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From 1880 to today: what is the surprising evolution of electroactive polymers?
Electroactive polymers (EAPs) are polymers that can change size or shape in response to electric field stimulation. The most typical applications of this type of material are actuators and sensors. A notable property of EAPs is that they can withstand large deformations while being subjected to large forces. In the past, actuators were mostly made of ceramic piezoelectric materials, which, although capable of withstanding large forces, often deformed by less than one part per thousand. By the late 1990s, studies showed that some EAPs could achieve strains as high as 380%, far exceeding any ceramic actuator. An important application of EAP in robotics is the development of artificial muscles, and electroactive polymers are therefore often referred to as artificial muscles.
Historically, the study of electroactive polymers began in 1880, when Wilhelm Roentgen designed an experiment to examine the effect of electrostatic fields on the mechanical properties of natural rubber.
An electric charge is applied from the air to a rubber band with one end fixed, and the change in its length is observed. In 1925, the first piezoelectric polymer (dielectric) was discovered, and this research laid the foundation for the future of EAP. The material is made by mixing carnaba wax, resin and beeswax and cooling it under an applied DC voltage. Over time, the response of polymers to environmental conditions has also become a focus of this research area. In 1949, Kacharsky et al. demonstrated that collagen fibers exhibit volume changes in acid or alkaline solutions, which also triggered research on other stimuli.
In 1969, Kawai confirmed that polyvinylidene fluoride (PVDF) had a strong piezoelectric effect, which sparked researchers' interest in developing other polymers with similar effects.
In 1977, the first batch of conductive polymers were discovered by Hideki Shiokawa and others. The conductivity of polyacetylene can be increased by eight orders of magnitude by doping with iodine vapor. With the invention of ionomer-metal composites (IPMCs) in the early 1990s, the development of EAP entered a new stage. This material only requires one to two volts of voltage to produce deformation, a feature that shows that EAP has greater application potential.
In 1999, Yousef Bar-Kohan proposed the idea of EAP robot arms competing against humans, and the first competition was held at a conference in 2005. In 2002, Japan's Eamex produced the first commercial EAP artificial muscle device, a fish that could swim independently, which accelerated the development of EAP in practical applications. However, the actual progress of related technologies is still unsatisfactory. Research funded by DARPA in the 1990s led to the establishment of an artificial muscle company in 2003 and industrial production in 2008.
Types of Electroactive Polymers
EAPs can be simply divided into two categories based on their structure: dielectric and ionic.
Dielectric EAP
In dielectric EAPs, actuation is caused by electrostatic forces between electrodes. Dielectric elastomers are capable of very high strains and behave like a capacitor whose capacitance changes when a voltage is applied.
Piezoelectric polymers
This class of polymers uses the piezoelectric effect to create acoustic sensors and motor actuators and has a wide range of applications due to its intrinsic piezoelectric response.
Liquid Crystal Polymer
The main chain liquid crystal polymer has a chain structure, which can exhibit unique mechanical properties under thermal changes and has potential mechanical drive applications.
Ionic EAP
This type of polymer is driven by the displacement of ions within the polymer, which only requires a few volts but relatively high electrical power.
Future Development Direction
While the EAP field is still developing, many challenges remain to be addressed. On the one hand, improving the performance and long-term stability of EAP and designing a waterproof surface to prevent water evaporation will effectively improve its reliability in various environments. On the other hand, developing thermally stable EAPs to improve their ability to operate continuously at higher voltages is also one of the future research focuses.
Against this backdrop of continuous progress, EAP technology will have the opportunity to be integrated into more and more application areas in the future, especially at the interface between humans and machines. With the advancement of materials science and technology, coupled with the development of bio-mimicry technology, we can't help but wonder what kind of amazing changes electroactive polymers will bring in the future?
