Imaging-guided collision-free transport of multiple optically trapped beads

Automation of an optical tweezers system is critical to fully leverage its immense multiplexing capabilities, which allows for highly precise and reliable manipulation of microscopic dielectric objects. Although there has been substantial work in this context, automation has been limited to only transporting a few objects at a time, particularly while ensuring collision avoidances with other freely diffusing objects in the workspace. In this paper, we provide the first step in addressing this limitation by developing a new algorithm that combines a model predictive controller with a fast grid search-based path planning method. We validate the usefulness of the presented method through experiments with silica beads in water.

I. Introduction

Optical tweezers are micro-robotic manipulation tools where a strongly focused beam of laser light acts as an end effector. They can affect any dielectric object that has a different optical refractive index compared to the medium it is present in. Since their inception by Ashkin in 1970 [1], they have been used widely as they offer certain advantages over other forms of micro-scale manipulators such as microfluidic chambers and magnetic micro manipulators. They can be easily multiplexed through a programmable spatial light modulator, thereby providing us the ability to simultaneously and independently control multiple objects. They exert forces of the order of picoNewtons, and, hence, can be used to handle fragile and irregular-shaped objects like biological cells easily [2]. They are being increasingly used for various biophysical experiments such as cell sorting [3] and characterizing the mechanochemical properties of cells [4].