In situ polymerization into porous ceramics: a novel route to tough biomimetic materials

Generation of Tough, Stiff Polylactide Nanocomposites through the In Situ Nanofibrillation of Thermoplastic Elastomer

Polylactide (PLA) resins are among the most desirable biopolymers due to their biobased and compostable nature, excellent stiffness, and tensile strength. However, the widespread application of PLA has long been hindered by its inherent brittleness. While multiple routes have been successfully developed for the toughening of PLA, this toughening has always come at the cost of compromising the stiffness and strength of the matrix. In this work, we report a robust and scalable method for the development of PLA nanocomposites with an unprecedented combination of stiffness and toughness. Using the in situ nanofibrillation technique, we generated PLA composites containing nanofibrils of thermoplastic polyester elastomer (TPEE). Due to the high aspect ratio of these nanofibrils, they form physically percolated networks at low weight fractions (∼2.8 wt %) which dramatically change the mechanical behavior of the material. We found that, upon network formation, the material transitions from brittle to ductile behavior, dramatically increasing its toughness with only a marginal decrease in Young's modulus. We investigate the peculiar rheological behavior and crystallization kinetics of these blends, and propose an extension of the critical ligament thickness mechanism, wherein intrinsic toughening arises at the fiber-matrix interface in the presence of entangled elastomer networks.

Improvement of the fracture toughness of hydroxyapatite (HAp) by incorporation of carboxyl functionalized single walled carbon nanotubes (CfSWCNTs) and nylon

The potential of improving the fracture toughness of synthetic hydroxyapatite (HAp) by incorporating carboxyl functionalized single walled carbon nanotubes (CfSWCNTs) and polymerized ε-caprolactam (nylon) was studied. A series of HAp samples with CfSWCNTs concentrations varying from 0 to 1.5 wt.%, without, and with nylon addition was prepared. X-ray diffraction (XRD), Scanning Electron Microscopy (SEM), and Transmission Electron Microscopy (TEM) were used to characterize the samples. The three point bending test was applied to measure the fracture toughness of the composites. A reproducible value of 3.6±0.3 MPa.√m was found for samples containing 1 wt.% CfSWCNTs and nylon. This value is in the range of the cortical bone fracture toughness. Increase of the CfSWCNTs content results to decrease of the fracture toughness, and formation of secondary phases.

Toughening and functionalization of bioactive ceramic and glass bone scaffolds by biopolymer coatings and infiltration: a review of the last 5 years

Inorganic scaffolds with high interconnected porosity based on bioactive glasses and ceramics are prime candidates for applications in bone tissue engineering. These materials however exhibit relatively low fracture strength and high brittleness. A simple and effective approach to improve the toughness is to combine the basic scaffold structure with polymer coatings or through the formation of interpenetrating polymer-bioactive ceramic microstructures. The polymeric phase can additionally serve as a carrier for growth factors and therapeutic drugs, thus adding biological functionalities. The present paper reviews the state-of-the art in the field of polymer coated and infiltrated bioactive inorganic scaffolds. Based on the notable combination of bioactivity, improved mechanical properties and drug or growth factor delivery capability, this scaffold type is a candidate for bone and osteochondral regeneration strategies. Remaining challenges for the improvement of the materials are discussed and opportunities to broaden the application potential of this scaffold type are also highlighted.

Impregnation of β-tricalcium phosphate robocast scaffolds by in situ polymerization

Ring-opening polymerization of ε-caprolactone (ε-CL) and L-lactide (LLA) was performed to impregnate β-tricalcium phosphate (β-TCP) scaffolds fabricated by robocasting. Concentrated colloidal inks prepared from β-TCP commercial powders were used to fabricate porous structures consisting of a 3D mesh of interpenetrating rods. ε-CL and LLA were in situ polymerized within the ceramic structure by using a lipase and stannous octanoate, respectively, as catalysts. The results show that both the macropores inside the ceramic mesh and the micropores within the ceramic rods are full of polymer in either case. The mechanical properties of scaffolds impregnated by in situ polymerization (ISP) are significantly increased over those of the bare structures, exhibiting similar values than those obtained by other, more aggressive, impregnation methods such as melt-immersion (MI). ISP using enzymatic catalysts requires a reduced processing temperature which could facilitate the incorporation of growth factors and other drugs into the polymer composition, thus enhancing the bioactivity of the composite scaffold. The implications of these results for the optimization of the mechanical and biological performance of scaffolds for bone tissue engineering applications are discussed.

Potential Bone Replacement Materials Prepared by Two Methods

Natural and synthetic hydroxyapatite (HA) scaffolds for potential load-bearing bone implants were fabricated by two methods. The natural scaffolds were formed by heating bovine cancellous bone at 1325°C, which removed the organic and sintered the HA. The synthetic scaffolds were prepared by freeze-casting HA powders, using different solid loadings (20–35 vol.%) and cooling rates (1–10°C/min). Both types of scaffolds were infiltrated with polymethylmethacrylate (PMMA). The porosity, pore size, and compressive mechanical properties of the natural and synthetic scaffolds were investigated and compared to that of natural cortical and cancellous bone. Prior to infiltration, the sintered cancellous scaffolds exhibited pore sizes of 100 – 300 μm, a strength of 0.4 – 9.7 MPa, and a Young’s modulus of 0.1 – 1.2 GPa. The freeze-casted scaffolds had pore sizes of 10 – 50 μm, strengths of 0.7 – 95.1 MPa, and Young’s moduli of 0.1 –19.2 GPa. When infiltrated with PMMA, the cancellous bone- PMMA composite showed a strength of 55 MPa and a Young’s modulus of 4.5 GPa. Preliminary data for the synthetic HA-PMMA composite showed a strength of 42 MPa and a modulus of 0.8 GPa.

Flower-like agglomerates of hydroxyapatite crystals formed on an egg-shell membrane

Flower-like hydroxyapatite agglomerates formed on the upper side and lower side of an egg-shell membrane were intensively investigated using a uniquely designed crystallizer. First the ion driving force was calculated in theory. In addition the influences of various factors, such as temperature, pH value, and holding time, on the morphology and crystallinity of the agglomerates were studied in detail by means of FESEM, TEM and XRD. It was found that flower-like hydroxyapatite agglomerates with high crystallinity can be produced under higher temperature, larger pH value, and moderated holding time. The information generated is relevant to the formation process of bone.

The degradation properties of co-continuous calcium phosphate polyester composites: insights with synchrotron micro-computer tomography

This study investigates the in vitro degradation properties of composites consisting of a porous tricalcium phosphate (TCP) foam filled with degradable poly(dl-lactic acid) (PDLLA) via either in situ polymerization or infiltration. The motivation was to develop a material for bone repair that would be initially mechanically strong and would develop porosity during degradation of one of the components. A thorough analysis of the physical in vitro degradation properties has been conducted and reported by the same authors elsewhere. Synchrotron micro-computer tomography analysis (conducted at ID19, ESRF, Grenoble, France) allowed detailed insights to be gained into the process of the composites' degradation, which was discovered to be strongly influenced by the manufacturing method. The polymer phase of in situ-polymerized TCP-PDLLA degraded as a bulk sample, with faster degradation in the centre of the sample as a whole. In contrast, the polymer phase of infiltrated TCP-PDLLA degraded as individual polymer spheres with faster degradation in the centre of each sphere.