טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
Ph.D Thesis
Ph.D StudentEmiliya Gutman
SubjectDynamics and Motion Control of Miniature
Swimmers
DepartmentDepartment of Mechanical Engineering
Supervisor Professor Or Yizhar
Full Thesis textFull thesis text - English Version


Abstract

The behavior of microorganisms in nature has always been a subject of research interest. Understanding the motion of miniature swimmers can be useful for different purposes. For example, recent advances in medicine require finding solutions for fast diagnostics and treatment without invasive intervention, in order to reduce risk and patient discomfort. A possible solution can be development of micron-size robotic swimmers which are designed to maneuver inside the human body for performing various tasks such as targeted drug delivery and taking tissue biopsies. This requires formulation and understanding of the fundamentals of microswimmers' dynamics. Therefore, the goal of this research is analysis of the dynamics and motion control of miniature swimmers analytically, numerically and experimentally.

In order to do so, this work analyzes simple models of microswimmers consisting of rigid elongated links connected by rotary joints. One of the most famous models of a three-link microswimmer was described in the seminal work of Purcell in 1977. Our work studies extensions of this model in several directions. First, the dynamics of the swimmer is analyzed under direct control of the joint angles, where periodic shape changes, i.e. gaits, generate traveling wave-like motion, similar to biological flagellum. Our research focuses on finding gaits that generate net motion in desired directions by exploiting geometric symmetries of the swimmer and its dynamic equations, and on verification of the results by conducting motion experiments using a macro-scale model of the swimmer in a viscous fluid. Second, a modified model of the three-link swimmer with a single control input is considered, where one joint angle is periodically actuated while the other joint is passively driven by a torsion spring. This model is a simplified description of a microswimmer with a passive elastic tail. Its motion is no longer time-invariant as the classical model, and we explicitly study the dependence of the motion on actuation frequency and swimmer’s parameters.

In order to minimize the swimmer’s size for biomedical applications, a common solution is to actuate the swimmer externally by applying a time-varying magnetic field instead of internal motor actuation at the joints. For this type of actuation, a minimal microswimmer model has been considered, which consists of only two rigid links connected by a flexible rotary joint, where one or both links are made of magnetic material. A planar external magnetic field whose direction is oscillating periodically is applied to the swimmer, and the magnetic torques induce planar undulations that generate net forward motion.  We analyze this model theoretically by using the method of perturbation expansion, assuming small oscillations amplitude. The optimal actuation frequency, joint elasticity and ratio of links' magnetization that maximize average swimming speed or displacement per cycle are obtained. The results of this analysis were experimentally demonstrated as a part of our collaboration in manufacturing and actuating magnetic micro-robotic swimmers which have been built by the research group of B. Nelson at ETH University.