Low-temperature crystallization of calcium phosphate coatings synthesized by ion-beam-assisted deposition

Calcium Phosphate Bioceramics: A Review of Their History, Structure, Properties, Coating Technologies and Biomedical Applications

Calcium phosphate (CaP) bioceramics are widely used in the field of bone regeneration, both in orthopedics and in dentistry, due to their good biocompatibility, osseointegration and osteoconduction. The aim of this article is to review the history, structure, properties and clinical applications of these materials, whether they are in the form of bone cements, paste, scaffolds, or coatings. Major analytical techniques for characterization of CaPs, in vitro and in vivo tests, and the requirements of the US Food and Drug Administration (FDA) and international standards from CaP coatings on orthopedic and dental endosseous implants, are also summarized, along with the possible effect of sterilization on these materials. CaP coating technologies are summarized, with a focus on electrochemical processes. Theories on the formation of transient precursor phases in biomineralization, the dissolution and reprecipitation as bone of CaPs are discussed. A wide variety of CaPs are presented, from the individual phases to nano-CaP, biphasic and triphasic CaP formulations, composite CaP coatings and cements, functionally graded materials (FGMs), and antibacterial CaPs. We conclude by foreseeing the future of CaPs.

Structural and dynamical studies of acid-mediated conversion in amorphous-calcium-phosphate based dental composites

OBJECTIVE

To investigate the complex structural and dynamical conversion process of the amorphous-calcium-phosphate (ACP)-to-apatite transition in ACP based dental composite materials.

METHODS

Composite disks were prepared using zirconia hybridized ACP fillers (0.4 mass fraction) and photo-activated Bis-GMA/TEGDMA resin (0.6 mass fraction). We performed an investigation of the solution-mediated ACP-to-apatite conversion mechanism in controlled acidic aqueous environment with in situ ultra-small angle X-ray scattering based coherent X-ray photon correlation spectroscopy and ex situ X-ray diffraction, as well as other complementary techniques.

RESULTS

We established that the ACP-to-apatite conversion in ACP composites is a two-step process, owing to the sensitivity to local structural changes provided by coherent X-rays. Initially, ACP undergoes a local microstructural rearrangement without losing its amorphous character. We established the catalytic role of the acid and found the time scale of this rearrangement strongly depends on the pH of the solution, which agrees with previous findings about ACP without the polymer matrix being present. In the second step, ACP is converted to an apatitic form with the crystallinity of the formed crystallites being poor. Separately, we also confirmed that in the regular Zr-modified ACP the rate of ACP conversion to hydroxyapatite is slowed significantly compared to unmodified ACP, which is beneficial for targeted slow release of functional calcium and phosphate ions from dental composite materials.

SIGNIFICANCE

For the first time, we were able to follow the complete solution-mediated transition process from ACP to apatite in this class of dental composites in a controlled aqueous environment. A two-step process, suggested previously, was conclusively identified.

Calcium orthophosphate coatings, films and layers

In surgical disciplines, where bones have to be repaired, augmented or improved, bone substitutes are essential. Therefore, an interest has dramatically increased in application of synthetic bone grafts. As various interactions among cells, surrounding tissues and implanted biomaterials always occur at the interfaces, the surface properties of the implants are of the paramount importance in determining both the biological response to implants and the material response to the physiological conditions. Hence, a surface engineering is aimed to modify both the biomaterials, themselves, and biological responses through introducing desirable changes to the surface properties of the implants but still maintaining their bulk mechanical properties. To fulfill these requirements, a special class of artificial bone grafts has been introduced in 1976. It is composed of various mechanically stable (therefore, suitable for load bearing applications) biomaterials and/or bio-devices with calcium orthophosphate coatings, films and layers on their surfaces to both improve interactions with the surrounding tissues and provide an adequate bonding to bones. Many production techniques of calcium orthophosphate coatings, films and layers have been already invented and new promising techniques are continuously investigated. These specialized coatings, films and layers used to improve the surface properties of various types of artificial implants are the topic of this review.

Ultra-small-angle X-ray scattering-X-ray photon correlation spectroscopy studies of incipient structural changes in amorphous calcium phosphate-based dental composites

The local structural changes in amorphous calcium phosphate (ACP)-based dental composites were studied under isothermal conditions using both static, bulk measurement techniques and a recently developed methodology based on combined ultra-small angle X-ray scattering-X-ray photon correlation spectroscopy (USAXS-XPCS), which permits a dynamic approach. While results from conventional bulk measurements do not show clear signs of structural change, USAXS-XPCS results reveal unambiguous evidence for local structural variations on a similar time scale to that of water loss in the ACP fillers. A thermal-expansion-based simulation indicates that thermal behavior alone does not account for the observed dynamics. Together, these results suggest that changes in the water content of ACP affect the composite morphology due to changes in ACP structure that occur without an amorphous-to-crystalline conversion. It is also noted that biomedical materials research could benefit greatly from USAXS-XPCS, a dynamic approach.

Strontium-substituted hydroxyapatite coatings synthesized by pulsed-laser deposition: in vitro osteoblast and osteoclast response

The increasing interest in strontium incorporation into biomaterials for hard tissue repair is justified by the growing evidence of its beneficial effect on bone. We successfully synthesized hydroxyapatite (HA) thin films with different extents of strontium substitution for calcium (0, 1, 3 or 7 at.%) by pulsed-laser deposition. The coatings displayed a granular surface and a good degree of crystallinity, which slightly diminished as strontium content increased. Osteoblast-like MG63 cells and human osteoclasts were cultured on the thin films up to 21 days. MG63 cells grown on the strontium-doped HA coatings displayed normal morphology, good proliferation and increased values of the differentiation parameters, whereas the number of osteoclasts was negatively influenced by the presence of strontium. The positive effect of the ion on bone cells was particularly evident in the case of coatings deposited from HA at relatively high strontium contents (3-7%), where significantly increased values of alkaline phosphatase activity, osteocalcin, type I collagen and osteoprotegerin/TNF-related activation-induced cytokine receptor ratio, and considerably reduced values of osteoclast proliferation, were observed.

Biomimetic apatite coatings on titanium coprecipitated with cephradine and salviae miltlorrhizae

Incorporation of antibiotics in the apatite coatings on titanium surface is an effective step for surgical infections of titanium implant. In this work, antibiotics cephradine and traditional Chinese medicine salviae miltlorrhizae (SM) were incorporated in the simulated body fluid (SBF). The apatite coatings were coprecipitated with cephradine and SM by a biomimetic method. The research showed that in the biomimetic coprecipitation process cephradine containing carboxylic groups were beneficial to the apatite coprecipitation. X-ray diffraction patterns showed that cephradine had a positive effect on the crystal growth in terms of cystallinity. And also in the Fourier transformed infrared spectra, the organic groups corresponded with cephradine were also observed. However, little apatite formed on the titanium immersed in SBF with SM. Probably, SM was more absorbed on the surface when calcium and phosphate ions precipitated.

In vitro culture of mesenchymal cells onto nanocrystalline hydroxyapatite-coated Ti13Nb13Zr alloy

In this study we coated a new biocompatible, nanostructured titanium alloy, Ti13Nb13Zr, with a thin layer of hydroxyapatite nanocrystals and we investigated the response of human bone-marrow-derived mesenchymal cells. The coating was realized using a slightly supersaturated CaP solution, which provokes a fast deposition of nanocrystalline hydroxyapatite. A thin layer of deposition is appreciable on the etched Ti13Nb13Zr substrates after just 1.5 h soaking in the CaP solution, and it reaches a thickness of 1-2 mum after 3 h soaking. The coating seems thinner than that deposited on Ti6Al4V, which was examined for comparison, likely because of the different roughness profiles of the two etched alloys, and it is constituted of elongated HA nanocrystals, with a mean length of about 100 nm. Mesenchymal stem cells were seeded onto coated and uncoated Ti alloys and cultured for up to 35 days. Cell morphology, proliferation and differentiation were evaluated. The cells display good adhesion and proliferation on the uncoated substrates, whereas the presence of hydroxyapatite coating slightly reduces cell proliferation and induces differentiation of MSCs towards a phenotypic osteoblastic lineage, in agreement with the increase of the expression of osteopontin, osteonectin and collagen type I, evaluated by means of rt-PCR. Type I collagen expression is higher in Ti13Nb13Zr MSC culture compared to Ti6Al4V, standing for a more efficient extracellular matrix deposition.

Early bone apposition in vivo on plasma-sprayed and electrochemically deposited hydroxyapatite coatings on titanium alloy

Three different implants, bare Ti-6Al-4V alloy, Ti-6Al-4V alloy coated with plasma-sprayed hydroxyapatite (PSHA), and Ti-6Al-4V alloy coated with electrochemically deposited hydroxyapatite (EDHA), were implanted into canine trabecular bone for 6 h, 7, and 14 days, respectively. Environmental scanning electron microscopy study showed that PSHA coatings had higher bone apposition ratios than those exhibited by bare Ti-6Al-4V and EDHA coatings after 7 days; however, at 14 days after implantation, EDHA and PSHA coatings exhibited similar bone apposition ratios, much higher than that for bare Ti-6Al-4V. The ultrastructure of the bone/implant interface observed by transmission electron microscope showed that the earliest mineralization (6 h-7 days) was in the form of nano-ribbon cluster mineral deposits with a Ca/P atomic ratio lower than that of hydroxyapatite. Later-stage mineralization (7-14 days) resulted in bone-like tissue with the characteristic templating of self-assembled collagen fibrils by HA platelets. Though adhesion of EDHA coatings to Ti-6Al-4V substrate proved problematical and clearly needs to be addressed through appropriate manipulation of electrodepositon parameters, the finely textured microstructure of EDHA coatings appears to provide significant advantage for the integration of mineralized bone tissue into the coatings.

Biocompatible nanocrystalline octacalcium phosphate thin films obtained by pulsed laser deposition

We extended for the first time pulsed laser ablation to the deposition of octacalcium phosphate Ca8H2(PO4)6.5H2O (OCP) thin films. The depositions were performed with a pulsed UV laser source (lambda=248 nm, tau> or =20 ns) in a flux of hot water vapors. The targets were sintered from crystalline OCP powder and the laser ablation fluence was set at values of 1.5-2 J/cm2. During depositions the collectors, Si or Ti substrates, were maintained at a constant temperature within the range 20-200 degrees C. The resulting structures were submitted to heat treatment in hot water vapors for up to 6 h. The best results were obtained at a substrate temperature of 150 degrees C during both deposition and post-deposition treatment. High-resolution electron microscopy and XRD at grazing incidence indicated that the coatings obtained were made of nanocrystalline OCP. Cross-section TEM investigations showed that the coatings contained droplets stacked on Ti substrates as well as distributed across the entire thickness of the arborescence-like structure layers. The results of WST-1 assay, cell adherence, DNA replication, and caspase-1 activity confirmed the good biocompatibility of the coatings.