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| Development in Medical-Grade Materials |
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Medical device designers can choose materials based on information in a powerful new database or adopt innovative formulations with a variety of desirable properties.
By Shana Leonard and Norbert Sparrow
European Medical Device Manufacturer Magazine
October 2007
The vital role that materials research plays in medical technology can’t be overstated. To develop effective, safe devices, design engineers and manufacturers must have access to precise information on material properties. Granta design (Cambridge, UK; www.grantadesign.com), a materials information technology firm with its roots in Cambridge University, and its partner ASM International (Materials Park, OH, USA; http://asmcommunity.asminternational.org), the world’s largest professional society for materials engineering, have developed a tool to effectively manage data related to medical-grade materials.
Leveraging online resources, Granta Design has taken material information management systems initially used by the aerospace industry and adapted them to medical technology. “Aerospace and the medical industry have similar needs in terms of quality of information and traceability,” says Dave Cebon, director of Granta Design and professor at Cambridge University. “They use a lot of materials in common. The software system we buildt for aerospace apply very well to the medical sector.” Cebon is one of the authors of the materials for Medical Devices Database.
The idea for the database had its origin in Cebon’s work as a professor at Cambridge. “I was looking at the information needs of medical device designers—seeing what was available and trying to determine how to make it usable,” says Cebon. In the course of the research, he discovered an interesting aspect of materials science as it applies to medical technology.
“Medical device designers may be engineers, but they may be also clinicians,” says Cebon. Engineers don’t know that much about the biological aspect of materials, and clinicians don’t know much about the engineering aspects.” Materials can represent a point of convergence for these communities, notes Cebon. “Developing a programme that can meet the needs of engineers and clinicians was one of the objectives of the project.” The team at Granta Design and ASM achieved this goal by establishing different ways of accessing data.
If you are a clinician who wants to design a new device, one way to begin is to find [a predicate] device and determine how it is made,” says Cebon. “That information is in the database. Data for [existing] devices include information about the materials used, coating, drugs that might be embedded, the production processes, and so forth.” An engineer will need similar information, he adds, but he may come at it from a different direction. “His questions may be about fatigue strength or stiffness requirements, a material’s biocompatibility in particular environments or its processability,” says Cebon.
One of the system’s most powerful tools is the capability to search for materials based on biocompatibility. That is difficult property to deal with,” says Cebon, “because it’s not the materials that are approved by US FDA, but the device itself. Using this database, if you can search for materials used in devices that have been approved by US FDA or that passed specific ISO10993 tests.”
The first database module, devoted to implantable cardiovascular devices, is now available. The volume includes more than 250,000 words of literature reviews, notes Cebon, and references thousands of journal articles. The preliminary release of the second module, which will focus on orthopaedics, is scheduled to launch by the time this issue prints.
Two-Shot Moulding, Silicone Rubber work Hand in Hand
While Granta and ASM attempt to catalogue existing medical materials, this dynamic sector continues to expand as a new formulations are developed. Recent examples include a self- bonding silicone rubber that improves product performance and custome thermoplastic elastomers (TPEs) with enhanced oxygen-barrier properties.
BioSil SB self-bonding silicone rubber that bonds to thermoplastics reduces scrap and can enhance the overall performance of medical parts made from multiple materials, according to Saint-Gobain Performance Plastics (SGPPL; Charny, France). The company’s two-shot moulding capability enables the silicone to be efficiently processed with substrate materials such as polycarbonate, PEEK, polyester, polyimide and phenolic resin.
The comoulding process eliminates having to mould the silicone and thermoplastic parts in separate operations and assembling them manually. It also fosters creativity, according to the firm.
Moulding a multimaterial parts in one press, under one process, provides design engineers with greater design freedom compared with other methods, explains Sarah Gonnering, marketing coordinator for SGPPL in the United States. “Two-shot moulding also provides savings in validation costs and a reduction in vendors,” she says.
Seals, caps, septums and soft-touch instruments are among the medical applications suited for this particular moulding process. The technology is suited for any silicone and thermoplastic device component that is typically assembled or over moulded, as well as designs that demands complex assembly, according to Gonnering.
Custom TPEs Reach Elevated Oxygen-Barrier Threshold
Initially engineered for food packaging, custom-formulated TPEs that reportedly provide 10 times better oxygen-barrier performance than conventional TPEs may also have a future in healthcare applications. GLS Corp. (McHenry, IL, USA) previewed the materials at the MD&M East show in New York in June.
The products offer numerous advantages over thermosets, which have traditionally reigned as the materials of choice for barrier applications, says the firm. In addition to the TPEs’ barrier quality, their clarity and cleanliness are prized by med-tech OEMs. Cleanliness is ensured via elimination of the leachable halogens and heavy metals used to cure thermoset rubber.
Owing to their inert nature, TPEs can be used for tubing applications, and can act as a subtitute for silicone. Though valued for its rubbery properties and clarity, silicone is viewed as a poor material for barrier applications. Design engineers could use TPEs in lieu of silicone in order to improve barrier properties in devices, according to Raj Varma, commercial innovation manager for GLS. They are also a suitable replacement for butyl rubber, according to Varma.
Butyl rubber and TPEs share similar barrier properties, says Varma, but the TPEs do not require multiple production steps. Each operation compromises quality, he notes. Varma points out that if each step yields 99 % accuracy, quality and accuracy are diminished after each additional stage of a process. “But with these [GLS] materials, the part that you injection mould or extrude comes out usable. By overmoulding, you consolidate all the steps,” he says.
As [industry] goes more toward disposables, [these TPEs allow] you to complete your disposable product without having togo through various assembly steps.”
Because TPEs can be processed by means of high-pressure injection moulding, extrusion, and blow moulding, the materials provide a great deal of design flexibility. Unlike thermoset rubbers, TPEs do not require reinforcing fillers, thus allowing them to maintain low specific gravity and clarity. Moreover, they can bond to polypropylene substrates, which opens up opportunities for part consolidation and design innovation.
Tungsten-Filled Compounds
Another recent product introduction is of special interest to OEMs that manufacture diagnostic catheters and similar devices that require a smooth surface and radiopacity. Tungsten-filled thermoplastic urethanes, nylons, olefins, and other resins are available from New England Urethane Inc. (North Haven, CT, USA; www.neuinc.com), a compounder of engineering plastics and polyurethanes.
In addition to producing smooth surfaces, the resins are easy to process by means of injection moulding or extrusion. The materials resist solvents, soften at body temperature, and have high specific gravity. They are available in shore hardness ranges from 75A to 75D, and can be ordered in quantities ranging from 25 to 500,000 lb.
Breaking the colour Barrier
Standard lines of universal masterbatch and precoloured resin products meeting the test standards of ISO 10993-1 for biocompatibility are available from a global compounder of thermoplastics. The standard and custom colour solutions are suited for use in medical products and related packaging.
The products have passed tests for cytotoxocity (part 5), irritation and delayed-type hypersensitivity (part 10), and systemic toxicity (part11). The pretested materials can reduce time to market, and save time and money that may be expended resubmitting products for testing, according to RTP Co. (Beaune, France). “Adding our standard biocompatible colours to [a company’s own] USP Class VI resins makes changing colours to refresh a product line much easier,” says global colour general manager Jean Sirois.
Masterbatch and pretested colour products can be developed for use with almost any resin, including styrenics, olefins, elastomers, nylons, PVC, engineering resins, Radel R, and other high temperature resins.
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