Synthesis and characterization of a brush-like copolymer of polylactide grafted onto chitosan

Materials Based on Chitosan and Polylactide: From Biodegradable Plastics to Tissue Engineering Constructions

Abstract

The transition to green chemistry and biodegradable polymers is a logical stage in the development of modern chemical science and technology. In the framework of this review, the advantages, disadvantages, and potential of biodegradable polymers of synthetic and natural origin are compared using the example of polylactide and chitosan as traditional representatives of these classes of polymers, and the possibilities of their combination via obtaining composite materials or copolymers are assessed. The mechanochemical approach to the synthesis of graft copolymers of chitosan with oligolactides/polylactides is considered in more detail.

Strengthening mechanism of the mechanical properties of graft copolymers with incompatible pendant groups: nano-clusters and weak cross-linking

It is known that the adhesive property and mechanical strength of an apolar polymer can be improved by grafting with polar side chains, whereas the underlying mechanism is still elusive. In this work, the equilibrium structure and mechanical moduli of the melt of graft copolymers have been explored by dissipative particle dynamics. Due to the strong immiscibility of the non-polar backbone and polar side chains, nano-clusters of side chains formed and acted as physical crosslinkers. Moreover, non-affinity adsorption of polar side chains in the melt to the wall was observed, revealing an improvement in the adhesion property. Subjecting graft copolymers to cyclic deformation, the storage and loss moduli were acquired, and they grew with increasing grafting density. The melt strength in terms of the crossover frequency ascended with more side chains on the backbone. Our findings reveal that the strengthening of the mechanical properties of graft copolymers can be attributed to the formation of weakly cross-linked structures, thus offering an insight into the molecular design to aid the development of stronger graft copolymers.

Chitosan grafted/cross-linked with biodegradable polymers: A review

Public perception of polymers has been drastically changed with the improved plastic management at the end of their life. However, it is widely recognised the need of developing biodegradable polymers, as an alternative to traditional petrochemical polymers. Chitosan (CH), a biodegradable biopolymer with excellent physiological and structural properties, together with its immunostimulatory and antibacterial activity, is a good candidate to replace other polymers, mainly in biomedical applications. However, CH has also several drawbacks, which can be solved by chemical modifications to improve some of its characteristics such as solubility, biological activity, and mechanical properties. Many chemical modifications have been studied in the last decade to improve the properties of CH. This review focussed on a critical analysis of the state of the art of chemical modifications by cross-linking and graft polymerization, between CH or CH derivatives and other biodegradable polymers (polysaccharides or proteins, obtained from microorganisms, synthetized from biomonomers, or from petrochemical products). Both techniques offer the option of including a wide variety of functional groups into the CH chain. Thus, enhanced and new properties can be obtained in accordance with the requirements for different applications, such as the release of drugs, the improvement of antimicrobial properties of fabrics, the removal of dyes, or as scaffolds to develop bone tissues.

Multicomponent Non-Woven Fibrous Mats with Balanced Processing and Functional Properties

The mimicking of the architectonics of native tissue, biodegradable non-woven fibrous mats is one of the most promising forms of scaffolding for tissue engineering. The key properties needed for their successful application in vivo, such as biodegradability, biocompatibility, morphology, mechanical properties, etc., rely on their composition and appropriate 3D structure. A multicomponent system based on biodegradable synthetic (polycaprolactone, oligo-/polylactide) and natural (chitosan, gelatin) polymers, providing the desired processing characteristics and functionality to non-woven mats fabricated via the electrospinning technique, was developed. The solid-state reactive blending of these components provided a one-step synthesis of amphiphilic graft copolymer with an ability to form stable ultra-fine dispersions in chlorinated solvents, which could be successfully used as casting solvents for the electrospinning technique. The synthesized graft copolymer was analyzed with the aim of fractional analysis, dynamic laser scattering, FTIR-spectroscopy and DSC. Casting solution characteristics, namely viscosity, surface tension, and electroconductivity, as well as electrospinning parameters, were studied and optimized. The morphology, chemical structure of the surface layer, mechanical properties and cytocompatibility were analyzed to confirm the appropriate functionality of the formed fibrous materials as scaffolds for tissue engineering.

Design of hybrid molecular brushes with reversible surface adaptability on exposure to specific solvents

Hybrid molecular brushes (HMBs) are macromolecules made of a linear backbone and polymeric side chains that differ in their chemical nature. The authors developed a new method of synthesis of HMB with chitosan (CHI) backbone. In the first step, chitosan-graft-polylactide (CHI-g-PLA) was synthesized by interfacial ring opening polymerization of lactide initiated from CHI. CHI-g-PLA is characterized for its molecular weight and structure. In the second step, polyvinylpyrrolidone (PNVP) or polyacrylamide (PAAm) is grafted by radical polymerization from the CHI in CHI-g-PLA to form CHI-g-PLA-g-PNVP and CHI-g-PLA-g-PAAm. This results in the formation of HMB, with hydrophobic PLA and hydrophilic PNVP or PAAm side chains grafted to CHI. The chemical structure and thermal behavior of the HMBs are characterized. The morphology of CHI-g-PLA as well as the HMBs is determined using atomic force microscopy (AFM). Both the HMBs tethered to separate surfaces exhibit reversible switching between the hydrophilic and hydrophobic polymers on exposure to specific solvents. This is studied by AFM and water contact angle measurements. Hence, the authors developed a method for synthesis of HMB that can be applied for surface modification.

Graft Copolymerization of Chitosan with L-Lactide under Microwave Irradiation

Poly (L-lactide) was grafted onto chitosan by ring-opening copolymerization under microwave irradiation in the presence of tin octoate (Sn (Oct)2) as catalyst. The chemical structure and physical properties of the copolymers with different feeding ratios were discussed by FTIR, DTG and X-RD, taking pure chitosan as reference. The results showed that the application of microwave irradiation in the reaction could achieve the copolymer with high grafting percentage in a short time and low temperature. The thermal stability and crystallinity were decreased with the feeding ratio increasing, attributing to the increasing of grafting percentage.

Heparin binding-epidermal growth factor-like growth factor for the regeneration of chronic tympanic membrane perforations in mice

We aim to explore the role of epidermal growth factor (EGF) ligand shedding in tympanic membrane wound healing and to investigate the translation of its modulation in tissue engineering of chronic tympanic membrane perforations. Chronic suppurative otitis media (CSOM) is an infected chronic tympanic membrane perforation. Up to 200 million suffer from its associated hearing loss and it is the most common cause of pediatric hearing loss in developing countries. There is a need for nonsurgical treatment due to a worldwide lack of resources. In this study, we show that EGF ligand shedding is essential for tympanic membrane healing as it's inhibition, with KB-R7785, leads to chronic perforation in 87.9% (n=58) compared with 0% (n=20) of controls. We then show that heparin binding-EGF-like growth factor (5 μg/mL), which acts to shed EGF ligands, can regenerate chronic perforations in mouse models with 92% (22 of 24) compared with 38% (10 of 26), also with eustachian tube occlusion with 94% (18 of 19) compared with 9% (2 of 23) and with CSOM 100% (16 of 16) compared with 41% (7 of 17). We also show the nonototoxicity of this treatment and its hydrogel delivery vehicle. This provides preliminary data for a clinical trial where it could be delivered by nonspecialist trained healthcare workers and fulfill the clinical need for a nonsurgical treatment for chronic tympanic membrane perforation and CSOM.

Novel osteoinductive photo-cross-linkable chitosan-lactide-fibrinogen hydrogels enhance bone regeneration in critical size segmental bone defects

The purpose of this study was to develop and characterize a novel photo-cross-linkable chitosan-lactide-fibrinogen (CLF) hydrogel and evaluate the efficacy of bone morphogenetic protein-2 (BMP-2) containing a CLF hydrogel for osteogenesis in vitro and in vivo. We synthesized the CLF hydrogels and characterized their chemical structure, degradation rate, compressive modulus and in vitro BMP-2 release kinetics. We evaluated bioactivities of the BMP-2 containing CLF hydrogels (0, 50, 100 and 500ngml(-1)) in vitro using W-20-17 preosteoblast mouse bone marrow stromal cells and C2C12 mouse myoblast cells. The effect of BMP-2 containing CLF gels (0, 0.5, 1, 2 and 5μg) on bone formation was evaluated using rat critical size segmental bone defects for 4weeks. Fourier transform infrared spectroscopy spectra and scanning electron microscopy images showed chemical and structural changes by the addition of fibrinogen into the chitosan-lactide copolymer. The incorporation of fibrinogen molecules significantly increased the compressive modulus of the hydrogels. The in vitro BMP-2 release study showed initial burst releases from the CLF hydrogels followed by sustained releases, regardless of the concentration of the BMP-2 over 4weeks. Cells in all groups were viable in the presence of the hydrogels regardless of BMP-2 doses, indicating non-cytotoxicity of hydrogels. Alkaline phosphate activity and mineralization of cells exhibited dose dependence on BMP-2 containing CLF hydrogels. Radiography, microcomputed tomography and histology confirmed that the BMP-2 containing CLF hydrogels prompted neo-osteogenesis and accelerated healing of the defects in a dose-dependent manner. Thus the CLF hydrogel is a promising delivery system of growth factors for bone regeneration.

Studies on the orthogonal assembly of β-cyclodextrin-poly (ε-caprolactone) and ferrocene-poly (ethylene oxide)

A biodegradable multi-arm polymer β-cyclodextrin-poly (ε-caprolactone) (CD-PCL) with a ``jellyfish-like'' structure was obtained, in which flexible and hydrophobic PCL arms were selectively grafted to the wide side of the hydrophilic torus-shaped β-CD. The amphiphilic "jellyfish-like'' polymer with a hollow cavity and hydrophobic tails could orthogonally self-assemble into a new amphiphilic supramolecular copolymer CD-PCL/FcPEG with poly (ethylene oxide) end-decorated by ferrocene (FcPEG) in aqueous solution based on terminal hydrophobic interactions. The chemical structures of CD-PCL and CD-PCL/FcPEG were characterized by IR, NMR and UV and their self-assembled structures in water were investigated by transmission electron microscopy (TEM) and dynamic light scattering (DLS). CD-PCL alone self-assembled into nano vesicles in water, while CD-PCL/FcPEG into nanospheres. The supramolecular nanospheres were further investigated by cyclic voltammogram. The results indicated that the ferrocenyl groups which were embedded into the hydrophobic core of the supramolecular nanospheres could not transmit electrons or carry out electrochemical oxidation and reduction reaction.

A novel route for the production of chitosan/poly(lactide-co-glycolide) graft copolymers for electrospinning

Both chitosan and polylactide/polyglycolide have good biocompatibility and can be used to produce tissue engineering scaffolds for cultured cells. However the synthetic scaffolds lack groups that would facilitate their modification, whereas chitosan has extensive active amide and hydroxyl groups which would allow it to be subsequently modified for the attachment of peptides, proteins and drugs. Also chitosan is very hydrophilic, whereas PLGA is relatively hydrophobic. Accordingly there are many situations where it would be ideal to have a copolymer of both, especially one that could be electrospun to provide a versatile range of scaffolds for tissue engineering. Our aim was to develop a novel route of chitosan-g-PLGA preparation and evaluate the copolymers in terms of their chemical characterization, their performance on electrospinning and their ability to support the culture of fibroblasts as an initial biological evaluation of these scaffolds. Chitosan was first modified with trimethylsilyl chloride, and catalyzed by dimethylamino pyridine. PLGA-grafted chitosan copolymers were prepared by reaction with end-carboxyl PLGA (PLGA-COOH). FT-IR and(1)H-NMR characterized the copolymer molecular structure as being substantially different to that of the chitosan or PLGA on their own. Elemental analysis showed an average 18 pyranose unit intervals when PLGA-COOH was grafted into the chitosan molecular chain. Differential scanning calorimetry results showed that the copolymers had different thermal properties from PLGA and chitosan respectively. Contact angle measurements demonstrated that copolymers became more hydrophilic than PLGA. The chitosan-g-PLGA copolymers were electrospun to produce either nano- or microfibers as desired. A 3D fibrous scaffold of the copolymers gave good fibroblast adhesion and proliferation which did not differ significantly from the performance of the cells on the chitosan or PLGA electrospun scaffolds. In summary this work presents a methodology for making a hybrid material of natural and synthetic polymers which can be electrospun and reacts well as a substrate for cell culture.

Fibrous poly(chitosan-g-DL-lactic acid) scaffolds prepared via electro-wet-spinning

DL-lactic acid was grafted onto chitosan to produce poly(chitosan-g-DL-lactic acid)(PCLA) copolymers. These PCLAs were then spun into submicron and/or nanofibers to fabricate scaffolds using an electro-wet-spinning technique. The diameter of fibers in different scaffolds could vary from about 100 nm to around 3 microm. The scaffolds exhibited various pore sizes ranging from about 1 microm to less than 30 microm and different porosities up to 80%. Two main processing parameters, that is, the concentration of PCLA solutions and the composition proportions of coagulation solutions, were optimized for obtaining desired scaffolds with well-controlled structures. The tensile properties of the scaffolds in both dry and hydrated states were examined. Significantly improved tensile strength and modulus for these fibrous scaffolds in their hydrated state were observed. The data collected from in vitro rabbit-fibroblast/scaffold culture showed that there were no substantial differences in the viability, density and distribution of cultured fibroblasts between PCLA scaffolds and pure chitosan scaffolds.

Core-shell particles for the dispersion of small polar drugs and biomolecules in hydrofluoroalkane propellants

PURPOSE

Demonstrate the applicability of a novel particle-based technology for the development of suspensions of small polar drugs and biomolecules in hydrofluoroalkane (HFA) propellants for pressurized metered-dose inhalers (pMDIs).

MATERIALS AND METHODS

Emulsification diffusion was used to prepare core-shell particles. The shell consisted of oligo(lactide) grafts attached onto a short chitosan backbone. The active drug was arrested within the particle core. Colloidal Probe Microscopy (CPM) was used to determine the cohesive forces between particles in a model HFA propellant. The aerosol characteristics of the formulations were determined using an Anderson Cascade Impactor (ACI). Cytotoxicity studies were performed on lung epithelial and alveolar type II cells.

RESULTS

CPM results indicate that particle cohesive forces in liquid HFA are significantly screened in the presence of the polymeric shell and correlate well with the physical stability of suspensions in propellant HFA. The proposed formulation showed little or no cytotoxic effects on both Calu-3 and A549 cells.

CONCLUSIONS

Core-shell particles with a shell containing the lactide moiety as the HFA-phile showed excellent dispersion stability and aerosol characteristics in HFA-based pMDIs. This is a general strategy that can be used for developing novel suspension pMDIs of both small polar drugs and large therapeutic molecules.

Tuning of shell and core characteristics of chitosan-decorated acrylic nanoparticles

The aim of the work was to develop a new family of chitosan-coated acrylic nanoparticles to increase the specificity of absorption of drugs associated given by the mucosal route. To achieve this goal, techniques of radical and anionic emulsion polymerisation of isobutylcyanoacrylate (IBCA) were used. Changes in the shell composition were made by using chitosan of different molecular weight and thiolated chitosan to modify the particle surface properties in order to vary the mucosae-nanoparticle interactions. The core was also modified by the inclusion of methyl methacrylate (MMA) as second monomer potentially able to improve the control of drug release. Finally, the labelling of nanoparticles core with a fluorophore, methacryloxyethyl thiocarbamoyl rhodamine B (Polyfluor), was successfully achieved, necessary for the in vitro and in vivo evaluation of the systems created. Results showed that nanoparticle size varied from 200 to 500 nm, depending on the molecular weight of chitosan used. Positive surface charge values were obtained in all cases. In addition, evidences of the presence of thiol groups were obtained (0.03-0.16 x 10(-3)micromol/cm(2) of nanoparticle).

Development and in vitro evaluation of a thiomer-based nanoparticulate gene delivery system

Chitosan-thiobutylamidine was developed and evaluated as a novel tool for gene delivery. The conjugate, displaying 299.1+/-11.5 micromol free thiol groups per gram polymer, formed coacervates with pDNA at a mean size of 125 nm and a zeta potential of +9 mV. Thiol groups, being susceptible for oxidation, were immobilised on the polymeric backbone of chitosan in order to introduce the property of extracellular stability and intracellular pDNA release by forming reversible disulfide bonds. The integrity of the new particles was compared to unmodified chitosan under simulated physiological conditions. Within 10h, pDNA was completely released from chitosan-DNA particles while only 12% were released from the thiomer-based particles. At pH 7, the amount of thiol groups significantly (p<0.05) decreased by more than 25% within 6h. In contrast, in a reducing environment as found intracellularly, chitosan-thiobutylamidine-DNA nanoparticles dissociated continuously, liberating approximately 50% of pDNA within 3h. Transfection studies performed in a Caco2 cell culture evinced the highest efficiency for chitosan-thiobutylamidine-DNA nanoparticles in combination with a glycerol shock solution. The combination of improved stability, enhanced pDNA release under reducing conditions, and higher transfection efficiency identifies chitosan-thiobutylamidine as a promising new vector for gene delivery.

Elaboration and characterization of thiolated chitosan-coated acrylic nanoparticles

The aim of the present work was to investigate the use of thiolated chitosan in the development of polysaccharide-coated nanoparticles in order to confer specific functionality to the system. After chemical modification of commercial and hydrolysed chitosan (400,000 and 9400 g/mol respectively), thiolated chitosans were used to elaborate particles in the nano-range. They were characterized in terms of size and surface charge measurement. Both analysis showed nanoparticles of mean hydrodynamic diameter around 200 nm and positive zeta potential values, indicating the presence of the cationic polysaccharide at the nanoparticle surface. Moreover, the Ellman's reaction was used to demonstrate the presence of thiol groups at the particle surface. The observation of nanoparticles by scanning electronic microscopy (SEM) showed spherical nanoparticles for all formulations. This new system, combining both the advantages of thiolated polymers and colloidal particles can be proposed as an original drug carrier system for mucosal delivery of biotechnology products.

In vitro degradation of biodegradable blending materials based on poly(p-dioxanone) and poly(vinyl alcohol)-graft-poly(p-dioxanone) with high molecular weights

Amphiphilic biodegradable graft copolymer, poly(vinyl alcohol)-graft-poly(p-dioxanone) (PVA-g-PPDO), was used to prepare a new biodegradable material by blending with poly(p-dioxanone) (PPDO). The in vitro degradation properties of the copolymer and blends with different contents of PVA-g-PPDO were studied in phosphate buffer at 37 degrees C. The degradation processes of the PVA-g-PPDO and its blends with the PPDO were monitored by weight loss, viscosimetry, water uptake, differential scanning calorimetry (DSC), and scanning electron microscopy. The results of inherent viscosity and weight loss reveal that the PVA-g-PPDO has a different in vitro degradation behavior from that of PPDO, and the introducing of copolymer into the blending system may enhance the degradability of PPDO when the contents of copolymer is higher than 5%. The change of the degree of crystallization (Dc) of copolymer and blends derived from the DSC also shows that the copolymer and blends have faster degradation rates than the neat PPDO during the testing period. A degradation mechanism of the blends was postulated based on the results of the weight retention, inherent viscosity measurement, and DSC.

Bulk and surface modifications of polylactide

This article reviews various methods of modifying the bulk and surface properties of poly(lactic acid) (PLA) so that the polymer may be used as a drug carrier in a drug delivery system (DDS) and as a cell scaffold in tissue engineering. Copolymerization of lactide with other lactone-type monomers or monomers with functional groups such as malic acid, copolymerization of lactide with macromolecular monomer such as poly(ethylene glycol) (PEG) or dextran, as well as blending polylactide and natural derivatives and other methods of bulk modification are discussed. Surface modifications of PLA-type copolymers, such as surface coating, chemical modification, and plasma treatment are described. Cell culture technology proves the efficiency of bulk and surface modification and the potential application of PLA in tissue engineering.