Over the last 75 years, sensors have played an increasingly significant part in the advancement of medicine.
Medical sensors for monitoring people’s vital signs, including temperature, blood pressure, heart rate, and respiration rate have become increasingly sophisticated. But sensors are also useful for measuring the vital signs of medical equipment.
Glass bulb thermometers have been used for decades to take body temperature. In the 1970s, they were replaced by electronic versions with digital displays. These were minimally invasive devices and were required to be inserted somewhere into the patient’s body.
Today, the most common devices are thermopile-based non-contact temperature sensors, which operate like a tiny infrared camera. They measure the radiated thermal energy of the skin and provide an output signal proportional to skin temperature. They can take a reading in just a few seconds, and being non-contact, can help to avoid the spread of bacteria and viruses.
Packaged in a small TO-5 or TO-18 hermetically sealed enclosure, they can easily be mounted on a PC board. The thermopile is a miniature array of dozens or hundreds of thermocouple elements on a silicon chip. The chip is designed so the junctions on its topside are exposed to the incoming IR radiation while the backside is attached to a metal header and remains at ambient temperature. The thermocouples are connected in series so that their outputs add. The sum of the signals provides a usable output with an amplitude in the tens of millivolts.
A cutaway view of a typical thermopile is shown in Figure 2. The chip is mounted so it “looks” through a window that is transparent only to infrared wavelengths. This helps eliminate interference from visible light. As thermal energy from a heated (or cooled) object enters the window, it impinges onto the thermocouple array and changes its top surface temperature from ambient. A separate reference temperature sensor is attached to the metal header to measure ambient, so the differential signal between the thermopile and the reference can be used to calculate the actual temperature of the object being measured. With some downstream signal processing and compensation, accuracies in the ±1% – ±2% range are easily attainable.
Arterial Blood Pressure Monitoring
A practical blood pressure cuff was developed in 1905 and is used to this day. The modern sphygmomanometer is easy to use, but only provides an indirect measurement with a broad accuracy range. A manual BP cuff in the hands of a trained operator can achieve accuracies of nearly 98%. Electronic and digital blood pressure devices typically achieve a 70% accuracy. In both cases they only provide an average reading.
In recently developed medical procedures, it’s been discovered that a direct BP reading at the site of a surgery provides better data to the surgeon and better outcomes for the patient. TE Connectivity’s Sensors Business Unit has recently introduced their Intrasense micro blood pressure sensor. Its most striking feature is the extremely small size — overall dimensions are 800 µm L x 270 µm W x 70 µm H.
The Intrasense is a MEMS-based absolute pressure sensor with a clinical range of -300 mmHg to +500 mmHg. The half bridge design utilizes two piezoresistive elements on the MEMS die that change their resistance values as pressure is applied. The signal is delivered to an amplification and compensation PC board at the proximal end of the 300mm wire leads.
The sensor can be placed at the tip of a very fine catheter or guidewire, and then used in remote locations inside the body, such as heart chambers, inter-cranial arteries, or even inside the kidneys, during critical surgical procedures.
Heart Rate and Respiration Rate Monitoring
In collaboration with the Stanford Research Institute (SRI), TE Connectivity has developed a demonstration smart chair that can measure both the heart rate and respiration rate of a person who simply sits still in it. The chair has piezo polymer film elements strategically placed in the seat and back. These sensing elements detect both heartbeat and breathing.
Piezo polymer film is a unique material made from polyvinylidene fluoride (PVDF). With special manufacturing techniques, this film can be made piezoelectric, a property where materials generate an electric charge when they’re subjected to mechanical stress. The film is very thin (28μm), pliable, and will easily conform to, and detect, the stress loads inside the seat cushion as someone sits down.
During respiration, the body’s center of gravity moves slightly as the rib cage expands and contracts with each breath. The piezo elements in the chair detect these dynamic changes and provide a usable signal to the electronics. For the heartbeat, the sensors utilize ballistocardiography, which is the detection of a pressure wave normal to the skin that’s produced by arterial pulsing. Future versions of this demo will add load cells to the feet of the chair, so the occupant can be weighed. Adding several more piezo sensors will also help to detect the physical size of the occupant. Along with weight data, a body-mass index (BMI) can be calculated. This chair becomes an excellent tool for home healthcare applications because no trained medical professional is needed, and a mini “check-up” can be performed anytime — just sit down quietly and relax.
Medical Equipment Self-Monitoring
Therapeutic and surgical medical machines must perform with very high levels of accuracy. To ensure the instrument is operating properly, designers are now adding sensors that keep track of critical machine functions. These sensors can operate in one of two ways. They can be made part of a feedback and control loop that measures a parameter and then makes adjustments to the equipment to keep it operating within specified ranges. The sensor can also be used as a limit alarm. When a parameter goes out of specification or the machine malfunctions in some way, the sensor alerts the operator about the fault condition and can even shut down the machine to protect patient safety.
The sensors built into modern medical ventilators are good examples of this technique. The block diagram in Figure 5 shows the inner workings of a typical machine. Note that all but one of the sensors are monitoring the functions of the ventilator. The CO 2sensor is the only one monitoring a function of the patient.
There is a trend in medicine to move patients out of hospitals and into home healthcare settings. Patients are more comfortable at home. They get attention from familiar caregivers and recover more quickly from ailments. The sensors built into home healthcare equipment make the machines reliable, simple to operate, and eliminate the need for constant attention from medical professionals. The results are improved patient safety and better medical outcomes.
Work is underway throughout the medical industry to incorporate more sensors into the machines and procedures used by doctors and their teams. The addition of sensors helps keep an eye on the functions and performance of the equipment, which allows the medical professionals to turn their eyes toward the patient and create better clinical results. Whether being used to monitor a patient, or monitor the medical machine being used to treat patients, the growing use of sensors will bring significant benefits to the world of medicine.