18. Design and Rapid Prototyping of Fiber-optic Based Micro-force Sensors by Two-photon Polymerization

We are developing fiber-optic based micro-sensors for measurement of dynamically induced micro- to nano-Newton forces. The force-sensing transduction mechanism is based on a series of mechanical micro-springs and an optically reflective micro-plate with axial movement that are embedded at the distal end of single-mode optical fibers. Micro-sensors have low-stiffness, which is required for characterization of biomedical samples and soft materials, and are rapid prototyped by two-photon polymerization methods. Total-internal-reflection based micro-retroreflectors are designed to direct the fiber output laser beam back into its core for subsequent quantification of force-displacement induced phase changes that are measured by interferometric methods. After conducting investigations to characterize mechanical properties of solidified photoresists as a function of fabrication parameters, that include laser exposure, we are designing and tuning the springs to perform over specific ranges of forces and displacements. The mechanical springs are characterized and calibrated by MEMS micro-force sensing probes attached to motorized nano-positioners to detect forces from 100 nano-Newtons to 500 micro-Newtons. We report the performance of the newly developed micro-force sensors as applied to several applications, including measurements of acoustically induced forces on the Tympanic Membrane surface to better understand hearing and middle-ear mechanics.