(C-110) Silicone Elastomers for Soft Robotics: Controlling Strain-Limiting Functionality

This work focuses on providing soft robotics researchers with enhanced information of off-the-shelf silicones and their capabilities for strain-limiting functionality. Typically, additive strain-limiting layers are implemented by embedding paper into the silicone to maintain stiffness and by cutting slits into the silicone, either vertically or horizontally, which results in bending the silicone in a certain direction. Depending on the silicone used, the angle at which bending occurs will vary. In soft robotics, specific applications call for special flexibility and stiffness in the material. However, the approach of embedding paper is flawed in that there is an inconsistency in placing the paper within the silicone, therefore causing difficulty in meeting the need for a desired stiffness for a specific application. This is rectified by combining silicones together to satisfy both stiffness and bending needs while also knowing the exact shore-hardness value, ultimately allowing proper use of strain-limiting layers without paper.

This research project was conducted to determine the shore-hardness values of the various silicone combinations. The Mechanical Testing System (CellScale UniVert, University of Waterloo) was used to calculate the tensile force as well as the displacement in gauge length for at least 5 dog bone samples of each combination.

Due to the elastic nature of the silicone combinations, each dog bone sample was fixed in a taut position at the start of each tensile test using a 3D printed scissor-like contraption along with 3D printed clamps to hold the samples in place. Ultimately, this approach increased the elongation length beyond what the CellScale UniVert was capable of by 81%. This contraption was held in place using the clamps of the CellScale UniVert.

Silicone samples were made by mixing, pouring the mixture into a mold and into a dogbone shape using 5 different samples of commercially-available skin-safe silicone (Dragon Skin 10 Medium, Smooth Sil 945, Ecoflex Gel, Ecoflex 00-50, and Soma Foama 15) (Smooth-On; Macungie, PA, USA). To determine the number of samples that needed to be prepared from combinations of the 5 silicones, fractional factorial design was conducted to determine the total number of experiments, leading to 32 total combinations. All combinations used equal portions of the included silicones.

From this setup, Young’s modulus was determined by calculating the slope of the line. Furthermore, the ultimate tensile strength (UTS) was determined by elongating the samples until breakage, thus capturing the maximum stress.

To demonstrate how a silicone combination would best suit its use in a specific application of soft robotics, its elasticity, and strength were investigated for 3 samples. From our preliminary results, we can see the addition of Dragonskin to Pure Gel increases Young's modulus from 0.0911 ± 0.0458 MPa to 0.313 ± 0.0323 MPa. Further, adding Dragonskin and Pure Gel along with Soma Foama results in a Young’s Modulus of 0.399 ± 0.0410 MPa.

In this study, various silicone combinations were tested to assess the elasticity and Young’s Modulus. From the graph, it is observed that there is a similar max strain between Pure Gel and Dragon-Gel but the addition of Dragonskin causes an increase in Young’s modulus. Moreover, the Young’s Modulus between Dragon-Gel and Gel-Dragon-Soma share similar values due to the sole combination of Dragonskin and Gel, however, adding Soma Foama reduces the max shear strain.

Clearly, from the enhanced control over the functional properties of the different silicone combinations, we can see the potential for eliminating these additive strain-limiting layers (e.g., paper) and placing all functional designs in material selection. Using these comprehensive results drawn from this work a database can be compiled for researchers. Future work will emphasize the potlife of any combination of silicones for extrusion printing, specifically, the proper combination provides a quick potlife, thus allowing for the use of 3D printing while still maintaining stiff and elastic properties.