EP0000949B1

EP0000949B1 - Cardiac and vascular prostheses and methods of making the same

Info

Publication number: EP0000949B1
Application number: EP78100752A
Authority: EP
Inventors: Philip Nicholas Sawyer
Original assignee: Individual
Current assignee: Individual
Priority date: 1977-08-26
Filing date: 1978-08-25
Publication date: 1983-05-18
Also published as: CA1122353A; JPH0137154B2; US4167045A; DE2862259D1; JPS5463588A; EP0000949A1

Description

This invention relates to a cardiac and vascular prosthesis and a method of making the same.
The major emphasis in early research on implants was performed within the field of orthopedics. (Bechtol, C. D., Ferguson, Jr., A. B., and Laing, P. G., Metals and Engineering in Bone and Joint Surgery, Williams and Wilkins Co., Bait., Chapter 1, 1-18 (1959); Scales, J. T., Arthroplasty of the Hip Using Foreign Materials: A History, Paper 13, A Symposium, Institute of Mechanical Engineers, Vol. 181 Part 3J 63-84, (1967); Weisman, S., Metals for Implantation in the Human Body, Ann. N. Y. Acad. Sci. 146, 80-95, (1968)). The most significant addition to implant surgery in the last twenty years has been the development of prostheses for vascular and cardiac reconstruction (Wesolowski, S. A., Martinez, A., and McMahone, J. D., The Use of Artificial Materials in Surgery, "Current Problems in Surgery", Yea. Book Medical Publishers, Inc., Chicago (1966). A major problem involved in the use of prosthetic alloplastic materials has been severe occlusive thrombogenicity (Hufnagal, C. A., The Use of Rigid and Flexible Plastic Prostheses for Arterial Replacement, Surgery, 37:165, (1955)). The search for acceptable materials and designs for vascular prostheses was of great importance because of a number of problems which were encountered with the use of polymeric and metal prostheses, autologous, homologous and heterologous bypass grafts in man. (Johnson, W. D., Auer, J. E., and Tector, A. T., Late Changes in Coronary Vein Grafts, Am. Jour. of Cardiol., 26: 640, (1970) and Artegraft Conference, Johnson and Johnson, Toronto, Ontario, Canada, June 23, (1973)).
Following the development of open heart surgery, an enormous effort was made in the field of prosthetic heart valves. The first successful development in this area was made by Goodman, Berg and Stuckey (Wesolowski, S. A. et al., The Use of Artificial Materials in Surgery, "Current Problems in Surgery", Year Book Medical Publishers, Inc., Chicago (1966). They produced the first completely thromboresistant prosthetic heart valve. This valve was later improved by Dr. Albert Star and Edwards Laboratories (Edwards, W. S. and Smith, L. Aortial Valve Replacement with a Sub Coronary Ball Valve, Sur. Forum 9:309 (1958); Starr, A., Mitral Valve Replacement with a Ball Valve Prosthesis, British Heart Journal, 33 Supp. 47 (1971). A number of valves were further developed by several corporations. These included the Bjork-Shiley valve which is the most successful of the prosthetic heart valves available today. More recently, glutaraldehyde tanned porcine collagen prosthetic heart valves developed by the Hancock Laboratory (Sauvage, L. R., Wesolowski, A. S., Sawyer, P. N. et al., Prosthetic Valve Replacement, Ann. of the N. Y. Academy of Sci., 146:289 (1968)) have proven useful. They are resistant to intravascular thrombosis and have maintained their tensile strength following several years of implantation in man. Thus, a host of glutaraldehyde tanned collagen valvular prostheses have proven useful in man with respect to long term function and resistance to thrombosis.
Early attempts to replace blood vessels in man involved the use of rigid tubes of gold, silver, aluminum, magnesium, as well as the later development of polyethylene and polymethyl acrylic tubes (Szilagayi, D. E., Long Term Evaluation of Plastic Arterial Substitutes: An Experimental Study, Surg. 55:165 (1964); Woodward, S. C., Biological End Points for Compatibility, Plastics in Surgical Implants ASTM-STP 386, A Symposium on Surg. Implants, Indianapolis, Indiana (1964); Sawyer, P. N., Wu, K. T., Wesolowski, A. S., Brattain, W. H. and Boddy, P. J., Long Term Patency of Solid Wall Vascular Prostheses, Arch. Surg. 91:735, (1965). In the majority of cases, these prostheses did not function satisfactorily. A major breakthrough came in 1952 when Voorhees and Blakemore described their experiments with a cloth prosthesis of Vinyon. (Voorhees, A. B., Jr., Jaretski, 111, A. and Blakemore, A. H., Use of tubes constructed from Vinyon-"N" Cloth in bridging Arterial Defects, Ann. Surg. 135:332 (1952)). This material was easy to handle, preclottable and resistant to thrombosis following implantation, although tensile strength, in situ, was lost with the passage of time. Preclotting the graft with the patient's blood produced a compound prosthesis, whereby, the lumen was covered by a fibrous neo-intima and the adventitia was enclosed by a fibrous capsule, which resulted in a well tolerated graft. Presently, the Dacron graft is considered to be the most successful cloth type graft. (Sawyer, P. N. et al., Vascular Graft Symposium, Current Status and Future Trends, NIH (1976)).
Early attempts at vessel substitution with materials of biological origin were carried out using arteries from cadavers (Wesolowski, S. A. and Sauvage, L. R., Heterologus Aortic Hetergrafts with Special Reference to Recipient Site, Ethylene Oxide Freeze Dry Preparation and Specied of Origin, Ann. Surg. 145:187 (195. ). On the whole, these homografts functioned well for a period of time, but were soon replaced by fibrous tissue and calcium salts. This resulted in an inelastic structure, susceptible to thrombus, fracture, and aneurysm formation (Sauvage, L. R. and Wesolowski, A. S., Healing and Fate of Arterial Grafts, Surg. 38:1090 (1955)).
Following failure of the homografts, autografts from a patient's veins (saphenous) were used. Currently, the implantation statistics show that the saphenous vein is superior to synthetic implants (Sawyer, P. N. et al, Vascular Graft Symposium; Sauvage, L. R. and Wesolowski, A. S., Healing and Fate of Arterial Grafts, supra, and Vladamer, C. and Edwards, J. E. Pathological Changes in Aortocoronary Arterial Saphenous Vein Grafts. Circulation 44:719 (1971). When the saphenous vein is not available the question arises as to which implant should be used. Experience has shown that a vascular graft must: (I) have an appropriate porosity (10,000-20,000 ml of water/square cm/minute), Sawyer, P. N., Wu, K. T., Wesolowski, S. A., Brattain, W. and Boddy, P. J., An Aid in the Selection of Vascular Prosthesis, Proc. Natl. Acad. Sci., U.S.A., 53:1965), (ii) be blood compatible, (iii) possess tensile strength and (iv) be easy to handle with respect to sewing characteristics (Sawyer, P. N. and Srinivasan, S., New Approaches in the Selections of Materials Compatible with Blood, Artificial Heart Prog. Conf. Proc., Chapter 2, 1969).
Furthermore, the US A 3 914 802 discloses a non-thrombogenic prosthetic material with an elastomeric layer capable of oxygen-diffusion, the outer face of the layer being bonded to a fabric backing acting to reinforce the layer, the interstices of the fabric permitting cellular diffusion to lock the material in place after implantation, and with a lining bonded to the inner face of said layer and formed primarily of colloidal, negatively-charged silica particles to repel negatively-charged blood platelets in contact therewith. In particular the elastomeric material is silicone rubber or urethane rubber and the fabric is formed of woven polyester yarns, preferably of polyethylene terephthalate.
US A 4 038 702 discloses a prosthetic device with a metal structure and an electrolytically treated metal surface to render the electrochemical surface more negative and homogeneous.
US A 3 927 422 proposes the use of collagen for making a prosthesis and the negative surface charge of the intimal surface thereof is increased by a succinylation reaction. Thus the collagen is chemically modified at least on the surface.
It is an object of this invention to provide a prosthesis with improved compatibility with the tissue and the blood in order to reduce the susceptibility to thrombosis and the like, and to provide a method for making the same. These objects are solved by the invention as claimed.
When implanted, these prostheses reveal a striking tendency to remain thromboresistant and maintain their tensile strength as opposed to other commercially available collagen grafts that possess a limited degree of patency with early loss of tensile strength. The copolymeric grafts according to the invention have remained patent in the femoral-popliteal, carotid and coronary positions.
As to why these types of grafts perform so well, conclusive answers are not yet available. Scanning electron microscopic studies indicate that when the grafts fail, they become occluded by an atypical thrombus. It is therefore obvious that the success of this type of graft depends upon (i) the surface modification characteristics of the surface material, (ii) blood interfacial reactions occurring, (iii) structural aspects of the surface itself, and/or the supporting prosthesis means.
According to the invention Dacron@ (a Polyester of terephthalic acid and ethylene glycol manufactured by DuPont de Nemours Et Co., Inc., Wilmington, Delaware, U.S.A.) hybrid grafts of several types have been produced. The application of this graft is aimed at specific clinical problems where the saphenous vein or conventional heterografts would be considered unsatisfactory. The hybrid graft can also be applied to the total artificial heart and bypass pump chambers and Dacron skirt of heart valves providing a more satisfactory blood compatible surface.
Dacron@ vascular grafts have been altered, in accordance with the invention, by coating them and filling their interstitial spaces with a variety of agents, in a variety of ways, to produce grafts with superior anti-thrombogenic characteristics.
The techniques used include: (1) copolymerization of albumin, or gelatin, or a combination thereof, to the Dacron@ with glutaraldehyde and subsequent negative charging, (2) the deposit of substances which may accept from a bloody environment materials which are compatible with the environment.
The prosthesis means may be, for example, of woven Dacron@. Thereupon may be deposited a surface material in the form of albumin, combined with gelatin. The thusly coated prosthesis may, for example, thereafter be treated with glutaraldehyde. The material deposited on the prosthesis may be applied preferably in about a 3% solution.
According to a further feature of the invention, a thusly coated prosthesis may be further treated with a succinic acid compound.
According to a further aspect of the invention, the surface material may be applied by immersing the prosthesis means in a slurry of agar.
The above and further objects and features of the invention will be found in the detailed description thereof as follows hereinafter.
In the drawing,

Figure 1 (A) is a scanning electron microphotograph group of a polarized protein;
Figure 1 (B) is a collection of scanning electron micrographs of a protein-gelatin Dacron@ hybrid graft;
Figure 2(A) shows scanning electron micrographs of a gelatin-Dacron® hybrid graft;
Figure 2(B) is a group of scanning electron micrographs of a gelatin-Dacron@ hybrid graft;
Figure 3(A) shows the structure of an agar coated graft;
Figure 3(B) shows the implantation results of a protein-gelatin Dacron@ hybrid graft;
Figure 4 shows scanning electron micrographs of an agar coated Dacron hybrid graft.

As has been stated hereinabove, the invention relates to the provision of a prosthesis having the capability of functioning in a bloody environment. The prosthesis may be of varying types, but it can be, for example, a vascular prosthesis consisting of a tube adapted to function as a connection for bypass or a valve functioning to operate in the cardiac environment.
The invention is based in general on having the prosthesis exhibit a periodic negative surface charge characteristic in the preferred case approximating one electron per 10 mm² of surface in a regular three dimensional pattern. Under certain circumstances, the periodic surface charge may be positive.
A surface material will generally be employed to exhibit the above-noted characteristic. For this reason, the present invention is concerned with hybrid prostheses which are prostheses consisting of a basic material forming a prosthesis means so that it is capable of performing the desired function and a surface material which exhibits the aforenoted charge.
In case the basic material is incapable of crosslinking with the surface material, an adaptive material may be provided to offer structural continuity and cross-link the surface material and the supporting material by being adapted to cross-link itself respectively to each of these materials.
The invention offers copolymerization for a form of albumin and/or gelatin deposition or agar application in the form of a uniform coating. The invention offers further selective processing of the above materials by treatment with succinic acid or the like or, for example, by treatment with glutaraldehyde.
The following are details of materials and methods which may be employed in accordance with the above: Copolymerization (a method of protein deposition) Fraction V Albumin (purified bovine or other mammalia albumin) purchased from Sigma Corporation, St. Louis, Mo., USA was dissolved (1, 3, 5 and 10 grams in 100 ml of water) to produce several solutions of 1%-10% with distilled and deionized water Gelatin (cutaneous), purchased from J. T. Baker Chemical Co., Phillipsburgh, New Jersey, U.S.A. was dissolved at room temperature in water, producing solutions ranging in concentration (1, 3, 5 and 10 grams in 100 ml of water) from 1 %―10%.
A Microknit® knitted or woven vascular prosthesis (Golaski Labs. Inc. of Philadelphia, Penna. USA) possessing 18 ridge to the inch (50 interstices per inch) was cut into 10 cm segmen (collapsed length) and cleaned in distilled and deionized water. It is then immersed in 10-20% (preferably 20%) glutaraldehyde (purchased from Electron Microscope Science Corporation of Fort Washington Penna., USA) with distilled and deionized water (40 ml of 50% glutaraldehyde in 60 ml of water) and was allowed to stand for three hours.
The graft materials were then immersed in the appropriate protein (albumin and/or gelatin,) solution until thin (1 to 2,000 microns) coats of polymerized protein surrounded the Dacron tube. They were then suspended in air at room temperature and rinsed with water at room temperature for a minimum of 2 minutes. Care was taken to rinse the lumen of the grafts with water. The graft-protein "hybrid" prostheses were then reintroduced into fresh (20%) glutaraldehyde (40 mi of 50% glutaraldehyde in 60 ml of water) for 5-15 minutes. The process was repeated several times (three to ten times) until thin (1 to 2000 microns), uniformly coated grafts with their interstitial spaces filled were produced.
Several types of grafts and varying concentrations of albumin and/or gelatin were utilized to determine optimal reaction condition. They included 3 and 6 mm. I.D. (2-20 mm is the range), 18 and 32 ridges 2.54 cm (10-32 ridges would be the range), and 1, 3, 5 and 10% albumin and/or gelatin solutions. It was found that the grafts possessing 18 ridges 2.54 cm (half crimp) were the optimal type of graft to modify, as they possessed the best surface configuration to provide for an even deposition of albumin and/or gelatin. The grafts having 32 ridges 2.54 cm (full crimp) had to be stretched on a glass rod of the same I.D. as the grafts in order to coat them properly. This prevented intra lumen coating and produced cracking when the rod was removed and the grafts regained their original shape. It was further found that a 3% solution of albumin and/or gelatin provided the most effective coating both in optimizing time to coat and in thickness. One percent took a much longer time period to coat (greater than 1 hour). A 10% coating was achieved within 30 seconds but was very thick and did not permit obtaining the amount of flexibility required. In the aforegoing, the range of depth of the crimps is about 1 to 2 mm and the wall thickness of the grafts is about 1/2 to 1 mm.
Using the techniques described above (i.e., 3% albumin and/or gelatin solutions and half crimped grafts) it was possible to coat both 3 mm and 6 mm I.D. grafts without difficulty.
The albumin and/or gelatin copolymerized hybrid grafts were further modified to enhance their net negative surface charge. The end terminal amino acid residues were covalently coupled to succinic acid under acetylation reaction conditions (see US-Patent No. 3,927,422). The grafts were rinsed thoroughly with distilled deionized water, and placed into a 10% (8-10% range) sodium bicarbonate buffer at a pH 8.5 (8.2-8.7 range) (8-10 grains in 100 ml of water). The grafts were allowed to equilibrate with the buffer (5-30 minutes at 17-27°C) then ground solid succinic anhydride was added in 5 mg portions (range 2-5 grams) to the buffer such that the final succinate solution concentration would be 1 M, (range: 1 to 5 molar) (2 to 8 micromoles of succinate per gram of albumin and/or gelatin. The reaction was sufficiently vigorous to perfuse the lumen of the grafts. Several aliquots (range: 3 to 5) of the solid anhydride were added directly into the lumens of the grafts which were then placed into the buffer. At the conclusion of the reaction (i.e. cessation of effervescence), the grafts were retested on a Zarb hydrostatic testing apparatus to ascertain structural integrity of copolymerization. The grafts were recoated if necessary and then stored and sterilized in 10% (range 3-10%) glutaraldehyde.
A 3% agar-water mixture (Difco Labs, Detroit, Mich., U.S.A.) was made and heated to 100°C until a uniform (aggregate free) slurry was evident. The graft material, washed three to ten times in distilled and deionized water, was immersed in the agar slurry until it was uniformly coated (100-2,000 microns). It was then kept at 4°C (range: 4-10°C) until the agar congealed. The graft was removed, washed and stored in 40% ethanol-H₂0 until used.
All hybrid grafts underwent hydrostatic in vitro testing using a Zarb hydraulic pressure tester. The criterion used was the ability to withstand 30 mm (range 20-50) of Hg pressure without showing evidence of air leaks through the graft pores.
If a graft failed because of leaks during the test procedure, it was recoated by the process described above until there was no evidence of leakage. The hybrid grafts were stored in 10% glutaraldehyde until used.
Implantation studies were divided into chronic and short term evaluation of hybrid graft materials. Three and four mm hybrid grafts were sutured (implanted) into the carotid and femoral arteries of dogs for periods ranging from 1 second to 2 months. (1 second, 2 hours, 1 week, 1 month, 2 months).
Six mm hybrid grafts were implanted into the abdominal aorta of dogs for time periods ranging from 1 month to 1 year (1 month, 6 months, 9 months, 1 year).
The hybrid grafts were removed from their sterile solutions and placed into a sterilized 500 ml beaker and sequential washed with 250 ml of sterile saline 20x. The grafts were then placed into a sterile stainless steel bin containing 1.5 liters of sterile saline, until placed in a sterile field and implanted.
Prior to implantation, the end pieces were cut to provide an even surface for anastomosis and samples for controls, culturing and SEM studies to determine efficiency of coating and function.
End-to-end anastomosis was carried out using a running Carrell suture with 4-0 cardiovascular Dacron@ or other material. Precise anastomosis was carried out in all cases.
Percent patency has been determined using a semiquantative scale. This has been determined by taking the cross sectional view of a graft and dividing it into equal concentric circles of 1/4 D from the wall of the vessel. Thus, it is now possible to quantify the amount of thrombus formed by measuring the depth of the thrombus then relating it to the total cross sectional area. The numbers generated by this technique are expressed in terms of percentage of the entire vessel.
In post implantation studies the graft-hybrid materials were analyzed using (1) scanning electron microscopy (SEM), (2) light microscopy and (3) histological staining. Patency was determined using the aforementioned semi-quantitative technique.
SEM's were performed on the grafts before implantation and after removal using a Phillip's scanning electron microscope at 200x, 10 Kx and 20 Kx magnifications. With these magnifications, it is possible to resolve and differentiate platelets, erythrocytes, and leukocytes, and white blood cells and to visualize protein (including fibrin) deposition.
The SEM's were used to detect (1) efficiency of hybridization (to determine structural flaws in the coatings) and (2) to determine visible evidence of the type of blood prostheses interfacial reactions.
In histological and light microscopic evaluations the graft sections were evaluated by the use of several standard histological stains including Hemotoxylan and Eosin (H&E), Van Geisen (Elastic VG stain), PTAH and Trichrome. Each stain specifically detects the type of material adhering to the graft or removal from the animal by the colors which are produced when the staining solution is in contact with the biologic material. H&E is used to visualize cellular entities and to demonstrate if an inflamatory response has occurred and if polynucleated cells or lymphocytes are present. EVG stain is used to illustrate fibroblastic invasion and to dye the collagen present. For instance, Trichrome stains collagen blue. PTAH stain is used to demonstrate the presence of fibrin.
The following table illustrates the patency levels of the various hybrid grafts.
Table I relates the type of hybridization, time of implantation and percent patency. Column 1 is the type of "coating" and subsequent negative charging. The next 4 columns are the time parameters at which they were analyzed. The numbers represent the % of patency. An "X" indicates that the graft is still functioning in-vivo. A "-" indicates that that parameter was not measured. As support Mikroknit® was used.
The following figures are SEM studies of the following hybrid grafts before and after implantation, negative charged and unmodified, of (i) Albumin (A) coated grafts, (ii) Gelatin (G) coated grafts, (iii) a 50/50 mixture of albumin and gelatin a concentration of 3% each (AG), and (iv) agar-agar coated grafts (AGR). Fig. 1 (a) shows scanning electron micrographs of a polymerized albumin Dacron@ hybrid graft. This series of photographs demonstrates the homogenicity of albumin on Dacron@ surface.

Fig. 1 (b) shows scanning electron micrographs of a protein-gelatin Dacron@ hybrid graft. The photographs show a homogeneous surface prior to and after exposure of the surface to blood for 1 second.
Fig. 2(A) shows scanning electron micrographs of a gelatin-Dacron@ hybrid graft. These photographs reveal minor cracking of the polymerized surface at high (10 kx) magnification.
Fig. 2(B) shows scanning electron micrographs of a gelatin-Dacron® hybrid graft. Following polymerization of the hybrid, the surface was chemically treated (succinylating) to produce a net negative surface charge. These micrographs indicate that the negative surface at 1 second, 2 hours and 1 month is far less reactive than the unmodified hybrid surface. The unmodified surface has a dense amount of cellular protein deposition thereon, while the negatively charged surface attracts fewer cellular aggregates. The patency rate for the negatively charged grafts was approximately 90% as opposed to 60% for the unmodified surface at 2 hours.
Fig. 3(A) shows the smooth structure of an agar coated Microknit@ Dacron@ graft and Fig. 3(B) shows implantation results of a protein-gelatin Dacron@ hybrid graft. The SEM photographs reveal that the protein-gelatin negatively charged hybrid is less reactive at 1 second, 2 hours and 1 month than the unmodified graft. Many erythrocytes entrapped in a protein matrix can be seen in the unmodified surface while the negatively charged surface possesses few red cells and no visible platelets. Patency rates for the negatively charged grafts was approximately 90% as opposed to 70% for the unmodified grafts at 2 hours.
Fig. 4 shows scanning electron micrographs of an agar coated-Dacron@ hybrid. Implantation results indicate a reactive surface with cellular and protein deposition. Patency rates were 80% at 1 second and 40% at 2 hours.

The different histological responses to the various types of hybrid grafts are described in Table II.

Claims

1. A prosthesis suitable for implantation in a bloody environment, particularly for cardiac and vascular implants, said prosthesis comprising a surface material and prosthesis means supporting said surface material and adapted to perform a prosthetic function in said environment, the surface material and/or prosthesis means being adapted such that the surface material can exhibit a periodic surface charge characteristic in said environment, characterised in that the surface material is albumin, or gelatin, or a combination thereof, or agar.

2. A prosthesis as claimed in claim 1, characterised in that the prosthesis means is of a material normally incapable of cross-linking to said surface material and that an adaptive material is used to cross-link to the material of said prosthesis means and to said surface material to provide structural continuity therebetween while preserving said surface charge characteristic.

3. A prosthesis as claimed in claim 2, wherein said adaptive material is glutaraldehyde or a succinic acid compound.

4. A prosthesis as claimed in any of claims 1 to 3, wherein said surface charge is equivalent to approximately one electron per 10 nm² in a regular three-dimensional pattern.

5. A prosthesis as claimed in any of claims 1 to 4, wherein said prosthesis means is a polyester of terephthalic acid and ethylene glycol.

6. A prosthesis as claimed in claim 5, wherein said polyester is of a woven structure.

7. A method of preparing a prostheses for functioning in a bloody environment with minimal thrombogenicity, said method comprising depositing on the surface of a prosthesis means a surface material, the surface material and/or prosthesis means being adapted such that the surface material can exhibit a periodic surface charge characteristic in said environment, characterised in that the surface material is albumin, or gelatin, or a combination thereof, or agar.

8. A method as claimed in claim 7, wherein said surface material is albumin, or gelatin, or a combination thereof, and is applied in about 3% solution.

9. A method as claimed in claim 7 or 8, characterised in that said surface material is normally incapable of cross-linking to said prosthesis means and in that an adaptive material is cross-linked to the surface material and to the prosthesis means.

10. A method as claimed in claim 9, wherein said adaptive material is glutaraldehyde or a succinic acid compound.

11. A method as claimed in any of claims 7 to 10, wherein the prosthesis means is of a woven polyester of terephthalic acid and ethylene glycol.