Note: This is the third article of a three-part series covering medical plastics (1) trends, (2) material/process advances and (3) applications.

The U.S. market for the medical sector is the largest and most sophisticated in the world. As a rule of thumb, the U.S. medical device market and most of its segments represent about half of the world market (the European market representing 25 percent).

This market is expected to grow at a compound annual rate of 8 percent over the next three years. The medical fields having the greatest impact on durable medical devices across these five areas are cardiovascular, orthopedics, minimally-invasive surgery and diagnostics.

Of particular significance to the durable medical device market are the five following areas:

  • surgical equipment and instruments
  • diagnostic systems
  • general hospital technologies
  • therapeutic devices
  • self-care and rehabilitation products

Blood therapy and cardiovascular management devices are an interesting starting point for durable medical applications. There are more than 1 million blood therapy treatments annually in North America, Asia and Japan.

In some cases, the single-use systems involve four or five molded components. Highly publicized studies in recent years on the potential health effects of bisphenol A (BPA) — a feedstock used to make polycarbonate (PC) are controversial, though the U.S. FDA has made no ruling that would restrict the use of polycarbonate in medical applications.

However, there is an increased focus on extractables in medical plastics applications. As the issue of extractables in healthcare heats up, some manufacturers of blood therapy and management equipment are exploring conversions from polycarbonate to copolyester compounds to avoid potential contamination from BPA.

Tritan copolyester for blood therapy surgical and blood management devices have been successfully commercialized by Eastman Chemical Company. The copolyester can be used in many blood therapy devices, including surgical devices such as oxygenators, hematological reservoirs, hematological filters, cardioplegia filters and bubble trap devices.

Eastman Chemical
Blood therapy management device (left), and Tritan copolyester vs. polycarbonate gamma sterilization comparison (right).

Additionally, it has proved suitable and made commercial inroads into such blood management devices as blood separation cassettes, blood micro filters or centrifugal devices. The material is BPA-free and also has:

  • Good clarity — allowing easier detection of air bubbles, blood clotting and leakage.
  • Chemical resistance — remains aesthetic and functional after exposure to blood, lipids and aggressive disinfectants. This also means improved environmental stress crack (ESC) resistance during solvent bonding that uses cyclohexanone or other solvents.
  • Color stability
  • Good processability
  • Toughness — this performance feature of Tritan copolyester compares favorably with Polycarbonate (PC) and offers significantly better impact strength than other common thermoplastics.
  • Eliminates annealing required for PC — has a lower inherent residual stress compared to PC.
  • Retains glasslike transparency after gamma or electron beam radiation and sterilization with ethylene oxide gas — Tritan has maintained its strong reputation as a virtual material drop in for PC.

Next, let's take a look at the use of PEEK (polyetheretherketone) hip socket development that targets metal failures. Surgical hip replacement is one of the most frequent operations carried out in Europe and North America (200,000 per year in Germany and 285,000 per year in the U.S.).

Some hip replacements metal-on-metal implants in particular that are not positioned optimally are often susceptible to premature failure. Some physicians are calling for a ban on artificial joints made of cobalt-chromium alloys in which the joint's metal ball rubs against its metal socket.

Poorly designed or positioned metal-on-metal implants can lead to higher wear rates and that release elevated cobalt-chromium ion levels that spread through the blood and lymph, potentially damaging organs and triggering inflammation. Metal ions are also suspected carcinogens.

The Fraunhofer Institute IPA partnering in the European Commission-funded "ENDURE" (ENhanced DUrability Resurfacing Endoprosthesis) project has developed a novel kind of hip implant a metal-free solution that has bone-like elasticity. The high-tech composite hip socket is made of carbon-fiber-reinforced PEEK, a high-strength, wear resistant, biocompatible polymer composite.

For the femoral head, ceramic is used. A hydroxylapatite coating at the bone interface helps ensure bone tissue will fuse with the implant surface structure.

Fraunhofer Institute
Fraunhofer's experimental artificial hip.

Cobalt-chromium implants still in use are rigid, and load transfer to the bone is nonoptimal, leading to potential adverse bone adaptation. By using this new material combination, force transmission through the PEEK plastic hip socket to the pelvic bone replicates natural conditions, and there are no metal ions released.

The development team was able to confirm the good wear resistance in initial tests of the novel hip replacement using a robot that simulated various series of movements.

The ENDURE implants follow the bone-preserving principle using thin-walled shells that replace the bearing surface of the joint articulation alone, instead of employing large metal stems for support, which require a substantial volume of bone to be removed.

Partners in the EC-funded project are Aurora Medical, Medicoat, Hunt Developments, Ala Ortho, CeramTec, Invibio, Biomatech, and the Universities of Gothenburg and Southampton.

Elsewhere, researchers at the University of Glasgow are also developing an orthopedic implant using PEEK. The implant is covered with a special surface consisting of tiny pits 120 nanometers in diameter. The pitted surface encourages stem cells to differentiate into bone cells and fuse to the implant, creating a much stronger bond between bone and implant.

Finally, from a self-care and rehabilitation perspective enhanced wheelchair safety is becoming a product development priority. The healthcare industry's first latching device, originally developed by Transfer Solutions LLC, secures the wheelchair during transfers to a fixed object like a bed, toilet or shower. It is the first product to address wheelchair tips during transfers.

The patent-pending device is made of Ixef 1022 polyarylamide (PARA) from Solvay Specialty Polymers. Translock features a "lock and dock" design with a pivotal arm clamp activated by a lever button or contact and pressure during a docking operation.

Transfer Solutions
Wheelchair lock-and-dock design (left) with a pivotal arm clamp (right).

Approximately, 80 percent of the lock-and-dock components installed on the wheelchair and related fixed objects (i.e., bed, toilet, shower, wall, etc.) are injection-molded of Ixef PARA. Ixef 1022 is a 50 percent glass-filled PARA compound that combines high stiffness and an ultrasmooth finish. The material is said to process well, despite its high glass loading.