Nanoscale materials for artificial muscles
In robotics, orthopedics, and other fields, there is a need for electrically controllable ‘artificial muscles.’ As a result, there is a lot of interest in electro-active polymer actuators, which convert electrical energy into linear mechanical motion. Researchers are trying to find light-weight materials and structures based on soft polymers (elastomers) that will permit large deformation. One example, the dielectric-elastomer actuator, is like a soft capacitor and provides very high motion when voltage is applied. Although actuators have been made that can stretch more than 100%, reliable and repeatable actuation levels seem to be around 10–20%, with applied voltages of several thousand volts. Since high voltage levels are impractical and unsafe, researchers are seeking materials that can operate at voltages below a few hundred volts. An actuator's activity can be improved by raising its ability to store electrical energy or ‘dielectric constant.’ However, many existing approaches lead to reduced mechanical properties and strongly reduce the ‘electrical-breakdown field’ above which the material will suffer catastrophic electrical failure.
Elastomers consist of polymer chains linked to each other to form a flexible network. Adding filler material can improve the dielectric constant, and both insulating and conducting fillers have previously been investigated.1–24 For both, surface-interaction effects between particle and matrix become very prominent when the particles are nano-sized.7,25–27
We explored how altering the way the conducting or insulating filler is combined with a thermoplastic elastomer affects the dielectric constant and electrical-breakdown strength. We found that chemically grafting the filler to the elastomer, regardless of whether it consisted of insulating metal-oxide nanoparticles or a conducting polymer, made it possible to enhance the dielectric constant without reducing the electrical-breakdown strength.
We first experimented with insulating titanium dioxide (TiO2) nanoparticles in a thermoplastic elastomer. Larger particles (300nm) led to enhancements in the dielectric constant that could be reconciled with classical mixing rules, while smaller particles (15nm) displayed enhancements far above the classical expectations. The relative surface area of smaller particles is much larger than for larger particles, and so this is a strong indication that the interface between particle surface and elastomer material directly influences dielectric properties. When we added a common surfactant to the composite with 15nm particles, the enhancement factor was reduced and approached the level of the dielectric properties found for the larger particles. Despite their increased dielectric constant, the poor mechanical properties of the 15nm TiO2 composites (with or without surfactants) prohibited their use for actuators.11 Later experiments, in which we chemically grafted the surfactant to the nanoparticles' surface using organosilane coupling agents, showed that the mechanical and breakdown properties were unaltered, resulting in dielectric-elastomer actuators operating at half the voltage level.28
Conducting particles such as metals, or carbon-based particles such as carbon black or nanotubes, are known to show remarkable enhancements in the dielectric constant.10–24 Our experiments on composites of carbon-black nanoparticles in elastomer confirmed that the dielectric constant could be improved by several orders of magnitude. Unfortunately, we found that strong enhancements in the local electric field at certain sites inside the material also led to a drastic lowering of the electrical-breakdown field.24
Several researchers have found that conducting filler particles could be harnessed for improvements.10,12,15,18 The most promising results were found with conducting polymer material, which could be either blended18 or chemically grafted15 to the elastomer network. However, the resulting materials all had a reduced breakdown field, thus limiting the maximum achievable actuation strain.
In our final experiments, we chemically grafted the conducting polymer filler, polyaniline, to the elastomer network. We recently observed similarly enhanced electromechanical properties in a polyaniline and thermoplastic elastomer composite, in which chemical bonding (grafting) was achieved. We did not find any reduction in breakdown strength (see Figure 1), which was accompanied by high improvements of both dielectric and actuation properties (see Figure 2). This strongly encouraging result could lead to materials operating at highly reduced voltage levels, making them interesting for a much broader range of applications.
In our attempts to reduce the operating voltage of dielectric-elastomer actuators, we found that improvements are possible with both insulating and conducting nanoparticles, although our results on grafted conducting-polymer-composite blends appear to lead to the most useful improvements. This general grafting principle can be exploited to improve the actuation properties of softer elastomers as well, possibly providing higher actuation strains. With the new materials it will be possible to build new actuators operating at lower voltages, which we are currently attempting.
Guggi Kofod received his PhD on dielectric-elastomer actuators from the Danish Technical University in 2001. He subsequently worked for one year at the Danish Institute of Fundamental Metrology and spent two years as an assistant professor at Ateneo de Manila University in the Philippines. He currently leads a group researching new materials for dielectric-elastomer actuators.
Denis McCarthy received his PhD from Trinity College Dublin (Ireland) in 2007, on the topic of carbon-nanotube composites. He is currently working on modifying the properties of dielectric-elastomer actuators using insulating particles, with an emphasis on micrometer-scale actuation for miniature devices.
Hristiyan Stoyanov received his MSc on co-axial dielectric-elastomer actuators from the University of Potsdam in 2007. He has since then focused on composites based on conducting particles, working on both classical percolation and new grafted blends. He submitted his PhD thesis to the University of Potsdam in May 2010.
