New Paper : On the intrinsic sterility of 3D printing

As a biologist with a 3D printer, one of the questions I get most often about 3D printed parts is, “Can you autoclave these things?” As it turns out, no, not really. There are only a handful of thermoplastics that can survive the autoclave process, and most of them are not very good for 3D printing. With few exceptions, only polypropylene and blends of polypropylene hold up to repeated autoclave cycles, and polypropylene is, unfortunately, very a difficult material to print. It shrinks a lot when it cools, which causes a lot of warping during printing, and it is very difficult to get molten polypropylene to bond strongly to cooler, solid polypropylene.

It turns out that this is all unnecessary. Fused deposition modeling (FDM) 3D printing involves shoving a rod of thermoplastic into a hot nozzle until it melts and squirts out the nozzle. For most popular 3D printing plastics like ABS and PLA, the nozzle temperature is somewhere between 180C and 260C, and the plastic stays at that temperature for around a minute, depending on what the toolpath looks like. It’s actually a lot like Pasteurization, except way overkill. Get it? Overkill?

Anyway, here’s how FDM 3D printing compares to various Pasteurization (in black) and autoclave (in red) protocols :

FDM 3D printing compared to pasteurization (black) and autoclave (red) protocols.

Earlier this year, I was wondering about this, and my friend Emily Tung convinced me to just try it. So, on her advice, I heated up the nozzle of my 3D printer to 220C and placed a beaker of sterile liquid LB media under it. I then extruded some plastic until the blob fell into the media. I then incubated it, along with a positive an negative control, at 37C until… well, until nothing. Only the positive control (a short length of raw plastic feedstock) grew anything.

Preliminary sterility experiment after 96 hours at 37C : From left to right, extruded filament, raw thermoplastic feedstock, negative control.

I tweeted this little experiment as it went along, and suddenly I had two collaborators, Kaitlin Flynn in Michele Swanson lab at the University of Michigan, and Luis Zaman in Richard Lenski’s lab at Michigan State University. Kaitlin immediately started suggesting different growth conditions I should try, and Luis, who also has a 3D printer, replicated all of my experiments and invented new variations to try. Because Kaitlin didn’t have a 3D printer of her own, Luis and I 3D printed a bunch of little test parts for her to try out, and mailed them to her. In her spare time, Kaitlin tried culturing the parts under all sorts of different conditions, including with mouse macrophages.

The results are detailed in our new paper On the intrinsic sterility of 3D printing, which is now available as a PeerJ PrePrint as of yesterday.

The tl;dr is that yes, 3D prints are sterile after printing. They are not sterile after touching them with your fingers or dropping them on the floor.

2 thoughts on “New Paper : On the intrinsic sterility of 3D printing

  1. I think the biggest issue of sterility with 3D printing is the uneven surface (due to the “dotty” nature of the construction) leaves a lot of voids for microscopic dirt to gather and bacteria to breed in. I.e. after you’ve touched it with your hands or dropped it on the floor, you can’t clean it, and I’m not just talking about clinical sterilisation — you can’t even clean it the same as you would clean a child’s plastic toy.

    1. It depends a lot on the material and on how it was printed. It is possible to achieve a huge variety of different surface properties by tweaking print speed, temperature, cooling rates, layer heights, wall thickness and nozzle diameter. A lot of these are trade-offs between how good the finish looks, how fast the part prints, and how brittle the part is. Usually, tweaks that make the part look better result in weaker, more porous parts. Also, the way the infill is computed could matter for cleanliness — a porous part with a closed infill pattern could get of yucky if exposed to water.

      There are post-print techniques for surface treatment that work pretty nicely, and give 3D printed parts a finish exactly like a plastic toy. It’s maybe not something casual home users would want to do, though. Acetone vapor isn’t something you want to play with in your kitchen.

      Nevertheless, porous surfaces an’t unique to 3D printing. Lots of toys have bumpy plastic or rubber surfaces to make them easier to hold, and as smooth plastic surfaces get scuffed, scratched and chipped, they become just as porous as a 3D printed surface. See, for example, the wheels on skateboard, or the forehead of a doll that gets dragged around by its foot, or a tub of Legos that get stirred over and over again as I hunt for that one black Technic 1×6 brick.

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Russell Neches

A microbiology graduate student at UC Davis, working with Jonathan Eisen @phylogenomics . Studies evolution & ecology. Advocate of Open Hardware & Open Access.