Designer’s guide: Sensors for medical applications

Over the past few years, there have been remarkable breakthroughs in the field of medical technology, leading to a significant transformation in patient care and diagnostic procedures. The significance of sensors is seen in the continually evolving world of medical technology, where these devices serve as a fundamental component of electronic health systems that measure and monitor diverse physiological data, such as blood sugar levels, blood pressure and respiration rate.

The field of medical applications necessitates a significant degree of precision and dependability. The design process necessitates meticulous consideration of various elements, including sensor accuracy, response time, battery consumption and compatibility with the human body. In addition, the sensors must conform to rigorous requirements to guarantee the safety of patients and compliance with regulations.

Types of sensors in medical applications

Depending on the specific capability they implement, or the physical attribute they measure, medical sensors can be grouped into the following main classes:

• Electrochemical sensors: These sensors are extensively used to measure several parameters, including glucose levels and blood gases. The functionality of these sensors is based on the chemical reaction that occurs between the target analyte and electrodes, resulting in the generation of an electrical signal that is directly proportional to the concentration of the analyte. Electrochemical sensors are commonly used in continuous glucose monitoring (CGM) systems, able to detect instantaneous glucose-level fluctuations and warn users of imminent hypo- or hyperglycemic events.
• Pressure sensors: These play a critical role in several applications, such as monitoring blood pressure and measuring respiration rate. Catheter-based pressure sensors for cardiovascular monitoring and blood pressure cuffs exemplify two distinct approaches to measuring pressure—namely, invasive and non-invasive methods, respectively. The use of microelectromechanical systems (MEMS) technology has made substantial contributions toward the reduction in size and performance improvements of pressure sensors.
• Flow sensors: In devices involving fluid dynamics, such as infusion pumps, flow sensors are indispensable. They ensure the precise and regulated delivery of fluids, which contributes to the safety and efficacy of medical treatments. Mass airflow sensors are primarily used in ventilation systems and respiratory care, as well as other medical applications that require high accuracy, repeatability and excellent signal-to-noise performance.
• Temperature sensors: Temperature sensors in medical applications encompass tasks like monitoring body temperature and regulating thermal therapy devices. In the field of temperature measurement, infrared sensors are frequently employed for non-contact applications, whereas implantable devices may use miniature thermocouples or thermistors to monitor internal temperatures.
• Optical sensors: These sensors employ light to quantify diverse physiological characteristics. Pulse oximeters, for example, use optical sensors to assess the oxygen saturation levels in the bloodstream by the analysis of light absorption at various wavelengths. Optical sensors are also used in identifying biomarkers and monitoring tissue oxygenation.

Key sensor features

The success of a medical device heavily relies on the quality and performance of the sensors. Here are some of the key features that engineers should consider in their design.

Accuracy and reliability

It is critical to ensure the precision and dependability of medical sensors. Protocols for calibration, rigorous testing procedures and continuous monitoring are fundamental aspects of sensor design. It is necessary to implement strategies for mitigating factors that could compromise accuracy, such as environmental conditions and signal interference. For example, blood glucose monitors for diabetic patients require sensors with a high degree of precision to ensure the correct insulin dosage.

In the case of wearables, thermal self-heating needs to be managed, as these devices are tiny and worn on the body. This, in turn, can impact accuracy in measuring vital signs like temperature, heart rate and blood-oxygen saturation (SpO₂). To maintain accuracy over time, medical sensors should have mechanisms for easy and reliable calibration.

Power consumption

Medical sensors often operate in resource-constrained environments, necessitating energy-efficient design and miniaturization. For protracted use, implantable and wearable devices require low power consumption, and miniaturization allows for unobtrusive integration with medical equipment and patient-worn devices. Frequently, medical devices must operate for lengthy durations, sometimes with limited access to power sources. Energy efficiency is essential for wearable devices and remote monitoring systems. Low-power sensors extend the battery life of battery-operated medical devices.

Biocompatibility

Because many medical sensors are placed near the human body, biocompatibility is of the utmost importance. To prevent unwanted reactions, the components used in the building of sensors need to be made of non-reactive and non-toxic materials. This is a key consideration for wearable technologies that are intended to maintain a continuous touch with the skin.

Security and privacy

Because medical sensors produce sensitive data about patients, it is necessary to implement comprehensive data security and privacy measures. Encryption techniques, secure data transmission protocols and compliance with healthcare data protection rules are all necessary for designers to implement into their products to keep patient information from being accessed inappropriately or stolen.

Key components in medical sensor design

While a medical device, comprised of one or more sensors, may adopt an architecture specifically tailored for the application, a range of components is required in most of these designs, including signal-conditioning circuits, processors and wireless transceivers.

Signal-conditioning circuit

The process of signal conditioning plays a vital role in the amplification, filtration and processing of signals obtained from various sensors. This is particularly important in cases where sensor signals are weak (small amplitude) or when establishing connections with microcontrollers (MCUs) and other processors. The proper functioning of obtained data relies heavily on the presence of analog front-end (AFE) circuits, which include instrumentation amplifiers and filters. These circuits are important in maintaining the integrity of the data.

For example, in a CGM device, an AFE is needed to interface with both the glucose electrochemical sensor(s) and MCU unit, usually provided with some sort of wireless connectivity, on which dedicated firmware is running. The MAX30131 from Analog Devices Inc. is an ultra-low–power AFE IC designed to interface with electrochemical sensors. Available in one, two or four independent channel versions, the MAX30131 has both DC current and electrochemical impedance spectroscopy (EIS) measurement capabilities.

In addition to high accuracy and precision, achieved through up to four 12-bit voltage DACs, up to four 16-bit current ADCs and one 16-bit EIS ADC, the MAX30131 features low current absorption (3.5 μA), an SPI interface and voltage monitoring for safety and compliance in a small package.

For vital sign sensing, one example is the ams Osram AS7038RB, which provides both a low-noise AFE for electrocardiogram (ECG) signal acquisition and a low-noise optical front end for blood-related measurement.

The AS7038RB uses algorithms to convert photoplethysmography (PPG) and ECG readings into digital high-resolution manometry, heart rate variability, continuous blood pressure and SpO₂ values. In addition, the detector provides interfaces for measuring skin temperature and skin resistivity using external sensors.

ams Osram’s AS708RB block diagram (Source: ams Osram)

MCUs and processors

MCUs and processors serve as the central processing units in medical equipment. Signal processing, data storage and connectivity with other devices or networks are among the tasks that they undertake. The selection of an MCU with adequate processing capabilities, minimal power consumption and the requisite interfaces is essential to ensure the smooth functioning of medical sensors.

Wireless transceivers

There is a current shift in the field of medical equipment toward the adoption of wireless connectivity, enabling the real-time transfer of data and facilitating remote monitoring. Bluetooth, Wi-Fi and other wireless communication modules enable the smooth integration of healthcare systems, granting patients and healthcare professionals immediate access to vital data.