Development of controlled strength-loss resorbable beta-tricalcium phosphate bioceramic structures

Dual therapeutic cobalt-incorporated bioceramics accelerate bone tissue regeneration

Bone grafting on defects caused by trauma or tumor stimulates bone regeneration, a complex process requiring highly orchestrated cell-signal interactions. Bone vascular growth is coupled with osteogenesis, but less is known about the interplay between angiogenesis and osteogenesis. Understanding this relationship is relevant to improved bone regeneration. Here, tricalcium phosphate (TCP) scaffolds doped with varying concentration of cobalt (Co-TCP) were designed to investigate the dosage effect of vascularization on bone formation. The surface structure, phase composition, mechanical features, and chemical composition were investigated. Co doping improved the mechanical properties of TCP. Co-TCP, particularly 2% and 5% Co-TCP, boosted cell viability of bone marrow stromal cells (BMSCs). The 2% Co-TCP promoted alkaline phosphatase activity, matrix mineralization, and expression of osteogenic genes in BMSCs in vitro. However, excessive Co doping decreased TCP-induced osteogenesis. Meanwhile, Co-TCP dose-dependently favored the growth and migration of human umbilical vein endothelial cells (HUVECs), and the expression of vascular endothelial growth factor (VEGF). The 2% Co-TCP significantly shrank the defect area in rat alveolar bone compared with TCP. Smaller bone volume and more abundant blood vessels were observed for 5% Co-TCP compared with 2% Co-TCP. The CD31 immunostaining in the 5% Co-TCP group was more intense than the other two groups, indicating of the increment of endothelium cells. Besides, 5% Co-TCP led to mild inflammatory response in bone defect area. Overall, TCP doped appropriately with Co has positive effect on osteogenesis, while excessive Co suppressed osteoblast differentiation and bone formation. These data indicate that vascularization within a proper range promotes osteogenesis, which may be a design consideration for bone grafts.

Calcium phosphate/microgel composites for 3D powderbed printing of ceramic materials

Composites of microgels and calcium phosphates are promising as drug delivery systems and basic components for bone substitute implants. In this study, we synthesized novel composite materials consisting of pure β-tricalcium phosphate and stimuli-responsive poly(N-vinylcaprolactam-co-acetoacetoxyethyl methacrylate-co-vinylimidazole) microgels. The chemical composition, thermal properties and morphology for obtained composites were extensively characterized by Fourier transform infrared, X-ray photoelectron spectroscopy, IGAsorp moisture sorption analyzer, thermogravimetric analysis, granulometric analysis, ESEM, energy dispersive X-ray spectroscopy and TEM. Mechanical properties of the composites were evaluated by ball-on-three-balls test to determine the biaxial strength. Furthermore, initial 3D powderbed-based printing tests were conducted with spray-dried composites and diluted 2-propanol as a binder to evaluate a new binding concept for β-tricalcium phosphate-based granulates. The printed ceramic bodies were characterized before and after a sintering step by ESEM. The hypothesis that the microgels act as polymer adhesive agents by efficient chemical interactions with the β-tricalcium phosphate particles was confirmed. The obtained composites can be used for the development of new scaffolds.

Bioresorbable β-TCP-FeAg nanocomposites for load bearing bone implants: High pressure processing, properties and cell compatibility

In this paper, the processing and properties of iron-toughened bioresorbable β-tricalcium phosphate (β-TCP) nanocomposites are reported. β-TCP is chemically similar to bone mineral and thus a good candidate material for bioresorbable bone healing devices; however intrinsic brittleness and low bending strength make it unsuitable for use in load-bearing sites. Near fully dense β-TCP-matrix nanocomposites containing 30vol% Fe, with and without addition of silver, were produced employing high energy attrition milling of powders followed by high pressure consolidation/cold sintering at 2.5GPa. In order to increase pure iron's corrosion rate, 10 to 30vol% silver were added to the metal phase. The degradation behavior of the developed composite materials was studied by immersion in Ringer's and saline solutions for up to 1month. The mechanical properties, before and after immersion, were tested in compression and bending. All the compositions exhibited high mechanical strength, the strength in bending being several fold higher than that of polymer toughened β-TCP-30PLA nanocomposites prepared by the similar procedure of attrition milling and cold sintering, and of pure high-temperature sintered β-TCP. Partial substitution of iron with silver led to an increase in both strength and ductility. Furthermore, the galvanic action of silver particles dispersed in the iron phase significantly accelerated in vitro degradation of β-TCP-30(Fe-Ag) nanocomposites. After 1month immersion, the composites retained about 50% of their initial bending strength. In cell culture experiments, β-TCP-27Fe3Ag nanocomposites exhibited no signs of cytotoxicity towards human osteoblasts suggesting that they can be used as an implant material.

Synthesis and evaluation of novel glass ceramics as drug delivery systems in osteomyelitis

In this study, a new generation of bioactive glass ceramics were developed using the wet chemical method. The synthetic conditions were strictly controlled to obtain the materials of a nanometric scale. As evaluated by scanning electron microscopy, bone-like apatite layers were produced in large amounts and completely covered their surfaces after immersion in phosphate buffer saline. On the basis of X-ray diffraction, X-ray fluorescence, and Fourier transform infrared spectroscopy results, PO₄³⁻ groups of hydroxyapatite (HA) were partially substituted by SiO₄⁴⁻ species. The defective chemical structures introduced provided materials that were more biologically active, compared with the parent HA. For effective treatment of infected bones, scaffolds containing the bioactive ceramics were prepared by chitosan cross-linking, and loaded with vancomycin (VCM). The drug-loaded scaffolds were not toxic to bone cells. About 75%-80% of the entrapped drug was released in a controlled pattern and the release was sustained over a 12-day period. The concentration of drug released was determined to be above 20 times the half maximal effective concentration of VCM on Staphylococcus aureus, and was sufficient for killing bacteria growing as biofilm. In summary, the synthesized bioceramics exhibited many of properties associated with an ideal material for implantable drug delivery system, and were suitable for testing the ability to cure bone diseases including osteomyelitis.

An evaluation of the processing conditions, structure, and properties (biaxial flexural strength and antibacterial efficacy) of sintered strontium–zinc–silicate glass ceramics

The use of artificial bone grafts has increased in order to satisfy a growing demand for bone replacement materials. Initial mechanical stability of synthetic bone grafts is very advantageous for certain clinical applications. Coupled with the advantage of mechanical strength, a material with inherent antibacterial properties would be very beneficial. A series of strontium-doped zinc silicate (Ca–Sr–Na–Zn–Si) glass ceramics have been characterized in terms of their crystalline structure, biaxial flexural strength and antibacterial efficacy based on the identification of optimum sintering conditions. All three glass ceramics, namely, BT110, BT111, and BT112 were found to be fully crystalline, with BT111 and BT112 comprising of biocompatible crystalline phases. The biaxial flexural strengths of the three glass ceramics ranged from 70 to 149 MPa and were shown to be superior to those of clinically established ceramics in dry conditions and following incubation in simulated physiological conditions. The bacteriostatic effect for each glass ceramic was also established, where BT112 showed an inhibitory effect against three of the most common bacteria found at implantation sites, namely, Enterococcus faecalis, methicillin-resistant Staphylococcus aureus (MRSA), and Pseudomonas aeruginosa. The results of the evaluation suggest that the materials studied offer advantages over current clinical materials and indicate the potential suitability of the glass ceramics as therapeutic bone grafts.

The effect of sintering temperature on the microstructure and mechanical properties of a bioceramic bone scaffold

Micro and nanostructural properties are believed to play a critical role in the osteoinductive capacity of bioceramic bone scaffolds. Physical characteristics also play an important role for optimum biological performance, including osteoconductivity and strength. In this study microstructural and nano-mechanical properties of a bioceramic bone scaffold were investigated as a function of the sintering temperature in the range of 950-1150 °C, through the use of scanning electron microscopy (SEM), X-ray diffraction (XRD) and nanoindentation testing. Although the samples presented the same crystallographic phase, an increase in sintering temperature resulted in increased grain size, density and crystallite size. The intrinsic mechanical properties were measured by nanoindentation testing and analyzed with the Oliver-Pharr method. The nanoindentation tests consisted of a series of fourteen partial unload tests (n=14 per treatment) of twelve steps ranging from 1 to 12 mN. Statistically significant increases in hardness and elastic modulus were measured for increasing sintering temperature. These results support the development of clinically successful bioceramic scaffolds with mechanical properties that encourage bone ingrowth and provide structural integrity.

Sintering of biphasic calcium phosphates

Biphasic calcium phosphate (BCP) discs were fabricated and then sintered using two different sintering programs to establish whether the phases present could be controlled at low and high sintering temperatures. X-ray diffraction (XRD) was used to establish the phases present after sintering and scanning electron microscopy (SEM) determined the microstructure. Sintering program 1 involved a simple heating and cooling schedule and temperatures of 1100, 1250, 1275 and 1300 degrees C. It produced samples containing an additional alpha-tricalcium phosphate (alpha-TCP) phase at temperatures above 1100 degrees C. The original ratio of hydroxyapatite/beta-tricalcium phosphate (HA/beta-TCP) could not be maintained above this temperature. Sintering program 2 combined the heating and cooling schedules of the first program with a 900 degrees C hold stage to allow alpha-TCP to beta-TCP conversion to take place. At temperatures of 1250 and 1275 degrees C, this program was successful in completely removing the alpha-TCP phase and preserving the HA:beta-TCP ratio. The SEM results show that the surface morphology of the discs was not greatly affected by choice of sintering program.