Product Update
Sensors
You might think that the drive to engineer innovative sensing systems that can perform a variety of tasks at a high level of accuracy would be compromised by an equally intense effort to contain costs. But cutting quality corners simply is not an option, according to suppliers of sensors to medical device manufacturers. "To remain competitive in this market," says Harold Joseph, director of marketing at EuroSensor (London), "you have to lower the cost unit and improve performance."
Photo courtesy of Vaisala Oy
The trend toward outpatient and in-home health care is a case in point. "There's a focus on less expensive and less complicated versions of the monitoring equipment used in hospitals for the home environment," says Joseph. "That means some reduction in the size of sensors as well as a push in performance." As devices move out of the traditional hospital setting, portability also becomes a major factor. "That means low power consumption, single-voltage supply, . . . things that connect easily into other systems," adds Paul Krill, technical director at Icor Instruments AB (Bromma, Sweden). And that creates another set of challenges, points out Joseph. "These devices may be battery operated, so the sensors don't require a lot of current consumption, but then you've got a wider temperature issue to deal with."
Micromachined silicon sensor elements, also known as microelectromechanical systems (MEMS), represent one proven technology that has enabled the makers of sensors to meet many of these demands. Silicon is a very easy material to work with because it is almost perfectly elastic, and since it will not give or creep over time, it is especially suited for medical applications where output reliability is critical. Meanwhile, batch processing of the sensing elements in a wafer fab has contributed to a significant decrease in production costs. MEMS, which first found medical industry applications in the 1980s, continues to be the technology of choice for silicon pressure, accelerometer, and custom microstructure applications. The most recent applications are a force sensor for the noninvasive monitoring of infusion pumps developed by SenSym Inc. (Milpitas, CA, USA) and distributed in Europe by Sensortechnics GmbH (Puchheim, Germany) and a disposable blood pressure sensor designed by EuroSensor (London).
Medical applications for silicon-based sensing devices will continue to broaden, according to Joseph. A temperature sensor for an inner-ear thermometer and pressure sensors designed into drug-delivery inhalers to control the release of medication are two examples that have great growth potential, he notes. "What typically happens in the device industry," says Joseph, "is that you begin working with a customer who is developing a new segment of the market--like the inhalers I mentioned. Then a couple of other companies start working in that field." Once the technology is feasible, adds Joseph, then the question becomes, how can manufacturers get the costs down?
Neglecting cost factors, even when the technology represents a significant improvement, is not wise. "Yes, our customers tend to want a more sophisticated product, but it had also better be smaller and lower in cost," says SenSym marketing director Duncan Miller. "We had a very accurate digital output transducer that gave a direct signal, so you didn't have to deal with all the analogue circuitry. It was in a pretty big package and it cost $100. Our customers told us, 'I could buy your basic sensor with temperature compensation for $20. I might pay you $5 more for the digital circuitry and high accuracy, but not $75.'"
The companies whose products are profiled on the following pages are among the leading suppliers of sensors to industry. As you will discover while reading these brief descriptions, their resolve to contain costs has not diminished their commitment to explore new technologies in sensor-system design and to improve existing product offerings.
Kistler Instrumente AG
Systems that typically are used to monitor extrusion pressure and temperature often experience ruptured diaphragms and fluid leakage. In addition, they tend to have a short service life. Kistler Instrumente AG (Winterthur, Switzerland) claims to have overcome these problems with its Type 4090B sensors. "The basic innovation in this product is that it operates without hydraulic fluid and lasts a very long time," says marketing director Rolf Sonderegger. The sensor's silicon measuring element is located behind a thick steel diaphragm (which is also available in abrasion-resistant Ultimet). Because it does not rely on hydraulic fluids to transmit pressure data, the sensor is exceptionally reliable, notes Sonderegger.
Another recent company innovation involves the use of force sensors to measure a patient's gait continuously and over an extended period. The sensors are used in the Gaitway treadmill to analyze the individual vertical force and centre of pressure exerted by each foot. This ability to capture multiple foot-strike analyses over a long time without imposing constraints on the subject's gait constitutes a breakthrough technology, states Sonderegger. "When you are monitoring a patient after an accident, for example, you can't accurately measure purely by eye whether or not walking ability is improving. If it hurts to walk a certain way, the body will progressively modify its way of walking so that it no longer feels any pain," Sonderegger explains. Eventually, he adds, this could cause secondary bone damage. "A force plate is often the only way to detect this phenomenon."
Kistler, which introduced the first commercially available force plate in 1969, can also supply complete systems that include electronic components and software.
Foxboro/ICT
A force/pressure transducer manufactured by Foxboro/ICT (San Jose, CA, USA) uses a solid-state silicon sensor to monitor pressure in infusion pumps, syringe pumps, anaesthesia equipment, and related devices. The component's resistive elements are made by diffusing or implanting boron into the silicon lattice structure to form a fully active Wheatstone bridge. A cavity is then micromachined beneath the resistors to create a pressure-sensing diaphragm. The absence of bonds or mechanical linkages in the resistors combined with silicon's elastic properties result in a sensor with long-term stability, repeatability, and zero pressure hysteresis, according to the company. The Model 1865 force/pressure transducer is made of medical-grade materials and can monitor pressure ranges from 0-10 through 0-30 psig. It can be specified to work either with current or voltage excitation, and its output can be signal-conditioned or amplified. Temperature compensation is achieved by means of a laser-trimmed board. The company has more than 20 years of experience designing pressure sensors; in addition to a range of standard products, it can design sensors to suit specific customer applications.
Vaisala Oy
In humidity sensors, a thin polymer film absorbs or exudes water vapour as the relative humidity in the ambient air rises or drops. Variations in humidity provoke changes in the dielectric properties of the film, which leads to changes in the sensor's capacitance. The capacitance is measured by electronic instruments that convert these data into a humidity reading. Capacitive thin-film sensors are especially popular among pharmaceutical manufacturers for monitoring dryness, according to Jan Grönblad, product manager at Vaisala Oy (Helsinki). "These are very stable sensors. The reading you get today will be the same you got six months ago," says Grönblad, "and that's extremely important in relation to process conditions." For humidity-measurement applications where extreme accuracy is desired, Grönblad recommends the Humicap sensor, which attains an accuracy of ±1% RH. The company also manufactures the Intercap, which is reportedly the world's first interchangeable capacitive humidity sensor. There is no need to adjust or recalibrate the device after replacement of the sensor, and its accuracy is rated at ±5% RH over two years.
Current research at Vaisala revolves around refining sensor sensitivity to chemicals, according to Grönblad. "The sensor might think it's measuring the water molecule when in fact it's measuring something else," he says. "We are trying to develop thin-film sensors that will tolerate all types of chemicals while remaining sensitive only to water." That's of great importance in medical and pharmaceutical manufacturing, adds Grönblad, which may involve thousands of different chemicals.
EuroSensor
A piezoresistive silicon pressure sensor for use in invasive blood pressure monitoring was recently introduced by EuroSensor (London). The component has a typical sensitivity of 5.0 µV/V/mmHg and a frequency response of 1.2 kHz. Featuring a standard operating pressure range of 30 to 300 mmHg, the Model 1620 can be positioned within a disposable blood pressure housing. The unit's performance specifications and low cost make it especially attractive to the medical industry, says marketing director Harold Joseph. "The sensor uses a new silicon die element that is smaller and less costly [than previous models], and it is shipped on a plate that is designed to feed production lines. It can be dropped directly into a disposable blood pressure housing," says Joseph. The component's pressure-sensing element is mounted on a ceramic substrate. Thick-film resistors on the substrate are laser trimmed for compensation and calibration, and a plastic cap attached to the substrate facilitates assembly attachment and protects the sensing element. A dielectric gel is placed over the sensor to provide electrical and fluid isolation. The products are batch manufactured in a 10 x 12-element array and are shipped in antistatic shipping containers compatible with automated production equipment. The sensor conforms to the specifications for blood pressure transducers published by the Association for the Advancement of Medical Instrumentation.
Icor Instruments AB
In addition to small, lightweight sensors that measure the concentration of halogenated agents during inhalation anaesthesia, Icor Instruments AB (Bromma, Sweden) supplies analogue and digital sensor and instrument boards as well as a transducer assembly. All are specifically designed for incorporation into a broad range of gas analyzers, anaesthesia machines, and related instruments. The assemblies feature stable sensitivity, single supply voltage, low power consumption, and automatic zeroing. Icor Instruments has supplied OEMs with miniaturized anaesthetic-agent sensors for more than a decade, notes technical director Paul Krill. "We have quite a bit of know-how when it comes to building gas analyzers and predicting the types of problems that are likely to occur," says Krill. "We are very willing to share that knowledge with our customers." One problem area in which Icor has expertise is gas absorption, according to Krill. "When you are dealing with anaesthetic agents, for example, you must exercise caution in the use of materials, because many of them will absorb the gases." Icor Instruments assists customers by providing linearization and compensation tables, he adds, and can supply printed circuit boards with proprietary software developed according to client specifications. "Fewer and fewer manufacturers produce their own hardware. They prefer to buy prefabricated tested modules," notes Krill.
LasIRvis Technology Europe Ltd.
Background disturbances associated with ambient light do not affect the performance of a photosensor developed by LasIRvis Technology Europe Ltd. (Sandy, Beds, UK)."This high-precision optosensor component is designed to overcome many of the problems associated with the transmission of signals in high levels of ambient light and optical noise," says marketing manager Ian Bulavs. The iC-WQ photosensor amplifies sensitive-signal photocurrent changes at the first stage while electronically suppressing dc photocurrents caused by background light. A high-pass filter screens out constant light and low-frequency alternating light; a low-pass filter reduces high-frequency noise. The iC-WQ operates with visible or near-infrared light with a maximum sensitivity of approximately 100 kHz. The output signal typically is amplified by 50 dB in the current stage. "The iC-WQ's monolithic design with integrated detector, band-pass filters, and current amplifier provides a single-component solution for OEMs," says Bulavs. The photosensor, he adds, exhibits improved reliability and lower power consumption compared to discrete components. The sensor is available as a four-pole TO18 metal package with glass window or as an SMD SO8 package for surface mounting. It is suited for remote control and robotic applications in the medical industry.
SenSym Inc.
Load cells based on a micromachined silicon sensor element were recently introduced by SenSym (Milpitas, CA, USA) for use in infusion pumps, enteral feeding pumps, and drug-delivery systems. The sensors are distributed throughout Europe by Sensortechnics GmbH (Puchheim, Germany). "A key characteristic of the FS series," says SenSym marketing director Duncan Miller, "is that it allows you to perform noncontact measurement of the solution pressure simply by placing the tubing across the SenSym transducer." One advantage of this technology, says Miller, is that it does not require thin-walled tubing, a balloon, or other special requirements to control and monitor infusion flow rates. In addition to measuring delivery efficiency, the sensors can also detect upstream or downstream occlusions. Two products are currently available: The FS01 has an operating force of 0-1 lb while the FS03 has a range of 0-3 lb. Both sensors offer a full-scale span of 3 V with an offset shift over temperature of ±2.5 mV. SenSym manufactures a variety of sensors, including absolute, differential, and gauge types, for 0-0.1- and 0-5000-psi pressure ranges.
Interface Inc.
Specializing in force-measurement technology, Interface Inc. (Scottsdale, AZ, USA) incorporates proprietary alloys into all of its load cells. These materials eliminate the need for additional resistors to compensate for temperature effect on elastic modulus, resulting in improved performance in the presence of thermal gradients or transients and increased sensitivity and fatigue life. In addition, constant voltage or current excitation can be used. This technology has been applied to a line of minibeam strain-gauge load cells which have no moving parts and cannot wear out or fall out of adjustment. The line is also available with a moisture-resistant coating to protect the 25--250-lb load cells from high humidity and periodic exposure to condensation. Applications include force and tensile testing, thrust measurements, and OEM scales.
Terwin Instruments Ltd.
Two-wire loop-powered pressure transmitter systems feature detachable pressure sensors and a self-check calibration feature that enables users to maximize accuracy even when process temperatures reach 450°C. These transmitters were initially designed to convert a low-level millivolt output into a process signal of 4-20 mA dc in the 2000-series extruder pressure transducers manufactured by Terwin Instruments Ltd. (Grantham, Lincs, UK). Because of the flexible design, however, the instruments can be used with any 350-(omega) strain-gauge pressure transducer or load cell. The detachable pressure sensors are available in four stem lengths--92.5, 152.5, 228.6, and 305 mm--and they feature an abrasion- and corrosion-resistant diaphragm. Pressure ranges are 0-35 to 0-2000 bar. Thermocouples can be incorporated into the sensors for dual pressure and temperature readings. The stainless-steel transmitters conform to IP65 (NEMA 4) as well as the Electromagnetic Compatibility Directive.
