You may have heard that the skin of a shark is not smooth, but instead is rough like sandpaper. If you get a piece of shark skin (perhaps at a seafood market) you can feel it yourself and verify this. But what your fingers cannot feel (nor can your unaided eyes see) is the actual shape of those tiny structures on the skin - called denticles - that create that sandpapery texture. The denticles are far too small for our hands to touch only a single one of them to discern their shape. Until now…
This instructable will explain how to use some cutting-edge technology (CT scanning and 3D printing) to make this untouchable feature of shark anatomy touchable.
Why make 3D printed shark skin?
The denticles on a shark play a very important role in the survival of these amazing creatures, and we wanted an effective way to tell their story. Swimming efficiency is crucial for a shark, and that efficiency starts at the interface between themselves and the water - their skin. Research has shown that the denticles on the skin significantly increase the hydrodynamic properties of the shark. Engineers are experimenting with shapes and textures inspired by shark denticles, applying them to the surfaces of watercraft and aircraft to see if humans can use this solution to save fuel - a great example of biomimicry. This video (above) offers more details about the function and importance of shark denticles, and shows our 3D printed versions in a live experiment. To help people to better understand this amazing feature of sharks, we wanted to create denticles large enough to allow even a single one of them - as small as a grain of sand - to be touched and examined.
Continue reading to learn how to do this.
Disclosure: This Instructable is for those with experience in 3D printing and 3D modeling software. Because there are a number of software options that can be employed, and users of each program often have their own preferred workflow, the intent here is to describe the overall process used to get from actual shark skin to a 3D printed denticle, rather than to offer "click by click" instructions on the use of any specific software. (Translation: I'll explain how the house was built, but not how to use a hammer and a saw.)
Some of the equipment needed is very costly, requires specialized training to operate, and is generally the domain of university or commercial scientific facilities. (Translation - you're probably not going to be able to do this whole process in your garage.) However, the only equipment used here that I personally own is the computer and the microscope, so in theory anyone else who arranges for access to the rest could also follow this process.
What you'll need:
Shark (skin is all that is needed, but it might be easier to get the whole thing)
Tongue depressor or wooden craft stick
Microscope (digital or standard)
3D printer 3D printing software (we used Cura, which is free)
3D modeling/editing software (Meshlab and Blender are also free)
Step 1: Get a Shark.
Being in Montana, this was not something we could just go out and catch, but depending on your location, perhaps that's an option. Thanks to the miracle of online shopping, you can order a shark nearly anywhere (delivery in 30 minutes or less is NOT guaranteed...) from a biological supply company.
We used a preserved specimen of Squalus acanthius, commonly known as dogfish. These are used regularly for dissection and study in high school and college biology classes, and even though they're not so awe-inspiring as "Jaws", they still have denticles, like all sharks do. Unless you're planning on a thorough study, just order the "base model" since you'll only be needing the skin.
Step 2: Collect and Mount Skin Sample.
Using a scalpel, remove a patch of skin about 2 x 3 inches in size. To be sure you get a good sample from this patch, examine it under the microscope to find a spot which has a nice field of intact denticles. (This image is of our skin sample.) Using the scalpel again, cut a small sample from the desired area of the patch. Because the denticles are so tiny, only a very small amount of skin is needed - less than half a square centimeter is sufficient to contain dozens of denticles. Be sure the skin is free of dirt or debris, (check with the microscope) then dry the skin overnight, putting something flat on top of it to prevent curling (we used a ruler). Mount the skin sample securely to something flat and stable so that it can be handled without touching it. Avoid mounting it on anything metal, as this will cause problems in the next step. We glued ours onto a portion of a wooden tongue depressor, because the wood has a fairly low density, which is helpful in the next step. Our adhesive was Vinac, a museum-grade polyvinyl acetate, but you can use Elmer's glue.
Step 3: Micro-CT Scanning.
Send your sample to a lab equipped with a micro-CT scanner. (Or, if you've got a few hundred thousand dollars laying around, perhaps you can buy your own...) Be sure to package it in such a way that allows it to remain clean and free of debris. (Anything that gets on the sample will get scanned and become part of the resulting scan images.) A small zip-lock bag taped between two pieces of cardboard and placed into a FedEx envelope will do. When sending samples to a lab for scanning, it is important to inform the technicians which specific area or tissue type of the sample is of interest, as the settings on the scanner can be optimized accordingly.
Much like a hospital CT scanner, a micro-CT scanner uses X-rays to capture the internal and external geometry of the specimen, but unlike the hospital machines, a micro-CT scanner is made to scan tiny objects, so it scans at much higher resolution than a hospital CT scanner. The technicians (Audrey and Olivia in the lab shown in the video above) position the specimen on the turntable and take some test images to calibrate the settings on the scanner. Once they're happy with what they're seeing, scanning begins. A scan can take several hours as a new X-ray image is taken for each fraction of a degree of rotation. For some specimens, a complete 360 degree rotation has over 500 steps. The resulting series of images is processed into a 3D reconstruction of the specimen, which can be used to create virtual models, animations, and in our case 3D prints.
Step 4: Convert CT Data to a 3D Model.
Using some fancy software, the resulting X-ray images are processed into a 3D model of the specimen. This is not necessesarily as easy as it sounds, especially if only a portion of a specimen is desired. The individual scan images can be edited to cut away unwanted bits. This "segmentation" can be a partially automated process, but sometimes not, and some costly software and serious computer power is often needed. Unless you're a 3D imaging guru, ask the lab if they can convert your scan data to an .STL file for you. You can edit/modify the .STL model yourself using free software such as Blender or Meshlab.
Here is our field of shark denticles rendered as an .STL file, shown after some clean-up in Meshlab. Because the density of the denticles is greater than the skin from which they grow, (and greater than the wooden stick the skin was mounted on) there wasn't too much unwanted "junk" in the CT reconstruction, so the .STL file provided by the lab was already pretty clean and fairly easy to edit.
Step 5: Edit File to Prepare for Printing.
In this case, our goal is to isolate a single denticle and enlarge it many times so that it can be touched for tactile examination and seen in complete detail with the unaided eye. Using Meshlab (shown here) or Blender, a single denticle can be isolated, and all other unwanted portions of the model can be digitally cut away. Here is our single denticle, showing the sleek and curvy shape of these amazing structures. Their "built for speed" form inspires visions of hydroplanes and Formula 1 race cars!
Step 6: Embiggening and Enhugenation!
With the final .STL model ready, it's time to bring it from the digital domain to large-scale physical reality with 3D printing! There are a number of 3D printing software programs available to scale the model up to a useful size, and your printer may prefer a specific program. Shown here is Cura, a popular 3D printing software (bonus - it's free!) that is compatible with a number of printers.
After downloading, open Cura and import your .STL file. Notice that when our model is first imported into Cura, it looks like nothing has happened. However, it's important to remember how small our object really is. Zooming in (a lot!) on the print bed reveals that the model is there, it's just very, very small. (Note the dimensions at the bottom of the screen.) In order to scale it up and position it correctly for optimum printing, the model must first be selected, but it's so tiny that you can't even click on it without zooming waaaaaaay in on it first. It's tiny, but not for much longer...
Using the Scale tool, you can enlarge the model to several times original size - we enlarged ours by 15,000%, which makes our 3D printed denticle 150 times the size of the real denticle.
Step 7: 3D Printing.
Once you've scaled up your model and have oriented it correctly for optimal printing (the ideal orientation may vary depending on what printer you're using), save your code and upload it to your printer. We used an Ultimaker 2 to print our denticle. Printing at a layer height of 60 microns required about 5 hours of printing time at this scale.
We call the result of this process a macromodel. Unlike an artistic sculpture, our macromodels are a precise volumetric enlargement of the actual specimen. The shape is the same as that of the real thing, only the size and the material are different. This enables us to make many untouchably tiny structures touchable. So far, we’ve done spider fangs, porcupine quills, scorpion stingers, and many others. All are being produced for a natural history museum exhibition, with some being made available for use as science classroom teaching aides. More info at: