The Evolutions of Medical Sensor Design

TE Connectivity

To keep pace with innovations and trends in medical device design, sensor technology continues to evolve and address the demands of a growing and increasingly connected medical device industry.

As more and more devices incorporate Internet of Things (IoT) technology and the ability to make decisions and analyze data using artificial intelligence (AI), selecting the right sensors is critical. Today’s medical devices use both invasive and non-invasive sensor technologies. Invasive sensors require specialized packaging and compatibility, usually in a disposable package, for applications such as arterial blood pressure monitoring during surgery, or measuring temperatures within a catheter assembly. Non-invasive and non-contact sensors have a broader use-case, with both mechanical and electrical design considerations.

These trends and design considerations for non-contact medical sensing applications are guiding component selection in the latest generation of medical device designs.

Survivability
The sensing elements that measure temperature, vibration, position and other properties are small and delicate. Survivability is a critical consideration for non-contact sensor selection. In addition, sensors can be packaged within an assembly through non-contact measurement for position sensing. Demand for anisotropic magneto-resistive (AMR) position sensors has increased with their ability to be packaged and sealed inside a device while measuring a magnetic scale outside the assembly.

For example, this type of sensor can be used in a prosthetic joint to measure the rotation of a user’s knee or ankle, in order to compensate for movement and create a more natural gait pattern. Because that sensor is packaged within the knee assembly, it is protected from the environment and from general wear and tear; the assembly allows independent rotational movement between the sensor and the moving limb without physical contact.

Sterilization
These sealed assemblies also address sterilization concerns. A sensor that is designed within a device remains protected against extreme temperatures and moisture at the component level.

For instance, non-contact sensors used for motion control can be used in assemblies that keep the sensor submerged in a non-conductive hydraulic oil. This further allows the sensor to be embedded within the assembly without external components being exposed.

Wearable medical devices face a different set of challenges as they may be exposed to sweat, chlorinated water and other environmental factors that can cause corrosion issues and/or electrical shorts without proper protection. Specialty coatings can be applied to this category of sensing devices without impacting their ability to detect and measure accurately.

Miniaturization
In a survey conducted by TE Connectivity, 85 percent of participants overwhelmingly agreed that miniaturization is a critical design consideration. Miniaturization has already allowed sensors to be smaller and lighter than in previous generations of medical devices, freeing up board space and enabling more compact devices overall.

Increased Accuracy with Digital Sensors
Sensors used in medical applications must be highly accurate, a consideration that makes digital sensors more desirable on account of their more precise and robust outputs.

For example, digital thermopile temperature sensors can deliver high accuracy, ±1 degree C readings within temperature ranges from 0 to 100 C. When customized to accommodate a wider range of applications for intensely harsh environments, these sensors can deliver high accuracy of ±4.5 degrees C at up to 300 C.

Despite the lower cost of analog sensor components, the design and configuration of digital sensors provide lower overall costs along with more options – for example, multiple output signals from the same device, thus reducing the size of the circuit board and/or the entire design.

Digital Signals and Scalability
At the same time, digital signal processing plays a role in scalability – another fundamental consideration in today’s medical devices. Traditional analog output signals require some level of conversion for modern electronics to read and process the data. However, onboard digital signal processing reduces the calibration time during system or device manufacture, leading to greater accuracy. Since the sensor manufacturer’s calibration equipment is designed to test and qualify the sensor, OEMs are required to invest less to duplicate the system requirements needed to manage signal processing from analog to digital.

SMT Technology and Scalability
While the medical industry in the U.S. is pivoting to reduce hospital visits, admissions and admission times, the demands of patient support and the need for monitoring equipment both remain consistent. Sensors designed for scalable, automated production are the more desired option in the medical space today.

Surface-mount technology enables design engineers to embed sensors within the electronics of the assembly. Photo optic sensors offer traditional lead-frame designs which are hand-soldered into assemblies and are now packaged for SMT designs. The reflow-solderable packaging allows engineers to design sensors into assemblies that are embedded through pick-and-place machines, increasing overall quality while reducing manufacturing time and cost.

This article is adapted from the TE Connectivity white paper “Considerations for Medical Sensor Technology Design.”