Opto-mechanical chiral microrobot for out-of-plane rotation

Optical tweezers use tightly focused laser beams to trap and manipulate microscopic particles by balancing scattering forces with a gradient force at the beam’s focal point. This enables precise and contact-free particle control, making optical tweezers a powerful tool to activate microrobots (optobots) that give multiple degrees of freedom. However, realizing out-of-plane rotation, which is demanded for medical applications such as cellmanipulation and drilling, remains a challenge. Here, we present an optobot design leveraging chirality to achieve full-cycle out-of-plane rotation. This optobot has an elongated structure with dual optical handles and a chiral helix aligned on the long axis of the robot. The optical handles are used to be trapped by the optical tweezers beams to keep the long axis of the optobot and the helix perpendicular to the laser light propagation, while its chiral helical part generates out-of-plane rotation around its long axis under laser excitation due to broken axial parity. Finite element analysis was conducted to simulate the interaction of the chiral helix with the laser beam. The scattered beam distribution was visualized to see how it is altered by the chiral structure. The optical torque was also calculated to show that it is unidirectional following chirality. For experimental demonstration, the optobot was fabricated via two-photon lithography. The optical manipulation experiments using an optical tweezer system showed that the optobot demonstrates versatile actuation capabilities, achieving on-demand repetitive rotation, controlled-speed translation, and combined rotational-translational motion.