Sagar Chowdhury

Real-Time Path Planning for Coordinated Transport of Multiple Particles Using Optical Tweezers

Automated transport of multiple particles using optical tweezers requires real-time path planning to move them in coordination by avoiding collisions among themselves and with randomly moving obstacles. This paper develops a decoupled and prioritized path planning approach by sequentially applying a partially observable Markov decision process algorithm on every particle that needs to be transported. We use an iterative version of a maximum bipartite graph matching algorithm to assign given goal locations to such particles. We then employ a three-step method consisting of clustering, classification, and branch and bound optimization to determine the final collision-free paths. We demonstrate the effectiveness of the developed approach via experiments using silica beads in a holographic tweezers setup. We also discuss the applicability of our approach and challenges in manipulating biological cells indirectly by using the transported particles as grippers.

Automated Manipulation of Biological Cells Using Gripper Formations Controlled By Optical Tweezers

The capability of noninvasive and precise micromanipulation of sensitive, living cells is necessary for understanding their underlying biological processes. Optical tweezers (OT) is an effective tool that uses highly focused laser beams for accurate manipulation of cells and dielectric beads at microscale. However, direct exposure of the laser beams on the cells can negatively influence their behavior or even cause a photo-damage. In this paper, we introduce a control and planning approach for automated, indirect manipulation of cells using silica beads arranged into gripper formations. The developed approach employs path planning and feedback control for efficient, collision-free transport of a cell between two specified locations. The planning component of the approach computes a path that explicitly respects the nonholonomic constraints of the gripper formations. The feedback control component ensures stable tracking of the path by manipulating the cell using a set of predefined maneuvers. We demonstrate the effectiveness of the approach by transporting a yeast cell using four different types of gripper formations along collision-free paths on our OT setup. We analyzed the performance of the proposed gripper formations with respect to their maximum transport speeds and the laser intensity experienced by the cell that depends on the laser power used.

Survey on indirect optical manipulation of cells, nucleic acids, and motor proteins

Optical tweezers have emerged as a promising technique for manipulating biological objects. Instead of direct laser exposure, more often than not, optically-trapped beads are attached to the ends or boundaries of the objects for translation, rotation, and stretching. This is referred to as indirect optical manipulation. In this paper, we utilize the concept of robotic gripping to explain the different experimental setups which are commonly used for indirect manipulation of cells, nucleic acids, and motor proteins. We also give an overview of the kind of biological insights provided by this technique. We conclude by highlighting the trends across the experimental studies, and discuss challenges and promising directions in this domain of active current research.

Indirect pushing based automated micromanipulation of biological cells using optical tweezers

In this paper, we introduce an indirect pushing based technique for automated micromanipulation of biological cells. In indirect pushing, an optically trapped glass bead pushes a freely diffusing intermediate bead that in turn pushes a freely diffusing target cell towards a desired goal. Some cells can undergo significant changes in their behaviors as a result of direct exposure to a laser beam. Indirect pushing eliminates this problem by minimizing the exposure of the cell to the laser beam. We report an automated feedback planning algorithm that combines three motion maneuvers, namely, push, align, and backup for micromanipulation of cells. We have developed a dynamics based simulation model of indirect pushing dynamics and also identified parameters of measurement noise using physical experiments. We present an optimization-based approach for automated tuning of planner parameters to enhance its robustness. Finally, we have tested the developed planner using our optical tweezers physical setup and carried out a detailed analysis of the experimental results. The developed approach can be utilized in biological experiments for studying collective cell migration by accurately arranging the cells in arrays without exposing them to a laser beam.

Automated Cell Transport in Optical Tweezers-Assisted Microfluidic Chambers

In this paper, we present a physics-aware, planning approach for automated transport of cells in an optical tweezers-assisted microfluidic chamber. The approach can be used for making a uniform distribution of cells inside the chamber to allow the study of a variety of biological processes, including cell signaling. Fluid forces inside the chamber, modeled using computational fluid dynamics, are incorporated into the widely used Langevin equation to simulate the motion of cells. The developed simulator was used for building a map that contains probabilities of a cell successfully reaching one of the outlets of the chamber from different locations under the influence of the fluid flow. The developed planner not only generates collision-free paths that exploit the fluid flow inside the chamber but also utilizes the offline generated simulation data to decide suitable locations for releasing the cells. This ensures fast and robust cell transport, while minimizing the required laser power and operational time. The planner is based on the heuristic D* Lite algorithm that employs a specific cost function for searching over a novel state-action space representation. The effectiveness of the planning algorithm is demonstrated using both simulation and physical experiments in a microfluidics-optical tweezers hybrid manipulation setup.

Optical Tweezers: Autonomous Robots for the Manipulation of Biological Cells

Optical tweezers (OTs) are a popular tool for manipulating biological objects, especially cells [1], [2]. Using a tightly focused laser beam, they exert sufficient forces to tweeze, i.e., hold (trap) and move, freely diffusing cells in the vicinity of the beam focus. The beam can be focused at any point in the workspace, which is typically a liquid-filled glass slide. The trapped cell can, thus, be translated and rotated (transported) in three dimensions by changing the beam focus position. OTs provide certain advantages over other cell-manipulation techniques. They are able to manipulate cells with a greater degree of precision as compared with microfluidic flow. Significant contact forces are not exerted on the cells, unlike in mechanical manipulation, thereby avoiding damages due to contact friction or surface chemistry. The cells are also easily released at the end of the manipulation by simply switching off the laser beam. Hence, OTs have been extensively used for mechanical characterization of cells by measuring their viscoelastic properties to distinguish between normal and diseased cells [3]. They have also been used for separating cells of different types [4] and investigating the response of cells to external stimuli [5]. However, manual or teleoperated control of the laser beam has limited their applicability for multicellular studies.

Gripper synthesis for indirect manipulation of cells using Holographic Optical Tweezers

Optical Tweezers (OT) are used for highly accurate manipulations of biological cells. However, the direct exposure of cells to focused laser beam may negatively influence their biological functions. In order to overcome this problem, we generate multiple optical traps to grab and move a 3D ensemble of inert particles such as silica microspheres to act as a reconfigurable gripper for a manipulated cell. The relative positions of the microspheres are important in order for the gripper to be robust against external environmental forces and the exposure of high intensity laser on the cell to be minimized. In this paper, we present results of different gripper configurations, experimentally tested using our OT setup, that provide robust gripping as well as minimize laser intensity experienced by the cell. We developed a computational approach that allowed us to perform preliminary modeling and synthesis of the gripper configurations. The gripper synthesis is cast as a multi-objective optimization problem.

Towards Independent Control of Multiple Magnetic Mobile Microrobots

In this paper, we have developed an approach for independent autonomous navigation of multiple microrobots under the influence of magnetic fields and validated it experimentally. We first developed a heuristics based planning algorithm for generating collision-free trajectories for the microrobots that are suitable to be executed by an available magnetic field. Second, we have modeled the dynamics of the microrobots to develop a controller for determining the forces that need to be generated for the navigation of the robots along the trajectories at a suitable control frequency. Next, an optimization routine is developed to determine the input currents to the electromagnetic coils that can generate the required forces for the navigation of the robots at the controller frequency. We then validated our approach by simulating an electromagnetic system that contains an array of sixty-four magnetic microcoils designed for generating local magnetic fields suitable for simultaneous independent actuation of multiple microrobots. Finally, we prototyped an mm-scale version of the system and present experimental results showing the validity of our approach.

The interplay of cell-cell and cell-substrate adhesion in collective cell migration.

Collective cell migration often involves notable cell–cell and cell–substrate adhesions and highly coordinated motion of touching cells. We focus on the interplay between cell–substrate adhesion and cell–cell adhesion. We show that the loss of cell-surface contact does not significantly alter the dynamic pattern of protrusions and retractions of fast migrating amoeboid cells (Dictyostelium discoideum), but significantly changes their ability to adhere to other cells. Analysis of the dynamics of cell shapes reveals that cells that are adherent to a surface may coordinate their motion with neighbouring cells through protrusion waves that travel across cell–cell contacts. However, while shape waves exist if cells are detached from surfaces, they do not couple cell to cell. In addition, our investigation of actin polymerization indicates that loss of cell-surface adhesion changes actin polymerization at cell–cell contacts. To further investigate cell–cell/cell–substrate interactions, we used optical micromanipulation to form cell–substrate contact at controlled locations. We find that both cell-shape dynamics and cytoskeletal activity respond rapidly to the formation of cell–substrate contact.

Optical micromanipulation of active cells with minimal perturbations: direct and indirect pushing

Abstract. The challenge to wide application of optical tweezers in biological micromanipulation is the photodamage caused by high-intensity laser exposure to the manipulated living systems. While direct exposure to infrared lasers is less likely to kill cells, it can affect cell behavior and signaling. Pushing cells with optically trapped objects has been introduced as a less invasive alternative, but the technique includes some exposure of the biological object to parts of the optical tweezer beam. To keep the cells farther away from the laser, we introduce an indirect pushing-based technique for noninvasive manipulation of sensitive cells. We compare how cells respond to three manipulation approaches: direct manipulation, pushing, and indirect pushing. We find that indirect manipulation techniques lessen the impact of manipulation on cell behavior. Cell survival increases, as does the ability of cells to maintain shape and wiggle. Our experiments also demonstrate that indirect pushing allows cell–cell contacts to be formed in a controllable way, while retaining the ability of cells to change shape and move.