Squishy Robots: Build an Air-Powered Soft Robotic Gripper
AbstractWhen you think about robots, chances are they are contraptions that you have seen on TV, in movies, or even in real life — and they are usually made of metal. What if you could make a soft robot that could bend, twist, or squirm like an octopus or an earthworm? Researchers at Harvard University have done exactly that, developing soft robots made of rubber and powered by air instead of electricity. In this project you will use their designs to build a soft robotic gripper of your own.
Ben Finio, Ph.D., Science Buddies
The soft robot technology used in this project was originally developed in the Whitesides Research Group at Harvard University. These do-it-yourself instructions were developed in the Creative Machines Lab at Cornell University. The work at Cornell was sponsored by the National Science Foundation (DRL-1030865) and the Motorola Foundation. Special thanks to Prof. Robert Shepherd for helping to develop and test the project at Cornell, and to the Ithaca Generator and Sciencenter for providing audiences to help test this project.
- PacBot is a registered trademark of iRobot Corporation.
- Ecoflex is a registered trademark of Smooth-On, Inc.
- Solidworks is a registered trademark of Dassault Systems.
ObjectiveBuild a soft robotic gripper and use it to pick up different objects.
Robots come in all shapes and sizes. Some move on wheels, some walk on legs, some fly with wings, some are as big as a horse, and some can fit on your fingertip! Figure 1 shows several completely different robots, but they all have one thing in common. Can you guess what it is?
Figure 1. Several completely different robots, from movies and real life: R2-D2 from the "Star Wars" movie franchise (top left); EVE and WALL-E from the animated film "WALL-E" (top right); ASIMO, a humanoid robot made by Honda (middle left); 510 PackBot®, a bomb-disarming robot made by iRobot (middle right); BigDog, a military cargo-carrying robot developed by Boston Dynamics (bottom left); and RoboBee, an insect-sized flying robot developed at Harvard University (bottom right).
Have you figured out what all of these robots have in common yet? They are all made of hard materials — either metals or plastics. Robots made of soft materials, like rubber, are much less common, mainly because engineers are used to making mechanical parts out of metal and plastic. Recently, however, engineers have started to look at things in nature (like animals) to get ideas for how to build robots. This is called biologically inspired engineering (or bio-inspired engineering for short). It turns out that many animals have soft features — for example, human skin is soft and stretchy, which makes it resistant to damage. Some soft animals can do amazing things — watch this video of an octopus escaping a box by squeezing its entire body through a 1-inch hole:
So, what if we could build soft robots that bend, squirm, expand, and contract? Some advantages could result — for example, soft robots might be able to pick up fragile objects easily, or interact with humans more safely. In fact, researchers at Harvard University have started doing that by designing air-powered robots made of rubber. The robots work just like balloons — they are hollow in the middle, so when air is pumped into them, they inflate and change shape. But instead of just expanding symmetrically like a regular party balloon, the robots' unique shapes let them perform different motions like bending and twisting.
The first robot the Harvard researchers designed is a gripper that can wrap its fingers around a variety of different objects and even pick up a raw chicken egg without breaking it. This contrasts to most metal robotic hands, which are specifically designed to pick up a single type of object and would have difficulty picking up fragile objects like eggs without breaking them.
The researchers even used the same technology to design a walking robot:
How do scientists go about designing and building these soft robots? First, they use a computer-aided design (or CAD) program to design a mold in the shape of the robot they want to build. Then, they use a 3D printer to print a plastic copy of the mold. Unlike a regular printer, which prints ink on two-dimensional sheets of paper, a 3D printer can actually create three-dimensional objects by printing solid plastic. Figure 2 shows a CAD model of a mold and an actual 3D-printed plastic mold.
Figure 2. A digital model (left) of a soft robot mold in a computer-aided design program, and a physical, 3D-printed copy (right) of the mold. This particular mold was designed in a program called Solidworks and printed on a 3D printer called the UP! Plus, but there are many different types of 3D printers and CAD programs.
This video shows a mold being printed:
The mold is then filled with liquid silicone rubber. Eventually the rubber solidifies (this process is called curing) but is still soft and stretchy, which enables the robot to inflate like a balloon with air. The unique shape of the mold creates appendages that can bend, instead of just expanding like a regular balloon. This lets the appendages function as "fingers" for a gripping robot or "legs" for a walking robot, like the ones you saw in the videos.
In this robotics project, you will follow this process to build your own robotic gripper. Remember: the robots in the videos were designed and built by professional, Ph.D. scientists at Harvard University! Your robot might not work as well as the gripper in the video, but this should still be a fun project!
Terms and Concepts
- Biologically inspired engineering
- Computer-aided design
- 3D printer
- What are some different kinds of robots? Can you list other robots not pictured in Figure 1 above?
- What are some potential advantages and disadvantages of soft robots?
- What is 3D printing? What can 3D printing be used for?
- Wood, J. (2012, May 3). Octopus escaping through a 1-inch hole. Retrieved July 18, 2013.
- GMWGroupHarvard. (2011, July 8). Soft Robotic Gripper. Retrieved July 18, 2013.
- IEEE Spectrum. (2011, November 29). Soft Robot Walking and Crawling. Retrieved July 18, 2013.
The following two references are technical publications from the Whitesides Research Group. The text may be difficult to understand, but the pictures might help you get a better sense of how the robots work.
- Ilievski, F., A. D. Mazzeo, R. F. Shepherd, X. Chen, and G. M. Whitesides. (2011, January 20). Soft Robotics for Chemists. Angewandte Chemie International Edition. Retrieved October 2, 2013.
- Shepherd, R.F., F. Ilievski, W. Choi, S. A. Morin, A. A. Stokes, A.D. Mazzeo, X. Chen, M. Wang, and G. M. Whitesides. (2011, October 15). Multi-Gait Soft Robot. Proceedings of the National Academy of Sciences (PNAS). Retrieved October 2, 2013.
Materials and Equipment
- Ecoflex® 00-30 (one "trial kit" is enough to make five to ten robots depending on size).
- (Optional): Ecoflex 00-50, which is stiffer than Ecoflex 00-30. Using both materials (00-30 for the top layer and 00-50 for the bottom layer, as explained in the Procedure) can help the robot bend more easily when inflated, but this is not required and will increase the cost of your project. Using both is recommended only if you plan to make a large number of robots (e.g., for an after-school program or summer camp), and need to purchase two or more Ecoflex kits.
- (Optional): Food coloring. The default color of cured Ecoflex is off-white, but you can use food coloring to customize your robots.
- 1/16 inch ID, 1/8 inch OD polyethylene tubing (1 foot). Part number 5181K15 at McMaster-Carr.
- Note: minimum order size for this tubing is 25 feet. You can use the extra tubing to make more robots.
- 1/8 inch ID, 1/4; inch OD opaque silicone rubber tubing (1 inch). Part number 5236K83 at McMaster-Carr.
- Note: minimum order size for this tubing is 10 feet. You can use the extra tubing to make more robots.
- Squeeze bulb: We recommend the Polaroid Super Blower with Hi Performance Silicon Squeeze Bulb, available at Amazon.com
- 3D-printed mold (See note below.)
- Plastic cafeteria tray or metal baking tray (Metal tray required only if you plan to use an oven; see below.)
- Disposable rubber gloves (one pair for each person doing the project)
- Plastic or paper cups (3)
- Wooden popsicle sticks (3)
- Paper towels for cleanup
- (Optional) Toaster oven. Do not use an oven that you also use to heat food. An oven can be used to speed up the curing of the silicone rubber, but it is not required to do the project.
- Important note regarding 3D printer materials: this project was originally tested using ABS plastic with an UP! Plus 3D printer. Due to the wide variety of 3D printer materials, we cannot guarantee that this project will work with every type of material. In particular, we have found issues with laser-sintered nylon and UV photopolymers, which are commonly available from online 3D printing services. Laser-sintered nylon tends to have a rough, porous surface, which makes it impossible to remove the silicone rubber from the mold. UV photopolymers can prevent the silicone rubber from curing completely, leaving it with a slimy, sticky surface. If possible, we recommend using an ABS mold. If you cannot obtain an ABS mold, the makers of Ecoflex (Smooth-On Inc.) recommend spraying your mold with a clear acrylic lacquer like Krylon Crystal Clear.
- If you own or have access to a 3D printer, you can print your own mold.
- If you do not have access to a 3D printer, you can order a mold from an online 3D printing service.
- Download either the big gripper or the mini gripper STL file.
- Choose an online 3D printing service. There are many available, including Shapeways, i.Materialize, and Sculpteo. You can find others by doing a web search for "3D printing service."
- Upload the STL file of your choice to the 3D printing service's website (this may require creating an account). Follow the website's directions to choose a material and order a part.
- Important: The dimensions of the STL files are in millimeters (mm). Many 3D printing services will ask you to select between millimeters and inches for the part.
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Important: before you begin, remember that it may take you several tries to get a working robot. Do not get frustrated if your first gripper does not work! You may have to tweak your process slightly and try again. Learning from your mistakes is an important part of the engineering design process!
- Obtain a 3D printed mold (see Materials tab for details).
- Prepare your work area.
- Safety Note: Ecoflex® is a brand of silicone rubber used for molding made by Smooth-On, Inc. Ecoflex comes in two bottles, containing part A and part B. Both parts are liquids — when mixed together, they will solidify and form silicone rubber (in four hours at room temperature, or ten minutes at 150 °F). The materials are nontoxic and harmless once cured. However, according to the Material Safety Data Sheet (or MSDS), "repeated or prolonged" exposure to the unmixed materials (part A and part B) can cause mild skin irritation. Wear disposable gloves when handling unmixed material. If you accidentally get either part A or part B on your skin, just wash it off with soap and water.
- Ecoflex can be messy. Work on a hard, flat surface that is easy to clean up. (Doing this project in a carpeted area is not recommended, in case you spill an ingredient on the floor). Covering your work area with a disposable tablecloth or working on a plastic cafeteria tray can also be good options. If you spill Ecoflex Parts A or B, you can clean them up with a paper towel. If you spill mixed Ecoflex, it will be easier to clean up if you wait 4 hours for it to solidify.
- Prepare mixed Ecoflex in a cup.
- Open your containers of Ecoflex 00-30 parts A and B (see Figure 3).
- Fill one of your plastic or paper cups about three-quarters full with equal amounts of parts A and B (it is OK to eyeball it; you do not need to measure exactly). How much you need will depend on the size of your mold, and whether you purchased Ecoflex 00-50. You can always mix more if you do not prepare enough.
- If you did not purchase Ecoflex 00-50, then you will need enough Ecoflex 00-30 to fill your plastic mold, and to make a flat "pancake" layer several millimeters thick that is slightly wider than your mold.
- If you did purchase Ecoflex 00-50, then you will only need enough Ecoflex 00-30 to fill your plastic mold.
- Optional: Add several drops of food coloring to the cup if you want to colorize your robot. Otherwise your robot will be off-white.
- Use your wooden stick to stir the Ecoflex in the cup for about two minutes. Be sure to thoroughly mix the Ecoflex. Food coloring may require extra vigorous mixing.
Figure 3. Ecoflex 00-30 parts A and B are mixed in a paper cup. Note: This picture shows a coffee stirrer, but it will be much easier to use a popsicle stick.
- Pour the mixed Ecoflex 00-30 into your mold. This will form the top half of your robot.
- Slowly pour the mixed Ecoflex out of the cup and into your 3D-printed mold, as shown in Figure 4. If you pour too quickly, the Ecoflex will spill out of the mold.
- Fill the mold all the way to the top. It is OK if you accidentally use a little too much Ecoflex and it overflows the edges of the mold — this will not affect the performance of your robot. However, if you use too little Ecoflex and do not fill the mold up all the way to the brim, your robot may not work very well.
Figure 4. Pour the Ecoflex 00-30 into the 3D printed mold.
- Prepare the bottom half of your robot.
- Optional: If you purchased Ecoflex 00-50, mix parts A and B in a new cup the same way you did in step 3, and then use it instead of Ecoflex 00-30 for this step.
- Pour your remaining Ecoflex from step 4 into the middle of your cafeteria tray or baking sheet. You should form a puddle slightly larger than your plastic mold, as shown in Figure 5. If necessary, mix more Ecoflex and add it to the puddle.
Figure 5. Uncured Ecoflex in the plastic mold and on a baking tray.
- Let the Ecoflex cure.
- Ecoflex will solidify, or cure, in four hours at room temperature. You can go do something else during this time, as you do not need to watch over the Ecoflex while it cures.
- Optional: You can use an oven to cure the Ecoflex in ten minutes at 150 °F. First check your mold for visible air bubbles in the Ecoflex, and wait five to ten minutes for all the air bubbles to disappear before putting your mold in the oven. You can speed the process up by popping the bubbles with a pin or toothpick. Be sure to follow the safety information listed when using an oven.
- Ask an adult to supervise.
- Do not use an oven that you also use to cook food in. You can buy a small toaster oven as a dedicated "science experiment oven" that is not used for cooking food.
- Double-check the safe temperature range for your 3D printed mold material before putting it in an oven. You may need to check with the manufacturer of your 3D printer or the company you ordered your mold from. Some plastics may melt or give off toxic fumes at 150 °F.
- Work in a well-ventilated area, preferably with open windows, a fan, or an exhaust system to blow any fumes outside. If at any time you notice fumes or a strange smell, turn the oven off and move to a well-ventilated area.
- Only use metal baking trays in an oven, not plastic cafeteria trays.
- Do not set the oven higher than 150 °F. This will not speed up the process notably, but it greatly increases the risk of melting your mold.
- Use oven mitts when removing hot objects from an oven.
- Check that the Ecoflex has solidified.
- After four hours at room temperature (or ten minutes in the oven), the Ecoflex should be solid. You can check this by touching it with your finger. (Note: If you used a toaster oven, use oven mitts to remove the mold and tray, and wait for them to cool to room temperature before touching them with your bare hands.) If the Ecoflex is soft and rubbery, then you can proceed to step 8. If it is still gooey or sticks to your finger, then it needs more time to cure.
- Remove the top half of the robot from the plastic mold.
- Gently peel the cured Ecoflex out of the plastic mold, as shown in Figure 6. Do not jerk the material or pull too hard, because it might rip.
- This will be easier if you start toward the outside of each "finger", and pull them up and inward toward the center of the robot, as shown in the first two panels of Figure 6.
Figure 6. Remove the top half of the robot from the mold.
- Apply the "glue layer" to bond the two halves of your robot together.
- Mix a small batch of fresh Ecoflex 00-30 in a new cup. You need only enough Ecoflex to apply a thin coating to the bottom layer of your robot.
- Apply a thin, even "glue" layer of fresh Ecoflex to your cured bottom layer, as shown in Figure 7. The glue layer should be approximately 1 mm thick. This thickness of the glue layer is critical because:
- Too much Ecoflex will clog the air channels in the top half and prevent your robot from inflating.
- Too little Ecoflex will create a weak bond between top and bottom halves and cause air leaks.
- If you accidentally pour on too much Ecoflex, you can soak it up with a paper towel.
Figure 7. Apply a thin, even "glue layer" of fresh Ecoflex about 1 mm thick on top of the cured bottom layer. It helps to use a coffee stirrer or popsicle stick to spread the layer.
- Place the top half of your robot onto the glue layer.
- The top half of your robot has two sides — one side is smooth, and the other side has ridges (see Figure 8).
- Place the side with the ridges facing down so it comes in contact with the glue layer (as shown in Figure 8).
Figure 8. Place the side of the top layer with ridges (pictured on the left) down onto the bottom layer.
- Seal the outer perimeter of your robot.
- Use a popsicle stick or coffee stirrer to apply a "sealant layer" of Ecoflex 00-30 around the outer perimeter of your robot, as shown in Figure 9. You can use any Ecoflex left over from step 9.
- Put a blob of Ecoflex on the top of your robot directly above the central air chamber, and also above any visible air bubbles, as shown in Figure 9 (this will help prevent them from popping open later).
Figure 9. Apply a "sealant layer" to seal around the outer perimeter of the robot (highlighted with red lines), above the central air chamber of the robot, and above any visible air bubbles.
- Let the "sealant layer" cure.
- Wait 4 hours at room temperature (or ten minutes in a toaster oven at 150 °F — follow the safety notes from Step 6) for the "sealant layer" to solidify, bonding the top and bottom halves of the robot together.
- Check that the new Ecoflex is solid and rubbery (no longer gooey/liquid-y) before proceeding to the next step.
- Remove your robot from the tray.
- Carefully peel both top and bottom halves of the robot off of the tray, as shown in Figure 10. If you used an oven, remember to wait for the tray to cool down.
Figure 10. Carefully peel both top and bottom halves of your robot (which should now be bonded together) off the tray.
- Cut out your robot.
- Use scissors to carefully cut around the outer perimeter of your robot, as shown in Figure 11.
- Be careful not to cut into the robot itself — this will cause air leaks. It is OK to leave a little extra material around the edges of the top and bottom halves.
Figure 11. Using scissors, cut out the perimeter of the robot.
- Puncture the robot with an air tube.
- Cut a roughly 1-foot (ft.) section of the polyethylene tubing. Cut one end of the tubing at a 45-degree angle so it is pointy — this will make it easier to insert the tubing into the robot.
- Use the pointy end of the tubing to puncture the robot from the side, at a 45-degree angle between two of the appendages, as shown in Figure 12. Aim the end of the tube for the circular central chamber.
- This step may be easier if you hold the robot up in front of a bright light (this will enable you to see the air channels inside the robot).
Photo and diagram of a cross-shaped silicon robot shows an air tube inserted into a central chamber inside the silicon robot. Four small channels lead from the central chamber of the robot to the end of each arm.
Figure 12. Puncture the robot with polyethylene tubing to provide the air supply.
- Connect the squeeze bulb.
- Cut a 1-inch section of rubber tubing.
- Use this piece as an adapter to connect the tip of your squeeze bulb to the polyethylene tubing (Figure 13).
- Note: if you are using the Polaroid Super Blower from the Materials section, remove the small rubber piece at the end of the plastic nozzle, and the 1/8-inch ID rubber tubing should fit directly onto the plastic nozzle. If you are using a different brand of squeeze bulb, you may need to use an additional piece of rubber tubing with a different interior diameter as an adapter (such as the one pictured in Figure 13).
Figure 13. Attach the squeeze bulb to the polyethylene tubing using rubber tubing as an adapter.
- Test your brand-new robotic gripper!
- Squeeze the bulb, and your robot gripper should begin to inflate (if not, see the Troubleshooting section).
- You may need to pump the squeeze bulb several times to get the robot to inflate fully.
- Try using your gripper to pick up different objects (see Figure 14).
- For your science project, make a table of objects that the gripper can and cannot lift.
- Determine the weight of the heaviest object your robot gripper can lift. Does the shape of the object matter? (For instance, can you test multiple objects with different shapes that weigh the same? What about objects with the same shape and different weights?)
- Note: If your polyethylene tube falls out of the robot gripper while you are testing it, re-insert the tube into the same hole (this will be easier if you hold the robot up to a light). Do not poke a new hole or your robot will leak.
- You can also try tying a string around your robot to support its weight, instead of hanging it from the polyethylene tube, as in this video of the original gripper (notice how the air tube goes off to the left side of the robot, whereas the whole robot and the object it grips are suspended from a string).
Figure 14. Test a soft robotic gripper by inflating it (left) and lifting different objects (middle and right).
For troubleshooting tips, please read our FAQ: Squishy Robots: Build an Air-Powered Soft Robotic Gripper.
Ask an Expert
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- If you have access to a CAD program, try designing and printing your own mold. Can you improve on the gripper design and pick up new objects that this project's robot gripper could not? (Note: Professional CAD programs can be very expensive — search online for "free CAD program" to find free or trial versions of consumer CAD programs.)
- Change the surface texture of your gripper to increase its gripping ability. For example, what if you embed the bottom layer of the gripper with sand or pencil erasers to give it more texture? Does that improve its gripping ability?
- Make the gripper from a different material instead of Ecoflex. Do different materials perform differently, even when you used the same mold? Warning: Always read the Material Safety Data Sheet (or MSDS) before handling new materials, and get adult supervision when using hazardous materials.
Frequently Asked Questions (FAQ)
- Check for air leaks. If your gripper doesn't inflate at all, listen and feel for air leaks when you squeeze the bulb. If you find air leaks, patch them with a fresh layer of Ecoflex. Let the patch cure, then try again.
- Check for clogged channels. Hold your gripper up to a bright light and look at the interior air channels. Can you see them in all four legs? If you can only see parts of the channels, or can't see them at all, they are probably clogged, which means you used too much "glue" in step 9. Unfortunately, you cannot repair this — you will have to start from the beginning. Luckily, you should have plenty of Ecoflex left over, and your mold, squeeze bulb, and tubing are all re-usable.
- Make sure the tip of the air tube is in the central chamber. Hold your gripper up to a light and look for the tip of the air tube. Did it make it all the way in to the central chamber? If you didn't fully puncture the outer wall of the robot, or pushed the tube too far and into the opposite wall, then the end will be sealed shut by rubber, and it will prevent air from getting into the channels. Wiggle the air tube until the tip is free in the central chamber.
- Some legs inflate better than others. If all four of your legs don't inflate to the same size and shape, first check each individual leg for clogged air channels, as described in the second troubleshooting tip. If none of the channels are clogged but the legs still don't inflate symmetrically, add a new, thin layer of silicone on top of the legs that are inflating the most. This should make it harder for them to inflate, allowing more air pressure to divert to the other legs. The material for your gripper robot has "nonlinear" behavior: instead of always inflating gradually with air pressure, it could not inflate at all and then suddenly "jump" and inflate. So, if one leg is slightly weaker than the others (due perhaps to some slight differences in wall thickness from the molding process), it might inflate first while the others don't inflate at all. Strengthening this leg with additional layers of silicone may prevent this.
- Gripper inflates like a balloon but the "fingers" don't bend. In order for the robot's appendages to bend, one half of the robot has to be stiffer (harder to stretch) than the other half. If your gripper's fingers don't bend, try making the bottom half thicker by repeating steps 5 to 10 in the Procedure (bonding your complete robot onto a newly formed bottom half). This will make the bottom half stiffer, and it should make your robot "fingers" bend more.
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