Biodegradable three-dimensional networks of poly(dimethylamino ethyl methacrylate). Synthesis, characterization and in vitro studies of structural degradation and cytotoxicity.

In ophthalmology, there is a need for novel degradable biomaterials for e.g. controlled drug release in the vitreous body. These degradable materials should feature both excellent biocompatibility, and well-defined kinetics of degradation. In most cases, poly(D,L-lactic acid), or poly(lactic-co-glycolic acid) are used. These materials, however, suffer from some serious drawbacks, since the degradation kinetics are difficult to control, especially since the so-called 'burst-degradation' occurs. Here, we describe a set of novel polymeric networks which largely consist of poly(dimethylamino ethyl methacrylate) (poly(DMAEMA)); these materials are crosslinked via a dimethacrylate molecule that contains two carbonate groups. This system is susceptible to hydrolytic scission. The degradation products do not exert a catalytic effect on the ongoing degradation reaction (i.e. there is no burst effect). We describe the synthesis of three of these materials, which differ merely with regard to the crosslinker content. These materials were characterized through DMTA, 1H NMR and FT-IR spectroscopy, and scanning electron microscopy. The reaction DMAEMA + 2-hydroxyethyl methacrylate (HEMA) was studied in detail, using 1H NMR spectroscopy, and these experiments revealed that the reaction of DMAEMA and HEMA produces a random (Bernouillian-type) copolymer. From this, we contend that the new materials have more or less uniform distribution of the crosslinks throughout their volume. Structural degradation of the three materials was studied in vitro, at pH 7.4, 9.1 and 12.0. It is found that the materials exhibit smooth hydrolysis, which can be controlled via the crosslink density and the pH, as was expected a priori. It should be noted that degradation of these materials produces non-hydrolysable, but water-soluble, oligo(DMAEMA) and poly(DMAEMA) molecules. We subsequently performed in vitro studies on the biocompatibility of these materials. The MTT cytotoxicity assay revealed that the materials were cytotoxic to chondrosarcoma cells. This is most probably due to local increase of the pH due to the basic character of the pending dimethylamino groups. Cytotoxicity remained virtually unchanged after extended washing with water. This indicates that the cytotoxicity is an intrinsic property of the material and was not caused by remnants of free monomer. Cytotoxicity was also seen in cell cultures (human fibroblasts isolated from donor corneas) which were grown in contact with the materials. It is concluded that the new materials have attractive degradation characteristics, but their cytotoxicity makes them unsuitable for applications in ophthalmology.

The use of polymer-based electrospun nanofibers containing amorphous drug dispersions for the delivery of poorly water-soluble pharmaceuticals.

Electrostatic spinning was applied to the preparation of drug-laden nanofiber for potential use in oral and topical drug delivery. While this technique is in its infancy with regard to pharmaceutical applications, a number of recent publications suggest that it may be of high value in the formulation of poorly water-soluble drugs by combining nanotechnology and solid solution/dispersion methodologies. The purpose of this article is to describe some of these recently published applications. For immediate release oral application, a water-soluble cellulose polymer was selected (i.e., hydroxypropylmethylcellulose, HPMC) while for topical application, a nonbiodegradable, water-insoluble polymer was investigated (i.e., a segmented polyurethane, SPU). Solutions of the polymer and the drugs in appropriate solvents could be spun across various potentials (16-24 kV) generating nanofibers with diameters ranging from 300 to 2000 nm. Dissolution studies found that the non-woven fabrics derived from HPMC and containing itraconazole dissolved over a time course of minutes to hours depending on the formulation used as well as the drug/polymer ratios. Drug release from the SPU samples was dependent on the incorporated drug as well as nanostructure obtained.

Passage rate and total clearance rate from the rumen of cows fed on grass silages differing in cell-wall content.

Four non-lactating, rumen-fistulated cows were fed ad lib. on two grass silages (first cut (FC) and second cut (SC) harvested at different growth stages, resulting in different crude-protein (CP) and neutral-detergent-fibre (NDF) contents (FC, 152g CP/kg, 515g NDF/kg and SC, 210g CP/kg, 442g NDF/kg). Voluntary intake and rumen contents, total as well as organic matter were higher for silage FC. Fractional passage rate from the rumen, calculated from the logarithmic decline in Cr-NDF rumen pool, was higher for silage FC (0.0395/h and 0.0446/h for silages SC and FC respectively). When fractional passage rates from the rumen were calculated by dividing the intake of indigestible organic matter by the mean rumen pool of this fraction, the same differences between silages were found, although the actual levels were much lower (0.0258/h and 0.0300/h for silages SC and FC respectively). The results from the present experiment suggest that disappearance rate from the rumen of particles with a size between 1.25 and 0.071 mm is the rate-limiting step in the control of rumen fill.

Absorption of volatile fatty acids from the rumen of lactating dairy cows as influenced by volatile fatty acid concentration, pH and rumen liquid volume.

The effect of rumen liquid volume, pH and concentration of volatile fatty acids (VFA) on the rates of absorption of acetic, propionic and butyric acids from the rumen was examined in lactating dairy cows. Experimental solutions introduced into the emptied, washed rumen comprised two different volumes (10 or 30 l), four levels of pH (4.5, 5.4, 6.3, 7.2) and three levels of individual VFA concentrations (20, 50 or 100 mM-acetic, propionic or butyric acid). All solutions contained a total of 170 mM-VFA and an osmotic value of 400 mOsmol/l. Absorption rates were calculated from the disappearance of VFA from the rumen corrected for passage with liquid phase to the omasum. An increase in initial fluid pH caused a reduction in fractional absorption rates of propionic and butyric acids. Increasing the initial pH from 4.5 to 7.2 reduced fractional absorption rates of acetic, propionic and butyric acids from 0.35, 0.67 and 0.85 to 0.21, 0.35 and 0.28/h respectively. The fractional absorption rates of all VFA were reduced (P < 0.05) by an increase in initial rumen volume. The fractional absorption rate of acetic acid was lower (P < 0.05) at an initial concentration of 20 mM than of 50 mM. The fractional absorption rate of propionic acid tended (P < 0.10) to decrease as the level of concentration increased while fractional absorption rate of butyric acid was not affected by butyric acid concentration. These results indicate that relative concentrations of VFA in rumen fluid might not represent relative production rates and that attempts to estimate individual VFA production from substrate digestion must take account of pH and VFA concentration.

Tailoring of new polymeric biomaterials for the repair of medium-sized corneal perforations.

The aim of this study was to investigate whether polymeric biomaterials can be designed such that they become suitable for surgical closure of medium-sized perforations in the cornea, the transparent tissue in the front of the eye. Such a biomaterial must meet stringent requirements in terms of hydrophilicity, strength, transparency, flexibility, and biocompatibility. Four different copolymers of n-butyl methacrylate (BMA) and hexa(ethylene glycol) methacrylate (HEGMA) were prepared and characterized. Poly(BMA) was made as a reference material. Physicochemical properties were measured (contact angles, glass-transition temperatures, thermal degradation, water uptake and swelling), and cytotoxicity in vitro was assessed with a MTT test. Moreover, the interaction between the materials and cultured human corneal epithelial cells was studied. The copolymers may be useful for temporary closure of corneal perforations.

New biodegradable networks of poly(N-vinylpyrrolidinone) designed for controlled nonburst degradation in the vitreous body.

Polymers of N-vinylpyrrolidinone (NVP) are known to have excellent biocompatibility when implanted in the vitreous body or used as a vitreous substitute. Although poly(NVP) is capable of absorbing relatively large amounts of water, it is not prone to hydrolysis. Yet intraocular degradation of several crosslinked poly(NVP) hydrogels has been reported recently, but some ambiguity remains about the exact mechanism of degradation of these materials. To date there is no biomaterial that combines the excellent intraocular biocompatibility on the one hand and controlled kinetics of degradation on the other hand. We attempted to design and prepare such materials through the chemical synthesis of a novel dimethacrylate crosslinker molecule. The essential feature of this molecule is that its core contains two carbonate groups, which are evidently susceptible to hydrolytic scission. We studied a series of 3-dimensional networks of poly(NVP), which were crosslinked by this molecule. This approach offers several advantages: the hydrolysis of the carbonate groups in the crosslinks leads to liberation of poly(NVP) and/or oligo(NVP) chains that can probably be cleared from the eye via phagocytosis; hydrolysis generates two alcohols and CO(2) (i.e., there is no catalytic burst effect); when these materials are implanted in dry form, swelling and degradation will progress from the exterior of the material toward its interior. Therefore, these materials can be designed such that surface degradation rather than bulk degradation occurs; the hydrolysis rate can be controlled via the crosslink density or through synthesis of other crosslink molecules with either more (>2) or less (1) carbonate groups or alternatively with one or more other labile groups. We report on the chemical synthesis of the crosslinker molecule, as well as the preparation and degradation of a series of poly(NVP)-based hydrogels in vitro and in vivo (rabbit eyes). We found that these materials indeed displayed excellent biocompatibility in the rabbit eye. Further, the experiments confirmed that degradation occurs without the burst effect. The results are in line with the idea that the rate of intraocular swelling and degradation depends on the crosslink density, but this is only a preliminary conclusion that must be strengthened by much more experimental work. Nonetheless, we foresee several applications of these or related materials in ophthalmology, for example, as biodegradable matrix materials for controlled drug delivery of ganciclovir in the vitreous body.