Thermoforming 3D Printed PLA for Use in Prostethics

One of the biggest challenges in low-cost 3D printed prosthetic devices is achieving the proper fit. If the loads on the device are not properly distributed across the skin and underlying tissue, it can lead to irritation, bruising, or rashes. In the case of the body-driven prosthetic devices that the e-NABLE community designs, there are two body-interfacing components: the palm and the gauntlet. The palm serves as a "pocket" into which the user slips their residual hand and the gauntlet anchors the device to the forearm. When it comes to fitting, one solution is to 3D scan the residual limb to which the device will be fitted and model the gauntlet in software to conform to the surface of the scan. When printing, the resulting geometry often requires significant support material and has weak regions due to the poor matching of the slicing plane to the planes of action in the geometry.

A more analog solution to the fit problem is thermoforming. Similar to the process by which mouth guards are formed, PLA thermoforming allows for custom-fitting directly to the body. Additionally, the fit can be adjusted as needed after the initial forming.

Print the flat gauntlet in PLA on a FFF/FDM machine of your choice. It's important to use PLA for two reasons:

1. PLA has a discrete glass transition temperature of 60C, which is what makes the thermoforming process possible. ABS doesn't have a distinct glass transition temperature which means that it won't suddenly become soft and rubbery as you increase the temperature.
2. PLA as a polymer is non-toxic. The pigments, melt-flow modifiers, and opacifiers that may be mixed in with the base polymer to make filament should be evaluated carefully. "Natural" PLA filament is the straight polymer without any pigments or opacifiers and is probaby the best choice for those concerned about skin contact. All Makerbot PLA filament is made with FDA approved pigments and may be a good runner up. In general, stay away from bright yellows and bright greens as these colors are more likely to contain toxic pigments.

Print with 2-4 shells to ensure that the resulting object is water-tight.

Once you have printed the flat gauntlet form, you will need to heat it to soften the material. I have tested the following methods and have ranked them in order of preference:

1. Toaster oven. This method is my favorite because it keeps the gauntlet dry and it is easier to handle without being covered in hot water. Set the toaster oven to a medium heat (200-300 F) and check frequently with a fork for flexibility and softness.
2. Hot air gun/hair dryer. It's a little trickier to get even heating using forced hot air, but a hair dryer is very useful for tweaking the fit and re-working the form to get a perfect fit.
3. Microwave with water. As PLA does not convert microwaves to heat very well, it is necessary to submerge it in a shallow bowl or plate of water to heat it. Microwave until the water is boiling and then remove with a fork or tongs when soft.

The PLA will gradually stiffen over the course of a minute or so after it has been removed from its heat source, so you will have a reasonable window in which to form the gauntlet. And the gauntlet can be re-formed at any time by repeating the heating steps. Prepare the arm by draping a piece of cloth over the area to which the gauntlet is to be fitted. Quickly remove the gauntlet from the heat source and drape over the arm and apply pressure to form the gauntlet to the arm. Check that the wrist pin hole are parallel and are at a comfortable location. You can either continue to hold the gauntlet in position until the PLA firms up, or you can hold the arm and gauntlet in place in a sink and run cold water over both to set the form of the gauntlet.

With the gauntlet finished, add padding, tensioners, and attach it to a palm of your choice. Sources of hardware and padding can be found on the e-NABLE resources page.