MATERIALS
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A composite material for implantable devices exhibits the strength of metal, but the biocompatibility and imaging characteristics of a polymer.
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A composite material suited for implantation couples the strength of a metal alloy with the imaging and biocompatibility properties of a polymer. The carbon-reinforced thermoplastic material, called Endolign, is offered by Invibio Ltd. (Thornton Cleveleys, Lancs, UK).
The composite is the product of a marriage between the PEEK-Optima polymer matrix—the company’s flagship product—and high levels of continuous carbon fibre. Rather than the chopped fibres traditionally used, the material incorporates 50 to 60% continuous carbon fibres. This results in improved strength, stiffness, and fatigue resistance compared with materials that have a lower fibre content, according to Invibio president Michael Callahan.
“There are certain applications where standard polymers have only a certain amount of strength,” Callahan says. “Our core product PEEK-Optima serves a certain market segment. But when you start looking at true load-bearing applications that have very high strength or stiffness requirements . . . there really isn’t a product like [our composite].”
Introducing higher levels of carbon to PEEK-Optima increased the product’s flexural strength from 150 to >1000 Mpa (when tested in moulded unidirectional-rod form) and improved its stiffness from 3.5 to 150 GPa, according to the company.
In addition to increased strength, the material’s combined properties also affect its imaging characteristics. Although Endolign features the mechanical properties of metals such as cobalt chromium, titanium, and stainless steel, it is compatible with MRI because of its polymer composition. Owing to its radiolucent nature, the composite also enables clear visualization without artifacts through such imaging methods as computed tomography and x-ray.
“The other advantage is that with metal, there are sometimes concerns with metal-ion sensitivities and allergic reactions,” Callahan adds. “We don’t have that.”
Endolign is offered in preimpregnated-tape form, which can be moulded into rods or other shapes. The material can withstand repeated steam, gamma, and EtO sterilization.
Suited for load-bearing applications involving blood, bone, or tissue contact for more than 30 days, the composite can be employed in such devices as translaminar fixation pins, spinal cages, bone fracture plates, and intermedullary nails. The material has received the CE mark and US FDA approval.
