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animals morphology biomechanics 3D printing

3D Printing for Fun and Science

Almost any conceivable object can now be 3D printed into reality. The process is fun and futuristic, but it’s also opening new doors for biological research.

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Fig. 1 A 3D printed macaque monkey skull (left) next to the original bone specimen (right). Source: Abby Vander Linden.

If the ability to construct any object you can think of sounds like science fiction, then welcome to the 3D printed future. 3D prints are everywhere, including this print of a macaque monkey skull currently on my desk. Let’s look at how this exciting method works and what you can do with it.

In general, 3D printing uses a printer with a small nozzle to extrude thin amounts of material in precise layers. By building up the layers of this material according to the specified design, the printer can create three-dimensional shapes out of plastic, nylon, food materials, and even living cells.

If you want to print out a replacement kidney or a human ear, you need a breathtakingly expensive precision machine and a multimillion dollar research facility. However, for just over $1000 you can buy your very own (much less precise) desktop 3D printer, download digital files of 3D objects from somewhere like Thingiverse, and print little plastic shapes to your heart’s content.

The technology is becoming widespread, but my own fascination with 3D printing hasn’t diminished. Never mind that in order to get a 3D print someone has to spend many frustrating hours to create, clean, and smooth a digital model of the specimen, and the 3D printed copy is often smaller and less detailed than the real thing. You can print tiny hyena skulls in neon colors! I want to 3D print everything!

But is there a point to making models of different parts of animals, apart from the fact that they’re fun to have? After all, science isn’t a dead collection of facts, it’s a process—observing the world, asking questions, formulating hypotheses, and designing experiments to test how biological life works. So can you use 3D printing to actively do science?

As a matter of fact, you can. Many researchers are beginning to use 3D printed models to test hypotheses about why animals evolved to be certain shapes and what kinds of physical forces they encounter in their environments. Here’s a recent example of how scientists are using 3D printed objects in their research.

Scientists from Friday Harbor Laboratories at the University of Washington were interested in how durophagous predators (animals that eat mollusks, crustaceans, and other very hard prey) can crack hard shells without breaking their teeth. In their paper, “How best to smash a snail: the effect of tooth shape on crushing load,” researchers Stephanie Crofts and Adam Summers wanted to test how effective different tooth shapes were at crushing marine snail shells [1]. They milled a variety of tooth shapes out of aluminum—some convex, some concave, some with a sharp cusp of varying widths—and used a materials testing machine to apply a controlled amount of force with the aluminum “tooth” onto a snail shell to test how much force each shape required to crack the shell.

image alt text Fig. 2 Experimental shapes used to manufacture aluminum “tooth” models and test how well different teeth can break open hard prey. Source: Crofts and Summers 2014.

How easily a shell breaks has a lot to do with its shape and thickness, and the researchers needed many identical shells to perform controlled, repeated experiments so they could compare results between different tooth shapes. Real shells are far from uniform, so the researchers mass-produced plastic shells for testing using a 3D printer and digital models created from two real species of snail. In total, 750 plastic shells were printed and smashed.

image alt text Fig. 3 3D printed plastic replicas of two species of intertidal snails, (a) Nucella ostrina, and (b) Nucella laminosa. Source: Crofts and Summers 2014.

Crofts and Summers found that teeth with a convex surface took less force to break both kinds of shell. Adding a tall, narrow cusp to the tooth helped to concentrate the force over a smaller area, resulting in a tooth shape that required less total force to smash snails. However, in nature this concentration of force would also mean the tooth cusp is more likely to break—and the “ideal” shell-crushing tooth shape doesn’t really exist in nature. This indicates a functional trade-off for teeth of animals that eat hard prey: the tooth shape must be efficient at cracking shells, but strong enough to withstand hard forces.

Because the plastic snail shells have different material properties than real shells made of calcium carbonate (and squishy snail innards), this study was not about measuring how much actual force it takes to crush snails. Rather, by making hundreds of copies of the same shells, the researchers were able to take differences in snail size and shape out of the equation and test how different tooth shapes performed under identical conditions. This kind of controlled mechanical experiment would have been extremely difficult without access to a 3D printer.

3D printed models of animal bones and shells are fun to look at and play with, but they can also play an important role in the process of experimental science! And even though it’s not pulling its weight in the scientific process, I love having my plastic macaque skull on my desk to remind me that we are living in the 3D printed future.

References:

[1] Crofts, Stephanie B. and Adam P. Summers. “How best to smash a snail: the effect of tooth shape on crushing load.” Journal of the Royal Society Interface 11 (2014): 20131053. (http://rsif.royalsocietypublishing.org/content/11/92/20131053.short)

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