Microfabricated sieve for the continuous sorting of macromolecules

Efficient methods to separate and analyze the different components of a mixture of biological macromolecules are of paramount importance, both for research and in biomedical applications. To meet this need, a concerted effort is being made to develop miniaturized bioanalytical devices [1], using the technology of microfabrication. These devices aim to supersede the present methods, which are time consuming and difficult to automate. The separation of DNA molecules, for example, is currently accomplished by gel electrophoresis [2]; molecules of different size migrate at different speeds through the gel, and can be distinguished from one another after a certain time has elapsed. Using a gel to sieve the DNA (a necessity, since the electrophoretic mobility m0 in free solution is independent of size) complicates the procedure: A new matrix must be made each time, and recovery of the DNA is awkward, since the gel must either be sliced or blotted [2]. In this Letter, we propose a fresh approach to separating biological macromolecules, based on the technology of microfabricated arrays introduced by Volkmuth and Austin [3]. Lithography is used to etch a pattern of obstacles on a silicon chip, which is then sealed to make a quasi-two-dimensional “sieve” through which a solution of molecules can be electrophoresed. One obvious benefit compared to gels is the regularity of the sieve. The major advantage, however, is that the pattern of obstacles can be chosen at will. This allows an alternative method of electrophoretic separation. By disposing the obstacles so that the molecules are deflected away from the field direction, a separation can be effected in the direction transverse to the field. Quite generally, this can be done by choosing a periodic array of obstacles, each of which is asymmetric with respect to reflection in the field direction. The combination of the spatial asymmetry and the broken time-reversal symmetry (imposed by the flow) causes the Brownian motion of the molecules to be rectified. Since the effect depends on the thermal motion, molecules with different diffusion coefficients are deflected by different amounts, and, consequently, a mix