Improvement of Stability and Cell Adhesion Properties of Polyelectrolyte Multilayer Films by Chemical Cross-Linking

Effect of thiocyanate counterion condensation on poly(allylamine hydrochloride) chains on the buildup and permeability of polystyrenesulfonate/polyallylamine polyelectrolyte multilayers

In this study, we investigate the buildup of PEI-(PSS-PAH)(n) polyelectrolyte multilayers at pH 7.4 in the presence of either NaCl or NaSCN as a supporting electrolyte. It appears that in the presence of increasing thiocyanate concentrations (from 0.1 to 0.5 M), the thickness increment, obtained from optical waveguide lightmode spectroscopy experiments, increases whereas it stays practically constant for increasing sodium chloride concentrations (between 0.1 and 0.5 M). The hydration of the films differs also markedly between both electrolyte solutions. The differences in the construction of the polyelectrolyte multilayers in the presence of both supporting electrolytes are rationalized in terms of strong SCN(-) condensation on the PAH chains. The occurrence of this ion condensation is indirectly demonstrated by means of zeta potential measurements and directly demonstrated by means of attenuated total internal reflection infrared spectroscopy on the multilayer films. Moreover when the films are built up in the presence of SCN(-), these ions are only slowly exchanged by the Cl(-) ions introduced in the bulk. Conversely the thick films obtained from 0.5 M NaSCN solutions do not deswell when the buffer solution is replaced by a 0.5 M NaCl containing buffer. The permeability of the films constructed in the presence of both sodium salts is also studied by means of cyclic voltametry and is found to be markedly different in the case of films made from five bilayers at 0.5 M salt concentration. This difference is due to the different morphology and porosity of the films constructed in the presence of 0.5 M NaCl and 0.5 M NaSCN.

Encapsulated living cells on microstructured surfaces

The immobilization of cells in defined arrays (cell patterning) is a key step towards cell-based biosensors or other cell-based devices. While cell patterning is usually achieved by modifying the surface on which only the cells should adhere and leaving the cells unmodified, we present here a different approach in which cells are first coated with polyelectrolytes and subsequently immobilized on patterned surfaces. By coating, the cells are protected and their interactions with the substrate are modified such that patterning is simplified. We used microcontact printing of polyelectrolytes to structure surfaces such that regions of opposite charges and the same charge as the cell coating were present and found that we can thus achieve patterning of the coated yeast cells. In accordance with prior work, we find that coating does not kill the cells and coated GFP-expressing cells still function after immobilization, which we checked by fluorescence microscopy.

Multicompartment films made of alternate polyelectrolyte multilayers of exponential and linear growth

The layer by layer deposition process of polyelectrolytes is used to construct films equipped with several compartments containing "free polyelectrolytes". Each compartment corresponds to a stratum of an exponentially growing polyelectrolyte multilayer film, and two consecutive compartments are separated by a stratum composed of a linearly growing multilayer that acts as a barrier preventing polyelectrolyte diffusion from one compartment to another. We use hyaluronic acid/poly(L-lysine) as the system to build the compartments and the poly(styrene sulfonate)/poly(allylamine) system for the barrier. Using confocal microscopy, it is shown that poly(L-lysine) diffuses only within the compartment in which it was initially introduced during the film construction and is thus unable to cross the barriers. Using fluorescein isothiocyanate as a pH indicator, it is also shown that although poly(styrene sulfonate)/poly(allylamine) multilayers act as a barrier for polyelectrolytes, they do not prevent proton diffusion through the film. Such films open the route for multiple functionalization of biomaterial coatings.

Bioinert solution-cross-linked hydrogen-bonded multilayers on colloidal particles

Bioinert polyelectrolyte multilayers comprised of poly(acrylic acid) and polyacrylamide were deposited on colloidal particles (1.7 microm in diameter) at low pH conditions by layer-by-layer assembly using hydrogen-bonding interactions. The multilayer films were coated uniformly on the colloidal particles without causing any flocculation of the colloids, and the deposited films were subsequently cross-linked by a single treatment of a carbodiimide aqueous solution. The lightly cross-linked multilayer films show excellent stability at physiological conditions (pH 7.4, phosphate-buffered saline), whereas untreated multilayer films dissolved. The multilayer-coated surfaces, both on flat substrates and on colloidal particles, exhibit excellent resistance toward mammalian cell adhesion. With this new solution-based cross-linking method, bioinert H-bonded multilayer coatings offer potential for biomedical applications.