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Biodegradable and Bioactive Porous Polymer/Inorganic Nanocomposites Scaffolds for Biomedical Applications
Full text linkWith the aging of populations and prolonged life expectancy, there is an increasing demand for bone grafts or synthetic materials that can potentially replace, repair or regenerate lost, injured or diseased bone. Tissue engineering (TE) is one of the approaches being investigated to tackle this problem. In common TE strategies, a three-dimensional structure, termed “scaffold”, fabricated from a suitable artificial or natural material.
In bone tissue engineering, a scaffolding material is used either to induce formation of bone from the surrounding tissue or to act as a carrier or template for implanted bone cells or other agents. To serve as a scaffold, the material must be biocompatible, osteoconductive, and osteointegrative, and have enough mechanical strength to provide structural support during the bone growth and remodeling. Several attempts have been successfully made to construct porous scaffolds with desired porosity and appropriate mechanical performance from inorganic materials such as bioactive ceramics and glasses, from biodegradable polymers and their composites.
The focus of biomaterial design for tissue engineering applications has recently been directed towards bioactive components that facilitate biomaterial integration and native tissue regeneration at the implant site. During the last four decades, various materials known as ‘bioactive materials’ such as glasses, sintered hydroxyapatite, glass ceramics, composite materials, etc., have been synthesized and developed for medical applications. A significant characteristic of bioactive materials is their ability to bond with living bone through the formation of a hydroxyapatite (HA) interface layer. A recognized method to estimate the bone-bonding potential ability of material is simulated body fluid method (SBF), which involves immersing materials into SBF for bone-like apatite formation on its surface according to Kokubo et al. In other words, the behavior in vivo could be predicted by using SBF method in vitro.
One remarkable success of bioactive ceramics as implant materials is the clinical use of sintered hydroxyapatite (HA) due to its bioactivity and osteoconductivity. However, the low fracture toughness of HA ceramic limits the scope of clinical applications. In recent years, more attentions have been focused on developing novel bioactive ceramics with improved properties. More recently, extensive interests have been shown in developing new bioactive inorganic materials containing CaO–SiO2 component for biomedical applications.
Calcium silicate-based ceramics have received great attention as materials for bone tissue regeneration due to their excellent bioactivity. Compared to phosphate-based bioceramics, silicate bioceramics possess a wide range of chemical compositions and crystal structures, which contribute to their adjustable physicochemical properties, such as mechanical strength, bioactivity and degradation, providing them with suitable characteristics to be used as biomaterials. However, a major drawback of the CaSiO3 ceramics is their high dissolution rate, leading to a high pH value in the surrounding environment, which is detrimental to cells, which can be modified by incorporation of different elements such as Zn, Mg, Sr, Ti and Zr. In any case, the proposed approach can be extended to those more complex bioceramic compositions.
In particular, due to the difficulties with sintering, silicate ceramics are generally obtained by complex techniques, such as the hydrothermal method, devitrification of glass, sol–gel processing, spark plasma-sintering, solution combustion processes etc. The sol–gel method is well suited for the preparation of complex ternary and quaternary silicate ceramics, as it allows for a precise control of the stoichiometry of the starting materials. However, it is of difficult industrialization, in the case of the fabrication of bulk components, because of the cost of the raw materials, the presence of large amounts of solvents and the associated drying problems.
The current project is aiming at developing and fabricating of bioactive silicate-based ceramics from preceramic polymers (commercially available polymethylsiloxanes, silicones), and fillers (commercially available MgO, CaO, ZnO, TiO2, nano- and/or micro-particles), in the form of tablets, foams and 3D printed structures using additive manufacturing technology, to be used as bioactive scaffolds and biomaterials, thereby confirming that the proposed approach can be used to obtain components suitable for bone tissue regeneration.
The incorporation of fillers, that generally can be passive or active, into the preceramic system is considered one of the most effective strategies to produce the silicate ceramics with different composition and structures as well as, to decrease the shrinkage and the formation of macro-defects in the produced ceramics. The approach of adding different oxide precursors (such as CaO and/or CaO, MgO and TiO2) as fillers enabled developing of different silicate bioactive ceramics (such as wollastonite (CaSiO3), hardystonite (Ca2ZnSi2O7), diopside (CaMgSi2O6) and sphene (CaTiSiO5)) as a result of the reactions between the preceramic polymers and these reactive fillers, occurring during the ceramization process and leading to the formation of specific crystalline phases with highly phase assemblage, that are known to be difficulty achievable by the conventional synthesis methods. A particular attention will be given to the production of open-celled porous components, to be employed as scaffolds for bone tissue engineering. These components will be prepared by various techniques, including unconventional direct foaming of silicone mixtures and additive manufacturing technology. Once the ceramic materials and scaffolds will be prepared, they will be fully characterized in terms of crystalline phase assemblage, physical and mechanical properties as well as microstructure analysis. The remarkable bioactivity of these scaffolds will be the main object of current investigations
Effect of particle size distribution and printing parameters on alumina ceramics prepared by Additive Manufacturing
No full textVat photopolymerization-based additive manufacturing is a promising technology for the preparation of ceramic parts, owing to its short fabrication cycle and low manufacturing cost. However, its application is limited due to the low mechanical properties and deformation of ceramic parts. To improve the properties of ceramic parts, changing particle size and printing parameters have been found to be useful. Herein, alumina ceramic parts were prepared using three different powders with different particle size (Powder A: D50 = 1.3 mu m, D90 = 6.1 mu m; Powder B: D50 = 3.4 mu m, D90 = 10.0 mu m; Powder C: D50 = 1.3 mu m, D90 = 2.8 mu m), different layer thickness (50 mu m, 75 mu m, 100 mu m), and different curing times (1 s, 3 s, 5 s, 8 s). The ceramics prepared with Powder A and Powder C, which possessed the same D50, had almost the same flexural strength, indicating that the flexural strength is closely related to the particle size and its distribution. With the increase in layer thickness, the flexural strength was increased. When the layer thickness was 100 mu m, the flexural strength reached 18.5 MPa when samples were prepared with Powder A and Powder C. At the same time, the flexural strength firstly increased and subsequently decreased with increasing curing time. Based on the flexural strength and shrinkage of the sintered ceramics, using Powder A, layer thickness of 50 mu m, and curing time of 5 s were regarded as the best fabrication conditions. The results indicate that adjusting powder particle size distribution, layer thickness, and curing time are promising methods for the fabrication of 3D printed ceramics with optimized properties
Highly porous cordierite ceramics from engineered basic activation of metakaolin/talc aqueous suspensions
No full textThe cellular structure, in alkali activated metakaolin-based suspensions, foamed by intensive mechanical stirring, is stabilised by the viscosity increase caused by gelation, in a condition of ‘inorganic gel casting’. The approach is so flexibile that it may be applied to mixtures embedding fillers, such as reactive γ-Al2O3 powders, playing a fundamental role upon ceramic conversion. The present study is dedicated to highly porous cordierite foams, obtained including talc as further component, and applying a heat treatment at 1200−1250 °C, in air. A key intermediate step is represented by the removal of Na+ ions from ‘green’ foams, by ion exchange in ammonium nitrate solution (24 h), before ceramization. Direct ceramization is also feasible, if the gelation is achieved by reaction with tetra-methyl-ammonium hydroxide, instead of NaOH. The new gels are effective in yielding phase-pure cordierite, otherwise feasible, with NaOH activation, by means of much longer ion exchange treatment (120 h)
Polymer-derived sphene biocoating on cpTi substrates for orthopedic and dental implants
No full textSphene coatings were prepared by a novel process involving the use of a preceramic polymer containing nanosized
and micro-sized active fillers as precursors for the formation of the desired ceramic phase. A commercially
available airbrush was used to cold-spray the suspension on the cpTi substrate, and the samples were heat
treated to transform the precursor and fillers mixture into a ceramic coating. The processing conditions were
optimized in order to obtain cracks free coatings, characterized by good adhesion to the substrate and a desired
phase assemblage
Use of cryogenic machining to improve the adhesion of sphene bioceramic coatings on titanium substrates for dental and orthopaedic applications
No full textHighly porous mullite ceramics from engineered alkali activated suspensions
Full text linkAir may be easily incorporated by vigorous mechanical stirring, with the help of surfactants, of activated geopolymer-yielding suspensions. The cellular structure is stabilized by the viscosity increase caused by curing reactions, configuring an inorganic gel casting. The present paper is aimed at extending this approach to mullite foams, obtained by the thermal treatment of engineered alkali activated suspensions. Green foams were first obtained by gel casting of a suspension for Na-geopolymer enriched with reactive -Al2O3 powders. Sodium was later extracted by ionic exchange with ammonium salts. In particular, the removal of Na+ ions was achieved by immersion in ammonium nitrate solution overnight, with retention of the cellular structure. Finally, the ion-exchanged foams were successfully converted into pure mullite foams by application of a firing treatment at 1300 degrees C, for 1hour. Preliminary results concerning the extension of the concept to mullite three-dimensional scaffolds are presented as well
Bioactive glass-ceramic scaffolds from novel 'inorganic gel casting' and sinter-crystallization
Full text linkHighly porous wollastonite-diopside glass-ceramics have been successfully obtained by a new gel-casting technique. The gelation of an aqueous slurry of glass powders was not achieved according to the polymerization of an organic monomer, but as the result of alkali activation. The alkali activation of a Ca-Mg silicate glass (with a composition close to 50 mol % wollastonite50 mol % diopside, with minor amounts of Na2O and P2O5) allowed for the obtainment of well-dispersed concentrated suspensions, undergoing progressive hardening by curing at low temperature (40 degrees C), owing to the formation of a C-S-H (calcium silicate hydrate) gel. An extensive direct foaming was achieved by vigorous mechanical stirring of partially gelified suspensions, comprising also a surfactant. The open-celled structure resulting from mechanical foaming could be frozen' by the subsequent sintering treatment, at 900-1000 degrees C, causing substantial crystallization. A total porosity exceeding 80%, comprising both well-interconnected macro-pores and micro-pores on cell walls, was accompanied by an excellent compressive strength, even above 5 MPa
3D printing of polymer-derived SiOC with hierarchical and tunable porosity
Full text linkA facile and controllable method for the fabrication of SiOC ceramic components with hierarchical porosity is reported in this work, using additive manufacturing, sacrificial polymeric microbeads and a silicone resin. Particles-containing inks with suitable rheology enabled the fabrication by Direct Ink Writing of scaffolds with mm-scale macropores between filaments, and μm-scale and/or nm pores within the filaments. Isotropic shrinkage and an appropriate pyrolysis program enabled to obtain scaffolds without defects and shape distortions. Total porosity and compression strength depended on the content and size of the PMMA particles in the inks, enabling the fabrication of SiOC ceramic with components possessing a remarkable strength up to 2.92 MPa for a total porosity of 86.5 vol% by the addition of 5 μm PMMA particles
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