Patent Publication Number: US-2021170069-A1

Title: Biocompatible polyisobutylene-fiber composite materials and methods

Description:
This application is a divisional of U.S. application Ser. No. 15/959,894, filed Apr. 23, 2018, which claims the benefit of U.S. Provisional Application No. 62/489,655 filed Apr. 25, 2017, the contents of which are herein incorporated by reference in their entirety. 
    
    
     FIELD 
     Embodiments herein relate to biocompatible composite materials. More specifically, embodiments herein relates to biocompatible polyisobutylene-fiber composite materials and related methods. 
     BACKGROUND 
     Many different implantable medical devices rely upon animal-derived or fully synthetic materials for their construction. As one example, the leaflets of heart valves have been formed from various materials, but are currently typically formed from animal tissues. Tissue valves, also known as biological or bioprosthetic valves, can be derived from animal tissue such as porcine (pig), bovine (cow) and equine (horse) models, and then fixed with a preserving solution before being mounted on a flexible frame to assist in deployment during surgery. 
     In comparison to mechanical valves, tissue valves offer avoidance of lifelong warfarin therapy to prevent the development of blood clots. However, a disadvantage of animal tissue valves is relatively poor durability compared to mechanical valves, with many requiring a re-operation in 10 to 20 years. 
     SUMMARY 
     Aspects herein relate to biocompatible polyisobutylene-fiber composite materials and related methods. 
     In a first aspect, a biocompatible composite material is included. The biocompatible composite material can include a network of fibers comprising one or more polymers to form a substrate and a continuous, interpenetrating polyisobutylene matrix that is non-porous and completely surrounds the electrospun fibers. 
     In addition to one or more of the preceding or following aspects, or in the alternative to some aspects, in a second aspect the network of fibers includes fibers oriented with a bias in a direction. 
     In addition to one or more of the preceding or following aspects, or in the alternative to some aspects, in a third aspect the network of fibers are electrospun fibers and are randomly oriented fibers. 
     In addition to one or more of the preceding or following aspects, or in the alternative to some aspects, in a fourth aspect the network of fibers comprise one or more of polyether-polyurethane copolymers (PE-PUR), high durometer polyisobutylene-polyurethane (PIB-PUR), polyamide, polyester, or linear polyethylene. 
     In addition to one or more of the preceding or following aspects, or in the alternative to some aspects, in a fifth aspect the fibers are disposed in a center of the polyisobutylene matrix. 
     In addition to one or more of the preceding or following aspects, or in the alternative to some aspects, in a sixth aspect the fibers are disposed biased towards an edge of the polyisobutylene matrix. 
     In addition to one or more of the preceding or following aspects, or in the alternative to some aspects, in a seventh aspect the biocompatible composite material is shaped as a prosthetic heart valve leaflet. 
     In addition to one or more of the preceding or following aspects, or in the alternative to some aspects, in an eighth aspect the composite material is substantially planar. 
     In addition to one or more of the preceding or following aspects, or in the alternative to some aspects, in a ninth aspect the network of fibers defines pores, wherein at least about 80% of the pores by volume are filled by the continuous, interpenetrating polyisobutylene matrix. 
     In addition to one or more of the preceding or following aspects, or in the alternative to some aspects, in a tenth aspect the composite is elastomeric. 
     In addition to one or more of the preceding or following aspects, or in the alternative to some aspects, in an eleventh aspect a method of manufacturing a biocompatible composite material is included. The method can include providing a network of electrospun fibers comprised of one or more thermoplastic polymers to form a substrate, adsorbing to the surface of the network of electrospun fibers a cross-linkable polyisobutylene composition comprising a polyisobutylene monomer, macromer, or polymer and a free radical initiator, and initiating cross-linking of the polyisobutylene polymer to create a continuous, interpenetrating thermoset polyisobutylene matrix that completely surrounds the electrospun fibers. 
     In addition to one or more of the preceding or following aspects, or in the alternative to some aspects, in a twelfth aspect the polyisobutylene polymer comprises methacrylate-endcapped telechelic PIB (PIB-DMA), acrylate-endcapped telechelic PIB (PIB-DA), monofunctional methacrylate-PIB (PIB-MA), or a star-PIB derivative having 3 or more branched side chains. 
     In addition to one or more of the preceding or following aspects, or in the alternative to some aspects, in a thirteenth aspect the free radical initiator comprises a photo-initiator. 
     In addition to one or more of the preceding or following aspects, or in the alternative to some aspects, in a fourteenth aspect at least one compound in the cross-linkable polyisobutylene composition comprises a cross-linkable vinylidene moiety. 
     In addition to one or more of the preceding or following aspects, or in the alternative to some aspects, in a fifteenth aspect the method can also include shaping the network of electrospun fibers into a prosthetic heart valve leaflet. 
     In addition to one or more of the preceding or following aspects, or in the alternative to some aspects, in a sixteenth aspect a method of manufacturing a biocompatible elastomeric composite material is included. The method can include providing a network of fibers comprised of one or more polymers to form a substrate, adsorbing to the surface of the network of fibers a cross-linkable polyisobutylene composition comprising a polyisobutylene (HR-PIB), maleic anhydride, and a free radical initiator and initiating cross-linking between the HR-PIB and maleic anhydride to form alternating PIB and maleic anhydride chains to create a continuous, interpenetrating thermoset polyisobutylene matrix that completely surrounds the electrospun fibers. 
     In addition to one or more of the preceding or following aspects, or in the alternative to some aspects, in a seventeenth aspect the polyisobutylene composition comprises PIB-diallyl, itaconic anhydride, and a free radical initiator. 
     In addition to one or more of the preceding or following aspects, or in the alternative to some aspects, in an eighteenth aspect the HR-PIB comprises a telechelic HR-PIB. 
     In addition to one or more of the preceding or following aspects, or in the alternative to some aspects, in a nineteenth aspect the HR-PIB comprises a monofunctional PIB. 
     In addition to one or more of the preceding or following aspects, or in the alternative to some aspects, in a twentieth aspect the method further includes the step of initiating a condensation reaction using a primary polyamine to create bis-maleimide bridges between cross-linked polyisobutylene and maleic anhydride chains. 
     This summary is an overview of some of the teachings of the present application and is not intended to be an exclusive or exhaustive treatment of the present subject matter. Further details are found in the detailed description and appended claims. Other aspects will be apparent to persons skilled in the art upon reading and understanding the following detailed description and viewing the drawings that form a part thereof, each of which is not to be taken in a limiting sense. The scope herein is defined by the appended claims and their legal equivalents. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
       Aspects may be more completely understood in connection with the following drawings, in which: 
         FIG. 1  is a schematic view of a biocompatible elastomeric composite material and components thereof including electrospun fibers and a polyisobutylene composition in accordance with various embodiments herein. 
         FIG. 2  is a schematic cross-sectional view of a biocompatible elastomeric composite material in according with various embodiments herein. 
         FIG. 3  is a schematic cross-sectional view of a biocompatible elastomeric composite material in according with various embodiments herein. 
         FIG. 4  is a schematic cross-sectional view of a biocompatible elastomeric composite material in according with various embodiments herein. 
         FIG. 5  is schematic view of a polymeric fiber network substrate in the shape of a valve leaflet. 
         FIG. 6  is schematic view of a biocompatible elastomeric composite material in the shape of a valve leaflet. 
         FIG. 7  is a schematic view of a prosthetic heart valve in accordance with various embodiments herein. 
     
    
    
     While embodiments are susceptible to various modifications and alternative forms, specifics thereof have been shown by way of example and drawings, and will be described in detail. It should be understood, however, that the scope herein is not limited to the particular embodiments described. On the contrary, the intention is to cover modifications, equivalents, and alternatives falling within the spirit and scope herein. 
     DETAILED DESCRIPTION 
     Many different implantable medical devices rely upon animal-derived or fully synthetic materials for their construction. As one example, the leaflets of heart valves have been formed from various materials, but are currently typically formed from animal tissues. 
     Aspects herein relate to fully synthetic materials that can be used for the construction of implantable medical devices. Fully synthetic materials as described herein can offer advantages including, but not limited to, high biocompatibility, little or no calcification of the material in vivo, tunable mechanical properties including elasticity, thin profiles for devices such as TAVR or mitral valves, and broad applications including use in other areas such as gastric, biliary, and urinary applications. 
     Referring now to  FIG. 1 , shown is a schematic of the biocompatible elastomeric composite material  100  in accordance with the embodiments herein.  FIG. 1  shows a network of electrospun fibers  102  deposited as a substrate. The network of electrospun fibers can contain pores  104  of various shapes and sizes that result between adjacent fibers during the electrospinning process. The individual fibers  106  within the network can be oriented with a bias in a particular direction or the fibers can be randomly oriented. For example, in some embodiments individual fibers can be oriented with a bias parallel to, perpendicular to, or at a specific desired angle to a particular axis of significance in the article or device ultimately formed such as the major lengthwise axis, an axis of maximum strain, a particular axis of flexion, or the like. 
     The network of electrospun fibers  102  can be completely coated with a cross-linkable polyisobutylene composition  108 , such as a flowable cross-linkable polyisobutylene composition. The polyisobutylene composition  108  can include at least a polyisobutylene polymer or derivative thereof and a free radical initiator, as discussed further below. The polyisobutylene composition  108  can be added to the network of electrospun fibers  102  by various methods, including spray coating, dip coating, or any other suitable method used to achieve complete coating of the network of electrospun fibers  102  and complete filling (or partially complete) of the pores  104  present therein. 
     Cross-linking of the polyisobutylene composition  108 , such as with a photoinitiator, results in a continuous, interpenetrating thermoset polyisobutylene matrix  110  completely surrounding the network of electrospun fibers  102 . The resulting composite fiber network and polyisobutylene matrix can create a nonporous biocompatible elastomeric composite material  100  as described herein. However, in some embodiments, such as where the polyisobutylene composition does not fully saturate spaces within the network of fibers, the resulting material can include both nonporous portions and porous portions (e.g., the network of fibers can remain porous in areas, if any, where the polyisobutylene composition does not penetrate). In various embodiments, the nonporous portions can be surface portions and the porous portions can be interior portions. 
     Referring now to  FIGS. 2 through 4 , shown are schematic cross-sectional representations of the biocompatible elastomeric composite material  100  in accordance with the embodiments herein.  FIG. 2  shows the biocompatible elastomeric composite material  100  having a network of electrospun fibers  102  disposed essentially throughout the thickness  202  of the surrounding thermoset polyisobutylene matrix  110 . The thickness  202  can vary depending on the specific application. However, in some embodiments, the thickness  202  can be about (or at least about) 50 μm, 75 μm, 100 μm, 125 μm, 150 μm, 175 μm, 200 μm, 225 μm, 250 μm, 275 μm, 300 μm, 400 μm, 500 μm, 1 mm, 2 mm, 3 mm, 5 mm or 10 mm. In some embodiments, the thickness  202  can be in a range wherein any of the foregoing numbers can serve as the upper or lower bound of the range, provided that the upper bound is greater than the lower bound. 
       FIG. 3  shows another example of the biocompatible elastomeric composite material  100  having a network of electrospun fibers  102  disposed in the center of the surrounding thermoset polyisobutylene matrix  110 . In this view, there is a portion  304  of the composite material  100  that includes both the electrospun fibers  102  and the polyisobutylene matrix  110 . There is also a portion  302  of the composite material  100  that includes the polyisobutylene matrix  110 , but is substantially free of the electrospun fibers. 
     In some embodiments, the portion  304  that includes both the electrospun fibers  102  and the polyisobutylene matrix spans about (or at least about) 1, 5, 10, 20, 30, 40, 50, 60, 70, 80, 85, 90, 95, 98, 99, or 100 percent of the thickness of the composite material  100 . In some embodiments, the portion  304  that includes both the electrospun fibers  102  and the polyisobutylene matrix spans a percentage of the thickness of the composite material in a range wherein any of the foregoing percentages can serve as the upper or lower bound of the range, provided that the upper bound is greater than the lower bound. 
     In some embodiments, the electrospun fibers can be centered within the composite with respect to the overall thickness thereof. However, in other embodiments, the electrospun fibers can be offset towards one side or the other of the composite creating an asymmetrical composite.  FIG. 4  shows an example of a biocompatible elastomeric composite material  100  where the network of electrospun fibers  102  is disposed to one side the surrounding thermoset polyisobutylene matrix  110 . 
     In still another embodiment, not shown, a portion of the fibers can extend outward beyond the polyisobutylene matrix. 
     In some embodiments, the density of the fibers can be the same throughout the composite with respect to the thickness, the length or the width of the composite. However, in other embodiments, the density of the fibers can vary and be denser in some places and less dense in others. 
     Polymeric Fiber Network Substrates Polymeric fiber network substrates described herein can be formed of various polymeric materials. The fibers can include a polymeric material such as a polymer, or a blend of polymers. Polymers herein can include homopolymers, copolymers, terpolymers, and the like. A “monomer” is a polymerizable molecule. Typically, the polymeric materials comprise polymer molecules having a median number of monomers that numbers in the tens (10 to 99), in the hundreds (100 to 999), in the thousands (1,000 to 9,999), or in the tens of thousands (10,000 to 99,999) as well as a mixture of polymers having different median numbers of monomers. The polymeric materials can comprise polymer molecules having a median number of monomers that is 100,000 or more. 
     Polymeric fiber network substrates described herein can be created from polymers that are cross-linked or uncross-linked, linear or branched, natural or synthetic, thermoplastic or thermosetting, and may be biostable, biodegradable, bioabsorbable, biodisintegrable, or dissolvable. Polymeric fiber network substrates described herein can specifically include those exhibiting hydrolytic stability in the context of long-term (chronic) implantation. 
     Polymers used to create the polymeric fiber network substrates herein can include those that are capable of being electrospun. Exemplary polymers can include, but are not limited to, polyether-polyurethane (PE-PUR) copolymers, high durometer polyisobutylene-polyurethane (PIB-PUR), polyamides such as nylons, polyesters, polyisobutylene (PIB), poly(ethylene oxide), polyethylene, poly(styrene-block-isobutylene-block-styrene (SIBS), polypropylene, polystyrene, polyvinylchloride, poly(styrene) polyurethanes, polyvinylidene difluoride, poly(methyl methacrylate), polyethylene glycol, polyanilines, polypyrroles, polythiophenes, polyphenols, polyacetylenes, polyphenylenes, polyacrylonitriles, polylactic acids, polycaprolactone, polyglycolides, polyvinyl acetates, cellulose acetate and copolymers including one or more of these. Polymers can also include biological polymers such as chitosan, proteins, carbohydrates, and the like. 
     In some embodiments, polymers used to create the polymeric fiber network herein are cross-linked. In other embodiments, polymers used to create the polymeric fiber network herein are not cross-linked. 
     The polymeric fiber network substrate can have various thicknesses. In some embodiments, the polymeric fiber network substrate can have a thickness of about (or at least about) 50 μm, 75 μm, 100 μm, 125 μm, 150 μm, 175 μm, 200 μm, 225 μm, 250 μm, 275 μm, 300 μm, 400 μm, 500 μm, 1 mm, 2 mm, 3 mm, 5 mm or 10 mm. In some embodiments, the thickness can be in a range wherein any of the foregoing numbers can serve as the upper or lower bound of the range, provided that the upper bound is greater than the lower bound. 
     Fiber Formation 
     The polymeric fibers herein can be formed in various ways including, but not limited to, electrospinning, electrospraying, spinning, centrifugal spinning (force spinning), drawing, template synthesis, phase separation, melt spinning, melt-blowing, self-assembly and the like. In some embodiments, the polymeric fibers can be a woven or non-woven polymeric fabric. 
     In some examples, the polymeric fibers can specifically be formed through an electrospinning (or electrostatic fiber formation or electrospraying) process. Electrospinning is a fiber production method which uses electric force to draw charged threads of polymer solutions or polymer melts. When a sufficiently high voltage is applied to a liquid droplet, the body of the liquid becomes charged, and electrostatic repulsion counteracts the surface tension and the droplet is stretched. At a critical point, a stream of liquid erupts from the surface. This point of eruption is known as the Taylor cone. If the molecular cohesion of the liquid is sufficiently high, stream breakup does not occur and a charged liquid jet is formed. As the jet dries in flight, the mode of current flow changes from ohmic to convective as the charge migrates to the surface of the fiber. The jet is then elongated by a whipping process caused by electrostatic repulsion initiated at small bends in the fiber, until it is finally deposited on the grounded collector. The elongation and thinning of the fiber resulting from this bending instability leads to the formation of substantially uniform fibers with nanometer-scale diameters. 
     The two principal parameters that control behavior of the Taylor cone are the viscosity and voltage at the nozzle. Exemplary methods of creating ultra-thin fibers for use in creating a fiber network involve electro-spinning. Electro-spinning methods are described in Shin, Hohman, Brenner, and Rutledge, “Experimental Characterization of electrospinning: the electrically forced jet and instabilities”, Polymer 42, 9955-9967, (2001), incorporated herein by reference in its entirety. Fibers that are micrometers in diameter can be created by melt spinning or gel spinning, i.e., they are formed out of a gel or a molten melt. 
     One exemplary method of depositing the fiber network, is to use a process referred to as flow-limited field-injection electrostatic spraying (FFESS). FFESS is a form of electrospraying which offers a very high degree of control over shape and flow regimes, and which allows spinning a fiber-network on top of a medical device, such as an endoprosthesis, with a glass spray nozzle. The nozzle generates a charge at the liquid meniscus that enables successful electrospray. The two principal differences between conventional electro-spraying (CES) and FFESS are first that FFESS sprays a polymer/solvent solution from a smooth glass capillary whereas CES uses a metal hypodermic needle, and second that FFESS uses a sharpened tungsten needle inside the capillary, whereas CES has no analogous structure. The overall effect of the FFESS apparatus is to improve jet stability and uniformity of the polymer sprayed by FFESS relative to that from CES. 
     Using the FFESS method for electro-spinning creates a fiber network in which the one or more fibers have a highly controlled fiber diameter. In particular, as would be understood by one of ordinary skill in the art, by controlling the voltage, flow-rate, concentration of polymer in the spray fluid, the viscosity of the spray fluid, and the distance of the nozzle from the surface of the underlying structure (e.g., a mold or a medical device frame, or a pocket within a medical device frame), the diameter of the fibers formed during the spinning process can be controlled. For exemplary descriptions of the various factors, see, e.g., “Electrostatic Spinning and Properties of Ultrafine Fibers”, Rutledge, et al., National Textile Center Annual Report, M01-D22, (November 2001), incorporated herein by reference. It is also consistent with the fiber network that the diameter of the fibers can be changed during deposition. 
     A further advantage of FFESS is thus that, because of the high degree of control of the fiber diameter, if the weight of the fiber network as well as the density of the polymer material for a given fiber diameter are known, the total surface area of the network can be precisely calculated. Thus, the surface area of a fiber of diameter d, and of length l, assuming a uniform perfectly cylindrical constant cross-section along its length, is πdl, ignoring contributions from the ends of the fibers. FFESS is further described in “Controlling surface nano-structure using flow-limited field-injection electrostatic spraying (FFESS) of poly(d,l-lactide-co-glycolide)”, Berkland, Pack, and Kim, Biomaterials, 25: 5649-5658, (2004) and U.S. Patent Application Publication No. 2004/0022939, both of which are incorporated herein by reference in their entirety. 
     Solvents used during the electrospinning process can affect various aspects such as fiber morphology. Solvents used can include, but are not limited to, dichloromethane, chloroform, methanol, tetrahydrofuran, ethyl acetate, ethanol, methyl ethyl ketone, dichloroethane, water, dimethylformamide, and combinations including one or more of these. In some examples, the solution conductivity can be manipulated in order to impact fiber diameter and morphology. By way of example, various salts (including but not limited to sodium chloride and phosphate salts) can be added with the solvent in order to change the solution conductivity. 
     In some embodiments, the fiber network can be deposited directly onto a structure or form having the desired shape of the final product. For example, in some embodiments, the fiber network can be deposited onto a mold or a portion thereof having a shape of a prosthetic heart valve leaflet as seen in  FIGS. 5-6 . In some embodiments, the fiber network can be deposited onto a build surface or plate that is substantially flat and the desired form of the fiber network can be controlled by manipulating the spray stream and/or the device creating the spray stream. 
     Beyond electrospinning, it will be appreciated that polymeric fiber networks herein can be deposited and/or formed into components of a biocompatible material in other ways. For example, in some examples, fibers can be woven. In some examples fibers can be woven to form a fibrous matrix forming at least part of a biocompatible material. 
     Diameters of the polymeric fibers used herein can be greater than about 5, 10, 20, 30, 50, 100, 150, 200, 250, 500, 750, or 1000 nanometers. In some examples the diameter of the polymeric fibers herein can be greater than about 1, 2, 3, 4, 5, 6, 7, or 8 micrometers. Diameters of the polymeric fibers used herein can be less than about 20, 18, 16, 14, 12, 10, 8, 6, 4, 2 or 1 micrometer. In some examples, diameters of the polymeric core of fibers used herein can be less than about 1000, 900, 800, 700, 600, 500, 400, 200, 100, or 50 nanometers. Diameters of the polymeric fibers used herein can be within a range wherein any of the foregoing numbers can serve as the lower or upper bound of the range, provided that the lower bound is less than the upper bound. In some examples, the average diameter of the polymeric core can be from about 10 nanometers to about 10 micrometers. 
     The fiber portion of the composite material can be greater than about 5, 10, 15, 20, 25, 30, 35, 40, 45, 50, 55, 60, 65, 70, 75, 80, 85, 90, or 95 weight percent. In some embodiments, the fiber portion of the composite material can be in a range wherein any of the foregoing numbers can serve as the upper or lower bound of the range provided that the upper bound is greater than the lower bound. 
     As references above, the fibers (fiber substrate or fiber mat) can include a substantial number of pores. However, when combined with the polyisobutylene composition that is later turned into an interpenetrating polyisobutylene matrix, the pores are filled by the polyisobutylene material. The amount of the pores in the fiber substrate that are filled by the polyisobutylene material can vary depending on various factors including the amount of the polyisobutylene material used compared with the amount of the fibers, how long the polyisobutylene material is allowed to penetrate the pores before undergoing a chemical reaction such as polymerization and/or cross-linking, the initial viscosity of the polyisobutylene material and the like. However, in various embodiments, about (or at least about) 10, 20, 30, 40, 50, 60, 70, 80, 90, 95, 98, 99, or 100 percent of the pores (as measured based on volume) are filled by the polyisobutylene matrix in the composite material. In some embodiments, the percentage of the pores that are filled can be in a range wherein any of the foregoing percentages can serve as the upper or lower bound of the range, provided that the upper bound is greater than the lower bound. 
     Polyisobutylene Compositions 
     In accordance with the embodiments herein, a polyisobutylene composition, which can include at least a polyisobutylene (monomer such as isobutylene, macromer, or polymer) or a derivative thereof and a free radical initiator, can be polymerized and/or cross-linked to form a continuous, interpenetrating polyisobutylene matrix that is non-porous and completely surrounds an electrospun fiber network. In some embodiments, the polyisobutylene composition can be a polyisobutylene fluid composition. In some embodiments, the polyisobutylene composition can be a flowable polyisobutylene fluid composition. In some examples the continuous, interpenetrating polyisobutylene matrix is either thermoplastic or thermoset. In some examples the continuous, interpenetrating polyisobutylene matrix is cross-linked and in other examples it is not cross-linked. 
     Pure homopolymeric polyisobutylene can be formed by the polymerization of isobutylene monomers having the chemical formula —[C(CH 3 ) 2 CH 2 ] n —, where n can be any number between 10 and 100,000 or more. Derivatives of polyisobutylene suitable for use herein can include those having symmetrical or asymmetrical functional end groups at the termini of a polyisobutylene chain. In some embodiments, the functional end groups can include, but not be limited to acrylate or methacrylate. In some embodiments, the functional end groups can include exo-olefinic groups. An exo-olefinic group for use herein can have the following formula: 
     
       
         
         
             
             
         
       
     
     where R is the polyisobutylene-based polymer chain. In some embodiments, the exo-olefinic group can be methyl vinylidene. To obtain even higher cross-link densities within the polyisobutylene matrix, in some embodiments “star” derivatives of polyisobutylene (star-PIB) can also be used. A star-PIB as used herein can be a derivative of polyisobutylene having three or more branches of functionalized polyisobutylene chains available for cross-linking. 
     The polyisobutylene compositions herein can include polyisobutylenes and derivatives thereof, including, but not limited to acrylate or methacrylate end-capped telechelic polyisobutylene, monofunctional polyisobutylene methacrylate, highly reactive polyisobutylene (HR-PIB), highly reactive telechelic polyisobutylene, diallyl polyisobutylene, monofunctional highly reactive polyisobutylene, or any other polyisobutylene derivative in accordance with the embodiments herein. As used herein, the term “telechelic” refers to any polymer that contains two or more reactive end groups, where the reactive end groups can be used in cross-linking to promote polymerization of the polyisobutylene matrix. 
     In some embodiments, polyisobutylenes and derivatives thereof consistent with the embodiments herein can be viscous compositions at room temperature and can range in molecular weight from 1,000 to 10,000 grams per mole (g/mol). In some embodiments, polyisobutylenes and derivatives thereof can have a molecular weight anywhere from 2,000 to 3,000 g/mol. In some embodiments, the polyisobutylenes and derivatives thereof used herein can have a molecular weight of less than or equal to 1,000, 2,000, 3,000, 4,000, 5,000, 6,000, 7,000, 8,000, 9,000, or 10,000 g/mol, or in a range between any of the foregoing numbers. 
     The polyisobutylene compositions herein can have viscosities ranging from 1,000 centipoise (cP) to 25,000 cP at room temperature. In some embodiments, the polyisobutylene compositions can have a viscosity of 10,000 cP at room temperature. In some embodiments, polyisobutylene compositions can have a viscosity of less than or equal to 25,000, 20,000, 15,000, 10,000, 8,000, 7,500, 5,000, 4,000, 3,000, 2,000, or 1,000 cP at room temperature. Viscosity of the polyisobutylene compositions used herein can be within a range wherein any of the foregoing numbers can serve as the lower or upper bound of the range, provided that the lower bound is less than the upper bound. 
     Many suitable free radical initiators can be used in the polyisobutylene compositions herein and can include, but are not limited to, thermal, ambient redox, and photo free radical initiators. Free radical initiators can include, but are not limited to, organic free radical initiators such as organic peroxides and azo compounds, or inorganic free radical initiators such as inorganic peroxides 
     Other free radical initiators herein can include photoinitiators. Examples of photoinitiators suitable for use herein include compounds from the α-hydroxyketone class of compounds, such as 1-[4-(2-hydroxyethoxy)-phenyl]-2-hydroxy-2-methyl-1-propane-1-one (known commercially as Irgacure® 2959, BASF, Florham Park, N.J.). Other examples of suitable photoinitiators include those from the family of α-amino ketones, acyl phosphine oxides, or benzophenones. 
     Other reagents for use in radical polymerization of the polyisobutylene polymers used herein can include maleic anhydride, itaconic anhydride, and the like. 
     Polyisobutylene compositions herein, after polymerization and/or cross-linking, can specifically exhibit hydrolytic stability in the context of long-term (chronic) implantation. 
     Methods of Making Biocompatible Elastomeric Composite Materials 
     Biocompatible elastomeric composite materials embodied herein can be created using various techniques. In some embodiments, electrospun fibers can be formed first into a substrate (with or without a specific shape) and then a polyisobutylene fluid can be applied to the substrate (through spraying, dip coating, brush coating, immersion, or the like) followed by cross-linking and/or polymerization. In some embodiments, the electrospun fiber is deposited onto a structure and the polyisobutylene composition is spray coated onto the network of electrospun fibers. London forces between adjacent PIB polymers and fibers provide enough attraction to allow the polyisobutylene composition to take the shape of the underlying structure. 
     However, in some embodiments, electrospun fibers are first sprayed into a volume of polyisobutylene fluid followed by cross-linking and/or polymerization of the polyisobutylene fluid. 
     A particular shape can be imparted to the composite in many different ways. In some embodiments, a form or mold having a cavity with the desired end shape can be used. In such a case, the electrospun fibers can be deposited into the form or mold and assume the shape of the cavity into the form or mold. Then a polyisobutylene fluid can be added into the form or mold followed by cross-linking and/or polymerization. 
     Alternatively, the polyisobutylene fluid can be disposed into the form or mold first and then the electrospun fibers can be deposited into the polyisobutylene fluid. In some cases, the electrospun fibers can be deposited onto a build plate or base plate or other surface (such as a flat surface) that does not become part of the finished structure. The electrospun fibers can be deposited in a manner so that a shape is formed either through movement of the portion of a device emitting the electrospray or another component of the spraying apparatus. Then the polyisobutylene fluid can be applied onto the deposited electrospun fibers (through various techniques) followed by cross-linking and/or polymerization. The build plate or base plate can be separated from the electrospun fibers either before or after the polyisobutylene fluid is applied onto the electrospun fibers. 
     In still other cases, the electrospun fibers can be deposited onto a material that does become part of the finished structure such as an underlying material or support layer. 
     In some embodiments, a particular shape can be created by first forming the composite into a substantially planar layer and then cutting the planar layer to the desired shape and dimensions. However, while not intending to be bound by theory, such a cutting step after composite formation can be less desirable for some applications as it is more likely to result in cut-ends of the electrospun fiber exposed on the surface of the composite. As such, in some embodiments herein, the composite lacks cut-ends of electrospun fibers on the surfaces of the composite. In some embodiments herein, the electrospun fibers are completely covered by the polyisobutylene material on the surfaces of the composite. 
     Biocompatible elastomeric composite materials as embodied herein can be created through various free radical polymerization reactions. In some embodiments, creating a thermoset polyisobutylene matrix around a network of electrospun fibers can include using a polyisobutylene composition including a telechelic polyisobutylene and a free radical initiator, such as a photoinitiator. In one example, telechelic polyisobutylene-dimethacrylate (PIB-DMA), a telechelic polyisobutylene having a methacrylate group on each end, is mixed with the photo initiator 1-[4-(2-hydroxyethoxy)-phenyl]-2-hydroxy-2-methyl-1-propane-1-one (known commercially as Irgacure® 2959, BASF, Florham Park, N.J.). The photo initiator can be activated by illuminating the polyisobutylene composition with a UV/VIS light source at the appropriate wavelength (approximately 260-290 nm, with an absorption maximum at 276 nm for Irgacure® 2959). PIB-DMA can react with Irgacure® 2959 and form cross-links to other PIB-DMA polymers to form a thermoset polyisobutylene matrix completely surrounding the polymeric fiber network substrate. 
     In some embodiments, creating a thermoset polyisobutylene matrix around a network of electrospun fibers can include using a polyisobutylene composition including a highly reactive polyisobutylene (HR-PIB), an anhydride such as maleic anhydride or itaconic anhydride, and a free radical initiator. In some embodiments, the HR-PIB can include vinylidene, or exo-terminated, end groups at either end of the PIB chain. In one example, a HR-PIB such as exo-olefin (methyl vinylidene)-terminated PIB can be mixed with maleic anhydride and a free radical initiator (R′) to yield a network of polymerized chains having alternating maleic anhydride and vinylidene PIB endgroups. Suitable free radical initiators for use herein are discussed above. 
     In some embodiments, creating a thermoset polyisobutylene matrix around a network of electrospun fibers can include using a polyisobutylene composition including a monofunctional HR-PIB, having only one exo-terminated end group, and maleic anhydride. For example, monofunctional HR-PIB can be radically polymerized with maleic anhydride to yield chains having alternating PIB and maleic anhydride groups. In some embodiments, the alternating PIB and maleic anhydride groups can be further cross-linked via a condensation reaction using a diamine to create bis-maleimide bridges between the chains. Examples of suitable diamines, including primary polyamines having the general formula H 2 N—CH 2 —[CH 2 ] n —CH 2 —NH 2 , where n can be any number greater than 1, and can include at least putrescine, cadaverine, and hexamethylenediamine. 
     Applications 
     The biocompatible elastomeric composite materials described herein can be used in many applications including, but not limited to, medical and biological applications. In some embodiments, the biocompatible elastomeric composite materials can be used as the basis for prosthetic heart valve leaflets. 
     In some embodiments, a polymeric fiber network substrate can be formed into the shape of a valve leaflet. Referring now to  FIG. 5 , a polymeric fiber network substrate  502  can be formed into the shape of a valve leaflet. The polymeric fiber network substrate  502  can include a plurality of fibers  504 . 
     Referring now to  FIG. 6 , a prosthetic heart valve leaflet  600  is shown comprised of biocompatible elastomeric composite materials in accordance with the embodiments herein. Prosthetic heart valve leaflet  600  can include a root portion  602  and an edge portion  604 . In some embodiments, the prosthetic heart valve leaflet  600  can be formed from a biocompatible elastomeric composite material as described herein, where the network of electrospun fibers is uniformly distributed throughout the valve leaflet. In other embodiments, the prosthetic heart valve leaflet  600  can be formed from a biocompatible elastomeric composite material as described herein, where the network of electrospun fibers is distributed as a gradient, the concentration of electrospun fibers being highest near root portion  602  and the concentration of electrospun fibers being lowest near edge portion  604 . 
     The thickness of biocompatible elastomeric composite material described herein can be tuned depending on the application. For example, biocompatible elastomeric composite material used for prosthetic heart valve leaflet  600  can be designed such that the maximum thickness of the leaflet is 100 μm or less. In some embodiments, prosthetic heart valve leaflet  600  can be designed having a decreasing thickness in a direction extending from root portion  602  towards edge portion  604 . The decreasing thickness in a direction extending from root portion  602  towards edge portion  604  can be achieved through a decreasing thickness in the network of electrospun fibers present in prosthetic heart valve leaflet  600  or through a decreasing thickness in the thermoset polyisobutylene matrix, or both. 
     While  FIGS. 5-6  show a particular example of (and configuration for) a heart valve leaflet, it will be appreciated that many other physical configurations of heart valve leaflets are contemplated herein as well as valves other than heart valves.  FIGS. 5-6  show a two-dimensional layout for a valve leaflet that would later be sewn (or otherwise attached) to a frame. But, other types of valve leaflet configurations are also included herein. For example, heart valve leaflets that are initially formed in their final three-dimensional shape are also included herein. Heart valve leaflets that are formed integrally with a frame are also included herein. 
     It will be appreciated that biocompatible elastomeric composite materials described herein can used to form various medical devices and/or portions thereof. In some embodiments, the biocompatible elastomeric composite material described herein can be used in in vivo environments where long-term implantation of an elastic material having anti-thrombogenic and anti-calcification properties is required. 
     In some embodiments, the biocompatible elastomeric composite material described herein can be used in in vivo environments to sequester encapsulated material within a body cavity or other in vivo region of the body. 
     As merely one specific example, the biocompatible elastomeric composite material described herein can be used to form a prosthetic heart valve or a portion thereof.  FIG. 7  provides a schematic view of the prosthetic heart valve  700  of  FIG. 7  having inflow end  706  and an outlet end  708 . Prosthetic heart valve  700  has a substantially tubular body  720 , a plurality of leaflets  740 , anchor elements  760  and a tubular seal  780 . The tubular body  720  can be a radially expandable member, e.g. annular frame or stent, having an annular cavity. As shown in  FIG. 7 , the heart valve  700  can have three heart valve leaflets  740  coupled to the tubular body  720  within the annular cavity. Each leaflet  740  can be secured to an adjacent leaflet  740  and to the expanding tubular body by stitches. Three anchor elements  760  positioned within the annular cavity of the tubular body can each secure the heart valve leaflets  740  to the tubular body  720 . Each anchor elements  760  can be coupled to the tubular body  720  with an anchoring element  760  and also be coupled to the leaflets  740  with a clamping element. The tubular seal  780  can be disposed about at least a portion of the tubular body  720 . In particular, the tubular seal  780  can have an inflow end portion  706  secured to bottom edges of the plurality of leaflets  740  and have an outflow end portion  708  disposed about an outer surface of the tubular body  720  to restrict blood flow around the leaflets  740 . 
     In use, prosthetic heart valve  700  is implanted (e.g., surgically or through transcatheter delivery) in a mammalian heart. The edge portions of the polymeric leaflets  740  move into coaptation with one another in a closed position to substantially restrict fluid from flowing past prosthetic heart valve  700  in a closed position. The edge portions of the leaflets  740  move away from one another to an open position permitting fluid to flow past prosthetic heart valve  700 . Movement of the leaflets  740  between the closed and open positions can substantially approximate the hemodynamic performance of a healthy natural valve. 
     Prosthetic heart valve  700  can be made of various materials. Synthetic materials of the prosthetic heart valve  700  can include polymeric materials, metals, ceramics, and combinations thereof. In various embodiments herein, however, the leaflets  740  can be formed of a biocompatible composite material as described herein. 
     The embodiments described herein are not intended to be exhaustive or to limit the invention to the precise forms disclosed in the following detailed description. Rather, the embodiments are chosen and described so that others skilled in the art can appreciate and understand the principles and practices. However, it should be understood that many variations and modifications may be made while remaining within the spirit and scope herein. 
     All publications and patents mentioned herein are hereby incorporated by reference. The publications and patents disclosed herein are provided solely for their disclosure. Nothing herein is to be construed as an admission that the inventors are not entitled to antedate any publication and/or patent, including any publication and/or patent cited herein. 
     It should be noted that, as used in this specification and the appended claims, the singular forms “a,” “an,” and “the” include plural referents unless the content clearly dictates otherwise. Thus, for example, reference to a composition containing “a compound” includes a mixture of two or more compounds. It should also be noted that the term “or” is generally employed in its sense including “and/or” unless the content clearly dictates otherwise. 
     It should also be noted that, as used in this specification and the appended claims, the phrase “configured” describes a system, apparatus, or other structure that is constructed or configured to perform a particular task or adopt a particular configuration to. The phrase “configured” can be used interchangeably with other similar phrases such as arranged and configured, constructed and arranged, constructed, manufactured and arranged, and the like.