Applications, Electronics & Scientific Guide

MEMS Sensors for Biomedical Applications

Especially in medicine components cannot be small enough. Therefore, sensors based on MEMS technology find their way into this field, for example by implanting it in the body of the patient. There, they then record medical values early, which allows doctors to make an early diagnosis. Also planned are nerve prostheses. But they also often occur outside of the body in medicine.
The birth of microelectromechanical systems (MEMS) dates back decades, but some experts believe that the potential of these tiny components is far from exhausted. The integration of electrical circuits and mechanical structures on a single substrate, the size of which is in the micrometer range, provides the underlying technology for most wearables and is essential for many smartphone functions.

MEMS also have far-reaching implications for biotechnology development as they allow scientists to explore and influence biological and chemical compounds in the body and its many complex subsystems. This may open up new possibilities in the future for the diagnosis and prevention of diseases as well as chronic diseases. Currently, the use of MEMS technology in neural prostheses is being explored, which would allow people with visual impairments to regain their eyesight or regain mobility for physically disabled persons.

MEMS are a revolutionary innovation in medical diagnostics and healthcare and are already being used successfully in pressure sensors. Today, physicians implant MEMS-based pressure sensors that communicate wirelessly into the body, gaining valuable insights into the condition of organs and arteries. Recent progress in MEMS packaging has been crucial.

But not only for implantation into the body is the small, inconspicuous form factor of MEMS pressure sensors of significant advantage, but also when used in devices: it allows smaller dimensions and thus ensures better portability of the devices. Also, since the costs are relatively low, the feasibility of disposable sensors becomes more likely. These could be used, among other things, in blood pressure monitors and ventilators for patients in hospitals and emergency rooms. They can also be used in dialysis machines and infusion pumps.

The integration of MEMS inertial sensors into consumer electronics also influences the way care providers can provide patients with increased fall risk by monitoring patients’ movements and alerting caregivers at an early stage when increased attention is needed. Also in the treatment of scoliosis, doctors now rely on this technology. It measures how often and how closely patients wear corsets to overcome barriers to progress.

A body in motion

Many manufacturers are already well positioned for the development and production of MEMS sensors because their output is similar to that of traditional semiconductors. At the 2017 Medical MEMS and Sensors Conference in Santa Clara, California, Mark da Silva, Ph.D., senior engineer in high-performance sensors at Analog Devices, held a presentation titled “Ultra Low Power Implantable Inertial MEMS Sensors.” E.g., implantable MEMS inertial sensors with ultra-low energy consumption). In it, he explained in which direction semiconductor manufacturers are moving. For example, implantable MEMS inertial sensors could monitor the movement of extremities in real time; however, similar sensors could be implanted in the back to stimulate the spinal cord for pain relief.

Low-energy MEMS accelerometers such as Analog Devices’ ADXL362 series are already available in the market and are used in a wide range of applications, including hearing aids and home care equipment. These three-axis accelerometers capture both dynamic and static accelerations, consuming only 10 nA in sleep mode and 270 nA in motion-controlled active mode. Because these sensors do not subsample unlike some other MEMS sensors, sampling at 8 and 12-bit output resolution occurs across the entire bandwidth of the sensor. This resolution can be 1 mg / LSB in the range ± 2 g. The standard noise level for this product family is 500 μg / √Hz. However, there is also a reduced noise mode that reduces this value to 175 μg / √Hz. As shown in Figure 1, the ADXL362 also features anti-aliasing filters, a temperature sensor, an analog-to-digital converter (ADC), SPI and digital logic.

All functions of the ADXL362 can be evaluated using the Arduino-compatible Eval-ADXL362 ARDS shield, which has an LC screen that displays sensed sensor data, such as slope or temperature.

TDK Invensense’s ICM-20948 is a nine-axis motion detection tracking module that includes a three-axis gyroscope, a three-axis accelerometer, a three-axis compass, and a Digital Motion Processor (DMP) a multi-chip module with dimensions of 3 × 3 × 1 mm are integrated. Each sensor has a self-test mode and the integrated motion processor handles the calibration and motion processing algorithms, which relieves the host processor.

Negative pressure

As mentioned above, MEMS are used in the medical field, especially in pressure sensors. For example, they are often used in ventilators for monitoring the respiratory rate, in dialysis machines for measuring and regulating blood pressure or eye surgery. For example, MEMS pressure sensors can detect blood oxygen, carbon dioxide, calcium, potassium, and glucose levels and provide the right pressure in infusion pumps. Also, they are essential for CPAP ventilators for the treatment of sleep apnea syndrome and wound healing vacuum therapy.

In non-invasive blood pressure monitoring, the system uses the well-known blood pressure cuff to measure systolic and diastolic blood pressure. The cuff applies pressure to the arm and pushes the artery in a controlled manner until blood flow ceases. After that, the pressure drops in a controlled manner until the blood flow begins again, which is usually audible with a stethoscope – this gives the systolic value. If the pressure in the cuff decreases again, the blood flow in the artery is no longer perceptible to the doctor – now the diastolic pressure is reached. With the help of MEMS-based sensors, non-invasive blood pressure monitoring can be mostly automated. Measured by volume, this is one of the most significant applications for MEMS pressure sensors.

Silicon Microstructures is a leader in the design and manufacture of MEMS pressure sensors, and the SM4421 is an excellent example of a pressure sensor used in this field. A SOIC-16 package houses a MEMS sensor element and an ASIC chip for signal conditioning to deliver a 14-bit temperature compensated output through an I 2 C interface. Available in gauge, differential pressure and asymmetric configurations, the SM4x21 series instruments operate in force ranges from 0.17 to 1.03 bar and can be used in a variety of medical applications.

The future of MEMS in medical technology

MEMS have the potential to turn different areas of medicine upside down completely. The concept of a lab-on-a-chip system paves the way for fast, cost-effective diagnostics in remote locations and developing countries. The technologies of MEMS devices are also used in the development of nanotechnology – two areas that are closely related. In the future, research in the field of microfluidics and microelectronics will merge even further, and the introduction of new materials such as carbon nanotubes and graphene opens up even more potential.

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