Formulation of Next-Generation Multicompartment Microcapsules by Reversible Electrostatic Attraction.

The combination of several active substances into one carrier is often limited due to solubility, stability and phase-separation issues. These issues have been addressed by an innovative capsule design, in which nanocapsules are assembled on the microcapsule surface by electrostatic forces to form a pH-responsive hierarchical capsule@capsule system. Here, melamine-formaldehyde (MF) microcapsules with a negative surface charge were synthesized and coated with a novel MF-polyethyleneimine (PEI) copolymer to achieve a positive charge of ζ=+28 mV. This novel coating procedure allows the electrostatic assembly of negatively charged poly-l-lactide (PLLA, ζ=-19 mV) and poly-(lactide-co-glycolide) (PLGA, ζ=-56 mV) nanocapsules on the microcapsule surface. Assembly studies at pH 7 gave a partial surface coverage of PLLA nanocapsules and full surface coverage for PLGA nanocapsules. The pH-responsive adsorption and desorption of nanocapsules was shown at pH 7 and pH 3.

Enzymatically active ultrathin pepsin membranes.

Enzymatically active proteins enable efficient and specific cleavage reactions of peptide bonds. Covalent coupling of the enzymes permits immobilization, which in turn reduces autolysis-induced deactivation. Ultrathin pepsin membranes were prepared by facile interfacial polycondensation of pepsin and trimesoyl chloride. The pepsin membrane allows for simultaneous enzymatic conversion and selective removal of digestion products. The large water fluxes through the membrane expedite the transport of large molecules through the pepsin layers. The presented method enables the large-scale production of ultrathin, cross-linked, enzymatically active membranes.

Challenges and advances in the field of self-assembled membranes.

Self-assembled membranes are of vital importance in biological systems e.g. cellular and organelle membranes, however, more focus is being put on synthetic self-assembled membranes not only as an alternative for lipid membranes but also as an alternative for lithographic methods. More investigations move towards self-assembly processes because of the low-cost preparations, structural self-regulation and the ease of creating composite materials and tunable properties. The fabrication of new smart membrane materials via self-assembly is of interest for delivery vessels, size selective separation and purification, controlled-release materials, sensors and catalysts, scaffolds for tissue engineering, low dielectric constant materials for microelectronic devices, antireflective coatings and proton exchange membranes for polymer electrolyte membrane fuel cells. Polymers and nanoparticles offer the most straightforward approaches to create membrane structures. However, alternative approaches using small molecules or composite materials offer novel ultra-thin membranes or multi-functional membranes, respectively. Especially, the composite material membranes are regarded as highly promising since they offer the possibility to combine properties of different systems. The advantages of polymers which provide elastic and flexible yet stable matrices can be combined with nanoparticles being either inorganic, organic or even protein-based which offers pore-size control, catalytic activity or permeation regulation. It is therefore believed that at the interface of different disciplines with each offering different materials or approaches, the most novel and interesting membrane structures are going to be produced. The combinations and approaches presented in this review offer non-conventional self-assembled membrane materials which exhibit a high potential to advance membrane science and find more practical applications.

Tuning the Morphology and Gas Separation Properties of Polysulfone Membranes.

The present work deals with the modification of casting solutions for polysulfone gas separation membranes fabricated by wet-phase inversion. The aim was to fabricate membranes with thin gas separation layers below one micrometer of thickness and a sponge-like support structure. With decreasing thicknesses of the separation layers, increasing permselectivities were observed. For the first time, we could show that permeabilities and diffusion coefficients of certain gases are orders of magnitude lower in separation layers of membranes below 500 Å of thickness compared to separation layers with a thickness above 1 micrometer. These results indicate that the selection of the solvent system has a huge impact on the membrane properties and that the permeability and diffusion coefficient are not material-related properties. Thus, they cannot be applied as specific indicators for gas-separating polymers. In this publication, scanning electron microscopy and gas permeation measurements were carried out to prove the gas separation properties and morphologies of polysulfone membranes.

Native supported membranes on planar polymer supports and micro-particle supports.

To bridge soft biological materials and hard inorganic materials is an interdisciplinary scientific challenge. Despite of experimental difficulties, the deposition of native biological membranes on supports is a straightforward strategy. This review provides an overview of advances in the fabrication and characterization of native biological membranes on planar polymer supports and micro-particles.

A Biocatalytically Active Membrane Obtained from Immobilization of 2-Deoxy-d-ribose-5-phosphate Aldolase on a Porous Support.

Aldol reactions play an important role in organic synthesis, as they belong to the class of highly beneficial C-C-linking reactions. Aldol-type reactions can be efficiently and stereoselectively catalyzed by the enzyme 2-deoxy-d-ribose-5-phosphate aldolase (DERA) to gain key intermediates for pharmaceuticals such as atorvastatin. The immobilization of DERA would open the opportunity for a continuous operation mode which gives access to an efficient, large-scale production of respective organic intermediates. In this contribution, we synthesize and utilize DERA/polymer conjugates for the generation and fixation of a DERA bearing thin film on a polymeric membrane support. The conjugation strongly increases the tolerance of the enzyme toward the industrial relevant substrate acetaldehyde while UV-cross-linkable groups along the conjugated polymer chains provide the opportunity for covalent binding to the support. First, we provide a thorough characterization of the conjugates followed by immobilization tests on representative, nonporous cycloolefinic copolymer supports. Finally, immobilization on the target supports constituted of polyacrylonitrile (PAN) membranes is performed, and the resulting enzymatically active membranes are implemented in a simple membrane module setup for the first assessment of biocatalytic performance in the continuous operation mode using the combination hexanal/acetaldehyde as the substrate.

Orientation-selective incorporation of transmembrane F0F1 ATP synthase complex from micrococcus luteus in polymer-supported membranes.

We report the vectorial incorporation of a highly asymmetric F0F1 ATP synthase complex from Micrococcus luteus into polymer-supported membranes. Dynamic light scattering and cryo electron microscopy confirm that the use of weak surfactants (bile acid) allows for the non-disruptive protein incorporation into lipid vesicles. Spreading of vesicles with ATP synthase onto a cellulose support results in a homogeneous distribution of proteins, in contrast to a patchy image observed on bare glass slides. The orientation of ATP synthase can be identified using an antibody to the ATP binding site as well as from topographic profiles of the surface. The method to "align" transmembrane proteins in supported membranes would open a possibility to quantify protein functions in biomimetic model systems.

Selective deposition of native cell membranes on biocompatible micropatterns.

We establish two methods to deposit native biomembranes (human erythrocyte membranes and sarcoplasmic reticulum membranes) selectively onto biocompatible microtemplates. The first method utilizes UV photolithography to micropattern the regenerated cellulose, while the second uses the "stamping" of protein barriers onto homogeneous cellulose supports. The relatively simple methods established here allow for the position selective spreading of three-dimensional native cells into two-dimensional films, retaining the orientation and lateral density of transmembrane proteins in their native state.