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Researchers Create Dynamic Self-Assembly Process for Building Mobile Micromachines

Shape-encoded assembly of magnetic microactuators in the form of a microvehicle.

A micromachine has the potential to maniupulate and probe the microscopic world, and can be made up of multiple chemistries, materials, or parts in order to address different functions, such as actuation, delivery, sensing, and transport. Its performance and functional modes can be commanded by the interaction and organization of its variable constituents, and it can be 3D printed, but it’s difficult to build programmable structural assemblies into mobile micromachines.

A group of researchers from the Max Planck Institute for Intelligent Systems and ETZ Zurich published a study in Nature Materials, titled “Shape-encoded dynamic assembly of mobile micromachines,” in which they introduced a new directed, dynamic self-assembly process of building mobile compound micromachines, with specific configurations.

The process use pre-programmed physical interactions between structural and motor units, and is driven by dielectrophoretic interactions (DEP) that are encoded in the 3D shape of individual parts. These DEP forces modulate the part’s 3D geometry in order to “encode precisely controlled distribution of electric field gradients around a body.”

Spatial encoding of DEP attraction sites by modulating the 3D geometry.

First, the researchers – Yunus Alapan, Berk Yigit, Onur Beker, Ahmet F. Demirörs, and Metin Sitti – programmed field gradients around a construct, so they could use DEP interactions to “drive the assembly of micromachine parts” at specific locations.

“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,” a Phys.org piece states.

The team needed a way to program local gradients, and looked into how they could modulate non-electric fields around different geometries. Then, they were able to control the mobile micromachine’s self-assembly, which was influenced by electric fields, using a microvehicle as an example. It had a large, spherical, non-magnetic dielectric, body, with several smaller magnetic actuators surrounding it. When an electric field was applied in the Z axis, the large body was able to generate enough local electric field gradients so as to attract smaller microactuators; these acted as wheels, and the researchers could steer the microvehicle by simply changing the direction of the magnetic field.

Assembly and translation of a compound microvehicle with magnetic actuators.

When they increased the number of microactuators, the microvehicle’s velocity also grew, but when the voltage was increased, the velocity went down. The researchers think this is due to increased mechanical coupling, during DEP interactions, between the microparticles and the substrate.

At lower voltages, small DEP forces “led to a loose lubrication-based coupling phenomenon” that made it possible for microactuators to move freely around the pole. This means it’s possible to regulate the strength of the DEP forces between the microactuators and passive body to adjust their mechanical coupling, in order to control the microvehicle’s rotational degrees of freedom.

Reversible assembly of magnetic microactuators with a non-magnetic body using DEP forces.

The researchers used shape-encoded physical interactions to make programmable self-assembling mobile micromachines by developing frames that had specific 3D geometries to help generate electric field gradients. The framework, made with two-photon lithography, attracts microactuators to specific locations on the frame. In one example, they made a microcar with four-wheel pockets that generated DEP forces and helped guide the magnetic microactuators into said pockets. Within just second of applying an electric field, the microcar completed an on-demand self-assembly: the magnetic wheels inside the pockets went into a free rotation due to the vertically rotating magnetic field.

The prototype was expanded in order to build reconfigurable micromachines, which are run by self-propelled micromotors. Self-propelled Janus silica (SiO2) microparticles with a gold cap were used to assemble these micromachines, and their DEP response and frequency-dependent self-propulsion made it possible to create mobile micromachines that featured self-repair and reconfigurable spatial organization. Then, the researchers defined the physical interactions between these mobile micromachines by expanding the shape-encoded DEP interactions in a two-level hierarchical assembly:

  • Level 1: self-propelled actuators assembled with two microstructure units form mobile micromachines with linear translation
  • Level 2: generation of low electric fields cause second and first units to assemble together laterally

R-L: Shape-encoded reconfigurable assembly of micromachines with self-propelled microactuators for frequency-tunable locomotion; Hierarchical assembly of multiple micromachines via shape-encoded DEP interactions.

The research team was able to extend their current design into the manipulation of 3D microactuators, and micromachine assembly, and say that it even has the potential to be used with lab-on-a-chip devices for digital manipulation, sorting, continuous transport, and microfluidic flow generation.

“The site-selective assembly strategy was versatile and could be demonstrated on different, reconfigurable, hierarchical and 3D 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.”

This design strategy makes it possible to achieve programmable self-assembly through the use of micromachines’ shape-directed dynamic assembly, which will give scientists more control over functions and dynamics. Because the team was able to incorporate heterogenous components for actuation, cargo loading, and sensing in a single step, their work may make it possible for others to engineer multifunctional, multimaterial microrobots.

The researchers will now focus on optimizing and expanding on “the irreversible assembly of micro-components” for better performance in applications that don’t use electric fields, such as in vivo biomedical applications.

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[Source/Images: Phys.org]

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Study Looks at Soft Materials in 3D Microprinting With 2 Photon Lithography

3D printing on the microscale is becoming more common as tools and materials become more sophisticated. Currently, however, most materials used in this type of 3D printing are rigid, glassy ones used in stereolithography. In a new paper entitled “Materials Overview for 2-Photon 3D Printing Applications,” a research team from Cornell University discusses the development of new materials for microscale 3D printing, particularly soft ones.

Several materials are explored and discussed in the study, including : S1) a chemically amplified photoresist, poly(tetrahydropyranyl
methacrylate-co-methyl methacrylate) with a 2-photon photoacid generator;  S2) Dow Corning Silgard 182 with (n5 -cyclopentadienyl-methyl)-trimethylplatinum as photoactive material; and S3) Norland NOA 63 photocurable resin with a non-ionic 2-photon photoacid generator.

“We have also explored the use of 2-photon absorbing molecules to enhance the activity of conventional photoactive compounds that alone do
not have sufficient 2-photon photosensitivity,” the researchers state. “These include: S4) hydrogels based on poly(ethylene glycol methacrylate) and hydroxyethyl methacrylate with 2,2-dimethoxy2-phenyl acetophenone and a 2-photon sensitizer, S5) a phenolic molecular glass crosslinked with acid catalyzed TMMGU with a photoacid generator and a 2-photon photosensitizer, and S6) SU8 epoxy and thiirane analogs with an ionic PAG with a coumarin-7 2-photon photosensitizer.”

The goal of the research is to build a strong 3D structure with specific properties and structures in the range of a few microns or less. Materials were selected with regards to mechanical properties, optical characteristics, resistance to solvents including water, and surface modification characteristics. Materials needed to be able to produce a thick “film” that could be extruded smoothly but without losing its shape once extruded. Soft materials, the researchers continue, need to be handled differently than rigid materials.

“The PDMS used in S2 is a very viscous oil which contains a photoactive hydrosilylation catalyst,” they continue. “Process challenges come from the mobility of the uncrosslinked polymer and the need to develop and remove the unreacted oligomer in the negative tone process. This aspect of processing can lead to significant swelling and place large mechanical stresses on the materials.”

Representative examples of 3D patterned materials produced using 2-photon lithography a) PDMS; b) dry poly(hydroxyethyl methacrylate); c) effect of hydration on hydrogel marked with fluorescent dye

Existing photopolymer systems can be used for printing these materials, but the researchers note that the photoactive compounds (PAC) are not sufficiently active in two-photon conditions. They recommend the addition of a sensitizer that has two-photon absorbing characteristics at near-IR wavelengths.

3D printing of hydrogels can be carried out, but the reactants must be water soluble. This creates, as the researchers describe it, a complicated situation in which typically water insoluble photoactive compounds and sensitizers must be used. In the S4 system, water soluble vinyl monomers were combined with benzophenone PAC and a two-photon sensitizer that was extremely hydrophobic. To introduce these aqueous materials, surface-active compounds such as non-ionic surfactants and cyclodextrins were used.

The researchers conclude that the prospects for high speed manufacturing of 3D microprinting and complex nanostructures are improving as new two-photon-based 3D printing systems are developed. However, the materials currently lag behind the tools.

“Liquid systems are simpler to create and work with but lack the resolution of dry systems,” the researchers state. “There are not yet standard 2-photon PACs and patterning materials the way chemically amplified photoresists have consistent standardized strategies and chemistries. As these new materials and concepts develop, 3D microprinting will become much more important than it is today and will become integrated into nanomanfacturing.”

Authors of the paper include Christopher K. Ober, Ziwei Liu, Roselynn Cordero and Alicia Cintora.

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