In an article recently published in the journal Communications Engineering, researchers proposed a high-throughput soft microrobot fabrication technique to fabricate soft microrobots with programmable magnetic and structural anisotropy by combining droplet-based microfluidics and simulation-guided design.
Study: High-Throughput Soft Microrobot Fabrication with Programmable Anisotropy. Image credit: Audrius Merfeldas/Shutterstock
Background
Soft microrobots have gained significant attention for biomedical functions, including drug delivery and biosensing, owing to their ability to imitate human blood cells while navigating the cardiovascular (CV) system and their deformability. Magnetic fields are primarily utilized as wireless-transmittable stimuli to control the microrobots’ function and state as the fields possess exceptional tissue penetration and are biocompatible.
Rotating magnetic fields (RMFs) are specifically advantageous due to their ability to generate powerful torques, which enable motion over long distances within the body. Swarms of soft microrobots controlled by minimally invasive magnetic fields have demonstrated significant potential as biomedical agents.
The collective behavior of such soft microrobot swarms depends on the properties of the individual constituents and is governed by hydrodynamic and magnetic interactions. Thus, strategies for pre-programming the individual microrobots to regulate their interactions with others following external stimuli are required to control the collective behavior.
Introducing structural and magnetic anisotropy into microrobots can expand the possibilities for predetermining and tailoring interactions and collective behaviors. However, the existing methods for large-scale soft microrobot production often result in isotropic properties. For instance, emulsion-based synthesis, a large-scale magnetic microrobot production method, typically produces isotropic microrobots, which limit the scope of preprogrammed swarm behavior and interactions.
Although more complex methods, such as additive manufacturing and three-dimensional (3D)/four-dimensional (4D) photolithography, yield sophisticated anisotropic structures for complex control, they have several disadvantages, including reduced biocompatibility, high costs, and low production rates.
A new soft microrobot fabrication approach
In this study, researchers proposed a high-throughput, versatile technique for producing soft microrobots with programmable magnetic and structural anisotropy by combining droplet-based microfluidics and simulation-guided design.
Specifically, the proposed strategy can assist in mass-producing and designing soft microrobots using external magnetic field-guided photopolymerization and droplet-based microfluidics. Researchers integrated simulation-guided design and computational modeling for tailoring the microrobots’ anisotropy and controlling their response to external magnetic torques.
Magnetic nanoparticles (MNPs) and hydrogen were selected as core constituents to fabricate biocompatible and programmable magnetic microrobots with magnetic and structural anisotropy. Spheres, ellipsoids, and doublets with internal magnetic supra-domains consisting of homogenously distributed MNPs, bundles, disks, and chains were fabricated.
A priori particle-based simulation model was developed to predict and analyze magnetic interactions and magnetization of soft microrobots with various magnetic and structural anisotropies to guide fabrication. Subsequently, droplet-based microfluidics was utilized to integrate controlled photopolymerization under static uniform/rotating/no magnetic fields. The fabricated microrobots contained iron oxide nanoparticles organized into supra-domain structures and entrapped in a hydrogel matrix, which could be elongated independently of its magnetic properties.
Significance of this approach
The encoded magnetization controlled the interactions and applicable torque between entities in microrobot assemblies. Researchers assessed the collective dynamics of microrobot swarms. Distinct torques and forces induced motion when the robots were exposed to dynamic magnetic fields due to interactions with neighboring entities and their surroundings. Multiple distinct collective behaviors, including variable crystal, gas-like, and heterogeneous motions, were generated by tuning magnetic and hydrodynamic interactions.
The variable crystal and gas-like behaviors were distinguished based on the collective motion patterns displayed by microrobot swarms. Additionally, interdependent movement behaviors were also observed among individuals within the swarm.
Magnetic and hydrodynamic interactions were regulated through individual magnetic supradomains, with a reducing encapsulated MNP concentration weakening the magnetic interactions. Dipole-dipole interactions became negligible in multi-chain microrobots below the threshold MNP concentration and only possessed a weak influence on collective behavior, while hydrodynamic interactions dominated, leading to a gas-like nature under RMF.
Unlike the multi-chain microrobots, multi-bundle microrobots possessed higher MNP concentrations and robust magnetic interactions. The aligned magnetic supra-domain pattern enabled anisotropic magnetic interactions. Individual microrobots displayed self-rotation around their axis and global rotation of the entire assembly/variable crystal behavior when the aligned pattern was combined with moderate magnetic interactions. This analysis demonstrated that magnetic interaction-guided microrobot design governs collective behavior and dynamic self-assembly, yielding several collective modes.
Conclusion
The variable and complex physiological environments require the customization of microrobots based on desired tasks and specific needs, which makes the programming of collective behaviors crucial. In this study, the proposed approach based on the simulation-guided design of soft microrobots and the incorporation of polymerization and droplet-based microfluidics under dynamic and static magnetic fields could successfully enable the fabrication of monodisperse microrobots with distinct magnetic and structural anisotropy, facilitating enhanced control and locomotion of swarm dynamics.
Journal reference:
- Yan, Y., Song, C., Shen, Z., Zhu, Y., Ni, X., Wang, B., Christiansen, M. G., Stavrakis, S., Lintuvuori, J. S., Chen, B., DeMello, A., Schuerle, S. (2024). Programming structural and magnetic anisotropy for tailored interaction and control of soft microrobots. Communications Engineering, 3(1), 1-11. https://doi.org/10.1038/s44172-023-00145-5, https://www.nature.com/articles/s44172-023-00145-5