Miniaturized PCR chips for nucleic acid amplification and analysis: latest advances and future trends

Zhang, Chunsun; Xing, Da

Sample evaporation is often problematic because PCR volumes are usually very small. Especially as denaturation temperatures approach 1008C the sample evaporation is so rapid that the sample would dry up quickly under standard atmospheric pressure. To circumvent such evaporation, a number of measures can be taken, but all have their respective advantages and disadvantages. A mineral oil cover layer is frequently used as a vapor barrier to prevent evaporation (17, 26, 27, 29, 54, 77, 79, 80) . The mineral oil is a suitable liquid cover because it has a boiling point far above 1008C and a density slightly below 1.0 g/cm 3 . However, its applicability is questionable for highly integrated PCR systems. Another approach is using a solid cover or valve to resist the internal pressure generated during PCR (10, 19, 20, 31, 32, 35, 43, 51, 52) . It is known that the evaporation rate decreases with the increase of the gas pressure around a liquid. Recently, Cheng et al. extended this concept to the oscillating-flow PCR chip (64) . In their approach, a single opening serves for both sample loading and syringe pump port. When the sample plug is pumped to high-temperature zones, the internal pressure increases by six times and thus the sample evaporation is greatly reduced. Noteworthy is that within the continuous-flow PCR chip, the relative sample evaporation loss can be decreased because of the decrease in the free surface area of the liquid. In addition, sample evaporation is also affected by the PCR chip substrates. For example, the water diffusion/vapor loss property of PDMS could lead to the sample evaporation loss and thus special approaches such as vapor barrier (52, 53) should be considered to reduce the sample loss at elevated temperatures.