Apparatus and Method for Electrostatic Endothelial Cell Seeding and DNA Transfection in a Vascular Prosthesis

The invention herein relates to methods and devices designed to facilitate the attachment of cells to a surface. More particularly, the invention relates to such methods and devices for the seeding of endothelial cells on the luminal wall of small diameter vascular prostheses. Specifically, the invention relates to a method and apparatus for enhancing endothelial cell adhesion and DNA transfection to the luminal surface by temporarily altering the surface charge to a more positive state.

Heretofore it has been known to use synthetic prostheses in experimental operations for the repair of diseased or damaged, small diameter (<6 mm) blood vessels in humans. A major problem with use of synthetic small diameter vascular prostheses is poor long-term patency. The endothelial cell (EC) seeding of these vascular prostheses has led to an increased patency in humans and has led to complete healing in lab animals. The reason for high failure rates of small diameter vascular prostheses is due to the formation of acute, spontaneous thrombosis and chronic intimal hyperplasia. The thrombosis is initiated by platelets reacting with any non-endothelialized, foreign surface to initiate a platelet plug. This plug then acts as a template for the blood coagulation proteins. Over time, this platelet plug continues to grow resulting in occlusion, or failure, of the small diameter vascular graft. Thus, there is a recognized need for a better EC seeding technique of vascular prostheses to provide long-term patency and eventual healing.

Under normal circumstances, platelets circulate through the vascular system in a non-adherent state. This non-adherence is accomplished by the endothelial cells lining the vascular system. The EC have several factors which contribute to their non-thrombogenic mechanism. These are the negative surface charge, the heparin sulfate in their glycocalyx, the production and release of prostacylin, adenosine diphosphate, endothelium-derived relaxing factor, and thrombomodulin. Knowing this about EC, attempts have been undertaken to "disguise" artificial grafts by seeding autologous EC onto the graft prior to implantation. Research has shown that the adherence of more EC to the graft during the seeding process leads to enhanced healing times and reduced failure rates.

Chronic failure of the synthetic vascular grafts is due to intimal hyperplasia which principally occurs at the anastomosis. The exact cause leading to intimal hyperplasia has not been evident in research so as to explain the long-term failure of vascular prostheses. Intimal hyperplasia can apparently be initiated by many factors, including native vessel-prosthetic compliance mismatch and EC denudation due to increased shear stresses or mechanical damage which promotes the EC to synthesize growth factors to initiate smooth muscle cell (SMC) proliferation.

Intimal thickening is created by a combination of cellular proliferation and extracellular matrix accumulation. Myofibroblasts generate fibrous cells within the media to lead to intimal thickening of the adjacent native vessel. The SMC proliferation from the native vessel to the prosthetic vessel intima also contributes, to intimal hyperplasia. This SMC proliferation seems to be initiated and controlled by the EC present. Studies have shown that SMC proliferation occurs only underneath EC and does not occur where EC are absent. The proliferation slows down as EC repair progresses, except in the anastomotic region.

A number of EC seeding techniques have been used. One of these is a single-staged technique using a four step preclotting technique. The other techniques are: forcible injection, static adhesion seeding-preclotting, vacuum cell seeding, and subendothelial extracellular matrix seeding.

The single-staged technique for EC seeding is conducted during the preclotting of the graft. The preclotting process is designed to take full advantage of the reaction of thrombin and fibrinogen to preclot the graft. The first step of the process is to form thrombin. This is accomplished by injecting autologous blood into the graft lumen. Clotting of the blood in the graft indicates the presence of thrombin. Also during this time, fibrin is forming in the graft interstices.

The graft is then injected with unheparinized autologous blood to expose the graft for several seconds. Excess blood is wiped from the outside of the graft with a sponge. Then another portion of unheparinized, autologous blood is injected into the graft lumen to expose the graft for several more seconds. The outside of the graft is again wiped with a sponge to remove any excess blood. The objective of these last two steps is to utilize thrombin generated in the first step to form fibrin and seal the graft interstices.

The final step is to place the graft in heparin and inflate the graft with heparinized autologous blood repeatedly until hemostasis has been achieved. In this step, the open end of the graft is clamped with a non-crushing clamp. The objectives of this final step are to seal completely the graft interstices and to neutralize all the thrombin within the graft. The neutralization of the thrombin is done by a reaction with heparin. This neutralization is necessary because the thrombogenicity of the vascular grafts has been attributed to residual thrombin. The EC seeding during this preclotting technique is accomplished by placing harvested autologous EC into the first three aliquots of blood used in the process.

In the static adhesion seeding-preclotting technique the harvested EC are resuspended in an appropriate suspending medium, such as a saline solution, or blood plasma, and placed within the graft lumen. Both ends of the graft are clamped. The filled graft is then placed horizontally and periodically rotated over a period of several minutes. The unadhered cells are rinsed away by a gentle rinse with a saline solution. The graft is then ready for implantation.

In the forcible injection seeding-preclotting technique harvested EC are resuspended in a suspension media, such as Hank's solution. This suspension is divided into several aliquots of heparinized autologous blood. This blood-EC mixture is then injected forcibly with a plastic syringe into the graft. Both ends of the graft are clamped. These preclotting-seeding injections continue until hemostasis occurs. One problem with this type of preclotting along with the static adhesion preclotting techniques is that they are not adequate for fully heparinized patients. Another problem with these techniques is that after preclotting the surface may be rough and thrombogenic. These are two major limitations of the preclotting-seeding techniques described above.

The vacuum cell EC seeding technique was developed to induce a rapid EC coverage on the vascular prosthesis prior to implantation. This technique uses a special chamber in which a graft is placed. The graft is clamped at one end and the other end attached to a syringe filled with a washing medium and a syringe filled with an EC suspension. A vacuum is then applied to the system and the washing medium pulled through the graft. After pressure equilibrium, the cell suspension is pulled through the graft. One of the prerequisites of this method is to have a microporous graft. This method leads to an evenly distributed EC layer on the graft lumen. One benefit of this technique is that the total EC seeding time is significantly reduced. One limitation of this technique is that the EC at this point were mainly in the spheroid adhesion phase with the flattening of the adhered EC occurring over a two hour incubation period.

The subendothelial extracellular matrix seeding technique begins by coating the graft lumen with a uniform, naturally produced subendothelial extracellular matrix. This extracellular matrix (ECM) leads to a more suitable vascular prosthesis lumen for EC adhesion, growth, and differentiation when compared to grafts coated with fibronectin or basement membrane. An advantage of this technique is that it contains adhesive glycoproteins, proteoglycans (heparin sulfate), and EC growth factors. The limitation of this technique is the amount of culture time required. The first culture period is required to produce the ECM on the graft lumen. The second culture period is for the EC seeding process required to produce a lumen which resembles a native vessel.

Previous platelet adhesion experiments have verified that platelet adhesion can be controlled by the platelet-surface interaction potential energy. This potential energy is due to the interaction of the two surface charges and their complimentary surface potentials, or total double layer potentials. This double layer potential is developed by ions within the suspending medium interacting with the surface charges. EC experiments have also indicated a charge interaction in the adhesion process.

Platelet experiments have shown that the adhesion process is affected by temporarily altering the surface charge. If the surface charge is made more negative, the platelet adhesion process is inhibited. The spreading also seems to be slowed under these conditions. When the surface charge is temporarily made more positive, the platelet adhesion process is enhanced. Likewise, the spreading is also enhanced under a more positive surface charge.

Experiments conducted on EC adhesion on varying surface charges show similar results to those obtained for platelets. The EC adhesion process was reduced with an increasingly negative surface charge. The increasingly positive surface charge led to an enhanced EC adhesion. The main problem with these experiments is that the surface charges are a function of the material and permanent. Although providing a more positive charge on the surface wall enhances EC adhesion, the positive charge causes the prosthesis or graft to become highly thrombogenic. This leads to increased platelet adhesion and graft failure by thrombosis.

The other similarity among these experiments is the actual adhesion process once the cells are in contact with a surface. Once the cells settle to the surface, the platelets, as well as the EC, then initiate adhesion by extending pseudopods. The central portion of the cell then follows these pseudopods to flatten and adhere to the surface. This portion of the adhesion process resembles "webbing" which connects the pseudopods. Under normal circumstances, the adhesion and spreading process for EC takes anywhere from several minutes to several hours depending on the surface conditions.

As previously discussed, altering the surface charge can increase or decrease the spreading time. Increased spreading of the seeded EC is an important aspect of the process. Studies have shown that EC adherence to vascular graft material under flow conditions is dependent on the EC-surface interaction and the time allowed for adhesion and spreading prior to the initial exposure to shear stress. This is because EC which are not tightly adhered or fully spread upon exposure to fluid shear stresses are washed away upon implantation. As many as 70% of the EC are removed from implanted EC seeded grafts over the first several minutes with 3.7%/hr lost over the period from 30 minutes to 24 hours using the forcible injection EC seeding technique.

The morphology of the adhered EC on the graft surface after the proposed procedure will be circular and flattened. It is assumed that the circular, flattened EC will change morphology to a more elongated, flattened shape upon initial exposure to the implanted shear stresses. Many studies have shown that EC will realign parallel to the direction of flow in response to local shear stresses.

Studies have also concluded, by scanning and transmission electron microscopy, that the surface free energy determines the number of contact sites between the graft and the flattened fibroblasts. A high surface free energy promotes cell adhesion, spreading, and contact sites. A low surface free energy inhibits cell adhesion, spreading, and contact sites. Also large contact regions, adhesion sites, between surfaces and fibroblast cells can be obtained on less negatively charged surfaces. It has further been shown that increasing the surface negativity leads to decreased contact regions, thus, showing the importance of the cell-surface electrostatic interaction in the cell adhesion process.

As discussed previously one problem with small diameter vascular grafts is their tendency to become occluded. It is known that DNA transfection, that is incorporating beneficial DNA strands into a medium, enhances desirable properties of the medium. Genetic alteration of seeded ECs may enhance the anti-thrombogenic properties as well as the smooth muscle anti-proliferative properties of the ECs to prevent occlusion. It is known how to use DNA transfection or electroporation in vitro within containers, such as cuvetes or petri dishes, by applying an electric potential across the container. Unfortunately, there is no known method for electrostatic or electroportation transfection of DNA while seeding endothelial cells.

Accordingly, it is most desirable to obtain an apparatus and method for the electrostatic seeding of endothelial cells on a non-thrombogenic graft surface without making the graft surface thrombogenic. Moreover, it is desirable to temporarily alter the surface charge of a negatively charged material to enhance EC adhesion to a less negatively charged surface and then return the surface to its original negative surface charge. There is also a need in the art to simultaneously transfect DNA plasmids and seed endothelial cells on the luminal wall of a vascular graft.

In light of the foregoing, it is an object of the invention to provide an endothelial cell (EC) seeding technique and apparatus to enhance the EC adhesion during seeding, prior to implantation of small diameter (<6 mm) synthetic vascular prostheses.

Another object of the invention is the provision of such an EC seeding technique and apparatus which uses electrostatic forces with gravitational forces to enhance endothelial cell adhesion and maturation.

A further object of the invention is the provision of such an EC seeding technique and apparatus which provides enhanced endothelial cell adhesion onto a graft surface which is nonthrombogenic without making it thrombogenic.

Yet another object of the invention is the provision of such an EC seeding technique and apparatus which takes nonthrombogenic surfaces and temporarily makes them more positive to cause them to be more attractive to the endothelial cells during seeding and after seeding allows the graft material to revert back to its natural negative surface charge which is non-thrombogenic.

Still a further object of the invention is the provision of such an EC seeding technique and apparatus which prevents thrombosis which would occlude a small diameter graft and also allows healing of any non-endothelialized areas resulting in increased long term patency of the prosthesis.

Still another object of the invention is the provision of a DNA transfection and an EC seeding technique to further enhance the long term patency rates of small diameter vascular prostheses.

The foregoing and other objects of the invention which will become apparent as the detailed description proceeds are achieved by an apparatus for modifying a lumen surface of a vascular graft, comprising: conductor means for temporarily altering an electrical charge on a lumen surface to be modified; and support means for maintaining a suspension containing at least one of endothelial cells and DNA plasmids adjacent to the luminal surface, wherein the alteration of the electrical charge transfers said at least one endothelial cells and DNA plasmids from the suspension to the luminal wall.

Still another object of the present invention is achieved by an apparatus for seeding endothelial cells and transfecting DNA plasmids on the luminal wall of a vascular prosthesis, the apparatus comprising: an external conductor having a bore extending the length thereof; an internal conductor adapted to be received within a vascular prothesis which is received within the bore; and a voltage source for applying an electrical potential across the external and internal conductors; whereby the vascular prosthesis may be filled with a suspension comprising endothelial cells and DNA plasmid and is sealingly mounted on the internal conductor, and maintained in a spaced relationship from the internal conductor within the bore of the external conductor, and where the voltage source is adapted to apply an electrical potential across the conductors, thereby temporarily altering the surface charge of the prosthesis to facilitate adhesion of the endothelial cells and transfection of the DNA plasmid thereto.

Another object of the present invention is achieved by a method for seeding endothelial cells and transfecting DNA on the luminal wall of a vascular prosthesis, comprising the steps of: passing an internal conductor wire through the lumen of the vascular prosthesis; sealing one end of the prosthesis around the internal conductor wire; suspending the internal conductor wire and prosthesis in an upright position in a filling apparatus; filling the prosthesis with a suspension of endothelial cells and DNA plasmid; sealing the other end of the prosthesis; providing an external conductor with a bore; placing the internal conductor and prosthesis into the bore; rotating the prosthesis; and applying temporarily an electrical potential across the internal and external conductors while the prosthesis is being rotated.

For a complete understanding of the objects, techniques and structure of the invention reference should be made to the following detailed description and accompanying drawings wherein:

FIG. 1 is a graphic depiction of the results of parallel plate capacitor adhesion experiments where the resuspending solution contained antioxidants;

FIG. 2 is a graphic depiction similar to FIG. 1 where the resuspending solution contained no antioxidants;

FIG. 3 is an elevational side view of the apparatus according to the invention;

FIG. 4 is a top plan view of the apparatus of FIG. 3;

FIG. 5 is a cross-sectional view of the apparatus of FIG. 4 taken along the line 5--5;

FIG. 6 is a cross-sectional view of the apparatus of FIG. 4, taken along the line 6--6;

FIG. 7 is an elevational side view of the filling apparatus according to the invention;

FIG. 8 is a flow chart depicting the steps of one method according to the invention; and

FIG. 9 is a flow chart depicting the steps of another method according to the invention.

With the foregoing background in mind, experiments were conducted on human umbilical vein endothelial cells (HUVEC), using a parallel plate capacitor, to determine the effects of altering the surface charge of flattened expanded polytetra fluoroethylene (e-PTFE) graft material with regard to HUVEC adhesion. After the HUVEC were cultured and harvested a cell count was performed to determine the number of HUVEC/ml. The experiments were accomplished by placing a static pool of HUVEC suspended in Dulbecco's Phosphate Buffered Saline (DPBS) solution in a Teflon.RTM. (Teflon is a Registered Trademark of E. I. du Pont de Nemours & Co.) ring held in position on the flattened e-PTFE graft.

Three experiments were performed on the HUVEC. The first was the control experiment to determine the amount and quality of EC adhesion on a graft having a non-induced surface charge for comparison of EC morphology. This experiment was conducted both with and without antioxidants.

The second experiment was to determine the amount and quality of EC adhesion on the graft when placed between the plates of the capacitor while an electrical potential was applied thereto.

The third experiment was like the second, but included an antioxidant in the HUVEC-DPBS solution.

Because EC tend to be more active when in the non-confluent state and less active in the confluent state, each of the experiments was performed using both confluent and non-confluent HUVEC. Contact interaction leads to proliferation inhibition and membrane stability. The non-confluent case (very little contact between EC) leads to an active EC state during proliferation. Thus, the presence of an antioxidant in experiments one and three was to protect the EC membrane during the active state.

Experiments two and three were conducted using applied voltages of 0.75 V, 1.50 V and 10.0 V.

The number of HUVEC in the spheroid, discoid, and flattened state was then determined for each of the experiments and compared to determine the effect of applied voltage, confluent v. non-confluent state, and effect of antioxidants.

The results obtained confirmed that endothelial cell adhesion and spreading on e-PTFE is affected by the surface charge. The results further indicate that increasing the magnitude of induced positive charge on the e-PTFE material increases: (1) the total number of HUVEC adhered; (2) the total and percentage of HUVEC adhered in the flattened phase; and, (3) the total HUVEC adhered in the discoid phase.

The results also indicate a trend in the data between confluent and non-confluent HUVEC. As is apparent from FIGS. 1 and 2, the percentage and total discoid HUVEC adhered is larger for the non-confluent, while the percentage and total spheroid HUVEC is higher for the confluent. However, no noticeable trend can be seen between the antioxidant and no-antioxidant experiments. Accordingly, the present invention was developed to take advantage of the experimental results discussed above for endothelial cell seeding of small diameter vascular prostheses.

Referring now to the drawings, and more particularly to FIGS. 3-7, it can be seen that an apparatus for electrostatic seeding of endothelial cells is designated generally by the numeral 10. The apparatus 10 is comprised generally of an external conductor 11, an internal conductor 12, a support assembly 14, a drive mechanism 15, a filling apparatus 16, and a voltage source 18.

The external conductor 11 is a hollow cylindrical member having a concentric internal bore 19. The external conductor 11 is preferably fabricated from a round length of an appropriate machinable metal such as 303 stainless steel. It is desirable that the conductor 11 be coated with an appropriate conductive polymer or, alternatively, fabricated entirely from such a conductive polymer, to prevent heavy metal contamination of the graft. In an alternative embodiment, a dielectrc coated, non-conductive, polymer may be employed. In an alternative embodiment, a dielectric coated, non-conductive, polymer may be employed. The conductor 11 includes a pair of set screw grooves 20a and 20b in its outer surface 21, proximal to ends 22a and 22b, respectively, thereof. Similarly, a drive groove 23 is located in the surface 21 of the conductor 11, approximately midway between the grooves 20.

With particular reference to FIG. 5 it can be seen that the internal conductor 12 is comprised of a strand of electrically conductive wire 24, preferably AWG-22 silver-plated copper coated with GORE-TEX.RTM., which is a registered trademark of W. L. Gore & Associates, Inc. The ends 26 of the wire 24 are each secured to an end terminal 27. The end terminals 27 are ball bearing swivels preferably of a nickel-plated steel. The conductor 12 further includes a pair of spaced silicon rubber plugs 28 which are molded directly on the wire 24. For reasons which will become apparent as the description continues the plugs 28 have an outside diameter approximating the inside diameter of the graft to be seeded. Similarly, the plugs 28 are spaced apart from one another along the wire 24 at a distance which is slightly less than the length of the graft to be seeded. Those skilled in the art will recognize that the plugs 28 may be molded from a resilient silicon compound such as Sylgard.RTM., which is a registered trademark of Dow Corning Corp, using sectional molds made of Teflon.RTM., which is a registered trademark of E. I. du Pont de Nemours & Co, or another appropriate material.

Referring again to FIGS. 3 and 4 the support assembly 14 comprises a base plate portion 30 having legs 31 affixed thereto. A pair of end supports 32 are disposed at opposite ends of the base plate 30. The end supports 32 are blocks fabricated preferably of Teflon.RTM.. A first end support 32a includes a somewhat larger plug receiving aperture 36 substantially centered therein. A second end support 32b includes a threaded aperature 34b for the receipt of a screw eye 35b. As shown, the end supports 32 are mounted to the base plate 30 by way of notches 38 in the sides 39 of the base plate 30. A support plug 37 is adapted to be received in the plug receiving aperture 36. The support plug 37 is similarly fabricated from an appropriate material such as Teflon.RTM. and includes a generally cylindrical head portion 41 and a reduced diameter body 45. A threaded aperture 34a is provided in the end face 49 of the plug 37 for the receipt of a screw eye 35a. It should thus be recognized that the diameter of the body portion 41 of the plug 37 is slightly smaller than the diameter of the plug receiving aperture 36 of the end support 32a. As shown a coil spring tensioner 53 is fitted on the screw eye 35a.

The support assembly 14 further includes a pair of bearing mounts or pillow blocks 40 for rotatably journaling the external conductor 11. Accordingly, the external conductor 11 is fitted within a pair of bearings 42 and secured thereto by tightening set screws 43 in the bearing locking collars 44 so as to engage the set screw grooves 20 in the outer surface 21 of the conductor 11. The bearings 42 are mounted in the pillow blocks 40, which are, in turn, mounted on the base plate 30. A non-conductive gasket 46 is interposed between the pillow blocks 40 and the base plate 30 to insulate electrically the pillow blocks 40 from the base 30. It should be noted that the bore 19 of the external conductor 11 is axially aligned with the apertures 34 and 36 of the end supports 32a and 32b respectively.

As depicted in FIG. 7, the filling apparatus 16 is comprised of a support section 47 having a pair of end sections 48 disposed at each end thereof. The end section 48a includes a bore 50 to accommodate the support plugs 37 previously described with respect to the support assembly 14, while the end section 48b is provided with a screw eye 35c. As shown, the end sections 48 are disposed at opposite ends of the support section 47 such that the plug receiving bore 50 is axially aligned with the screw eye 35c.

With reference now to FIGS. 4 and 6, the drive mechanism 15 includes a motor support base 51 which is affixed to the base plate portion 30 of the support assembly 14. A motor 52 is mounted to the motor support base 51 by way of a mounting bracket 54. The motor 52 is preferably a synchronous A.C. motor operating at 115 V/60 Hz and capable of running at approximately 1/12 RPM in a clockwise rotation. A drive sheave 55 is mounted on the motor shaft 56. The sheave 55 may be made of a reinforced nylon or other appropriate material. A drive belt 58 is fitted in the drive groove 23 of the external conductor 11 and drivingly engages the sheave 55.

The power supply 18 may be a conventional multi-range voltage calibrator capable of generating a 0-20 V output with an accuracy of .+-.0.05% of the full scale reading. The positive lead 59 is electrically connected to the external conductor 11 by way of the pillow block 40, while the negative lead 60 is connected to the internal conductor 12 by way of the screw eye 35a. Accordingly, an electrical potential may be applied across the prosthesis. As discussed previously, a more positive charge on the luminal wall of the prosthesis facilitates the adhesion of ECs thereto. Once the electrical potential is removed, the graft returns to its originally charged state which prevents the attraction of platelets to minimize the coagulation thereof.

In operation a synthetic small diameter vascular prosthesis or graft 70 to be seeded with homologous EC is fitted on the internal conductor 12 by passing an end 26 of the wire 24 through the lumen of the graft 70. The graft 70 is positioned over the plugs 28 and secured to the conductor 12 by seating the graft 70 on the lower plug 28a using 2-O silk suture or the like. This creates a seal between the graft 70 and the plug 28a to prevent fluid leakage during the seeding process. The ends 26 of the wire 24 are then affixed to the screw eye 35a and spring 53 respectively of the end section 48a and support plug 37. The support plug 37 is in turn mounted in the end section 48b of the filling apparatus 16 such that the open end 71 of the graft 70 is facing upwardly. The graft 70 may then be slowly filled with the EC-Saline solution to the point of overflow so as to eliminate air in the graft 70. Once the graft 70 is filled, the open end 71 is then secured to the upper plug 28b using silk suture in a similar manner as was done for the lower plug 28a.

After the graft 70 has been filled and closed the internal conductor 12 is removed from the filling apparatus 16 and transferred to the support assembly 14. The internal conductor 12 is fitted within the external conductor 11 by passing an end 26 of the wire 24 through the bore 19 using a lead wire (not shown). The end terminals 27 of the internal conductor 12 are then secured to the end supports 32 as shown.

Seeding of EC in the graft 70 is thus accomplished by energizing the motor 52 so as to initiate rotation of the external conductor 11. Simultaneously, an electrical potential is applied across the apparatus 10 by the power supply 18. Those skilled in the art will recognize that the internal and external conductors act as a cylindrical capacitor while the graft material acts as a dielectric interposed therebetween. As such, the surface charge of the graft lumen is temporarily altered to a more positive state so as to provide a more suitable environment for EC adhesion. After the seeding procedure is completed, and the electrical potential removed, the surface charge of the graft returns to its original, more negative, non-thrombogenic state. The graft may then be removed from the internal conductor by removing the silk suture. The graft is ready for implantation after being gently rinsed to remove the excess EC-Saline solution. While the method of the present invention should now be apparent in view of the foregoing discussion of the operation of the apparatus, the preferred method will now be discussed briefly with reference to FIG. 8. The first step involves the selecting of a synthetic vascular prosthesis of an appropriate size and quality for the implant procedure. Next plugs of an appropriate size to fit within the prosthesis are molded on the internal conductor. Homologous endothelial cells are harvested and suspended in a suspension medium. The internal conductor is fitted to the prosthesis by passing the internal conductor through the lumen thereof, and sealing one end of the prosthesis to one of the molded plugs. The internal conductor, carrying the prosthesis is then suspended upright in the filling apparatus by connecting the end terminals thereof to the screw eye of the first end section and the spring of the support plug respectively. With the prosthesis suspended upright it may be filled to overflow with the suspended endothelial cell solution. Once the prosthesis is filled, the open end thereof may be sealed to the remaining molded plug.

After the filled prosthesis is sealed it is removed from the filling apparatus and transferred to the seeding apparatus by disconnecting the end terminal from the screw eye of the first end section and removing the support plug from the second end section. The prosthesis and internal conductor are mounted in the seeding apparatus by passing the disconnected end terminal through the bore of the external conductor and pulling the prosthesis into the bore. The disconnected end terminal is then connected to the screw eye of the first support block while the support plug is fitted to the aperture of the second support block. With the internal conductor and prosthesis mounted in the seeding apparatus rotation of the external conductor is initiated by energizing the motor. Simultaneously, an electrical potential is applied across the conductors by energizing the power source. As such, the surface charge of the luminal wall of the prosthesis is temporarily altered to a more positive state allowing the endothelial cells to adhere thereto in a more flattened state.

In another embodiment of the present invention it will be appreciated that the apparatus 10 may also be employed to transfect DNA along with the EC cells. By concurrently transfecting DNA plasmids and seeding EC cells to a graft 70, the long term patency of the graft is extended. As those skilled in the art will appreciate, specific DNA plasmids may be selected to incorporate anti-coagulant properties into the lumen wall of the graft to ensure non-adhesion of the platelets. Moreover, DNA plasmids may be transfected into a graft to facilitate the delivery of disease or cancer fighting agents to a particular area of the body.

As seen in FIGS. 3-5, a second voltage source 100 may be connected in parallel with the first voltage source 18. The voltage source 100, which in the preferred embodiment may be a Hoefer progenerator pulse controller which can deliver square wave pulses at a selected voltage value for a selected duration, performs an electrical shock of the graft 70 to initiate the transfection of DNA into the graft 70. As will become apparent in the detailed discussion below, the pulse voltage and duration thereof are critical for the successful transfection as this can only be accomplished in a narrow voltage and duration range. Those skilled in the art will appreciate that the voltage sources 18 and 100 may be embodied in a singular voltage source which performs the necessary application of voltage to seed the endothelial cells and transfect the appropriate DNA.

To accomplish the concurrent endothelial cell transfection and seeding the ECs may be suspended in Dulbecco's Phosphate Buffered Saline (DPBS) containing antioxidants (1.times.10.sup.-4 M sodium astorbate, 3.6.times.10.sup.-5 M glutathione and 1.3.times.10.sup.-3 M dimethyl sulfoxide) or other like solutions. The antioxidants are present to minimize cellular membrane damage upon electrical stimulation and subsequent free radical generation. The concentration of ECs within the DPBS will be about 5.times.10.sup.6 EC/ml. This concentration was determined to be optimum in EC seeding of e-PTFE graft material and is currently being used in electrostatic EC seeding procedures. Of course, other concentrations may be employed depending upon the size and material used for other types of grafts. This concentration of ECs during electroporation using a cell suspension in a cuvette is in the optimum range of about 10.sup.6 -10.sup.7 cells/ml. As those skilled in the art will appreciate, a lower concentration of cells shows poor cell recovery after shock while at higher concentrations, an undesirable cellular fusion takes place. Also included with the solution is the DNA to be transfected which is usually in the form of a linearized DNA plasmid. A linear DNA plasmid is required to obtain a stable transfection during an electroporation. In the preferred embodiment, the linear DNA concentration in the seeding/transfection solution or suspension may be in the range of 1-2 .mu.g/.mu.L. This solution or suspension is then placed in the graft 70 using the filling apparatus 16 and the method described above. Likewise, the loading of the internal conductor within the graft 70 into the external conductor follows the procedure used in the previous embodiment.

Once the graft 70 is filled with the EC-DPBS-plasmid solution, the internal conductor 12 and graft 70 are placed within the external conductor 11. To initiate transfection of the ECs, an electroporation shock is applied to the cells. This is accomplished by the voltage generator 100 which applies a voltage of up to 2,000 volts across the internal and external conductors 12 and 11, respectively, for a duration of up to about 8 microseconds which may be repeated up to 9 times over a five second period. Of course, the voltage values, the durations and the number of applications of the electric shock may be adjusted depending upon the size and material of the graft 70. The through current generated by the voltage source 100 may also be limited within the apparatus 10 to a maximum amount to prevent cellular damage. Additionally, damage may be reduced by maintaining the temperature of the apparatus at a nominal value of around 72.degree. Fahrenheit. To initiate the seeding of the endothelial cells, the voltage source 18 applies voltage across the external and internal conductors 12 and 11, respectively, for a time period which has been determined to be optimum in seeding ECs on particular graft materials and diameters. This electrical potential is up to 20 volts and applied for a duration of anywhere between one minute to two hours. This applied electrical potential not only enhances EC adhesion, it also aids in the transfection efficiency by attracting the DNA plasmids onto the seeded, shocked ECs on the luminal surface of the graft.

As in the previous embodiment, upon completion of the transfection-seeding procedure, the graft 70 is removed from the external conductor 11 and then removed from the internal conductor 12. The seeded-transfected graft is then rinsed with sterile DPBS to remove any loose cells or plasmids. Ater rinsing, the graft is ready for implantation. It will be appreciated, however, that the EC seeding procedure--the application of the lower value voltage--may accrue prior to the application of the higher voltage values. Additionally, depending upon the particular graft material employed and the characteristics of the DNA, various combinations of application of electrical potential may be used to facilitate the seeding/transfection procedure.

Based upon the foregoing, the advantages of the concurrent transfection/seeding procedure should be readily apparent. Primarily, this apparatus and related method of use facilitates the genetic alteration of seeded ECs to enhance the anti-thrombogenic properties as well as the smooth muscle anti-proliferative properties of the ECs. As such, use of the disclosed apparatus may reduce or eliminate the complications and failures currently encountered in the use of small diameter vascular grafts which currently reduce their patency rate and prevent them from being widely used in clinical trials. Another advantage of the present embodiment is that the induced positive surface charge maintained at the graft luminal surface wall, which enhances EC adhesion, works to increase transfection efficiency as well. This increased efficiency may be brought about by the attraction of negatively charged DNA to the surface in close contact with the ECs. Moreover, the induced luminal surface charge may also maintain the porosity induced by the first high electrical shock to allow passage of even more foreign DNA into the cells adhered on the luminal surface.

Thus, it can be seen that the objects of the invention have been satisfied by the structure presented above. While in accordance with the patent statutes only the best mode and preferred embodiment has been presented and described in detail, it is to be understood that the invention is not limited thereto or thereby. Accordingly, for an appreciation of the true scope and breadth of the invention reference should be made to the following claims.