ALLPCB
Overview
Point-of-care (PoC) molecular diagnostics growth is driven by high infectious disease prevalence, wider acceptance of personalized medicine, and advances in molecular techniques that improve accuracy and portability. PoC molecular diagnostic systems enable clinicians to make diagnostic and treatment decisions at the patient's first visit without waiting days for results, improving care delivery. This article briefly describes the testing approach and highlights practical electronic components used in key instrument modules.
Sample Preparation and Amplification
Clinical biological samples may contain too little target DNA to be detected directly by optical fluorescence methods. DNA amplification (replication) is therefore required before analysis. Two principal amplification techniques are polymerase chain reaction (PCR) and loop-mediated isothermal amplification (LAMP).
PCR and LAMP require heating and temperature control elements. PCR uses a thermoelectric cooler (TEC) to perform thermal cycling among three temperature stages: heating the sample to 95°C, cooling to 50–56°C, and holding at 72°C. Repeating these cycles produces billions of DNA copies. LAMP holds the sample at a constant 60–65°C; avoiding thermal cycling can speed the reaction but requires a more complex set of primers.
Sensor Front-end and TEC Unit
The front-end is commonly based on a current-input analog-to-digital converter such as the DDC112, combined with current drivers, precision amplifiers, and temperature sensors.
The TEC unit requires high temperature accuracy to control the heating and cooling steps of nucleic acid amplification. The TMP117 digital temperature sensor achieves typical accuracy of ±0.1°C and maximum ±0.2°C over a ?40°C to 100°C range. It integrates a 16-bit ADC and communicates with digital components via I2C or SMBus. TMP117 is suitable for battery-powered systems, with a 150 nA shutdown current and about 3.5 μA supply current per 1 Hz conversion.
Power regulation and motor/actuator drive are also important. A buck regulator providing a constant current (for example, 1.5 A) can be used to power a driver such as the DRV8873, which runs efficient heating and cooling elements that drive the TEC. The DRV8873 integrates four N-channel MOSFETs to bidirectionally drive motors with up to 10 A peak current and includes integrated current sensing, eliminating the need for two external parallel resistors and reducing BOM and board space.
Optical Detection and Signal Conditioning
During amplification, fluorescent probes targeting pathogen sequences are excited by a light source. A single photodiode or a photodiode array can detect the emitted fluorescence. Signal levels change with amplification time or cycles, indicating the initial concentration of the target in the sample. Early-stage detection with only a few target DNA fragments shortens the time to a positive result.
Devices such as the DDC112 can sample currents from 1 to 256 photodiodes and integrate the current amplifier and ADC in one package. These devices offer very low input-referred noise (in the femtoampere RMS range), low input bias current (about 0.1 pA), and high-linearity ADCs with up to 24-bit resolution.
Conclusion
The detection workflow spans multiple scientific disciplines, and this article does not attempt an exhaustive treatment. The component-level details above are intended to help engineers select key electronic parts when designing PoC molecular diagnostic instruments.
