Reduction of water evaporation in polymerase chain reaction microﬂuidic devices based on oscillating-ﬂow
Entry by Max Darnell, AP 225, Fall 2011
Title: Reduction of water evaporation in polymerase chain reaction microﬂuidic devices based on oscillating-ﬂow
Authors: A. Polini, E. Mele, A. G. Sciancalepore, S. Girardo, A. Biasco, A. Camposeo, R. Cingolani, D. A. Weitz, and D. Pisignano
Journal: Biomicrofluidics 4 036502 (2010)
Keywords: microfluidics, polymerase chain reaction, diffusion, hydrogel
Polymerase Chain Reaction (PCR) is a technique used for DNA replication that involves thermal cycling. One of the major issues of on-chip PCR and other biochemical reactors is that the high temperatures involved in the reactions lead to evaporation of the working fluid and diffusion into the surrounding hydrogel chip. Using glass capillary tubes inside the elastomer-based microfluidic channels creates a diffusion barrier between the fluid and surrounding hydrogel, limiting evaporation of the fluid at high temperatures.
There exist two types of PCR chips. This first is known as a stationary device, in which the entire chip and contents are exposed to different temperatures. The second, and that examined in this paper, is dynamic chip, in which the PCR mixture flows through the chip, being heated in different regions. All devices were fabricated from PDMS using standard soft lithography. Figure 1a) shows a typical microfluidic device, with different wells in the PDMS for different temperature regions. Figure 1b) is the modified version, with a glass capillary in place of the PDMS wells. The capillary is simply treated as part of the mold during the PDMS curing process.
The use of the capillary tube lowers the diffusion coefficient of the fluid through the channel. In order to probe the degree to which the glass capillary reduces evaporation, a two-phase oil-water mixture was used as the working fluid and the outlet fluid volume was measured at different time steps. Figure 4 a) shows that fluid viscosity has only a limited positive impact on reducing evaporation, and that water molecules can still diffuse through the oil and into the PDMS. Figures 4 b) and c) show that considerable fluid evaporation is evident at 20°C and 95°C, respectively. Finally, Figure 4 d) shows that despite repeated PCR cycles, fluid volume is not appreciably reduced in the glass capillary chip, confirming the notion that glass acts as a barrier to diffusion by greatly increasing the diffusion coefficient between the fluid and boundary.
In order to verify the efficacy of this method in practice, PCR was carried out on the cappillary-containing chip. Figure 5 shows the gel analysis results from PCR. The results are comparable to those achieved with a standard thermal cycler.
Connection to Soft Matter
In soft matter physics, the interplay at the solid-liquid interface is an extremely important area. In fact, one could argue that such phenomena that occur at such an interface define many aspects of the field. In this paper, the solid-liquid interface is engineered in one of the most basic senses, by engineering the diffusion coefficient in the system. Although completely fundamental and seemingly simplistic compared to the exploitation of other soft matter properties, this paper shows this significance of even basic soft matter principles and demonstrates the efficacy in leveraging such a knowledge.