Polymeric Biomaterials

Medical Device & Diagnostic Industry Magazine | MDDI Article Index

An MD&DI April 1997 Feature

MEDICAL PLASTICS

A limited number of biomaterials are available for long-term implantable applications.

The traditional definition of biomaterial is a systemic, pharmacologically inert substance designed for implantation or incorporation into the human body. Materials used must nearly duplicate the properties of the tissues they replace in order not to trigger an immune response in the body. Polymeric materials typically used for such applications include polyurethane, polypropylene, and polyimide. Typical applications include catheters, contact lenses, and hip joints. On yet another level, pacemaker leads and blood vessel replacements present additional challenges.

"More and more researchers are selecting core materials for their mechanical properties and then modifying the surface to fit a particular environment," says Karin Caldwell, a biomaterials researcher at the University of Utah (Salt Lake City). She explains that most manufacturers now choose a core material that exhibits the rigidity, tensile strength, or other characteristics they desire, then modify it to work within the environment the device is being designed for.

And, although many polymer manufacturers have begun to restrict their materials for use in only critical medical devices, some companies continue to develop and manufacture new biomaterials for long-term implants, namely, those specifically intended to remain in the body for longer than 29 days.

The Polymer Technology Group (Emeryville, CA). The firm produces the polycarbonate-based polyurethane Bionate, achieving oxidative stability by replacing susceptible ether groups with carbonate linkages adjacent to hydrocarbon groups. Such biomaterials are used in applications that have a potential mode of degradation, such as pacemaker leads. The company also uses the polycarbonate urethanes as base polymers for its surface modification technology, known as surface modifying end groups (SMEs). The SMEs are incorporated during polymer manufacture and can permanently modify surface properties, such as blood compatibility, abrasion resistance, coefficient of friction, and resistance to degradation in implants. The resulting material contains covalently bonded surface-active end groups and provides a measurable difference in interfacial tension compared with identical polymers that do not have SMEs.

One of the Polymer Technology Group's services is the continuous casting of polyurethane films or membranes for use in medical devices.

Applied Silicone Corp. (Ventura, CA). The company has introduced an implant-grade liquid silicone rubber system. The HS-series system is supplied in two-component kits that contain equal amounts of pure dimethyl silicone elastomers engineered for use in liquid-injection molding systems that produce high-strength molded parts. The liquid silicone rubber is a pumpable, colorless, translucent paste, so that when the two components are mixed together in equal portions the liquid cures to a tough, rubbery elastomer using addition-cure chemistry.

According to the company, these liquid silicone rubbers provide improved clarity and cure faster at elevated temperatures than peroxide-catalyzed systems. In addition, they produce no peroxide residues and no volatile by-products. They are designed specifically for long-term implantable devices, including catheter strain relief junctions, encapsulated electronic parts, and needle septum ports. The company recommends that device manufacturers use airless mixing, metering, and dispensing equipment for production operations to ensure the best combination of the two components. The company cautions manufacturers against using this material with polymer systems that contain traces of amines, sulphur, nitrogen oxide, organotin compounds, or carbon monoxide because the combination can interfere with curing.

The company has also released a high-consistency addition-cure silicone elastomer system for extrusion or molding. The health-care-grade high-consistency addition-cure extrudable systems are two-part platinum-catalyzed silicone elastomers designed for use in applications that require high-strength extruded or molded parts. The extrusion materials are formulated for rapid extrusion vulcanization in short-dwell cure ovens. The molding materials are formulated for extended-mix-life transfer and compression molding operations. Both contain 100% (by weight) of dimethyl silicone elastomer when cured. The products, which are supplied in equal amounts, are strained through a 25-µm screen to ensure freedom from particulate contamination. Both systems contain no volatile cure by-products and no peroxide residues.

Glasflex (Stirling, NJ). Glasflex produces an implantable-grade polymethyl methacrylate (PMMA) suitable for such applications as intraocular lenses and cement spacers for orthopedic prostheses. The material, a virgin methyl methacrylate monomer, is tested for residual monomers, molecular weight distribution values, spectral curve requirements, and other physical and chemical properties to ensure biocompatibility. The company's intraocular lens materials are available in UV-absorbing or UV-transmitting formulations with either a finite or infinite molecular weight. Using UV absorbers, cross-linking agents, and other specialized chemicals, the company develops proprietary implantable PMMA materials.

Thermedics (Woburn, MA). The firm manufacturers a line of elastomeric polyurethanes developed for tissue and blood-contact situations. Tecoflex aliphatic polyurethane was originally developed for use in the company's artificial heart program. Because it produces minimal tissue reaction and blood clotting, its use has been expanded for other implantable devices requiring biocompatibility. The aliphatic resins are reaction products synthesized of methylene bis(cyclohexyl) diisocyanate, poly(tetramethylene ether glycol), and 1,4 butane diol chain extender. According to the company, the resins can be used in many soft elastomer medical applications, such as indwelling catheters, because of easy processing and high tensile strength. Other common uses for the materials include gastric feeding, vascular access, cardiac pacing, and dialysis devices. In such applications, the materials exhibit minimal acute or chronic inflammation in short- and long-term use compared with other materials. The polyurethanes can be loaded with radiopaque materials for detection on x-ray or fluoroscope and col-ored for product identification or coding. They have relatively low melt temperatures and can be extruded or molded in a variety of durometers (hardness). The materials are tailored for prototyping and casting complicated configurations. Accord-ing to the company, the materials' composition eliminates the danger of forming methylene dianiline, a known carcinogen, which can occur in improperly processed or overheated aromatic polyurethanes.

Limited availability of materials poses a challenge for manufacturers of long-term implantables. Much research is under way to graft materials onto core polymers to increase the choice of materials for these critical applications. Currently, hydrophilic polyethylene oxide, which is unique in its ability to provide a stealth coating, is being used to covalently link to polypropylene oxide, producing a seaweedlike surface, the University of Utah's Caldwell says. Other researchers are seeking to develop polymer networks that do not absorb blood proteins for such applications as contact lenses.

Sherrie Steward is a senior editor for MD&DI.

Copyright © 1997 Medical Device & Diagnostic Industry