Title of Invention: Remodelable Prosthetic Valve Abstract: Prosthetic medical devices with delayed valve function and in situ remodeling of a portion thereof are provided, as well as methods of manufacturing the medical devices and methods of treatment using the medical devices. The medical devices may be adapted to initially maintain a remodelable material within a body vessel in a first configuration suitable to contact the remodelable material with a body fluid contacting the remodelable material for a first time period or a second configuration adapted to regulate the flow of body fluid contacting the remodelable material. The medical devices may include a releasable holding member configured to retain the remodelable material in the first configuration. Detailed Description: RELATED APPLICATIONS This application claims the benefit under 35 U.S.C. § 119(e) of U.S. Provisional Application Ser. No. 60/988,981, filed Nov. 19, 2007. The co-pending U.S. patent application Ser. No. 11/616,159, entitled “Implantable Graft Material,” filed Dec. 26, 2006 by Johnson et al., is incorporated herein by reference in its entirety. TECHNICAL FIELD The present invention relates to methods of treatment and medical devices for implantation in a body vessel. More particularly, preferred embodiments of the present invention relate to implantable medical devices including a remodelable material moveable from a first configuration to permit remodeling of the remodelable material to a second configuration functioning as a valve within the body vessel. BACKGROUND Many vessels in animals transport fluids from one body location to another. Frequently, fluid flows in a substantially unidirectional manner along the length of the vessel. Native valves within the heart and veins function to regulate blood flow within the body. Heart valves positioned within the heart direct the flow of blood to and from other organs and pump oxygenated blood to the rest of the body. Venous valves are typically bicuspid valves positioned at varying intervals within veins to permit substantially unidirectional blood to flow toward the heart. Body vessels such as veins transport blood to the heart and arteries carry blood away from the heart. Occasionally, congenital defects or injury to valves within a body vessel can result in an undesirable amount of retrograde fluid flow across a valve therein, and compromise the unidirectional flow of fluid across the valve. Various implantable medical devices are advantageously inserted within various portions of the body. Minimally invasive techniques and instruments for placement of intraluminal medical devices have been developed to treat and repair undesirable conditions within body vessels, including treatment of conditions that affect blood flow such as venous valve insufficiency. Various percutaneous methods of implanting medical devices within the body using intraluminal transcatheter delivery systems can be used to treat a variety of conditions. One or more intraluminal medical devices can be introduced to a point of treatment within a body vessel using a delivery catheter device passed through the vasculature communicating between a remote introductory location and the implantation site, and released from the delivery catheter device at the point of treatment within the body vessel. Intraluminal medical devices can be deployed in a body vessel at a point of treatment and the delivery device subsequently withdrawn from the vessel, while the medical device retained within the vessel to provide sustained improvement in valve function or to increase vessel patency. For example, an implanted medical device can improve the function of native valves by blocking or reducing retrograde fluid flow. Alternatively, a prosthetic valve can be implanted to replace the function of damaged or absent native valves within the body. One challenge for development of a prosthetic valve with the venous system is mitigating thrombus formation that can occlude the vessel and/or lead to loss of functionality of the valve structures that regulate blood flow. In contrast to the arterial system, the lower flow rates in some body vessels such as the deep veins of the legs and feet can lead to stagnation of blood in the pockets about the bases of the leaflets or valve structure due to the inability of the blood to be flushed and refreshed thereabout. The pockets can fill with thrombus that compromises the ability the leaflets or valve structure to open and close in response to antegrade and retrograde flow (i.e., pressure differentials across the valve). For example, fibrinogen absorbed on to the surface of an implanted prosthetic valve can form a layer that triggers the biochemical pathway leading fibrin deposition, platelet aggregation, and thrombus formation. Remodelable materials, such as extracellular matrices (ECM), can be used to provide a thrombo-resistant surface in an implantable prosthetic valve. Prosthetic valves desirably include valve leaflets formed from a remodelable material such that, upon implantation, the remodelable material can be used to form a permanently non-thrombogenic leaflet surface. Small intestinal submucosa (SIS) is a commercially available ECM material (Cook Biotech Inc., West Lafayette, Ind.) derived from a porcine source and processed to retain remodelability. When implanted in some vessels such as veins, however, remodelable material is often subjected to intermittent fluid flow with intervals of blood stagnation. Changes in the flow rate, flow direction or fluid pressure of intraluminal fluid across an implanted remodelable material have the potential to disrupt or slow the remodeling process. The intraluminal fluid flow can be characterized by parameters such as pressure, direction, composition and flow rate across the interface. Intraluminal fluid flow in a vascular environment is subject to regular modulations in pressure and fluid flow due to respiratory and calf muscle function. The remodeling process itself may be linked to the flow of fluid across the remodelable surface. Recent investigations have shown that SIS-based remodeling of implanted medical devices can occur by recruitment of cells directly from intraluminal circulation. See Brountzos, et al, “Remodeling of suspended small intestinal submucosa venous valve: an experimental study in sheep to assess the host cells' origin,” J. Vasc. Interv. Radiol., 14(3), 349-356 (March 2003). While the ability of valve leaflets made of ECM materials to remodel has been demonstrated clinically, the surface of the newly-implanted ECM materials such as SIS can be vulnerable to thrombus formation, particularly in sinus regions near valves. Because remodeling is a process that can take days, weeks or longer, depending on the environment, thrombogenicity has remained a clinical issue to be addressed when using remodelable biomaterials. In particular, implantable valves having valve leaflets comprising a remodelable material may thicken or undergo thrombotic deposition during the remodeling period within the body vessel. This may be the result of progressive fibrin deposition due to hemodynamics of blood flow contacting a portion of the valve leaflet near the wall of the body vessel. For example, blood may stagnate in the sinus defined by a valve leaflet and an adjacent portion of the body vessel wall. As a result, during remodeling of the valve leaflet after implantation, the thickness of the valve leaflet may increase on the upstream side of the valve leaflet upon endothelialization and the downstream side of the leaflet may sustain fibrin deposition and progressive thrombogenic deposition without significant endothelialization. The resulting thickening of the valve leaflet and/or fibrotic deposition thereon may reduce or compromise valve function of the remodeled valve leaflet within a period of about 3-6 weeks. What is needed are prosthetic valves that provide for remodeling of at least a portion of the valve while reducing exposure of the remodelable material to conditions that may undesirably permit or promote the deposition of thrombogenic material on the remodelable material or thickening of the valve leaflet during remodeling. A prosthetic valve configured to permit contact of a remodelable valve leaflet with blood or tissue that promotes remodeling for a time period effective for remodeling prior to configuring the valve leaflet as an operable configuration to regulate fluid flow would be particularly desirable. SUMMARY Prosthetic medical devices may be configured to maintain a remodelable material within a body vessel in at least two different configurations: a first configuration suitable to contact the remodelable material with a body fluid for a first time period and a second configuration adapted to regulate the direction of body fluid contacting the remodelable material. Preferably, the body fluid flow is blood flow across the remodelable material within a body vessel, such as a venous valve or a heart valve. In the first configuration, the remodelable material is preferably in contact with the fluid flow while being positioned in a manner permitting blood flow to remain in moving contact with the remodelable material. For example, the remodelable material in the first configuration may be oriented as a substantially flat sheet substantially parallel to the longitudinal axis of a body vessel, preferably without contacting the wall of the body vessel, to permit blood flow to pass around and contact the remodelable material. The medical device may be configured and adapted to maintain the remodelable material in the first configuration for a period of time effective to remodel (i.e., a remodeling-effective period of time) at least a portion of the remodelable material by contact with the body fluid. For example, the time period may be several days, weeks or months (e.g., 14-90 days). In moving from the first configuration to a second configuration, the remodelable material may change shape, configuration or orientation within the body vessel to extend across at least a portion of the body vessel and provide a valve function to regulate fluid flow within the body vessel in a substantially unidirectional manner while in the second configuration. The remodelable material in the second configuration may be configured as one or more valve leaflets forming a heart valve or venous valve. In a first embodiment, an implantable prosthetic valve assembly may include at least one valve leaflet comprising a remodelable material attached to a support frame and a releasable holding member configured to restrain the valve leaflet in the first configuration for a desired period of time. The period of time is preferably effective to provide a desired degree of remodeling within the valve leaflet by contact with a body tissue or fluid, such as blood. The support frame may have any suitable structure. For example, a support frame may define a lumen extending from a proximal end to a distal end along a longitudinal axis and be configured for implantation within a body vessel. The valve leaflet may be attached to the support frame and oriented in a first configuration within the lumen permitting a bidirectional fluid flow through the lumen in a first direction along the longitudinal axis and a second direction opposite the first direction Preferably, the valve leaflet is moveable from the first configuration to a second configuration permitting fluid flow through the lumen in the first direction while substantially limiting fluid flow through the lumen in the direction opposite the first direction. The release member may simultaneously prevent the valve leaflet from moving to the second configuration. The valve leaflet is preferably moveable to the second configuration within the lumen upon release of the leaflet holding member The releasable holding member may include a bioabsorbable polymer selected to dissipate within a body vessel within the period of time for a desired extent of remodeling to occur within the valve leaflet in contact with a body fluid, such as 14 to 90 days. The valve leaflet may be maintained in the first configuration inside a body vessel or other body cavity that is the same or different from the location of the valve leaflet in the second configuration. In one aspect, the valve leaflet may be implanted within a body cavity in a first configuration for a period of time to permit a desired degree of remodeling of the remodelable material therein, subsequently explanted and placed at a second point of treatment within the body of the same or a different subject. The valve leaflet may be moved from the first configuration to a second configuration prior to or after placement at the second point of treatment. The second configuration is preferably adapted to provide a valving function within the body vessel. More preferably, the prosthetic valve assembly is placed at a single point of treatment within a blood vessel in the first configuration. Upon release of the valve leaflet holding member, the valve leaflet may change from the first configuration that does not regulate fluid to the second configuration providing a substantially unidirectional fluid regulating function within the body vessel. For example, a valve leaflet in the second configuration may extend from a base portion contacting a portion of the support frame to a free edge portion distal to the base portion. The free edge may be flexibly moveable within the lumen to define at least a portion of a valve orifice moveable therein to permit the fluid flow through the lumen in the first direction while substantially limiting fluid flow through the lumen in the direction opposite the first direction. The valve leaflets in the first configuration may be oriented in a manner permitting fluid flow across the surface of the remodelable material without providing stagnation of fluid contacting the remodelable material. The prosthetic valve assembly is preferably adapted to permit one or more valve leaflets to change orientation from the first configuration to a second configuration upon contact with a body fluid or tissue for a desired period of time, preferably a remodeling-effective period of time. For example, release of the leaflet holding member may result in the change in orientation of one or more valve leaflets from the first configuration to the second configuration. In the second configuration, the valve leaflets are preferably oriented to regulate fluid flow and may contact a portion of a body vessel wall. In the second configuration, static fluid may contact portions of the valve leaflets during periods of retrograde fluid flow, as fluid collects between the closed valve leaflets and the wall of the body vessel. Preferably, the support frame comprises a self-expanding material, such as a nickel titanium alloy, spring metals, stainless steel and shape memory polymers, configured to urge the valve leaflet from the first configuration toward the second configuration in the absence of the releasable leaflet holding member. A valve leaflet in the first configuration may be a sheet substantially parallel to the longitudinal axis of the body vessel or support frame. For instance, the valve leaflet in the first configuration may be configured as a substantially flat sheet contained within the lumen defined by the support frame. The valve leaflet(s) can provide a valve function to the implantable medical device when the valve leaflet(s) are in the second configuration, with one or more edges of the leaflet(s) preferably defining at least a portion of the valve orifice of a prosthetic valve. A valve leaflet can provide a valve function by moving between the first and second configurations relative to one another. When the valve orifice is in an open position, the leaflet substantially blocks the lumen of the vessel; in the closed position, the lumen is substantially blocked. Thus, the leaflet(s) permits fluid to flow through the vessel in a first direction when in the first (open) configuration, and substantially prevents fluid flow through the vessel in a second, opposite direction when in the second (closed) configuration. The valve leaflet(s) can be formed of a remodelable material, such as small intestine submucosa (SIS) or other extracellular matrix (ECM) material. The valve leaflet(s) may also be formed from a non-remodelable material having a remodelable material or remodeling encouraging material attached to the non-remodelable material. The valve leaflet(s) may be attached to a support frame in any suitable manner. The one or more leaflets are optionally attached to a support structure that is moveable from a radially compressed to a radially expanded configuration. The support frame can comprise any suitable support frame, including self-expandable, mechanically expandable, wire, tube, metal, polymeric, composite and other types of support frames. In a second embodiment, methods of treatment may include the steps of implanting the medical devices according to the first embodiment are provided. The medical devices are preferably adapted for transcatheter percutaneous delivery in a body vessel, and can radially expand at a point of treatment. The medical devices can have any suitable configuration, but preferably include one or more valve leaflets attached to the support frame in the first configuration. In a third embodiment, methods of manufacturing the implantable medical devices of the first embodiment are provided. The methods may include the step of attaching a valve leaflet including a remodelable material to a support frame extending along a longitudinal axis and being configured for implantation within a body vessel. The valve leaflet may be attached to the support frame oriented in a first configuration permitting a bidirectional longitudinal fluid flow past the leaflet along the longitudinal axis of the support frame, the medical device and/or a body vessel in both a first direction and a second direction opposite the first direction. In addition, the valve leaflet is preferably moveable from the first configuration to a second configuration permitting fluid flow along the longitudinal axis of the medical device in the first direction while substantially limiting fluid flow through the lumen in the direction opposite the first direction. The methods may also include the step of attaching to the support frame a releasable holding member for restraining the valve leaflet in the first configuration. Preferably, the releasable holding member simultaneously prevents the valve leaflet from moving to the second configuration. The valve leaflet is preferably moveable to the second configuration within the lumen upon release of the holding member. The invention includes other embodiments within the scope of the claims, and variations of all embodiments, and is limited only by the claims made by the Applicants. Additional understanding of the invention can be obtained by referencing the detailed description of embodiments of the invention, below, and the appended drawings. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1A is a perspective view of a first prosthetic valve assembly having two leaflets in a first configuration. FIG. 1B is cross-sectional view of the native valve along line A-A′ shown in FIG. 1A. FIG. 2A is a perspective view of the first prosthetic valve assembly of FIG. 1A having two leaflets in a second configuration. FIG. 2B is cross-sectional view of the first prosthetic valve assembly along line B-B′ shown in FIG. 2A. FIG. 3A is side view of a second prosthetic valve assembly positioned within a body vessel with a pair of valve leaflets in a planar configuration. FIG. 3B is side view of the second prosthetic valve assembly shown in FIG. 3A positioned within the body vessel with the pair of valve leaflets in a second configuration and oriented to prevent retrograde fluid flow through the body vessel. FIG. 3C is side view of the second prosthetic valve assembly shown in FIG. 3B positioned within the body vessel with the pair of valve leaflets in the tilted second configuration and oriented to permit antegrade fluid flow through the body vessel. DETAILED DESCRIPTION The following detailed description and appended drawings describe and illustrate various exemplary embodiments of the invention. The description and drawings serve to enable one skilled in the art to make and use the invention. DEFINITIONS As used herein, the term “implantable” refers to an ability of a medical device to be positioned at a location within a body, such as within a body vessel. Furthermore, the terms “implantation” and “implanted” refer to the positioning of a medical device at a location within a body, such as within a body vessel. The term “biocompatible” refers to a material that is substantially non-toxic in the in vivo environment of its intended use, and that is not substantially rejected by the patient's physiological system (i.e., is non-antigenic). This can be gauged by the ability of a material to pass the biocompatibility tests set forth in International Standards Organization (ISO) Standard No. 10993 and/or the U.S. Pharmacopeia (USP) 23 and/or the U.S. Food and Drug Administration (FDA) blue book memorandum No. G95-1, entitled “Use of International Standard ISO-10993, Biological Evaluation of Medical Devices Part-1: Evaluation and Testing.” Typically, these tests measure a material's toxicity, infectivity, pyrogenicity, irritation potential, reactivity, hemolytic activity, carcinogenicity and/or immunogenicity. A biocompatible structure or material, when introduced into a majority of patients, will not cause an undesirably adverse, long-lived or escalating biological reaction or response, and is distinguished from a mild, transient inflammation which typically accompanies surgery or implantation of foreign objects into a living organism. A large number of different types of materials are known in the art which may be inserted within the body and later dissipate. The term “bioabsorbable” is used herein to refer to materials selected to dissipate upon implantation within a body, independent of which mechanisms by which dissipation can occur, such as dissolution, degradation, absorption and excretion. The terms “bioabsorbable,” “absorbable,” or “biodegradable” are used synonymously herein, unless otherwise specified, to refer to the ability of the material or its degradation products to be removed by biological events, such as by fluid transport away from the site of implantation or by cellular activity (e.g., phagocytosis). Only the term “bioabsorbable” will be used in the following description to encompass absorbable, absorbable, bioabsorbable, and biodegradable, without implying the exclusion of the other classes of materials. “Non-bioabsorbable” material refers to a material, such as a polymer or copolymer, which remains in the body without substantial bioabsorption. The terms “remodelable” or “bioremodelable” as used herein refer to the ability of a material to allow or induce host tissue growth, proliferation or regeneration following implantation of the tissue in vivo. Remodeling can occur in various microenvironments within a body, including without limitation soft tissue, a sphincter muscle region, body wall, tendon, ligament, bone and cardiovascular tissues. Upon implantation of a remodelable material, cellular infiltration and neovascularization are typically observed over a period of about 5 days to about 6 months or longer, as the remodelable material acts as a matrix for the ingrowth of adjacent tissue with site-specific structural and functional properties. The remodeling phenomenon which occurs in mammals following implantation of submucosal tissue includes rapid neovascularization and early mononuclear cell accumulation. Mesenchymal and epithelial cell proliferation and differentiation are typically observed by one week after in vivo implantation and extensive deposition of new extracellular matrix occurs almost immediately. The term “non-remodelable” refers to a material that is not a remodelable material that is not selected or configured to promote or induce tissue growth upon contacting living tissue. A non-remodelable material preferably does not contain biological molecules (such as growth factors) that promote tissue ingrowth, angiogenesis, and other growth processes within the material. Non-remodelable materials may include biostable or bioabsorbable polymers, as well as forms of collagen or other biomolecules configured or treated to slow tissue ingrowth. For example, a cross-linked extracellular matrix material configured and treated to substantially retard or prevent tissue ingrowth can also be used as a non-remodelable material. Non-remodelable materials may also be provided with materials such as ligands or ECM attached to the nonremodelable material to promote remodeling. As used herein, the term “body vessel” means any body passage lumen that conducts fluid, including but not limited to blood vessels, esophageal, intestinal, billiary, urethral and uretheral passages. The term “alloy” refers to a substance composed of two or more metals or of a metal and a nonmetal intimately united, for example by chemical or physical interaction. Alloys can be formed by various methods, including being fused together and dissolving in each other when molten, although molten processing is not a requirement for a material to be within the scope of the term “alloy.” As understood in the art, an alloy will typically have physical or chemical properties that are different from its components. The term “mixture” refers to a combination of two or more substances in which each substance retains its own chemical identity and properties. The medical devices of the embodiments described herein may be oriented in any suitable absolute orientation with respect to a body vessel. The recitation of a “first” direction is provided as an example. Any suitable orientation or direction may correspond to a “first” direction. The medical devices of the embodiments described herein may be oriented in any suitable absolute orientation with respect to a body vessel. For example, the first direction can be a radial direction in some embodiments. The terms “frame” and “support frame” are used interchangeably herein to refer to a structure that can be implanted, or adapted for implantation, within the lumen of a body vessel. As used herein, a “support frame” is any structure that is attached to the remodelable material, for example to hold a remodelable leaflet in place within a body vessel, including an interior portion of a blood vessel, lymph vessel, ureter, bile duct or portion of the alimentary canal. A “valve support frame,” as used herein, refers to a support frame that forms a portion of a valve means for modifying fluid flow within a body vessel. The valve support frame can have any suitable configuration, but is preferably a radially expandable structure comprising a plurality of struts and bends and enclosing an interior lumen. Preferably, one or more valve leaflets can be attached to the valve support frame. As used herein, “retrograde flow across a valve” refers to fluid flow in a direction other than the primary (antegrade) direction of fluid flow when the valve is open and functioning correctly. Retrograde flow typically proceeds in a direction opposite the direction of fluid through the open valve. Retrograde flow can occur when the valve is in the open or closed position, through a valve orifice or through other apertures in a valve surface. For example, for a valve in a vein, retrograde flow proceeds in the direction away from the heart. For a heart valve, retrograde flow can occur when the valve is in a closed position and can lead to various medical complications. As used herein, “valve orifice” refers to an opening in a valve moveable between an open position permitting fluid flow through the valve orifice, and a closed position that substantially prevents fluid flow through the valve orifice. The valve orifice can be defined by the opposably positioned edges of one or more valve leaflets. The valve orifice can be defined by any suitable number of valve leaflets. Preferably, the valve orifice can be defined by two or three valve leaflets to form a bicuspid or tricuspid valve, respectively. As used herein, “retrograde valve aperture” refers to an opening in the surface of the valve other than the valve orifice that permits retrograde fluid flow therethrough. Apertures can include holes placed in previously implanted prosthetic valve leaflets to promote remodeling of an extracellular matrix leaflet material. Implantable Medical Devices In a first embodiment, prosthetic medical devices are provided. Preferably, the medical devices are adapted to maintain a remodelable material in at least two configurations. The material itself may be remodelable (such as an extracellular matrix material) or may be formed from a non-remodelable material (such as a polymeric material) provided with a material such as a ligand or extracellular matrix material, attached to the nonremodelable material to promote remodeling. The medical device may be configured to reorient or reposition the remodelable material from a first configuration permitting remodeling of the remodelable material within a body vessel, to a second configuration permitting the remodelable material to function as a valve leaflet operable to regulate fluid flow through the body vessel. Preferably, the medical device is configured to move the remodelable material from the first configuration to the second configuration without intervention after a period of time suitable for remodeling of at least a portion of the remodelable material. The medical device may include a releasable holding member for maintaining the remodelable material in the first configuration and may be configured to move the remodelable material to the second configuration upon release of the holding member. The medical devices may be adapted for implantation within a blood vessel and permit contact between a remodelable material attached to a support frame and blood within the body vessel. In the first configuration, the remodelable material may be a sheet of extracellular matrix material or a sheet of nonremodelable material having a remodelable material attached thereto maintained within a body vessel in contact with dynamic body fluid flow, substantially without contact between the remodelable material and static body fluid. Also preferably, the prosthetic medical device is configured to initially maintain the remodelable material without contacting the body vessel upon implantation. FIG. 1A is a perspective view of a prosthetic valve assembly having two leaflets in a first configuration. A prosthetic valve assembly 100 includes a support frame 114, a first valve leaflet 110 and a second valve leaflet 112. The support frame 114 defines a lumen 116 extending from a proximal end 102 to a distal end 104 along a longitudinal axis 101. The support frame 114 can have any suitable configuration configured for implantation within a body vessel. For example, the support frame 114 may be formed from a biocompatible metal or alloy configured as a plurality of interconnected struts and bends. A series of sinusoidal hoop members may be longitudinally spaced apart, axially aligned along the longitudinal axis 101 and interconnected by a plurality of connecting members to form the support frame 114, shown as a cylinder in FIG. 1A. The prosthetic valve assembly 100 is shown in a first configuration in FIG. 1A, having the two valve leaflets 110, 112 oriented substantially parallel to one another and symmetrically stacked on either side of the longitudinal axis 101 within the lumen 116. FIG. 1B is cross-sectional view of the prosthetic valve along line A-A′ shown in FIG. 1A, showing the first valve leaflet 110, the second valve leaflet 112, the lumen 116 and the support frame 114. The first valve leaflet 110 and the second valve leaflet 112 in the first configuration may be oriented to permit fluid to flow through the lumen in contact with the both sides of the valve leaflets 110, 112 without reducing the rate of fluid flow in the first direction 106 relative to fluid flow in the second direction 108, thereby permitting bidirectional flow through the lumen 116. Preferably, each valve leaflet includes a remodelable material and is maintained in contact with blood while the prosthetic valve assembly 100 is in the first configuration. A remodelable material can undergo biological processes such as angiogenesis when placed in communication with a living tissue, such that the remodelable material is biologically transformed into material that is substantially similar to said living tissue in cellular composition. Unless otherwise specified herein, a “remodelable material” can include a single layer material, or multiple layers of one or more materials that together undergo remodeling when placed in communication with living tissue. The remodelable material can provide an acellular scaffold or matrix that can be populated by cells. One example of a remodeling process is the migration of cells into the remodelable material. Migration of cells into the remodelable material can occur in various ways, including contact with blood. Preferably, a remodelable valve leaflet is placed within the lumen 116 of the support frame 114 in the first configuration shown in FIGS. 1A and 1B without contacting the wall of a body vessel upon implantation. During blood contact with the remodelable material, recruitment of cells from tissue at a remote location may occur when blood contacts the remodelable material, permitting in situ remodeling of the valve leaflet upon implantation. See, e.g., Brountzos, et al, “Remodeling of suspended small intestinal submucosa venous valve: an experimental study in sheep to assess the host cells' origin,” J. Vasc. Interv. Radiol., 14(3), 349-356 (March 2003), this disclosure of which pertaining to the formation of implantable prosthetic valves and the implantation of the valves within a body vessel is incorporated herein by reference. The migration of cells into the remodelable material can impart improved function to the remodelable material and reduce incidence of thrombosis. In some embodiments, the remodelable material itself can be absorbed by biological processes. Non-limiting examples of remodelable materials, their preparation and use are also discussed herein. Preferably, a remodelable valve leaflet material remains in the first configuration within a body vessel while undergoing a desired degree of remodeling upon contact for about 90 days or less with living tissue of the type present at an intended site of implantation, such as the interior of a body vessel. The valve leaflets 110, 112 of the prosthetic valve assembly 100 are moveable to a second configuration shown in FIGS. 2A and 2B. FIG. 2B is an end view of the prosthetic valve assembly 100 shown in FIG. 2A along line B-B′ showing the prosthetic valve assembly 100 in a second, closed configuration. The prosthetic valve assembly 100′ (in the second configuration) may have one or more valve leaflets 110, 112 shaped and oriented in any suitable manner. However, while the valve leaflets 110, 112 in the first prosthetic valve assembly configuration are preferably shaped and oriented to permit fluid to flow through the lumen 116 in both a first direction 106 and a second direction 108 opposite the first direction 106, the valve leaflets 110, 112 in the second configuration are preferably shaped and oriented to permit fluid flow in only the second direction 108 (while blocking or reducing the rate of fluid flow in the first direction 106). The first direction 106 and the second direction 108 may be oriented along the longitudinal axis 101. Compared to the first configuration shown in FIGS. 1A and 1B, the valve leaflets 110, 112 in the second configuration 100′ are re-oriented relative to one another and/or relative to the support frame 114 so as to provide a valve function. Preferably, the second configuration 100′ is adapted to permit fluid flow through the lumen 116 in the first direction 106 while substantially limiting fluid flow through the lumen in the second direction 108 opposite the first direction 106. For example, the proximal edges of the first valve leaflet 110 and the second valve leaflet 112 near the proximal end 102 of the support frame 114 are tilted toward one another in the second configuration to form a valve orifice 124. The distal edges of the first valve leaflet 110 and the second valve leaflet 112 near the distal end 104 of the frame 114 are tilted away from each other in the second configuration, and away from the longitudinal axis 101, to sealably engage a portion of the support frame 114. Each valve leaflet 110, 112 may be flexible enough to move toward or away from the longitudinal axis 101 in response to fluid flow contacting each valve leaflet 110, 112 within the lumen 116. Fluid flow in the first direction 106 contacts the proximal side of the valve leaflets 110, 112 and urges the proximal edge of the valve leaflets 110, 112 toward the longitudinal axis 101, closing the valve orifice 124 as shown in FIGS. 2A and 2B, and preventing fluid flow in the first direction 106. Conversely, fluid flow in the second direction 108 contacts the distal side of the valve leaflets 110, 112 and urges the proximal edge of the valve leaflets 110, 112 away from the longitudinal axis, opening the valve orifice 124 and permitting fluid flow in the second direction 108. Preferably, a prosthetic valve assembly is configured to move a remodelable material from a first configuration to a second configuration within a body vessel. The prosthetic valve assembly may be configured to resist fluid flow through a lumen defined by a support frame in a first longitudinal direction relative to a second longitudinal direction opposite the first direction to a greater degree when the remodelable material is in the second configuration than when the remodelable material is in the first configuration. Preferably, the resistance to fluid flow in at least one direction is at least 20% (preferably about 20-100%) greater in the second configuration compared to the first configuration. For example, the prosthetic valve assembly 100 has two valve leaflets 110, 112 oriented substantially parallel to one another in a first configuration shown in FIGS. 1A and 1B, and tilted toward one another at the proximal edges of the valve leaflets 110, 112 to form a valve orifice 124 in the second configuration shown in FIGS. 2A and 2B. The resistance to fluid flow through the lumen 116 of the prosthetic valve assembly 100 in the first direction 106 compared to the second direction 108 is greater when the valve leaflets 110, 112 are in the second configuration. In one aspect, the valve leaflets 110, 112 in the second configuration may be configured to prevent or substantially prevent fluid flow in the first direction 106. The prosthetic valve assembly preferably includes a releasable holding member configured to restrain one or more leaflets having a remodelable material in the first configuration. The releasable holding member may be formed from materials that are bioabsorbable as described in more detail below. The releasable leaflet holding member may simultaneously prevent the valve leaflet(s) from moving to the second configuration. The releasable leaflet holding member may include a bioabsorbable means for restraining the valve leaflet and may comprise one or more bioabsorbable polymers or metals and is absorbable within a remodeling-effective period of time, such as about 14 days to 60 days after implantation within a blood vessel. The releasable holding member may also include mechanical devices that may be activated either at the delivery site or externally after the prosthetic valve has been implanted for a period of time with the valve leaflet(s) in the first configuration. For example, the mechanical device may include a latch connecting a portion of the frame to a portion of the leaflet that has a friable connection that may be released using an energy source to release the connection and allow the leaflets to move to the second configuration. Other structures such as temporary magnets or wires may also be used. For some mechanical devices, the releasable holding means may be released within the body lumen, for example by cutting a suture. Other types of mechanical holding mechanisms know to one skilled in the art may also be used. Examples provided below describe a bioabsorbable means for restraining the leaflet(s) in the first configuration. One skilled in the art will understand that mechanical releasable holding member may also be used in the devices described below. By way of non-limiting example, the prosthetic valve assembly 100 includes one or more valve leaflet(s) 110, 112 moveable to the second configuration within the lumen upon release of the holding member. For example, the releasable leaflet holding member may be a first and a second bioabsorbable support frame portions 120, 122. The support frame 114 may include a radially self-expanding portion configured to return the valve leaflets 110, 112 in the second configuration absent the bioabsorbable support frame portions 120, 122. The valve leaflets 110, 112 may be attached to a portion of the support frame formed in the second configuration shown in FIGS. 2A and 2B. The valve leaflets 110, 112 may be moved to the first configuration shown in FIGS. 1A and 1B and the bioabsorbable support frame portions 120, 122 may be introduced to or attached to the support frame 114 to retain the valve leaflets 110, 112 in the first configuration. The valve leaflet in a first configuration may be configured as a substantially flat sheet contained within the lumen defined by the support frame. The valve leaflet in the second configuration may extend from a base portion contacting a portion of the support frame to a free edge portion distal to the base portion. The free edge may be flexibly moveable within the lumen to define at least a portion of a valve orifice moveable therein to permit the fluid flow through the lumen in the first direction while substantially limiting fluid flow through the lumen in the direction opposite the first direction. A second prosthetic valve assembly 200 may include a first support frame portion 202 that may be securely positioned within a second frame portion 214 before or after implantation in a body vessel 204. FIG. 3A is side view of a second prosthetic valve assembly 200 positioned within a body vessel 204 with a pair of valve leaflets 210, 212 in a planar configuration. FIG. 3B is side view of the second prosthetic valve assembly 200 shown in FIG. 3A positioned within the body vessel 204 with the pair of valve leaflets 210, 212 in a tilted configuration and oriented to prevent retrograde fluid flow through the body vessel 204. FIG. 3C is side view of the second prosthetic valve assembly 200 shown in FIG. 3B positioned within the body vessel 204 with the pair of valve leaflets 210, 212 in the tilted configuration and oriented to permit antegrade fluid flow through the body vessel 204. The first support frame portion 202 may be formed from a radially self-expanding biocompatible material configured as a plurality of alternating struts and bends to form a sinusoidal hoop member. The particular first frame portion 202 may be formed as a single member, for example, of shape memory metal (e.g., a superelastic nickel titanium alloy), biocompatible metal, or shape memory polymer, configured as an alternating array of a alternating series of four bends and four struts continuously connected in a pair of parallel connected “V” shapes. A first valve leaflet 210 and a second valve leaflet 212 are tensionably attached across opposing struts in the first frame portion 202, defining an orifice 223 at the proximal end of the first support frame portion 202. The first frame portion 202 is configured to resiliently expand to an expanded configuration shown in FIGS. 3B and 3C in the absence of a means for restraining the first frame portion 202 with the valve leaflets 210, 212 in the planar configuration of FIG. 3A. The second frame portion 214 may be a tubular structure adapted to receive the first frame portion 202 within a lumen defined between a pair of hoop members 214a and a plurality of longitudinal connecting members 214b extending therebetween. The hoop members 114a may have a ring-like structure as shown in FIG. 3A, or may be formed as one or more sinusoidal undulating ring structures. The second frame portion 214 need not be tubular as shown in the preferred embodiment of FIG. 3A, but may alternatively be formed in any manner suitable to securely receive the first frame portion 202 with a fixed orientation within the body vessel 204. The second prosthetic valve assembly 200 may be assembled prior to, during or after implantation of the second frame portion 214. For example, a “cartridge” comprising the valve leaflets 210, 212 attached to the first frame portion 202 may be seated within the second frame portion 214 inside the body vessel. The first frame portion 202 may be positioned within the second frame portion 214 with the longitudinal axes of the first frame portion 202 and the second frame portion 214 aligned with the longitudinal axis of the second prosthetic valve assembly 200. Upon implantation within the body vessel 204, the longitudinal axis of the prosthetic valve assembly 200 may be aligned parallel to the longitudinal axis of the body vessel 204. The prosthetic valve assembly 210 may be assembled within the body vessel 204, or prior to implantation therein. In the first instance, a second frame portion 214 may be implanted at a point of treatment within a body vessel. The cartridge comprising the first frame portion 202 and valve leaflets 210, 212 may be secured to the second frame portion 214 inside the body vessel at the time the second frame portion 214 is implanted, or in a subsequent procedure performed after implanting the second frame portion 214. Optionally, the cartridge comprising a first valve leaflet 210 formed from a remodelable material may be contacted with blood, tissue or other biological material prior to implantation of the cartridge within the second frame portion 214. One or more of the valve leaflets 210, 212 comprising remodelable material may be contacted with a biological material to induce remodeling in the valve leaflet attached to the cartridge prior to placement of the cartridge within the second frame portion 214 within a body vessel 204. For example, published US patent application US2007/0154515 A1, filed Dec. 26, 2006 by Johnson et al., describes a method of contacting a remodelable material with biological material (e.g., blood, tissue, and the like) to induce remodeling in the material prior to placing the material at a preferred treatment site within a body vessel. The cartridge comprising the first frame portion 202 preferably maintains the valve leaflets 210, 212 in a parallel planar configuration shown in FIG. 3A. The cartridge may include a self-expandable first support frame portion 202 configured for implantation within a body vessel and having a longitudinal axis. The first valve leaflet 210 and the second valve leaflet 212 are shown in FIG. 3A in the parallel configuration permitting a bidirectional fluid flow along the longitudinal axis of the first support frame portion 202 in both a first longitudinal direction and a second longitudinal direction opposite the first direction. The first valve leaflet 210 and the second valve leaflet 212 may each include a remodelable material and extend from a leaflet base to a leaflet free edge distal to the leaflet base. The free edges are oriented to form a flexible orifice 224 in both the planar and tilted configurations. The first valve leaflet 210 and the second valve leaflet 212 may each be separately attached to the first support frame portion 202 in a substantially planar configuration oriented substantially parallel to each other along the longitudinal axis of the body vessel 204 and/or the first support frame portion 202. The first support frame portion 202 is configured to permit the first valve leaflet 210 and the second valve leaflet 212 to move from the planar configuration to a tilted configuration shown in FIGS. 3A and 3B. The tilted configuration provides the free edge of the first valve leaflet 210 in contact with the free edge of the second valve leaflet 212 to form at least a portion of a valve orifice 224. The base of the first valve leaflet 210 and the base of the second valve leaflet 212 are each positioned a greater distance from the longitudinal axis of the first support frame portion 202 in the tilted configuration than in the planar configuration. The prosthetic valve assembly 200 including the valve leaflets 210, 212 in the tilted configuration permits fluid flow along the longitudinal axis of the prosthetic valve assembly 200 in the first longitudinal direction while substantially limiting fluid flow in the second longitudinal direction opposite the first direction. The cartridge with the valve leaflets 210, 212 attached to the first support frame portion 202 may be configured to resiliently expand to the tilted configuration absent a means for restraining the valve leaflets in the planar configuration. Preferably, the cartridge includes a bioabsorbable means for restraining the valve leaflets 210, 212 in the planar configuration, and/or simultaneously preventing each valve leaflet from moving to the tilted configuration. The valve leaflets 210, 212 are preferably moveable to the tilted configuration upon release of the leaflet holding member. For example, the bioabsorbable means for restraining the valve leaflets 210, 212 in the planar configuration may be one or more bioabsorbable threads 223 wrapped around the first support frame portion 202 with the leaflets 210, 212 in the planar configuration. Upon bioabsorption of the bioabsorbable threads 223 in situ, the first support frame portion 202 radially self expands outward to place the valve leaflets 210, 212 in the tilted configuration. In addition, the second prosthetic valve assembly 200 may include additional or alternative releasable holding member configured to restrain the valve leaflets 210, 212 in the planar configuration. For example, the second support frame portion 214 may be formed of a bioabsorbable material and be configured as a bioabsorbable member configured to the valve leaflets 210, 212 for a desired period of time. Upon bioabsorption of the second support frame portion 214, the first support frame portion 202 radially expands to place the valve leaflets 210, 212 in the tilted configuration. The cartridge may be implanted within a bioabsorbable second support frame portion 214 as shown in FIG. 3A. In one aspect, a first support frame portion 202 comprises a self-expanding material attached to the first valve leaflet 210, the first support frame portion 202 being configured to urge the first valve leaflet 210 from the planar configuration to the tilted configuration by self-expansion of a portion of the first support frame portion 202 after release of the leaflet holding member (e.g., a bioabsorbable second support frame portion 214 and/or a bioabsorbable thread 223). The holding member resists the self-expansion of the support frame and may include at least one structure selected from the group consisting of: a bioabsorbable suture, a portion of the support frame comprising a bioabsorbable polymer or bioabsorbable metal and a member comprising a bioabsorbable material connecting two portions of the support frame.
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