Lightweight yet resistant, composites can be a competitive alternative to metals and alloys for the fabrication of devices.
By: N. Bernet, Composites Busch SA, Porrentruy, Switzerland
Reinforced plastics and advanced composites
Major advances have been made in the development of polymer matrix composites during the past three decades. These types of materials consist of organic polymers reinforced with short or continuous fibres to impart strength and stiffness.
Polymer matrix composites are usually divided into two categories: reinforced plastics and advanced composites.
Advanced composites, also called high-performance composites, contain a large percentage (approximately 60% by volume) of highly resistant continuous fibres usually composed of carbon, glass or aramid materials. The aerospace industry in Europe and the United States has been the predominant driver of research in advanced composite materials. High-performance composites are attractive to the aerospace industry because of their stiffness and strength coupled with their light weight. Weight savings lead to improved fuel efficiency and allow the construction of larger aircrafts, capable of transporting more passengers with increased cargo payloads at greater speeds. Other beneficial features of composites include corrosion and fatigue resistance. Consequently, the aerospace industry, public institutions and academia have poured substantial R&D resources into this class of materials. Now that composites have been well characterised, and given the broad spectrum of polymers and reinforcements that are available, the materials are being used in other industries, and notably in the medical technology sector.
Benefits for medical uses
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Figure 1: Comparison of physical and mechanical properties between composites and metals used in implants, surgical instruments and orthopaedic devices.
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As in aerospace applications, composites used in medical instruments must meet stringent standards and exact end-use requirements. Composites increasingly have become a competitive alternative to the metals and metallic alloys that are traditionally used in medical devices. In order to compete with materials such as aluminium, stainless steel and titanium, special composites (principally based on carbon fibres) have been developed for hospital equipment, surgical instruments, orthopaedic products and biocompatible implants. Composites have several advantages over metals. In particular, composites are:
- lightweight and very resistant (see Figure 1)
- anisotropic (allowing different properties and functionalities to be obtained in different part areas or directions)
- easily designed as wanted (allowing, for instance, the elastic stiffness of an implant to be tailored to the stiffness of the bone to which it is attached, so that the bone continues to bear load and does not resorb because of an absence of mechanical loading)
- unlikely to cause allergic reactions by the release of metal ions
- x-ray translucent (permitting optimal visualisation and interpretation of radiographic images)
- pleasant to the touch (providing a warm feeling in contact with the skin)
- and aesthetically appealing.
Moreover, composite parts that withstand repeated sterilisation cycles without deterioration of their performance, aesthetical aspect or geometrical accuracy can be obtained through an appropriate selection of the polymer matrix and its constituents.
Breaking down manufacturing costs
While the aforementioned advantages of composite materials may be compelling for the patient and surgeon, the commercial motivation to switch to composites remains a critical practical issue. Cost is often the biggest barrier to the use of composites. Because of high manufacturing costs, composite components in many cases are more expensive than metallic ones. It is therefore very important to think about manufacturing-related cost-reduction strategies early in the development phase.
| Table I: The relative cost and properties of two polymer matrices used in composites for medical applications. |
| Property |
Epoxy Thermoset |
PEEK Thermoplastic |
| Material Price |
Low |
High |
| Processing Temperature |
Medium |
High |
| Sterilisation Resistance |
High |
High |
| Biocompatibility—short-term tissue contact |
High |
High |
Biocompatibility—long term
tissue contact |
Low |
High |
| Impact Resistance |
Medium |
High |
| Colour Transparency |
High |
Low |
|
The manufacturing costs for composites can be broken down into four subcategories: materials, processing, assembly and inspection.6
The material costs for medical products tend to be high. This is primarily a result of the high cost of the fibres, especially when high-modulus carbon fibres are used. The cost of the polymer matrix can vary widely, depending on the nature of the polymer needed to fulfil the requirements of a specific medical application (see Table I).
Processing costs also can vary widely. For a given process, costs are driven by part design (size and complexity), production volumes and how the process is run. Strategies to reduce processing costs may include shortening the polymer cure cycle for thermoset composites or increasing the heating or cooling rate before or after shaping for thermoplastic composites, and automating labour-intensive steps.
Process automation also can help reduce inspection costs by generating fewer quality variations.
As regards assembly costs, they can be drastically reduced or even eliminated by a redesign of the part that results in the integration of several components. This has been the key to success in such processes as injection moulding, compression moulding and resin transfer moulding, where complex-shaped parts may be fabricated in one shot. Table II gives an overview of the influence of the selected process, fibre type and fibre length on the finished part in terms of shape complexity, stiffness and radiolucency.
The risk factor
Once benefits and costs have been assessed, and both technical and economic feasibility have been demonstrated, realistic and profitable implementation of composites is conceivable. But there is one more important parameter that needs to be considered for the successful incorporation of a particular composite material, associated with a particular composite manufacturing technique, into a desired medical application: the risk factor.
| Table II: Material and process selection criteria. |
| Fibre Type |
Fibre Length |
Manufacturing
Process |
Part Shape
Complexity |
Intrinsic Stiffness |
X-Ray Visibility |
| Glass |
Long and Continuous |
Compression Moulding |
High |
Low |
Radio-Opaque |
| Carbon |
Long and Continuous |
Compression Moulding |
High |
High |
Radiolucent |
| Resin Transfer Moulding |
High |
Medium |
Radiolucent |
| Filament Winding |
Limited |
Medium |
Radiolucent |
| Pultrusion |
Limited |
High |
Radiolucent |
| Long and Discontinuous |
Compression Moulding |
High |
Medium |
Radiolucent |
| Short |
Injection Moulding |
Very High |
Low |
Radiolucent |
The risk of introducing a new technology must be acknowledged, understood, assessed and perfectly managed, especially when it comes to public health. Improved product performance and quality assurance help to minimise risk, but at a cost premium. Advantages, costs and risk thus are interconnected and all will have to be considered, not only from the technology sphere (mechanical performance, cytotoxicity, durability and reliability, disposal and so forth), but also from a strategic perspective (engineering skills, supply chain management, market competitiveness and so forth). Hence, the desire to use composites in a new application will have wide-ranging implications. The advice of specialists from composite manufacturing companies can play an essential role in successfully implementing the use of composites in the medical industry.
References
1. Technical data sheet, AK Steel Corp., West Chester, OH, USA
2. Technical data sheet, Aubert & Duval, Gennevilliers, France
3. Technical data sheet, Fine Tubes Ltd, Plymouth, Devon, UK
4. Technical data sheet, Alcoa Inc., Pittsburgh, PA, USA
5. Internal data
6. T.G. Gutowski, Advanced Composites Manufacturing, Wiley-Interscience, New York, NY, USA (1997).
Nicolas Bernet, PhD,
is Medical Business Manager at
Composites Busch SA
CH-2900 Porrentruy, Switzerland
tel: +41 32 465 7030
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