Dielectric elastomer actuators (DEAs) are a class of soft actuators that have been known as artificial muscles due to their large actuation strains, rapid speeds, and high energy densities. However, the soft materials behind these devices tend to be mechanically weak and susceptible to damage from large tensile loads or sharp objects. After damage, the material loses its functionality and is discarded, generating waste. While previous works have achieved self-healing DEAs that can recover their performance after damage, these devices do not have high strength and toughness that aids in resisting external damages. This is attributed to the increased stiffness that is often accompanied by mechanical enhancements that impede actuation strains or raise the driving electric field requirements. To address this, we introduce the concept of co-stimulation in pursuit of a resilient DEA with high actuation performance.
What is co-stimulation?
Co-stimulation refers to applying two stimuli concurrently to achieve a particular function. This can be seen in athletes warming up their muscles before an exercise. While muscles are activated by neural electrical signals, the addition of a co-stimuli to raise the body temperature results in increased blood flow to the muscles, improving their performance.
Inspired by this, we applied co-stimulation of NIR light and electric fields to achieve resilient DEAs with high actuation performance. Resilient DEAs were based on polyurethane elastomers that were functionalized with carboxyl groups to form dynamic physical crosslinks through hydrogen bonding. As such, the elastomer could achieve high toughness (96.5 MJ m-3) and tensile strength (18.5 MPa) suitable for resisting damage. Subsequently, liquid metal nanoparticles (LMNPs) were introduced to the matrix due to their unique ability to increase dielectric constants and concurrently impart photothermal capabilities to the elastomer, enhancing responses to electric field and NIR light respectively.
LMNPs enhanced the resilient nature of DEAs in the following manner.
1) Mechanical toughness of nanocomposites was enhanced due to effective load transfer from the polymer chains to LMNPs through interfacial interactions.
2) When illuminated by NIR light, these nanocomposites realized self-healing in a contactless manner as photothermal effects accelerated the reformation of broken hydrogen bonds (Fig. 1a).
3) Nanocomposite can be recycled at lower temperatures and shorter times compared to the pristine, as hydrogen bonds between the nanoparticles and the matrix lowered energy barriers for topological re-arrangement (Fig. 1b). With recycling procedures, the DEA could recover its actuation performance even after being cut into many pieces or electrical failure (Fig. 1c,d), achieving area strains of 57.1% and 34.2% respectively.
Based on these properties we achieved both the toughest and best-performing DEA with recoverable actuation area strains.
Effects of co-stimulation
During co-stimulation, NIR light is applied to induce photothermal effects of LMNPs that soften the elastomer. The degree of modulus reduction can be controlled by tuning the NIR light intensity while simultaneously applying the electric field. As the DEA is softer, lower driving electric fields are required to achieve high actuated area strains.
When configured to a bilayer structure, the actuator can respond to both electric fields and NIR light, based on Maxwell pressure and asymmetrical thermal expansion mechanisms respectively. Under co-stimulation, a synergistic effect is realized between both mechanisms that allow the actuator to bend at larger degrees compared to the expected additive effects when driven by electric field and NIR light individually. When the bilayer structure was used for a soft robotic crawler, photothermal effects enabled longer strides and increased the flattening of its body for enhanced shape adaptability to move across narrow spaces (Fig. 2). With our co-stimulation strategy, resilient elastomers can now be applied to soft robots and devices to achieve longer operational lifetimes and improved actuation capabilities.
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