Surface modification of organic polymers with bioactive titanium oxide without the aid of a silane-coupling agent

Polyethylene (PE), polyethylene terephthalate (PET), ethylene-vinyl alcohol copolymer (EVOH), and poly(ε-caprolactam) (Nylon 6) were successfully modified with a thin crystalline titanium oxide layer on their surfaces by a simple dipping into a titanium alkoxide solution and a subsequent soak in hot HCl solution, without the aid of a silane-coupling agent. The surface modified polymers formed a bone-like apatite layer in a simulated body fluid (SBF) within a period of 2 days. PE, PET, and Nylon 6 formed an apatite layer faster and had a higher adhesive strength to the apatite. Three-dimensional fabrics with open spaces in various sizes containing such surface modified polymer fibers are expected to be useful as bone substitutes, since they may be able to form apatite on their constituent fibers in the living body, and thus, integrate with living bone.     

Biomimetic apatite formation on Ultra-High Molecular Weight Polyethylene (UHMWPE) using modified biomimetic solution

Modifications were performed on a biomimetic solution (SBF), according to previous knowledge on the behavior of ions present in its composition, in order to obtain apatite coatings onto Ultra-High Molecular Weight Polyethylene (UHMWPE) without having to use polymer pre-treatments that could compromise its properties. UHMWPE substrates were immersed into a 30% H₂O₂ solution for a 24-h period and then submitted to a biomimetic coating method using standard SBF and two other modified SBF solutions. Apatite coatings were only obtained onto UHMWPE when the modified SBF solutions were used. Based on these results, apatite coatings of biological importance (calcium-deficient hydroxyapatite—CDHA, amorphous calcium phosphate—ACP, octacalcium phosphate—OCP, and carbonated HA) can be obtained onto UHMWPE substrates, allowing an adequate conciliation between bonelike mechanical properties and bioactivity.     

Biomimetic apatite formation on a CoCrMo alloy by using wollastonite,bioactive glass or hydroxyapatite

A biomimetic method was used to promote a bioactive surface on a CoCrMo alloy (ASTM F75). To enhance the nucleation of apatite on the metallic substrate, wollastonite ceramics (W), bioactive glass (BG) or hydroxyapatite (HA) were used in the biomimetic method. Metallic samples were chemically treated and immersed for 7 days in SBF on a bed of bioactive material (W, BG or HA) followed by an immersion in 1.5SBF for 7 or 14 days without bioactive system.A bonelike apatite layer was formed on the surface of all the samples tested. The samples treated with wollastonite showed a higher rate of apatite formation and the morphology of the layer was closer to that of the existing bioactive systems. A higher crystallinity of the apatite layer was also observed by using wollastonite. The pH of the SBF, the Ca/P ratio and the thickness of the layer on the samples treated with wollastonite and bioactive glass increased as increasing the immersion time. The thickness of the layer on the samples treated with hydroxyapatite also increased with time, but the pH of the SBF and the Ca/P ratio changed with no a defined trend.     

Composition and structure of apatite formed on organic polymer in simulated body fluid with a high content of carbonate ion

Apatite layer was formed on polyethyleneterephthalate (PET) substrate by the following biomimetic process. The PET substrate was placed on granular particles of a CaO, SiO₂-based glass in simulated body fluid (SBF) with ion concentrations nearly equal to those of human blood plasma to form apatite nuclei on their surfaces. The apatite nuclei was then grown into a continuous layer by subsequently soaking the substrate in SBF under air or CO₂ atmosphere in which CO₂ partial pressure in the ambient was adjusted to 14.8 kPa to increase the content of carbonate ion to a level nearly equal to that of blood plasma. The increase in the content of carbonate ions in SBF changed the Ca/P atomic ratio of the apatite from 1.51 to 1.63, content of CO₃ ^2- ions from 2.64 to 4.56 wt %, and lattice constants a from 94.32 to 94.23 nm and c from 68.70 to 68.83 nm, respectively. The Ca/P ratio and lattice constants of the apatite formed in SBF under CO₂ atmosphere were approximately identical to those of bone apatite, i.e. Ca/P atomic ratio 1.65, content of CO₃ ^2- ion 5.80 wt % and lattice constants a 94.20 and c 68.80 nm. This indicates that an apatite with composition and structure nearly identical to those of bone apatite can be produced in SBF by adjusting its ion concentrations including the content of carbonate ions to be equal to those of blood plasma.     

Apatite coated on organic polymers by biomimetic process: improvement in adhesion to substrate by HCl treatment

A dense, uniform and highly biologically active bone-like apatite layer can be formed in arbitrary thickness on any kind and shape of solid substance by the following biomimetic method at normal temperature and pressure: first, a substrate is set in contact with particles of CaO-SiO₂-based glass soaked in a simulated body fluid (SBF) with ion concentrations nearly equal to those of human blood plasma. Second, the substrate is soaked in another solution with ion concentrations 1.5 times those of SBF (1.5 SBF). In the present study, organic polymer substrates were treated with 1 m HCl solution, then subjected to the above mentioned biomimetic process. The induction periods for the apatite nucleation on polyethyleneterephthalate, polymethylmethacrylate, polyamide 6 and polyethersulfone substrates were reduced from 24 to 12 h with the HCl treatment. The adhesive strength of the formed apatite layer to the polyethyleneterephthalate, polymethylmethacrylate and polyamide 6 substrates were increased from 3.5 to 7.0 MPa from 1.1 to 2.8 MPa and from 0.6 to 3.1 MPa, respectively, with the HCl treatment. It is supposed that highly polar carboxyl group formed by the HCl hydrolysis reaction of ester group in polyethyleneterephthalate and polymethylmethacrylate or amide group in polyamide 6 increased the affinity of the substrates with a silicate ion to decrease the induction period, and also increased the affinity of the substrate with the apatite to increase the adhesive strength. The apatite-organic polymer composites thus obtained are expected to be useful as bone-repairing materials as well as soft-tissue-repairing materials.     

PET fiber fabrics modified with bioactive titanium oxide for bone substitutes

A rectangular specimen of polyethylene terephthalate (PET) was soaked in a titania solution composed of titanium isopropoxide, water, ethanol and nitric acid at 25 °C for 1 h. An amorphous titanium oxide was formed uniformly on the surface of PET specimen, but did not form an apatite on its surface in a simulated body fluid (SBF) within 3 d. The PET plate formed with the amorphous titanium oxide was subsequently soaked in water or HCl solutions with different concentrations at 80 °C for different periods of time. The titanium oxide on PET was transformed into nano-sized anatase by the water treatment and into nano-sized brookite by 0.10 M HCl treatment at 80 °C for 8 d. The former did not form the apatite on its surface in SBF within 3 d, whereas the latter formed the apatite uniformly on its surface. Adhesive strength of the titanium oxide and apatite layers to PET plate was increased by pre-treatment of PET with 2 wt% NaOH solution at 40 °C for 2 h. A two-dimensional fabric of PET fibers 24 μm in diameter was subjected to the NaOH pre-treatment at 40 °C, titania solution treatment at 25 °C and subsequent 0.10 M HCl treatment at 80 °C. Thus treated PET fabric formed the apatite uniformly on surfaces of individual fibers constituting the fabric in SBF within 3 d. Two or three dimensional PET fabrics modified with the nano-sized brookite on surfaces of the individual fibers constituting the fabric by the present method are believed to be useful as flexible bone substitutes, since they could be integrated with living bone through the apatite formed on their constituent fibers.     

Ceramic–metal and ceramic–polymer composites prepared by a biomimetic process

A biomimetic process was developed to prepare apatite–metal and apatite–polymer composites. A variety of metals and organic polymers incorporated surface functional groups such as Si–OH, Ti–OH or Ta–OH to induce formation of a biologically active bonelike apatite by chemical treatment or physical adsorption. Subsequent immersion in a simulated body fluid (SBF) with ion concentrations nearly equal to those of human blood plasma or 1.5 SBF led to the formation of a dense and uniform bonelike apatite layer on the surface. Apatite–metal and apatite–polymer composites prepared in this way are believed to be very useful as artificial bone substitutes.     

Apatite–organic polymer composites prepared by a biomimetic process: improvement in adhesion of the apatite layer to the substrate by ultraviolet irradiation

A dense and uniform layer of highly bioactive apatite can be formed in arbitrary thickness on any kind and shape of organic polymer substrates by the following biomimetic process. The substrate is first placed in contact with granular particles of CaO, SiO2-based glass soaked in a simulated body fluid with ion concentrations nearly equal to those of human blood plasma for forming apatite nuclei, and then soaked in another fluid highly supersaturated with respect to the apatite for making the apatite nuclei grow. In the present study, the polymer substrates were pretreated with ultraviolet (UV) light, and then subjected to the biomimetic process described above. By UV irradiation, the induction period for the apatite nucleation of poly(ethylene terephthalate) (PET), poly-ether sulphone (PESF), polyethylene (PE), poly(methyl methacrylate) (PMMA) and polyamide 6 (N6) substrates were reduced form 24 h to 10 h. The adhesive strengths of the apatite layer to the substrates increased from 2.5–3.2 MPa to 4.5–6.0 MPa for PET, PESF and PMMA, and from about 1.0 MPa to 4.0–6.5 MPa for PE and N6 substrates. These results have been explained by assuming that silicate ions, which induce apatite nucleation, are easily adsorbed on the substrates due to the formation of polar groups, with an improved hydrophilic nature, on the polymer surfaces by UV irradiation. © 1998 Chapman & Hall     

Effect of preparation conditions on the properties of bioactive glasses for testing SBF

A simulated body fluid (SBF) with ion concentrations similar to body fluid, proposed by Kokubo et al., is widely used to evaluate bone-bonding potential through the formation of an apatite layer. To be confident of the evaluation of the potential for the apatite formation in SBF, standard substrates are required. Although Na₂O–CaO–SiO₂ glasses have been focused upon as candidate standard substrates, it has not been clarified whether the preparation conditions of the glasses affect their apatite formation potential in SBF. In this study, Na₂O–CaO–SiO₂ glasses were prepared by a conventional melting–quenching method with different melting periods and annealing processes to examine their properties, including apatite formation in SBF. The Na₂O–CaO–SiO₂ glasses show reproducible apatite-forming ability when prepared using moderate melting and annealing processes, and can be useful substrates to test the reproducibility of SBF.     

Apatite formation abilities and mechanical properties of hydroxyethylmethacrylate-based organic–inorganic hybrids incorporated with sulfonic groups and calcium ions

Apatite formation in the living body is an essential requirement for artificial materials to exhibit bone-bonding bioactivity. It has been recently revealed that sulfonic groups trigger apatite nucleation in a body environment. Organic–inorganic hybrids consisting of organic polymers and the sulfonic groups are therefore expected to be useful for preparation of novel bone-repairing materials exhibiting flexibility as well as bioactivity. In the present study, organic–inorganic hybrids were prepared from hydroxyethylmethacrylate (HEMA) in the presence of vinylsulfonic acid sodium salt (VSAS) and calcium chloride (CaCl₂). The bioactivities of the hybrids were assessed in vitro by examining the apatite formation in simulated body fluid (SBF, Kokubo solution). The hybrids deposited on the apatite after soaking in SBF within 7 days. Tensile strength measurements showed a tendency to increase with increases in VSAS and CaCl₂ content. It was assumed that this phenomenon was attributed to the formation of cross-linking in the hybrids.     

Apatite deposition on polyamide films containing carboxyl group in a biomimetic solution

The development of organic–inorganic hybrids composed of hydroxyapatite and organic polymers is attractive because of their novelty in being materials that show a bone-bonding ability, i.e. bioactivity, and because they have mechanical properties similar to those of natural bone. The biomimetic process has received much attention for fabricating such a hybrid, where bone-like apatite is deposited under ambient conditions on polymer substrates in a simulated body fluid (SBF) having ion concentrations nearly equal to those of human extracellular fluid or related solutions. It has been shown that the carboxyl group is effective for inducing heterogeneous nucleation of apatite in the body. In the present study, apatite deposition on polyamide films containing various numbers of carboxyl groups was investigated in 1.5 SBF, which had ion concentrations 1.5 times those of a normal SBF. The effect of incorporation of calcium chloride on the formation of apatite was examined. Polyamide films containing 33 mol % CaCl₂ did not form apatite, even after soaking in 1.5 SBF for 7 days, and even when the polymer film contained 50 mol % carboxyl group. On the other hand, those modified with 40 mass % CaCl₂ formed apatite on their surfaces in 1.5 SBF. The ability of the modified film to form an apatite layer increased, and the adhesion of the apatite layer bonded to the film improved, with increasing carboxyl group content. It is concluded that novel apatite–polyamide hybrids can be prepared by a biomimetic process.     

Hybrid structure in PCL-HAp scaffold resulting from biomimetic apatite growth

Polymer–ceramic composites are favourite candidates when aiming to replace bone tissue. We present here scaffolds made of polycaprolactone-hydroxyapatite (PCL-HAp) composites, and investigate in vitro mineralisation of the scaffolds in SBF after or without a nucleation treatment. In vitro bioactivity is enhanced by HAp incorporation as well as by nucleation treatment, as demonstrated by simulated body fluid (SBF) mineralization. Surprisingly, we obtained a hybrid interconnected organic-inorganic structure, as a result of micropore invasion by biomimetic apatite, which results in a mechanical strengthening of the material after two weeks of immersion in SBF×2. The presented scaffolds, due to their multiple qualities, are expected to be valuable supports for bone tissue engineering.     

Biomimetic coating of an apatite layer on poly(<Emphasis Type="SmallCaps">l</Emphasis>-lactic acid); improvement of adhesive strength of the coating

A biodegradable polymer coated with a bonelike apatite layer on its surface would be useful as a scaffold for bone tissue regeneration. In this study, poly(l-lactic acid) (PLLA) was treated with oxygen plasma to produce oxygen-containing functional groups on its surface. The plasma-treated specimen was then alternately dipped in aqueous CaCl₂ and K₂HPO₄·3H₂O solutions three times, to deposit apatite precursors onto the surface. The surface-modified specimen then successfully formed a dense and uniform bonelike surface apatite layer after immersion for 24 h in a simulated body fluid with ion concentrations approximately equal to those of human blood plasma. The adhesive strength between the apatite layer and the specimen surface increased as the power density of the oxygen plasma used increased. The maximum adhesive strength of the apatite layer to the specimen was significantly higher than that to the commercially available artificial bone, HAPEX^TM. The resultant bonelike apatite–PLLA composite would be useful as a scaffold for bone tissue regeneration.     

In vitro apatite formation on organic–inorganic hybrids in the CaO–SiO<Subscript>2</Subscript>–PO<Subscript>5/2</Subscript>–poly(tetramethylene oxide) system

The osteoconduction potential of artificial materials is usually evaluated in vitro by apatite formation in a simulated body fluid (SBF) proposed by Kokubo and his colleagues. This paper reports the compositional dependence of apatite formation on organic–inorganic hybrids in the CaO–SiO₂–PO_5/2–poly(tetramethylene oxide) system, initiated from tetraethoxysilane (TEOS), triethyl phosphate (OP(OEt)₃), calcium chloride (CaCl₂) and poly(tetramethylene oxide)(PTMO) modified with alkoxysilane. Formation of an apatite layer was observed on the surface of the organic–inorganic hybrids with molar ratios of TEOS/OP(OEt)₃ ranging from 100/0 to 20/80. The rate of apatite formation remarkably decreased when the hybrids were synthesized with TEOS/OP(OEt)₃ ratios of 40/60 or less. Hybrids without TEOS showed no apatite formation in SBF for up to 14 days. Addition of small amounts of OP(OEt)₃ to TEOS in the hybrids led to the high dissolution of calcium and silicate, while addition of large amounts of OP(OEt)₃ decreased the dissolution of calcium and silicate ions and resulted in reduced apatite formation regardless of the dissolution of phosphate ions from the hybrids.     

Preparation and preliminary cytocompatibility of magnesium doped apatite cement with degradability for bone regeneration

In the present study, we fabricated magnesium doped apatite cement (md-AC) with rapid self-setting characteristic by adding the mixed powders of magnesium oxide and calcium dihydrogen phosphate (MO–CDP) into hydroxyapatite cement (HAC). The results revealed that the md-AC with 50 wt% MO–CDP could set within 6 min and the compression strength could reach 51 MPa after setting for 1 h, indicating that the md-AC had highly initial mechanical strength. The degradability of the md-AC in Tris–HCl solution increased with the increase of MO–CDP amount, and the weight loss ratio of md-AC with 50 wt% MO–CDP was 57.5 wt% after soaked for 12 weeks. Newly flake-like apatite could be deposited on the md-AC surfaces after soaked in simulated body fluid (SBF) for 7 days. Cell proliferation ratio of MG₆₃ cells on md-AC was obviously higher than that of HAC on days 4 and 7. The cells with normal phenotype spread well on the md-AC surfaces and attached intimately with the substrate, and alkaline phosphatase (ALP) activity of the cells on md-AC significantly improved compared with HAC on day 7. The results demonstrate that the md-AC has a good ability to support cell proliferation and differentiation, and indicate a good cytocompatibility.     

Fabrication and biological characteristics of β-tricalcium phosphate porous ceramic scaffolds reinforced with calcium phosphate glass

The fabrication process, compressive strength and biocompatibility of porous β-tricalcium phosphate (β-TCP) ceramic scaffolds reinforced with 45P₂O₅–22CaO–25Na₂O–8MgO bioglass (β-TCP/BG) were investigated for their suitability as bone engineering materials. Porous β-TCP/BG scaffolds with macropore sizes of 200–500 μm were prepared by coating porous polyurethane template with β-TCP/BG slurry. The β-TCP/BG scaffolds showed interconnected porous structures and exhibited enhanced mechanical properties to those pure β-TCP scaffolds. In order to assess the effects of chemical composition of this bioglass on the behavior of osteoblasts cultured in vitro, porous scaffolds were immersed in simulated body fluid (SBF) for 2 weeks, and original specimens (without soaked in SBF) seeded with MC3T3-E1 were cultured for the same period. The ability of inducing apatite crystals in simulated body fluid and the attachment of osteoblasts were examined. Results suggest that apatite agglomerates are formed on the surface of the β-TCP/BG scaffolds and its Ca/P molar ratio is ~1.42. Controlling the crystallization from the β-TCP/BG matrix could influence the releasing speed of inorganic ions and further adjust the microenvironment of the solution around the β-TCP/BG, which could improve the interaction between osteoblasts and the scaffolds.     

Bioactivity of calcium phosphate coatings prepared by electrodeposition in a modified simulated body fluid

The purpose of this study was to investigate bioactivity of calcium phosphate coatings prepared by electrodeposition in a modified simulated body fluid (SBF). Calcium phosphates were electrodeposited on commercially pure titanium substrates in the modified SBF at 60 °C for 1 h maintaining the cathodic potentials of − 1.5 V, − 2 V, and − 2.5 V (vs. SCE). Subsequently, the calcium phosphate coatings were transformed into apatites during immersion in the SBF at 36.5 °C for 5 days. The apatites consisted of needle-shaped crystallites distributed irregularly with different grain sizes. As the coatings were electrodeposited at higher cathodic potential, the crystallite of the apatites got denser and the grain sizes of the apatites became bigger during subsequent immersion in the SBF. However, as the coatings were electrodeposited at higher cathodic potential, the coatings were transformed into apatites with lower crystallinity and the Ca/P atomic ratio of the apatites got higher than 1.67, that of stoichiometric hydroxyapatite, after subsequent immersion in the SBF. In addition, CO₃²⁻ ions contained in the modified SBF were incorporated in the calcium phosphate coating during electrodeposition and had an influence on transforming the calcium phosphate into bonelike apatite during subsequent immersion in the SBF showing that CO₃²⁻ incorporated in the apatites disturbed crystallization of the apatites. These results revealed that the coating electrodeposited at − 2.0 V (vs. SCE) in the modified SBF containing CO₃²⁻ ions was the most bioactive showing transformation into carbonate apatite similar to bone apatite.     

Mineral phase deposition on pectin microspheres

The nucleation of apatite and calcium phosphates onto natural polysaccharides containing carboxyl groups can be obtained by immersion in biomimetic solutions. The deposition of hydroxyapatite has been recently described also on pectin, and pectin–apatite hybrid gels were proposed as scaffolds for bone tissue regeneration. In this work, injectable calcium phosphate/pectin microspheres were prepared to promote both the natural process of biomineralization and the controlled release of encapsulated cells, genes, enzymes, proteins or drugs in the pathological situ. Pectin microspheres were prepared with different formulations from aqueous solutions using an extrusion system, and incubated in H₂O or SBF for 14 days. The presence of an excess of CaCl₂, deriving from the preparation process, was determinant in promoting the deposition of calcium phosphates from SBF, whereas no mineral deposition was detected on pectin microspheres extensively purified after preparation. The nucleation of calcium phosphates occurred on pectin microspheres incubated in SBF for 14 days and was evaluated through ESEM–EDS analysis and FT-IR spectroscopy.     

Formation of calcium phosphates on low-modulus Ti–7.5Mo alloy by acid and alkali treatments

This study investigated the hydroxyapatite (HA) coating on metal implants in order to enhance their bioactive properties. In this study, HA coatings were formed on the surfaces of commercially pure titanium (c.p. Ti) and Ti–7.5Mo which were acid-etched and subsequently alkali-treated before samples were soaked in simulated body fluid (SBF). Specimens of c.p. Ti and Ti–7.5Mo were etched in either H₃PO₄ or HCl, and subsequently treated in NaOH. The surfaces of acid-etched c.p. Ti showed a porous structure, whereas those of acid-etched Ti–7.5Mo showed some grinding marks, but no porosity. After subsequent alkali treatment in NaOH, the surfaces of both the c.p. Ti and Ti–7.5Mo substrates exhibited microporous network structures. The specimens were then immersed in SBF at 37 °C for 28 days. Apatite began to deposit on acid-etched and NaOH-treated Ti–7.5Mo within 1 day after immersion in the SBF. After 28 days of immersion in the SBF, a dense and uniform layer was produced on the surfaces of all samples. The HA formation rate was the highest for HCl and NaOH-pretreated samples, and the results of EDS and XRD showed that much more intensive peaks of HA appear on the specimens of HCl and NaOH-treated Ti–7.5Mo than on any other sample. Thus, this method of apatite coating Ti–7.5Mo appears to be promising for artificial bone substitutes or other hard tissue replacement materials with heavy load-bearing applications due to their desirable combination of bioactivity, low elastic modulus, and low processing costs.     

A novel bioactive porous bredigite (Ca<Subscript>7</Subscript>MgSi<Subscript>4</Subscript>O<Subscript>16</Subscript>) scaffold with biomimetic apatite layer for bone tissue engineering

The aim of this study was to develop a novel bioactive, degradable and cytocompatible bredigite (Ca₇MgSi₄O₁₆) scaffold with biomimetic apatite layer for bone tissue engineering. Porous bredigite scaffolds were prepared using polymer sponge method. The bredigite scaffolds with biomimetic apatite layer (BTAP) were obtained by soaking bredigite scaffolds in simulated body fluid (SBF) for 10 days. The porosity and in vitro degradability of BTAP scaffolds were investigated. In addition, osteoblast-like cell morphology, proliferation and differentiation on BTAP scaffolds were evaluated and compared with β-tricalcium phosphate (β-TCP) scaffolds. The results showed that BTAP scaffolds possessed 90% of porosity. The degradation of BTAP scaffolds was comparable to that of β-TCP scaffolds. Cells on BTAP scaffolds spread well and presented a higher proliferation rate and differentiation level as compared with those on β-TCP scaffolds. Our results indicated that BTAP scaffolds were degradable and possessed the function to enhance cell proliferation and differentiation, and might be used as bone tissue engineering materials.