RESUMEN
Typically, laterally patterned films are fabricated by lithographic techniques, external fields, or di-block copolymer self-assembly. We investigate the self-patterning of polyelectrolyte multilayers, poly(diallyldimethylammonium) (PDADMA)/poly(styrenesulfonate) (PSS)short. The low PSS molecular weight (Mw(PSSshort) = 10.7 kDa) is necessary because PSSshort is somewhat mobile within a PDADMA/PSSshort film, as demonstrated by the exponential growth regime at the beginning of the PDADMA/PSSshort multilayer build-up. No self-patterning was observed when the PDADMA/PSS film consisted of only immobile polyelectrolytes. Atomic force microscopy images show that self-patterning begins when the film consists of seven deposited PDADMA/PSSshort bilayers. When more bilayers are added, the surface ribbing evolved into bands, and circular domains were finally observed. The mean distance between the surface structures increased monotonously with the film thickness, from 70 to 250 nm. Scanning electron microscopy images showed that exposure to vacuum resulted in thinning of the film and an increase in the mean distance between domains. The effect is weaker for PSSshort-terminated films than for PDADMA-terminated films. The mechanism leading to domain formation during film build-up and the effect of post-preparation treatment are discussed.
RESUMEN
Electrostatic forces at the cell interface affect the nature of cell adhesion and function; but there is still limited knowledge about the impact of positive or negative surface charges on cell-material interactions in regenerative medicine. Titanium surfaces with a variety of zeta potentials between -90 mV and +50 mV were generated by functionalizing them with amino polymers, extracellular matrix proteins/peptide motifs and polyelectrolyte multilayers. A significant enhancement of intracellular calcium mobilization was achieved on surfaces with a moderately positive (+1 to +10 mV) compared with a negative zeta potential (-90 to -3 mV). Dramatic losses of cell activity (membrane integrity, viability, proliferation, calcium mobilization) were observed on surfaces with a highly positive zeta potential (+50 mV). This systematic study indicates that cells do not prefer positive charges in general, merely moderately positive ones. The cell behavior of MG-63s could be correlated with the materials' zeta potential; but not with water contact angle or surface free energy. Our findings present new insights and provide an essential knowledge for future applications in dental and orthopedic surgery.
RESUMEN
Surface forces are used to investigate the polymer conformation and the surface charge of polyelectrolyte multilayers. Films are prepared from strong polyelectrolytes with low and high linear charge density at 0.1 M NaCl, namely poly(diallyldimethylammonium) (PDADMA) and poly(styrenesulfonate) (PSS). The multilayer has two growth regimes: in the beginning, the film can contain as many positive as negative monomers. After about 15 deposited layer pairs, a linear growth regime characterized by an excess of cationic PDADMA monomers occurs. Independent of the film composition, at preparation conditions, the film surface is flat, uncharged and partially hydrophobic. Surface force measurements at decreased ionic strength provide insight. For PSS-terminated films electrostatic forces are found. At the beginning of multilayer formation, the surface charge density is negative. However, in the linear growth regime it is positive and low (one charge per 200-400 nm2). This reversal of surface charge density of PSS-terminated films is attributed to excess PDADMA-monomers within the film. PDADMA terminated films show steric forces, chains protrude into the solution and form a pseudobrush, which scales as a polyelectrolyte brush with a low grafting density (1900 nm2 per chain). We suggest a model of polyelectrolyte multilayer formation: PDADMA with its low linear charge density adsorbs with weakly bound chains. Monovalent anions within the film compensate PDADMA monomer charges. When PSS adsorbs onto a PDADMA-terminated multilayer, PSS monomers replace monovalent anions. While electrostatic bonds are formed and dissolved within the polyelectrolyte multilayer, the surface charge density remains zero.
