Laser Tweezers Transport Cell-Size Liposomes

Scott M. Aldis-Wilson

Researchers at Kyoto University in Japan have demonstrated a method to trap and transport cell-size liposomes with laser tweezers. Liposomes can carry chemicals such as enzymes, DNA or RNA and remain stable for weeks. The work may pave the way for the development of microlaboratories for biochemistry and molecular biology.

Experiments at Kyoto University showed that cell-size liposomes can be trapped and transported without damage if the inner solution has a higher refractivity than the outer. In this photo, the trapping potential was improved by 30 times in the lower experiment. Courtesy of Kyoto University.

Liposomes, or closed phospholipid bilayer membranes, are used to model cell membranes, but vesicles less than 0.1 µm in diameter have received the most study. Researchers have paid less attention to 1-µm-diameter and larger cell-size liposomes, and the phospholipid membranes have contained the same solutions in which they were immersed.

The new experiments demonstrate that the laser tweezer technique is suitable for cell-size liposomes if the inner solution has a higher refractivity than the external solution. The university researchers prepared 0.1- to 10-µm-diameter vesicles by the hydration of lipid films. For the optical tweezers, they used the 1064-nm output of an Nd:YAG laser, reflected by a dichroic mirror and focused by a microscope and an objective lens with a numerical aperture of 1.30. A laser power meter monitored the output, and a CCD camera that was connected to a video recorder through an image processor captured microscope images of the process.

To investigate optical transport, the researchers employed two scenarios. In the first one, they used pure water for the solution both inside and outside the liposome. In the second, the solution inside contained 100 mmol/l glucose and the solution outside had 50 mmol/l sodium chloride.

In the first, with identical refractivity inside and outside, the liposomes frequently escaped during transport and suffered deformation by the focused laser. In the second, where the inner solution had a higher refractive index than the outer, the liposome was trapped and transported without failure or serious deformation, and the effective attractive potential increased by an order of magnitude.