Cobalt iron oxide-infused silicone nanocomposites: Magnetoactive materials for remote actuation and sensing

2.1 Materials

RTV silicone, containing silicone and activator (separate), was purchased from Contenti (USA), with a specific gravity of 1.1, a tensile strength of 4.6 MPa, and a hardness of 28 Shore A, as per the manufacturer’s product details. Cobalt iron oxide (CoFe₂O₄, abbreviated as CIO) nanoparticles, averaging 30 nm in size with 99% purity, and silicone oil (SO) with a viscosity of 100 mPa s and a density of 0.967 g/mL, were obtained from Sigma Aldrich (USA). The raw materials were utilized as supplied, without any additional pre-processing or purification.

2.2 Silicone/CIO nanocomposite synthesis

Silicone/CIO MAPCs were prepared using the mechanical mixing method (illustrated in Figure 2), a widely recognized technique for producing stable and homogeneously reinforced polymer composites [46,47]. Four compositions were formulated by varying the nanoparticle weight percentages (0.25, 0.5, 1, and 3%). CIO nanoparticles were incrementally introduced into 10 mL of SO in a glass beaker under continuous mechanical stirring. The SO/CIO solution was mixed for 15 min to achieve the homogenous dispersion of the nanoparticles within the SO. Mechanical mixing was employed as the preliminary dispersion step, which allowed the coarse but uniform CIO nanoparticle distribution within the SO, ensuring the appropriate wetting of CIO nanoparticles by SO and resulting in a homogenous starting mixture. Followed by an ultrasonication process for 5 min, which allowed the breaking of CIO nanoparticles agglomerates through high-frequency energy, further improving the dispersion. SO allowed a stable suspension of CIO nanoparticles for a homogenous dispersion within the silicone matrix. The synergic effect of both techniques, i.e. mechanical mixing and ultrasonication, significantly contributed to the enhanced dispersion of CIO nanoparticles, which affected the mechanical and magnetic properties of MAPCs, as discussed in the subsequent sections. The pre-defined weight of silicone (as detailed in Table 1) was added to another glass beaker. The prepared SO/CIO solution was then mixed thoroughly with the silicone base through rigorous mechanical mixing for 30 min to obtain a silicone/SO/CIO mixture. Finally, a curing agent (activator) was added to the mixture as prescribed by the manufacturer (10:1 weight ratio with respect to silicone), and mechanically mixed for another 15 min for a homogenous mixing of the crosslinking agent. The curing agent contains a tin-based catalyst that facilitates the cross-linking reaction between hydroxyl-terminated polysiloxane chains and silane functional groups. The curing process occurs at room temperature and releases alcohol as a byproduct, characteristic of condensation-type curing.

Figure 2

The synthesis process of silicone/CIO MAPCs involves the mixing of CIO nanoparticles in SO (mechanical mixing) followed by ultrasonication. The resulting mixture was added to the silicone base (mechanical mixing) followed by the addition of a crosslinking agent. The final mixture was poured into moulds for curing with (for anisotropic samples) and without (for isotropic samples) a magnetic field.

The final solution was then poured into a 3D-printed mould for curing to produce specimens for testing. The mould was designed using Solidworks to produce samples for tensile testing and magnetic actuation response of silicone/CIO MAPCs, followed by 3D-printing of it using Markforged Mark Two 3D printer using Onyx^® material (from Markforged, USA). The silicone/CIO MAPCs were then left at room temperature for 36 h to cure completely. Compression testing specimens were prepared using Petri dishes to produce the disc samples measuring 33.5 mm in diameter and 9.5 mm in height. Silicone/CIO MAPC samples were then removed from the mould for chemical, thermal, mechanical, and magnetic characterization.

2.3 Magnetization of MAPCs

A custom setup was designed and 3D-printed for the magnetization of MAPCs (as reported in Figure 3). This setup was also 3D-printed using a Markforged Mark Two 3D printer using Onyx^® material (from Markforged, USA). The setup contains slots for placing two permanent magnets (N42 neodymium disc magnets with a diameter of 2 in, a thickness of 0.5 in, and an approximate pull force of 87.5 lb) on each side and a platform for sample placement during the curing process to align the CIO nanoparticles along the magnetic field direction. The slots were secured with removable plates to avoid the magnet interaction with the surroundings.

Figure 3

Custom-designed 3D-printed setup for magnetization of MAPCs contains two slots on either side for placement of permanent magnets (N42 neodymium disc magnets), and a platform between the magnets to place the mould during the curing process.

The setup allowed a homogenous magnetic field between the magnets with a magnetic field strength of 711 G at the centre and was used to produce anisotropic samples for tensile and magnetic actuation experiments. For each silicone/CIO MAPC composition, the moulds were placed in the magnetic fields during the curing process to produce anisotropic MAPCs; however, no magnetic field was applied to isotropic MAPCs. However, only isotropic samples were prepared for compression testing.

2.4 Material characterization

The morphology of silicone/CIO MAPCs was analysed using a field emission scanning electron microscope (FE-SEM, FEI Quanta650FEG) operated at an acceleration voltage of 20 kV and a working distance of 0.9 cm. SEM analysis assessed the dispersion of CIO nanoparticles and the influence of the applied magnetic field on anisotropic MAPCs. Fourier transform infrared (FTIR) spectroscopy was conducted on a Nicolet iS50 FT-IR spectrometer (Thermo Scientific, USA) equipped with an attenuated total reflectance (ATR) accessory featuring a diamond crystal plate. This technique was used to identify the molecular structure of MAPCs, utilizing 32 scans per sample in the 500–4,000 cm⁻¹ spectral range with a resolution of 4 cm⁻¹. Thermal properties were investigated through thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) using a Thermal Analyzer (SDT 650, TA Instruments, USA). The silicone/CIO MAPCs were carefully placed in a ceramic crucible and heated from 30 to 900°C at a constant rate of 10°C/min under a nitrogen atmosphere.

2.5 Mechanical testing

Tensile testing specimens for silicone/CIO MAPCs were mould-cast according to ASTM D638 Type-IV standards [48]. Fully cured tensile testing specimens are reported in Figure 4(a). Tensile testing was performed using a Mark 10 material testing machine with a 250 N load cell at a 500 mm/min displacement rate. At least three samples were tested for each material composition and particle orientation (i.e. isotropic and anisotropic) to evaluate the impact of CIO nanoparticle concentration and their orientation on the mechanical response of MAPCs. Load vs displacement data were post-processed to evaluate stress vs strain behaviour. Compression testing specimens were prepared using Petri dishes to produce the disc samples measuring 33.5 mm in diameter and 9.5 mm in height, as shown in Figure 4(b). Compression testing was also performed using a Mark 10 material testing machine with a 250 N load cell at a 2 mm/min displacement rate.

Figure 4

Fully cured silicone/CIO MAPC samples at different CIO nanoparticle concentrations. (a) Dog-bone-shaped samples for tensile testing and (b) disc samples for compression testing.

2.6 Magnetic properties and actuation response of MAPCs

The magnetic properties of silicone/CIO MAPCs were analysed using a vibrating sample magnetometer (VSM, Quantum Design Dynacool PPMS) within a magnetic field range of 0–10 kOe at room temperature. Hysteresis loops were recorded for all samples to determine the values of saturation magnetization (M _s), coercivity (H _c), and remanence (M _r). To evaluate the magnetic actuation response, silicone/CIO MAPCs were tested in the form of mould-cast strips with dimensions of 70 mm × 15 mm × 2 mm. MAPC strips were clamped at one end and placed vertically, hanging (free at the other end) to apply an external magnetic stimulus. N42 Neodymium disc magnet of 2 in. diameter and 1 in. thickness with a flux density (Br) of 13,150 G was used to actuate the MAPC strips. The magnet position was adjusted manually to record the threshold (1 mm), 5 mm, and 10 mm actuation in response to the applied magnetic field for each MAPC composition and configuration (isotropic and anisotropic). The distance (x) of the magnet from the strip was recorded for each actuation displacement, and the magnetic field (B _x ) at a distance x from the magnet was calculated using the following equation [49,50]:

where T is the magnet thickness and R is the magnet radius.

3.1 Scanning electron microscopy (SEM)

SEM was conducted on silicone/1% CIO MAPCs in both isotropic and anisotropic configurations to examine the dispersion of CIO nanoparticles within the silicone matrix and assess the effects of magnetic field-assisted curing. Before analysis, samples were coated with a 10 nm layer of gold to enhance surface conductivity for both surface and cross-sectional imaging. Due to the relatively low concentration of CIO nanoparticles (1%) in the MAPCs, only a limited number of nanoparticles were detected on the surface of the samples. However, the cross-sectional images provided clearer insights into the distribution of CIO nanoparticles, as illustrated in Figure 5. For the isotropic MAPCs, SEM images (Figure 5(a) and (b)) revealed a uniform and homogenous dispersion of CIO particles throughout the silicone matrix with some aggregates (ranging from 1 to 10 µm). These aggregates disrupt the uniform stress distribution within the silicone matrix and act as stress concentrators, negatively impacting the mechanical performance of MAPCs, especially at higher concentrations, i.e. 3%. In contrast, the anisotropic MAPCs (Figure 5(c) and (d)) exhibited alignment of the CIO nanoparticles along the direction of the applied magnetic field during curing, confirming the magnetic field’s influence on the nanoparticle orientation within the matrix.

Figure 5

SEM analysis. (a) Silicone/1% CIO MAPCs (isotropic configuration) analysed at 250× magnification. (b) Silicone/1% CIO MAPCs (isotropic configuration) analysed at 500× magnification shows homogenously dispersed CIO particles within silicone matrix with some aggregates (ranging from 1–10 µm). (c) Silicone/1% CIO MAPCs (anisotropic configuration) analysed at 250× magnification. (d) Silicone/1% CIO MAPCs (anisotropic configuration) analysed at 500× magnification shows CIO nanoparticles aligned in the applied magnetic field direction.

3.2 FTIR spectroscopy

FTIR spectroscopy was used to investigate the chemical structure and functional groups of pure silicone and silicone-based MAPCs (for isotropic configuration – with randomly aligned nanoparticles) reinforced with CIO nanoparticles at varying concentrations (0.25, 0.50, 1, and 3%). The FTIR spectra were recorded within the wavenumber range of 500–4,000 cm⁻¹, and the results are presented in Figure 6.

Figure 6

FTIR spectra of pure silicone and silicone/CIO MAPCs show characteristic peaks of pure silicone recorded at around 700–800 cm⁻¹ (Si–CH₃ stretching vibrations), 1,000–1,100 cm⁻¹ (Si–O–Si stretching vibrations), 1,150–1,260 cm⁻¹ (CH₃ stretching of methyl group), and 2,900–2,950 cm⁻¹ (C–H stretching of methyl group). No significant changes in characteristic peaks were observed with the addition of CIO nanoparticles.

The FTIR spectrum of pure silicone exhibited characteristic absorption peaks corresponding to its siloxane backbone and methyl groups. Notable peaks were observed around 700–800 cm⁻¹ (attributed to Si–CH₃ stretching vibrations) and 1,000–1,100 cm⁻¹ (attributed to Si–O–Si stretching vibrations), which are consistent with the typical structure of polydimethylsiloxane (PDMS). In addition, the characteristic peaks were observed at around 1,150–1,260 cm⁻¹ (attributed to CH₃ stretching of methyl groups) and 2,900–2,950 cm⁻¹ (attributed to C–H stretching of methyl groups). With the addition of CIO nanoparticles, no significant changes in the intensity and position of the characteristic silicone peaks were observed, likely due to the formation of weak physical or chemical bonds between the CIO nanoparticles and the silicone matrix. Moreover, no new significant peaks were observed in MAPCs, indicating that the CIO nanoparticles primarily interact with the existing silicone bonds without introducing new functional groups. FTIR analysis revealed that the absence of new functional groups indicates that the CIO nanoparticles are well-dispersed within the matrix and do not form new chemical bonds, making MAPCs promising for maintaining the chemical integrity of silicone while enhancing its mechanical and magnetic properties.

3.3 TGA

TGA shows that both pure RTV silicone and silicone/CIO MAPCs undergo a single-step thermal degradation process, with the weight reduction beginning around 300°C (Figure 7(a)). As the temperature increases beyond this point, a sharp and significant weight loss is observed, corresponding to the thermal degradation of the RTV silicone matrix, likely involving the breakdown of the siloxane backbone and organic groups. Despite the incorporation of CIO, there is no significant shift in the onset of degradation, indicating that the initial thermal stability of the silicone matrix is largely unaffected. The degradation process continues rapidly up to around 550–600°C, after which the weight stabilizes, leaving a substantial residual mass. This final residue is higher than typical for pure silicone, indicating the presence of inorganic CIO nanoparticles that do not decompose at high temperatures.

Figure 7

(a) TGA and (b) DSC of silicone and silicone/CIO MAPCs.

In the DSC analysis of RTV silicone and silicone/CIO MAPCs, the incorporation of CIO does not significantly shift the degradation onset temperature but influences the heat flow trends, as shown in Figure 7(b). As the temperature increases, a broad endothermic peak appears, which is typically associated with molecular relaxation or softening within the silicone matrix, reflecting increased chain mobility. This is followed by a noticeable exothermic peak at higher temperatures, suggesting additional crosslinking or post-curing reactions. The presence of the nanoparticles causes a slight shift of the thermal events towards higher temperatures compared to pure silicone, indicating a small improvement in thermal resistance. An increasing CIO content intensifies these exothermic reactions, suggesting complex interactions between the CIO filler and the silicone matrix, which influence the material’s thermal behaviour.

3.4 Mechanical testing

Mechanical properties of silicone/CIO MAPCs loaded with varying concentrations of CIO were evaluated under both compressive and tensile loading conditions, and the results are summarized in Table 2. Silicone/CIO MAPCs were tested for their compression modulus in isotropic configuration, while tensile strength, failure strain, and elastic modulus were measured in both isotropic and anisotropic configurations. The compressive behaviour of silicone/CIO MAPCs was evaluated until 25% of deformation relative to the initial sample height. In this range, all silicone/CIO MAPC compositions revealed a linear behaviour without any failure, as shown in Figure 8(a). The compression moduli of silicone/CIO MAPCs were measured from the slopes of these strain vs stress curves. The pure silicone sample exhibited a compression modulus of 0.412 MPa. With the addition of 0.25% CIO, the modulus decreased to 0.199 MPa, which might be due to weak interactions between the silicone matrix and the CIO nanofillers at low concentrations. As the CIO content increased from 0.50 to 3%, the compression modulus recovered, reaching 0.274, 0.322 and 0.340 MPa, for silicone/0.50% CIO, silicone/1% CIO, and silicone/3% CIO MAPCs, respectively, which indicates that higher CIO content improves the stiffness of silicone/CIO MAPCs due to better particle dispersion and increased silicone/CIO interaction. The stiffening effect at higher filler concentrations results from the reinforcing action of the magnetic nanoparticles, although the rate of increase slows down at higher CIO concentrations (i.e. 3%), potentially due to particle agglomeration.

Figure 8

Mechanical properties of silicone/CIO MAPCs. (a) Strain vs stress curves for compression testing. (b) Strain vs stress curves for tensile testing of isotropic MAPCs. (c) Strain vs stress curves for tensile testing of anisotropic MAPCs. (d) Comparison of tensile strength for different materials and CIO nanoparticle orientation. (e) Comparison of failure strain for different materials and CIO nanoparticle orientation. (f) Comparison of tensile strength for different materials and CIO nanoparticle orientation. (g) Compression and tensile testing specimens. (h) Visual representation of CIO nanoparticles within isotropic and anisotropic MAPCs.

Tensile testing was conducted on dog-bone-shaped specimens until failure, and displacement vs load data were post-processed to obtain strain vs stress curves, as shown in Figure 8(b) and (c). Figure 8(b) and (c) shows the strain vs stress behaviour of both isotropic and anisotropic silicone/CIO MAPCs. The tensile response of MAPCs followed a similar trend, as observed in compression testing, where an increase in the tensile strength was observed with increasing CIO content; however, the tensile strength decreased at 3% CIO concentration.

The tensile strength of pure silicone was observed to be around 1.356 MPa, whereas the addition of CIO nanoparticles reduced the tensile strength to 0.737 MPa for silicone/0.25% CIO. This reduction may be attributed to poor dispersion or weak interactions between the CIO particles and the silicone matrix at a low filler content. Further increasing the CIO concentrations resulted in improved tensile strength, i.e. 0.775 and 1.250 MPa for silicone/0.50% CIO and silicone/1% CIO, respectively. This improvement in the tensile strength can be attributed to the reinforcing effect of the CIO nanoparticles, which enhance the material’s ability to resist deformation under tensile load. Finally, at a higher CIO concentration (i.e. 3%), the tensile strength decreases potentially due to nanoparticle agglomeration as a result of high loading, which can create stress concentration points and reduce the material’s ability to resist tensile loads.

In the anisotropic samples, where the CIO particles were aligned in a magnetic field, the tensile strength was consistently higher compared to the isotropic counterparts, indicating the beneficial effect of particle alignment. Silicone/0.25% CIO MAPCs exhibited a tensile strength of 0.925 MPa, increasing to 0.975 MPa for the silicone/0.50% CIO and 1.356 MPa for the silicone/1% CIO, reflecting the improved load-bearing capacity provided by the alignment of CIO nanoparticles, which enhances stress transfer between the silicone matrix and CIO nanofillers. However, the tensile strength dropped to 0.857 MPa for the silicone/3% CIO, likely due to the same particle agglomeration effect observed in the isotropic configuration. Figure 8(d) presents a visual comparison of tensile strengths for isotropic and anisotropic silicone/CIO MAPCs.

The failure strain, or elongation at break, of pure silicone was 11.88 mm/mm, as depicted in Figure 8(e). The failure strain increased to 14.17 mm/mm for the silicone/0.25% CIO, suggesting improved ductility at lower concentrations of nanoparticles, which could be due to the nanoparticles acting as spacers within the silicone matrix, allowing for more significant elongations before failure. Failure strain decreased with further increases in CIO concentrations, with 12.67, 12.04, and 12.46 mm/mm of failure strains for silicone/0.50% CIO, silicone/1% CIO, and silicone/3% CIO, respectively. In the anisotropic configuration, the failure strain was generally higher than in the isotropic configuration, indicating that the alignment of the CIO nanoparticles not only improves strength but also enhances the MAPCs’ ability to deform under load. The silicone/0.50% CIO exhibited the highest failure strain (15.25 mm/mm), which decreased slightly as the CIO content increased, reaching 13.50 mm/mm for the silicone/1% CIO and 12.29 mm/mm for the silicone/3% CIO. The CIO nanoparticle alignment improves ductility; however, at higher CIO loadings, potential agglomerations can reduce the material’s ability to deform before failure.

The elastic modulus, which reflects the stiffness of the material, was measured through uniaxial tensile tests performed on MAPCs, and for pure silicone, it was 0.153 MPa. Upon the addition of 0.25% CIO, the elastic modulus decreased to 0.065 MPa, likely due to weak interactions between the CIO nanoparticles and silicone matrix at low concentrations. However, as the CIO content increased, the modulus improved to 0.081 MPa for the silicone/0.50% CIO and 0.141 MPa for the silicone/1% CIO composite. The increased stiffness with higher CIO nanoparticle content is a result of the reinforcing effect, which restricts polymer chain mobility and enhances the material’s resistance to deformation. For silicone/3% CIO, the modulus slightly decreased to 0.109 MPa, potentially due to particle agglomerations, which could create weak points in the MAPC structure. In the anisotropic configuration, the elastic modulus was improved for MAPCs at lower filler concentrations, as illustrated in Figure 8(f). Elastic modulus increased steadily with CIO content, reflecting the improved stiffness due to particle alignment. The silicone/0.25% CIO had an elastic modulus of 0.078 MPa, which increased to 0.091 MPa for the silicone/0.50% CIO. The aligned particles likely restrict polymer chain mobility more effectively, resulting in a stiffer material. However, for silicone/1% CIO and silicone/3% CIO, the elastic modulus dropped slightly to 0.128 and 0.089 MPa, again likely due to agglomeration effects that reduce the efficiency of particle-matrix interactions.

As the CIO nanoparticle content increased, the tensile strength and elastic modulus generally improved up to 1% of CIO concentrations, reflecting the reinforcing role of CIO nanoparticles in the silicone matrix. The particles provide additional load-bearing capacity and restrict polymer chain mobility, leading to increased stiffness and strength. However, at higher concentrations (3% CIO), particle agglomeration likely occurred, which reduces the reinforcing effect and leads to a decrease in both tensile strength and elastic modulus. The anisotropic samples, where CIO nanoparticles were aligned using a magnetic field, consistently outperformed the isotropic samples in terms of tensile strength and elastic modulus up to 1% of CIO concentrations. The CIO nanoparticle alignment enhances stress transfer and load-bearing capacity, as the aligned nanoparticles can more effectively resist deformation and distribute applied loads. The alignment also improved failure strain, allowing for greater ductility before failure.

A key observation from the mechanical testing results is the trade-off between the tensile strength and failure strain. While the addition of CIO nanoparticles increases stiffness and strength, it also reduces the material’s ability to deform, particularly at higher filler concentrations, which is a common phenomenon in reinforced polymers, where the increased stiffness typically comes at the cost of reduced ductility. At low CIO concentrations, poor particle dispersion likely weakens the silicone/CIO MAPCs, as observed in the low elastic modulus of the silicone/0.25% CIO. As the concentration increases, better dispersion enhances the mechanical properties, but beyond a certain threshold (e.g. 3% CIO), particle agglomeration occurs, which introduces stress concentration points and weakens the composites.

3.5 Magnetic properties of silicone/CIO MAPCs

Magnetic properties of the synthesized silicone/CIO MAPCs were evaluated using a VSM under the magnetic fields from 0 to 10 kOe at ambient temperature by recording the hysteresis loop and measuring saturation magnetization (M _s), coercivity (H _c), and remanence (M _r). The VSM results are illustrated in Figure 9(a) and summarized in Table 3. CIO nanoparticles, being ferromagnetic in nature, exhibit remarkable magnetic properties, including saturation magnetization (M _s) of 52.56 EMU/g, coercivity (H _c) of 2438 Oe, and remanence (M _r) of 30.8 EMU/g. For a detailed analysis of the magnetic properties of CIO nanoparticles, readers are referred to our previous study [51].

Figure 9

Magnetic properties and magnetic actuation response of silicone/CIO MAPCs. (a) Magnetic field vs magnetization profiles of silicone/CIO MAPCs represent an increased magnetic saturation and remanence with the increase in CIO nanoparticles concentration. (b) Magnetic field required to achieve threshold (1 mm), 5 mm, and 10 mm displacements for isotropic (iso) and anisotropic (aniso) configurations of silicone/CIO MAPCs. (c) Magnetic actuation of different configurations and compositions of silicone/CIO MAPCs.

For silicone/CIO MAPCs, M _s increases with higher CIO content, demonstrating the proportional contribution of magnetic nanoparticles to the MAPCs’ overall magnetization. H _c remains consistent, reflecting stable magnetic anisotropy and particle size distribution, with minimal influence from changes in CIO concentration. Finally, M _r increases with increasing CIO content due to stronger dipolar interactions. The saturation magnetization (M _s) of the MAPCs increased as the concentration of CIO nanoparticles in the matrix increased. The M _s values ranged from 0.087 EMU/g for the silicone/0.25% CIO to 1.057 EMU/g for the silicone/3% CIO due to the increasing amount of CIO nanoparticles embedded in the silicone matrix. As more CIO nanoparticles are incorporated, the overall magnetic moment of the MAPC increases due to the intrinsic magnetism of the CIO. The increase in M _s with CIO concentration follows the expected behaviour, where a higher content of magnetic nanoparticles results in a stronger collective magnetization under an external magnetic field, which indicates an effective dispersion of the nanoparticles within the matrix, allowing the magnetization to develop throughout the material without significant agglomeration, which could have otherwise reduced the magnetic response due to dipole–dipole interactions.

The coercivity (H _c) of MAPCs remained relatively constant across all materials tested, with values around 2350 Oe, indicating that the inclusion of different concentrations of CIO did not significantly alter the energy required to demagnetize the material. The stability of coercivity could be due to the particle size of CIO in the nanoscale regime contributing to a consistent coercivity, as the magnetization reversal mechanism does not drastically change with concentration. Furthermore, the intrinsic properties of CIO, such as its magneto-crystalline anisotropy, dominate the coercive behaviour. Thus, the MAPCs inherit this stability, with minimal influence from the polymer matrix or particle loading.

The remanent magnetization (M _r) also revealed a dependence on the CIO content, increasing from 0.054 EMU/g for the silicone/0.25% CIO to 0.625 EMU/g for the silicone/3% CIO. Remanence reflects the ability of the material to retain magnetization after the external magnetic field is removed. The increase in M _r suggests that with higher concentrations of CIO, the MAPCs retain a more significant magnetic moment due to the higher volume fraction of CIO nanoparticles, which is closely linked to the magnetic dipole–dipole interactions among nanoparticles. At lower concentrations, the nanoparticles are more isolated, and their interactions are weaker, leading to lower remanence. As the concentration increases, the nanoparticles are closer together, enabling stronger dipole–dipole interactions and leading to enhanced remanence. However, the lack of a sharp increase in M _r at higher CIO loading indicates that agglomeration effects might be limited, preventing excessive magnetic particle clustering, which could otherwise reduce the remanent magnetization by creating closed-loop magnetic domains.

The changes in magnetic properties can be attributed to dipolar interactions; as the CIO content increases, the magnetic dipoles within the material experience stronger interactions, leading to higher magnetization (M _s and M _r). The increase in M _s and M _r is gradual and controlled, indicating uniform dispersion of CIO particles in the silicone matrix without significant agglomeration. In addition, nanoparticles typically exhibit surface spin disorder due to a large surface-to-volume ratio, which slightly reduces the total magnetization compared to bulk materials, especially at lower CIO concentrations. However, this effect is minimized at higher concentrations where collective magnetic behaviour dominates. Finally, the uniformity in coercivity (H _c) suggests that the particle size distribution and magnetic anisotropy of CIO nanoparticles are maintained across all MAPCs, resulting in stable coercive behaviour. This indicates that the synthesis and processing of the MAPCs achieved a consistent particle size and orientation, critical for reliable magnetic performance.

The synthesized silicone/CIO MAPCs outperformed similar materials reported in the literature in terms of their magnetic properties. Polyurethane (PU) nanocomposites with 3% magnetite nanoparticle (MNP) concentration revealed an M _s of 1.99 EMU/g, M _r of 0.033 EMU/g, and H _c of 15.37 Oe [49]. Another study reported an M _s of 1.719 EMU/g, M _r of 0.024 EMU/g, and H _c of 11.855 Oe for 3% magnetite nanoparticle-reinforced poly(butylene succinate-co-butylene adipate) (PBSA) MAPCs [50]. However, the M _s of silicone/CIO MAPCs reported in this study is lower than the similar materials (with the same nanoparticle concentrations, i.e. 3%) reported in the literature. The M _r and H _c for silicone/CIO MAPCs are significantly higher than the reported materials, indicating stronger magnetic retention and higher resistance to demagnetization, making them an excellent candidate for smart devices requiring precise control and sustained magnetic performance.

3.6 Magnetic actuation of silicone/CIO MAPCs

The magnetic actuation response of silicone/CIO MAPCs containing different concentrations of CIO nanoparticles (i.e. 0.25, 0.5, 1, and 3%) was investigated under isotropic and anisotropic configurations, as shown in Figure 9(b) and (c). Increasing the CIO concentration in both isotropic and anisotropic samples reduced the magnetic field required to achieve the same actuation distances (i.e. threshold (1 mm), 5 mm, and 10 mm), which indicates that a higher CIO content enhances the magnetic responsiveness of MAPCs due to stronger internal magnetic interactions. Silicone/0.25% CIO isotropic samples required 2,223  G to achieve a threshold (1 mm) displacement, increasing to 3258  G for 10 mm displacement. However, anisotropic samples needed lower fields (1769  G for 1 mm and 2,933G for 10 mm displacements). For silicone/0.50% CIO isotropic samples, the required magnetic field dropped further (1,873 G for 1 mm and 2,779 G for 10 mm displacements). As expected, anisotropic configurations revealed improved actuation efficiency, with lower magnetic fields (1,491 G for 1 mm and 2,631  G for 10 mm). A similar pattern was observed for silicone/1% CIO and silicone/3% CIO MAPCs, with the lowest magnetic fields observed in anisotropic samples (703G and 439  G for 1 mm for 1% CIO and 3% CIO, respectively). The reduction in required fields with increasing CIO content is due to enhanced magnetic interactions that facilitate actuation at lower applied magnetic fields. Therefore, a higher CIO content coupled with particle alignment significantly improved magnetic actuation efficiency, minimizing the magnetic field requirements for achieving desired displacements.

Across all silicone/CIO MAPCs, anisotropic samples consistently required lower magnetic fields than isotropic samples for the same actuation displacement owing to the alignment of magnetic nanoparticles along the direction of the magnetic field during sample preparation, facilitating more efficient magnetic actuation. The lower magnetic field requirements in anisotropic samples can be attributed to the alignment of particles, which provides a preferred pathway for the magnetic flux, reducing magnetic reluctance and allowing the MAPCs to respond more easily to the magnetic field. The difference in the magnetic field requirements between isotropic and anisotropic configurations decreased with increasing CIO content. At higher concentrations (1 and 3%), the magnetic interaction within the isotropic samples was strong enough to partially compensate for the lack of alignment, resulting in a less pronounced difference compared to lower concentrations. The magnetic actuation results highlight the importance of optimizing both the concentration and distribution of magnetic particles to enhance the performance of MAPCs for remote actuation applications.