by Randy Frank, Contributing Editor
In medical and healthcare applications, sensors cover a variety of aspects from routine to critical patient measurements and more.
There are several different sensing techniques that specifically address well being and health monitoring. The increasing importance of sensors is noted by the 2012 market research report’s (World Sensors Market in Healthcare Applications 2012-2017) projection that sensors in healthcare applications will reach $13.11 billion by 2017. This analysis includes the use of temperature, pressure, chemical, flow, level, position, image and biosensors.
Many sensors are increasingly connected wirelessly to remote monitoring equipment. To participate in this exciting and growing area, sensor manufacturers strive to provide sensors that address the measurement requirements of healthcare applications better than previous products and, in many cases, other industrial applications benefit from these improvements as well.
Temperature is number 1
For medical applications, temperature sensing devices such as digital thermometers have increased the use of temperature sensors. Measuring a person’s body temperature allows a caregiver or patient to determine if they are ill or if a prescribed medication is working. While 98.6° F is the average normal body temperature, it can vary by a number of factors including, age, person, time of day, where on the body the temperature was taken and other factors. Since a temperature reading of 1 to 1.5° F above normal indicates a fever, the accuracy of the reading is quite important. However, other sensor parameters must be satisfied.
“In portable medical applications, the critical parameters of concern are power and solution size,” said Emmy Denton, an applications engineer at Texas Instruments.
Texas Instruments’ LMT84 analog output temperature sensor targets these critical areas. The CMOS integrated circuit (IC) uses the temperature sensitive properties of a transistor’s base to emitter junction as the based sensing technique. Consuming less than 8.6 µW without external support circuitry, the sensor maintains better than 1° C accuracy from -50° C to 150° C and comes in a small 2×2 mm SC70 package.
Medical pressure measurements range from high pressures such as hyperbaric oxygen that can be 6 bar (87 psi) to low pressures for respirators and ventilators that can be 4 kPa or less. Knowing the pressure range is only one of the criteria for selecting the right pressure sensor.
“What is most often overlooked when selecting a pressure sensor is the bandwidth to signal to noise (S/N) tradeoff,” said Tim Shotter, Director of New Product Development and Applications at All Sensors Corp. “It is as fundamental a consideration as the gain bandwidth product term when selecting an op amp.”
Along with temperature, pressure provides one of baseline measurements for patient health. In addition to blood pressure, other common medical pressure measurements include:
The tradeoff considerations are especially important in medical and other applications where dynamic signal analysis is performed. As with most analog systems, the greater the bandwidth, the lower the S/N ratio becomes. While this is generally understood, the impact of the compensation method on the bandwidth to S/N curve is less obvious.
As Shotter explained, “In general, a basic sensor (which has no factory compensation), has the greatest potential for the highest performance when viewed in terms of the bandwidth to S/N curve.” However, this design approach also has the greatest cost and effort to amplify, calibrate and compensate for temperature effects.
To solve the problem, users cannot settle for the standard amplification that most MEMS pressure sensor companies supply. In most cases, for high performance applications, users will have to perform the amplification and signal conditioning themselves to get the performance they need. An All Sensors solution to this problem is provided in its BLVR Series Basic series and Low Pressure Millivolt Output family.
General purpose amplified and digital sensors offer excellent calibration and thermal compensation; however, their S/N tends to be lower (compared to a Basic or Millivolt sensor) due to the use of lower power op amps and quantization noise. “Also, digital devices may suffer from factory prescribed update rates (bandwidth) which may or may not follow the application requirement unless the factory includes appropriate options for the update rate,” said Shotter.
A trimmed Millivolt sensor is a good compromise when considering the bandwidth to S/N. In this case, the part only needs user provided amplification and the amplifier noise can be tailored to the application. The output level of the Millivolt sensor is generally lower than a basic sensor, so the overall S/N curve is impacted compared to a Basic sensor even if the same op amp is used for both.
Targeting medical applications and others, All Sensor’s BLVR Series Basic Sensor and Low Pressure Millivolt Output products are based on a dual die technology to reduce all output offset or common mode errors. Both series also incorporate another sensor-level design technology to reduce the overall supply voltage while maintaining comparable output levels to traditional equivalent basic sensing elements. The Low Pressure Millivolt Output family addresses 0- to 0.5-in. H2O to 0- to 30-in. H2O pressure ranges and the BLVR Series Basic Sensor series covers 0- to 1-in. H2O to 0- to 30-in. H2O pressure ranges.
Air in line sensing
When fluids flow in tubes for patients, the presence of air in the line can create significant problems. For example, an air bubble in an intravenous (IV) line can cause an embolism if the bubble gets into the blood stream. However, the size of the bubble is important and false/nuisance warnings can be a problem, too. To detect the presence of air, Morgan Technical Ceramics (MTC) uses piezo ceramics capability in its ultrasonic sensors.
“Our air in line sensors are a critical component in drug delivery and dialysis equipment, detecting bubbles of air in the delivery lines through ultrasonic waves created and interpreted by the sensor,” said Rich Miles, R&D Manager of Morgan Technical Ceramics Ltd.
The Air in Line (AIL) sensor family includes a bubble detector for a 4-5 mm tube. Operating at a nominal resonant frequency of 1.45 MHz, the Model No: 09168/000 bubble detector can detect a bubble as small as 0.5 μL. Mounted in any orientation, the sensor is designed to be dry-coupled to PVC or silicone tubing. For some of its sensors, bubble sizes down to 4 µL in volume or just 2 mm in diameter can be detected. Targeted medical applications for the AIL sensors include infusion pumps, enteral feeding pumps and dialysis equipment.
To simplify the design-in and extend the use of this technology, MTC offers ultrasonic air-in-line sensors and ultrasonic bubble detectors as stand alone units or complete with integrated control electronics and additional integrated sensors. One possible option is an air-in-line sensor combined with optical sensor to detect that the housing is correctly closed, for fail-safe operation.
Accelerometers beyond smart phones
Besides pressure sensors, MEMS technology is also used for sensors that address other measurement requirements in the medical/healthcare market.
“Accelerometers have become more prevalent in medical and wellness applications,” said Jeannette Wilson, Product Line Manager for the Gyro and Combo Sensors Sensor and Actuator Solutions Division at Freescale Semiconductor.
Used in smart phones and tablets for enabling basic functions such as portrait/landscape orientation, MEMS accelerometers have several existing and potential applications in medical and wellness equipment with graphical displays. “They can also be used for safety of patients such as monitoring the tilt in hospital patient beds and are currently being tested in clinical studies for pain delivery systems,” said Wilson.
Freescale’s Xtrinsic MMA9559L provides an intelligent motion sensing platform for medical and other consumer applications. The sensor embeds a high precision 3-axis accelerometer and a 32-bit ColdFire V1 microcontroller (MCU) to provide autonomous sensing with local compute capability to reduce parts count and overall system cost for medical applications.
With its embedded MCU, the +2g, +4g, +8g 3-axis digital accelerometer can also handle the inputs from a gyroscope, as well as pressure, touch and magnetic sensors and up to 12 sensors components. In addition to monitoring the tilt of hospital patient beds, other accelerometer uses in medical and wellness applications include:
• Measure depth of cardiopulmonary resuscitation (CPR) chest compressions
• Clinical trials for implants to fine tune delivery of electrical stimulation to nerves in the spine to reduce chronic pain (posture based)
• Virtual nurses (robots)
• High-end blood glucose meters with graphical displays (portrait/landscape)
• High-end heading aids (using motion to turn on hearing aid or change volume)
• Patient activity monitors
• Sports watches
• Gait analysis
• Concussion detection
Along with these five measurement techniques, sensor companies are developing the next generation of sensors in many other areas including chemical, flow, level, position, image and biosensors The ongoing advances in a great variety of sensors promise to provide a higher level of awareness in healthcare. Hopefully, this translates into a healthier population.
Morgan Technical Ceramics