1.1. MSMPs Filled with Fe3O4
Schmidt
[1] tried to make remotely controllable shape memory polymers. She made a shape memory polymer with incorporated magnetic particles (Fe
3O
4). These magnetic particles are used to heat the shape memory polymer by induction heating the particles by an AC field. The Massart and Cabuil method was employed for the experimental part
[2]. Magnetite nanoparticles between 2 and 12 wt% were used for magnetic heating. She succeeded in remotely activating the shape memory effect with the magnetic particles (see
Figure 1), while also maintaining basic thermal and mechanical properties of the polymer matrix.
Figure 1. Deformed magnetic shape memory polymer, induction heated magnetic shape memory polymer in the process of recovering its memorized shape, recovered memorized shape
[1].
Mohr, et al.
[3] also performed research to remotely activate the shape memory effect of a shape memory polymer. Shape changing from a temporary shape (corkscrew like spiral) to a permanent shape (plane stripe) happened within 22 s (see
Figure 2). In this study, they made two types of MSMPs. One had a base of polyetheruerthane (TFX) and the other a biodegradable multi block copolymer (PDC). For the magnetic micro particles, Fe
3O
4 was used. They found that the 88 °C maximum temperatures reached by induction heating were dependent on the added magnetic particles and the shape made with the MSMP. All experiments were done with a constant cooling and heating rate of 10 K/min. Moreover, the shape recovery capabilities of induction-heated MSMPs with an AC magnetic field seemed very similar to that for conventional shape memory polymers activated by an environmental heat change, which makes them a promising candidate for smart implants and controlled medical instruments.
Figure 2. Shape memory effect through induction heating
[3].
Instead of using randomly distributed magnetic micron particles, Leng, et al.
[4] fabricated a MSMP with the magnetic powder laying in chains by applying a 0.03 T field by an AC magnetic field. They confirmed the formation of clear chain structure in chained samples. These chains increase the electrical conductivity in the shape memory polymer. Due to this, a lower electric voltage is needed to heat the MSMP and activate the shape change. Furthermore, they reported that alignment of magnetic particle chains significantly improves the mechanical properties of the SMP.
In addition, Yakacki, et al.
[5] did further research on the influence of the percentage of magnetic particles (Fe
3O
4) incorporated in the polymer matrix. There is a direct influence of the percentage of the added particles on the induction heating rate of the material by an AC magnetic field. The polymer did not influence the heating of the particles. Higher percentages of the magnetic powder also lowered the glass temperature of the material. Materials with a lower percentage of magnetic particles had more crosslinks and therefore had higher glass temperature. Finally, when a large percentage of magnetic particles was added to the polymer matrix, the material had some strains that could not be repaired and the material was more brittle.
Razzaq et al.
[6] also investigated the thermal and electrical performance of a MSMP with various wt% of Fe
3O
4 particles. They found that the thermal conductivity of the polymer matrix increased with the increase of magnetic particle fraction. The thermal conductivity improved from 0.19 to 0.60 W/mK with the increase of magnetite volume fraction but the polymer specific electrical resistivity showed a reversed trend. The changes of resistivity could be due to a conversion of the morphology of the polymer matrix which leads to a faster separation of magnetic particles.
Yu, et al.
[7] developed a MSMP made from poly(ε-caprolactone) (c-PCL) with incorporated Fe
3O
4 nanoparticles (5–25 wt%). One part of their study consisted of researching the results of crosslinking the polymer. These crosslinks increased the shape recovery of the polymer up to 70%. The MSMP was also activated by both hot water and by an alternating magnetic field.
Figure 3 gives an intuitive contrast of the shape-recovery process in both an AC magnetic field and hot water. They observed that both magnetic nanoparticles and hot water stimulated the shape recovery of the helicoidal specimens. However, shape recovery time was significantly different. Thus, this revealed that the reactivity of c-PCL/Fe
3O
4 was better in hot water compared to an AC magnetic field. They believed that the faster recovery time was due to higher thermal transfer and lower heat loss in hot water.
Figure 3. (above, (
a)) The shape memory effect activated in a heated environment (hot water), (below (
b)) the shape memory effect remotely activated by an alternating magnetic fields
[7].
Weigel, et al.
[8] studied the effect of different parameters on the shape memory effect of MSMPs initiated by induction heating through an AC magnetic field. Parameters that were studied were the properties of the magnetic particles, what they were made of, how they were distributed in the polymer matrix, the heat transport conditions, and the surface to volume ratio. The investigated materials were polyetherurethane and a biodegradable multiblock copolymer (PDC) with integrated Fe
3O
4 particles. They found that with the addition of 10 wt% of magnetic particles in the polymer, the shape memory improved and the mechanical properties of the polymer were not changed.
Instead of making a one-way shape recovery MSMP, Kumar, et al.
[9] made a MSMP that was triple-shape and could memorize two shapes; the shape memory effect was activated by an AC magnetic field. The magnetic triple-shape polymer was made of poly(3-caprolactone) (PCL) and poly(cyclohexyl methacrylate) (PCHMA) with incorporated Fe
3O
4 particles. The best triple-shape memory effect was achieved by a composite with 40 wt% PCL.
Yang, et al.
[10] made a MSMP out of a polynorbornene copolymer and magnetite (Fe
3O
4) with a size of 55 nm. The shape memory effect was activated at a temperature of 51 °C, by the induction heating of magnetite by an AC magnetic field. Through the addition of the magnetic particles, the shape recovery of the material declined. The recovery time for the polymer was 186 s. The added magnetic particles also increased the thermal conductivity of the polymer.
Cai, et al.
[11] also made a MSMP out of poly(ε-caprolactone)-polyurethane with incorporated Fe
3O
4. The nanocomposites were made by in situ polymerization at a molar ratio of PCL:MDI:BDO (1:4:3) with various content of Fe
3O
4 nanoparticles. The magnetic particles were homogeneously distributed in the polymer matrix. The shape memory effect was tested both by the activation of hot water and by the induction heating of the magnetic particles by an AC magnetic field (see
Figure 4). It was found that the shape recovery started around 40 °C. The best shape recovery was of 97%. Their experiments confirmed that the shape change was faster in hot water compared to the ones heated by magnetic fields. In addition, they reported that shape recovery rate decreased with the more magnetic particles whereas shape recovery time increased.
Figure 4. (
left) The shape recovery process in hot water, (
right) the shape recovery process in an alternating magnetic field
[11].
The development of models to further predict how the magnetic particles influence the shape change of the MSMPs was further studied by Yu, et al.
[12]. They used a finite element model that was previously developed to model shape memory polymers. Parameters that they studied were particle size, particle volume fraction, particle heating temperature, and rate to magnetically induce shape recovery behavior. The overall outcome was that an increased volume percentage of the magnetic particles or the smaller the size of the particles increased the heating of the material which stimulated the shape memory effect. They also found that there was a limit to the added volume fraction of particles. When adding particles above this volume percentage, there was no increase in the heating rate of the material.
Liu, et al.
[13] developed a dual-response shape memory polymer, which is the same as a MSMP. In this research, the carboxylic styrene butadiene rubber (XSBR)/ferriferrous oxide (Fe
3O
4)/zinc dimethacrylate (ZDMA) was synthesized and tested. The authors investigated how the shape memory effect of the polymer behaved through the two different types of actuation, a heated environment or by induction heating through an AC magnetic field of the magnetic particles (Fe
3O
4) (see
Figure 5). The shape recovery ratio of both shape memory activation methods was 100% but with different response times. It is noticeable that the shape recovery rate under magnetic fields was lower at the initial stage and higher in the last phase. This might be due to the fact that heating by an AC magnetic field is a slow process in the early stage and fast in the end phase.
Figure 5. (
Left) shape memory activated by a heated environment, (
right) shape memory activated by an AC magnetic field
[13].
Zhang, Wang, Zheng, Liu and Leng
[14] made a MSMP out of PLA with incorporated Fe
3O
4 particles, that is activated by an AC magnetic field. For this material, the shape memory behaviors were studied through a lens for biomedical applications. It was found that the developed bone support structure (see
Figure 6) could very quickly return to its remembered shape, in a few seconds, and the surface temperature of the induction heated material was uniform and around 40 °C, which makes the material safe to use inside the human body. They reported that the shape recovery rate was about 96%. The bones could recover their initial shape within 100 s under a magnetic field. However, they confirmed that the shape recovery time in a hot water was much faster (about 10 s).
Figure 6. The shape recovery of a printed structure (above (
a1–
a4)), the simulation of the shape recovery of a bone support structure in a bone (below, (
b1–
b4))
[14].
Zhao, et al.
[15] also studied bone tissue scaffolds made out of PLA with incorporated Fe
3O
4 particles that were activated by an AC magnetic field. They mixed 20 wt% Fe
3O
4 with PLA. One of the noticed advantages of MSMPs is that in the context of biomedical design, these structures can be implemented in the body with minimally invasive surgery. Furthermore, this study found that the mechanical structure of the scaffolds was stable and could improve the attachment.