Introduction to biochip technology
Scientists and engineers are borrowing miniaturization, integration, and parallel-processing techniques from the computer industry to develop laboratory devices and procedures that will fit on a wafer or microchip. Biochip is a broad term indicating the use of microchip technology in molecular biology and can be defined as arrays of selected biomolecules immobilized on a surface. DNA microarray is a rapid method of sequencing and analyzing genes. An array is an orderly arrangement of samples. The sample spot sizes in microarray are usually less than 200 microns in diameter. It is comprised of DNA probes formatted on a microscale plus the instruments needed to handle samples (automated robotics), read the reporter molecules (scanners) and analyze the data (bioinformatic tools). Hybridization of RNA or DNA-derived samples on chips allows the monitoring of expression of mRNAs or the occurrence of polymorphisms in genomic DNA (Jain 2001).
In a DNA chip, an array of oligonucleotides or peptide nucleic acid probes is synthesized either in situ (on-chip) or by conventional synthesis followed by on-chip immobilization. The array is exposed to labeled sample DNA, hybridized, and the identity/abundance of complementary sequences is determined (Jain 2000). For practical purposes, the terms 'DNA microarray' and 'DNA chip' are used as synonyms although there are some technical differences as already indicated.
A microarray is a collection of miniaturized test sites arranged on a surface that permits many tests to be performed simultaneously, or in parallel, in order to achieve higher throughput. The average size of test sites in a microarray and the spacing between them defines the array's density. Higher density increases parallel processing throughput. In addition to increasing the throughput, higher density reduces the required volume for the sample being tested, and thereby lowers costs. Currently, the principal commercially available ways to produce microarrays include mechanical deposition, bead immobilization, inkjet printing and photolithography.
There are essentially three developments going on that aim to improve the presently available technique and to fulfill the requirements for second generation microarray systems: (1) new methods for label-free detection of the hybridization signals; (2) automatable flow-through systems; and (3) powerful data-processing software. Several biochip technologies will be mentioned later on in relation to nanobiotechnology.
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