Build an Air-Powered Soft Robotic Gripper

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! Most of them are made of hard materials.

Replica models of WALL-E and EVE from the Pixar movie WALL-EA flying robobee resting on a fingertip

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, it has been found that it is  possible to design 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.

 

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
    • 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
    • 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,
  • 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)
  • Scissors
  • 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: 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!

  1. Obtain a 3D printed mold (see Materials tab for details).
  2. Prepare your work area.
    1. 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.
    2. 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.
  3. Prepare mixed Ecoflex in a cup.
    1. Open your containers of Ecoflex 00-30 parts A and B (see Figure 3).
    2. 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.
      1. 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.
      2. If you did purchase Ecoflex 00-50, then you will only need enough Ecoflex 00-30 to fill your plastic mold.
    3. Optional: Add several drops of food coloring to the cup if you want to colorize your robot. Otherwise your robot will be off-white.
    4. 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.
A blue and yellow container of two part silicon rubber next to a paper cup
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.
  1. Pour the mixed Ecoflex 00-30 into your mold. This will form the top half of your robot.
    1. 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.
    2. 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.
Liquid silicon is poured into a red cross-shaped mold
Figure 4. Pour the Ecoflex 00-30 into the 3D printed mold.
  1. Prepare the bottom half of your robot.
    1. 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.
    2. 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.
A red cross-shaped mold filled with silicon next to a puddle of silicon on a baking sheet
Figure 5. Uncured Ecoflex in the plastic mold and on a baking tray.
  1. Let the Ecoflex cure.
    1. 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.
    2. 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.
      1. Ask an adult to supervise.
      2. 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.
      3. 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.
      4. 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.
      5. Only use metal baking trays in an oven, not plastic cafeteria trays.
      6. 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.
      7. Use oven mitts when removing hot objects from an oven.
  2. Check that the Ecoflex has solidified.
    1. 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.
  3. Remove the top half of the robot from the plastic mold.
    1. 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.
    2. 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.
Three photos show cured silicon being pulled from a red cross-shaped mold
Figure 6. Remove the top half of the robot from the mold.
  1. Apply the “glue layer” to bond the two halves of your robot together.
    1. 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.
    2. 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:
      1. Too much Ecoflex will clog the air channels in the top half and prevent your robot from inflating.
      2. Too little Ecoflex will create a weak bond between top and bottom halves and cause air leaks.
      3. If you accidentally pour on too much Ecoflex, you can soak it up with a paper towel.
Two photos show a thin layer of uncured silicon spread over a larger sheet of cured silicon in a baking sheet
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.
  1. Place the top half of your robot onto the glue layer.
    1. The top half of your robot has two sides — one side is smooth, and the other side has ridges (see Figure 8).
    2. Place the side with the ridges facing down so it comes in contact with the glue layer (as shown in Figure 8).
Two photos show ridges on a cross-shaped silicone mold being pressed down onto a large sheet of silicon in a baking sheet
Figure 8. Place the side of the top layer with ridges (pictured on the left) down onto the bottom layer.
  1. Seal the outer perimeter of your robot.
    1. 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.
    2. 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).
Photo of uncured silicon added around the edges and in the center of a cross-shaped silicon mold
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.
  1. Let the “sealant layer” cure.
    1. 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.
    2. Check that the new Ecoflex is solid and rubbery (no longer gooey/liquid-y) before proceeding to the next step.
  2. Remove your robot from the tray.
    1. 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.
Two photos show a silicon sheet being peeled off of a baking sheet
Figure 10. Carefully peel both top and bottom halves of your robot (which should now be bonded together) off the tray.
  1. Cut out your robot.
    1. Use scissors to carefully cut around the outer perimeter of your robot, as shown in Figure 11.
    2. 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.
Scissors cut out a cross-shaped silicon robot from the center of a larger sheet of silicon
Figure 11. Using scissors, cut out the perimeter of the robot.
  1. Puncture the robot with an air tube.
    1. 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.
    2. 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.
    3. 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 of a cross-shaped silicon robot to the left of a diagram for a silicon robot
Figure 12. Puncture the robot with polyethylene tubing to provide the air supply.
  1. Connect the squeeze bulb.
    1. Cut a 1-inch section of rubber tubing.
    2. Use this piece as an adapter to connect the tip of your squeeze bulb to the polyethylene tubing (Figure 13).
    3. 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).
Rubber tubing connects the center of a cross-shaped silicon robot to a squeeze bulb
Figure 13. Attach the squeeze bulb to the polyethylene tubing using rubber tubing as an adapter.
  1. Test your brand-new robotic gripper!
    1. Squeeze the bulb, and your robot gripper should begin to inflate
    2. You may need to pump the squeeze bulb several times to get the robot to inflate fully.
    3. Try using your gripper to pick up different objects (see Figure 14).
    4. For your science project, make a table of objects that the gripper can and cannot lift.
    5. 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?)
    6. 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.
    7. You can also try tying a string around your robot to support its weight, instead of hanging it from the polyethylene tube Three photos of an inflated cross-shaped silicon robot curling in on itself and grabbing objects
      Figure 14. Test a soft robotic gripper by inflating it (left) and lifting different objects (middle and right).

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