Key Engineering Materials Vol. 977

Abstract: In this study, the major objective was to investigate the mechanical and electrical properties, and strain sensibility of the carbon nanotube (CNT) filled epoxidized natural rubber (ENR) nanocomposite. The second filler, cellulose nanofibers (CNFs), at various proportions was incorporated into the CNT-filled ENR nanocomposites. The preparation of ENR nanocomposite was carried out using a latex mixing process. The CNT:CNF hybrid filler was pre-dispersed in deionized water before being added to the ENR latex. The ratios of CNTs to CNFs varied from 1:0 to 1:0.05, 1:0.5, 1:1, 1:1.25, and 1:1.5. Although the presence of CNFs enhanced the stiffness of the substance, its negative effect on the tensile strength was noted. From the evaluated electrical properties, the outcomes demonstrated that the presence of CNFs with suitable proportions can have a positive effect on the performance of the substance when used as a stain-sensitive substance. The electrical conductivity of the hybrid ENR nanocomposite initially increased with the increase of CNF proportion up to 0.5. Beyond this proportion, the conductivity declined gradually. Moreover, the CNT:CNF_1:0.5 filled ENR nanocomposite had the highest recoverable piezoresistive property. From this finding, it can be inferred that the CNT:CNF_1:0.5 filled ENR nanocomposite is suitable to be used as a strain sensor device.

Abstract: Transition metal dichalcogenides (TMDCs) nanomaterials, in particular Molybdenum disulfide (MoS₂), have been employed frequently as a basis for flexible gas sensors due to their extreme sensitivity to gas molecules, super mechanical and electrical properties, and large surface area. This work aims to study the behavior of the flexible gas sensor made of 2D-MoS₂ under exposure to nitrogen dioxide (NO₂) gas at the part per million (ppm) level. The mono-layered MoS₂ was successfully synthesized by Chemical Vapor Deposition (CVD). The formation of MoS₂ layers was confirmed by Raman spectroscopy and Photoluminescence (PL). Two different gas-sensing devices were fabricated by transferring two MoS₂ samples (obtained from two positions inside the CVD tube) onto paper substrates. Specifically, upstream sample S_up was obtained from an area near the MoO₃ source, and downstream sample S_down was obtained from an area far from the MoO₃ source. Both sensors showed a good response to a concentration as low as (1.5 ppm) of NO₂. Although a high response of 62.8% along with a fast response of 9 sec were recorded by S_down, the sensor showed a slow recovery time of 42 sec. On the other hand, S_up showed good stability with an appropriate response of 36.8% along with a reasonable response time and recovery times of 20 and 27 sec, respectively. Such behavior could be accredited to the difference in the reactivity in both MoS₂ samples. This work opens the way for further improvements in manufacturing MoS₂-based flexible gas sensors.

Abstract: Composites of natural fiber-reinforced thermoplastic and thermoset polymers have been studied for developing prosthetic socket materials. This study investigated the abaca fiber (AF)/carbon fiber (CF)/epoxy (EP) hybrid composite properties: i.e., tensile, flexural, impact, thermal, and water absorption, by varying AF and CF ratios of 1: 0, 0: 1, 2: 1, 3: 1, and 4: 1 with 80 vol% epoxy resin. The cracks formed in bending test specimens were characterized with an optical microscope, whereas the tensile fracture surface was characterized by scanning electron microscopy (SEM). The results confirmed that the mechanical properties of the CF/EP composite are the highest. The higher the AF/CF ratio, the lower the hybrid composite's mechanical properties and the higher the water absorption. The hybrid composite with a 2:1 AF/CF ratio achieved the highest tensile and flexural strengths of 70 MPa and 103 MPa, respectively, and the lowest water absorption of 7.89%. Based on the experimental results, a simulation of the prosthetic socket was performed using Autodesk Inventor 2019 integrated with ANSYS Workbench 2019 R1, resulting in von Mises stress of 2.14 MPa and deformation of 0.015 mm. Besides, its thermal gravimetric analysis (TGA) resulted in good thermal stability.

Abstract: Scaffold Carbonated Hydroxyapatite/Honeycomb/Polyethylene Oxide (CHA/HCB/PEO) has been obtained by freeze-drying. The bioceramic CHA used in this study was synthesized from oyster shells using precipitation. HCB and PEO were added as reinforcement materials that affect the crystallographic properties of the scaffold. This study aimed to determine the characteristics of the scaffolds for bone tissue engineering. CHA and scaffolds were characterized using Fourier Transform Infrared Spectroscopy (FTIR), X-Ray Diffractometer (XRD), and Scanning Electron Microscopy (SEM). FTIR spectra and XRD graphs confirmed that the CHA produced was B-type. FTIR spectra of the scaffold showed the presence of HCB and PEO in the scaffold, which means they were homogeneously bound in the scaffold solution. XRD test results show that scaffolds' crystallinity and crystallite size tends to decrease compared to CHA. This was good because they could make cells easier to proliferate. A small-scale pore structure (micropore) was also formed in the scaffold. The porosity and pore size of the scaffold were affected by the concentration of CHA. The presence of the micropores can increase the permeability of the scaffold and facilitate cell migration. Thus, the composition of CHA/HCB/PEO scaffolds can be a good candidate material in bone tissue engineering.

Abstract: Hydroxyapatite Carbonate (CHA) is a material that is found to have a composition more similar to bone, with a higher bioactivity than Hydroxyapatite (HA). CHA was synthesized using precipitation and hydrothermal methods using (NH₄)₂HPO₄ as a phosphate source, NH₄HCO₃ as a carbonate source, and Pokea shells as a calcium source. In this study, the Pokea shells were crushed, calcined, and characterized based on physicochemical tests. CaO from Pokea shell contains 74.33% calcium. CHA was successfully produced by precipitation method at room temperature and hydrothermal at 120 C for 8 h. Sample characterization was carried out using X-Ray Diffraction (XRD), Fourier Transform Infrared (FTIR), and Scanning Electron Microscope Energy Dispersive X-Ray Spectroscopy (SEM-EDX). Based on XRD data, there are differences in the crystal size of CHA produced via precipitation and hydrothermal methods, where the crystal sizes of Precipitation CHA-1 and Hydrothermal CHA-2 are 6.388 nm and 25.969 nm. The FTIR results of both CHA show the functional groups typical of CHA, namely OH-, CO, CaO, PO₄^3-, and CO₃^2-. From the Ca/P EDX data results, Precipitation CHA-1 and Hydrothermal CHA-2 do not differ much, namely 1.71 and 1.69, and this value indicates that CHA has been formed.

Abstract: Micro EDM is unconventional metal removing technique that is effective in machining hard-to-cut conductive materials. It has a big potential in modifying surfaces of metallic bone implants for better biocompatibility by providing proper surface topography to ease cell adhesion. However, it is still important to study machining performance. This paper investigates material removal rate (MRR) and surface roughness (SR) of micro WEDM on Ti-6Al-4V alloy. Three level Taguchi’s design was implemented to observe the effect of capacitance and gap voltage. Moreover, analysis of variance (ANOVA) and grey relation analysis (GRA) allowed to investigate contribution of each parameter and find their best combination for multiple output optimization. Results showed that highest MRR of 1.72*10^-2 mm³/s can be achieved at 10 nF and 90 V values, while smallest SR of 0.309 µm can be achieved at 1nF and 90 V. In addition, the contribution and significance of capacitance on MRR and SR was considerably higher than the effect of gap voltage. Lastly, the optimal parameters for multiple output performance were calculated at 10 nF and 90 V values.

Abstract: Irreversible hydrocolloid impression material is extensively used in dentistry to record and duplicate the details of tooth structure, surrounding tissue, and other intraoral structures. The materials allow dentists to work more easily and perform better diagnosis and treatment planning. When used, the material is exposed to oral microorganisms, some are pathogens, from the patient’s teeth, mucosa, blood, and saliva. These pathogens can be absorbed into the material and transmitted to dentists and other dental workers. Hydrocolloid impression material incorporated with vanillin has been shown to exhibit antimicrobial potential, however, its physical properties have not been performed. Therefore, this study aims to investigate the physical properties of a vanillin-incorporated irreversible hydrocolloid impression material on four different physical properties: working time, setting time, elastic recovery, and strain-in compression. The impression powder was mixed with varying concentrations of vanillin (0.1%, 0.5%, and 1.0% w/w) using the electric vacuum mixer. The impression material without vanillin was used as a control. All the tests were done following the ISO specification 21563: 2021 for dental materials with 15 specimens for each test. The result showed that at concentrations of 0%, 0.1%, and 0.5% w/w vanillin, no significant change in the four physical properties has been observed. However, the working time and setting time values of impression material with 1% vanillin were reduced significantly and did not meet the ISO standard. In conclusion, the incorporation of 0.1%, 0.5%, and 1% w/w vanillin into irreversible hydrocolloid impression material showed a significant reduction in setting time and working time. However, when considering the ISO standard specifications, the 0.1% and 0.5% w/w vanillin-added materials can still be used due to their acceptable values of all physical properties (setting time, working time, elastic recovery, and strain-in compression). These impression materials will be valuable for use in clinical settings to reduce the disinfection procedure and the risk of cross-contamination.

Abstract: Dental caries, or tooth decay, is one of the most widespread chronic and multifactorial diseases affecting people worldwide. It is described as the localized destruction of tooth structure by acids produced from bacteria fermentation of edible carbohydrates. The disease process begins within the bacterial biofilm on the surfaces of the teeth. Streptococcus sanguinis, Sreptococcus gordonii, and Streptococcus mutans are the main organisms in the biofilm associated with health or disease conditions in the oral cavity. Streptococcus mutans is believed to be the primary cause of enamel demineralization and the development of dental caries. S. gordonii and S. sanguinis belong to a group of streptococci considered health-related commensal oral bacteria, crucial for forming a biofilm on oral hard tissues. Several types of materials with multipurpose characteristics have been incorporated into dental adhesives. Recently, the calcium salt of an acidic monomer (calcium salt of 4-methacryloxyethyl trimellitic acid, or CMET) has been included in dental adhesive and demonstrated to suppress cariogenic biofilm formation of S. mutans. However, using a single-species model may not mimic the intricate antagonistic and synergistic relations that occur in oral biofilms. Therefore, the effect on cariogenic multi-species biofilm was assessed in this study. Dental adhesive consisting of CMET and 10-methacryloyloxydecyl dihydrogen calcium phosphate (MDCP) (Bio-Coat CA) was spread over the flat-bottom surface of the 96-well plate and LED light-cured. Then it was coated with sterile saliva at 37 °C for 45 min to form an acquired pellicle for microbial attachment. The multi-species bacterial suspension containing Streptococcus mutans ATCC 25715, Streptococcus sanguinis ATCC 10556, and Streptococcus gordonii ATCC 10558 was prepared and added to the saliva-coated well. Then, the plate was incubated at 37°C in a 5% CO2 atmosphere for 24 h, 48 h, and 72 h to support the biofilm formation. The number of vital bacteria in the biofilm was determined with the WST-8 Microbial Cell Counting Kit (Dojindo Molecular Technologies, USA). All tests were done in triplicate and repeated three times. For statistical analysis, Kruskal-Wallis and Dunn’s tests were employed. The results showed that at 24 h, 48 h, and 72 h, dental adhesive with CMET could inhibit the biofilm formation of multi-species bacteria significantly compared with controls. The percentages of biofilm inhibition were 29.1%, 34.7%, and 33.2% at 24 h, 48 h, and 72 h, respectively. Dental adhesive containing CMET displayed favorable multi-species biofilm-inhibiting effects up to 72 h of biofilm growth. It is a promising adhesive for use to prevent secondary caries at the sites of restorations.

Abstract: Controllable mechanical properties of highly performed Poly (L-lactide acid) (PLLA) monofilaments with oriented molecular structure could widen their applications, especially in biomedical field. Herein, different heat treatments were applied to regulate the degree of molecular relaxation of oriented PLLA monofilaments to tune their mechanical properties. These filaments were manufactured by melting spun and solid-state drawing processes. Then, they were processed by different heat treatments, including annealing, normalizing, and quenching. As the cooling time extension, an obvious molecular orientation loss in filaments happened and increased regularly, and it could reach up to about 35.1% maximumly. However, molecules only in crystal phase were limitedly affected. As a result, mechanical performances of these filaments exhibited a corresponding change after heat treatments. Young’s modulus and elongation at break were promoted after all kinds of post-processes and increased with longer cooling time gradually. But breaking strength showed a contrast change. It means that different heat treatments could be effective avenues to control mechanical properties of oriented PLLA materials by altering the orientation structure.

Abstract: Stent implantation is the mainstream treatment for high-incidence vascular diseases. The stent is implanted into the blocked vessel with minimal trauma to restore blood flow. Polymer braided stents with superior biocompatibility and flexibility have broad application prospects in stent implantation. An ideal polymer stent should have suitable radial supporting capacity to withstand the cyclic radial load from vessels. Especially in the case of accelerated vasoconstriction caused by emotional excitement, drinking or fever, etc. However, there are currently limited studies on the mechanical properties of stent at rapid radial loading. In this work, the radial supporting capacity and fatigue properties of polymer stent at rapid loading rate were investigated by experiment and simulation. With the increase of radial loading rate, the stent has different deformation tendency and fatigue resistance. This study is helpful to study the structural changes of the polymer braided stent at different loading rates, and provide ideas for the evaluation and the optimization of the polymer braided stent.