When swimming at low Reynolds numbers, inertial effects are negligible and reciprocal movements cannot induce net motion. Instead, symmetry breaking is necessary to achieve net propulsion. Directed swimming can be supported by magnetic fields, which simultaneously provide a versatile means of remote actuation. Thus, we analyze the motion of a straight microswimmer composed of three magnetizable beads connected by two elastic links. The swimming mechanism is based on oriented external magnetic fields that oscillate in magnitude. Through induced reversible hysteretic collapse of the two segments of the swimmer, the two pairs of beads jump into contact and separate nonreciprocally. Due to higher-order hydrodynamic interactions, net displacement results after each cycle. Different microswimmers can be tuned to different driving amplitudes and frequencies, allowing for simultaneous independent control by just one external magnetic field. The swimmer geometry and magnetic field shape are optimized for maximum swimming speed using an evolutionary optimization strategy. Thanks to the simple working principle, an experimental realization of such a microrobot seems feasible and may open new approaches for microinvasive medical interventions such as targeted drug delivery.

翻译：在低雷诺数条件下游泳时，惯性效应可忽略不计，且往复运动无法产生净位移。相反，必须通过对称性破缺来实现净推进。定向游泳可由磁场驱动，磁场同时提供了一种多功能的远程驱动方式。因此，我们分析了一种由三个可磁化球体通过两条弹性连杆连接而成的直线型微型游泳器的运动。其游泳机制基于强度振荡的定向外部磁场。通过游泳器两个连杆的可逆磁滞收缩，两对球体以非往复方式实现接触与分离。由于高阶流体动力学相互作用，每个运动周期后会产生净位移。不同的微型游泳器可调谐至不同的驱动振幅与频率，仅通过单一外部磁场即可实现同步独立控制。采用进化优化策略对游泳器几何结构与磁场形态进行优化，以实现最大游泳速度。得益于其简单的工作原理，此类微型机器人的实验实现具有可行性，或可为靶向给药等微创医疗干预开辟新途径。