A Continued Study of Manufacturing Methods for PEEK/HA

Polyether ether ketone (PEEK) is a thermoplastic possessing many qualities such as biocompatibility, non-toxicity, radiolucency, rigidity, and an elastic modulus similar to bone. Its primary drawback of bio-inertness can be mitigated when supplemented with hydroxyapatite (HA) to create a composite material, but this complicates its manufacturing procedures. This research explores three different ways to create PEEK/HA material specifically for the purpose of 3D printing mechanically characterizable samples for applications such as designing bone scaffolds or implants.

Hello, my name is Elijah Pointer and my research this semester was a continued study of manufacturing methods for PEEK/HA under Dr. Bagheri.

In this video, I will first explain what PEEK/HA is and how it can be applied. Next, I will describe the filament extrusion process and the different manufacturing methods of the material. Lastly, I will provide my results, difficulties, and progress over the semester.

PEEK/HA is a composite material composed of polyether ether ketone and hydroxyapatite. In short, a composite material is a combination of different materials with the intent of combining or improving specific attributes. PEEK is a high performance polymer with characteristics such as, but not limited to: biocompatibility, non-toxicity, radiolucency, and a bone-like elastic modulus. One of its few drawbacks is its inherent biological inertness. Supplementing it with an additive such as HA, however, reduces PEEK’s inertness and can encourage bone growth. This in particular opens up the possibilities of utilizing PEEK/HA in orthopedics, and, given its plasticity, even 3D printing bone scaffolds and implants. My research focused on different ways to produce this material and test its mechanical characteristics.

To use PEEK/HA material in 3D prints, the composite material is often made into pellets and drawn into filament using an extruder such as this one. Pellets are dropped into the funnel, where an auger pushes them further into the barrel, where they are melted down and pushed out of the nozzle as a thin filament.

The first manufacturing method I employed to create PEEK/HA material was dry mixing. This method involves mixing the PEEK pellets with HA powder one to two times at 3000 rpm for 30 seconds. I would then extrude the pellets into filament. After two previous semesters of hard work, I was finally able to produce testable PEEK/HA samples using this method. The figure on the right compares the quality between regular PEEK samples and PEEK/HA samples.

The second method, re-extrusion, attempts to make use of dry mixed filament which could not be utilized for 3D printing due to its poor surface quality or inconsistent diameter. Any dry mixed filament with these characteristics would be cut back into pellet sized pieces and run through the extruder one to two more times to better mix the composite material in the hopes of producing a higher quality filament. Despite obtaining slightly higher quality filament compared to previous semesters, it is still far too brittle to 3D print with. The figure on the right compares the ideal surface quality of dry mixed filament, with the highest quality sample I obtained via re-extrusion.

Melt mixing is a novel method of creating specifically composite filament. I wanted to try adding HA powder directly onto PEEK filament while it was still warm and malleable from the extruder. To do this, I 3D modeled and printed a plate which could attach directly to the extruder as seen in the figure on the left. This would allow me to either pour HA onto the plate, which the filament would brush against or directly sprinkle HA onto the incoming filament. Unfortunately, in my early tests with PLA instead of PEEK, the filament stuck more intensely to the plate than the actual HA, which can be seen in the figure on the right. However, I think the concept of applying HA to warm, malleable filament is still worth pursuing with a possible redesign.

Using a Newton Test Machine to mechanically characterize the dry mixed samples, the elastic modulus I obtained for samples tested to tensile failure was about 2,135 MPa or 2.135 GPa. The elastic modulus I obtained for regular PEEK samples was about 2,992 MPa or 2.992 GPa, which is around 40% higher. This is indicated by the graph on the right for plain PEEK, having a higher stress-strain ratio or slope than the graph on the left for PEEK/HA. This can be more easily seen when scaling down the x axis for the graph on the right.