Surface Or Lining Compatible With Blood Or Other Tissue

Abstract

A surface or lining containing microcavities for anchoring pseudointimal growth within blood handling prostheses such as vascular grafts, heart assist pumps, artificial hearts, extracorporeal devices, and the like to form a thin, stable autologous lining. The surface is also compatible with other living tissue and promotes tissue adhesion to percutaneous leads, catheters, cannulae, etc., which inhibits bacterial penetration and consequent infection. A method of forming a lining or surface containing microcavities.

Description

BACKGROUND OF THE INVENTION

This invention relates generally to surfaces and linings compatible with blood or other living tissue and to a method of making the same, and more particularly to a lining for blood-carrying prostheses and tissue-adhering surfaces for implanted devices.

The majority of blood-handling devices (catheters, blood pumps, hemodializers, blood oxygenators) have historically been fabricated from materials (e.g., silicone rubber, polypropylene, polytetrafluoroethylene) which are made very smooth and inert to avoid blood damage and thrombus formation. Such materials or surfaces are classified as antithrombogenic substances. Much effort has been devoted in the past and is currently being expended to develop improved, satisfactory antithrombogenic surfaces but with only limited success. The effective prevention of all thrombus formation, thrombo-emboli, and blood trauma in devices formed of such materials has not been realized. Other materials (hydrogels, anionic cellulose) produce less complications, but have physical properties which are inadequate for many applications.

On the other hand some success has been achieved with the use of porous materials (primarily Dacron and Teflon) fabricated as woven or knitted tubes and used as arterial grafts (replacement segments of blood vessels) in patients with vascular disease or aortic aneurisms. These prostheses develop a relatively thin pseudointimal lining over a period of weeks which remains stable and satisfactory for an indefinite period in many patients. The lining is formed by the ingrowth of tissue through the porous material and the deposition of fibrin and cellular material from the blood. It covers these grafts in a few days with a biologic lining which is blood-compatible and prevents significant blood trauma and thrombus or thrombo-embolic formation.

Recently, in the past few years, non-porous or impermeable prostheses such as arterial grafts and blood pumps have also been fabricated with anchoring surfaces of a fibrous nature to promote the formation of a pseudointimal lining. It has been observed that fibrin cells from the blood are deposited on the anchoring surface, forming small areas of pseudointima which gradually increase in size. Such linings are compatible with blood to the extent that blood trauma has been tolerable and thrombo-embolism has been virtually eliminated in well-designed prostheses. However, in devices having complex shapes as with blood pump valves or flexing surfaces the biologic material may not deposit uniformly. Thicker deposits occur in portions where excessive growth takes place. These thicker deposits may die from lack of nourishment and detach to form thromboemboli or may accumulate at the valves and interfere with the valve function. The thick lining may also reduce blood flow in the smaller passages. In prosthetic devices which have flexing elements, the thick biologic deposits may prevent proper flexure and eventually cause failure. These problems occur despite the use of large quantities of anticoagulating drugs in an effort to prevent the formation of a thicker layer. The anchoring substrates found in prior art have comprised loose-knit Dacron cloth backed with silicone rubber, or Nylon and Dacron velour backed with silicone rubber. Another type of substrate which has been used employs Dacron fibers embedded in a polyurethane adhesive to form a flocked surface.

The methods of anchoring biologic materials in the prior art have not been entirely satisfactory. Velour and flocking fibers from relatively thick substrates and as a result the fibrin-cellular lining is also thick since it continues to form and does not stabilize until the fibers are essentially completely covered. A much thinner but adherent anchoring surface or substrate is highly desirable.

A very thin substrate results in a more rapid coverage by biologic materials (cells and fibrin) which stabilizes early and which differentiates or forms pseudointima much more quickly. A thin pseudointimal lining which forms rapidly is desirable to shorten or avoid the period of anticoagulant treatment. The underlying cells nearest the prosthesis are adequately nourished from the blood stream if the biologic lining is thin, and the shear and bending (tensile and compressive) stresses in the lining are reduced. A rapidly-developing pseudointima on a thin anchoring substrate would considerably reduce the interim risks of blood trauma, thrombus formation, and thromboemboli as well as the problem of potential failure resulting from thick cellular deposits on critical surfaces (e.g., pump bladder, valve shunt). Adherence to the biologic lining is dependent on both surface chemical and surface mechanical properties. The configuration is most critical, yet no presently used surfaces are satisfactory in this respect; the knitted or woven cloths or flocked surfaces cannot be made sufficiently thin and still retain good adhesive characteristics. The woven, knitted, or matted fibrous surfaces are extremely difficult to apply uniformly to irregular surfaces such as the valves and tubing of blood pumps. They are also difficult to apply satisfactorily to flexing surfaces as in blood pumping chambers because their indistensibility along the surface leads to shear and tensile stresses which can cause the pump chambers to fail or the fibers to separate from the chamber surface.

Percutaneous leads or prosthetic devices in contact with tissue have a history similar to that for bloodcarrying devices. These devices have until recently used smooth, inert flexible or rigid surfaces in contact with tissues (skin, subcutaneous tissue, or internal organs or tissue). One problem with this approach is that adhesion of the tissue to the surface had been inadequate, resulting in relative motion and a consequent substantial risk of complete removal of percutaneous leads (catheters, cannulae) or damage to internal organs. Another problem is the substantial risk and incidence of infection with percutaneous leads by bacteria penetrating the interface between tissue and lead into the patient, with localized infections frequently followed by more serious systemic infections if the local infections are not diagnosed or adequately treated.

Recently various fibrous materials (e.g., Dacron velours, Teflon felt) have been adhered to the percutaneous leads in an effort to solve both of these problems. This has generally resulted in improved adhesion and fixation, but only moderately decreased risks of infection. The bacteria have been able to penetrate the interface between the tissue and fibrous anchoring surface and also the one between the lead and anchoring surface either because the tissue has not been able to penetrate the fibrous layer and block bacterial influx, or because tissue adhesion to the fibrous layer and lead has been inadequate to prevent bacterial penetration. As with the pseudointimal lining, a thin layer of tissue on the prosthesis anchoring surface must be made to penetrate and adhere to all exposed surface materials by proper surface treatment and by the mechanical configuration of the surface. Existing surfaces are generally either too thick, such as the flocking type, or do not lend themselves to fabrication of complex shapes, such as the woven fabric type.

SUMMARY OF THE INVENTION AND OBJECTS

In accordance with the present invention, there is provided a lining or surface containing a plurality of microcavities or pockets which is compatible with blood and living tissue and forms a tenacious base or anchor for pseudointimal growth and tissue ingrowth and yet provide normal metabolic processes to the cells. The microcavities are formed by providing particles or fibers in the base material and thereafter removing the particles or fibers leaving microcavities.

It is an object of the present invention to provide a surface which forms a thin substrate capable of tenacious anchoring of subsequent cell deposition and ingrowth.

The foregoing and other objects will be more clearly apparent from the following description when taken in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A-1E show the steps of forming microcavities in a substrate using fibrous material.

FIGS. 2A and 2B show the steps of forming a microcavity in a substrate using granular particles.

FIG. 3 shows a regularly shaped enclosed volume having a surface formed in accordance with the present invention.

FIGS. 4A-4E show the steps of another process for forming a surface containing microcavities.

FIG. 5 shows an enlarged view of a portion of skin and subcutaneous tissue together with a percutaneous lead device according to the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

In general, the surface or lining containing microcavities or pockets in accordance with the present invention is formed by applying fibrous, or particulate or granular material to the surface which is to contain microcavities, while the surface is soft, causing the surface to set up, cure or harden with the fibers or granules partially or wholly incorporated therein, and thereafter using a solvent which dissolves the fibers or particles leaving a surface which contains microcavities at the location where the fibers or particles were embedded. The size and shape of the microcavities can be controlled by the selection of the size and shape of the fibers, particles or granules. The density or surface distribution of these pores or pockets can be controlled in the manufacturing process by regulating the particle distribution.

Referring particularly to FIG. 1, there are shown the steps of forming a surface containing microcavities. In FIG. 1A there is shown a substrate which forms the prosthetic device. A layer 12, for example, silicon rubber, heat curing polyurethane or solvent evaporation polyurethane is applied and adheres to the surface of a substrate, FIG. 1B. The layer 12 is in its soft or tacky state but can be hardened by a curing ambient depending upon the material used. Fibers may be flocked or otherwise applied to the tacky surface 12 as shown in FIG. 1C whereby the fibers are randomly embedded in the layer 12. Thereafter, the layer 12 is hardened by solvent evaporation, irradiation heating or the like, FIG. 1D. Finally a solvent which serves to selectively etch or dissolve the fibers leaving the layer 12 is applied. The fibers are dissolved leaving a plurality of microcavities 14 in the layer or surface 12. The surface is then sterilized as, for example, by washing and autoclaving.

The materials used for the substrate, base material or layer, fibers and solvents are not necessarily inert or tissue compatible. By way of example, the substrate may be a metal or metal alloy such as stainless steel, aluminum, ferrous-nickel, titanium, a rigid plastic such as polypropylene, Teflon, Nylon and polycarbonate or an elastomer such as silicon rubber, polyurethane and natural rubber.

The adhesive layer 12 may be silicon rubber, polyurethane, or solvent-evaporation polyurethane as previously described.

The fibers applied may be Nylon, Dacron or acetate. In the event that Nylon or Dacron is employed, acetic acid may be employed for selectively dissolving the Nylon or Dacron while the acetate may be selectively dissolved by acetone or methyl ethyl ketone.

It will, of course, be apparent that the foregoing are merely examples of suitable material for the substrate and for the flocking fibers. The selection of the substrate as being either rigid or an elastomer depends upon the use for which the prostheses or implanted device is to be put. Furthermore, the layer is selected whereby it is compatible with the blood or other living cells with which it will be in intimate contact and for which it will provide the base for growth of cells or depositions and adhesion of organized cellular linings.

In FIG. 2 there is shown a method of forming microcavities by employing particles or granules 16 rather than fibrous material 13. For example, the particles 16 may be NaCl crystals of a selected size or shape distribution. The embedded particles or granules may be dissolved with distilled water leaving a plurality of irregularly shaped microcavities 17. The layer is then sterilized.

Typically, the depth of the microactivities is between 0.002 and 0.020 inches. This provides a relatively thin pseudointima for adequate diffusion of nutrients from the flowing blood, minimal tissue stress during operation. In a blood pump this thin lining will not interfere with proper pump valve function, and presents minimal tissue or lining stress due to pumping chamber flexure.

The above described processes are particularly suitable for use in devices which have irregular or enclosed surfaces. For example, in FIG. 3 there is shown a sketch of a device including an enclosed volume. The steps in forming such a device would be to form the outer wall 21 of the device which, in essence, is the substrate, applying an elastomer or other layer 22, embedding, then hardening and then dissolving the particles to leave a plurality of microcavities 23.

In FIG. 4 there is shown another process for forming a surface including microcavities. A mold 26 is provided with a polymer adhesive layer 27 which, in its soft condition, is flocked with suitable flocking material such as fibers 28. The layer 27 is thereafter set up or hardened and is prevented from adhering to the mold by suitable mold release compound. When the layer is hardened, the outer surface layer 29 is applied over the surface. Thereafter, the molded object 29 is removed from the mold along with layer 27 and the embedded particles 28 which then, in turn, are dissolved leaving a plurality of internal microcavities 31.

Thus, it is seen that there has been provided an improved surface or lining and method of forming same with microcavities which are controllably spaced and interconnected. Such cavities permit the supply of nutrients from the adjacent cells or from the bloodstream to penetrate easily through the lining and into the cavities to nourish the cells within the cavities from many directions, for example, three or more directions whereby to provide an improved anchoring surface for use in prostheses or any type of device which is in contact with blood.

The depth and size of the microcavities can be controlled by the fiber length and size or the particle size and shape. The openings are of uniform density but may be randomly angled with some holes interconnecting below the surface. This structure provides good adhesion to the non-cellular coagulum and fibrin-cellular constituents which form the pseudoendothelium.

This type of formation of microcavities has several advantages. Whereas in prior art methods the flocked fibers are not tightly adhered to the surface and some may separate and form emboli or nucleate thromboembolisms, the negative flock or cavities are an integral part of the prostheses device. Good cellular interface adhesion is obtained by pseudointimal growth or ingrowth of internal tissue or skin and subcutaneous tissue. Therefore no interface separation in the prostheses will occur. Furthermore, good tissue adhesion in a percutaneous lead device provides an interface sealed against bacterial infection. The microcavity surface can be made extremely thin without the difficulties encountered using the flock, woven, knitted, or other fiber techniques, where very small diameter or very short fibers must be used and which are difficult to work with.

The microcavity surface of the present invention is particularly well suited as an external coating for percutaneous lead devices such as tubes, shunts, cannulae insulated wire and various other tubular or mass, energy, and information transport devices which provide an external connection to a point beneath the skin. Such lead devices are used as access to the circulating blood and as linkages to implanted devices such as blood pumps. The skin and subcutaneous tissue grows into the microcavities to provide a tenacious adhesion with the exposed surface of the lead device. This results in safety, appearance, and comfort for the patient and also in the formation of the bacterial seal at the interface of the tissue and microcavity surface which is extremely effective in the prevention of infection caused by bacteria penetrating the body along the interface.

Referring to FIG. 5, one embodiment of the percutaneous lead device of the invention is illustrated in the form of a hollow lead tube 32 in an implanted position after penetration through skin 33 and subcutaneous tissue 34. Tube 32 includes an outer wall containing a plurality of microcavities 36 of the aforementioned type. Microcavities 36 may either be incorporated directly into the outer surface of tube 32 or may be applied in an adhesive layer. The lead tube material is preferably flexible to reduce tissue shear and tensile stresses caused by body movement or external binding of the tube. Shear stresses may be further reduced by orienting the tube generally parallel to the skin surface for a substantial extent in the subcutaneous tissue.

Another application of the microcavity surface of the present invention, not shown, is as an external lining on the surfaces of implanted prostheses which encourages the formation of a thin adherent tissue envelope. Although the tissue forming this envelope has somewhat different characteristics than skin and subcutaneous tissue, it grows into the lining in essentially the same manner. The envelope prevents damage to surrounding organs and tissue, is fully compatible with the body and body fluids, and substantially lowers the risk of infection at the prosthesis surface by preventing stagnant pockets of fluid at the interface. Such tissue encapsulation also improves fixation and support of the prosthesis and protects the same against damage.

Claims

I claim:

1. A prosthetic device having a tissue or blood-compatible flexible surface adapted to receive ingrowth of living cells from an interfacing region of tissue or blood, said surface including a plurality of adjacent substantially discrete pockets defined by walls which extend into said surface, said pockets having openings which face outwardly in the direction of said region of tissue or blood to provide means to accommodate a number of living cells, the walls of said pockets extending inwardly to a depth in the range of .002 to .020 inches to provide anchoring but not of such shape or size as to prevent essentially normal transfer of nutrients to said living cells in said pockets from said adjacent area of tissue or blood.

2. A prosthetic device as in claim 1 in which said prosthetic device is a hollow blood handling device with said surface in contact with the blood whereby blood cells entering into said pockets are nourished from the blood adjacent said surface.

3. A prosthetic device as in claim 1 in which the entire device is flexible.

4. A prosthetic device as in claim 1 wherein said device is rigid and the surface is part of a layer applied to the device.

5. A prosthetic device as in claim 3 wherein the surface is part of a layer applied to the flexible device.

6. A prosthetic device as in claim 1 wherein said device is an elongated device with said surface on the outside adapted to penetrate the skin and receive ingrowth of cells.

7. A prosthetic device as in claim 1 wherein the size and shape of said pockets have a predetermined distribution.