# Thermoresponsive Behavior of Magnetic Nanoparticle Complexed pNIPAm-_co_-AAc Microgels
## 1 Introduction  
Stimuli-responsive hydrogels, that undergo the volume phase transition in response to external stimuli, have great value in the development of biocompatible smart materials for use in biosensors [1; 2; 3; 4; 5], artificial tissues [6], and many other utilities [2; 4; 7]. The responsivity can be carefully designed to cause the swelling/deswelling of the hydrogels and most common cases are temperature and pH responsive behaviors [8; 9; 10; 11]. To achieve such responsivity, poly(_N_-isopropylacrylamide) (pNIPAm) and acrylic acid (AAc) are widely used due to the lower critical solution temperature (LCST) and the pK\({}_{\text{a}}\), respectively [12; 13]. Above the LCST, the polymer network is favorable to be collapsed by chain-chain interactions, while above the pK\({}_{\text{a}}\) the network tends to swell due to Coulombic repulsion among the deprotonated acid groups [14; 15; 16]. Colloidal hydrogel particles (microgels) are commonly prepared by aqueous free-radical precipitation polymerization due to the low polydispersity andaccurate size control [13; 17]. These microgels have been extensively investigated for their fast response rate and nanoscale structure in comparison to those of bulk hydrogels [18; 19]. The development of multi-responsive hydrogels often requires different kinds of monomers for copolymerization, and even post-modifications by conjugating interesting molecules to the hydrogels [20; 21]. Therefore, efficient methods for the formation of such advanced hydrogels are highly desired.  
Nanoparticles including gold nanoparticles (AuNPs) and magnetic nanoparticles (MNPs) have been widely applied in the development of functional nanosystems for optical modulators [5; 22], biosensors [23; 24], imaging agents [25], and biological interfacing technology [26]. This suggests that complexation of the nanoparticles with responsive hydrogels allows the development of multipotent materials in a wide range of applications [27; 28; 29; 30; 31; 32]. For instance, AuNPs and MNPs are coupled with hydrogels via DNA hybridization, which shows photothermal and magnetic-field responsive activities, respectively [27; 28]. In addition, copolymerization in which AuNPs and MNPs are used as a seed material has been used to yield hybrid nanoparticle-hydrogels in core-shell structure. Although this method allows for many interesting applications of the nanoparticle-hydrogels, it also requires a careful optimization for synthetic procedures [29; 30; 31; 32]. Thus, there is a need for simple, fast, yet useful hybridization methods for the nanoparticle-hydrogel complexation, while retaining the hydrogel's responsivity. In this study, we aim to demonstrate that the pNIPAm-_co_-AAc microgels are aptly complexed with MNP, and the hybridized MNP-microgels show the characteristic thermoresponsivity in a repeatable manner. Our finding is distinguished from previous studies that utilize copolymerization of MNP-hydrogels by its simplicity and rapidity, allowing responsive pNIPAm-_co_-AAc microgels to acquire additional merits of magnetic nanoparticles for magneto-induced applications.