Novel Higher Sensitivity Silicone Based Capacitive Pressure Sensors For Biomedical Applications

Pressure measurement is an important tool in the diagnosis and treatment of injuries and disease. For example, after a serious accident, monitoring intracranial pressure can give information on haemorrhaging that may occur. In minimally invasive surgery, pressure sensors are used to compensate for the surgeons loss of tactile sensation. Thick films and similar technologies can be used for the fabrication of cost effective sensors for a wide range of medical applications. Research in this area has recently focused on the implementation of capacitive sensors with silicone rubbers as the dielectric layer. However it is noted that a continuous elastomer layer is not sensitive enough and more complex sensor designs are commonly required, thus affecting the cost of fabrication. In this work an investigation into the pressure sensing properties of a RTV silicone rubber (Notcutt Ltd) was carried out. The silicone was used as the dielectric material of a metal-insulator-metal capacitive design. Small amounts of Carbon Black (CB) are added to the silicone during fabrication so that its sensitivity to pressure may be improved. It was thought that this should eliminate the need for complexity in sensor design. It is found that adding CB to the material resulted in changes of both electrical and mechanical properties. Adding 6 wt.% CB increases the relative permittivity of the silicone by a factor of two without causing undesirable increases in dissipation factor. Similar improvements in the pressure sensing properties of these materials have been observed when tested using a Lloyd Instruments LR50K over the range 0- 80 kPa. For samples with 6 wt.% CB a four-fold increase in sensitivity has been recorded. The improved sensitivity may eliminate the need for complexity in the sensor design.