Stability of embossed PEI-(PSS–PDADMAC)20 multilayer films versus storage time and versus a change in ionic strength

The use of microstructured films increased markedly in many areas of science and technology, notably in the design of microfluidic channels and in the design of parallel biosensing arrays. The concept of imprinting polyelectrolyte multilayer films (PEMs) has been introduced recently [C. Gao, B. Wang, J. Feng, J. Shen, Macromolecules 37 (2004) 8836]. These irreversibly imprinted films, obtained by plastic deformation, have to keep their size and shape after contact with fluids having physicochemical properties comparable to those of biological fluids in order to be used as microfluidic channels. We demonstrate herein that PEI-(PSS–PDADMAC)20 PEMs built-up by the spray deposition from NaCl 1 M solutions and subsequently imprinted with polydimethylsiloxane stamps keep their morphology over time (up to 9 months) when stored in the dry state. In addition the depth of the imprinted channels does not change over this time duration. When the embossed films are immersed in NaCl 0.15 M solutions, mimicking biological fluids, the depth of the imprinted channels also does not significantly change. But, when the imprinted films prepared in the presence of 1 M NaCl are subsequently dipped in a 4 M NaCl solution, partial film loss and subsequent disappearance of the imprinted channels are observed. An explanation for these findings is furnished by means of FTIR spectroscopy in the attenuated total reflection mode (ATR-FTIR). These observations should offer large opportunities for the use of the imprinted multilayer films as microfluidic channels.