Stefano Palagi

Design and development of a soft magnetically-propelled swimming microrobot

A novel approach for the design of magnetically-propelled microrobots is proposed as an effective solution for swimming in a liquid medium. While intrinsic neutral buoyancy of a microrobot per se simplifies propulsion in the liquid environments, softness makes it compliant with delicate environments, such as the human body, thus guaranteeing a safe interaction with soft structures. With this aim, two groups of soft microrobots with paramagnetic and ferromagnetic behaviors were designed, fabricated and their features were experimentally analyzed. In agreement with the theoretical predictions, in the performed trials the ferromagnetic microrobots showed orientation capabilities in response to the magnetic field that could not be achieved by the paramagnetic one. Moreover, it was observed that the ferromagnetic microrobot could reach higher speed values (maximum value of 0.73 body length/s) than the paramagnetic prototype.

Propulsion of swimming microrobots inspired by metachronal waves in ciliates: from biology to material specifications

The quest for swimming microrobots originates from possible applications in medicine, especially involving navigation in bodily fluids. Swimming microorganisms have become a source of inspiration because their propulsion mechanisms are effective in the low-Reynolds number regime. In this study, we address a propulsion mechanism inspired by metachronal waves, i.e. the spontaneous coordination of cilia leading to the fast swimming of ciliates. We analyse the biological mechanism (referring to its particular embodiment in Paramecium caudatum), and we investigate the contribution of its main features to the swimming performance, through a three-dimensional finite-elements model, in order to develop a simplified, yet effective artificial design. We propose a bioinspired propulsion mechanism for a swimming microrobot based on a continuous cylindrical electroactive surface exhibiting perpendicular wave deformations travelling longitudinally along its main axis. The simplified propulsion mechanism is conceived specifically for microrobots that embed a micro-actuation system capable of executing the bioinspired propulsion (self-propelled microrobots). Among the available electroactive polymers, we select polypyrrole as the possible actuation material and we assess it for this particular embodiment. The results are used to appoint target performance specifications for the development of improved or new electroactive materials to attain metachronal-waves-like propulsion.

How does buoyancy of hydrogel microrobots affect their magnetic propulsion in liquids

Gravity compensation is a key requirement for achieving three-dimensional navigation of magnetic microrobots in fluids. Here we present a brief theoretical introduction to the issue of gravity compensation in the case of magnetic “pulling” propulsion, explicitly highlighting the constraints it introduces. Therefore, we evaluate the advantages that quasi-neutral buoyancy gives to hydrogel microrobots, demonstrating that despite their relatively weak magnetic properties, for certain size/velocity ranges they could be more easily and efficiently propelled than state-of-the-art metal microrobots. Hence, our analysis endorses the adoption of smart polymers, such as stimuli-responsive hydrogels, for developing truly multifunctional magnetic microrobots.

Wireless swimming microrobots: Design and development of a 2 DoF magnetic-based system

In this work, the design and development of an integrated platform for the steering of swimming microrobot is reported. The system consists of: a near-spherical soft and buoyant magnetic microrobot (with a diameter of about 500 μm) conceived for operation in liquid; a wireless magnetic steering system, including a compact magnetic field generator based on two pairs of Helmholtz and Maxwell coils; an electronic system for their driving; a control software; a joypad physical user interface; and, the micro-arena as working environment. The platform design fulfills the requirements for the “Mobility Task” of the 2011 NIST Mobile Microrobotics Challenge. The results obtained from preliminary validation experiments confirm that the microrobots can move in a fully controlled way, successfully accomplishing an intricate eight-shape path, as required, in the water filled micro-arena. In particular we achieved a maximum average speed of 0.71 mm/s and an exceptionally smooth motion.

Navigation of Magnetic Microrobots With Different User Interaction Levels

Micro-technologies based on wirelessly powered and manoeuvred submillimeter device, i.e.,microrobots, are attracting growing attention. Their application in lab-on-a-chip systems, such as micromanipulation and in vitro cell sorting, is expected to steeply increase. However, the actuation, powering and control of microrobots are challenges that still need concrete solutions. Magnetic fields generally enable wireless navigation of microrobots, but proper control architectures and magnetic navigation systems are needed, depending on the specific task and on the level of interaction required to the user. Here we present a magnetic navigation platform intended for lab-on-a-chip applications and we address its usability with different levels of human involvement by using two control architectures: teleoperated and autonomous. We perform an experimental analysis to demonstrate that both architectures, enrolling different levels of interaction by the user, lead to reliable execution of the microrobotic task. First, we validate the open-loop response of the microrobotic system, and second, we evaluate the performance of the system by testing both control architectures with a standard mobility task. The results show that users can teleoperate the microrobot with 100% success rate, in 14.4±1.9s with a normalized spatial mean error of 0.60±0.13. Moreover, results show a fast decaying learning curve for the users involved in the study. Compared to this, when the navigation task is performed by the autonomous control, 100% success rate, a time of 8.0±0.5s and a normalized spatial mean error of 0.50±0.05 are obtained. Finally, we quantitatively demonstrate how both control methodologies enable very smooth movements of the microrobot, suggesting application for any task where repeatable and dexterous movements in liquid microenvironments are key requirements.

A Power-efficient Propulsion Method for Magnetic Microrobots

Current magnetic systems for microrobotic navigation consist of assemblies of electromagnets, which allow for the wireless accurate steering and propulsion of sub-millimetric bodies. However, large numbers of windings and/or high currents are needed in order to generate suitable magnetic fields and gradients. This means that magnetic navigation systems are typically cumbersome and require a lot of power, thus limiting their application fields. In this paper, we propose a novel propulsion method that is able to dramatically reduce the power demand of such systems. This propulsion method was conceived for navigation systems that achieve propulsion by pulling microrobots with magnetic gradients. We compare this power-efficient propulsion method with the traditional pulling propulsion, in the case of a microrobot swimming in a micro-structured confined liquid environment. Results show that both methods are equivalent in terms of accuracy and the velocity of the motion of the microrobots, while the new approach requires only one ninth of the power needed to generate the magnetic gradients. Substantial equivalence is demonstrated also in terms of the manoeuvrability of user-controlled microrobots along a complex path.

Acoustic Fabrication via the Assembly and Fusion of Particles

Acoustic assembly promises a route toward rapid parallel fabrication of whole objects directly from solution. This study reports the contact-free and maskless assembly, and fixing of silicone particles into arbitrary 2D shapes using ultrasound fields. Ultrasound passes through an acoustic hologram to form a target image. The particles assemble from a suspension along lines of high pressure in the image due to acoustic radiation forces and are then fixed (crosslinked) in a UV-triggered reaction. For this, the particles are loaded with a photoinitiator by solvent-induced swelling. This localizes the reaction and allows the bulk suspension to be reused. The final fabricated parts are mechanically stable and self-supporting.

Novel Smart Concepts for Designing Swimming Soft Microrobots

Abstract The development of mobile un-tethered microscale robots could revolutionize the future of medicine, since they can be conceived to move in micro-structured liquid environments, such as in inaccessible districts of the human body for performing in vivo diagnosis and therapy. However, power supply and actuation are still open issues in microrobotics, because of the lack of power sources and actuators at these scales. Considering the amazing levels of functionality exhibited by microorganisms, bioinspiration is an attractive approach to address the development of innovative solutions. The demonstration of efficient methods for building, powering and steering microscale robots are thus the first crucial steps towards such advanced systems.

Soft continuous microrobots with multiple intrinsic degrees of freedom

One of the main challenges in the development of microrobots, i.e. robots at the sub-millimeter scale, is the difficulty of adopting traditional solutions for power, control and, especially, actuation. As a result, most current microrobots are directly manipulated by external fields, and possess only a few passive degrees of freedom (DOFs). We have reported a strategy that enables embodiment, remote powering and control of a large number of DOFs in mobile soft microrobots. These consist of photo-responsive materials, such that the actuation of their soft continuous body can be selectively and dynamically controlled by structured light fields. Here we use finite-element modelling to evaluate the effective number of DOFs that are addressable in our microrobots. We also demonstrate that by this flexible approach different actuation patterns can be obtained, and thus different locomotion performances can be achieved within the very same microrobot. The reported results confirm the versatility of the proposed approach, which allows for easy application-specific optimization and online reconfiguration of the microrobots behavior. Such versatility will enable advanced applications of robotics and automation at the micro scale.

Controlled Magnetic Propulsion of Floating Polymeric Two-Dimensional Nano-Objects

Soft-bodied magnetically actuated microrobots could be developed by including controlled dispersions of magnetic nanoparticles into polymeric micro-fabricated structures. The characterization and actuation of magnetically active soft-bodied microrobots by tailored magnetic fields is, thus, a key issue for the design, full control and further development of these mobile micro-systems. In this work, the authors demonstrate the predictable and controllable transportation of polymeric flexible nanofilms embedding super-paramagnetic nanoparticles, by developing and quantitatively validating a model of the magnetic force acting on such structures, thus paving the way towards the wireless magnetic actuation of soft-bodied microrobots. The magnetic forces generated in our experimental conditions range from about 10 –9 to 10–6 N, with typical velocities for the nanofilms ranging between about 0.1 and 2.3 mm/s. For the entire range, a good agreement between theoretical model predictions and measured data is obtained (average normalized error δ =3. 65%). The proposed approach for microrobotic development targets challenging environments, where keywords are liquid or wet micro-structured environments, like in biomedical applications.