Silicone Rubber

Our primary supplier for castable silicone rubber is Smooth-On. These are two-part products with a variety of hardnesses and working times. These products are fairly safe and easy to use.

Smooth-On has a lot of useful technical literature on the site, including:

Candidate Fabrication Strategies

A principal challenge is producing silicone parts with complex internal cavity geometry for pneumatic pathways.

  1. Casting silicone into one or two-part 3D-printed plastic molds.

  2. Rigid cores embedded in highly elastic silicone (e.g. beads on wires removed after curing).

  3. Lost-wax casting in which a shaped wax core is melted out.

  4. Multiple layers of partial-cure silicone (e.g. Smooth-On Rebound brushable silicone).

  5. Silicone bonding (needs research and product identification). Follow-up: silicone-on-silicone, silicone-release.

Follow-up resources:

Fabrication Literature

A quick survey of techniques documented in the literature follows.

  • A Recipe for Soft Fluidic Elastomer Robots [R45] (2015). This paper has considerable process detail.

    • soft lithography in which multiple molded parts are glued to form closed cavities

    • laminated structures including laser-cut constraint layers

    • lost-wax cores to create complex cavities without gluing

  • Design and Fabrication of Soft Artificial Skin Using Embedded Microchannels and Liquid Conductors [R55]

    • EcoFlex 00-30 silicone cast into 3d-printed molds to form multiple layers with microchannels

    • layers are bonding using the same liquid silicone by spin-coating

    • cavity holes are plugged during bonding

    • to finish their sensors: eutectic Gallium-Indium metal is injected into the microchannels

  • George Whitesides lab at Harvard, including Multigait soft robot [R65]

    • soft lithograpy (as per [R78], [R59])

    • uses pneu-nets style actuators (as per [R26])

    • uses Smooth-On Ecoflex 00-30 or 00-50 for actuating layer

    • uses Dow Corning Sylgard 184 PDMS for strain-limiting layer

    • layers cast separately and bonded

  • PneUI [R79]

    • Figure 11: silicone cast around suspended threads of beads, which are pulled out after curing

    • several other techniques described

  • Bubble casting soft robotics [R27]

    • process creates long silicone tubes in designed shapes

    • a two-part mold is filled with elastomer, than air injected into the interior

    • the bubble creates a tube with an asymmetric thickness

    • a sample final product is a long thin tube which coils under pressure to act as a contractile actuator

  • Elastomeric Origami [R47]

    • process embeds inextensible fibers within a silicone pneumatic net structure

    • a flexible actuation layer (Ecoflex 00-30) is cast which contains pneumatic channels

    • a paper layer soaked in uncured silicone is bonded to it

    • bending, contracting, twisting, pleating, extending modes are possible

Fabrication Strategies Requiring CNC Tooling

All of these examples involve a custom CNC machine and are likely out of the reach of this class.

  • An integrated design and fabrication strategy for entirely soft, autonomous robots [R77] (2016).

    • cast elastomer (so far so good)

    • fugitive and catalytic inks injected into matrix, e.g. using CNC needle (not easy for us)

    • evacuation of inks leaves open channels

    • powered by monopropellant decomposition (fully autonomous)

  • MIT Self-Assembly Laboratory [R85], [R69].

    • Rapid Liquid Printing: 3D-printing liquid silicone directly into a final form within a supporting gel (not easy for us).

  • Embedded 3D Printing of Strain Sensors within Highly Stretchable Elastomers [R52].

    • embedded 3D printing (e-3DP): CNC injection of conductive ink directly into uncured elastomer

  • bioPrint: a liquid deposition printing system for natural actuators [R80]

    • uses CNC router with custom tool to deposit solution-based natural stimuli-responsive material on a thin flat substrate to create hygromorphic biohybrid films

Other Soft Robotics Fabrication Strategies

These are directly related to silicone casting or fabrication, but within the space of soft robotics.

  • Laser cutting as a rapid method for fabricating thin soft pneumatic actuators and robots [R1]. Laser-welding of thin TPU sheets to form complex pneumatic actuators.

  • Towards Printable Robotics: Origami-Inspired Planar Fabrication of Three-Dimensional Mechanisms [R54]. Laser-engraved polymer films which are then manually folded.