Dae Joon Kim

Preparation and characterization of nano-sized hydroxyapatite/alginate/chitosan composite scaffolds for bone tissue engineering

The aim of this study was to develop chitosan composite scaffolds with high strength and controlled pore structures by homogenously dispersed nano-sized hydroxyapatite (nano-HAp) powders. In the fabrication of composite scaffolds, nano-HAp powders distributed in an alginate (AG) solution with a pH higher than 10 were mixed with a chitosan (CS) solution and then freeze dried. While the HAp content increased up to 70 wt.%, the compressive strength and the elastic modulus of the composite scaffolds significantly increased from 0.27 MPa and 4.42 MPa to 0.68 MPa and 13.35 MPa, respectively. Higher content of the HAp also helped develop more differentiation and mineralization of the MC3T3-E1 cells on the composite scaffolds. The uniform pore structure and the excellent mechanical properties of the HAp/CS composite scaffolds likely resulted from the use of the AG solution at pH 10 as a dispersant for the nano-HAp powders.

Influence of processing parameters on pore structure of 3D porous chitosan–alginate polyelectrolyte complex scaffolds†

Fabrication of porous polymeric scaffolds with controlled structure can be challenging. In this study, we investigated the influence of key experimental parameters on the structures and mechanical properties of resultant porous chitosan-alginate (CA) polyelectrolyte complex (PEC) scaffolds, and on proliferation of MG-63 osteoblast-like cells, targeted at bone tissue engineering. We demonstrated that the porous structure is largely affected by the solution viscosity, which can be regulated by the acetic acid and alginate concentrations. We found that the CA PEC solutions with viscosity below 300 Pa.s yielded scaffolds of uniform pore structure and that more neutral pH promoted more complete complexation of chitosan and alginate, yielding stiffer scaffolds. CA PEC scaffolds produced from solutions with viscosities below 300 Pa.s also showed enhanced cell proliferation compared with other samples. By controlling the key experimental parameters identified in this study, CA PEC scaffolds of different structures can be made to suit various tissue engineering applications.

Initial bacterial adhesion on resin, titanium and zirconia in vitro

PURPOSE The aim of this in vitro study was to investigate the adhesion of initial colonizer, Streptococcus sanguis, on resin, titanium and zirconia under the same surface polishing condition. MATERIALS AND METHODS Specimens were prepared from Z-250, cp-Ti and 3Y-TZP and polished with 1 µm diamond paste. After coating with saliva, each specimen was incubated with Streptococcus sanguis. Scanning electron microscope, crystal violet staining and measurement of fluorescence intensity resulting from resazurin reduction were performed for quantifying the bacterial adhesion. RESULTS Surface of resin composite was significantly rougher than that of titanium and zirconia, although all tested specimens are classified as smooth. The resin specimens showed lower value of contact angle compared with titanium and zirconia specimens, and had hydrophilic surfaces. The result of scanning electron microscopy demonstrated that bound bacteria were more abundant on resin in comparison with titanium and zirconia. When total biofilm mass determined by crystal violet, absorbance value of resin was significantly higher than that of titanium or zirconia. The result of relative fluorescence intensities also demonstrated that the highest fluorescence intensity was found on the surface of resin. Absorbance value and fluorescence intensity on titanium was not significantly different from those on zirconia. CONCLUSION Resin specimens showed the roughest surface and have a significantly higher susceptibility to adhere Streptococcus sanguis than titanium and zirconia when surfaces of each specimen were polished under same condition. There was no significant difference in bacteria adhesion between titanium and zirconia in vitro.

Bone formation around zirconia implants combined with rhBMP-2 gel in the canine mandible

OBJECTIVE The aim of this study was to estimate the effects of zirconia implants and recombinant human bone morphogenetic protein-2 (rhBMP-2) gel on the acceleration of local bone formation and osseointegration in the canine mandible. MATERIALS AND METHODS Four groups of 48 implants with identical geometry were installed in the mandibles of beagle dogs: alumina-blasted zirconia implants applied with rhBMP-2, alumina-blasted zirconia implants applied with demineralized bone matrix (DBM), alumina-blasted zirconia implants, and resorbable blast media-treated titanium (Ti) implants. For the first two groups, zirconia implants were inserted after the surgical sites were filled with rhBMP-2 or DBM gel. For the other two groups, zirconia or Ti implants were installed with no adjunctive treatment. Fluorescent bone markers were administered to monitor bone remodeling at weeks 2, 4, and 5 postimplantation. After healing periods of 3 weeks and 6 weeks, the animals were sacrificed, and fluorescent microscopy, histology, and histomorphometric analyses were performed. RESULTS Fluorescent microscopy showed that bone formation around the zirconia implants installed with rhBMP-2 gel was the most prominent at 2 weeks postimplantation, while the Ti implants acquired bone apposition mainly at week 5. No significant differences were found in bone area among the groups (P > 0.05). The zirconia implants showed similar bone-to-implant contact to the Ti implants. There were no significant differences in bone-to-implant contact between the zirconia implants with rhBMP-2 gel and those with DBM (P > 0.05). CONCLUSIONS The zirconia implants with alumina-blasted surfaces may achieve osseointegration in much the same manner as the well-established Ti implants. The area influenced by rhBMP-2 gel, including the alveolar crest, may cause active remodeling and early bone formation.

Initial in vitro bacterial adhesion on dental restorative materials

Objectives The purpose of this study was to evaluate initial bacterial adhesion on several restorative materials with similar roughness. Materials and Methods Sixty cylindrical slabs were prepared from four restorative materials: zirconia (Zr), alumina-toughened zirconia (Al-Zr), type III gold alloy (Au), and cp-titanium (Ti). All the materials were polished until a mirror-like shine was achieved. The average surface roughness and topography were determined by atomic force microscopy. Contact angles were measured to calculate surface free energy by the sessile drop technique. After the formation of a salivary pellicle, S. sanguinis, S. gordonii, and S. oralis were inoculated onto the specimens and incubated for 4 h. Quantification of the adherent bacteria was performed by crystal violet staining technique and resazurin reduction assay. One-way ANOVA and Tukeys post hoc test were adopted for statistical analysis. The level of significance was 0.05. Results The Ra values determined with atomic force microscopy for all specimens were lower than 5 nm. Surface free energy increased in the order of Al-Zr, Zr, Ti, and Au. Differences were significant between the investigated materials in both crystal violet absorbance and fluorescence intensities. Gold alloy showed the highest values for all bacterial strains (p<0.05). Conclusions Zirconia, alumina-toughened zirconia, and titanium may be more suitable than gold alloy as an abutment material with respect to the initial bacterial adhesion and subsequent advance of peri-implantitis.

Zirconia/Alumina Composite Dental Implant Abutments

(Y,Nb)-TZP/20 vol% Al 2O3 composite was utilized as dental implant abutments. Biaxial strength and fracture toughness of the composite were 820 MPa and 8.5 MPa m, respectively. A total of 55 ceramic abutments were connected to implant fixtures f o ingle and short span fixed prostheses for a period of 2 years. Neither fracture nor screw l oosening of abutments was observed so far. Introduction Dental implants have been successfully used as one of the viable tre tment modalities in dentistry [1]. The dental implant system consists of a fixture and an abutment. The fixtur part is placed into the bone bed surgically and left for several months of undisturbed healing per iod. After second surgery, the abutments are connected to fixtures to replace the crown portion of the teeth. Traditionally, titanium and titanium alloy have been used for the abutments. Recently , however, ceramic abutments are introduced to solve esthetic problems of metal abutments [2]. A lumina has been used for this purpose, but it has limited applications due to the fracture during prepar ation for desired shapes and connection procedures as a result of relatively inferior mechanical properties. Although zirconia possesses higher mechanical properties than alumina, it experiences the low temperature degradation during steam sterilization and its color is too white to be esthet ic in he dental application. On the other hand, zirconia/alumina composites have suitable mechanical properti es, biocompatibility, and esthetics for the implant abutments [3]. The purpose of this study wa s to evaluate the clinical performance of the loaded single and short span fixed prostheses supported by th zirconia/alumina composite abutments. Materials and Methods Powder preparation procedure of the zirconia/alumina composite abutments (ZirAce, Acucera, Korea) was delineated elsewhere [3]. Zirconia/ 20 vol% alumina com posite powders were die-pressed into disks, and then isostatically pressed at 140 MPa. The g reen compacts were sintered for 2 h at 1600oC in air. Both flexural strength and fracture toughness of disk specimens were measured using a flat-on-three-ball biaxial-fixture [4] and toughnes s was determined by the indentation-strength method [5]. Low temperature degradation was deter mined by detecting m-ZrO 2 phase on aged specimens using x-ray diffractometer. Key Engineering Materials Online: 2003-12-15 ISSN: 1662-9795, Vols. 254-256, pp 699-702 doi:10.4028/www.scientific.net/KEM.254-256.699

Influence of Two-Step Sintering Variables on Phase Stability of Hydrothermally Prepared HAp Nano Powders

Two-step sintering was performed to prepare hydroxyapatite (HAp) with a high density and a fine microstructure utilizing hydrothermally synthesized nano powders. Its effectiveness was dependent on starting chemicals for precursors of HAp, powder processing condition, initial temperature in the two-step sintering, heating rate to the initial temperature, and holding time at final temperature. HAp powder, hydrothermally treated after aging of precipitate of Ca(OH)2 and H3PO4 for 48 h, led to HAp having a high phase stability at elevated temperatures, a high density, and a fine microstructure after two-step sintering with heating rate of 10°C/min to 1275°C.

Acoustic Emission Characteristics of Single-Edge-Notched Glass Fiber/Aluminum Hybrid Laminates

Acoustic emission (AE) characteristics have been studied for single-edge-notched monolithic thin aluminum (Al) plates and glass fiber/Al hybrid laminates. Traveling microscope was used for observing the plastic deformation and damage zone around the initial notch tip. Frequency characteristics of AE signals processed by fast Fourier transform (FFT) from monolithic Al could be classified into two different types. Type I signal had a relatively low frequency band of 96~260kHz, while Type II signal had broad band frequencies of 192~408kHz. In case of glass fiber/Al hybrid laminates, AE signals with high amplitude (>80dB) and long duration (>2msec) were additionally confirmed on FFT frequency analysis, which corresponded to macro-crack propagation and/or delamination between aluminum layer and glass fiber layer. Also, distributions of the first and the second peaks in frequency spectrum were related with local fracture behaviors of the hybrid laminates. AE source location determined by signal arrival time showed the extent of fracture zones. On the basis of the above AE analysis, characteristic features of fracture processes of single-edge-notched glass fiber/aluminum laminates were elucidated according to different fiber orientations.

Low Temperature Stability of Zirconia/Alumina Hip Joint Heads

Low temperature degradation-free zirconia/alumina composite was de velop d by mixing the tetragonal zirconia solid solution containing 5.31 mol% Y 2O3 and 4.45 mol% Nb 2O5 with 10 vol% alumina. No degradation was observed after annealing the composite for 5 h a t 2 0oC and 3.97 MPa water vapor pressure. The composite showed flexural strength of 880 MP a and fracture toughness of 8.9 MPam and was machined into 28 mm diameter heads with sphericity of 1.0 μm and roughness (Ra) of 0.02 μm.

Wear of UHMWPE against Zirconia/Alumina Composite

The sliding wear behavior of ultra high molecular weight polyethylene (UHMWPE) was examined on a novel low temperature degradation-free zirconia/alumina composite material and on the conventional ceramics (alumina and zirconia) used for a femoral head in total hip joint replacement. The wear of UHMWPE pins against these ceramic disks was evaluated by performing linear reciprocal sliding and repeat pass rotational sliding tests for one million cycles in a bovine serum. The weight loss of polyethylene against the novel low temperature degradation-free zirconia/alumina composite disks was much less than conventional ceramics for all tests. The mean weight loss of the polyethylene pins was more in the linear reciprocal sliding test than in the repeat pass rotational sliding test for all kinds of disk materials. Neither the coherent transfer film nor the surface damage was observed on the surface of the novel zirconia/alumina composite disks during the test. In conclusion, the novel zirconia/alumina composite leads the least wear of polyethylene among the tested ceramics and demonstrates the potential as the alternative materials for femoral head in total hip joint replacement.