Assunta Borzacchiello

Hyaluronic acid hydrogel in the treatment of osteoarthritis

In order to overcome the problem of rapid clearance of the polysaccharide hyaluronic acid (Hyal) in the treatment of osteoarthritis (OA), a 50% cross-linked Hyal hydrogel (Hyal 50%) was synthesised. The 50% refers to the amount of COOH groups of the polysaccharide involved in the cross-linking reaction. i.e. 50% of the total amount. The rheological behaviour of the Hyal 50% hydrogel, and in particular the possibility to inject it through a needle, was studied. The results obtained demonstrated that the hydrogel injected through the needle still behaved like a gel, although it showed a reduction of the dynamic moduli. The most appropriate sterilisation technique for this kind of hydrogel was also evaluated. Liophilised Hyal 50% samples were sterilised by steam, Ethylene Oxide (EtO) and gamma-rays. EtO and gamma-rays did not modify the characteristics of the hydrogel in terms of swellability and morphology. Lastly, the in vivo effect of Hyal 50% hydrogel in the treatment of chondral defect in rabbit knee was also studied. The results obtained showed the Hyal 50% injections improved chondrocytes density and matrix appearance. Furthermore, the permanence in situ of the hydrogel was longer than that of the linear Hyal.

A novel poloxamers/hyaluronic acid in situ forming hydrogel for drug delivery: Rheological, mucoadhesive and in vitro release properties

The influence of hyaluronic acid (HA) on the gelation properties of poloxamers blends has been studied with the aim of engineering thermosensitive and mucoadhesive polymeric platforms for drug delivery. The gelation temperature (T(gel)), viscoelastic properties and mucoadhesive force of the systems were investigated and optimised by means of rheological analyses. Poloxamers micellar diameter was evaluated by photon correlation spectroscopy (PCS). Moreover in order to explore the feasibility of these platforms for drug delivery, the optimised systems were loaded with acyclovir and its release properties studied in vitro. By formulating poloxamers/HA platforms, at specific concentrations, it was possible to obtain a thermoreversible gel with a T(gel) close to body temperature. The addition of HA did not hamper the self assembling process of poloxamers just delaying the gelation temperature of few Celsius degrees. Furthermore, HA presence led to a strong increase of the poloxamer rheological properties thus indicating possible HA interactions with micelles through secondary bonds, such as hydrogen ones, which reinforce the gel structure. These interactions could also explain PCS results which show, in systems containing HA, aggregates with hydrodynamic diameters much higher than those of poloxamer micelles. Mucoadhesion experiments showed a rheological synergism between poloxamers/HA gels and mucin dispersion which led to a change of the flow behaviour from a quite Newtonian one of the separate solutions to a pseudoplastic one of their mixture. In vitro release experiments indicated that the optimised platform was able to prolong and control acyclovir release for more than 6h.

Perspectives on: PEO-PPO-PEO Triblock Copolymers and their Biomedical Applications

Thermo-reversible polymeric gels represent an interesting class of materials that can be tailored for a wide range of applications. The triblock poly(ethylene oxide) (PEO) and poly(propylene oxide) (PPO) based systems, PEO-PPO-PEO, show thermoreversible gelation around body temperature and, therefore, are particularly suitable for biomedical applications such as drug delivery, gene therapy and tissue engineering. The PEO-PPO-PEO tri-block copolymers have amphiphilic characteristics and self-assemble into micelles to form a variety of close packed structures. By varying the block composition (PEO/PPO ratio) and the molecular weight, it is possible to tailor the final properties of these systems to meet the specific application needs. In this report the thermodynamic basis of micellization of PEO-PPO-PEO systems is described. The factors influencing the micelles formation are discussed along with the methods used to investigate the micellization process and morphology as well as with the main applications of these systems in biomedical fields.

Viscoelastic properties of rabbit vocal folds after augmentation.

BACKGROUND: Vocal fold function is closely related to tissue viscoelasticity. Augmentation substances may alter the viscoelastic properties of vocal fold tissues and hence their vibratory capacity. OBJECTIVES: We sought to investigate the viscoelastic properties of rabbit vocal folds in vitro after injections of various augmentation substances. STUDY DESIGN AND SETTING: Polytetrafluoroethylene (Teflon), cross-linked collagen (Zyplast), and cross-linked hyaluronan, hylan b gel (Hylaform) were injected into the lamina propria and the thyroarytenoid muscle of rabbit vocal folds. Dynamic viscosity of the injected vocal fold as a function of frequency was measured with a Bohlin parallelplate rheometer during small-amplitude oscillation. RESULTS: All injected vocal folds showed a decreasing dynamic viscosity with increasing frequency. Vocal fold samples injected with hylan b gel showed the lowest dynamic viscosity, quite close to noninjected control samples. Vocal folds injected with polytetrafluoroethylene showed the highest dynamic viscosity followed by the collagen samples. CONCLUSIONS: The data indicated that hylan b gel in short-term renders the most natural viscoelastic properties to the vocal fold among the substances tested. This is of importance to restore/preserve the vibratory capacity of the vocal folds when glottal insufficiency is treated with injections.

Chitosan-based hydrogels: Synthesis and characterization

Chitosan (CHI) is a polysaccharide of β-1,4-linked 2-amino-2-deoxy-D-glucopyranose derived by N-deacetylation of chitin in aqueous alkaline medium. The shells of crustaceans such as crabs, shrimp, and lobster are the current source of chitosan. It is known to be non-toxic, odourless, biocompatible in animal tissues and enzymatically biodegradable. For these reasons much research interest has been paid to its biomedical, ecological, and industrial applications over the past decade. However, its rigid crystalline structure, poor solubility in organic solvents and poor processability have limited its use. To broadening its range of applications, a growth research effort has been devoted to explore ways of modifying Chitosan. Here it has been reported on the synthesis of new hydrogels, obtained by self-curing chitosan with acrylic acid (AA) and methyl acrylate (MA). The hydrogels were characterized by FTIR, swelling and rheological analysis. The results of this study showed that the swelling and mechanical properties of chitosan are highly improved by the presence of poly acrylate. The swelling degree of these materials does not depend upon the ratio MA/AA. It is possible to improve and modulate the mechanical properties of the hydrogels by changing the relative MA/AA ratio.© 2001 Kluwer Academic Publishers

Structural and Mechanical Properties of UV-Photo-Cross-Linked Poly(N-vinyl-2-pyrrolidone) Hydrogels

Biocompatible poly( N-vinyl-2-pyrrolidone) (PVP) hydrogels have been produced by UV irradiation of aqueous polymer mixtures, using a high-pressure mercury lamp. The resulting materials have been characterized by a combination of experimental techniques, including rheology, small-angle neutron scattering (SANS), electron paramagnetic resonance (EPR), and pulsed gradient spin-echo nuclear magnetic resonance (PGSE-NMR), to put in evidence the relationship between the microstructural properties and the macrofunctional behavior of the gels. Viscoelastic measurements showed that UV photo-cross-linked PVP hydrogels present a strong gel mechanical behavior and viscoelastic moduli values similar to those of biological gels. The average distance between the cross-linking points of the polymer network was estimated from the hydrogels elastic modulus. However, SANS measurements showed that the network microstructure is highly inhomogeneous, presenting polymer-rich regions more densely cross-linked, surrounded by a water-rich environment. EPR and PGSE-NMR data further support the existence of these water-rich domains. Inclusion of a third component, such as glycerol, in the PVP aqueous mixture to be irradiated has been also investigated. A small amount of glycerol (<3% w/w) can be added keeping satisfactory properties of the hydrogel, while higher amounts significantly affect the cross-linking process.

Viscoelasticity of rabbit vocal folds after injection augmentation.

Objectives/Hypothesis Vocal fold function is related to the viscoelasticity of the vocal fold tissue. Augmentation substances used for injection treatment of voice insufficiency may alter the viscoelastic properties of vocal folds and their vibratory capacity. The objective was to compare the mechanical properties (viscoelasticity) of various injectable substances and the viscoelasticity of rabbit vocal folds, 6 months after injection with one of these substances.

RHEOLOGICAL STUDY ON HYALURONIC ACID AND ITS DERIVATIVE SOLUTIONS

ABSTRACT Rheological measurements were performed on Hyaluronic acid (HA) and its derivative solutions to evaluate steady flow viscosity and dynamics response with the aim to correlate the materials properties to the concentration, molecular weight and chemical structure. At low molecular weight and concentration, the HA solutions behaved as viscous liquid, whereas a soft-gel response was evident at higher molecular weight and concentration due to chains entanglement. Increasing the molecular weight was more effective than increasing concentration in promoting entanglement of molecular chains of HA. Comparing the behavior of HA solutions with that of Hyaluronic acid derivatives, it is showed that it is possible to modulate the rheological properties of HA based solutions by chemical modification preserving the bio-compatibility of the materials. The results of the rheological analysis provide a valuable tool to properly design optimal substitutes for specific biomedical application.

Synthesis, chemical and rheological characterization of new hyaluronic acid-based hydrogels

New hyaluronic acid-based hydrogels have been synthesized. The carboxylate groups of hyaluronan were activated in order to bind the amino terminal groups of the di-amine cross-linking reagent. Different hydrogels were obtained according to the different di-amine cross-linking agents (1,3-diaminepropane, 1,6-diaminohexane, PEG500 di-amine, and PEG800 di-amine). The crosslinked polymer (C.L.Hyal) was then sulphated (C.L.HyalS) by a heterogeneous reaction using sulphur trioxide pyridine complex (SO3-Py). The thermo-mechanical properties and swelling degree were evaluated and are discussed in relation to the chemical structure and the hydrophilic character of the gels. The different behaviours of C.L.Hyal and C.L.HyalS indicate the important role of sulphated groups.

Rheological Characterization of Hyaluronic Acid Derivatives as Injectable Materials Toward Nucleus Pulposus Regeneration

Nucleus pulposus (NP) is the soft center of the intervertebral disc (IVD), able to resist compressive loads, while the annulus fibrosus withstands tension and gives mechanical strength. NP function may be altered as consequence of several pathologies or injury and when a damaged IVD does not properly play its role. In the past years, a great effort has been devoted to the design of injectable systems as NP substitutes. The different synthetic- and natural hydrogel-based materials proposed, present many drawbacks and, in particular, they do not seem to mimic the required behavior. In the search for natural-based systems a dodecylamide of hyaluronic acid (HA), HYADD3®, has been proved as bioactive and suitable vehicle to carry cells for NP tissue engineering, while a crosslinked HA ester, HYAFF120® showed interesting results if used as injectable acellular material. Even though these derivatives showed appropriate biological behavior up to now, data on mechanical behavior of these derivatives are still missing. In this frame, the aim of this study was to provide a rheological characterization of these HA derivatives to asses their biomechanical compatibility with the NP tissue. To this, the rheological properties of these derivatives were studied through dynamic shear tests before and after injection through needles used in the current surgical procedure. Both HA derivatives showed a ‘gel-like’ rheological behavior similar to the native NP tissue and this behavior was not altered by injection.