GB2187463A

GB2187463A - Graft forming material

Info

Publication number: GB2187463A
Application number: GB08708757A
Authority: GB
Inventors: Jr Harmon Hoffman; Kemal Schankereli
Original assignee: Meadox Medicals Inc
Current assignee: Boston Scientific Scimed Inc
Priority date: 1984-01-30
Filing date: 1987-04-13
Publication date: 1987-09-09
Also published as: IL74179A; BE901610A; SE8500421D0; SE464058B; AU577826B2; GB2153685B; NL193264B; IL74179A0; NL8500240A; GB2187463B; FR2558719B1; DE3503127A1; JPS60203264A; SE8500421L; GB8708757D0; AU3819585A; CA1264207A; JPH0636818B2; FR2558719A1; GB8502157D0

Abstract

A collagen-coated impregnated vascular graft is made by treating a porous synthetic vascular graft substrate (formed e.g. by knitting or weaving) at least three times with collagen fibrils, cross-linking e.g. by exposure to formaldhyde vapor) and drying. Treatment is by an aqueous slurry containing 0.5-5% collagen fibrils and 4-12% of a biocompatible plasticiser.

Description

GB 2 187 463 A 1

SPECIFICATION

Collagen-coated synthetic vascular graft Backgroundof the invention 5

This invention relatesto a synthetic vascular graft, and more particularlyto a synthetic vascular graft having a series of plasticized collagen fibril layers which rendersthe graft blood-tight without the need to be preclotted.

The replacement of segments of human blood vesseiswith synthetic vascular grafts is well accepted inthe art. Synthetic vascular grafts havetaken a widevariety of configurations and are formed of a widevarietyof 10 materials. Among the accepted and successful vasculargraft implants arethosewhich areformedfrom a biologically compatible material which retains an open lumen to permit blood to flowthrough thesynthetic graftafter implant. The grafts may be madefrom biologically compatible fibers, such as Dacron andTeflon, may be knitted orwoven and may be of a mono-filiment yarn multi-filiment yarn or stapleyarn.

An importantfactor in the selection of a particular graft substrate isthe porosity of thefabricwall of which 15 the graft isformed. Porosity is significant because it controls the tendency to hemorrhage during and after implantation and controlsthe ingrowth of tissue into thewall of the graft. It is desirable that the vasculargraft substrate be sufficiently blood-tightto preventthe loss of blood during implant,yetthe structure must be sufficiently porousto permit ingrowth of fibroblast and smooth muscle cells in orderto attach the grafttothe hosttissue. Synthetic vascular grafts of thetype described in United States Patents No. 3,805,301 and No. 20 4,047,252, assigned tothe assignee of the subject application, are elongated flexibletubular bodies formed of a yarn such as Dacron. In the earlier patent,the graft is a warp knitted tube and in the latter issued patent it isa double-velour synthetic graft marketed under the trademark Microvel. Thesetypes of grafts have sufficiently porous structuresto permit ingrowth of hosttissue. The general procedurefor implantation includesthe step of pre-clotting, wherein the graft is immersed in the blood of the patient and allowed to stand fora period of 25 time sufficientfor clotting to insue. After pre-clotting, hemorrhaging does not occurwhen the graft is implan ted and growth of tissue is not impeded. However, it is desirableto avoid pre-clotting as it takes valuable time during surgery.

Blood-tight absorbable collagen reinforced grafts have been proposed in United States PatentNo.

3,272,204. Thetype of collagen disclosed is obtained from the deep flexor tendon of cattle. Tendon-derived 30 collagen is generally highly cross-linked and difficuitto process bythe enzyme digestion procedure descri bed in the patent. An additional reinforced vascular prosthesis is described in United States PatentNo.

3,479,670which discloses an open mesh cylindrical tubewrapped byan outerhelical wrapping offused polypropylene mono-filament which may befilledwith collagen fibrils to renderthe prosthesis impermeable to bacteria andfluids. The collagenfibrils utilized arethe same as described in Patent No. 3,272,204. 35 The synthetic vascular grafts suggested bythe priorartare claimedto besuitablefor many applications.

However, it remains desirableto provide a flexible vascular graft which exhibits virtually zero porosity,yet remains sufficiently receptiveto ingrowth of hosttissue andwhich may be more easiiyprocessed thanthe teachings of the priorart.

40 Summary of the invention

Acollagen coated synthetic vascular graftformed from atubular porous structure of a biocompatible filimentary material having a cross-iinked coating of at leastthree layersof collagen fibrils admixedwith a plasticizer which rendersthe graft blood-tight without preclotting. The porous graft substrate may bea tubular vascular graft formed of a Dacron material and maybewoven orknit. 45 Thecollagen source is preferablyfrom bovineskin which has been processed byan acid digestionto result in afibril dispersion of high purity.An aqueous purified collagen slurry including a plasticizer is applied to the synthetic vascular graft by massageto coverthe entiresurface areato provide aflexible graftwith good hand.Afterat leastthree repeated coating and drying applicationsthe collagen is cross-linked byexposureto formaldehyde vapor. Graft porosityis reducedto lessthan about 1%of the porosity& graft beforecoating. 50 Accordingly, itis an objectof the invention to providean improved synthetic vascular graft.

Anotherobjectof the invention isto provide an improved blood-tight synthetic vascular graft.

Afurther object of the invention isto providean improved collagen-coated synthetic vascular graft.

It is anotherobjectof the invention to providean improved bovineskin derived collagen-coating.

It is a further object of the invention to providean improved methodforthe preparation of acollagen- 55 coated synthetic vascular graft.

Still anotherobjectof the invention isto provide an improved process for coating a synthetic vascular graft with collagento renderthegraft blood-tight.

Still otherobjects and advantages of the invention will in partbeobvious andwill in part be apparentfrom the specification. 60

The invention accordingly comprises the article possessing the features, properties and the relation of elements and the several 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.

65 2 GB 2 187 463 A 2 Brief description of the drawing

For a fuller understanding of the invention, reference is had to thefollowing description taken in connec tion with the accompanying drawings, in which:

Figure 1 is a partial cross-sectional view of a collagen-coated synthetic vascular graft in accordancewith theinvention; 5 Figure2 is a partial cross-sectional view of a branched tubular graft of the type illustrated in Figure 1; and Figure 3 is a graph showing the reduction of porosity after a series of collagen coating in accordance with the invention.

Description of thepreferredembodiments 10

A synthetic vascular graft 10 constructed and arranged in accordancewith the invention is shown in Figure 1. Graft 10 includes atubular substrate portion 12which isformed of a biologically compatible filamentary synthetic material, preferably a polyethyleneterephthalate, such as Dacron. Substrate 12 is a porous Dacron warp knitfabric having an inner and outervelour surface of thetype described in U.S. Patent 4,047,252. While tubular portion 12 isformed of Dacron, any biocompatiblefilimentary material may be used forthesubstrate 15 provided it may befabricated into a porous structurewhich will permittissue ingrowth and maintain an open lumen forflow of blood.

Tubular portion 12 has on the innersurface a coating of collagen shown as 16. Collagen coating 16 is formed from at leastthree layers of an aqueous collagen fibril and plasticizer dispersion which has been cross-linked byexposureto formaldehyde vapor. Figure 2 shows a bifurcated collagen-coated graft20. Graft 20 includes a main tubular portion 22 and two branches 24. Main tubular portion 22 and bifurcated portions 24 areformed from a Dacron knitsubstrate 26 having an innersurface coating of a collagen coating 28 formed from at leastthree layers of collagen fibrils.

Porousvascular graftsubstrates suitablefor use in accordancewith the invention, preferablyare produced from Dacron mu Iti-filiment yarns by knitting orweaving processeswhich are commonly used in manufacture 25 of these products. Generally, the porosity of the Dacron substrate rangesfrom about2,000to 3,000 milmin cm'(purified waterat 120mm Hg). The innercoating of cross-linked collagen is applied byfilling atubular substrata with a collagen and plasticizer slurry and massaging manually, removing the excess and per mitting the deposited dispersion to dry. Afterthefinal application,the collagen coating is cross-linked by exposureto formaldehyde vapor, air dried andthen vacuum dried to remove excess moisture and excess 30 formaldehyde. The coated grafts in accordancewith the invention have essentiallyzero porosity.

Thefollowing examples are setforth to illustratethe method of preparing purified collagen from bovine skin and coated grafts in accordance with the invention. The examples are setforth for purposes of illustra tion and not intended in a limiting sense.

35 Example 1

Fresh calf skinswere mechanically stripped from young calves, fetuses or stillborns and washed ina rotating vessel with cold running water until the water was observed to befreefrom surface dirt, blood and/ortissues. The subcutis was mechanically cleaned to remove contaminating tissues, such asfatand bloodvessels. Subsequently, the skins were cut in the longitudinal direction into strips about 12 cm wide and 40 wereplaced in awood orplasticvessel ascommonlyused inthe leather industry.

The skins were deha i red byusing a flusher solution of 1 M Ca(Offi2for25 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 V X V' and were washed in cold water.

Following washing, 120 Kg of the bovine skin was placed in a vessel having 260 Lwater, 2 L NaOH (50%) 45 and 0.4 L H202 (35%). The components were mixed slowlyfor 12 to 15 hours at 4'C and washed with an excess of tap waterfor 30 minutes to provide partially purified skins. The partially purif ied skins were treated in a solution of 260 Lwater, 1.2 L NaOH (50%) and 1.4 Kg CaO for 5 minutes with slow mixing. This treatmentwas continued twice dailyfor 25 days. Following this treatment, the solution was decanted and discarded andthe skins werewashed with an excess of tap waterfor 90 minutes under constant sti rri ng. 50 The skins were acidified bytreatmentwith 14 kg HCI (35%) and 70 Lwaterwhile subjecting the skinsto 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 waterfor 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 55 mesh size. The final productwas a white homogeneous smooth paste of pure bovine skin-derived collagen.

In orderto impart adequate pliabilityto the grafts in the dry state, a plasticizer such as glycerine, sorbitol or other biologically acceptable plasticizer is added to an aqueous collagen slurry before application. In a col lagen slurry containing between about 0.5to 5.0% collagen byweight, the plasticizer is present between about4 and 12 weight percent. Between about 10 to 25 percent ethanol may be present to hasten evaporation 60 of the water.

The most important property obtained when coating a synthetic vascular graft with collagen and plast icizer coatings in accordance with the invention is reduction of porosity of the porous su bstrate to about zero.

For comparison, the porosity of twenty randomly selected uncoated Meadox Microvel synthetic vascular grafts had a mean porosity of water of 1796 mi/min_CM2 at 120 mm Hg and a standard deviation of 130. After 65 3 GB 2 187 463 A 3 coating in accordance with the invention, the porosity is reduced to zero. The following example illustrates the method of coating the graft substrate in accordance with the invention.

Example2

A 50 cc syringe is filled with an aqueous slurry of 2% purified bovine skin collagen prepared in accordance 5 with Example 1. The collagen slurry includes 8% glycerol, 17%ethanol and the remainder water and a visco sity of 30,000cps. The syringe is placed into one end of a Meadox Medical Microvel Dacron graft 8mm in diameter by approximately 12 cm in length. The slurry is injected into the lumen of the Microvel graft and it is massaged manually in order to cover 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 hours at 10 room temperature. The coating and drying steps were repeated three more times.

Following the fourth coating application, the collagen coating was crosslinked by exposure to form aldehyde 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.

15 Example3

The blood-tightness of a col lagen-coated vascular grafts prepared in accordance with Example 2 was tes ted as follows. A Microvel graft 8mm x 12cm was attached to a blood reservoir at a pressure of 120mmHg due to the height of the reservoir. Heprin stabilized blood was passed through the graft. Blood collected through the graft was determined and expressed in mi per min-cm 2 The porosity of 5 runs was determined to 20 be 0.04,0.0,0.0,0.04 and 0.03. This represents a mean porosity of 0.022 mi/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 uncoated Microvel, the experiment was repeated using an uncoated graft fabric. The mean porosity was 36mi/min_CM2.

25 Example 4

The porosity of a collagen coated fabric graft is reduced to less than about 1 % afterthree coatings demo nstrated as follows. A standard water porositytest used to measure water porosity of a graft is as follows. A column of water equivalentto 120 mm Hq pressure is allowed to flowthrough a one-half CM2 orifice having a sample of graft overthe orifice for one minute. The amount of water collected was measured. The milliliters 30 of water collected per minute per CM2 squared area was calculated. Several readings are taken for each sample. The porosity is reported as follows:

porosity = mi/min/cM2 Thewater porosity of a Microvel graftfabricwas about 1,900 mi/min/cm 2 The porosity after coating was as follows: 35 Number of Coatings Porosity 0 1,900 1 266 40 2 146 3 14 4 5 2 6 0 45 in each case the collagen coating was a bovine skin derived-plasticized slurry prepared in accordance with the composition described in Example 2. These results are set forth in the graph of Figure 3. Based on this, it is preferable to provide a col lagen coating of at least three layers off ibrils, and most preferably four orfive layers with drying between each application and cross-linking after the final layer to fix the coating to the 50 substrate.

In addition to reduced porosity, collagen coated vascular grafts in accordance with the invention exhibit reduced thrombogenicity compared to uncoated grafts. Thefollowing examples demonstrate significantly less thrombogenicity of collagen impregnated vascular compared to controlled.

55 Example 5

Antithrombogenicity was evaluated in vitro bythe method of lmai and Nose (J. Biomed, Mater Res. 6,165, 1972). In accordance with the procedure, a volume of 0.25 m] of ACD blood (citric acid stabilized) was mixed with25ulofO.1 m CaC12 and placed onto the inner surface of a collagen coated Microvel graft prepared in accordance with Example 2. A similar volume was placed on an uncoated Microvel graft as a control. The 60 same geometry of the blood spot was observed after 5, 10 and 15 minutes. The clotting reaction was stopped by adding 5 m 1 distilled water to the test samples. Striking differences were observed between the two tested grafts and the following semi-quantitative parameters were ascertained:

65 4 GB 2 187 463 A 4 Collagen Impregnated Plain Rate of soaking the graft matrix with blood fast slow 5 Thrombus formation in 5 min 0 + + 10min +...

min + +..

Acomparison of thrombus formation onto the inner surface of the collagen impregnated Microvel graft 10 and control Microvel graftwas asfollows. In the collagen impregnated grafttherewas nofibrin clotformation in 5 minutes. At 15 minutesthe cloton the collagen impregnated graftwas much lessthan inthe corresponding plain grafts.

The surface of the Microvel graft in contactwith the drop of blood behaved almost as hydrophobic. Ittook between about 10 and 15 seconds beforethe blood soaked into thefabric of the Dacron knitted graft.This 15 contrasts with the collagen impregnated graftwith blood soaked intothe graft matrix evenly and fast.

After 5 minutestherewas no thrombus residue detected on the collagen coated graft. Atthe sametime, a thin but definite thrombus was present on the surface of the plain control Microvel graft. At 10 and 15 min utes,thetotal volume of thrombus present on the graft innersurfacewas less in the collagen coated graft than in the controls. 20 Based on the above observations under in vitro conditions, with no blood flow,the collagen coated Dacron knitted Microvel graftsoaked quicklywith blood, without forming anythrombus within 5 minutes. Atthis time,the control graft showed thrombus formation. Later after 10 and 15 minutesthe amountof thrombus was less in collagen impregnated graftsthan in plain control Dacron grafts.

25 Example 6

Thethrombogenicity of collagen impregnated Microvel graftswas tested in vitro (dogs) asfollows. A femoral arterial venous (AV) shuntwas installed in greyhounds in deep anesthesia. A 5 centimeter long prosthetic material was adapted at both ends with plasticconical tubing for better handling. This allowed easy insertion of a test graft segment into the arterial shunt. After insertion, a venous clamp wasslowly 30 removed with the arterial end released slowly thereafter. The blood flowwas circulated through the implant for 10 minutes or 30 minutes. Then, both ends of the shuntwere clamped again and the inserted prosthesis removed. The excess of blood was drained off and theweight established. The presence of graftsurface adhering thrombi was observed macroscopically. The graftswere then washed (threetimes) in excess dis tilled water and weighed again. 35 As a control a standard 6 mm diameter Dacron Microvel graftwas used. This graftwas pre-clotted before insertion into the shunt. Accordingly, this test provided both visual and objective gravimetric evidence of the thrombogenicity on the surface tested. The weight of blood oozing across the graftwall was also ascertained to record the difference between the tested samples.

No bleeding at all wasfound with collagen impregnated grafts. 40 After insertion of a pre-clotted control graft into the AV shunt, an average of 30 mi blood/5 cm long graft was lost in the first 5 minutes. In the next 5 minutes, only 3-5 mi blood was lost. In one of the control grafts tested for 30 minutes, minimum bleeding of 1 milmin/5 cm through the pre- clotted graft continued forthe entiretest.

The collagen impregnated grafts implanted for 10 or 30 minutes showed the same pattern of resistanceto 45 thrombus formation observed macroscopically. Athin smooth layer of glistening proteinaceous material covers the collagen layer. Afterwashing repeatedly in distilled water a continuous film of proteins (fibrin) is seen in most prosthesis. Atypical clotwas not observed in the sample prosthesis.

Of the five tested pre-clotted plain Dacron graft implants, three exhibited distinct multiple thrombi. These were located transverseto the direction of blood f low crossing 1/3 to 1/2 of the circumference. In the remain- 50 ing two prosthesis, a similar skin proteinaceous layer covered the inner surface. The outer surfaces of each control graft contained large thrombi due to continuous bleeding through thewall.

Based on these observations, thrombogenicity of collagen impregnated vascular Dacron graftswas signifi cantly less than in control pre-clotted grafts. This could be due to either reduced thrombus because of the collagen coating orto the thrombus formed in the control grafts due to the need for pre-clotting. As blood 55 clotting is an event participating in excessive cell reaction with the fibrotic replacement, it is advantageousto reduce th rom bus 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 porousvascular graftsubstrate 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, butare 60 not limited to elimination of the necessity for pre-clotting. Conventional porous grafts, although proven to be necessaryfor long-term patqncy, made it necessaryforthe surgeon to pre- clot the graftwith the patient's blood in orderto prevent excessive blood loss atthetime 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 a collagen coating to eliminate the need for pre-clotting synthetic grafts altogether. 65 GB 2 187 463 A 5 The porous synthetic vascular graft substrates provide a ideal matrix for tissue ingrowth while eliminating the need for pre-clotting. Additionally, the significantly lower thrombogenicity 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 coatings in accordance with the invention also provides a vascular g raftwhich remains flexible with good hand. 5 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 maybe made in carrying out the above process and in the article set forth without departing from the spirit and scope of the invention, it is intended that al I matter contained in the above description and shown in the accompanying drawings shal I be interpreted as illustrative and not in a limiting sense. 10 It is also to be understood thatthe following claims are intended to coverall of the generic and specific features of the invention herein described and all statements of the scope of the invention which, as a matter of language, might be said to fall therebetween.

Particularly it isto be understood that in said claims, ingredients or compounds recited in the singular are intended to include compatible mixtures of such ingredients whereverthe sense permits. 15

Claims

1. A collagen fibril slurry forforming a blood-tight syntheticvascular graft comprising about 0.5 to 5.0 percent collagen fibril, 4.0 to 12.0 precent of a biologically compatible plasticizer and the balance water. 20

2. The slurry of claim 1, wherein the collagen fibrils are obtained by acid digestion of bovine skin.

3. The slurry of claim 1, wherein the plasticizer is selected from the group consisting of sorbitol and glycerine.

Printed for Her Majesty's Stationery Office by Croydon Printing Company (L1 K) Ltd,7187, D8991685.

Published by The Patent Office, 25 Southampton Buildings, London WC2A l AY, from which copies maybe obtained.