Femtosecond laser writing of ant-inspired reconfigurable microbot collectives

The collective behavior of organisms, enabling complex tasks through self-organization, inspires the development of microbot collectives. Existing microbot collectives often suffer from unstable connections, reliance on continuous external stimuli, and imprecise individual control. While collectives formed through direct contact are unstable, those formed through interlocking, like ant colonies, exhibit greater stability. This research aims to create a stable, reconfigurable microbot collective mimicking the interlocking behavior of ants, addressing the limitations of current approaches. The development of advanced micro/nanofabrication and materials science has led to the creation of stimulus-responsive microswimmers, but these primarily focus on single units. This work bridges the gap by creating a multi-unit system with stable, reversible connections.

The literature review highlights two main types of biological collectives: those with direct contact between individuals (e.g., bees, fish, birds), which are inherently unstable, and those with interlocking mechanisms (e.g., ants, shrimps), resulting in more stable structures. Previous research has explored microbot collectives using magnetic, light, electric, and ultrasound actuation, but these systems lack precise individual control and require continuous stimuli. The authors focus on a stable assembly strategy inspired by the interlocking mechanism observed in ant colonies, which can provide more robust and versatile systems.

The researchers fabricate ant-shaped microbots using two-photon polymerization (TPP). Each microbot consists of a magnetic photoresist body, two hydrogel joints acting as flexible mandibles, and silver nanoparticles (Ag NPs) deposited on the head for photothermal actuation. The hydrogel joints, composed of varying crosslink densities, exhibit asymmetric deformation upon laser heating. This allows for controlled opening and closing of the mandibles. The magnetic photoresist body enables remote control via external magnetic fields. The fabrication process involves three steps: 1) TPP of the magnetic photoresist body; 2) TPP of the asymmetrical hydrogel joints; and 3) photoreduction of Ag NPs on the mandibles. The hydrogel's properties, particularly its shrinkage ratio, are precisely controlled through parameters in the TPP process. The Ag NPs provide rapid photothermal actuation of the mandibles (opening in 8ms, closing in 12ms), and the magnetic properties of the body allow for remote positioning and orientation via an external magnetic field. The researchers systematically investigate the effects of different processing parameters on the microbot's functionality and demonstrate its controlled opening and closing. The characterization includes SEM, EDS, and VSM analyses.

The researchers demonstrate the controlled and reversible assembly of ant microbots into various morphologies (e.g., 90° and 180° assemblies) using coordinated magnetic and light fields. The mechanical interlocking through the mandibles allows for stable assemblies without the need for continuous external energy input. The study shows the ability of these assembled collectives to traverse gaps, adapt to constrictions, and transport microcargo. Experiments using two and three microbots demonstrate the gap traversal capability. The robustness of the assembly is evaluated under vibration, showcasing the stability of both 90° and 180° configurations. The versatility of the approach is demonstrated by assembling different numbers and shapes of microbots into various complex structures, including J, L, O, V, S, and W shapes. The method also allows for assembly of magnetic and non-magnetic units. The application of the microbot collectives for maze navigation and drug delivery (using doxorubicin hydrochloride) is successfully demonstrated. Fluorescence imaging confirms drug delivery to HeLa cells.

The findings address the limitations of previous microbot collectives by demonstrating a robust, reconfigurable system with stable connections and controlled individual actuation. The ant-inspired design, combined with the precise control afforded by the magnetic and light fields, enables complex collective behaviors like gap traversal and cargo manipulation. The ability to assemble both magnetic and non-magnetic units opens up a wide range of applications. The successful drug delivery experiment showcases its potential in targeted therapies. The integration of magnetic and optical control offers advantages in various scenarios such as microfluidic devices, where they can serve as dynamic microvalves or operate in confined spaces.

This research presents a novel approach to building reconfigurable microbot collectives inspired by the interlocking mechanism of ants. The system demonstrates stable and reversible assemblies, enabling complex tasks such as gap traversal and cargo transport. Its potential in biomedical applications, particularly targeted drug delivery, is promising. Future research could explore the integration of more sophisticated control systems, the scaling up to larger collectives, and the exploration of new functionalities in diverse environments.

The current study focuses on relatively small collectives (up to three microbots for the most complex tasks). Scaling up the system to larger, more complex collectives while maintaining precise control will be a challenge. The reliance on external magnetic and light fields limits the autonomy of the system. Further research could investigate ways to enhance the system's autonomy and adaptability.