Even minute alterations in a cell's intracellular scaffolds, i.e. the cytoskeleton, which organize a cell, result in significant changes in a cell's elastic strength since the cytoskeletal mechanics nonlinearly amplify these alterations. Light has been used to observe cells since Leeuwenhoek's times and novel techniques in optical microscopy are frequently developed in biological physics. In contrast, with the optical stretcher we use the forces caused by light described by Maxwell's surface tensor to feel cells. Thus, the stretcher exemplifies the other type of biophotonic devices that do not image but manipulate cells. The optical stretcher uses optical surface forces to stretch cells between two opposing laser beams, while optical gradient forces, which are used in optical tweezers, play a minor role and only contribute to a stable trapping configuration. The combination of the optical stretcher's sensitivity and high throughput capacity make a cell's "optical stretchiness" an extremely precise parameter to distinguish different cell types. This avoids the use of expensive, often unspecific molecular cell markers. This technique applies particularly well to cells with dissimilar degrees of differentiation, as a cell's maturation correlates with an increase in cytoskeletal strength. Because malignant cells gradually dedifferentiate during the progression of cancer, the optical stretcher should allow, the direct staging from early dysplasia to metastasis of a tumor sample obtained by MRI-guided fine needle aspirations or cytobrushes. With two prototypes of a microfluidic optical stretcher at our hands, we prepare preclinical trials to study its potential in resolving breast tumors' progression towards metastasis. Since the optical stretcher represents a basic technology for cell recognition and sorting, an abundance of further biomedical applications can be envisioned.
In an optical stretcher, infrared laser light is used to exert surface stress on biological cells, causing an elongation of the trapped cell body along the laser beam axis. These optically induced deformations characterize individual cells and cell lines. When integrated within a microfluidic chamber with high throughput, this enables diagnosis of diseases, on a cellular level, that are associated with cytoskeletal processes. Additionally, it allows sorting of cells with high accuracy in a non-contact manner. To determine the surface stress on the cell, ray optics calculations as well as the system transfer operator (T-matrix) approach with an appropriate incident field are used. The latter approach allows a more accurate modeling of the cell in the optical stretcher and reveals a more detailed stress profile acting on the cell surface. Analyzing the deformation behavior of normal and malignantly transformed fibroblasts, significant differences in axial elongation even for sample sizes as low as 30 cells are already measurable on a time scale of 0.1s. Here, malignant transformation of cells is discussed as an example of how any process that affects the cell's optical or mechanical properties allows classification with the optical stretcher.