Ferdinand Brandl

Hydrogel-based drug delivery systems: Comparison of drug diffusivity and release kinetics

Hydrogels are extensively studied as matrices for the controlled release of macromolecules. To evaluate the mobility of embedded molecules, these drug delivery systems are usually characterized by release studies. However, these experiments are time-consuming and their reliability is often poor. In this study, gels were prepared by step-growth polymerization of poly(ethylene glycol) (PEG) and loaded with fluoresceine isothiocyanate (FITC) labeled dextrans. Mechanical testing and swelling studies allowed prediction of the expected FITC-dextran diffusivity. The translational diffusion coefficients (D) of the incorporated FITC-dextrans were measured by fluorescence recovery after photobleaching (FRAP) and pulsed field gradient NMR spectroscopy. Because the determined values of D agreed well with those obtained from release studies, mechanical testing, FRAP, and pulsed field gradient NMR spectroscopy are proposed as alternatives to release experiments. The applied methods complemented each other and represented the relative differences between the tested samples correctly. Measuring D can therefore be used to rapidly evaluate the potential of newly developed drug delivery systems.

Enzymatically degradable poly(ethylene glycol) based hydrogels for adipose tissue engineering

Adipose tissue engineering requires biomaterials that promote the differentiation of seeded adipocytes. Here, we report on the development and characterization of in situ forming, poly(ethylene glycol) (PEG) based hydrogels for soft tissue augmentation. Branched PEG-amines were modified with collagenase-sensitive peptides and cross-linked with branched PEG-succinimidyl propionates without the use of free-radical initiators (enzymatically degradable hydrogels). Alanine-modified PEG-amines were used for the preparation of non-degradable gels. Depending on the used polymer concentration, the strength of degradable gels after swelling ranged from 1708 to 7412 Pa; the strength of non-degradable hydrogels varied between 1496 and 7686 Pa. Enzyme mediated gel degradation occurred within 10, 16, and 19 days (5%, 10%, and 15% initial polymer content). To evaluate their suitability as scaffold materials for adipose tissue engineering, the hydrogels were functionalized with the laminin-derived adhesion peptide YIGSR, and seeded with 3T3-L1 preadipocytes. Compared to a standard two-dimensional cell culture model, the developed hydrogels significantly enhanced the intracellular triglyceride accumulation of encapsulated adipocytes. Functionalization with YIGSR further enhanced lipid synthesis within differentiating adipocytes. Long-term studies suggested that enzymatically degradable hydrogels furthermore promote the formation of coherent adipose tissue-like structures featuring many mature unilocular fat cells.

Hydrogels in ophthalmic applications

More and more people worldwide are affected by severe eye diseases eventually leading to visual impairment or blindness. In most cases, the treatment involves the application of ophthalmic dosage forms such as eye drops, suspensions or ointments. Unfortunately, some of the therapeutic approaches have major shortcomings, especially in the treatment of the posterior segment of the eye, where many vision-threatening diseases originate. Therefore, research focuses on the development of new materials (e.g., for vitreous substitution) and more advanced drug delivery systems. Hydrogels are an extremely versatile class of materials with many potential applications in ophthalmology. They found widespread application as soft contact lenses, foldable intraocular lenses, in situ gelling formulations for ophthalmic drug delivery and ocular adhesives for wound repair; their use as vitreous substitutes and intravitreal drug delivery systems is currently under investigation. In this article, we review the different applications of hydrogels in ophthalmology with special emphasis placed on the used polymers and their suitability as ocular drug delivery systems.

Biodegradable hydrogels for time-controlled release of tethered peptides or proteins

Tethering drug substances to a gel network is an effective way of controlling the release kinetics of hydrogel-based drug delivery systems. Here, we report on in situ forming, biodegradable hydrogels that allow for the covalent attachment of peptides or proteins. Hydrogels were prepared by step-growth polymerization of branched poly(ethylene glycol). The gel strength ranged from 1075 to 2435 Pa; the degradation time varied between 24 and 120 h. Fluorescence recovery after photobleaching showed that fluorescently labeled bovine serum albumin (FITC-BSA) was successfully bound to the gel network during gel formation. Within 168 h, the mobility of the tethered molecules gradually increased due to polymer degradation. Using FITC-BSA and lysozyme as model proteins, we showed the potential of the developed hydrogels for time-controlled release. The obtained release profiles had a sigmoidal shape and matched the degradation profile very well; protein release was complete after 96 h.

The Diels–Alder reaction: A powerful tool for the design of drug delivery systems and biomaterials

Click reactions have the potential to greatly facilitate the development of drug delivery systems and biomaterials. These reactions proceed under mild conditions, give high yields, and form only inoffensive by-products. The Diels-Alder cycloaddition is one of the click reactions that do not require any metal catalyst; it is one of the most useful reactions in synthetic organic chemistry and material design. Herein, we highlight possible applications of the Diels-Alder reaction in pharmaceutics and biomedical engineering. Particular focus is placed on the synthesis of polymers and dendrimers for drug delivery, the preparation of functionalized surfaces, bioconjugation techniques, and applications of the Diels-Alder reaction in nanotechnology. Moreover, applications of the reaction for the preparation of hydrogels for drug delivery and tissue engineering are reviewed. A general introduction to the Diels-Alder reaction is presented, along with a discussion of potential pitfalls and challenges. At the end of the article, we provide a set of tools that may facilitate the application of the Diels-Alder reaction to solve important pharmaceutical or biomedical problems.

Protein Compatibility of Selected Cross-linking Reactions for Hydrogels

The compatibility of selected cross-linking reactions with lysozyme is investigated. Michael-type additions of nucleophilic amino acids to maleimide, vinyl sulfone and acrylamide groups are detected by gel electrophoresis. The degree of modification depends on the polymer and the pH. Complete modification with more than five PEG chains is observed after incubation with mPEG5k-vinyl sulfone at pH 9, whereas 96% of the protein remains unmodified after incubation with mPEG5k-acrylamide at pH 4. Incubation with mPEG5k-thiol results in thiol-disulfide exchange reactions. Hydrogel preparation is simulated by using polymer mixtures. Protein modifications are detected, which may affect the protein structure, decrease activity and bioavailability, and increase the risk for immune responses.

New insights into the cross-linking and degradation mechanism of Diels–Alder hydrogels

Eight-armed poly(ethylene glycol) was functionalized with furyl and maleimide groups. The two macromonomers were cross-linked by Diels-Alder (DA) reactions and the degradation behavior of the formed hydrogels was investigated. UV spectroscopy showed that maleimide groups were subject to ring-opening hydrolysis above pH 5.5, with the reaction rate depending on the pH and temperature. As a result of this, the gelation kinetics and stiffness of DA hydrogels were dependent on the temperature and the pH of the cross-linking medium, as demonstrated by rheological experiments. The gel time varied between 87.8 min (pH 3.0, 37 °C) and 374.7 min (pH 7.4, 20 °C). Values between 420 Pa (pH 9.0, 37 °C) and 3327 Pa (pH 3.0, 37 °C) were measured for the absolute value of the complex shear modulus. Hydrogel swelling and degradation were influenced by the same parameters. With increasing pH and temperature the degradation time was reduced from 98 days (pH 7.4, 20 °C) to 2 days (pH 7.4, 50 °C); no degradation was observed at pH 3.0 and 5.5. Molecular modeling studies of the DA and retro-Diels-Alder (rDA) moieties revealed that hydrogel degradation occurred by rDA reaction followed by OH--catalyzed ring-opening hydrolysis of maleimide groups to unreactive maleamic acid derivatives.

Ocular tissue engineering

In the early 1990s, tissue engineering emerged as a new concept to overcome the problem of tissue and organ failure. It proposed to supply engineered, yet biological, organ and tissue substitutes. It was anticipated that this technology would soon allow us to overcome donor shortages and graft rejection, the major limitations of tissue and organ transplantation. Tissue engineering approaches that were developed on the basis of this paradigm relied on the use of cells and stem cells, preferably of autologous origin, the application of growth factors and cytokines, the design of biodegradable scaffolds and bioreactor technology1, 2.

Diels-Alder Hydrogels for Controlled Antibody Release: Correlation between Mesh Size and Release Rate.

Eight-armed PEG, molecular mass 10 kDa, was functionalized with furyl and maleimide groups, respectively; the obtained macromonomers were cross-linked via Diels-Alder chemistry. The mesh size (ξ) of the prepared hydrogels was determined by swelling studies, rheology, and low field NMR spectroscopy. The in vitro release of fluorescein isothiocyanate labeled dextrans (FDs) and bevacizumab was investigated. The average mesh size (ξavg) increased from 5.8 ± 0.1 nm to 56 ± 13 nm during degradation, as determined by swelling studies. The result of the rheological measurements (8.0 nm) matched the initial value of ξavg. Low field NMR spectroscopy enabled the determination of the mesh size distribution; the most abundant mesh size was found to be 9.2 nm. In combination with the hydrodynamic radius of the molecule (Rh), the time-dependent increase of ξavg was used to predict the release profiles of incorporated FDs applying an obstruction-scaling model. The predicted release profiles matched the experimentally determined release profiles when Rh < ξavg. However, significant deviations from the theoretical predictions were observed when Rh ≥ ξavg, most likely due to the statistical distribution of ξ in real polymer networks. The release profile of bevacizumab differed from those of equivalently sized FDs. The delayed release of bevacizumab was most likely a result of the globular structure and rigidity of the protein. The observed correlation between ξ and the release rate could facilitate the design of controlled release systems for antibodies.

Hydrogels for Tissue Engineering

Today’s tissue engineering approaches rely on two different types of polymeric cell carriers. First, there are the well-established solid scaffolds, such as poly(α-hydroxy esters), which are generally based on lipophilic but hydrolytically degradable polymers that were originally designed as degradable sutures or drug-releasing matrix materials. Alternatively, new and promising strategies rely on hydrophilic polymer networks; these are based on hydrogels, which are also degradable polymers. Hydrophilic polymer network systems are suitable for many of the soft tissue engineering applications that do not require the strong mechanical support of solid scaffolds, but rather a flexible material that mimics the extracellular matrix (ECM). Highly hydrated hydrophilic polymer networks contain pores and void space between the polymer chains (Fig. 37.1); this provides many advantages over the common solid scaffold materials, including an enhanced supply of nutrients and oxygen for the cells. Pores within the network provide room for cells, and after proliferation and expansion, for the newly formed tissue. Their formation can be controlled using chemical modifications of the hydrogel network.