Biodegradable polymer-lipid monolayers as templates for calcium phosphate mineralization.

A combination of poly([R]-3-hydroxy-10-undecenoate) (PHUE), a biodegradable polymer from the group of polyhydroxyalkanoates (PHAs), and lipids of different head groups was used to support the growth of calcium phosphate, the main component of mammalian bones. Crystallization took place under two-dimensional films (Langmuir monolayers). The addition of a negatively charged lipid, 1,2-dioleoyl-sn-glycero-3-phospho-l-serine, to a PHUE film led to the formation of lipid domains (rich in negative charge), and resulted in excellent mineralization control: crystals with uniform size and morphology were formed. The results show that carefully optimized combinations of materials can lead to better control of calcium phosphate crystallization compared to one-component organic scaffolds.

Membrane protein distribution in composite polymer-lipid thin films.

We present a model system to demonstrate that the positioning of biomolecules (membrane proteins) in a nonnative, complex thin film environment can be regulated by the phase behavior of film components. Partial separation between an amphiphilic polymer and a lipid drives the protein to a fluid phase, mechanically more similar to a cellular bilayer.

Interactions of biodegradable poly([R]-3-hydroxy-10-undecenoate) with 1,2-dioleoyl-sn-glycero-3-phosphocholine lipid: a monolayer study.

Polyhydroxyalkanoates (PHAs) are biodegradable, biocompatible polyesters and very attractive candidates for biomedical applications as materials for tissue engineering. They have a hydrophobic character, but some are able to spread at the air-water interface to form monomolecularly thin films (Langmuir monolayers). This is a very convenient model to analyze PHA self-assembly in two dimensions and to study their molecular interactions with other amphiphilic compounds, which is very important considering compatibility between biomaterials and cell membranes. We used the Langmuir monolayer technique and Brewster angle microscopy to study the properties of poly([R]-3-hydroxy-10-undecenoate) (PHUE) films on the free water surface in various experimental conditions. Moreover, we investigated the interactions between the polymer and one of the main biomembrane components, 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC). The addition of lipid to a polymer film does not change the monolayer phase behavior; however, the interactions between these two materials are repulsive and fall in two composition-dependent regimes. In summary, this is the first systematic study of the monolayer behavior of PHUE, thus forming a solid basis for a thorough understanding of material interactions, in particular in the context of biomaterials and implants.

Calcium phosphate growth beneath a polycationic monolayer at the air-water interface: effects of oscillating surface pressure on mineralization.

The self-assembly of the amphiphilic block copolymer poly(butadiene)-block-poly[2-(dimethylamino)ethyl methacrylate] at the air-water interface and the mineralization of the monolayers with calcium phosphate was investigated at different pH values. As expected for polyelectrolytes, the subphase pH strongly affects the monolayer properties. The focus of the current study, however, is on the effect of an oscillating (instead of a static) polymer monolayer on calcium phosphate mineralization. Monitoring of the surface pressure vs. mineralization time shows that the monolayer is quite stable if the mineralization is performed at pH 8. In contrast, the monolayer at pH 5 shows a measurable decrease of the surface pressure already after ca. 2 h of mineralization. Transmission electron microscopy reveals that mineralization at low pH under constant oscillation leads to small particles, which are arranged in circular features and larger entities with holes of ca. 200 nm. The larger features with the holes disappear as the mineralization is continued in favor of the smaller particles. These grow with time and form necklace-like architectures of spherical particles with a uniform diameter. In contrast, mineralization at pH 8 leads to very uniform particle morphologies already after 2 h. The mineralization products consist of a circular feature with a dark dot in the center. The increasing contrast of the precipitates in the electron micrographs with mineralization time indicates an increasing degree of mineralization vs. reaction time. The study therefore shows that mechanical effects on mineralization at interfaces are quite complex.

Calcium phosphate mineralization beneath a polycationic monolayer at the air-water interface.

The self-assembly of the amphiphilic block copolymer poly(n-butyl methacrylate)-block-poly[2-(dimethylamino)ethyl methacrylate] at the air-water interface has been investigated at different pH values. Similar to Rehfeldt et al. (J. Phys. Chem. B 2006, 110, 9171), the subphase pH strongly affects the monolayer properties. The formation of calcium phosphate beneath the monolayer can be tuned by the subphase pH and hence the monolayer charge. After 12 h of mineralization at pH 5, the polymer monolayers are still transparent, but transmission electron microscopy (TEM) shows that very thin calcium phosphate fibers form, which aggregate into cotton ball-like features with diameters of 20 to 50 nm. In contrast, after 12 h of mineralization at pH 8, the polymer film is very slightly turbid and TEM shows dense aggregates with sizes between 200 and 700 nm. The formation of calcium phosphate is further confirmed by Raman and energy dispersive X-ray spectroscopy. The calcium phosphate architectures can be assigned to the monolayer charge, which is high at low pH and low at high pH. The study demonstrates that the effects of polycations should not be ignored if attempting to understand the colloid chemistry of biomimetic mineralization. It also shows that basic block copolymers are useful complementary systems to the much more commonly studied acidic block copolymer templates.

Monolayer interactions between lipids and amphiphilic block copolymers.

Interactions in binary mixed monolayers from lipids 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) and 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) and amphiphilic poly(2-methyloxazoline)-block-poly(dimethylsiloxane)-block-poly(2-methyloxazoline) block copolymers were studied by using the Langmuir balance technique and Brewster angle microscopy. It is shown that monolayers from the saturated lipid (DPPC) are more sensitive to the presence of polymers in the film, resulting in phase separation and the formation of pure lipid domains at high surface pressure. The morphology and composition of such phase-separated lipid-polymer films were studied by fluorescence microscopy and ToF-SIMS. In contrast, in DOPC-containing monolayers, the polymers tend to phase-separate at low surface pressures only and homogeneous films are obtained upon further compression, due to higher lipid fluidity. The analysis of excess energy of mixing shows that while the separation effect in densely packed DPPC-containing films is strongly dependent on the polymer size (with the larger polymer having a much stronger influence), in the case of monolayers with DOPC much smaller effects are observed. The results are discussed in terms of the monolayer composition, lipid fluidity, and polymer size.

Calcium phosphate mineralization beneath monolayers of poly(n-butylacrylate)-block-poly(acrylic acid) block copolymers.

Amphiphilic poly(acrylic acid)-block-poly(n-butylacrylate) block copolymer films at the air-water interface have been mineralized with calcium phosphate. The polymers form stable monolayers at the free surface. Their stability is virtually independent of ion strength or pH changes in the subphase. The outcome of calcium phosphate crystallization depends on the calcium and phosphate concentrations, the stirring rate of the subphase, and the subphase pH. At low calcium and phosphate concentrations (2 mM), uniform polymer-calcium phosphate hybrid films form. Higher calcium and phosphate concentrations yield less ordered films, which often contain large blocks of material beneath the polymer monolayer. Occasionally, also filaments similar to samples described by Peytcheva et al. (Colloid Polym. Sci., 2002, 280, 218) are observed. Films mineralized at pH values below ca. 6 contain particles that are a few nanometers apart and the resulting films retain some flexibility. Films grown above pH 6 have a higher degree of mineralization. They are stiff and tend to break upon mechanical stress. Overall, the paper illustrates that low calcium phosphate supersaturation in the subphase and a well-defined (but not crystalline) interface are crucial for controlling calcium phosphate mineralization. As a result, the current study could serve as a model for biological mineralization which is more closely related to Nature than films made from e.g. detergents or low molecular mass compounds.

Synthesis of compounds presenting three and four anthracene units as potential connectors to mediate infinite lateral growth at the air/water interface.

We report the synthesis of molecular sheets based on the photochemically initiated dimerization of monomers with lateral anthracene units. The film thickness and composition were investigated by ellipsometry and X-ray photoelectron spectroscopy (XPS). The mechanical stability of the film was sufficient to span it over 45x45 microm-sized holes. Several model reactions were performed to illustrate the underlying chemistry and to assist in analysis. The reported experiments are considered first steps towards the ultimate goal of the rational synthesis of laterally "infinite", one-monomer-unit-thick molecular sheets with a long-range positional order and a periodic covalent-bonding pattern. Such sheets are referred to as 2D polymers and are considered a prime goal of chemical synthesis with intriguing applications.

Phase behavior of mixed Langmuir monolayers from amphiphilic block copolymers and an antimicrobial peptide.

The behavior of binary monolayers from PMOXA-PDMS-PMOXA triblock copolymers and alamethicin, an antimicrobial peptide, was investigated in the context of formation of novel biocomposite nanostructured materials. The properties of mixed monolayers were studied by surface pressure-area isotherms and Brewster angle imaging. As reported previously, functionality of alamethicin relies on its aggregation properties in lipid mono- and bilayers. This is also the case in polymer matrixes, however, here the mixing properties differ from lipid-peptide systems due to the polymers' structural specificity. The peptide influence on the polymer films is provided in detail for the first time, and supported by the compressibility data to asses the elastic properties of such composite membranes.

Structural, volumetric, and thermodynamic characterization of a micellar sphere-to-rod transition.

The thermotropic sphere-to-rod transition of nonionic surfactants was characterized in terms of a large set of parameters: the transition temperature and width, the partial volume, coefficient of thermal volume expansion, enthalpy, isobaric heat capacity, and structural parameters, such as radius of gyration and hydrodynamic radius. Data were recorded as a function of concentration of surfactants in H2O and in D2O. To this end, pressure perturbation calorimetry (PPC), small angle neutron scattering (SANS), dynamic light scattering (DLS), differential scanning calorimetry (DSC), and isothermal titration calorimetry (ITC) were applied in a study of aqueous solutions containing myristyl, tridecyl, and lauryl maltoside and heptaethyleneglycoltetradecyl ether (C14EO7). Small changes in the thermodynamic and volumetric parameters (e.g., the partial volume change is approximately +2 per thousand) are discussed in detail as the result of three effects governing the transition. (i) Reduction of the water accessible hydrophobic surface area (ASA(ap)) drives the transition. (ii) Shrinking in headgroup size by thermal dehydration triggers the transition. (iii) Hypothesized gradual ordering of the chains may control the effect of chain length on the transition.