The ability to design reliable, accurate sensor functionality within an ever-shrinking footprint is critical in a wide range of applications—and a difficult requirement.

　　The ability to design reliable, accurate sensor functionality within an ever-shrinking footprint is critical in a wide range of applications—and a difficult requirement. However, among other benefits, miniaturization (including downsizing modular systems) can free up valuable board space and reduce interference by offering designers greater flexibility in placing components on a printed circuit board (PCB).

　　The medical industry remains a major driver of sensor miniaturization and ultra-reliability, as internal body applications require extremely small, precise sensors due to their significant application potential in medical procedures. Arthroplasty (force sensors for joint replacement) and spinal fusion. In external use, sensor suppliers must provide flexible mounting options to ensure that the sensor is placed as close as possible to the patient and/or fluid (e.g., medication, blood, or water) for accurate and precise measurements.

　　The development of small, portable medical devices to fit into the confined spaces of hospitals also enables caregivers to quickly and easily move devices from one patient to another. Sensor miniaturization helps achieve this goal in designs such as mobile medical devices, wearable technology, and handheld instruments.

　　Many elements must come together to achieve this size reduction. For example, in addition to the physical and packaging requirements that fit into the smaller end product itself, there are budgetary and technical issues, such as continuously reducing BOM costs by integrating multiple sensor functions into a single, smaller package.

　　Putting the development of medical sensors into context, let's briefly review the history of the pacemaker.

　　Colombian physician Alberto Vejarano Laverde collaborated with Colombian electrical engineer Jorge Reynolds Pombo to build the first pacemaker. This weighed 45 kg or 99.2 lbs. and was powered by a 12-volt car battery, with electrodes connected to the heart and used to keep 70-year-old priest Gerardo Florez alive. Once silicon transistors entered the picture, things became somewhat easier. The first fully implantable pacemaker was in 1958 in Sweden. It was connected to electrodes connected to the heart myocardium. It lasted three hours. The second implanted device made it two days. The first implantable pacemaker recipient, Arne Larsson, actually received a total of 26 devices and died at age 86.

　　As with most electronics innovations, pacemakers have become smaller and more functional. For example, recent advances include leadless pacemaker devices that perform as well as their traditional counterparts and are 10% the size of traditional devices, with the miniaturized device placed directly inside the patient's heart. This is accomplished via a catheter without surgery. The integration is high, even in its small state, as it contains activity sensors, metabolic sensors, and dual sensors capable of measuring temperature, posture, and the consequences of arrhythmias.

　　Pacemakers and other heart-related technologies have been constantly evolving because heart disease is a leading cause of death in developed countries, and early diagnosis, treatment, and maintenance are critical. Biomedical engineers have successfully developed miniature instruments to open blocked arteries and treat cardiovascular diseases. However, these tools remain susceptible to infection. Miniature nanotechnology sensors will be able to sense and monitor biological signals such as the release of proteins or antibodies in the heart or inflammatory events.

　　Beyond pacemakers, other announcements targeting medical goals have also regularly made news headlines recently. Recently, the University of Twente unveiled a prototype of the world's smallest hand force sensor1 for measuring the motor function of rehabilitation patients. It is smaller than a fingertip and can measure the force exerted by your hand. This small sensor can be built into gloves and prosthetic devices, or designed into bicycle pedals, shoe soles, or touchscreens. In addition to measuring total force, the sensor can also measure the direction of the applied force, which is important for a wide range of applications. The device measures the load involved in manual labor and the performance of athletes or rehabilitation patients as a means of improving skills, and it addresses the challenge of having a unit small enough to measure the force exerted between fingers and the object being grasped.

　　Current developments in miniaturization of sensors for medical applications are rapidly moving out of the laboratory and into the everyday working world. Consider human respiration, which requires measuring the airflow inside and/or outside the human lungs during breathing. Monitoring and assessing patient respiratory function during conscious sedation is important, which is a pharmacologically induced state of relaxation where the patient remains awake and cooperative during, for example, dental treatment. Understanding the respiratory cycle can also be used to detect sleep apnea. Typically, these techniques indirectly measure flow in a breathing tube by sensing the pressure difference caused by flow in a shunt configuration at two ports located along the side of the tube.

　　Pressure sensors can be integrated into many designs because they offer many mating connector and packaging options (surface mount, DIP, SIP), as well as selectable outputs (analog or digital). All Sensors Miniature series pressure sensors are used in medical instruments and respiratory breathing, saving space and providing high performance. The H-GRADE is a high-accuracy version of the company's millivolt output pressure sensor (Figure 1). They offer calibrated millivolt output with excellent output characteristics, reducing output offset errors due to factors such as temperature changes. The supplier also notes that the sensor provides stability during warm-up and stability over long periods. In addition, the sensor uses a silicon, micromachined, stress-concentrated enhanced structure to provide a linear output for measuring pressure.

　　This series of sensors is suitable for non-corrosive, non-ionic working fluids such as air, dry gases, etc. The output of the device is proportional to the supply voltage and can accept any DC supply voltage up to +16 V operation. Features include 0 to 4"H2O to 0 to 100 PSI pressure ranges, 0.5% linearity, temperature compensation, and calibrated zero and span.

　　Another example of a sensor for traditional medical applications is the MLX90615SSG-DAA-000-TU ultra-miniature Melexis Technologies intelligent contactless infrared thermometer (Figure 2). Used in ear thermometers and fever thermometers, as well as continuous temperature monitoring, the device is considered the world's smallest "intelligent" infrared thermometer, with a diameter of only 4.7 mm (0.185 inches) and a height of 2.7 mm (0.106 inches).

　　The signal processing chip is integrated into the transistor-style package, offering a plug-and-play advantage for a fully calibrated thermometer. With its low-noise amplifier, 16-bit ADC, and its DSP unit, high accuracy and high resolution of the thermometer are achieved; for example, accuracy up to ±0.1˚C in the critical range of 30-40˚C.

　　In the medical field, accelerometers are used to monitor movement. An example of a miniature sensor for motion and fitness devices is the ADI's 3-axis ADXL 335 accelerometer (Figure 3).

　　The ADXL335 is a complete 3-axis accelerometer system. It has a measurement range of at least ±3 g and includes a polysilicon surface micromachined sensor built on a silicon wafer and signal conditioning circuitry to achieve an open-loop acceleration measurement architecture. The output signal is an analog voltage proportional to acceleration.

　　Polysilicon springs suspend the structure above the wafer surface and provide the ability to resist acceleration forces. The deflection of the structure is measured using differential capacitors, which consist of independent fixed plates and plates attached to the moving mass block. The fixed plates are driven by a 180° out-of-phase square wave. Acceleration causes the moving mass to deflect and unbalances the differential capacitors, producing a sensor output whose magnitude is proportional to the acceleration. Phase-sensitive demodulation techniques are then used to determine the magnitude and direction of the acceleration.

　　The demodulator output is amplified and brought off-chip via a 32kΩ resistor. The user sets the device's signal bandwidth by adding a capacitor. This filtering improves measurement resolution and helps prevent aliasing. The device can measure static gravitational acceleration in tilt sensing applications, as well as dynamic acceleration caused by motion, shock, or vibration.

　　Summary

　　As we have seen, sensors play a crucial role in many medical applications, and more sensors are being developed based on the ongoing trend of miniaturization. The transition from a very large pacemaker to one that fits within the heart is a remarkable feat, combining advancements in medicine and sensor technology.

　　While small size doesn't always translate to cost-effectiveness, it often translates to high integration without compromising performance, which is crucial in medical applications where size, ease of use, portability, reliability, accuracy, and cost are paramount.

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