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This paper demonstrates closed-loop control of self-propelled microjets using feedback extracted from B-mode ultrasound images. Previous work on control of self-propelled microrobots has mainly employed video cameras equipped with microscopic lenses in order to obtain the required feedback. Nonetheless, in medical applications such as targeted drug delivery, the use of video cameras might be unsuitable for localizing microrobots that navigate within the human body. This issue is a major obstacle for transferring medical microrobotic technologies into the clinic. On that account, the first reported methods and results on control of self-propelled microjets using ultrasound equipment are provided herein. In order to exploit the microjets' self-propulsion mechanism, their motion is directed towards a predefined target by exerting magnetic torques to steer them. Binary image analysis techniques are used to estimate the microjet's position from the ultrasound images. Two air-cored coils are used to generate the steering torques within a plane. Coil currents are calculated using the estimated position error. Results show that our system employing ultrasound images allows control of microjets at an average velocity of 156±35.1 μm/s and with an average tracking error of 250.7±164.7 μm. As a reference, when microscopic image feedback is used in the setup, an average velocity and tracking error of 207±25.9 μm/s and 183.2±84.31 μm, respectively, are observed.

We experimentally demonstrate that using oscillating weak magnetic fields a sperm-shaped microrobot (which we refer to as MagnetoSperm) can swim using flagellar propulsion and slide on a surface under water. The sperm morphology allows the MagnetoSperm to mimic the locomotion mechanism of a living sperm cell. The MagnetoSperm is designed and developed with a magnetic head and a flexible tail to provide a magnetic dipole moment and propulsion, respectively. The head oscillates under the influence of controlled oscillating weak magnetic fields (∼5 mT). This oscillation generates a thrust force in the flexible tail, and hence allows the MagnetoSperm to overcome the drag and friction forces during swimming and sliding on a surface, respectively. Point-to-point open-and closed-loop control of the MagnetoSperm are accomplished using an electromagnetic system under microscopic guidance. This motion control is done in two cases, i.e., swimming in water and sliding on a surface. At oscillating magnetic field of 5 Hz and 45 Hz, the MagnetoSperm swims at an average swimming speed of 32 µm/s (0.1 body lengths per second) and 158 µm/s (0.5 body lengths per second), respectively. At the same frequencies, the MagnetoSperm slides on the bottom of a petri-dish at an average speed of 21 µm/s (0.07 body lengths per second) and 6 µm/s (0.02 body lengths per second), respectively.978-1-4799-6934-0/14/$31.00 ©2014 IEEE

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