Energy Efficient Hydraulic Actuation of a Soft Total Artificial Heart

Abstract

Heart failure affects millions of people worldwide, creating a critical need for effective Total Artificial Hearts (TAHs) as a bridge to heart transplantation. Soft robotics offer a promising approach for creating TAH's with a more natural, physiological pumping motion. However, current soft TAHs (sTAHs) rely on pneumatic actuation, requiring bulky external compressors and skin-penetrating tubes that pose severe infection risks and limit patient mobility. This thesis explores the transition toward a fully implantable, hydraulically actuated sTAH. Because hydraulic fluid is significantly denser and more viscous than air, this shift introduces substantial energy efficiency challenges. To address these challenges, this research utilizes lumped-parameter models and physical testing within a mock circulatory loop, actuating the sTAH with a custom-designed hydraulic pump. The primary focus of this thesis is the geometrical redesign of the existing pneumatic LIMO (Less In More Out) sTAH prototype. The LIMO prototype utilizes a circular array of pouch motors which contract a central ventricle upon inflation. To minimize hydraulic resistance, the redesigned 'HYDRO LIMO' prototype maximizes cross-sectional flow area and integrates fluid flow paths directly into these pouches, significantly reducing viscous losses. Operating under pulmonary conditions, the HYDRO LIMO achieved an average cardiac output of 6.7 L/min at the 25 W continuous power limit of the proposed Transcutaneous Energy Transfer (TET) system, surpassing the average resting cardiac output of 5-6 L/min. Furthermore, the prototype demonstrated a competitive peak capacity of 8 L/min average at 35 W. Alongside geometric optimization, two theoretical energy-saving strategies were evaluated: resonant actuation and actuation waveform optimization. Simulation results indicate that resonant actuation is not a viable energy-saving strategy for sTAHs. The heavily overdamped nature of the cardiovascular system causes viscous forces to completely dominate the system dynamics and the introduction of an elastic element yields a net electrical energy penalty. Furthermore, while waveform optimization (sine, triangular, square) yielded negligible efficiency differences for the current low-resistance HYDRO LIMO prototype, simulations reveal that rapid diastolic unloading (e.g., a square wave) is beneficial to maximize stroke volume under the restrictive filling conditions as a result of future system miniaturization. Ultimately, this thesis demonstrates that the transition to a hydraulically actuated sTAH is highly viable when prioritizing the minimization of internal flow resistance and optimizing diastolic filling conditions. The HYDRO LIMO establishes a strong, energy-efficient foundation for the future development of fully implantable, TET-powered soft artificial hearts.