Novel fabrication of hierarchically porous hydroxyapatite scaffolds with refined porosity and suitable strength

Based on extrusion deposition and foaming technique, a novel method for biological hydroxyapatite (HA) scaffolds was introduced in this paper. The scaffolds were primarily characterised by interconnected and hierarchically porous structures with high porosity, adjustable distribution of pore sizes, as well as considerable mechanical strength. In order to confirm that fine control of bulk porosity and mechanical strength was possible and feasible, further analysis of obtained scaffolds was carried out by field emission scanning electron microscope (FESEM), compressive test and calculation of volumetric shrinkage; in particular, the additional porosity resulting from the introduction of pore former was evaluated. The results indicated that this method can have a great potential to construct HA scaffolds of suitable quality for spongy bone in bone tissue engineering.     

From CT Scan to Ceramic Bone Graft

An indirect fused deposition process was used to fabricate controlled-porosity alumina bone grafts using a computer-aided-design file created from computed tomography (CT) scans of a horse's short pastern bone. Structures with both uniform and gradient porosity were fabricated to show the effectiveness of this process for the fabrication of custom orthopedic implants. Cytotoxicity and cell proliferation studies were conducted with different cell lines to show that these bone grafts are biocompatible. Uniaxial compression tests were also conducted to understand the influence of porosity on the mechanical properties of these structures.     

Bioactive monetite‐containing whisker‐like fibers reinforced chitosan scaffolds

The aim of this work was to develop bioactive chitosan scaffolds reinforced with monetite‐containing whisker‐like fibers. The fibers synthesized by homogeneous precipitation were characterized as monetite/hydroxyapatite short fibers (MAFs), using XRD, FTIR and SEM. The pure chitosan and MAFs/chitosan composite scaffolds were produced by freeze‐drying, and characterized with respect to porosity, pore size, swelling behavior, compressive strength and modulus, and in vitro bioactivity. The incorporation of MAFs in chitosan matrices led to increase the pore size, according to the evaluation by FE‐SEM, and decrease the porosity of composite scaffolds. The swelling ratio decreased as MAFs content of scaffolds increased. The compressive strength and modulus of scaffolds were improved by an increase in MAFs content. The noncross‐linked scaffolds with a chitosan: MAFs weight ratio of 1:1 (CW3) showed a porosity of 75.5%, and the strength and modulus of 259 kPa and 2.8 MPa in dry state, respectively. The crosslinking by glutaraldehyde resulted in improved mechanical properties. The strength and modulus of cross‐linked CW3 scaffolds in wet state reached to 345 kPa and 1.8 MPa, respectively. The in vitro bioactivity of the reinforced scaffolds, evaluated by FE‐SEM/EDS, XRD, and ATR‐FTIR, was confirmed by the formation of a carbonated apatite layer on their surfaces when they soaked in simulated body fluid (SBF). The results of this initial study indicate that the monetite‐containing whisker‐like fibers may be an appropriate reinforcement of chitosan scaffolds.     

Direct ink writing of silicone/filler mixtures for sphene scaffolds with advanced topologies

Pastes based on preceramic polymers have a great potential for direct ink writing of bioceramic scaffolds. In this paper, we discuss the fabrication of phase pure sphene (CaTiSiO₅) bioceramics, developed by firing, at 1300°C, of silicone-based printed scaffolds containing CaCO₃ and TiO₂ as active fillers. As a proof of the flexibility of the combination of preceramic polymers and additive manufacturing technologies, several lattice geometries of increasing complexity were successfully explored. In particular, the approach allowed the fabrication of sphene scaffolds with gyroid-like structure exhibiting an impressive compressive strength, given the high porosity. Moreover, different lattice topologies of sphene scaffolds were compared also in terms of permeability.     

A novel fabrication procedure for producing high strength hydroxyapatite ceramic scaffolds with high porosity

Hydroxyapatite (HA) scaffolds were fabricated using the space holder method with a pressureless sintering process in a systematically developed manner at different fabrication stages to increase the strength of the scaffold at high porosity. Polyvinyl alcohol (PVA) and Polymethyl methacrylate (PMMA) were used as binders and space holder agents, respectively. The physical properties of the HA scaffolds were characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM), linear shrinkage test, and porosity measurements. The mechanical properties of the HA scaffolds were analyzed using compressive strength measurements. The results revealed that the HA scaffold met the expected quality requirements with a compressive strength of 2.2 MPa at a porosity of 65.6% with pore sizes distributed in the range of 126–385 μm. The shrinkage of the scaffold diameter occurred by 20.27%, this diameter shrinkage predominantly to the shrinkage of the HA scaffold caused by sintering. Besides, suspect that a higher PMMA concentration causes pore size shrinkage upon sintering. The formation of pore interconnections was evidenced by SEM observations and the ‘translucent light method’ developed in this study. The results of the scaffold phase test using XRD showed that the final scaffold consisted only of the HA phase, as the PVA and PMMA phases burned out during the sintering process.     

Effect of magnesium silicate on 3D gel-printing of hydroxyapatite ceramic composite scaffold

The hydroxyapatite scaffolds doped with MgSiO₃ were prepared by 3D gel-printing technology in this study. The effect of MgSiO₃ on the scaffold characteristics has been studied, and the composite ceramic scaffold with higher strength was obtained. The viscosity of different MgSiO₃ weight percentages slurry decrease with the shear rate increasing. When the solid loading was 50 vol% and the shear rate was 100 S⁻¹, the viscosity of the slurry decrease with weight percent of MgSiO₃ increasing. The scaffold has interconnected internal structures and the sintered body has a pore size of about 350-620 μm. The porosity of the scaffolds with different weight percents of MgSiO₃ is kept at 65%, and the scaffold with 3% MgSiO₃ has the highest compressive strength of 93.15 MPa. As increasing weight percent of MgSiO₃, the compressive strength of the scaffold gradually decreases and remains at 40 MPa eventually. The ceramic scaffold with 3% MgSiO₃ has better degradability than that of other weight percents of MgSiO₃. After soaking in Tris-HCl solution for 5 weeks, the weight loss of the scaffold reached 9.91%.     

Formation of porous PLGA scaffolds by a combining method of thermally induced phase separation and porogen leaching

A novel method for the fabrication of porous poly(L -lactide-co-glycolide) (PLGA) scaffolds by combining thermally induced phase separation and porogen leaching is presented in this article. Big pores with about 75–400 μm diameters in the obtained scaffolds were generated by the porogen, sucrose particles, while small pores with diameters less than 20 μm induced via phase separation. Extraction of the solvent, chloroform by ethanol at cool temperatures could reduce the scaffold toxicity. Effects of PLGA concentration, freezing temperature, volume fraction of porogen, and introduction of β-tricalcium phosphate (β-TCP) on morphology, porosity, and compressive properties of the scaffolds were systematically discussed. Results showed that the size of small pores decreased by decreasing the polymer concentration and reducing the freezing temperature, whereas the interconnectivity of the scaffolds was improved by increasing the porogen fraction. The compressive modulus and strength were significantly lowered by increasing the scaffold porosity, that is, by increasing porogen fraction, or decreasing the polymer concentration, or reducing the freezing temperature. Addition of β-TCP into the scaffolds did not influence the compressive modulus significantly but tended to decrease the compressive strength. The obtained scaffolds with diverse pore sizes would be potentially used in bone tissue engineering. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Experimental relationship between splitting tensile strength and compressive strength of GFRC and PFRC

This paper describes an experimental investigation into the relationship between the splitting tensile strength and compressive strength of glass fiber reinforced concrete (GFRC) and polypropylene fiber reinforced concrete (PFRC). The splitting tensile strength and compressive strength of GFRC and PFRC at 7, 28 and 90 days are used. Test results indicate that the addition of glass and polypropylene fibers to concrete increased the splitting tensile strength of concrete by approximately 20-50%, and the splitting tensile strength of GFRC and PFRC ranged from 9% to 13% of its compressive strength. Based on this investigation, a simple 0.5 power relationship between the splitting tensile strength and the compressive strength was derived for estimating the tensile strength of GFRC and PFRC.     

Modeling of tensile strength in polymer particulate nanocomposites based on material and interphase properties

In this work, a simple model is presented to determine tensile/yield strength in polymer nanocomposites containing spherical nanofillers based on material and interphase properties. The accuracy of the proposed model is estimated by comparing with the experimental strength of several samples from the literature. In addition, the effects of thickness (t) and tensile strength (σ_i) of the interphase as well as the radius (R) and volume fraction ( ) of the nanoparticles on the tensile strength are explained according to the proposed model. The high level of nanoparticle strength (more than 100 GPa) commonly leads to overestimates of the tensile strength of nanocomposites, whereas the assumption of correct interphase properties produces accurate calculations. The tensile strength of nanocomposites does not change at σ_i < 38 MPa, while it increases by 140% at t = 20 nm and σ_i = 90 MPa. However, a maximum 14% growth in tensile strength is obtained with the optimum values of = 0.04 and R = 10 nm. Therefore, the concentration and size of the nanoparticles have minor effects on the tensile strength of nanocomposites, but the major influences of interphase thickness and strength are pronounced. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 44869.     

A simple relationship between the compressive strength and porosity of hydrated portland cement

Previous work has shown that, for a series of experimental autoclaved aerated concretes with porosities ranging from 0.48 to 0.78, compressive strength is linearly related to the solid/pore volume ratio determined by helium pycnometry. In the work described here, this type of relationship has been applied to experimental data from hydrated Portland cement specimens, prepared using initial water/cement ratios of 0.35 to 0.50 and curing times of 2 to 28 days, with porosities ranging from 0.26 to 0.45. The relationship was found to provide a good fit using data obtained both by varying the water/cement ratio at constant curing times and by varying the curing time at constant water/cement ratios.     

Direct ink writing of vancomycin-loaded polycaprolactone/ polyethylene oxide/ hydroxyapatite 3D scaffolds

Novel inks were formulated by dissolving polycaprolactone (PCL), a hydrophobic polymer, in organic solvent systems; polyethylene oxide (PEO) was incorporated to extend the range of hydrophilicity of the system. Hydroxyapatite (HAp) with a weight ratio of 55–85% was added to the polymer-based solution to mimic the material composition of natural bone tissue. The direct ink writing (DIW) technique was applied to extrude the formulated inks to fabricate the predesigned tissue scaffold structures; the influence of HAp concentration was investigated. The results indicate that in comparison to other inks containing HAp (55%, 75%, and 85%w/w), the ink containing 65% w/w HAp had faster ink recovery behavior; the fabricated scaffold had a rougher surface as well as better mechanical properties and wettability. It is noted that the 65% w/w HAp concentration is similar to the inorganic composition of natural bone tissue. The elastic modulus values of PCL/PEO/HAp scaffolds were in the range of 4–12 MPa; the values were dependent on the HAp concentration. Furthermore, vancomycin as a model drug was successfully encapsulated in the PCL/PEO/HAp composite scaffold for drug release applications. This paper presents novel drug-loaded PCL/PEO/HAp inks for 3D scaffold fabrication using the DIW printing technique for potential bone scaffold applications.     

Investigating the effect of printing conditions and annealing on the porosity and tensile behavior of 3D-printed polyetherimide material in Z-direction

Fused filament fabrication process presents drawbacks in mechanical properties observed when printing in the build direction (Z-direction). Such anisotropic properties will affect the part's performances and have to be minimized during fabrication. This study aims to evaluate the effects of nozzle temperature, printing speed and specimen state (annealed or as-printed) on porosity percentage and tensile properties for 3D printed polyetherimide (PEI) (ULTEM 1010) parts in Z-direction. The results demonstrated that print speed is the most influential process parameter that should be adjusted in consideration with the other printing parameters. The specimens' state did not reveal a noticeable influence, as the amorphous nature of PEI is considered less receptive to annealing. The optimization method to achieve the best results yielded values of 360°C and 30 mm s⁻¹ as printing conditions, followed by heat treatment. This was confirmed by porosity measurements, tensile testing, and scanning electron microscopy observations. The best performances of PEI material were 3425.5 MPa, 102 MPa, and 4.30% for Young's modulus, tensile strength, and elongation at break, respectively.     

Effect of the network topology on the tensile strength of natural rubber vulcanizate at elevated temperature

The influence of network topology on tensile strength at elevated temperature of natural rubber compounds vulcanized with different systems was studied. The tensile strength behavior of natural rubber at high strain is attributed to the capacity to crystallize on stretching. Variation of this property with the temperature and/or crosslink density was related, not only with the nature of the crosslinks, but also with their spatial distribution. In this sense, measurements of the freezing point depression of cyclohexane imbibed in the rubber matrix were used to evaluate the spatial distribution of the crosslinks. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 98: 1219–1223, 2005     

焦炭强度指标间关系探讨

焦炭的冷态强度测定周期较短?成本低廉,而热态性能测定周期相对较长?成本昂贵。因此,利用焦炭冷态强度预测热态性能,不但对指导焦化生产具有一定的意义,而且利用各强度指标间的关系可以检验实际测量数据是否存在异常值。     

Preparation of di‐butyryl‐chitin scaffolds by using salt leaching method for tissue engineering and their characteristics

Scaffolds are used as support material in treatment of damaged tissues such as cartilage and bone. With the help of scaffolds, damaged tissues can be cured in shorter period with less pain. Chitin is one of the most important scaffold materials curing the damaged tissues while providing a support for related part of the body during healing period. It is biocompatible and biodegradable; however it can not be solved by common solvents leading to the major drawback for this kind of applications. Therefore di‐butyril‐chitin (DBC), which is a chitin derivative and can be solved easily in solvents like acetone, ethanol, and methanol, is preferred for scaffold production instead of chitin. In this study, DBC scaffolds were produced for orthopedic applications and their structural and mechanical properties such as porosity, elasticity, compressibility, and strength were tested to confirm their suitability for such end‐uses. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Investigating the structure and water permeation of membranes modified with natural and synthetic additives using tensile,porosity, and glass transition temperature studies

This article reports the results of a study on the effect of using different additives (lignin, polyethylene glycol [PEG], and polyvinyl pyrrolidone [PVP]) to fabricate ultrafiltration polysulfone (PSf) membranes. The main focus of this study was on the difference in permeation properties brought about by the absence or presence of a fabric when fabricating the membranes. Differential scanning calorimetry was used to characterize the thermal properties and was also used to predict the other membrane properties. An Instron machine was used to evaluate the mechanical properties. The bulk porosity of lignin and PVP‐modified membranes was observed to be higher than that of the membranes modified with PEG. There was a strong negative correlation between the bulk porosity and the glass transition temperature irrespective of the additive used. Membranes cast on a fabric showed higher flux compared with membranes cast on glass. There was a strong positive correlation between the bulk porosity and the observed permeability regardless of whether the membrane was cast on a nonwoven fabric or on a glass plate. Pore‐size distribution results showed that lignin and PVP‐modified membranes had a narrow pore‐size distribution ranging between 10 and 25 nm when compared with PEG‐modified membranes with a pore‐size distribution ranging between 2.5 and 20 nm. These results indicate that thermal, bulk porosity, and mechanical properties can be used to probe the membrane structure. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 40616.     

Preparation and characterization of new nano-composite scaffolds loaded with vascular stents

In this study, vascular stents were fabricated from poly (lactide-ɛ-caprolactone)/collagen/nano-hydroxyapatite (PLCL/Col/nHA) by electrospinning, and the surface morphology and breaking strength were observed or measured through scanning electron microscopy and tensile tests. The anti-clotting properties of stents were evaluated for anticoagulation surfaces modified by the electrostatic layer-by-layer self-assembly technique. In addition, nano-composite scaffolds of poly (lactic-co-glycolic acid)/polycaprolactone/nano-hydroxyapatite (PLGA/PCL/nHA) loaded with the vascular stents were prepared by thermoforming-particle leaching and their basic performance and osteogenesis were tested in vitro and in vivo. The results show that the PLCL/Col/nHA stents and PLGA/PCL/nHA nano-composite scaffolds had good surface structures, mechanical properties, biocompatibility and could guide bone regeneration. These may provide a new way to build vascularized-tissue engineered bone to repair large bone defects in bone tissue engineering.     

Investigations on the relationship between porosity,structure and strength of hydrated portland cement pastes I. Effect of porosity

In a series of cement paste specimens made with different water-cement ratios and hydrated for different times the relationship between porosity and strength was determined. For a range of porosities between 5 and 28 per cent this relationship can be best expressed in the form of a linear plot. At equal porosities strengths of specimens obtained by pressing lie distinctly below those obtained by casting.     

Impact and biocompatibility studies on Mg2+-substituted Apatite-derived 3D porous scaffolds for hard tissue engineering

The objective of this study was to develop Mg²⁺-substituted Apatite scaffolds by slip-casting method. The Apatite scaffolds were prepared as engineering constructs with interconnected pore structure with a pore size of 128-194 μm range. The physicochemical properties such as crystalline phase, functional group, microstructure, pore size distribution, and elemental compositions of the scaffolds were characterized. The bioactivity of the developed porous scaffolds was investigated in Simulated Body Fluid (SBF) for various time periods (3 and 7 days). In vitro bioactivity results confirm the hydroxyl Apatite layer formation of the scaffolds and results suggest that the developed microporous scaffold could be used as suitable candidates in bone tissue engineering.     

Digital light processing stereolithography of hydroxyapatite scaffolds with bone-like architecture,permeability, and mechanical properties

This work deals with the additive manufacturing and characterization of hydroxyapatite scaffolds mimicking the trabecular architecture of cancellous bone. A novel approach was proposed relying on stereolithographic technology, which builds foam-like ceramic scaffolds by using three-dimensional (3D) micro-tomographic reconstructions of polymeric sponges as virtual templates for the manufacturing process. The layer-by-layer fabrication process involves the selective polymerization of a photocurable resin in which hydroxyapatite particles are homogeneously dispersed. Irradiation is performed by a dynamic mask that projects blue light onto the slurry. After sintering, highly-porous hydroxyapatite scaffolds (total porosity ~0.80, pore size 100-800 µm) replicating the 3D open-cell architecture of the polymeric template as well as spongy bone were obtained. Intrinsic permeability of scaffolds was determined by measuring laminar airflow alternating pressure wave drops and was found to be within 0.75-1.74 × 10⁻⁹ m², which is comparable to the range of human cancellous bone. Compressive tests were also carried out in order to determine the strength (~1.60 MPa), elastic modulus (~513 MPa) and Weibull modulus (m = 2.2) of the scaffolds. Overall, the fabrication strategy used to print hydroxyapatite scaffolds (tomographic imaging combined with digital mirror device [DMD]-based stereolithography) shows great promise for the development of porous bioceramics with bone-like architecture and mass transport properties.