Influences of Extrusion and Silver Content on the Degradation of Mg-Ag Alloys In Vitro and In Vivo.

Binary magnesium-silver (Mg-Ag) alloys were designed as antibacterial materials for biomedical implant applications. In the present study, we focused on the effects of extrusion (extrusion ratio (ER): 1, 7.1, and 72.2) and Ag content (Ag = 0, 3, and 6 wt.%) on the degradation of Mg-Ag alloys in vitro and in vivo via microstructure characterization and corrosion/degradation measurements. The results showed that the Ag promoted a galvanic reaction with the Mg matrix to accelerate degradation or formed a protective oxide mesh texture to inhibit degradation, especially in vivo. Ag might also be beneficial for product crystallization, biomineralization, and organic matter deposition. For pure Mg, extrusion produced a more refined grain and decreased the degradation rate. For the Mg-Ag alloys, a low extrusion ratio (7.1) accelerated the degradation caused by the increase in the proportion of the precipitate. This promoted the release of Mg2+ and Ag+, which led to more deposition of organic matter and calcium phosphate, but also more H2 bubbles, which led to disturbance of product deposition in some local positions or even inflammatory reactions. Extrusion at a higher ratio (72.2) dissolved the precipitates. This resulted in moderate degradation rates and less gas production, which promoted osteogenesis without an obvious inflammation reaction.

Redox/pH-Responsive Biodegradable Thiol-Hyaluronic Acid/Chitosan Charge-Reversal Nanocarriers for Triggered Drug Release.

Biodegradable nanoparticles and micelles are promising nanosystems for the targeted delivery of potent anticancer drugs. By using specialized polymers as nanocarriers, targeted drug delivery and release can be developed. We developed thiol-hyaluronic acid (HA-SH)/chitosan (CS) nanoparticles with redox/pH dual-responsiveness via electrostatic self-assembly followed by spontaneous chemical cross-linking. The nanoparticle surface charges were reversible through different HA-SH and CS mass ratios. Doxorubicin (DOX) was used as a model drug. Dual cross-linked nanoparticles with diameters of approximately 300 nm exhibited superior stability under physiological conditions compared with nanoparticles without disulfide cross-linking. DOX was loaded more efficiently into negative nanoparticles (45.7 wt%) than positive nanoparticles (14.2 wt%). Drug release from negative nanoparticles (ζ potential of approximately -20) was higher (87.8 wt%) at pH 4.5 and in the presence of 10 mM glutathione. Positive nanoparticles (ζ potential of approximately +20) showed the same trend, but the release rate was slower than that of negative nanoparticles. DOX-loaded HA-SH/CS particles were taken up by human breast cancer cells (SKBR3), and the loaded drug was released, exhibiting potential antitumor efficacy. The HA-SH/CS nanoparticles in this study were stable under physiological conditions and are promising candidates for the targeted delivery and release of anticancer drugs.

Development of porous chitosan/tripolyphosphate scaffolds with tunable uncross-linking primary amine content for bone tissue engineering.

The primary amine along the chitosan backbone plays a key role in biomedical applications. Although chitosan-based porous scaffolds have been widely used in tissue engineering, it remains very challenging to regulate uncross-linking primary amine content (CN) in scaffolds in order to realize particular mechanical and biological properties. In the present study, chitosan/tripolyphosphate (TPP) scaffolds with controlled CN (i.e., degree of cross-linking) were prepared based on the ionic-dependent solubility of chitosan together with the freezing process. The effects of the concentration of TPP (CTPP) and NaCl (CNaCl) in the cross-linking solution on CN were studied by infrared spectroscopy, ninhydrin assays and elemental analysis. The results showed that CN decreased with increasing CTPP and decreasing CNaCl. CN affected physicomechanical properties such as swelling behavior and the mechanical strength of the chitosan/TPP scaffolds. The uncross-linking primary amine in scaffolds can be used for chemical and biological modifications. The protein loading of the scaffolds demonstrated that the pH-responsive adsorption and release behavior was influenced by CN. Cell experiments also illustrated that CN affected the proliferation of bone marrow mesenchymal stem cells (BM-MSCs). All of these results indicate that these porous chitosan/TPP scaffolds containing uncross-linking primary amines are potentially useful for applications in regenerative bone medicine.

Design and fabrication of porous chitosan scaffolds with tunable structures and mechanical properties.

Chitosan-based porous scaffolds are of great interest in biomedical applications because of their biodegradability and biocompatibility. However, the poor mechanical properties of these scaffolds hinder their broad utility. In the present study, a novel compression method was developed to fabricate chitosan scaffolds with high mechanical strength and tuneable topography, based on the ionic strength and pH-dependent solubility of chitosan. When the compressive ratio increases from 1 to 8, the compressive elastic modulus of the scaffold increases from 5.2kPa to 520kPa and the porosity decreases from 94.1% to 82.5%. Furthermore, the number of human adipose-derived stem cells adhering to the scaffolds increases as the compressive ratio increases, owing to the high density of the chitosan fibres. This method does not require external cross-linker agent, sophisticated instrumentation and/or technical proficiency and could be extended to other polysaccharides.

Design and fabrication of a chitosan hydrogel with gradient structures via a step-by-step cross-linking process.

The development of scaffolds to mimic the gradient structure of natural tissue is an important consideration for effective tissue engineering. In the present study, a physical cross-linking chitosan hydrogel with gradient structures was fabricated via a step-by-step cross-linking process using sodium tripolyphosphate and sodium hydroxide as sequential cross-linkers. Chitosan hydrogels with different structures (single, double, and triple layers) were prepared by modifying the gelling process. The properties of the hydrogels were further adjusted by varying the gelling conditions, such as gelling time, pH, and composition of the crosslinking solution. Slight cytotoxicity was showed in MTT assay for hydrogels with uncross-linking chitosan solution and non-cytotoxicity was showed for other hydrogels. The results suggest that step-by-step cross-linking represents a practicable method to fabricate scaffolds with gradient structures.

Dynamic fatigue performance of implant-abutment assemblies with different tightening torque values.

Implant-abutment assemblies are usually subject to long-term cyclic loading. To evaluate the dynamic fatigue performance of implant-abutment assemblies with different tightening torque values, thirty implant-abutment assemblies (Zimmer Dental, Carlsbad, CA, USA) were randomly assigned to three tightening groups (24 Ncm; 30 Ncm; 36 Ncm), each consisted of 10 implants. Five specimens from each group were unscrewed, and their reverse torque values recorded. The remaining specimens were subjected to a load between 30 N~300 N at a loading frequency of 15 Hz for 5 × 10(6) cycles. After fatigue tests, residual reverse torque values were recorded if available. In the 24 Ncm tightening group, all the implants fractured at the first outer thread of the implant after fatigue loading, with fatigue crack propagation at the fractured surface showed by SEM observation. For the 30 Ncm and 36 Ncm tightening groups, a statistical significant difference (p<0.05) between the unloaded and loaded groups was revealed. Compared with the unloaded specimens, the specimens went through fatigue loading had decreased reverse torque values. It was demonstrated that insufficient torque will lead to poor fatigue performance of dental implant-abutment assemblies and abutment screws should be tightened to the torque recommended by the manufacturer. It was also concluded that fatigue loading would lead to preload loss.

[Effect of Polident denture cleansers on the properties of heat-polymerized denture base acrylic resin].

OBJECTIVE: To study the change in properties of heat-polymerized acrylic resin after using one denture cleanser. METHODS: In the study, 20 disk-shaped (50.0 mm×0.5 mm) and 40 rectangular (34.0 mm×13.5 mm×1.3 mm) specimens prepared from heat-polymerized acrylic resin were randomly divided into four groups. The specimens were exposed to one of the three treatments as follows: Group 1 was without any treatment, Group 2 was exposed to air, Group 3 to distilled water, and Group 4 to Polident. The 4 exposures lasting 8 hours were conducted daily and repeated for 30 days. The color stability of heat-polymerized acrylic resin was determined by visual methods. The flexural strength and bonding strength of acrylic resin were measured using a universal testing machine. All the results were analyzed using ANOVA. RESULTS: The flexural strength test of acrylic resin demonstrated significant differences between Group 4 and Group 1 (P<0.05), there were no significant differences among Group 1, Group 2 and Group 3. No significant differences were found in other properties of the test materials. CONCLUSION: Long-term use of polident could alter the physical and mechnical properties of heat-polymerized acrylic resin. It may be related to the accelerating aging of resins caused by certain chemicals in denture cleansers. Polident may have some adverse effects on denture materials for decreasing flexural strength of heat-polymerized acrylic resin after 30-day immersion.