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Soft actuators offer opportunities for drug delivery, medical surgery, design of active deformable clothing, soft human-computer interaction, and refreshable Braille displays The basic components for constructing a soft actuator include the nature of the soft material as a body and the mechanism used to actuate that soft material. The soft actuator material should be flexible and stretchable as well as reliable, fast, repeatable and dynamically stable in deformation. It transforms the input stimulus into a useful mechanical output. To build soft actuators, smart materials and structures such as shape memory alloys, dielectric elastomers, ionic polymer-metal composites, fibers, hydrogels, phase change, and magnetically responsive materials have been used. In particular, phase change materials (PCMs) can change their state when triggered by heat. Their mechanisms can be thermal expansion/contraction (e.g., wax-based materials), liquid-to-solid transition (e.g., silicone polymer elastomers), ordered-to-disordered phase transitions (e.g., liquid crystal elastomers), and thermal transitions from an amorphous to a crystalline state (e.g., shape memory polymers). Phase change polymers containing crystallizable side chains have been shown to combine shape memory and dielectric elastomer driving. These bistable electroactive polymers (bsep) are expected to be used for large strain rigid-to-rigid actuation; however, their operation requires a highly stretchable Joule heating electrode (JHE) to manage temperature changes.
The main point of this paper
Stretchable JHEs materials:
Carbon nanotubes (CNTs), carbon fibers, graphene, MXene, metallic nanomaterials, liquid metals, and conductive polymers are used to fabricate stretchable Joule heating elements (JHEs).
Low-voltage Joule heating challenges:
High electrical conductivity is required, but the ability to maintain highly reversible deformations with low resistance at all strains is difficult.
Material Characterization:
Carbon-based nanomaterials and conducting polymers are moderately conductive and require high voltages (>10 V).
Metallic nanomaterials are highly conductive but lose their conductive network at high strains and have poor thermal and oxidative stability.
Liquid metal electrodes are low-power, but complex to fabricate and challenging for long-term stability.
Solution:
Use deformable geometries (pleats, serpentine patterns, kirigami).
Use of composite nanomaterials (graphene and silver nanowires, carbon nanotubes and silver nanoparticles).
High resolution patterning challenges:
Realizing high-resolution patterning in stretchable hybrid electrode systems is challenging.
Stretchability limitations:
Most deformable JHEs can only bend and few can maintain their resistance at strains >100%.
Innovation of this study:
A new stretchable JHE for low-pressure heating and high thermal stability up to 100% at various strains is presented.
A hybridized layer of carbon nanotubes and silver nanowires (AgNWs) embedded in a waterborne polyurethane (WPU) matrix was used as the electrode.
On-site pre-stretching (OPS) technique is introduced to form wrinkled electrode layers and serpentine traces.
Application example:
Fabrication of multicellular refreshable braille (MCRB).
MCRB displays can maintain low operating voltage for more than 20,000 cycles.
Importance of Joule Heating Electrodes (JHEs):
JHEs are a necessary component of thermally actuated systems.
Introduction of novel JHEs:
A highly stretchable, patternable and low-voltage operated JHE based on a hybrid layer of silver nanowires and carbon nanotubes is reported.
Fabrication Process:
A conductive layer was coated on a locally pre-strained bistable electroactive polymer (BSEP) film to form a folded conductive surface.
Serpentine traces are formed by laser engraving.
Resistance Stability:
The resistance of the resulting JHE electrodes remains nearly constant at 80-90% area strain.
Heating performance:
Applying a voltage of 7-9 V, the temperature of the BSEP film rises to about 60°C, much higher than its phase transition temperature of 46°C.
The temperature increase results in a 103-fold decrease in the BSEP modulus.
Electronic Braille Device Applications:
An electronic Braille device was developed based on the JHEs on the BSEP membrane.
The electrodes were designed as 3 × 2 individually addressable pixels.
By Joule heating the pixels and locally expanding the BSEP membrane using pneumatic pressure, the pixels are deformed out of plane by more than 0.5 mm to display Braille letters.
Durability:
Braille content can be refreshed 20,000 times at the same operating voltage.
A stretchable, mapable, low-voltage JHE was developed using a carbon nanotube/AgNWs/carbon nanotube hybrid layer.The OPS method is effective in locally generating Joule heaters in the active region of BSEP membranes. The combination of flexural structure and serpentine patterning of the hybrid layer ensured strain-invariant stretchability. Furthermore, embedding CNTs/AgNWs/CNTs electrodes in the WPU substrate enhances their adhesion to the substrate and allows for reversible stretchability of the electrodes. At the same voltage, the Joule heater exhibits consistent performance in both driven and relaxed states, withstanding more than 20,000 driven cycles. An array of 1 × 10 Braille cells was demonstrated using localized Joule heating at a single point combined with globally applied pneumatic pressure.
