm in a direction perpendicular to the first axis and each mount of the second set of mounts can extend about 50 mm to about 90 mm in a direction perpendicular to the second axis. In some embodiments, each mount of the first set of mounts and the second set of mounts is substantially cylindrical with an outer diameter of about 6 mm to about 25 mm. 
     In some embodiments, each mount of the first and second set of mounts includes mitered end portions. For example, the mitered end portions of each mount of the first set of mounts can be complementary to the mitered end portions of each mount of the second set of mounts. 
     In certain embodiments, the first and second actuators are each movable along the respective first and second axes to transmit each respective set stress load substantially simultaneously to a piece of tissue. 
     In some embodiments, the first and second actuators each comprise at least one spring. For example, the at least one spring of each first and second actuator can be compressible to set the respective stress load and expandable transmit the respective stress load to the piece of tissue mechanically coupled to the first and second set of mounts. The at least one spring of the first actuator and the at least one spring of the second actuator can have substantially similar spring constants. Additionally or alternatively, the at least one spring of the first actuator and the at least one spring of the second actuator can have spring constants of about 5 N/m to about 50 N/m. Additionally or alternatively, the at least one spring of the first actuator and the at least one spring of the second actuator can each be compressible from respective equilibrium positions by a distance of about 5 mm to about 35 mm. 
     In certain embodiments, each mount of the first and second set of mounts includes at least one surface in slidable contact with the respective first and second guides along the respective first and second axes. For example, the at least one surface of each of the first and second sets of mounts in slidable contact with the respective first and second guides is polytetrafluoroethylene. 
     In some embodiments, each mount of the first and second set of mounts comprises a bearing in rolling contact with a respective first and second guide. For example, each bearing can be polyoxymethylene or polytetrafluoroethylene. Additionally or alternatively, each bearing can be a flange bearing. 
     In certain embodiments, each mount of the first and second set of mounts comprises a plurality of hooks extending away from each respective mount. For example, each of the plurality of hooks can be substantially axially aligned with one another along each respective mount. Additionally or alternatively, the plurality of hooks can be substantially evenly spaced along the axis. 
     In another aspect, a tissue modification system includes a tissue stretcher and a jig. The tissue stretcher includes a first guide and a second guide, a first set of mounts and a second set of mounts, a first actuator and a second actuator. The first guide defines a first axis, and the second guide defines a second axis intersecting the first axis. The mounts of first set of mounts are movable relative to one another along the first axis, and the mounts of the second set of mounts are movable relative to one another along the second axis. The first actuator and the second actuator are mechanically coupled, respectively, to the first and second set of mounts. The jig includes a base, a support extending from the base, and a plurality of locking members. The first guide and/or the second guide are mechanically couplable to the base such that the first and second guides are stationary relative to the base. The plurality of locking members are mechanically couplable to the base and movable with respect to the first and second actuators to allow movement of the first and second actuators along the respective first and second axes to transmit a respective stress load to a piece of tissue mechanically coupled to the first and second set of mounts. 
     In some embodiments, the first and second actuators are movable relative to the base to set respective stress loads and the plurality of locking members are movable to hold the first and second actuators at the respective set stress loads. 
     In certain embodiments, the support and the locking members are simultaneously mechanically decouplable from the tissue stretcher. 
     In some embodiments, the first guide includes a first pair of rods substantially parallel to one another and the second guide includes a second pair of rods substantially parallel to one another. For example, the first pair of rods can be substantially perpendicular to the second pair of rods to form a substantially cruciform frame with an aperture defined by the first pair of rods and the second pair of rods and portions of each of the first and second pair of rods extending away from the aperture. At least a portion of the support can be engageable with the aperture to hold the first and second pair of rods stationary relative to the base. Additionally or alternatively, the first actuator and the second actuator can be disposed on the respective portions of the first and second pair of rods extending away from the aperture. Additionally or alternatively, the first set of mounts and the second set of mounts can be disposed on the respective portions of the first and second pair of rods extending away from the aperture. 
     In another aspect, a tissue modification method includes moving a first pair of mounts toward one another along a first axis to set a first load, moving a second pair of mounts toward one another along a second axis, intersecting the first axis, to set a second load, mechanically coupling a substantially planar sheet of tissue to the first pair of mounts and to the second pair of mounts, moving the first pair of mounts away from one another along the first axis to apply the set first load to the substantially planar sheet of tissue, and moving the second pair of mounts away from one another along the second axis to apply the set second load to the substantially planar sheet of tissue. 
     In some embodiments, the first axis is substantially perpendicular to the second axis. In certain embodiments, the first stress load is substantially equal to the second stress load. In some embodiments, the first stress load and the second stress load are applied parallel to the substantially planar surface of the tissue. In certain embodiments, the first and second set stress loads are applied substantially simultaneously. 
     In certain embodiments, moving the first and second pair of mounts toward one another along the respective first and second axes includes compressing one or more springs mechanically coupled to the first and second pair of mounts. Additionally or alternatively, moving the first and second pair of mounts away from one another along the respective first and second axes can include at least partially releasing the compression of the one or more springs mechanically coupled to the first and second pair of mounts. 
     In another aspect, a tissue modification method includes setting a first and a second stress load to apply to a piece of tissue, applying the first stress load to the piece of tissue along a first axis, and applying the second stress load to the piece of tissue along a second axis substantially perpendicular to the first axis, wherein the first and second stress loads are applied to the tissue substantially simultaneously. 
     In certain embodiments, setting the first and second stress loads includes compressing respective first and second springs. Additionally or alternatively, applying the first and second stress loads includes at least partially releasing the respective compressed first and second springs. 
     In some embodiments, the first stress load and the second stress load are substantially equal. 
     In certain embodiments, the piece of tissue is a substantially planar sheet and the first and second axes are parallel to the plane defined by the substantially planar sheet. Additionally or alternatively, the piece of tissue includes biological tissue. For example, the piece of tissue can be one of bovine pericardium, equine pericardium, and porcine pericardium. 
     In some embodiments, the piece of tissue is exposed to a glutaraldehyde solution. Additionally or alternatively, the piece of tissue can be exposed to the glutaraldehyde solution during at least a portion of the exposure of the piece of tissue to the first and second stress loads. For example, the piece of tissue can be exposed to glutaraldehyde solution for about one day to about two weeks. The first and second stress loads can be each applied to the piece of tissue for about 30 minutes to about 120 minutes. 
     In another aspect, a tissue modification method includes arranging a substantially planar patch of pericardial tissue in a stationary position relative to a base, and moving a shaver relative to the substantially planar patch of pericardial tissue to remove tissue along at least a portion of a substantially planar surface of the substantially planar patch of pericardial tissue. 
     In some embodiments, the substantially planar patch of pericardial tissue has a first substantially planar surface rougher than a second substantially planar surface. The shaver can remove at least a portion of the first substantially planar surface. 
     In certain embodiments, the substantially planar patch of pericardial tissue is bovine pericardium, equine pericardium, or porcine pericardium. 
     In some embodiments, vacuum pressure is applied to the substantially planar patch of pericardial tissue such that the substantially planar patch of pericardial tissue is drawn toward the base. For example, the vacuum pressure is applied to the substantially planar patch of tissue by drawing air through a plurality of orifices defined by the base. Additionally or alternatively, a saline solution can be applied to the substantially planar piece of tissue. For example, the saline solution and the vacuum pressure can be simultaneously applied to the substantially planar piece of tissue. 
     In certain embodiments, the substantially planar piece of tissue is fixed and the shaver is moved relative to the stationary piece of tissue during or after fixing the substantially planar piece of tissue. 
     In another aspect, a tissue modification method includes forming a substantially planar leaflet from a piece of pericardial tissue, arranging the substantially planar leaflet in a stationary position relative to a base, and removing tissue from at least a portion of a substantially planar surface of the substantially planar leaflet. The substantially planar leaflet includes a coaptation portion, an arcuate edge substantially opposite the coaptation portion, the arcuate edge having a first end and a second end, and a belly extending from the arcuate edge to an axis defined by the first and second ends of the arcuate edge. 
     In certain embodiments, removing at least a portion of the substantially planar surface of the substantially planar leaflet includes removing tissue from the belly of the leaflet. Additionally or alternatively, removing at least a portion of the substantially planar surface of the substantially planar leaflet includes moving a laser (e.g., a femtosecond laser) over a portion of the stationary, substantially planar leaflet. 
     In another aspect, a method of prosthetic heart valve preparation includes storing a prosthetic heart valve in a first solution, the prosthetic heart valve comprising a biological tissue isotonic to the first solution, exposing the biological tissue of the prosthetic heart valve to a second solution hypertonic to the biological tissue such that water flows out of the biological tissue, and moving a sheath distally over the prosthetic heart valve to contract the prosthetic heart valve for intraluminal delivery to a mammalian heart. 
     In some embodiments, the second solution is about 75 percent to about 80 percent water. In certain embodiments, the tonicity of the second solution is about 339 mOsm/L to about 12.3 Osm/L. Additionally or alternatively, the biological tissue of the prosthetic heart valve can be exposed to the second solution for about 30 seconds to about 15 minutes. Additionally or alternatively, the first solution and the second solution can be at substantially the same temperature. 
     In certain embodiments, the biological tissue is sterilized (e.g., in a terminal sterilization procedure) after the biological tissue of the prosthetic heart valve is exposed to the second solution hypertonic to the biological tissue. For example, sterilizing the biological tissue can include exposing the biological tissue to electron beam (e-beam) radiation and/or exposing the biological tissue to ethylene oxide (EtO). 
     In another aspect, a method of prosthetic heart valve preparation includes exposing a prosthetic heart valve in a first solution, the prosthetic heart valve comprising a biological tissue isotonic to the first solution, exposing the biological tissue of the prosthetic heart valve to a second solution including alcohol or ethylene glycol such that water flows out of the biological tissue, and moving a sheath distally over the prosthetic heart valve to contract the prosthetic heart valve for intraluminal delivery to a mammalian heart. In some embodiments, exposing the biological tissue of the prosthetic heart valve to the second solution includes exposing the biological tissue to a series of graded alcohol solutions or a series of graded ethylene glycol solutions. For example, the series of graded alcohol or ethylene glycol solutions includes about 50%, about 60%, about 70%, about 90%, about 95%, and about 100% alcohol or ethylene glycol solutions. 
     In some embodiments, the biological tissue of the prosthetic heart valve is exposed to the second solution for about 30 seconds to about 15 minutes. Additionally or alternatively, the first solution and the second solution can be at substantially the same temperature. 
     In certain embodiments, the biological tissue is sterilized (e.g., in a terminal sterilization procedure) after the biological tissue of the prosthetic heart valve is exposed to the second solution including alcohol or ethylene glycol. For example, sterilizing the biological tissue can include exposing the biological tissue to electron beam (e-beam) radiation and/or exposing the biological tissue to ethylene oxide (EtO). 
     In still another aspect, a method of prosthetic heart valve preparation includes exposing a biological tissue to a first solution, exposing the biological tissue to a second solution such that water flows out of the biological tissue, and applying biaxial stress loads to the biological tissue. The biological tissue is isotonic to the first solution. The second solution includes alcohol or ethylene glycol. The biaxial stress load can be applied to the biological tissue after the biological tissue is exposed to the second solution. 
     Embodiments can include one or more of the following advantages. 
     In some embodiments, stress loads to be transmitted to tissue can be set prior to being applied to a respective first axis and a second axis. Setting the stress loads in this manner can allow biaxial stress to be repeatably and reliably applied to multiple, different pieces of tissue. This repeatable and reliable application of stress load can improve the amount of tissue that can be processed, as compared to methods that require more manual manipulation by an operator. 
     Moreover, setting the stress load in these embodiments results in a set stress to the tissue and variable amount of strain applied to pieces of tissue. For example, in these embodiments, pieces of tissue having different stress-strain characteristics will be stretched different distances under the same set stress load. Setting the stress load and allowing the resulting strain in the tissue to vary can improve the uniformity of the mechanical properties (e.g., stiffness along a first axis and a second axis) across several pieces of tissue. This improved uniformity can facilitate tissue matching for leaflets used in a prosthetic heart valve, where matching mechanical properties of the tissue used for the leaflets can improve the load distribution over the leaflets and, thus, improve hemodynamic performance and/or improve the durability of the prosthetic heart valve. 
     In certain embodiments, set stress loads are applied substantially simultaneously along the respective first and second axes. As compared to biaxial stress load that is not applied substantially simultaneously, the application of substantially simultaneous biaxial stress load to tissue to apply a substantially simultaneous stress to the tissue can improve the mechanical properties of the tissue. For example, exposing tissue to substantially simultaneous biaxial stress loading can improve the similarity between stiffness of the tissue along the first and second axes. In a prosthetic heart valve including leaflets made from tissue, the improvements in the similarity in stiffness characteristics along the first and second axes of the tissue can result in improved load distribution and/or durability in the valve. 
     In some embodiments, a shaver is moved relative to a planar patch of pericardial tissue to remove tissue along at least a portion of the substantially planar surface of the substantially planar patch of pericardial tissue. This removal of tissue can improve the uniformity of thickness of the planar patch of tissue. In instances in which the tissue or a portion of the tissue is part of an intraluminally delivered prosthetic heart valve, the improved uniformity of thickness of the tissue can result in reduced forces associated with sheathing the prosthetic heart valve for intraluminal delivery. 
     In certain embodiments, the substantially planar patch of pericardial tissue has a first substantially planar surface rougher than a second substantially planar surface and the shaver is moved relative to the planar patch of pericardial tissue to remove tissue from the rougher surface. Removal of tissue from the rougher substantially planar surface of the pericardial tissue can decrease the thickness profile of the tissue while substantially maintaining the overall mechanical properties of the tissue. 
     In certain embodiments, a laser is moved relative to a substantially planar leaflet to remove tissue from at least a portion of a substantially planar surface of the substantially planar leaflet. By selectively removing material from portions of the substantially planar leaflet (e.g., the belly of the leaflet), the laser can be used to achieve a local reduction in thickness of the tissue. This local reduction in the thickness of the tissue can reduce the forces associated with sheathing a prosthetic heart valve for intraluminal delivery while maintaining the thickness of the leaflet in other areas to facilitate attachment and/or to improve load distribution. 
     In certain embodiments, biological tissue of a prosthetic heart valve is exposed to a solution hypertonic to the biological tissue such that water flows out of the biological tissue. In some embodiments, biological tissue of a prosthetic heart valve is exposed to a solution including alcohol or ethylene glycol such that water flows out of the biological tissue. Such removal of water from the biological tissue reduces one or more dimensions of the biological tissue such that forces associated with sheathing the prosthetic heart valve are reduced. Additionally or alternatively, such reduction of one or more dimensions of the biological tissue can facilitate reduction of the overall outer diameter of the inraluminal delivery system for the prosthetic heart valve. 
     In certain embodiments, the tissue can be dehydrated (e.g., by exposure to a hypertonic solution, alcohol, or ethylene glycol) during the final manufacturing steps of a prosthetic heart valve such that the tissue undergoes terminal sterilization in a dehydrated state. As compared to tissue in a hydrated state, the tissue in a dehydrated state can better withstand exposure to forms of terminal sterilization (e.g., e-beam and ethylene oxide sterilization) that tend to damage and/or alter biological tissue in a hydrated state. Additionally or alternatively, the dehydration of the biological tissue can facilitate sterilization and storage of the prosthetic heart valve in a sheathed or unsheathed position. This can, for example, facilitate the sterilization of the entire prosthetic heart valve assembly (including the valve, valve delivery system, and handle, etc.) in a single package, without the need, for example, for storing the valve in liquid. 
     The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other aspects, features, and advantages of the invention will be apparent from the description and drawings, and from the claims. 
    
    
     
       DESCRIPTION OF DRAWINGS 
         FIG. 1  is a partially exploded isometric view of a tissue modification system. 
         FIG. 2  is a top view of a tissue stretcher of the tissue modification system of  FIG. 1 . 
         FIG. 3  is a top view of a jig of the tissue modification system of  FIG. 1 . 
         FIG. 4  is a top view of the tissue modification system of  FIG. 1  in a compressed state. 
         FIG. 5  is a top view of the tissue modification system of  FIG. 1  in a compressed state and mounted with tissue. 
         FIG. 6  is a top view of the tissue stretcher of  FIG. 1  in an expanded state and mounted with tissue. 
         FIG. 7  is a side view of a mount of a tissue stretcher. 
         FIG. 8  is a top view of a tissue modification system mounted with tissue. 
         FIG. 9  is a cross-sectional view of the tissue modification system of  FIG. 8  taken along the line A-A in  FIG. 8 . 
         FIG. 10  is a top view of a tissue modification system mounted with a tissue leaflet. 
         FIGS. 11A-D  are schematic representations of the process of sheathing a prosthetic heart valve including biological tissue. 
     
    
    
     Like reference symbols in the various drawings indicate like elements. 
     DETAILED DESCRIPTION 
     Referring to  FIGS. 1-6 , a tissue modification system  1  includes a tissue stretcher  10  and a jig  50 . In use, as described in further detail below, the tissue stretcher  10  is mechanically couplable to the jig  50  such that at least a portion of the tissue stretcher  10  remains fixed in place relative to the jig  50  and a substantially planar piece of tissue  70  can be mounted on the tissue stretcher  10 . As also described in further detail below, the tissue stretcher  10  can be decoupled from the jig  50  such that at least a portion of the tissue stretcher  10  expands to stretch the tissue  70  biaxially. 
     Referring now to  FIGS. 1-2 , the tissue stretcher  10  includes first mounts  26   a,b  disposed along substantially parallel first guides  12   a,b  and second mounts  28   a,b  disposed along substantially parallel second guides  16   a,b . First actuators  32   a,b,c,d  are disposed on the first guides  12   a,b  and second actuators  34   a,b,c,d  are disposed on the second guides  16   a,b . In use, the first actuators  32   a,b,c,d  can move the first mounts  26   a,b  away from one another along a first axis  14  defined by one of the first guides  12   a,b , and the second actuators  34   a,b,c,d  can move the second mounts  28   a,b  away from one another along a second axis  18  defined by one of the second guides  16   a,b . The relative movement of the first mounts  26   a,b  away from one another along the first axis  14  and the relative movement of the second mounts  28   a,b  away from one another along the second axis  18  results in the application of a biaxial stress load to tissue (e.g., the tissue  70  in  FIGS. 5-6 ) mounted on the tissue stretcher  10 . 
     As used herein, “stress load” is the force (measured, e.g., in N) applied to the tissue. For example, in some embodiments in which the first actuators  32   a,b,c,d  and the second actuators  34   a,b,c,d  are springs, the stress load is proportional to the spring constant and the displacement of the spring from its equilibrium position. In these embodiments, the stress load can be mathematically expressed by Hooke&#39;s law (F=−k*x, where F is the stress load, k is the spring constant, and x is the displacement of the spring&#39;s end from it equilibrium position). As used herein, “stress” is the force (i.e., stress load) per unit area of the tissue and can be measured, for example, in N/mm 2 . Accordingly, the stress applied to the tissue is a function of the stress load and the cross-sectional area of the tissue. For example, for a given stress load, the stress applied to the tissue may vary with the thickness of the tissue. 
     The first guides  12   a,b  and the second guides  16   a,b  are coupled (e.g., welded, interference fit) to one another at connection regions  24  defined by the intersection of the first guides  12   a,b  with the second guides  16   a,b  (e.g., the intersection of the first axis  14  and the second axis  18 ). The mechanical coupling between the first guides  12   a,b  and the second guides  16   a,b  can facilitate the application of biaxial stress load to a piece of the tissue  70  ( FIGS. 5-6 ) mounted on the tissue stretcher  10 . In some embodiments, the connection regions  24  are the point of origin of the respective actuation forces transmitted by the respective force actuators  32   a,b,c,d  and  34   a,b,c,d  to the respective first mounts  26   a,b  and second mounts  28   a,b.    
     In some embodiments, the first and second guides  12   a,b ,  16   a,b  include at least one polished surface and/or at least one passivated surface. The polished surface can, for example, reduce friction associated with the movement of the respective first and second mounts  26   a,b ,  28   a,b  along the respective first and second guides  12   a,b ,  16   a,b . The passivated surface can, for example, improve resistance of corrosion of the first and second guides  12   a,b ,  16   a,b  that could otherwise occur through exposure of the first and second guides  12   a,b ,  16   a,b  to saline and/or glutaraldehyde solutions. 
     The first guides  12   a,b  are substantially perpendicular to the second guides  16   a,b  in a plane substantially parallel to a substantially planar surface of the tissue  70  ( FIGS. 5-6 ) mechanically coupled to the first and second set of mounts  26   a,b ,  28   a,b . This substantially perpendicular orientation of the first guides  12   a,b  to the second guides  16   a,b  forms a cruciform frame  20  with an aperture  22  defined by at least a portion of the first and second guides  12   a,b  and  16   a,b . At least a portion of each of the first guides  12   a,b  and the second guides  16   a,b  extend away from the aperture. As described in further detail below, the first actuators  32   a,b,c,d  and the first set of mounts  26   a,b  are disposed on the portions of the first guides  12   a,b  extending away from the aperture  22 . Analogously, the second actuators  34   a,b,c,d  and the second set of mounts  28   a,b  are disposed on the portions of the second guides  16   a,b  extending away from the aperture  22 . As also described in further detail below, the aperture  22  is releasably engageable with at least a portion of the jig  50  such that the cruciform frame  20  remains fixed relative to the jig  50  to facilitate movement of the first and second set of mounts  26   a,b ,  28   a,b  as the respective stress loads are set. 
     Each of the first guides  12   a,b  and the second guides  16   a,b  can be a rod having an outer diameter greater than or equal to about 0.75 mm and less than or equal to about 13 mm. For example, each of the first guides  12   a,b  and the second guides  16   a,b  can be a stainless steel rod with an outer diameter of about 3.2 mm. The use of such a rod for the first guides  12   a,b  and the second guides  16   a,b  can facilitate sizing of the tissue stretcher  10  for manual manipulation by an operator while providing sufficient rigidity for the application of loads to the tissue to be modified. 
     A collar  36  can be disposed on one or more of the first guides  12   a,b  and the second guides  16   a,b . The collar  36  can be coupled (e.g., welded, interference fit) in a stationary position relative to one or more of the first guides  12   a,b  and the second guides  16   a,b  to limit the range of motion of the respective first and second set of mounts  26   a,b ,  28   a,b . For example, the collar  36  can be coupled to an end portion of the respective first and second guides  12   a,b ,  16   a,b.    
     Each of the first and second mounts  26   a,b ,  28   a,b  is substantially cylindrical (e.g., a right circular cylinder) with a substantially uniform cross-sectional area along the length of the cylinder. For example, each mount  26   a,b ,  28   a,b  can have an outer diameter of about 6 mm to about 25 mm (e.g., about 13 mm). Additionally or alternatively, each mount  26   a,b ,  28   a,b  can have a length of about 50 mm to about 150 mm (e.g., about 75 mm, about 125 mm). The length of each of the first mounts  26   a,b  is perpendicular to the first guides  12   a,b , and the length of each of the second mounts  28   a,b  is perpendicular to the second guides  16   a,b.    
     Each mount  26   a,b ,  28   a,b  includes a plurality of hooks  40  extending away from an outer surface of each respective mount  26   a,b ,  28   a,b . For example, each hook  40  can extend about 1.5 mm from the surface of each respective mount  26   a,b ,  28   a,b . This hook size can facilitate, for example, secure attachment to the tissue  70  ( FIGS. 5-6 ) while providing a low profile to reduce the likelihood of unintended snagging during handling. The hooks  40  can be substantially axially aligned with one another along the length of the respective first and second mounts  26   a,b ,  28   a,b . Additionally or alternatively, the plurality of hooks  40  can be substantially evenly spaced along the length of each respective mount  26   a,b ,  28   a,b . This axial alignment and substantially uniform spacing of the hooks  40  can result, for example, in substantially uniform application of stress loads to the tissue  70 . In some embodiments, each hook  40  formed from a cut mandrel. 
     Each of the first and second mounts  26   a,b ,  28   a,b  has mitered end portions  38  such that the respective mitered end portions  38  of each of the first mounts  26   a,b  is complementary to the mitered end portions  38  of each of the second mounts  28   a,b . These mitered end portions  38  can facilitate setting the biaxial loads to be applied by the tissue stretcher  10  by, for example, allowing the first mounts  26   a,b  and the second mounts  28   a,b  to be moved into close proximity with one another (e.g., by abutting the complementary mounts) during loading. 
     Additionally or alternatively, the length of each of the first mounts  26   a,b  can be greater than the length of each of second mounts  28   a,b . For example, each of the first mounts  26   a,b  can have a length of about 100 mm to about 150 mm (e.g., about 125 mm), and the second mounts  28   a,b  can have a length of about 50 mm to about 90 mm (e.g., about 75 mm). This relative difference in the lengths of the first and second mounts  26   a,b ,  28   a,b  can also facilitate setting the biaxial loads to be applied by the tissue stretcher  10  by, for example, allowing the first mounts  26   a,b  and the second mounts  28   a,b  to be moved into close proximity with one another during loading. 
     Each mount  26   a,b ,  28   a,b  includes at least one surface in slidable contact with the respective first and second guides  12   a,b ,  16   a,b  such that the first mounts  26   a,b  are slidable relative to one another along the first axis  14  and the second mounts  28   a,b  are slidable relative to one another along the second axis  18 . In some embodiments, the at least one surface of the respective mounts  26   a,b ,  28   a,b  in slidable contact with the respective first and second guides  12   a,b ,  16   a,b  is polytetrafluoroethylene (PTFE), which can result in low friction between the first and second mounts  26   a,b ,  28   a,b  and the respective first and second guides  12   a,b ,  16   a,b . Such low friction can be useful for the efficient and consistently uniform transmission of stress loads from the first and second actuators  32   a,b,c,d ,  34   a,b,c,d  to the respective mounts  26   a,b ,  28   a,b  and, thus, ultimately to the tissue  70  ( FIGS. 5-6 ) to be modified. 
     The first and second actuators  32   a,b,c,d ,  34   a,b,c,d  are each movable respectively along first and second guides  12   a,b ,  16   a,b . For example, each of the first and second actuators  32   a,b,c,d ,  34   a,b,c,d  can be moved toward the aperture  22  to a fixed position relative to the aperture  22 , with this fixed position corresponding to a set (e.g., predetermined) stress load to be applied to the tissue  70  ( FIGS. 5-6 ). As described in further detail below, the first and second actuators  32   a,b,c,d ,  34   a,b,c,d  can be released from this fixed position such that the respective set stress loads are applied to the tissue  70  mounted to the first and second mounts  26   a,b ,  28   a,b . In some embodiments, the relative position of the first and second actuators  32   a,b,c,d ,  34   a,b,c,d  with respect to the aperture  22  is proportional to the respective set stress load that will be applied by each actuator to the tissue  70  coupled to the first and second mounts  26   a,b ,  28   a,b.    
     In some embodiments, the first and second actuators  32   a,b,c,d ,  34   a,b,c,d  are substantially simultaneously movable along the respective first and second guides  12   a,b ,  16 , a,b  to transmit biaxial stress loads to the tissue  70  ( FIGS. 5-6 ). For example, each of the first and second actuators  32   a,b,c,d ,  34   a,b,c,d  can be a spring such that the stress load is set through compression of each spring relative to the equilibrium point of the spring. Each compressed spring can be allowed to expand toward the equilibrium position to move the respective first mounts  26   a,b  away from one another and/or move the respective second mounts  28   a,b  away from one another and, thus, apply respective stress loads to the tissue ( FIGS. 5-6 ). Each spring of the first and second actuators  32   a,b,c,d ,  34   a,b,c,d  can have a spring constant of about 5 N/m to about 50 N/m. Additionally or alternatively, springs of the first and second actuators  32   a,b,c,d ,  34   a,b,c,d  are each compressible from respective equilibrium positions by a distance of about 5 mm to about 35 mm. 
     In some embodiments, the first and second actuators  32   a,b,c,d ,  34   a,b,c,d  are springs having substantially similar spring constants such that the stress loads applied along the first axis  14  and the second axis  18  are substantially equal. In certain embodiments, the first actuators  32   a,b,c,d  have a spring constant greater than the spring constant of the second actuators  34   a,b,c,d  such that a larger stress loading can be applied along the first axis  14  than along the second axis  18 . 
     Referring now to  FIGS. 1, 3, and 4 , the jig  50  includes a support  54  extending from a substantially planar base  52 . The base  52  defines a plurality of locking apertures  58 . Locking members  56  (e.g., pins) are mechanically couplable to the base  52  (e.g., by interference fit with one or more of the locking apertures  58 ) and extend away from the base  52  on the same side of the base  52  as the support  54 . It should be appreciated that the locking members  56  can be disposed in any of the locking apertures  58 , as required to set a desired stress load by restricting axial movement of the first and second actuators  32   a,b,c,d ,  34   a,b,c,d . It should be further appreciated that moving the locking members  56  away from the tissue stretcher  10  (e.g., removing the mechanical coupling between the locking members  56  and the first and second actuators  32   a,b,c,d ,  34   a,b,c,d ) can allow the respective stress loads to be applied to tissue to be biaxially stretched. 
     The planar surface of the base  52  can be about as wide and as long as the overall length of the first and second guides  12   a,b ,  16   a,b  such that the first and second guides  12   a,b ,  16   a,b  can remain supported on the base  52  to provide stability while an operator mounts tissue on the first mounts  26   a,b  and the second mounts  28   a,b . In some embodiments, the base  52  is a substantially rigid material that can be easily cleaned and/or sterilized and resists corrosion over time. For example, the base  52  can be poly(methyl methacrylate) (PMMA). 
     The support  54  is mechanically couplable to the aperture  22  defined by the first and second guides  12   a,b ,  16   a,b  such that the first and second guides  12   a,b ,  16   a,b  are stationary relative to the base  52 . For example, the support  54  can be complementary to the aperture  22  of the tissue stretcher  10  such that the support  54  is movable through the aperture  22  to hold the tissue stretcher  10  fixed in place along the substantially planar surface of the base  52 . The support  54  can be dimensioned relative to the aperture  22  such that the support  54  holds the tissue stretcher  10  in place but is removable from the aperture  22  of the tissue stretcher  10  by the application of manual force by an operator. 
     Referring now to  FIGS. 4-6 , certain methods of biaxially stretching tissue using the tissue stretcher  10  and the jig  50  are described. The tissue stretcher  10  is mounted to the jig  50  by positioning the support  54  of the jig  50  through the aperture  22  defined by the tissue stretcher  10 . This positioning allows the tissue stretcher  10  to be positioned relative to the locking members  56  extending from the base  52 . Using gripping portions  30  of the respective first and second mounts  26   a,b ,  28   a,b , an operator can push each of the first and second mounts  26   a,b ,  28   a,b  toward one another along the respective first and second guides  12   a,b ,  16   a,b  such that the first and second mounts  26   a,b ,  28   a,b  are disposed within the area circumscribed by the locking members  56 . This has the effect of compressing the respective first and second actuators  32   a,b,c,d ,  34   a,b,c,d  such that respective first and second stress loads are set. 
     The tissue  70  is mounted over hooks  40  extending from the respective first and second mounts  26   a,b ,  28   a,b  such that a substantially planar surface of the tissue  70  is parallel to a plane along which the first and second actuators  32   a,b,c,d ,  34   a,b,c,d  move. The tissue  70  can be bovine pericardium, equine pericardium, or porcine pericardium. In some embodiments, the tissue  70  is a patch cut from a pericardial sac. In certain embodiments, the tissue  70  has an initial thickness of about 0.1 mm to about 0.7 mm. 
     With the tissue  70  mounted on the first and second mounts  26   a,b ,  28   a,b , the jig  50  can be moved relative to the tissue stretcher  10  such that the support  54  and the locking members  56  are decoupled from the tissue stretcher  10 . For example, the jig  50  can be moved relative to the tissue stretcher  10  such that the support  54  and the locking members are substantially simultaneously decoupled from the tissue stretcher. 
     The decoupling of the jig  50  from the tissue stretcher  10  allows the first actuators  32   a,b,c,d  to move the first mounts  26   a,b  away from one another along the first axis  14  to apply a first set stress load to the tissue  70 . Similarly, the decoupling of the jig  50  from the tissue stretcher  10  allows the second actuators  34   a,b,c,d  to move the second mounds  28   a,b  away from one another along the second axis  18  to apply a second set stress load to the tissue  70 . In some embodiments, the first mounts  26   a,b  and the second mounts  28   a,b  move away from each other substantially simultaneously such that the first and second set stress loads and, thus, first and second stresses are applied to the tissue  70  substantially simultaneously. Additionally or alternatively, the first and second set stress loads applied to the tissue  70  through the relative movement of the first and second mounts  26   a,b ,  28   a,b  such that the resulting stresses applied to the tissue  70  can be less than the elastic limit of the tissue  70 . In some embodiments, the first and second stress set stress loads is about 0.1 N to about 2 N. In certain embodiments, the first and second set stress applied to the tissue  70  is about 0.01 N/mm 2  to about 2 N/mm 2 . 
     In some embodiments, the first and second set stress loads are applied to the tissue  70  by the tissue stretcher  10  for about 30 minutes to about 120 minutes (e.g., in a glutaraldehyde solution). In certain embodiments, the tissue can be removed from the tissue stretcher  10  and exposed to a glutaraldehyde solution for about one day to about two weeks. Additionally or alternatively, the tissue  70  can be mounted on the tissue stretcher  10  and exposed to a non-cross linking-solution (e.g., phosphate-buffered saline or saline) for about 30 minutes to about 120 minutes prior to exposure to a glutaraldehyde solution for about one day to about two weeks. The exposure of the tissue  70  to the non-cross linking solution could be carried out between about 4° C. to about 37° C. (e.g., about 20° C.). The exposure of the tissue  70  to the non-cross-linking solution, while the tissue  70  is being biaxially stressed, can allow the tissue  70  to respond to the stress and reorient prior to locking that structure in place with a cross-linking solution such as glutaraldehyde. 
     As the tissue stretcher  10  acts on the tissue  70  to apply the first and second set stress loads, the tissue stretcher  10  and the tissue  70  can be exposed to a glutaraldehyde solution  80 . This exposure can range in duration from about 10 minutes to about 3 hours (e.g., about 30 minutes to about 120 minutes). Such exposure of the tissue  70  to the glutaraldehyde solution  80  can facilitate crosslinking of the tissue  70  such that the tissue  70  will hold the stretched position after the first and second stress loads are removed (e.g., after the tissue  70  is removed from the tissue stretcher  10 ). In some embodiments, the tissue  70  is held in the stretched position such that the average thickness of the tissue  70  is reduced. For example, the average thickness of the tissue  70  held in the stretched position can be about 0.1 mm to about 0.4 mm. In some implementations, biaxial stretching of tissue and fixing the tissue results in little to no increase in thickness in the tissue. In these embodiments, as compared to fixing tissue under uniaxial stress loading or no stress loading, biaxial stretching can result in thinner fixed tissue. In implementations in which the tissue  70  is part of an intraluminally delivered prosthetic heart valve, such a reduction in the thickness of the tissue  70  can, for example, reduce the sheathing forces associated with the prosthetic heart valve and/or reduce the overall profile of the prosthetic heart valve for easier delivery to the implantation site. 
     While certain embodiments have been described, other embodiments are possible. 
     For example, while the first and second mounts  26   a,b  and  28   a,b  have been described as being movable relative to the respective first and second guides  12   a,b ,  16   a,b  through low-friction slidable contact, other embodiments are additionally or alternatively possible. In some embodiments, referring to  FIG. 7 , a mount  26   a ′ includes bearings  42   a,b  that can form at least a portion of the moving interface between the mount  26   a ′ and the first guides  12   a,b . For example, the bearings  42   a,b  can be polyoxymethylene or polytetrafluoroethylene such that the bearings  42   a,b  are grease-free to reduce the likelihood of contamination of the tissue  70 . Additionally or alternatively, each bearing  42   a,b  can be a flange bearing to facilitate placement of the bearings  42   a,b  on the mount  26   a′.    
     As another example, while the thickness of the tissue  70  has been described as being reduced through the application of set biaxial stress loads, other embodiments are additionally or alternatively possible. In some embodiments, referring to  FIGS. 8-9 , a tissue modification system  90  includes a downdraft table  98 , clamps  92 , and a planar shaver  94 . The clamps  92  can secure the tissue  70  (e.g., a substantially planar patch of pericardial tissue) to the downdraft table  98  such that a relatively smooth substantially planar surface of the tissue  70  is disposed toward the table  98  while a relatively rough substantially planar surface of the tissue  70  is disposed toward the planar shaver  94 . Vacuum pressure is applied through vents  100  defined by the downdraft table  98 . The vacuum pressure draws the tissue  70  toward the substantially planar surface of the table  98  while the planar shaver  94  moves over the tissue  70 , for example, in the direction  95  indicated in  FIG. 8  such that a blade  96  of the planar shaver  94  removes tissue along at least a portion of the relatively rough substantially planar surface of the tissue  70 . Such movement of the planar shaver  94  over the tissue  70  can result in a global reduction of the thickness of the tissue  70 . Saline solution can be applied to the tissue  70  to keep the tissue  70  moist throughout the process of mounting the tissue  70  to the table  98  and moving the planar shaver  94  over the tissue  70 . 
     In certain embodiments, referring to  FIG. 10 , a tissue modification system  102  includes a laser  112  (e.g., a femtosecond laser) proximate to the table  98  to direct a laser beam across at least a portion of a leaflet  104  clamped by clamps  92  to the table  98 . For example, the leaflet  104  can be a leaflet of a prosthetic heart valve, and the leaflet can include a coaptation portion  106  substantially opposite an arcuate edge  105  having a first end  107  and a second end  109 . The leaflet  104  can also include a belly  108  extending from the arcuate edge  105  to an axis  110  defined by the first and second ends  107 ,  109  of the arcuate edge  105 . 
     The laser  112  can, for example, direct the laser beam across the leaflet  104  to remove tissue locally along a portion of the leaflet  104  to achieve a thickness profile in which portions of the leaflet  104  may be thinner than portions of the leaflet  104 . For example, the laser  112  can direct the laser beam across the leaflet  104  to remove tissue from the belly  108  of the leaflet  104 . Such local removal of tissue from the leaflet  104  can, for example, reduce the forces associated with sheathing a prosthetic heart valve including the leaflet  104  as compared to a leaflet without material removed from its respective belly. 
     As still another example, while tissue stretching and tissue removal have been described for reducing the forces associated with sheathing a prosthetic heart valve including one or more tissue leaflets, other embodiments are additionally or alternatively possible. For example,  FIGS. 11A-11D  illustrate a prosthetic heart valve  112  that includes tissue  114  (e.g., one or more leaflets movable between open and closed positions to permit and restrict, respectively, the flow of blood through the heart) and a sheath  120  that can be used for intraluminal delivery of the prosthetic heart valve  112 . The prosthetic heart valve  112  can be stored initially in a first solution  116  that is isotonic with the tissue  114 . For example, the prosthetic heart valve  112  can be shipped in the first solution  116 . The prosthetic heart valve  112  can be exposed to a second solution  118  (e.g, in the operating room just prior to implantation of the prosthetic heart valve  112 ) such that water in the tissue  114  flows out of the tissue  114  to reduce at least one dimension of the tissue  114 . 
     For example, the second solution  118  can be a solution hypertonic to the tissue  114 . In some embodiments, the second solution  118  is about 75 percent to about 80 percent water. Additionally or alternatively, the tonicity (e.g., hypertonicity) of the second solution  118  is about 339 mOsm/L (110% of isotonic, ˜10 g/L) to about the limit of solubility of NaCl in water, 12.3 Osm/L (40× isotonic, 359 g/L). 
     Additionally or alternatively, the second solution  118  can include alcohol. For example, the exposure of the tissue  114  to the second solution  118  can include exposure of the tissue  114  to a series of graded alcohol solutions (e.g., about 50%, about 60%, about 70%, about 90%, about 95%, about 100%) to remove water and thin the tissue  114  prior to loading the tissue. Additionally or alternatively, exposure of the tissue  114  to the second solution  118  can include exposures of the tissue  114  to a series of graded ethylene glycol solutions (e.g., about 50%, about 60%, about 70%, about 90%, about 95%, about 100%) to remove water and thin the tissue  114  prior to loading. In some embodiments, the tissue  114  is exposed to alcohol or ethylene glycol solutions prior to stress loading the tissue  114 . 
     In some embodiments, the tissue  114  can be exposed to the second solution  118  as an acute rinse (e.g., for about 30 seconds to about 15 minutes). In certain embodiments, the tissue  114  can be stored in the second solution  118  to eliminate, for example, the need for a rinse. The first solution  116  and the second solution  118  can be at substantially the same temperature (e.g., room temperature) which, as compared to techniques that require exposing tissue of a prosthetic heart valve to solutions at different temperatures, can reduce the need for the end-user to maintain a controlled difference in temperature to achieve a desired reduction in the size of tissue of a prosthetic heart valve. 
     In some embodiments, removal of water from the tissue  114  through exposure to the second solution  118  is done immediately prior to sheathing the valve  112 . In certain embodiments, removal of water from the tissue  114  through exposure to the second solution  118  is done some time prior to sheathing the valve  112 . For example, the tissue  114  can be exposed to the second solution  118  during the final manufacturing of the valve  112  that includes the tissue  114 . Through exposure of the tissue  114  to the second solution  118  during the final manufacturing of the valve  112 , the tissue  114  can, for example, undergo terminal sterilization in a dehydrated state. As compared to tissue  114  in a hydrated state, the tissue  114  in a dehydrated state can better withstand exposure to forms of terminal sterilization that tend to damage and/or alter tissue  114  in a hydrated state. For example, terminal sterilization of the tissue  114  in the dehydrated state can include exposure of the dehydrated tissue to electron beam (e-beam) and/or ethylene oxide (EtO). Additionally or alternatively, with the tissue  114  in the dehydrated state, the valve  112  could be sheathed or unsheathed during sterilization and storage, and the entire assembly (including the valve  112 , a delivery system, and a handle) could be sterilized in one package, without the need for storing the valve in liquid. 
     With the size of the tissue  114  reduced through exposure to the second solution  118 , the sheath  120  can be advanced distally to sheath the prosthetic heart valve  112  for intraluminal delivery to an implantation site in a patient. As compared to tissue that has not been exposed to the second solution  118 , the force required to sheath the tissue  114  will be reduced. Upon implantation and exposure to blood in the patient, the tissue  114  will absorb water to return to its original size. 
     A number of embodiments of the invention have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the invention. For example, the first and second guides can each be a single rod or three or more rods. Accordingly, other embodiments are within the scope of the following claims.