Soft, high-speed sensors are crucial for real-time detection of mechanical de-formation in brain-like materials, particularly in studying mild traumatic brain injury (mTBI). This work presents the design, fabrication, and validation of stretchable optical fiber sensors using thermoplastic polyurethane waveguides paired with infrared optoelectronics. A repeatable process was established, pro- ducing sensors with consistent mechanical stiffness and electrical sensitivity un- der tensile loads. Dynamic testing was performed using ballistic and blunt im- pacts in hydrogel, silicone, and preserved sheep brain tissue. Results confirmed millisecond-scale responsiveness and sensitivity to both local deformation and impact propagation. Sensor performance under compressive and constrained conditions was also evaluated, revealing limitations due to pre-strain and ma- terial swelling. A localization framework based on LASSO regression was de- veloped and validated on a 2D sensor matrix. While complete 3D localization was not implemented, the system demonstrated feasibility for integration into future diagnostic platforms for mTBI detection and soft-tissue impact sensing.