Henning Menzel

Encapsulation of proteins in hydrogel carrier systems for controlled drug delivery influence of network structure and drug size on release rate

Novel hydrogels based on hydroxyethyl starch modified with polyethylene glycol methacrylate (HES-P(EG)₆MA) were developed as delivery system for the controlled release of proteins. Since the drug release behavior is supposed to be related to the pore structure of the hydrogel network the pore sizes were determined by cryo-SEM, which is a mild technique for imaging on a nanometer scale. The results showed a decreasing pore size and an increase in pore homogeneity with increasing polymer concentration. Furthermore, the mesh sizes of the hydrogels were calculated based on swelling data. Pore and mesh size were significantly different which indicates that both structures are present in the hydrogel. The resulting structural model was correlated with release data for bulk hydrogel cylinders loaded with FITC-dextran and hydrogel microspheres loaded with FITC-IgG and FITC-dextran of different molecular size. The initial release depended much on the relation between hydrodynamic diameter and pore size while the long term release of the incorporated substances was predominantly controlled by degradation of the network of the much smaller meshes.

Mixed silane self assembled monolayers and their in situ modification

Mixed self assembled silane monolayers were prepared by coadsorption of Br and methyl terminated trichlorosilanes. By subsequent in situ modification the Br groups were converted into amino groups which can be used to tether polyglutamates to the surface. Here we describe the characterization of the monolayers by means of contact angle measurements and X-ray photoelectron spectroscopy (XPS). Both methods clearly indicate that the Br terminated silane is preferentially adsorbed from the solution. Following the in situ modification by XPS a yield of approximately 80% was determined for the substitution of the bromine by azide, whereas the yield for subsequent reduction to the amine is close to 100%.

A review of developments in electrospinning technology: New opportunities for the design of artificial tissue structures

Purpose As a technology for the production of micro- and nano structured scaffold materials, electro-spinning has gained widespread acceptance in the medical research community over the last decade. The process generates a non-woven fiber mat consisting of one continuous filament with diameters ranging from the micron to the nanometer range. Because of its similarity to the filamentous microenvironment in native tissues, it is most often used as scaffold material in tissue engineering applications. This similarity promotes a more positive cell response to the generated fibers than to bulk material alone. However, because current electrospinning equipment does not yet fully utilize the potential of the process technology, the reproducibility of the scaffold structure is often limited. It is thus the goal of this review to describe the current state of electrospinning process technology with respect to the design of artificial tissue structures. Method This review is based on a comprehensive examination conducted in 2007 and 2008 of patents filed with the European Patent Organization and other scientific publications. Results It describes selected technical developments in electrospinning related to the production of non-woven fiber scaffolds and their implications in the design of artificial tissue structures. Specifically, it describes techniques for the production of aligned fiber structures, multilayered, multiscaled and multifiber scaffolds, fiber modification and functionalization, and useful advances in process control. It also presents technical sketches showing in detail how to implement the techniques presented into electrospinning equipment setups. Finally, it discusses remaining limitations that pertain to the design of scaffold materials.

PHOTOINDUCED ALIGNMENT OF AZOBENZENE MOIETIES IN THE SIDE-CHAINS OF POLYGLUTAMATE FILMS

Abstract The ‘trans↔cis’ reversible photoisomerization process tends to align azobenzene derivatives perpendicular to the polarization direction of the pumping beam. It is shown that in the trans→cis optical transition the cis state is aligned perpendicular to the pumping light polarization. This is shown for spin-coated films of ‘hairy-rod’ polyglutamate with azobenzene moieties in the side chains. Photoselection in both trans and cis states is demonstrated using the attenuated total reflection method.

Self-Assembled Antimicrobial and Biocompatible Copolymer Films on Titanium

Copolymers of 4-vinyl-N-hexylpyridinium bromide and dimethyl(2-methacryloyloxyethyl) phosphonate self-assemble to form ultrathin layers on titanium surfaces that show antimicrobial activity, and biocompatibility. The copolymer layers are characterized by contact angle measurements, ellipsometry and XPS. Antibacterial activity is assessed by investigation of adherence of S. mutans. Biocompatibility is rated based on human gingival fibroblast adhesion and proliferation. By balancing the opposing effects of the chemical composition on biocompatibility and antimicrobial activity, copolymer coatings are fabricated that are able to inhibit the growth of S. mutans on the surface but still show attachment of gingival fibroblasts, and therefore might prevent biofilm formation on implants.

Hydroxyethyl starch-based polymers for the controlled release of biomacromolecules from hydrogel microspheres.

Hydrogels are promising delivery systems for the controlled release of biomacromolecules. Based on previous studies, hydrogels were prepared from crosslinkable hydroxyethyl starch with new linker groups to improve mechanical and release properties of the resulting hydrogels. Polyethylene glycol methacrylate with two different spacer lengths was used to obtain polymers (HES-P(EG)(n)MA) with increased hydrophilicity and degradability, whereas a polymer with methacrylate linker directly at the starch backbone (HES-MA) resulted in a less degradable polymer. Hydrogel disks were obtained by UV crosslinking and characterized by swelling and rheological measurements. The hydrogel strength was strongly influenced by the polymer concentration. Using a water-in-water emulsion process, hydrogel microspheres were prepared. The influence of the type of the linker, the degree of substitution and the phase ratio in the production process on the properties of the microspheres was investigated. Depending on the preparation parameters, particles with narrow particle size distribution and encapsulation efficiencies of up to more than 80% for FITC-dextran 70 kDa (FD70) were obtained. Incorporated FITC-labeled IgG showed a faster release from hydrogel microspheres than FD70. The release rate of incorporated FD70 could be adjusted by using different polymers (HES-P(EG)(10)MA>HES-P(EG)(6)MA>HES-MA).

Chemical properties of polyamines with relevance to the biomineralization of silicaElectronic Supplementary Information (ESI) available: detailed information about the synthesis and the kinetic investigations. See http://www.rsc.org/suppdata/cc/b3/b310201g/

Polyamines mimicking substances which occur naturally in biosilicas have been synthesized and show an accelerating effect on silica condensation, which depends on the chemical nature, the architecture (linear or branched), and the degree of polymerization.

Comparison of in vitro and in vivo protein release from hydrogel systems

Hydrogel systems based on hydroxyethyl starch-polyethylene glycol methacrylate (HES-P(EG)(6)MA) or hydroxyethyl starch methacrylate (HES-MA) were used to assess the protein release behavior. Here, we analyzed the in vitro release of FITC-anti-human antibodies incorporated in either HES-P(EG)(6)MA or HES-MA hydrogel delivery systems in PBS or human serum. In addition, hydrogel disks and microparticles prepared from the two polymers were subcutaneously implanted in BALB/c mice. The in vivo release of FITC-IgG was non-invasively monitored by an in vivo imaging system (IVIS 200) over a time period of up to 3 months. The imaging system allowed to asses individual animals over time, therefore only a small number of animals was required to obtain high quality data. The reduction in fluorescence intensity at the site of administration was compared to in vitro release profiles. These investigations demonstrated a sustained release from HES-MA hydrogel disks compared to rapidly degrading HES-P(EG)(6)MA disks and microparticles. The sustained release from HES-MA disks could be further optimized by using increased polymer concentrations. Human serum as in vitro release medium reflected better the in vivo release from HES-P(EG)(6)MA systems than PBS, suggesting that the presence of organic substances like proteins or lipids may play a significant role for the release kinetics.

Influence of polymeric additives on biomimetic silica deposition on patterned microstructures

Microstructured polymer films prepared by photochemical grafting of different polymers were used as restricted reaction areas in silica deposition experiments. Linear and branched poly(alkyleneimines) and poly(allylamine hydrochloride) in pure aqueous or phosphate-containing solutions were used as additives to silica precursor solutions. The silica deposits obtained by spin-coating these solutions onto microstructured polymer films were investigated by scanning electron microscopy and atomic force microscopy. Experiments with poly(alkylene imines) in the silica precursor solution show the deposition of smooth and granular silica structures that closely mimic the natural patterns. The structure formation can be explained by physicochemical processes. Hypotheses that have been made for the natural silification processes can be evaluated on this basis.

Screening of photochemically grafted polymer films for compatibility with osteogenic precursor cells.

Surfaces of biomaterials often do not have the ideal properties for direct application in vivo. Although titanium and its alloys show a good biocompatibility, in some applications there is still need to improve the osteoblast adhesion to titanium implants. A polymeric surface coating is an ideal solution because the polymer can be adjusted to the needs of the application and can be bound to the surface by the photochemical grafting method. Therefore, 22 different polymers were tested for their compatibility using a murine mesenchymal progenitor cell line and three polymers were identified for which more elaborate investigations are reasonable. It was investigated whether or not the results of the cell culture test can be correlated with, e.g., the wetting properties. Indeed it was found that a contact angle above approx. 45° was necessary for good cell adhesion and proliferation. However, otherwise no clear correlation between the contact angle hysteresis or the functionalities of the polymers and the cell growth was observed.