Effect of hybrid polymer coating of Bioglass® foams on mechanical response during tensile loading

A simplified two-dimensional finite elements model was created for a polyvinyl alcohol (PVA) coated Bioglass^® strut undergoing tensile stresses (loading mode I). The strengthening contributions due to the infiltration of coating into surface cracks and coating's stiffness were evaluated in terms of stress intensity factor K_I and tensile stresses σ_yy in the proximity of the crack tip. The infiltration of the coating until the crack tip resulted as the most effective criterion for the struts strengthening. Bioglass^® based scaffolds were dip coated into PVA and PVA/microfibrillated cellulose (MFC) aqueous solutions and tested in tensile load. Coated samples exhibited remarkably higher tensile strength than non-coated ones, which further raised with the increased amount of MFC. Contact angle θ and linear viscosity η measurements of PVA/MFC solutions showed that MFC caused a reduction in θ and a drastic increase in η, indicating that a balance between these two effects must be achieved.     

Crack-free silicate bioceramics from preceramic polymers

As a case study, the present paper illustrates an innovative processing method employing a preceramic polymer containing different fillers, which can be used to manufacture various ceramic components for biomedical applications. Crack-free wollastonite (CaSiO₃) ceramics were successfully produced, with high phase purity, after heating at 900°C in air starting from a silicone resin containing CaCO₃ micro-sized particles as ‘active filler’. As ‘passive filler’, wollastonite preceramised powders as well as commercially available wollastonite fibres were added. Their presence reduces the gas evolution occurring due to the decomposition of the calcium carbonate active filler and the polymer-to-ceramic conversion, reducing the stresses that generate in the component during heating. The resulting samples exhibited improvements in terms of the morphology and the mechanical strength, with respect to samples not containing any passive fillers, without significant modification of the final phase assemblage.     

Preparation of hydroxyapatite-carbon nanotube composite nanopowders

Here we report a simple preparation of composite nanopowders made of hydroxyapatite (HA) and carbon nanotube (CNT). In particular, CNTs were ionically modified to dissolve homogeneously in a series of organic solvents, including tetrahydrofuran and ethanol. The addition of HA nanopowders within the CNTs solution resulted in rapid precipitation of the composite HA-CNTs nanopowders. High resolution electron image revealed individual CNTs were evenly distributed within the cluster of HA nanoparticulates. A maximal concentration of CNTs to be organized within the HA nanopowder was highly dependent on the physicochemical characteristics of HA powders. A pilot biological assessment of the composite powders demonstrated favorable adhesion and growth of tissue cells. The developed HA-CNTs composite nanopowders may be potentially useful as an initial powder source for HA-CNT nanocomposites or coatings in biomedical applications.     

Calcium and potassium addition to facilitate the sintering of bioactive glasses

Nowadays bioactive glasses are diffused in medical practice due to their excellent bioactivity. However high temperature treatments, which are commonly required in several processing routes, may induce the glass to crystallize into a glass-ceramic, with possible negative effects on its bioactivity. In this work a new bioactive glass composition, inspired by the widely used Bioglass® 45 S5, was formulated by increasing the calcium content and substituting the sodium oxide with potassium oxide. The novel glass can be treated at a relatively low temperature (800 °C) and it is characterized by a reduced tendency to crystallize with excellent effects in terms of bioactivity, according to in vitro tests. Therefore, the new composition opens intriguing scenarios whenever a thermal treatment is required to apply or to sinter the glass, such as in the production of scaffolds or the deposition of coatings.     

Bioceramic composites for orthopaedic applications: A comprehensive review of mechanical,biological, and microstructural properties

Bioceramics have been widely utilized for orthopaedic applications in which the biocompatibility and mechanical properties of the materials are vital characteristics to be considered for their clinical use. Till date, extensive studies have been devoted to developing a range of scientific ways for tailoring the microstructure of bioceramics in order to attain the trade-off of mechanical properties and biocompatibility of the final product. Owing to low reactivity, earlier stabilization and longer functional life of bioceramic, the developed implants are capable of replicating the mechanical behaviour of original bone. As the safety of the patient and its ultimate functionality are the ultimate goal of the selected implant material hence, the present literature survey investigates and brings forth the important aspects associated to the mechanical, biological and microstructural characteristics of bioceramics employed in orthopaedic applications. The review paper majorly focuses on effective utilization of various materials as an additive in bioceramics and processing techniques used for enhancement of properties, enabling the use of material in orthopaedic applications. The influence of various additives on the microstructure, mechanical properties and biological performance of developed bioceramics orthopaedic implants has been elaborately discussed. Furthermore, future prospects are proposed to promote further innovations in bioceramics research.     

Evaluation of the pore morphology formation of the Freeze Foaming process by in situ computed tomography

The so-called Freeze Foaming method aims at manufacturing ceramic cellular scaffolds for diverse applications. One application is dedicated to potential bone replacement material featuring open, micro and interconnected porosity. However, the main challenges of this foaming method is to achieve a homogeneous pore morphology. In a current project, the authors throw light on the bubble/pore and strut formation of this process by in situ computed tomography. This allows for evaluating varying process parameter’s effects on the growth of the ceramic foam during the foaming process. As first result and basis for CT analysis, a stable and reproducible model suspension was developed which resulted in reproducible foam structures. In dependence of selected process parameters like pressure reduction rate or air content in the ceramic suspension resulting Freeze Foams became adjustable with regard to their pore morphology. Pore size and distribution data as well as the porosity were characterized and evaluated accordingly.     

Effect of sintering temperature on the morphology,crystallinity and mechanical properties of carbonated hydroxyapatite (CHA)

Effect of sintering temperature on the physical and mechanical properties of synthesized B-type carbonated hydroxyapatite (CHA) over a range of temperature in CO₂ atmosphere has been investigated. The B-type CHA in nano size was synthesized at room temperature by using a direct pouring wet chemical precipitation method. The synthesized CHA powders were subsequently consolidated by sintering treatment from 800 to 1100 °C. The sintered CHA samples were evaluated using X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectrometry, X-ray fluorescence (XRF), carbon-hydrogen-nitrogen-sulfur-oxygen (CHNS/O) elemental analyzer, Field emission scanning electron microscopy (FESEM), and Vicker's indentation technique. The results obtained from XRD and FESEM indicated that the synthesized B-type CHA powders were nanometer in size. The crystallinity and crystallite size of the sintered CHA samples were increased due to increasing sintering temperature. The heat treatment between 800 °C and 1000 °C has resulted in coarsening and increased hardness of the sintered CHA samples. However, these properties began to deteriorate when sintering beyond 1100 °C due the formation of calcium oxide.     

Phase equilibria in CaNaPO4-CaKPO4 system and their influence on formation of bioceramics based on mixed Ca-K-Na phosphates

An investigation of the two-component phase diagram of the CaNaPO₄- CaKPO₄system performed using various analysis techniques is reported. The continuous solid solution series of α-CaMPO₄ existing above 700 °C undergoes eutectoid decomposition during cooling to β-CaMPO₄-based solid solutions enriched with Na and K, and to an intermediate nonstoichiometric compound with an ideal composition of CaK_0.6Na_0.4PO₄. All three compounds exhibit significant volumetric effects associated with first-order phase transitions, with positive volume changes under cooling for the intermediate compound. Increased K content in ceramics based on CaK_yNa_1-yPO₄ compositions enhances the strength properties of those ceramics, including their fracture toughness, which is associated with increased density. Increased K content also has a smaller effect of inducing phase transformations accompanied by strong volume changes.     

Vital role of hydroxyapatite particle shape in regulating the porosity and mechanical properties of the sintered scaffolds

The effect of particle shape on the porosity and compressive strength of porous hydroxyapatite (HA) scaffolds was investigated by sintering the mixture of rod-shaped HA (r-HA) and spherical HA (s-HA) with polyacrylamide used as the sacrificial template. It was found, for the first time, that addition of r-HA into s-HA could exponentially decrease the porosity of sintered HA scaffolds and enhance their compressive strength with the increase of r-HA content. The mechanism, according to the results from scanning electron microscopy and X-ray diffraction, lies in the restriction of s-HA to the grain formation and growth of r-HA during sintering and results in the fusion of r-HA with s-HA. These findings suggest that mixture of r-HA and s-HA might provide a new and facile way to improve the compressive strength of porous HA scaffolds.