Over the years, people have done numerous experiments to confirm this theory. This idea is so captivating that it even influenced the great George Lucas. Star Wars movies featured famous Lightsabers that utilized the special properties of the light to create a powerful Jedi weapon. But we have not seen such fantastic spectacles of light and matter interaction in our everyday macroscale world. Light has very small momentum. So moving a heavy couch by shining a laser on it remains in the realm of science fiction. Unfortunately, if you make the laser too powerful, it will simply evaporate the couch and set your house on fire.
A different picture emerges at the microscale. It is certainly possible to move tiny objects by shining a laser on them. But this mode of interaction does not offer much control. Ashkin in 1986 figured out a better way. He created optical traps that were able to hold tiny particles in place. The basic idea was to bend and focus a laser beam tightly using an objective lens. Once the object enters the laser beam, the laser starts interacting with it and pushing it towards the focal point where it gets trapped.
We can imagine the laser as a collection of rays. These rays are reflected and refracted by objects that intercept them. As the rays are bent, their momentum changes and they exert force on the object. This phenomenon can be visualized as interactions between a stationary ball and a moving ball. The direction of the motion of the moving ball changes as it strikes the stationary ball. Hence it exerts a force on the stationary ball. Ashkin found out that once the effect of all the rays in the laser beam was accounted for, the direction of the resultant force was such that the object was pushed towards the focal point of the beam. So as the object entered the laser beam, it was simply pushed towards the focal point and once it reached that point it remained there. In essence, the focused laser beam created a particle trap. A trapped particle can be moved by moving the laser beam. Thus, the laser has been turned into a tweezing tool for grabbing small particles and moving them. Moving optical traps are often referred as optical tweezers.
Numerous groups have used optical traps to manipulate biological cells and study them. In fact, many important discoveries in biology have been made using optical traps. Biologists are primarily interested in fundamental scientific discoveries. So they are happy to create and move optical traps using tele-operation. Just like tele-operated robots, tele-operated optical traps have many inherent limitations. They are slow, require significant expertise, and limit what kind of manipulation is possible.
I was introduced to optical tweezers in 2004 during my sabbatical at the National Institute of Standards and Technology. Thank you Arvind Balijepalli and Tom LeBrun! I am interested in robotics. So once I learned about optical traps, I became interested in turning them into robotic hands for automatically manipulating biological cells. In many situations, directly trapping biological cells can cause complications. Cells might have an irregular shape and they might be susceptible to damage due to direct exposure to the laser.
We decided to take a different path. Rather than building robotic optical tweezers, we wanted to build robotic optical hands. The idea was to use the laser to trap and move microspheres made out of silica or polystyrenes. These microspheres can serve as “fingers” for gripping or pushing cells. So in our idea, the laser would act as an optical hand (i.e., hand of a ghost) and microspheres would act as “fingers”. This idea enabled free floating “fingers” with no physical hand attached to them. This would be truly an alien hand with no biological counterpart on Earth! We could have as many “fingers” as we wanted by splitting the laser beam to create multiple traps. We could also have multiple hands if we wanted! It was a crazy idea. But it removed many constraints associated with conventional microscale robotic grippers and offered several new possibilities. Soon we were hooked to make this idea a reality.
There were numerous challenges. Microspheres and cells float in the liquid medium and exhibit Brownian motion. We had to detect these objects in the scene, plan the next trap location, and make sure that microspheres and cells moved the way we wanted them to move. However, we only had a few milliseconds to do image analysis, planning, and control. Images are noisy and the environment has significant uncertainty. Moreover, the motions of all the hands and fingers need to be exquisitely coordinated. So this was a really tough robotics problem. +ashis banerjee , +Sagar Chowdhury , +Petr Svec , and +Atul Thakur worked incredibly hard to solve the challenging planning, perception, and control problems to realize this vision. They built upon the basic software capability provided by Andrew Pomerance. Wolfgang Losert and Chenlu Wang provided valuable help in conducting the experiments. Thank you National Science Foundation for supporting this work!
Our adventures in this area began by concurrently trapping multiple microspheres and moving them into an ensemble. We then used that ensemble to hold a cell and move it. We also developed the capability to move the cell into its desired location by pushing on it using a microsphere. If a cell is very sensitive to the laser, then we can use an intermediate microsphere as a tool, so that the microsphere “finger” being trapped by the laser does not touch the cell and ensures physical separation between the cell and the laser. Please see below the video of our robotic optical hand.
Hopefully, our colleagues in biology and medicine would be able to think about new scientific theories that can be enabled by the above described robotic optical hands and their variants. Possibilities range from understanding the behavior of cancer cells to understanding how cells communicate.
Robotics solutions that enable precise automated manipulation of individual cells are expected to revolutionize medicine and biology. Our explorations in this area have taught us that robotics at the microscale requires out-of-the-box thinking. We are now busy coming up with even crazier ideas to marry robotics and biology. So stay tuned for updates.