FR2558719A1 - SYNTHETIC VASCULAR GRAFT, PROCESS FOR THE PREPARATION OF SUCH A COLLAGEN-COATED GRAFT AND BLOOD SEALING AND SLURRY OF COLLAGEN FIBRILLA FOR ITS PREPARATION - Google Patents

Abstract

THE INVENTION RELATES TO A SYNTHETIC VASCULAR GRAFTING. ACCORDING TO THE INVENTION, IT INCLUDES A FLEXIBLE AND POROUS TUBULAR SUBSTRATE 12, THE SUBSTRATE HAVING, AT LEAST ON ITS INTERNAL SURFACE, A RETICULATED COATING 16 OF AT LEAST THREE LAYERS OF COLLAGEN FIBRILLES IN MIXED WITH A QUANTITY OF EFFICIENT ' A PLASTICIZING AGENT TO MAKE THE GRAFT BLOOD-TIGHT AND FLEXIBLE. THE INVENTION APPLIES IN PARTICULAR TO BLOOD VESSEL GRAFTS.

Description

25587 19

  The present invention relates to a synthetic vascular graft, and more particularly to a synthetic vascular graft having a series of layers of plasticized collagen fibrils that make the graft tight to blood without requiring precoagulation. The replacement of segments of human blood vessels with synthetic vascular grafts is well accepted in practice. Synthetic vascular grafts have taken a wide variety of configurations and are made of a wide variety of materials. Among accepted and successful vascular graft implants,

  those which are formed of a biological material

  compatible which maintains an open port to allow blood to flow through the synthetic graft after implant. The grafts may be made of biologically compatible fibers, such as Dacron and Teflon, may be knitted or woven and may be single-filament yarn, multi-filament yarn

or a discontinuous yarn.

  An important factor in selecting a substrate for a particular graft is the porosity of the fabric wall from which the graft is formed. Porosity is important because it controls the tendency for hemorrhage during and after implantation and controls the growth of tissue in the graft wall. It is desirable that the vascular graft substrate is sufficiently blood tight to prevent loss of blood during implantation, yet the structure must be sufficiently porous to allow growth of fibroblasts and smooth muscle cells to attach the transplant to the host tissue. Synthetic vascular grafts of the type disclosed in US Pat. Nos. 3,805,301 and 4,047,252, assigned to the present applicant, are flexible and elongated tubular bodies formed of a wire such as Dacron. In the first patent, the graft is a knit tube and in the last published patent is a double velvet synthetic graft marketed under the trade name Microvel, These types of graft have sufficiently porous structures to allow the growth of tissue hurry. The general process for implantation involves the precoagulation step, where the graft is immersed in the patient's blood and left for a time sufficient for coagulation to occur. After precoagulation, haemorrhages do not occur when the graft is implanted and tissue growth is not impaired. However, it is desirable to avoid precoagulation because it takes a good time

during the surgery.

  Enhanced blood-tight absorbable collagen grafts have been proposed in US Pat. No. 3,272,204. The type of collagen disclosed is obtained from the deep flexor tendon of cattle. Collagen derived from the tendon is generally highly crosslinked and difficult to treat by the enzym digestion process described in the patent. An additional reinforced vascular prosthesis is disclosed in U.S. Patent No. 3,479,670 which discloses an open mesh cylindrical tube surrounded by an outer helical wrap of a molten polypropylene monofilament which may be loaded with collagen fibrils to render the prosthesis impervious to bacteria and fluids. The collagen fibrils used are the same as those described in the patent

No. 3,272,204.

  The synthetic vascular grafts suggested by the prior art are claimed to be suitable for many applications. However, it remains desirable to produce a flexible vascular graft having virtually no porosity, yet remain sufficiently receptive to growth of the host tissue and which can be more easily treated.

  than the teachings of the prior art.

  A collagen coated synthetic vascular graft is formed of a tubular porous structure of a biocompatible filamentary material having a crosslinked coating of at least three layers of collagen fibrils in admixture with a plasticizing agent which

  makes the graft tight to the blood without precoagulation.

  The porous substrate of the graft may be a tubular vascular graft formed of a Dacron material

and can be woven or knitted.

  The collagen source is preferably bovine skin which has been treated by acid digestion resulting in a dispersion of the high purity fibrils. A purified aqueous slurry of collagen containing a plasticizer is applied to the synthetic vascular graft by massage to cover the entire surface to produce a flexible graft with a good feel. After at least three repeated applications of coating and drying the collagen is crosslinked by exposure to a formaldehyde vapor. The porosity of the graft is reduced to less than about 1% of the porosity of the graft before

coating.

  Accordingly, the subject of the present invention is

  an improved synthetic vascular graft.

  Another subject of the present invention is a graft

  Advanced synthetic vascular and blood tight.

  Another subject of the present invention is an improved synthetic vascular graft coated with

of collagen.

  Another object of the present invention is an improved coating of collagen derived from

skin of the cattle.

  Another object of the present invention is an improved method of preparing a graft

  synthetic vascular coated with collagen.

  Another object of the present invention is an improved method for coating a synthetic collagen vascular graft to make the graft tight.

to the blood.

  The invention will be better understood, and other purposes, features, details and advantages thereof

  will become clearer during the description

  explanatory text which will follow with reference to the accompanying schematic drawings given only by way of example illustrating several embodiments of the invention, and in which: - Figure 1 is a partial cross sectional view of a coated synthetic vascular graft collagen according to the invention; FIG. 2 is a partial cross-sectional view of a branched tubular graft of the type illustrated in FIG. 1; and FIG. 3 is a graph showing the porosity reduction after a series of collagen coatings according to the invention, the number of collagen layers being indicated on the axis of the collagen layers.

  abscissa and porosity on the ordinates.

  A synthetic vascular graft 10 constructed

  and arranged according to the invention is shown in FIG.

  The graft 10 comprises a tubular substrate portion 12 which is formed of a biologically compatible filamentary synthetic material, preferably a polyethylene terephthalate such as Dacron. Substrate 12 is a porous knitted Dacron knit fabric having an inner and outer velvet surface of the type disclosed in US Pat. No. 4,047,252. While the tubular portion 12 is formed of Dacron, any biocompatible filamentary material may be used. for the substrate provided it can be made into a porous structure that will allow tissue growth and maintain an open port for blood flow. The tubular portion 12a, on the inner surface, a collagen coating shown at 16. The collagen coating 16 is formed of at least three layers of an aqueous dispersion of collagen and plasticizer fibrils, which has been crosslinked by exposure to formaldehyde vapor. Figure 2 shows a bifurcated graft coated with collagen. The graft 20 includes

  a main tubular portion 22 and two branches

  24. The main tubular portion 22 and the bifurcated portions 24 are formed of a Dacron knitted substrate 26 having an inner surface coating of a collagen coating 28 formed from

  minus three layers of collagen fibrils.

  Porous vascular graft substrates suitable for use according to the invention are preferably produced from Dacron multifilament yarns by knitting or weaving processes which are

  commonly used in the manufacture of these products.

  In general, the porosity of the Dacron substrate is between about 2000 and 3000 ml / min-cm (purified water at 160 m bar). The internal coating of crosslinked coagulene is applied by filling a tubular substrate with a slurry of collagen and plasticizer and massaging by hand, removing the excess and allowing the deposited dispersion to dry after the final application. the collagen coating is crosslinked by exposure to formaldehyde vapor, air-dried and then dried under vacuum to remove excess moisture and excess formaldehyde. The grafts coated according to the present invention have

  essentially zero porosity.

  The following examples are given to illustrate the process for the preparation of purified collagen from

  bovine skin and grafts coated according to the invention.

  Examples are given to illustrate the invention

without in any way limiting it.

EXAMPLE 1

  Young calves, fetuses or dead animals were mechanically skinned from fresh calf skin and the skins were washed in a rotating container with cold running water until it was observed that water was free of surface dirt, blood and / or other dermis The dermis were mechanically cleaned to remove contaminating tissues such as fat and blood vessels. Subsequently, the skins were cut longitudinally into strips of about 12 cm wide and placed in a wooden or plastic container such as

  currently uses it in the leather industry.

  The skins were depilated using a

  1M Ca (OH) 2 scanning solution for 25 hours.

  Alternatively, the skin can be depilated by mechanical means or by a combination of mechanical and chemical means. After unstacking, the skins were cut into pieces of small dimensions of about 2.54 cm.

  x 2.54 cm and were washed with cold water.

  Following washing, 120 kg of bovine skin were placed in a vessel having 260 L of water, 2 L of 50% NaOH and 0.41 H 2 O 2 (35%). The components were slowly mixed for 12 to 15 hours at 4 ° C and washed with excess tap water for 30 minutes to produce partially purified skins. The partially purified skins were treated in a solution of 260 L of water, 1.2 L of NaOH (50%) and 1.4 Kg of CaO for 5 minutes with a slow mixture. We continued this treatment

  twice daily for 25 days.

  Following this treatment, the solution was decanted and discarded and the skins were washed with excess tap water for 90 minutes with constant agitation. The skins were acidified by treatment with 14 kg of HCl (35%) and 70 liters of water while subjecting the skins to vigorous stirring. The acid was left

  penetrate for about 6 hours. As a result of the acidification

  The skins were washed in an excess of tap water for about 4 hours or until a pH of 5.0 was reached. The pH of the skins was readjusted to 3.3-3.4. using acetic acid with a 0.5% preservative. The purified skin was then passed through a meat grinder and pressurized through a series of constantly decreasing mesh size filter screens. The final product was a smooth, homogeneous paste of pure collagen derived from

bovine skin.

  In order to impart adequate flexibility to the grafts

  in the dry state, a plasticizer such as glycerine.

  Sorbitol or other biologically compatible polyhydric plasticizer is added to an aqueous slurry of collagen prior to application. In a collagen slurry containing between about 0.5 and 5.0% collagen by weight, the plasticizer is present at about 4 to 12% by weight. Between about 10 and 25% ethanol can be

  present to accelerate the evaporation of water.

  The most important property obtained in coating a synthetic vascular graft with collagen and plasticizer coatings according to the invention is the reduction of the porosity of the porous substrate to about zero. By way of comparison, the porosity of twenty random and randomly selected Meadow Microvel synthetic vascular grafts had a mean porosity of 1796 ml / min-cm2 at 160 mbar and a standard deviation of 130. After coating according to the porosity is reduced to zero. The following example illustrates the method of coating the substrate of the graft according to the invention.

EXAMPLE 2

  A 50 cc syringe is filled with an aqueous slurry of 2% purified collagen of bovine skin prepared according to Example 1. The collagen slurry contains 8% glycerol, 17% ethanol and the remainder is water and has a viscosity of 30,000 cps. The syringe is placed in one end of a Meadox Microvel Medical Dacron graft 8 mm in diameter and about 12 cm long. The porridge is injected into the opening of the Microvel graft and massaged by hand to cover

  the entire internal surface of the collagen porridge.

  Any excess collagen slurry is removed by one of the open ends. The graft is allowed to dry for about 1/2 hour at room temperature. The coating and drying steps were repeated three times

Furthermore.

  Following the fourth coating application, the collagen coating was crosslinked by exposure to formaldehyde vapor for 5 minutes. The cross-linked graft was then air-dried for 15 minutes and then dried under vacuum for 24 hours to

  remove moisture and any excess formaldehyde.

EXAMPLE 3

  The blood-tightness of vascular grafts coated with collagen prepared according to Example 2 was tested as follows. An 8 mm × 12 cm Microvel graft was attached to a blood reservoir at a pressure of m bars due to the height of the reservoir. The blood was passed through the graft with hepine stability. The blood collected through the graft was determined and expressed in ml per mm-cm. The porosity of 5 tests was determined to be 0.04, 0.0, 0.0, 0.04 and 0.03. This represents an average porosity of 0.022 ml / min-cm 2 considered to be zero because the

  value is in the experimental error of the study.

  In order to compare this result to blood loss for uncoated Microvel, the experiment was repeated using an uncoated graft cloth. The

  average porosity was 36 ml / min-cm.

EXAMPLE 4

  The porosity of a collagen-coated fabric graft is reduced to less than about 1% after three coatings shown below. A standard water porosity test used to measure the water porosity of a graft is as follows. A column of water equivalent to 160 m bar of pressure is allowed to flow through a half-cm hole having a

  graft sample on the orifice for a minute.

  The amount of water collected was measured. The milliliters of water collected per minute per cm of area were calculated. Several readings are taken for each sample. The porosity is reported as follows:

porosity = ml / min / cm2.

  The water porosity of a Microvel graft fabric was about 1900 ml / min / cm 2. The porosity after coating was as follows: Number of coatings Porosity

0 1,900

1,266

2,146

3 14

4 5

2

6 0

  In each case, the collagen coating was a slurry of a plasticizer derived from bovine skin prepared according to the composition described in Example 2. These results are shown by the graph of FIG. 3. Based on this , it is preferable to produce a collagen coating of at least three layers of fibrils, and better four or five layers with drying between each application and crosslinking

  after the final layer to fix the coating to the substrate.

  In addition to the reduced porosity, the vascular grafts coated according to the invention have reduced thrombogenicity compared to uncoated grafts. The following examples demonstrate significantly lower thrombogenicity of vascular grafts

  impregnated with collagen compared to the control.

EXAMPLE 5

  Antithrombogenicity has been evaluated in vitro by the method of Imai and Nose (J. Biomed, Mater Res., 6, 165, 1972). According to the procedure, a volume of 0.25 ml of ACD (citric acid stabilized) blood was mixed with 25 μl of 0.1 m CaCl 2 and placed on the inner surface of a microvel graft coated with prepared collagen. according to Example 2. A similar volume was placed on an uncoated Microvel graft as a control. The same geometry of the blood spot was observed after 5, 10 and 15 minutes. The coagulation reaction was stopped by adding 5 ml of distilled water to the test samples. Striking differences were observed

  between the two grafts tested and the semi-

  Quantitative data that follow were determined: Impregnated with speed of soaking of collagen Simple matrix of fast blood transplantation slow formation of thrombus in 5 minutes O ++ 10 mn + +++ mn ++ ++++ A comparison of Thrombus formation on the inner surface of the collagen-impregnated Microvel graft and the control Microvel graft was as follows. In the collagen-impregnated transplant, there was no

  no formation of the fibrin clot in 5 minutes.

  At 15 minutes, the clot on the collagen-impregnated graft was much lower than in the corresponding single grafts. The surface of the Microvel graft in contact with the blood drop behaved almost as hydrophobic. It took about 10 to 15 seconds before the blood got into the fabric of the Dacron knitted graft. This is in contrast to the collagen-impregnated graft with soaked blood

  in the transplant matrix regularly and quickly.

  After 5 minutes there was no residue

  of thrombus detected on collagen coated graft.

  At the same time, a thin but definite thrombus was present on the surface of the single control Microvel graft. At 15 minutes, the total volume of thrombus volume present on the internal surface of the graft is smaller in the collagen-coated graft than in the controls. Based on the above observations under in vitro conditions, with no blood flow, the Dacron knitted Microvel graft coated with collagen was rapidly soaked with blood, with no formal thrombus in 5 minutes. At this point, the control graft showed thrombus formation. After 10 and 15 minutes, the amount of thrombi was lower in the collagen-impregnated grafts than in the

control grafts in simple Dacron.

EXAMPLE 6

  Thrombogenicity of Microvel grafts impregnated with collagen was tested in vitro (dogs) as follows. A venous bridge of the femoral artery (AV) was installed on greyhounds under deep anesthesia. A prosthetic material 5 cm long was fitted at both ends with plastic conical tubes for better handling. This allowed the easy insertion of a trial graft segment into the arterial bridge. After insertion, a venous forceps was slowly removed with the arterial end released slowly thereafter. The flow of blood was put

  circulating through the implant for 10 or 30 minutes.

  Then, the two ends of the bridge were locked again and the inserted prosthesis removed. The excess blood was drained and the weight established. The presence of thrombi adhering to the surface of the graft was observed macroscopically. The grafts were then washed (three times) in excess distilled water

and weighed again.

  As a control, a standard Dacron Microvel graft 6 mm in diameter was used. This graft was precoagulated insertion into the bridge. As a result, the test gave both visual and objective gravimetric evidence of thrombogenicity on the test surface. The weight of the blood oozing through the wall of the graft was also determined for

  record the difference between the samples tested.

  We will not find any bleeding with the

grafts impregnated with collagen.

  After insertion of a pre-coagulated control graft into the AV bridge, an average of 30 ml of blood / 5 cm of graft length was lost in the first 5 minutes. During the next 5 minutes, only 3-5 ml of blood was lost. In one of the control grafts tested for 30 minutes, a minimum bleeding of 1 ml / min / 5 cm through the precoagulated graft continued

during the whole test.

  Collagen impregnated grafts implanted during or 30 minutes showed the same pattern of resistance

  to the formation of the thrombus observedemacroscopically.

  A smooth, thin layer of scintillating proteinaceous material covers a layer of collagen. After washing repeatedly in distilled water, most of the prostheses are seen as a continuous film of protein (fibrin). A typical clot is not observed in the

prosthesis sample.

  Of the five pre-coagulated single Dacron grafted textured implants, three had distinct multiple thrombi. They were placed transversely in the direction

  of the flow of blood crossing 1/3 to 1/2 of the circumference

  ence. In the two remaining prostheses, one layer

  Protein-like skin covered the inner surface.

  The external surfaces of each control graft contained significant thrombi due to continuous bleeding at

through the wall.

  Based on these observations, the thrombogenecity of Dacron-transplanted collagen-grafted collagen was

  significantly lower than in the previous transplants

  coaguiated witness. This may be due to either reduced thrombus due to the collagen coating or to the formed thrombus

  in transplants because of the need for

  coagulation. Since blood coagulation is an event involved in excessive cell reaction with fibrotic replacement, it is advantageous to reduce thrombus formation in the matrix of Dacron graft leading to a lower risk of embolism. By applying at least three layers of the plasticizer and collagen fibrils to a porous synthetic vascular graft substrate according to the invention, desirable specific improvements are obtained when the graft is surgically placed in a human patient as a vascular replacement. . The

  anticipated benefits include, without limitation, the elimination of

  the need for pre-coagulation. Conventional porous grafts, although found to be necessary for long-term treatment, force the surgeon to pre-coagulate the graft with the patient's blood to prevent excessive loss of blood at the time of implantation. Typically, the pre-coagulation step is time consuming and requires a certain address. As a result, the primary goal of a collagen coating is to eliminate the need for pre-coagulation

synthetic grafts.

  The porous synthetic vascular graft substrates form an ideal matrix for tissue growth while eliminating the need for pre-coagulation. In addition, the considerably lower thrombogenicity of synthetic vascular grafts impregnated with collagen reduces the risk of embolism. The coating of a synthetic vascular graft with a collagen and plasticizer slurry in a series of coatings according to the invention also produces a vascular graft which

stays flexible with good touch.

Claims (17)

R E V E N D I C A T I ONS   A synthetic vascular graft characterized in that it comprises: a flexible and tubular porous graft substrate (12); said substrate having, on at least its inner surface, a crosslinked coating (16) of at least three layers of collagen fibrils mixed with an effective amount of a plasticizer to render   the blood-tight and flexible graft.   2. The graft of claim 1, characterized in that the porous substrate is polyethylene terephthalate. 3. Graft according to claim 2, characterized   in that the porous substrate is knitted.   4. Graft according to claim 2, characterized   in that the porous substrate is woven.   5. Graft according to claim 1, characterized in that the inner and outer surfaces of the substrate have a velvet surface.   6. Graft according to claim 1, characterized   in that the collagen fibrils are crosslinked by.   exposure to formaldehyde evaporator.   7. Graft according to claim 1, characterized   in that the plasticizer is a poly-plastic material Biologically compatible water.   8. Graft according to claim 7, characterized   in that the plasticizer is sorbitol.   9. The graft of claim 7, characterized   in that the plasticizer is glycerine.   The graft of claim 1, characterized in that the coating layers are formed from a deposited aqueous slurry having about 0.5 and 5.0 percent collagen fibrils and about 4 to 12 percent of the collagen fibrils. plasticizer by weight 11. The graft according to claim 1, characterized in that the collagen fibrils are derived bovine skin.   12. The graft according to claim 1, characterized in that the inner and outer surfaces. external of the grafts are coated.   13. A method for preparing a collagen-coated and blood-tight synthetic vascular graft characterized in that it comprises: forming a tubular and porous flexible synthetic graft substrate, placing an aqueous slurry of collagen fibrils and a plasticizer on at least the inner surface of the substrate; massaging said substrate to ensure intimate mixing of the collagen fibrils on the porous structure of the substrate; to dry collagen; placing a second layer of collagen slurry and the plasticizer on the first layer of the substrate and drying; placing a third layer of collagen and plasticizer slurry on the substrate and drying, cross-linking the collagen coating by exposure to formaldehyde vapor; and vacuum drying to remove excess formaldehyde. A porridge of collagen fibrils for forming a blood-tight synthetic vascular graft characterized in that it comprises 0.5 to 5.0 percent collagen fibrils, 4.0 to 12.0 percent a biologically compatible plasticizer, the rest being water.   15. The porridge according to claim 14, characterized in that the collagen fibrils are obtained by   acid digestion of the bovine skin.   16. The porridge as claimed in claim 14, characterized in that the plasticizing agent is chosen from the group   consisting of sorbitol and glycerin.   17. A synthetic vascular graft characterized in that it comprises: a flexible and porous tubular graft substrate of polyethylene terephthalate; the inner surface having a coating of at least five layers of crosslinked collagen fibrils mixed with a plasticizer; and the layers being formed of an aqueous slurry containing between about 1.5 and 4.0 percent by weight of collagen fibrils and between about 6 and about   percent by weight of a plasticizer.