Paul Brochu

Advances in Dielectric Elastomers for Actuators and Artificial Muscles

A number of materials have been explored for their use as artificial muscles. Among these, dielectric elastomers (DEs) appear to provide the best combination of properties for true muscle-like actuation. DEs behave as compliant capacitors, expanding in area and shrinking in thickness when a voltage is applied. Materials combining very high energy densities, strains, and efficiencies have been known for some time. To date, however, the widespread adoption of DEs has been hindered by premature breakdown and the requirement for high voltages and bulky support frames. Recent advances seem poised to remove these restrictions and allow for the production of highly reliable, high-performance transducers for artificial muscle applications.

Compliant Silver Nanowire-Polymer Composite Electrodes for Bistable Large Strain Actuation

A new compliant electrode-based on silver nanowire-polymer composite has been developed. The composite electrode has low sheet resistance (as low as 10 Ω/sq), remains conductive (10(2) -10(3) Ω/sq) at strains as high as 140%, and can support Joule heating. The combination of the composite and a bistable electroactive polymer produces electrically-induced, large-strain actuation and relaxation, reversibly without the need of mechanical programming.

Highly stretchable, conductive, and transparent nanotube thin films

We have studied the electrical and optical properties of transparent and conductive nanotube thin films subjected to extremely large strains, both isotropic and anisotropic. The films maintain electrical conductivity for strains up to 700% and the eventual loss of conductivity is due primarily to the buildup of cracks in the nanotube films. We also measured the change in optical transmittance and explain the observed haziness of the films by considering the micrometer sized cluster. This study of transparent nanotube films as stretchable electrodes is crucial for many applications, in particular, for medical implantation of electronic devices.

Large-strain, rigid-to-rigid deformation of bistable electroactive polymers

Thermoplastic poly(tert-butyl acrylate) (PTBA) is reported as an electroactive polymer that is rigid at ambient conditions and turns into a dielectric elastomer above a transition temperature. In the rubbery state, a PTBA thin film can be electrically actuated to strains up to 335% in area expansion. The calculated actuation pressure is 3.2 MPa. The actuation is made bistable by cooling to below glass transition temperature. The PTBA represents the bistable electroactive polymer (BSEP) that can be actuated to various largely strained, rigid shapes. The application of the BSEP for refreshable Braille display, an active tactile display, is also demonstrated.

Bistable Large-Strain Actuation of Interpenetrating Polymer Networks

The bistable electroactive polymer is a new smart material capable of large strain, rigid-to-rigid actuation. At the rubbery state of the polymer heated to above its glass transition, stable electrically-induced actuation is obtained at strains as large as 150%. Electromechanical instability can be effectively overcome by the formation of interpenetrating polymer networks. An application as a refreshable braille display is demonstrated.

Long lifetime, fault-tolerant freestanding actuators based on a silicone dielectric elastomer and self-clearing carbon nanotube compliant electrodes

We explore the effect of pre-stretch and application of mechanical loads on a soft polydimethylsiloxane (PDMS) elastomer to obtain high linear strain freestanding dielectric elastomer actuators. It is shown that when the mechanical loads are properly applied, large linear actuation strains of 120% and work density of 0.5 J cm−3 can be obtained due to a transition from pure-biaxial to pure-uniaxial actuation conditions. Furthermore, we demonstrate that when coupled with single wall carbon nanotube (SWNT) compliant electrodes, fault-tolerance is introduced via self-clearing leading to significantly improved operational reliability. Cycling actuation tests reveal that even after more than 30 self-cleared electrical breakdown events the actuators maintain a high level of performance. Driven at moderate electric fields, the actuators display relatively high linear actuation strain (25%) without degradation of the electromechanical performance even after 85000 cycles.

Dielectric elastomer transducers with enhanced force output and work density

We demonstrate that the force output and work density of polydimethylsiloxane (PDMS) based dielectric elastomer transducers can be significantly enhanced by the addition of high permittivity titanium dioxide nanoparticles. The nanocomposites are capable of maintaining the actuation strain performance of the pure PDMS at relatively low electric fields while increasing the force output and work density due to mechanical reinforcement. A model relating the Maxwell stress to the measured force from the actuator was used to determine the dielectric permittivity at high electric fields thus providing results that can be directly correlated to device performance. This approach toward higher work density materials should enable smaller, lighter, and less intrusive actuator systems ideal for biomedical and robotic devices in particular.

All-silicone prestrain-locked interpenetrating polymer network elastomers: free-standing silicone artificial muscles with improved performance and robustness

We present a novel all-silicone prestrain-locked interpenetrating polymer network (all-S-IPN) elastomer for use as a muscle-like actuator. The elastomer is fabricated using a combination of two silicones: a soft room temperature vulcanizing (RTV) silicone that serves as the host elastomer matrix, and a more rigid high temperature vulcanizing (HTV) silicone that acts to preserve the prestrain in the host network. In our novel S-IPN fabrication procedure we co-dissolve the RTV and HTV silicones in a common solvent, cast thin films, and allow the RTV silicone to cure before applying prestrain and finally curing the HTV silicone to lock in the prestrain. The free-standing prestrain-locked silicones show a performance improvement over standard free-standing silicone films, with a linear strain of 25% and an area strain of 45% when tested in a diaphragm configuration. We show that the process can also be used to improve electrode adhesion and stability as well as improve the interlayer adhesion in multilayer actuators. We demonstrate that, when coupled with carbon nanotube electrodes, fault-tolerance through self-clearing can be observed. We use the fault-tolerance and improved interlayer adhesion to demonstrate stable long-life (>30 000 cycles at >20% strain) actuation and repeated high-performance actuation (>500 cycles at ~40% strain) of prestrained free-standing multilayer actuators driving a load.

Refreshable tactile displays based on bistable electroactive polymer

Refreshable tactile displays can significantly improve the education of blind children and the quality of life of people with severe vision impairment. A number of actuator technologies have been investigated. Bistable Electroactive Polymer (BSEP) appears to be well suited for this application. The BSEP exhibits a bistable electrically actuated strain as large as 335%. We present improved refreshable tactile display devices fabricated on thin plastic sheets. Stacked BSEP films were employed to meet the requirements in raised dot height and supporting force. The bistable nature of the actuation reduces the power consumption and simplifies the device operation.

Factors influencing the performance of dielectric elastomer energy harvesters

We present a simplified dielectric elastomer energy harvesting model to explore the effects of various materials and operating parameters on both the amount of energy generated and the efficiency of dielectric elastomer generators and show that high energy output and efficiency can be obtained in various materials systems. The amount of energy generated increases with increasing values of bias voltage and applied stretch while the efficiency is shown to depend strongly on bias voltage but only weakly on stretch. We show that increasing the dielectric constant can have significant impacts on the amount of energy generated in certain systems and that stiffening the elastomer has the main effect of shifting the regions of high efficiency to lower strains and larger voltages. Using these results as a basis we explore one particular material system experimentally and compare with the results from our model. The impacts of electrode resistance and elastomer viscoelasticity are also explored.