Ultra-miniature sensors are enabling advanced procedures and treatments across a wide range of medical devices, from catheters and neuro interfaces to wearables. But as electromagnetic sensors get smaller, trade-offs begin to emerge — lower sensitivity, less tolerance for environmental influences, and greater susceptibility to interference — underscoring the need for robust testing to ensure accurate, reliable tracking.
Testing in isolation isn’t enough. Ultra-miniature sensors must be tested under real-world conditions to measure how the sensors truly perform in clinical and surgical settings.
This article explores common sensor testing methodologies, reveals where test plans often break, and outlines what medical device manufacturers and OEMs must do to ensure their sensors perform as expected to achieve desired patient outcomes and commercial success.
Advantages and Limitations of Proxy Testing
Testing ultra-miniature sensors within fully assembled devices is the gold standard, but it’s not always practical — especially when sensors and the devices they go into are often developed in parallel at different facilities.
Gantry systems, where sensors are moved through thousands of locations in space, are the next best alternative, but they’re time-consuming and cost-prohibitive. It’s important to work with a contract development and manufacturing organization (CDMO) partner who uses the gantry system, as Forj Medical does, because it may be the best choice for testing highly complex projects.
In most cases, however, medical device manufacturers rely on proxy tests to measure sensor performance. For example:
Sensitivity and linearity testing: Isolates the sensor in a Helmholtz coil to measure field strength.
Inductance and resistance testing: Confirms the correct number of turns on the microcoil and directly correlates to sensitivity.
In this scenario, inductance and resistance testing serves as a proxy for sensitivity, which in turn serves as a proxy for tracking performance.
This is a highly accurate methodology for predicting and confirming sensor performance outside the device: custom electromagnetic (EM) sensors are designed to achieve a specific range of sensitivity and gain, and if tests show a sensor is within those ranges, designers know the sensor will reliably track.
However, proxy testing has inherent limitations: when sensors are not tested in alignment with use cases, even well-designed proxy test plans can break down.
Why Test Plans Break
Testing in isolation is not analogous to testing in a device — and there is a difference between sensor performance and device performance. A sensor can meet every requirement in a controlled test environment, yet the device can fail once the sensor is fully integrated during design verification (DV). The impact can be monumental: in one extreme case, an OEM’s project was delayed an entire year because its sensor failed in DV.
The core problem is that integration testing often occurs too late, when design changes are costly — not just in redesign expense, but also in delayed product launch and lost market opportunity. Test plans break down when sensors aren’t tested in environments that simulate their real-world use cases.
Key considerations include:
Materials: Understand and measure how adjacent materials impact sensor performance. For example, if a sensor is housed inside or nearby magnetic material, it can interfere with the signal and impede accuracy.
Location: Sensor placement matters in small, flexible devices like catheters. Bend locations can create structural integrity issues, as repeated flexing can damage delicate sensors, degrading tracking performance over time or causing total signal loss.
Dimensional constraints: Understand the trade-offs between sensor size and sensitivity. For example, a sensor might need to be elongated to achieve the desired sensitivity, or sensitivity requirements might need to be adjusted to meet size constraints.
Field generator: Sensors must be tested with the same field generator that will be used in a clinical setting or performance tests will give inaccurate results.
Electrical interference: Electronics located within the device can introduce noise that disrupts signal quality.
Vibration and motion: Mechanical components on the device — such as motors that oscillate at high speeds — can introduce vibrations and magnetic fields that impact tracking accuracy.
While large sensors are highly sensitive and can detect magnetic fields in noisy environments, ultra-miniature sensors are far less sensitive and therefore much more susceptible to any potential interference. When sensors are only tested in isolation, test plans break because they do not account for their in-device environments.
How to Ensure Sensor Accuracy in Clinical Settings
The most common failure point is not the testing method itself but the timing of integration. Testing a sensor outside of the device does not measure its performance in a true clinical setting, yet integration testing is often delayed until later development stages when issues are the most expensive to fix.
A better approach is to test ultra-miniature sensors under real-world conditions as early as possible. That doesn’t mean you need a fully assembled device for testing, but it does mean you should simulate the sensor environment. It means taking a more holistic view of sensor performance, one that accounts not only for isolated tracking results but also device materials, sensor location, electrical noise, vibration, and other factors early in the design process.
Rapid iteration is also key. At Forj Medical, for example, it’s not uncommon to build and test a sensor on a Monday, then redesign and retest by the end of the week. OEMs that are developing sensor-driven devices should evaluate their CDMO’s approach to sensor testing: are they solely focused on performance in isolation, or are they designing, developing, and testing for how the sensor will perform within your device, over the long term? The difference can determine whether a project is commercially viable or fails later, when the stakes are highest.
Understanding sensor performance in the context of its intended use case is critical, and early testing — and adjustments — not only ensure tracking accuracy and patient safety, but also accelerate time to market, reduce overall costs, improve clinical outcomes, and increase the likelihood of commercial success.
This article was written by Dan Hanson and Aaron Harpster, Product Development Engineers at Forj Medical. Headquartered in St. Paul, MN, Forj Medical helps OEMs bring high-stakes medical devices to market through a design for manufacturing philosophy and highly scalable end-to-end production. For more information, visit here .
