Toward Automated Formation of Composite Micro-Structures Using Holographic Optical Tweezers

TL;DR

This work addresses automated assembly of composite micro-structures using holographic optical tweezers by leveraging multiplexed annular and line traps, real-time bead detection, and a wavefront-based path planner. The proposed framework couples SLM-based phase pattern generation with collision-free trajectory planning in a discretized grid to move multiple traps from predefined starts to goals while handling obstacles. Experimental demonstrations with $5 μm$ beads show successful flower- and P-shaped structures within tens of seconds, and runtime analyses indicate real-time feasibility for scaling to larger trap arrays. The study highlights the practicality of automated HOTs for micro-assembly and discusses opportunities to improve robustness, scalability, and 3D capabilities for future work.

Abstract

Holographic Optical Tweezers (HOT) are powerful tools that can manipulate micro and nano-scale objects with high accuracy and precision. They are most commonly used for biological applications, such as cellular studies, and more recently, micro-structure assemblies. Automation has been of significant interest in the HOT field, since human-run experiments are time-consuming and require skilled operator(s). Automated HOTs, however, commonly use point traps, which focus high intensity laser light at specific spots in fluid media to attract and move micro-objects. In this paper, we develop a novel automated system of tweezing multiple micro-objects more efficiently using multiplexed optical traps. Multiplexed traps enable the simultaneous trapping of multiple beads in various alternate multiplexing formations, such as annular rings and line patterns. Our automated system is realized by augmenting the capabilities of a commercially available HOT with real-time bead detection and tracking, and wavefront-based path planning. We demonstrate the usefulness of the system by assembling two different composite micro-structures, comprising 5 $μm$ polystyrene beads, using both annular and line shaped traps in obstacle-rich environments.

Figures (6)

• Figure 1: Phase mask and light intensity distribution on the focal plane. Annular trap phase mask (a), line trap phase mask (b), annular trap light intensity distribution using a topological charge $l=15$(c), line trap light intensity distribution (d).
• Figure 2: Illustration of the problem formulation. $S_i$ and $G_i$ denote the start and goal configurations, respectively. Red circles denote the obstacles with buffer regions around them. Gray beads indicate the objects of interest for path planning.
• Figure 3: Heatmap of the primary wavefront planner to construct a P-shaped micro-structure. Yellow and green dots represent the start and goal locations, respectively. Red dots represent the obstacle beads and solid blue regions denote the obstacle buffer zones and unexplored areas.
• Figure 4: Heatmap of the secondary wavefront planner to construct a P-shaped micro-structure. Note the addition of the first path as obstacles.
• Figure 5: Time lapse images of a flower-shaped structure generation using two multiplexed optical traps. The time elapsed is 35 seconds and the frames are taken in 7 second intervals. The green circles are active traps and the blue circles are the detected beads.