Fiber-Reinforced Actuators

 Fiber-reinforced actuators are a class of soft actuator that have been developed by a number of research groups around the world in recent years. 

The basic design consists of an elastomer bladder wrapped with inextensible reinforcements.

The inner bladder acts like any typical balloon; when inflated it tries to expand in all directions.

Wrapping the bladder with inextensible fibers constrains it from expanding radially; when inflated it can only expand in the axial direction.
Adding a sheet of inextensible material prevents the actuator from expanding in the region of that sheet; since one side expands axially and one doesn't, the actuator bends when inflated.

This documentation set contains files and instructions to support the design, fabrication, modeling, and testing of a bending actuator; however it is possible to use the approach described here to make actuators with a wide range of motions. 

The placement of the reinforcements defines the type of motion achieved when the bladder is inflated. By selecting the appropriate placement, it is possible to achieve combinations of bending, extension, and twisting. This page explains how to design different motion types into your actuator. It is even possible to achieve different behaviors along the length of a single actuator. This documentation set includes a case study of a thumb assist device that incorporates an actuator that undergoes bending, twisting, and extension when inflated.

Compared to PneuNets bending actuators, the fiber-reinforced actuators have some advantages and disadvantages. On the one hand, the fiber-reinforced actuators can achieve a wider range of motions and are more robust as they have a smaller "seam" area: PneuNets have a seam around the entire perimeter where the top piece is bonded to the base layer, while fiber reinforced actuators only have a small seam at the end caps. This reduces the risk of delamination failure. On the other hand, the fabrication process for fiber-reinforced actuators is a lot more time- and resource-intensive: whereas a PneuNets actuator can be cast and assembled in less than an hour using three 3D-printed mold pieces, casting a fiber-reinforced actuator can take up to 5 days and requires six 3D-printed mold pieces. The selection of which type of actuator to use will depend upon your application, time constraints, and available resources.

Some of the information contained in this web site includes intellectual property covered by both issued and pending patent applications. It is intended solely for research, educational and scholarly purposes by not-for-profit research organizations. If you have interest in specific technologies for commercial applications, please contact us here.


The fiber-reinforced bending actuator documented here consists of a core bladder reinforced with a strain-limiting layer and inextensible fibers. This section of the documentation describes some of the design considerations involved in making your own actuator, followed by a detailed tutorial on designing the required molds. Solid model files of the actuator and molds can be downloaded here (SolidWorks 2013) and .stl files of the molds can be downloaded here.

Since design decisions are closely tied to the fabrication process, here we provide a brief overview of the steps involved (a more detailed explanation is available in the Fabrication section).

1. The actuator core is a hollow semi-cylindrical tube molded out of elastomer.

2. The inextensible, strain-limiting layer is a sheet of fiberglass attached to the flat face of the actuator core, preventing that face from lengthening when inflated.

3. The entire actuator is wrapped with inextensible Kevlar thread, which restricts radial expansion.

4. This thread wrapping is secured in place by molding an additional outer "skin" layer which encapsulates the threads and the actuator core.

5. Finally, the actuator is plugged at both ends, forming a closed chamber. A vented screw is installed in one of the end caps to provide access for an air source to inflate the chamber.

This fabrication approach involves a multi-step molding process. The actuator core is cast on a steel half-round which serves multiple purposes: providing the hollow central chamber, aligning the actuator within the molds, and providing structural reinforcement during the attachment of the strain limiting layers.

SRT_FR Mold SolidWorks Files.zip17.95 MB
SRT_FR Mold STL Files.zip2.01 MB

Variation: Motion

By varying the configurations of the strain-limiting components of the actuator (the inextensible layer and fiber wrapping), different motions can be achieved when the actuator is inflated. Here, we discuss how to achieve three types of motion: bending, twisting, and extension. These motion types can be combined, for example it is possible to design an actuator that will bend and twist when inflated. Furthermore, it is possible to vary the strain-limiting configuration along the length of a single actuator so that different behaviors can be achieved at different locations. See the case study for an example application of this approach.


By wrapping the fiber reinforcement in a symmetrical, double-helix configuration, the actuator is prevented from expanding radially and can only expand axially. The addition of a strain-limiting sheet of inextensible material prevents this expansion on one side of the actuator, resulting in an overall bending motion.


Again, a symmetrical, double-helical fiber wrapping limits radial expansion. However, in this configuration we do not add an inextensible layer and as a result the actuator will expand in the axial direction when inflated.


By using a single helical wrapping instead of a symmetrical double helical wrapping like in the above 2 cases, a twisting motion can be achieved. As the elastomer core expands, the thread helix also increases in diameter. But since the thread is inelastic and fixed in length, the diameter increase must be compensated by a reduction in the number of coils – resulting in a twisting motion opposite to the direction of thread wrapping.

Twisting and Bending

Single-helical fiber wrapping combined with a strain-limiting layer results in twisting and bending.

Twisting and Extending

Single-helical fiber wrapping with no strain-limiting layer results in bending and axial expansion.

Combining multiple behaviors

The actuator can be divided into segments, with each segment having a different motion type. This enables the design of more complex and customized actuator behaviors. In the example shown, when the actuator is inflated the left-hand segment will bend, the central segment will extend in the axial direction, and the right-hand segment will twist and bend. The ability to "pre-program" complex behaviors like this is one of the primary advantages of soft robotics over traditional rigid robotics.

Variation: Morphology

Cross-section type

Three common cross-sectional shapes seen in soft actuators are rectangular, circular, and semi-circular. Assuming identical wall thicknesses and cross-sectional areas, we have found the following:

  • The circular actuator is capable of applying the most bending torque for a given pressure, but it also has a high resistance to bending, making it the least efficient of the three shapes.
  • The rectangular and semi-circular shapes have roughly similar efficiency. However, the rectangular cross-section deforms into a quasi-circular shape when pressurized, while the other two cross-section types maintain their original shapes.
  • The sharp corners of the rectangular actuator may also increase the potential for stress concentrations and resulting fatigue/failure.

Radius, wall thickness, length

a) Increasing the radius of the actuator's semi-circular cross-section decreases the pressures required for the actuator to achieve a given amount of bending (in degrees). In addition, actuators with a larger radius (all other parameters being equal) achieve higher tip forces at a given pressure.

b) Longer actuators require lower pressures to achieve the same amount of bending as their shorter counterparts. However, length did not greatly affect the force exerted at the actuator tip for a given pressure.

c) Increasing wall thickness increases the pressures required for the actuator to achieve a certain amount of bending

Fiber wrapping spacing

As the spacing of the fiber wrapping gets closer together (increasing turn number n), lower pressures are needed to achieve the same amount of bending. At low turn numbers with fibers spaced far apart, significant bulging and radial expansion is observed in the area between threads.

Variation: Material

Elastomer options

As discussed in the PneuNets bending actuator documentation, materials stiffness defines the amount of pressure required to actuate and the output forces attainable. Low durometer (high strain) materials will bend at lower pressures but will produce lower output forces.

We have found Elastosil M4601 silicone from Wacker Chemie AG or Dragon Skin 30 from Smooth-On Inc. to be useful for the core bladder of the actuator. Lower-durometer materials could be substituted to change the behavior of the actuator.

For the outer skin layer, which is applied to hold the reinforcements in place, we use a lower durometer material such as Ecoflex 20 or Dragon Skin 20. Using a higher-durometer material for the outer skin would make the actuator stiffer and require higher pressures to actuate.

Radial constraint options
During the development of these actuators, various methods of adding radial constraints have been tried. For example, metal pinch rings can be used, but this leads to bulging in the actuators.
Thread wrapping, though more labor intensive, allows for the most customization. It is possible to use different thread materials, but Kevlar is preferred for its strength.

Inextensible layer options

In this documentation set we use fiberglass fabric as the strain-limiting layer. However, this can get messy, and in cases where the fiberglass is molded concurrently with the main chamber, stray fibers can create leaks. Any thin, inelastic material should work, including office paper or Tyvek. However, as these materials are less porous than fiberglass, they do not embed into silicone very well. To solve this, you can cut holes/slits into the material so that the elastomer can reach both sides.

Mold Design CAD Tutorial

This tutorial contains step-by-step instructions for making a solid model of a fiber-reinforced bending actuator mold in SolidWorks 2013. The SolidWorks part files can be downloaded here. If you prefer to use a different software package, you should be able to apply the general tutorial steps to most solid modeling environments.


The inner diameter of the actuator is defined by the steel rod used in the casting process. This tutorial and the associated part files assume that a 1/2" (12.7mm) diameter half-round rod will be used in the fabrication process.

The thickness of the actuator walls is defined by the difference between the rod diameter and the mold diameter. Here, we will create molds that will result in an actuator inner bladder with 2mm wall thickness.

Finally, the overall length of the actuator described here will be 170mm (6.7").

Of course, you can adjust these dimensions to suit your own design by modifying the relevant steps in the tutorial, or by editing the provided solid model files.


We will follow these steps to make our mold for the inner core of the actuator:

  1. Make the actuator body, an extruded semicircular tube (with filleted edges)
  2. Make one set of thread grooves with a swept cut. The sweep profile is a small circle, and the path is a helix.
  3. Mirror the swept cut to get the 2nd set of thread grooves.
  4. Make the main mold: create a base mold block, use an assembly to position the block relative to the actuator, then use the Cavity feature to subtract the actuator from the mold block.
  5. Repeat for the thin mold top, this time subtracting both the actuator and the main mold.
  6. Make the mold cap.

After making these molds, we can make the molds for the outer skin using the exact same process, even re-using the same solid model files:

  1. Make a copy of all the files in a new folder by opening the interim assembly for the top mold (which contains all the parts) and using File > Pack and Go. You should use the "Add Prefix" option to differentiate the new files. Do not just copy & paste files in Finder/Windows Explorer, as this will mess up your references!
  2. In the newly copied folder, open the actuator file. Get rid of the thread grooves by deleting the Mirror and Swept Cut features (check the "Also delete absorbed features" option to clean up the associated sketches).
  3. Increase the wall thickness by however thick you want the outer skin to be (1mm recommended). Just change the smart dimensions accordingly and rebuild the part.
  4. Open the interim assembly for the top mold, rebuild, and save. The main and top mold pieces will update themselves to fit the new actuator.
  5. If you feel that the top mold piece is too thin after the change, you can thicken the pieces.  In the interim assembly, double click on the top mold. The smart dimensions will appear; double click the dimension corresponding to thickness and increase it. Next, double click on the bottom ledge of the main mold, and again modify the smart dimension so that the ledge width is increased by the same amount. Rebuild the assembly and save all the parts.
  6. The mold cap needs to be updated manually, but it is very easy: open the mold cap file, change the wall thickness smart dimensions in the sketch, and rebuild.
SRT_FR Mold SolidWorks Files.zip17.95 MB

Make actuator body

Note: Your document units should be in the MMGS system! If they aren't, change your settings by going to Options, selecting the Document Properties tab, and clicking on Units in the side menu.

Draw base shape

[Video: Make actuator body]

In the top plane, draw a 12.7mm diameter circle, centered at the origin.

Draw a line across the diameter of the circle.

Use the Trim tool to remove half of the circle’s perimeter, leaving a semicircle.

Offset for actuator wall thickness

Now give the actuator 2mm wall thickness.

First, use Offset Entities to offset the sketch arc by 2mm. Make sure the “Select chain” option is unchecked so that the flat bottom is not offset.

We manually offset the bottom by drawing 3 lines to connect the arc endpoints, keeping everything at right angles.

Use Smart Dimension to make the bottom 2mm thick as well.


Exit the sketch, and Extrude the shape 170 mm.



[Video: Fillet edges]

Add a 1.1 mm radius Fillet on the two long edges of the actuator’s flat face.

Make thread grooves

Draw helix path

[Video: Make helix]

The thread grooves will be made using the Swept Cut feature. First, we have to make the path for the cut to follow, which is a helix. To create this helix, go to Curves > Helix and Spiral.

Select the bottom face of the actuator, and draw the base circle for the helix, which is 20mm in diameter and also centered at the origin.


Exit the sketch and set the helix parameters.

  • Defined By: Height and Pitch
  • Constant pitch
  • Height: 168mm (2mm shorter than actuator length to leave room for cap)
  • Pitch: 3.6mm
  • Start angle: 90degrees
  • Counterclockwise

If the helix is going in the wrong direction (not surrounding the actuator), then check “Reverse Direction.” The result should look like the image on the right.

Swept cut

[Video: Swept cut for thread grooves]

Now that we have the helix path, we can draw the profile for the swept cut. In the plane corresponding to the flat face of the actuator’s inner cavity, make a new sketch.

Draw a 0.4mm diameter circle, with its center aligned with the outer bottom edge of the actuator.


Using Swept Cut, use the sketches you just made and define them as the sweep profile and path. This will cut a series of grooves spiraling up the front of the actuator.



[Video: Mirror for symmetrical thread wrapping]

To get the other set of grooves, we can simply Mirror the swept cut we just made over the mid plane of the actuator, which corresponds to the Right Plane in this tutorial.

The result should look like this:

Make main mold

To make the molds, we use the Cavity feature, which enables us to do subtraction with bodies in an assembly. We have the molded actuator CAD part, but we also need the mold block we are going to subtract the rod from. The following section will cover making the main mold – we will make the top mold and cap piece later.

Make main mold block

[Video: Make main mold body]

The main mold starts out with a 29mm x 12.5mm x 169mm rectangular block which has 3mm x 3mm ridges to help with mold alignment and sealing.

In the top plane, sketch a rectangle and Smart Dimension it to 29mm x 12.5mm. Next, draw a smaller attached rectangle and smart dimension it to 3mm x 3mm, then draw a rectangle of matching height on the other side (use the dotted line guide to make sure the heights are equal).

Select the widths of the 2 small rectangles (hold down CTRL and click both segments). The properties panel should appear. Select "Equal" under the Add Relations section. The two rectangles should now be identical.

Use the Trim tool to delete the borders between rectangles so that you have one contiguous shape.

Exit the sketch and Extrude it 169mm.

The result should look like this:

Add ledge

[Video: Add ledge to mold body]

On the bottom face of the mold block, draw a rectangle that matches the width of the block and is 19mm high.

Extrude the rectangle 10mm, creating a ledge.

The result should look like this:

Add hole for inner rod

[Video: Add rod slot to mold]

Now we have to cut a hole for the inner rod to lock into.

Make a sketch on the newly created “ledge” at the bottom of the actuator.

Draw a Point at the midpoint of the ledge.

Use this point as the center of a 12.7mm diameter circle.

Using the same method as before (draw a diameter line and Trim), make this into a semicircle.

Next, we use Offset Entities to add 0.1mm for tolerance. This time we can offset everything, so use “Select chain.” Also check “Make base construction.”

Using Extruded Cut, cut in both directions from the sketch plane: 6mm down and 2mm up.

The result should look like this: 

Align mold block and actuator

[Video: Make interim assembly for main mold]

Save the mold block part, then create a new assembly. This will be the interim assembly used for the Cavity feature.

In the assembly, add the mold block and actuator. Position them with 3 mates:

  • Coincident: bottom face of actuator and bottom lip of mold
  • Concentric: inner actuator cavity and rod hole in mold block
  • Parallel: flat side of actuator, flat of mold block

The final assembly should look like this:

Subtract actuator from mold block

[Video: Subtract actuator using the cavity feature to get main mold]

Now that the parts are properly positioned relative to each other, we can subtract the rod from the mold block with the Cavity feature.

Save the interim assembly (but don’t close it yet). Click on the mold block in the FeatureManager design tree, then click the “Edit Part” button in the menu that pops up.

Now you are editing the mold block within the assembly.

Now, use the Cavity feature (Insert > Features > Cavity). If it's not in the menu, go to Customize Menu and add it first.

Select the Design Component, which is the part to subtract from the part you’re currently editing (the mold block). In this case, you want to subtract the actuator. To select it, expand the FeatureManager design tree and select the part corresponding the actuator.

Click the check mark to complete the Cavity. A dialog will pop up asking about bodies to keep. This is because in addition to the mold block, there is another body created by the hollow space inside of the actuator. However, we don’t need this body for anything, so we select only the body corresponding to the mold block to keep.

Now that this mold piece is done, click the “Edit Component” button in the corner to return to the assembly.

Make top mold

Now we will make the 2nd mold half, once again using the Cavity feature. We make a solid rectangular block, then subtract both the actuator and the main mold piece from it. Once again, we make the basic part, then assemble it to properly position everything relative to each other, then use Cavity.

Make top mold block

[Video: Make top mold block]

In the top plane, draw a 6.5 mm x 29 mm rectangle. Extrude it 169mm to get a long, thin rectangular prism. Save the part.


Align all parts

[Video: Make interim assembly for top mold]

Create another new assembly and save it as the interim assembly for the mold top. Insert 2 components: the thin block you just created, and the previous interim assembly. Position the components with 3 coincident mates (top, side, front).

The result should look like this:

Subtract actuator and main mold

[Video: Cavity for top mold]

Once again, click on the mold part in the design tree and click the “Edit Part” button.

Use Cavity (Insert > Features > Cavity). Once again, expand the design tree and select the component you want to subtract, in this case it’s the first interim assembly for the main mold. However, since you cannot select assemblies, expand the tree further and select the two parts (main mold and actuator) that make up the main mold assembly.


Click the checkmark to complete the cavity, and click the “Edit Component” button to return to the assembly.

Make mold cap

The final part needed for the mold is the cap piece, which aligns the top of the inner rod during molding.

Get sketch for mold cap

[Video: Sketch for mold cap]

To make the sketch for the cap, we can reuse the sketch from the actuator part file:

Open the actuator part file and expand the design tree so that you can see the sketch used to extrude the actuator body.
Select the sketch, then press CTRL+C to copy it.

Create a new part file, and press CTRL+V to paste the sketch.

Finalize sketch

Edit the pasted sketch.

First, if it doesn’t show up already, smart dimension the 2mm wall thickness offsets to strictly define the sketch so we do not accidentally modify these values while making the following edits. Next, add a rectangle surrounding the sketch, which defines the edges of the mold cap.

The rectangle doesn't have to be perfectly centered and dimensioned for the mold cap to work -- just make sure that there is a comfortable margin around the lip.

However, if you want the cap to look nice and flush with the rest of the mold, then dimension the rectangle to 29mm x 19mm, distance the inner flat face of the actuator 6.5mm from the bottom, and use Add Relations to vertically align the center point of the rod arc with the midpoint of the bottom edge of the rectangle.


Extrude selected contours

[Video: Extrude mold cap]

As this sketch has multiple closed contours, when Extruding it we only extrude sections of it at a time using the “Selected Contours” options.

First, select the “D” shaped contour and extrude it 1.5mm.

Select the sketch again in the design tree (it is now located under an extrude feature), then click Extrude in the Features toolbar to reuse it.

This time in "Selected Contours", select both the ‘D’ and the outer region of the sketch. Extrude it 3.5mm in the opposite direction from before (click the button with arrows next to “Blind” to reverse direction).

Complete the extrude. The final mold cap should look like this (front and back):

Fillet the outer corners of the lip, to match the fillet of the actuator (and corresponding cavity in the mold) so that the cap will fit. Alternatively, these corners can easily be cut or filed off after printing.

Make outer skin molds

We can make the molds for the outer skin repeating same process we just followed, but adjusting some of the dimensions. Instead of remaking everything, the easiest option is to make copies of the solid model files we just created, and then modify some of the dimensions:

  1. Make a copy of all the files in a new folder by opening the interim assembly for the top mold (which contains all the parts except for the mold cap) and using File > Pack and Go. You should use the "Add Prefix" option to differentiate the new files. Do not just copy & paste files in Finder/Windows Explorer, as this will mess up your references!
  2. In the newly copied folder, open the actuator file. Get rid of the thread grooves by deleting the Mirror and Swept Cut features (check the "Also delete absorbed features" option to clean up the associated sketches).
  3. Increase the wall thickness by however thick you want the outer skin to be. A 1mm increase is recommended, with a 1.5mm increase on the bottom to accommodate the fiberglass layer. Change the smart dimensions accordingly and rebuild the part.
  4. Open the interim assembly for the top mold, rebuild, and save. The main and top mold pieces will update themselves to fit the new actuator.
  5. The mold cap needs to be updated manually, but it is very easy: open the mold cap file, change the wall thickness smart dimensions in the sketch, and rebuild.

Fabrication: 3D Printed Mold

This section contains step-by-step instructions for fabricating a fiber-reinforced bending actuator. If you are making a different fiber-reinforced actuator, e.g. one that twists or extends or combines different motions along its length, the overall steps will stay the same; the only changes you will need to make will be during the second step when the strain limiting layer and fiber reinforcements are added. This should be straightforward, using this page as a guide.

Because of the multiple molding steps, the fabrication of these actuators takes much longer than PneuNets. However, you can make multiple actuators in parallel.

There are two processes described in the following pages: the fast curing process and the slow curing process. The overall steps are the same, the only difference is the amount of time spent on curing. Leaving the cast elastomer to cure at room temperature leads to better bonds between layers, but takes about 12 hours. Alternatively, an oven can be used to speed up the curing process, but can lead to weaker bonds and can cause the finished actuator to have a pre-bent neutral position (as some of the layers contract when cured in the oven). Each of the steps described here ends with the elastomer being left to cure. At these points in the instructions, the relevant steps for each process are described.

If you choose to follow the fast curing process, please read the tips for heat curing first.

To fabricate our actuator, we will follow these steps:

  • Step 1: Mold the actuator body (a semilunar tube) out of Elastosil M4601 (image A above).
  • Step 2: Demold the actuator, but leave the inner rod in to hold the shape. Glue on the strain-limiting layer (B).  Wrap with Kevlar thread (C).
  • Step 3:  Coat an outer skin of Dragon Skin to hold the wrapping in place, then remove the inner rod (D) and dip the actuator in Elastosil to plug one end (E).
  • Step 4: Install a vented screw into the plugged end and seal with silicone glue (see image below). Plug the other end of the actuator.
  • Step 5: Seal the newly plugged end with silicone glue. Once this is cured, the actuator is complete. Attach the actuator to an air source and pressurize it to make it bend.

Bill of Materials

This section will give a list of items that are used in this project with selected links to suppliers. You can download a more detailed Bill of Materials sheet here.

Note: Many of the items listed are just examples and you can use your own discretion to substitute parts which are easier or cheaper to obtain. 

Mold Components

Steel half-round rod*, 2 sets of molds (one for actuator body, one for outer skin coating)
Note: only one set of molds is shown in the image.

* The diameter and length of the half-round rod depends on the size of the actuator you want to make. In the fabrication and CAD tutorials shown here, we use a 1/2" diameter rod (supplier link), which has been cut with a hacksaw or bandsaw to be 3-4 cm longer than the planned actuator length. After cutting the rod, be sure to deburr and file the ends so that they are smooth and will fit well into the corresponding slots in the molds.

Click here to download the .STL files  needed to 3D print the molds for an actuator that is compatible with a 1/2" diameter rod, has 2mm wall thickness, and is 170mm long.

If you would like to modify the molds (for example if you want to use a rod of different dimensions) click here to download the SolidWorks part files.

Polymer Materials

Dragon Skin 10 (or 20) 2-part silicone rubber (datasheet)

Elastosil M4601 2-part silicone rubber (datasheet)

(Note: Dragon Skin 30 is a feasible alternative to Elastosil M4601)

Silicone sealant, Wacker Elastosil E951 (datasheet) OR Smooth-On Sil-Poxy (datasheet)

Other Materials

#10-32 vented screw (supplier link) + nut Laser-cut washers, 3mm acrylic (download DXF file) Kevlar thread (supplier link)
PTFE (teflon) pipe thread sealing tape Fiberglass fabric Isopropanol (IPA)

Tools & Hardware

Thin metal rod for piercing Quick-grip spring clamps Long** hex key (compatible with vented screw, 5/32" hex)

Lab gloves X-acto knife Female threaded hex standoff (supplier link) & tubing-to-male threaded pipe adapter (supplier link)***

** The length of the hex key also depends on the planned length of your actuator. It should be at least as long as your actuator so it can reach all the way inside. Here we use a 9" long hex key (supplier link), which is 5/32" hex size to be compatible with the vented screw we listed above.

*** Hardware components for connecting to air source (alternative options discussed here)


Vacuum chamber Benchtop vise Mass scale
Lab oven Centrifugal mixer Mixing cups
SRT_FR Mold STL Files.zip2.01 MB
SRT_FR Mold SolidWorks Files.zip17.95 MB
SRT_FR Laser Cutter DXF Files for Washers.zip6 KB
Fiber Reinforced Actuators Detailed BOM.xlsx14 KB

Step 1: Mold actuator body

Prepare elastomer

Elastosil needs to be mixed in a 1:9 ratio of Part A:Part B, by weight. The total amount you need depends on your mold. Here, we will make 50g total of Elastosil, so we need 5g of Part A.

(If using a different material see the vendor instructions on mixing ratios)

Take the bottle of Elastosil Part A (red fluid) and shake it vigorously, as it tends to settle. Measure out 5g in the cup, making sure to pour slowly so you don’t overshoot. If you do overshoot, you can pour some back into the bottle, or add more part B later to compensate and maintain the 1:9 ratio. [Video]

Now add 45 g (5g x 9) Elastosil Part B (white). Being accurate within 2-3 grams is okay. If you pour too much, you can remove some material with a spatula. [Video]

Place the Thinky cup with its contents inside the cupholder/adapter. Weigh this assembly, then adjust the mixer counterbalance (spin the dial) to match this weight. Press the ‘Start’ button to run the mixing program [Video]. The program should be set to 30s @ 2000 RPM (mixing) and 30s @ 2200 RPM (defoaming). Remove the cup of mixed elastomer from the mixer [Video].

Place the mixing cup containing the elastomer into the vacuum chamber and turn on the vacuum pump to remove any trapped gas. After about 5 minutes turn the pump off and remove the mixing cup. Pop any bubbles on the surface of the elastomer.

Pour elastomer into mold

Prepare your mold parts as shown. Insert the rod into the mold cap.

Pour a layer of the mixed Elastosil into the main body of the mold, going slowly to avoid spillage and trapped air pockets. Do not pour right to the open end of the mold (where the cap will go); stop about a centimeter from the edge.

When you have finished pouring, take the rod and come in at an angle to insert it into the hole at the base of the main mold.

Once the end is in the hole, slowly lower the rest of the rod into the elastomer, then slide the end cap forward to match the mold.

Make sure to push the rod really hard into the hole because the elastomer in there will resist the rod from going in and locking properly.

Now pour another layer of Elastosil to cover the rod.

Allow the Elastosil to settle for a few seconds and pop any visible bubbles.

Position the final part of the mold and gently push it into place. Clean up any spillage with paper towels.

Attach clamps to hold the mold together. Attach the first clamp at the bottom of the mold.

Attach another clamp near to the top of the mold, and on the opposite side so that the mold can balance while standing upright.

Clean any spillage using paper towels. The cap will probably have popped up when the clamps were attached. Push it back down until it snaps into the hole.


Stand the mold upright on a paper towel or petri dish to catch any leaking elastomer.

If following the slow curing process, leave the mold to cure overnight at room temperature. Tip: leave the mixing cup containing the remainder of your mixed elastomer beside the mold so that you can see when it’s cured.

If following the fast curing process: Leave the mold standing at room temperature for at least an hour, then remove the clamps and place in the oven (lying on its side with the flat side of the rod facing upwards) for at least 30 minutes at 65°C.

Step 2: Add strain limiting materials


In this step we will first remove the rod, with the actuator main body attached, from the mold. Then we will attach the strain limiting layer, which causes the actuator to bend. Finally, we will wrap the actuator with Kevlar thread to limit its radial expansion.

If using the slow curing approach, remove the clamps and place the mold in the oven for about 15 minutes, until it is warm and pliable. This will make opening the mold easier, and prevent the mold from snapping.

If using the fast curing approach, remove the mold from the oven.

Remove the mold cap, then peel away the thin flat mold piece. You can use a screwdriver or other tool to initially lever off the parts.

Prop the inner rod on an solid, elevated surface, flat side facing up. Near the top of the mold, press down on either side of the actuator until the mold pops off. Continue down the actuator until it is almost completely separated, then gently remove the actuator, taking care not to break the bottom of the mold.

Attach strain-limiting layer

Cut a strip of fiberglass slightly larger than your actuator (as the fiberglass will fray). Wearing lab gloves, use your finger to spread a thin layer of silicone glue along the flat side of the actuator.

Place the strip of fiberglass on top of the layer of silicone glue and press until all parts are glued down. Trim any excess fiberglass.

Variation: if you want your actuator to expand axially at some points along its length, do not attach a strain limiting layer at those points. See this page for more details.


Place the free end of the rod in a vise and tighten to secure. At the end of the actuator closest to the vise, wrap the Kevlar thread a couple of times and tie a knot.

Following the grooves, wrap in one direction until you reach the opposite end. 

Wrap the actuator just tight enough to stay in the groove; if it is too tight it will deform when the rod is removed.

When you reach the other end of the actuator, wrap a few times, then switch into the other set of grooves and wrap back to the start.

Variation: To make a twisting actuator, the thread should be wrapped in only one direction for the length of the section you want to twist. See this page for more details.

When you reach the start, wrap a couple of times and knot again (use a one-ended knot like a constrictor knot).

Rub silicone glue on the two knot areas to reinforce them. Allow the silicone glue to cure – if you used SilPoxy, this should only take 15 minutes.

Step 3a: Encapsulate

Mold outer skin

The next step is to mold a thin skin on the actuator, to lock the fiber reinforcements in place.

Make sure that the Sil-Poxy for gluing the fiberglass is completely cured -- it should not smell like vinegar at all. Otherwise, uncured Sil-Poxy may interfere with the curing of the skin.

Make 20g of DragonSkin 20, which should be mixed in a 1:1 ratio. After mixing, place the DragonSkin in the vacuum chamber for 5 minutes to remove any bubbles.

As before, prepare your mold parts and slide the cap onto the rod. Note that the cap for this step is different to the one used when molding the actuator body.

Pour a layer of elastomer into the main body of the mold, and then insert the rod and push the cap into place.

Again, make sure to push the rod in very forcefully so that it goes all the way into the hole. There will be a lot of resistance from the very viscous elastomer you have to displace. If you don't push in the rod far enough, you will not be able to close the end cap of the mold because the actuator is sticking out the top.

Pour another layer of elastomer to cover the rod and actuator main body.

Gently push the final part of the mold into place.

As before, attach clamps to the mold, working from the bottom up and balancing the clamps so that the mold can stand on end.


Stand the mold upright on a paper towel or petri dish to catch any leaking elastomer.

If following the slow curing process, leave the mold to cure overnight at room temperature. Tip: leave the mixing cup containing the remainder of your mixed elastomer beside the mold so that you can see when it’s cured.

If following the fast curing process: Leave the mold standing at room temperature for at least an hour, then remove the clamps and place in the oven (lying on its side with the flat side of the rod facing upwards) for at least 30 minutes at 65°C.

Using the slow curing approach is encouraged for this step as it results in a better bond between the actuator body and the outer skin.

Step 3b: Plug one end


As before, remove the clamps and clean the outside of the mold with a paper towel. Warm the mold in the oven, then peel it open using the same method as before.

Remove inner rod from actuator

Place the end of the rod in a vise, so that the actuator is not caught in the vise.

Twist the actuator 90 degrees – this reduces the contact area between the actuator and the rod, and therefore the friction.

Inject IPA for lubrication.

Pull, with one hand behind the actuator and one hand gripping the front half.


Cap one end of the actuator

Wrap Teflon tape around one end of the actuator – starting at the edge and continuing up about an inch. Do not cover the end face.


Mix 20g of Elastosil (1:9 ratio). Leave the Elastosil in the mixing cup, and dip the Teflon tape-wrapped end of the actuator in it. You can use a laser-cut guide plate to hold the actuator upright in the cup, or strategically lean it against something.


Leave overnight to cure. It is possible to put it in the oven to cure faster, but this seems to lead to a weaker bond, and the actuator will fail at this seam.

Trim cap of actuator

Cut away the mixing cup, leaving the actuator standing in a blob of Elastosil.

Trim away most of the Elastosil using an X-acto blade, leaving a thin island around the actuator.

Peel this layer down so that it’s sticking beyond the end of the actuator, like flipping an umbrella inside-out, and trim this excess away.

Peel off the Teflon tape, then cut a few mm off the end of the actuator to make it nice and smooth and to make sure your vented screw can go all the way through the cap (i.e. cap isn’t too thick compared to screw length).


Step 4: Install vented screw

Prepare actuator and screw

Pierce the actuator end using a thin rod and push it through until it’s sticking out the far end of the actuator.

Place the hex screwdriver in a vise, pointing upwards. Place the vented screw and small laser cut washer on the tip of the hex screwdriver.

Insert screw

Push the piercing rod into the hole at the end of the vented screw. Using the rod to guide the screw into place, slide the actuator down over the entire assembly until the screw comes through the hole at the far end.

If there is a lot of resistance, the screw has probably lost contact with the guide rod and is trying to puncture the wrong location – retract the assembly and try again.

Place the other laser cut washer over the screw, adding some more glue between the actuator end and the washer. Finally, add the nut and hand-tighten it.


Remove the actuator from the vise assembly and tighten the nut further using pliers. Do not over-tighten as the end cap will start to compress and bulge.


Paint the washer and actuator end with a thin layer of silicone glue. Leave it to cure.

Step 5: Cap other end and finish

Cap other end

Follow the exact same capping procedure as before to cap the remaining open end of the actuator:

  • Wrap Teflon tape around 1” of the actuator’s end.
  • Mix 20g of Elastosil (9:1 ratio) and leave it in the cup.
  • Dip the Teflon-wrapped end of the actuator into the cup, brace it so that it stays upright, and leave it to cure overnight at room temperature.

Paint the newly capped end with silicone glue to reinforce it. Let it cure.

Attach air source and actuate

The attachment method depends on the air source you use. If the air source has a threaded output, simply use threaded adapters to connect it to the #10-32 threaded vented screw. Depending on the air source's thread size and type (female/male), you can use female x male pipe adapters or coupling nuts. If the air source's thread is male and also #10-32, you can use female threaded standoffs, which are generally cheaper than coupling nuts and have more options available - just make sure the standoffs you select are fully threaded.

If the air source output is tubing, use threaded barbed connectors. Insert a tubing-to-male threaded pipe adapter into the air source, and connect it to the vented screw using a coupling nut/standoff.

A simpler but more expensive option is a tube-to-female threaded pipe adapter, attached to the vented screw and the barbed end inserted into the air source's tubing. 

Be careful not to over-inflate the actuator, as it may spring a leak or burst. These actuators can typically be pressurized up to 40-50 psi.

If the connection to the air source is leaking, you can wrap the threaded sections of the connector components 1-2 times with PTFE (Teflon) thread sealing tape before screwing them together. If the leak is in the actuator body itself, patch the leaking area with silicone adhesive.

Fabrication: Cardboard Mold


A 3D printer is an invaluable tool in the fabrication of soft robots. Unfortunately, these printers are expensive and not generally considered household objects. Those who do not have the luxury of a 3D printer are able to purchase printer time. However, both the cost and the time constraints required to ship 3D print jobs can deter new soft robotics hobbyists from entering the field. This mold is designed for those interested in entering soft robotics. It is used to produce fiber-reinforced actuators and is easily constructed using readily attainable, affordable and often household materials such as cardboard and hot glue.     

Overall Design 

This mold is perfect for beginners because of its simple geometry and low cost. The mold is constructed out of two distinct parts. The first part of the mold is made up of a central block and a surrounding wall. This produces the flexible top half of the robot. The second part of the mold is created by layering hot glue on a flat surface. It is used to fabricate the bottom inextensible half of the robot.  To assemble the robot, both halves are bonded together and wrapped with ribbon.

This type of mold is not limited to individual actuators. The mold geometry can be modified to make more complex robots such as this soft robotic gripper.

Bill of Materials

Materials and Equipment

               Cardboard     Ecoflex 00-30 silicon rubber                          (datasheet)  Hot glue gun and hot glue sticks
Tape 1/8" OD tubing Curling ribbon
          Printer paper

Step 1: Prepare Cardboard

Make Cardboard Mold Components

To make this mold you will need to cut 7 rectangles out of cardboard.

2 - 5" x 0.5"
2 - 6" x 075"
2 - 1" x 0.75"
1 - 6" x 12"


Glue the two 5” pieces together to form the central block.

To prevent rubber from flowing inside the cardboard, seal the block with tape. Wrap the block with a strip of tape and fold down the edges.

Smear hot glue on the top and bottom to hold the tape down. Seal the individual 6” wall pieces the same way.

Step 2: Assemble Mold

Assemble Top Mold

Glue the central block to a sheet of cardboard and trace the bottom with hot glue to prevent rubber from flowing underneath.
Build the surrounding wall around the central block, with a 0.25” gap between the block and the corresponding side walls and a 0.5” gap between the short sides of the block and the end walls.
To prevent the mold from leaking, seal all joints with hot glue. This should be done at least twice to ensure there are no leaks.

Assemble Bottom Mold

Make the bottom mold by tracing a 7” by 1.5” block with hot glue. After the glue cools trace the box again to add a second layer of hot glue. Repeat until the hot glue is stacked about a 0.25” high.

This is what the mold looks like when completed. (The paper is used in the fabrication of the actuator.)

Step 3: Fabricate Actuator

Actuator Fabrication

Prepared Cardboard Molds                                                                               
Ecoflex 00-30 
Printer Paper
1/8" OD tubing
Curling Ribbon                                                                                                                                      

Pour Elastomer Into Top Mold

To make the flexible top portion of the robot, fill the top mold with prepared Ecoflex so that the center block is submerged beneath roughly an 1/8" of rubber.

Fabricate Strain Limiting Layer

To make the bottom flat inextensible layer, cut a piece of office paper slightly larger than the top mold. Then spread a thin layer of Ecoflex across the inside of the bottom mold and place the paper inside. Flatten the paper and fill the mold with additional Ecoflex . Wait four hours for the Ecoflex to cure.


Assemble Actuator

After the rubber has cured, remove both pieces from the molds. Inspect the inside of the top half for imperfections. If imperfections are found, paint the inside with Ecoflex to fill them. Allow the rubber to cure.

Bonding the two halves of the actuator together

Assemble the actuator by bonding the two halves together with a layer of Ecoflex. Make sure that Ecoflex is on both the outside walls of the actuator in addition to the bottom.


Wrap Actuator

Image of a fully wrapped actuator After the actuator has cured, wrap it in an inextencible ribbon such as curling ribbon. Wrap the actuator both clockwise and counterclockwise. Once the wrapping is complete, puncture one end of the actuator with the tubing.


Connect the actuator to a pressure source and inflate. It is recommended to add another layer of rubber to the bottom of the robot to prevent the wrapping from moving during operation.




Finite Element Modeling

This section describes a Finite Element Method (FEM) model for a particular type of fiber-reinforced actuator using the Abaqus software suite. This actuator differs from the fiber-reinforced actuators discussed in the rest of the documentation, as it consists of a cylindrical elastomeric tube, with circular cross section, and with fibers wrapped in a helical pattern around the outside of the tube. The fibers constrain the motion of the actuator, as they have a much higher stiffness than the elastomer. The angle between the fibers and the horizontal is referred to as the fiber angle α.

When pressurized, this type of actuator is capable of four types of motion: radial expansion, axial extension, twisting about its axis, or bending. When the elastomeric tube is of uniform stiffness, some combination of expansion, extension, and twisting occurs upon pressurization. When the elastomeric tube is made of two different materials (with different stiffnesses), pressurization produces a bending motion.


The effect of the fiber angle on the behavior of extending, expanding, twisting actuators is illustrated in the video below.

This tutorial describes the procedure for creating an Abaqus model of a fiber-reinforced actuator which extends, expands, or twists upon pressurization. The Abaqus cae file created in this tutorial can be downloaded here: FR_tutorial.cae. To create a model of a bending actuator, follow the main tutorial, and take note of the alternative steps in italics, which describe the modifications which need to be made to produce a bending actuator.

Overview of model components

  • A Fluid-Structure Interaction (FSI) problem treated with a standard/implicit FEM using Abaqus/CAE (SIMULIA, Dassault Systèmes)
  • Materials:
    • Elastosil M4601 silicone rubber: neo-Hookean strain energy potential defined by the coefficient C10 = 0.12MPa.
    • Kevlar fiber: elastic material with Young’s modulus 31067MPa and Poisson’s ratio 0.36.
    • For a bending actuator: 
      • Dragon Skin 10 silicone rubber: neo-Hookean strain energy potential defined by the coefficient C10 = 0.0425MPa.
      • Smooth-Sil 950 silicone rubber: neo-Hookean strain energy potential defined by the coefficient C10 = 0.34MPa.
      • Kevlar fiber: elastic material with Young’s modulus 31067MPa and Poisson’s ratio 0.36.
  • Sections:
    • Elastosil (uniform solid), assigned to the main body and caps
    • Kevlar assigned to the fibers
    • For a bending actuator: 
      • Smooth-Sil 950 (uniform solid), assigned to half of the main body and to the caps
      • Dragon Skin 10 (uniform solid), assigned to half of the main body
      • Kevlar assigned to the fibers
  • 1 load:
    • Pressure, acting at all internal faces of the actuator cavity
  • Tie constraints between the fibers and the outer wall of the actuator

Overview of FEM steps

  1. Create parts
    1. Elastomeric tube and caps
    2. Fibers
  2. Create and assign materials
  3. Create surfaces and loads
    1. Create surface for applying pressure
    2. Create step
    3. Define boundary conditions
    4. Apply load
  4. Mesh the parts
  5. Add tie constraint
  6. Run job and view results

Scripting in Abaqus

More advanced users of Abaqus may note that all of the above can be done using the scripting interface. For a useful introduction to the Abaqus scripting interface see learnabaqusscriptinonehour.pdf. The scripts corresponding to the model developed in this tutorial can be downloaded here: and To run the script (equivalent to creating the model and running the job), go to File-> Run script, and browse to the file


Acknowledgements and References

The content of this tutorial is based on the following work:

  1. F. Connolly, P. Polygerinos, C. J. Walsh, and K. Bertoldi. “Mechanical programming of soft actuators by varying fiber angle”  Soft Robotics, 2015 vol 2 pp 26-32.

Many research groups have worked on this type of actuator, and further interesting references include:

  1. G Krishnan, J Bishop-Moser, C Kim, and S Kota, "Kinematics of a generalized class of pneumatic artificial muscles", J Mech Robot 2015 doi:10.1115/1.4029705
  2. S Hirai, P Cusin, H Tanigawa, T Masui, S Konishi and S Kawamura, "Qualitative synthesis of deformable cylindrical actuators through constraint topology" IROS 2000 pp. 197-202
  3. K Suzumori, S Iikura, and H Tanaka, "Flexible microactuator for miniature robots", Proceedings of Micro Electro Mechanical Systems 1991 pp. 204-209
learnabaqusscriptinonehour.pdf257 KB
fr_scripts.zip14 KB
fr_tutorial.zip986 KB

Step 1a: Create Parts

In this step, we will create the elastomeric tube and fibers that make up the actuator. Make sure to download the file, as we will use that to create the fibers later.
For a bending actuator, download the file

Create tube

To begin, open a new project in Abaqus CAE. You should see a screen like below:

We form the main body of the actuator by creating a tube and then capping it at each end. To make the tube, double click on Parts. Name the part ‘tube’ and make it a 3d deformable solid extrusion.


Now sketch the cross section of the tube. Click on Create Circle: Center and Perimeter

Enter (0,0) as the center of the circle, and enter (6.35,0) as the perimeter point. Make a second circle with (0,0) as the center of the circle, and (8.35,0) as the perimeter point. Click cancel procedure and done. Enter 165 as the depth, and click OK

This gives us a tube with inner diameter 6.35mm, outer diameter 8.35mm, and length 165mm.

Create Caps

Next, create the geometry for the caps.

  • Double click on Parts again.
  • Name the part ‘cap’ and make it a 3d deformable solid extrusion like the tube.
  • Click on Create Circle: Center and Perimeter.
  • Enter (0,0) as the center of the circle and (6.35,0) as the perimeter point.
  • Click cancel procedure and done.
  • Enter 5 as the depth, and click OK

Add parts to assembly

Now we need to merge the tube and the caps to make one part. To do this, we first need to add instances of each part to the assembly. Expand the assembly tree and double click on Instances

In the Create Instance box, make sure Dependent (mesh on part) is selected. Double click on ‘tube’ to add an instance of this to the assembly. Double click on ‘cap’ twice to add two instances of this to the assembly (one for each end). Click on Cancel.

Both of the caps are now positioned at the same end of the actuator, so we need to translate one of them to position it at the other end. In the main toolbar, click on Instance->Translate

Click on Instances in the bottom right corner of the screen.

Select ‘cap-2’ and click OK in the instance selection box.

Enter (0,0,0) as the start point for the translation vector and hit enter. Enter (0,0,160) as the end point, hit enter and then click OK to confirm the position of the instance.

Now there is a cap positioned at each end of the actuator.


Merge parts

To merge the caps and the tube, go to Instance->Merge/Cut

Name the new part ‘Merged_Actuator’, select Original Instances: Delete, and click Continue

When prompted to choose which instances to merge, click on Instances in the bottom right corner. Hold the ctrl key, select the three instances and click OK.

Now we have created a new part called ‘Merged_Actuator’, and an instance of this part has been added to the assembly.

For a bending actuator, we need to partition the actuator so we can assign different materials to the top half and bottom half of the actuator.

Double click on 'Merged_Actuator' under ‘Parts’ in the model tree. This will allow you to edit the part.

In the main toolbar, go to Tools->Datum. Select Plane, Offset from principal plane. Choose the X-Y plane, and enter 5 (the cap thickness) as the offset amount. Repeat, choosing 160 as the offset amount.

Now go to Tools->Partition. Select Cell and Use datum plane. Click on the first datum plane you created, and click Create partition, to complete the partition definition.

Now click on the main body of the actuator (the cell to be partitioned), click ‘Done’, select the second plane you created and click Create Partition.

Now the caps have been partitioned from the rest of the actuator. 

To partition the actuator into its top and bottom halves, define another datum plane: go to Tools->Datum. Select Plane, Offset from principal plane. Choose the X-Z plane, and enter 0 as the offset amount.

Again, go to Tools->Partition. Select Cell and Use datum plane. Select the middle portion of the actuator as the cell to be partitioned, and select the datum plane you just created, and click Create partition, to complete the partition definition.

Now we can assign different materials to the top, bottom and caps of the actuator.



Step 1b: Create Fibers

Create fibers

The second major component of the actuator is the set of fibers around the outside. To create these fibers, double click on Part again. Name the part ‘fibers’ and make it a 3d Point. Hit enter to accept the default of (0.0,0.0,0.0) as the coordinates.


Go to File->Run Script. Browse to the file and run it. This file creates a helical wire structure, with radius 8.35mm, height 163mm, and fiber angle 3°. An actuator with this fiber angle will extend and twist upon pressurization.

Note: the fiber angle can be changed by modifying the parameter ALPHA in

For a bending actuator, run the file This will create a symmetric arrangement of fibers (fibers at equal and opposite angles).


To view the fibers, click on Apply Iso View.

Add fibers to assembly

Now we will make a set containing the fibers, as we will use this later in the model. In the model tree, expand Parts->Fibers and double click on Sets.

Name the set FiberSet and click Continue

Click on the fibers to select them as the geometry for the set, and click Done.  

To add an instance of the fiber to the assembly, expand the assembly tree, double click on Instances, select ‘fibers’ in the Create Instance box, and click OK.

Step 2: Create and assign materials

Create and assign materials

Next we will create the two materials used in this model: Elastosil (for the elastomeric tube) and Kevlar (for the fibers).

To create the first material, double click on Materials in the model tree. Name the material ‘Elastosil’. This is a nonlinearly elastic material, so we assign it the property Mechanical->Elasticity->Hyperelastic. We will model it using a neo-Hookean strain energy potential. Choose ‘Coefficients’ as the input source, with C10=0.12MPa and D1=0.0. Click OK.

For a bending actuator, instead of defining the Elastosil material, define the materials Smooth-Sil 950 and Dragon Skin 10, as follows:
Double click on Materials in the model tree. Name the material ‘DS10’. This is a nonlinearly elastic material, so we assign it the property Mechanical->Elasticity->Hyperelastic. We will model it using a neo-Hookean strain energy potential. Choose ‘Coefficients’ as the input source, with C10=0.0425MPa and D1=0.0. Click OK.
Make a second material and name it 'SS950'. Again, we will model it using a neo-Hookean strain energy potential. This time set C10=0.34MPa and D1=0.0.

To make the material for the fibers, we double click on Materials again. Name this material ‘Kevlar’. We model it as a linearly elastic material, with properties Mechanical->Elasticity->Elastic. Enter 31067 MPa for the Young’s modulus and 0.36 for the Poisson’s ratio. Click OK

Double click on Sections in the model tree.

Name the section elastosilSection. Click Continue, choose Elastosil as the material, and click OK

For a bending actuator, instead of defining the section elastosilSection, use the materials DS10 and SS950 to define DS-section and SS-section.

Double click on Sections again and name this section kevlarSection. Select Beam as the Category and as the Type, and click Continue

In the ‘Edit Beam Section’ dialog box that appears, click on Create Beam Profile

Name the profile fiberProfile, and select ‘Circular’ as the shape.

Click Continue, enter 0.0889 as the radius ‘r’ and click OK. This models the fiber as a beam structure with a circular cross section.

In the ‘Edit Beam Section’ dialog box, select ‘Kevlar’ as the Material name and enter 0.36 as the Section Poisson’s ratio. Click OK.  

Now we need to assign a material to each of the parts. Expand Merged_Actuator under Parts and double click Section Assignments. Select the actuator and click Done.

Click OK in the ‘Edit Section Assignment’ box.

For a bending actuator, assign SS-section to the caps and the bottom half of the actuator. Assign DS-section to the top half of the actuator.

Expand fibers under Parts and double click Section Assignments. Select the fiber and click Done.

Click OK in the ‘Edit Section Assignment’ box.

In the main toolbar, click Assign->Beam Section Orientation. Select the fiber and click Done.

Accept the default n1 direction of (0,0,-1) by hitting enter. Click OK


Step 3: Create surfaces and loads

Create surface for applying pressure

We will want to apply a pressure load to the internal surface of the actuator, so we first need to create this surface. Expand Merged_Actuator under  Parts, and double click on Surfaces. Name the surface ‘InnerSurf’.

In order to access the inner surfaces, we need to deselect the Select Entity Closest to Screen icon.

Now hold the shift key and select the inner surfaces of the actuator (the curved surfaces and the two flat surfaces) and click Done. To check that the new surface has been defined correctly, you can go to Surfaces and click once on ‘InnerSurf’, and check that the correct surfaces are highlighted.

Later, we will also need to refer to the outer surface of the actuator, so under Parts->Merged_Actuator, double click on Surfaces again. Reselect Select Entity Closest to Screen. Name the surface ‘OuterSurf’. Select the outer curved surface of the actuator and click Done.

Create Step

In the model tree, double click on Steps, to create the step in which we will apply the pressure load. Accept the default options of a static general step.

In the ‘Edit Step’ dialog box, select Nlgeom ‘On’ under the Basic tab. Under the Incrementation tab, set 1000 as the Maximum number of increments, 0.01 as the Initial increment size, 1E-5 as the Minimum increment size, and 0.01 as the Maximum increment size. Click OK

Define Boundary Conditions

For our boundary condition, we will fix the actuator completely at one end. Double click on BCs in the model tree. Choose Displacement/Rotation as the type of boundary condition and click Continue.

Click on Rotate View to rotate the actuator so you can select the actuator face located at (0,0,0), and click Done

We want to impose zero displacement on this actuator face, so check the boxes next to U1, U2 and U3, and accept the default displacement values of 0.0.

To control the rate at which the pressure load is applied, we create an amplitude. Double click on Amplitudes, and select ‘Equally spaced’. In the ‘Edit Amplitude’ dialog box, enter [0,1] in the coumn under ‘Amplitude’. This will apply the load linearly as a function of time.

Apply load

Now we define the pressure load, by double clicking on Loads. Select ‘Pressure’ as the load type, and click Continue.

When prompted to select a surface for the load, click on the Surfaces button at the bottom right of the screen.

Select ‘Merged_Actuator-1.InnerSurf’.
Enter 0.06 as the magnitude in the ‘Edit Load’ dialog box, and select ‘Amp-1’ as the amplitude.

Step 4: Mesh


Double click on Parts->Merged_Actuator, then double click on Parts->Merged_Actuator->Mesh.

Go to Mesh->Controls. Select ‘tet’ as the element shape and click OK.

Go to Mesh->Element Type. Select ‘Quadratic’ as the Geometric Order, and under the Tet tab, check the box for ‘Hybrid formulation’.

Go to Seed->Part, and choose 2.0 as the ‘Approximate global size’. The appropriate size to enter here depends on the size and geometry of the part being modeled, and the amount of deformation it undergoes.

Go to Mesh->Part, and click Yes to confirm it’s ok to mesh the part.

Under fibers in the model tree, double click on Mesh. Go to Mesh->Element Type. In the ‘Element Type’ dialog box, choose quadratic beam elements (B32) and click OK.

Go to Seed->Part, enter 1.0 as the ‘Approximate global size’, and click OK.

Finally, go to Mesh->Part to mesh the fibers.

Step 5: Add tie constraint

Add tie constraint

The fibers and the elastomeric tube need to be connected to each other in some way, and this connection is modeled as a ‘tie constraint’, which ties the fibers to the surface of the actuator. To apply this constraint, double click on Constraints in the model tree. Accept the default type of ‘tie’ and click Continue.

  • Choose ‘Surface’ as the master type.
  • Click on Surfaces in the bottom right corner, select ‘Merged_Actuator-1.OuterSurf’, and click Continue.
  • Choose ‘Node region’ as the slave type. Select ‘fibers-1.FiberSet’ and click Continue.
  • Click OK in the ‘Edit Constraint’ dialog box to impose the constraint.  

Step 6: Create and run job

Create and run job

In the model tree, double click on Jobs. Give the job a name, and accept all the default settings. In the model tree, right-click on the job you just created, and select Submit.

View Results

When the job has completed, right-click on the job name and select Results.

The default result displayed is the von Mises stress. This can be changed using a dropdown menu. For example, you can choose to visualize U3, the displacement in the z direction, to see how much the actuator is extending.

The navigation buttons at the top right of the screen can be used to scroll through the step increments.

 To view an animation of the solution, click on the animate button.

Segmented Actuators: Design Tool

Designing a fiber-reinforced actuator to perform a specific task can be a challenging problem and often involves a lot of trial and error. To speed up the process and find the optimal actuator design for the task at hand, we can use mathematical models which relate actuator geometry and materials to actuator output motion.

For example, we may wish to design an actuator which upon pressurization replicates the motion of the thumb or index finger. To achieve this, we put (1) the kinematics of the desired motion and (2) analytical models of actuator motions into an optimization algorithm. The algorithm then outputs a recipe for designing an actuator which will achieve the required motion.

More details on our design strategy can be found in the following paper:
F. Connolly, C.J. Walsh and K. Bertoldi, Automatic Design of Fiber-Reinforced Soft Actuators for Trajectory Matching, PNAS 2016, doi:10.1073/pnas.1615140114

The Matlab scripts which implement the design strategy can be downloaded below. The main file is run_optimization.m, and the other files are functions called by the main script.


segmented_design_scripts.zip6 KB


A thin bend sensor can be integrated into the actuator, and in conjunction with an analytical model can give actuator bending angles and interaction forces. The sensor is first embedded separately in a thin layer of low-durometer silicone rubber (i.e. Ecoflex 00-30), and this sensing layer is then bonded to the flat face of the actuator using silicone glue.

Fiber reinforced actuators can be characterized by using an evaluation platform like the one used to test PneuNets. As they are both bending actuators, similar tests can be conducted, i.e. measuring forces at the actuator tip using a 6-axis force/torque sensor.

For position tracking of simple in-plane bending, image analysis can be used. However, since fiber reinforced actuators, unlike PneuNets, are capable of twisting and other out-of-plane motions, more advanced position tracking methods may be needed. One such option is electromagnetic 3-D tracking.

Above,  E/M sensors (3D Guidance TrakSTAR, Ascension Technology Corp.) are attached at various points along the actuator bottom face to characterize actuator behavior along its length, not just at the tip. Positional data is collected using the software provided by the manufacturer, and data can be analyzed and visualized in MATLAB.

Case Studies

Soft Wearable Device for Thumb Rehabilitation

This case study describes the design of a wearable device for thumb rehabilitation, and is based on work carried out by Maeder-York et al. (2014)

Soft Prosthetic Hand for Amputees

This case study describes the design and construction of a soft prosthetic hand for amputees designed by a group of students in the Global Immersion Summer Program in India.

Soft Wearable Device for Thumb Rehabilitation

This case study describes the design of a wearable device for thumb rehabilitation, and is based on work carried out by Maeder-York et al. (2014)

Clinical Need

As discussed in the PneuNets glove case study, loss of grasping ability is a significant problem which negatively impacts quality of life in patients. It is possible to recover some lost function through intensive physical therapy, which typically involves the use of repetitive task practice (RTP). However, RTP requires a therapist to assist the patient and guide the hand through the correct motions. As RTP also needs to be performed continuously for several hours, this results in high therapy costs and limits the number of patients a therapist can see. 

The aim of the project described here was to develop a wearable device that replicates and restores correct thumb motor function for opposition grasp for patients with neurologically caused hand disabilities.


The design consists of a multi-segment fiber-reinforced actuator, mounted to a a conformable neoprene-padded aluminum attachment with Velcro straps, and tethered to a portable control system.

In order to determine the types of motion required from the actuator, electromagnetic (EM) trackers (3D Guidance TrakSTAR from Ascension Technology Corp.) were used to study the motion of the thumb during opposition grasp. This study yielded values for the amount of bending, twisting, and extension required of the actuator. Following the guidelines described on this page, the project team was able to design an actuator which, when inflated, would mimic the motion of the thumb.

In any wearable device design, methods of attaching to the patient are a non-trivial issue. In this project, one end of the actuator was mounted to a conformable aluminum plate which could wrap around the patient's hand. Velcro straps along the length of the actuator attached it to the patient's thumb. The full assembly was sewn into a glove.

The portable control box for the device was a modified version of the Soft Robotics Toolkit control board, as described on this page.
This video gives a demonstration of the final device in operation.

Soft Robotic Prosthetic Hand for Amputees

This case study describes the design and construction of a soft prosthetic hand for amputees designed by a group of students in the Global Immersion Summer Program in India.


Clinical Need

Diabetes, peripheral artery disease and trauma cause hundreds of thousands of upper limb amputations worldwide per year. For amputees, restoring the utility of a missing hand is a major factor in being able to do activities of daily living (ADLs) like eating, bathing and dressing themselves. Gaining the ability to do these basic tasks is extremely helpful to the patient achieving a higher quality of life. Current prosthetics including body powered and myoeletric devices give a lot of this function back to the patient, but can be tiring to use, expensive, stiff, or have a limited range of motion.



The team’s solution consists of 4 fiber reinforced bending actuators that mimics the index, middle, ring and little fingers. The thumb is an aluminum mechanism actuated by pneumatic artificial muscles.

The overall goals of the project were to design a soft prosthetic hand that was: cost effective, aesthetic, provided sufficient grip strength for ADLs, low maintenance, and light weight. The design of the hand was broken up into 4 separate modules: Thumb, Palm, Fingers, and Control. These 4 modules were determined to need to come together to into a few potential grip patterns that would be useful to complete ADLs, including pinching, open palm, pointing and a power grip.

The thumb design needed to be rigid and able to transmit forces in multiple different orientations for the different grip patterns. To achieve these grip patterns, the thumb was designed as a hinged aluminum mechanism. For more information on the thumb's design and actuation visit the case study page under the Pneumatic Artificial Muscles section.

The four fingers consisted of fiber reinforced actuators fabricated exactly as laid out in this section. The four fingers were attached to a palm, which was a passive hinged mechanism. The students experimented with materials such as wood and acrylic before deciding on aluminum for the palm.

The fiber reinforced actuators were controlled by a version of the Open Source Control Board described on this site.

The following video shows the hand in action:


Files for Fabrication

Files for Modeling

Some of the information contained in this web site includes intellectual property covered by both issued and pending patent applications. It is intended solely for research, educational and scholarly purposes by not-for-profit research organizations. If you have interest in specific technologies for commercial applications, please contact us here.

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