Vein Cover

Abstract

Provided is a vein cover capable of preventing intimal thickening by delivering the blood flowing through the lumen to the downstream vein while gradually buffering the blood pressure, pulse pressure, and blood flow.　A tubular vein cover (10) placed in a vein (4) anastomosed with an artery (3) or an artificial blood vessel (5), the tubular vein cover (10) having a portion (A) that has an elastic index (10% elastic index) of 25 N or lower when the inner diameter of the vein cover (10) is expanded 10% from the natural state.

Description

[Technical field]

[0001] The present invention relates to a vein cover for use at an anastomosis where blood vessels are anastomosed, and more particularly to a vein cover that can be used at the anastomosis, for example, when an artery is anastomosed to a vein in the construction of a shunt, or when an artificial blood vessel anastomosed to an artery is anastomosed to a vein. [Background technology]

[0002] For patients with serious kidney diseases, including renal failure, hemodialysis is regularly performed. Blood is taken from the patient's body, and waste products, excess water, minerals, etc. are removed using a dialyzer, and then the blood is returned to the patient's body. When performing hemodialysis, a special needle is usually inserted into a vein. At this time, since the blood flow in a normal vein is not sufficient to perform dialysis, an artery is anastomosed to a vein. This significantly reduces peripheral resistance, and the venous blood flow increases significantly, so it is necessary to make the venous blood vessels suitable for dialysis. Such blood vessels are called shunts, and a shunt is usually created by exposing the artery and vein by making a small incision in the artery and anastomosing the vein thereto, and directing part of the arterial blood flow to the vein. At this time, the vein may be anastomosed directly to the artery as shown in Figure 1, or an artificial blood vessel may be anastomosed to the small incision in the artery and the other end of the artificial blood vessel may be anastomosed to the vein to create an artificial blood vessel between the artery and the vein as shown in Figure 2.

[0003] At the site of shunt construction, the elasticity of arteries and veins differs significantly, so when pulsating high-pressure blood flows into veins that have extremely high extensibility at low pressure and low elasticity at high pressure, blood turbulence and stress changes occur in the venous wall. As a result, intimal thickening occurs at the anastomosis site and outflow vein, and pathological changes such as stenosis, occlusion, and thrombus formation easily occur. If the shunt blood flow state cannot be adjusted while still placing a great burden on the body, it can cause more widespread local (aneurysm formation and stenosis in downstream veins, steal syndrome due to excessive shunt blood flow, etc.) or systemic (heart failure due to a significant increase in venous return, etc.) pathological conditions.

[0004] If conditions are favorable, appropriate remodeling occurs due to changes in the elasticity of the venous wall as a defensive adaptive response of the body, and in some cases, stenosis or blockage due to intimal thickening can be avoided, and the shunt blood flow state can be self-regulated to a state that places no burden on the body. However, if local conditions such as shunt blood flow rate and anastomosis shape, or systemic conditions (diabetes, hypertension, arteriosclerosis, normal blood, etc.) are poor, the range in which an appropriate defensive adaptive response occurs will be exceeded, resulting in a pathological biological reaction, causing local and systemic pathology.

[0005] In order to suppress the initial rapid increase in blood flow immediately after surgery, vascular banding is performed to prevent excessive blood pressure in the inner vein and the resulting overstretching and blood turbulence by reinforcing the venous wall from the outside (Non-Patent Document 1). Patent Document 1 discloses a covering for reinforcing natural veins used as surgical implants, which is a woven mesh net covering made by forming a seamless, tubular, and substantially pileless knitted fabric. Patent Documents 2 and 3 disclose that an arteriovenous graft (AVG) wrapped with a restrictive fiber matrix of a biodegradable polymer exhibited pulsatile radial deviation similar to that of the carotid artery.

[0006] However, the above-mentioned vascular banding has not been able to sufficiently prevent lesions such as intimal hyperplasia. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Special Publication No. 2004-535896 [Patent Document 2] Special Publication No. 2010-516437 [Patent Document 3] Special Publication No. 2013-509258 [Non-patent literature]

[0008] [Non-Patent Document 1] Hiroaki Haruguchi, "I. Blood Access: Problems Associated with Blood Flow Insufficiency," Journal of the Japanese Society for Dialysis Therapy, Vol. 15, No. 1, 68-70, 2000 Summary of the Invention [Problem to be solved by the invention]

[0009] In conventional vascular banding, the reinforced vein wall is modified (arterialized) to have a structure similar to that of an arterial wall only under certain conditions, but when blood flows from the reinforced area to an unreinforced vein, the blood pressure and pulsation are not buffered and are delivered downstream as is, so the cause of intimal thickening is not fundamentally resolved. To resolve this, it is necessary to gradually reduce the blood pressure and pulsation from the anastomosis downstream, so that the most downstream vein side is only subjected to a low pressure without pulsation.

[0010] The present invention has been made in consideration of the above-mentioned circumstances, and has an object to provide a vein cover that can prevent intimal thickening by gradually reducing the blood pressure, pulse pressure, and blood flow rate of blood flowing through the lumen while delivering it to a downstream vein. [Means for solving the problem]

[0011] The vein cover of the present invention, which solves the above problems, has the following configuration. [1] A tubular vein cover to be placed in a vein anastomosed to an artery or an artificial blood vessel, said vein cover having a portion (A) whose elasticity index when the inner diameter of said vein cover is expanded by 10% from its natural state, as measured using the measurement method described below (hereinafter referred to as 10% elasticity index), is 25 N or less. [Measurement method] A portion of 5 mm in the axial direction is cut out from one end of the vein cover to prepare a cylindrical sample with an axial length of 5 mm. A first pin and a second pin with a diameter d of 0.75 mm are inserted into the inner cavity of the cylindrical sample in parallel with the axial direction of the cylindrical sample. The first pin is fixed, and the second pin is pulled radially outward of the cylindrical sample with a force F. When the distance between the first pin and the second pin is L, the force F when πd+2L is 1.1 times the circumference of the cylindrical sample in its natural state is calculated. 1.1 Divide by the strain [(1.1-1.0) / 1.0] The value obtained is the 10% elasticity index.

[0012] Furthermore, the vein cover according to the present invention preferably also includes the following configurations [2] to [9]. [2] The vein cover described in [1], wherein the 10% elasticity index of part (A) is 0.1 mN or more. [3] A vein cover as described in [1] or [2], in which the portion from the midpoint between one end and the other end of the vein cover to the one end is defined as one part of the vein cover, and the portion from the midpoint to the other end is defined as the other part of the vein cover, and the portion (A) is positioned on one part of the vein cover. [4] The vein cover according to any one of [1] to [3], wherein the axial length of the portion (A) is 50% or more of the outer diameter of the artery or artificial blood vessel to which it is anastomosed. [5] The vein cover according to any one of [1] to [4], wherein the 10% elasticity index of the other part of the vein cover is smaller than the 10% elasticity index of the one part of the vein cover. [6] A vein cover described in any of [1] to [5], wherein the elasticity index of the above part (A) when the inner diameter is expanded by 20% from the natural state is 32 N or less when measured using the measurement method described below (hereinafter referred to as the 20% elasticity index). [Measurement method] A portion of 5 mm in the axial direction is cut out from one end of the vein cover to prepare a cylindrical sample with an axial length of 5 mm. A first pin and a second pin with a diameter d of 0.75 mm are inserted into the inner cavity of the cylindrical sample in parallel with the axial direction of the cylindrical sample. The first pin is fixed, and the second pin is pulled radially outward of the cylindrical sample with a force F. When the distance between the first pin and the second pin is L, the force F when πd+2L is 1.2 times the circumference of the cylindrical sample in its natural state is calculated. 1.2 The value obtained by dividing by the strain [(1.2-1.0) / 1.0] is the 20% elasticity index. [7] The vein cover described in any one of [1] to [6], having an axial length of 5 mm or more. [8] The vein cover according to any one of [1] to [7], which is made of a biodegradable material. [9] A vein cover according to any one of [1] to [8], which has at least one of a woven fabric, a nonwoven fabric, or a porous material as a partial or entire component.

[10] The vein cover according to any one of [1] to [9], which is integrally formed with an artificial blood vessel. Effect of the Invention

[0013] The vein cover of the present invention, with its above-mentioned configuration, gradually changes the wall structure from the anastomosis to the downstream vein, and within the covered interior, the shear stress, pressure perpendicular to the wall, blood flow rate, flow velocity, and the range of change associated with pulsation change in a gradient, thereby suppressing incompatibility of vascular wall elasticity, turbulence, and high flow rate, and preventing intimal thickening. The following are thought to be the reasons why the vein cover of the present invention has such effects.

[0014] Both arteries and veins are composed of the intima, media, and adventitia, and in arteries, the media consists of a smooth muscle layer rich in smooth muscle cells and an elastic fiber layer containing collagen fibers. Arteries have thick smooth muscle layers and elastic fiber layers so that even when pressure from pulsating intraluminal blood flow is applied, there is little pulsation change in the vascular wall and no turbulence or friction stress fluctuation occurs. On the other hand, veins have thin vascular walls and do not have thick smooth muscle layers and elastic fiber layers like arteries. When arterial blood flows directly into such veins through an arteriovenous shunt, lesions such as intimal thickening occur due to the significant difference in elasticity between arteries and veins as described above. To prevent this, in the vein at the shunt construction site, 100% pulsatile arterial pressure is applied to the anastomosis site, i.e., the most upstream part of the vein, but the blood pressure, pulsatility, blood flow rate, and maximum flow velocity gradually decrease toward the downstream of the vein, and the most downstream vein is required to be remodeled into a vessel that can provide low pressure without pulsation, i.e., a buffer vessel.

[0015] The vein cover of the present invention having the above-mentioned configuration can remodel the vein at the shunt site into a buffer system blood vessel by forming a two-layer structure consisting of a smooth muscle layer containing elastic fibers thicker than the smooth muscle layer of a normal vein and an elastic fiber layer containing collagen fibers thicker than the smooth muscle layer on the outside of the smooth muscle layer in an inclined manner on the vein wall of the vein at the shunt site. In this way, the vein at the shunt site is remodeled into a buffer system blood vessel having an elastic fiber layer containing collagen fibers thicker than the smooth muscle layer, and the pulsatile blood flow with high arterial pressure at the arteriovenous anastomosis site and the artificial blood vessel venous anastomosis site is gradually buffered downstream and finally transferred to venous blood flow. As a result, blood turbulence and pulsatile changes in the vein wall are suppressed, and intimal thickening can be prevented.

[0016] The above buffer system blood vessels are different from normal arteries. Blood vessels are composed of three layers: the intima, the media, and the adventitia. Of these, the intima contributes greatly to anticoagulation, but its mechanical contribution is extremely small. The mechanical elements of the arteries of the limbs, which are normal arteries used in artificial dialysis, are composed of the media, which contains a small amount of elastic fiber and abundant smooth muscle, and the adventitia, which is composed of elastic fiber and collagen fiber, etc., making up a large proportion of the media. In other words, these arteries have a very large amount of smooth muscle and relatively few elastic fibers (a "smooth muscle > elastic fiber" structure). Elastic fibers, due to their elasticity, have a buffer function like a rubber tube that resists and relieves the high blood pressure caused by the pulsation of the artery. On the other hand, smooth muscle, being a muscle, also has a more active mechanical function, and while resisting arterial blood pressure, has the active and proactive function of delivering high pulsatile arterial blood pressure to the periphery without attenuating it. Due to the pressure transmission function of the abundant smooth muscle in arteries, blood pressure is almost the same whether the inner diameter is a centimeter-scale aorta or a small artery of a fraction of a millimeter. In other words, normal arteries (composed of "smooth muscle > elastic fiber") do not have the function of buffering pulsatile, high arterial pressure due to the action of abundant smooth muscle.

[0017] On the other hand, when the veins in the shunt area are remodeled into buffer system vessels, the ratio of elastic fiber to smooth muscle is the opposite of that in normal arteries, with elastic fiber abundant and smooth muscle relatively thin (the "elastic fiber > smooth muscle" structure). Therefore, in buffer system vessels, the pressure buffering function is dominant over the pressure delivery function. In buffer system vessels, while maintaining the above-mentioned "elastic fiber > smooth muscle" structure, i.e., while maintaining the buffering function, the entire vessel gradually becomes thinner and transitions into a vein, that is, as the buffered pressure decreases, it gradually transitions into a normal vein.

[0018] If the venous wall were to change into a normal artery-like state, this would be arterialization (remodeling into an artery), not remodeling into a buffer system vessel. If this arterialization were to gradually weaken and naturally transition downstream into a completely normal vein, this would mean that the vein would gradually become thinner while retaining its "smooth muscle > elastic fiber" structure and transitioning into a vein. Since it would not have a buffer function, high pulsatile blood pressure would act on the venous wall, resulting in pathological changes in the downstream vein. This is a clear functional difference between the case where a buffer system vessel gradually thins and transitions into a normal vein, and the case where a normal artery-like change gradually thins and transitions into a normal vein.

[0019] It should also be noted that in the definition of buffer system blood vessels, the morphological change of the abovementioned "elastic fiber > smooth muscle" structure is a necessary but not sufficient condition. In order to be able to define a vessel as a buffer system blood vessel, it is necessary to prove by actual observation that it buffers blood pressure and pulsation by measuring blood flow. [Brief description of the drawings]

[0020] [Figure 1] 1 shows a schematic diagram of an example of a shunt construction site. [Diagram 2] 13A and 13B show schematic diagrams of other examples of shunt construction sites. [Diagram 3] 1 illustrates a perspective view of a vein cover according to one embodiment of the present invention; [Figure 4] 13 illustrates a perspective view of a vein cover according to another embodiment of the present invention; FIG. [Diagram 5] 13 illustrates a perspective view of a vein cover according to another embodiment of the present invention; FIG. [Figure 6] 13 illustrates a perspective view of a vein cover according to another embodiment of the present invention; FIG. [Figure 7] 1 shows a perspective view of a sample for measuring elasticity index. [Figure 8] 8 shows a plan view of the measurement sample shown in FIG. 7 as viewed from above. [Figure 9] FIG. 1 illustrates a perspective view of a venous cover placed at an anastomosis in accordance with one embodiment of the present invention. [Figure 10]FIG. 13 illustrates a perspective view of a venous cover placed at an anastomosis according to another embodiment of the present invention. [Figure 11] FIG. 13 illustrates a perspective view of a venous cover placed at an anastomosis according to another embodiment of the present invention. [Figure 12] FIG. 13 illustrates a perspective view of a venous cover placed at an anastomosis according to another embodiment of the present invention. [Figure 13] FIG. 13 illustrates a perspective view of a venous cover placed at an anastomosis according to another embodiment of the present invention. [Figure 14] 1 shows photomicrographs of vein wall cross sections stained with Elastica-van Gieson. [Figure 15] 13 is a graph showing the relationship between the axial distance from the anastomosis and the thickness of the smooth muscle layer and the elastic fiber layer containing collagen fibers in a cross section of the venous wall. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0021] The present invention will be described below based on the embodiments, but the present invention is not limited to the following embodiments, and can be modified as appropriate within the scope of the above and below, and all of these are included in the technical scope of the present invention. In addition, hatching and component symbols may be omitted in each drawing for convenience, but in such cases, the specification and other drawings should be referred to. Furthermore, the dimensions of various components in the drawings may differ from the actual dimensions, since priority is given to helping understand the features of the present invention.

[0022] The vein cover of the present invention is a cylindrical vein cover placed on a vein anastomosed to an artery or an artificial blood vessel, and has a portion (A) in which the elasticity index (hereinafter referred to as 10% elasticity index) is 25N or less when the inner diameter of the vein cover is expanded by 10% from the natural state. Unlike the conventional idea of ​​reinforcing the vein wall by vascular banding, in the present invention, a two-layer structure consisting of a smooth muscle layer containing elastic fibers and an elastic fiber layer containing collagen fibers thicker than the smooth muscle layer is formed in the covered vein by loosely covering the vein with a vein cover having a portion (A) in which the 10% elasticity index is 25N or less, and the vein is remodeled into a buffer system blood vessel, thereby preventing lesions such as intimal thickening. The vein cover of the present invention can prevent lesions such as intimal thickening both when the vein is anastomosed to an artery and when the vein is anastomosed to an artificial blood vessel connected to an artery.

[0023] A vein cover according to an embodiment of the present invention will be described below with reference to Figures 1 to 6. Figure 1 is a schematic diagram of a case where an autologous vein is anastomosed to a small incision in an artery at a shunt creation site, and Figure 2 is a schematic diagram of a case where one end of an artificial blood vessel is anastomosed to the small incision in the artery and the other end of the artificial blood vessel is anastomosed to a vein. Figures 3 to 6 are perspective views of vein covers according to different embodiments.

[0024] As shown in Figures 1 to 6, the vein cover 10 is a tubular cover to be placed on a vein 4 that is anastomosed to an artery 3 or an artificial blood vessel 5, and has a portion (A) in which the elasticity index (hereinafter referred to as the 10% elasticity index) when the inner diameter of the vein cover 10 is expanded by 10% from its natural state is 25 N or less.

[0025] 1, when a patient's own vein 4 is anastomosed to a small incision in an artery 3 at a shunt construction site 1, blood flows from the artery 3 through the anastomosis 6 to the vein 4 in the blood flow direction shown by arrow B. The vein cover 10 is preferably positioned from the most upstream part of the vein 4 on the anastomosis 6 side.

[0026] 2, when one end of an artificial blood vessel 5 is anastomosed to a small incision in an artery 3 at a shunt creation site 1 and a vein 4 is anastomosed to the other end of the artificial blood vessel 5, blood flows from the artery 3 through the artificial blood vessel 5 and further through the anastomosis 6 to the vein 4 in the blood flow direction shown by arrow B. The vein cover 10 is preferably positioned from the most upstream portion on the anastomosis 6 side of the vein 4.

[0027] Although not shown, regardless of whether it is anastomosed to the artery 3 or the artificial blood vessel 5, the vein cover 10 may be positioned so as to cover not only the vein 4 but also a portion of the artery 3 and the artificial blood vessel 5 on the anastomosis site 6 side.

[0028] The vein cover 10 may have any shape as long as it is tubular, and may have a joint, for example, formed by rolling a flat member into a cylindrical shape and joining it by sewing. In this case, it is preferable that the joint such as the stitching is formed on the outer surface of the vein cover 10. Alternatively, a molded member or knitted fabric may be used to form a seamless tubular member.

[0029] The vein cover 10 has an axial direction X and one end 10a and the other end 10b. When the vein cover 10 is placed at the shunt creation portion 1 as shown in Figures 1 and 2, in this specification, the one end 10a is placed upstream of the blood flow direction B, i.e., on the anastomosis portion 6 side of the vein 4, and the other end 10b is placed downstream of the blood flow direction B.

[0030] The 10% elasticity index is determined for a sample of vein cover 10 having a length of 5 mm in axial direction X when the inner diameter expands by 10% from the natural state. The method for measuring the elasticity index will be described with reference to Figures 7 and 8. Figure 7 shows a perspective view of a cylindrical sample 100 for the elasticity index, and Figure 8 shows a plan view of cylindrical sample 100 shown in Figure 7 as viewed from above. [Measurement method] A portion of the vein cover 10 extending 5 mm in the axial direction X from one end 10a is cut out to prepare a cylindrical sample 100 having a length of 5 mm in the axial direction X. The cylindrical sample 100 is cut out along a cut surface perpendicular to the axial direction, i.e., a cutting line in the circumferential direction, so that a cylindrical sample 100 having an axial length of 5 mm and continuous over the entire circumference is obtained. A first pin 101 and a second pin 102 having a diameter d of 0.75 mm are inserted into the inner cavity of the cylindrical sample 100 in parallel with the axial direction X of the cylindrical sample 100. The first pin 101 is fixed, and the second pin 102 is pulled radially outward of the cylindrical sample 100 with a force F. When the distance between the first pin 101 and the second pin 102 is L, the force F when πd+2L is 1.1 times the circumferential length of the cylindrical sample 100 in its natural state is calculated. 1.1 The value obtained by dividing by the strain [(1.1-1.0) / 1.0] is the 10% elasticity index.

[0031] Here, the distance L between the first pin 101 and the second pin 102 is the distance from the center of the first pin 101 and the second pin 102 as shown in Fig. 8. The inner diameter of the cylindrical sample 100 is equal to the sum of 1 / 2 the circumference πd of the first pin 101, 1 / 2 the circumference πd of the second pin 102, and twice the distance L between the first pin 101 and the second pin 102, i.e., πd+2L. Therefore, the force F pulling the second pin 102 when the inner diameter of the cylindrical sample 100 expands by 10% from the natural state, i.e., when πd+2L becomes 1.1 times the circumferential length of the cylindrical sample 100 in the natural state, is 1.1 Dividing by the strain [(1.1-1.0) / 1.0] gives 10% The elasticity index can be obtained.

[0032] If the length in the axial direction X of the vein cover 10 to be measured is less than 5 mm, multiple vein covers made in the same manner and having the same elasticity index can be prepared, and the multiple vein covers can be arranged in the axial direction X to form a sample with a total length in the axial direction X of 5 mm, and the elasticity index can be measured using the above method.

[0033] The portion (A) having a 10% elasticity index of 25 N or less may be provided continuously or separately. The 10% elasticity index of portion (A) is preferably 24 N or less, more preferably 21 N or less, even more preferably 17 N or less, and may be 14 N or less. By having portion (A) with a 10% elasticity index within the above range, the vein 4 covered by the vein cover 10 can be remodeled into a buffer system blood vessel.

[0034] When the vein cover 10 is placed at the shunt creation site 1, it may be positioned so as to cover not only the vein 4 but also the artery 3 on the anastomosis site 6 side and part of the artificial blood vessel 5, but it is preferable that part (A) is positioned in the part that covers the vein.

[0035] The inner diameter of the vein cover 10 can be set to an appropriate value depending on the diameter of the blood vessel to which it is applied, and is preferably, for example, 1 mm or more, more preferably 2 mm or more, even more preferably 3 mm or more, and may be 4 mm or more. The inner diameter of the vein cover 10 is preferably, for example, 10 mm or less, more preferably 9 mm or less, and even more preferably 8 mm or less. The vein cover 10 may have different inner diameters depending on the axial direction X.

[0036] The 10% elasticity index of portion (A) is preferably 0.1 mN or more. The 10% elasticity index of portion (A) may be 0.2 mN or more, 0.5 mN or more, or 1 mN or more. By having portion (A) with a 10% elasticity index in the above range, the vein 4 covered by the vein cover 10 can be remodeled into a buffer system blood vessel.

[0037] 3 to 6, when the portion from the midpoint M between one end 10a and the other end 10b of the vein cover 10 to one end 10a is defined as the vein cover one part 11, and the portion from the midpoint M to the other end 10b is defined as the vein cover other part 12, it is preferable that part (A) is disposed in the vein cover one part 11. If part (A) is disposed in the vein cover one part 11, when one end 10a of the vein cover 10 is disposed upstream of the blood flow direction B of the vein 4, it is gradually remodeled from the upstream side into a buffer vascular system, buffering the transmural pressure, shear stress, pulsatile pulse pressure, flow velocity, and blood flow rate, and bringing about changes from upstream to downstream.

[0038] The length in the axial direction X of the portion (A) is preferably 50% or more of the outer diameter of the anastomosed artery 3 or artificial blood vessel 5. The length in the axial direction X of the portion (A) is more preferably 60% or more, further preferably 80% or more, and may be 100% or more of the outer diameter of the anastomosed artery 3 or artificial blood vessel 5. The upper limit of the length in the axial direction X of the portion (A) is not particularly limited, but may be 2000% or less, 1750% or less, or 1500% or less of the outer diameter of the anastomosed artery 3 or artificial blood vessel 5. When the length in the axial direction X of the portion (A) is within the above range, remodeling of the vein 4 into a buffer vascular system is facilitated.

[0039] It is preferable that the 10% elasticity index of the other vein cover part 12 is smaller than the 10% elasticity index of the one vein cover part 11. When the one vein cover part 11 is disposed upstream of the blood flow direction B of the vein 4, the 10% elasticity index is large on the upstream side and small on the downstream side, so that the vein 4 is gradually remodeled into a buffer vascular system, which can buffer the transmural pressure, shear stress, pulsatile pulse pressure, flow velocity, and blood flow rate, and bring about changes from upstream to downstream.

[0040] The 10% elasticity index of the width of 5 mm from the midpoint M to the other end 10b is preferably 0.1 to 0.98 times, 0.96 times or less, or 0.1 to 0.9 times the 10% elasticity index of the width of 5 mm from one end 10a. The 10% elasticity index of the width of 5 mm from the midpoint M to the other end 10b may be 0.2 times or more, or 0.3 times or more, the 10% elasticity index of the width of 5 mm from one end 10a. The 10% elasticity index of the width of 5 mm from the midpoint M to the other end 10b may be 0.8 times or less, or 0.7 times or less, the 10% elasticity index of the width of 5 mm from one end 10a. Since the 10% elasticity index of the vein cover 10 5 mm from the midpoint M to the other end 10b, i.e., the portion covering the downstream part of the vein 4, is in the above range compared to the 10% elasticity index of the vein cover 10 5 mm wide from one end 10a, i.e., the portion covering the upstream part of the vein 4, when one end 10a of the vein cover 10 is positioned upstream in the blood flow direction B of the vein 4, it is gradually remodeled from the upstream side into a buffering vascular system, buffering transmural pressure, shear stress, pulsatile pulse pressure, flow velocity, and blood flow rate, and bringing about changes from upstream to downstream.

[0041] The 10% elasticity index of the width of 5 mm from the other end 10b in the axial direction X is preferably 0.1 to 0.98 times, 0.96 times or less, or 0.1 to 0.9 times, of the 10% elasticity index of the width of 5 mm from the one end 10a in the axial direction X. The 10% elasticity index of the width of 5 mm from the other end 10b in the axial direction X may be 0.2 times or more, or 0.3 times or more, of the 10% elasticity index of the width of 5 mm from the one end 10a in the axial direction X. The 10% elasticity index of the width of 5 mm from the other end 10b in the axial direction X may be 0.8 times or less, 0.7 times or less, or 0.6 times or less, of the 10% elasticity index of the width of 5 mm from the one end 10a in the axial direction X. Since the 10% elasticity index of the vein cover 10 of the width of 5 mm from the other end 10b, i.e., the portion covering the most downstream part of the vein 4, is smaller than the 10% elasticity index of the vein cover 10 of the width of 5 mm from one end 10a, i.e., the portion covering the most upstream part of the vein 4, as in the above range, when one end 10a of the vein cover 10 is positioned upstream of the blood flow direction B of the vein 4, it is gradually remodeled from the upstream side into a buffering vascular system, buffering transmural pressure, shear stress, pulsatile pulse pressure, flow velocity, and blood flow rate, making it easier to bring about changes from upstream to downstream.

[0042] The elasticity index of portion (A) when the inner diameter is expanded by 20% from the natural state (hereinafter referred to as 20% elasticity index) is preferably 32N or less. The 20% elasticity index of portion (A) is more preferably 29N or less, further preferably 27N or less, and may be 25N or less. Having portion (A) with a 20% elasticity index within the above range makes it easier for the vein 4 covered by the vein cover 10 to be remodeled into a buffer system blood vessel. The 20% elasticity index is measured by calculating the force F when πd+2L becomes 1.2 times the circumference of the cylindrical sample 100 in the natural state. 1.2 The method is the same as that for measuring the 10% elasticity index, except that

[0043] The outer diameter of the vein cover 10 within 5 mm from the other end 10b is preferably expandable by 40% or more from its natural state, more preferably by 50% or more, and even more preferably by 60% or more. If the outer diameter of the vein cover 10 within 5 mm from the other end 10b is expandable within the above range, the vein 4 covered by the vein cover 10 can be easily remodeled into a buffer system blood vessel.

[0044] The vein cover 10 may be formed in a straight shape with the inner diameters of one end 10a and the other end 10b being approximately equal, as shown in Fig. 3 for example. The 10% elasticity index and the position where portion (A) is provided can be adjusted, for example, by changing the density of the knitted fabric constituting the vein cover 10. If the vein cover 10 has a straight shape, the vein 4 can be covered over the entire length of the axial direction X of the vein cover 10 without being affected by the shape of the vein cover 10.

[0045] The vein cover 10 may be formed in a tapered shape in which the inner diameter increases from the one end 10a side toward the other end 10b side, as shown in Fig. 4, for example. By forming the vein cover 10 in a tapered shape, it becomes easy to adjust the 10% elasticity index and the position where the portion (A) is provided. In addition, since the inner diameter of the other end 10b side is larger than that of the one end 10a, the vein cover other part 12 can cover the vein 4 more loosely, and when the one end 10a side of the vein cover 10 is disposed upstream of the blood flow direction B of the vein 4, it is gradually remodeled from the upstream side into a buffer vascular system, buffering the transmural pressure, shear stress, pulsatile pulse pressure, flow velocity, and blood flow rate, and bringing about changes from upstream to downstream.

[0046] As shown in Fig. 5, a bellows structure may be formed on one end 10a. The elasticity index of the bellows structure can be adjusted by the density of the bellows, so that it is easy to adjust the 10% elasticity index and the position where the portion (A) is provided. For example, the elasticity index can be gradually changed by providing a portion with a high-density bellows, a portion with a low-density bellows, and a portion without a bellows structure in this order. One end of the bellows portion may be arranged so as to cover not only the vein 4 but also part of the artery 3 and the artificial blood vessel 5 on the anastomosis portion 6 side when the vein cover 10 is arranged at the shunt creation portion 1.

[0047] The vein cover 10 may be formed as a bellows structure as a whole, for example as shown in Fig. 6, and the density and amplitude of the bellows may be different depending on the axial direction X. In this case, it is preferable that the bellows is formed at one end 10a with a high density, and the density of the bellows is decreased toward the other end 10b. In this case, it is easy to make the vein cover 10 have a smaller 10% elasticity index of the other part 12 of the vein cover than the 10% elasticity index of the one part 11 of the vein cover. When the one end 10a of the vein cover 10 is disposed upstream of the blood flow direction B of the vein 4, the vein cover 10 is gradually remodeled from the upstream side into a buffer vascular system, buffering the transmural pressure, shear stress, pulsatile pressure, flow velocity, and blood flow rate, and bringing about changes from upstream to downstream.

[0048] The vein cover 10 preferably has a length in the axial direction X of 5 mm or more. The length in the axial direction X is more preferably 10 mm or more, even more preferably 20 mm or more, particularly preferably 30 mm or more, and may be 40 mm or more. The length in the axial direction X is preferably 120 mm or less, more preferably 100 mm or less, and even more preferably 90 mm or less. If the length in the axial direction X is within the above range, it becomes easy to remodel the vein 4 into a buffer system blood vessel.

[0049] The vein cover 10 is preferably made of a knitted fabric. Knitted fabrics have excellent elasticity and flexibility, and are therefore suitable for obtaining a desired elasticity index. The type of knitted fabric is not particularly limited, and may be warp knitted or weft knitted. Examples of warp knitting structures include half knitting, back half knitting, queen's coat knitting, and satin knitting. Examples of weft knitting structures include circular knitting and flat knitting, and examples of weft knitting structures include flat knitting, rib knitting, double-sided knitting, Milano rib knitting, and jacquard knitting. In terms of excellent elasticity, it is preferable that the knitted fabric is made of weft knitting.

[0050] The vein cover 10 may be made of a woven fabric such as a plain weave. Alternatively, the vein cover 10 may be made of a nonwoven fabric or a porous body produced by any method such as a melt-blowing method, a needle punching method, or a spunlace method. That is, the vein cover 10 may have at least one of knitted fabric, woven fabric, nonwoven fabric, and a porous body as a component constituting at least a part or the entire structure of the vein cover. In addition, the vein cover 10 may be made of a combination of two or more different materials, for example, a part of the vein cover 10 may be made of a knitted fabric and the other part of the vein cover 10 may be made of another material, such as a porous body.

[0051] The elasticity of the vein cover 10 may be due to the material, the structure, or both. For example, if the material is highly elastic, the desired elasticity index can be obtained by simply forming the material into a sheet such as a woven or nonwoven fabric that does not have elasticity due to its structure and forming the sheet into a cylindrical structure. If the material is less elastic, the desired elasticity index can be obtained by forming the material into a structure that can provide elasticity, such as a knitted fabric.

[0052] The yarns forming the knitted fabric, woven fabric, and nonwoven fabric are preferably made of a resin material having biocompatibility, such as polyolefin resins such as polyethylene and polypropylene, polyamide resins such as nylon, polyester resins such as polyethylene terephthalate and polybutylene terephthalate, aromatic polyether ketone resins such as PEEK, polyether polyamide resins, polyester elastomers such as polyester polyol, polyurethane resins, polyimide resins, fluorine resins such as PTFE, PFA, and ETFE, polyvinyl chloride resins, and synthetic resins such as silicone resins. The yarns forming the knitted fabric and woven fabric can also be made of resin materials used in artificial blood vessels (e.g., polyester, PTFE, and polyurethane), and specifically include ePTFE, which is PTFE stretched, and Dacron (registered trademark), a polyester fiber from DuPont. The vein cover 10 may be made of a biodegradable material, for example, aliphatic polyesters such as polylactic acid, polycaprolactone, polylactic acid / polycaprolactone copolymer, polyglycolic acid, polybutylene succinate, polyhydroxyalkanoic acid, etc.; aliphatic polyethers such as polyethylene glycol; polyvinyl alcohol, etc. The yarn forming the knitted or woven fabric may be made of natural fibers such as silk or cotton, or may be made of a combination of a resin material, a biodegradable material, and a natural fiber.

[0053] It is also preferable that the vein cover 10 is integrally formed with the artificial blood vessel 5. For example, in the shunt creation site 1 shown in Fig. 2, the autologous vein 4 is end-to-end anastomosed to the artificial blood vessel 5 which is in turn anastomosed side-to-end to the autologous artery 3, and because the vein cover 10 is integrally formed with the artificial blood vessel 5, it becomes easy to cover the portion downstream from the anastomosis site 6 with the vein cover 10, and inconveniences such as the vein cover 10 being displaced or detached can be avoided.

[0054] The elasticity index of the vein cover 10 may be adjusted by changing the density of the knitted or woven fabric, by partially overlapping the knitted or woven fabric, or by modifying the shape of the vein cover 10.

[0055] Whether or not the vein 4 has been remodeled into a buffer system blood vessel can be evaluated by confirming whether a two-layer structure consisting of a smooth muscle layer containing elastic fibers and an elastic fiber layer containing collagen fibers thicker than the smooth muscle layer has been formed on the outside of the smooth muscle layer. Specifically, the vein 4 at the shunt construction site 1 is excised, and special staining such as hematoxylin-eosin (HE) staining and Elastica van Gieson (EvG) staining is performed, and the cross section of the vein wall is observed under a microscope. For example, with EvG staining, smooth muscle is stained cloudy yellow, elastic fiber is stained deep purple, and collagen fiber is stained dark red, so that the smooth muscle layer containing elastic fibers and the elastic fiber layer containing collagen fibers can be observed and their respective thicknesses can be confirmed.

[0056] This application claims the benefit of priority based on Japanese Patent Application No. 2020-36211, filed on March 3, 2020. The entire contents of the specification of Japanese Patent Application No. 2020-36211, filed on March 3, 2020, are incorporated by reference into this application. EXAMPLES

[0057] The present invention will be described below with reference to examples. The present invention is not limited to the following examples, and can of course be modified within the scope of the above and below-described aims, and all such modifications are within the technical scope of the present invention.

[0058] The measurements and evaluations in the examples were carried out by the following methods.

[0059] (1) 10% Elasticity Index A cylindrical sample having an axial length of 5 mm and continuous over the entire circumference was prepared by cutting the vein cover from one end to a section 5 mm in the axial direction along a cut surface perpendicular to the axial direction, i.e., a cutting line in the circumferential direction, all around. A first pin and a second pin having a diameter of 0.75 mm were inserted into the inner cavity of the cylindrical sample parallel to the axial direction of the cylindrical sample. The first pin was fixed, and the second pin was pulled outward in the radial direction of the cylindrical sample. The 10% elasticity index was obtained by dividing the force when the sum of twice the distance between the first pin and the second pin and (π×0.75 mm) became 1.1 times the circumference of the cylindrical sample in its natural state by the strain [(1.1-1.0) / 1.0].

[0060] (2) 20% Elasticity Index Using a method similar to that described in (1) above, the elasticity index required to expand 20% from the natural state was obtained by dividing the force required when the sum of twice the distance between the first pin and the second pin and (π × 0.75 mm) became 1.2 times the circumference of the cylindrical sample in its natural state by the strain [(1.2-1.0) / 1.0].

[0061] (3) Animal experiments on covering the shunt Beagle dogs (male and female, weighing 9-14 kg) were used. A shunt with an anastomosis form shown in Table 1 was constructed between the carotid artery and the jugular vein, and a venous cover with the shape, material, inner diameter, axial length, and elasticity index shown in Table 1 was placed over the vein at the shunt construction site and observed over time. In cases where a non-absorbable material was used, observation was performed for 12 weeks or more to ensure that no adverse events occurred over the long term. On the other hand, in cases where a bioabsorbable material was used, the animals were sacrificed and evaluated after 28 and 56 days in order to confirm the completion of remodeling into a buffer system vessel macroscopically and microscopically before the mechanical strength of the bioabsorbable material deteriorates.

[0062] (4) Observation of the smooth muscle layer and the elastic fiber layer containing collagen fibers After the observation in (3) above, the beagle dogs were euthanized and the vein at the site of the shunt was extracted. The extracted vein was stained with Elastica van Gieson (EvG) staining, and the cross sections including the area 5 mm in the direction of blood flow from just below the anastomosis, the area 5 mm in the direction of blood flow near the midpoint of the vein cover, and the area 5 mm from the other end of the vein cover were observed under an optical microscope.

[0063] (5) Measurement of the thickness of the smooth muscle layer and the elastic fiber layer containing collagen fibers From the micrographs obtained in (4) above, the thickness of each layer at each distance from the venous anastomosis shown in Table 2 was determined.

[0064] (6) Assessment of remodeling of the buffer vasculature After the follow-up observation in (3) above, the blood flow state was measured by Doppler blood flow measurement, and further, diagnosis was performed using a color Doppler ultrasound imaging diagnostic device. Based on these measurements and diagnoses, as well as the results in (4) and (5) above, we evaluated whether the vein at the site of shunt construction had remodeled into a buffer vascular system, using the following criteria. <Evaluation criteria> If all of the following (a) to (e) were met, the vein at the shunt creation site was evaluated as having remodeled into a buffer vascular system (○ in Table 1); otherwise, it was evaluated as not having remodeled (× in Table 1). (a) Macroscopic findings at autopsy were that the vascular lumen was patent, the vascular wall was smooth and free of varicose veins or unnatural irregularities, and there was no pathological finding of intimal hyperplasia, stenosis, or thrombus formation that would affect blood flow. (a) In the observation in (4) above, a two-layer structure is observed, with an inner smooth muscle layer that is clearly thicker than the smooth muscle layer and contains elastic fibers, and an outer elastic fiber layer that contains collagen fibers that are thicker than the smooth muscle layer, and there is no intimal thickening or thrombus formation. (c) In the measurement in (5) above, the thickness of the outer elastic fiber layer containing collagen fibers is greater than the inner smooth muscle layer. (e) In the above Doppler blood flow measurement, blood flow is antegrade and arterial pulsatile blood flow is buffered (pulsatility gradually disappears from upstream to downstream). (E) In diagnosis using the above-mentioned color Doppler ultrasound imaging diagnostic device, the vascular lumen is patent, the vascular wall is smooth, and there is no pathological intimal thickening or thrombus formation that affects blood flow.

[0065] Example 1 A vein cover having an axial length of 50 mm and an inner diameter of 5 mm was prepared using a polyester artificial blood vessel (hereinafter, referred to as "Dacron (registered trademark) artificial blood vessel") (Maquet Cardiovascular, Hemashield Gold Knitted Vascular Graft). The 10% elasticity index and 20% elasticity index of the prepared vein cover were measured. A shunt was constructed in the neck of a beagle dog by anastomosis of an artery and a vein, and the vein cover was placed on the vein so that one end of the vein cover was placed on the anastomosis as shown in FIG. 9, and the dog was observed for 105 days. The vein was then evaluated by Doppler blood flow measurement and diagnosis using a color Doppler ultrasound imaging device. The beagle dog was euthanized, and the vein at the shunt construction site was extracted. The smooth muscle layer and the elastic fiber layer containing collagen fibers of the extracted vein were observed and their thicknesses were measured, and remodeling evaluation into a buffer vascular system was performed. The preparation conditions and elasticity index of the vein cover and the evaluation results are shown in Table 1. An optical microscope photograph of a cross section of the venous wall parallel to the blood flow direction at a distance of 2.5 mm from the anastomosis is shown in Figure 14, and the thickness measurements of the smooth muscle layer and the elastic fiber layer containing collagen fibers are shown in Table 2 and Figure 15. In the optical microscope photograph of Figure 14, the lower side is the outer wall side of the venous wall, and the upper side is the inner wall side of the venous wall. The arrow on the inner wall side indicates the thickness of the smooth muscle layer, and the arrow on the outer wall side indicates the thickness of the elastic fiber layer containing collagen fibers.

[0066] [Table 1]

[0067] [Table 2]

[0068] As shown in Figure 14, a smooth muscle layer and an elastic fiber layer containing thicker collagen fibers were observed. In addition, as shown in Table 2 and Figure 15, the thickness of the smooth muscle decreased downstream from the anastomosis. The result of evaluation based on the above criteria (A) to (E) was ○.

[0069] Example 2 Instead of Dacron (registered trademark) artificial blood vessel, a nylon monofilament (φ3μm) was bundled and woolly processed to produce a bellows-shaped vein cover with an axial length of 40mm and an inner diameter of 6mm. The 10% elasticity index and 20% elasticity index of the produced vein cover were measured. A shunt was constructed in the neck of a beagle dog by anastomosis of an artery and a vein, and the vein cover was placed on the anastomosis as shown in FIG. 10, and the dog was observed for 154 days. The vein was then evaluated by Doppler blood flow measurement and diagnosis using a color Doppler ultrasound imaging device. The beagle dog was euthanized and the vein at the shunt construction site was extracted. The smooth muscle layer and elastic fiber layer containing collagen fibers of the extracted vein were observed and their thicknesses were measured, and remodeling to a buffer vascular system was evaluated. The production conditions and elasticity index of the vein cover and the evaluation results are shown in Table 1. The evaluation results based on the above criteria (A) to (E) were ○.

[0070] Examples 3 to 8 A vein cover was prepared under the preparation conditions shown in Table 1, and the 10% elasticity index and 20% elasticity index were measured. A shunt was constructed in the anastomosis form as shown in Figures 9 and 10 to 12, as shown in Table 1, and observation and evaluation were performed in the same manner as in Example 1. The values ​​before and after the arrows of the inner diameter for Examples 5, 7, and 8 in Table 1 indicate the minimum and maximum inner diameters of the tapered vein cover. The elasticity index and evaluation results are shown in Table 1. The results of evaluation based on the above criteria (A) to (E) were all rated as ○.

[0071] Example 9 A vein cover having a straight shape with an axial length of 28 mm and an inner diameter of 5.2 mm was produced using a knitted fabric made of polyglycolic acid (Neoveil (registered trademark) manufactured by Gunze Co., Ltd.). The 10% elasticity index and 20% elasticity index of the produced vein cover were measured. A shunt was constructed in the neck of a beagle dog by anastomosing an artificial blood vessel made of ePTFE (Gore-Tex (registered trademark) Stretch Vascular Graft manufactured by WL Gore & Associates) end-to-end with an artery, and the vein cover was placed on the vein so that one end of the vein cover was placed at the anastomosis as shown in FIG. 13, and the follow-up observation was performed for 56 days. The elasticity index and the results of evaluation performed in the same manner as in Example 1 are shown in Table 1. The results of evaluation based on the above criteria (a) to (e) were all ◯.

[0072] Example 10 A vein cover having an axial length of 25 mm, an inner diameter of 5 mm on one end, and an inner diameter of 8 mm on the other end was prepared using a nonwoven fabric made of polylactic acid / caprolactone copolymer (25% polylactic acid / 75% caprolactone). The 10% elasticity index and 20% elasticity index of the prepared vein cover were measured. A shunt was constructed in the neck of a beagle dog by anastomosing an artificial blood vessel made of ePTFE (WL Gore & Associates, Gore-Tex (registered trademark) Stretch Vascular Graft) to an artery end-to-end with a vein, and a cover was placed on the vein so that one end of the vein cover was placed at the anastomosis as shown in FIG. 12, and the follow-up observation was performed for 28 days. The elasticity index and the results of evaluation performed in the same manner as in Example 1 are shown in Table 1. The results of evaluation based on the above criteria (A) to (E) were all ○.

[0073] Comparative Example 1 A straight vein cover with an axial length of 30 mm and an inner diameter of 4.5 mm was prepared using a Dacron (registered trademark) artificial blood vessel (Japan Lifeline Co., Ltd., J-Graft, Shield Neo S). A shunt was constructed in the neck of a beagle dog by anastomosing an artificial blood vessel made of ePTFE (Bard Peripheral Vascular Co., Ltd., Distaflo (registered trademark) Bypass Grafts) joined to an artery with a vein, and the vein cover was placed on the vein so that one end of the vein cover was placed at the anastomosis, and the dog was observed for 84 days. Evaluation was performed in the same manner as in Example 1. The elasticity index and evaluation results are shown in Table 1.

[0074] The smooth muscle layer and the thicker elastic fiber layer containing collagen fibers were not observed, and intimal thickening occurred at the anastomosis with the vein, forming a thrombus, which extended into the ePTFE artificial blood vessel and caused obstruction. The result of evaluation based on the above criteria (A) to (E) was ×.

[0075] Comparative Examples 2 to 4 A vein cover was produced under the production conditions shown in Table 1, and a shunt was constructed in the anastomosis form as shown in Figures 9 and 13 as shown in Table 1, and evaluation was performed in the same manner as in Example 1. The elasticity index and evaluation results are shown in Table 1. The results of evaluation based on the above criteria (A) to (E) were all x. [Explanation of symbols]

[0076] 1: Shunt site 2: Arms 3: Artery 4: Veins 5: Artificial blood vessel 6: Anastomosis 10: Vein cover 10a: One end of the vein cover 10b: The other end of the vein cover 11: One part of the vein cover 12: Other part of vein cover 100: Cylindrical sample 101: 1st pin 102: 2nd pin M: Axial midpoint of the vein cover B: Blood flow direction F: Force pulling the second pin

Claims

1. A cylindrical vein cover to be placed in a vein anastomosed to an artery or an artificial blood vessel, the vein cover having a portion (A) having an elasticity index of 25 N or less when the inner diameter of the vein cover is expanded by 10% from its natural state by the following measurement method (hereinafter, referred to as 10% elasticity index), A vein cover in which the 10% elasticity index of the other part of the vein cover is smaller than the 10% elasticity index of the one part of the vein cover, when the portion from the midpoint between one end and the other end of the vein cover is defined as one part of the vein cover, and the portion from the midpoint to the other end is defined as the other part of the vein cover. [Measurement method] The vein cover is cut out by 5 mm in the axial direction to prepare a cylindrical sample having an axial length of 5 mm. A first pin and a second pin having a diameter d of 0.75 mm are inserted into the inner cavity of the cylindrical sample in parallel with the axial direction of the cylindrical sample. The first pin is fixed, and the second pin is pulled outward in the radial direction of the cylindrical sample with a force F. When the distance between the first pin and the second pin is L, the force F when πd+2L becomes 1.1 times the circumference of the cylindrical sample in its natural state is calculated. １．１ The value obtained by dividing by the strain [(1.1-1.0) / 1.0] is the 10% elasticity index.

2. 2. The vein cover according to claim 1, wherein the 10% elasticity index of the portion (A) is 0.1 mN or more.

3. A vein cover as described in claim 1 or 2, wherein the part (A) is arranged on one part of the vein cover.

4. 4. The vein cover according to claim 1, wherein the axial length of the portion (A) is 50% or more of the outer diameter of the artery or artificial blood vessel to which it is anastomosed.

5. 5. The vein cover according to claim 1, wherein the elasticity index of the portion (A) when the inner diameter is expanded by 20% from its natural state is 32 N or less when measured using the measurement method described below (hereinafter referred to as 20% elasticity index). [Measurement method] The vein cover is cut out by 5 mm in the axial direction to prepare a cylindrical sample having an axial length of 5 mm. A first pin and a second pin having a diameter d of 0.75 mm are inserted into the inner cavity of the cylindrical sample in parallel with the axial direction of the cylindrical sample. The first pin is fixed, and the second pin is pulled outward in the radial direction of the cylindrical sample with a force F. When the distance between the first pin and the second pin is L, the force F when πd+2L becomes 1.2 times the circumference of the cylindrical sample in its natural state is calculated. １．２ The value obtained by dividing by the strain [(1.2-1.0) / 1.0] is the 20% elasticity index.

6. The vein cover according to any one of claims 1 to 5, wherein the axial length is 5 mm or more.

7. The vein cover according to any one of claims 1 to 6, which is made of a biodegradable material.

8. The vein cover according to any one of claims 1 to 7, comprising at least one of knitted fabric, woven fabric, nonwoven fabric and porous material as a partial or entire component.

9. The vein cover according to any one of claims 1 to 8, which has an integral structure with the artificial blood vessel.

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