Double shot peening is the development of shot peening by shooting large media as a first shot and re-shooting again with smaller media as a second shot in order to achieve high residual compressive stress and hardness at the surface, while the in-depth effect can still be maintained. This research aims to examine the effect of media type and media size when used in the second shot of double shot peening on hardness, roughness, and residual stress to identify the suitable conditions and compare them with single shot peening, such as conventional shot peening and fine shot peening, which was used as the first shot and second shot. Ti–6Al–4V was used as the substrate material, while various diameter sizes of silica and SUS304 media were selected as the media for the second shot in the process. The results showed that in the case of the larger size of silica media in the second shot of double shot peening, the hardness and residual compressive stress on the surface clearly increased more than with the smaller media due to the higher Almen intensity, which affected impact energy. On the other hand, when shooting with SUS304 media as a second shot, the increment of residual compressive stress and hardness, including roughness reduction on the surface, showed less effect than was the case for silica media, due to the lower Almen intensity, which affected the impact energy transfer. This research found that the condition of shooting with 80 μm of silica media as the second shot could generate the highest hardness and residual compressive stress on the surface, which increased by 14% and 53%, respectively, while roughness was decreased by 20% when compared with single shot peening.

The residual stress and hardness generated by the nitriding process are important parameters for increasing the bending fatigue strength to Ti-6Al-4V. Therefore, this research is focused on the analysis of residual stress and hardness, including surface morphology and microstructure generated by the nitriding process at different times and temperatures. The plasma nitriding at temperatures of 750 °C and 800 °C with times of 5 h and 10 h were selected in this research. After plasma nitriding, the material would have residual compressive stress and higher hardness, including changes in the surface morphology and microstructure. The results also indicated that higher temperature and processing times generated more surface roughness and thickness in the compound layer, resulting in higher surface hardness. Moreover, higher time and temperature could generate deeper residual compressive stress and case depth hardness. This research revealed maximum hardness in the cross-sectional analysis of 643 HV and residual compressive stress of −65.3 MPa. In conclusion, the depth of the residual stress and case depth hardness were well compatible with the depth of the diffusion layer of plasma-nitrided Ti-6Al-4V, which confirmed the effect of plasma nitriding.

Objectives: To address the high surface roughness and poor optical properties of three-dimensional (3D) printed orthodontic clear retainers, an alternative post-processing protocol was investigated with the goal of achieving improved surface, optical, and mechanical properties while preserving dimensional accuracy. Materials and methods: Samples were prepared from two biocompatible methacrylate-based 3D-printing resins (Formlabs Dental LT Clear V2, NextDent OrthoFlex) and one thermoplastic material (Duran). For the 3D-printed resins, one group was post-processed by rinsing in isopropyl alcohol, while another group was centrifuged before post-curing in glycerine. Three different testing conditions were used: dry, wet (24-h water immersion), and aged (thermocycling for 10,000 cycles). Surface characteristics were evaluated qualitatively and quantitatively. Optical properties were assessed for transparency and colour stability, while mechanical properties were elicited from tensile and microhardness tests. Water sorption and solubility were calculated. Samples mounted on a dental model were scanned by micro-computed tomography to measure thickness and gap width. Results: 3D-printed samples post-processed by centrifugation showed significantly decreased surface roughness and improved visible light transmission, colour stability, tensile strength, and hardness. The centrifuged samples showed significantly increased thickness, while designing an offset equal to this thickness improved the adaptation. Conclusions: Post-processing by centrifugation produces surface coating that enhances the surface and optical properties of the 3D-printed orthodontic retainers, while curing in an oxygen-free environment improves their mechanical properties. Design modifications may be necessary for this protocol to ensure proper adaptation to the dentition. Clinical relevance: Proper design and post-processing protocols are necessary to achieve the desired properties of orthodontic clear retainers.

Nickel-Titanium (NiTi) springs have been increasingly used in orthodontics; however, no optimum condition of heat treatment has been reported. Therefore, this research was conducted to determine the optimum heat-treatment temperature and duration for the fabrication of NiTi-closed coil springs by investigating their effects on thermo-mechanical properties. As-drawn straight NiTi wires of 0.2 mm diameter were used to fabricate closed coil springs of 0.9 mm lumen diameter. The springs were heat-treated at three different temperatures (400, 450, and 500 °C) with three different durations (20, 40, and 60 min). Electron Probe Micro-Analysis (EPMA) and Differential Scanning Calorimetry (DSC) were used to investigate element composition and thermo-mechanical properties, respectively. Custom-made NiTi closed coil springs composed of 49.41%-Ti and 50.57%-Ni by atomic weight, where their DSC curves of 500 °C presented the obvious endothermic and exothermic peaks, and the austenite finish temperature (A_f) were approximately 25 °C. With increasing temperature, deactivation curves presented decreased plateau slopes generating higher superelastic ratios (SE ratios). At 500 °C, closed coil springs showed superelastic tendency with lower stress hysteresis. The thermo-mechanical properties were significantly influenced by heat-treatment temperature rather than duration. The optimum parameter appeared to be 500 °C for 40 min to produce appropriate force delivery levels, relatively low plateau slope, and lower hysteresis for orthodontic use.

Oligomeric amyloid-β 1–42 (Aβ_1–42) has a close correlation with neurodegenerative disorder especially Alzheimer’s disease (AD). It induces oxidative stress and mitochondrial damage in neurons. Therefore, it is used to generate AD-likein vitromodel for studying neurotoxicity and neuroprotection against amyloid-β. A low-level light therapy (LLLT) is a non-invasive method that has been used to treat several neurodegenerative disorders. In this study, the red wavelength (660nm) and near infrared wavelength (810nm) at energy densities of 1, 3, and 5 J/cm²were used to modulate biochemical processes in the neural cells. The exposure of Aβ_1–42resulted in cell death, increased intracellular reactive oxygen species (ROS), and retracted neurite outgrowth. We showed that both of LLLT wavelengths could protect neurons form Aβ_1-42-induced neurotoxicity in a biphasic manner. The treatment of LLLT at 3 J/cm²potentially alleviated cell death and recovered neurite outgrowth. In addition, the treatment of LLLT following Aβ_1–42exposure could attenuate the intracellular ROS generation and Ca²⁺influx. Interestingly, both wavelengths could induce minimal level of ROS generation. However, they did not affect cell viability. In addition, LLLT also stimulated Ca²⁺influx, but not altered mitochondrial membrane potential. This finding indicated LLLT may protect neurons through the stimulation of secondary signaling messengers such as ROS and Ca²⁺. The increase of these secondary messengers was in a functional level and did not harmful to the cells. These results suggested the use of LLLT as a tool to modulate the neuronal toxicity following Aβ_1–42accumulation in AD’s brain.