Electromechanically Active Polymers

摘要

Electromechanically Active Polymers (EAPs) form a broad family of ‘smart materials’ capable of transducing energy from the electrical to the mechanical form, and vice versa. As such, they are used for electromechanical actuation and mechanoelectrical sensing, as well as mechanical energy harvesting to generate electricity1-5. While EAPs are traditionally known as ElectroActive Polymers, this denomination does not capture the specificity of the family, consisting of the ability to undergo significant deformation and/or stress changes in response to suitable electrical stimuli. Accordingly, that term is not used here, to explicitly exclude any other type of electrically induced/regulated ‘activity’, wherein the mechanical form is not the ultimate (or the initial) stage of the energy transformation chain of interest (as in the case, for instance, of electrically semiconducting or conducting polymers for plastic electronics). Today, EAPs represent a well established and promising scientific field of research and development. EAP materials are commonly classified in two major families: ionic EAPs, activated by an electrically-induced transport of ions and/or solvent, and electronic EAPs, activated by electrostatic forces1-5. Ionic EAPs include polymer gels6, ionic polymer metal composites7 (including later evolutions to ionic polymer conductor composites, and the variant of interpenetrating polymer networks8), conjugated polymers9, and carbon nanotubes10. Electronic EAPs include piezoelectric polymers11, electrostrictive polymers12, dielectric elastomers13, liquid crystal elastomers14, along with what is proposed here as the latest entry, represented by carbon nanotube aerogels15. While each EAP category shows specific electromechanical properties, typically suitable for different needs and applications, general features include high mechanical compliance, low density, ease of processing, inherent responsiveness to electrical stimuli, as well as low cost. As a result, EAP transducers, in general, are flexible, light-weight, structurally-simple, versatile, scalable, and cheap; additionally, when used as actuators they have sizable electromechanical performance and integrated force/stroke feed-back, and are noiseless and heat-free [1–5]. Although several EAP materials and their properties have been known for many decades, they have found very limited applications. Such a trend has changed recently, as a result of an effective synergy of at least three main factors: key scientific breakthroughs being achieved in some of the existing EAP technologies; unprecedented electromechanical properties being discovered in materials previously developed for different purposes; and a higher concentration of efforts for real exploitation of EAP materials. As an outcome, after several years of basic research, today the EAP field is just starting to undergo transition from academia into commercialization, with significant investments from large companies. EAP actuators are being developed for applications that so far have been precluded to conventional actuation technologies (mainly electric/electromagnetic, hydraulic/pneumatic and thermo-chemical motors). Usage spans from the micro- to the macro- scale in different sectors, such as medical and haptic devices, consumer electronics, and automation and robotic systems. Reported examples include micro-pumps and micro-valves for micro-fluidic systems (e.g. for lab-on-a-chip devices)1, 5, controlled release of active compounds for medical therapeutic devices (e.g. insulin release in blood stream)1, 5, miniaturized surgical tools for medical interventional systems (e.g. steerable catheters)1, 5, miniaturized implantable actuators as components of artificial organs (e.g. mechanical stimulators of cardiac tissue)1, 5, 16, robotic systems (including medical and industrial robots)1, 4, 16-18, variable-stiffness devices (e.g. safe actuators for robots interacting with humans and vibration dampers for vehicles)