In recent years, robot technology has gained significant traction across various fields such as aerospace, education, healthcare, and scientific exploration. The motion of robots primarily stems from external stimuli, and compared to robots driven by temperature, pressure, magnetic fields, light, or fluids, those powered by electric fields offer advantages like rapid response, high control precision, and flexibility. Among electrically driven robots, those composed of rigid components suffer from limited degrees of freedom and adaptability, making them unsuitable for complex environments and safe human-robot interactions. With advancements in new materials and manufacturing techniques, flexible polymer materials that respond to external stimuli have been widely adopted in soft actuators and robot technology. Polymer-based electrically driven soft robots, with their continuously deformable structures, higher degrees of freedom, and design versatility, can better interact with unstructured spaces. Their low density also reduces costs in aerospace applications, showcasing broad potential. However, current electro-responsive smart materials face issues like low energy efficiency, insufficient driving precision, and stability problems. At the device level, challenges include simplistic actuation methods, limited application scenarios, and a lack of multimodal motion capabilities in complex environments. This article reviews the driving mechanisms of polymer-based electrically driven soft robots from a material perspective, summarizes performance enhancement methods, explores their motion behaviors in various environments, and discusses the integration of driving functions with multi-physical fields to broaden applications. Finally, we address current challenges and future trends in robot technology.
Electrically driven actuators based on polymers are categorized into electroactive polymers (EAPs) and electro-thermal actuators (ETAs). EAPs are smart materials that convert electrical energy into mechanical energy, offering benefits like low density, elasticity, large strain, and fast response. They are further divided into electronic EAPs and ionic EAPs based on their driving mechanisms. Prominent electronic EAPs include dielectric elastomers (DEs) and piezoelectric polymers, while ionic EAPs include ionic polymer-metal composites (IPMCs) and electroactive hydrogels (EAHs). ETAs rely on the Joule heating effect to transform electrical energy into thermal energy and then mechanical motion, primarily through bilayer ETAs with different thermal expansion coefficients or thermally triggered shape memory polymers (SMPs). This section delves into the driving mechanisms of DEs, piezoelectric materials, IPMCs, EAHs, and electro-thermal polymers, along with strategies for improving their performance in robot technology.
Dielectric elastomers (DEs) are a key material in electronic EAPs, known for their high power density, fast response, and driving stability. The electro-deformation mechanism of DEs involves electro-polarization and electro-elastic effects. When an electric field is applied to a DE film sandwiched between electrodes, molecular polarization occurs, leading to thickness compression and in-plane expansion due to Maxwell stress. The Maxwell stress \( p \) and thickness strain \( s \) can be expressed as:
$$ p = \varepsilon_0 \varepsilon_r E^2 $$
$$ s = -\frac{\varepsilon_0 \varepsilon_r E^2}{Y} $$
where \( \varepsilon_0 \) and \( \varepsilon_r \) are the vacuum and relative permittivity, \( E \) is the electric field strength, and \( Y \) is the elastic modulus. The dielectric constant, elastic modulus, and breakdown strength are critical factors influencing DE performance. Traditional DE materials like silicone rubber, polyurethane, and acrylates often fall short in driving performance, prompting enhancements through filler incorporation and chemical modification. For instance, adding high-dielectric or conductive fillers, such as carbon dots or barium titanate nanoparticles, can significantly boost the dielectric constant and driving displacement. Chemical modifications, like introducing polar groups or forming interpenetrating polymer networks, also improve dielectric properties without increasing stiffness, enabling large actuation strains at low voltages. These advancements are crucial for developing efficient soft actuators in robot technology.
Piezoelectric polymers are another class of materials used in electrically driven soft actuators, leveraging the inverse piezoelectric effect to convert electrical energy into mechanical force. Unlike DEs, where deformation stems from Maxwell stress, piezoelectric materials exhibit deformation due to piezoelectric and electrostrictive effects. The piezoelectric effect is linear with the electric field, while electrostriction is quadratic. Poly(vinylidene fluoride) (PVDF)-based polymers are widely used due to their flexibility, low density, and ease of processing. However, their piezoelectric constants are relatively low compared to inorganic materials. Enhancements involve modulating the amorphous phase, crystal-amorphous interfaces, and crystalline phases. For example, introducing fluorinated alkyne monomers into PVDF-TrFE-CFE terpolymers can significantly increase the piezoelectric coefficient \( d_{33} \) to over 1000 pm/V, rivaling ceramic oxides. These improvements expand the application of piezoelectric polymers in precision actuation for robot technology, such as in walking and flying robots.
Ionic polymer-metal composites (IPMCs) belong to ionic EAPs and operate through ion migration under an electric field, resulting in bending motions. A typical IPMC consists of an ion-exchange polymer film coated with noble metal electrodes. When a voltage is applied, cations move toward the cathode, causing swelling and bending toward the anode. IPMCs offer advantages like low driving voltage (≤5 V) and suitability for aqueous environments, but they suffer from low blocking force and energy efficiency. Enhancements focus on the ion-exchange membrane, electrodes, and electrolyte. For instance, incorporating nanomaterials or optimizing electrode adhesion can improve performance. IPMCs are promising for underwater robots and microscale devices in robot technology, such as synthetic cilia systems for fluid manipulation or microscale origami robots for medical applications.
Electroactive hydrogels (EAHs) are ionic EAPs that undergo volume or shape changes under an electric field due to ion redistribution and osmotic pressure gradients. They are formed by incorporating conductive polymers, metal nanoparticles, or graphene derivatives into hydrogel networks. Strategies to enhance electroactivity include doping with suitable agents, adding functional nanoparticles, introducing cross-linkers, or forming interpenetrating networks. However, EAHs typically have poor mechanical properties (Young’s modulus below 100 kPa), leading to low blocking forces. Composite approaches, such as embedding cellulose nanocrystals or using micelle-based copolymers, can improve mechanical strength. EAHs are valuable for underwater soft robots, drug delivery systems, and biomimetic engineering in robot technology, as they respond to multiple stimuli like pH, temperature, and electric fields.
Electro-thermal actuators (ETAs) utilize the Joule heating effect to induce shape changes. In bilayer ETAs, materials with different thermal expansion coefficients are layered with a heating element, causing bending upon heating. Shape memory polymers (SMPs) can be electro-thermally triggered to undergo phase transitions and recover programmed shapes. Although SMP actuators offer programmable shapes and low energy consumption, they often lack reversible actuation. Solutions include developing semi-crystalline SMPs with broad melting ranges or designing bilayer structures for reversible motion. Challenges remain in achieving fully reversible drives without external forces. ETAs are commonly used in microvalves and soft grippers in robot technology.
| Material Type | Advantages | Challenges | Operating Environment |
|---|---|---|---|
| Dielectric Elastomer | High power density, fast response, large deformation | High driving voltage (kV range), short lifespan due to breakdown | Aquatic, terrestrial, aerial soft robots |
| Piezoelectric Polymer | Anti-magnetic interference, high efficiency, high response frequency | Hysteresis, creep, temperature sensitivity | Walking and flying robots |
| Ionic Polymer-Metal Composite | Low driving voltage, works in wet and dry conditions | Low blocking force | Underwater robots, microscale robots |
| Electroactive Hydrogel | Low driving voltage, works in aqueous environments | Low blocking force, requires electrolyte | Underwater robots, microvalves |
| Electro-thermal Polymer | Shape programmable | Requires external force for reversible actuation | Microvalves |
| Material Type | Driving Voltage (V) | Blocking Force (mN) | Displacement Range (mm) | Energy Density | Efficiency |
|---|---|---|---|---|---|
| Dielectric Elastomer | 450 | 9 | ≈3 | 0.02 kJ/kg | ≈26% |
| Dielectric Elastomer | 3800 | N.A. | 67.5 | – | – |
| Piezoelectric Polymer | 200 | N.A. | 200 | – | – |
| Ionic Polymer-Metal Composite | ≈3.5 | 0.86 | ≈19 | 2.44 kJ/m³ | 2.5–3% |
| Ionic Polymer-Metal Composite | ≈2.5 | N.A. | ≈14 | – | – |
| Electroactive Hydrogel | <10 | N.A. | ≈44 | – | – |
| Electroactive Hydrogel | 6–15 | N.A. | ≈36 | – | – |
| Electro-thermal Polymer | ≈3 | N.A. | ≈2 | – | – |
| Electro-thermal Polymer | ≈36 | 677 | ≈5 | – | – |
Polymer-based electrically driven soft robots exhibit diverse motion behaviors in various environments, enabled by anisotropic material properties and biomimetic structural designs. Gait studies focus on coordinating limb movements with body motion to achieve specific locomotion patterns. Researchers have developed robots capable of crawling, walking, jumping, climbing, underwater movement, and flight, each adapted to non-structured environments. This section summarizes these motion behaviors, highlighting how robot technology leverages material innovations for enhanced performance.
Crawling and walking robots are constrained to two-dimensional planar motion, typically consisting of a torso and limbs that generate friction with the ground for propulsion. For example, a fast-crawling soft robot using PVDF as the driving material and passive PET achieved speeds of 20 body lengths per second, with robust climbing and load-bearing capabilities. However, early designs lacked precise directional control, leading to innovations like electrostatic footpads for differential steering, enabling maze navigation in 5.6 seconds. Further improvements involved limb structure optimization and onboard power sources for untethered operation. Inspired by cheetahs and kangaroos, robots with double-helix and single-helix resonators achieved maximum speeds of 42.8 body lengths per second and turning rates of 482 °/s, enhancing mobility and environmental adaptability. Dielectric elastomers have also been used in crawling robots; for instance, a soft crawler with chiral grid feet and DE muscles could switch between forward, backward, and circular motions by adjusting voltage frequency, allowing access to any planar position. Multi-legged robots, such as tripedal designs with stacked DE films, operate below 500 V and carry five times their weight, while hexapodal robots with 3D-printed legs can climb stairs. Integrating sensing capabilities is another advancement; a soft robot with a biomimetic spine used the direct and inverse piezoelectric effects for self-sensing and adaptation, efficiently traversing grass and gravel. Similarly, DEA-based robots combined actuation and sensing to detect and avoid obstacles, showcasing the integration of robot technology with intelligent systems.
Jumping robots mimic animals like frogs and grasshoppers to overcome obstacles by briefly leaving the ground. Their mechanisms involve rapid mechanical deformation or instantaneous energy release. Electrically driven jumpers often use DEs for large deformations or high forces. A DE jumping actuator adhered to a rigid membrane utilized residual stress and Maxwell stress under 5 kV to achieve jumping upon voltage removal. For multimodal mobility, a 3.5 g DE robot demonstrated forward, backward, and jumping motions: at 5 kV, it moved forward at 1–9 Hz, backward above 10 Hz, and jumped over 5 mm obstacles with high voltage and duty cycle. These designs highlight the versatility of robot technology in dynamic environments.
Climbing robots extend motion to vertical spaces, requiring adhesion to counteract gravity. Electrostatic adhesion is commonly employed in electrically driven soft climbers. A DE-based wall-climbing robot used voltage applied to limbs for adhesion and complex motions, scaling 90° vertical walls at 0.75 body lengths per second and moving horizontally at 1.04 body lengths per second. Inspired by caterpillars, a two-degree-of-freedom DEA with flexible footpads enabled crawling, climbing, and bending, achieving speeds of 2.38 mm/s and 2.30 mm/s, respectively, with turning modes for tunnel inspection. Transitions between horizontal and vertical planes are challenging; a PVDF robot with electrostatic footpads climbed 60° inclines and 3-body-height steps, while a graphene-liquid crystal elastomer robot with temperature-stiffness joints achieved 90° transitions. These innovations demonstrate the potential of robot technology in inspection and maintenance applications.
Underwater soft robots emulate swimming organisms like frogs, jellyfish, and fish. Frog-inspired DE robots with two actuators weighing 14 g swam at 19 mm/s under 5 kV and 0.25 Hz, while a similar design with adaptive feet reached 77 mm/s at 4.8 kV and 1.5 Hz, with a 70 mm turning radius. Jellyfish-like robots using frameless DEs achieved untethered swimming at 3.2 mm/s, and octopus-inspired robots with asymmetric stiffness arms peaked at 314 mm/s. DE and ion hydrogel-based electronic fish swam at 0.69 body lengths per second, leveraging water as a power source for up to 3 hours. A DE-powered soft robot inspired by deep-sea snailfish operated at 10,900 m depth, showcasing pressure resistance and swimming performance for deep-sea exploration. These advances underscore the role of robot technology in aquatic environments.
Flying soft robots achieve aerial locomotion, with flapping-wing designs being prominent at small scales due to sufficient lift generation under low Reynolds numbers. Motor-driven flapping robots demonstrated multimodal capabilities, such as a 35.4 g tailless robot that flew at 5 m/s, crawled, self-righted, and took off horizontally, with 8.2 minutes of flight and over 60 minutes of crawling. Smart material-driven flappers primarily use piezoelectric materials and DEs. An early piezoelectric flapper with PZT-5 series actuators weighing 60 mg and a 3 cm wingspan generated double its weight in lift at 110 Hz resonance. Later, an 80 mg decoupled flapper achieved untethered flight. DE-based insect-scale robots utilized multilayer actuators; a 100 mg DEA resonating at 500 Hz with a power density of 600 W/kg enabled robust flight, while a 143 mg DEA produced 0.36 N lift and 1.15 mm displacement at 400 Hz, achieving a high lift-to-weight ratio of 3.7 and 20 s hover time. A dragonfly-inspired robot with interacting wings saw a 19% lift increase from synchronized flapping. Integrating additional functions, such as embedded luminescent particles in DEAs, enabled visual tracking and closed-loop flight. These developments highlight the evolution of robot technology in aerial domains.
| Operating Environment | Driving Material | Mass | Motion Speed |
|---|---|---|---|
| Crawling and Walking | PVDF | 0.024 g | 20 body length/s |
| Crawling and Walking | PVDF | 0.041 g | 76 body length/s |
| Crawling and Walking | PVDF | 0.058 g | 42.8 body length/s |
| Underwater | DE | 14.3 g | 7.7 cm/s |
| Underwater | DE | – | 31.4 cm/s |
| Flying | DE | 0.155 g | – |
| Flying | – | 35.4 g | 5 m/s |
Integrating driving functions with multi-physical fields expands the application scope of soft actuators beyond mere motion. For instance, a electro-caloric cooling device using PVDF-based polymer films leveraged electro-thermo-mechanical synergy to achieve self-sustained cooling cycles under AC stimulation, reducing electronic chip temperatures by 17.5 K compared to passive cooling. Another example is a self-regenerative heat pump based on dual-functional relaxor ferroelectric polymers, which used field-driven electrostriction for efficient heat transfer, cooling to 8.8 K below ambient in 30 s with a specific cooling power of 1.52 W/g. Such couplings of electromechanical effects with other physical fields open new avenues for smart devices in robot technology, enabling functionalities like thermal management and energy harvesting.
In summary, polymer-based electrically driven soft robots offer rapid response, light weight, and simple structures, with significant progress in material performance and structural design. However, challenges remain in accurate control due to the hyperelastic nature of electro-responsive materials and response hysteresis. The integration of rigid-soft hybrid actuators and AI tools may address precision issues. Manufacturing complexities across scales require high-precision techniques like lithography, but cost-effectiveness must be considered. Biocompatibility is crucial for biomedical applications, necessitating non-toxic, absorbable materials to avoid immune responses. Most robots are still experimental, with practical applications limited by accuracy, cost, and biocompatibility. Future trends should focus on enhancing sensing modules using machine learning and neural networks for intelligent robot technology, enabling simultaneous actuation and perception in complex environments. The continued advancement of robot technology will rely on overcoming these hurdles through interdisciplinary innovations.