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.

This page has general information on the material and related processes. For specifics on our own lab please see Silicone Rubber Procedure, Casting Equipment, Design Example: Open Molded Soft Silicone Flexure, Design Example: Open Molded Soft Silicone Actuator, and Design Example: Laser-Cut Open Mold.

Smooth-On Technical Information

The following Smooth-On product videos offer a brief overview of silicone fabrication:

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

We normally stock the following silicone rubber materials:

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 [R25] (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 [R39]

    • 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 [R46]

    • soft lithograpy (as per [R59], [R41])

    • uses pneu-nets style actuators (as per [R14])

    • 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 [R61]

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

    • several other techniques described

  • Bubble casting soft robotics [R17]

    • 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 [R28]

    • 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 [R57] (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 [R69], [R47].

    • 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 [R33].

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

  • bioPrint: a liquid deposition printing system for natural actuators [R62]

    • 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 [R36]. Laser-engraved polymer films which are then manually folded.