Piezoresistive sensors provide real-time output signals for enhanced functionality and offer a variety of configurations for design flexibility.
Memo Romero, Raúl Figueroa, and Chad Madden
Pressure sensors are becoming increasingly central to medical equipment design. They play key roles, for example, in blood pressure monitoring systems, vital-signs monitoring equipment, and respiratory-health maintenance applications. As used for devices in the last category, pressure sensors historically have been mechanical in design and have performed a simple switching function. Respiratory products such as spirometers, oral drug delivery systems, and oxygen therapy equipment all involve pressure sensing of a fluid in some manner.
|Medical-grade pressure sensor packages are temperature compensated with a built-in resistive network and are available in disposable units.|
Now, solid-state pressure sensing devices, which can provide more functionality than a simple switch, are taking the place of mechanical sensors in this product class. Some sensor manufacturers have developed devices especially tailored for the medical equipment industry. The small size and low cost of solid-state sensors, as well as their ability to provide real-time output signals, make it possible for designers to create more-sophisticated products. With solid-state devices, values such as transient pressure, flow rate, and total fluid volume can be calculated by the application of basic mechanical engineering principles.
Solid-State Pressure Sensors
Many pressure sensors used in medical applications are made with piezoresistive semiconductor technology and are therefore considered solid-state devices. The manufacture of these devices involves bulk micromachining and surface etching of silicon wafers to create a chip that produces a voltage output that is fairly linearly dependent on the applied pressure. This linear dependency equates to a very short response time, the precondition for time-varying output signals that effectively map the applied pressure. Figure 1 depicts a typical cross section of a pressure sensor. The performance curve in Figure 2 shows the typical linear relationship between output voltage and applied pressure.
Pressure sensor manufacturers generally offer three types of pressure sensors&151;uncompensated, temperature compensated and calibrated, and integrated&151;each of which provides specific characteristic output signals. Depending on the requirements of a particular design, an engineer may specify any one of the three.
Figure 1. Cross section of a typical solid-state pressure sensor.
An uncompensated pressure sensor is a bare-bones part, usually chosen to minimize cost in an application where accuracy is not a critical factor. It requires signal conditioning, such as amplification and noise filtering, to produce usable output signals.
Temperature compensated and calibrated devices are manufactured with minimal offset and are compensated for temperature with a built-in resistive network. As with uncompensated devices, these products require signal conditioning, because output voltages are in the low-millivolt range.
Figure 2. Typical pressure sensor performance curve.
Integrated pressure sensors are temperature compensated and calibrated as well, but they are also signal conditioned, producing a signal of anywhere between 0 and 5 V. This type of output can be fed readily into an analog-to-digital (A/D) signal convertor or a microcontroller. These devices are popular from an integration standpoint and may present the lowest system-level cost.
Temperature-compensated and integrated pressure-sensing devices are the ones most widely used in medical devices. When very specific signal conditioning and resolution is required, a temperature-compensated device affords the designer full signal control. For ease of implementation and reduction of board space requirements, an integrated device is the appropriate choice.
Mounting for Implementation
Figures 3 a, b, and c depict three mechanical techniques commonly used to mount pressure sensors in fluid-storage and flow environments. Each has variants that can be employed for a specific application or medical device.
Figure 3a. Measuring pressure in a nonmoving fluid.
Figure 3a shows a simple method often used to measure the static pressure of liquids. And typically, the fluid being measured with such a setup is a liquid, as opposed to a gas. To protect pressure sensors used in this arrangement from potentially corrosive materials, a column of air is often confined between the sensor and the liquid.
Figure 3b. Measuring pressure in a moving fluid using a flow constriction tube.
Figure 3b shows a diagram of a flow-constriction tube with an orifice, a geometry often used to measure the pressure of a moving fluid. Cross sections A1 and A2 are areas of unrestricted and constricted flow, respectively. This measurement setup has the advantage of requiring only that pressure be sensed at one location. Another benefit is that this type of tube is relatively easy to manufacture. A drawback of this approach is the substantial and nonrecoverable pressure drop that occurs across the constriction orifice.
Figure 3c. Measuring pressure in a moving fluid using a venturi tube.
Another approach is to use a venturi flow tube, such as is shown in Figure 3c. This configuration has been used for many years in a variety of flow applications. The basic advantage of a venturi tube is that there is relatively little pressure drop from the inlet to the outlet of the flow tube, and thus a minimal requirement for upstream pressure. To gain this advantage may be absolutely necessary for very-low-pressure applications. The disadvantages of a venturi flow tube are that it is more difficult to manufacture, and pressure sensing at two locations is required to get the largest signal. Figure 4 shows a typical pressure curve for a venturi tube.
Figure 4. Venturi tube pressure curve.
Fluids under different conditions present different challenges for accurate pressure measurement. Sometimes, solid-state sensors are essential.
Stationary Fluids. Pressures in stationary fluids, as are found in a tank, are the simplest to measure, whether the pressure is constant or changing with respect to time. In applications where the pressure of a gas such as oxygen is being measured, the placement of the pressure sensor is not critical. The sensor can be mounted adjacent to the gas container or can work remotely via a tube. Both methods are used extensively. In applications involving a liquid, typically the head pressure of the liquid is exploited, as in Figure 3a.
Moving Fluids. Steady-state and transient pressures in moving fluids are more difficult to sense accurately. Flow tubes are typically employed, and the sensors are placed as close to the fluid as possible without disturbing the flow pattern (see Figures 3b and 3c. In steady-state fluid flow, whether gas or liquid, patterns of flow are fully developed and will be either laminar (fairly smooth) or turbulent (displaying vortices and eddies). The voltage signal from the pressure sensor will be relatively constant in either case, implying a constant average fluid velocity and pressure. In the case of transient fluid flow, the pressure varies with the velocity of the fluid, a circumstance that will be reflected in rising and falling voltages. One of the benefits of solid-state pressure sensors is their ability to accurately capture real-time fluctuations in pressure. Because the sensing element's mass is so small, its response time is extremely short, on the order of a few microseconds. This can be especially useful in switching functions where other circuitry is activated immediately on the basis of pressure values.
Figure 5. Application circuit schematic for very-low-pressure sensing applications.
Very Low Pressures. Pressures well below 1 kPa must be detected in some medical devices. In such applications, even an integrated sensor rated at 1 kPa will produce a relatively low voltage at these pressures. Therefore, special signal-amplification circuitry must be employed to produce an output voltage suitable for microcontrollers or for A/D conversion. A commonly used circuit is illustrated schematically in Figure 5.
Calculating Flow Rates and Volumes
Flow Rate. Students of fluid dynamics know that the volume flow rate of a fluid is proportional to the square root of the difference between the pressure in the fluid and that of the ambient condition. This is true for both steady-state and transient flows that satisfy certain assumptions of incompressibility and subsonic velocity.
Figure 6. Transient pressure curve.
The relationship is expressed in the following equation:
In this equation, Q is the flow rate; �P is the pressure difference between the fluid pressure at the cross section of interest and that at another reference point, typically the ambient or another cross section in the flow tube; and K is the constant of proportionality, which depends only on cross-sectional areas and on the density of the fluid. For a flow tube such as that shown in Figure 3b, flow analysis using Bernoulli's equation and the principle of flow continuity will yield the following relationship:
Thus, the cross-sectional areas must be chosen carefully in order to produce appropriate values of K for the expected pressures.
Volume. In certain applications, the volume of air transferred over a particular time duration is of interest. Figure 6 shows a transient pressure curve typical of some medical devices, such as spirometers. Also depicted is a corresponding flow-rate curve as would be calculated using equations 1 and 2.
The volume, V, in this case can be calculated by integrating the flow-rate curve over time. Therefore, expressed mathematically,
V = (integral) Q(t) dt.
This integral can be evaluated from time t1 to time t2. Recall that Q is a function of pressure and geometry; the volume can therefore be calculated simply by measuring pressure as a function of time and using equations 1–3. Depending on the particular boundary conditions of an application, the closed-form solution of equation 3 can be a very difficult integral to solve. Thus, well-known numerical integration methods using discrete time steps are often employed.
Respiratory Applications for Solid-State Sensors
Solid-state pressure sensors are the defining components in several classes of respiratory-health devices in growing healthcare markets. The following paragraphs characterize the current and prospective future uses to which these fluid-pressure-sensing devices can be put.
Spirometry. A spirometer is a device used to measure lung capacity during inhalation or exhalation. It is also used to determine peak flow rate (PFR), a measure of the forcefulness with which a person can exhale. These measurements and the relationships among them can help detect whether a patient is suffering from any number of lung disorders, including asthma and chronic obstructive pulmonary disease. Patients with these and other conditions routinely use spirometers, either at home or with a physician, to monitor their lung performance. Asthmatics in particular can often predict the onset of an asthma attack by closely monitoring changes in their PFR. According to Clinica Reports, the spirometer market generated $160 million in 1998 and is projected to be at $210 million by 2003.1
A solid-state pressure sensor can be used in these devices, with appropriate amplification circuitry, to measure respiratory pressure as a function of time. PFR and lung capacity can then be calculated as described in the mathematical section of this article.
Sleep Apnea. Some 4% of men and 2% of women throughout the world are estimated to have moderate to severe obstructive sleep apnea (brief interruptions of breathing during sleep). The overall market for treating this disorder is valued at $370 million this year, according to the Clinica report cited above. The average growth in this market over the past three years has been more than 14% a year.
Though more research is needed to fully understand the causes and variations of this disorder, it has been established that many patients can benefit from devices that maintain a continuous positive airway pressure (CPAP) in their trachea. A solid-state pressure sensor can be used not only to monitor airway flow rate and pressure but also to provide a profile of the pressure inside a patient's airway during sleep. This feedback can reveal clues about what may trigger or aggravate a sleep apnea condition.
Drug Delivery. Pharmaceutical companies have developed drug-delivery products in recent years that make it easier for patients to take respiratory medication orally. These products, known as nebulizers and inhalers, are able to sense a very slight inhalation and responsively deliver medication into the person's lungs. Oral inhalers used by asthmatics are one such example. For these devices to operate satisfactorily, inhalation pressures as low as 0.5 in.H2O must be detectable. The sensor in such a case functions as a low-pressure switch and activates circuitry to perform the actual drug delivery.
Flow-constriction or venturi tube geometry as in Figures 3b and 3c can be employed effectively in drug delivery, using appropriate cross-sectional areas that will generate a reasonable voltage output at very low pressures. The advantage of a solid-state pressure sensor in this application is that, in addition to performing the switching function that mechanical pressure sensors can perform, it can extract additional information about a patient's inhalation patterns. For example, varying and precise quantities of medication can be delivered depending on the strength of a person's inhalation. Additionally, devices can be developed to properly train an individual to inhale for maximum benefit. The medical industry is looking closely at using solid-state pressure sensors in many portable, handheld, and bedside drug-delivery devices.
Other Medical Devices. In addition to spirometry, CPAP, and oral drug delivery, solid-state pressure sensors can be used in other respiratory applications. Two examples are oxygen therapy equipment (ventilators, concentrators, and conservers) and humidifiers. Oxygen therapy equipment is used to increase the oxygen content of the air that is delivered to the lungs. It is used to treat illnesses such as emphysema. These devices take many forms, but all require careful monitoring of pressure in order to ensure the maintenance of proper airflow rates. Humidifiers increase the water content of the atmosphere surrounding a patient. This atmospheric modification can serve to soothe respiratory mucous membranes in patients who have suffered damage in those areas. As with CPAP and oxygen therapy equipment, airflow and pressures must be measured and controlled accurately.
The real-time performance of solid-state pressure sensors makes them very suitable for application in respiratory medical devices. Indeed, the utility of many devices can be enhanced by virtue of the information these sensors can provide. Solid-state pressure sensors are typically smaller than their mechanical counterparts, have no moving parts, and, because they are electrical devices, are readily incorporated into other circuitry. They are designer friendly. By shortening design time and thus accelerating time-to-market, they are business friendly as well.
1. "Respiratory Devices: The World Markets," Report CBS-761 (Richmond, UK: Clinica Reports, 1998).
Memo Romero and Raúl Figueroa are systems and applications engineers for Motorola's Sensors Products Div., Transportation Systems Group (Phoenix). Chad Madden is marketing communications specialist for the company.