Kinetic and mechanistic considerations in the gelation of genipin-crosslinked gelatin

Biodegradable polymer scaffold, semi-solid solder, and single-spot lasing for increasing solder-tissue bonding in suture-free laser-assisted vascular repair

We recently showed the fortifying effect of poly-caprolactone (PCL) scaffold in liquid solder-mediated laser-assisted vascular repair (ssLAVR) of porcine carotid arteries, yielding a mean ± SD leaking point pressure of 488 ± 111 mmHg. Despite supraphysiological pressures, the frequency of adhesive failures was indicative of weak bonding at the solder-tissue interface. As a result, this study aimed to improve adhesive bonding by using a semi-solid solder and single-spot vs. scanning irradiation. In the first experiment, in vitro ssLAVR (n=30) was performed on porcine abdominal aorta strips using a PCL scaffold with a liquid or semi-solid solder and a 670-nm diode laser for dual-pass scanning. In the second experiment, the scanning method was compared to single-spot lasing. The third experiment investigated the stability of the welds following hydration under quasi-physiological conditions. The welding strength was defined by acute breaking strength (BS). Solder-tissue bonding was examined by scanning electron microscopy and histological analysis was performed for thermal damage analysis. Altering solder viscosity from liquid to semi-solid solder increased the BS from 78 ± 22 N/cm(2) to 131 ± 38 N/cm(2) . Compared to scanning ssLAVR, single-spot lasing improved adhesive bonding to a BS of 257 ± 62 N/cm(2) and showed fewer structural defects at the solder-tissue interface but more pronounced thermal damage. The improvement in adhesive bonding was associated with constantly stronger welds during two weeks of hydration. Semi-solid solder and single-spot lasing increased welding strength by reducing solder leakage and improving adhesive bonding, respectively. The improvement in adhesive bonding was associated with enhanced weld stability during hydration.

Biopolymer-based hydrogels as scaffolds for tissue engineering applications: a review

Hydrogels are physically or chemically cross-linked polymer networks that are able to absorb large amounts of water. They can be classified into different categories depending on various parameters including the preparation method, the charge, and the mechanical and structural characteristics. The present review aims to give an overview of hydrogels based on natural polymers and their various applications in the field of tissue engineering. In a first part, relevant parameters describing different hydrogel properties and the strategies applied to finetune these characteristics will be described. In a second part, an important class of biopolymers that possess thermosensitive properties (UCST or LCST behavior) will be discussed. Another part of the review will be devoted to the application of cryogels. Finally, the most relevant biopolymer-based hydrogel systems, the different methods of preparation, as well as an in depth overview of the applications in the field of tissue engineering will be given.

Gelatin microspheres crosslinked with genipin for local delivery of growth factors

A main challenge in tissue engineering and regenerative medicine is achieving local and efficient growth factor release to guide cell function. Gelatin is a denatured form of collagen that cells can bind to and degrade through enzymatic action. In this study, gelatin microspheres were used to release bone morphogenetic protein 2 (BMP2). Spherical microparticles with diameters in the range of 2-6 µm were created by an emulsification process and were stabilized by crosslinking with the small molecule genipin. The degree of crosslinking was varied by controlling the incubation time in genipin solution. Loading rate studies, using soy bean trypsin inhibitor as a model protein, showed rapid protein uptake over the first 24 h, followed by a levelling off and then a further increase after approximately 3 days, as the microspheres swelled. Growth factor release studies using microspheres crosslinked to 20%, 50% and 80% of saturation and then loaded with BMP2 showed that higher degrees of crosslinking resulted in higher loading efficiency and slower protein release. After 24 h, the concentration profiles produced by all microsphere formulations were steady and approximately equal. Microspheres incubated with adult human mesenchymal stem cells accumulated preferentially on the cell surface, and degraded over time in culture. BMP2-loaded microspheres caused a three- to eight-fold increase in expression of the bone sialoprotein gene after 14 days in culture, with more crosslinked beads producing a greater effect. These results demonstrate that genipin-crosslinked gelatin microspheres can be used to deliver growth factors locally to cells in order to direct their function.

Analysis of gelatin plasma substitutes in blood based on detection of hydroxyproline derivatives

The gelatin plasma substitute is often polydisperse and heterogenous, making it difficult to determine the elimination rate and half-life in the body. In this study, one method was developed based on quantitative determination of hydroxyproline derivatives. Two plasma substitutes were prepared by succinylation and genipin-crosslinking, respectively. After transfusion, the blood samples were hydrolyzed and derivatized, and then analyzed by HPLC. A two-phase exponential association equation was used for fitting the time-concentration curves. The results indicated that this method could be used for quantitative determination of gelatin in blood, and the pharmacokinetic parameters such as elimination rate and half-life.

Gelation studies of a cellulose-based biohydrogel: the influence of pH, temperature and sterilization

The present paper investigates the rheological properties of silated hydroxypropylmethylcellulose (Si-HPMC) biohydrogel used for biomaterials and tissue engineering applications. The general property of this modified cellulose ether is the occurrence of self-hardening due to silanol condensation subsequent to a decrease in pH (from 12.4 to nearly 7.4). The behavior of unsterilized and sterilized Si-HPMC solutions in diluted and concentrated domains is first described and compared. In addition, the influence of physiological parameters such as pH and temperature on the rate of the gelation process is studied. In dilute solution, the intrinsic viscosity ([eta]) of different pre-steam sterilization Si-HPMC solutions indicates that macromolecular chains occupy a larger hydrodynamic volume than the post-steam sterilization Si-HPMC solutions. Although the unsterilized Si-HPMC solutions demonstrate no detectable influence of pH upon the rheological behavior, a decrease in the limiting viscosities (eta(0)) of solutions with increasing pH is observed following steam sterilization. This effect can be explained by the formation of intra- and intermolecular associations during the sterilization stage originating from the temperature-induced phase separation. The formation of Si-HPMC hydrogels from injectable aqueous solution is studied after neutralization by different acid buffers leading to various final pHs. Gelation time (t(gel)) decreases when pH increases (t(gel) varies from 872 to 11s at pH 7.4 and 11.8, respectively). The same effect is observed by increasing the temperature from 20 to 45 degrees C. This is a consequence of the synergistic effect of the increased reaction rate and acid buffer diffusion. pH and temperature are important parameters in the gelation process and their influence is a key factor in controlling gelation time. By adapting the gel parameters one could propose hydrogels with cross-linking properties adapted to clinical applications by controlling the amount of pH of neutralization and temperature.

Injection temperature significantly affects in vitro and in vivo performance of collagen-platelet scaffolds

Collagen-platelet composites have recently been successfully used as scaffolds to stimulate anterior cruciate ligament (ACL) wound healing in large animal models. These materials are typically kept on ice until use to prevent premature gelation; however, with surgical use, placement of a cold solution then requires up to an hour while the solution comes to body temperature (at which point gelation occurs). Bringing the solution to a higher temperature before injection would likely decrease this intra-operative wait; however, the effects of this on composite performance are not known. The hypothesis tested here was that increasing the temperature of the gel at the time of injection would significantly decrease the time to gelation, but would not significantly alter the mechanical properties of the composite or its ability to support functional tissue repair. Primary outcome measures included the maximum elastic modulus (stiffness) of the composite in vitro and the in vivo yield load of an ACL transection treated with an injected collagen-platelet composite. In vitro findings were that injection temperatures over 30 degrees C resulted in a faster visco-elastic transition; however, the warmed composites had a 50% decrease in their maximum elastic modulus. In vivo studies found that warming the gels prior to injection also resulted in a decrease in the yield load of the healing ACL at 14 weeks. These studies suggest that increasing injection temperature of collagen-platelet composites results in a decrease in performance of the composite in vitro and in the strength of the healing ligament in vivo and this technique should be used only with great caution.

Evaluation of cross-linking methods for electrospun gelatin on cell growth and viability

The creation of a tissue engineering scaffold via electrospinning that has minimal toxicity and uses a solvent system composed of solvents with low toxicity and different cross-linking agents was investigated. First, a solvent system of acetic acid/ethyl acetate/water (50:30:20) with gelatin as a solute was evaluated. The optimum system for electrospinning a scaffold with the desired properties resulted from a gelatin concentration of 10 wt %. Several different methods were used to cross-link the electrospun gelatin fibers, including vapor-phase glutaraldehyde, aqueous phase genipin, and glyceraldehyde, as well as reactive oxygen species from a plasma cleaner. Because glutaraldehyde at high concentrations has been shown to be toxic, we explored other cross-linking methods. Using reactive oxygen species from a plasma cleaner is an easy alternative; however, the degradation reaction dominated the cross-linking reaction and the scaffolds degraded after only a few hours in aqueous medium at 37 °C. Glyceraldehyde and genipin were established as good options for cross-linking agents because of the low toxicity of these cross-linkers and the resistance to dissolution of the cross-linked fibers in cell culture medium at 37 °C. MG63 osteoblastic cells were grown on each of the cross-linked scaffolds. A proliferation assay showed that the cells proliferated as well or better on the cross-linked scaffolds than on traditional two-dimensional polystyrene culture plates.