# Magnetic-Responsive Microparticles that Switch Shape at 37 \({}^{\circ}\)[PERSON]
## 4 Discussion  
We have been demonstrating the potential of SMPs in the area of tissue engineering, mechanobiology, and micro-electro-mechanical systems in the past decade [26; 27]. We have recently reported shape-memory surfaces fabricated from PCL films with on-demand, tunable nano-grooves [28; 29; 30; 31]. The key technology behind these studies is a rational method to control the shape switching temperature because PCL, for example, has a \(T_{\text{m}}\) around 60 which is very high for biological experiment. To achieve this purpose, we proposed a rational approach to modulate the \(T_{\text{m}}\) by mixing linear telechelic and four-branched PCLs in 2006 [15]. Other researchers also demonstrated successfully decreasing the \(T_{\text{m}}\) of PCL by a similar strategy based on regulation of molecular weight [20; 32]. Despite the advancements of various SMPs; however, fabrication of shape-memory particles still remains scarce. In the current study, we demonstrated that PCL-based shape-memory microparticles exhibited shape recovery from a disk-like temporal shape to spherical shape at 37 \({}^{\circ}\)C. Incorporation of MNPs enabled the remote and reversible control of capture/release of the particles. It is noteworthy that incorporation of MNPs into the PCL microparticles did not affect the shape-memory ability, while incorporation of some impurities can affect polymer crystallinity in principle. We used chemical crosslinking (covalent bond) instead of physical crosslinking to introduce netpoints into PCL networks in this study, because chemically crosslinked polymer networks should provide a higher content of switching phase and high form stability. Those, in turn, allowed the system  
Figure 4: **(a)** Top view SEM image of shape-memory PCL microparticles containing MNPs after size separation by sedimentation technique; **(b)** Photographs of dispersion of purified shape-memory PCL microparticles containing MNPs before and after applying an external magnetic field; **(c)** Time course of magnetophoretic mobility for purified shape-memory PCL microparticles containing MNPs with or without external magnetic field. The measurement was conducted in distilled water; **(d)** Repeatability of magnetophoretic mobility and dispersion of shape-memory PCL microparticles containing MNPs. Stirring (blue) and applying with externa magnetic field (red) were repeated alternately every 60 s for 15 min. **(c,d)** Magnetophoretic mobility was estimated by measuring the optical transmittance at 500 nm. to have sharp responsiveness and better shape memory performance [22]. Furthermore, they should also prevent erosion of loaded MNPs. [PERSON] and [PERSON] have reported that 100% of shape recovery for chemically crosslinked shape-memory particles occurred within temperature change between 40 and 43 \({}^{\circ}\)C [22], while the shape recovery rate for physically crosslinked ones was still 90% at 90 \({}^{\circ}\)C [23]. In our previous study, crosslinked samples showed approximately 99% and 90% of the shape fixity and shape recovery rate, respectively [29]. When the magnetic field is applied, MNP-loaded particles were moved toward the magnet. The capture and release behavior reversibly occurred in response to an external magnetic field. This result indicates that magnetophoretic mobility can be controlled with the particle shape because the size and morphology are known to affect the diffusivity of MNPs [33]. [PERSON] et al. reported that the diffusivity of individual MNPs decreases as the particle size becomes larger. In contrast, the magnetophoretic velocity of individual MNPs increases significantly as the particle size becomes larger [34]. Therefore, we can control the magnetic-responsive behavior of the particles with their shape. Dynamic manipulation of particle properties has been attracting attention as it has great potential to precisely manipulate particle interactions with cells.