Q.L. Feng

A mechanistic study of the antibacterial effect of silver ions on Escherichia coli and Staphylococcus aureus

To investigate the mechanism of inhibition of silver ions on microorganisms, two strains of bacteria, namely Gram-negative Escherichia coli (E. coli) and Gram-positive Staphylococcus aureus (S. aureus), were treated with AgNO(3) and studied using combined electron microscopy and X-ray microanalysis. Similar morphological changes occurred in both E. coli and S. aureus cells after Ag(+) treatment. The cytoplasm membrane detached from the cell wall. A remarkable electron-light region appeared in the center of the cells, which contained condensed deoxyribonucleic acid (DNA) molecules. There are many small electron-dense granules either surrounding the cell wall or depositing inside the cells. The existence of elements of silver and sulfur in the electron-dense granules and cytoplasm detected by X-ray microanalysis suggested the antibacterial mechanism of silver: DNA lost its replication ability and the protein became inactivated after Ag(+) treatment. The slighter morphological changes of S. aureus compared with E. coli recommended a defense system of S. aureus against the inhibitory effects of Ag(+) ions.

Antimicrobial effects of metal ions (Ag+, Cu2+, Zn2+) in hydroxyapatite

The antimicrobial ceramics (AC) based on hydroxyapatite (HA) were made in a wet chemical process with additions of AgNO3, Cu(NO3)2 · 3H2O and Zn(NO3)2 · 6H2O. The ACs were composed of metal-ion substituted hydroxyapatite and nitrate–apatite, which was identified by X-ray diffraction. The viable count and turbidity measurement was adopted to observe the antimicrobial effects of the various ACs. The aerobic Escherichia coli was used in the study. An obvious antimicrobial effect against E. coli was observed in Ag+ AC. In contrast to Ag+ AC, it was difficult to ascertain any bactericidal effect in the case of Cu2+ and Zn2+ AC. The bactericidal effect of Ag+ was observed using a dialysis tube experiment. This suggests that Ag+ dissolved out and reacted with E. coli, thus inhibiting its growth.

A simple biomimetic method for calcium phosphate coating

Abstract A biomimetic calcium phosphate coating is expected to enhance the bioactivity and bone inductivity of human implants. This study presents a very simple, highly effective biomimetic method to obtain a calcium phosphate coating on a titanium substrate. Using NaH 2 PO 4 , a stable solution was prepared with high calcium and phosphate ion concentrations. This solution turned to a supersaturated calcification solution (SCS) when NaHCO 3 was added. The addition of NaHCO 3 elevated the pH value of the solution gradually and steadily. A uniform coating approximately 40 μm thick was found on the substrates after 24 h immersion. The compositions of the coatings were adjustable from hydroxyapatite (HA) to HA/dicalcium phosphate dihydrate (DCPD). The calcium phosphate deposits were investigated by means of scanning electron microscopy (SEM), X-ray diffraction (XRD) and Fourier transformed infrared (FT-IR) spectroscopy.

Crystal orientation, toughening mechanisms and a mimic of nacre

Abstract Based on the investigations of crystal structure of nacre using SEM, TEM and XRD, it is proposed that there exists a domain structure of crystal orientation in the nacre. The orientation domain consists of continuous 3–10 tablets along the direction perpendicular to nacreous plane, and 1–5 tablets in a single lamina. The tablets in a domain are crystallographic identical in three dimensions. From the crack morphologies, it is found that the crack deflection, fibre pull-out and organic matrix bridging are the three main toughening mechanisms acting on nacre. The organic matrix plays an important role in the toughening of this biological composite. The biomimetically synthesized composite made of alumina and kevlar showed significant increase in the fracture energy compared with the single ceramics. The soluble proteins extracted from nacre can induce aragonite and the one from prism can induce calcite grown with a preferred orientation of [104]. The insoluble proteins control the nucleation site and thus lead to a finer crystallization of CaCO3.

Ag-substituted hydroxyapatite coatings with both antimicrobial effects and biocompatibility

Q. L. FENG, F. Z. CUILaboratory of Advanced Materials, Department of Materials Science and Engineering, Tsinghua University,Beijing 100 084, People’s Republic of ChinaE-mail: zjz-dms@mail.tsinghua.edu.cnT. N. KIMDivision of Advanced Materials Engineering, Paichai University, Taejon 302-735, KoreaJ. W. KIMDepartment of Biochemistry, Paichai University, Taejon 302-735, Korea

The modulation of collagen on crystal morphology of calcium carbonate

The modulation function of proteins was studied during biomineralization in vitro. Morphological development of calcium carbonate precipitation in collagen solutions at different concentrations has been examined using scanning electron microscope. X-ray diffraction is used to determine the polymorph of calcium carbonate crystals. Its data indicate that only calcite crystals are formed. The experimental results also show that the calcite growth is more and more inhibited as collagen is increasing in concentration. A simple crystal model is employed to explain these phenomena. Different crystal planes with different affinities to collagen play a crucial role in changing the morphology of calcite crystals.

Controlled crystal growth of calcium phosphate on titanium surface by NaOH-treatment

A simple supersaturated calcification solution (SCS) was used to deposit calcium phosphate (Ca-P) on the surface of NaOH-treated titanium (NaOH-Ti), in order to study the corresponding deposition mechanism of calcium phosphate (Ca-P). No Ca-P coating from SCS was found on titanium without NaOH-treatment, while NaOH-Ti possessed the capability to induce a Ca-P coating on the titanium surface. Octacalcium phosphate (OCP) crystals were first grown on an NaOH-Ti surface, followed by hydroxyapatite (HA) with a [0 0 1] preferential orientation on OCP. It is found that two factors controlled the growth of Ca-P crystals on NaOH-Ti from SCS. First, the surface morphology of NaOH-Ti characterized with crevices seems to be beneficial for inducing a Ca-P coating from SCS; second, the basic hydroxyl, Ti-OH, radical has increased in NaOH-Ti with the increase of treating time and concentration, which facilitate the nucleation of Ca-P crystals.

Oriented growth of hydroxyapatite on (0001) textured titanium with functionalized self-assembled silane monolayer as template

A highly (0001) textured hydroxyapatite [Ca 10 (PO 4 ) 6 (OH) 2 , HA] coating on polycrystalline titanium plate is successfully synthesized by a biomimetic process mimicking biomineralization. To simulate the first stage of biomineralization, that is, supramolecular preorganization, a template surface with highly organized arrangement of carboxyl (–COOH) and alcoholic hydroxyl (–1OH) groups is prepared through self-assembly of vinyltriethoxysilane [(C 2 H 5 O) 3 SiCH=CH 2 , VTS] on hydroxylated titanium with strong (0001) texture, followed by oxidation of the vinyl groups (–1CH=CH 2 ) with dilute KMnO 4 solution into alcoholic hydroxyl and then into carboxyl groups. The functionalized substrate can induce oriented nucleation and growth of HA with (0001) planes parallel to the substrate surface from supersaturated HA solution through interfacial molecular recognition. The mechanisms of molecular recognition are also discussed.

Influence of solution conditions on deposition of calcium phosphate on titanium by NaOH-treatment

The present paper demonstrated a biomimetic method to coat calcium phosphate (Ca-P) on the surface of titanium induced by NaOH-treatment from a simple supersaturated hydroxyapatite solution (SHS). The influence of pH value and calcium ions concentration on the precipitation process was investigated. It is necessary for the solution to be supersaturated than the critical concentration of octacalcium phosphate (OCP) to get Ca-P coatings on titanium surface. In the precipitating process, it seems that amorphous calcium phosphate (ACP) precipitated first, then OCP, and finally hydroxyapatite (HA). The system was in continuous evolution and the phase transitions occurred in sequence.

Highly adhesive hydroxyapatite coatings on alumina substrates prepared by ion-beam assisted deposition

Abstract Ion-beam assisted deposition (IBAD) was carried out to deposit hydroxyapatite (HAp) on alumina substrate. The amorphous coating layer on alumina substrate is crystallized to the dense HAp after a post-treatment process of annealing at 500 °C for 2 h and immersion in deionized water for 72 h at room temperature. The homogeneous and dense HAp coating is observed by scanning electron microscopy and the thickness of the coating is measured to be about 638 nm. The hardness of the coating layer in the as-deposited and post-treated samples is 475 and 508, respectively. The adhesive strength between HAp and α-alumina substrate was measured by a scratch test. The ion-beam sputtered coatings were used as control samples. The adhesion was found to be worse for ion-beam sputtering deposition (IBSD) than for IBAD.