Innovation in Methods for Incorporating Magnetite into Biocellulose for Electromagnetic Interference Shielding Effectiveness Applications

Abstract

Materials with magnetic properties are essential in various electric sector technologies. However, the generation of pollutants is of concern, increasing the interest in developing new sustainable, low-cost magnetic materials. These materials have notable applications in protecting against electromagnetic interference (EMI), which can lead to health problems as well as environmental pollution. Therefore, the aims of the present study were to produce a sustainable magnetic polymer using different methods of magnetite incorporation, investigate its magnetic properties, and determine its EMI shielding potential. The magnetic BC materials were obtained via in situ and ex situ magnetic incorporation in processed BC membranes and BC hydrogels. Analyses were carried out using XRD, FTIR, SEM, and VSM, and tests were performed to assess electromagnetic interference shielding effectiveness (EMI SE). The results revealed that the magnetite incorporation method influences the final size of nanoparticles, the arrangement among BC fibers, and the magnetic properties. Materials produced from processed BC had a higher percentage of incorporated magnetite and greater magnetic saturation, whereas those containing nanoparticles with a larger diameter had a stronger coercive field. Although samples did not have high EMI SE, magnetite increased the wave reflection and absorption of the material. This biomaterial can drive important innovations in the energy sector, particularly in efficient and ecological electrical infrastructure.

1. Introduction

The study and development of electromagnetic technologies are fundamental for the electrical sector, as they provide the basis for the generation, transmission, and distribution of electrical energy [1]. Advanced knowledge in this area allows the creation of more efficient devices such as transformers, motors, and generators, which are essential for modern electrical infrastructure [2]. Electromagnetic technologies also enable innovation in renewable energy systems, such as solar and wind energy, contributing to a more sustainable future [3]. Therefore, advances in this field have a positive influence on energy efficiency and reduce environmental impact.

Magnetic materials are necessary to produce various modern equipment and devices such as sensors, electronics, magnetic resonance systems, analytical equipment, transformers, etc. [4]. Thus, several lines of research have sought the creation of novel magnetic materials with a lower production cost and greater sustainability, including the preparation of new biocomposites using biocellulose (BC) [5,6,7].

BC has been widely explored as support for the production of new materials due to its biocompatibility, nanometric fibers, high degree of purity, flexibility (which enables the production of devices in various formats, such as boxes and foldable structures), and ability to be produced through bacterial fermentation, thus eliminating the effect of seasonality and the need for large spaces for production [8,9,10]. The cost of the fermentation process can be lowered by using agro-industrial waste products in the production medium. After this step, BC is obtained in the form of a hydrogel that can hold a large quantity of water [11,12,13]. Numerous studies have used BC hydrogels for the incorporation of dopants, giving BC new characteristics. Such compounds include iron oxides, such as magnetite, maghemite, and ferrites, which have magnetic properties [14,15].

Most studies involving the incorporation of iron oxides in BC have employed in situ and ex situ coprecipitation methods, generally showing satisfactory results in terms of magnetic characteristics of the new materials [16,17]. Some authors state that in situ coprecipitation allows for a better distribution of particles within BC [18,19,20,21], whereas others prefer ex situ synthesis of magnetic particles, describing it as a simple method that enables greater control of particle production variables [22,23,24]. Ex situ coprecipitation is also preferred for the production of particles that require critical conditions. For instance, Salidkul et al. [25] synthesized barium ferrite nanoparticles at a temperature above 800 °C, which would degrade BC if in situ synthesis was performed.

One of the applications of novel magnetic materials is the shielding of the electromagnetic interference (EMI) of waves generated by equipment involved in the generation, transmission, and distribution of electrical energy. With the continuous search for devices operating at higher frequencies, such as wireless systems, satellites, and military devices, the high quantity of energy stored in electromagnetic fields spread throughout the atmosphere can reach a ‘tipping point’ [26,27,28].

Electromagnetic pollution can be harmful to electronic devices, the environment, and animals, as electromagnetic waves can cause overheating of electronic systems, skin burns, cell damage, interference in medical devices, and even tumors [29,30,31,32,33,34,35]. There is also the Intentional Electromagnetic Interference, aimed at damaging electronic systems, often in cyberattacks, robustness testing, and scientific research. This interference can be caused by a conducted susceptibility, such as injecting noise into power supplies, or a radiated susceptibility, such as emitting electromagnetic pulses [36,37]. Therefore, many innovative studies, like the present one, are presently focusing on Electromagnetic Interference Shielding Effectiveness (EMI SE).

EMI SE quantifies a material’s ability to attenuate the strength of an electromagnetic field within a specific space. Measured in decibels (dB), EMI SE indicates the proportion of electromagnetic energy that penetrates through a surface over time. Shielding occurs through reflection, absorption, or transmission of waves when they encounter the material. The reflection coefficient (R), absorption coefficient (A), and transmission coefficient (T) represent the power fractions of these respective phenomena, adhering to the principle of energy conservation, i.e., R + A + T = 1. New materials are being developed and tested to contain pollution [38,39,40,41,42]. Conductive, semiconductive, and magnetic materials, such as MXene, carbon nanotubes, and derivatives of nickel and magnetite, have shown good wave reflection. However, producing these materials faces challenges, including pollution during manufacturing and the need to produce them in small sizes without losing efficiency [43,44,45].

Biocellulose (BC) and magnetite-derived materials can be innovative, sustainable alternatives for producing EMI SE materials since the BC matrix can be flexible and compact, and magnetite possesses good magnetic properties. Therefore, the present work aimed to produce magnetic BC via in situ and ex situ coprecipitation methods for incorporating magnetite in BC hydrogels, compare the membranes obtained, and determine their ability to shield from electromagnetic waves.

2. Materials and Methods

2.1. Maintenance of Microorganism

The bacterium Komagataeibacter hansenii UCP1619, used for the production of BC, was acquired from the Culture Bank of the Núcleo de Pesquisas e Ciências Ambientais e Biotecnologia (NPCIAMB) of the Catholic University of Pernambuco (UNICAP). The microorganism was maintained in the HS medium described by Hestrin and Schramm [46] and adapted by Hungund and Gupta [47], which is composed of 2.0% (m/v) glucose, 0.5% (m/v) yeast extract, 0.5% (m/v) peptone, 0.27% (m/v) Na₂HPO₄, and 0.15% (m/v) citric acid.

2.2. Preparation of Pre-Inoculum

This step followed the method described by Costa et al. [48] and Galdino et al. [49]. The bacterium was activated and inoculated in HS agar medium, followed by incubation at 30 °C for 48 h. The activated cells were then used to prepare the pre-inoculum, which was grown in liquid HS medium for 48 h at 30 °C.

2.3. Biocellulose Production by Fermentation

For the fermentative production of BC, we used the production medium described by Costa et al. [48] and Galdino et al. [49], namely, 1.5% glucose, 2.5% corn steep liquor, 0.27% Na₂HPO₄, and 0.15% citric acid, pH 5, to which 3% of the pre-inoculum was added. BC films were produced using the static method at 30 °C during 10 days of growth. After production, BC films were removed from the fermented broth, cleaned, purified by immersion in a 4% NaOH solution for 2 h, and washed with deionized water.

2.4. Magnetite Synthesis and Incorporation Methods

To compare the methods employed in an equal manner, magnetite (Fe₃O₄) was incorporated in BC hydrogels using the same physical and chemical variables, the only difference being where nanoparticles were precipitated. The in situ process involved coprecipitation within the BC hydrogels, whereas the ex situ synthesis involved the production of particles in an aqueous medium, followed by incorporation into the hydrogels. BC films with and without mechanical processing in a blender were used. Thus, four different types of samples were obtained, as shown in

Table 1. Description of biocellulose (BC) samples produced with magnetite.

Sample	State of BC Used	Type of Fe3O4 Synthesis
BC-MAG	Not processed	in situ coprecipitation
BCP-MAG	Processed	in situ coprecipitation
BC-COMAG	Not processed	ex situ coprecipitation
BCP-COMAG	Processed	ex situ coprecipitation

BC-MAG = Magnetic bacterial cellulose prepared by ex situ coprecipitation; BCP-MAG = Magnetic bacterial cellulose prepared by ex situ coprecipitation and processing; BC-COMAG = Magnetic bacterial cellulose prepared by in situ coprecipitation; BCP-COMAG = Magnetic bacterial cellulose prepared by in situ coprecipitation and processing.

.

BC was processed with a blender (model FPSTHB2610R017, Oster, Milwaukee, WI, USA). Unprocessed hydrogels had a solid, gelatinous appearance, whereas processed hydrogels had a creamy, gel-like consistency (Figure 1).

The magnetite coprecipitation process in all cases followed a model similar to that described by Chanthiwong et al. [19] and Salles et al. [24], with adaptations, from which the following chemical equations can be proposed:

Fe²⁺ + 2 Fe³⁺ + 8 OH⁻ → Fe₃O₄ + 4 H₂O

2 FeCl₃ + FeCl₂ + 8 NH₃ + 4 H₂O → Fe₃O₄ + 8 NH₄Cl

2.4.1. In Situ Coprecipitation

The BC hydrogels were first immersed in an iron (II) and iron (III) chloride solution at a 1:2 molar ratio and remained at rest. After 48 h, the hydrogels were immersed in a 28% NH₄OH (v/v) aqueous solution for 30 min, taking on a darker color indicating the formation of Fe₃O₄ particles. After coprecipitation, the membranes formed were dried in a forced-air-circulation oven at 60 °C for 48 h to obtain the BC-COMAG and BCP-COMAG samples.

2.4.2. Ex Situ Coprecipitation

An iron (II) and iron (III) chloride solution at a 1:2 molar ratio was first prepared at room temperature. A 28% NH₄OH (v/v) aqueous solution was added slowly and gradually for a 30 min period. The reaction occurred according to the above chemical equations, resulting in an aqueous magnetite solution. After the addition, agitation was interrupted, the magnetite nanoparticles were left to settle, the supernatant was removed, and the particles were collected. Particles were then re-suspended in distilled water into which the hydrogels were placed, remaining at rest for 48 h. The material formed by BC impregnation was collected and dried in a forced-air-circulation oven at 60 °C for 48 h to obtain the BC-MAG and BCP-MAG samples.

2.5. Analysis of Results

2.5.1. Incorporation Rate

To determine the percentage of nanoparticles incorporated into each material, a method was used based on the gain in mass of each sample (M_mag), as described by Hebeish et al. [50], with adaptations. The BC hydrogels were weighed before and after the incorporation of nanoparticles, and the relating masses were inserted into Equation (1):

$$M_{mag}\left(\%\right)=\frac{M_{Total}-M_{FibresI}}{M_{Total}}\times 100$$

(1)

in which M_Total is the final mass of the film and M_FibresI is the mass of the BC fibers, taking into account the initial weight of cellulose and percentage of water retained in it (98%).

2.5.2. Physicochemical and Structural Characterization

The compositions and structures of samples were characterized by X-ray diffraction (XRD) analysis, Fourier transform infrared (FTIR) spectroscopy, and scanning electron microscopy (SEM). The conditions and equipment used for each analysis are described in the Supplementary Material.

2.5.3. Vibrating Sample Magnetometry (VSM)

Magnetic hysteresis was determined using a vibrating-sample magnetometer (model 3482-70 Electromagnet, MicroSense, Hamm, Germany) at room temperature, with the application of a magnetic field that ranged from −20 to 20 kOe. The equipment operated with a sensitivity of 10⁻⁵ emu/g. Coercive field (Hc) values were calculated using the equations found in Phan et al. [51].

2.5.4. Magnetic Shielding

The electromagnetic interference shielding effectiveness (EMI SE) is a parameter aimed at estimating the percentage of power transmitted through a certain material. Physically, it is measured by the transmission coefficient (T) expressed in decibels (dB) and can be categorized as shielding due to reflection and shielding due to absorption.

For the experiment, a two-port Vector Network Analyzer (VNA) was used through which we could determine the scattering parameters (S-parameters) to calculate T and the reflection coefficient (R). The S-parameters S₁₁ and S₂₂ measure reflection, while the S₁₂ and S₂₁ parameters measure transmission according to the following formulae [52,53]:

R = |S₁₁|² = |S₂₂|²

(2)

A = 1 − |S₁₁|² − |S₂₁|²

(3)

T = |S₁₂|² = |S₂₁|²

(4)

SE_R (dB) = −10 log₁₀ (1 − R) = −10 log₁₀ (1 − |S₁₁|²) = −10 log₁₀ (1 − |S₂₂|²)

(5)

SE_A (dB) = 10 log₁₀ [(1 − R)/T] = 10 log₁₀ [(1 − |S₁₁|²/|S₂₁|²]

(6)

SE_T (dB) = −10 log₁₀ (T) = −10 log₁₀ (|S₁₂|²)= −10 log₁₀ (|S₂₁|²)

(7)

The experiment was conducted using the microwave frequency range of the X-band (8–12 GHz), employing two waveguides operating within the specified band. A chamber was inserted at the waveguide junction to better fix the samples, as shown in Figure 2.

More details about this methodology can be found in the Supplementary Material.

3. Results and Discussion

3.1. Incorporation Rate

The incorporation of magnetite (Fe₃O₄) nanoparticles into biocellulose (BC) through the selected in situ and ex situ methodologies led to samples whose visual aspect can be seen in Figure 3 and whose percentages of incorporated magnetite are listed in

Table 2. Percentage of Fe₃O₄ incorporated into each sample of magnetic bacterial cellulose.

Sample	Percentage of Magnetite in Sample (%)
BC-MAG	78.57
BCP-MAG	82.92
BC-COMAG	80.39
BCP-COMAG	83.62

BC-MAG = magnetic bacterial cellulose prepared by ex situ coprecipitation; BCP-MAG = magnetic bacterial cellulose prepared by ex situ coprecipitation and processing; BC-COMAG = magnetic bacterial cellulose prepared by in situ coprecipitation; and BCP-COMAG = magnetic bacterial cellulose prepared by in situ coprecipitation and processing.

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Methods involving BC hydrogel processing before incorporation allowed higher magnetite (Fe₃O₄) uptake due to larger BC surface area and then enhanced nanoparticles fixation. Of the methods without processing, the in situ method enabled slightly greater Fe₃O₄ uptake than the ex situ one, likely due to the smaller size of Fe ions than Fe₃O₄ nanoparticles. Moreover, the higher incorporation of iron ions within BC may have been due to their increased attachment to cellulose hydroxyl groups.

However, since the incorporation rate was not so different between the methods, both were considered effective; so, selecting one or the other should depend on the final application of the material and the availability of raw materials.

3.2. X-ray Diffraction (XRD)

The results of XRD analysis (Figure 4) showed for BC samples impregnated with Fe₃O₄ the peaks corresponding to the characteristic diffractometric planes of both cellulose (−1 1 0 and 2 0 0) [19,49] and magnetite (2 2 0, 1 0 4, 3 1 1, 4 0 0, 4 2 2, 5 1 1 and 4 4 0) [54,55,56,57,58,59,60,61,62]. It is noteworthy that, regardless of the synthetic route, such as coprecipitation [58,59,60] like in this work and alternative syntheses [61,62], the characteristic peaks are preserved.

As shown in

Table 3. Mean size of magnetite crystallites formed and impregnated in bacterial cellulose hydrogels.

Sample	Mean Crystallite Size (nm)
Free magnetite	20.20
BC-MAG	18.60
BCP-MAG	31.03
BC-COMAG	22.17
BCP-COMAG	8.02

BC-MAG = magnetic bacterial cellulose prepared by ex situ coprecipitation; BCP-MAG = magnetic bacterial cellulose prepared by ex situ coprecipitation and processing; BC-COMAG = magnetic bacterial cellulose prepared by in situ coprecipitation; and BCP-COMAG = magnetic bacterial cellulose prepared by in situ coprecipitation and processing.

, the mean crystallite size of free magnetite formed by the coprecipitation synthesis (20.2 nm) was slightly larger than those obtained by Yazdani and Seddigh [54] and Chanthiwong et al. [19] using a similar methodology.

Of the samples produced using the ex situ coprecipitation method, BC-MAG had smaller magnetite crystallites than BCP-MAG. This size difference may have occurred as a result of nanoparticle diffusion in the medium containing BC and water. Although in BC-MAG many of the particles permeated the hydrogel, its intact fiber structure may have prevented the passage of larger particles. Instead, the BCP-MAG structure was fragmented due to BC processing, thus changing the spacing among fibers and allowing larger particles to pass through and cluster together.

As for the samples produced using the in situ method, BC-COMAG contained Fe₃O₄ nanoparticles with a larger mean crystallite size than those found in BCP-COMAG. During BC-COMAG preparation, iron and ammonium ions, which are smaller than magnetite nanoparticles, permeated the BC fibers and formed magnetite crystals in the available interstices. Compared to BC-MAG, BC-COMAG magnetite nanoparticles did not need to penetrate the hydrogel structure selectively, which allowed for the presence of larger nanoparticles inside, as they formed in the interstices among fibers. In the case of BCP-COMAG, iron and ammonium ions were able to permeate the processed hydrogel in a sequential manner. The paste formed by BC processing primarily coated the iron ions. This may have affected the interaction with the subsequently added ammonium ions, leading to slower, more controlled Fe₃O₄ precipitation and producing smaller particles.

The literature reports magnetite particles of different sizes prepared via synthesis by coprecipitation with iron (II) and (III) chlorides. Yazdani and Seddigh [54] and Chanthiwong et al. [19] reported nanoparticles with crystallite sizes (10.03 and ≤14 nm, respectively) close to those found in BCP-COMAG (8.02 nm) and BC-MAG (18.06 nm). However, although the pH in the present study was kept between 8 and 10, the synthesized Fe₃O₄ nanoparticles were smaller than those described by Riaz et al. [63] for the same pH range (25–100 nm).

These results also indicate that magnetite nanoparticles in BCP-MAG had a mean size smaller than the critical radius of the material (30 ± 5 nm), leading to the formation of magnetic monodomains. In contrast, the mean particle size in BC-MAG, BC-COMAG, and BCP-COMAG was 20 ± 5 nm, with magnetite exhibiting a superparamagnetic behavior [64].

3.3. Fourier Transform Infrared (FTIR) Spectrometry

The results of the FTIR analysis illustrated in Figure 5 showed for BC-MAG, BCP-MAG, BC-COMAG, and BCP-COMAG the characteristic peaks corresponding to the BC structure. Likewise, Chaabane et al. [55] and Usawattanakul et al. [65] reported, for BC with magnetite nanoparticles, vibrational peaks of asymmetric C-H, CH₂, C-O, C-C, and O-H bonds at 2914, 1415, 1080, 839, and 3122 cm⁻¹, respectively. Moreover, the four samples exhibited the characteristic vibrational peaks of Fe-O and Fe-O-C of magnetite already reported by Chaabane et al. [55] and Niranjana et al. [66] in the wavenumber range of 586 to 598 cm⁻¹.

The graphs illustrated in Figure 5 show some fluctuations from sample to sample, probably caused by differences in homogeneity and arrangement of their components, which will be further discussed in the following session. Nonetheless, the type of bond between BC and magnetite, i.e., the hydrogen bridge, did not alter the chemical structure of the biopolymer, but ensured the adhesion of nanoparticles to the support [17,20].

3.4. Scanning Electron Microscopy (SEM)

The structures of materials prepared from BC and magnetite are shown with large magnification in the SEM micrographs depicted in Figure 6.

Although the four materials had the same composition (BC and Fe₃O₄), the arrangement of these components varied according to the preparation method employed. A dense formation of clusters of nanoparticles was found in BC-MAG and BC-COMAG (Figure 6A,C), similar to that observed by Zhou et al. [67] and Salles et al. [24]. In contrast, fibers covered by nanoparticles, making the samples thicker, were found in BCP-MAG and BCP-COMAG (Figure 6B,D), similar to those described by Chaabane et al. [55] and Chanthiwong et al. [19].

BCP-MAG and BCP-COMAG samples, for having been processed, had fibers that were cut and reorganized in conjunction with the reagents and/or Fe₃O₄ added, taking on a more homogeneous conformation that led to surfaces composed of fibers covered with nanoparticles. On the other hand, BC-MAG and BC-COMAG were composed of the same materials but without the rearrangement of fibers. Therefore, the magnetite nanoparticles were lodged and clustered in the BC pores, covering the existing structures and forming a thicker layer on the material surface, as evidenced by the denser appearance in SEM images.

Interestingly, micrometric-sized iron chloride crystals were found in the BC-COMAG sample (Figure 6C), which stand out as a result of the brightness emitted due to the interaction between iron and electron beam of the SEM device. The presence of such crystals indicates that not all reagents were converted into magnetite during the material preparation, which may be a disadvantage of this synthesis and incorporation method.

The results of XRD, FTIR, and SEM analyses proved the presence and incorporation of Fe₃O₄ nanoparticles within the BC hydrogels.

3.5. Vibrating Sample Magnetometry (VSM)

The magnetic behavior of each BC-based sample is shown in the VSM graphs of Figure 7 and Figure 8.

These results enabled the determination of each sample’s saturation magnetization (Ms) and coercive field (B), whose values are summarized in

Table 4. Saturation magnetization and coercive field of each sample.

Sample	Saturation Magnetization (emu/g)	Coercive Field (Oe)
Fe3O4	29	87.57
BC-MAG	8	3.80
BCP-MAG	10	22.07
BC-COMAG	3	6.23
BCP-COMAG	17	1.04

BC-MAG = magnetic bacterial cellulose prepared by ex situ coprecipitation; BCP-MAG = magnetic bacterial cellulose prepared by ex situ coprecipitation and processing; BC-COMAG = magnetic bacterial cellulose prepared by in situ coprecipitation; and BCP-COMAG = magnetic bacterial cellulose prepared by in situ coprecipitation and processing.

.

The samples produced with processed BC (BCP-COMAG and BCP-MAG) had higher Ms values than the non-processed ones (Figure 7 and Table 4). This was due to the percentage of magnetic material incorporated into the hydrogels (Table 2), which was the factor mainly responsible for the Ms increase in samples. Being a diamagnetic material, BC had a negative interference with magnetization. Thus, the increase in the concentration of Fe₃O₄ particles increased Ms, confirming the observations of Sriplai et al. [68].

The distribution of magnetite particles in the material was another factor that influenced magnetization. As seen in the SEM images illustrated in Figure 6 and discussed in Section 3.4, each sample took on an arrangement between the particles and BC fibers according to the incorporation method employed. Samples produced with processed BC (BCP-MAG and BCP-COMAG) had greater homogeneity between nanoparticles and fibers, ensuring the presence of Fe₃O₄ in both the outer and inner parts of the material. Instead, in those without prior BC processing (BC-MAG and BC-COMAG), magnetite nanocrystals were deposited on the outer part of samples due to the difficult access caused by the fibers arrangement. Thus, electromagnetic induction during the VSM analysis was greater in BCP-MAG and BCP-COMAG than in BC-MAG and BC-COMAG, in which induction occurred only on the surface.

In the case of the BC-COMAG sample, which had less saturation magnetization, the non-reacted fraction of iron chloride evidenced by SEM (Figure 6C) interfered with the analysis, leading to a lower Ms than that of the BC-MAG sample, which had a lower percentage of magnetite.

The mean size of magnetite crystals incorporated into each sample strongly influenced the respective coercive field values. In fact, as shown in Figure 9, the coercive field increased exponentially with the increase in Fe₃O₄ crystal size.

This increase occurred because the size of nanocrystals approached the critical diameter of magnetite for the formation of magnetic monodomains (in the case of BCP-MAG) and for the adoption of superparamagnetic behavior (in the case of BC-MAG, BC-COMAG, and BCP-COMAG), as discussed in Section 3.3. Superparamagnetic particles respond more quickly to a change in direction of the external magnetic field and cannot retain magnetism, thus having a coercive field that tends towards zero. In contrast, larger nanoparticles have more ferrimagnetic characteristics, which enables the material to magnetize with the external magnetic field, thus having a higher coercive field [64,69,70].

Due to considerable saturation magnetization and large coercive field, BCP-MAG may be used in data storage systems [25], sound amplifiers, and electronic components [71], due to its behavior similar to ferrimagnetism. In contrast, BCP-COMAG, which has high magnetization and a quick response to changes in the magnetic field, may be used in drug delivery systems [24], contrasts for magnetic resonance exams [20], and treatment of dyes-polluted wastewater [22].

3.6. Electromagnetic Interference Shielding Efficiency

As mentioned above, EMI SE depends strongly on the square modulus of the S-parameters. In particular, the larger the square modulus of S₂₁, the lesser the shielding, as more energy is transmitted over time. A square modulus of 0.01 for S₂₁ provides 20 dB, the minimum accepted by the standards for commercial shielding devices [72].

Figure 10 shows the behavior of S-parameters for all the samples. The proximity of the transmission parameters (both real and imaginary parts) represents a virtually close SE_T value for all samples, and the high S₂₁ values denote limited shielding capacity, revealing low EMI SE. The small S₁₁ values may indicate that only a small percentage of the shielding process is produced by reflection.

The literature suggests that materials with good reflection parameters (good electrical conductors, such as metals and semiconductors and magnetic materials) are used to improve shielding mechanisms. Bacterial cellulose does not fit into these cases. Therefore, an approach to dope it with magnetic materials was taken to improve its shielding [1,2,3,4,5].

Figure 11A,B show the EMI SE due to reflection and absorption, respectively. Shielding due to absorption was often threefold greater than that due to reflection. This important characteristic may constitute a safer type of environmental shielding, as reflection processes can produce secondary pollution.

As shown in Figure 12 and Figure 13, BC with a thickness of 0.3 mm had poor shielding characteristics. At its highest, shielding effectiveness was 1 dB at 9.3 GHz, offering feeble protection against electromagnetic interference. Conversely, the smallest value was recorded at 0.1 dB but at lower frequency (8.2 GHz). On average, BC maintained a shielding effectiveness of 0.5 dB, demonstrating insufficient and unreliable performance. The standard deviation of 0.2 dB suggests a level of consistency in shielding capability, making BC a material unsuitable for applications that require electromagnetic shielding.

The fluctuations generated in the measurement of scattering parameters, illustrated in Figure 10, Figure 11 and Figure 12, were caused by the experiment’s inaccuracy. Since internal reflections cannot be obtained in the calculation, this caused an increase in the standard deviation.

As for the materials produced without processing, BC-MAG had a thickness of 1.1 mm and proved to be a more robust contender in the domain of electromagnetic shielding. At its peak, which was the highest one among all the samples studied, it achieved a moderate shielding effectiveness of 4.5 dB at 8.0 GHz, demonstrating a better capacity to thwart electromagnetic interference. It maintained an appreciable shielding effectiveness of 1.8 dB at 11.6 GHz at its minimum, and a mean value of 2.9 dB, thus attesting to its overall effectiveness. Moreover, a standard deviation of 0.6 dB revealed a satisfactory degree of consistency in measure. On the other hand, BC-COMAG, with a thickness of 0.3 mm, exhibited its highest and lowest shielding effectiveness values of 3.8 and 1.9 dB at frequencies of 8.0 and 11.6 GHz, respectively, as well as a mean value of 2.8 dB with a standard deviation of 0.4 dB.

As for the materials produced with processing, BCP-MAG, with a thickness of 1.0 mm, showed the highest and lowest values of 3.1 and 1.5 dB at 8.0 and 11.6 GHz, and a mean value of 2.1 dB with standard deviation of 0.4 dB, while BCP-COMAG, with almost the same thickness (0.9 mm), showed significantly lower shielding effectiveness (highest value of 2.7 dB at 8.0 GHz, smallest value of 1.0 dB at 11.6 GHz, and mean value of 1.8 dB), with the same standard deviation.

Since the overall EMI SE did not reach the standards for commercial shielding devices [72], it could not compete with other shielding materials [73,74]. However, the high values of SE_A and SE_T displayed in Figure 13 suggest potential applications for blocking secondary pollution created by devices with a high SE_R [75,76,77]; so, BC could be used on multilayered substrates for EMI SE. Reaching a mean normalized SE (EMI SE/thickness) of 9.3 dB/mm (Figure 14), BC-COMAG may be suitable for shielding as a substrate, as it allows considerable flexibility and has the ability to withstand temperature changes. Moreover, the incorporation of other materials besides magnetite may enable the use of BC as an EMI shielding material, as suggested by Peng et al. [78].

Table 5. Comparison of the main characteristics of organic materials used for EMI SE with those of BC-MAG and BC-COMAG.

Material	MXene/BC	MXene/ Leather	CNT/ BC-BP	MWCNT@Fe3O4/CMC	BC-MAG	BC-COMAG
Mean EMI SE (dB)	41.0	30.1	24.0	0.23	2.9	2.8
Thickness (mm)	0.015	19	0.036	0.028	1.1	0.3
EMI SE/Thickness (dB/mm)	2733	1.6	667	8.2	2.6	9.3
Frequency (GHz)	8–12	8.2–12.4	5.85–8.2	8–12	8–12	8–12
Reference	[79]	[80]	[81]	[82]	This work	This work

MXene/BC = MXene with bacterial cellulose; MXene/Leather = MXene with leather; CNT/BC-BP = carbon nanotubes with bacterial cellulose and buckypaper; MWCNT@Fe₃O₄/CMC = multi-walled carbon nanotubes with magnetite and carboxymethyl cellulose; BC-MAG = magnetic bacterial cellulose made by ex situ coprecipitation; and BC-COMAG = magnetic bacterial cellulose made by in situ coprecipitation.

lists the main characteristics of organic materials used for EMI SE as well as those of BC-MAG and BC-COMAG. Compared to other compounds, the present results did not meet the initial expectations, which must be seen as an opportunity for valuable learning and improvements. Nonetheless, these findings can guide future approaches towards more effective strategies. The constructive insights gained from this outcome will undoubtedly contribute to the ongoing pursuit of excellence.

4. Conclusions

The method of magnetite nanoparticle incorporation into bacterial cellulose (BC) directly influenced the final characteristics of magnetic BC materials. The methods tested achieved similar degrees of magnetite incorporation in the BC hydrogels, but the selected variables (in situ vs. ex situ synthesis and processed vs. unprocessed BC) affected the size of incorporated particles and their arrangement in the hydrogels, giving the samples different magnetic properties and electromagnetic interference shielding effectiveness (EMI SE). BC processing prior to incorporation gave the final material a higher percentage of particles and a more homogeneous distribution of magnetite among the fibers. The coercive field of samples increased exponentially with the increase in the size of incorporated particles.

Although the samples produced in the present study did not show satisfactory EMI SE, incorporating magnetite increased the wave transmission, absorption, and reflection levels of BC, potentially benefiting the electricity sector. Based on complementary research, this technology can be applied to improve communication between electrical equipment, reducing interference and increasing the reliability of networks, using wave shielding. Furthermore, the use of BC as a renewable material makes this approach more sustainable than the current ones.