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

Zhang, Chunsun; Xing, Da

The organization of this article is as follows. First, several important topics on the microfluidic PCR chips will be presented. Those topics, which are crucial in the development of PCR chips, include chip substrates and surface treatments, PCR chip architecture, on-chip PCR reaction volume and reaction speed and approaches to eliminating cross-contamination. Then, the temperature and fluidic controls and measurements in PCR chips are discussed, which include thermal insulation, evaporation and gas-bubble formation and measures to counteract these phenomena, semi-invasive or noninvasive temperature and fluidic measurements and numerical simulation of temperature and fluid fields in PCR chips. Finally, product detection methods used in PCR chips, e.g. off-line and on-line detection, are covered, followed by integration of functional components in PCR chips, biological samples used in PCR chips and potential applications of PCR chips, as well as practical issues related to the development of PCR chips. (69) , cyclic olefin copolymer (COC) (65) , Gene Frame Õ (70), perfluoroalkoxy-modified polytetrafluoroethylene (PFA) (13, (71) (72) (73) (74) (75) (76) , LiNbO 3 (77) and 317 stainless steel (78) , have also been used in PCR microfluidic devices. Each substrate has different properties and therefore different advantages and disadvantages. The superior thermal conductivity of silicon makes rapid PCR cycling possible. Silicon fabrication processes are well developed, and thus precise and complex chip structures can be achieved (4) . However, silicon can be problematic: bare silicon inhibits PCR; its high thermal conductivity requires thermal insulation and therefore results in structural complexity (14, 23, 25, 36, (79) (80) (81) ; its opacity limits optical detection; and its electrical conductivity makes it difficult to combine micro PCR with micro capillary electrophoresis (CE) (26, 27, 31, 35, 40) on a single silicon chip. Transparent glass is suitable for optical detection. The electro-osmotic-flow (EOF) property of glass allows the integration of PCR and CE on a monolithic chip (26, 27, 31, 35, 40) . However, the PCR chips made from silicon or glass cannot be disposed due to the high cost of fabrication. The use of polymers as substrates may overcome these disadvantages. PDMS, an inexpensive elastomeric polymer, has emerged as a promising substrate. It exhibits high flexibility, better optical transparency, lower cost of fabrication and better biocompatibility than silicon. The flexibility of the polymers gives rise to the highly integrated PCR chips incorporating PDMS micropumps and/or microvalves (19, 20, 26, 27, 31, 35, 40, 46, 48, 52, 53) . PDMS adsorbs less PCR sample than other materials and thus can be used to coat glass surfaces to avoid the possible inhibitory effect of glass on PCR (39, 50, 54) . However, the permeability of PDMS can result in diffusional loss of biological sample, and implantation of polyethylene (PE) vapor barrier in the PDMS has been proposed to prevent this fluid loss (53) . Air bubbles can be formed on PDMS surface during sample loading due to its hydrophobicity (51) .