All-Fiber Electrofluidic Muscles—Soft Actuators with Unprecedented Design Freedom

Soft fluidic muscles are ideal for active wearables and textile-based robots. Thin McKibben muscles, with diameters around 1 mm and lengths of several meters, can be woven into artificial textile muscles. However, most fluidic muscles need external, noisy, and bulky pumps, limiting integration, wearability, and portability.<br/><br/>We report the integration of ion-drag ElectroHydroDynamic (EHD) fiber pumps with McKibben fiber muscles, both in a compatible form factor (1-2 mm diameter) with textile fabrication techniques. We developed an antagonistic system with two thermoplastic polyurethane (TPU)-core McKibben muscle units and a TPU-based fiber pump unit in between. This results in an all-fiber electro-fluidic muscle that actuates when voltage is applied: the pump transfers liquid from the antagonist muscle, extending it, while the pressurized agonist muscle contracts. Optimizing electro-fluidic muscle performance requires careful system design to maximize mechanical output within the pressure and flow rate limits of fiber pumps.<br/><br/>We outline design considerations based on our analytical model and present robotic demonstrators in knitted, woven and bundled forms. The integrated muscle weighing less than 5 grams promises wearable robots with rapid response (&lt; 1 sec) and power density (&gt; 25 W/kg) comparable to human muscles. Achieving these performances require specific material, geometrical, and configuration choices.<br/><br/>Various electrode materials for EHD pumps are comparatively evaluated showing that platinum electrodes offer the highest stability and performance, achieving a 700 % increase (8 bars/m) over previous results at 10 V/μm. This substantial enhancement is due to the selection of electrode material, a larger electrode surface area in contact with the dielectric fluid, and the application of bias pressure for higher electric field operation.<br/><br/>From our analytical model, we found the maximum pressure the coupled system can reach is limited by cavitation on the pump's low-pressure side. For a 1 bar bias pressure, the safe operation is at 4 bars of relative pressure generated by the pump. This led to an optimal pump length of 350 mm and a maximum safe operation voltage of 8 kV.<br/><br/>We modeled and evaluated the coupled system in a symmetric configuration and proposed methods to increase cavitation thresholds, such as varying fiber muscle lengths and asymmetric loading, enhancing stroke and power density.<br/><br/>Utilizing these principles, we developed several robotic demonstrators:<br/>- An assistive glove was knitted enabling fingers' movement when the fiber muscles contract and relax. Two tendons are actuated by symmetric muscle pairs powered by a forearm-wrapped fiber pump. Activating the pump contracts a 1.5 mm diameter muscle and elongates the other, bending a bionic hand finger of 60° in &lt; 1 second.<br/>- A woven biceps and triceps muscle pair was asymmetrically configured, mimicking human anatomy. The woven structure consists of McKibben muscles as the warp and fiber pumps as the weft. A 10 McKibben biceps muscle exhibits 22% strain at 8 kV, bending the forearm nearly 90° in 1 second.<br/>- An agonist 8-McKibben (1.75 mm diameter, 280 mm length) bundle with 2 pumps wound around it, in fluidically parallel configuration, was coupled with a symmetrical antagonist pair. The two parallel pump pairs are then fluidically connected in series, doubling the generated pressure and flow rate of the system. Activation results in the agonist muscle contracting by 40 mm and the antagonist extending by 30 mm, achieving a relative stroke of 70 mm in under 2 seconds.<br/><br/>The integration of EHD fiber pumps with thin McKibben muscles facilitates compliant, untethered, silent and power-dense fluidic systems ideal for wearable robotic applications in assistive devices and exoskeletons. Future work will extend these integration principles to more complex all-fiber fluidic circuits, including entire robotic garments with distributed control of individual pump-actuator ensembles.