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Claims  |
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What is claimed is:
1. A method for forming an implantable device for medical use, which device
is treated to substantially prevent in vivo cracking thereof, the method
comprising:
providing a non-woven shaped substrate of an implantable medical device
having a biocompatible polymeric surface that is elastomeric and
susceptible to cracking when subjected to implantation under in vivo
conditions for substantial time periods, said in vivo conditions including
those that promote crack-forming degradation of said biocompatible
polymeric surface, wherein said biocompatible polymeric surface is a
polyurethane;
applying a crack preventative composition to said elastomeric biocompatible
polymeric surface, said crack preventative composition including a
silicone material being a siloxane including
##STR5##
groups and, when polymerized, being elastomeric and having a predetermined
surface tension, said applying step including efecting adsorption of the
silicone material onto the biocompatible polymeric surface and bonding
between said elastomeric silicone material and said elastomeric
biocompatible polymeric surface so as to effect grafting of said
polymerized elastomeric silicone material onto said elastomeric
biocompatible polymeric surface; and
said providing step includes selecting said elastomeric biocompatible
polymeric surface to have a surface tension that is greater than the
predetermined surface tension of said silicone material.
2. The method according to claim 1, wherein said silicone rubber material
of the crack preventative composition is a siloxane condensation reaction
product of reactants including:
(a) a silicone moiety of the formula:
##STR6##
wherein n has an average value of greater than 100; and (b) an acetoxy
silane crosslinker,
##STR7##
whereby the reaction product is a poly(dimethyl siloxane).
3. The treatment method according to claim 1, wherein said crack
preventative composition includes a solvent that does not detreimentally
affect said biocompatible polymeric surface.
4. The treatment method according to claim 1, wherein said applying step
includes imparting lubricating properties to the biocompatible polymeric
surface.
5. The method according to claim 1, wherein said silicone rubber material
of the crack preventative composition is a siloxane which, prior to
curing, has the formula:
##STR8##
wherein each R group is an organic moiety selected from the group
consisting of one or more ester moieties, acetoxy moieties, acrylic
moieties and alcohol moieties, wherein each R.sub.3 and R.sub.4 group is
an organic group selected from the group consisting of aliphatic groups
and substituted aliphatic groups having from 1 to about 12 carbon atoms,
and aromatic groups and substituted aromatic groups having from 6 to about
20 carbon atoms, and wherein each R.sub.1 group and R.sub.2 group is an
organic component selected from the group consisting of R, R.sub.3 and
R.sub.4.
6. The method according to claim 1, wherein said silicone material of the
crack preventative composition is poly(dimethyl siloxane).
7. A method for forming an implantable device for medical use, which device
is treated to substantially prevent in vivo cracking thereof, the method
comprising:
providing a non-woven shaped substrate of an implantable medical device
having a biocompatible polymeric surface that is elastomeric and
susceptible to cracking when subjected to implantation under in vivo
conditions for substantial time periods, said in vivo conditions including
those that promote crack-forming degradation of said biocompatible
polymeric surface, wherein said shaped substrate includes one or more
extruded fibers that are shaped by winding over a mandrel;
applying a crack preventative composition to said elastomeric biocompatible
polymeric surface, said crack preventative composition including a
silicone material being a siloxane including
##STR9##
groups and, when polymerized, being elastomeric and having a predetermined
surface tension, said applying step including effecting adsorption of the
silicone material onto the biocompatible polymeric surface and bonding
between said elastomeric silicone material and said elastomeric
biocompatible polymeric surface so as to effect grafting of said
polymerized elastomeric silicone material onto said elastomeric
biocompatible polymeric surface; and
said providing step includes selecting said elastomeric biocompatible
polymeric surface to have a surface tension that is greater than the
predetermined surface tension of said silicone material.
8. A method for forming an implantable device for medical use, which device
is treated to substantially prevent in vivo cracking thereof, the method
comprising:
providing a non-woven shaped substrate of an implantable medical device
having a biocompatible polymeric surface that is elastomeric and
susceptible to cracking when subjected to implantation under in vivo
conditions for substantial time periods, said in vivo conditions including
those that promote crack-forming degradation of said biocompatible
polymeric surface, wherein said shaped substrate includes one or more
strands of said biocompatible polymer;
applying a crack preventative composition to said elastomeric biocompatible
polymeric surface, said crack preventative composition including a
silicone material being a siloxane including
##STR10##
groups and, when polymerized, being elastomeric and having a predetermined
surface tension, said applying step including effecting adsorption of the
silicone material onto the biocompatible polymeric surface and bonding
between said elastomeric silicone material and said elastomeric
biocompatible polymeric surface so as to effect grafting of said
polymerized elastomeric silicone material onto said elastomeric
biocompatible polymeric surface; and
said providing step includes selecting said elastomeric biocompatible
polymeric surface to have a surface tension that is greater than the
predetermined surface tension of said silicone material.
9. An implantable device for medical use under in vivo conditions,
comprising:
a shaped substrate of an implantable medical device having a biocompatible
polymeric surface that is elastomeric and susceptible to degradation
cracking when subjected to implantation under in vivo conditions for
substantial time periods;
a crack preventative adsorbed to and grafted onto said substrate so that
there is bonding between said elastomeric substrate and said crack
preventative, said crack preventative being an elastomeric silicone
material having a predetermined surface tension, said crack preventative
being a siloxane including recurring
##STR11##
groups; said biocompatible polymeric surface has a surface tension greater
than the predetermined surface tension of the crack preventative; and
said elastomeric biocompatible polymeric surface is a polyurethane.
10. The implantable device according to claim 9, wherein said crack
preventative is a siloxane which, prior to curing, has the formula:
##STR12##
wherein each R group is an organic moiety selected from the group
consisting of one or more ester moieties, acetoxy moieties, acrylic
moieties and alcohol moieties, wherein each R.sub.3 and R.sub.4 group is
an organic group selected from the group consisting of aliphatic groups
and substituted aliphatic groups having from 1 to about 12 carbon atoms,
and aromatic groups and substituted aromatic groups having from 6 to about
20 carbon atoms, and wherein each R.sub.1 group and R.sub.2 group is an
organic component selected from the group consisting of R, R.sub.3 and
R.sub.4.
11. The implantable device according to claim 9, wherein said silicone
material of the crack preventative composition is poly(dimethyl siloxane).
12. The implantable device according to claim 9, wherein said shaped
substrate is a pacemaker lead insulator having a generally smooth surface
of said elastomeric biocompatible polymeric material having said crack
preventative bonded thereto.
13. An implantable device for medical use under in vivo conditions,
comprising:
a shaped substrate of an implantable medical device having a biocompatible
polymeric surface that is elastomeric and susceptible to degradation
cracking when subjected to implantation under in vivo conditions for
substantial time periods, wherein said shaped substrate includes one or
more strands of said biocompatible polymer;
a crack preventative adsorbed to and grafted onto said substrate so that
there is bonding between said elastomeric substrate and said crack
preventative, said crack preventative being an elastomeric silicone
material having a predetermined surface tension, said crack preventative
being a siloxane including recurring
##STR13##
groups; and said biocompatible polymeric surface has a surface tension
greater than the predetermined surface tension of the crack preventative.
14. An implantable device for medical use under in vivo conditions,
comprising:
a shaped substrate of an implantable medical device having a biocompatible
polymeric surface that is elastomeric and susceptible to degradation
cracking when subjected to implantation under in vivo conditions for
substantial time periods, wherein said shaped substrate includes one or
more extruded fibers that are shaped by winding over a mandrel;
a crack preventative adsorbed to and grafted onto said substrate so that
there is bonding between said elastomeric substrate and said crack
preventative, said crack preventative being an elastomeric silicone
material having a predetermined surface tension, said crack preventative
being a siloxane including recurring
##STR14##
groups; and said biocompatible polymeric surface has a surface tension
greater than the predetermined surface tension of the crack preventative.
15. An implantable device for medical use under in vivo conditions,
comprising:
a shaped substrate of an implantable medical device having a biocompatible
polymeric surface that is elastomeric and susceptible to degradation
cracking when subjected to implantation under in vivo conditions for
substantial time periods;
a crack preventative adsorbed to and grafted onto said substrate so that
there is bonding between said elastomeric substrate and said crack
preventative, said crack preventative being an elastomeric silicone
material having a predetermined surface tension, said crack preventative
being a siloxane including recurring
##STR15##
groups; said biocompatible polymeric surface has a surface tension greater
than the predetermined surface tension of the crack preventative; and
said shaped substrate is a graft including wound strands of extruded fiber
that overlie and intersect one another, and said extruded fiber is said
elastomeric biocompatible polymeric surface having said crack preventative
bonded thereto.
16. An implantable device for medical use under in vivo conditions,
comprising:
a shaped substrate of an implantable medical device having a biocompatible
polymeric surface that is elastomeric and susceptible to degradation
cracking when subjected to implantation under in vivo conditions for
substantial time periods;
a crack preventative adsorbed to and grafted onto said substrate so that
there is bonding between said elastomeric substrate and said crack
preventative, said crack preventative being an elastomeric silicone
material having a predetermined surface tension, said crack preventative
being a siloxane including recurring
##STR16##
groups; said biocompatible polymeric surface has a surface tension greater
than the predetermined surface tension of the crack preventative; and
said shaped substrate is a suture including an extruded fiber that is
composed of said elastomeric biocompatible polymeric surface having said
crack preventative bonded thereto.
17. An implantable device for medical use under in vivo conditions,
comprising:
a shaped substrate of an implantable medical device having a biocompatible
polymeric surface that is elastomeric and susceptible to degradation
cracking when subjected to implantation under in vivo conditions for
substantial time periods;
a crack preventative adsorbed to and grafted onto said substrate so that
there is bonding between said elastomeric substrate and said crack
preventative, said crack preventative being an elastomeric silicone
material having a predetermined surface tension, said crack preventative
being a siloxane including recurring
##STR17##
groups; said biocompatible polymeric surface has a surface tension greater
than the predetermined surface tension of the crack preventative; and
said elastomeric biocompatible polymeric surface is a polyurethane, and
said crack preventative bonded thereto is acetoxy terminated poly(dimethyl
siloxane).
18. A method for forming an implantable device for medical use, which
device is treated to substantially prevent in vivo cracking thereof, the
method comprising:
providing a non-woven shaped substrate of an implantable medical device
having a biocompatible polymeric surface that is elastomeric and
susceptible to cracking when subjected to implantation under in vivo
conditions for substantial time periods, said in vivo conditions including
those that promote crack-forming degradation of said biocompatible
polymeric surface, wherein said elastomeric biocompatible polymeric
surface is a polyurethane, and wherein said crack preventative bonded
thereto is acetoxy terminated poly(dimethyl siloxane);
applying a crack preventative composition to said elastomeric biocompatible
polymeric surface, said crack preventative composition including a
silicone material being a siloxane including
##STR18##
groups and, when polymerized, being elastomeric and having a predetermined
surface tension, said applying step including effecting adsorption of the
silicone material onto the biocompatible polymeric surface and bonding
between said elastomeric silicone material and said elastomeric
biocompatible polymeric surface so as to effect grafting of said
polymerized elastomeric silicone material onto said elastomeric
biocompatible polymeric surface; and
said providing step includes selecting said elastomeric biocompatible
polymeric surface to have a surface tension that is greater than the
predetermined surface tension of said silicone material. |
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Claims |
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Description |
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BACKGROUND AND DESCRIPTION OF THE INVENTION
The present invention generally relates to implantable prostheses and to
methods for making and treating same in order to substantially prevent
cracking or crazing thereof when they are implanted, the treatment
including applying a silicone rubber material to an implantable polymeric
surface of a medical prosthesis, the polymeric surface being one that will
crack when subjected to implantation for substantial time periods if it is
not thus treated with the silicone rubber material. The implantable
polymeric surface is made of a material that has a surface tension which
is greater than that of the silicone rubber, the silicone rubber being a
substantially non-polar material which is conveniently applied by
immersing the medical prosthesis to be treated into a composition
containing the silicone rubber.
Several biocompatible materials which are quite suitable for use in making
implantable medical devices that may be broadly characterized as
implantable prostheses exhibit properties that are sought after in such
devices, including one or more of exceptional biocompatibility,
extrudability, moldability, good fiber forming properties, tensile
strength, elasticity, durability and the like. However, some of these
otherwise highly desirable materials exhibit a serious deficiency when
implanted within the human body or the like, such deficiency being the
development of strength reducing and unsightly cracks which, for
prostheses components having relatively thin strands or members, cause a
complete severance of a number of those strands or members. Often, surface
fissuring or cracking occurs after substantial exposure, which may be on
the order of one month or more or shorter time periods depending upon the
materials and the conditions, to body fluids such as are encountered
during in vivo implantation and use. Many implantable prostheses are
intended to be permanent in nature and should not develop any substantial
cracking during years of implantation.
Several theories have been promulgated in attempting to define the cause of
this cracking phenomenon. Proposed mechanisms include oxidative
degradation, hydrolytic instability, enzymatic destruction, thermal and
mechanical failure, immunochemical mechanisms and imbibition of lipids.
Prior attempts to control surface fissuring or cracking upon implantation
include incorporating antioxidants within the biocompatible polymer and
subjecting the biocompatible polymer to various different annealing
conditions, typically including attempting to remove stresses within the
polymer by application of various heating and cooling conditions. Attempts
such as these have been largely unsuccessful.
A particular need in this regard is evident when attempting to form
prostheses with procedures including the extrusion or spinning of
polymeric fibers, such as are involved in winding fiber-forming polymers
into porous vascular grafts, for example as described in U.S. Pat. No.
4,475,972, the subject matter thereof being incorporated by reference
hereinto. Such vascular grafts include a plurality of strands that are of
a somewhat fine diameter size such that, when cracking develops after
implantation, this cracking often manifests itself in the form of complete
severance of various strands of the vascular graft. Such strand severance
cannot be tolerated to any substantial degree and still hope to provide a
vascular graft that can be successfully implanted on a generally permanent
basis whereby the vascular graft remains viable for a number of years.
Numerous vascular graft structures that are made from spun fibers appear to
perform very satisfactorily insofar as their viability when subjected to
physical stress conditions that approximate those experienced during and
after implantation, including stresses imparted by sutures and the like.
For example, certain polyurethane fibers, when subjected to constant
stress under in vitro conditions, such as in saline solution at body
temperatures, do not demonstrate cracking that is evident when
substantially the same polyurethane spun vascular graft is subjected to in
vivo conditions. Accordingly, while many materials, such as polyurethanes,
polypropylenes, polymethylmethacrylate and the like, may appear to provide
superior medical devices or prostheses when subjected to stresses under in
vitro conditions are found to be less than satisfactory when subjected to
substantially the same types of stresses but under in vivo conditions.
There is accordingly a need for a treatment which will impart crack
preventative properties to polymers that experience surface fissuring or
cracking under implanted or in vivo conditions and which are otherwise
desirable and advantageous in connection with the formation of medical
devices or prostheses that must successfully thwart the cracking
phenomenon even after implantation for months and years, in many cases a
substantial number of years. Exemplary medical devices or prostheses for
which such a treatment would be significantly advantageous include
vascular grafts, intraocular lens loops or haptics, pacemaker lead
insulators, permanent sutures, diaphragms for artificial hearts,
prosthetic heart valves, and the like. Moreover, experience has shown that
crack prevention that is successful under in vitro conditions is not
necessarily successful under in vivo conditions.
Objectives of this type are met by the present invention which achieves a
successful treatment of biocompatible polymers including polyurethanes,
polypropylenes, polymethylmethacrylate and the like to the extent that
these polymers do not exhibit the surface fissuring, cracking or crazing
phenomenon which they would otherwise exhibit under in vivo conditions.
The invention includes treating such polymers with a crack preventative
material that includes a silicone rubber, typically a siloxane. The
treatment can be carried out by a procedure as straightforward as dipping
the prosthesis into a container including the silicone rubber material and
a crosslinker or curing agent, preferably followed by taking steps to
insure that the silicone rubber material adsorbs into and on the
biocompatible surface of the prosthetic device at least to the extent that
the crack preventative is secured to the biocompatible surface of the
implantable device or medical prosthesis. Alternatively, the biocompatible
surface can be pretreated with primer or other material or radiation that
provides the surface with chemical functionality with which the silicone
rubber material can react and to which it can bond.
It is accordingly a general object of the present invention to provide an
improved implantable device, method of its production, and crack
prevention treatment.
Another object of this invention is to provide an improved vascular graft
that is made from spun fibers and that exhibits an exceptional ability to
prevent the formation of cracks and strand severances upon implantation
for substantial time periods such as those experienced in generally
permanent implantation procedures.
Another object of the present invention is to provide an improved crack
preventative treatment procedure for biocompatible polymers having a
surface tension greater than that of a silicone rubber crack preventative
agent.
Another object of the present invention is to provide an improved
production method, treatment method and treated product that imparts in
vivo crack prevention properties to biocompatible polymeric materials that
exhibit desirable medical properties but experience cracking in in vivo
applications.
Another object of this invention is to provide an improved treatment
method, product and process for preparation thereof which dramatically
improves the crack resistance properties of a biocompatible material while
also imparting lubricating or friction reduction properties thereto.
These and other objects, features and advantages of this invention will be
clearly understood through a consideration of the following detailed
description.
BRIEF DESCRIPTION OF THE DRAWINGS
In the course of this description, reference will be made to the attached
drawings, wherein:
FIG. 1 is a photomicrograph of a section of a vascular graft made by
spinning a polyurethane not treated in accordance with this invention and
after subcutaneous implantation; and
FIG. 2 is a photomicrograph of another section from the spun polyurethane
graft used in FIG. 1 but which was treated in accordance with this
invention prior to subcutaneous implantation.
DESCRIPTION OF THE PARTICULAR EMBODIMENTS
Crack preventative materials in accordance with the present invention are
of the silicone rubber type and are most typically and readily provided
within crack preventative compositions that include a silicone rubber type
of material and a solvent therefor. Often, the crack preventative
composition includes a catalyst and a crosslinker or other system for
curing the silicone rubber type of material. A convenient manner of
incorporating the silicone rubber type of material into the crack
preventative composition is to provide same in dispersion form. Thinning
agents, actinic radiation, coupling agents and primer coatings for the
silicone rubber type of material may also be utilized.
With more particular reference to the crack preventative agent itself, this
silicone rubber type of component is preferably a siloxane having groups
which can be generally represented by the formula:
##STR1##
A representative siloxane component, prior to curing, can be represented by
the formula:
##STR2##
wherein each of R, R.sub.1 and/or R.sub.2 can be a group such as an ester
moiety, an acetoxy moiety, an alcohol moiety, an acrylic moiety and the
like that are involved in the crosslinking, curing or polymerizing of the
siloxane component. R.sub.3 and R.sub.4, as well as R.sub.1 and R.sub.2,
can each be aliphatic or aromatic groups such as methyl, ethyl, propyl,
phenyl, or substituted aliphatics or aromatics containing halogen moieties
or other groups, for example 3,3,3-trifluoropropylmethyl moieties. This
general formula represents a siloxane component that can react with itself
with or without the presence of moisture and/or a catalyst in order to
crosslink or polymerize into the silicone elastomer. If at least the R
groups are alcohol moieties, the silicone elastomer can be formed by
reaction with a suitable crosslinking component.
An exemplary silicone elastomer or rubber is a siloxane condensation
reaction product, the principal reactants of which include a silicone
moiety:
##STR3##
wherein n has an average value of greater than 100. Another principal
reactant is an acetoxy silane crosslinker of the formula:
##STR4##
wherein p is 1, 2 or 3. The exemplary siloxane of this type is
poly(dimethyl siloxane). Other siloxane polymers include poly(ethylmethyl
siloxane), poly(3,3,3-trifluoropropylmethyl siloxane) and copolymers of
these types of siloxanes with poly(dimethyl siloxane). Polymeric siloxanes
are generally known and are available commercially, for example, from Dow
Corning Corporation. Siloxanes are generally described in U.S. Pat. No.
3,434,869, the subject matter of which is incorporated by reference
hereinto. These materials are hydrophobic and substantially non-polar.
Usually these silicone rubber or silicone resin materials will be applied
in accordance with this invention while dispersed or dissolved in a
solvent that will not detrimentally affect the surface of the implanted
device that is being treated. Typically acceptable solvents in this regard
include heptane, hexamethyldisiloxane, trichloroethane, polyhalogenated
hydrocarbons and the like. Exemplary polyhalogenated hydrocarbons include
materials available under the Freon trademark, including
trichlorofluoromethane, dichlorodifluoromethane,
1,2-dichloro-1,1,2,2-tetrafluoroethane and octafluorocyclobutane. Certain
of these polyhalogenated hydrocarbons exhibit atmospheric boiling points
below room temperature, and these solvents can be advantageously used by
maintaining the application composition at an appropriate elevated
pressure and/or decreased temperature so that the solvent is liquid during
application and readily evaporates thereafter.
These silicone rubbers perform best when crosslinked, and crosslinking is
facilitated by a suitable catalyst, although curing at room temperature
and at elevated temperatures with ultraviolet or gamma radiation can also
be practiced. Suitable crosslinkers containing catalysts are available
commerically, for example from Dow Corning Corporation and are typically
combined with the silicone polymer dispersion at a ratio of on the order
of 50:1 silicone rubber dispersion to crosslinker/catalyst, the ratio
being by weight. Exemplary catalysts are those including platinum, benzoil
peroxide, tin, and the like.
Usually, especially when the implantable device being treated consists of
one or more fine strands, the crack preventative will adequately adhere to
the bicompatible surface of the implantable device without the need of any
pretreatment thereof. In those instances where the implantable device has
a relatively large and smooth surface area, such as would be the case for
a cardiac pacer lead insulator or an artificial heart diaphragm which
present a unitary surface area that is generally smooth and without any
significant undulations or porosity. In these instances, it can be
advantageous to pretreat the surface of the implantable device with a
coupling agent or primer coating such as the priming solution that is
described in U.S. Pat. No. 3,434,869, including the primers specified
therein which are reaction products of aminoorganosilicon compounds and
epoxy resins. Others include mercaptosilanes. Typical coupling agent or
primer coating compositions include such primers in solution or dispersion
with solvents including isopropanol, acetone, water and the like.
Components such as methyltrimethoxy silane can be added to enhance the
priming properties. A thin film of the coupling agent or primer
composition is typically sitable and may be applied by brushing, dipping,
spraying or the like.
Bonding between the substrate and the silicone polymer, or grafting of the
silicone onto the substrate can also be facilitated by exposing the
substrate/silicone system to a suitable free radical initiator. Exemplary
silicones for this application include those containing acrylic functional
groups or crosslinkers with acrylic functionality. Typical initiators
include actinic radiation such as ultraviolet light, RF Plasma sources,
gamma radiation, high temperature, or the like or combinations of the
above. Known radiation initiators include oxidizing agents such as ceric
ammonium nitrate and the like.
Whether or not pretreatment is performed, the implantable device or portion
thereof to be rendered resistant to in vivo crack development is contacted
with the crack preventative composition, typically by immersion into a
bath of the crack preventative composition, although spraying, brushing or
the like could also be used. After such application of the crack
preventative composition, it is usually preferred to physically manipulate
the device in order to remove excess crack preventative composition and to
assist in directing the crack preventative into interstices or undulations
of the device. Exemplary physical manipulations in this regard include one
or more of squeezing the device such as between rollers or presses,
utilizing a vacuum system or a centrifuge device, use of increased
quantities of diluents in the crack preventative composition, or the like.
The diluents and/or solvents should be evaporated or otherwise removed.
Thorough coverage may require repetitive coatings if so desired.
Application of the crack preventative can include or be closely followed by
a curing operation, whether in conjunction with a suitable catalyst with a
crosslinker as herein described or in association with a vulcanizing type
of procedure at ambient or at elevated temperatures. Moisture levels can
also contribute to the effectiveness of such post-treatment procedures.
The crack preventative treatments according to this invention are used to
form or provide implantable devices having biocompatible surfaces that are
substantially completely crack-free and that will not crack or sever after
having been implanted for extensive time periods. Implantable medical
devices that are especially appropriate products according to this
invention are vascular grafts that are spun from extruded fibers on an
apparatus including an elongated mandrel and a spinnerette assembly that
rotate with respect to each other while the spinnerette traverses a
pathway generally along the elongated mandrel. Other especially
appropriate products include permanent sutures, especially since crack
development in such products can lead to breakage of the permanent suture
and a diminishment of its intended implanted function. Other especially
appropriate products include the loops or haptics of intraocular lens
implants. The products according to this invention can also include items
such as the external insulator sheath of cardiac pacemaker leads,
artificial heart diaphragms, artificial heart valve leaflets and sewing
cuffs, and the like. These products also exhibit a beneficial additional
property of typically exhibiting reduced surface friction, which can be
especially evident when suturing a vascular graft treated according to
this invention wherein the suture readily slides with respect to strands
of the vascular graft through which it is passed in order to facilitate
the ability of the surgeon to slide the vascular graft to its desired
surgical implant location by pulling on the suture.
Materials out of which these implantable medical devices may be made
include substantially any biocompatible material, typically a polymeric
material, which has a surface tension that is greater than that of the
crack preventative material, primarily in order to effect proper
adsorption of the crack preventative onto the biocompatible material. For
example, a typical siloxane has a surface tension of approximately 18
Dynes/cm, and biocompatible materials having a surface tension greater
than this value would be suitable. Included are various polyurethanes,
polypropylenes and acrylate polymers such as polymethylmethacrylate.
Exemplary polyurethanes include polyurethanes and poly(fluorosilicone
urethane) copolymers, which are described in my copending application
entitled "Polyurethanes". Such materials may be subjected to annealing
conditions prior to treatment with the crack preventative composition.
EXAMPLE
A polyurethane dissolved in dimethyl acetamide was spun onto a rotating
mandrel in the manner generally described in U.S. Pat. No. 4,475,972 in
order to form a vascular graft having individual fibers with a diameter of
approximately 8 microns. The resulting graft was annealed for 48 hours
under alternating atmospheres of nitrogen and vacuum at 80.degree. C. and
then cut into separate pieces or segments. Two of these segments were not
further modified and served as unmodified controls. Another two of these
separate segments were subjected to the crack preventative procedure in
accordance with this invention.
More particularly, the two treated segments were subjected to silicone
impregnation which included forming a crack preventative composition
including a 50:1 by weight mixture of a Dow Corning silicone dispersion
with the recommended Dow Corning catalyst. These two graft segments were
then submerged into this crack preventative composition, and the mixture
was evacuated for two to ten minutes to remove air bubbles and to force
the crack preventative composition into the interstices of the graft,
after which the two segments of graft were squeezed between two rollers in
order to remove excess crack preventative composition. The thus treated
graft segments were then dried and cured in a laminar flow hood at room
temperature (24.degree. C.) and humidity (40% relative humidity) for 24
hours.
The two unmodified control graft segments and the two crack preventative
treated segments were ethylene oxide sterilized and implanted
subcutaneously in a dog. After one month, all four of the samples were
explanted, cleaned in sodium hydroxide and sodium hypochloride solution
and then examined under a scanning electron miroscope for fiber breakage
and cracking. FIG. 1 is a photomicrograph of the scanning electron
microscopic reproduction of a typical unmodified control graft that is not
in accordance with this invention, from which obvious cracking and strand
breakage are evident, even though such grafts had been subjected to
annealing conditions in an effort to reduce cracking and breakage. FIG. 2
is a photomicrograph of the scanning electron microscopic image that is
typical of one of the graft sections treated in accordance with this
invention. Fiber breakage is essentially non-existent, and no surface
cracking can be seen.
Sections of the explanted graft were also manually pulled until breakage
was evident, and their relative respective tensile strengths were
observed. The unmodified sections exhibited what amounted to an almost
complete loss of tensile strength, while the tensile strength of the
explanted sections that had been treated in accordance with this invention
demonstrated a tensile strength substantially the same as the tensile
strength of a section of the graft that was neither implanted nor treated.
It will be understood that the embodiments of the present invention which
have been described are illustrative of some of the applications of the
principles of t | | |
