Many medical applications require heat for optimal performance. For patient comfort, effective treatment, and a variety of diagnostic processes, equipment and fluids must be raised to, or kept at, specific operating temperatures. Medical heating applications can have strict requirements including fast warm up, uniform heating both over time and across a surface, compact design, and high reliability.

Traditional approaches to thermal control couple some sort of heating technology with external sensors and controllers. While these multicomponent solutions can provide the necessary heat and thermal equilibrium, they can be expensive, bulky, and complicated to assemble. They can also have potentially vulnerable points of failure that negatively impact efficacy of the end product. In addition to the risks of heating loss due to breakdown, these designs can be subject to dangerous overheating in the case of malfunctions such as delamination.

The ideal heater for many of these medical applications would be one that is self-regulating without the need for external sensors or controllers. It would heat quickly, reduce power as the target temperature is neared, and cut power entirely before the target temperature is exceeded to eliminate the risk of overshoot. It would do all this in a compact package without additional connections that could complicate assembly or break down in the field.

Self-Limiting Heater Technology

There is a technology that can do all of this, eliminating the need for both sensors and controllers by integrating those functions into the thin film of the heater itself, controlling temperature at the molecular level by modulating power at an infinite number of points across the heating surface. The heater film consists of a thin silicone matrix loaded with conductive carbon particles. Electrical current moves between the carbon particles via quantum tunneling of electrons through the non-conductive silicone material.

The electrical resistance of the material and the current it can carry at any point over its surface is determined by the spacing between the carbon particles, and that spacing changes with temperature.

As the silicone warms, thermal expansion drives the carbon particles farther apart and increases resistance point-by-point over the surface of the heater; at a designed set point temperature, the heater effectively becomes an electrical isolator, draws negligible current, and no longer produces heat. Conversely, if the silicone cools via environmental or load variations, the carbon particles pull closer together, reducing resistance, allowing current to flow and the heater to produce heat as needed to maintain thermal equilibrium.

As a result, the technology works without external instrumentation and control systems. By controlling the composition of the carbon-silicone matrix in production, the heater is designed to approach but not exceed a specific temperature set point. When the heater is powered, it quickly warms to its designed temperature set point and maintains that temperature within a narrow band.

Simpler in Design. This technology is of particular value in medical device development for several reasons. First, it is far simpler than traditional heating technologies. With no external sensor or controller, it performs both of those functions at the molecular level within the heater itself. The heater is just 0.40 mm thick and can be integrated into an end product using just two connections.

Fig. 1 - Self-regulating heaters offers engineers a simpler design process.

Figure 1 shows that with fewer connections and no need for external components, this technology simplifies a product developer’s bill of materials. At the same time, with fewer potential points of failure, it provides a more robust installation. As an all-in-one heater, this technology requires virtually no maintenance and is far less likely to require repair in the field than a traditional heater and control system. No user programming is required because the technology is manufactured to maintain a specific temperature set point within the heater itself.

Stable in Operation. In medical applications, reliable temperature control can be critical, and temperature control with this technology is extremely reliable. From a cold start, the carbon-silicone matrix initially provides maximum power to speed warm-up to its designated operating temperature. As the heater approaches its set point, its resistance increases and current draw approaches zero to slow heating.

Once thermal equilibrium is achieved, the unit responds to any changes in the environment to maintain the target temperature across the entire device surface. The heater adjusts power at each point across its surface to maintain uniform temperature. These intrinsic features allow the heater to quickly achieve and maintain thermal equilibrium while preventing undesirable temperature overshoot that can occur in traditional control systems.

Set point temperature is reliably maintained as power is automatically adjusted in response to variations in ambient environment temperature. The technology is self-tuning in dynamic environments and maintains the target temperature across the entire device surface even if the heat sink exhibits variation. If, for example, one region of the heat sink surface is cooled more than the rest, the heater increases power output in that region only to maintain even temperature.

This is very different from a conventional resistive heater, the power output of which is typically controlled by a single sensor feedback loop. Similarly, where a conventional heater would have to be “zoned” (designed with specific regions of differing power density) to handle differences in heat loss across its surface, the carbon-silicone matrix responds to these differences via point-wise power modulation.

Fig. 2 - SmartHeat SLT heater intentionally damaged by a hole punch (left, visual image). Heater is shown to continue operating in areas surrounding the damage (right, thermal image).

Safer in Use. In many medical applications, failure is not an option. When heating is lost, critical, even life-saving, processes can be affected. This risk is greatly reduced with a this type of self-regulating heater. Even if the heater is physically damaged, it will rarely be affected at any point beyond the location of the damage. Figure 2 shows a heater with a hole punched through the body. The accompanying infrared photo shows that the heater is continuing to function over most of its surface. In most cases, this provides enough heat to maintain the device’s designed function.

The opposite of heating loss can also happen; a heater can delaminate or otherwise fail resulting in dangerous overheating that can damage equipment or hurt patients, for instance, in a medical fluid warming application in which fluid may not always be present. When there is no fluid to warm, a traditional heater can easily overheat. A self-regulating heater, on the other hand, simply reduces its power output wherever a thermal load (i.e., fluid) is not present.

Overheat protection applies to each point across the heater surface. Because current flow is controlled locally at the molecular level, even if the heater delaminates — pulling away from the heat sink — current to the delaminated segment is reduced to prevent even local overheating, protecting both the heater itself and the surrounding materials. These capabilities make this technology a safe solution for medical applications in which heater failure or overheating must be avoided.

Heater Construction. Design construction of the self-limiting heater does not differ greatly from that of a traditional thin film heater. The difference is the material used for a self-limiting, “smart” heating element. The multilayer construction of a SmartHeat SLT heater, for example, begins with the carbon-silicone matrix itself. By adjusting the density of carbon particles within the silicone layer, the material is manufactured to have a specific temperature set point. When a set point temperature is reached, the carbon particles are spaced such that the heater becomes an electrical isolator, preventing any chance of thermal runaway. If temperature drops below the set point, current flow to that area resumes maintaining the set point temperature.

Medical Design Briefs Magazine

This article first appeared in the November, 2017 issue of Medical Design Briefs Magazine.

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