Originally Published MPMN November 2006

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Biocompatible Material Has Properties of Both Metals and Polymers

Shana Leonard

As the demand for advanced implantable devices increases, the debate rages on as to which material is optimal for these in vivo applications. Biomaterial provider Invibio (Greenville, SC) has created a composite for implantables that boasts the strength of a metal alloy coupled with the imaging properties and biocompatibility of a polymer.

The Endolign composite is a carbon-reinforced thermoplastic material resulting from a marriage between the PEEK-Optima polymer matrix—the company’s flagship product—and high levels of continuous carbon fiber. By using continuous carbon fibers, rather than traditionally used chopped fibers, at rates of 50–60%, the product exhibits improved strength, stiffness, and fatigue resistance over materials with lower percentages of the fibers, 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 need to start looking at truly load-bearing applications that have very high strength or stiffness requirements and concerns with loading and fatigue, there really isn’t another product out there that offers the benefits that ours does.”

Introducing higher levels of carbon to PEEK-Optima increases the product’s flexural strength when tested in molded unidirectional rod form from 150 to more than 1000 MPa, and its stiffness from 3.5 to 150 GPa, according to the company.

In addition to increased strength, a benefit of the combined properties of the material is that of its imaging characteristics. Although Endolign features 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.

“Another advantage is that with metal, there are sometimes concerns with metal-ion sensitivities and allergic reactions; we don’t have that,” Callahan adds.

Endolign is offered in preimpregnated-tape form, which can be molded into rods or other shapes. Invibio also counts among the product’s strengths its ability to be repeatedly sterilized, using such methods as steam, gamma radiation, and EtO processes, without detriment.

Suited for load-bearing applications requiring 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 FDA approval.

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