Robotics and Biology Laboratory

RBO Hand3 Compilation

Welcome to the RBO Hand 3(RH3) Compilation. The goal of this page and the accompanying folder here is to get you started building the RH31, as developed at the Robotics and Biology Lab at Technische Univeristät Berlin. To simplify the rather extensive process, we'll break it down into pieces wherever possible.

There are four types of components needed to create the RH3:

  • 3D printed pieces
  • silicone molded fingers,
  • lasercut pieces
  • and the bellow actuators.

We'll go through the creation of each of these types of pieces and discuss assembly. We also have a video tutorial on how to cast the molds and assemble the connector plates on youtube, which you can find here.

 1. Steffen Puhlmann, 2020

RBO Hand3 in Action


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Guide to File Formats

2D Files: .AI, .SVG, .DXF
All files to be processed with the lasercutter are Vector Graphics files: *.ai, *.svg or *.dxf. These are 2D files to be used on flat materials. You can open them in Illustrator, Lightburn, Inkscape, or most other Vector Graphics programs.
.AI files are native to Adobe Illustrator.
.SVG [Scalable Vector Graphic] is one of the more versatile formats for editing Vector Graphics, especially with non-proprietary software like Inkscape.
.DXF is an industry standard file format for 2D CAD designs.

3D Files: .STL

.STL is an industry standard for files that need printing with 3D printers. You can open them with Fusion360, Solidworks, AutoCAD, TinkerCAD, Blender, Cura or any other program that supports 3D models.

Bill Of Materials

  • PLA/ABS (or equivalent) FDM Prints
  • TPU FDM Prints
  • SmoothOn Dragonskin 10 (minimum 250g)
  • SmoothOn EcoFlex 30 (minimum 300g)
  • Acrylic Glas 4mm (minimum 20x100 mm)
  • 4x M3 10mm
  • 3x M3 Nuts
  • 4x M3 Sleeve nut with countersunk head
  • 3x M2 20mm
  • 8x M2 10mm
  • Thread (we use polyester, specifically AMANN's saba thread)
  • Passive layer fabric (SEFAR PET 1500 54/137-64W PW)
  • Connector plate fabric (two sided wool fabric from coatmaking, the important feature is that superglue doesn't soak through both sides because the softer, wooly side that connects with the silicone must remain soft)
  • TPU coated Nylon (210den, TPU-coated one side, 275g/sqm, heat-sealable, from e.g.
  • TPU sheet (for added heat-sealability)
  • Baking paper
  • Superglue
  • Sil-Poxy
  • Silicone tubes 1.2 x 1.2mm (minimum 3meters)
  • Silicone tubes <2mm (the exact size is not important, it's used as a permeable spacer in a pneumatic system, we use 1.2mm outside diameter) (<0.5m)
  • Hard tube connectors for the silicone tubes (for example
  • Office Binder Clips
  • Vaccuum Chamber


  • SuperSeal by SmoothOn to seal the molds
  • Silicone Dye (the RBO lab uses SmoothOn Silc Pig Blue)

3D Printing

There are 12 pieces that need to be 3D printed: While you can print a single version of each mold, you'll need to use the finger mold four times for each hand, so making multiple fingers at once with multiple molds can be helpful. Please pay attention that you print the two pieces marked (TPU) (Median Scaffold and Thumb Scaffold) out of flexible TPU material. Our recommendation is to print the following pieces out of PLA or equivalent. The material just needs to be durable, and not chemically inhibit the curing process.

  • 4x Finger Mold Top.stl
  • 4x Finger Mold Bottom.stl
  • 1x Thumb Mold Top.stl
  • 1x Thumb Mold Bottom.stl
  • 1x Radial Palm Scaffold Plate.stl
  • 1x Radial Palm Scaffold.stl
  • 1x Ulnar Palm Scaffold Plate.stl
  • 1x Ulnar Palm Scaffold.stl

These next ones need printing out of TPU:

  • 1x Median scaffold (TPU).stl
  • 1x Thumb Scaffold (TPU).stl

We use an external 3D printing service to print our parts, as many of them have small details that would get lost in standard 3D printers. We don't recommend using a resin printer for the molds due to chemical interference in the vulcanization process, but if you do go for that method, be sure to give your mold an isolating coat before use.




We use Lasercutting for two processes: In manufacturing the connector plates and the bellow actuators.

Lasercutting Connector Plates

To get the connector plates, our goal is to cut small arch-shaped disks of acrylic in two variations: One with a circular hole for a screw, an indentation for the hexagonal screw head, two circular holes for the 4mm silicone tubes, and an engraved surface roughened to a thickness of 0.1-0.7 mm to aid adhesion in the next steps. The other in the same set-up with 3 indentations and holes, instead of one, this time designated for the nuts of screws.

To achieve these three types of surfaces (cutting the shape and holes, engraving the indentation, and engraving the surface), we have made files that have lines for the cuts, and two different shades of gray for engraving. Depending on the laser used, the settings necessary will vary. While lasercutting connector plates is cheap, it's also possible to 3D print these, though we don't have an STL file for that, you'll need to generate your own from the .ai/svg files you can find in the Connector Plate folder. Relevant dimensions for the Connector Plate:

  • connector plate is 4mm thick,
  • the indentation for screwheads or nuts need to be at least 2mm deep
  • the roughened surface can be very shallow (0.1-0.7mm), the goal is to give the super glue grip. This step takes experience to get right, so don't be disappointed if your first results don't turn out quite right.)

Lasercutting Bellow Actuators

The bellow actuators are a compact assembly composed of layers. Bellow actuators come in 5 variants: The Abduction Bellows (abd for short) The Palm Bisection Bellow (palm for short) and the 3 Thumb Bellows: Thumb Bellow 1 (T1 for short) Thumb Bellow 2 (T2 for short) Thumb Bellow 3 (T3 for short)

 Each bellow is made of layers:

  • TPU coated Nylon Outer layer
  • TPU sheet Outer layer
  • a smaller inset baking paper
  • TPU sheet Outer layer
  • TPU coated Nylon Outer layer

The T1, T2, T3 and Abduction Bellows also have a thin silicone tube on the baking paper to avoid the airflow being restricted once the bellow is folded into its final form. In the folder Bellow Actuators, there are two variations for each Bellow: the Inner and Outer shapes. The Inner files are used to cut the baking paper. The TPU coated Nylon and the TPU sheets are cut using the Outer files. The files have small bridges that connect the cut shapes to the blank canvas so that the laser does not suck them into the fume extractor.

[In order to make cutting faster, you can fold the baking paper a number of times and cut multiple layers at the same time. Add a press-jig or weight around the edges to stop it from unfolding in the cutter.]

Connector Plate Assembly

Most of the process to Assemble to connector plates once they're cut is glueing. First, prime the engraved surface with the primer, then glue the nut or screw flat into the indentation using superglue. Screws should go through the plate, the threading coming out the other side. Prep a strip of the two sided wool fabric wide enough to account for all the connector plates you're making, and high enough to fully cover the hight of the connector plate. 30x120mm should well cover it. Once the screws and nuts are in place, add superglue onto the engraved surface, and clamp the connector plate onto the black side of the fabric. Be careful with the amount of glue you use, it should cover only the surface of the connector plate; if it comes out at the sides, widening the plate, you've used too much. Once it's dried, cut the fabric down to exactly match the shape as the connector plate. It's important to cut as close to the acrylic as possible, as any excess fabric will make the connector plates tough to slot into the molds later. This process is also documented in the Youtube Video

Finger Fabrication


Molding as documented in the Youtube Video

Molding the fingers requires the molds to be printed, and ideally to have been treated with Sealing Agent twice(We use Super Seal by SmoothOn). The super seal comes in a big canister or bottle, so we pour it into a temporary container large enough to comfortably contain the molds. Into this we then pour enough Sealing Agent to submerge the Molds, which we do twice: dip in the molds, let drip dry for an hour, then dip them in again. Then pour the Sealing Agent back into it's bottle for next time.

The Top and Bottom slot into each other with a ridge lip down both sides. Take the passive layer fabric, and cut into an arch approx 17mm high and 15mm wide.There is also a file in the Connector Plates folder, so you can lasercut the shape using that file if you prefer. The passive layer arches slot between the two ridges in the bottom of the Mold.

Then carefully combine the mold pieces making sure to not bend or dislodge the passive layer piece, and use office clips to clamp them together. Use more than one clip per side, a snug seal is necessary to avoid silicone leaking out. If you have a Mold Holder, slot the molds onto that now, otherwise, use tape to raise a ridge around the top of the mold.

Be quick when working with silicone: the molding should be complete within 40 minutes max. Despite using slow drying silicone, if you wait too long or process it too slow, the silicone will turn too thick and you wont get usable results. We cast the fingers using Dragon Skin 10 Slow by SmoothOn, which is a slow air-curing Silicone with Shore strength A10.
We have rough estimates for how much silicone each mold needs, as follows:

  • RH3 Fingers 30g
  • RH3 Thumbs 20g

When casting multiple fingers and thumbs, we round up, as there are small losses during the mixing and pouring process: Some silicone inevitably remains in the mixing pot, and some silicone is often spilled while pouring.
To give our fingers their distinctive blue colour, we add silicone dye Silc Pig Blue, of which only a very very small amount is necessary. Using a small wooden stick, add as small an amount as possible, then stir, then add more. Stop when the colour stops getting darker through more dye. We want a deep dark blue colour, and you should never need more than a maximum of 1g of dye to reach that. The dye also helps us see when the two silicone components (A&B) have been thoroughly combined.
The Silicone is then put into a vacuum chamber to remove all the air bubbles generated while mixing. If you watch the silicone, bubbles will rise to the surface, and the whole mix will expand significantly. Once all but very few bubbles have disappeared, leaving a flat surface, you can remove the silicone from the vacuum chamber and slowly pour up to half of the silicone into the finger molds. Give the silicone a moment to settle into the mold, and then place the molds into the vacuum chamber. Here again the silicone will bubble, and eventually calm. When you release the vacuum, the silicone will retreat into the finger mold, leaving a cavity for you to add more silicone into. Repeat the process with the vacuum chamber until the silicone no longer retreats into the chamber. Interrupt and Finish this process by knocking the molds a couple of times hard against your work surface to shake out any bubbles that may be lingering inside.

To summarize:

  • Fill the molds 3/4 full
  • Place in vacuum chamber
  • Fill molds
  • Place in vacuum chamber
  • pour in the rest of the silicone
  • knock against the table to loosen bubbles, then wait a minute or two and repeat.

add connector plates

The shape of the Finger Molds accommodate the connector plates with a small ridge: the connector plates are inserted into the top of the finger molds, firmly into the silicone, the screws pointing up. Experience shows that it's worth laying the plates down onto the silicone for a moment before pushing them into place. This is to give the connector plate fabric time to soak up the silicone. Once the connector plates are firmly lodged at the top of the molds, place in an undisturbed location to set for at least 8 hours, or the curing time of your silicone. After the silicone has set, open the molds, if necessary using a screwdriver to lever the pieces apart. Check the silicone for bubbles larger than 1mm. If there are still significant air bubbles in the silicone, the finger is likely to break very soon when pressurized. We recommend disposing of fingers with large air bubbles over committing to the effort of finishing a faulty finger. Bubbles that create a hole throught the entire membrane are fatal to fingers. Pockets that are merely on the outside surface can be filled up with Sil-Poxy and used.

Once you have molded the finger base, there are four more steps before the fingers are finished:

  • Tubing
  • Adding the passive layer
  • Threading and Sealing
  • Pulps

The Pulps are molded in the same way as the Fingers. The differences are the mold type (Pulp Molds for Pulps, predictably) and Silicone type: We use Eco Flex 30 for Pulps, which is a softer, more elastic silicone. This gives the Fingers a better grip-surface to hold objects with.


Passive Layer as documented in the Youtube Video

RH3 Fingers have two air chambers visible in the finger base. Tubes must be laid to both of these, via the connector plate. The air chamber closer to the connector plate in tubed through the upper hole in the connector plate. The longer tube goes through the lower hole, lying close to the passive layer (which is going to be added in the next step) and goes through the barrier between the air compartments. To do this, we use a hole-punch to make a 4mm hole through the fabric-reinforced barrier. In order to get the tubes through the connector plate, the acrylic holes and silicone need to be drilled to 3.5, or 4mm diameter. The Tubes are then threaded through the holes. An airtight seal is ensured on all three holes through the addition of Sil-Poxy. Add Sil-Poxy to the outside of the tubes, and then pull the coated tubes through the connector plate, so that the Sil-Poxy is drawn into the hole:

Dont use too much SilPoxy sealant, as it decreases the size of the chamber and can influence the fingers behaviour. Amounts in the diagram are exagerated for visibility. It's very important to let the fingers dry overnight after the tubing step, as SilPoxys Curing process inhibits the curing process of Silicone chemically. Make sure SilPoxy is fully cured before proceeding.

Adding Passive Layer Text

Passive Layer as documented in the Youtube Video

Adding the passive layer means sealing the fingers by closing the open side with a silicone-infused sheet of fabric mesh. This mesh restricts the elasticity of the finger on this side, causing a controlled curve in the extension of the finger.

The passive layer is prepared on a plastic tray using Dragon Skin 10 Silicone, half of which is poured and then spread out on the tray using a rectangle of plastic, like a card, or a piece of acrylic. A rectangle of passive layer material is laid down over the silicone, and then covered with the other half of the silicone, which is spread smoothly across the fabric.
The Fingers are then carefully placed onto this sheet, opening facing down, about 2mm apart, and held down by weights. There should be segment of at least 1 centimeter of passive layer at the connector plate base, known as the 'flap'. The weights placed on top to secure the fingers into the silicone should not be heavy enough to deform the finger.

After the silicone is set, it is peeled off of the tray, and turned over. It's important to check for bubbles that turned up between the finger walls and the passive layer, as these mean failure: the fingers' airtight seal is fragile and could break. Another thing to check for is patches where the mesh comes out of the silicone. This usually happens when the lowest layer of silicone was not consistent. This can be fixed using a thin layer of sil-poxy.

The Fingers are then cut from this sheet using scissors, following the shape of the finger and fingertip and as straight and as accurately as possible. Uneven cutting at this step can have significant impact on the morphology of flexion later.

Threading & Sealing

Threading & Sealing as documented in the Youtube Video

The last steps in finishing RH3 Fingers are to thread and seal them. Test the fingers first though by pumping air into them (either manually with a big syringe or pump or using a pneumatic system), to check that the chambers expand and contract independantly of each other and there are no leaks (leaks are easy to recognise by air bubbles if you submerge the finger under water). Threading means wrapping thread around the finger in a consistent pattern, following the insets. Starting at the base with a thread-end secured with Sil-Poxy, the thread is wound up the finger, filling one valley, then skipping one, alternatingly until the end of the finger. On the way back down the finger, the empty valleys are filled, with the crossover points of the thread being set on the left and right side of the finger. Dont thread too tightly, as the finger will start to curve, it's better to wind the thread lightly, just tight enough to avoid it slipping around on the finger. Sealing the finger in the next step will help to keep the thread where it needs to be. Once back at the base, the thread is sealed in place using a very thin layer of DragonSkin 10, or in a pinch, Sil-Poxy will do.


Pulps, once removed from the mold, need to be shortened of the excess silicone at the top so they lay neatly down the length of the finger. Once the Fingers are threaded and sealed, pulps are slotted onto them over the fingertip, and secured in place with Sil-Poxy.


The bellow actuators, commonly referred to as "pouches", are small nylon pockets designed to be inflated and deflated with pneumatics. They consist of a piece of non-stick paper (like baking paper) in the desired shape (files are marked "inner") sandwiched between two nylon layers. The sealant is TPU, so we use TPU coated Nylon. Experiments have shown that two additional layers of TPU are beneficial. Both Nylon and TPU Shape files are marked "outer". As a result, we arrive at the structure previously stated:

  • TPU coated Nylon Outer layer
  • TPU sheet Outer layer
  • a smaller inset baking paper
  • TPU sheet Outer layer
  • TPU coated Nylon Outer layer

 Some pouches have 3 air chambers, and are folded on themselves to extend their actuation space. The folds can cause the unequal distribution of air, so a small tube is added across the bellow. The stack of layers is then sealed together using heat. We add a non-stick paper layer above and below the stack, and then use a standard clothes iron to firmly seal the layers for 10-15 seconds. Exposure to heat for too long may cause damage to the material.


Once all the pieces have been manufactured, we can start assembly. First, the bellow actuators are mounted into the TPU Scaffold pieces: The palm actuator is added to the Median Scaffold, and screwed into position with M2 10mm screws. The thumb actuators are folded up and added to the Thumb scaffold, according to shape, and also screwed into place with M2 10mm screws. The Ulnar and Radial palm Scaffolds are connected using the Median Scaffold, the Thumb Scaffold is also slotted into place and then they're all secured in position with the appropriate Scaffold Plates, and M3 Screws & Nuts. The fingers are then added into place. each finger has one hole designed for the screw, and two holes or valleys designed to accommodate the tubes. The screws are tightened into place using the countersunk head nuts.

Folder Overview

Bellow Actuators

This folder contains all the vector files to cut the layers of the bellow actuators. Each actuator is a combination of the Outer layers for the Nylon & TPU, and the Inner layers that are used to cut the non-stick paper. Each Actuator has two appropriately named folders.

  • abd_inner
    The inner layer of nonstick paper for the Abduction Actuators, screwed into place between the fingers using the screw protruding from the base of the finger.
    • abd_inner.dxf
    • abd_inner.svg
  • abd_outer
    The outer layer of TPU coated Nylon and TPU for the Abduction Actuators.
    • abd_outer.dxf
    • abd_outer.svg
  • all_inner
    A collection of all inner layers, cut out of nonstick paper.
    • all_inner.dxf
    • all_inner.svg
  • all_outer
    A collection of all outer layers, cut of TPU coated Nylon and TPU for the Abduction Actuators.
    • all_outer.dxf
    • all_outer.svg
  • palm_inner
    The inner layer of nonstick paper for the Palm Actuator, which is placed in the TPU printed palm scaffold which connects the two halves of the hand.
    • Palm_G2_Inner.dxf
    • Palm_G2_Inner.svg
  • palm_outer
    The outer layer of TPU coated Nylon and TPU for the Palm Actuator.
    • Palm_G2_Outer.dxf
    • Palm_G2_Outer.svg
  • t1_inner
    The inner layer of nonstick paper for the T1 Actuator, which is placed in the lowest Slot on the Thumb Scaffold, closest to the base.
    • T1_G2_Inner.dxf
    • T1_G2_Inner.svg
  • t1_outer
    The outer layer of TPU coated Nylon and TPU for the T1 Actuator.
    • T1_G2_Outer.dxf
    • T1_G2_Outer.svg
  • t2_inner
    The inner layer of nonstick paper for the T2 Actuator, which is placed in the middle slot of the Thumb Scaffold, and secured in place using M2 screws.
    • T2_G2_Inner.dxf
    • T2_G2_Inner.svg
  • t2_outer
    The outer layer of TPU coated Nylon and TPU for the T2 Actuator.
    • T2_G2_Outer.dxf
    • T2_G2_Outer.svg
  • t3_inner
    The inner layer of nonstick paper for the T3 Actuator, which is placed in the furthest slot of the Thumb Scaffold, just below the connector plate.
    • T3_G2_Inner.dxf
    • T3_G2_Inner.svg
  • t3_outer
    The outer layer of TPU coated Nylon and TPU for the T3 Actuator.
    • T3_G2_Outer.dxf
    • T3_G2_Outer.svg

Connector Plates

The Connector Plates for Fingers and thumb. Greyscale has been used to denote levels of engraving, and can be used that way directly by many lasercutters.

  • connector_plate_finger.dxf
  • connector_plate_finger.svg
  • connector_plate_thumb.dxf
  • connector_plate_thumb.svg


This Folder contains the 3D files for the finger molds, as well as a shape reference for the piece of passive layer fabric that reinforces the silicone between the air chambers.

  • Passive Layer shape.dxf
  • Passive Layer shape.svg
    • Molds
      The 3D files for the finger molds, each finger mold is made of two parts the "top" and "bottom".
      • Finger Mold Bottom.stl
      • Finger Mold Top.stl
      • Thumb Mold Bottom.stl
      • Thumb Mold Top.stl


This folder contains all the reference pictures in this document.


Here you can find the Files that need 3D printing for the structure of the Hand (called 'Scaffold'). The files marked TPU need the flexibility of TPU to bend in the way needed. The folder also contains the folder 'Glove Molds' which has the 3d files that can be 3d printed to make the Mold for the Glove, the silicone skin that goes over the scaffold for grip and aesthetic purposes.


This PDF is an expanded view of the assembly instructions as seen in the paper. It helps as a reference guide when combining all the fabricated pieces.


We provide this tutorial as is, in the hope that this technique for creating robots is adopted and developed by other researchers or hobbyists. We would like to hear of your experiences and applications, so drop us an email! Please take care when working with chemicals. Read the Material Safety Data Sheets provided by the manufacturers and evaluate whether our instructions are compatible with your lab safety rules! We will not take responsibility for your actions.

Linking, Citing and Contact

We welcome linking to this site, and we will maintain it under the URL If you want to copy or reuse all or parts of this content, please contact the RBO Lab for permission.

If you want to cite our work, please use:

Raphael Deimel and Oliver Brock. A Novel Type of Compliant and Underactuated Robotic Hand for Dexterous Grasping. The International Journal of Robotics Research 35(1-3):161-185, 2016.

S. Puhlmann, J. Harris and O. Brock, "RBO Hand 3: A Platform for Soft Dexterous Manipulation," in IEEE Transactions on Robotics, doi: 10.1109/TRO.2022.3156806.