Applications of optical tweezers and an integrated force measurement module for biomedical research

Optical tweezers are useful for manipulating biological samples and measuring biological forces. In the present study, we have integrated a forward scatter analysis (FORSA) module into the `single-beam gradient force optical tweezers'. The entire set-up was then incorporated onto an inverted microscope. In the FORSA module, a Helium-Neon probing laser was spotted (at a slightly out-of-focus way) onto the object being trapped by the infrared laser-based tweezers and generated a diffraction pattern. Images of the diffraction pattern were captured by a charge-coupled device (CCD), and digitized and processed by a computer. We demonstrated that tracking the `amplified' diffraction pattern was much more precise to determine the movement of the object within the trap than analyzing the minute motion of the object itself. Displacement of the object could then be translated into the force being applied by the tweezers. Also, using an algorithm developed in the lab, we were able to follow the movement of the scattering pattern at a temporal resolution close to video rate. We have used this system to investigate the binding force associated with cell-cell interactions and molecular interactions. In these studies, a cell was carefully positioned to make contact with another cell or a microparticle coated with proteins of interest by optical tweezers in a well-controlled manner. During these events, we noted a progressive increase of cell adhesion at the immediate early period (i.e., a few minutes after initial contact) of cell-cell interactions. Also, binding of a disintegrin, rhodostomin, and its mutant to the counterpart integrin on the cell surface could be assessed with great convenience and accuracy. Our results demonstrated that addition of the forward scatter analysis module to conventional optical tweezers provides an effective and convenient way for monitoring biological activities in situ and measuring changes of biological forces with precision.