Polymers are highly attractive for their inherent properties of mechanical flexibility, light weight, and easy processing. In addition, some polymers exhibit large property changes in response to electrical stimulation, much beyond what is achievable by inorganic materials. This adds significant benefit to their potential applications.

The focus of this issue of MRS Bulletin is on polymers that are electromechanically responsive, which are also known as electroactive polymers (EAPs). These polymers respond to electric field or current with strain and stress, and some of them also exhibit the reverse effect of converting mechanical motion to an electrical signal.

There are many types of known polymers that respond electromechanically, and they can be divided according to their activation mechanism into field-activated and ionic EAPs. The articles in this issue cover the key material types used in these two groups, review the mechanisms that drive them, and provide examples of applications and current challenges. Recent advances in the development of these materials have led to improvement in the induced strain and force and the further application of EAPs as actuators for mimicking biologic systems and sensors. As described in this issue, the use of these actuators is enabling exciting applications that would be considered impossible otherwise.

Field-activated electroactive polymers (FEAPs) are a class of electroactive polymers that are insulating and exhibit coulombic interaction with and dipole formation in response to external electric signals. There are many polarization mechanisms in insulating polymers, from the molecular to the mesoscopic and even the macroscopic level, which couple strongly with mechanical deformation and can be used to create polymer actuators and sensors. FEAPs feature fast response speed limited by the polymer dielectric and elastic relaxation time, a very large strain level (to more than 100% strain), high electromechanical efficiency, the ability to operate down to micro/nanoelectromechanical devices, and a highly reproducible strain response under electric fields. One challenge in FEAP actuators and electromechanical devices is reducing the operation voltage to below 100 V or even 10 V while achieving an electromechanical conversion efficiency comparable with that of inorganic electroactive materials.

This article introduces and considers the fundamental understanding of ionic polymer–metal composites (IPMCs) functioning as electroactive actuators and sensors. IPMCs consist of ion-exchange polymers acting as base materials and metal layers functioning as electrodes. The actuation and sensing abilities of IPMCs are dependent upon the components of ion-exchange polymers (ionic groups and cations) and electrode materials. In order to improve the bending and sensing performance of the IPMCs, an integral, two-step electroplating technique and a requisite dispersion agent are used during fabrication. Electroding materials also play a key role in determining the properties of IPMCs, and numerous methods in electroding have been tried, making use of various metals, carbon nanotubes, and composites. So far, IPMCs have been adapted as robotic actuators, artificial muscles, and electrical sensors. In the future, it is expected that IPMCs will broadly spread their roles from small-sized biomedical devices to large-scale actuators for aerospace as well as many industrial applications.

Conjugated polymer artificial muscles fill a unique niche in the electroactive polymer portfolio. They combine high strength, low voltage, and reasonable speed with versatile fabrication and design. This article reviews the actuation mechanism in these materials and presents some of the designs that have been developed for applications such as Braille displays, catheters, and bioMEMS devices.

Gels, soft polymeric or composite materials that have a high fraction of water, are often found as structural materials and actuators in nature but have so far not found many uses when fabricated synthetically. We first examine some natural systems such as jellyfish, sea anemones, starfish, legumes, and human tissue, all having interesting ways of moving or otherwise reacting to the surrounding environment. Then we discuss swelling and cross-linking of hydrogels, followed by a look at actuation by electrically, thermally, and chemically stimulated gels, noting that electrical stimulation needs a chemical intermediary to show substantial actuation (comparable to human muscle, for instance). Electroactive gels have great potential as sensors and actuators but their actual uses are mainly restricted to passive drug delivery and matrices for sensors. For most applications as artificial muscles, electrically driven actuators are too weak, but chemically driven actuators look very promising. Better ways of coupling electrical energy to chemically driven gels are needed.

Carbon nanotubes (CNTs) with macroscopically ordered structures (e.g., aligned or patterned mats, fibers, and sheets) and associated large surface areas have proven promising as new CNT electroactive polymer materials (CNT-EAPs) for the development of advanced chemical and biological sensors. The functionalization of CNTs with many biological species to gain specific surface characteristics and to facilitate electron transfer to and from them for chemical- and bio-sensing applications is an area of intense research activity.

Mechanical actuation generated by CNT-EAPs is another exciting electroactive function provided by these versatile materials. Controlled mechanical deformation for actuation has been demonstrated in CNT mats, fibers, sheets, and individual nanotubes. This article summarizes the current status and technological challenges for the development of electrochemical sensors and electromechanical actuators based on carbon nanotube electroactive materials.

Artificial molecular machines capable of converting chemical, electrochemical, and photochemical energy into mechanical motion represent a high-impact, fast-growing field of interdisciplinary research. These molecular-scale systems utilize a “bottom-up” technology centered upon the design and manipulation of molecular assemblies and are potentially capable of delivering efficient actuation at length scales dramatically smaller than traditional microscale actuators. As actuation materials, molecular machines have many advantages, such as high strain (40%–60%), high force and energy densities, and the capability to maintain their actuation properties from the level of a single molecule to the macroscale. These advantages have inspired researchers to develop molecular-machine–based active nanomaterials and nanosystems, including electroactive and photoactive polymers. This article will discuss the structures and properties of artificial molecular machines, as well as review recent progress on efforts to move molecular machines from solution to surfaces to devices.