A Stimuli-Responsive Hybrid Platform for the On-Demand Delivery of Vitamin B₁₂

Featured Application

A hybrid smart platform was designed for the magnetically triggered release of B₁₂ as a model molecule. This proof-of-concept study demonstrates a heat-responsive nanocomposite capable of modulating the delivery of molecules in response to an externally applied magnetic field and the resulting thermal stimulus.

Abstract

Physically triggered drug release is an emerging field focused on developing materials capable of modulating release kinetics in response to external stimuli. In this work, we present a strategy for the fabrication and evaluation of heat-mediated drug release from electrospun fibers composed of a polyacrylonitrile (PAN) and poly (methyl vinyl ether-alt-maleic acid) (PMA-MVE) blend, encapsulating vitamin B₁₂ (B₁₂-NFs) as a model. Following thermal treatments at 90, 120, and 180 °C, results from SEM, TGA, DSC, and FTIR confirmed that increasing the crosslinking temperature promoted the formation of a more hydrophobic matrix (contact angle > 150°), which effectively reduced spontaneous drug leakage. As a proof-of-concept, we evaluated the sensitivity of the elaborated B₁₂ to heating in aqueous media using UV-Vis spectrometry. The results indicate that the release kinetics followed a sigmoidal profile governed by the dissolution Gompertz model. This laboratory-scale evaluation establishes the fundamental mechanisms for magnetically triggered platforms based on polymeric blends, providing a robust framework for the design of remotely activated, non-invasive drug delivery platforms.

1. Introduction

Innovative drug dosage forms and delivery systems have been developed in recent years to enhance the therapeutic efficacy of active agents for treating a wide range of high-impact conditions, including metabolic, autoimmune, degenerative, and infectious diseases [1,2,3,4]. From colloidal suspensions to three-dimensional scaffolds, both organic and inorganic nanomaterials have served as essential building blocks for the implementation of such technologies [5,6,7,8,9,10].

For the development of robust systemic delivery strategies, nanostructures in colloidal dosage forms have been developed to facilitate active and selective transport. Specific organs or damaged cells can be targeted through selective functionalization, improving therapeutic precision [11,12,13,14,15]. In parallel, localized delivery approaches based on polymeric nanofibers have gained increasing attention due to their ability to cover large surface areas while offering compositional versatility [16,17,18,19].

The scaffolds based on electrospun polymeric nanofiber (EPN) are particularly well-suited for applications in tissue engineering and localized drug delivery due to their ability to mimic the extracellular matrix. These scaffolds support biocompatibility, promote cell adhesion, and allow for precise modulation of dosage, enabling applications ranging from bone repair to skin regeneration [20,21,22,23,24,25]. The intrinsic properties of EPNs can be tailored through polymer chemistry to meet the specific requirements of localized release, including responsiveness to external or physiological stimuli at targeted stages of treatment [26,27,28,29,30,31].

Blends of synthetic copolymers have been engineered to serve as the basis for multifunctional smart platforms [32,33,34]. Such polymeric blends offer the advantage of accommodating multiple components within a matrix governed by viscoelastic, physical, and chemical compatibility. Furthermore, drug molecules can be co-loaded with imaging agents, such as fluorescent or radioactive tracers, facilitating real-time monitoring of biodistribution and pharmacokinetics in preclinical or clinical settings [35,36,37,38]. Additionally, incorporation of sensitizing agents (such as photosensitizers for photodynamic therapy or radiosensitizers for radiotherapy) can synergistically enhance treatment efficacy by increasing the sensitivity of pathological tissues to external therapeutic stimuli [39,40,41,42].

In this context, poly (maleic acid-alt-methyl vinyl ether) (PMA-MVE) presents a promising candidate for the development of versatile drug delivery matrices. This alternating copolymer, consisting of maleic acid (or maleic anhydride) and methyl vinyl ether units, has attracted significant attention due to its physicochemical properties and potential applications in stimuli-responsive delivery systems [43,44,45,46,47]. Its solubility in both aqueous and organic media, together with its biocompatibility and degradability into non-toxic byproducts, make PMA-MVE a valuable component for formulating environmentally friendly and safe drug carriers. Moreover, its chemical structure allows for further functionalization, including the attachment of targeting ligands, enabling the design of complex, entangled architectures for controlled drug delivery [48,49,50,51,52].

Upon contact with aqueous environments, PMA-MVE hydrolyzes into poly (methyl vinyl ether-alt-maleic acid), a water-soluble derivative that is readily cleared and has been shown to be non-toxic in both in vitro [53] and in vivo models [54]. PMA-MVE nanoparticles exhibit no significant in vivo toxicity or inflammatory response, improving intestinal absorption [54]. Furthermore, its biocompatibility as a matrix for drug delivery has been extensively documented in the development of materials for theranostics of Diabetes, evaluated on INS-1 cells delivery systems [55].

In this study, by incorporating vitamin B₁₂ as a model into PMA-MVE -based electrospun scaffolds, the suitability for a drug release platform was evaluated. The design leverages the chemical interaction between the carboxylic acid groups of the polymer and the amine functionalities of the cargo, integrated within a thermally crosslinked polyacrylonitrile (PAN) matrix. By modulating the crosslinking temperature, the molecular architecture and aqueous solubility of the nanofibers were modified towards drug release modifications.

The performed characterization confirmed that these structural modifications enable both drug retention and triggered release under specific stimuli. As a proof-of-concept of the drug release platform, we demonstrate that the release profiles are sensitive to heat-mediated induction using water as a liquid carrier. Furthermore, the ability to modulate the thermal response through magnetic field parameters (frequency and intensity) establishes these hybrid materials as promising candidates for smart, on-demand drug delivery platforms capable of remote, non-invasive activation towards fine control of the drug dosage.

2. Materials and Methods

2.1. Reagents

Poly (maleic acid-alt-methyl vinyl ether) (PMA-MVE) (anhydrous, Mw ~1,080,000, Mn ~311,000 (average)), PAN (average Mw 150,000 (Typical)), N,N-Dimethylformamide (DMF, ReagentPlus^®, ≥99%), and vitamin B₁₂ (cyanocobalamin ≥ 98%) were purchased from Merck (KGaA, Darmstadt, Germany) and used as received.

2.2. Materials Elaboration

Figure 1 schematically illustrates the strategy for the fabrication and evaluation of the stimuli-responsive hybrid platform developed in this study. The polymer blend contains water-soluble PVM/MA and water-insoluble PAN, leveraging their capacity for thermal cross-linking to modulate the solubility and structural integrity of the matrix. This smart delivery platform is designed for the controlled release of vitamin B₁₂ in aqueous media, utilizing alternating magnetic fields (AMF) to provide non-invasive inductive heating. The inductive heating activation facilitates the dissolution and diffusion of the encapsulated vitamin B₁₂, enabling remote-controlled, on-demand drug release.

2.2.1. Polymer Blend Elaboration

For polymeric blend preparation, 0.4 g of PMA-MVE was dissolved in DMF (10 mL). Then, vitamin B₁₂ (10 mg) was added, and the suspension was heated to 60 °C. Later, PAN (1 g) was dissolved in the solution and stirred for 2 h, after which the heating was turned off. The blend was continuously stirred for 12 h to promote entanglement of the PAN polymeric chains and electrostatic interactions between carboxylate anions from PMA-MVE and amine groups from vitamin B₁₂.

2.2.2. Electrospinning

The as-prepared solution was loaded into a 10 mL syringe and fed (0.7 mL/h) to the electrospinning setup using a rotative collector, working distance of 15 cm, and an applied high voltage of 8 kV. The obtained B₁₂-NFs mat was dried (60 °C, 12 h) and prepared for further characterization and evaluation.

2.2.3. Thermal Treatment

Thermal crosslinking was studied at 90, 120, 160, and 180 °C, considering the well-known transition temperatures of PAN where the nitrile groups are available to interact (less than 220 °C) with the carboxylic acid groups (-COOH) in PMA-MVE through hydrogen bonding to entrap amine groups. The samples were labeled as B₁₂-NFs-90, B₁₂-NFs-120, B₁₂-NFs-160, and B₁₂-NFs-180, respectively.

2.3. Materials Characterization

2.3.1. Electron Microscopy

To determine the physical properties of the polymeric mats, electron microscopy was performed using a Thermo Fisher Scientific INSPECT-F50, Waltham, MA, USA Scanning Electron Microscope for imaging, along with energy-dispersive X-ray spectroscopy (EDS) for compositional analysis. Samples were coated with carbon after the analysis to ensure surface conductivity.

2.3.2. Thermal Analysis

Thermal analysis was performed using Differential Scanning Calorimetry (DSC) with a DSC822e Module (Mettler Toledo, Greifensee, Switzerland) from 50 to 500 °C at a heating rate of 1 °C/minute under N₂ atmosphere. Thermo Gravimetric Analysis (TGA) was conducted using a Mettler Toledo TGA SDTA851 analyzer from 50 to 800 °C at a heating rate of 10 °C min⁻¹ and an N₂ purge of 60 mL min⁻¹ in a ceramic pan.

2.3.3. FTIR Spectroscopy

Normal vibration modes of the functional groups in the samples were detected using Attenuated Total Reflectance Fourier Transform Infrared spectroscopy (ATR FT-IR); the spectra from 4000 to 600 cm⁻¹ were acquired using a Bruker VERTEX 70v FT-IR Spectrometer (Bruker, Billerica, MA, USA).

2.3.4. Contact Angle Measurement

After the deposition of a drop (20 µL), digital images were taken (Dino-Lite AM3111 digital microscope, AnMo Electronics Co., Taipei, Taiwan), and the complementary angles between the drops and the surface of B₁₂-NFs mats were measured using DINOCAPTURE 2.0 software.

2.4. Drug Release Platform Evaluation

For the determination of the modifications in the drug release profiles promoted by the materials modifications using thermal treatment, the experimental setup presented in Figure 2 was employed to monitor the amount of released B₁₂, using as a reference the characteristic 510 nm absorbance band over time, using the continuous system reported in [56] with Milli-Q water flowing in the system in all experiments.

To evaluate spontaneous release kinetics from the different B₁₂-NFs samples, the profiles were evaluated at room temperature (20 °C) and 30 °C, controlling the temperature in the sample chamber using recirculating water flow in a thermalized bath. Three disks of 10 mm diameter, trimmed before thermal treatment, were used. The disks were placed in a sample holder for the evaluation of the release profiles. Magnetically triggered release kinetics of samples B₁₂-NFs-90 to B₁₂-NFs-180 were evaluated under stimulation by an alternating magnetic field (AMF, f = 366 kHz, 350 G), where the three disks of B₁₂-NFs were placed between two inductive heating electrospun magnetic nanofibers, elaborated as reported in [56]. After 10 min of spontaneous release, the samples were stimulated with an AMF and evaluated. Once the steady state was reached, the AMF was applied again, and the release profile curves were analyzed. This experiment was conceived to demonstrate the capacity of the B₁₂-NFs to modify the release profile as a consequence of localized inductive heating.

The experimental release profiles were fitted using the Gompertz dissolution model (Equation 1), and the temperature impact on the delivery kinetics was determined by comparing the release rate (k), according to Equation (1) [57,58]:

$$y=\mathrm{A}{e}^{e^{k(t-t_c)}}$$

(1)

where the parameters are defined as follows:

• y = concentration reached at time t [mg/mL].
• A = maximum released concentration [mg/mL].
• k = release rate constant [s⁻¹].
• t = time [s].
• t_c = time to reach 63% of the release [s].

3. Results

3.1. B₁₂-NFs Properties

The physical and chemical properties obtained from the B₁₂-NFs characterization confirm the successful fabrication of drug-loaded fibers with controlled size and morphology. Visually, the electrospun fibers appeared as a homogeneous pink mat, resulting from the combination of the characteristic red color of vitamin B₁₂ and the white hue of the polymeric blend.

As shown in Figure 3A–C, the fibers were continuous, solid, and rough, with an average diameter of approximately 1.50 ± 0.35 µm. The electron microscopy images (Figure 3D) revealed compact, randomly oriented B₁₂-NFs with a textured surface, suggesting that vitamin B₁₂ was distributed both within and on the surface of the fibers. This highlights the successful integration of the drug into the nanofiber matrix.

Thermal analysis results were summarized in Figure 4. TGA thermograms revealed slight differences between blank electrospun fibers (PMA-MVE/PAN) and the B₁₂-NFs (Figure 4A,C). Figure 4B,D show the DSC thermograms, where the thermal transitions were slightly shifted. The electrospun PMA-MVE/PAN/B₁₂ fibers thermograms show weight loss ($\approx$10%) at 213 °C and a higher glass transition temperature (Tg) compared with the blank sample, suggesting interactions among the components as temperature increases, but associated with B₁₂ degradation, which was not found up to 200 °C. These results allow us to conclude that below 200 °C, the samples exhibited modifications related to the polymeric blend, ensuring that thermal treatment within the working interval (90–180 °C) does not degrade the B₁₂ molecule.

The normal vibration modes of the molecules that compose the fabricated materials were analyzed after the thermal treatment at 90, 120, 160, and 180 °C, and compared to non-treated samples (NT). As shown in Figure 5, the FTIR spectra revealed interactions of the main functional groups of the different samples. The spectral modifications are associated with conformational modifications of the polymeric chains, mainly attributed to chain reorganization produced by the nitrile group volatilization [59].

Table 1. Main vibrational normal modes detected in the samples. The labeled frequencies ($\overline{\nu }$) are associated with the corresponding functional group.

$\overline{\mathit{\nu }}$ (cm−1)	Functional Group
3400	O-H
2900	CH2
2400–2200	C≡N
1700, 1550	NH2
1550	NH
1750	C=O
1725	C=C
1425	C-H
1240, 1100	C-O
780–580	C-H

summarizes the characteristic vibrational modes identified in the FTIR spectra of the analyzed samples. The bands corresponding to the amine functional groups of the B₁₂ molecule (at ~1700 and 1550 cm⁻¹) appeared more intense and better resolved compared to those observed in the composite samples (Figure 5). This difference may be attributed to modifications in the polymeric matrix, which can restrict the mobility and accessibility of the amine groups at the surface. Such constraints likely alter the vibrational degrees of freedom of the B₁₂ molecule, resulting in band shifts or intensity changes within the complex system.

The effects of thermal treatment on the wettability of the produced B₁₂-NFs were evaluated by analyzing the contact angle. The evolution of the contact angle as a function of increasing thermal treatment temperature is shown in Figure 6. Superhydrophobic behavior (contact angle > 150°) was observed in samples treated above 120 °C; in contrast, for the samples treated at 90 °C, it was not possible to register the contact angle due to immediate wetting.

3.2. Drug Release Response

Figure 7 shows representative release profiles from the evaluated samples at room temperature (20 °C). The profile from the sample treated at 90 °C (Figure 7A) shows a steady state after 25 s, while the hydrophobic samples (Figure 7B–D) released less B₁₂ and increased time for the steady state. Reduced wettability may limit interactions between the liquid carrier (Milli-Q water) and the B₁₂-NFs, thereby decreasing the system’s ability to release the molecule. This suggests that the dissolution profile of the samples is modified as a function of thermal treatment.

The observed spontaneous release profiles were adjusted to the Gompertz dissolution model (Figure 7, red lines). The triplicated measurements results showed good fitting with the selected model, and the summary of parameters as a result of the fitting can be observed in

Table 2. Parameters from the Gompertz model adjusted release kinetics from thermally treated B₁₂-NFs (90, 120,160, and180 °C). The mass of the B₁₂-NFs (mg) is reported. Maximum release concentration (A [mg/mL]), release rate constant (k [s⁻¹]), and time to reach 63% of the release (t_c [s]) from the release profiles obtained at room temperature (20 °C). R² of each release profile adjustment is provided. Average and standard deviation (σ) were calculated for triplicated measurements.

Samples	B12-NFs (mg)	Measure	A (mg/mL)	k (s−1)	tc (s)	R2
90 °C	2.2	1	0.024	0.273	4.4	0.89745
	2.7	2	0.029	0.378	4.3	0.89531
	2.7	3	0.027	0.360	2.6	0.80021
		Average	0.027	0.337	3.8
		σ	0.003	0.056	1.0
120 °C	2.7	1	0.029	0.009	104.3	0.99763
	2.1	2	0.024	0.271	4.9	0.96525
	2.4	3	0.026	0.011	78.6	0.99793
		Average	0.026	0.097	62.6
		σ	0.002	0.151	51.6
160 °C	2.3	1	0.010	0.014	75.5	0.94767
	1.4	2	0.006	0.019	8.2	0.88721
	2.0	3	0.008	0.043	15.5	0.86573
		Average	0.008	0.025	33.1
		σ	0.002	0.015	36.9
180 °C	0.8	1	0.001	0.0163	90.1	0.89397

. As can be observed, the experiment for the 180 °C samples was performed only once as a reference for this thermal treatment temperature. This is because the release was near the detection limits of the equipment, and the temperature was close to the degradation of B₁₂. However, the release profile from 180 °C samples still has a good fit with the Gompertz profile (R² = 0.89397).

The water bath temperature was subsequently raised to maintain 30 °C within the sample holder (Figure 2 and Figure 4), allowing for the evaluation of the thermal sensitivity of the spontaneous release. Representative release profiles from the B₁₂-NFs (90, 120, 160, and 180 °C) samples are presented in Figure 8A–D, and the Gompertz fitting is represented by red lines. It can be observed that all the profiles showed superior release capacity from B₁₂-NFs evaluated at 30 °C, compared with 20 °C (Figure 7). These results suggest a dependency of the capacity to deliver B₁₂ on the temperature of the surrounding media.

The fitting parameters for the Gompertz dissolution model for the obtained release profiles are summarized in

Table 3. Parameters from the Gompertz model adjusted release kinetics from thermally treated B12-NFs (90, 120, 160, and 180 °C). The mass of the B₁₂-NFs (mg) is reported. Maximum release concentration (A [mg/mL]), release rate constant (k [s⁻¹]), and time to reach 63% of the release (t_c [s]) from the release profiles obtained at room temperature (30 °C). R² of each release profile adjustment is provided. Average and standard deviation (σ) were calculated for triplicated measurements.

Samples	B12-NFs (mg)	Measure	A (mg/mL)	k (s−1)	tc (s)	R2
90 °C	2.8	1	0.028	0.183	6.0	0.93309
	2.6	2	0.024	0.493	18.8	0.93093
	2.4	3	0.024	0.361	5.5	0.92125
		Average	0.026	0.346	10.1
		σ	0.002	0.155	7.5
120 °C	2.8	1	0.028	0.030	26.3	0.97767
	2.2	2	0.025	0.466	18.2	0.81065
	2.3	3	0.026	0.194	16.6	0.97527
		Average	0.026	0.230	20.4
		σ	0.001	0.220	5.2
160 °C	1.6	1	0.014	0.006	24.4	0.97156
	2.5	2	0.024	0.002	150.0	0.98988
	2.0	3	0.020	0.004	107.9	0.98872
		Average	0.019	0.004	94.1
		σ	0.005	0.002	63.9
180 °C	1.0	1	0.006	0.004	−109.7	0.95152

. Variability of the results can be attributed to the changes in the mass of the samples evaluated. The three disks employed can have variability in weight and B₁₂ loading. On the other hand, the dependence of the release rate on the increase in temperature can be observed in the differences in the release rate (k) between the samples evaluated at 30 °C and those evaluated at 20 °C. The obtained values of R² for fitting are superior as well, supporting the hypothesis of dissolution mechanism in the release response of the elaborated materials.

SEM images of the membranes used to evaluate the magnetically triggered drug release platform are shown in Figure 9. B12-NFs (Figure 9A) were placed between compacted magnetic fibers (PAN/FeMnO), which served as an inductive heating source, and were stimulated using an alternating magnetic field (AMF) at 28 kA/m (≈350 G). The representative drug release response from the evaluated B12-NFs is presented in Figure 9C, where the influence of the magnetic stimulus on the release profile can be observed. AMF was activated after 10 min of spontaneous release, upon reaching steady state. Figure 9D displays the temperature increase induced by AMF activation, confirming the localized heating effect of the magnetic platform.

The magnetically triggered response of the materials is summarized in

Table 4. Parameters from the Gompertz model adjusted release kinetics from thermally treated B12-NFs (90, 120, 160, and 180 °C). Maximum release concentration (A [mg/mL]), release rate constant (k [s⁻¹]), and time to reach 63% of the release (t_c [s]) from the release profiles obtained using magnetic stimulus (up to 60 °C). The mass of the B12-NFs (mg) and magnetic fibers is reported. R² of each release profile adjustment is provided. Average and standard deviation (σ) were calculated for triplicated measurements.

					Fiber’s Mass (mg)
Samples	Measure	A (mg/mL)	k (s−1)	tc (s)	B12-NFs	Magnetic	R2
90 °C	1	1.719	1.819	9.0	1.4	8	0.06445
	2	2.442	0.719	7.0	2.2	10.4	0.87578
	3	1.463	1.312	8.9	1	9.5	0.95898
	Average	1.875	1.283	8.3
	σ	0.508	0.551	1.1
120 °C	1	1.548	0.813	8.5	1.1	8.6	0.90973
	2	2.866	0.952	8.1	2.1	8	0.94088
	3	2.056	0.953	7.9	1.3	9.2	0.90484
	Average	2.157	0.906	8.2
	σ	0.665	0.081	0.3
160 °C	1	1.552	1.010	8.4	2.6	8.1	0.97793
	2	1.263	1.207	8.9	2.4	10.3	0.87452
	3	1.776	1.207	8.7	1.8	9.8	0.90015
	Average	1.530	1.141	8.7
	σ	0.257	0.114	0.3
180 °C	1	0.781	0.757	9.0	2.1	9.5	0.88692

. The obtained results correspond to the release after the steady state was reached after 10 min (Figure 9C). Once the spontaneous release was constant, the AMF was switched on, and the modifications to the release profile were recorded using the continuous flow UV-Vis monitor. The platform showed improved release rates compared with 30 °C and 20 °C. The maximum release concentration (A) and the release rate (k) increase significantly.

4. Discussion

The observed surface features suggest that the critical solubility point of vitamin B₁₂ in the polymeric precursor solution was not reached during the fabrication. The solubility of vitamin B₁₂ in the precursor solution plays a crucial role in determining its homogeneity, drug loading efficiency, and the final morphology of the nanofibers. Exceeding the solubility threshold could result in phase separation or surface aggregation, potentially contributing to the decorated-like texture observed in the microscopy images.

For the drug-loading process in the B₁₂-NFs system, both maleic acid and methyl vinyl ether units in PMA-MVE can engage in hydrogen bonding interactions with DMF molecules. This interaction can further enhance the solubility of the polymer and the potential interaction with amine-based drugs. When PMA-MVE is dissolved in a basic solvent, the protonation state of its carboxylic acid groups can change, leading to alterations in solubility, conformation, and potentially, chemical reactivity.

Additionally, carboxylate anions (COO⁻) and amine groups (NH₂) can form hydrogen bonds with each other. The oxygen atom in the carboxylate group carries a partial negative charge, while the nitrogen atom in the amine group carries a partial positive charge. This charge separation allows for hydrogen bonding between the two functional groups. The hydrogen bond formation enhances the stability of the interaction between carboxylate anions and amine groups.

Carboxylate anions and amine groups may interact through ion–dipole interactions. The ionic nature of the carboxylate and the polarity of the amine nitrogen create a strong electrostatic attraction between them. This interaction is like the attraction between ions and polar molecules in solution, ensuring the B₁₂ encapsulation and integration on the fibers.

Thermal treatment is a straightforward strategy to control the drug release capacities of B₁₂-NFs. By controlling the crosslinking temperature, material properties such as hydrophilicity, swelling behavior, and degradation rate can be tailored to suit specific applications. Temperature affects the reaction rate of the crosslinking reaction, influences the degree and uniformity of crosslinking, which determines the properties of the final polymer network. Low temperature may lead to incomplete crosslinking or irregular network formation, affecting the mechanical strength and stability of the B₁₂-NFs. However, excessive temperature can cause over-crosslinking, leading to brittleness or degradation of the B₁₂-NFs.

Selecting the right temperature prevents thermal degradation of the polymer or drug embedded in the fibers. It also ensures the formation of a well-organized and homogeneous crosslinked network, balancing flexibility and rigidity. Avoiding elevated temperatures helps to remove residual solvents or byproducts from the crosslinking reaction, ensuring a clean and stable network. According to the FT-IR spectra (Figure 3), the band at 1450 cm⁻¹ is associated with CH₂ bending (scissoring), and at 1170 cm⁻¹, it is associated with the unsaturated esters and carboxylic acids [60,61]. It is observed that the vibrations corresponding to the -OH groups only appear in the membranes with a temperature treatment lower than the boiling temperature of water. The vibrational modes of the functional groups listed in Table 1 represent the main interactions of the polymer chain. In the sharp peak centered at 2250 cm⁻¹ corresponding to the nitrile group (C≡N), there are no significant changes, but the rest of the band loses intensity as the treatment temperature increases. As the temperature increases in the heat treatment, the nitrile groups begin to dissociate, which initiates the cross-linking process.

Almost all the bands that appear within the 1800–600 cm⁻¹ interval are due to the functional groups provided by vitamin B₁₂, being able to identify both primary and secondary amines [62] (Table 1). However, it is important to highlight the C=C interactions that are observed centered at 1750 and 1725 cm⁻¹, which is proof that the polymer chains undergo conformational changes following treatment at 120 °C. These conformational changes directly impact the physicochemical properties of the material. With this characterization, it is possible to predict more evident changes in the behavior of the material after 120 °C of heat treatment.

The samples subjected to the highest temperature (B₁₂-NFs180) were the most hydrophobic, with a contact angle of 167 °C. All of them can be considered superhydrophobic, surpassing the superhydrophobic threshold (150°) with 154° for B₁₂-NFs120 and 157° for B₁₂-NFs160, respectively. In the case of the membrane being treated at 90 °C, since the water drop was expanded instantly and wetted all the mat, it was classified as hydrophilic. The thermal treatment of B₁₂-NFs leads to cross-linking via inter-macromolecular esterification and anhydride formation controlled by temperature and thermal treatment time.

The cross-linked B₁₂-NFs materials are able to deliver vitamin B₁₂ in water, and the release rate can be modified by temperature and magnetic field stimulation using magnetic fibers. The thermal treatment of the prepared B₁₂-NFs determines the fiber’s mat properties, while the heating stimulus on the evaluated B₁₂-NFs in water promotes release enhancement. The elaborated platform showed evidence of modified release profiles using the intrinsic properties (cross-linking) of the materials and the magnetic stimulus (frequency and intensity) as control parameters.

Once a steady state of the spontaneous release was ensured (10 min), the magnetic stimulus was applied to increase the local temperature. The observed behavior described in Figure 9 is associated with an increment in the release rate because of the enhanced dissolution of the B₁₂-NFs once the magnetic stimulation is applied.

From the comparison of Table 4, it is possible to establish that the involved mechanism is the dissolution of the fibers, supported by the observed Gompertz response, associated with this phenomenon. Higher cross-link temperature promotes stronger chemical interactions that impact the degree of solubility of the B₁₂-NFs and the observed spontaneous release. The AMF, as a non-invasive stimulus, is capable of modifying the release profiles from the B₁₂-NFs, modifying the solubility based on the local heating.

While the current study successfully establishes the fundamental release mechanisms of the [PMA-MVE/PAN] matrix under controlled laboratory conditions, it is important to acknowledge the limitations of using Milli-Q water as the release medium. In a biological environment, physiological factors such as ionic strength and pH fluctuations (e.g., in Phosphate-Buffered Saline, pH 7.4) are expected to influence the electrostatic interactions between the polymer network and the therapeutic cargo. Furthermore, the inevitable formation of protein corona upon exposure to biological fluids may alter the surface porosity and diffusion pathways of the nanofibers, potentially modulating the release kinetics in vivo. Consequently, the results presented here serve as a necessary physicochemical baseline, providing the structural and thermal evidence required to advance toward future evaluations in simulated physiological media and complex biological environments.

The future outlook for magnetically triggered electrospun platforms lies in the transition from proof-of-concept to in vitro and in vivo evaluation within living systems. While this study establishes the fundamental sensitivity of B₁₂-NFs to inductive heating, future research should focus on the implementation of Curie-limited nanoparticles to ensure intrinsic thermal safety at physiological thresholds (41–45 °C). Additionally, the AMF conditions (intensity and frequency) play an important role in the control of the temperature, opening the gates for multiparametric control of the spatiotemporal molecule release. Furthermore, the integration of these hybrid fibers into theranostic frameworks could revolutionize personalized medicine, as the magnetic signal serves as both a release trigger and a contrast agent for real-time imaging. Addressing the long-term biocompatibility and the influence of the protein corona in complex media will be the final steps in translating these non-invasive, remote-controlled scaffolds from the laboratory to clinical implementation.

5. Conclusions

This study successfully demonstrates a hybrid electrospun platform capable of precisely modulating the release of a model drug through thermal crosslinking and remote magnetic induction. The thermal processing temperature was identified as a critical parameter in governing the release kinetics; higher temperatures fostered a more compact and hydrophobic matrix, effectively suppressing spontaneous drug leakage. Under magnetic actuation, the system exhibited a remarkable transition from passive to active delivery. Notably, for the B₁₂-NFs treated at 90 °C, the application of an alternating magnetic field (AMF) increased the release rate constant from 0.346 s⁻¹ to 1.283 s⁻¹, while the maximum concentration reached 1.875 mg/mL, representing more than a 70-fold enhancement compared to passive diffusion, 0.026 mg/mL. These results confirm that inductive heating can effectively overcome diffusion barriers within the nanofiber matrix, restoring high release rates on demand. These results establish a robust physicochemical framework for the development of smart scaffolds. The ability to tune the thermal response via magnetic field parameters offers a promising strategy for future therapeutic systems requiring high spatial and temporal control over dosage. Further studies in physiological media will be essential to translate this proof-of-concept into clinical applications.