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Sensors for Medical Pump Applications PAGE 3 MEDICAL SENSORS / WHITE PAPER There are many pump variables that need to be considered within medical pump applications. These variables include the volume of fluid but also the mode of infusion, which can be continuous, intermittent or patient-controlled. To ensure a correct flow of the fluid, force sensors are integrated into the pump to detect possible occlusions. The force sensor is typically installed under a portion of the tubing that is carrying the IV fluid. When a blockage occurs in a pump the tubing expands. A force sensor placed where the tubing meets the housing can detect this expansion by monitoring the force applied to the sensor by a section of the tubing. The sensor can trigger an alarm to alert the user if this expansion is detected. The same principles of operation can apply in infusion pumps and devices designed for operation by medical professionals in settings such as hospitals, hospices, and home-care. Expansion by monitoring the force applied to the sensor by a section of the tubing. The sensor can trigger an alarm to alert the user if this expansion is detected. The same principles of operation can apply in infusion pumps and devices designed for operation by medical professionals in settings such as hospitals, hospices, and home-care. Although medical sensors for external use, such as in infusion pumps, tend to pose less of a technological challenge than implantable sensors, they still must meet tough requirements. Even as devices become smaller for increased portability, users still expect that these systems maintain functionality and accuracy. To design small yet accurate medical devices, component parts such as sensors must also offer robust features at a low cost. Additionally, reliability is essential for most medical pump applications, especially where critical fluids are being infused. Force Sensor Technology and Principles of Operation A force sensor is a sensor whose properties change when a force, pressure or mechanical stress is applied. A range of technologies can be utilized in the design and manufacture of a force sensor. TE Connectivity (TE) designs and manufactures force sensors for applications that require high performance or unique packaging, including medical devices and equipment. TE's force sensors are based on a proprietary Piezoresistive silicon strain gage (Microfused) technology, that combines durability and long-term stability in extremely low-cost packages. Force sensors (also referred to as load cells) are used in a variety of medical applications including infusion pumps. Other applications in the medical market include physical therapy, hospital beds (patient weight), surgical staplers, cardiopulmonary-resuscitation (CPR) assist machines for emergency medical treatment, as well as for oxygen tank monitoring. Principles of Operation The Piezoresistive effect is widely used in sensor technology. The Piezoresistive effect is a change in the electrical resistivity of a semiconductor or metal when a mechanical strain is applied. A Piezoresistive force sensor contains several thin wafers of silicon embedded between protective surfaces. The surface is connected to a Wheatstone bridge, a device for detecting small differences in resistance. The Wheatstone bridge runs a small amount of current through the sensor. When the resistance changes, the current passing through the sensor also changes. The Wheatstone bridge detects this change and reports a change in pressure. The proprietary TE Microfused manufacturing technology is based on micromachining silicon Piezoresistive strain gages and fusing them in a high-temperature glass-bonding process to a high- performance stainless-steel substrate. This mature and reliable process has delivered millions of force sensors and is a proven technology in a wide variety of medical applications. Microfused technology eliminates age- sensitive organic epoxies that are used in traditional load cell designs, thus providing excellent long-term span and zero stability. The cells measure force directly and are therefore not subject to lead-die fatigue issues common with competitive designs, which use a pressure capsule embedded within a silicone gel- filled cavity. Operating at very low strains, Microfused technology provides an essentially unlimited cycle life expectancy, superior resolution, high frequency response and high over-range capabilities. Overall, Microfused force sensors have demonstrated their ruggedness and reliability in a wide range of medical devices. DIAGRAM OF A PIEZOERESISTIVE FORCE SENSOR WITH A WHEATSTONE BRIDGE AND AMPLIFIER