Balco/nickel etched foil sensors: Next-generation resistance temperature detectors for medical applications

Editor's Note: This article originally appeared in the August 2025 print edition of Today's Medical Developments under the headline “Etched Balco/nickel foil: a great choice for medical devices”.

As diagnostic and therapeutic technologies grow more advanced, so does the need for accurate and responsive thermal control systems. Often these systems must also be compact and lightweight. Traditionally, thermal sensing has relied on single point sensing solutions such as resistance temperature detectors (RTDs), thermistors, and thermocouples. However, as designs become smaller, more complex, and lighter weight, etched foil circuits, specifically those made from Balco foil or nickel, have emerged as a versatile and high-performing alternative.

Balco and nickel foil circuits, when integrated into flexible heaters, allow thermal sensing and heating functionality in a single, cohesive design. This approach enables temperature monitoring over larger surface areas, making it ideal for a variety of applications in today’s advanced medical and diagnostic devices.

What are etched Balco/nickel foil circuits?

Etched Balco and nickel foil circuits are made from Balco, a specialized alloy composed of iron, manganese, and nickel; or nickel, specifically nickel 201, which is 99.6% pure. The circuits are fabricated by chemically etching a precise circuit pattern into a thin sheet of foil. These patterns are custom designed to cover a specific surface area for each application.

How do they work?

These foils operate based on a fundamental electrical principle – the thermal coefficient of resistance (TCR). A low TCR means a material’s resistance won’t change as it gets hotter (like Inconel, Constantan, or Cupron). However, materials with a high TCR (like Balco and nickel) have a resistance that changes predictably as the temperature changes.

Once a Balco or nickel circuit is exposed to heat, the resistance of the etched foil responds to changes in temperature. These resistance changes are measured by a control system, which references a predefined resistance-to-temperature lookup table. For example, a resistance of 120Ω might correspond to 50°F, while 125Ω might indicate 60°F. This allows the system to calculate the average surface temperature in real time.

This measurement allows these materials to function as RTDs, but with a flexible, thin-film design that facilitates surface-level temperature monitoring over large or irregular areas.

How are they different from heater circuits?

Due to the difference in TCR, Balco and nickel circuits react differently than typical heater circuits made from materials such as Inconel, Constantan, or Cupron. They do produce some heat when powered by electricity, but they naturally produce less wattage as they heat up and resistance rises. Heater circuits, on the other hand, are optimized for consistent thermal output. These metals have a much lower TCR, ensuring their resistance remains stable as temperature increases. This stability allows the heater to deliver consistent power output, even as the surrounding environment heats up.

How do they differ from traditional RTDs, thermistors, and thermocouples?

What sets Balco and nickel circuits apart from traditional sensors is their form and footprint. Each of the traditional sensors (RTDs, thermistors, and thermocouples) are suited for pinpoint temperature measurements, but don’t provide a comprehensive reading across a broader area. In contrast, an etched foil sensor can be spread across a heater surface, allowing for average temperature readings giving a full picture of the thermal environment. This approach is especially useful in applications like fluid warming, where uniform heating and sensing are critical.

If pinpoint measurement is what you’re seeking, a thermistor is often the best option in medical devices, with its high sensitivity around body temperature. At those temperatures, the resistance changes greatly per degree change in temperature, enabling extremely fast and accurate measurement.

Another difference with etched Balco and nickel circuits is their ability to seamlessly integrate into heater assemblies, preserving space and weight. Their low-profile design is useful in applications where compactness, efficiency, and reliable thermal feedback are key requirements.

Combining etched sensors and etched heaters on Kapton Flexible Heaters

One of the most innovative uses of etched Balco and nickel foil is in combination with flexible Kapton heaters. These heaters can integrate a heating circuit and a sensing circuit into a single Kapton laminate.

Traditionally, the heating and sensing layers are etched on separate foil panels, which are then laminated together. However, with advancements in laser direct imaging (LDI) technology, it’s now possible to etch both circuits simultaneously on either side of a single Kapton sheet. Optical recognition tools ensure precise alignment between the two layers, enhancing functionality while reducing complexity and bulk.

This dual-layer or dual-sided approach results in compact thermal systems monitoring their own output across a wide area, providing superior thermal control in sensitive medical applications.

Key benefits of using Balco/nickel foil in a medical diagnostic device

Etched foil sensors offer several performance and design benefits for medical and diagnostic devices:

• Uniform temperature monitoring: Unlike point sensors, etched foil sensors measure temperature across a larger area, giving an accurate average reading.
• Low profile and lightweight: The circuits are thin and flexible, contributing little mass and conserving space in the application.
• Flexible and conformable: These sensors can be bonded to curved or irregular surfaces, enabling outstanding design freedom.
• Integrated sensing and heating: The ability to combine heater and sensor circuits in a single unit streamlines product architecture.

Real-world examples of Balco/nickel foil in medical and diagnostic devices

There are quite a few medical applications where Balco or nickel foil make a great option. One notable application involves microfluidic heating systems in diagnostic equipment. In these systems, fluid flows through coiled tubing encased in copper sleeves. Etched flexible heaters, bonded to the sleeve, maintain precise fluid temperatures. By aligning an etched Balco sensor circuit with the heater, manufacturers can monitor the temperature across the entire heating surface in real time.

Another example is thermal cyclers in polymerase chain reaction (PCR) tests which require extremely precise temperature control across a series of cycles. This equipment benefits from the accuracy of sensor foil over the entire heated area, ensuring optimal control and more accurate results.

Notably, Balco and nickel foil are used in a variety of diagnostic devices requiring the heating of blood samples to accurately analyze samples. The lightweight, low-profile, and flexible nature of these foils allows them to be used in areas where other RTD sensors cannot.

Key takeaways

Medical device manufacturers looking to improve temperature control across an area of their device while reducing product footprint should consider etched Balco and nickel foil circuits as an alternative to traditional RTD single-point sensors. When paired with Kapton flexible heaters, these foils offer a low-profile, integrative solution providing design flexibility without sacrificing performance.

Birk Manufacturing designs and produces high-quality flexible heaters, temperature sensors, and thermal solutions for medical devices and diagnostics. We have expertise in finding the best solution for your application – whether using Balco or nickel foil, or another RTD sensor. Our engineering team is available to work with you on simple projects to the most complex.

Birk Manufacturing
https://www.birkmfg.com

About the author: Michael Blair is the engineering manager at Birk Manufacturing.