Mechanical and Surface Integrity of 3-D Printed PLA-HA Composite

The materials currently used to construct replacement hip and knee joints for humans are stainless steel and titanium. Whilst these materials are strong and fairly biocompatible, they are expensive and can cause adverse long term side effects such as stress shielding. The aim of this project was to construct and evaluate an alternative to these materials, specifically 3-D printed Polylactic Acid and Hydroxyapatite composite (PLA-HA). PLA-HA is a polymer composite of Polylactic Acid (PLA) and Hydroxyapatite (HA). PLA-HA is relatively cheap, very biocompatible, and can be easily adapted for use in a 3-D printer. This vastly simplifies the process of manufacturing unique parts with complex geometries such as a replacement joint. Raw PLA-HA was created in-lab using two different manufacturing methods. These manufacturing methods were Dry Speed Mixing and Magnetic Stirring. The raw PLA-HA was then converted into filament for use in a 3-D printer using a uniaxial filament press. The PLA-HA filament was then used to 3-D print three different samples for mechanical testing for both manufacturing methods. These samples were subjected to different tests to characterize their mechanical and surface properties. These tests included Energy Dispersive Spectroscopy (EDS) and micro-indentation. These mechanical and surface properties were then evaluated to determine if they are sufficient for a joint replacement application. The different manufacturing methods will also be compared against one another to determine which method produces samples with the desired mechanical properties and distribution of hydroxyapatite throughout its matrix.

Slide 1:

Hello! My name is Zan Stuart, I’m a Senior Mechanical Engineering Student and today I’ll be talking about my project, which is a mechanical and surface characterization study of 3-D printed PLA-HA Composite.

Slide 2:

So the motivation for this project is the materials currently used for hip and knee joint replacements, such as stainless steel and titanium, are adequate, but they have some significant issues, such as the fact that they’re expensive, they’re difficult to machine into these complex shapes that are necessary for this application, and they can cause some adverse long term side effects such as stress shielding, which basically wears away the bone material around the replacement joint.

Polylactic acid, or PLA, and Hydroxyapatite, also known as HA, composite is a potentially viable alternative. PLA is a very common plastic used for 3-D printing, and hydroxyapatite is a mineral that’s commonly found in your bone structure.

PLA-HA is pretty cheap, it’s biocompatible, and it’s easily adapted for 3-D printing.

Very few studies exploring the properties of PLA-HA in general exist, and fewer, if any, study the properties of 3-D printed PLA-HA.

Slide 3:

To briefly state the objectives, we wanted to manufacture the PLA-HA composite using different manufacturing methods, we wanted to process that material into a form that useable with a 3-D printer, and we wanted to 3-D print some samples and test their properties, and study how the manufacturing method effects the properties of 3-D printed PLA-HA.

Slide 4:

Two different manufacturing methods were used, one is dry speed mixing, which basically involves placing the raw PLA pellets in a container, mixing them with hydroxyapatite powder, and spinning it very very fast in a dry film until the mixture is roughly homogenous and then turning that into filament.

The other is magnetic stirring, where you basically dissolve the PLA pellets in a solvent, and then put them in a magnetic stirrer, which mixes them up as you can imagine, mixing that viscous dissolved PLA mixture with hydroxyapatite powder and then a silane coupling agent and then forming that into raw PLA-HA after it cures, which you can see in the middle of the slide.

Slide 5:

That raw material is then placed into a filament extruder which you can see on the right side of the slide here, where you put the raw material in the hopper up top, it is then compressed and fed through a heated nozzle at the end where it spits out something resembling a plastic wire which is then fed to a 3D printer.

Slide 6:

Here is just a basic schematic of how 3-D printing actually works, like I said, the plastic string once again gets fed through a heated nozzle, the bed upon which the part that you’re building is formed on moves around along with the nozzle so you get the proper geometry as you desire.

Slide 7:

Here are a couple of examples of the completed samples I made for this project. On the right is a magnetic stirred sample as well as a failed one, and here’s an example of a dry speed mixed sample.

Slide 8:

Two different testing methods were used, one was microindentation which I won’t cover in extreme depth here, but basically it involves pressing a very small diamond tipped pin into the surface of the material, and you use the data collected from that to observe the mechanical properties. In this case we wanted to find the elastic modulus and contact creep performance.

Creep performance is a measure of how much a material deforms with time with a constant load. As you can imagine that’s something you want to keep to a minimum. And the SEM image here is just an example of one of the indentations that was performed on a magnetic stirred sample.

Slide 9:

The other method used was Energy Dispersive Spectroscopy or EDS. Here’s the basic working principle of it, you’re basically shooting an electron beam at the surface of the sample, that excites the atoms and displaces electrons at lower energy states. Following the principle of conservation of energy, when an outer shell electron moves to fill that vacancy, because it’s at a higher energy state that excess energy has to go somewhere, in this case it goes to X-rays. These X-Rays can then be received. Each element actually emits a unique X-Ray during this process, so by figuring out what the wavelengths of the x-rays are and receiving them we can figure out what the composition of the sample is.

Slide 10:

Here are the results of EDS that you can actually see, the highlighted areas or the colored areas indicate the presence of the element given on the left. Hydroxyapatite is composed of calcium oxygen, phosphorus, carbon, and hydrogen. Calcium is not present in the base material of PLA, so if we see calcium or phosphorus we know we’re seeing HA.

We can see that HA was actually integrated successfully using both manufacturing methods. As far as the concentration goes it’s hard to say much, as the area that we imaged was very small. However, this had to be done because the hydroxyapatite powder used is only visible on the nanoscale, so it’s hard to make a solid observation other than the fact that we know that HA is present in the PLA material.

Slide 11:

And here are the results of the mechanical testing, you can see that the contact creep was significantly higher for the magnetic stirred samples which is less desirable, and the elastic modulus of the dry speed mixed samples was significantly higher than that of the magnetic stirred samples.

A two sample t test was conducted to determine the significance of manufacturing method on these two properties, and you can see that yes, the manufacturing method significantly influenced both properties.

Slide 12:

In conclusion, the manufacturing method significantly altered both the elastic modulus and creep performance, and hydroxyapatite was successfully integrated into the PLA matrix using both manufacturing methods.

If I was to continue this project in the future, I would produce and test more samples, and use different manufacturing methods such as wet speed mixing and re-extrusion.

I would also conduct more and different mechanical tests, such as wet and dry wear tests to see what the friction properties of the material are, as well as tensile tests to more thoroughly evaluate the elastic modulus.

Slide 13:

Thank you for your time and attention, and I hope that you have a great day!