Radiolucent Structural Materials for Medical Applications

Originally Published MDDI June 2001

Medcial Plastics and Biomaterials

Radiolucent Structural Materials for Medical Applications

For devices that are transparent to x-rays, thermoplastic and carbon-fiber composites provide properties that are competitive with traditional metals.

Barry Chadwick and Chris Toto

TRADITIONAL MATERIALS OF CONSTRUCTION

Traditionally, metals such as aluminum, stainless steel, and titanium have been used for structural components in the medical device industry. But these materials are radiopaque—that is, they obstruct x-rays. Accordingly, a metal device located in front of a trauma region would restrict x-ray visibility to the region.

Plastics are inherently radiolucent; with mechanical properties generally inferior to those of metals, however, plastics normally cannot directly replace structural metal components. For example, flexural modulus—a measure of material stiffness—is typically several hundred thousand pounds per square inch (psi) for many unreinforced thermoplastics; in contrast, the flexural modulus of aluminum is 10 million psi. Many applications require the rigidity of metals and thus cannot use these unreinforced thermoplastics. For instance, external fixators used to support bone fractures during the healing process must be rigid enough to maintain bone alignment despite the rigorous pressures and forces placed on them by patients—forces that would deform traditional plastics beyond acceptable limits and potentially result in poor healing of the trauma area.

A NEW BREED OF MATERIALS

A growing number of device applications—including halos, nail guides, x-ray equipment accessories, and others—demand both the physical properties of metals and the radiolucency of plastics. To meet new engineering challenges, medical manufacturers are turning toward radiolucent structural materials, which are transparent to x-rays and provide the necessary mechanical performance for the structural components used in surgical equipment and devices (Figure 1). These materials—generally composites—can provide mechanical properties competitive with those of some metals, featuring flexural moduli as high as 8 to 17 million. They are also much lighter than traditional metals, with attainable densities as little as one-half that of aluminum and one-sixth that of stainless steel. For a product such as a fixator, which patients wear following surgery, reduced weight is a major benefit.

A COMPOSITE SOLUTION

The most recent varieties of radiolucent structural materials combine the toughness of a thermoplastic matrix with the strength of carbon-fiber reinforcements. Carbon has strength and stiffness properties that, in many cases, exceed those of metals. Carbon is also inherently radiolucent. Unfortunately, carbon itself is brittle and difficult to form into complex shapes. As a fiber, however, carbon can provide substantial reinforcing properties to plastics.

Fiber reinforcement of plastics is a long-established practice. The two most common reinforcements are glass fiber and carbon fiber. Glass-fiber reinforcements are typically used in general-purpose applications because of the good balance they provide between mechanical performance and cost. Although carbon is often more expensive, it is generally more rigid and has a lower density than glass. This has resulted in carbon-fiber-reinforced plastics being the material of choice in many high-performance structural applications, such as those found in the aerospace industry. For medical uses, there is another substantial difference between carbon and glass fibers: carbon is far more radiolucent, and thus becomes the reinforcement of choice for applications that require x-ray transparency.

Regarding the plastic matrix, reinforcements can be added to either thermosets or thermoplastics. One key distinction between these two types of materials is the reprocessibility of thermoplastics. Simply stated, thermosets, such as phenolics and epoxies, cannot be remelted once formed into the desired part; but thermoplastics—including a wide range of materials such as nylons, polycarbonates, and polyketones—can be remelted and re-formed.

There are other distinctions between these material families that are more pertinent to the medical industry, however. For example, thermoplastics tend to exhibit better toughness properties than thermosets. For applications requiring high impact resistance, such as nail guides or x-ray cassettes, this can be a very important distinction. In other specific cases, thermoplastics can be selected for their superior resistance to moisture degradation, which makes them suitable for applications requiring sterilization. Additionally, thermoplastics generally have an unlimited shelf life, thus avoiding the storage and refrigeration problems associated with thermosetting polymers.

ANATOMY OF A RADIOLUCENT COMPOSITE

The broadest definition of a radiolucent composite includes the entire family of plastics that contain a fiber reinforcement to increase structural properties yet still maintain transparency to x-rays. For reasons previously discussed, the trend in medical radiolucent composites increasingly focuses on thermoplastic resins with carbon-fiber reinforcement.

Several material variables affect the performance of these thermoplastic composites. Three of the most important are the thermoplastic resin matrix, the carbon-fiber, and the manufacturing method type and orientation used.

OVERVIEW OF THERMOPLASTIC RESINS

A wide variety of thermoplastic resins are used in the medical industry. The selection process for a resin invariably begins with application requirements. Most applications have multiple requirements that must be met, which narrows the list of candidate materials considerably. Some of the most common selection criteria include chemical resistance, temperature resistance, impact resistance, elongation or flexibility, strength, stiffness, clarity, dimensional stability, and biocompatibility, among others.

Resin selection is a complicated and challenging process of matching material capabilities with design requirements. There are at least two important aspects of material selection that should always be considered during this process. First, material property enhancements often involve trade-offs. For example, the selection of a stiffer resin material frequently means the material will have less elongation.

Second, performance often has a price. The higher the performance capabilities of the material—especially if it has multiple property attributes—the higher the price. The price of the thermoplastic matrix, however, is only one component in the overall price of a composite. The cost of carbon fiber and of processing can substantially affect the price of the finished material. Relative to radiolucent composites, it is often helpful to consider polymers in four broad categories for the purpose of pricing and performance:

Commodity polymers, which include materials such as polypropylene and polyethylene, are not typically used for carbon-fiber-reinforced composites.

General-purpose polymers tend to be moderately priced while offering a good combination of properties, especially when reinforced. For instance, polyamides, commonly referred to as nylons, come in a wide variety of formulations, all of which generally exhibit a good combination of strength, toughness, chemical resistance, and low friction.

Performance polymers include materials that are higher in price yet offer unique attributes in selective performance properties. Polyetherimides, for example, exhibit a good combination of high-performance properties, including temperature and chemical resistance, dimensional stability, toughness, strength, and chemical resistance. Polyphenylene sulfides demonstrate excellent chemical and temperature resistance, electrical properties, and high strength, but they typically do not withstand impact very well.

High-performance polymers, although the highest priced of the thermoplastics, can frequently offer properties unavailable even with metals or other types of materials. An example of a high-performance polymer would be polyaryletherketone, which exhibits very high temperature and chemical resistance and has excellent toughness and strength.

OVERVIEW OF CARBON-FIBER REINFORCEMENTS

In most composite applications, mechanical strength and stiffness requirements are key factors in material selection decisions. The structural properties of thermoplastics can be significantly enhanced by the fibers added to the matrix, and there are several factors associated with the fiber that affect these properties. Fiber type, length, quantity, and orientation are a few of the important variables related to fiber selection.

Fiber Type. There are a wide variety of carbon fibers, as a result of differing manufacturing methods and end-use application requirements. Generally similar fibers can in fact demonstrate substantially different properties in areas ranging from mechanical to electrical performance. Additionally, some fibers are specially treated to enhance bonding to the polymer matrix. This bonding is sometimes necessary to maximize the mechanical characteristics of the overall composite.

Fiber Length and Orientation. The relationship between fiber length and diameter (L/D) is referred to as the aspect ratio. Fibers with higher aspect ratios typically offer improved mechanical properties when added to thermoplastics. Higher aspect ratios can be achieved by increasing the length or reducing the diameter of the fiber. For structural components, fiber length is frequently the focus for enhancing mechanical properties, and can vary from chopped to continuous.

A polymer containing chopped fibers will be considerably stronger compared with the same resin in an unreinforced state. The percentage of chopped fibers added is generally from 10 to 40% by volume. Amounts exceeding 40% may be difficult to process because of the absence of sufficient resin for flow properties. Percentages less than 10% typically do not provide adequate reinforcement. Chopped fibers are most often randomly oriented. One of the key benefits of this random orientation is the relative uniformity of properties in all directions.

Continuous-fiber reinforcement offers the highest possible strength and stiffness. The percentage of continuous fibers used is typically in the 50-65% range by volume. On the upper end, this is primarily limited by the proportional relationship of the fiber and resin, since there must again be sufficient resin to maintain a strong matrix. Continuous fibers can be oriented in different ways to achieve desired structural properties. The most common continuous-fiber orientations are unidirectional and bidirectional.

Unidirectional Continuous Fibers. The structural properties of composites with unidirectional fiber orientation vary drastically when measured parallel (0°) as opposed to perpendicular (90°) to the fibers (Table I). In the parallel direction, the overall material properties most closely resemble those of the fiber, whereas in the perpendicular direction, the properties most closely resemble those of the resin. Thermal expansion also varies depending on fiber direction. One of the more common uses for unidirectional fiber composites is in filament-wound or fiber-placed tubes.

Bidirectional Continuous Fibers.This type of fiber orientation offers a good balance of properties in two directions (50% in the parallel and 50% in perpendicular direction). Stacking unidirectional plies on top of one another with each layer turned 90° from the previous layer can form a bidirectional fiber structure. Alternatively, the fibers or groups of fibers can be woven into symmetrical patterns to form layers. These layers can then be stacked on one another (Figure 2).

MANUFACTURING OPTIONS

One of the primary benefits of the process is that it is fairly quick and can be used for plates or custom shapes that are later machined to finished form. Compression molding is often recommended for small to moderate production runs.

Fiber Placement. This process places fiber-reinforced, unidirectional tape—also known as prepreg—upon a mandrel. As the tape is laid down, it is heated and subjected to a consolidating force. Individual layers are built up until the desired thickness is achieved. Normally, this method is used to make tubes, although more-complex geometries are also possible. Fiber placement is a highly automated process that can be tightly controlled for high repeatability and precision.

Injection Molding. This injection molding process melts resin and forces material to flow into a mold to form finished parts. Because of the equipment used for the process, only chopped fibers can be employed for injection molding. This method offers higher physical properties when compared to chopped-fiber, compression-molded parts and is a good approach for large quantities or precision-tolerance parts.

Extrusion. Processing via extrusion produces shapes of standard cross sections. As with injection molding, the equipment can manage only chopped-fiber reinforcements. But unlike injection molding, which typically involves high injection pressures, extrusion uses lower pressures, which enhances the mechanical performance of the parts. Accordingly, the physical properties of chopped-fiber-reinforced parts made by extrusion are typically lower than those of similar parts made with injection molding. Generally, the structural performance of these components lies between that of injection-molded and compression-molded parts made from the same material. The extrusion process can be used to make large, rectangular plates and some standard cross-section shapes. In addition, extrusion is an economical approach for prototype development.

MATERIAL SELECTION PARAMETERS

Selecting the right radiolucent composite requires an understanding of the inherent advantages and limitations of the different fiber types and processing methods. Again, some fiber types are compatible only with certain processing methods (Table II). In turn, these processing methods can have an impact on part economics (Table III). The processing method and fiber type can also have a substantial effect on the characteristics of the finished part (Table IV).

The most advanced radiolucent composites combine high-performance polymers and bidirectional carbon fibers to form uniformly sound structural parts able to withstand aggressive use as well as multiple sterilization cycles. Many of these components can be designed for reuse in surgical tools. Table V compares the properties of one such material with those of aluminum.

CONCLUSION

Innovative materials are increasingly among the critical elements required for product development and technology breakthroughs in the medical industry. A clear example of this can be found in the advanced procedures incorporating the latest x-ray technology that have created the need for radiolucent structural materials. These composites of thermoplastic polymers and carbon fibers are transparent to x-rays and provide structural properties that, in many cases, are competitive with those of metals. With a thorough understanding of the candidate polymers, fiber types, and processing methods, designers can select a radiolucent composite to satisfy many of their daunting materials-related challenges.

Photos courtesy of GREENE, TWEED, AND CO. (KULPSVILLE, PA)

Copyright ©2001 Medical Device and Diagnostic Industry