Time-of-flight optophoresis analysis of live whole cells in microfluidic channels.
Optophoresis is a non-invasive cell analysis technique that is based on the interaction of live whole cells with optical gradient fields, typically generated by a near-infrared laser. The magnitude of the interaction depends upon the intrinsic physical properties of the cells, such as their refractive index, composition, size, and morphology. Time-of-flight (TOF) optophoresis is an implementation of this technique in a microfluidic environment. It measures cell travel times through a fixed distance with and without irradiation from a laser beam. The magnitude of the optical force from the laser, and therefore the change in transit time introduced by the presence of the infrared laser provides a signature for the cell. By accumulating such measurements for a population of cells (typically 200-300 cells per population), different cell types, drug treatments, or biological states can be compared quantitatively without the need for external labels or markers. An integrated TOF system has been constructed and characterized. The system typically uses square capillaries with 50-100 microm internal diameter and uses a syringe-pump-based flow system that generates initial bulk flow velocities between 200 and 600 microm/sec. Using this TOF technique, we have been able to consistently detect significant differences between normal skin and melanoma cell lines, CCD-1037 and A375, respectively. We have also been able to measure consistent differences in a cell differentiation model (HL60 cell line with DMSO treatment). These early results indicate the potential biological sensitivity of the TOF measurement technique for cellular analysis and cancer diagnostic applications.