Feeling and Influencing Active Intracellular Polymer Networks with Light

The cytoskeleton, a compound of highly dynamic polymers and active nano-elements inside biological cells, mechanically senses a cell's environment and generates cellular forces sufficiently strong to push rigid AFM-cantilevers out of the way. These forces are generated by molecular motor-based nano-muscles, and by polymerization through mechanisms similar to Feynman's hypothetical thermal ratchet. Light has been used to observe cells since Leeuwenhoek's times; however, we use the forces caused by light described by Maxwell's surface tensor to feel cells. The optical stretcher exploits the nonlinear, thus amplified response of a cell's mechanical strength to small changes between different cytoskeletal proteomic compositions as a high precision cell marker that uniquely characterizes different cell types. Consequentially, the optical stretcher detects tumors and their stages with accuracy unparalleled by molecular biology approaches. This precision allows us to isolate adult stem cells for regenerative medicine without contamination through molecular markers. In addition to probing cytoskeletal structure, optical gradient forces can also influence cytoskeletal activity allowing us to manipulate neuronal growth. The specific opto-molecular interactions are complex since cells, which cannot modulate diffusion by the parameters found in the Einstein equation (temperature, viscosity, molecular size), exhibit rich multifaceted behavior including ballistic transport and anomalous diffusion.