Fabricating a soft robot or an actuator with conventional molding methods is a laborious and time-consuming process. Most of the manufacturing steps involved, heavily depend on manual handling that causes fabrication variability and limitations to scientific repeatability. These conventional methods are lamination casting (also known as soft lithography), retractable pin casting, lost wax casting, and rotomolding. All of the molding techniques, restrict possible geometrical shapes, complexity, and scale of the manufactured soft robots.

Therefore, researchers have turned their focus to additive manufacturing (AM) (aka 3D Printing) which requires neither a multi-step fabrication process nor human intervention. Compared to conventional soft robot fabrication techniques AM offered design freedom, automation and repeatability.  First examples of the 3D printed soft robots emerged with the usage of stereolithography (SLA) [1]. Later with commercial 3D printers and flexible materials, other AM methods such as fused deposition modeling (FDM) [2], and poly-jetting [3] are successfully used for fabricating more complex soft robots. However,  these techniques are limited by the used materials. Commercially available 3D print materials are subjected to high Shore hardness values ranging between 30A and 95A, while commonly used soft robot manufacturing material polydimethylsiloxane (PDMS) has values around 10A [4]. As a result, 3D printed soft robots fail due to high strain [1] and tear apart. Basically, current 3D printers are not able to 3D print soft robots as soft as their molded counterparts.  

To overcome strain limitations and use viscoelastic PDMS materials, researchers focused on direct ink writing (DIW) techniques. A research group developed a micro-scale active mixing system for two-part materials and successfully 3D print PDMS objects [5], [6], but they did not demonstrate the fabrication of soft actuators or robots. Instead of using two-part PDMS materials, another research group used moisture-cured silicone elastomers [7] but, their technique limited the achievable geometry. Previously, our group 3D printed a single channel soft actuator [8]. However, the used technique was limited by the 15-minute pot-life.

In contrast to research activities for DIW of PDMS materials, there are also successful industrial 3D printers. A company called PICSIMA have developed a 3D printing system based on sub-surface catalyzation [9]. Even though PICSIMA can 3D print high-quality silicone objects with multiple Shore hardnesses starting from 10A, it fails to 3D print internal structures. Furthermore, the companies Structur3D [10] and ViscoTec [11] have introduced extrusion head systems for two-part materials, but there has not been an open demonstration of these systems for 3D printing two-part platinum cure PDMS materials. 

In summary, considering the limitations present in current state of the art PDMS printing, we have yet to match the performance of the molded functional soft robots made out of PDMS materials with 3D printing technology. Considering the introduced soft material limitations inherited from the available 3D printing technologies in the areas of research and industry so far, one can easily see that there still exists a knowledge gap for matching the performance and dimensional quality of functional soft actuators and robots made out of two-part platinum cure silicone rubbers with the laborious and time-consuming conventional molding processes. 

In this research project, we have addressed this gap by developing an extrusion system that combines aspects of the two previous works: 1) an active mixer [5], and 2) controlled heat treatment [8]. We have developed a 3D printer with an enhanced extruder mechanism capable of fabricating soft functional robots with a two-part platinum cure silicone material. The layer-by-layer 3D DIW technique, in contrast to soft lithography and lost wax casting, requires no human intervention, introduces fewer dimensional errors, and reduces the fabrication time by more than 50%. The 3D printed soft robots performed better or matched the performance of their molded counterparts while being more stronger and reliable [12].  

Fabrication time benefit

In addition to this background section, we also would like to show our findings about fabrication time comparison. In the figure below the time spent per fabrication step is illustrated. Since the time range of the steps which involve manual work may change depending on the experience, we timed the length of the fabrication process based on the individual most experienced with molding in our research group. In both 4 channel tentacle and pneu-net actuator (robot pictures can be seen in the introduction section), the fabrication with 3D DIW method took significantly less time and steps without any human intervention. In contrast, the lost wax casting (4 channel tentacle) and lamination casting (pneu-net actuator) required multiple steps with skilled labor.

The fabrication times measured for single unit production. Mass production is not considered since its out of our scope.

References

[1] B. N. Peele, T. J. Wallin, H. Zhao, and R. F. Shepherd, “3d printing antagonistic systems of artificial muscle using projection stereolithog- raphy,” Bioinspiration & Biomimetics, vol. 10, no. 5, p. 055003, 2015.

[2] H. K. Yap, H. Y. Ng, and C.-H. Yeow, “High-Force Soft Printable Pneumatics for Soft Robotic Applications,” Soft Robotics, vol. 3, no. 3, pp. 144–158, Sep. 2016.

[3] D. Drotman, S. Jadhav, M. Karimi, P. deZonia, and M. T. Tolley, “3d printed soft actuators for a legged robot capable of navigating unstructured terrain,” in 2017 IEEE International Conference on Robotics and Automation (ICRA), May 2017, pp. 5532–5538.

[4] Dragonskin series, high performance silicone rubber. https://www.smooth-on.com/product-line/dragon-skin/

[5] T. J. Ober, D. Foresti, and J. A. Lewis, “Active mixing of complex fluids at the microscale,” Proceedings of the National Academy of Sciences, vol. 112, no. 40, pp. 12 293–12 298, Oct. 2015.

[6] J. O. Hardin, T. J. Ober, A. D. Valentine, and J. A. Lewis, “Microflu- idic Printheads for Multimaterial 3d Printing of Viscoelastic Inks,” Advanced Materials, vol. 27, no. 21, pp. 3279–3284, Jun. 2015.

[7] J. Plott and A. Shih, “The extrusion-based additive manufacturing of moisture-cured silicone elastomer with minimal void for pneumatic actuators,” Additive Manufacturing, vol. 17, no. Supplement C, pp. 1–14, Oct. 2017.

[8] J. Morrow, S. Hemleben, and Y. Menguc, “Directly Fabricating Soft Robotic Actuators With an Open-Source 3-D Printer,” IEEE Robotics and Automation Letters, vol. 2, no. 1, pp. 277–281, Jan. 2017.

[9] F. D. Ltd., “Silicone 3d printing.” [Online]. Available: http://www.picsima.com/how-picsima-works

[10] S. Printing, “Discov3ry 2.0.” [Online]. Available: https://www.structur3d.io/

[11] Viscotec, “3d extruders for pastes and fluids, based on endless piston principle.”

[12] Osman Dogan Yirmibesoglu, John Morrow, Stephanie Walker, Walker Gosrich, Reece Aidan Canizares, Hansung Kim, Uranbileg Daalkhaijav, Chloe Fleming, Callie Branyan and, Y. Mengüç, "Direct 3D Printing of Silicone Elastomer Soft Robots and Their Performance Comparison with Molded Counterparts " IEEE-RAS International Conference on Soft Robotics (ROBOSOFT), April 2018.

The design of the 3D silicone printer consists of 3 main components: 3D printer base, silicone extruder and pump system. Each component have their subsections in this documentation. First, we will start with selecting our 3D printer base. We will provide 3D printer base options with the shopping links so that you guys can check them out. Later, we will teach you how to create the parts for the silicone extruder and how to assemble it. Lastly, we will finalize with pump and cartridge selection. If you are familiar with 3D printers and how they work, this project will be very easy to complete for you.

Here is the representation of the 3 main components of this project:

Let's start making our first 3D silicone printer!!!!

It is time to prepare the silicone inks for 3D printing. We have a formulation for turning the silicone material into 3D printable version. You need to use the additive materials in different compositions to achieve 3D silicone printing ink. The values for the syringe preparations are for the first syringe 0.02 gr of dye (red), 0.5 gr of thi-vex, and 49.5 gr of dragon skin very fast part A. And for the second syringe 0.3 gr (yellow), 0.5 gr of thi-vex, and 49.5 gr of dragon skin very fast part B. Here is a summary table.

Shopping list

1. Colorant (Dye)
2. Thi-vex
3. Dragon Skin Very Fast 10 part A, B
4. Syringes
5. Luer lock plugs: part number 51525K315
6. 3D print the helping hand

Preperation steps

We have prepared a step by step youtube video for you guys to follow. Please watch the video from the link down below.

Here is the step by step summary of the youtube video:

Step 1: Get your cup, place it on top of the scale, zero the scale and put your dye. (you can use the suggessted dye amounts in the table above. Also you can experiment with the amounts to get more color spectrum. Just be careful to not go over 1gr of dye per each syringe, it will cause your mixture to not cure resulting failed prints)

Step 2: Zero the scale, put 0.5 gr of thivex with a small helping syringe

Step 3: Zero the scale, put 49.5 gr of dragon skin very fast 10 part A

Step 4: Mix the the cup at high speed

Step 5: Transfer the mixed material inside the 60 ml syringes

Step 6: Insert the plug for the syringe

Step 7: Repeat the steps starting from step 1 for the part B of the silicone material

When these steps are finalized install the syringes into syringe pump mechanism as shown in the picture down below.

Rigth now, we are ready in terms of hardware. Let's move into the software section before we start our first test print!

After completing all the hardware setup it is time to install our software and make everything ready for 3D silicone printing. First of all you need to know couple general knowledge about 3D printing. After you get your .stl file, for a 3D printer print it, the file has to be converted into gcode commands. To do this there are many softwares. For this project we will use software called "Sli3r". It is a gcode generator for 3D printer. Go to their website from this link and download the software into your desktop: http://slic3r.org/ 

Sli3r initial setup

Here are the 9 step to setup the slic3r software right after you open it. Please follow them from 1 to 9 and insert the same parameters as it can be seen in the pictures.

When you are done, our software is ready to import the 3D silicone printing profile.

Sli3r silicone printing profile import

Right now we are going to import our 3D printing profile. For this section first download the profile bundle attached to the end of this page as "Slic3r_config_bundle.ini"

After downloading the profile follow these steps as seen in the figures 1-3 down below.

At the end of the profile import, you must see the same drop-down menus as seen in the red box above. And your system is almost ready to start 3D printing. 

Marlin update and prontorface

Our 3D printer base has a controller software inside. But you will need to update it for better results. By following this youtube video you can simply update your firmware into Marlin which will be a better fit into our project: 

After updating your firmware into Marlin right now please download the 3d printer controller software prontorface from this link: http://www.pronterface.com/  This software is very straightforward and easy to implement. Please follow the documentations in their website. Prontorface will allow you to control the motors of the 3D printer in x,y,z directions in the way that you want. And you will need to connect your computer into Anet A8 with a usb cable. And that's pretty much it. Prontorface will directly communicate with your printer over COM ports after the Marlin update. 

Right now your 3D printer is ready to go! Let's start out first print together!

It is time to fabricate our first silicone object. Our system is ready regarding hardware and software. You will see that fabricating soft robots with additive manufacturing is much easier than the molding techniques. Here are our fabrication steps:

Fabrication steps:

Step 1: First of all, get your 3D robot design and turn it into .stl file format. Later, open that stl file in slic3r software and generate gcode file. Finally, upload that gcode file into the 3D printer's micro sd card. Down below you can see the slic3r software gcode generation steps.

Step 2: Make sure that you home the all axis and be careful with the Z-axis. Since we changed the nozzle it might be homed above the bed or below the bed. To avoid any problems relocate the z axis limit switch. See the pictures below.

Step 3: After we finalize our homing sequence right now we will start our silicone pump. First, raise the z height of the extruder over prontorface and put a small cup under the extruder. 

Step 4: Start the mixer motor and then run the pump with 26 ml/hr and make sure that silicone flows through the clear tubes into the nozzle

Step 5: Wait until the two part silicone material to mix. It will take around 5 minutes to get good composition. You will understand the mix with the color change inside the mixer. In this case my mixed material looks like light red / pink~ish. See the picture below. 

Step 6:  After the compotion ready open the heater controller system and set the fan speed up to 80% and set temperature to 50C

Step 7: Hit the "print from sd" button from prontorface and remove the cup.

Step 8: If your part is taller than 3 cm, increase the heater wings temperature up 80C after 3cm height

Step 9: Wait until your print is done. (Also give it some time before you remove your object from the heated bed. Top most section will still need time to cure. Around 5 minutes or 10)

Step 10: !!!!!!!!!!!!!!! CELEBRATE !!!!!!!!!!!!!!!

Congratulations

Your 3D Silicone Printer is ready for the next awesome soft robot fabrications!

After finalizing our 3D silicone printer, we have 3D printed soft robots. These robots are: a 4 channel tentacle, a pneu-net actuator and a quadrupedal soft robot. You can see their pictures down below. 

Don't forget to watch our youtube video for the operation of these robots: 

We used fluidic control board from soft robotics toolkit to operate our robots: https://softroboticstoolkit.com/book/control-board You can also use 60 ml syringes to pump air into them but to make quadrupedal soft robot to walk you will need to code the fluidic control board.

The stl files of the soft robot designs and the code for the simple walking gait can be downloaded from the zip file located at the end of this page named as "Silicone robots and control script"

Soft quadrupedal robot was characterized for its performance regarding maximum hold, lift, and walking speed. The robot was 3D printed in 7 hours without any human intervention. Immediately after the print, we installed the tubing for actuation. The robot was 170 mm long, 80 mm wide, and 40 mm tall. The quadruped is capable of lifting the maximum weight of 0.61 kg and holding 1.05 kg at 51.7 kPa pressure. It can travel at a rate of 0.34 m/h by using a simple walking gait. 4 channel tentacle and pneu-net actuator were characterized for blocked force values and bend angles in response to a given pressure. The 4 channel tentacle printed in 1 hour. It was 95 mm tall and 35 mm wide in x and y directions. The tentacle is capable of applying maximum 5 N force toward the blocking surface at 88 kPa, and is able to bend 70 degrees at the same pressure. The pneu-net actuator printed in 3 hours. It was 124 mm long, 25 mm wide, and 32.5 mm tall. Pneu-net actuator is capable of applying maximum 1.2 N blocked force at 43.2 kPa pressure, and it is able to bend 170 degrees at 39.2 kPa. Down below you can see the figures from our experiments.

If you want to learn more about the our experiments and their results please check out our research paper. 

Research Paper

Direct 3D Printing of Silicone Elastomer Soft Robots and Their Performance Comparison with Molded Counterparts

Osman Dogan Yirmibesoglu, John Morrow, Stephanie Walker, Walker Gosrich, Reece Aidan Canizares, Hansung Kim, Uranbileg Daalkhaijav, Chloe Fleming, Callie Branyan and, Y. Mengüç, IEEE-RAS International Conference on Soft Robotics (ROBOSOFT), April 2018. 

After your first print, you will probably want to print more and see what are the other possibilities with this printer. Yes, there are many possibilities and lots of objects to 3D print. However, there are some rules in 3D silicone printing that will help you to get successful prints. Because not all the designs are 3D printable with our silicone printer. Compared to conventional FDM printers our ink is affected by gravity a lot. 

A successful print depends on an understanding of the 3D printer limitations and designing the CAD file of the print accordingly.
With this system, the print process had to be fine-tuned. The silicone material had a major role to play in this. Every G-code path generated by the slicer software used must be considered for print success. Disconnection between print paths and jumps between print spaces will cause distortions in the printed shape. The following design rules have been determined from our print processes. Design rules are marked from D1 to D9 and slic3r settings marked from S1 to S19 in the tables down below.

In terms of design rules, the printer have 9 main limiting constraints. Rule D1 basically indicates that if the designer came up with 1 mm thick feature in the CAD file printer will print it as 1.5mm due to nozzle size. It is currently a hard problem to reduce nozzle size because the silicone material cures faster with a smaller nozzle diameter. Rule D2 brings a limitation to the layer height. For example, if the feature has 3mm thickness, the printer will print it in two layers, which may lead a weak spot. As a note, a 3 mm wall thickness is optimal for pneu-net walls. 

According to rule D3, slope of the wall should be in between 70 and 90 degrees. If the designer came up with a 45 degree slope in the CAD file, since the printer is not using any support material, the feature will fail to exist. The gap spanning (Rule D4) is the one of the important feature that shows the quality of a 3D printer. In silicone printing case, spanning 2 mm gap without overhang is a great success. The designer can come up with a CAD file that has 2 mm ceilings over air chambers. D5 and D6 are one of the very important rules and they are connected. If the print feature is too small, this will mean that the extruder will move on the next layer in a very short time which will happen before extruded layer cured well. Due to this fact the limitation D5 or D6 must be followed for thin and tall structures. Rule D7 is caused because of attraction between extruded silicone lines. When extruder head follows the circle path, the edge of the dispensing tip pulls the silicone material close to each other. Due to this fact we observed a limitation in the achievable circle diameter. In terms of creating soft robots, wall thickness is really important due to inner air chambers for actuation. Outcome of our experiments lead us to rule D8 for minimum wall thickness in CAD design. The last rule emerged by combining rules D1-D3 and D8 to come up with horizontal air chamber designs. These chambers are vital for a soft robot to operate and by following the rule D9 it is easy to 3D print them.

Software settings

After following the design rules, it is necessary to check the settings of your gcode software called "sli3er". Effective print settings in Slic3r is a challenge because all currently available slicer programs were designed for printing thermoplastics. To avoid main problems such as over-extrusion, cloaking, under extrusion and gaps on the corners these software settings will be your guide. Please check the table below. (We also provide the troubleshooting guide at the very last section of this project)

When over extrusion problems occur the printed shape expands and mismatch between CAD file and real object is inevitable. To prevent overflow the setting S1, S5, S7-S14 should be followed. The most important rule is S14. If the pump flow rate is too high, the shape will become bulky. Certain desired settings, such as a larger extrusion width relative to the nozzle diameter to compensate for silicone flow, were not allowed in the software. This requires clever manipulation of settings to get to the desired settings. 

When cloaking problems occur, the silicone flow will from the nozzle will start to slow down and get thinner, in couple minutes flow will  stop completely and pumps will stall. To prevent cloaking the settings S12 - S19 should be followed. Sharp turns over 90 degree can often lead to holes in the print layer because adjacent silicone roads will not come in contact and extruded silicone lines will pull each other. There are several workarounds for this to prevent the gap problem for the sharp corners of the designed shape, the setting S2, S3, S7-S9, S14, S15 should be followed. 

The settings from S17-S19 are not located within the software. You will need to arrange them manually in the control panel of the heater wings.

These settings are not necessarily the final settings. What you need to do is to experiment with them and see which parameters are working the best for you. Feel free to ask any questions over the contact info section.

As all projects evolve, our 3D silicone printer project is advencing too. Currently, as mLab team (https://www.mlabrobotics.com/) we are working on increasing the print capabilities of the 3D silicone printers and pushing the boundaries of the silicone printing research. On the right handside you can see our tallest 3D silicone printer for fabricating big soft robots. In my Ph.D. research I am looking for making soft robots bigger and stronger with different design techniques and 3D silicone printing is my the most valuable tool. My colleuge Steph Walker is also working with her 3D silicone printing project too. She is looking into high resolution small silicone objects and the science behind. 

I hope you guys enjoyed this project and your 3D silicone printers are working properly. Hopefully towards the end of this year our kickstarter project will take place and fully constructed 3D silicone printers and their kits will be available.

For the news and upcoming announcements don't forget to subscribe into my youtube channel ( https://www.youtube.com/osdoyi ). I will be preparing more videos our ongoing research and keep you guys updated!

Troubleshooting is the key to successful prints. Since our 3D silicone printing material is behaving differently compared to PLA or ABS we need to be aware of common problems. Some of them have easy solutions. Here is the list of common problems, their names and the solutions that we suggest. 

Contact info

Congratulations! You have made it. Which means that you assembled the 3D printer and make it work and you already 3D printed your first silicone robot or object. And at this point you need more help, because something went wrong. Or maybe you have an idea to share with us. 

If so here are my contact information:

• Email: yirmibeo@oregonstate.edu
• Youtube: https://www.youtube.com/user/osdoyi
• Instagram: https://www.instagram.com/osdoyi/