**4. Conclusions**

A reverse sol–gel transition behavior of microgel dispersion has long been studied in the biomedical research field. In particular, a p(NiPAm)-based microgel dispersion was widely studied due to a moderate volume phase transition temperature around human body temperature. In this report, we showed the unique phase transition behavior of p(NiPAm)-based microgel dispersion is maintained in the presence of nanoparticle additives. When the p(NiPAm-co-HEMA) microgel was mixed with silica nanoparticles at a 1:1 weight ratio, it showed a stable phase transition behavior. Although some micro aggregation between microgels and silica nanoparticles was observed, a reversible global phase transition behavior was also observed. It is noted that a bulk gel strength was affected by the existence of additives as the jammed colloidal nanoparticle reinforced the microgel networks. The result implies that the microgel system can be potentially feasible for versatile applications that demand a complex colloidal system. For example, one can introduce the conducting colloids to microgel dispersion to provide self-healing properties to electronic materials. By incorporating plasmonic or fluorescent colloids with microgels, optical signals can be amplified or reduced because the sol–gel transition also drives the volume change of the system. Rheological property can also be tuned by gelation, which can be applied for the formulation of a slurry composed of a complex colloidal system. Therefore, we believe that the microgel–colloid composite system can be a strong candidate to be applied for designing a smart and functional colloidal system in the future.

**Supplementary Materials:** The following are available online at https://www.mdpi.com/1996-194 4/14/5/1212/s1, Figure S1: (a) UV-vis spectrum of p(NiPAm-co-HEMA) microgel dispersion with molar ratio between NiPAm and HEMA of 7:3 at various pH values. The increase of transmittance at acidic of pH 4 and alkalic of pH 10 is attributed to the sedimentation of microgels. (b,c) photograph images of p(NiPAm-co-HEMA) microgel dispersion at (b) pH 4, and (c) pH 10. Photos were taken after slightly shaking the sedimented microgels solution for the visualization.; Figure S2: Microscopic images of p(NiPAm-co-HEMA) microgel and silica nanoparticle composite. The composite was 1:5 in weight ratio. (a) Small aggregates at room temperature. (b) local aggregation as the temperature increases. (c) Global aggregation and gelation of the composite with 1:5 weight ratio.; Figure S3: Evolution of dynamic modulus of 1:5 weight ratio composite. Turnover of G(storage modulus) and G"(loss modulus) was observed 51.3 ◦C.; Video S1: Supporting Movie S1; Video S2: Supporting Movie S2.

**Author Contributions:** Conceptualization, J.S.K. and T.S.S.; Investigation, J.S.K.; Methodology, B.S.H.; Project administration, T.S.S.; Supervision, J.M.K. and T.S.S.; Writing—original draft, B.S.H. and T.S.S.; Writing—review & editing, J.S.K., J.M.K. and T.S.S. All authors have read and agreed to the published version of the manuscript.

**Funding:** This study was supported by the Ajou University research fund, gran<sup>t</sup> number [S-2019- G0001-00498].

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** All data in this study are available from the corresponding author upon reasonable request.

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