Physical Gelation of Chitosan in the Presence of β-Glycerophosphate: The Effect of Temperature

Thermosensitive chitosan-gelatin-glycerol phosphate hydrogels as a cell carrier for nucleus pulposus regeneration: an in vitro study

Injectable hydrogel is one of the great interests for tissue engineering and cell encapsulation. In the study, the gelatin molecules were added to the thermosensitive chitosan/beta-glycerol phosphate (C/GP) disodium salt hydrogels to form chitosan/gelatin/beta-glycerol phosphate (C/G/GP) disodium salt hydrogels which were applied as a cell carrier for nucleus pulposus (NP) regeneration. The gelation temperature, gelation time, and gel strength of the C/G/GP hydrogels were analyzed by the rheometer. NP cells were then harvested from the intervertebral discs of the adult New Zealand white rabbits and cultured in monolayer or in C/G/GP hydrogel, respectively. The cell viability, material-mediated cytotoxicity, cell proliferation, production of sulfated glycosaminoglycans, anabolic/catabolic gene expressions, and extracellular matrix-related gene expressions of the NP cells were demonstrated. The results show that the sol/gel transition temperature of the C/G/GP hydrogel was in the range of 31.1-33.8 degrees C at neutral pH value, the gelation time was shortened, and the gel strength also improved at body temperature when compared with the C/GP hydrogel. Among those, C/GP with 1% gelatin addition showed the most promising gelation time and gel strength and were utilized in the later experiments. From the results of cell activity, cytotoxicity, and cell proliferation assays, NP cells cultured in C/G/GP hydrogel had normal cell viability and cell proliferation that indicated the hydrogel was noncytotoxicity. The amounts of sulfated glycosaminoglycans of NP cells cultured in C/G/GP hydrogels were significantly higher than monolayer cultured. Considering the extracellular matrix-related gene expression, type II collagen and aggrecan of NP cells cultured in the hydrogels greatly increased than those in monolayer culture. On the contrary, the unfavorable gene expression, such as that of type I collagen, was decreased significantly. The results reveal that gelatin added into C/GP hydrogel significantly shortened the gelation time and improved the gel strength without influencing the biocompatibility. NP cells cultured in the C/G/GP hydrogel also displayed better gene expressions when compared with the monolayer culture. This study indicates that using chitosan/gelatin hydrogel for NP cell culture is feasible and may apply in minimal invasive intervertebral disc surgery in the future.

Development of a Novel Cell Encapsulation System Based on Natural Origin Polymers for Tissue Engineering Applications

Cells microencapsulated in biocompatible semi-permeable polymeric membranes are effective as cell delivery systems while protecting the host against immune responses. In this study, cell encapsulation membranes were prepared based on carrageenan and alginate, two natural cationic polymers. Different formulations/conditions were explored to optimize the microcapsules which were characterized with respect to their morphology, mechanical stability, and cytotoxicity. Spherical-shaped microcapsules were obtained from all the polymeric systems. The iota-carrageenan/sodium alginate microcapsules exhibited the best stability and permeability, and therefore, these were selected for the cell encapsulation. These capsules provided an environment that supported cell proliferation and have the potential for tissue engineering as well as other cell-based therapy applications.

A chitosan/beta-glycerophosphate thermo-sensitive gel for the delivery of ellagic acid for the treatment of brain cancer

We report here the development of a chitosan/beta-glycerophosphate(Ch/beta-GP) thermo-sensitive gel to deliver ellagic acid (EA) for cancer treatment. The properties of the Ch/beta-GP gels were characterized regarding chemical structure, surface morphology, and viscoelasticity. In vitro EA release rate from the EA loaded Ch/beta-GP gel and chitosan degradation rate were investigated. The anti-tumor effect of the EA loaded Ch/beta-GP gel on brain cancer cells (human U87 glioblastomas and rat C6 glioma cells) was evaluated by examining cell viability. Cell number and activity were monitored by the MTS assay. The Ch/beta-GP solution formed a heat-induced gel at body temperature, and the gelation temperature and time were affected by the final pH of the Ch/beta-GP solution. The lysozyme increased the EA release rate by 2.5 times higher than that in the absence of lysozyme. Dialyzed chitosan solution with final pH 6.3 greatly reduced the beta-GP needed for gelation, thereby significantly improving the biocompatibility of gel (p < 0.001). The chitosan gels containing 1% (w/v) of ellagic acid significantly reduced viability of U87 cells and C6 cells compared with the chitosan gels at 3 days incubation (p < 0.01, and p < 0.001, respectively).

Thermogelling chitosan and collagen composite hydrogels initiated with beta-glycerophosphate for bone tissue engineering

Chitosan and collagen type I are naturally derived materials used as cell carriers because of their ability to mimic the extracellular environment and direct cell function. In this study beta-glycerophosphate (beta-GP), an osteogenic medium supplement and a weak base, was used to simultaneously initiate gelation of pure chitosan, pure collagen, and chitosan-collagen composite materials at physiological pH and temperature. Adult human bone marrow-derived stem cells (hBMSC) encapsulated in such hydrogels at chitosan/collagen ratios of 100/0, 65/35, 25/75, and 0/100 wt% exhibited high viability at day 1 after encapsulation, but DNA content dropped by about half over 12 days in pure chitosan materials while it increased twofold in materials containing collagen. Collagen-containing materials compacted more strongly and were significantly stiffer than pure chitosan gels. In monolayer culture, exposure of hBMSC to beta-GP resulted in decreased cell metabolic activity that varied with concentration and exposure time, but washing effectively removed excess beta-GP from hydrogels. The presence of chitosan in materials resulted in higher expression of osterix and bone sialoprotein genes in medium with and without osteogenic supplements. Chitosan also increased alkaline phosphatase activity and calcium deposition in osteogenic medium. Chitosan-collagen composite materials have potential as matrices for cell encapsulation and delivery, or as in situ gel-forming materials for tissue repair.

Comparison of chitosan nanoparticles and chitosan hydrogels for vaccine delivery

In this work the potential of chitosan nanoparticles (CNP) and thermosensitive chitosan hydrogels as particulate and sustained release vaccine delivery systems was investigated. CNP and chitosan hydrogels were prepared, loaded with the model protein antigen ovalbumin (OVA) and characterised. The immunostimulatory capacity of these vaccine delivery systems was assessed in-vitro and in-vivo. Particle sizing measurements and SEM images showed that optimised OVA-loaded CNP had a size of approximately 200 nm, a polydispersity index &lt; 0.2, and a positive zeta-potential of approximately 18 mV. The amount of OVA adsorbed onto CNP was high with an adsorption efficacy of greater than 96%. Raman spectroscopy indicated conformational changes of OVA when adsorbed onto the surface of CNP. Uptake of the dispersions and immunological activation of murine dendritic cells in-vitro could be demonstrated. Investigation of the release of fluorescently-labelled OVA (FITC-OVA) from CNP and chitosan hydrogels in-vitro showed that approximately 50% of the total protein was released from CNP within a period of ten days; release of antigen from chitosan gel occurred in a more sustained manner, with &lt; 10% of total protein being released after 10 days. The slow release from gel formulations may be explained by the strong interactions of the protein with chitosan. While OVA-loaded CNP showed no significant immunogenicity, formulations of OVA in chitosan gel were able to stimulate both cell-mediated and humoral immunity in-vivo.

Synthesis and structural characterization of chitosan nanogels

Colloidal physical gels of pure chitosan were obtained via an ammonia-induced gelation in a reverse phase emulsion. The water weight fraction and the chitosan concentration in the water phase were optimized so as to yield nanogels with controlled particle size and size distribution. The spherical morphology of the nanogels was established by transmission electron microscopy with negative staining. Wide-angle X-ray scattering experiments showed that these gels were partially crystalline. The electrophoretic mobilities of the particles remained positive up to pH 7, above which the particles aggregated due to the charge neutralization. From the investigation on the colloidal stability of these nanogels in various conditions (pH, salt concentration, temperature), an electrosteric stabilization process of the particles was pointed out, related to the conformation of mobile chitosan chains at the gel-liquid interface. Therefore, the structure of the nanogels was deduced as being core-shell type, a gelified core of neutralized chitosan chains surrounded by partially protonated chains.

Chitosan-dibasic orthophosphate hydrogel: a potential drug delivery system

Injectable thermo-activated hydrogels have shown great potential in biomedical applications including use in therapeutic delivery vehicles. In addition to their biocompatibility, the feasibility of these delivery systems is significantly contributed by their ability to gel at physiological conditions and to release entrapped molecules in a sustained manner. In this study, parameters affecting the gelling behavior and the release characteristics of a neutral hydrogel system based on chitosan and an inorganic orthophosphate salt have been investigated. Monobasic and tribasic phosphate salts were not effective in inducing gelation of chitosan solution. However, in the presence of dibasic phosphate salt such as dipotassium hydrogen orthophosphate (DHO), the acidic chitosan solution was neutralized and gelling at temperature and time regulated by varying chitosan and salt concentrations in the formulation. The release rate of the entrapped macromolecules depended on chitosan concentration, DHO concentration, structural conformation and molecular weight of entrapped agents. The relationship between the morphology of the hydrogel and the release profiles are discussed. Chitosan/DHO (Chi/DHO) hydrogels were found to be cytocompatible as evaluated in an in vitro study using a human cell line. These results indicate the potential of Chi/DHO hydrogels as delivery systems for different therapeutic agents with controlled release kinetics.

Nucleus pulposus tissue engineering: a brief review

Symptomatic intervertebral disc degeneration is associated with several spinal diseases, which cause losses of life quality and money. Tissue engineering provides a promising approach to recover the functionality of the degenerative intervertebral disc. Most studies are directed toward nucleus pulposus (NP) tissue engineering because disc degeneration is believed to originate in NP region, and considerable progress has been made in the past decade. Before this important technique is utilized for clinical treatment of disc degeneration, many challenges need to address including in all three principal components of tissue engineering, i.e., seed cells, signals and biomaterial scaffolds. This article briefly gives certain aspects of state of the art in this field, as well as pays a little more attention to our work published in the past 5 years, on growth and differentiation factor-5 (GDF-5), adipose-derived stem cells (ADSCs) and heparin functionalization of scaffold. We suggest that combinatorial application of ADSCs, GDF-5, heparin functionalization and injectable hydrogels will be advantageous in NP tissue engineering.

Heat-induced transfer of protons from chitosan to glycerol phosphate produces chitosan precipitation and gelation

Recently, chitosan dissolved in solutions containing glycerol phosphate (GP) were found to undergo a sol-gel transition when heated and the proposed gelling mechanism was based on increasing hydrophobic interactions with temperature. Subsequently, an investigation of ionization and precipitation behavior of chitosan, including dependencies on temperature, added salt, and fraction of deacetylated monomers (fD) was performed. This latter study revealed important differences in the temperature dependence of pKa of chitosan versus GP and led us to propose an alternative hypothesis for the mechanism of gelation in chitosan-GP systems whereby heat induces transfer of protons from chitosan to glycerol phosphate thereby neutralizing chitosan and allowing attractive interchain forces to form a physical gel. To investigate this specific molecular thermogelling mechanism, temperature ramp experiments on dilute chitosan-GP solutions were performed. Chitosans with fD of 0.72 and 0.98 were used to prepare solutions with a range of molar ratios of GP to chitosan glucosamine monomer of 1.25 to 10 and with 0 or 150 mM added monovalent salt. Light transmittance measurements were performed simultaneously to indicate precipitation in these dilute systems as a surrogate for gelation in concentrated systems. Measured temperatures of precipitation ranged from 15 to 85 degrees C, where solutions with less GP (used in a disodium salt form) had lower precipitation temperatures. A theoretical model using acid-base equilibria with temperature dependent pKa's, including the electrostatic contribution from the polyelectrolyte nature of chitosan, was used to calculate the degree ionization of chitosan (alpha, the fraction of protonated glucosamine monomer) as a function of temperature and showed a significant decrease in alpha with increased temperature due to proton transfer from chitosan to GP. This heat-induced proton transfer from chitosan to GP was experimentally confirmed by 31P NMR measurements during temperature ramp experiments since the chemical shift of 31P of GP is an indicator of its level of protonation. By assuming average temperature independent values of alpha p that were calculated from measured T(p), the model was able to accurately predict measured temperatures of precipitation (T(p)) of all chitosan-GP mixtures. The resulting alpha(p) were temperature independent but increased with increased chitosan fD and with increased salt. Measurements and theory revealed that T(p) can be adjusted in a predictable manner by changing the chitosan-GP molar ratio and thereby systematically tailored to obtain a large range of precipitation temperatures. Finally, similar temperature ramp experiments using inorganic phosphate and MES in place of GP demonstrated that the temperature-induced precipitation of chitosan also occurs with these buffers, confirming that the key feature of the buffer used with chitosan is its ability to absorb heat-stimulated release of chitosan protons and facilitate chitosan neutralization. A theoretical expression for the variation of chitosan ionization degree with temperature in a system composed of two titratable species (chitosan and buffer) was derived and allowed us to establish the required characteristics of the buffer for efficient heat-stimulated proton transfer between a chitosan and the buffer. These results provide a useful explanation for the mechanism of heat-induced gelation of chitosan-based systems that could be exploited for numerous practical applications.

Viscoelastic behavior and in vivo release study of microgel dispersions with inverse thermoreversible gelation

The dispersion of microgels with two interpenetrating polymer networks of poly( N-isopropylacrylamide) and poly(acrylic acid) (PNIPAM-IPN-PAAc) has been studied for its viscoelastic behavior, biocompatibility, and in vivo release properties. The IPN microgels in water had an average hydrodynamic radius of about 85 nm at 21 degrees C, measured by dynamic light scattering method. The atomic force microscope image showed that the particles were much smaller after they were dried but remained spherical shape. The storage and loss moduli ( G' and G'') of dispersions of IPN microgels were measured in the linear stress regime as functions of temperature and frequency at various polymer concentrations using a stress-controlled rheometer. For dispersions with polymer concentrations of 3.0 and 6.0 wt % above 33 degrees C, the samples behave as viscoelastic solids and the storage modulus was larger than the loss modulus over the entire frequency range. The loss tangent was measured at various frequencies as a function of temperature. The gelation temperature was determined to be 33 degrees C at the point where a frequency-independent value of the loss tangent was first observed. At pH 2.5, when heated above the gelation temperature, IPN microgels flocculate by pumping a large amount of water from the gel. When the pH value was adjusted to neutral, deprotonation of -COOH groups on PAAc made the microgel keep water even above the gelation temperature. Using an animal implantation model, the biocompatibility and drug release properties of the IPN microgel dispersion were evaluated. Fluorescein as a model drug was mixed into an aqueous microgel dispersion at ambient temperature. This drug-loaded liquid was then injected subcutaneously in Balb/C mice from Taconic Farms. The test results have shown that the IPN microgels did not adversely promote foreign body reactions in this acute implantation model and the presence of gelled microgel dispersion substantially slowed the release of fluorescein.

Hydrogels for tissue engineering and delivery of tissue-inducing substances

This manuscript presents hydrogels (HGs) from a tissue engineering perspective being especially written for those who are approaching this field by offering a concise but inclusive review of hydrogel synthesis, properties, characterization methods, and applications.

Polylysine-functionalised thermoresponsive chitosan hydrogel for neural tissue engineering

Foetal mouse cortical cells were cultured on 2D films and within 3D thermally responsive chitosan/glycerophosphate salt (GP) hydrogels. The biocompatibility of chitosan/GP 2D films was assessed in terms of cell number and neurites per cell. Osmolarity of the hydrogel was a critical factor in promoting cell survival with isotonic GP concentrations providing optimal conditions. To improve cell adhesion and neurite outgrowth, poly-D-lysine (PDL) was immobilised onto chitosan via azidoaniline photocoupling. Increase in PDL concentrations did not alter cell survival in 2D cultures but neurite outgrowth was significantly inhibited. Neurons exhibited a star-like morphology typical of 2D culture systems. The effects of PDL attachment on cell number, cell morphology and neurite outgrowth were more distinct in 3D culture conditions. Neurones exhibited larger cell bodies and sent out single neurites within the macroporous gel. Immobilised PDL improved cell survival up to an optimum concentration of 0.1%, however, further increases resulted in drops in cell number and neurite outgrowth. This was attributed to a higher cell interaction with PDL within a 3D hydrogel compared to the corresponding 2D surface. The results show that thermally responsive chitosan/GP hydrogels provide a suitable 3D scaffolding environment for neural tissue engineering.