Micro/nanomotors (MNMs) are synthetic active devices achieving self-propulsion through converting various types of energy into mechanical motion. Earlier works on enzyme-powered MNMs have demonstrated the motion of single particles, as well as proof-of-concept studies in drug delivery and sensing. Nonetheless, recent reports have shifted focus to the collective motion of these particles. Recently, it was found that enzymatic nanomotors exhibit emergent collective behaviors in fuel-rich environments. Furthermore, the active collective dynamics, combined with advanced imaging technologies, position them as promising tools in the field of biomedicine.
- Why study this? Enzymatic nanomotors assemble into ordered groups, exhibiting intriguing collective behavior akin to the bioconvection observed in aerobic microorganismal suspensions. This collective behavior presents several advantages over individual nanomotors, such as expanded coverage and prolonged propulsion duration, demonstrating their potential in biomedical applications. Despite these promising attributes, the physical mechanisms driving their collective motion remain unclear. This study investigates the formation of swarming enzymatic nanomotors through experimental analysis and computational modeling.
- Seeking insights from bioconvection. To clarify the underlying mechanism, we established a side-view camera setup and observed the swarming behavior from both side and top views. We discovered that the intriguing collective phenomenon resembles bioconvection, a self-organized and self-sustained vortex motion that arises naturally in suspensions of microorganisms. In bioconvection, buoyant microorganisms accumulate, creating unstable density gradients. Some gravitactic algae or aerotactic bacteria swim upward, forming a thin boundary layer of dense, microorganism-rich fluid at the top. This layer eventually becomes unstable, forming descending plumes. Inspried by this natural phenomenon, we proposed a buoyancy-driven model for the collective dynamics of enzymatic nanomotors. In this model, a swarm of nanomotors rises within a denser fluid environment, generating a convective flow in a closed, fuel-rich space. As the nanomotors reach the upper boundary, they spread to balance the mean upward force, forming a layer of unstable particle-rich fluid. The layer then sinks in the form of falling plumes.
- Experimental and simulation-based validation of mechanisms. To verify our proposed mechanisms, we studied the effects of three primary control factors on collective behavior: particle concentration, fuel concentration, and viscosity (mediated by hyaluronic acid concentration). The results revealed that both fuel concentration and particle concentration significantly influence collective dynamics by affecting buoyancy. Additionally, increasing fuel viscosity impacts the dynamics as well. Further supporting the role of buoyancy, we vertically confined the enzymatic motors and observed that the confinement controlled swarm behavior by modulating convective fluid flows. We verified that chemical reaction products increase the density difference between the particulate and the fuel media, accelerating nanomotor movement compared to passive particles. We performed computational modeling of the effect of three main control factors and vertical confinement on collective behavior. The results align well with experimental findings, supporting the buoyancy-driven convection mechanism.
- Why does this matter? The buoyancy-driven convective flow enables the collective movement of enzymatic nanomotors and promotes a more homogeneous particle distribution. In a fuel-rich environment, collective behavior occurs naturally due to buoyancy and chemical reactions, without requiring external forces. This buoyancy-driven dynamics can be harnessed to design future protocols for large tissue and organ volumes, such as the bladder and joints. It allows overcoming the limitations of current cancer treatments, including sedimentation and poor dispersion in small volumes, thereby facilitating mass transport, accumulation, penetration, and effective diffusivity of individual motors.
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