A second focus of the Project House is VESTAKEEP^®. “In chemical terms this is a biocompatible polyetheretherketone,” says Evonik expert Marc Knebel, who is responsible for Medical Devices & Systems in the High Performance Polymers Business Line. “It is used in orthopedic implants that will remain in the mouth, jaw, skull, and, in particular, the spine over the long term.”

A typical VESTAKEEP^® product is an intervertebral disc implant. These look like small cages with a central cavity that is important for correct functioning; that’s why they are also known as spine cages. For a slipped disc, doctors may advise removing the disc and replacing it with an implant of this type to restore the natural distance between the vertebrae,” says Knebel. “Over time the bone material then grows into the cage, bridging neighboring vertebrae.”

As Knebel explains, VESTAKEEP^® is particularly suitable here due to the properties of the material: “The elasticity of VESTAKEEP^® is comparable to that of bone, so that implants of this material prevent stress shielding of adjacent vertebrae.”

However, a challenge with spine cages is that they may migrate, or subside, in the post-implantation phase. “Until ossification is sufficiently well advanced, the ridged surface of the implant ensures adequate stability through its clamping action,” says Knebel. The VESTAKEEP^® experts have even devised a trick to accelerate the healing process: A composite of VESTAKEEP^® and the biomineral hydroxyapatite is expected to facilitate the growth of bone cells on the implant surface, allowing the vertebral bodies to fuse faster.

Here again, the temperature that is required in the production process presents a major obstacle, since it must be above 400°C. The solution is a modified hydroxyapatite that is easily processed at high temperatures and ensures very good bone growth. The process is currently being ramped up to commercial scale, and the launch of the new composite is slated for 2019. Its name, VESTAKEEP^® Osteoconductive, indicates the ability of the composite to act as a scaffold and facilitate natural bone growth.

Evonik’s researchers in Birmingham are also pursuing the idea of developing printable RESOMER^® polymers for medical technology. Should a patient with, for instance, skull or facial trauma require an implant, computed tomography can be employed to determine its exact form. Software then sends the data to a printer that fabricates the implant. A patient-specific implant is thus available within a matter of hours and the patient can undergo surgery.

However, this remains a vision. “Currently, the surgeon selects the most appropriate implant from a number of standard sizes: Individually fabricated polymer implants for individual patients are not available,” says Prabhu. This is a serious drawback, but no implantable polymer materials have been available so far that give consistently high print quality and also meet the stringent regulatory and property requirements for medical technology. For example, polylactic acids now available on the market for industrial 3D printing contain additives that make the material printable in the first place, but these very additives prohibit its use in medical devices.

“We’re working on making our biodegradable RESOMER^® polymers usable for 3D printing in medical devices,” says Dr. Thomas Riermeier, head of the Pharma Polymers & Services Product Line at Health Care. The aim is to bring to market materials with the appropriate documentation for the most common printing processes: powders for selective laser sintering (SLS) and filaments for fused deposition modeling (FDM).

With FDM, a filament of the polymer is fed to an extruder in the 3D printer, heated to melting point, and extruded through a nozzle. The component is built up layer by layer. “It’s vital that the filament has a stable geometry suitable for the purpose and does not change chemically when melted. And it must deliver reproducible results, even on printers from different manufacturers,” says Riermeier about the requirements of the material.

With SLS, by contrast, a laser moves over a powder bed, sintering only certain defined areas of the uppermost particle layer on the surface of a powder bed. These areas solidify upon cooling. After each layer is completed, the powder bed is lowered, a new layer of material is applied, and the process is repeated until the complete part has been produced, layer by layer. “For optimal results the polymer powder should have a suitable particle diameter and good flowability. This latter requirement is especially challenging for us: While free-flowing additives are commonly used in industrial applications, this is of course impossible in medical devices because they are not approved for this purpose,” says Riermeier.

The project house researchers have already achieved some success with both printing methods. They have developed processes that produce RESOMER^® powder and RESOMER^® filaments of the right type. The first test parts have been printed to investigate how the material behaves during printing and to assess the properties of the printed component. The next step will be to provide manufacturers of medical devices with research samples, enabling them to carry out their own printing tests.

But there are regulatory hurdles to be overcome before the first printed implants can be deployed in healthcare environments. Who will print the device—the hospital, the printer manufacturer, or a service provider? How will consistent quality be assured? Who approves a freshly manufactured implant? “Our aim is to be ready with the appropriate products as soon as these issues have been resolved,” says Riermeier. On the basis of the results achieved so far, the chances are good that this goal will be achieved.