**4. Conclusions**

In this study, MRE samples with and without nanoparticles were successfully fabricated, and the overall properties, related to the magnetic, rheological and electrical resistance properties, were experimentally investigated. It has been shown from the micrographic analysis that the CIPs and nanosized Ni-Mg cobalt-ferrites are randomly dispersed in the MRE samples. More specific results achieved from this work are summarized as follows.

(1) The MRE with nanoparticles as a filler possesses an enhancement of 3% in the saturation of magnetization and decreased the remanence of the nanosized Ni-Mg cobalt-ferrite itself by 4000%.

(2) The storage modulus and loss factor of the MRE with the nanosized Ni-Mg cobalt-ferrites showed an enhancement of almost 66% as compared to the control MRE. The results revealed that the addition of 1.0 wt.% of nanosized Ni-Mg cobalt-ferrites as filler altered and enhanced the interaction of particles in the MRE, and thus led to an increase in the magnetic and rheological properties.

(3) The electrical resistance of the nanosized Ni-Mg cobalt-ferrites decreased with the increase in the applied weight. The MRE with a higher content of Mg in nanosized Ni-Mg cobalt-ferrites exhibited higher electrical resistance in the range of 1% to almost 400% in the <sup>o</sup>ff- and on-state conditions due to the easier movement of the particles as a result from the deformation that occurred in the MRE matrix when exposed to an external magnetic field.

It is interesting to note that this work demonstrates that the use of a small concentration (1.0 wt.%) of nanoparticles—which acted as a filler—is capable of altering the magnetic, rheological and electrical resistance behavior of the MRE. This capability directly indicates that MREs with appropriate nanoparticles can be used as actuators or sensors. For example, the stiffness of the flexible structure can be tuned by the MRE (actuator), and the strain (or force) of the flexible structures due to the external force can be measured using the electrical signal generated from the MRE (sensor). Future work will include the investigation of the optimum performance of the electrical resistance of larger Mg concentration and various weights of the nanosized Ni-Mg cobalt-ferrites for MRE samples. In addition, research in terms of the practical aspect of the proposed MREs with the addition of nanoparticles for actuator and/or sensor applications will be undertaken.

**Author Contributions:** Conceptualization, S.A.A.A., S.A.M., U.U., M.K.S., N.A.Y. and S.-B.C.; Data curation, S.-B.C. and R.M.R.; Formal analysis, S.A.A.A., U.U., R.M.R. and M.K.S.; Funding acquisition, S.A.M., U.U., N.A.Y. and N.A.N.; Investigation, S.A.A.A.; Methodology, S.A.A.A., S.A.M., M.K.S. and R.M.R.; Project administration, S.A.A.A. and S.A.M.; Resources, S.A.A.A. and N.A.N.; Software, S.A.A.A. and U.U.; Supervision, S.A.M.; Validation, S.A.A.A., S.A.M., U.U., M.K.S., S.-B.C. and R.M.R.; Visualization, S.-B.C.; Writing—original draft, S.A.A.A.; Writing—review and editing, S.A.M., U.U., M.K.S., N.A.Y., N.A.N. and S.-B.C.

**Funding:** This study was financially supported by the Universiti Teknologi Malaysia under Transdisciplinary Research Grant (Vot No: 06G77), and Professional Development Research University gran<sup>t</sup> (Vot No: 04E02). This work and the APC was also supported and funded by Universiti Teknologi Petronas under Yayasan Universiti Teknologi Petronas-Fundamental Research Grant (YUTP-FRG), Cost Center. 015LC0-135. The research is also partially funded by USAID through Sustainable Higher Education Research Alliances (SHERA) Program- Center for Collaborative (CCR) National Center for Sustainable Transportation Technology (NCSTT) with Contract No. IIE00000078ITB-1, as well as Universitas Sebelas Maret (UNS) through Hibah Mandatory 2019.

**Acknowledgments:** This study was financially supported by the Universiti Teknologi Malaysia under a Transdisciplinary Research Grant (Vot No: 06G77), and Professional Development Research University gran<sup>t</sup> (Vot No: 04E02). This work was also supported by Universiti Teknologi Petronas under Yayasan Universiti Teknologi Petronas-Fundamental Research Grant (YUTP-FRG), Cost Center. 015LC0-135. The authors also thank the Ministry of Higher Education, Research, and Technology, Indonesia for a mandatory research gran<sup>t</sup> through Hibah WCR (World Class Research) 2019–2020.

**Conflicts of Interest:** The authors declare no conflict of interest.
