Using Circuit Board Materials for Thermal Control In

USING CIRCUIT BOARD MATERIALS FOR THERMAL CONTROL IN MEDICAL DIAGNOSTICS Circuit board materials and structures can be cleverly adapted to become thermal control components for medical diagnostic systems. Printed circuit board (PCB) fabrication techniques simplify design by providing an easily customizable source from which heat (and cooling) can be generated, distributed, measured and dispersed. Through iterative simulation and physical prototyping, PCB techniques can rapidly advance designs for precision thermal management in the tight spaces and dynamic climates of microfluidic cartridge architectures. Conveniently, PCB fabrication techniques also easily scale from rapid prototyping to mass production, speeding design transfer. Key Tech has employed PCB techniques for thermal management whether or not there is already a need for electronics on a diagnostic cartridge, and these PCB-based thermal managers can live on the cartridge or near it on the instrument. We’ve dramatically shortened thermal design time using PCB-based modules, and solved some unique thermal management challenges along the way. USING CIRCUIT BOARD MATERIALS FOR THERMAL CONTROL IN MEDICAL DIAGNOSTICS KEYTECHINC.COM 2 INTRODUCTION In Vitro Diagnostic (IVD) assays almost applications with fast thermal cycle times, but they will always require some type of thermal control increase design scope and the unit cost of cartridges. of a fluidic sample. Heating may be needed to assist with lysis during sample prep, thermocycling is At Key Tech, we have been leveraging Printed Circuit required to amplify DNA, and certain detection methods Board (PCB) fabrication techniques to create custom thermal control components, where the thermal masses require the sample to be at a precise temperature and heat conduits are built right into the PCB. PCBs when measurements are taken. Many IVD systems are constructed by alternating layers of electrically are also using microfluidic lab-on-a-chip technologies conductive copper traces and insulating substrates like to increase speed, reduce device size, and bring these FR4, a common class of glass fiber/epoxy composite instruments out of the lab and closer to the point of used in PCB’s. These same materials that are used care. Thermal control in these microfluidic applications for their beneficial electrical properties are also good thermal conductors and insulators, so they are optimal can present many challenges, including: for creating thermal control components. There are a number of benefits with using this approach: • Maintaining highly precise temperatures • Maintaining tight temperature uniformity, • Leveraging a common fabrication technique especially when there are multiple zones at allows for quick prototyping and also easily different temperatures in close proximity scales up to mass production. • Achieving fast rise and cool times, • Thermal management components embedded in especially while maintaining temperature uniformity the PCB can be customized over a wide range • Accurately measuring temperature on of shapes and sizes. a small scale where traditional sensors are too • Multiple components and functions large and would influence the measurement can be built into the same PCB (e.g., power and control circuitry, resistors for heat So how do you quickly and economically design generation, thermistors or RTDs for temperature thermal control for microfluidic IVD applications with measurement, electrochemical detection circuitry, etc.). these challenges? For macro heating applications, • The low thermal mass compared to some cartridge heaters embedded in blocks of metal alternative approaches allows for faster contacting the vials or cartridges containing the sample temperature cycling. will do the trick, but in small scale applications, the precise heat control required and the small space Using this approach can be especially beneficial if the limitations usually make this approach impractical. Thin microfluidic cartridge or device already requires a PCB film heaters can work in some cases, but customizing for other functions. Because of the other benefits listed above, this approach can also be beneficial even if a these heaters to challenging applications can be tricky. PCB isn’t already required for other functions. Custom heating elements built into the microfluidic cartridges can also be a promising option, balancing Key Tech has designed a number of thermal control heat management requirements with cartridge cost components from PCBs over the years. Below is a guideline for designing these types of applications targets. We’ve worked on applications where traces based on our experience: of metals like platinum are bonded to microfluidic chips to create custom heating and temperature sensing elements. These implementations allow for total customization and may be required for demanding USING CIRCUIT BOARD MATERIALS FOR THERMAL CONTROL IN MEDICAL DIAGNOSTICS KEYTECHINC.COM 3 HEAT GENERATION Generating heat is one of the most basic Another approach is to use copper traces to create functions of thermal control. The simplest way a heating element inside the PCB. Even though to generate heat on a PCB is by soldering a power copper is very conductive, it still has some resistance, resistor to it. Resistors generate plenty of heat, but so a long thin trace (e.g., 10-15cm) can be used to because they are a point source, the PCB design must form a heating element within the PCB (Figure 3). sufficiently spread the heat out. Otherwise a metal This approach may not generate as much heat as a heat spreader may need to be attached. Custom dedicated resistor, but it can provide a more uniform elements like metal heat spreaders are typically heated area without the need for a separate heat more useful in an instrument than on a microfluidic spreader. Additionally, since the trace that creates cartridge. Figure 1 shows a cross section of a PCB illustrating how this approach can be implemented. the heat is formed as part of board fabrication and A power resistor is placed on the bottom of a PCB no additional components are needed, the heating and then Vertical Interconnect Access holes (VIAs) element comes “free” as part of the design. This is transfer the heat to the top side where a copper plane very appealing for on-cartridge heating applications, spreads the heat out over the desired area to maintain especially if a PCB is already needed in the a uniform temperature. Figure 2 shows a PCB cartridge for other functions. Even if the cartridge where this approach was implemented to allow for 64 doesn’t already require a PCB for other functions, independently heated zones. for demanding applications it may still be worth Copper plane on Vias for heat transfer considering integrating a PCB into the microfluidic top surface to through the PCB spread heat cartridge just to implement this approach. It may be Copper more cost effective than designing custom heating FR-4 elements onto the cartridge (e.g., layering platinum heaters onto the plastic cartridge components). Resistor V+ for resistor Ground plane also Winding a trace of facilitates heat transfer copper across the heated zone allows for a long trace length Figure 1: Construction of PCB with a resistive heater on the bottom and copper top plane to spread heat on the temperature controlled top side Zones formed by copper planes isolated by FR4 Figure 3: Trace layout of a heater made with a thin winding trace of copper Peltier elements can create heat and can also cool, so if active cooling is also needed, a Peltier can be Figure 2: Thermal Management PCB with 64 independently connected to a PCB. More details on Peltier elements controlled temperature zones heated by resistors attached to the are discussed in the cooling section below. underside of the PCB USING CIRCUIT BOARD MATERIALS FOR THERMAL CONTROL IN MEDICAL DIAGNOSTICS KEYTECHINC.COM 4 TEMPERATURE MEASUREMENT In order to provide accurate closed-loop much as possible. The low thermal mass of miniature temperature control, the temperature of thermocouples can also allow for fast response times, the thermal control component must be which can be especially helpful when measuring measured. A thermistor soldered onto a PCB is one transient applications. of the simplest methods for measuring temperature. Thermistors can provide precise measurement at Another challenge with verifying temperature temperatures up to 150C. Like a power resistor, a performance during development is to obtain accurate thermistor provides a point-source measurement, surface temperature measurements. Non-contact so the design must accurately relate the thermistor infrared (IR) measurement can be a helpful tool; while temperature to the temperature of interest. The goal in IR cannot give a highly accurate absolute temperature diagnostics is to control the fluid sample as it proceeds measurement (+/- 2C typically), it can provide through assay steps on the cartridge, so the thermistor very accurate differential measurement of surface must measure a temperature as close as possible to temperature (within a few tenths of a degree C). For the fluid sample temperature. Offset temperatures this reason IR sensors can be especially helpful between thermistors and the fluid sample can when verifying the temperature uniformity of a heated sometimes be successfully characterized (e.g., sample surface. When using the IR approach you need line-of- temperature is 1C less than measured thermistor sight to the surface being measured, and the emissivity temperature

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