Introduction: Air-Powered Soft Robots With LEGOs

About: Writer for Science Buddies ( and lecturer at Cornell University's Sibley School of Mechanical and Aerospace Engineering.
This project is a follow-up to my Air-Powered Soft Robotic Gripper project, based on research by the Whitesides Group at Harvard University. I've gotten a lot of positive feedback on that project, but one repeated question - "What if I don't have a 3D printer?" While 3D printers are continually getting cheaper and more available, not everyone has easy access to one. There are also numerous online 3D printing services, but we've run into compatibility issues between some of their common material choices (particularly laser-sintered nylon and any UV-cured photopolymers) and the silicone rubber used to make the robots.

So, I set out to reproduce the project using a more readily-available mold material: LEGOs! The plus side is that means the project no longer requires a 3D printer. The down side is that the smallest 1x1 LEGO "brick" unit is considerably larger than the resolution of even cheap consumer 3D printers - meaning the entire robot mold has to be scaled up, so the cost for silicone rubber will be higher. You'll also need a much bigger air supply than the tiny squeeze bulbs or syringes used in the first Instructable - but luckily a bike pump, generally readily available, will work nicely.

In this Instructable I'll go over the basic procedure to build a single, manually-powered "gripper" robot. If you'd like to try something more advanced like a walking, electronically-controlled, or even autonomous robot using these methods, you should check out the publications of the Microrobotics Lab and the Whitesides Group at Harvard, where this technology got started.


  • Ecoflex 00-30 for robot's top layer. Trial kit should be enough for 1-2 robots depending on the size of your mold, gallon unit is more cost effective if you're doing a camp, makerspace activity etc. and need to make lots.
  • Ecoflex 00-50 for robot's bottom layer. Same notes apply about quantity.
  • Bicycle pump with needle attachment (for inflating footballs, soccer balls etc)
  • 1 or 2 large LEGO base plates (see picture above). 2 plates means you can make 2 molds and cure each half of the robot simultaneously, so the project will go faster.
  • Assorted small LEGO bricks and plates (see Step 2). Exactly what you need depends on the shape of the mold you want to build. You can get individual bricks from the LEGO Pick a Brick shop.
  • Recommended: small level, to make sure the mold is level when curing
  • Optional: plastic tubing with an inner diameter that will fit over your bike pump's needle. This makes it easier to attach the pump to the robot, but is not required (see step 12). I used 1/16" ID polyethylene tubing.
  • Plastic cups and popsicle sticks for mixing ecoflex
  • Highly recommended, especially if doing this activity with kids: paper towels, disposable rubber gloves, and plastic cafeteria trays or a disposable table cloth. Ecoflex is non-toxic* but can be messy and kids tend to spill it everywhere.
*The MSDS for the Ecoflex series says that Ecoflex contains no hazardous ingredients, but that it can be an eye and skin irritant so protective equipment should be worn. I have gotten Ecoflex on my hands and never had any issues, but with kids it can't hurt to be extra safe.

Step 1: Build the LEGO Mold

The basic idea behind the mold is the same as the mold in the original Instructable. There are two layers to the robot, a top layer and a bottom layer. The bottom layer is a solid piece, but the top layer has tiny air channels inside it. When you pump air into these channels, they inflate and cause the robot's appendages to bend. So, you need to make two molds - whether you do these simultaneously or one at a time depends on whether or not you have two LEGO base plates:

1) A bottom mold that just has the outer perimeter of your robot. Use standard LEGO bricks (3 units high) for the outer walls.

2) A top mold that has the outer perimeter and interior air channels. Use regular bricks (3 units high) for the outer walls, and smaller plates (only 1 unit high) for the air channels.

Make sure the outer perimeters of the two molds match. You can follow my design above to make a four-fingered "gripper," or come up with your own.

Step 2: Mix Ecoflex 00-30

Mix enough Ecoflex 00-30 to fill your top mold (equal parts A and B by volume - no need to measure exactly, you can just eyeball it). Pour parts A and B into a plastic cup and stir for a few minutes until you're sure they are well-mixed.

Note: it can be hard to pour the Ecoflex without dribbling some down the sides of the containers - have some paper towels handy for cleanup.

Step 3: Pour Ecoflex 00-30 Into Top Mold

If you have a level available, do your best to make sure your mold is sitting on a flat, level surface. If the mold is tilted at all, that will result in the Ecoflex curing in a layer without uniform thickness, and this can severely affect the final performance of the robot - usually one leg that inflates way before the others.

Now, slowly and carefully pour the Ecoflex into the mold until it is just starting to spill over the top edge. It's much better to over-fill the mold than to under-fill it. You can always use scissors to trim away excess material later if the Ecoflex spills over the edges. However, if you don't use enough, the robot could be too weak, and is more likely to pop or develop an air leak.

Watch the Ecoflex for a few minutes - you should see air bubbles slowly floating up and working their way out of it. You can accelerate this by popping the bubbles with a pin or toothpick. Ideally you would do this with a vacuum chamber, but that type of equipment isn't typically available at home.

If you only have one LEGO base plate available, now you'll need to wait 4 hours for the Ecoflex to cure at room temperature. Ecoflex will cure in only 10 minutes at 150 degrees Fahrenheit, but I did not want to find out what happens when I stick my LEGOs in an oven.

Step 4: Mix Ecoflex 00-50 and Pour Into Bottom Mold

Repeat steps 2-3 with Ecoflex 00-50 and the bottom mold.

If you only have one mold, you'll need to remove the top half of your robot from the mold first (see Step 6), then remove all the interior air channel pieces from your top mold to convert it into a bottom mold.

Step 5: Let Ecoflex Sit for 4 Hours

Let the Ecoflex cure for 4 hours at room temperature. Again, it will cure in 10 minutes at 150 F - but I'm not going to recommend putting LEGOs in an oven. While ABS shouldn't melt at that temperature, I can't guarantee that it won't give off any fumes or leave any residue that you wouldn't want in an oven that you also use for food. For the 3D printed ABS molds we used small "no food" toaster ovens in a well-ventilated area - so if you have a bigger science-only oven that you can try this out in, give it a shot and leave a comment about whether it worked.

Step 6: Remove Top Layer From Mold

Starting with the outer edges, carefully peel your top layer out of the mold. Ecoflex is pretty tough and stretchy, so you shouldn't need to worry too much about breaking it, but it can't hurt to be careful. You should be able to clearly see the air channels on the underside of the mold - make sure none of the tiny LEGO plates got stuck in them.

Important: do NOT remove the bottom layer from its mold yet.

Step 7: Apply "glue" Layer of Ecoflex 00-30

This is the most critical step in determining whether or not your robot will work well, so be careful!

Mix a fresh batch of Ecoflex 00-30, the same way you did in Step 2. The quantity can be much smaller - you just need enough to apply a "glue" layer, not to fill an entire mold.

Keeping the bottom layer in its mold, carefully pour and spread a thin, even layer of fresh Ecoflex onto the top side. The thickness of this layer is critical. If you use too much, you will clog the air channels, so your robot will not inflate. If you use too little, the top and bottom layers will not bond well, and are much more likely to peel apart or develop an air leak. It's difficult to measure, but experience indicates that you should use a layer about 1 millimeter thick. So, make sure you have a complete, even coating with no gaps; but also make sure that you don't go overboard and just dump on a ton of Ecoflex. If you accidentally use too much, you can always use a paper towel to soak some up.

Step 8: Place the Top Layer Face-down on the Bottom Layer

Carefully place the top layer onto the bottom layer with the air channels facing down (and the smooth side facing up). Make sure the edges of the two layers line up.

Step 9: Apply Glue Around the Perimeter

Apply some additional glue around the entire outer perimeter by using a popsicle stick. This can help create a better seal between the two layers and prevent any air leaks.

Step 10: Let Sit for 4 Hours

Let the entire robot sit for another 4 hours so the glue layer can cure. The same warnings about using an oven apply - do so at your own risk!

Step 11: Remove Robot From Mold

Similar to what you did in Step 6, carefully remove the entire robot from the mold.

Step 12: Attach Air Supply

Now you need to puncture the side wall of your robot with your air supply line, and make sure the tip of the line reaches the interior air channels. You can do this directly with a needle attachment for a bike pump, but this can be a bit unwieldy since the hose attached to the bike is rather thick and not very flexible.  Important - don't poke more than one hole in the robot! This will cause a leak - if your air supply line falls out, or you need to start over, make sure you re-use the same hold. It can be easier to see if you hold the robot up to a bright light, since cured Ecoflex is slightly translucent.

Step 13: Inflate!

Pump it up! Start using your bike pump slowly at first and you should see your robot gradually inflate. Listen closely for air leaks, especially around where you inserted the air tube. As you can see from the sequence of images here, it looks like I ran into one of the problems I warned about - I had an area that didn't have a strong enough glue layer. This causes a "bubble" that makes one of the legs inflate before the others, so the robot isn't symmetric.

Unfortunately there's no good way to fix this if you have de-bonding issues inside the robot. If you have an air leak though, all you need to do is mix up some fresh Ecoflex and patch it up. Remember that this technology is still a work in progress, actively being developed by scientists at a major research university - so don't feel bad if your robot doesn't work perfectly on the first try! I don't have better pictures because I used all of my Ecoflex on this rather large mold (I actually only used 00-30, but would have gotten much better results with 00-30 and 00-50), and didn't want to spend the money on a second batch. If you manage to do better, please post pictures in the comments!

If you run into any other issues or have questions about this or any of my other Instructables, please don't hesitate to leave a comment below.

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