ac/dc Magnetic Fields for Enhanced Translation of Colloidal Microwheels

Magnetically powered microwheel thrombolysis of occlusive thrombi in zebrafish

Tissue plasminogen activator (tPA) is the only FDA-approved treatment for ischemic stroke but carries significant risks, including major hemorrhage. Additional options are needed, especially in small vessel thrombi which account for ~25% of ischemic strokes. We have previously shown that tPA-functionalized colloidal microparticles can be assembled into microwheels (µwheels) and manipulated under the control of applied magnetic fields to enable rapid thrombolysis of fibrin gels in microfluidic models of thrombosis. Transparent zebrafish larvae have a highly conserved coagulation cascade that enables studies of hemostasis and thrombosis in the context of intact vasculature, clotting factors, and blood cells. Here, we show that tPA-functionalized µwheels can perform rapid and targeted recanalization in vivo. This effect requires both tPA and µwheels, as minimal to no recanalization is achieved with tPA alone, µwheels alone, or tPA-functionalized microparticles in the absence of a magnetic field. We evaluated tPA-functionalized µwheels in CRISPR-generated plasminogen (plg) heterozygous and homozygous mutants and confirmed that tPA-functionalized µwheels are dose-dependent on plasminogen for lysis. We have found that magnetically powered µwheels as a targeted tPA delivery system are dramatically more efficient at plasmin-mediated thrombolysis than systemic delivery in vivo. Further development of this system in fish and mammalian models could enable a less invasive strategy for alleviating ischemia that is safer than directed thrombectomy or systemic infusion of tPA.

Reversible Microwheel Translation Induced by Polymer Depletion

For in vivo applications, microbots (μbots) must move, which is a need that has led to designs, such as helical swimmers, that translate through the bulk fluid. We have previously demonstrated that, upon application of a rotating magnetic field, colloidal particles in aqueous systems can be reversibly assembled from superparamagnetic particles into μbots that translate along surfaces using wet friction. Here, we show that high-molecular-weight polymers of a size that approaches the length scale of the gap between the μbot and surface can be excluded, impacting μbot transport. Using xanthan gum as a convenient high-molecular-weight model, we determine that polymer depletion imparts only a weak effect on colloid-surface interactions but has a significant influence on local viscosity, which is an effect great enough to induce a reversal in the μbot translation direction.

Size-Dependent Locomotion Ability of Surface Microrollers on Physiologically Relevant Microtopographical Surfaces

Controlled microrobotic navigation inside the body possesses significant potential for various biomedical engineering applications. Successful application requires considering imaging, control, and biocompatibility. Interaction with biological environments is also a crucial factor in ensuring safe application, but can also pose counterintuitive hydrodynamic barriers, limiting the use of microrobots. Surface rolling microrobots or surface microrollers is a robust microrobotic platform with significant potential for various applications; however, conventional spherical microrollers have limited locomotion ability over biological surfaces due to microtopography effects resulting from cell microtopography in the size range of 2-5 µm. Here, the impact of the microtopography effect on spherical microrollers of different sizes (5, 10, 25, and 50 µm) is investigated using computational fluid dynamics simulations and experiments. Simulations revealed that the microtopography effect becomes insignificant for increasing microroller sizes, such as 50 µm. Moreover, it is demonstrated that 50 µm microrollers exhibited smooth locomotion ability on in vitro cell layers and inside blood vessels of a chicken embryo model. These findings offer rational design principles for surface microrollers for their potential practical biomedical applications.

Magnetically Powered Microwheel Thrombolysis of Occlusive Thrombi in Zebrafish

Tissue plasminogen activator (tPA) is the only FDA approved treatment for ischemic stroke but carries significant risks, including major hemorrhage. Additional options are needed, especially in small vessel thrombi which account for ~25% of ischemic strokes. We have previously shown that tPA-functionalized colloidal microparticles can be assembled into microwheels (µwheels) and manipulated under the control of applied magnetic fields to enable rapid thrombolysis of fibrin gels in microfluidic models of thrombosis. Providing a living microfluidic analog, transparent zebrafish larvae have a highly conserved coagulation cascade that enables studies of hemostasis and thrombosis in the context of intact vasculature, clotting factors, and blood cells. Here we show that tPA-functionalized µwheels can perform rapid and targeted recanalization in vivo. This effect requires both tPA and µwheels, as minimal to no recanalization is achieved with tPA alone, µwheels alone, or tPA-functionalized microparticles in the absence of a magnetic field. We evaluated tPA-µwheels in CRISPR-generated plasminogen (plg) heterozygous and homozygous mutants and confirmed that tPA-µwheels are dose-dependent on plasminogen for lysis. We have found that magnetically powered µwheels as a targeted tPA delivery system are dramatically more efficient at plasmin-mediated thrombolysis than systemic delivery in vivo. Further development of this system in fish and mammalian models could enable a less invasive strategy for alleviating ischemia that is safer than directed thrombectomy or systemic infusion of tPA.

The mismatch between experimental and computational fluid dynamics analyses for magnetic surface microrollers

Magnetically actuated Janus surface microrollers are promising microrobotic platform with numerous potential biomedical engineering applications. While the locomotion models based on a "rotating sphere on a nearby wall" can be adapted to surface microrollers, real-world dynamics may differ from the proposed theories/simulations. In this study, we examine the locomotion efficiency of surface microrollers with diameters of 5, 10, 25, and 50 µm and demonstrate that computational fluid dynamics simulations cannot accurately capture locomotion characteristics for different sizes of microrollers. Specifically, we observe a significant mismatch between lift forces predicted by simulations and opposite balancing forces, particularly for smaller microrollers. We propose the existence of an unaccounted force component in the direction of lift, which is not included in the computational fluid dynamics simulations. Overall, our findings provide a deeper understanding of the physical mechanisms underlying surface microroller locomotion and have important implications for future applications in biomedical engineering.

Magnetically Powered Chitosan Milliwheels for Rapid Translation, Barrier Function Rescue, and Delivery of Therapeutic Proteins to the Inflamed Gut Epithelium

Inflammatory bowel disease (IBD) is mediated by an overexpression of tumor necrosis factor-α (TNF) by mononuclear cells in the intestinal mucosa. Intravenous delivery of neutralizing anti-TNF antibodies can cause systemic immunosuppression, and up to one-third of people are non-responsive to treatment. Oral delivery of anti-TNF could reduce adverse effects; however, it is hampered by antibody degradation in the harsh gut environment during transit and poor bioavailability. To overcome these shortcomings, we demonstrate magnetically powered hydrogel particles that roll along mucosal surfaces, provide protection from degradation, and sustain the local release of anti-TNF. Iron oxide particles are embedded into a cross-linked chitosan hydrogel and sieved to produce 100-200 μm particles called milliwheels (m-wheels). Once loaded with anti-TNF, these m-wheels release 10 to 80% of their payload over 1 week at a rate that depends on the cross-linking density and pH. A rotating magnetic field induces a torque on the m-wheels that results in rolling velocities greater than 500 μm/s on glass and mucus-secreting cells. The permeability of the TNF-challenged gut epithelial cell monolayers was rescued in the presence of anti-TNF carrying m-wheels, which both neutralized the TNF and created an impermeable patch over leaky cell junctions. With the ability to translate over mucosal surfaces at high speed, provide sustained release directly to the inflamed epithelium, and provide barrier rescue, m-wheels demonstrate a potential strategy to deliver therapeutic proteins for the treatment of IBD.

Accelerating the Design of Self-Guided Microrobots in Time-Varying Magnetic Fields

Mobile robots combine sensory information with mechanical actuation to move autonomously through structured environments and perform specific tasks. The miniaturization of such robots to the size of living cells is actively pursued for applications in biomedicine, materials science, and environmental sustainability. Existing microrobots based on field-driven particles rely on knowledge of the particle position and the target destination to control particle motion through fluid environments. Often, however, these external control strategies are challenged by limited information and global actuation where a common field directs multiple robots with unknown positions. In this Perspective, we discuss how time-varying magnetic fields can be used to encode the self-guided behaviors of magnetic particles conditioned on local environmental cues. Programming these behaviors is framed as a design problem: we seek to identify the design variables (e.g., particle shape, magnetization, elasticity, stimuli-response) that achieve the desired performance in a given environment. We discuss strategies for accelerating the design process using automated experiments, computational models, statistical inference, and machine learning approaches. Based on the current understanding of field-driven particle dynamics and existing capabilities for particle fabrication and actuation, we argue that self-guided microrobots with potentially transformative capabilities are close at hand.

Reduced rotational flows enable the translation of surface-rolling microrobots in confined spaces

Biological microorganisms overcome the Brownian motion at low Reynolds numbers by utilizing symmetry-breaking mechanisms. Inspired by them, various microrobot locomotion methods have been developed at the microscale by breaking the hydrodynamic symmetry. Although the boundary effects have been extensively studied for microswimmers and employed for surface-rolling microrobots, the behavior of microrobots in the proximity of multiple wall-based "confinement" is yet to be elucidated. Here, we study the confinement effect on the motion of surface-rolling microrobots. Our experiments demonstrate that the locomotion efficiency of spherical microrollers drastically decreases in confined spaces due to out-of-plane rotational flows generated during locomotion. Hence, a slender microroller design, generating smaller rotational flows, is shown to outperform spherical microrollers in confined spaces. Our results elucidate the underlying physics of surface rolling-based locomotion in confined spaces and present a design strategy with optimal flow generation for efficient propulsion in such areas, including blood vessels and microchannels.

Position control of a wheel-based miniature magnetic robot using neuro-fuzzy network

Untethered small-scale robots can accomplish tasks which are not feasible by conventional macro robots. In the current research, we have designed and fabricated a miniature magnetic robot actuated by an external magnetic field. The proposed robot has two coaxial wheels and one magnetic dipole which is capable of rolling and moving on the surface by variation in the direction of magnetic field. To generate the desired magnetic field, a Helmholtz electromagnetic coil is manufactured. To steer the robot to the desired position, at first the robot dynamics is investigated, and subsequently a controller based on a neuro-fuzzy network has been designed. Finally, the proposed controller is implemented experimentally and the performance of the control system is demonstrated.

Multimodal microwheel swarms for targeting in three-dimensional networks

Microscale bots intended for targeted drug delivery must move through three-dimensional (3D) environments that include bifurcations, inclined surfaces, and curvature. In previous studies, we have shown that magnetically actuated colloidal microwheels (µwheels) reversibly assembled from superparamagnetic beads can translate rapidly and be readily directed. Here we show that, at high concentrations, µwheels assemble into swarms that, depending on applied magnetic field actuation patterns, can be designed to transport cargo, climb steep inclines, spread over large areas, or provide mechanical action. We test the ability of these multimodal swarms to navigate through complex, inclined microenvironments by characterizing the translation and dispersion of individual µwheels and swarms of µwheels on steeply inclined and flat surfaces. Swarms are then studied within branching 3D vascular models with multiple turns where good targeting efficiencies are achieved over centimeter length scales. With this approach, we present a readily reconfigurable swarm platform capable of navigating through 3D microenvironments.

An experimental design for the control and assembly of magnetic microwheels

Superparamagnetic colloidal particles can be reversibly assembled into wheel-like structures called microwheels (μwheels), which roll on surfaces due to friction and can be driven at user-controlled speeds and directions using rotating magnetic fields. Here, we describe the hardware and software to create and control the magnetic fields that assemble and direct μwheel motion and the optics to visualize them. Motivated by portability, adaptability, and low-cost, an extruded aluminum heat-dissipating frame incorporating open optics and audio speaker coils outfitted with high magnetic permeability cores was constructed. Open-source software was developed to define the magnitude, frequency, and orientation of the magnetic field, allowing for real-time joystick control of μwheels through two-dimensional (2D) and three-dimensional (3D) fluidic environments. With this combination of hardware and software, μwheels translate at speeds up to 50 µm/s through sample sizes up to 5 × 5 × 5 cm3 using 0.75 mT-2.5 mT magnetic fields with rotation frequencies of 5 Hz-40 Hz. Heat dissipation by aluminum coil clamps maintained sample temperatures within 3 °C of ambient temperature, a range conducive for biological applications. With this design, μwheels can be manipulated and imaged in 2D and 3D networks at length scales of micrometers to centimeters.

Multifunctional surface microrollers for targeted cargo delivery in physiological blood flow

Mobile microrobots offer great promise for minimally invasive targeted medical theranostic applications at hard-to-access regions inside the human body. The circulatory system represents the ideal route for navigation; however, blood flow impairs propulsion of microrobots especially for the ones with overall sizes less than 10 micrometers. Moreover, cell- and tissue-specific targeting is required for efficient recognition of disease sites and long-term preservation of microrobots under dynamic flow conditions. Here, we report cell-sized multifunctional surface microrollers with ~3.0 and ~7.8-micrometer diameters, inspired by leukocytes in the circulatory system, for targeted drug delivery into specific cells and controlled navigation inside blood flow. The leukocyte-inspired spherical microrollers are composed of magnetically responsive Janus microparticles functionalized with targeting antibodies against cancer cells (anti-HER2) and light-cleavable cancer drug molecules (doxorubicin). Magnetic propulsion and steering of the microrollers resulted in translational motion speeds up to 600 micrometers per second, around 76 body lengths per second. Targeting cancer cells among a heterogeneous cell population was demonstrated by active propulsion and steering of the microrollers over the cell monolayers. The multifunctional microrollers were propelled against physiologically relevant blood flow (up to 2.5 dynes per square centimeter) on planar and endothelialized microchannels. Furthermore, the microrollers generated sufficient upstream propulsion to locomote on inclined three-dimensional surfaces in physiologically relevant blood flow. The multifunctional microroller platform described here presents a bioinspired approach toward in vivo controlled propulsion, navigation, and targeted active cargo delivery in the circulatory system.