Accurate temperature measurement is needed in laboratory applications such as polymerase chain reaction (PCR) analysis, which is used to identify bacteria and viruses in biological samples. Although this type of research has gained attention recently in the battle to control COVID-19, PCR analysis has a key role in projects aiming to overcome a wide variety of serious infections and genetic diseases.
PCR reactions amplify the DNA in tiny samples to help researchers study pathogens in detail. Here, controlling the temperature at which the reactions take place is critical. However, measuring temperature using traditional thermometers that rely on contact with the specimen faces several challenges.
For a start, the equipment must allow new specimens to be setup quickly to maximize utilization, which is not well suited to direct contact methods and prevents accurate measurement. Next, direct contact risks cross-contamination between specimens. In addition, the temperature sensor needs to be calibrated accurately.
Figure 1 Temperature sensors for laboratory environments mandate highly accurate measurement. Source: Melexis
Temperature control in PCR analysis
PCR analysis comprises three processes, beginning with a high-temperature step called denaturation. This is performed between 94-98°C to cut the DNA double-helix structure into two single-stranded DNA template molecules. In the next step, annealing, DNA primer molecules are forced to bind selectively to the two DNA templates. The temperature here must be precisely controlled to ensure the primers attach correctly. The final stage is called elongation and is typically done at 72°C. Here, nucleotides—the building blocks for DNA/RNA—react with the template and primer to create double-stranded molecules.
Together, these three steps convert one DNA molecule into two such double-stranded molecules (Figure 2). Repeating the cycle, doubling the number of molecules every time, quickly creates a vast quantity of molecules—after, say, 40 cycles there will be 2^40 = 1.099.511.627.776 molecules—thereby easing further detection by other means.
Figure 2 PCR quickly amplifies the DNA of viruses and bacteria to ease medical diagnoses. Source: Melexis
Non-contact temperature sensing
Thermal cyclers are a key component of the apparatus used to facilitate the temperature-sensitive biochemical reactions. Each cycler contains one or multiple thermal blocks featuring holes where the tubes holding the reactants can be inserted. The thermal cycler exposes the tubes to a pre-determined temperature program and is designed to allow fast and accurate temperature sweeps. Some models allow a controlled temperature gradient over the thermal block, exposing different samples to different temperatures. This feature is mostly used in the research phase to optimize certain critical steps of the temperature cycle.
Tight control loops rely on accurate sensor inputs. When the specimens are replaced frequently, it can be very difficult for the manufacturers to reliably measure the tubes through a direct contact method. Here, infrared sensors deliver a significant advantage by enabling non-contact temperature measurement. Furthermore, the risk of cross-contamination between specimens is greatly reduced by avoiding direct contact.
To help makers of laboratory equipment overcome these challenges, Melexis has developed far-infrared sensors such as the through-hole MLX90614 and surface-mount MLX90632. The sensors are factory-calibrated and designed to maintain high accuracy in noisy and thermally dynamic environments.
Factory-calibrated sensors like the MLX90614 allow easy implementation into the thermal cycler. The sensor offers plug-and-play convenience that enables the user to start work immediately without going through a calibration process. The MLX90614 is offered with various field of view (FoV) options, including 90°, 35°, 12°, 10°, and 5°. Narrowing the FoV allows for a greater maximum distance between the sensor and the sample.
With the tendency toward making equipment more portable and easier to use in diverse scenarios, the surface-mount MLX90632 sensor offers the benefit of a heavily reduced form factor that can save both size and weight (Figure 3). Packaged as a 3 mm x 3 mm x 1 mm QFN device, the sensor is responsive to temperature change while preserving accuracy and stability. Miniaturizing such sensors also facilitates integration into a sensing matrix that enables multipoint temperature control.
Figure 3 The MLX90632 temperature sensor is especially suitable in thermally dynamic environments and when available space is limited. Source: Melexis
By improving the speed and accuracy of thermal cyclers, infrared sensors such as the MLX90632 and MLX90614 devices have a vital role in enhancing the accuracy and productivity of PCR testing in numerous areas of medicine in addition to the battle against COVID-19. For instance, the small size of the MLX90632 boosts the prospects for affordable, portable PCR analysis close to the point of care.
It is worth noting that medical-grade versions of both sensors are available that display increased accuracy, within ± 0.2°C, in the human body-temperature range from 35-42°C. These are well suited to use in mass screening systems, where a narrow field of view enables extended detection distance and personal temperature monitors such as wearable devices and patches that require sensors to be highly miniaturized and responsive yet stable and repeatable.
Tight temperature control
Recent events have highlighted the need for accurate and quick diagnostics. Small infrared sensors play a crucial role in enabling laboratory equipment to meet the need for rapid analysis of biological samples.
The latest infrared temperature sensors combine small size and immunity to thermal interference to overcome the key technical challenges to non-contact sensing. These advancements deliver a multitude of benefits including greater accuracy, enhanced ease of use, and tight temperature control for biochemical processes such as PCR analysis that offers rapid and reliable diagnostics capabilities.
Joris Roels is marketing manager for temperature sensors at Melexis.
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