July 15, 2019 feature
Shape-encoded dynamic assembly of mobile micromachines
Field-directed and self-propelled colloidal assembly can be used to build micromachines to perform complex motions and functions, although their integration as heterogenous components with specified structures, dynamics and functions within micromachines is challenging. In a recent study on Nature Materials, Yunus Alapan and co-workers at the departments of physical intelligence and complex materials in Germany and Switzerland described the dynamic self-assembly of mobile micromachines with desired configurations using preprogrammed physical interactions between structural and motor units.
They drove the assembly using dielectrophoretic interactions (DEP) encoded in a three-dimensional shape (3-D) of individual parts. They followed the protocol by assembling the new micromachines with magnetic and self-propelled motor parts for reconfigurable locomotion and additional degrees of freedom hitherto unrealized with conventional monolithic microrobots. The site-selective assembly strategy was versatile and could be demonstrated on different, reconfigurable, hierarchical and three-dimensional (3-D) mobile micromachines. The scientists anticipate the design principles presented in the work to advance and inspire the development of more sophisticated micromachines integrated in multiscale hierarchical systems.
Mobile micromachines offer significant potential to probe and manipulate the microscopic world and create functional order/assemblies at the micro- and meso-scale. A micromachine can be composed of multiple parts, materials or chemistries to address multiple functions, including actuation, sensing, transport and delivery. Functional modes and performance of a micromachine can be dictated by the collective organization and interaction of its constituents.
For instance, magnetic particles interacting under rotating magnetic fields can assemble into chains or wheels capable of moving close to solid surfaces. Similarly, scientists have developed light-activated microswimmers within living crystals and allowed self-rotation by regulating the chemical consumption. To design higher complexity, bioengineers and materials scientists must allow programmable physical interactions into individual parts for shape- and material-specific actions under external influences. Examples include the development of composite microstructures assembled as colloids using virtual electric and magnetic molds.
While new approaches have shown promise to build programmable structural assemblies, these remain to be translated into mobile micromachine assemblies. In the present work, Alapan et al. introduced a directed assembly process to build mobile compound micromachines using dielectrophoretic (DEP) forces to encode precisely controlled distribution of electric field gradients around a body by modulating its 3-D geometry.
The results showed site-selective and directional microactuators with a versatile shape-encoded assembly strategy. They showed the possibility of improved strengthening between the actuators and the body by tuning the DEP forces to provide control on rotation. Alapan et al. implemented a new design strategy of directed assembly to control operational dynamics between functional components using shape-encoded DEP forces. The experimental work will provide a rich design space to develop functional micromachines and mobile microbots to perform complex tasks.
Alapan et al. first programmed the field gradients around a construct to drive the assembly of micromachine parts at desired locations using DEP interactions. The working principle of the device under electric fields relied on the shape-dependent regulation of electric fields around polarizable bodies of the assembled micromachine. To program the local gradients, they investigated how non-electric fields could be modulated around different geometries.
The scientists then demonstrated controlled self-assembly of mobile micromachines influenced by electric fields, where they first focused on the assembly of a simple microvehicle. The experimental microvehicle contained a large non-magnetic dielectric spherical body and multiple smaller magnetic microactuators organized around the larger body. When they applied an electric field in the Z axis, the non-magnetic body generated local electric field gradients to attract smaller microactuators around its poles. The newly assembled magnetic actuators served as propelling wheels and Alapan et al. could steer the microvehicle by changing the magnetic field direction by applying a vertically rotating magnetic field.
While they increased the velocity of the microvehicle by increasing the number of microactuators, when the voltage in the system increased—the velocity of the microvehicle decreased instead. The scientists presumed this to be due to increased mechanical coupling between the microparticles and the substrate during DEP interactions. The researchers used the method to capture randomly distributed, non-magnetic particles with magnetic microactuators by applying an electric field, then translated them to a new position using a rotating magnetic field for release upon turning the electric field off.
To control the rotational degrees of freedom of the microvehicle, Alapan et al. could regulate the strength of the attractive DEP forces between the passive body and microactuators to tune their mechanical coupling. For instance, at low voltages, the small attractive DEP forces led to a loose lubrication-based coupling phenomenon allowing microactuators to move freely around the pole. The rotatory joints developed in the study can become crucial for specific biological systems during the development of synthetic molecular, nano- and micromachines for application in the mechanical transmission of energy.
The researchers then realized programmable self-assembly of mobile micromachines with shape-encoded physical interactions by designing micromachine frames with specific 3-D geometries to generate electric field gradients. The 3-D framework selectively attracted microactuators to desired locations on the micromachine frame itself that the scientists fabricated using two-photon lithography. For the first design, the scientists created a microcar with four-wheel pockets to generate DEP forces and guide the assembly of magnetic microactuators into the pockets. They performed on-demand self-assembly of the microcar within seconds of applying an electric field for free rotation of magnetic wheels inside the pockets as a result of a vertically rotating magnetic field. When they turned on the electric field to a high value, the magnetic microactuators self-assembled into the docking sites for rigid coupling between the micro-rotor frame and magnetic microactuators. When they applied a horizontally rotating magnetic field, the micromotor assembly rotated as a rigid body.
The scientists expanded the prototype to build reconfigurable micromachines powered by self-propelled micromotors. For this, they designed micromachines assembled with self-propelled Janus silica (SiO2) microparticles with a gold cap (Au). The frequency-dependent self-propulsion and the DEP response of the Janus microparticles allowed them to design mobile micromachines with reconfigurable spatial organization and kinematics. This experimental setup also demonstrated a form of self-repair.
Alapan et al. expanded the observed shape-encoded DEP interactions to define physical interactions between the mobile micromachines by paving the way for hierarchical multi-machine assemblies. As a proof of principle, they designed a two-level hierarchical assembly between constituent micromachines.
- At the first level; self-propelled actuators assembled with two microstructure units to form mobile micromachines that translated linearly
- In the second level; the second unit assembled laterally with unit 1 due to the generation of low electric fields
The scientists extended the design introduced in the present work into 3-D micro-actuator manipulation and the assembly of micromachines with significant potential for application on lab-on-a-chip devices to facilitate continuous transport, sorting, digital manipulation of micro-objects and microfluidic flow generation. In this way, Yunus Alapan and co-workers designed and implemented programable self-assembly using shape-directed dynamic assembly of micromachines from modular structural and motor subunits to provide unprecedented control on dynamics and functions. The method can provide a solution to engineer multifunctional/material microrobots since the scientists succeeded to incorporate the heterogenous components for sensing, cargo loading and actuation together in a single step.
For applications without electric fields such as in vivo biomedical applications, the scientists will aim to optimize and build on the irreversible assembly of micro-components for optimized performance. The work holds significant potential to develop multifunctional, reconfigurable micromachines and life-inspired complex hierarchical systems in materials science for applications in microrobotics, colloidal science, medicine and autonomous microsystems.
Stefano Palagi et al. Bioinspired microrobots, Nature Reviews Materials (2018). DOI: 10.1038/s41578-018-0016-9
Antoine Aubret et al. Targeted assembly and synchronization of self-spinning microgears, Nature Physics (2018). DOI: 10.1038/s41567-018-0227-4
Bartosz A. Grzybowski et al. The nanotechnology of life-inspired systems, Nature Nanotechnology (2016). DOI: 10.1038/nnano.2016.116
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