Enhancing Medical Device Performance with Nanocomposite Polymers

Originally Published MDDI May 2002

MEDICAL PLASTICS

Advances in compounding medical plastics with nanoclay fillers are pushing the materials envelope for minimally invasive devices.

Lawrence A. Acquarulo Jr. and Charles J. O'Neil

If ever there was an example of the union of art and science, it is in today's most advanced medical devices. Highly engineered devices allow physicians to perform procedures noninvasively that were previously possible only through surgery.

Because so much sensitivity is required to maneuver these devices, engineers have been exploring methods to modify responsiveness to pressure and torque—sometimes even when deployed over long distances, such as with catheters. For designers of medical devices, the challenge is to create devices with improved feel while continually striving to reduce device size.

To date, design and manufacturing techniques have played major roles in the advance of precision medical devices; however, engineers are now recognizing how modifications in a device's material can provide significant enhancements and increased precision in performance.

Polymers are key building blocks in the development of medical devices, and dozens of polymers offer unique properties for specific applications. An increased matrix of material options is available through the use of polymer compounds. These compounds incorporate performance-enhancing fillers, which provide properties not possible with ordinary polymers alone (e.g., bending stiffness, tensile strength, elongation, or torque).

In many medical applications the enhancements required are mechanical in nature. Traditional polymer-reinforcing fillers include glass, carbon, and other fibrous materials. Advanced medical procedures, however, may involve such devices as catheters and stent-delivery balloons, which require extraordinarily thin walls and smooth surfaces. The traditional fillers are far too large to provide homogenous compounds suitable for these thin sections. Thus, many medical manufacturers have not taken advantage of the benefits of traditional polymer reinforcement technology to enhance their product's performance.

MATERIAL LIMITATIONS

Previous attempts to reduce the particle size of traditional fillers (to accommodate thin sections) have had limited success. The critical factor in the success of polymer reinforcement is the aspect ratio (or the length-to-thickness ratio). Additives with aspect ratios of less than 20:1 do not generally provide sufficient strength enhancements to be considered reinforcing agents. Thus, reduction in traditional fiber size through length reduction alone is insufficient. When reinforcing agents are reduced substantially in all directions, there is often an agglomeration between the small particles; this creates difficulty dispersing the filler in the polymer melts. Ultimately, poor dispersion of the reinforcement can create unpredictable and often deficient performance in the finished part.

NEW RESOURCES

Most composite manufacturers are well aware of the benefits of high-aspect-ratio fillers for reinforcing polymers. But only recently have they become aware of the opportunities available through the addition to the polymers of nanometer-size particles with high-aspect ratios; these polymers are referred to as nanocomposites.

Although nanocomposites were first referenced as early as 1950 and polyamide nanocomposites were reported as early as 1976, it wasn't until Toyota research labs began working with polymer-layered silicate-clay mineral composites in the early 1990s that nanocomposites became more widely studied. And only in the past several years have these compounds become commercially available.

Today there are a variety of nanofillers for use in nanocomposites. Cost and availability continue to change, as the field is relatively new and several of these fillers are still being developed. The most common types of nanofillers for plastics, and their approximate costs, include the following:

As the number of larger-volume applications increase in future years, improved supplier economies of scale will likely decrease the costs of nanofillers. Applications are now being evaluated in packaging, automotive, and wire and cable that could result in substantial demand for these fillers.

NANOCLAYS

Due to cost and availability, there is currently a great deal of focus on nanoclay fillers. These fillers are slightly more expensive than glass, yet generally much less expensive than carbon. Additionally, the small amount of nanofillers required to enhance properties enables these materials to compete more effectively with traditional glass-fiber reinforcements.

Nanoclays are minerals with a high-aspect ratio and with at least one dimension of the particle in the nanometer range. Reinforcements in the nanometer size range closely approach the molecular size of the polymer. This makes for an intimate encounter between the two materials. When properly modified, the filler particles and polymer interact to create constrained regions at the particle surface. This immobilizes a portion of the polymer chain, creating a reinforcement effect.

Purity and cation-exchange capacity are two characteristics critical to nanoclays' success as polymer-reinforcing agents. Purity is important in achieving maximum increases in mechanical properties; impurities act as stress concentrators, resulting in poor impact and tensile properties. Cation-exchange capacity provides the surface activity necessary for acceptance of surface treatments to the clay. Such treatments are essential to allow the small particles to be effectively accepted and dispersed in the polymer matrix.

NANOCLAY REINFORCEMENTS

The most important factor in the success of polymer reinforcement is the aspect ratio of the clay particle. Clays with a platy structure and a thickness of <1 nm are optimal. The length and width of these choice clays are in the micron range, with aspect ratios between 300:1 and 1500:1.

The surface area of the exfoliated platelets is usually in the range of 700 m²/g. The nanoclays of commercial interest to date are hydrotalcite, montmorillonite, mica fluoride, and octasilicate. Hydrotalcite and octasilicate have limits of use both from a physical and a cost standpoint. Mica fluoride is synthetic clay, while montmorillonite is natural.

Montmorillonite. Montmorillonite has the widest acceptability for use in polymers. It is a type of smectite clay that can absorb water, and it is a layered structure, with aluminum octahedron sandwiched between two layers of silicon tetrahedron. Each layered sheet is slightly less than 1 nm thin (10 Å), with surface dimensions extending to about 1 µm or 1000 nm. The aspect ratio is about 1000 to 1 and the surface area is in the range of 750 m²/g.

Montmorillonite clays are relatively common throughout the world. Deposits of commercial clays are referred to as bentonite, which generally contains in excess of 50% montmorillonite. Conventional purification methods are adequate for the clays used in most common applications, such as binders for metal casting, well-drilling legs, and cosmetics.

Because montmorillonite clay is hydrophilic, it is not inherently compatible with most polymers and must be chemically modified to make its surface more hydrophobic. The most widely used surface treatments are ammonium cations, which can be exchanged for existing cations already on the surface of the clay. The treatments work on the clay to minimize the attractive forces between the agglomerated platelets.

The process of separating the nanoclay platelets is referred to as the intercalation process (see Figure 1). Without this separation, the nanoclay would not be capable of allowing the polymer to penetrate the platelet layers.

In the exfoliated form, nanofillers have a very small flexible-platelet-type structure. The thickness of the platelet is in the nanometer range, while the length and width are between 0.1 and 2 µm. Because of this, a single gram of exfoliated nanoclay will contain over a million individual particles.

NANOCOMPOSITE PRODUCTION

Because a nanofiller contains so many individual particles in such a small amount of material, it requires very low loading to obtain a high concentration of constrained areas within the polymer. At 5% loading, this leads to reinforcing effects equal to about 12 to 15% glass fiber. The nanofiller also creates a tortuous path for the penetration of gaseous vapors and liquids into the polymer. In turn, this leads to better chemical and moisture resistance.

Previously, most nanoclay research involved the incorporation of a nanoclay into a polymer during the polymerization process. With this method, exfoliation or dispersion of the clay particles is more approachable. But the batch sizes required for polymerization—and therefore the cost—has resulted in challenging economics for many commercial applications. To date, only a few commercially available compounds have been made using this method.

Melt Compounding. An alternative approach involves melt compounding the nanoclay and polymer. This method offers significant promise, since the modifying of plastic compounds is a custom business and well suited for medical device application variations. In combination with the pretreatment of the clay itself, the compounding parameters and mixing-screw profile are important variables. Since the clay is an aggregate of thousands of individual platelets, it is critical that those platelets be separated prior to compounding. If the platelets are not separated, it is unlikely that the shear forces generated during the compounding process will be sufficient to overcome the forces holding the aggregates together. Hence, less-than-optimum exfoliation or dispersion will result. The process of opening up the spaces between the plates, which are known as galleries, is called intercalation.

The right clay treatment is required to allow for intercalation; after that, it is necessary to make the intercalate compatible with the host polymer so some properties are enhanced without sacrificing others. This allows for improvement in the strength without embrittling the plastic. Secondly, melt compounding must occur such that the prepared clays get maximum dispersion without degrading the polymer. Today, several nanoclay compounds are commercially available and have been used successfully.

BENEFITS OF NANOCOMPOSITES

Today's nanocomposites typically demonstrate unique improvements in material properties, including rigidity, strength, and barrier characteristics, while maintaining a level of transparency and offering the potential for recyclability.

Increased Rigidity. As shown in Figure 2, nanocomposite polymers offer increased rigidity and stiffness while maintaining a high degree of the elongation inherent in the base polymer. Increased stiffness without brittleness is essential for many catheter applications, which require torque and push/pull strength without kinking. In addition, new dilation balloons are required to withstand higher pressures without tearing, and may also be excellent candidates for improvement in mechanical performance from nanocomposite technology.

Permeation Resistance. The platelet structure of the reinforcing fillers in nanocomposites may offer improvement in barrier properties of the polymer compound. To date, much commercial interest in nanocomposites barriers has come from packaging companies.

Transparency. With nanocomposites, low loadings and filler dispersion create compounded materials that maintain inherent polymer transparency in thin sections. This is an additional benefit for packaging and film applications.

Recyclability. A major difference in compounding nanoclays versus other types of reinforcement fillers is that with nanoclays, typical fibers are broken down during the high-shear compounding operation but nanoparticles are not affected or degraded during the process. This allows nanocomposites to be recycled and reprocessed without seriously affecting the physical properties.

Performance enhancements with nanoclays vary among polymers. For instance, nylons will accept nanoclays more readily than will polypropylene, in which dispersion is currently difficult to achieve.

NANOCOMPOSITES IN MEDICAL DEVICE APPLICATIONS

Although nanocomposites have been around for a number of years, it has only been recently that researchers discovered the nanoclay purification requirements for maximizing the performance of these compounds. This is due to the high levels of amorphous silica present in nanoclays; the silica substantially degrades the impact and elongation properties of the composite and also increases the haze in film. The true value of nanocomposites in medical applications is only beginning to be discovered.

To date, many healthcare applications for nanocomposites have involved nylons, due to the prolific use of nylons and the substantial mechanical benefits provided by the nanoclay additives (see Table I). Other materials currently being evaluated for such applications include polycarbonates, polyolefins, polyurethanes, and thermoplastic elastomers. As medical device companies begin to learn more about the advantages nanocomposites offer in tailoring material properties, the number and diversity of polymer formulations will grow substantially.

Medical products receiving notoriety as candidates for nanocomposites are shafts, balloons, catheter luers, and similarly precise device components. Many of these applications require precise volumes of material. The application of nanocomposites in large-volume applications, such as laparoscopic tubing, would likely further improve the economies of these materials in the medical device market.

CONCLUSION

Much of the discussion of nanocomposites is of a micro nature, since the additives involved are extremely small. Yet the macro view of these materials is perhaps the most appealing to medical device designers. Due to the effective dispersion of nanosize particles, these materials offer more homogenous performance than traditional compounds that use larger fillers.

Each new nanocomposite formulation is likely to be viewed as a uniquely different polymer with its own appearance, processing, and performance characteristics. To the designer tailoring medical devices, this will effectively mean there will be an increasing portfolio of polymers to choose from. This increased selection of medical materials could create a new wave in the development of precision devices.

Lawrence Acquarulo Jr. is the president and Charles O'Neil is the chief R&D engineer of Foster Corp. (Dayville, CT). Both are principals in Foster's applied development group.

Copyright ©2002 Medical Device & Diagnostic Industry