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The layer-by-layer method is a robust way of surface functionalization using a wide range of materials, e.g. synthetic and natural polyelectrolytes (PEs), proteins and nanoparticles. Thus, this method yields films with applications in diverse areas including biology and medicine. Sequential adsorption of different oppositely charged macromolecules can be used to prepare tailored films with controlled molecular organization. In biomedical research, electrically conductive coatings are of interest. In manuscript 1, we investigated films sequentially assembled from the polycation poly (diallyldimethyl-ammonium) (PDADMA) and modified carbon nanotubes (CNTs), with CNTs serving as the electrically conductive material. We assume that charge transport occurs through CNT contacts. We showed that with more than four CNT/PDADMA bilayers, the electrical conductivity is constant and independent of the number of CNT/PDADMA bilayers. A conductivity up to 4∙10^3 S/m was found. It is possible to control the conductivity with the CNT concentration of the CNT deposition suspension. A higher CNT concentration resulted in thicker CNT/PDADMA bilayers, but in a lower conductivity per bilayer. We suspect that an increased CNT concentration leads to a rapid CNT adsorption without the possibility to rearrange themselves. If PDADMA then adsorbs on the disordered CNTs in the next deposition step, the average thickness of the polymer layer is thicker than on the more ordered CNT layer from the dilute solution. This leads to an increased PE monomer/CNT ratio and lower conductivity. More polycations between the CNT layers leads to less CNT contacts. Thus, the controlled composition of films can be used to fulfill specific requirements.
For many applications of polyelectrolyte multilayers (PEMs), cheap PEs with a broad distribution of molecular weights are used. It was unknown whether the distribution of molecular weights of the PE in the adsorption solution is maintained during the adsorption process and hence in the film. To investigate this, the PSS adsorption solution in article 2 consisted of a binary mixture of short and long poly (styrene sulfonate) (PSS). A good model system to study layered films in terms of composition are PDADMA/PSS multilayers. Neutron reflectivity and in-situ ellipsometry measurements were carried out to determine the PSS composition in the film and the growth regimes. At a mole fraction of long PSS of 5 % or more in solution, the exponential growth (which is characteristic of short PSS) is totally suppressed, and only long PSS is deposited in the resulting multilayer. Variation of adsorption time of PSS showed that short PSS first adsorbs to the surface but is displaced by long PSS. Between 0 and 5 % of long PSS in the adsorption solution exponential growth occurs. The fraction of short PSS in the film continuously decreases with the increase of long PSS in the adsorption solution. In the assembly of films prepared from binary PSS mixtures, the short PSS leaves the film through adsorption/desorption steps both during PSS adsorption and during PDADMA adsorption (as PDADMA/PSS complexes). Both techniques show that the composition of the film does not correspond to that of the deposition solution. The composition and thus the properties of the resulting multilayer are influenced by the choice of adsorption time. Moreover, we conclude that a multilayer grown from a polydisperse polyelectrolyte contains fewer mobile low molecular weight polymers than the deposition solution.
In manuscript 1 and article 2, the composition of multilayers was studied. In manuscript 1 adsorption kinetics were important for the arrangement of CNTs on the surface. In article 2, the adsorption kinetics, i.e. the diffusion of the polyelectrolytes to the surface, was also investigated. In article 3, we investigated the influence of the composition of the film as well as the preparation condition on the mobility of PEs in the film. The molecular weight of the polycation PDADMA and the NaCl concentration of the deposition solution were varied. The vertical PSS diffusion constant D_PSS within the PDADMA/PSS multilayers was measured using neutron reflectivity. The salt concentration of the preparation solution defines the polymer conformation during deposition. The molecular weight of the polycation determines the degree of intertwining. Together, both parameters determine the polyanion-polycation coupling and thus the PSS mobility within the network. Log−log display of D_PSS vs the molecular weight of PDADMA and fits to two power laws (D_PSS ∝ X_n(PDADMA)^(-m) ∝ M_w(PDADMA)^(-m)) reveals for films built from 10 or 200 mM NaCl a kink. Below and above the kink, the dependence of D_PSS on M_w(PDADMA) can be described by different power laws. For Χ_n(PDADMA) < X_n,kink(PDADMA) ≈ 288, the exponents are consistent with the predictions of the sticky reptation model. X_n(PDADMA) ≈ 288 is the entanglement limit. For Χ_n(PDADMA) > X_n,kink(PDADMA) ≈ 288, the decrease of D_PSS with M_w(PDADMA) is larger than below the entanglement limit, which is indicative of sticky reptation and entanglement. The PSS diffusion constant of films built from 100 mM NaCl drops three orders of magnitude when increasing the molecular weight of PDADMA from 45 kDa to 72 kDa. To figure out if an immobile PSS fraction exists in the film built from 72 kDa PDADMA (beyond the entanglement limit), the film was annealed at different conditions in article 4: both temperature and salt concentration were varied. For data analysis, the simplest model with two PSS fractions with different diffusion constants was used. These diffusion constants increase as the temperature of the surrounding solution is increased. As assumed in article 3, an immobile PSS fraction exists when annealing at room temperature. At higher annealing temperatures, at least two diffusion processes must be distinguished: the diffusion of the highly mobile PSS fraction through the entire film and a slow PSS fraction, mowing in a limited way. The choice of preparation conditions determines whether a polyelectrolyte multilayer can intermix completely. It is not clear if complete intermixing will ever occur for films built with PDADMA beyond the entanglement limit. It is possible that the diffusion is more complex. Long-term measurements will clarify this question. Calculating scattering length density profiles with subdiffusive behavior would be interesting and is a challenge for the future. Furthermore, immobile fractions are only visible with long annealing times. We hypothesize that an immobile or nearly immobile fraction is present whenever the dependence of D_PSS on the molecular weight of PDADMA cannot be described by the sticky reptation. To verify this hypothesis, further studies are necessary.
All results presented and discussed in the manuscript and articles show that by varying the preparation conditions, tailored films can be built. The composition of the film is also determined by the adsorption kinetics. In addition, the mobility of the PEs within the multilayers can be controlled by varying the conformation, mingling and entanglement of the chains within the film. The influence of the salt concentration in the preparation solution on the growth regimes during film formation is part of our future research. It is planned to investigate films built of different PDADMA molecular weights under varied annealing conditions to better understand the mobile and immobile fractions.
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.