Bias Pressure Enables Untethered, Power-Dense Fluidic Muscles Driven by Integrated Soft Pumps

Fiber pumps are mm-sized flexible tubes that create their own pressure and flow rate without the need of an external pump. Soft and solid-state, these pumps solve the long-standing problem of integrating pressure generation in portable systems, such as robots and wearables.<br/><br/>We developed untethered fluidic muscles made of fiber-form fluidic actuators (i.e., thin McKibben muscles) in antagonistic configuration, connected by internal fiber pumps. We discovered that when using an internal pump instead of an external pressure source fluidic actuators need to be operated with a well-defined bias pressure. We studied both analytically and experimentally the effects of bias pressure on closed fluidic systems and discovered that it is not only required, but also increases their global power density by a factor of 5 to 10.<br/><br/>Fiber pumps are made of two materials: a thermoplastic polymer tubing with two metal wire electrodes arranged in a spiral inside the tube. They work based on ion-drag EHD (ElectroHydroDynamics): dielectric liquid molecules get ionised at one electrode, accelerated by Coulomb force in between the electrodes, and de-ionised at the opposite electrode. During their motion from one electrode to the other, they exchange momentum with the rest of the liquid, putting it in motion.<br/><br/>We connected fiber pumps with thin McKibben muscles (OD=1-2 mm) to develop for the first time all-fiber fluidic actuators with integrated pressure generation. These actuators convert electrical energy from a power supply into fluid pressure in the pumps, and then fluid pressure into mechanical work in the thin McKibben muscles. The result is an electro-fluidic muscle fiber that contracts when voltage is applied to it. We designed an antagonistic configuration, where one McKibben muscle acts as a reservoir for its antagonist. When voltage is applied to the fiber pump one muscle contracts and the antagonistic one relaxes.<br/><br/>Our study demonstrates that such systems cannot be operated by filling them with liquid at atmospheric pressure. An extra volume of liquid and a bias pressure is required for closed electro-fluidic muscles to work. We studied the effect of bias pressure and report the following findings:<br/>- Optimal bias pressure increases the contraction by &gt;100% and reduces the response time by &gt;50% by leveraging nonlinearities in elastomer-based fluidic actuators.<br/>-It prevents liquid cavitation, which would make closed fluidic actuators not viable (liquid on the low-pressure side would reach its vapor pressure and evaporate locally).<br/>-It increases the pressure generated by the fiber pumps enabling them to reach higher operating electric fields: from 4 bar/m at 7 V/µm with no bias pressure to 8 bar/m at 10 V/µm with 1 bar of bias pressure.<br/><br/>Integrated all-fiber artificial muscles open wide opportunities for robots and wearables. To fully leverage their performance, these coupled systems need to be carefully designed to maximize the muscles’ output given the finite pressure and flow that fiber pumps can generate. Bias pressure is an essential element in the design of these systems.<br/><br/>We developed an analytical model that describes the behavior of the coupled pump-actuator system and provide a design tool to select pumps, actuators and bias pressure for a given robotic task, for example a muscle bundle for lifting 1 kg of weight or a glove for hand rehabilitation.<br/><br/>The combined muscle-pump-muscle ensemble with an optimized bias pressure showed impressive performance in our preliminary tests. Two individual McKibben muscles with one pump lift a 200 g load over 40 mm of stroke in less than 1 second. A bundle of 8 muscles with an overall weight of 40 g can lift over 1 kg with a stroke of 7 cm and in 2 seconds.<br/><br/>Such performance is comparable with skeletal muscles. Using our findings and analytical design tools researchers will be able to deepen the optimization of these novel artificial muscles and achieve a wide adaptation to different form factors and robotic tasks.