Dissertation Title:  Millimeter-Scale Electrofluidic Actuators for Soft Mechatronic Systems

Abstract:

Actuators are to robots what muscles are to animals: they convert energy into mechanical work through physical interactions. Below the centimeter scale, the challenge of delivering useful force and mechanical work within a compact volume motivates alternative actuation physics. Achieving energy-dense linear actuation within slender, compliant structures challenges conventional motor–transmission assemblies, particularly when many actuators must fit within a compact body. This thesis investigates a “system in a tube” approach that uses electrostatic forces to generate fluid flow and pressure to drive muscle contraction. The resulting electrofluidic fiber muscles (EFMs) integrate pressure generation, fluid transport, and liquid storage within a self-contained circuit, making the fluidic power source and reservoir part of the fiber-format muscle ensemble.

Electrohydrodynamic fiber pumps are coupled with millimeter-scale fluidic actuators, using a dielectric liquid as a conduit for energy transfer between electrical excitation and mechanical contraction. Within an antagonistic pair, the pump transfers liquid into the contracting agonist as the elongating antagonist releases it. This reciprocal arrangement incorporates liquid storage into the muscles themselves, eliminating the need for a separate external pump, compressor, or reservoir.

An electrohydromechanical analytical model couples pump pressure–flow behavior with nonlinear actuator mechanics through fluid-volume conservation to predict muscle performance at different bias pressures and external loads. Experimental measurements validate these predictions. The model establishes how the pump-to-actuator ratio governs dynamic response and how arranging muscles in series and parallel determines assembly-level stroke and force, providing a basis for configuring task-specific actuator assemblies. Robotic demonstrations include a lever arm for rapid launch, a muscle bundle lifting multi-kilogram loads, and muscle sheets arranged as a biceps–triceps pair to drive a backdrivable joint. Across operating conditions, we report contractile strains approaching 20% from a biased baseline, specific mechanical work of approximately 20–45 J/kg, and a response time of 0.3 s.

Building on the first-generation architecture, the thesis investigates strategies to improve electrohydraulic conversion efficiency and enable reliable bidirectional pumping. These investigations examine how reducing electrode spacing, increasing geometric asymmetry, and using dissimilar electrode materials influence pressure generation and flow. To overcome the unidirectional pumping of first-generation EFMs, a second-generation tri-electrode fiber-pump architecture uses optoswitches to reverse fluid transport between the agonist–antagonist pair. This transition enables either muscle to actively drive joint motion under load. Active antagonistic operation expands joint range of motion, shortens return time relative to passive relaxation, and, with closed-loop control, enables compensation for the effects of actuator hysteresis.

Complementary studies of architected fluidic and liquid crystal elastomer fibers extend the investigation of fiber-format actuation to continuous manufacturing and spatially defined contractile regions within monolithic soft transducers. Together, these contributions advance artificial muscles for soft mechatronic systems through the introduction of a new class of electrofluidic fiber actuators and advances in manufacturing principles, modeling, and system integration at the millimeter scale.

Hiroshi Ishii
Jerome B. Wiesner Professor of Media Arts and Sciences
MIT Media Lab

Ritu Raman
Eugene Bell Career Development Associate Professor
MIT Mechanical Engineering

Herbert Shea
Full Professor, Soft Transducers Laboratory
LMTS, École polytechnique fédérale de Lausanne (EPFL)