Chapter 4 Biomedical elastomers

Abstract

Elastomers are described as materials that possess pronounced elasticity and rebound. They can be tough, relatively impermeable to air and water and exhibit resistance to cutting, tearing and abrasion. Often they are modified by compounding to increase their hardness and strength. Or, conversely, they can be soft, compliant and absorbent to water if the need exists. In some instances their properties can closely simulate that of the tissues which they must contact. As biomedical materials they may have originated from commercial formulations or been custom designed from basic chemistry. Those that have been judged as biocompatible have made significant contributions towards the development of successful medical devices. Literally, every basic elastomer has been evaluated at some time for its possible suitability in contact with the body. This would include such materials as natural rubber, styrene rubber, polybutyl rubber, silicone rubber, acrylate rubber, Hypalon®, polyurethanes, fluorinated hydrocarbon rubbers, polyvinyl chloride, thermoplastic vulcanizates and others. Of these, only special medical grade formulations of silicone, polyurethane, polyvinyl chloride and thermoplastic elastomer have continued to be commercially successful.

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4.1 Introduction

Elastomers are described as materials that possess pronounced elasticity and rebound. They can be tough, relatively impermeable to air and water and exhibit resistance to cutting, tearing and abrasion. Often they are modified by compounding to increase their hardness and strength. Or, conversely, they can be soft, compliant and absorbent to water if the need exists. In some instances their properties can closely simulate that of the tissues which they must contact. As biomedical materials they may have originated from commercial formulations or been custom designed from basic chemistry. Those that have been judged as biocompatible have made significant contributions towards the development of successful medical devices. Literally, every basic elastomer has been evaluated at some time for its possible suitability in contact with the body. This would include such materials as natural rubber, styrene rubber, polybutyl rubber, silicone rubber, acrylate rubber, Hypalon®, polyurethanes, fluorinated hydrocarbon rubbers, polyvinyl chloride, thermoplastic vulcanizates and others. Of these, only special medical grade formulations of silicone, polyurethane, polyvinyl chloride and thermoplastic elastomer have continued to be commercially successful.

There are important differences between materials and differences among similar materials within a given generic class. These differences are due to the chemical composition of the polymer, the molecular configuration of the polymer and the presence of functional groups. For instance, polyurethanes of a polyester base were initially tried and found unstable for implantation whereas polyether based polyurethanes were decidedly more stable. Elastomers with aromatic structures behave differently than the polymer having aliphatic structure. Not every material is suitable for every application. Some have been found to perform successfully under static conditions but fail or perform undesirably under dynamic situations. Often, the design of a device and the demands upon it will determine if the elastomer chosen is the proper selection. Therefore the material and its use are inseparable. They must be evaluated together. Merely passing an array of physical and biological tests do not confer success. Biocompatibility is an essential element of medical grade elastomers. A set of compatibility tests determine the general physiological acceptability of an elastomer. These consist of passing USP Class VI tests. Additional testing may be required depending upon the device, its area of application and the time it is in contact with tissues. A Master File is often registered by the manufacturer of the basic elastomer with the FDA to attest to its properties, composition and response to biological testing. Demands on medical device manufacturers have never been more stringent. Regulatory pressures, more indepth testing, the threat of litigation plus the constraints of health care cost containment are affecting all aspects of the design and development process and the availability of some biomedical elastomers. A variety of elastomeric materials are available to meet the design challenges presented by medical devices. However, there is still a need for even better materials.

The elastomers that are listed here should be considered in light of their suitability for a specific application. The properties tables should serve as a guide to design options for those in the early stages of the development process. Keep in mind that these properties listed in the tables and the compatibility standings are only indicative of the performance characteristics that an elastomer may exhibit.

4.2 Types of Elastomer

Biomedical elastomers can be classified as to whether they are thermoplastic or thermosetting in nature. Thermoplastic biomedical elastomers are gaining in commercial importance and in some cases replacing traditionally used vulcanized versions. Thermosetting elastomers are irreversibly crosslinked and have had the longest history of medical use. Both groups will be described citing representative medical elastomers that are either commercially available or that may replace elastomers that have been recently withdrawn from the market.

4.2.1 Thermoplastic elastomers

Thermoplastic elastomers (TPEs) are a special class of materials that process similarly to other thermoplastic polymers, yet possess many of the desirable properties of thermoset elastomers. Some TPEs are elastomeric alloys consisting of crosslinked particles of rubber surrounded by a thermoplastic matrix. Others consist of block copolymers and are typified by polyurethanes and styrene polymers.

Depending upon which thermoplastic elastomer is chosen, physical properties can vary over a wide range. They can be either hard, or soft, flexible or stiff, elastic or rigid. For the most part, they are smooth to the touch, yet will form tight seals to surfaces they contact. They can be processed using conventional techniques and equipment and in automated modes. Medical applications consist of such examples as pacemaker lead wire coatings, artificial hearts, and catheters. A wide variety of sundry uses have contributed to patient care and consists of bulbs and bladders, serum caps and tubes, cushions, diaphragms, electrical connectors, flexible medical wire coatings, gaskets, needle shields, pharmaceutical closures, seals, stoppers, tubing, and valves. Most of the TPEs can be sterilized using gas, steam and radiation with very little change in their molecular structures or properties (Table 4.13).

Thermoplastic vulcanizates

Thermoplastic vulcanizates are a separate class of thermoplastic elastomers (TPEs) with Santoprene® as the representative biomedical elastomer.

Santoprene®

This thermoplastic vulcanizate is an olefin based elastomer; an elastomeric alloy. It is totally synthetic and does not contain any natural rubber thereby avoiding many of the allergic reaction problems associated with natural rubber latex. It exhibits outstanding flex-fatigue resistance, low temperature flexibility (-40 °C) and resistance to tearing and abrasion. Its resistance to plastic deformation under tensile and compression stress is another of its features. Santoprene® is reported to be superior to natural rubber in some situations and replaces silicone elastomers in others. It has found use in peristaltic pump tubing, syringe plungers, seals, and caps, tracheal and enteral tubing, vial closures and pump seals, disposable anesthetic hoses, intravenous delivery systems, and other hospital devices. Santoprene® has met USP Class IV requirements for in vivo biological reactivity and conforms to the Tripartite Biocompatibility Guidance standards. However, the manufacturer does not recommend Santoprene® for use as part of human implants. The material may be injection molded, extruded, blow molded and thermoformed. For details on physical properties, processing and biocompatibility see Tables 4.1, 4.2, 4.13 and 4.14.

Table 4.1 Typical Properties of Thermoplastic Vu1canizates

Table 4.2 Typical Properties of Copolyester Elastomers, PCCE

Table 4.3 Typical Properties of Polyurethane-based Elastomers

Table 4.4 Typical Properties of Polycarbonate-based Polyurethane

Copolyester ether elastomer

Ecdel ^™

This copolyester ether TPE is essentially polycyclohexanedimethyl-cyclohexanedicarboxylate (PCCE). It is reported to possess the chemical resistance, toughness and inertness yet exhibits elastic flexibility over a broad temperature range. Ecdel™ is an unusual elastomer since it has a crystalline structure. Quenching during molding can reduce its crystallinity and impart increased clarity. The material is being used for uniquely designed intravenous bags with built-in bottle necks and fasteners. The material can be injection or blow molded and extruded into film or sheet; but only Ecdel™ 9967 may be processed into tubing. This TPE is also manufactured under the name CR3 by Abbott Labs (Tables 4.2, 4.12, 4.13, and 4.14).

Table 4.5 Typical Properties of Polypropylene-based Elastomers

Polyurethane-based elastomers

Polyurethanes are another class of TPEs. They are a large family of chemical compounds that can consist of ether-based, ester-based, polycarbonate-based or polypropylene-based varieties. A number of copolymers are also included;. polyurethanes are combinations of macroglycols and diisocyanates that have been polymerized into tough and elastic materials. TPE polyurethanes have been used for peristaltic pump tubing, parenteral solution tubing and catheters. The tables list the majority of those that are commercially available. Among others are those either of limited supply, available for proprietary use only or that have been successful, but recently discontinued such as:

• Hemothane Sarns Div. of 3M. Restricted to proprietary use.

• Biomer Ethicon, Inc. No longer available through this source.

• Surethane Cardiac Control Systems, Inc. Redissolved Lycra® thread. Some formulations may have a few percent PDMS blended with it. Limited availability.

• Pellethane™ 2360 Dow Chemical, USA. This material is no longer available for medical implant use (see also Pellethane™).

• Angioflex ABIOMED, Danvers, Mass. Restricted to proprietary use.

• Cardiothane Kontrol, Inc. A silicone-urethane interpenetrating polymer network. Limited availability.

Internationally, polyurethanes for medical use have been developed by Italy, China and Japan.

Biospan®

This TPE is a segmented polyurethane and is reported to be not significantly different from biomer in chemistry and molecular weight. It is a polytetra-methyleneoxide-based aromatic polyurethane urea with mixed aliphatic and cycloaliphatic diamine chain extenders. A copolymer of diisopropylamino-ethyl methacrylate and decyl methacrylate are added as a stabilizer. The material is supplied as 25% solids in dimethyl acetamide solvent (Tables 4.3, 4.12, 4.13, and 4.14).

Biospan®-S

This is a silicone modified analog of Biospan® with a different stabilizer. It possesses a silicone-rich surface to enhance thromboresistance while maintaining the bulk properties of Biospan® (Tables 4.3, 4.12, 4.13, and 4.14).

Biospan®-D

This is another version of Biospan® with surface modification by an oligomeric hydrocarbon covalently bonded to the base polymer during synthesis. The additive has a pronounced effect on the polymer surface chemistry but little effect on the bulk properties of the base polymer according to the manufacturer (Tables 4.3, 4.12, 4.13, and 4.14).

Hydrothane™

Hydrothane™ is a TPE hydrogel belonging to the polyurethane family of polymers. Hydrothane™ is an aliphatic material with water absorption capabilities ranging from 5 to 25% by weight while still maintaining high tensile strength and elongation. Because of its water absorption capacity, Hydrothane™ is reported to be bacteria-resistant and lubricious. The polymer can be processed by conventional extrusion and injection molding techniques. It can also be dissolved in dimethyl acetamide solvent to produce a 25% solids solution suitable for dip-coating and other solution processing techniques (Tables 4.3, 4.12, and 4.13).

Medicaflex™

The Lambda series of Medicaftex is a polyurethane-based TPE polymer that exhibits low modulus characteristics with high tear strength and abrasion resistance. Those listed in the tables have passed USP Class VI compatibility tests and have been used as replacements in some natural rubber latex and silicone rubber applications. The polymer has been applied to uses such as catheters, tubing and films where softness, low durometer hardness, low modulus or high elongation are needed (Tables 4.3, 4.12, and 4.13).

Pellethane™ polyurethane elastomers

The 2363 series Pellethane™ TPE elastomers have a wide range of durometer hardness and are noted for their high tensile and tear strength and abrasion resistance. Chemically they are classed as polytetramethylene glycol ether polyurethanes. The ether series is the most widely used for medical applications although polyester versions of Pellethane™ are useful for some applications. None of these polymers have the disadvantage of containing plasticizers which can migrate out of the polymer over time resulting in reduction in physical properties. Medical tubing made from Pellethane™ polymer is widely used. These TPEs are unaffected by ethylene oxide gas, gamma radiation and electron beam sterilization procedures. Pellethane™ can be processed by injection molding and extrusion. For details on physical properties, processing and biocompatibility (Tables 4.3, 4.12, and 4.13).

Notice Regarding Long-Term Medical Implant Applications

The Dow Chemical Company does not recommend Pellethane™ elastomers for long-term medical implant applications in humans (more than 30 days). Nor do they recommend the use of Pellethane™ elastomers for cardiac prosthetic devices regardless of the time period that the device will be wholly or partially implanted in the body. Such applications include, but are not limited to, pacemaker leads and devices, cardiac prosthetic devices such as artificial hearts, heart valves, intra-aortic balloon and control systems, and ventricular bypass assist devices. The company does not recommend any non-medical resin (or film) product for use in any human implant applications.

PolyBlend™ polyurethane

This TPE has been described as an aromatic elastoplastic polyurethane alloy. It possesses a low coefficient of friction, low extractables, and dimensional stability. Hardness ranges from 65 to 75 Shore D. The material is classified for short-term (29 days or less) implantation. Clear and radiopaque formulations are available. Tubing should be annealed at 80°C for four hours to reduce crystallinity (Tables 4.3, 4.4, 4.12, and 4.14).

Tecoflex® polyurethane

Tecoflex is an aliphatic polyether-based polyurethane that is available in clear and radiopaque grades. They are reaction products of methylene bis (cyclohexyl) diisocyanate (HMDI), poly (tetramethylene ether glycol) (PTMEG), and 1,4 butane diol chain extender. The manufacturer claims that the aliphatic composition of Tecoflex® eliminates the danger of forming methylene dianiline (MDA) which is potentially carcinogenic. MDA can be generated from aromatic polyurethanes if they are improperly processed or overheated. Tecoflex has been reported to crack under stress when implanted, long-term, in animals. An advantage of Tecoflex is that it softens considerably within minutes of insertion in the body. This feature can offer patient comfort for short-term applications such as catheters and enteral tubes; it is also reported to reduce the risk of vascular trauma (Tables 4.3, 4.12, and 4.13).

Tecothane®

Tecothane® is an aromatic polyether-based TPE polyurethane polymer. It has processibility and biocompatibility characteristics similar to Tecoflex® except that it is an aromatic rather than an aliphatic polyurethane. Tecothane® is synthesized from methylene diisocyanate (MDI), polytetramethylene ether glycol and 1,4 butanediol chain extender. By varying the ratios of the reactants, polymers have been prepared ranging from soft elastomers to rigid plastics. The manufacturer of Tecoflex® and Tecothane® point out that there is not much difference between medical-grade, aliphatic and aromatic polyether-based polyurethanes with regard to chemical, mechanical and biological properties. However, they caution that with improper processing of Tecothane® (e.g., high moisture content or steam sterilization) it is possible to form measurable amounts of methylene dianiline (MDA), a listed carcinogen. The use of ethylene oxide or gamma radiation are suitable sterilizing agents that do not affect the chemical or physical properties (Tables 4.3, 4.12, and 4.13).

Texin™

There are four basic polymer formulations of Texin polyurethane TPE that may be suitable for medical applications. They range in hardness and flexural modulus. Texin elastomers are produced by the reaction of diisocyanate with a high molecular weight polyester or polyether polymer and a low molecular weight diol. The polyethers (products 5286 and 5265) offer greater hydrolytic stability and stress crack resistance. The polyesterbased polyurethane (product 5187) and the polyester polyurethane/ polycarbonate blend (product 5370) possess high impact strength and high stiffness along with useful low-temperature properties. Texin is not recommended for implants of greater than 30 days duration. Texin should not be sterilized by autoclave or use of boiling water. Other advantages offered by Texin TPUs are that plasticizers are not necessary to achieve flexibility, the amount of extractables are low, and they possess high tensile strength, high tear strength, and high abrasion resistance. Texin polyurethanes are hydroscopic and will absorb ambient moisture. They can be processed by extrusion and injection molding if thoroughly dried beforehand. As with all chemical systems, the proper use and handling of these materials can not be over-emphasized (Tables 4.3, 4.12, and 4.13).

Texin™ 5370 is a blend of polyester-based polyurethane and polycarbonate. It offers high impact strength and high stiffness. Steam sterilization or boiling should be avoided (Tables 4.3, 4.12, and 4.13).

Polycarbonate-based polyurethanes

Carbothane ^™

This medical grade TPE polyurethane is the reaction product of an aliphatic diisocyanate, a polycarbonate-based macrodiol, and a chain terminating low molecular weight diol (Tables 4.4, 4.12, and 4.13).

ChronoFlex™ AR.

Available as a dimethyl acetamide solution, this segmented, aromatic, polycarbonate-based TPE polyurethane was designed to mimic Ethicon Corporation’s Biomer. The polymer is made from the addition of diphenylmethane 4,4’-diisocyanate to a polycarbonate diol followed by addition of a mixture of chain extenders and a molecular weight regulator. The polymer is believed to be resistant to environmental stress cracking such as that experienced by other polyurethanes coated onto pacemaker leads (Tables 4.4, 4.12, and 4.13).

Coremer™

Specifically designed as an 80 Shore A durometer TPE, this is a diamine chain extended version of Corethane®. Coremer™ solution cast films have a low initial modulus and high flex fatigue life. Information as to long-term biostability is not available at this time (Tables 4.4 and 4.13).

Corethane®

A polycarbonate TPE polyurethane that claims biostability is achieved through its replacement of virtually all ether or ester linkages with carbonate groups. The soft segment is composed of a polycarbonate diol formed by the condensation reaction of 1,6-hexanediol with ethylene carbonate. The polycarbonate diol is converted to a high molecular weight polyurethane by the reaction with 1,4-methylene bisphenyl diisocyanate (MDI) and 1,4-butanediol. It is reported to be resistant to environmental stress cracking as experienced with insulation on pacemaker lead wires. The polymer can be extruded, injection molded or compression-molded, and can be bonded with conventional urethane adhesives and solvents (Tables 4.4, 4.12, 4.13, and 4.14).

Corhesive™

Corhesive™ is a solvent-free, two-component reaction adhesive system for use with polyurethanes, plasma treated silicones and certain metals (Tables 4.4, 4.12, 4.13, and 4.14).

Polypropylene-based elastomers Sarlink®

This is a polypropylene-based TPE that has been used as a replacement for medical stoppers previously made from butyl rubber. Sarlink® has the characteristics typical of rubber vulcanizates such as elasticity, flexibility, high coefficients of friction and softness. Sarlink® combines gas impermeability without concern for contamination of biological medium. Applications for medical grade Sarlink® are inserts on syringe plungers, reusable injection caps, vacuum assisted blood sampling tubes, plus flexible grade tubing. The number of stoppers produced from Sarlink annually number in the billions. The material can be injection molded, blow molded, extruded, calendered, and thermoformed on standard processing equipment. It can be thermal bonded or adhesive bonded (Tables 4.5, 4.12, and 4.13).

Polyvinyl chloride elastomers

Polyvinyl chloride polymer is polymerized from vinyl chloride monomers. It is a hard material which can be made soft and flexible through the addition of a plasticizer or a copolymer. As such, it resembles an elastomer and can be included with other TPEs. Also optionally added to PVC are fillers, stabilizers, antioxidants and others. A typical PVC plasticizer for medical products is di(2-ethylhexyl) phthalate (also known as dioctyl phthalate, DOP). Some producers of PVC also offer non-phthalate formulations. PVC has been used extensively for blood bags, blood tubing, endotracheal tubes, catheters and fittings, urology tubes, intravenous tubing, respiratory devices and dialysis sets. Leaching of the plasticizer can offer difficulties if the application is not short-term. Medical grade PVC is available from B.F. Goodrich under the name Geon® RX, Elastichem™ PVC, Ellay™ PVC, Multichem™ PVC, Teknor™ PVC, AlphaGary and others. PVC polymers have also been incorporated as additives to polyurethane to alter the properties of the latter.

Elastichem™ PVC.

This polyvinyl chloride compound family is highly elastomeric and exhibits a dry non-tacky surface even at hardnesses as low as 40 Shore A durometer. Their rubber-like resilience, high elongation and low permanent set and fatigue resistance offer advantages over conventional formulations (Tables 4.6, 4.12, and 4.13).

Table 4.6 Typical Properties of Plasticized Polyvinyl Chloride

Ellay™ PVC.

Compounds from Ellay Corp. are available with Shore hardness ranges from 55 A to 100 A. The polymers have been applied to medication delivery systems, blood collection, processing and storage, gastro-urological devices and collection systems. Product numbers ending in ‘R’ are special radiation resistant grades (Tables 4.6, 4.12 and 4.13).

Geon® PVC.

Geon® PVC is associated with vinyl examination gloves. For this use, Geon® recommends a combination of Geon® 121 AR and 213. For a more ‘latex type’ feeling, Goodtouch 250x100 is recommended. Typical film samples have passed patch insult tests when worn against the skin for extended periods (Tables 4.6, 4.12 and 4.13).

Multichem™ PVC

This line of PVC polymers consist of alloys of PVC in combination with other polymers. They display notable dynamic properties and resistance to migration and extraction. These non-toxic PVC compounds (includes Multichem™ and Elastichem™) have over 25 years of experience in the medical field (Tables 4.6, 4.12 and 4.13).

Teknor™ Apex PVC

This extrudable PVC has found use as tubing for blood transport and delivery systems, dialysis and enteral feeding systems, oxygen delivery systems, catheters, and drainage systems. Product numbers containing an R are special radiation resistant grades (Tables 4.6, 4.12 and 4.13).

Styrene-based elastomers

C-Flex ^® TPE.

C-Flex® thermoplastic elastomers are based on styrene/ethylene-butylene/styrene block copolymers. C-Flex® polymers designated as ‘medical grade’ are clear and can be processed using conventional extrusion and injection molding equipment. They have been tested using Good Laboratory Practices and have successfully passed USP Class VI, biocompatibility tests. Translucent versions have high rebound values at ultimate elongation. Medical tubing, ureteral stents, blood pumps, feeding tubes and nephrostomy catheters are successful uses of this material (Tables 4.7, 4.12 and 4.13).

Table 4.7 Typical Properties of Styrene-based Thermoplastic Elastomers

Kraton®

Kraton® elastomer consists of block segments of styrene and rubber monomers and are available as Kraton® D and G series. The D series is based on unsaturated midblock styrene-butadiene-styrene copolymers whereas the G series is based on styrene-ethylene/butylene-styrene copolymers with a stable saturated midblock. Listed among the attributes of both series are such features as low extractables, dimensional stability, vapor and gas transmission properties, ease of sterilization, softness and clarity. They exhibit elastomeric flexibility coupled with thermoplastic processibility (Tables 4.7, 4.12, 4.13).

4.2.2 crosslinked elastomers

Natural rubber

Natural rubber (cis-polyisoprene) is strong and one of the most flexible of the elastomers. The material has been used for surgeon's gloves, catheters, urinary drains and vial stoppers. However, because it has the potential to cause allergic reactions thought to be due to the elution of entrapped natural protein, this elastomer is being used less now than in the past. Safer substitutes are being selected.

Silicone elastomers

Silicone elastomers have a long history of use in the medical field. They have been applied to cannulas, catheters, drainage tubes, balloon catheters, finger and toe joints, pacemaker lead wire insulation, components of artificial heart valves, breast implants, intraocular lenses, contraceptive devices, burn dressings and a variety of associated medical devices. A silicone reference material has been made available by the National Institutes of Health to equate the blood compatibility of different surfaces for vascular applications. This material is available as a silica-free sheet. Contact the Artificial Heart Program, NHBLI, NIH, Bethesda, Md. for further information.

The silicone elastomers most commonly used for medical applications are the high consistency (HC) and liquid injection molding (LIM) types. The former is most often peroxide cured and the latter platinum cured although there are variations. Both materials are similar in properties. LIM offers greater advantages to the medical device molder and is gaining in popularity. This form of silicone may become the molder’s material of choice within the next few years.

High consistency (HC) silicone elastomer

High consistency silicone elastomer consists of methyl and vinyl substituted silicones with aromatic and fluorinated functional groups in some formulations. For the most part, they are peroxide crosslinked. Items are usually compression or transfer molded (Tables 4.8).

Table 4.8 Typical Properties of High Consistency (HC) Silicone Elastomers

Liquid injection molding (LIM) silicone elastomer

Liquid injection molding (LIM) with liquid silicone rubber (LSR) is fast becoming the technique of choice for processing silicone elastomers. Modifications of conventional injection molding equipment are required. For example, pumps to handle two components being injected simultaneously are required. The heaters on the injection barrel and nozzle are replaced by water cooled jackets. The mold is heated in the range of 300 to 400°F. Because the (LSR) flows easily, injection pressures are low (800 to 3000 psi). Elastomeric items cure rapidly in the mold (e.g., a 7 gram part will crosslink in about 15 seconds at 350 °F). Many formulations rely on platinum as a crosslinker. Perhaps in the future, the majority of silicone rubber molded parts will be made in this fashion. Appropriate equipment is commercially available.

Tables 4.8, 4.9, 4.10 and 4.11 list the silicones made by Applied Silicone Corp., Dow Corning Corp., and NuSil Technologies. Table 4.12 lists their biocompatibility status and Table 4.13 recommended sterilization methods. Dow Corning no longer offers the following materials for general sale:

• Silastic MDX 4–4515

• Silastic MDX 4–4515

• Silastic Q7–2245

• Dow Corning Q7–2213

Table 4.9 Typical Properties of Liquid Injection Molding (LIM) Silicone Elastomers

Table 4.10 Typical Properties of Elastomeric Dispersions

Table 4.11 Typical Properties of Silicone Elastomeric Adhesives

Table 4.12 Biocompatibility of Various Elastomers

Table 4.13 Sterilization Methods for Elastomers

Table 4.14 Water Absorption of Various Elastomers

Further, they have discontinued the sale of all implant grade materials.

Other silicones

Silicones and polyurethanes have been used to produce denture liner materials and maxillofacial prostheses. Most of these materials are silicone based, e.g., Flexibase, Molloplast-B, Prolastic, RS 330 T-RTV, Coe-Soft, Coe-Super Soft, Vertex Soft, PERform Soft, and Petal Soft. Other custom made elastomers have been applied to maxillofacial prostheses, e.g., Cosmesil, Silastic® 4-4210, Silastic® 4-4515, Silicone A-102, Silicone A-2186, Silskin II, Isophorone polyurethane, and Epithane-3. Denture liners with acrylic and silicone include Coe-Soft, Coe Super-Soft, Vertex Soft, Molloplast-B and Flexibase.

Dispersions

Solvent solutions of polyurethane elastomers and silicone elastomers are given in Table 4.10. These materials are helpful in casting thin films and odd or complex shapes.

4.3 Establishing Equivalence

Specific polymeric materials traditionally used for medical applications have been recently withdrawn from the medical market. Silicone elastomers are among those withdrawn. To maintain continued supply of vital implants, methods of determining equivalence for withdrawn elastomers with new or existing ones has been adopted by the FDA in the form of an FDA Guidance Document.

4.3.1  FDA Guidance document for substitution of equivalent elastomers

The FDA will allow manufacturers to change sources of silicone elastomers (and others) if they can show that the replacement material is ‘not substantially different’ from materials described in existing approved applications. The device manufacturer is still required to certify that the processes of fabrication, cure and sterilization it uses in the manufacture of its device are appropriate for the new material and that the device will perform as intended. Premarket notification submission under section 510(k) of the Federal Food, Drug, and Cosmetic Act (21 USC 360(k) and 21 CFR 807.81(a)(3)(i), or a supplemental premarket approval application under 21 USC 360(k) section 515 and 21 CFR 814.39 is necessary when change could significantly affect the safety or effectiveness of the device. These submissions are required to be submitted and approved before the device may be marketed with the change.

There are a number of tests necessary for comparison of silicone elastomers as indicated by ‘Guidance for Manufacturers of Silicone Devices Affected by Withdrawal of Dow Corning Silastic® Materials’ (Federal Register, Vol. 58, No. 127, Tuesday, July 6, 1993/ Notices, 36207). They compare the physical, chemical and biological properties of the bulk polymers as they are received from the supplier and also compare the molded elastomer as it exists in the final medical device.

4.3.2 Equivalent silicone elastomers

Two manufacturers, NuSil Technology and Applied Silicone Corp., are providing equivalent silicone materials for the Dow Corning products that have been withdrawn. Tables 4.15 and 4.16 gives reported comparisons.

Table 4.15 Equivalent Silicone Elastomers for Existing Dow Corning Silicones

Table 4.16 Equivalent Silicone Elastomers for Withdrawn Dow Corning silicones

4.4  Sterilization of Elastomers

4.4.1 Sterilization methods

Not all materials respond alike when subjected to various means of sterilization. Some are heat sensitive, some will absorb sterilization fluids, some will be affected by molecular changes when subjected to radiation sterilization and others will absorb and hold irritating gases for extended periods of time. Table 4.13 gives sterilization methods that have been judged most appropriate for each elastomer. The consequences of using an inappropriate method can be loss in physical properties and an adverse biological response.

4.5 Relevant ASTM Standards

Standard methods of testing elastomers used for medical applications are given by specific ASTM test methods. Physical and biological tests are provided here to serve as references for the data cited in the tables and listed in Table 4.17. They are also designated in the FDA Guidance Document.

Table 4.17 Relevant ASTM Standards

4.6 Biocompatibility

Table 4.12 on biocompatibility of various elastomers is intended to show the status of in vitro and in vivo testing. The successful outcome of these tests can serve as guides to potentially acceptable performance of an elastomeric product in a medical device under development. However, the use of elastomeric products in medical devices is the responsibility of the device manufacturer who must establish their safety and efficacy with the FDA.

4.7 Sources

• AlphaGaryAlphaGary, Leominster, MA

• Applied SiliconeApplied Silicone Corp., Ventura, CA

• Biospan®Polymer Technology Group, Inc., Emeryville, CA

• C-Flex®Consolidated Polymer Technologies, Inc., Largo, FL

• Carbothane™Thermedics, Inc., Woburn, MA

• ChronoFlex™PolyMedica Industries, Inc., Woburn, MA

• Coremer™Corvita Corp., Miami, FL

• Corethane®Corvita Corp., Miami, FL

• Corhesive™Corvita Corp., Miami, FL

• Ecdel™Eastman Chemical Co., Kingsport, TN

• Elastichem™Colorite Plastics Co., Ridgefield, NJ

• Ellay™Ellay, Inc., City of Commerce, CA

• Geon®B.F. Goodrich Co., Chemical Group, Cleveland, OH

• Hydrothane™PolyMedica Industries, Inc., Woburn, MA

• Kraton®Shell Chemical Co., Oak Brook, IL

• Medicaflex™Advanced Resin Technology, Manchester, NH

• Multichem™Colorite Plastics Co., Ridgefield, NJ

• Natural rubberExxon Chem. Co., Buffalo Grove, IL Goodyear Tire and Rubber Co., Akron, OH

• NuSil SiliconeNuSil Technology, Carpinteria, CA

• Pellethane™Dow Chemical Co., Midland, MI

• PolyBlend™PolyMedica Industries, Inc., Woburn, MA

• Santoprene®Advanced Elastomer Systems, St Louis MO

• Sarlink®DSM Thermoplastic Elastomers, Inc., Leominster, MA

• SilasticDow Corning Corp., Midland, MI

• Tecoflex®Thermedics, Inc., Woburn, MA

• Tecothane®Thermedics, Inc., Woburn, MA

• Teknor™Teknor Apex Co., Pawtucket, RI

• Texin™Miles, Inc., Pittsburgh, PA