Development & Characterisation of Nanocomposites for Bone Tissue Engineering

The aim of this thesis was to develop a bioactive and resorbable nanoscale composite that mimics the properties of bone and will have the potential to regenerate bone. In conventional composites, the polymer phase can mask the bioactive phase and often degrades faster than the ceramic phase due to the weak interfacial bonding between the polymer and ceramic. Here in this thesis an organic/inorganic nanocomposite with stronger interfacial bonding between the two phases has been produced using the sol-gel route. Glasses containing SiO2 and CaO were used as the inorganic while the amino acid poly-γ−glutamic acid (γ−PGA) was used as the organic. This is the first time an inorganic/organic hybrid with enzymatically degradable polymer covalently crosslinked to the inorganic has been produced. Several factors contributed to the homogeneity of the nanocomposites; most important of all was the extent of integration (homogeneity and phase miscibility) of the organic into the inorganic sol. The main focus of this thesis was to synthesise this new material and to develop an understanding of the nanoscale interactions of the two phases. The chemical structure of the nanocomposites were characterised with Fourier transform infrared spectroscopy (FTIR) and nuclear magnetic resonance spectroscopy (NMR) and the nanostructure was characterised with scanning and transmission electron microscopy (SEM and TEM). Bioactivity studies of the nanocomposites in simulated body fluid (SBF) showed that the nanocomposites containing calcium were bioactive. Initial in vitro cell response studies also showed that the nanocomposites were not toxic to cells. Nanocomposites were also foamed to create the first porous bioactive inorganic/organic scaffolds with covalent bonding between the organic and inorganic. Micro-computed tomography (μCT) was used to non-destructively image and quantify the internal pore structure of the bioactive nanocomposite scaffolds. The three-dimensional images of the scaffolds show that the nanocomposites have large macropores with multiple connections between them giving a suitable pore structure for tissue engineering

Cellulosic materials as biopolymers and supercritical CO2as a green process: chemistry and applications

In this review, we describe the use of supercritical CO2 (scCO2) in several cellulose applications. The focus is on different technologies that either exist or are expected to emerge in the near future. The applications are wide from the extraction of hazardous wastes to the cleaning and reuse of paper or production of glucose. To put this topic in context, cellulose chemistry and its interactions with scCO2 are described. The aim of this study was to discuss the new emerging technologies and trends concerning cellulosic materials processed in scCO2 such as cellulose drying to obtain aerogels, foams and other microporous materials, impregnation of cellulose, extraction of highly valuable compounds from plants and metallic residues from treated wood. Especially, in the bio-fuel production field, we address the pre-treatment of cellulose in scCO2 to improve fermentation to ethanol by cellulase enzymes. Other reactions of cellulosic materials such as organic inorganic composites fabrication and de-polymerisation have been considered. Cellulose treatment by scCO2 has been discussed as well. Finally, other applications like deacidification of paper and cellulosic membranes fabrication in scCO2 have been reviewed. Examples of the discussed technologies are included as well

Bioglass®-derived glass-ceramic scaffolds for bone tissue engineering

Imperial Users onl

New Sol-Gel Derived Bioactive Glasses and Organic/Inorganic Hybrids for Bone Regeneration

Sol-gel derived bioactive glasses have been considered as one of the most promising materials for bone regeneration. However they are brittle, therefore composites are needed if bioactive materials are to share load with bone. One strategy for production of composites with tailored mechanical properties and congruent degradation rates is the development of inorganic/ organic hybrids. Hybrids are particular types of nanocomposite synthesised by introducing a polymer into the sol-gel process so that the silica and polymer chains interact at the nanoscale. Calcium must be incorporated into glasses and hybrids if they are to be bioactive (e.g. bone to bone). Calcium nitrate is conventionally used in the sol-gel process as the calcium source. However, there are many disadvantages of using it. Calcium nitrate causes inhomogeneity by forming calcium rich regions and it requires high temperature treatment (>400⁰C) to be incorporated into the glass network. Calcium nitrate cannot be used in the synthesis of hybrids where the highest temperature used in the process is approximately 60⁰C. Therefore, a different precursor is needed to improve homogeneity of glasses and for low temperature synthesis of hybrids. In this work, two alternatives were investigated and compared to the conventional approach of using calcium nitrate: calcium chloride, an alternative calcium salt, and calcium methoxyethoxide (CME), a calcium alkoxide. The structure of the gels and glasses were investigated over a range of final processing temperatures from 60⁰C to 800⁰C, corresponding to hybrid and glass process temperatures using advanced probe techniques such as solid state NMR. The temperature at which calcium was incorporated into the network was identified for 70S30C (70 mol% SiO2, 30 mol% CaO) and 58S (60 mol% SiO2, 36 mol% CaO, 4 mol% P2O5) compositions synthesised with the three different calcium precursors. Using calcium nitrate, calcium did not enter the silica network until temperatures greater than 380⁰C were reached. When calcium chloride was used, the calcium did not seem to enter the network at any of the temperatures. In contrast, calcium from CME entered the silica network at room temperature, indicating CME is an improved calcium source for low temperature synthesis. An aim of this work was to synthesise poly(γ-glutamic acid)/ silica hybrids, containing calcium, using calcium chloride and CME. Calcium incorporation was much improved when CME was used and mechanical properties were much improved compared a sol-gel glass or hybrids synthesised with other calcium sources. A hydroxycarbonated apatite (HCA) formed on the hybrids after immersion in simulated body fluid (SBF), indicating bioactivity. Polylactide was also trialled as the organic phase for hybrid synthesis, using polylactide-diol (PLAD). However, synthesis of PLAD/bioactive glass hybrid was not successful as it was difficult to incorporate the functionalised polymer into the sol. Calcium incorporation into the silica network using the sol-gel process is therefore possible but challenging

Development of novel 3D porous melt-derived bioactive glass scaffolds

The aim of this thesis is to develop a method to produce melt-derived bioactive glass scaffolds, without the glass crystallizing into a glass-ceramic, while establishing an interconnected pore network suitable for bone tissue regeneration. In order to achieve this, the scaffold must have the ability to closely mimic the porous structure of cancellous bone and its mechanical properties. Two bioactive glasses were used in this project, both are modified from the Bioglass® composition: ICIE 16 (49.46% SiO2, 36.27% CaO, 6.6% Na2O, 6.6% K2O and 1.07% P2O5, all in mol%) and ICIE 16M (49.46% SiO2, 27.27% CaO, 6.6% Na2O, 6.6% K2O, 3% ZnO, 3% MgO, 3% SrO and 1.07% P2O5, all in mol%). Gel-cast foaming produced improved pore networks over alternative methods for producing porous scaffolds. There are many variables in the process. An initial protocol was established and each of the variables assessed systematically. The relationships between each component, the gelling time and the foam body volume were evaluated to develop an optimized protocol for the process. The size of the glass powder was critical in determining the sintering efficiency. A suitable drying and sintering program was also determined to prevent crystallization of the glass and formation of crystal species from by-products of the process. The scaffolds were characterized in terms of the interconnect size, the rate of hydroxycarbonate apatite (HCA) formation in simulated body fluid (SBF) solutions, the ion release rate and the compressive strength. The results showed that ICIE 16M sintered better and was stronger than ICIE 16, however in the bioactivity aspect of view, the rate of HCA formation in SBF was faster for ICIE 16 than ICIE 16M

Reshapable polymeric hydrogel for controlled soft-tissue expansion: In vitro and in vivo evaluation

Tissue expansion is the process by which extra skin is generated using a device that applies pressure from underneath the skin. Over the course of weeks to months, stretching by this pressure creates a flap of extra tissue that can be used to cover a defect area or enclose a permanent implant. Conventional tissue expanders require a silicone shell inflated either by external injections of saline solution or air, or by internal osmotic pressure generated by a hydrophilic polymer. In this study, a shell-free tissue expander comprised only of a chemically cross-linked biocompatible polymeric hydrogel is developed. The cross-linked network of hydrophilic polymer provides for intrinsically controlled swelling in the absence of an external membrane. The new type of hydrogel expanders were characterized in vitro as well as in vivo using a rat-skin animal model. It was found that increasing the hydrophobic polyester content in the hydrogel reduced the swelling velocity to a rate and volume that eliminate the danger of premature swelling rupturing the sutured area. Additionally, increasing the crosslinking density resulted in enough mechanical strength of the hydrogel to allow for complete post-swelling removal, without the hydrogel cracking or crumbling. No systemic toxicity was noted with the expanders and histology showed the material to be highly biocompatible. These expanders have an advantage of tissue expansion without requiring an external silicone membrane, and thus, they can be cut or reshaped at the time of implantation for applications in small or physically constrained regions of the body

Carriers in Cell-Based Therapies for Neurological Disorders

published_or_final_versio

Characterization of the Degradation of Shape Memory Polymers

Shape memory polymer (SMP) polyurethanes have been proposed for a variety of vascular devices due to their biocompatibility, stimuli-responsiveness, and tunable properties. While this technology shows promise, a primary limitation for translation of these SMPs into the clinic is a lack of understanding of the degradation behavior and stability. Characterization of the degradation of these SMPs revealed excellent hydrolytic stability although the presence of tertiary amines results in a rapid oxidatively-induced mass loss over time. The mechanism of degradation was found to be the scission of the tertiary amines, producing secondary amines, primary amines, aldehydes and carboxylic acids. This understanding of degradation was then used to assess the toxicity risks for SMP implants, and it was found that despite degradation these SMPs may possess minimal risks as vascular implants. One of the goals in medical devices is biostability, or relatively minimal mass loss over time. One method of tailoring oxidative mass loss was the use of antioxidants. Bulk inclusion of antioxidants resulted in tunability of pore size, mechanical properties, shape recovery kinetics, and oxidative resistance. A limitation of this method was the retention of antioxidants in the SMP matrix after cleaning the material post-synthesis, as determined from gas-chromatography mass spectrometry (GCMS). Synthesis and incorporation of polyurethane microparticles was used to improve the retention of antioxidants, determined by gravimetric comparisons. Additionally, while the inclusion of antioxidants resulted in various property changes, SMP composites containing the particles possessed similar scaffold pore size, shape recovery kinetics, and mechanical properties while displaying improved oxidative stability. Further increase of the SMPs’ biostability was achieved through chemical modification of the polymer structure, with glycerol and isocyanurate groups examined. Glycerol was found to improve the oxidative resistance, resulting in SMPs with lifespans of ca 5 years when sufficient concentrations of glycerol were used. Isocyanurate-based SMPs are predicted to have lifespans extending potentially to nearly 20 years. The presented work demonstrates the degradation behavior, toxicity risks, and mechanism for SMPs containing tertiary amines. This work is then applied for further tuning oxidation by the biostability through chemical and physical means, including new covalently added monomers and macromers, as well as composite synthesis. This work supports the concept of SMP-based vascular occlusion materials, and it is hoped that these studies will aid in the translation of such devices into clinics in the near future

Characterization of the Degradation of Shape Memory Polymers

Shape memory polymer (SMP) polyurethanes have been proposed for a variety of vascular devices due to their biocompatibility, stimuli-responsiveness, and tunable properties. While this technology shows promise, a primary limitation for translation of these SMPs into the clinic is a lack of understanding of the degradation behavior and stability. Characterization of the degradation of these SMPs revealed excellent hydrolytic stability although the presence of tertiary amines results in a rapid oxidatively-induced mass loss over time. The mechanism of degradation was found to be the scission of the tertiary amines, producing secondary amines, primary amines, aldehydes and carboxylic acids. This understanding of degradation was then used to assess the toxicity risks for SMP implants, and it was found that despite degradation these SMPs may possess minimal risks as vascular implants. One of the goals in medical devices is biostability, or relatively minimal mass loss over time. One method of tailoring oxidative mass loss was the use of antioxidants. Bulk inclusion of antioxidants resulted in tunability of pore size, mechanical properties, shape recovery kinetics, and oxidative resistance. A limitation of this method was the retention of antioxidants in the SMP matrix after cleaning the material post-synthesis, as determined from gas-chromatography mass spectrometry (GCMS). Synthesis and incorporation of polyurethane microparticles was used to improve the retention of antioxidants, determined by gravimetric comparisons. Additionally, while the inclusion of antioxidants resulted in various property changes, SMP composites containing the particles possessed similar scaffold pore size, shape recovery kinetics, and mechanical properties while displaying improved oxidative stability. Further increase of the SMPs’ biostability was achieved through chemical modification of the polymer structure, with glycerol and isocyanurate groups examined. Glycerol was found to improve the oxidative resistance, resulting in SMPs with lifespans of ca 5 years when sufficient concentrations of glycerol were used. Isocyanurate-based SMPs are predicted to have lifespans extending potentially to nearly 20 years. The presented work demonstrates the degradation behavior, toxicity risks, and mechanism for SMPs containing tertiary amines. This work is then applied for further tuning oxidation by the biostability through chemical and physical means, including new covalently added monomers and macromers, as well as composite synthesis. This work supports the concept of SMP-based vascular occlusion materials, and it is hoped that these studies will aid in the translation of such devices into clinics in the near future