Some aspects of the crosslinking of gelatin by dextran dialdehydes

Injectable polysaccharide hydrogels as localized nitric oxide delivery formulations

A series of injectable polysaccharide hydrogels were prepared with oxidized dextran and diethylenetriamine-modified carboxymethylcellulose or hyaluronic acid. Rheological evaluation revealed that carboxymethylcellulose-based hydrogels achieved the largest storage moduli (>1 kPa) when prepared from 5 wt. % solutions. However, carboxymethylcellulose-based hydrogels with storage moduli >100 Pa were prepared from solutions with concentrations as low as 2 wt. %. Hyaluronic acid-based hydrogels demonstrated smaller storage moduli but had swelling ratios more than four times that of the carboxymethylcellulose systems at the same polymer concentrations. The incorporation of N-diazeniumdiolate NO donors into the hydrogels resulted in reduced hydrogel storage moduli as a function of NO donor concentration. The impact of the hydrogel architecture on NO-release kinetics proved dependent on the identity of the NO donor. Hydrogel degradation over 14 d was measured at pH 5.4 and 7.4 and indicated that hyaluronic acid-based hydrogels degraded more rapidly than carboxymethylcellulose hydrogels and that the addition of NO to the hydrogels increased the rate at which they degraded. In vitro cytotoxicity of hydrogel extracts was evaluated against five cell lines, with no observed toxicity except for that of hyaluronic acid-based hydrogel extracts against human gingival fibroblasts. The diverse properties, versatility, and non-toxic characteristics of these injectable hydrogels should facilitate local delivery of nitric oxide for a range of biomedical applications.

Immobilization of Enzymes on Flexible Tubing Surfaces for Continuous Bioassays

Immobilized enzymes can be used to catalyze biochemical reactions in a batch process, however, it is more difficult to use them in a continuous process. Herein, we develop an enzyme immobilization technique for flexible tubing surfaces, which can be used to catalyze biochemical reactions in a continuous process. In this technique, the tubing is first treated with (3-aminopropyl)triethoxysilane at 50 °C and baked at 100 °C in vacuum to form a network of reactive amine functional group on the inner tubing surface. Subsequently, dextran polyaldehyde, a polymeric cross-linker, is used to immobilize crude protease extract and catalase for hydrolyzing casein and degrading H₂O₂, respectively, in a continuous process. The immobilized proteases are highly stable even after a long-term storage at 4 °C. After 12 weeks of storage, 90% of the original protease activity can be preserved. Meanwhile, the immobilized catalase is able to degrade 0.1% H₂O₂ solution flowing at 5 μL/min. The immobilization technique is potentially useful for bioassays and industrial wastewater treatments when continuous processes are preferred.

Preparation and characterization of in-situ crosslinked pectin-gelatin hydrogels

Crosslinked hydrogels were developed by in-situ reaction of periodate oxidized pectin (OP) and gelatin. The reaction takes place through the formation of Schiff bases between aldehyde groups of OP and amino groups of gelatin. The effect of various process parameters such as reaction time, reaction temperature, pH of the reaction and composition on the efficacy of the crosslinking was investigated. Field emission scanning electron micrsocopy (FESEM) revealed that homogenous, single phase systems are obtained after the crosslinking of OP and gelatin. The swelling characteristics of the hydrogels were monitored. The equilibrium swelling varies in the range of 195-324% with a variation in the gelatin content (10-40%). Glycerol, when used as a plasticizer, improved the flexibility and the handling characteristics of the crosslinked hydrogels. Plasticized films retained good tensile strengths in the range of 19-48 MPa. By proper selection of the reaction conditions, the efficiency of crosslinking can be controlled to obtain the optimum results.

Gelatin hydrogels cross-linked with bis(vinylsulfonyl)methane (BVSM): 1. The chemical networks

This paper deals with chemical gelation of gelatin in the presence of a cross-linker, bis(vinylsulfonyl)methane (BVSM), which is able to create covalent C-N bonds with amine groups. The investigation is performed at 40 degrees C, where no triple helices are present. Gelatin is in random coil conformation. The influence of various parameters (gelatin concentration, cross-linker concentration, and pH (number of reacting sites along the gelatin chain)) was examined. Gel formation was followed by rheological and thermodynamic measurements (microcalorimetry) versus time (kinetic measurements). Furthermore, the storage moduli were compared to the number of links formed in the course of gelation. The experiments show that, within the experimental range investigated, a fully homogeneous network is not reached; the chemical gels, even upon completion of the reactions, are still in the critical domain, near the threshold. A power law behavior was put in evidence for the shear modulus versus the distance to the gel point, expressed as the concentration of links per gelatin chain. The exponent (f = 3.4 +/- 0.3) is close to that expected for the vulcanization of long chains. The storage moduli can be superposed on a single curve where the abscissa is the product of the number of C-N links per unit volume and the gelatin concentration at an exponent equal to -0.76 +/- 0.03. This exponent suggests the role of entanglements for interchain cross-linking.

Gelatin hydrogels cross-linked with bisvinyl sulfonemethyl. 2. The physical and chemical networks

This paper deals with the physical and the chemical gelation of gelatin in the presence of a reactant, bisvinyl sulfonemethyl (BVSM). The strategy of this investigation is to separate the contributions of the two types of cross-links in order to deduce the resultant elasticity of the network. In addition, the question raised by several authors concerning an increase of the thermal stability of the triple helices in the presence of cross-links was examined by using several techniques. In this study, the concentration of gelatin and BVSM were kept constant, while the influence of the thermal protocols was put in evidence. The gel formation was followed by rheological, thermodynamic (microcalorimetry), and optical spectroscopy (optical rotation) measurements. The results demonstrate the large differences which arise on the storage moduli by changing the thermal protocols. Cross-linking of the networks in the presence of the triple helices induce a heterogeneous repartition of the bonds, which can form along the triple helices and at the end of the sequences. Consequently, the rubber like network obtained by denaturation of the triple helices is still reminiscent of the initial twist of the chains, and a large modulus is observed, as if rigid segments were still present (storage modulus 10 times larger than for random cross-linking). The hydrogels have an elastic modulus which is larger that the addition of the physical and chemical contributions. The interpretation of the network elasticity is based on the predominant role of the rigid rods of triple helices, where the BVSM cross-links can either modify the ratio between the apparent length and distance between rods, l/d, and/or increase the rigidity of the interchain connections, which are loose coils for the physical gels. The hydrogels investigated have a network which is still close to the percolation threshold of the physical gel, and therefore, the statistical models known for well developed networks cannot be directly validated in these experimental conditions.

Soft tissue adhesive composed of modified gelatin and polysaccharides

Although fibrin glue has been clinically used as a surgical adhesive, hemostatic agent, and sealant, it has the risk of virus infection because its components, fibrinogen and thrombin, are obtained from human blood. To circumvent this problem, we employed bioabsorbable gelatin and polysaccharides to prepare a safer hemostatic glue. Gelatin was modified with ethylenediamine using water-soluble carbodiimide to introduce additional amino groups into the original gelatin, while dextran and hydroxyethyl-starch were oxidized by sodium periodate to convert 1,2-hydroxyl groups into dialdehyde groups. Upon mixing of the two polymer components in aqueous solution, Schiff base was formed between the amino groups in the modified gelatin and the aldehyde groups in the modified polysaccharides, which thus resulted in intermolecular cross-linking and gel formation. The fastest gel formation took place within 2 s, and its bonding strength to porcine skin was about 225 gf cm(-2) when 20 wt% of an amino-gelatin (55% amino) and 10 wt% of aldehyde-HES (>84% dialdehyde) aqueous solutions were mixed. In contrast, the gelation time and bonding strength of fibrin glue was 5 s and 120 gf cm(-2), respectively.

Insulin delivery governed by covalently modified lectin-glycogen gels sensitive to glucose

A glucose-sensitive gel formulation containing concanavalin A and glycogen has been reported previously. Precipitation resulting from the addition of concanavalin A to glycogen has been documented, but the formation of glucose-sensitive gels based on lectin-glycogen interactions is novel and used here in our studies. An improved in-vitro self-regulating drug-delivery system, using covalently modified glucose-sensitive gels based on concanavalin A and a polysaccharide displacement mechanism, is described. The successful use of the covalently modified gels addresses a problem identified previously where significant leaching of the mitogenic lectin from the gel membranes of non-coupled gels was encountered. Concanavalin A was covalently coupled to glycogen by use of derivatives of Schiff's bases. The resulting gels, like the non-coupled gels, were shown to undergo a gel-sol transformation in response to glucose. Insulin delivery was demonstrated using this covalently modified system in conditions of repeated glucose triggering at 20 degrees C and 37 degrees C. The magnitude of the response was less variable than for the dextran-based gels studied previously. The performance of this system has been improved in terms of concanavalin A leaching. This could, therefore, be used as the basis of the design of a self-regulating drug-delivery device for therapeutic agents used to treat diabetes mellitus.

Studies on gelatin-containing artificial skin: II. Preparation and characterization of cross-linked gelatin-hyaluronate sponge

This study was conducted to develop a new sponge type of biomaterial to be used for either wound dressing or scaffold for tissue engineering. We were able to prepare an insoluble matrix composed of gelatin and sodium hyaluronate (HA) by dipping the soluble sponge into 90% (w/v) acetone/water mixture containing a small amount of cross-linking agent, 1-ethyl-3-3-dimethylaminoproplycarbodiimide hydrochloride, EDC. To characterize the sponge, Fourier-transformed infrared spectroscopy (FT-IR), scanning electron microscopy (SEM), and Instron analysis were performed. The obtained results indicate that the chemically cross-linked sponge shows a cross-linking degree of 10-35%, a mean pore size of 40-160 microm, porosity of 35-67%, and a tensile strength of 10-30 gf/cm(2). Especially, the porosity measured by image analysis showed a tendency to increase with HA content, resulting in an increased water uptake. The resistance to collagenase degradation in vitro increased for up to 2 days. Silver sulfadiazine (AgSD)-impregnated gelatin-HA sponge was also prepared and compared with conventional vaseline gauze by applying it onto a dorsal skin defect of wistar rat for 5, 12, and 21 days. Histological results showed an enhancement of wound healing in AgSD-impregnated gelatin-HA sponge.

Studies on gelatin-containing artificial skin: II. Preparation and characterization of cross-linked gelatin-hyaluronate sponge

This study was conducted to develop a new sponge type of biomaterial to be used for either wound dressing or scaffold for tissue engineering. We were able to prepare an insoluble matrix composed of gelatin and sodium hyaluronate (HA) by dipping the soluble sponge into 90% (w/v) acetone/water mixture containing a small amount of cross-linking agent, 1-ethyl-3-3-dimethylaminoproplycarbodiimide hydrochloride, EDC. To characterize the sponge, Fourier-transformed infrared spectroscopy (FT-IR), scanning electron microscopy (SEM), and Instron analysis were performed. The obtained results indicate that the chemically cross-linked sponge shows a cross-linking degree of 10-35%, a mean pore size of 40-160 microm, porosity of 35-67%, and a tensile strength of 10-30 gf/cm(2). Especially, the porosity measured by image analysis showed a tendency to increase with HA content, resulting in an increased water uptake. The resistance to collagenase degradation in vitro increased for up to 2 days. Silver sulfadiazine (AgSD)-impregnated gelatin-HA sponge was also prepared and compared with conventional vaseline gauze by applying it onto a dorsal skin defect of wistar rat for 5, 12, and 21 days. Histological results showed an enhancement of wound healing in AgSD-impregnated gelatin-HA sponge.