JP6967265B2 - Evaluation method and manufacturing method of medical base material - Google Patents

Description

The present invention relates to an evaluation method and a manufacturing method of a medical substrate.

The most important function of blood vessels is anticoagulant. The anticoagulant property is to prevent blood from coagulating in the blood vessel and impairing the flow of blood. Further, this anticoagulant property is borne by the intima layer existing on the outermost surface of the luminal surface of the blood vessel, and is maintained by the stable formation of this intima layer.

By the way, the conventional artificial blood vessel is made of a material that remains permanently in the living body (so-called bionon-absorbable material, for example, polyester, nylon, silk thread, etc.), for a certain period of time. Materials that decompose in the body (called biodegradable materials, such as polylactic acid, caprolactone, PGA, gelatin, collagen, etc.), or non-bioabsorbable materials and biodegradable materials Even if it was made by combining the above, when it was transplanted as an artery in the living body, it was not possible to maintain the state in which the intima layer was stably formed and maintained on the luminal surface.

Therefore, conventional artificial blood vessels cause various disorders such as thrombus formation, abnormal tissue growth, and abnormal morphology on the luminal surface of the wall, and there is a risk that the function as an artery deteriorates relatively early. rice field. In particular, relatively thin artificial blood vessels with a lumen diameter of 6 to 8 mm or less have insufficient anticoagulant properties, and when transplanted as arteries, thrombus formation and tissue overgrowth cause deterioration of vascular function within 6 months. It was easy (see Patent Documents 1 and 2 and Non-Patent Document 1).

Japanese Unexamined Patent Publication No. 2013-031595 Japanese Unexamined Patent Publication No. 2016-158765

"Artificial blood vessels", Japan Society for Artificial Organs, [online], [Search on April 24, 2017], Internet <URL: http://www.jsao.org/public/7.html>

The present invention is a medical substrate suitable for regeneration of the circulatory system, and is exposed to high pressure from a lumen such as a blood vessel of which a thrombus is easily formed, for example, a venous blood vessel or an artery, and has a lumen diameter. An object of the present invention is to provide an evaluation method and a manufacturing method of a medical base material, which makes it possible to provide a medical base material suitable for a relatively thin artificial blood vessel of 6 to 8 mm or less.

As a result of diligent studies, the inventors have made that a part or all of the layers constituting the wall of the artificial blood vessel have the specific strength described below in order for the intima layer to be formed and stably maintained in the artificial blood vessel. It has a layer made of a material with a deterioration period (this part is referred to as an outer layer below), and the outer layer has a certain range of rigidity (in other words, appropriate elasticity), and constitutes the outer layer. We found that the fiber spacing of the cloth needed to be wider than a certain range. Then, when the outer layer has a certain range of rigidity and fiber spacing, the intima layer resembling the intima layer of a natural blood vessel and the inner part of the media in contact with the outside (viewed from the luminal side). It was confirmed that a similar structure was formed in the lumen of the artificial blood vessel and that stable anticoagulability could be maintained for a long period of time. Then, it was confirmed that when these conditions were satisfied, it fulfilled a good function as an artificial blood vessel.

The material constituting the outer layer having a specific strength deterioration period is stereocomplex polylactic acid, an in vivo absorbable polymer having a slower strength deterioration in the living body, or a non-absorbable polymer (that is, the strength deterioration period is infinite). Large polymer). Specifically, a specific period of strength deterioration is (when an artificial blood vessel created with stereocomplex PLA is transplanted into an artery, it takes at least 6 months to 10 months or more for the strength to deteriorate. Based on the results of our experiments in which strength deterioration was observed from less than 6 months when normal PLLA was used in the same way), it refers to a strength deterioration period of 6 months or more to 10 months or more.

The method for evaluating a medical substrate of the present invention is a medical substrate having a sheet shape, a tube shape, or a combination thereof, which is transplanted into the body and used for regeneration of the circulatory system. It has a multi-layer structure including at least an inner layer arranged on the intima side of the circulatory system and an outer layer arranged on the outer membrane side of the circulatory system from the inner layer, and has a multi-layer structure outside the circulatory system rather than the inner layer. An index of rigidity determined by the following method for a medical substrate formed in a porous shape so that the layer arranged on the membrane side can reach the inner layer or penetrate into the vicinity of the inner layer. And also for the blood vessel to be transplanted, the index a of rigidity determined by the following method is calculated, and the index a of the rigidity of the outer layer of the tube-shaped medical substrate / the rigidity of the blood vessel to be transplanted. When the ratio of the index a of is within 7.5, it is evaluated that the medical substrate is suitable for the blood vessel to be transplanted.
Further, the method for producing a medical base material of the present invention has a sheet shape, a tube shape, or a shape in which these are combined, and is a medical base material that is transplanted into the body and used for regeneration of the circulatory system. It is a manufacturing method and includes at least an inner layer arranged on the inner membrane side of the circulatory system and an outer layer arranged on the outer membrane side of the circulatory system from the inner layer, and the outer membrane side of the circulatory system is more than the inner layer. In the step of forming a multi-layer structure formed into a porous shape so that the vasa vasorum reaches the inner layer or penetrates into the vicinity of the inner layer , and the blood vessel to be transplanted based on the above evaluation method. Includes a step to confirm suitability.
Hereinafter, the medical base material evaluated to be suitable for the blood vessel to be transplanted based on the evaluation method of the medical base material of the present invention may be referred to as "medical base material of the present invention".

[Method for determining rigidity index a]
(1) An L-shaped jig is sandwiched between the upper and lower chucks of the tensile tester, and the tube-shaped object to be measured is passed between the two L-shaped jigs to keep the cross section of the object to be measured round and circular. The object to be measured is pulled at a constant pulling speed from the state in which the object is being measured, and the inner walls facing each other are parallel to each other, but no force is applied. The length in the tensile direction is defined as D0, the length in the vertical axis direction of the object to be measured perpendicular to this is defined as L0, and the wall thickness of the object to be measured is defined as T0.
(2) From the start of measurement, the tension Nx, the length Dx in the tensile direction, the length Lx in the vertical direction of the object to be measured, and the wall thickness Tx are measured multiple times while pulling at the same tensile speed.
(3) Calculate the elongation rate X by the formula X = (Dx-D0) / D0 × 100 (%).
(4) When the stress σx in the tensile direction applied to the inner wall of the object to be measured is calculated by Eq. (4a), and it is assumed by Eq. (4b) that this stress is caused by the internal pressure of the object to be measured with an inner radius Rx. Calculate the tube internal pressure Yx corresponding to
(4a) σx ＝ Nx / (Tx × Lx × 2)
(4b) Yx = σx × Tx / Rx = π × Nx / 2 (Dx × Lx)
(5) The constants a and b are determined by the least squares method so that the obtained elongation rate X and the corresponding internal pressure Y approximate the linear function Y = aX + b (a and b are constants).

The ratio of the rigidity index a of the medical substrate to the rigidity index a of the transplanted blood vessel is preferably 5.5 or less. Further, the cloth constituting the outer layer is preferably a woven fabric or a knitted fabric when it is desired to make the diameter, arrangement, and distribution of the holes into which the feeding blood vessels enter as uniform as possible.

The inner layer of the medical substrate of the present invention is a group consisting of polyglycolic acid, a copolymer of lactic acid and caprolactone, L-polylactic acid, D-polylactic acid, a copolymer of glycolic acid and lactic acid, gelatin, collagen and elastin. It may be composed of at least one selected material. The inner layer may be made of a cloth made of a fiber material. Further, the cloth constituting the inner layer may be a non-woven fabric, a woven fabric or a knitted fabric.

Since the outer layer of the medical substrate of the present invention has appropriate rigidity, it can be used for regeneration of blood vessels such as arteries and veins, and the circulatory system such as heart and lymph vessels. Stable regeneration and maintenance of arteries (especially the intima and media) on which blood vessels are hung (especially the intima and media over a long period of time), and blood vessels in which blood clots are likely to form, such as venous blood vessels and lumen diameters of 6 to 8 mm or less Stable regeneration and maintenance of small arteries.

FIG. 1 is an external perspective view (a) and a sectional view (b) of the medical base material according to the present invention. FIG. 2 is a diagram for explaining a method of determining the index a of the rigidity of the outer layer constituting the medical substrate according to the present invention. FIG. 3 is an external perspective view (a) and a sectional view (b) of another medical base material according to the present invention. FIG. 4 is a drawing substitute photograph showing the result of transplanting the medical substrate (Example 1) according to the present invention into a dog. FIG. 5 is a drawing substitute photograph showing the result of transplanting the medical substrate (Example 2) according to the present invention into a dog. FIG. 6 is a drawing substitute photograph showing the result of transplanting the medical substrate (Example 3) according to the present invention into a dog. FIG. 7 is a drawing substitute photograph showing the result of transplanting the medical substrate (Example 4) according to the present invention into a dog. FIG. 8 is a drawing substitute photograph showing the result of transplanting the medical substrate (Example 5) according to the present invention into a dog. FIG. 9 is a drawing substitute photograph showing the result of transplanting a conventional medical base material (Comparative Example 1) into a dog. FIG. 10 is a drawing-substituting photograph showing the results of observing the aortic wall of a dog to which a medical substrate has not been transplanted.

1. 1. Medical base material The medical base material of the present invention has a sheet shape, a tube shape, or a shape in which these are combined, and is subjected to surgical operations such as artificial blood vessels, intravascular stents, and intravascular stent grafts. , Is transplanted into the body and used for regeneration of the circulatory system.

Therefore, the case where the medical substrate of the present invention is used as an artificial blood vessel will be described below with reference to the drawings. The use of the medical substrate of the present invention is not limited to artificial blood vessels.

FIG. 1 is an external perspective view (a) and a sectional view (b) of the medical base material 1 according to the present invention. As shown in this figure, the medical substrate 1 includes an outer layer 11 and an inner layer 12 arranged inside the outer layer 11. Here, the inner layer 12 may be integrated with the outer layer 11 or may be installed in multiple layers. In addition, such a medical base material 1 can be manufactured, for example, by manufacturing each layer separately and then fitting them by hand or by a known machine.

2. 2. Outer layer The outer layer 11 constituting the medical base material 1 of the present invention is a hollow tubular cloth made of a bionon-absorbable material, a biodegradable material, or a combination thereof, and has a constant rigidity index a described later. It is within the range of. Further, the outer layer refers to a part or all of the layers constituting the wall of the artificial blood vessel, which is made of a material having a specific strength deterioration period described below.

The material constituting the outer layer having a specific strength deterioration period is stereocomplex polylactic acid, an in vivo absorbable polymer having a slower strength deterioration in vivo, or a non-absorbable polymer (that is, the strength deterioration period is longer). Infinite polymer).

Specifically, a specific period of strength deterioration is (when an artificial blood vessel created with stereocomplex PLA is transplanted into an artery, it takes at least 6 months to 10 months or more for the strength to deteriorate. Based on the results of our experiments in which strength deterioration was observed from less than 6 months when normal PLLA was used in the same way), it refers to a strength deterioration period of 6 months or more to 10 months or more.

(1) Bio-non-absorbable material, bio-degradable material All bio-non-absorbable materials can be used for the outer layer. In addition to stereocomplex polylactic acid, biodegradable materials include polybutylene succinate, polyesteramide, copolyester, modified polyester, polyethylene succinate, polybutylene succinate, polyhydroxybutyrate, and polyvinyl. Alcohol, copolymers containing these, bacterial cellulose, etc. can be used. It is medically said that polyurethane does not cause strength deterioration in the living body, but in reality, strength deterioration due to hydrolysis is observed after 1 year or more of observation, so from the stereo complex PLA described here. It is contained in a bioabsorbable polymer whose strength deteriorates slowly in a living body.

When a biodegradable material or a material containing the same is used for the outer layer 11, the above-mentioned is performed for a certain period (6 months to 10 months or more) until the intima cells are replaced after the medical base material 1 is transplanted. The stiffness index a needs to be within a certain range. Therefore, when a biodegradable material is used, it is preferable to use a material having a long time of strength deterioration of 6 months to 10 months or more even if it is used for the wall of a blood vessel, for example, stereocomplex lactic acid.

(2) Cloth In the present specification, the cloth is a thin and wide plate processed from a large number of fibers, and the cloth is divided into woven fabric, knitted fabric, and non-woven fabric. As the fiber constituting the outer layer 11, a single fiber may be used, or a plurality of fibers may be blended and used. The fibers constituting the cloth may be monofilaments, twisted yarns, or roving yarns, but twisted yarns are preferable.

The cloth constituting the outer layer 11 is not particularly limited, and can be produced from a bioabsorbable material, a biodegradable material, or the like by a known method. Specifically, it may be manufactured as a woven fabric or knitted fabric (including mesh-like fabrics; the same shall apply hereinafter unless otherwise specified) by a known loom or knitting machine, and is known as an electrospinning method or a melt blow method. It may be manufactured as a non-woven fabric by the method of.

(3) Rigidity index a
The rigidity index a is an index showing the rigidity of the outer layer 11 and blood vessels, and is a value measured by a method described later. The index a of the rigidity of the outer layer portion of the artificial blood vessel is 1.0 to 30 mmHg, preferably 1.2 to 12 mmHg. The ratio of the rigidity index a of the outer layer portion of the artificial blood vessel to the rigidity index a of the blood vessel to be transplanted is 7.5 or less, preferably 5.5 or less.

(4) Method for determining rigidity index a FIG. 2 is a diagram for explaining a method for determining rigidity index a. With reference to this figure, the index a of the artificial blood vessel and the rigidity of the blood vessel is determined by the following procedures 1) to 5).

1) Place an L-shaped jig between the upper and lower chucks of the tensile tester. The outer layer or blood vessel of the artificial blood vessel is cut into a constant and sufficiently long length (for example, 100 mm) in the vertical axis direction to obtain an object to be measured. Pass the object to be measured between the two L-shaped jigs. From the state where the cross section of the object to be measured keeps a round circle, it is pulled at a constant pulling speed (for example, a sufficiently slow speed such as 10 mm / min), and as shown in FIG. 2A, the facing inner walls are parallel. However, the force was virtually not applied, and this was set as the start of measurement. In this state, the length in the vertical axis direction of the object to be measured is L0 (mm), the length in the tensile direction is D0 (mm), and the wall thickness of the object to be measured is T0 (mm).

2) From the start of measurement, while pulling at the same tensile speed, as shown in Fig. 2 (b), the tension Nx (N), the length Dx (mm) in the tensile direction of the object to be measured, and the object to be measured orthogonal to this. Measure the length Lx (mm) in the vertical direction of the object and the wall thickness Tx (mm) of the object to be measured multiple times.

3) The elongation rate x is calculated by the formula X = (Dx-D0) / D0 × 100 (%).

4) When the stress σx in the tensile direction applied to the inner wall of the object to be measured is calculated by Eq. (4a), and it is assumed by Eq. (4b) that this stress is caused by the internal pressure of the object to be measured having an inner radius Rx. Calculate the corresponding tube internal pressure Yx (mmHg).
(4a) σx ＝ Nx / (Tx × Lx × 2)
(4b) Yx = σx × Tx / Rx = π × Nx / 2 (Dx × Lx)

5) The constants a and b are determined by the least squares method so that the obtained elongation rate X and the corresponding internal pressure Y approximate the linear function Y = aX + b (a and b are constants). The unit of the obtained constant a is millimeter of mercury (mmHg).

(5) Fiber length, fiber diameter, fiber spacing, etc. When the cloth constituting the outer layer 11 is a cloth made of a fiber material such as a woven cloth, a knitted cloth, or a non-woven fabric, the fiber length, fiber diameter, and fiber of the fibers constituting the cloth are used. The ratio of the diameter to the length does not need to be particularly limited as long as the strength condition is satisfied. However, in consideration of the strength condition, the fiber diameter is preferably 0.1 to 50 μm, more preferably 0.5 to 30 μm in terms of the median value. When used as a vascular stent, 50 μm or more is preferable, and 100 μm or more is more preferable.

The distance from any one of the fiber stumps recognized in the cross section of the cloth to the stump of the adjacent fiber (hereinafter, abbreviated as fiber spacing) varies depending on the structure of the cloth and the thickness of the blood vessel. Specifically, in terms of the structure of the cloth, in the case of a non-woven fabric, the median value is preferably 5 μm to 1000 μm, more preferably 15 μm to 500 μm. If it is smaller than 5 μm, it becomes difficult for cells and especially feeding blood vessels to invade and settle, and it is difficult for the cloth to self-organize including blood vessels and stably regenerate and maintain the blood vessel wall structure. Further, if it is larger than 1000 μm, blood or the like may leak from the medical substrate 1 due to internal pressure such as blood pressure. For the same reason, in the case of woven fabrics and knitted fabrics, the fiber spacing is preferably 15 μm to 2000 μm, more preferably 30 μm to 1500 μm. In terms of the thickness of the blood vessel, it ranges from 20 μm to 1/4 of the outer peripheral length of the blood vessel, preferably 100 μm to 1/6 of the outer peripheral length of the blood vessel, and 250 μm to the outer peripheral length of the blood vessel. More preferably between 1/8 of the length of.

When the outer layer is layered a plurality of times around the inner layer, and the medical base material includes an intermediate layer described later, the fiber spacing of the fibers constituting the outer layer may be 1 mm to 4 mm.

The fiber length, fiber diameter, ratio of fiber diameter to length, and fiber spacing of the fibers constituting the cloth may be single, but it is preferable that there are variations. This is advantageous for cell proliferation and tissue regeneration by (a) imitating the fibers constituting the extracellular structure in the living body, and (b) if the deterioration rate differs depending on the variation, the strength of the regenerated tissue changes. This is because the risk of abnormal shape of regenerated tissue (abnormal expansion, rupture, stenosis, occlusion) and abnormal constituents (scarring, calcification, etc.) is reduced because the tissue changes gradually.

(6) Method for measuring fiber diameter and fiber spacing The fiber diameter and fiber spacing of the fibers constituting the cloth shown in (5) are median values measured as follows. The median value is one of the representative values and is a value located in the center when a finite number of data are arranged in order of size. When the number of data is an even number, it is the arithmetic mean value of two values near the center.

1) In the case of knitted fabric and woven fabric (a) Fiber diameter The fiber diameter of knitted fabric and woven fabric is obtained as follows. First, the cloth is cut and the cut surface is photographed with an optical microscope (20 to 100 times). Next, the captured image is taken into a computer image system, and the fiber diameter is measured using distance measurement software (theoretically, it can measure up to 0.01 μm).

The woven fabric and the knitted fabric are made by bundling a plurality of monofilament fibers into one weaving yarn or knitting yarn. Therefore, the fiber diameters of 50 monofilaments having a perfectly circular cross section selected at random are measured, and the median value thereof is taken as the fiber diameter of the cloth.

(B) Fiber spacing The fiber spacing of knitted fabric and woven fabric is determined by the following method. First, the surface of the cloth is photographed with a stereomicroscope (magnification of 10 times or less, light source irradiation from both the front side and the back side). The captured image is imported into a computer image system, and the captured image is measured for the fiber spacing using distance measurement software (theoretically, it can measure up to 0.01 μm).

The woven fabric and the knitted fabric are made by bundling a plurality of monofilament fibers into one weaving yarn or knitting yarn. Therefore, the fiber spacing is determined based on the size of the stitches formed between the adjacent weaving yarns (or knitting yarns) and the edges of the weaving yarns (or knitting yarns). Since this stitch has a substantially triangular shape, a square shape, or a pseudo-circular shape, how to obtain the fiber spacing in each case will be described below. If there are multiple textures (stitches) of multiple shapes and sizes, the description will be given for each.

If the texture (stitch) is approximately triangular, this is regarded as a triangle, and the average value of the three heights of the triangle is taken as the fiber spacing of this triangle. Then, the median value of the fiber spacing of 30 randomly selected triangles is taken as the fiber spacing of the cloth.

When the texture (stitch) is approximately square, the average value of the maximum and minimum values of the distance between the pair of opposing sides, and the maximum and minimum values of the distance between another pair of opposite sides. Calculate the average value of. Then, the weighted average of these four numerical values is taken as the fiber spacing of this quadrangle. Then, the median value of the fiber spacing of 30 randomly selected squares is used as the fiber spacing of the cloth.

When the weave (stitch) has a pseudo-circular shape, this is regarded as a circle, and the diameter of the circle is defined as the fiber spacing of this circle. Then, the median value of the fiber spacing of 30 pseudo-circular fibers randomly selected is taken as the fiber spacing of the cloth.

2) In the case of non-woven fabric (a) Fiber diameter The fiber diameter of the non-woven fabric is determined as follows. First, the non-woven fabric to be measured is frozen and cured in liquid nitrogen, and then cut. Next, the cut surface of the non-woven fabric is photographed with a scanning electron microscope. Then, 50 fibers are randomly selected from many fiber cross sections exposed on the cut surface of the non-woven fabric, and the fiber stump diameter is measured. The median value of the measured fiber diameter is defined as the fiber diameter of the non-woven fabric.

(B) Fiber spacing The fiber spacing of the non-woven fabric is determined as follows. First, the non-woven fabric to be measured is frozen and cured in liquid nitrogen, and then cut. Next, the cut surface of the non-woven fabric is photographed with a scanning electron microscope. Then, one fiber is randomly selected from many fiber cross sections exposed on the cut surface of the non-woven fabric, 30 other fibers are selected in order of proximity to the selected fiber, and the fiber spacing with the selected fiber is selected. And calculate the median value of the measured fiber spacing. Similarly, three medians are obtained for one nonwoven fabric, and the median of the obtained three medians is used as the fiber spacing of the nonwoven fabric.

3. 3. Inner layer The inner layer 12 constituting the medical base material 1 of the present invention is made of an easily biocompatible cloth, and does not maintain the outer shape of the medical base material 1 but promotes engraftment of endothelial cells and the like. It promotes self-renewal of the circulatory system such as the aorta, and is finally replaced with vascular endothelial cells and the like. In addition, the easy biocompatibility means that the affinity is richer than that of the material of the outer layer 11. Therefore, the inner layer 12 is rapidly absorbed by the living body, and is preferably absorbed by the living body in, for example, about 1 to 12 months.

(1) Easy biocompatible cloth The material of the easy biocompatible cloth constituting the inner layer 12 can be used without particular limitation as long as it is rich in biocompatibility. For example, polyglycolic acid, lactic acid and caprolactone can be used. Examples thereof include known bioabsorbable polymers such as copolymers of L-polylactic acid, D-polylactic acid, copolymers of glycolic acid and lactic acid, gelatin, collagen, and elastin.

The weight ratio of the monomers constituting the bioabsorbable polymer is not particularly limited as long as it satisfies the biocompatibility. Further, as long as it satisfies the easy biocompatibility, one kind of bioabsorbable polymer may be used alone, or two or more kinds of bioabsorbable polymers may be mixed and used.

The easily biocompatible cloth constituting the inner layer 12 can be produced by a known method without particular limitation as long as it satisfies the easy biocompatibility. Specifically, it may be manufactured as a woven fabric or a knitted fabric by a known loom or knitting machine, or may be manufactured as a nonwoven fabric by a known method such as an electrospinning method or a melt blow method.

When the cloth constituting the inner layer 12 is made of a fiber material such as a woven cloth, a knitted cloth, or a non-woven fabric, the fiber length, the fiber diameter, and the ratio of the fiber diameter to the length of the fibers constituting the inner layer 12 are easily biocompatible. As long as the sex is satisfied, there is no particular need to limit it. However, the median fiber diameter is preferably 20 μm or less, more preferably 10 μm or less. The reason is that if the fiber diameter is too large, turbulence of blood or the like is likely to occur, and the risk of closure of the vascular lumen due to thrombus formation increases.

The fiber spacing of the fibers of the biocompatible cloth constituting the inner layer 22 is preferably 100 μm or less, more preferably 60 μm or less in terms of the median value. When the inner layer 22 is a porous body, the fiber spacing (pore diameter) is preferably 200 μm or less, more preferably 100 μm or less in terms of its median value. When the fiber spacing (pore diameter) is large, when used for blood vessel regeneration, the risk of blood vessel obstruction due to thrombus formation increases, and it becomes impossible to prevent liquid such as blood from leaking from the blood vessel wall.

The fiber length, fiber diameter, ratio of fiber diameter to length, and fiber spacing of the fibers constituting the biocompatible cloth may be single, but it is preferable that there is a variation. The reason is the same as that of the outer layer 11. Further, the method for measuring the fiber diameter and the fiber spacing of the fibers constituting the biocompatible cloth is the same as that for the outer layer 11.

4. Intermediate layer The medical substrate of the present invention may be provided with an intermediate layer between the inner layer and the outer layer. FIG. 3 is an external perspective view (a) and a sectional view (b) of another medical base material 2 according to the present invention. As shown in this figure, the medical base material 2 includes an outer layer 21, an inner layer 22, and an intermediate layer 23 arranged between the outer layer and the inner layer.

In addition, such a medical base material 2 can be manufactured, for example, by manufacturing each layer separately and then fitting them by hand or by a known machine. Further, since the outer layer 21 and the inner layer 22 have the same configuration as the outer layer 11 and the inner layer 12 of the medical base material 1, respectively, the description thereof will be omitted.

The intermediate layer 23 is made of a biodegradable cloth, and while maintaining the outer shape of the medical base material 2 together with the outer layer 21, it assists in the regeneration of the vasa vasorum and the media, and is finally absorbed by the living body. (Hereafter, it is abbreviated as absorption condition.).

As the material of the cloth constituting the intermediate layer 23, known materials can be used without particular limitation as long as the absorption conditions are satisfied. Specifically, known bioabsorbable polymers such as polyglycolic acid, lactic acid and caprolactone copolymer, L-polylactic acid, D-polylactic acid, glycolic acid and lactic acid copolymer, gelatin, collagen, and elastin are available. Can be mentioned.

The weight ratio of the monomers constituting the bioabsorbable polymer is not particularly limited as long as the absorption conditions are satisfied. Further, if the absorption conditions are satisfied, one kind of bioabsorbable polymer may be used alone, or two or more kinds of bioabsorbable polymers may be mixed and used.

The cloth constituting the intermediate layer 23 can be produced by a known method without particular limitation as long as the absorption conditions are satisfied. Specifically, it may be manufactured as a woven fabric or a knitted fabric by a known loom or knitting machine, or may be manufactured as a nonwoven fabric by a known method such as an electrospinning method or a melt blow method.

When the cloth constituting the intermediate layer 23 is made of a fiber material such as a woven cloth, a knitted cloth, or a non-woven fabric, the fiber length, the fiber diameter, and the ratio of the fiber diameter to the length of the fibers constituting the intermediate layer 23 are absorption conditions. If the above is satisfied, there is no particular need to limit. However, the fiber diameter of the fibers constituting the cloth constituting the intermediate layer 23 is preferably 50 μm or less, more preferably 20 μm or less in terms of the median value.

In the case of a non-woven fabric, the fiber spacing of the cloth constituting the intermediate layer 23 is preferably 3 to 300 μm, more preferably 5 to 100 μm in terms of the median value. In the case of woven fabrics and knitted fabrics, the median value is preferably 15 μm to 1000 μm, more preferably 30 μm to 300 μm.

The fiber length, fiber diameter, ratio of fiber diameter to length, and fiber spacing of the fibers of the cloth constituting the intermediate layer 23 may be single, but it is preferable that there are variations. The reason is the same as the above-mentioned outer layer 11. The method for measuring the fiber diameter and the fiber spacing of the fibers constituting the cloth is the same as that for the outer layer 11.

The medical substrate of the present invention may have, for example, a sheet shape in addition to the tube shape shown in FIGS. 1 and 3. The sheet-shaped medical base material is, for example, wrapped around the affected area and used for its regeneration. In the case of this medical base material, the inner layer (innermost layer) arranged most inside the affected part promotes the regeneration of the affected part by engrafting endothelial cells and the like. On the other hand, the outer layer of the innermost layer, that is, the layer arranged on the outer membrane side of the circulatory system, maintains the strength of the medical substrate until the affected area is regenerated, and reaches or reaches the innermost layer of the vasa vasorum. Since it is formed in a porous shape so as to penetrate to the vicinity, it assists the growth of vasa vasorum and the growth of engrafted endothelial cells and promotes the regeneration of the affected area.

Further, the medical base material of the present invention may be provided with an outer layer, an inner layer and an intermediate layer shown in FIGS. 1 and 3, and may be provided with another layer as needed. For example, when a medical substrate is anastomosed to the aorta, a protective layer may be provided to protect the anastomosed portion. Further, the outer layer may be overlapped a plurality of times around the inner layer.

Further, the outer layer, the inner layer, and the intermediate layer may be a thin porous body other than the cloth, and when the porous body is a porous body, it can be produced by a known method without particular limitation.

In addition, instead of sewing the cloth into a cylindrical shape, the cloth may be manufactured in a tubular shape in advance by a manufacturing method such as electrospinning or a flat knitting machine.

The present invention will be described in more detail below based on examples and the like. The present invention is not limited to the following examples in any sense.

1. 1. Performance comparison The effect of differences in the materials that make up medical substrates on their performance was investigated by creating artificial blood vessels and transplanting them into the arteries of experimental animals. Specifically, the experiment was conducted as follows.

(1) Experimental animals and their acclimatization A non-pregnant female beagle dog weighing 7 to 10 kg or less and weighing 7 to 10 kg or less purchased from Shimizu Experimental Animal Co., Ltd. was used as an experimental animal (hereinafter abbreviated as dog). During the experiment, dogs were bred individually and bred under standard conditions for at least one week prior to the experiment, and were allowed to freely ingest standard dog feed and water.

(2) Preparation of artificial blood vessels Artificial blood vessels having different outer layer rigidity index a, Examples 1 to 5, and Comparative Examples 1 to 2 were prepared. The details of the artificial blood vessel produced will be described below.

1) Example 1
The outer layer and the intermediate layer cloth were overlapped, the overlapped cloth and the inner layer cloth were rolled into a tubular shape, and the walls were sewn with 6-0 polypropylene single thread suture to prepare a tube. These tubes were fitted by hand to prepare an artificial blood vessel (length 24 mm, inner diameter 5 mm to 6 mm). Finally, the stump of the artificial blood vessel was treated by heat melting, and the artificial blood vessel was further reinforced by applying a copolymer solution of polylactic acid / caprolactone. Artificial vessels were sterilized with ethylene oxide gas prior to use.

Outer layer:
Commercially available pantyhose fabric, non-run knitting Supporting yarn made by entwining monofilament nylon yarn with elastomer yarn Fiber spacing: Approximately 300 to 700 μm (because the mesh is irregular)
Number of rolls of cloth: Doubled 3 times (6 times)
Rigidity index a: 3.3mmHg

Middle class:
PLLA / CL (75% / 25%) Electrospinning non-woven fabric of copolymer fiber Fiber spacing: 32 μm (using spacer to expand fiber spacing)
Thickness: 200 (140-260, depending on the part) μm
Number of cloth turns: 3 times

Inner layer:
Polylactic acid electrospinning non-woven fabric Fiber spacing (average): 30 μm,
Fiber diameter (average): 3.9 μm
Thickness: 200 μm

2) Example 2
An artificial blood vessel (length 30 mm, inner diameter 6 mm) was prepared in the same manner as in Example 1.

Outer layer:
Knitting of stereo complex polylactic acid fiber Molecular weight of stereo complex polylactic acid: Approximately 200,000 Crystal melting point of stereo complex polylactic acid: 200 to 230 ° C
Monofilament diameter: 16-20 μm,
Number of monofilaments / twisted yarn: 78 false twisted yarns are used.
Number of cloth turns: 3 times Rigidity index a: 12mmHg

Middle class:
PLA / CL (75% / 25%) Electrospinning non-woven fabric of copolymer fiber Fiber spacing: 32 μm (using spacer)
Thickness: 200 (140-260, depending on the part) μm
Number of cloth turns: 3 times

Inner layer:
PLA / CL (50% / 50%) Electrospinning non-woven fabric of copolymer fiber Electrospinning non-woven fabric Fiber spacing: 11 μm
Fiber diameter: 0.8 μm
Thickness: Approximately 200 μm

3) Example 3
An artificial blood vessel (length 30 mm, inner diameter 6 mm) was prepared in the same manner as in Example 1.

Outer layer:
Stereo complex Polylactic acid fiber knitting cloth (plain knitting)
Molecular weight of stereocomplex polylactic acid: Approximately 200,000 Crystal melting point of stereocomplex polylactic acid: 200-230 ° C
Monofilament diameter: 16-20 μm
Number of monofilaments / twisted yarn: 42,
Plying spacing: Approximately 100-200 μm
Number of cloth turns: 3 times Rigidity index a: 8.5mmHg

Middle class:
PLA / CL (75% / 25%) Electrospinning non-woven fabric of copolymer fiber Fiber spacing: 32 μm (using spacer)
Thickness: 200 (140-260, depending on the part) μm
Number of cloth turns: 3 times

Inner layer:
PLA / CL (50% / 50%) Electrospinning non-woven fabric of copolymer fiber Fiber spacing: 6.5 μm
Fiber diameter: 0.8 μm
Thickness: Approximately 200 μm

4) Example 4
An artificial blood vessel (length 30 mm, inner diameter 6 mm) was prepared in the same manner as in Example 1.

Outer layer:
Stereo complex Polylactic acid fiber knitting cloth (plain knitting)
Molecular weight of stereocomplex polylactic acid: Approximately 200,000 Molecular weight of stereocomplex polylactic acid: Crystal melting point = 200-230 ° C
Monofilament diameter: 16-20 μm
Number of monofilaments / twisted yarn: 42,
Plying spacing: 700-1300 μm
Number of cloth turns: 2 times Rigidity index a: 1.2 mmHg

Middle class:
PLA / CL (75% / 25%) Electrospinning non-woven fabric of copolymer fiber Fiber spacing: 32 μm (using spacer)
Thickness: 200 (140-260, depending on the part) μm
Number of cloth turns: 2 times

Inner layer:
PLA / CL (50% / 50%) Electrospinning non-woven fabric of copolymer fiber Fiber spacing: 6.5 μm
Fiber diameter: 0.8 μm
Thickness: Approximately 200 μm

5) Example 5
A commercially available tubular artificial blood vessel was softly processed in its original shape and covered with an inner layer to prepare an artificial blood vessel (length 34 mm, inner diameter 7 mm). Finally, the stump of the artificial blood vessel was treated by heat melting and further reinforced by applying a copolymer solution of polylactic acid / caprolactone. Artificial vessels were sterilized with ethylene oxide gas prior to use.

Outer layer:
Commercially available artificial blood vessel (polyester cloth, manufactured by Terumo Corporation) tanned to soften the cloth Fiber spacing: 10.6 μm
Number of cloth turns: 1 Stiffness index a: 27mmHg

Middle class:
none.

Inner layer:
PLA / CL (75% / 25%) Electrospinning non-woven fabric of copolymer fiber Fiber spacing: 32 μm
Fiber diameter: 4.8 μm
Thickness: 300-350 μm

6) Comparative Example 1
An artificial blood vessel (length 20 mm, inner diameter 6 mm) was prepared. The following commercially available artificial blood vessel (made of polyester, inner diameter 7 mm) was used as an outer layer, and it was used by covering it with an inner layer (inner diameter 6 mm). Finally, the stump of the artificial blood vessel was treated by heat melting and further reinforced by applying a copolymer solution of polylactic acid / caprolactone. Artificial vessels were sterilized with ethylene oxide gas prior to use.

Outer layer:
A commercially available artificial blood vessel (made of polyester, made by Terumo, inner diameter 7 mm) was used as an outer layer in a hard state as it was.
Fiber spacing: 7.0 μm or less Rigidity index a: 53 mmHg

Middle class:
none.

Inner layer:
PLA electrospinning non-woven fabric Fiber spacing: 30 μm
Fiber diameter: 3.9 μm
Thickness: 440 μm (using spacer)

7) Comparative Example 2
An artificial blood vessel (length 25 mm, inner diameter 6 mm) was prepared. The following commercially available artificial blood vessel (made of polytetrafluoroethylene, inner diameter 7 mm) was used as an outer layer in a rigid state, and was used by covering it with an inner layer (inner diameter 6 mm). Finally, the stump of the artificial blood vessel was treated by heat melting and further reinforced by applying a copolymer solution of polylactic acid / caprolactone. Artificial vessels were sterilized with ethylene oxide gas prior to use.

Outer layer:
Use a commercially available artificial blood vessel (made of polytetrafluoroethylene, with outer wall reinforcement, manufactured by Nippon Gore Co., Ltd.) as it is in a hard state.

Middle class:
none.

Inner layer:
PLA electrospinning non-woven fabric Fiber spacing: 30 μm:
Fiber diameter: 3.9 μm
Thickness: 200 μm (using spacer)

(3) Experimental method All the following surgical procedures were performed by a single surgical team under aseptic conditions. The dog was basally anesthetized with 34 mg / kg pentobarbital intravenous anesthesia, a respiratory tube was infused into the dog's trachea, and general anesthesia was performed with 40% oxygen and inhalation anesthesia with sevoflurane or isoflurane. Under general anesthesia, the dog was fixed in the supine position and the abdominal hair was shaved. Skin was cleaned with 80% ethanol solution containing 5% chlorhexidine and disinfected with 10% povidone iodine solution.

A 15 cm laparotomy wound was placed in the middle of the abdomen and a retroperitoneal incision was made. The abdominal aorta on the peripheral side of the renal artery bifurcation was exposed to the common iliac artery bifurcation. During this detachment, the lumbar artery was ligated and dissected. Connective tissue around the artery was removed. After intravenous injection of 1000 units / kg body weight of parin into a small molecule, the aorta was grasped with two forceps. The aorta was resected over a length of 10 mm between the two forceps. The two stumps of the aorta were washed with heparin saline, an artificial blood vessel 25 mm long and 5-7 mm in diameter was placed between the two stumps of the aorta, and the aorta and tube were anastomosed end to end. That is, the tube stump and the aortic wall stump were sewn together with a 12-needle suture of 6-0 polypropylene single thread suture. The peritoneal incision edges were sewn together and the laparotomy wound was closed with a two-layer suture. Postoperatively, 2000 units of small molecule heparin and 100 mg aspirin or 1 mg warfarin anticoagulant therapy were given daily.

Ten to 18 months after surgery, dogs were euthanized by intravenous injection of 100 mg / kg pentobarbital. After reopening the abdomen, the aorta in the part where the artificial blood vessel was transplanted and the tissue around it were surgically resected as a mass, and the resected specimen was examined macroscopically and microscopically.

After macroscopically evaluating this excised specimen, it is fixed in a 10% neutral formalin solution to make a microscopic sliced specimen with a thickness of 4 μm by a standard method, and hematoxylin / eosin staining (HE staining) is applied to an optical microscope. Observed at. For comparison, the aortic wall of a (natural) dog without an artificial blood vessel was also observed with an optical microscope.

(4) Experimental Results The results of macroscopic observation of the transplanted portions of the artificial blood vessels of Examples 1 to 4 and Comparative Examples collected from dogs and the results of microscopic observation of the excised specimen are abbreviated below. In addition, the results observed with a microscope are shown in FIGS. 4 to 10.

1) Result of Example 1
Twelve months later, the dog was euthanized, and the artery at the part where the artificial blood vessel was transplanted was collected and observed. No abnormal findings such as thrombus formation, aneurysm or stenosis were observed by macroscopic observation. In addition, microscopic observation (FIG. 4) showed that the formation of the inner part of the intima and media was stable and good at 6 months, and it was very similar to the structure of the natural aorta shown in FIG. As described above, the overall evaluation of Example 1 was good.

2) Result of Example 2
Ten months later, the dog was euthanized, and the artery at the part where the artificial blood vessel was transplanted was collected and observed. No abnormal findings such as thrombus, aneurysm or stenosis were observed by macroscopic observation. In addition, microscopic observation (FIG. 5) showed that the formation of the inner part of the intima and media was stable and good. As described above, the overall evaluation of Example 2 was good.

3) Result of Example 3
After 17 months, the dog was euthanized, and the artery in the part where the artificial blood vessel was transplanted was collected and observed. No abnormal findings such as thrombus, aneurysm or stenosis were observed by macroscopic observation. In addition, microscopic observation (FIG. 6) showed that the formation of the inner part of the intima and media was stable and good. As described above, the overall evaluation of Example 3 was good.

4) Results of Example 4
Ten months later, the dog was euthanized, and the artery at the site where the arterial scaffold was transplanted was collected and observed. No abnormal findings such as thrombus, aneurysm or stenosis were observed by macroscopic observation. In addition, microscopic observation (FIG. 7) showed that the formation of the inner part of the intima and media was stable and good. As described above, the overall evaluation of Example 4 was good.

5) Results of Example 5
Ten months later, the dog was euthanized, and the artery at the site where the arterial scaffold was transplanted was collected and observed. No abnormal findings such as thrombus, aneurysm or stenosis were observed by macroscopic observation. In addition, microscopic observation (FIG. 8) showed that the formation of the inner part of the intima and media was stable and good. As described above, the overall evaluation of Example 5 was good.

6) Results of Comparative Example 1
Ten months later, the dog was euthanized, and the artery at the part where the artificial blood vessel was transplanted was collected and observed. Gross observation revealed that the lumen was completely obstructed by an organized thrombus. Further, in microscopic observation (FIG. 9), polyester fibers and PLLA fibers were observed in contact with the organized thrombus in the lumen. As described above, the overall evaluation of Comparative Example 1 was poor.

7) Results of Comparative Example 2
Ten months later, the dog was euthanized, and the artery at the part where the artificial blood vessel was transplanted was collected and observed.
As a result of macroscopic observation and actual microscopic observation, the formation of the intima and media on the lumen surface was unstable, and the artificial blood vessels were also exposed in a part of the lumen. As described above, the overall evaluation of Comparative Example 2 was poor.

In summary, after transplanting the artificial blood vessel of the example, when the transplanted part was visually observed, it was confirmed that the artificial blood vessel was functioning as a regenerated aorta. That is, it was confirmed that the transplanted part had atrium through which aortic blood flowed and had no thrombus. On the other hand, when the artificial blood vessel of the comparative example was transplanted, occlusion due to a thrombus was confirmed in the transplanted part.

Moreover, when the aorta transplanted with the artificial blood vessel of the example was observed under a microscope, the intima layer and the inner part of the media were regenerated, and the structure was very similar to the structure of the natural aorta. In addition, even if a part of the artificial blood vessel remained, the regeneration of a part of the intima layer and the media layer was good, and thrombus formation, abnormal tissue growth, stenosis / occlusion were not observed. On the other hand, when the artificial blood vessel of the comparative example was transplanted, the formation of the intima and media of the transplanted part was unstable, and the artificial blood vessel was also exposed in a part of the lumen of the transplanted part. ..

Furthermore, for Examples and Comparative Examples, the stiffness index a and its performance are summarized in Table 1. As shown in Table 1, the stiffness index a of Examples 1 to 4 was 1.2 to 12, and was distributed between 1/5 and 5.5 times the stiffness index a of the aorta to be transplanted. The stiffness index a of Example 5 was 27, which was 4.5 to 24 times the stiffness index a of the aorta to be transplanted. On the other hand, the stiffness indexes a of Comparative Examples 1 and 2 were 53 and 147, which were 8.8 to 67 times the stiffness index a of the transplanted aorta.

As described above, the ratio of the rigidity index a of the artificial blood vessel to the rigidity index a of the portion to which it is transplanted (here, the aorta) may be within 7.5 times (Example 5) on average, and is 0.2 to 5.5. It was found that doubling (all of Examples 1 to 4 were within this range) was more preferable. On the contrary, it was found that it is not preferable that the ratio is 8.8 times or more (Comparative Examples 1 and 2).

In addition, the effect of the size of the fiber spacing of the cloth constituting the outer layer of the artificial blood vessel on the transplantation result was also considered. As shown in Table 1, Example 1 (average 300 to 700 μm), Example 2 (average 400 to 1000 μm), Example 3 (about 100 to 200 μm), and Example 4 (700 to 1300 μm) having sufficiently large fiber spacing. ) Was evaluated well.

On the other hand, the evaluation result of Example 5 in which the maximum fiber spacing was only 10.6 μm was somewhat good, and the evaluation result of Comparative Example 1 in which the maximum fiber spacing was only 7.0 μm was poor. It was found that such a large fiber spacing is also related to the good evaluation result.

In addition to Examples 1 to 5, artificial blood vessels having various configurations are prepared by combining the outer layer cloth shown in Table 2, the intermediate layer cloth shown in Table 3, and the inner layer cloth shown in Table 4. Then, it was transplanted to a dog, and the change over time was observed with an ultrasonic diagnostic device for 6 months or more (up to 20 months later). The observation results are summarized in Table 5.

As shown in Table 5, the artificial blood vessels of the examples (38 in total) are arteries with an abnormal course such as arteriosclerosis, stenosis / occlusion, rupture, and aneurysmization during the observation period of 6 to 20 months. Only 4 regenerations were allowed. On the contrary, in the artificial blood vessels of the comparative example (18 in total), abnormalities were observed in 15 of them within the observation period (of these, 4 of them had abnormalities within 2 months).

Thus, by transplanting the artificial blood vessel of the present invention, the aorta could be regenerated with high probability. On the other hand, even with the same configuration, the artificial blood vessel (comparative example) having an outer layer whose rigidity index a deviated from a certain range could not properly regenerate the aorta. That is, it was found that the index a of the rigidity of the outer layer is related to the regeneration of the aorta.

The medical substrate of the present invention is a medical substrate suitable for regeneration of the circulatory system, and in particular, a lumen diameter that easily forms a thrombus when exposed to high pressure from the lumen such as an artery. It is suitable for relatively thin artificial blood vessels of 6 to 8 mm or less, venous and portal vein systems that easily form thrombi even if they are thick, and blood vessels for artificial dialysis shunts.

1, 2 Medical base material 11, 21 Outer layer 12, 22 Inner layer 23 Intermediate layer

Claims (6)

It has a sheet shape, a tube shape, or a combination of these, and is a medical base material that is transplanted into the body and used for regeneration of the circulatory system, and is arranged on the intima side of the circulatory system. an inner layer, an outer layer disposed on the outer film side of the circulatory system than the inner layer, has a multi-layer structure at least comprising a layer of the inner layer is disposed on the outer film side of the circulatory system, vasa vasorum For the medical substrate formed in a porous shape so that the blood vessel reaches the inner layer or penetrates into the vicinity of the inner layer, the index a of the rigidity determined by the following method is calculated, and the index a of the rigidity is calculated.
For the blood vessel to be transplanted, calculate the rigidity index a determined by the following method.
When the ratio of the rigidity index a of the outer layer of the tube-shaped medical substrate / the rigidity index a of the blood vessel to be transplanted is within 7.5, the medical substrate is suitable for the blood vessel to be transplanted. Evaluation method of medical base material to be evaluated.
[Method for determining rigidity index a]
(1) An L-shaped jig is sandwiched between the upper and lower chucks of the tensile tester, and the tube-shaped object to be measured is passed between the two L-shaped jigs to keep the cross section of the object to be measured round and circular. The object to be measured is pulled at a constant pulling speed from the state in which the object is being measured, and the inner walls facing each other are parallel to each other, but no force is applied. The length in the tensile direction is defined as D0, the length in the vertical axis direction of the object to be measured perpendicular to this is defined as L0, and the wall thickness of the object to be measured is defined as T0.
(2) From the start of measurement, the tension Nx, the length Dx in the tensile direction, the length Lx in the vertical direction, and the wall thickness Tx are measured multiple times while pulling at the same tensile speed.
(3) Calculate the elongation rate x by the formula X = (Dx-D0) / D0 × 100 (%).
(4) When the stress σx in the tensile direction applied to the inner wall of the object to be measured is calculated by Eq. (4a), and it is assumed that this stress is caused by the internal pressure of the object to be measured with the inner radius Rx by Eq. (4b). Calculate the tube internal pressure Yx corresponding to
(4a) σx ＝ Nx / (Tx × Lx × 2)
(4b) Yx = σx × Tx / Rx = π × Nx / 2 (Dx × Lx)
(5) The constants a and b are determined by the least squares method so that the obtained elongation rate X and the corresponding internal pressure Y approximate the linear function Y = aX + b (a and b are constants). When the ratio of the rigidity index a of the outer layer of the tube-shaped medical substrate / the rigidity index a of the blood vessel to be transplanted is within 5.5, the medical substrate is suitable for the blood vessel to be transplanted. evaluating the method of evaluating medical substrate of claim 1. Fabric, a woven or knitted, evaluation method of medical substrate of claim 2. At least one selected from the group consisting of polyglycolic acid, a copolymer of lactic acid and caprolactone, L-polylactic acid, D-polylactic acid, a copolymer of glycolic acid and lactic acid, gelatin, collagen, and elastin for the inner layer. It is made of material, method for evaluating medical substrate according to any one of claims 1 to 3. The inner layer is constituted by a readily biocompatible fabric made of fibrous material, method for evaluating medical substrate of claim 4. It is a method for manufacturing a medical base material that has a sheet shape, a tube shape, or a shape that combines these, and is transplanted into the body and used for regeneration of the circulatory system.
A layer having at least an inner layer arranged on the intima side of the circulatory system and an outer layer arranged on the outer membrane side of the circulatory system from the inner layer, and a layer arranged on the outer membrane side of the circulatory system rather than the inner layer. The process of forming a multi-layer structure formed into a porous shape so that the vasa vasorum reaches the inner layer or penetrates into the vicinity of the inner layer .
A step of confirming that the blood vessel is suitable for transplantation based on the evaluation method according to any one of claims 1 to 5.
A method for manufacturing a medical base material.

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