Liquid Handling
Small is beautiful: integrating liquid handling into PCR microchips
In the future, liquid handling may have to viewed through the microscope if it is to be seen at all. By integrating a novel liquid handling system into a microfluidic flow-through reactor for real-time PCR, Swiss researchers have developed a reliable and flexible solution for one of main challenges facing developers of biochips – namely, how to precisely control the movement and positioning of very small amounts of liquid.
Microfluidic devices are one of the most rapidly evolving areas in biotechnology, and recent engineering breakthroughs are now opening the door for novel microdevices that can be used in demanding, high-throughput applications. For PCR applications, two basic types of devices have been developed: chamber-type and continuous flow-type. In both cases, the goal of optimisation work carried out in the past few years has been to integrate additional functionalities into the chip so that as many operations as possible (such as cell lysis or sample pretreatment) can carried out within a single system. Although many possibilities exist in terms what can be done on the engineering level, it is difficult to design these more complex devices in such a way so as to keep fabrication costs low. Another challenge is to keep the liquid handling simple while avoiding the risk of cross-contamination, which for PCR is an important consideration.
In the October issue of Biomedical Microdevices, a team of Swiss investigators report the development of an autonomous microfluidic multi-channel biochip for real-time PCR with integrated liquid handling. The disposable, polymer-based microdevice was designed and developed by Olivier Frey, with the Institute of Microtechnology at the University of Neuchatel, together with colleagues from the Physical Electronics Laboratory at ETH Zurich. A key feature with regard to the liquid handling in this device is that very small amounts (nanoliters) of liquid are shuttled between different temperature zones through the use of a pneumatic pump.
The primary advantages of this microfluidic PCR device are (1) its cost-effective and straightforward fabrication using conventional processes and materials, (2) its rapid analysis times, (3) the ability to fine-tune all thermal cycling parameters, and (4) its capacity for high-throughput analysis at an acceptably low risk of cross-contamination. By using an externally actuated pneumatic pump in combination with microfluidic components in the channels, Frey and colleagues have created a device that enables robust and autonomous sample manipulation. Furthermore, the design of this biochip allows multiple channels to arranged parallel to each other, enabling large numbers of samples to be processed at the same time through simultaneous pump actuation.
It should be noted that although polydimethylsiloxane (PDMS) was used for the fabrication of these prototype devices, the known disadvantages of this polymer (hydrophobicity and absorption of biomolecules) render it unsuitable for large-scale production. For this purpose, the Swiss team plans to evaluate other polymers such as hybrid laminates with selective surface treatments. Further improvements to this prototype PCR chip include the integration of an optical readout, thereby creating an advanced liquid handling environment that enables the feedback control of sample placement. In addition, the integration of sample pretreatment units on the biochip would enable its use for point-of-care applications without additional, external sample manipulation steps.
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