A new type of vibration isolator based on magnetorheological elastomer

In this work, a new type of adaptive vibration isolator based on magnetorheological (MR) elastomer (MRE) is presented. A new method was adopted to develop such an isolator where both a magnetic field and a preload were applied simultaneously. The magnetic attraction force was utilized to change the preload in the single degree of freedom (DOF) system. The system has such a provision that when a magnetic field is applied the preload would be automatically acting to the MR elastomer. In such a combined loading condition, the natural frequency of a single DOF system promptly shifted to a higher frequency and the stiffness of the MR elastomer was significantly increased. The stiffness of the MR elastomer system was found to be increased as high as 730 times of its original stiffness when the magnetic field of 520 mT was applied, which is a significantly higher augmentation than those reported in the literature. The combined effect of the preload and the magnetic field was profound because the magnetic interaction among the magnetic particles was simultaneously boosted by both the magnetic field and the preloading effect. It is often a large difficulty to generate a higher magnetic field in most of the MRE-based isolators. Our study showed that when a suitable preload and a suitable magnetic field are applied together, a highly tunable isolator system can be developed even with the application of a relatively lower magnetic field strength.MOE (Min. of Education, S’pore

Dot-patterned hybrid magnetorheological elastomer developed by 3D printing

This article presents the development of dot-patterned magnetorheological (MR) elastomers (MREs) via 3D printing technology and their magnetorheological characterization. The 3D printed MR elastomer consists of three different materials; magnetic particles, magnetic particles carrier medium, and an elastomer. In such 3D printing, a controlled volume of MR fluid is encapsulated layer-by-layer within the elastomer matrix. The capability of 3D printing technology has been successfully demonstrated by developing the various dot patterns MR elastomers namely isotropic, anisotropic and configurations inspired from basic crystal structures such as BCC and FCC. The magneto-mechanical properties of such 3D printed MR elastomers (3DP-MREs) are studied using a cyclic compression and through a forced vibration testing. In the presence of a magnetic field, a clear change in stiffness of 3DP-MREs has been achieved. Moreover, the anisotropic behavior of 3DP-MREs has also been demonstrated. The experimental results suggested that the 3D printing method makes it possible to develop various structured MREs even without applying a magnetic field during the fabrication process.Ministry of Education (MOE)This work was supported by the Academic Research Funds (RG189/ 14) from the Ministry of Education, Singapore

Magnetic circuit analysis to obtain the magnetic permeability of magnetorheological elastomers

The magnetic permeability of magnetorheological elastomers must be known for their long-term use in the actual engineering systems. In this article, the magnetic permeability of both isotropic and anisotropic magnetorheological elastomers has been determined using a new method. The magnetic induction was measured and a closed magnetic circuit analysis was carried out to obtain the magnetic permeability of magnetorheological elastomers for both isotropic and anisotropic magnetorheological elastomers with 10%–50% volume concentration of carbonyl iron particles. The magnetic permeability was increased with increasing particle concentration for both isotropic and anisotropic magnetorheological elastomers as we could expect. The isotropic permeability is always lower than the anisotropic permeability. The maximum relative permeability value of 6.6 was obtained for 50% particle volume concentration. The experimental results also show a good agreement with theoretical predictions and previous investigations.MOE (Min. of Education, S’pore

Development of hybrid magnetorheological elastomers by 3D printing

Intelligent or smart materials have one or more properties that can be significantly changed in a controlled fashion by external stimuli, such as temperature, pH, electric or magnetic fields, etc. Magnetorheological (MR) materials are a class of smart materials whose properties can be varied by applying an external magnetic field. In this work, the possibility of employing a suitable 3D printing technology for the development of one of the smart MR materials, the magnetorheological elastomer (MRE) has been explored. In order to achieve such 3D printing, a multi-material printing is implemented, where a controlled volume of MR fluid is encapsulated within an elastomer matrix in the layer-by-layer fashion. The choice of printing materials determines the final structure of the 3D printed hybrid MR elastomer. Printing with a vulcanizing MR suspension produces the solid MR structure inside the elastomer matrix while printing with a non-vulcanizing MR suspension (MR fluid) results in the structures that the MR fluid is encapsulated inside the elastomer matrix. The 3D printability of different materials has been studied by measuring their rheological properties and we found that the highly shear thinning and thixotropic properties are important for 3D printability. The quality of the printed filaments strongly depends on the key printing parameters such as extrusion pressure, initial height and feed rate. The experimental results from the forced vibration testing show that the 3D printed MR elastomers could change their elastic and damping properties when exposed to the external magnetic field. Furthermore, the 3D printed MR elastomer also exhibits the anisotropic behavior when the direction of the magnetic field is changed with respect to the orientation of the printed filaments. This study has demonstrated that the 3D printing is viable for fabrication of hybrid MR elastomers with controlled structures of magnetic particles or MR fluids.MOE (Min. of Education, S’pore

A hybrid magnetorheological elastomer developed by encapsulation of magnetorheological fluid

A new hybrid MR elastomer was fabricated by encapsulating a magnetorheological fluid (MR fluid) within a UV-curable silicone elastomer. A strong magneto-deformation effect was observed where the hybrid MR elastomer changed its shape in the presence of a magnetic field. Furthermore, when a moderately strong magnetic field was applied, the elastic and damping properties of the hybrid MR elastomer changed obviously. The magnetic field strength, strain amplitude, strain rate, preload, and orientation of magnetic flux direction affected the behavior of the new hybrid MR elastomer. The hybrid MR elastomer also exhibited a higher MR effect when compared with conventional MR elastomer. The investigation also found that the combination of magnetic field strength and preload highly influenced the hybrid MR elastomer behavior. This MR fluid-encapsulated elastomer is expected to be a potential candidate for the tunable spring-damper element as well soft actuators.MOE (Min. of Education, S’pore

A 3D Printable and Mechanically Robust Hydrogel Based on Alginate and Graphene Oxide

Sodium alginate (SA) was used for the first time to noncovalently functionalize amino-graphene oxide (aGO) to produce the SA-functionalized GO, A-aGO. A-aGO was then filled into a double-network (DN) hydrogel consisting of an alginate network (SA) and a polyacrylamide (PAAm) network. Before UV curing, A-aGO was able to provide the SA/PAAm DN hydrogel with a remarkable thixotropic property, which is desirable for 3D printing. Thus, the A-aGO-filled DN hydrogel could be nicely used as an “ink” of a 3D printer to print complicated 3D structures with a high stackability and high shape fidelity. After UV curing, the 3D-printed A-aGO filled DN hydrogel showed robust mechanical strength and great toughness. For the function of A-aGO it was considered that A-aGO acted as a secondary but physical cross-linker, not only to give the hydrogel a satisfactory thixotropic property but also to increase the energy dissipation by combining the physical SA network and the chemical PAAm network. As an exciting result we successfully developed a 3D printable and mechanically robust hydrogel

3D/4D Printing of Polymers: Fused Deposition Modelling (FDM), Selective Laser Sintering (SLS), and Stereolithography (SLA)

Additive manufacturing (AM) or 3D printing is a digital manufacturing process and offers virtually limitless opportunities to develop structures/objects by tailoring material composition, processing conditions, and geometry technically at every point in an object. In this review, we present three different early adopted, however, widely used, polymer-based 3D printing processes; fused deposition modelling (FDM), selective laser sintering (SLS), and stereolithography (SLA) to create polymeric parts. The main aim of this review is to offer a comparative overview by correlating polymer material-process-properties for three different 3D printing techniques. Moreover, the advanced material-process requirements towards 4D printing via these print methods taking an example of magneto-active polymers is covered. Overall, this review highlights different aspects of these printing methods and serves as a guide to select a suitable print material and 3D print technique for the targeted polymeric material-based applications and also discusses the implementation practices towards 4D printing of polymer-based systems with a current state-of-the-art approach

3D printed silicone meniscus implants : influence of the 3D printing process on properties of silicone implants

Osteoarthritis of the knee with meniscal pathologies is a severe meniscal pathology suffered by the aging population worldwide. However, conventional meniscal substitutes are not 3D-printable and lack the customizability of 3D printed implants and are not mechanically robust enough for human implantation. Similarly, 3D printed hydrogel scaffolds suffer from drawbacks of being mechanically weak and as a result patients are unable to execute immediate post-surgical weight-bearing ambulation and rehabilitation. To solve this problem, we have developed a 3D silicone meniscus implant which is (1) cytocompatible, (2) resistant to cyclic loading and mechanically similar to native meniscus, and (3) directly 3D printable. The main focus of this study is to determine whether the purity, composition, structure, dimensions and mechanical properties of silicone implants are affected by the use of a custom-made in-house 3D-printer. We have used the phosphate buffer saline (PBS) absorption test, Fourier transform infrared (FTIR) spectroscopy, surface profilometry, thermo-gravimetric analysis (TGA), X-ray photoelectron spectroscopy (XPS), differential scanning calorimetry (DSC), and scanning electron microscopy (SEM) to effectively assess and compare material properties between molded and 3D printed silicone samples.National Research Foundation (NRF)Published versionThis research is supported by the National Research Foundation, Prime Minister’s Office, Singapore under its Medium-Sized Center Funding scheme and the NTU Start-Up Grant

Symptoms 6 months following SARS-CoV-2 infection in Nepali women.

In Nepal, over 1 million individuals have tested positive for SARS-CoV-2. We sought to describe the frequency of nonrecovery from this infection at 6 months and associated symptoms. We conducted a retrospective cohort study of 6142 women who had positive and negative PCR tests for this infection 6 months previously at 3 institutions in Kathmandu. In telephone interviews women provided information on 22 symptoms and their intensities, health status and history, and functional status. Of 3732 women who had tested PCR positive, 630 (16.9%) reported that they were unrecovered. These 630 unrecovered women were distinguished statistically from the 3102 recovered women by more frequent histories of allergies, rheumatoid disease, BCG immunization, Covid vaccination, strep throat and recent URIs, and both weight gain and weight losses of more than 5 kg in the 6 months following testing, and stressful events in the preceding year. Fatigue, pain, difficulty remembering, shortness of breath, heat and cold intolerance and unrefreshing sleep were reported in 41.9% to 10.5% of these 630 unrecovered women. Six months after confirmed SARS-CoV-2 infection 16.9% of Nepali women have long-COVID manifested as an immune, metabolic, and hormonal systems disruptive and dysfunction syndrome