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.