E. Vassileva

Crystallization of water in some crosslinked gelatins

Abstract Five gelatin samples crosslinked with 1,4-diisocyanatobutane, 1,6-diisocyanatohexane, 1,12-diisocyanatododecane, 1,3-butadiene diepoxide and 1,2,7,8-diepoxyoctane were used to investigate the behavior of the gelatin–water system. The crosslink densities were estimated quantitatively by the values of the molecular weight between crosslinks Mc. Differential scanning calorimetry indicated that the water was only partially crystallizable below 0°C, and the fraction of non-crystallizable water depends on the nature of the crosslinking agent. This fraction tends to zero for high overall water fraction, approaching unity. The overall water weight fraction at which crystallizable water forms was found to be of the order of 0.35 for four of the samples, whereas for the sample crosslinked with 1,3-butadiene diepoxide it is surprisingly high, specifically 0.68.

DSC and TGA studies of the behavior of water in native and crosslinked gelatin

Water molecules absorbed into gelatin are found to be only partially crys- tallizable. The fraction of noncrystallizable water depends on whether the gelatin is native or crosslinked, and on the crosslinking conditions as well. This dependence is explained by the Tg-regulation effect newly proposed by Rault and coworkers for water-swollen gelatin cooled below 0°C. According to this effect, a part of the frozen water cannot crystallize because during the cooling the amorphous gelatin-water phase becomes glassy before the water crystallization temperature is reached. During the heating of water-plasticized gelatin samples in a TGA cell, the crystallizable water separates from the gelatin, mainly in the temperature interval 50 -100°C, whereas the noncrystallizable water leaves the gelatin gradually over the entire temperature inter- val investigated, up to 300°C.

Biodegradation of chemically modified gelatin films in lake and river waters

Gelatin was chemically modified by crosslinking samples with one of a number of bifunctional reagents as was done earlier in a processing technique used to improve mechanical properties through chain orientation. The effects of this crosslinking on the biodegradability of the resulting films were evaluated in the laboratory by exposing them to lake and river waters for 10 days with or without inoculation with periphyton organisms. Biodegradabilities were assessed by weight losses of the films and by measurements of dehydrogenase activity of biomasses taken from their surfaces. The extent of biodegradation depended on the type of crosslinking agent and the presence or absence of the periphyton. The gelatin films crosslinked with formaldehyde, glyoxal, or glutaraldehyde were the slowest to biodegrade; complete degradation required 8-10 days. In contrast, the most biodegradable was the gelatin crosslinked with hexamethylene diisocyanate, which required only 3-4 days. The uncrosslinked gelatin and the gelatin crosslinked with butadiene diepoxide and diepoxyoctane were intermediate, degrading in 5-7 days. The dehydrogenase activity paralleled the weight losses but rapidly decreased when the amount of gelatin remaining was small.

Biodegradation of chemically modified gelatin films in soil

Gelatin films that had been chemically modified (crosslinked with formaldehyde, glyoxal, glutaraldehyde, hexamethylene diisocyanate, butadiene diepoxide, or diepoxyoctane) were tested for their biodegradability by soil burial testing in a laboratory environment under temperature and humidity control. The relationship between weight loss and time of biodegradation showed a linear behavior for all the samples, but the rate of biodegradation showed a dependence on the type of crosslinking agent. The most stable films were those crosslinked with aldehydes, and these biodegraded by the 10th day. The samples crosslinked with hexamethylene diisocyanate and diepoxides completely biodegraded by the fourth and sixth days, respectively. It was shown that the rate of biodegradation depended on the density of crosslinking, which was calculated by a modified Flory–Rehner equation. The biodegraded samples showed considerable changes in the fingerprint region of FTIR spectra, and, thus, these spectra could be used for evaluation of the soil burial biodegradation of chemically modified gelatin samples.

Mechanical properties of native and crosslinked gelatins in a bending deformation

Gelatin samples, native and chemically crosslinked with three different diisocyanates, were studied by bending-creep measurements. These samples were characterized by the number-average molecular weight of a chain segment between two points of crosslinkage Mc. The chemical network was found to contribute to a marked extent to the mechanical behavior of the samples. The dependence of the creep compliance on the time for different loads was determined. The experimental results were compared with calculated ones according to a model, comprising four parameters, to obtain a better understanding of structure–property relationships for these materials. A very good agreement between the model and experimental data was found. Two of the fitting parameters, however—the relaxation time and η (which is connected with the viscosity)—were found to strongly depend on the time of the experiment.

Enzymatic degradation of formaldehyde-crosslinked gelatin

A general method is proposed for characterizing the enzymatic degradability of formaldehyde-crosslinked gelatin which is simple and uses subtilisin as a readily available, commercial alkaline proteinase. Solubilization of gelatin involved dissolution of species not chemically bonded to the crosslinked network, as well as the soluble fractions resulting to the enzymatic degradation. There was a linear relationship between the complete solubilization time of the gelatin and its exposure time to the formaldehyde crosslinking procedure.

New aspects of thermal treatment effects on gelatin films studied by microhardness

Microhardness measurements were carried out using various treatment cycles, including heating and cooling during different treatment times at high temperatures. Two possible processes to explain the observed increase in the microhardness are proposed, namely crystallisation and crosslinking. In order to distinguish between these two alternatives, differential scanning calorimetry (DSC), wide-angle X-ray scattering (WAXS), and swelling kinetics measurements were performed. The observed decrease of crystallinity (from DSC and WAXS measurements) as well as the decrease of swelling ability with increasing temperature and duration of thermal treatment are in favour of the occurrence of crosslinking reactions during thermal treatment. It is suggested that the crosslinking, as a result of additional intra- and intermolecular condensation processes, leads to a denser chain packing in the amorphous gelatin and consequently to higher microhardness values.

Melting of Gelatin Crystals below Glass Transition Temperature: A Direct Crystal-Glass Transition as Revealed by Microhardness

Abstract In the present paper the effect of crystallization conditions on the melting temperature (Tm ) of chemically cross-linked gelatin is studied by means of differential scanning calorimetry (DSC) and microhardness (H) measurements as extension of our recent findings (Ref. [11]). A rather unusual situation when polymer crystals melt below the glass transition temperature (Tg ) is confirmed by DSC. This is highlighted on dry gelatin which is characterized by relatively high, and close to each other, Tg and Tm values (217 and 230°C, respectively). By depressing Tg , using water as plasticizer, rather imperfect crystallites are obtained which melt well below the Tm of dry gelatin. It is shown that H increases with temperature mostly due to the drying out of the room-conditioned gelatin. In the 160–180°C range H reaches values of about 390MPa. In this temperature range the imperfect crystallites melt (according to DSC), however without formation of a typical liquid phase (since H remains constant). It is...

Sonochemically born proteinaceous micro- and nanocapsules

The use of proteins as a substrate in the fabrication of micro- and nanoparticulate systems has attracted the interest of scientists, manufactures, and consumers. Albumin-derived particles were commercialized as contrast agents or anticancer therapeutics. Food proteins are widely used in formulated dietary products. The potential benefits of proteinaceous micro- and nanoparticles in a wide range of biomedical applications are indisputable. Protein-based particles are highly biocompatible and biodegradable structures that can impart bioadhesive properties or mediate particle uptake by specific interactions with the target cells. Currently, protein microparticles are engineered as vehicles for covalent attachment and/or encapsulation of bioactive compounds, contrast agents for magnetic resonance imaging, thermometric and oximetric imaging, sonography and optical coherence tomography, etc. Ultrasound irradiation is a versatile technique which is widely used in many and different fields as biology, biochemistry, dentistry, geography, geology, medicine, etc. It is generally recognized as an environmental friendly, cost-effective method which is easy to be scaled up. Currently, it is mainly applied for homogenization, drilling, cleaning, etc. in industry, as well for noninvasive scanning of the human body, treatment of muscle strains, dissolution of blood clots, and cancer therapy. Proteinaceous micro- and nanocapsules could be easily produced in a one-step process by applying ultrasound to an aqueous protein solution. The origin of this process is in the chemical changes, for example, sulfhydryl groups oxidation, that takes place as a result of acoustically generated cavitation. Partial denaturation of the protein most probably occurs which makes the hydrophobic interactions dominant and also responsible for the formation of stable capsules. This chapter aims to present the current state-of-the-art in the field of sonochemically produced protein micro- and nanocapsules, paying special attention to the proposed mechanisms for their formation, the factors that influence the capsules characteristics as well to the current applications of these particles. Current challenges in the field are also outlined as, for example, the ultrasound-protein interaction and other possible aspects of the mechanism of their formation.

SOLUBILIZATION AND BIODEGRADATION OF CROSS-LINKED GELATINS BY ALKALINE PROTEINASE

This investigation involves the chemical modification of gelatin, a biomaterial which has been widely studied because of its environmental friendliness, its ready availability from waste products, and its biodegradability. The focus was on cross-linking because it has been shown that the mechanical properties of gelatin can be improved by a sequence of processing steps involving cross-linking, swelling, orientation, and, finally, drying in the oriented state. Because cross-linking is required in this technique, the present study characterizes its possible effects on gelatin biodegradability, as gauged by the time required to solubilize the material. A series of cross-linking agents having various alkylene sequence lengths was used. The resulting cross-linked gelatins were relatively insoluble in phosphate buffer (pH = 8.2) at room temperature, but at higher temperatures, they became partially soluble. Full solubilization of cross-linked gelatins was successfully obtained, however, by the action of an alkaline proteinase, specifically subtilisin. Both the rate of the partial solubilization in buffer and the full solubilization with alkaline proteinase showed dependences on the type, concentration, and chain lengths of the cross-linking agent.