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

Zhang, Chunsun; Xing, Da

Thermal interaction or 'crosstalk' has emerged as an important issue as chip device size decreases. Most DNAbased assays [e.g. PCR, restriction endonuclease reaction, temperature gradient gel electrophoresis (TGGE) and PCR-sequence-specific oligonucleotide polymorphism (SSOP)] are highly temperature sensitive and require precise temperature control. When integrating these analytical components on a single chip, thermal crosstalk will deteriorate chip device's performance and the thermal insulation is often required. The thermal isolation should also be used to eliminate or weaken the thermal crosstalk between chambers within the PCR array chip because PCR conditions vary slightly from one gene target to another although DNA hybridization occurs at the same temperature for many different gene targets. In addition, the thermal isolation between the temperature zones and the substrate is also usually considered to prevent heat loss to the surroundings. For these reasons, there have been increasing attempts to develop thermal isolation solutions on PCR chips. Shih et al. recently proposed a novel technology using parylene-cross-linking structure to achieve air gap thermal isolation for on-chip continuousflow PCR (81) . This technology provides excellent thermal isolation efficiency. Its simplicity of integration with other analytical components also makes applications of micro total analysis systems (mTAS) feasible. Burn's group reported in detail two cost-effective thermal isolation techniques: the thermal conduit technique based on a selective conduction mechanism (32, 36) , and the silicon back-dicing technique based on a selective insulation mechanism (36) . They are inexpensive alternatives to the silicon back etching technique. In addition, most existing thermal isolation techniques also adopt thin substrate structures (e.g. cantilever beam (79, 80) or deep trenches (14, 23, 26, 63) ) to thermally insulate the PCR region. These structures can provide excellent thermal isolation due to their high thermal resistance, but they usually have the low mechanical stability and require complicated microfabrication processes. Recently, Zou et al. proposed a simple conductive polymer flip-chip bonding technique to accomplish the thermal isolation of multi-chip array to allow the PCR chambers to be thermally and independently controlled (25) .