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In 2003, Ou-Yang’s group became one of the first to use optical tweezers to study the interior of an endothelial cell noninvasively, without introducing foreign particles into or around the cell. Achieving a resolution of 0.5 nm, Ou-Yang uncovered new information about the rigidity of the cytoskeletons of live cells. He began quantitatively mapping the intracellular mechanical properties of the cells and determining how they are affected by extracellular physical or chemical influences.
Ou-Yang has utilized optical tweezers in several projects with engineers. At Lehigh, he works with Prof. Samir Ghadiali of mechanical engineering and mechanics to evaluate the mechanical responses of epithelial cells in the deep lung to the forces imposed by ventilating machines.
Working with Shu Chien, director of the Whitaker Institute of Biomedical Engineering and professor at UC-San Diego, Ou-Yang uses a tweezer-based cytorheometer to study the mechanisms by which biological cells sense and respond to their external mechanical environment.
Ou-Yang also collaborates with Miriam Rafailovich, director of the Garcia Center for Polymers at Engineered Interfaces at SUNY-Stony Brook. They are seeking to identify the critical parameters involved in engineering the mechanical properties of the extracellular matrix to control cell functions.
A bottle that harnesses light
While optical tweezers are ideal for studying micron-sized objects, the optical bottle is better suited for studying nanoparticles, says Ou-Yang.
“You can’t trap individual nanoparticles,” he says, “but you can study their interactions inside the optical bottle by imparting a mechanical force with light. The question is how much radiation you need.”
Determining how nanoparticles interact with each other, and with the light source, says Ou-Yang, requires deduction.
“We can deduce the nature of these interactions by measuring the extent to which the particles fill the bottle; this suggests particle-to-particle attraction. The reverse is true; a bottle not filled with many particles suggests a repulsion or lack of attraction.”
When large numbers of particles crowd into the bottle, says Ou-Yang, the electrical field of the infrared laser induces the formation of an electric dipole in each particle. The dipoles, in turn, interact with each other to form secondary structures. Ou-Yang plans to introduce a third laser to study the formation of these structures.
“The key here is that you can manipulate colloidal stability, either mechanically or optically, to change the natural state. Whether you end up with a magnetic, electrical or optical bottle, you are dealing with the same phenomenon.
“In a magnetic bottle, a high magnetic field confines the plasma used for nuclear fusion. In an electrical bottle, an electrical field confines dielectric particles to a localized region. In an optical bottle, light triggers or alters the dynamics of the interactions of colloidal nanoparticles, allowing us to study and analyze these interactions or control their transport behavior for manipulation purposes.
“The overriding question remains: How do you harness the mechanical force from photons and what do you do with that force in different regimes of colloidal science and engineering?”