SU-8 MEMS Fabry-Perot pressure sensor

The biocompatible polymer, SU-8, was used to microfabricate an interferometric pressure sensor designed for invasive biomedical applications. Tests of the sensor in air and liquid environments show promising results as well as the limitations of SU-8 as a critical material in microdevices. The sensor consists of a polymer cap with a reflective, pressure-sensing diaphragm mounted onto the end of a fiber optic cable. Diaphragm deflection was measured by analyzing the spectrum reflected from the Fabry-Perot interferometer formed between the diaphragm and the fiber end. The device is fast, simple, and inexpensive to manufacture. Its small dimensions (300 μm outer diameter) reduce the risk of inflammation and infection and allow for its insertion through a catheter into small vessels and cavities. The sensor showed a linear pressure response from 0 to 125 mmHg with 1–2 mmHg resolution. The cap swelled upon immersion in a manner consistent with Fickian diffusion of water into SU-8. The interferometric displacement transducer allowed a series of measurements to characterize the drift and hysteresis of the SU-8 sensors in different environments. These results provide guidance for the design and manufacture of SU-8 microdevices.