Magnetic control of self-propelled microjets under ultrasound image guidance

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.