Search PubMed⌕ Search

Biomedical subjects

Helmuth Möhwald

Publications and source records attributed to Helmuth Möhwald.

At least 73 records · Page 4Linked to original sources
PubMed2004

Self-assembly of human serum albumin (HSA) and L-alpha-dimyristoylphosphatidic acid (DMPA) microcapsules for controlled drug release.

Human serum albumin (HSA) and L-alpha-dimyristoylphosphatidic acid (DMPA) were applied as a pair to encapsulate ibuprofen microcrystals by means of a technique based on the layer-by-layer (LbL) assembly of oppositely charged species, for the purpose of controlling drug release. The successful adsorption of HSA and DMPA multilayers onto ibuprofen crystals was confirmed by optical microscopy. The drug release process, in a solution of pH 7.4, was monitored by optical microscopy and UV spectroscopy. The results revealed that the rate of release of ibuprofen from HSA/DMPA microcapsules decreased as the capsule wall thickness and drug crystal size increased, indicating that the permeability of the microcapsules can be controlled by simply varying the number of HSA/DMPA deposition cycles.

Capsules↗
PubMed2004

A study of kinetic molecular exchange processes in the medium frequency range by surface SHG on an oscillating bubble.

The dilatational properties of fluid surfaces and interfaces have been comprehensively investigated in recent years. For example, an improved oscillating bubble device provided experimental results that allow for critical testing of established surface models, such as the Lucassen/van den Tempel (LvdT) model. The comparison of the LvdT model with the oscillating bubble experiments demonstrates a mismatch between the model parameters. For example, near the CMC or the limit of solubility the calculated parameters of surfactant solutions become unrealistically large. The deviation can be explained by the introduction of more detailed surface models, in particular by the modification of the effective thickness of the surface layer, its internal structure and the molecular exchange processes between these structures. For the verification of such processes an experimental setup was realized which allows for an independent determination of the instantaneous adsorption state at the surface of an oscillating bubble inside a surfactant solution. The setup utilizes the Second Harmonic Generation (SHG)--effect at the air-solution interface generated by the light of a pulsed LASER. The set-up is described in detail, and the results of a first experimental series are presented and discussed in this paper. As system, aqueous solutions of the fluortenside F381 were used.

Journal Article↗
PubMed2004

Single particle light-scattering photometry--some fields of application.

Present developments in the field of bioscience will profit from the fabrication of multilayer films of polyelectrolytes on colloids as well as of nano- and microsized capsules. The single particle light-scattering technique is suitable for process control. This paper describes the mode of operation and efficiency of a laboratory-built single particle light-scattering photometer, which opens up new possibilities in applications, especially in the field of bioscience. This has been achieved by improving the measurement capabilities of organic particle sizes near 100 nm and precision of thickness changes of about 0.5 nm. Besides the classic applications, such as particle size distribution measurements at high resolution and the coagulation kinetics of colloids, the measurement of the layer-by-layer polyelectrolyte adsorption at colloid particle surfaces and the fabrication of colloidal carriers is introduced. The described method is also a precise method for controlling the removal of the latex core to obtain multilayer microcapsules.

Journal Article↗
PubMed2004

Self-assembly, optical behavior, and permeability of a novel capsule based on an azo dye and polyelectrolytes.

A novel capsule composed of an azo dye, Congo red (CR), and different polymers, including poly(styrenesulfonate, sodium salt) (PSS), poly(allylamine hydrochloride) (PAH), and poly(diallyldimethylammonium chloride) (PDDA), have been successfully fabricated by the layer-by-layer self-assembly technique. The stepwise linear deposition process was monitored by means of UV-visible absorption measurements. The formation of hollow capsules was verified by confocal laser scanning microscopy (CLSM) and scanning force microscopy (SFM). The resulting hollow PSS/PAH/CR/PDDA capsules displayed a sensitive response to visible light. Optical changes of the hollow capsules prior to and after the photoreaction were investigated in detail by means of UV-visible spectroscopy, CLSM, and SFM. It was found that the photochemical reaction of the assembled hollow capsules depends strongly on the matrix. Qualitative results on the permeability of the hollow capsule walls with CR as one component indicate that the permeability of the walls can be easily photo-controlled at varying irradiation time intervals without addition of external chemicals.

Azo Compounds↗
PubMed2004

Control of the water permeability of polyelectrolyte multilayers by deposition of charged paraffin particles.

We present a new way to protect polyelectrolyte multilayers from water, consisting in the adsorption and subsequent fusing of charged wax particles atop a multilayer. The formation of the wax layer is demonstrated by different techniques such as ellipsometry, contact angle measurements, and atomic force microscopy. The diffusion of water in protected and unprotected multilayers is studied by in situ neutron reflectometry. Whereas a top layer of wax crystals already allows substantial reduction of the diffusion, the fusion of this top layer leads to the dominating exclusion of water from the multilayers when dipped in water. This method opens up new interesting avenues for polyelectrolyte multilayers in practical applications where permeability of water, ions, or hydrophilic drugs is an issue.

Journal Article↗
PubMed2004

Engineering of layer-by-layer coated capsules with the prospect of materials for efficient and directed electron transfer.

Intermolecular electron transfer is investigated in a dye-doped polyelectrolyte (PE) multilayer film. Hollow PE capsules, with a mean diameter of 2 microm, were prepared by stepwise adsorption of a pyrene (PY)-labeled polyanion and various polycations onto charged colloids and subsequent dissolution of the colloidal core. The high concentration of dye molecules within the capsule wall and the control of the medium polarity on a nanometer length scale are proposed to facilitate light-induced charge separation over distances of a few nanometers. In particular, a PY-labeled poly(styrene sulfonate) (PSS-PY) has been synthesized and used as polyanion for the polyelectrolyte capsule preparation. A polarity gradient across the wall of the PE shells is assumed to be achieved by adsorbing diverse polycations at different film positions. The high effective film area followed by high optical density of the PE capsule solution enables time-resolved optical spectroscopy. Using pulsed excited state absorption (ESA) the transient absorption peaks of the radical anion and cation state of pyrene were measured, respectively. In the presence of additional electron donor (or acceptor) molecules in the capsule solution the pyrene anion (cation) is observed in the ESA spectra, while both transient states are seen if no additional molecules are present. These results are interpreted as an electron transfer from pyrene to the donor (acceptor) molecule or between two pyrene molecules. An asymmetry of the electron donor and electron acceptor efficiency was observed when multilayer shells were used that are supposed to carry an internal polarity gradient.

Journal Article↗
PubMed2004

Nanocapsules with functionalized surfaces and walls.

With the possibility of creating and handling nanometer-sized objects, it became popular to dream of future miniaturized tools allowing completely new applications in medicine and technology. Mother nature created during evolution many such self-assembled highly sophisticated and robust objects, and it is very tempting to modify them for other purposes: artificial cells as intelligent submarines floating in the bloodstream attacking cancer cells, single-molecule detection and manipulation allowing the bottom-up construction of devices, and so on. Nanobiotechnology became a hot topic and significant budgets have been attributed to it. However, caution has to be paid that unrealistic dreams might have a fairly negative effect on the taxpayer seeing unkept promises. In this paper, we explore the possibilities of using biological substances in material science.

Biocompatible Materials↗
PubMed2003

Phospholipase A2 hydrolysis of mixed phospholipid vesicles formed on polyelectrolyte hollow capsules.

Mixtures of the phospholipids L-alpha-dimyristoylphosphatidic acid (DMPA) and L-alpha-dipalmitoylphosphatidylcholine (DPPC) have been successfully adsorbed onto the charged surface of multilayer polyelectrolyte capsules to form a novel vesicle. Leaving such vesicles in phospholipase A(2) solution, we observed the hydrolysis reaction on the surface of the lipid/polymer vesicles and a permeability change before and after the reaction by confocal-laser scanning microscopy (CLSM). A capsule with adjustable permeability was constructed. This method may provide new features for drug-release vesicles.

1,2-Dipalmitoylphosphatidylcholine↗
PubMed2003

Direct observations of the cleavage reaction of an L-DPPC monolayer catalyzed by phospholipase A2 and inhibited by an indole inhibitor at the air/water interface.

The enzymatic hydrolysis of an L-dipalmitoylphosphatidylcholine (L-DPPC) monolayer at the air/water interface, catalyzed by phospholipase A(2) (PLA(2)), serves as a model for biospecific interfacial reactions. The cleavage of L-DPPC was investigated by Brewster angle microscopy. Different types of domain defects were observed to form in the coexisting liquid expanded and liquid condensed phases during the hydrolysis reaction. The adsorption of the enzyme was quantitatively recorded as the increase of the surface pressure over a fixed molecular area with time. In the case of L-DPPC, the surface pressure first increases and then starts to decrease, which indicates that a soluble product (lysolipid) is produced during the catalytic cleavage reaction. The increase and decrease of the surface pressure, which corresponds to the change of shape and number density of domains, indicated the occurrence of the following processes: adsorption of PLA(2), cleavage reaction, and rearrangement of substrate and product molecules at the interface. Addition of a PLA(2) inhibitor to the lipid monolayer leads to a fast surface pressure increase after enzyme injection. The surface pressure reaches a maximum value and then does not change for a long time. During this period, no change in the domain shape and number density was observed, which indicates that the enzyme is inhibited for a certain period of time. The experimental results provide the possibility of a direct way to prove inhibitor activity.

1,2-Dipalmitoylphosphatidylcholine↗
PubMed2003

Polymer-stabilized phospholipid vesicles formed on polyelectrolyte multilayer capsules.

Phospholipid vesicles on polyelectrolyte multilayer shells can be stabilized against ethanol by coating a single cationic polyelectrolyte. Confocal laser scanning microscopy (CLSM) proved that the lipids were stabilized by cationic polyelectrolytes and the permeability to small hydrophilic dyes was decreased. Measurements of fluorescence recovery after photo-bleaching (FRAP) with individual capsules enable quantification of release profiles.

Biopolymers↗
PubMed2003

Langmuir monolayers to study interactions at model membrane surfaces.

Langmuir monolayers at the liquid-air interface are well-defined interfacial systems and, therefore, excellent model systems to learn about interactions at interfaces beyond the classical DLVO description. Many parameters can be independently varied over a broad range and the structure can be analyzed with A precision. In the first part of the paper, the rich polymorphism in monolayers composed of amphiphilic molecules is demonstrated. Using homologues series generic phase diagrams can be derived. The delicate interplay of interactions causes a richness of phases which in turn can be used to measure fine variations in these interactions. Based on the understanding of the polymorphism in pure or mixed lipid monolayers, one can study the interaction of molecules dissolved in the subphase with monolayers. Samples presented are chemical reactions catalyzed by enzymes and coupling of polyelectrolytes to oppositely charged monolayers. To relate structure and reactivity, the activity of enzymes at the interface can be studied, predominantly combining X-ray diffraction and FTIR-spectroscopy. It is shown that the activity depends on monolayer structure. In one case, the reaction product leads to structural changes in the monolayer and stops the reaction, hence, indicating a subtle case of product inhibition via the membrane. On the other hand it has become possible to manipulate the organization of polyelectrolytes at interfaces via lipid charge density and ionic strength. In the most important case of DNA interacting with a membrane surface we show that DNA arranges at the interface in a lamellar manner, and the intermolecular distances, measured by Synchrotron X-ray diffraction can be varied by the lipid density.

Adsorption↗