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Description  |
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This present invention relates to flexible fabric grafts for prolonged or
permanent implantation in a living body, and, more particularly is
directed to flexible fabric grafts such as artificial vascular grafts and
artificial patch grafts having a carbon coating. Such materials may also
be used to advantage in flexible parts of artificial heart and cardiac
assist devices.
The employment of pyrolytic carbon coatings to produce biocompatible and
thromboresistant surfaces has produced substantial advancement in the
field of medical prosthetic devices, and is described for example, in U.S.
Pat. Nos. 3,526,005 issued Sept. 1, 1970 and 3,685,059, issued Aug. 22,
1972. These patents generally describe deposition of pyrolytic carbon
coatings, usually from a diluted hydrocarbon atmosphere at atmospheric
pressure. Various other techniques have been developed for depositing
vapor coatings, for example as by vacuum vapor deposition (VVD) which is
also sometimes referred to as vacuum metalizing, physical vapor deposition
or evaporative coating, sputtering or as by ion-plating techniques. [e.g.,
see Marinkovic, et al., Carbon, 14 329 (1976); cited references are
incorporated herein by reference]. Coatings deposited by such techniques,
which are generally referred to herein as vapor-deposited carbon coatings,
have been utilized in prosthetic devices, as described in U.S. Pat. No.
3,952,334. However, despite these advances, there are still deficiencies
in the provision of certain prosthetic elements such as artificial
vascular and patch grafts.
Conventionally, vascular grafts with diameters greater than six
millimeters, fabricated from a variety of synthetic materials, have been
used successfully for a number of years in reconstructive surgery. The
same degree of success has not been achieved with conventional grafts
having diameters smaller than six millimeters. Similarly, while various
degrees of success have been realized in respect of synthetic flexible
fabrics for patch grafts, there is a need for improved flexible fabrics
for reconstructive surgery.
Accordingly, it is an object of the present invention to provide small
diameter vascular grafts and materials therefore which are suitable for
prolonged or permanent implantation in a living body. It is a further
object to provide improved flexible fabrics for reconstructive surgery.
These and other objects of the invention will be readily apparent from the
following detailed description and the accompanying drawings of which
FIG. 1 is a perspective view of one embodiment of an elastic,
small-diameter fabric vascular graft in accordance with the present
invention;
FIG. 2 is a view of the weft knit structure of the vascular graft of FIG.
1;
FIG. 3 is an illustration of an embodiment of warp knit fabric for
reconstructive surgery; and
FIG. 4 is an illustration of a mesh knit fabric for reconstructive surgery.
Generally, the present invention is directed to flexible, biologically
compatible fabric prostheses suitable for prolonged or permanent
implantation in a living body. The fabrics may be provided in tubular form
for use as vascular grafts, and are particularly desirable for small
diameter grafts.
The flexible fabric prostheses comprise an array of a plurality of
carbon-coated, organopolymeric fibers of particular characteristics. The
term "fiber array" is meant to include woven and non-woven fabric
structures, including knitted and felted structures, with knitted
structures being particularly preferred. The organopolymeric fibers are of
relatively small diameter which are able to sustain the functional
stresses intended for the prosthetic fabric and provide for a desired high
degree of flexibility without straining more than about 5 percent. The
fibers should generally best have a major diameter dimension of less than
about 25 microns, and a minor diameter dimension of at least about 5
microns, although fibers as small as 1 micron might be used in certain
applications. By "major diameter dimension" is meant the widest dimension
of the fiber in a direction orthogonal to the longitudinal axis of the
fiber, and by "minor diameter dimension" is meant the narrowest dimension
of the fiber in a direction orthogonal to the longitudinal axis of the
fiber. Of course, for a fiber of circular cross section, the major and
minor dimensions will be the same, but it should be appreciated that the
invention does contemplate fibers of non-circular cross-section. However,
deviation from circular fiber cross-sections generally leads to stiffer
fabrics because of the increased interfibral friction and increased forces
required for bending and unbending of the fiber filaments.
In the prostheses of the present invention, the fibers provide a flexible
array in sheet or tubular form so that the prosthesis is provided with a
predetermined high degree of flexibility in a prosthetic system which also
has beneficial biologically compatible properties of a carbon coating.
Furthermore, a high degree of elasticity may be provided through bending
of the fibers of the array rather than through substantial tensile
elongation of the fibers.
As indicated previously, knit fabric arrays are particularly preferred
fiber structures, and in this regard the term "knit" is used generally to
include weft knit and warp knit fiber arrays. Weft knit fabric structures
(including double-knit structures) utilize interlocked fiber loops in a
filling-wise, or weft, direction, while warp knit structures utilize
fabric loops interlocked in a length wise, or warp, direction. Weft knit
structures generally are more elastic than warp knit structures, but the
resiliency of warp knit fabrics is satisfactory to provide a substantial
degree of elasticity, or resiliency, to the fabric structure without
substantially relying on tensile fiber elongation for such elasticity.
Weft knit fabrics generally have two dimensional elasticity (or stretch),
while warp knit fabrics generally have unidirectional (width wise)
elasticity. The different elasticity properties of the various knit or
woven structures may be beneficially adapted to the functional requirement
of the particular prosthetic application. In some cases, where little
elasticity is desired, the fabric may be woven to minimize in plane
elasticity but yet provide flexibility. For large diameter vascular grafts
(6 mm diameter or larger) and various reconstructive fabric applications,
polyethylene terephthalate fiber fabric arrays of suitably small fiber
size may be utilized as preformed substrate materials for subsequent
carbon coating. Commercially available woven and knitted fabrics of
medical grade Dacron fibers including, single and double velour graft
fabrics, stretch Dacron graft fabric and Dacron mesh fabrics, provided the
fibers have suitably small diameter and other properties, may be suitable
as substrates for application of a suitably thin, high tensile strain
carbon coating to provide prostheses in accordance with the present
invention. For smaller vascular graft applications (less than 6 mm
diameter), and for other applications for which suitable substrates of
desired structure are not commercially available, special manufacture will
be necessary.
The relatively high degree of flexibility and/or elasticity of the carbon
coated fiber arrays of the fabric prostheses is due primarily to the
bending of the fibers, rather than substrate fiber elongation which would
be incompatible with maintenance of the integrity of a carbon coating on
the fibers of the knit structure. The radius of curvature of the
individual fibers that will provide a degree of bending without fracture
of an adherent carbon coating having a tensile fracture strain of at least
5 percent is determined by the diameter of the fiber. The radius of
curvature is approximately
##EQU1##
For example, for a diameter of about 10 microns (=10.sup.-3 cm), the
allowable radius R of curvature is:
##EQU2##
Accordingly, the relatively small fiber diameters utilized in the fabric
substrate structures provide the prostheses with substantial flexibility
without cracking the carbon coating used in the prostheses, which is
provided in an isotropic form which can withstand at least about 5% strain
without fracture. Smaller fibers are preferred for increased flexibility,
and the lower limit of diameter is determined by handling and coating
parameters. In order to provide a high degree of flexibility and
resiliency (or elasticity), a fiber strand knit is preferred which
minimizes localization of individual fiber bending in response to flexure
of the fabric. The fiber diameter and modulus are important to assure
proper flexure of the fabric so that flexibility is achieved by
rearrangement of the fabric structure through bending and unbending of the
individual fibers.
Certain physical parameters characterize the substrate fiber of the fabric
prostheses, and in this connection, the fibers should be of an
organopolymeric material having a tensile strength of at least about
20,000 psi and should be fabricated of medical grade materials. Generally,
the fibers will best have a high degree of axial orientation. The modulus
is an important parameter, and the organopolymeric fibers should have a
tensile modulus of elasticity of about 2.times.10.sup.6 psi or more.
Polyethylene terephthalate fibers, such as those sold under the trade name
Dacron, are particularly preferred because of the biocompatability of such
polyester fibers ["Implants in Surgery," D. Williams, et al., W. B.
Saunders Company, Ltd., London (1973)], their strength (e.g., 50,000 to
99,000 psi breaking strength) and stiffness (e.g., modulus of elasticity
of about 2.times.10.sup.6 psi), which may be almost equal to that of the
isotropic carbon coating. Such a high modulus, high strength material can
support a relatively large load without straining more than 5% (such that
the carbon coating would break). Polyethylene terephthalate fibers may be,
for example, about three times tougher and five times stiffer than
poly(tetrafluorethylene).
In view of the small diameter of the substrate fibers, it will generally be
desirable in most applications of woven and knitted structures to utilize
strands of a plurality of fibers in the fiber array. Usually such strands
will have at least 5, and preferably at least 10 individual fibers, with
the strands being formed into the desired woven or knitted structure.
In cardiovascular fabric grafts, it is of course desirable that the fabric
have a controlled degree of porosity. For cardiopulmonary bypass
applications, a densely woven fabric structure with very low porosity such
as from about 30 to about 125 cc/minute/cm.sup.2 may be utilized as a
substrate.
For other cardiovascular applications, knit substrate fabrics having, for
example, higher porosities in the range of from about 1200 to about 4200
cc/minute/cm.sup.2 may be provided with a suitable, high-strain carbon
film. The resulting coated cardiovascular prostheses will generally be
preclotted in accordance with conventional practice to establish fluid
integrity necessary for the cardiovascular use.
In addition to poly(ethylene terephthalate), other suitable high strength,
high modulus organopolymeric substrate materials, provided their
biocompatability is demonstrated, include various so-called "high
temperature polymers" which have generally been developed in the last
decade, such as the high modulus and high tensile strength aromatic
polyimides and aromatic polyamides. High temperature polymer fibers which
may be used herein exhibit thermal stability at temperatures of
300.degree. C. and higher and are generally characterized as high
temperature, high molecular weight, aromatic, nitrogen-linked polymers.
Such polymers are well known in the polymer art, and examples of such high
temperature polymers include ordered aromatic copolyamides, such as the
reaction products of phenylenebis (amino-benzamide) and isophthaloyl
chloride, all-aromatic polybenzimidazoles, such as poly [2,2'
(m-phenylene)-5,5' (6,6' benzimidazole)], polyozadiazoles, poly (n-phenyl
triazoles), polybenzobenzimidazoles, polymides and poly (amide-imide)
resins. Of course, the biocompatability of such materials should be
tested, and medical implant grade materials should be used for prosthetic
implants. The preferred organopolymeric fibers contemplated for use herein
are medical grade polyethylene terephthalates, but various conventional
high temperature polymer fibers commercially available, such as fibers
sold under the name Kevlar by DuPont, and having a modulus of about
10.times.10.sup.6 psi may prove useful.
As previously indicated, the fiber array of the prostheses of the present
invention is provided with an adherent isotropic carbon coating of
particular properties, and in this connection, carbon, the organic
building block of all body matter, has shown outstanding tissue and blood
compatability for a variety of prosthetic device applications.
The carbon coatings may be provided by vapor-deposition techniques such as
described in the previously referenced U.S. Pat. No. 3,952,334 to produce
strongly adherent carbon coatings which provide a particularly desirable
biomedical interface between the prosthetic fabric and the implantation
site. In cardiovascular prostheses, the carbon coating should be applied
to at least the surface which is intended to be in contact with fluid
blood. Of course, for tubular cardiovascular prostheses, at least the
interior surface of the prosthesis will have the desired carbon coating.
In certain circumstances, it may be desirable to coat both surfaces of
cardiovascular and patch grafts of the invention.
The individual fibers will typically be about 10 microns in diameter. The
smaller the fiber, the smaller the radius of curvature it can sustain
without cracking the particular carbon coating, which can sustain at least
about 5% elastic strain before fracture, as previously discussed. In view
of the small diameter of the fibers used, it is a desirable advantage that
the carbon coating may be provided either by coating the individual fibers
or yarn strands, or by coating the assembled strand or fiber array.
However, it will be appreciated that weaving or knitting of previously
coated fibers may generally tend to introduce bending strain, while
coating of the finished fabric prosthesis produces a minimum of strain in
the composite structure and therefore is particularly preferred.
In any event, the entire exposed surface of the prosthetic fabric is
provided with a carbon layer of particular properties and may be applied
while using coating technology, such as described in U.S. Pat. No.
3,952,334. Further in this connection, the carbon coating should be at
least about 1000A degrees (0.1 micron) thick, should be adherent, and in
order to provide for large fracture strains, should have BAF (Bacon
Anistropy Factor) of about 1.3 or less and preferably about 1.2 or less.
Generally, a coating thickness of about 1000 to 7000A degrees and
preferably from about 3000 to about 5000A degrees of intermediate density
of carbon (at least about 1.6 gm/cm.sup.3) is employed; greater
thicknesses tend to crack and flake. Preferably, the vapor-deposited
carbon has a density of about 1.8 gm/cm.sup.3, and the density should not
exceed about 2.0 gm/cm.sup.3. Such vapor-deposited carbon exhibits
biocompatible properties and also may be provided with excellent adherence
to the small polymer fibers of the high modulus organopolymeric fiber
fabric. As a result, the coated fibers exhibit excellent properties for
use as a prosthetic fabric device and are considered to be fully
acceptable for implantation within the human body in flexible and tensile
service in a vascular or patch graft such as an artificial septal patch
graft or an aneurism patch graft, or the like. Such fabrics may be used
for tissue repair, for support in abdominal surgery and for general
reconstructive surgery.
Through the design provision of a limited tensile strain in the individual
substrate fibers of not more than 5%, under intended conditions of use,
the integrity of the carbon coating is preserved for prolonged or
permanent implantation service. In this regard, as previously indicated,
woven or knit arrays of small oriented polyethylene terephthalate fibers
(e.g., medical grade Dacron) having a high stiffness and high strength are
preferred. Other polymers such as aromatic polymers like Kevlar (tensile
modulus of 10.times.10.sup.6 psi) may also be useful in small fiber form.
Thus, an artificial vascular or septal prosthesis may be provided which
has a high degree of flexibility together with long-term biocompatability
and physical integrity.
Having generally described the flexible fabric prostheses of the present
invention, the invention will now be more particularly described with
respect to the particular embodiments illustrated in the drawings.
Illustrated in FIG. 1 is a side view of a portion of a small diameter
vascular graft 10. The small diameter vascular graft 10 comprises a knit
fabric tube 12 of a weft-knit dacron fiber substrate which is
substantially similar to a conventional dacron vascular prosthesis. The
vascular tube walls are constructed with a plurality of regularly spaced
pleats 14 of circumferential ridges 16 and valleys 18 to provide for
increased elasticity and extensibility along the axis of the prosthetic
vascular graft. In the illustrated embodiment, the vascular graft 10 has
an internal diameter (to the innermost interior surface of the valleys 18)
of 4 mm, and an external diameter of about 7 mm (to the outermost,
exterior surface of the ridges 16) in an unstretched condition. In a fully
axially stretched condition, the vascular graft has an internal diameter
of about 5 mm and an external diameter of about 5.5 mm. The length of the
prosthesis 10 will depend on the surgical repair objective, but it will
generally be at least about 5 cm to accommodate vascular attachment at the
ends of the artificial graft and may be up to 60 cm or more in length. Of
course, larger and smaller diameter vascular grafts may be provided.
Turning now to FIG. 2 in which the knit structure of the vascular graft is
shown in more detail, it may be seen that the flexible substrate fiber
tube is weft-knit in a jersey structure in tubular form from strands of a
plurality of individual small diameter fibers 20. The individual fibers 20
are of circular cross-section and are made of axially oriented
polyethylene terephthalate. The fibers have a diameter of about 10
microns, a tensile strength of about 40,000 psi and a tensile modulus of
about 2.times.10.sup.6 psi.
The fibers of the knit fiber array of the vascular graft 10 have an
adherent, carbon coating on the interior surface of the graft 10. In order
to insure complete coating of the pleats, the graft is coated in an
axially stretched condition, but returns to almost its original condition
after coating. The coating on the fibers of the vascular graft 10 is
isotropic carbon having a BAF of about 1.3 or less and a maximum thickness
of about 3000 Angstroms over fibers at the interior surface. Of course,
the coating thickness on the fibers decreases toward the exterior surface,
which does not have a carbon coating. Upon implantation, the vascular
graft 10 is flexible and fatigue resistant and is biologically compatible
in the implantation environment. Further, the knit structure of the graft
permits tissue ingrowth from the natural vascular tissue, to provide for
effective and natural fixation of the prosthesis. The interior surface of
the vascular graft has excellent compatability with blood.
Illustrated in FIG. 3 is an embodiment 30 of carbon-coated flexible fabric
prosthetic cloth 30 of knit polyethylene terephthalate fiber which is
particularly adapted for cardiovascular bypass utilization, for carotid or
intracardiac patch grafting, or for abdominal aortic aneurism repair. As
may be seen from the drawing, the fabric prosthesis is warp knit from
strands 32 of a plurality of small organopolymeric fibers 34 each having a
circular cross-section and a diameter of about 5 microns. The fiber
strands 32 are knit in a relatively dense warp knit structure which
provides substantial strength and fluid impermeability, while retaining
substantial flexibility. The knit substrate is coated in a manner such as
that of U.S. Pat. No. 3,952,334, and coating is carried out until a
thickness of about 3000 Angstroms of carbon is deposited on the individual
fibers. The carbon coating is smooth and uniform, and has a density of
about 1.8 gm/cm.sup.3, a BAF of about 1.2, and a tensile strain at
fracture which is greater than 5 percent.
Like the fibers of the embodiment of FIGS. 1 and 2, the substrate fibers of
the coated fiber array are individually coated with the carbon coating and
are not substantially bonded together. The individual fibers are thus free
to bend and glide over each other in the flexure of the prosthesis. The
graft has excellent biocompatability and provides for tissue ingrowth at
the tissue-joining edges of the fabric graft. The fabric further has
excellent compatability with blood.
While the previously described embodiments have illustrated relatively
densely knit materials of relatively low porosity, more loosely woven or
knit materials may also be used for prosthetic fabric applications. In
this connection, an embodiment 40 of carbon coated mesh knit Dacron fabric
is shown in magnified view in FIG. 4, which has a relatively open
structure useful in reconstructive surgery. The mesh structure is knit
from strands 42 of small diameter Dacron fibers. The fiber mesh substrate
has substantial elasticity and resiliency in both weft and warp directions
and has an adherent, isotropic carbon coating like that of the substrate
of FIG. 3.
It will be appreciated that in accordance with the present invention,
artificial cardiovascular and patch grafts have been provided which are
particularly adapted for prolonged or permanent implantation in a living
body, which are biologically inert, and which are capable of substantial
flexible motion in service.
Although the invention has been described with regard to certain preferred
embodiments, it should be understood that the scope of the invention is
defined in the appended claims.
Various of the features of the invention are set forth in the following
claims.
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