Variable stiffnes materials — Michelle C. Yuen

Active Variable-stiffness Fibers

… combine actuation (shape-memory alloy) and stiffness-changing (thermoplastic polymer) functions.

Composition of AVS fiber

NiTi wire coated in polymer (ABS or PLA) = variable-stiffness. Shape memory alloy wire = linearly-contracting actuator. SEBS is used to bind the two entwined ‘threads’ together.

Move-and-hold

AVS fibers are able to lift a weight and hold it passively. This gives it an advantage over a simple SMA actuator in applications that require shape-holding functionality.

Active variable-stiffness fibers

Actuation and stiffness-changing are entwined together to form active variable-stiffness (AVS) fibers. Shape-memory-alloy actuators (Nitinol) are programmed to contract into a tight helix when heated above their transition temperature. Variable-stiffness fibers consist of a heating wire coated with a thermoplastic; when heated above its glass-transition temperature, the polymer softens. By combining these two functions together, the fibers can perform move-and-hold operations which can allow for lower energy requirements for robotic applications. Moreover, their fiber-like form allows them to be integrated into fabric substrates to extend its utility from 1D to 2D planar substrates that can then be wrapped around 3D soft passive structures.

M.C. Yuen, R.A. Bilodeau*, R.K. Kramer. "Active Variable Stiffness Fibers for Multifunctional Robotic Fabric." IEEE Robotics and Automation Letters, Vol. 1, No. 2, pp. 708-715, 2016.

Solid-liquid phase-changing particles embedded into polymer matrices

Field's metal composites

Field’s metal particles mixed into epoxy (A) and silicone (B) matrices.

Field's metal particles

Bulk Field’s metal is melted and shear-mixed in hot water to create Field’s metal particles (5-212um dia.)

Field's metal/epoxy composite

FMEpoxy exhibits a wider range of stiffness compared to native epoxy (A). When used as the inextensible layer of a pneumatic bending actuator, the FMEpoxy can be used for move-and-hold operations (B).

Field's metal/silicone composite

FMSi can be used for stretch-and-hold applications which can be used to create on-demand inflatable displays (A), or control the orientation of a pneumatic actuator (B, C).

Close-up view of FMSi

Here, an FMSi sample was heated, stretched, and then allowed to cool while held stretched. The FMSi was then exposed to heat and we can observe the needles of Field’s metal melt, allowing the silicone to pull itself back to its unstretched length.

Field’s metal has a melting point of 62C, allowing access to solid-liquid phase transition near room temperature. Particles of Field’s metal were mixed into epoxy, which softens when heated above its glass-transition temperature, and silicone elastomer, which can stretch to >500% strain. In the FMEpoxy composite, the Field’s metal particles expanded the range of stiffnesses traversed by the material - allowing for higher load-bearing capacity without sacrificing softness at the other end of the spectrum. In the FMSi composite, the solid-liquid phase transition was leveraged to perform stretch-and-hold operations by cycling through heating to melt the particles, deforming the composite, and cooling to hold the new shape.

M. C. Yuen, T. L. Buckner*, S. Y. Kim, and R. Kramer‐Bottiglio, “Enhanced Variable Stiffness and Variable Stretchability Enabled by Phase-Changing Particulate Additives,” Advanced Functional Materials, vol. 29, no. 50, p. 1903368, Nov. 2019.

*co-first author

Bilayer construct

Silicone elastomer (blue) cast on top of FMSi (gray) creates an interesting bilayer construct that can be thermo-mechanically programmed to hold a range of helical shapes.

Stiffness characterization

The tensile modulus of FMSi when the Field’s metal particles are liquid (red) is much lower than when they are solid (blue). In fact, heated FMSi has a similar stiffness to native silicone (black).

Topography recording

FMSi can be used to record the fine details and broader contours of a toy racecar.

Trajectory control of a pneumatic bladder

Extending the bilayer demonstration, FMSi can be cast onto an inflatable pneumatic bladder. By thermo-mechanically programming the initial state of the FMSi, different trajectories can be accessed with the same pneumatic input.

Shape memory silicone using Field’s metal particles

A deeper dive into FMSi. This work explores additional applications of stretch-and-hold operations using Field’s metal silicone composites.

T. L. Buckner, M. C. Yuen, and R. Kramer-Bottiglio, “Shape Memory Silicone Using Phase-Changing Inclusions,” in 2020 3rd IEEE International Conference on Soft Robotics (RoboSoft), May 2020, pp. 259–265.