Directed transport of microcargoes is essential for living organisms as well as for applications in microrobotics, nanotechnology and biomedicine. Existing delivery technologies often suffer from low speeds, limited navigation control and dispersal by cardiovascular flows. In cell biology, these issues are largely overcome by cytoskeletal motors that carry vesicles along microtubule highways. Thus inspired, here we developed an artificial microtubule (AMT), a structured microfibre with embedded micromagnets that serve as stepping stones to guide particles rapidly through flow networks. Compared with established techniques, the microcargo travels an order of magnitude faster using the same driving frequency, and dispersal is mitigated by a strong dynamic anchoring effect. Even against strong fluid flows, the large local magnetic-field gradients enable both anchoring and guided propulsion. Finally, we show that AMTs can facilitate the self-assembly of microparticles into active-matter clusters, which then enhance their walking speed by bridging over stepping stones collectively. Hence, we demonstrate a unique strategy for robust delivery inside microvascular networks and for minimally invasive interventions, with non-equilibrium effects that could be equally relevant for enhancing biological transport processes.
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Self-folding soft-robotic chains with reconfigurable shapes and functionalities
Nature Communications Open Access 07 March 2023
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We thank X.-P. Wang and S. Pane for helping with the development of the fabrication process. We thank X. Chen for his help on the VSM test and E. Zuurmond for proofreading this article. The sample fabrication was performed using the cleanroom facilities at the FIRST at ETH Zurich. We thank the ETH Lab Supporting Group for the deposition process. Funding: This work was financially supported by the European Research Council Advanced Grant–Soft MicroRobots (SOMBOT, number 743217), Swiss National Science Foundation grant 200020B_185039 and an ETH grant (1916-1).
The authors declare no competing interests.
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Extended Data Fig. 1 Fabrication process of artificial microtubules.
a, Schematic illustration of the microfabrication processes of the AMTs. The details of the microfabrication can be found in Methods: Fabrication of AMTs. b, SEM images of the microtubule samples with different electroplating time. Under an optimal electroplating time, the nickel plates have the same height as the SU-8 fibre.
Extended Data Fig. 2 Magnetic properties of artificial microtubules.
We tested the AMT sample using the VSM. The testing sample was a segment of the AMT, with 92 embedded nickel plates with a length of 8.28 mm. The sample behaved as typical soft magnetic materials with a low coercivity of 90 Oe.
Extended Data Fig. 3 Diagram of magnetic microrobot transport on an artificial microtubule.
a, A spherical magnetic particle near the AMT. The red shaded areas represent the magnetic-field gradient. b, Modelling of magnetic interactions between a magnetic microrobot and embedded nickel plates.
Extended Data Fig. 4 Position and orientation tracking results of a microrod walking on AMTs.
a-c, Subfigures represent the relative x, y position and the orientation angle, respectively. The tracking time frame was 6 seconds, until the microrod reached the same reference x position. The solid lines are the moving average of the nearest 5 measurements.
Extended Data Fig. 5 Correlation of tracking results of x, y and angle within each period from the data used in Extended Data Fig. 4.
At low frequencies there are strong correlations between x,y, and the orientation angle, due to the periodic semicircle trajectories.
Extended Data Fig. 6 Externally driven fluidic flow simulation of the experimental set-up.
a, Channel geometry in the FEM simulation. The microtubule was fixed at the bottom of the substrate. The inlet and outlet were far from the AMT allowing the flow to be fully developed as a typical Couette flow. b, Simulation of fluid pressure along the channel length. c, Simulation of fluid flow along the x direction in the observation window near the microtubule. The average flow speed was measured at the position where the microrod was walking. The estimated flow speed was 200 μm/s at 20 μL/s, which agrees well with the experimental observation shown in Fig. 4c.
Extended Data Fig. 7 ‘Stick and slip’ behaviour against strong flow under 0.2 Hz rotating magnetic field.
At a low frequency of 0.2 Hz, the locomotion of the microrod is highly dependent on the magnetic-field direction within a period, showing ‘stick and slip’ behaviour, explaining the reason for the wide dispersion of the tracking speed. The video can be found in Supplementary Video 4.
Extended Data Fig. 8 Comparison of flocking microparticle transport on the microtubules at different frequencies.
The tilted stripes show the translation of the flocking particles over time, and the slope of the bright stripes represent the local transport speed. At 1 Hz, the transport was faster than at 0.5 Hz. At 2 Hz, we observed that slopes were different depending on the local density. The two bright stripes have a higher speed than the rest of the stripes.
Modelling of the dynamic locomotion of a rotating magnetic particle on the artificial microtubule.
Supplementary Video 1
A magnetic microrod walks on the artificial microtubule.
Supplementary Video 2
Walking microrod on the artificial microtubule with different frequencies.
Supplementary Video 3
Full dynamics of the magnetic microrod walking on the artificial microtubule.
Supplementary Video 4
Magnetic microrod walks on the artificial microtubule with an externally driven fluid flow.
Supplementary Video 5
Self-assembly and collective motion of magnetic microparticles.
Supplementary Video 6
Collective locomotion of magnetic microparticles under different frequencies.
Supplementary Video 7
Demonstration of microcargo delivery inside a microfluidic network.
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Gu, H., Hanedan, E., Boehler, Q. et al. Artificial microtubules for rapid and collective transport of magnetic microcargoes. Nat Mach Intell 4, 678–684 (2022). https://doi.org/10.1038/s42256-022-00510-7
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