Key Engineering Materials Vols. 284-286

Abstract: Struvite or magnesium ammonium phosphate MgNH4PO4 has been proposed as active component in setting surgical cements. The usual formulation is one in which the magnesium component in the powder is either magnesium hydrogen phosphate trihydrate or trimagnesium phosphate or a mixture of these two compounds. As the cement liquid a concentrated solution of diammonium phosphate is taken. To make the cement attractive as a bone substitute material a calcium phosphate filler is generally incorporated. Thus such materials are a type of pseudo calcium phosphate cements. This study was intended to find out which calcium phosphate and which magnesium compound are the most suitable. In the first series of experiments a mixture of 12 g Mg3(PO4)2 and 4 g MgHPO4.3H2O was used as the magnesium component in the powder. To that powder 30 g of either precipitated hydroxyapatite PHA or CaHPO4 or CaHPO4.H2O or b-TCP or a-TCP was added. The cement liquid was a 3.5 M solution of (NH4)2HPO4. At specific liquid/powder ratios L/P suitable setting times were obtained for the different formulations. However, the compressive strengths after immersion of the cements in 0.9% saline solution at 37°C varied over a large range. The best formulation was that with a-TCP which reached a compressive strength of 57 MPa after 18 h of immersion. In the second series of experiments 20 g of Mg3(PO4)2 was used as the magnesium component in the powder. Again 30 g of either of the above mentioned calcium phosphates was used as filler and again a 3.5 M solution of (NH4)2HPO4 was used as the cement liquid. At the appropriate L/P ratios the respective setting times were longer than in the first series of experiments but all five formulations appeared to result in good compressive strengths varying from 41 MPa for the formulation with b-TCP to 67 MPa for the formulation with PHA. In the third series of experiments 30 g a-TCP was taken as the calcium phosphate in the powder. As magnesium components mixtures of Mg3(PO4)2.8H2O and MgHPO4.3H2O and Mg3(PO4)2 were used. Again the cement liquid consisted of a 3.5 M solution of (NH4)2HPO4. The formulations with Mg3(PO4)2.8H2O had the shortest setting times and the lowest compressive strengths, whereas those with Mg3(PO4)2 had the longest setting times and the highest compressive strengths. Therefore, it is advantageous to use Mg3(PO4)2 as the magnesium component.

Abstract: The effect of silica content in the PMMA/silica nano-composite on the mechanical properties and the growth behavior of apatite crystals were investigated. The PMMA/silica nano-composites with different silica content were synthesized through the sol-gel reaction with triethoxysilane end-capped PMMA and tetraethyl orthosilicate (TEOS). The compressive strength showed its maximum value when the content of TEOS was 20 wt% while the elastic modulus showed its maximum value when the content of TEOS was 60 wt%. The growth behavior of the apatite crystals following the cell culture showed different response according to the silica content. As increasing the TEOS content, the shape of the apatite crystals changed from globule-like structure to fiber-like one.

Abstract: Silicon containing self setting bone cement has been prepared by adding silicon containing hydroxyapatite whisker to obtain a biomaterial having an improved resorption properties. Silicon containing calcium phosphate bone cement was composed of a-TCP: TeCP: DCPD: Si-HA whisker with a NH4H2PO4 as a setting liquid. From the XRD analysis, it was confirmed that calcium deficient hydroxyapatite phase appeared when it immersed in PBS solution. The dissolution rate of silicon containing calcium phosphate cement was measured in PBS solution and showed high dissolution rate. Based on in-vivo test, silicon containing self setting bone cement can be considered a useful material for bone bonding materials.

Abstract: Typical plasma-sprayed hydroxyapatite coatings work well on non-porous substrates but do not coat the inner surfaces of open-porous substrates. Solution deposition can produce consistent bioceramic coats of precise thickness on porous surfaces. The resultant “biomimetic” surface more closely mimics the trabecular pattern and biochemistry at the bone interface. This report compares bone response to porous surfaces with biomimetic hydroxyapatite coatings. Implants were manufactured as Ti6Al4V cylinders (5-mm diameter, 41-mm long) coated with c.p-Ti PorocoatÒ porous layer with a thickness of 750 (± 250 µm). Implants were divided into three groups based on surface treatments. The porous surfaces of control group implants did not receive any treatment. The porous surfaces of HA group implants were plasma sprayed with hydroxyapatite. The porous surfaces of BAp group implants were coated with a biomimetic apatite (BAp) coating using a lowtemperature solution-based process that mimics bone mineralization. BAp coating is pure apatite coating of uniform structure and composition, with a thickness of approximately 15 µm on the outer beads. Because of the reduced thickness, the BAp coating does not block the pores or alter the porous structure. Bilateral femurs in thirty-six rabbits were implanted with one of the above groups. Twelve rabbits each were euthanized at 2, 4, and 12 weeks. Osseointegration was measured by automated computerized histomorphometry of scanning electron microscopy images of sections taken through the implant. Bone ingrowth on the Control surface was 45 % at 2 weeks and 47% at 12 weeks. Bone ingrowth on the PS surface increased from 51% at 2 weeks to 67% at 12 weeks. Bone ingrowth on the BAp surface increased from 45 % at 2 weeks to 71% at 12 weeks. At both time points mean bone ingrowth on PS and BAp coated implants was significantly higher than the control uncoated implants (p < 0.01). By 12 weeks the PS hydroxyapatite coat began showing evidence of fragmentation and debris production on SEM. This was not evident in the BAp coat. This study supports the hypothesis that apatite coating benefits osseointegration. A biomimetic coat of solution deposited apatite may not show the disadvantages of coating delamination and particle generation. Biomimetic apatite coatings may be attractive alternatives for noncemented total hip arthroplasty.

Abstract: The reaction of animal bone to Ti6Al4V pins coated with ceramics of Ca2KNa(PO4)2 or Ca10[K/Na](PO4)7 as main crystalline phases were tested. It is shown that the new materials possess a very high resorption in comparison with hydroxyapatite (HA).

Abstract: Fibrous cellulose templates are attractive candidates for the use as tissue engineering scaffolds due to their biocompatibility and the adjustable porosity. Nevertheless, a direct bond between cellulose and bone is not formed under physiological conditions. A simulated body fluid solution with a high degree of supersaturation (5*SBF) was used to accelerate the biomimetic formation of bonelike apatite on cellulose templates. After generating calcium phosphate nuclei on the cellulose fibers in 5*SBF with high Mg2+and HCO3 - concentrations the cellulose templates were immersed in a modified 5*M-SBF which was optimized in respect to crystal growth kinetics by reduced Mg2+and HCO3- concentrations. After 48 hours a hydroxy carbonated apatite (HCA) layer with a thickness of 20 µm was obtained.

Abstract: The RF magnetron sputter technique was used to deposit Bioglass (BG) and hydroxyapatite (HA) coatings onto titanium substrates. In the current study, the physico-chemical and dissolution properties of various deposited coatings were investigated. X-ray diffraction demonstrated that the as-sputtered coatings had an amorphous structure, a heattreatment for 2 hours at 600°C changed only the HA coating into a crystalline apatite structure. Dissolution experiments demonstrated that all the amorphous coatings dissolved during the incubation for 4 weeks in simulated body fluid, while all the heattreated sputter coatings were still maintained. In contrast with the HA heattreated sputter coatings all the bioglass containing sputter coatings showed the formation of a crystalline apatite phase. Scanning electron microscopical examination of the sputtered coatings demonstrated that on all the heattreated BG/HG sputter coating a thick CaP precipitate was formed, while on the BG sputter coating occasionally a globular precipitate was observed.

Abstract: Electrospraying of nano-sized hydroxyapatite (HA) has been used as a technique to modify the surface of alumina in order to achieve the goal of improving bone integration. A porous HA coating on alumina was produced by heat treating electrsprayed HA. Preliminary in vitro studies shown that this porous HA coating provided a favourable surface for attachment and growth of HOB cells. The results indicate that electrospraying is a very promising technique to create thin HA coatings on a range of biomedical implants to improve interfacial bonding with the host tissue.

Abstract: Hydroxyapatite (HA) was coated onto titanium substrates using radio frequency sputtering, and the coated HA films were crystallized in an autoclave at 110 °C using a low temperature hydrothermal method. The crystallinity, the Ca/P ratio, and the surface of the films were observed using XRD, EDS, and SEM, respectively. From the XRD patterns, a sputtered film after the hydrothermal treatment had crystallized after 24 h, and the Ca/P ratio decreased from 2.43 ± 0.07 to 1.75 ± 0.11. The growth rate of osteoblast cells was used for cell culture. Among control, titanium and hydrothermally treated film, there was no significant difference in the cell growth rate. However, the growth rate was suppressed on the as-sputtered film.

Abstract: The recently developed Electrostatic Spray Deposition (ESD) technique was used in order to deposit calcium phosphate (CaP) coatings onto various substrates, since this technique enables deposition of inorganic thin films with a variety of morphological and chemical properties. In the present study, the relationship between various deposition parameters and the physicochemical properties of deposited coatings was investigated in order to be able to deposit CaP coatings with defined chemical and morphological properties using ESD. The results showed that the chemical characteristics of the coatings were determined by both the composition of the precursor solutions (solution acidity, absolute and relative precursor concentrations) and apparatus-related parameters, such as the liquid flow rate and the nozzle-tosubstrate distance. By varying these parameters, several crystal phases and phase mixtures were obtained (carbonate apatite, carbonated hydroxyapatite, a/b-tricalciumphosphate, monetite, b/g-pyrophosphate, metaphosphate, calcite/calcium oxide). Coating morphology was also shown to be strongly dependent on several deposition parameters. A wide range of coating morphologies was obtained, varying from relatively dense to highly porous, reticular coating surface architectures.