Fabrication of Carbon-Based Ionic Electromechanically Active Soft Actuators

This protocol describes the fabrication of carbon-based electromechanically active material for biomedical and soft robotics applications. The main advantage of this method is that it enables the reproducible fabrication of ionic actuators also in large quantities. Watching this video should give you a good understanding on how to fabricate and use ionic actuators.

The protocol is divided into five steps. First, an ion-conducting membrane is prepared. It is then covered with carbon-based electrodes to which the gold current collectors are attached.

After cutting the sample into shape, the material is ready for use. To begin, choose between a tri-PTFE membrane, the same membrane soaked in electrolyte, or a textile reinforced membrane. Every option results in a functional actuator.

See the text for help with the selection. The material is prepared using a frame. Take a high porosity PTFE sheet and place it over the frame.

Taut and fasten the sheet on the frame. Be careful not to damage the dry membrane. Once the membrane is ready, skip to the electrode fabrication step.

Take a large Petri dish and place a high porosity PTFE sheet inside. Add an excess of ionic liquid. Make sure that the whole sheet is covered.

Once the membrane is sufficiently soaked, remove the excess using a pipette. Carefully place the membrane between filter papers to remove any remaining ionic liquid that was not absorbed by the PTFE sheet. Repeat this process until the sheet is semitransparent but not wet.

Taut and fasten the soaked membrane on a plastic frame. Make sure to avoid any wrinkles and folds. Now that the membrane is ready, skip ahead to the electrode fabrication step.

Take a fabric with fine inert fibers and fasten it onto a frame. Make sure to taut it well. Trim any excess fabric using scissors.

Carefully remove any loose fibers. While working under the fume hood, cover the fabric with a thin layer of membrane solution. See the text for exact recipe.

Let the first layer dry completely. First, use the heat gun and later the heat gun together with a dedicated setup to speed up the drying process. Avoid using a too high spin rate on a completely wet membrane as it may result in the loss of active material.

See the text for further details. Inspect the membrane against backlight for pinholes. Keep applying coating layers until a defect-free membrane is obtained.

Add subsequent membrane layers with extreme caution. Apply thin layers as possible and never go over already wet surfaces twice. Apply layers on both sides.

This way, the reinforcement will remain in the middle of the composite. Let one layer dry before adding another. Once a defect-free membrane has been obtained, check its thickness using a thickness gauge.

Currently, it is 54 micrometers. In a sealed flask, dissolve the polymer in the solvent by stirring overnight at 70 degrees Celsius using a magnetic stirrer and a temperature controlled hot plate. See the text for exact recipe.

Into another flask, weigh the carbon powder, add ionic liquid, solvent, and a magnetic stir bar. Seal the flask and mix. Once the carbon suspension has homogenized and the polymer has dissolved, fix or remove the magnetic bead and pour the polymer solution into the carbon suspension.

Use 10 milliliters of solvent to remove polymer residue from the flask walls and add it to the carbon suspension. Homogenize the suspension using an ultrasonic probe. After that, the suspension is ready for use or storage.

Fill the reservoir of a spray gun or air brush with acetone. Test the flow on a piece of paper first. Make sure that the air brush is clean and free of blockages.

During storage, the suspension may turn into a gel. Mix it in a closed vessel at 70 degrees Celsius to obtain a liquid. Fill the reservoir of the air brush with the electrode suspension.

Test the suspension flow on a piece of paper first. Take the prepared membrane. Start moving the spray gun before starting to spray.

Keep the gun moving in straight strokes. Let one side dry before starting to spray on the other. Spray until the desired thickness has been reached.

Carefully remove the material from the frame. If the textile reinforced membrane was used, then align the cut with the fibers. Cut a four by three centimeter piece using a metal ruler and a scalpel.

This cut size is most convenient for small to mid-sized batches. However, it is not crucial for obtaining working actuators. Take a metal tube or pipe and fix the cut piece on it.

Try to overlap only about one millimeter of the actuator material with tape. Take a sheet of fine gold on transfer paper and cut it into four by four centimeter pieces. Place one of them on a tissue paper.

Spray the composite with a thin layer of glue. See the text for exact recipe. Quickly the store the air brush upright.

Roll the pipe over the gold leaf while the glue is still wet. No excessive pressure is needed for rolling. Remove the transfer and roll over the tissue paper again to make sure that the gold is properly attached.

Place the material to dry. Once dry, carefully remove the tape to release the material from the pipe. Clean the pipe with acetone.

Fix the material on the pipe, gold coated side facing the pipe. Then repeat the steps to attach the current collector on the other side too. Note the sides that were covered with tape.

Cut out rectangles or more complex shapes. A four by 20 millimeter sample is good for characterization. Align the sample length with a curving direction.

The soft gripper must be thermoformed first. Place the actuator into a glass vial mold to thermoform the gripper into shape. Once the actuator is inside the mold, place them both into an oven or use the infrared light.

The gripper is placed between gold contacts, the gold side facing the active material. Voltage steps are applied to handle the payload. Opening the gripper.

Closing the gripper. Lifting the payload by hand. Testing the grip.

And finally, releasing the payload. Kelvin clips are used for characterization. Place the actuator between clamps and monitor the angle alpha using a video camera.

In case of triangular signal, the current response of a functional actuator is capacitive. Whereas the response from a faulty sample closely follows the Ohm's Law and is resistive. Use scanning electron microscopy to describe the actuator structure.

The samples are freeze fractured using liquid nitrogen to obtain clean-cut cross-sections. Caution:Never close the lid of a liquid nitrogen container. The pressure buildup could cause serious injuries.

First, freeze the actuator for a few minutes in liquid nitrogen. Then use two sets of cool tweezers to break the frozen sample. Textile reinforced actuators might not break even in the frozen state.

Freeze a scalpel together with the actuator and chop the frozen sample into two pieces. This is a cross-section of a PTFE actuator showing two carbon electrodes, separated by a PTFE membrane, forming the actuator. The key point of this method is the inclusion of an inert reinforcement such as PTFE in the membrane layer.

This simplifies the manufacturing process significantly and enables to reproducibly manufacture the active materials in large scale. Our method shows a promising route towards industrial-scale fabrication of ionic actuators.