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Since their early stages of development, micro-electro-mechanical systems (MEMS) have shown potential for breakthroughs in the fabrication of medical tools. The miniaturization of various devices using MEMS technology has enabled minimally invasive treatments and in situ measurements. In this paper, we introduce two advancements in MEMS applications in the medical field: (1) microneedle devices for brain activity evaluation, a transdermal drug delivery system, and biological fluid sampling; and (2) miniaturized MEMS sensors for monitoring the conditions inside blood vessels and respiratory organs. In addition, we provide a summary of MEMS sensors used in developing new drugs, detecting vital signs, and other applications.

We developed a stent flow sensor using micro-electromechanical systems (MEMS) technology in order to measure airflow in the airways of laboratory animals. An airflow waveform contains a respiration signal produced by lung motion and heartbeats as these functions are physically connected. This enables both respiration and heart functions to be evaluated under unrestrained and unanesthetized conditions during drug development. Using MEMS technology, we fabricated a tube flow sensor based on a thermal principle, and the stent structure was produced by photolithography and wet etching. The fabricated flow sensor and stent were integrated via a connection area on both elements. In the proof-of-concept trial, we verified that the flow sensor was placed in the center of the tube by expanding the stent. We evaluated the flow detection characteristics, sensor output vs flow rate and sensor response and used the developed stent flow sensor to measure the airflow in the airway of a rat. The respiration and heartbeat signals were successfully identified from the airflow waveform by applying Fourier transform analysis.

Cross-sectional imaging of isolated rat lungs was demonstrated by ultrahigh-resolution optical coherence tomography using supercontinua at 800, 1060, and 1300 nm wavelengths. The detailed structure of the trachea, including cartilage, mucosa, and annular ligaments, were observed clearly. In the imaging of visceral pleura and alveoli, when phosphate-buffered saline was instilled into the lung, the penetration depth of imaging was improved, and clear images of the fine structure of the lung, including alveoli, were observed owing to the index-matching effect. The wavelength dependence of the light source was discussed for the observation of fine structure and imaging contrast.

We optimized a catheter-type thermal flow sensor to measure the reciprocating airflows in bronchial pathways. The electrical wiring in the outer tube was extracted to keep the symmetry of the temperature distribution on the heaters. First, a flexible sensor was fabricated on polyimide film with photolithography, and then the enameled wires were bonded to the contact pads on the film with anisotropic conductive film. The film sensor was finally assembled with a catheter configuration that had a diameter of 1.8 mm by applying a heat shrinkable tube. We experimentally confirmed that the sensor outputs under both forward and reverse flow conditions had the same output characteristics when a design with symmetrical heating elements was applied. The sensor outputs we obtained at a reciprocating flow frequency ranging from 1 to 3 Hz almost coincided, and they also fit that obtained under unidirectional airflow conditions. Finally, the aspirated- and inspired-air characteristics in the air passages of small laboratory animals (rats and mice) were measured. Due to our development of a flexible method of electrical wiring, we could try a new approach to intubation measurements in which the sensor was inserted into the air passage from the mouth with a fiberscope, and we confirmed that the sensor we developed was able to directly measure the breathing characteristics in air passages. Implanted measurements were also attempted in this study.