Implantable piezoresistive pressure sensors represent an important component in interventional medical tools such as catheters. With an increasing ageing population affected by chronic diseases, there is a growing demand for continuous monitoring of health status, and thus the need for in vivo reliable measurement of physiological conditions. Based on the information presented above, it is expected that the market for implantable biosensors will continue to raise in the near future.
Ultrananocristalline diamond (UNCD) in thin film form has shown to possess extraordinary mechanical, tribological, electrical, when properly doped, and biological properties. Importantly, UNCD has demonstrated to exhibit extraordinary resistance to biofoluing, one of the main issues for implantable biosensors. In addition, it has been already reported that the mechanical properties of diamond allow diamond to sustain higher pressure conditions than silicon. As a result, diamond represents a very promising material for a new generation of piezoresistive pressure sensors. Moreover, diamond provides a bionert material that is highly resistant to corrosion, as may occur when materials are exposed to fluids in the human body, an issue that silicon pressure sensors can only overcome by the use of special packaging, which increase the size and cost of implantable sensors. In this work, we report results from an R&D program to develop a novel implantable piezoresistive pressure sensor based on UNCD thin films grown by Hot Filament Chemical Vapor Deposition (HFCVD), using a unique chemistry involving a mixture of Ar (90 sccm), CH4 (2 sccm) and small amount of H2 (10 sccm), which produce films with grain 2-5 nm grain size characteristic of UNCD. Diaphragms based on UNCD films were fabricated using photolithography and wet chemical etching. Determination of maximum stress as a function of diaphragm area and thickness were calculated using an empirical formula. Based on these results, a set up for measuring diaphragm performance under maximum stress has been designed. First results of diaphragm fabrication and set up design are presented. Future work related to resistor design, and integration with the diaphragm element are discussed.
University of Texas Dallas, Massachusetts Institute of Technology
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