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Biomedical subjects

Helmuth Möhwald

Publications and source records attributed to Helmuth Möhwald.

At least 19 recordsLinked to original sources
PubMed2007

Multifunctional cargo systems for biotechnology.

One of the challenges in the field of bio-nanotechnology is the development of nano-sized delivery systems comprising different functionalities. These systems should carry bioactive substances to predefined site and unload them in controlled manner. Capsules assembled layer-by-layer have been intensively studied in the past few years owing to their ability to be modified, their capacity to encapsulate a wide range of chemicals, their responsiveness to different factors, and the variety of functionalities with which they can be enhanced. Current research focuses on the development of carriers with remote guiding and activation (optical, magnetic or ultrasound), thereby addressing unique in vivo applications with multifunctional biomaterials. Submicron-sized capsules are good models to mimic biochemical processes in a confined geometry that imitates cell organelles. Moreover, the cellular and tissue-targeted delivery of the capsules might serve as an intracellular reporter or enzymatic reactor. However, several obstacles still have to be overcome before capsule technology can be implemented. This article discusses possible solutions as well as promising applications.

Biotechnology↗
PubMed2007

Ordered binary arrays of Au nanoparticles derived from colloidal lithography.

By using angle-resolved colloidal lithography and O2-plasma etched bilayers of hexagonally packed spheres as templates, we succeeded in fabrication of highly ordered binary arrays of gold nanoparticles with varied shapes, for instance, with a shuttlecock-like shape composed of a small crescent-shaped nanoparticle and a big fan-shaped one. The size and shape of both small and big nanoparticles obtained were manipulated by the plasma etching period and the incidence angle of Au vapor flow. The subsequent thermal annealing led to binary arrays of round Au nanoparticles with a rather narrow distribution in terms of size and shape. Our approach should pave a simple and versatile colloidal way to form binary nanoparticle arrays, holding immense promise for technical applications such as nanoelectronics and nanophotonics.

Colloids↗
PubMed2006

Behavior of temperature-sensitive PNIPAM confined in polyelectrolyte capsules.

Layer-by-layer assembled polyelectrolyte microcapsules are of great interest because they can possibly be used as microcontainers and they show interesting stimuli-responsive properties, which have been recently investigated. Here, we exploit capsules which are made temperature-sensitive by encapsulating poly(N-isopropylacrylamide) (PNIPAM). PNIPAM has a cloud point in water at about 32 degrees C, above which it collapses and is insoluble in water. Further this temperature responsiveness can be tuned by addition of various ions at various concentrations. Here, we present the encapsulation of PNIPAM inside polyelectrolyte microcapsules, and describe the dependence of the lower critical solution temperature (LCST) on the nature and the amount of different salts added. With this information, we demonstrate the ability to tune and finely control the collapse of encapsulated PNIPAM. In this light, this system could be used as a microsensor or drug- delivery system.

Journal Article↗
PubMed2006

Thermal behavior of polyelectrolyte multilayer microcapsules: 2. Insight into molecular mechanisms for the PDADMAC/PSS system.

Polyelectrolyte multilayer capsules consisting of poly(diallyldimethylammonium chloride) (PDADMAC) and poly(styrene sulfonate) (PSS) were used as a model system to study the temperature-dependent behavior of polyelectrolyte multilayer films in aqueous media. Shells terminated with PSS shrink upon heating, whereas PDADMAC-terminated ones swell, independent of the nature of the first layer, as measured by means of confocal laser scanning microscopy (CLSM) and scanning electron microscopy (SEM). Elemental analysis shows that the initial exponential layer growth of the film leads to a nearly neutral overall charge in the first case or a high positive excess charge in the latter. Depending on this overall charge either surface tension, due to an unfavorable polymer-solvent interaction, or electrostatics dominates, resulting in a shrinkage or expansion of capsules, respectively. Thus, it is possible to swell temperature-shrunk capsules by coating them with an additional PDADMAC layer. Micro-DSC measurements prove that polyelectrolyte multilayers undergo a glass transition in water at which the wall material softens, allowing the rearrangements to occur. It is found that the thermal history has an influence on the temperature behavior of capsules, especially on those ones terminated with PDADMAC. Also, the molecular weight of the polyelectrolytes affects the rearrangement of capsules. The lower the molecular weight and thus the smaller the entanglement of chains, the easier polyelectrolytes can rearrange.

Journal Article↗
PubMed2006

Weak first-order tilting transition in monolayers of mono- and bipolar docosanol derivatives.

A systematic analysis of pressure-area isotherms and grazing incidence X-ray diffraction (GIXD) data of 22-methoxydocosan-1-ol (H3C-O-(CH2)22-OH, MDO), docosan-1-ol (H3C-(CH2)21-OH, DO), and docosyl methyl ether (H3C-(CH2)21-O-CH3, DME) monolayers on pure water between 10 and 35 degrees C is presented. All monolayers form fully condensed phases in the investigated temperature region. The GIXD data reveal that the monolayers exhibit the phase sequence -S at lower temperature and -LS at higher temperature. Phase diagrams have been established. Inserting a second hydrophilic group at the opposite end of the molecule (bipolar MDO) shifts the S/LS boundary to higher temperatures. All monolayers exhibit herringbone (HB) packing at lower temperatures. The "kink" in the isotherms observed at lower temperatures is replaced by a very small plateau region at higher temperatures. The entropy changes connected with this weak first-order tilting transition are much smaller compared with the first-order transition from liquid-expanded (LE) to condensed (LC). Additionally, this transition is endothermic in contrast to the LE/LC transition. The reason for the endothermic transition is the weaker positional correlation in the nontilted state compared with the tilted one. The appearance of the weak first-order endothermic transition can be connected with the changed phase sequence. X-ray photoelectron spectroscopy (XPS) measurements provide information about the polar group orientation. Considerations based on GIXD and XPS data as well as adhesion energy of the different terminal end groups lead to the conclusion that the hydroxyl group of the bipolar MDO is attached to the water surface while the methoxy group is in contact with air. The presented results show that the second hydrophilic group influences the monolayer properties in a mild way.

Fatty Alcohols↗
PubMed2006

Salt-induced swelling-to-shrinking transition in polyelectrolyte multilayer capsules.

We study the size of polyelectrolyte multilayer capsules as a function of ionic strength, temperature, and time. A dynamic micromechanical model is developed which successfully describes the experiments. The model includes the polymer-solvent surface tension, an electrostatic force which is strongly ionic strength dependent, and a temperature-dependent mobility parameter. The activation of >50 kT suggests that multiple ion pairs must be broken simultaneously in the process of chain rearrangement. In support of our physical model capsules can repeatedly swell and shrink by varying ionic strength.

Journal Article↗
PubMed2006

Redox-controlled molecular permeability of composite-wall microcapsules.

Many smart materials in bioengineering, nanotechnology and medicine allow the storage and release of encapsulated drugs on demand at a specific location by an external stimulus. Owing to their versatility in material selection, polyelectrolyte multilayers are very promising systems in the development of microencapsulation technologies with permeation control governed by variations in the environmental conditions. Here, organometallic polyelectrolyte multilayer capsules, composed of polyanions and polycations of poly(ferrocenylsilane) (PFS), are introduced. Their preparation involved layer-by-layer self-assembly onto colloidal templates followed by core removal. PFS polyelectrolytes feature redox-active ferrocene units in the main chain. Incorporation of PFS into the capsule walls allowed us to explore the effects of a new stimulus, that is, changing the redox state, on capsule wall permeability. The permeability of these capsules could be sensitively tuned via chemical oxidation, resulting in a fast capsule expansion accompanied by a drastic permeability increase in response to a very small trigger. The substantial swelling could be suppressed by the application of an additional coating bearing common redox-inert species of poly(styrene sulfonate) (PSS(-)) and poly(allylamine hydrochloride) (PAH(+)) on the outer wall of the capsules. Hence, we obtained a unique capsule system with redox-controlled permeability and swellability with a high application potential in materials as well as in bioscience.

Capsules↗
PubMed2006

Ultrasonically induced opening of polyelectrolyte microcontainers.

The effect of ultrasonic treatments of different intensity and duration on the integrity and permeability of polyelectrolyte capsules was investigated both in poly(allylamine)/poly(styrene sulfonate) and Fe(3)O(4)/poly(allylamine)/poly(styrene sulfonate) polyelectrolyte capsules. Ultrasonic treatment of polyelectrolyte capsules induces the destruction of the polyelectrolyte shell and the release of the encapsulated material even at short (5 s) sonification times. The presence of magnetite nanoparticles significantly improves the efficiency of the ultrasonically stimulated release of the encapsulated compounds and enables magnetically controlled delivery to the desired site before ultrasonic treatment. Release of the encapsulated compound induced at ultrasonic power comparable to those of ultrasonic generators applied in medicine, demonstrating practical application of the ultrasonically triggered capsule opening in medicine.

Allylamine↗
PubMed2006

Manipulation of aqueous growth of CdTe nanocrystals to fabricate colloidally stable one-dimensional nanostructures.

The present article is devoted to systematically exploring the influence of various experimental variables, including the precursor concentration, the ligand nature, the counterion type, the Cd-to-Te molar ratio, pH, and temperature, on the aqueous growth of CdTe nanocrystals. The growth may be divided into two stages: the early fast growth stage and the later slow growth stage. The later stage is found to be dominated by Ostwald ripening (OR), being strongly dependent on all experimental conditions. In contrast, the early stage is dominated by adding monomers to nanocrystals, which may be dramatically accelerated by lowering precursor concentrations and using ligands with a molecular structure similar to that of thioglycolic acid (TGA). This fast growth stage is similar to that observed during organometallic growth of nanocrystals in hot organic media. On the basis of this finding, one-dimensional wurtzite CdTe nanostructures can be directly prepared in aqueous media by storing rather dilute precursor solution (2.4 mM with reference to ligand) in the presence of TGA at lower temperature (from room temperature to 80 degrees C). A low growth temperature is used to suppress OR during crystal growth. In addition, the simultaneous presence of both TGA-like ligand and 1-thioglycerol or 2-mercaptoethylamine leads to formation of colloidally stable 1D CdTe nanostructures with controlled aspect ratios.

Journal Article↗
PubMed2006

Sonochemical nanosynthesis at the engineered interface of a cavitation microbubble.

This work demonstrates the potential of sonochemistry to become a most valuable tool for nanotechnology if the underlying complex processes are understood and controlled. It is shown that control of cavitation requires controlling of interfaces at the microsec time scale and thus a diversity of phenomena are observed using different surfactants. By means of concrete examples it is shown that non-equilibrium nanostructures, e.g. Ru/Pt core-shell nanoparticles can be prepared. Hollow microbubbles with nanoparticles in the wall, which may contain gases over weeks, can be constructed by sonication of the corresponding colloid solutions.

Microbubbles↗
PubMed2006

Equilibrium distribution of permeants in polyelectrolyte microcapsules filled with negatively charged polyelectrolyte: the influence of ionic strength and solvent polarity.

The effects of ionic strength and solvent polarity on the equilibrium distribution of fluorescein (FL) and FITC-dextran between the interior of polyelectrolyte multilayer microcapsules filled with negatively charged strong polyelectrolyte and the bulk solution were systematically investigated. A negatively charged strong polyelectrolyte, poly(styrene sulfonate) (PSS), used for CaCO3 core fabrication, was entrapped inside the capsules. Due to the semipermeability of the capsule wall, a Donnan equilibrium between the inner solution within the capsules and the bulk solution was created. The equilibrium distribution of the negatively charged permeants was investigated by means of confocal laser scanning microscopy as a function of ionic strength and solvent polarity. The equilibrium distribution of the negatively charged permeants could be tuned by increasing the bulk ionic strength to decrease the Donnan potential. Decreasing the solvent polarity also could enhance the permeation of FL, which induces a sudden increase of permeation when the ethanol volume fraction was higher than 0.7. This is mainly attributed to the precipitation of PSS. A theoretical model combining the Donnan equilibrium and Manning counterion condensation was employed to discuss the results.

Journal Article↗
PubMed2006

Microcapsules made of weak polyelectrolytes: templating and stimuli-responsive properties.

Hollow microcapsules composed of the weak polyelectrolytes poly(allylamine hydrochloride) (PAH) and poly(methacrylic acid) (PMA) are templated on silicon oxide particles using the layer-by-layer adsorption. The colloidal template is removed with a buffer system of hydrofluoric acid and ammonium fluoride. With this buffer system, the template can be dissolved in mild pH conditions, where the polymeric layers are still stable. The morphology and the thickness of the resulting capsules are investigated with atomic force microscopy. The resulting hollow capsules show pH-dependent properties. The shells are stable over a broad pH range and swell and immediately dissolve for pH values below 2.3 and above 11. If the molecular weight of the poly(methacrylic acid) is increased, the enhanced entanglement of the polymers results in a reversible swelling of the capsules at low and at high pH. The swelling degree is probed with confocal laser scanning microscopy. In addition to the pH-dependent size variations, the different ionization degree of poly(methacrylic acid) as a function of pH is used for the selective binding of calcium ions.

Binding Sites↗
PubMed2006

Directing the self-assembly of nanocrystals beyond colloidal crystallization.

This article gives an overview of recent progress in the self-assembly of nanocrystals. Classic self-assembly of nanocrystals, so-called colloidal crystallization driven by van der Waals interactions, is highlighted first with an emphasis on the recent realization of binary colloidal crystals. Next, new developments in the integration of nanocrystals into clusters based on electrostatic interactions, hydrogen bonding and dipole-dipole interactions are summarized, shedding light on the defined control of the interactions between the nanocrystals. Finally, the fabrication of heterogenous nanocrystals, obtained via either phase selective modification at the water/oil interface or facet-selective crystal growth on non-spherical nanocrystals is discussed. These last materials may provide significant building blocks for mimicking molecular self-assembly.

Adsorption↗
PubMed2006

Fluorination of the hydrophilic head accelerates the collapse of the monolayer but stabilizes the bilayer of a long-chain trifluoroethyl ether on water.

A comparison of the collapse of Langmuir monolayers of docosyl trifluoroethyl ether (DFEE) and docosyl ethyl ether (DEE) on water shows that in both films the 3D phase is formed layer-by-layer. The substitution of CH3 by a CF3 group in the hydrophilic head yields a more stable bilayer exhibiting lower equilibrium spreading pressure, pi(esp)(DFEE) < pi(esp)(DEE). Upon lateral compression, the DFEE bilayer fractures abruptly as a compact solid body whereas the DEE bilayer breaks down gradually as a polycrystalline material. A comparison of the collapse kinetics of the two films at the same constant supersaturation pi-pi(esp) = 7 mN/m shows that the fluorinated DFEE monolayer transforms more quickly, yielding a stable bilayer of closely packed upright molecules, whereas the DEE film undergoes a continuous monolayer-bilayer-multilayer transition. Brewster angle microscopy allows us to visualize different collapse mechanisms of the DFEE and DEE films; the domains of the fluorinated DFEE bilayer grow laterally at constant thickness and density, and the collapse of the nonfluorinated DEE monolayer occurs through a sequence of disordered stripelike and broken elongated textures. The characteristic molecular areas of the monolayer and bilayer collapse suggest that the 2D-3D transition in the DFEE and DEE films is accompanied by at least partial dehydration of their headgroups. The faster collapse of the fluorinated monolayer could result from a lower energy barrier due to the more hydrophobic CF3 group in the heads. The increased stability of the DFEE bilayer could be associated with the electrostatic attraction between the -C(F delta-)3 versus (H delta+)3C- terminals at the heads-to-tails contact plane of the top and the bottom layer, contrasting with the repulsion between the -C(H delta+)3 versus (H delta+)3C- terminals of the top-layer heads and the bottom-layer tails in the DEE bilayer.

Journal Article↗
PubMed2006

Microcontainers with electrochemically reversible permeability.

The present study demonstrates a novel application of polyelectrolyte microcapsules as microcontainers with electrochemically reversible flux of redox active materials into and out of the capsule volume. Incorporation of the capsules inside the conducting polymer (polypyrrole) film results in a new composite electrode combining electrocatalytic and conducting properties of the polypyrrole with the storage and release properties of the capsules. This electrode, if loaded with electrochemical fuels, can possess electrochemically controlled switching between "open/closed" states of the capsule shell and be of practical interest for a new type of chemically rechargeable batteries or fuel cells. A special explanation for the potential depending loading and unloading of the capsule inner volume may be related to the fact that the polyelectrolyte capsules experience a potential gradient in the polypyrrole matrix within which the polyions of the shell can be moved.

Journal Article↗
PubMed2006

Two-compartment micellar assemblies obtained via aqueous self-organization of synthetic polymer building blocks.

We synthesized a symmetric linear ABCBA pentablock copolymer consisting of poly(ethylene oxide), poly(gamma-benzyl l-glutamate), and a poly(perfluoro ether) (fluorolink). The different blocks are highly immiscible with each other and form two-compartment micelles of mainly cylindrical shape in aqueous solution with lengths in the range of 100 to 200 nm and diameters of about 24 nm. The poly(perfluoro ether) (C blocks) forms the liquidlike center of the micelles (d = 6 nm). This is surrounded by a first shell of ca. 2 nm thickness consisting of beta-sheets of poly(gamma-benzyl l-glutamate) (B blocks) and a second 7 nm shell of poly(ethylene oxide) (A blocks). The A blocks provide water solubility, and the B and C blocks form separated hydrophobic compartments. This work is a contribution to the development of multicompartment micelles devoted to mimic transport proteins such as serum albumins in long-term development.

Journal Article↗