CA1264207A

CA1264207A - Collagen synthetic vascular graft composite

Info

Publication number: CA1264207A
Application number: CA000473089A
Authority: CA
Inventors: Harmon Hoffman, Jr.; Kemal Schankereli
Original assignee: Meadox Medicals Inc
Current assignee: Boston Scientific Scimed Inc
Priority date: 1984-01-30
Filing date: 1985-01-29
Publication date: 1990-01-09
Also published as: SE8500421D0; GB8502157D0; GB2187463A; NL193264B; SE8500421L; IL74179A0; JPS60203264A; CH670379A5; NL8500240A; AU577826B2; GB2153685B; FR2558719A1; AU3819585A; FR2558719B1; NL193264C; BE901610A; SE464058B; GB2187463B; IL74179A; JPH0636818B2

Abstract

ABSTRACT OF THE DISCLOSURE

A collagen-treated vascular graft includes a porous synthetic vascular graft substrate formed by knitting or weaving with at least three layers of dispersed collagen fibrils.
Plasticizers are incorporated into the collagen slurry before application and the slurry is applied in separate treatments by massage and flow through techniques, cross-linked by exposure to formaldehyde vapor and dried.

Description

1'~6~Z~7 CoLLAGEN-coATED SYNTHETIC VASCULAR GRAFT

BAcxGRouND OF THE INVENTION
This invention relates to a synthetic vascular graft, and more particularly to a synthetic vascular graft having a series of plasticized collagen fibril layers which renders the graft blood-tight without the need to be preclotted.
The replacement of segments of human blood vessels with synthetic vascular grafts is well accepted in the art. Synthetic vascular grafts have taken a wide variety of configurations and are formed of a wide variety of materials. Among the accepted and successful vascular graft implants are those which are formed from a biologically compatible material which retains an open lumen to permit blood to flow through the synthetic graft after implant.
The grafts may be made from bioloyically compatible fibers, such as Dacron and Teflon*, may be knitted or woven and may be of a mono-filament yarn, multi filament yarn or staple yarn.
An important factor in the selection of a particular graft substrate is the porosity of the fabric wall of which the graft is formed. Porosity is significant because it controls the tendency to hemorrhage durin~ and after implantation and controls the ingrowth of tissue into the wall of the graft. It is desirable that the vascular graft substrate be sufficiently blood-tight to prevent the loss of blood during implant, yet the structure must be sufficiently porous to permit ingrowth of fibroblast and smooth muscle cells in order to attach the graft to the host tissue.
Synthetic vascular grafts of the type described in United States Patents NoO 3,805,301 and No. 4,047,252, assigned to the assignee of the subject application, are elongated flexible tubular bodies *Trade Mark sP~ 7 formed of a yarn such as Dacron. In the earlier patent, the graft is a warp knitted tube and in the latter is~ued patent it is a double-velour synthetic graft marketed under the trademark Microvel. These types of grafts have sufficiently porous structures to permit ingrowth of host tissue. The general procedure ~or implantation includes the step of preclotting, wherein the graft is immersed in the blood of the patient and allowed to stand for a period o~ time sufficient for clotting to insue. After pre-clotting hemorrhaging does not occur when the g~raft is implanted and growth of tissue is not impeded. However, it is desirable to avoid preclotting as it take~ valuable time during surgery.
Blood-tight absorbable collagen reinforced gra~ks have been proposed in United Stat~s Patent No. 3,272,204. The type of collagen disclosed is obtained from the deep fl~xor tendon of cattle. Tendon-derived collagen iæ generally highly cross-linked and di~ficult to process by the enzyme digestion procedure described in the patent. An additional r~.inforced vascular prosthesis is described in United States Patent No. 3,479,670 which discloses an open mesh cylindrical tube wrapped by an outer helical wrapping of fusad polypropylene mono-filament which may be filled wikh collagen ~ibrils to render the prosthesi impermeable to bacteria and fluid~. The collagen fibrils utilized are the same as described in Patent No. 3,272,204.
The synthetic vascular grafts.suggested by the prior art are claimed to be suitable ~or many applications. However, it remains desirable to provide a ~lexible vascular graft which exhibit~ virtually ~ero porosity, yet remains sufficiently ,~
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li~6~2r~7 receptive to ingrowth of host tissue ancl which may be more easily processed than the teachings of the prior ar~.

SUMMARY OF THE INV~ENTION
A collagen-impregnated synthet:ic vascular graft composite including a tubular flexible porous substrate having on the inner and outer surfaces and extending through the porous structure of the substrate cross-linked collagen fibrils admixed with a plasticizer is provided. The porous graft substrate may be a tubular vascular graft formed of a Dacron material and may be woven or knit. The collagen source is an aqueous fibril dispersion which may be of high purity bovine skin collagen including a plasticizer.
The collagen is applied to the graft substrate by massage to cover the entire inner surface and penetrate into the substrate to ensure intimate mixing of the collagen fibril complex into the porous structure of the substrate and extend over the outer surface thereof. The collagen is cross-linked preferably by exposure to formaldehyde vapor. The collagen graft is blood tight, flexible with good hand and is resolved by the body after implant.
Still other advantages of the in~ention will in part be obvious and will in part be apparent from the specification.
The invention accordingly comprises the article possessing the features, properties and the relation of elements and the se~eral steps and the relation of one or more of such steps with respect to each of the others, which are exemplified in the following detailed disclosure, and the scope of the invention will be indicated in the claims~

4 ~2642~7 BRIEF DESCRIPTION OF TH~ DRAWI~
For a fuller understanding of the invention, reference is had to the following description taken in connection with the accompanying drawing, in which:
FIG. 1 is a partial cross-sectional view of a collagen treated synthetic vascular graft in accordance with 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 reduction of porosity after a series of collagen treatments in accordance with the invention.

~ESCRIPTION OF THE P~EFERRED EMBODIMENTS
A synthetic vascular graft 10 constructed and arranged in accordance with the invention is shown in Fig. 1. Graft 10 includes 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 Dacron warp knit fabric having an inner and outer velour surface of the type described in U.S. Patent 4,047,252. While tubular portion 12 is formed of Dacron, any biocompatible filamentary material may be used for the substrate provided it may be fabricated into a porous structure which will permit tissue ingrowth and maintain an open lumen for flow of blood.
Tubular portion 12 has applied to its inner surface collagen as shown at 16. Collagen 16 i5 formed from at least three layers of an aqueous collagen fibril and plasticizer dispersion which has been cross-linked by exposure to formaldehyde vapor.
Fig. 2 shows a bifurcated collagen-treated graft 20. Graft 20 includes a main tubular portion 22 and two branches 24. Main ~Z6~7 tubular por~ion 22 and bifurcate~ portions 24 are formed from a Dacron knit substrate 26 having an inner surface treated with collagen 28 formed from at least three layers of collagen fibrils.
Poroug vascular graft substrates suitable for use in ac-cordance with the invention, preferably are produced from Dacron multi-filament yarns by knitting or weaving processes which are commonly used in manufacture of these products. Generally, the porosity of the Dacron su~strate ranges from about 2,000 to 3,000 ml/min-cm2 (purified water at 120 mmHg). The cross-linked collagen is applied by filling a tubular substrate with a collagen and plasticizer slurry and massaging manually, removing the excess and permitting the deposited dispersion to dry. After the final application, the collagen is cross-linked by exposure to formaldehyde vapor, air dried and then vacuum dried to remove excess moisture and excess formaldehyde. The treated grafts in accordance with the invention have essentially zero porosity.
The following examples are set forth to illustrate the method of preparing purified collagen from bovine skin and treated grafts in accordance with the invention. The examples are set forth for purposes of illu6tration and not intended in a limiting sense.
Example 1 Fresh calf skins were mechanically stripped from young calves, fetuses or stillborn~ and washed in a rotating vessel with cold running water until the water wa~ observed to be free from surface dirt, blood and/or tissues. The subcutis was mechanically cleaned to remove contaminating tissues, such as fat and blood vessels. Subsequently, the skins were cut in the longitudinal 6 ~;~6~LZO ~
direction into strips about 12 cm wide c~nd were placed in a wood or plastic vessel as commonly used in the leather industry.
The skin were dehaired by us:ing a flusher solution of 1 M Ca(OH)2 for 25 hours. Alternatively, the skin may be dehaired by mechanical means or by a combination of chemical and mechanical means. Following the dehairing, the skins were cut into small size pieces about 1" x 1" and were washed in cold water.
Following washinq, 120 Kg of the bovine skin was placed in a vessel having 260 L water, 2 L NaOH (50%) and 0.4 L H202 (35%~.
The components were mixed slowly for 12 to 15 hours at 40C and washed with an excess of tap water for 30 minutes to provide partially purified skins. The partially purified skins were treated in a solution of 260 L water, 1.2 L NaOH (50%) and 1.4 Kg CaO for 5 minutes with slow mixing. This treatment was continued twice daily for 25 days. Following this treatment, the solution was decanted and discarded and the skins were washed with an excess of tap water for 90 minute~ under constant stirring.
The skins were acidified by treatment with 14 kg HCl (35%) and 70 L water while subjecting the skins to vigorous stirring. The acid was allowed to penetrate the skins for about 6 hours. Following 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 extruded under pressure through a series of filter sieves of constantly decreasing mesh size. The final product was a white homogeneous smooth paste of pure bovine skin-derived collagen.

r ~6~ 7 In order to impart adequate pliability to the grafts in the dry state, a plasticizer uch as glycerine, sorbitol or other biologically acceptable plasticizer is added to an aqueous collagen slurry before application. In a collagen slurry containing between about 0.5 to 5.0% collagen by weight, the plasticizer is present between about 4 and 12 weight percent. Between about 10 to 25 percent ethanol may be present to hasten evaporation of the water.
The most important property obtained when treating a synthetic vascular graft with collagen and plasticizer in accordance with the invention is reduction of porosi y of the porous substrate to about zero. For comparison, the porosity of twenty randomly selected untreated Meadox Microvel synthetic vascular grafts had a mean porosity to water of 1796 ml/min-cmZ at 120 mm Hg and a standard deviation of 130. After treatment in accordance with the invention, the porosity is reduced to zero.
The following example illustrates the msthod of treating the graft substrate in accordance with the i~vention.
Example 2 A 50 cc syringe i8 filled with an aqueous slurry of 2%
purified bovine skin collagen prepared in accordance with ~xample 1. The collage~ filurry includes 8% glycerol, 17% ethanol and the remainder water and a viscosity of 30~000 cps. The syringe is placed into one end of a Meadox Medical Microvel Dacron graft 8 mm in diameter by approximately 12 cm in length. The slurry is injected into the lumen of the Microvel graft and it is massaged manually over the entire inner surface area with the collagen slurry. Any excess collagen slurry is removed through one of the open ends. The graft is permitted to dry for about 1/2 hour at 8 ~L~ 7 room temperature. The treating and drying steps were repeated three more times.
Following the fourth treating application, the collagen treating was cross-linked by exposure to formaldehyde vapor for 5 minutes. The cross-linked graft was then air dried for 15 minutes and then vacuum dried for 24 hours to remove moisture and any excess formaldehyde.
ExamPle 3 The blood-tightness of a collagen-treated vascular graft prepared in accordance with Example 2 was tested as follows.
Microvel* graft 8 mm x 12 cm was attached to a blood reservoir at a pressure of 120 mm ~g due to the height of the reservoir.
Heparin stabilized blood was passed through the graft. Blood collected through the graft was determined and expressed in ml per min-cm2. The porosity of 5 runs was determined to be 0.04, 0.0, o . o, n . 04 and 0.03. This represents a mean porosity of 0.022 ml/min-cm2 which was considered zero, as the value is within the experimental error of the study.
In order to compare this result with the blood loss for untreated Microvel, the experiment was repeated using an untreated graft fabric. The mean porosity was 36 ml/min-cm2.
Example ~
The porosity of a collagen treated fabric graft is reduced to less than about 1% after three applications which was demonstrated as follows. A standard water porosity test used to measure water porosity of a graft is as follows. A column of water equivalent to 120 mm Hg pressure is allowed to flow through *Trade Mark g lZ~ Q7 a one-half Cm2 orifice having a sample of graft over the orifice for one minute. The amount of water collected was measured. The milliliters of water collected per minute per cmZ squared area was calculated. Several readings are taken for each sample. The porosity is reported as follows:
porosity = ml/min-cm2.
The water porosity of a ~icrovel graf~ fabric was about 1,900 ml/min-cmZ. The porosity after treatment was as follows:

Number of TreatmentsPorositv 0 1,900

2 146

3 14

4 5 In each case the collagen was a bovine skin derived plasticizer slurry prepared in accordance with the composition described in Example 2. These results are set forth in the graph of Fig. 3. Based on this, it is preferable to provide collagen in at least three layers of fibrils, and most preferably four or five layers with drying between each application and cross-linking after the final layer to fix the collagen to the substrate.
In addition to reduced porosity, collagen treated vascular grafts in accordance with the invention exhibit reduced thrombogenicity compared to untreated grafts. The following examples demonstratP significantly less thrombogenicity of collagen impregnated vascular grats compared to controlled.

1~6gz~7 Example ~
Antithrombogenicity was evaluated in vitro by the method of Imai and Nose (J. Biomed, Mater Res. 6, 165, 1972). In accordance with the procedure, a volume of 0.25 ml of ACD blood (citric acid stabilized) was mixed with 25 ul of 0.1 m CaC12 and placed onto the inner surface of a collagen treatPd Microvel graft prepared in accordance wi~h Example 2~ A similar volume was placed on an untreated Microvel graft a6 a control. The same geometry of the blood spot was observed after 5, 10 and 15 minutes.
The clotting reaction was stopped by adding 5 ml distilled water to the test samples. Striking differences wQre observed between the two tested grafts and the following semi-quantitative parameters were ascertained:

Collagen Impreanated Plain Rate of soaking the graft matrix with blood fast slow Thrombus formation in 5 min O ++
10 min + +++
15 min ++ ++++
A comparison of thrombus formation onto the inner surface of the collagen impregnated Microvel graft and control ~icrovel graft wa~ a~ follows. In the collagen impregnated graft there was no fibrin clot formation in 5 minutes. A~ 15 minutes the clot on the collagen impregnated graft was much less than in the corresponding plain grafts.
The surface of the Microvel graft in contact with the drop of blood behaved almost as hydrophobic. It took between about 10 and 15 seconds before the blood soaked into the fabric of the 11 1.Z642~7 Dacron knitted graft. This contrasts with the collagen impregnated graft with blood soaked into the graft matrix evenly and fast.
After 5 minutes there was no thrombus residue detected on the collagen treated graft. At the same time, a thin but definite thrombus was present on the surface of the plain control Microvel graft. At 10 and 15 minutes, the total volume of thrombus present on the graft inner ~urface was less in the collagen treated graft than in the controls.
Based on the above observations under in vitro conditions, with no blood flow, the collagen treated Dacron knitted Microvel graft soaked quickly with blood, without forming any thrombus within 5 minut~s. At this time the control graft showed thrombus formation.
Later after 10 and 15 minutes the amount of thrombus was less in collagen impregnated grafts than in plain control Dacron grafts.
Example 6 The thrombogenicity of collagen impreqnated Microvel grafts was tested in vitro (dogs1 as follows. A femoral arterial venous (AV~ shunt was installed in greyhounds in deep anesthesia.
A 5 centimeter long prosthetic material was adapted at both ends with plastic conical tubings for better handling. This allowed easy insertion of a test graft segment into the arterial shunt.
After insertion, a venous clamp was slowly removed with the arterial end rel~ased slowly thereafter. The blood flow was circulating through the implant for 10 minutes or 30 minutes.
Then, both ends of the shunt were clamped again and the inserted prosthesis removed. The excess of blood was drained off and the 12 ~6~
weight established. The presence of graf~t surface adhering thrombi was observed macroscopically. The grafts were then washed (three times) in excess distilled water and weighed again.
As a control a standard 6 mm diameter Dacron Microvel graft was used. This graft was preclotted before insertion into the shunt. Accordingly, this test provided both visual and objective gravimetric evidence of ~he thrombogenicity on the surface tested. The weight of blood oozing across the graft wall was also ascertained to record the difference between the tested samples.
No bleeding at all was found with collagen impregnated grafts.
After insertion o~ a pre-clotted control graft into the AV shunt, an average of 30 ml blood/5 cm long graft was lost in the first 5 minutes. In the next 5 minutes, only 3-5 ml blood was lost. In one of the control grafts tested for 30 minutes, minimum bleeding of 1 ml/min/5 cm through the pre-clotted graft continued for the entire test.
The collagen impregnated grafts implanted for 10 or 3u minutes showed the same pattern of resistance to thrombus formation observed macroscopicall~. A thin smooth layer of glistening proteinaceous material covers the collagen layer. After washing repeatedly in distilled water a continuous film of protein~
(fibrin~ is seen in most prosthesis. A typical clot was not observed in the sample prosthesis.
Qf the five tested pre-clotted plain Dacron graft implants, three exhibited distinct multiple thrombi. These were located transverse to the direction of blood flow crossing 1/3 to 13 ~ Z~ 7 1/2 of the circumference. In the remaining two prosthesis, a similar skin proteinaceous layer covered the inner surface. Th~
outer surfaces of each control graft contained large thrombi due to continuous bleeding through the wall.
Based on these observations, thrombogenicity of collagen impregnated vascular Dacron grafts was significantly less than in control pre-clotted grafts. This could be due to either reduced thrombus because of the collagen or to the thrombus formed in the control grafts due to the need for pre-clotting. As blood clotting is an event participating in excessive cell reaction with the fibrotic replacement, it is advantageous to reduce thrombus formation within the matrix of the Dacron graft leading to a lesser risk of emboli.
By applying at least three layers of a collagen fibril and plasticizer layers to a synthetic porous vascular graft substrate in accordance with the invention specific desirable improvements are obtained when the graft is surgically placed in a human patient as a vascular replacement. The anticipated benefits include, but are not limited to elimination of the necessity for pre-clotting. Conventional porous grafts, although proven to b~ necessary for long-term patency, made it nece~sary for the surqeon to pre-clot the graft with the patient's blood in order to prevent excessive bloodloss at the time of implant.
Typically, the pre-clotting step is a time-consuming one requiring some practice and skill. Accordingly, it has been a primary objective of collagen treatment to eliminate the need for pre-clotting synthetic grafts altogether.

14 ~Z~;4207 The porous synthetic vascular gra~t substrates provide a ideal matrix for tissue ingrowth while eliminating the need for preclotting. Additionally, the signi~icantly lower thrombo-genicity of collagen impregnated synthetic vascular grafts reduce the risk of emboli. Coating a synthetic vascular graft with a collagen and plasticizer slurry in a series of treatments in accordance with the invention also provides a vascular graft which remains flexible with good hand.
It will thus be seen that the objects set forth above, among those made apparent from the preceding description, are efficiently attained and, since certain changes may be made in carrying out the aboYe process and in the article set forth without departing from the spirit and scope of the invention, it is intended that all matter contained in the above description and shown in the accompanying drawings shall ~e interpreted as illustrative and not in a limiting sense.
It is also to be understood tha~ the following claims are intended to cover all of the generic and specific features of the invention herein described and all ~tatements of the scope of the invention which, as a matter of language, might be said to ~all ther~between.
Particularly it is to be understood that in said claims, ingredients or compounds recited in the sinyular are intended to include compatible mixtures of such ingredients wherever the sense permits.

//

Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE PROPERTY OR
PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A synthetic vascular graft comprising a tubular flexible porous substrate formed of a synthetic fiber having a porosity of less than about 3,000 ml/min.-cm2 (purified water at 120 mm Hg);
the graft substrate having on the inner surface and extending through the porous substrate to the outer surface cross-linked-collagen fibrils admixed with a plasticizer for rendering the graft blood-tight and flexible, the collagen fibrils having been applied by application of an aqueous slurry of water-insoluble collagen fibrils which has been massaged through the substrate and dried and cross-linked after application.

2. The vascular graft of claim 1, wherein the porous substrate is polyethylene terephthalate.

3. The vascular graft of claim 2. wherein the porous substrate is knitted.

4. The vascular graft of claim 2, wherein the porous substrate is woven.

5. The vascular graft of claim 2, wherein the inner and outer surface of the substrate have a velour surface.

6. The vascular graft of claim 1, wherein the collagen fibrils are cross-linked by exposure to formaldehyde vapor.

7. The vascular graft of claim 1, wherein the plasticizer is a biologically compatible polyhydric material.

8. The vascular graft of claim 7, wherein the plasticizer is sorbitol.

9. The vascular graft of claim 7, wherein the plasticizer is glycerine.

10. The vascular graft of claim 1, wherein the collagen is from a deposited aqueous slurry of between about 0.5 to 5.0 percent collagen fibrils and about 4 and 12 percent plasticizer by weight, based on the total weight of the slurry.

11. The vascular graft of claim 1, wherein the collagen fibrils are derived from bovine skin.

12. A process for preparing a collagen synthetic vascular graft, comprising providing a porous tubular flexible synthetic graft substrate;
placing an aqueous slurry of collagen fibrils in the lumen of the substrate;
massaging the slurry into the substrate to ensure intimate mixing of the collagen fibril complex into the porous structure of the substrate;
drying the collagen;
cross-linking the collagen; and vacuum drying said graft.

13. The process of claim 12, wherein the steps of placing an aqueous slurry of collagen fibrils onto the substrate, massaging and drying is repeated three times.

14. The process of claim 12 wherein the collagen fibril slurry includes about 0.5 to 3.0 percent collagen fibrils, 4.0 to 12.0 percent of a biologically compatible plasticizer and the balance water.

15. The slurry of claim 14, wherein the collagen fibrils are obtained by acid digestion of bovine skin.

16. The process of claim 14, wherein the plasticizer is selected from the group consisting of sorbitol and glycerine.

17. A synthetic vascular graft comprising:
a tubular flexible porous polyethylene terephthalate graft substrate of initial porosity less than 3,000 ml/min.-cm2 (purified water at 120 mmm Hg);
the inner surface thereof having a coating of at least five layers of cross-linked collagen fibrils admixed with plasticizer;
the layers formed from an aqueous slurry containing between about 0.5 to 5.0 weight percent collagen fibrils and between about 6 to 10 weight percent plasticizer.

18. The vascular graft of claim 17, wherein the plasticizer is selected from the group of sorbitol and glycerine.

19. The vascular graft of claim 1, wherein the slurry of collagen is applied at least three times and dried between applications.

20. The process of claim 12, wherein formaldehyde vapor is used to cross-link the collagen.

21. The process of claim 12, wherein the flexible synthetic-graft substrate has an initial porosity of less than 3,000 ml/min.-cm2 (purified water at 120 mm Hg).