Overview
Point-of-care (PoC) molecular diagnostics market growth is driven by high infectious disease prevalence, increasing awareness and adoption of personalized medicine, and advances in molecular techniques that improve accuracy and portability. PoC molecular diagnostics enable clinicians to make diagnostic and treatment decisions at the patient's first visit, without requiring patients to wait days for results. This article briefly describes the detection approach and highlights practical electronic components used in key instrument modules.
Nucleic Acid Amplification
Biological samples often contain insufficient target DNA for direct optical fluorescence detection. Therefore, DNA amplification is required before analysis. The two main amplification methods are polymerase chain reaction (PCR) and loop-mediated isothermal amplification (LAMP).
PCR and LAMP require heating and cooling elements. PCR uses a thermoelectric cooler (TEC) to perform thermal cycling through three temperature steps: heating the sample to about 95°C, cooling to 50°C–56°C, and holding at 72°C. Repeating this cycle generates billions of DNA copies. By contrast, LAMP maintains the sample at a constant 60°C–65°C; avoiding thermal cycling can speed the reaction, but LAMP requires a more complex primer set.
Sensor Front End and TEC Control
Figure 2: Sensor front-end and TEC unit block diagram for a PoC molecular diagnostic analyzer
The TEC unit requires precise temperature control to support nucleic acid amplification. The TMP117 digital temperature sensor offers typical accuracy of ±0.1°C and maximum accuracy of ±0.2°C over a ?40°C to 100°C range. It integrates a 16-bit ADC and communicates via I2C or SMBus. The device is suitable for battery-powered systems, with 150 nA standby current and about 3.5 μA per 1 Hz conversion.
A switching regulator such as TPS54201 can supply a constant current (for example, 1.5 A) to drive a motor driver like DRV8873, which in turn drives the heating and cooling elements that power the TEC. DRV8873 uses four N-channel MOSFETs to bidirectionally drive loads with up to 10 A peak current, includes integrated current sensing, and eliminates the need for two external parallel sense resistors, saving board space and component count.
Optical Detection and Current Sampling
During amplification, fluorescent labels attached to pathogen target sequences are excited by a light source. A single photodiode or an array of photodiodes detects the fluorescence. Signal levels change over time or cycles as amplification proceeds, indicating the initial concentration of the target in the sample. Detectable signals can appear early in amplification when only a few target DNA copies are present, reducing time to a positive result.
TI DDC112 current-input analog-to-digital converters integrate current amplifiers and ADCs and can sample between 1 and 256 photodiodes. These devices provide very low input-referred noise in the femtoampere RMS range, low input bias current (about 0.1 pA), and high linearity with up to 24-bit resolution, which is valuable for sensitive fluorescence measurements.
Conclusion
Molecular diagnostic testing involves complex interactions across multiple scientific domains. This overview focuses on key electronic building blocks for PoC analyzers to aid designers in selecting components for temperature control, signal acquisition, and power delivery.
