PMSSO-Hydrogels as a Promising Carrier for B₁₂ Vitamin

Abstract

The development of novel dosage forms of vitamin B₁₂ is an urgent task for addressing vitamin deficiency in individuals with gastrointestinal diseases or those following stringent dietary limitations. The study illustrates the fundamental possibility of employing a non-toxic and biocompatible organosilicon hydrogel with significant sorption capacity for B₁₂ delivery. Research indicated that 40 min of incubation suffices for optimal loading efficiency, influenced by both external diffusion and intradiffusion factors. The release of B₁₂ in a medium that mimics the human gastrointestinal tract transpires almost entirely within a timeframe that aligns with physiological conditions. Consequently, organosilicon hydrogels serve as potential vehicles for the administration of vitamin B₁₂.

1. Introduction

In recent years, vegetarianism and its stricter variation, veganism, have been gaining increasing popularity among the population. In some cases, such a diet may be a necessary avoidance of animal-derived products to treat diseases and/or support medical treatments [1]. Therefore, an individual’s condition, including cardiovascular system parameters, is carefully monitored by physicians. They ensure the necessary balance of micro- and macro-elements, hormone levels, and vitamins. Consequently, this dietary system is designed to support and improve the individual’s well-being and facilitate the treatment process.

In other cases, voluntary vegetarianism has become popular among the general population due to growing awareness of its advantages. These include animal welfare, reduced risk of infections transmitted from animals to humans (such as helminthiasis), the widespread availability of plant-based foods in many regions, high fiber content in the diet, and other health benefits. However, the main drawback of voluntarily vegetarianism is the deficiency of compounds that can only be obtained from animal-derived foods, for instance, animal proteins, specific fatty acids, several trace elements (iron, zinc, selenium), and some vitamins. As a result, the World Health Organization (WHO) recommends that vegetarians take various supplements and dietary additives to compensate for the deficiency [2].

One of the most significant issues is vitamin B₁₂ deficiency among vegetarians. The daily requirement for B₁₂ in adults is 2–6 µg [3]. However, B₁₂ is found exclusively in animal-derived foods such as meat and fatty milk. Plant-based foods contain only trace amounts of vitamin B₁₂ [4]. As an enzyme cofactor, B₁₂ plays a key role in crucial metabolic processes; thus, the B₁₂ deficiency can lead to severe health problems.

On the other hand, vitamin B₁₂ deficiency can result from several factors: inadequate dietary intake, lack of intrinsic factor, limited bioavailability, surgical procedures, aging, and more. The treatment includes intramuscular injection (often at intervals of 2–3 months) or oral tablet forms with a high vitamin content. Oral vitamin delivery systems represent a viable alternative to intramuscular injections [5]. To date, no adverse effects have been identified in such cases, so there is no established upper safe limit for B₁₂ dosage [6,7]. However, numerous recent studies have focused on the rational use of B₁₂: the decrease of dosage by improving the vitamin’s stability and bioavailability in the gastrointestinal tract. This goal can be achieved using drug delivery systems [8,9].

Therefore, vegetarians and patients from the risk groups are advised to consume supplements containing the recommended daily amount of vitamin B₁₂ [10].

An effective approach to the development of non-animal-based dietary supplements involves encapsulation of precise vitamin B₁₂ dosage in drug delivery systems, for example, polymers that can effectively adsorb the bioactive compound and exhibit pH-responsive release from the matrix [11,12]. Recently, we demonstrated the successful sorption of iron compounds and their complexes with cyclodextrins by silicon-organic polymethylsilsesquioxane hydrogels (PMSSO). The promising results allowed us to highlight the other perspectives of PMSSO as a drug delivery system for more complicated molecules, such as B₁₂ [13,14]. Modified silicas are suitable carriers for vitamin B₁₂, as these materials are already widely applied in dietary additives, intestinal sorbents, cosmetic formulations, and pharmaceutical products [15,16]. The well-known physicochemical properties of silicas (a large specific surface area that ensures high sorption capacity; thermal stability; chemical inertness; biocompatibility) make the polymer an advantageous carrier for vitamin B₁₂ [17]. Compared to polypeptide- or lipid-based B₁₂ delivery systems, polymer carriers have shown prospective experimental results and offer broader possibilities for modification [8].

Here we investigate the patterns of sorption and desorption of vitamin B₁₂ in PMSSO-hydrogel to uncover its potential as an effective vitamin carrier for future application to the support and/or treatment of B₁₂ deficiency.

2. Materials and Methods

2.1. Materials

Sodium chloride (NaCl, 58.44 g/mol, purity 99.9%, Reakhim, Moscow, Russia), potassium phosphate monobasic (KH₂PO₄, 136.09 g/mol, purity ≥ 99%, Sigma Aldrich, St. Louis, MO, USA), sodium phosphate dibasic (Na₂HPO₄, 141.96 g/mol, purity ≥ 99%, Sigma Aldrich, St. Louis, MO, USA), Cyanocobalamin B₁₂ (0.5 mg/mL, Mosagrogen, Russia), 0.9% NaCl solution (saline solution for injection, Solopharm, St. Petersburg, Russia). PMSSO-hydrogel was provided by Dr. Alexandra Kalinina (1:1 MeSiO_1.5/SiO₂ links molar ratio, specific surface area 278.7 ± 4.5 m²/g) [14]. The structure of PMSSO-hydrogel is shown in Figure 1.

2.2. Methods

2.2.1. B₁₂ Sorption into PMSSO-Hydrogel

First, 100 mg of PMSSO-hydrogel was soaked in 1 mL of a vitamin B₁₂ solution (0.3 mg/mL). The system was thoroughly mixed and incubated at the temperature of 22 °C and a stirring speed of 150 rpm. Between 5 and 120 min, 20 μL of the supernatant solution was collected and diluted in 580 μL of physiological saline. Then, the UV absorption spectra of the resulting samples were recorded. Based on the obtained optical density at the maximum wavelength at 361 nm, the concentration of loaded B₁₂ was calculated. After that the sorption curve was analyzed.

2.2.2. B₁₂ Release

The B₁₂ release was carried out in three buffer systems that imitate digestive media in order to imitate the gastro-internal tract in vitro according to the American Pharmacopeia and previously published protocols [14]. The main pipeline of the experiment is presented on Scheme 1.

Briefly, 100 mg of PMSSO-B₁₂ was added to 2 mL of the gastric medium (pH 1.1), containing 0.08 M HCl and 0.03 M NaCl. After 2 h of incubation at 37 °C, the sample was transferred to the Intestinum Tenue medium (I.T.), which consisted of 0.05 M KH₂PO₄ and 0.05 M NaOH (pH 6.8), and incubated for 4 h, followed by incubation in the Intestinum Colon medium (I.C.), containing 0.07 M KH₂PO₄ and 0.07 M Na₂HPO₄ (pH 7.4), for 2 h. Every 20 min, a sample of 2 mL was collected, and a fresh portion of the medium was added to the gel. The UV absorption spectrum for the selected solutions was recorded to monitor the concentration of desorbed vitamin B₁₂.

2.2.3. Spectroscopy Studies

The UV-spectra were recorded with an Ultrospec 2100 pro instrument (Amersham Biosciences, Amersham, UK) within a wavelength range of 200–500 nm in a 1 mL quartz cell (Hellma Analytics, Müllheim, Germany).

2.2.4. Mathematical Processing of the Data

Sorption curves were studied using pseudo-first-order and pseudo-second-order kinetics, according to Equations (1) and (2).

$${\mathrm{A}}_{\mathrm{t}}{=\mathrm{A}}_{\infty }(1-{\mathrm{e}}^{{\mathrm{k}}_1\mathrm{t}})$$

(1)

$${\mathrm{A}}_{\mathrm{t}}={\displaystyle \frac{{\mathrm{A}}_{\infty }^2{\mathrm{k}}_2\mathrm{t}}{1+{\mathrm{A}}_{\mathrm{t}}{\mathrm{k}}_2\mathrm{t}}}$$

(2)

where ${\mathrm{A}}_{\mathrm{t}}$ is the amount of sorbed B₁₂ at time t, ${\mathrm{A}}_{\infty }$ is the amount of sorbed B₁₂ at equilibrium, and ${\mathrm{k}}_1$ and ${\mathrm{k}}_2$ are the rate constants of the sorption process.

Equations (3) and (4), representing the Boyd–Adamson model for external and internal diffusion, respectively, are as follows:

$$\ln (1-{\mathrm{A}}_{\mathrm{t}}/{\mathrm{A}}_{\infty })=-\mathrm{C} \times \mathrm{t}$$

(3)

$$\mathrm{A}=f\left(\sqrt{\mathrm{t}}\right)$$

(4)

where C is an empirical constant.

For the studies of intradiffusion processes, the Weber–Morris model was applied with Equation (5):

$${\mathrm{A}}_{\mathrm{t}}={\mathrm{k}}_{\mathrm{id}}\sqrt{\mathrm{t}}+\mathrm{c}$$

(5)

where ${\mathrm{k}}_{\mathrm{id}}$ is the rate constant of internal diffusion and c is a parameter related to the thickness of the boundary layer.

Drug release models were considered as follows. The zero-order release model is described using Equation (6):

$${\mathrm{Q}}_{\mathrm{t}}={ \mathrm{Q}}_{\infty }+{\mathrm{K}}_0\mathrm{t}$$

(6)

where ${\mathrm{K}}_0$ is the zero-order release constant, min⁻¹; ${\mathrm{Q}}_{\mathrm{t}}$ is the amount of substance released at time t; ${\mathrm{Q}}_{\infty }$ is the total amount of substance released.

The first-order release model corresponds to Equation (7):

$${\mathrm{Q}}_{\mathrm{t}}={\mathrm{Q}}_{\infty } \times {\mathrm{e}}^{-{\mathrm{K}}_1\mathrm{t}}$$

(7)

where ${\mathrm{K}}_1$ is the first-order release constant, min⁻¹.

The Korsmeyer–Peppas release is described using the ratio (8):

$${\displaystyle \frac{{\mathrm{Q}}_{\mathrm{t}}}{{\mathrm{Q}}_{\infty }}}=\mathrm{K} \times {\mathrm{t}}^{\mathrm{n}}$$

(8)

where n is the release index; $\mathrm{K}$ is the degree of release constant, min⁻¹.

The Higuchi release model is represented by a complex relationship (9):

$${\mathrm{Q}}_{\mathrm{t}}=\mathrm{A} \times \sqrt{{\displaystyle \frac{\mathrm{D} \times \mathsf{\epsilon }}{\mathsf{\tau }}} \times \left(2 \times \mathrm{C}-\mathsf{\epsilon } \times {\mathrm{C}}_{\mathrm{s}}\right) \times {\mathrm{C}}_{\mathrm{s}} \times \mathrm{t}}={\mathrm{K}}_{\mathrm{H}} \times \sqrt{\mathrm{t}}$$

(9)

where ${\mathrm{K}}_{\mathrm{H}}$ is the Higuchi dissolution constant, min^−1/2; A is the unit of surface from which the amount of substance ${\mathrm{Q}}_{\mathrm{t}}$ during time t; D is the diffusion coefficient, sm²/min; C is the initial concentration of the released substance; ${\mathrm{C}}_{\mathrm{s}}$ is the solubility of the substance in the matrix; $\mathsf{\epsilon }$ is the porosity of the matrix (the number of channels and pores); $\mathsf{\tau }$ is the tortuosity factor of the capillary system of the matrix (the value of the radius and branching of the channels and pores of the matrix).

2.2.5. Statistical Analysis

All experiments were triplicated, and the results were expressed as the mean value ± standard deviation, SD (n = 3). AtteStat 3.04 for Microsoft Excel was used for statistical analysis. Significance was analyzed using the Mann–Whitney test, with p ≤ 0.05 considered statistically significant.

3. Results and Discussion

3.1. B₁₂ Vitamin Sorption—Role of the Temperature

To investigate the vitamin B₁₂ sorption process in PMSSO-hydrogels, we applied UV-VIS spectroscopy, as vitamin B₁₂ exhibits a pronounced absorption band at 361 nm. Figure 2 illustrates the structure of vitamin B₁₂ and the calibration curve (saline solution), which exhibits linearity throughout a broad concentration range. The sensitivity coefficient is $(29.2 \pm 0.3) \times {10}^3 \mathrm{L}/(\mathrm{mol} \times \mathrm{cm})$, signifying the method’s great sensitivity to the concentration of vitamin B₁₂ in the solution. The detection limit is $2.0 \pm 1.0 \mathsf{\mu }\mathrm{g}/\mathrm{mL}.$ The determined extinction coefficient $\mathsf{\epsilon }=(29.2 \pm 0.3) \times {10}^3$ L/(mol × cm) aligns well with the literature values [18].

The incubation temperature can significantly affect the sorption processes of active compounds in gels. We investigated the kinetic of sorption equilibrium by incubating PMSSO-hydrogels in a vitamin B₁₂ solution for 120 min at 22 °C (Figure 3).

The analysis of the curve indicates that sorption equilibrium is attained promptly, after ≈40 min. The concentration of B₁₂ after sorption calculated using the UV-VIS calibration curve was $1.65 \times {10}^{-4}$ M, whereas the concentration of the initial solution was ${\displaystyle \frac{0.3}{1355.38}} \approx 2.22 \times { 10}^{-4}$ M. The amount of sorbed B₁₂ per 100 mg of the carrier was $(2.22-1.65) \times { 10}^{-4} \times 1000 \times 1355.38 \approx 77 \mu \mathrm{g}$.

Thus, the degree of B₁₂ extraction from the solution was 25.7% (10):

$$\alpha ={\displaystyle \frac{77}{0.3 \times 1 \times 1000}} \times 100\% \approx 25.7\%$$

(10)

The sorption capacity of the PMSSO-hydrogel can be calculated as the mass or amount of B₁₂ sorbed per 1 g of sorbent:

$$\mathrm{A}={\displaystyle \frac{77}{0.1000 \times 1000}} \approx 0.77 \mathrm{mg}/\mathrm{g}$$

(11)

$$\mathrm{A}={\displaystyle \frac{77 \times { 10}^{-6}}{1355.38 \times 0.1000}} \approx 5.7 \times { 10}^{-7} \mathrm{mol}/\mathrm{g}$$

(12)

Compared to the previously published data, the highest sorption capacity of the carrier for B₁₂ in the literature is characteristic of inorganic materials. For example, loading B₁₂ into nanolayers of montmorillonite yields 20–160 mg per 1 g of sorbent [19], while mesoporous carbon particles have achieved a record 486 mg of B₁₂ per 1 g of the delivery system [20]. For organic polymer carriers, apparently due to high hydrophilicity, the sorption capacity of the carriers is significantly lower. Thus, the highest value (15–25 mg/g) was found when encapsulating B₁₂ in crosslinked carboxymethyl pullulan [21]. However, other studies mention values close to our data, for example, 1% by weight for poly(acrylic acid) microspheres (modified or unmodified with cysteine) [22], or 0.21 mg/g for soy protein nanoparticles [23].

The initial increase in sorption, followed by a plateau as equilibrium is reached, may suggest that at the beginning of experiment, B₁₂ molecules transfer from the solution to the active sites of the gel and then bind to the sorbent’s surface (apparently to the –OH groups). The sorption curve may be divided into two segments: the initial rising region and the plateau. The mathematical analysis of both regions may provide detailed insight into the interaction mechanism between B₁₂ molecules and PMSSO [24].

3.2. B₁₂ Vitamin Sorption: Looking for the Mathematical Model

The kinetics of B₁₂ sorption into PMSSO were approximated using first- and second-order models, as shown in Equations (1) and (2), and the corresponding kinetic parameters are summarized in

Table 1. Rate constants of the first- and second-order of sorption kinetics (p = 0.95, n = 3).

α, %	At, mg/g	Pseudo-First Order			Pseudo-Second Order
25.7	0.77	At, mg/g	${\mathbf{k}}_{\mathbf{1}}$, min−1	R2	At, mg/g	${\mathbf{k}}_{\mathbf{1}}$, g/(mg × min)	R2
		0.766 ± 0.01	0.147 ± 0.008	0.99	0.83 ± 0.06	0.303 ± 0.012	0.98

.

Figure 4 shows the linear anamorphosis of the experimental kinetic data describing the pseudo-first and pseudo-second order. The pseudo-second-order kinetic equations describe experimental data over the entire time interval, while the pseudo-first-order model describes only the initial stages of the process. This can be considered as a sign of nonlinear sorption of active molecules when filling the main binding sites.

In order to study this process in detail, the sorption of B₁₂ molecules from the solution to the bulk phase of the PMSSO was analyzed using the Boyd–Adamson model for external and internal diffusion and the Weber–Morris model for internal diffusion [25]. The Boyd–Adamson model was applied using Equations (3) and (4), which describe external and internal diffusion, respectively. This approach has proven itself in a number of studies, for example, in the analysis of the sorption of copper compounds into the matrix of the natural polysaccharide chitosan [26].

The obtained data are shown in Figure 5A,B. Comparing these graphs, one could conclude that neither external nor internal diffusion are unambiguously limiting the sorption process, which may be due to the uneven distribution of the PMSSO pore size. At the initial stages of sorption, the limiting process is probably external diffusion (external diffusion mass transfer), and at the final stages, when most of binding sites are already loaded of sorption, internal.

For the further studies of intradiffusion processes, we have applied the Weber–Morris model according to Equation (5). The obtained data are shown in Figure 6.

The curve in the coordinates of the Weber–Morris model (Figure 6) is not linear, indicating the sorption process is not uniquely limited by external and internal diffusion, which agrees with the conclusions drawn from the Boyd–Adamson theory, similar to the previously published paper [26]. The multilinearity of the dependence of the Weber–Morris model indicates a multi-stage interaction of sorbate and sorbent.

In the sorption process (Figure 3), three successive stages of mass transfer of cyanocobalamin can be distinguished. The first linear section characterizes the diffusion of B₁₂ from the volume of the solution through the outer diffusion layer to the PMSSO surface, which is limited by the stage of diffusion through the boundary at the surface. After the breaking point, the role of external diffusion processes decreases, and the role of intra-diffusion processes rises. The second linear region corresponds to the diffusion of B₁₂ into PMSSO-hydrogel through a layer of already sorbed molecules deep into the gel through the systems of pores and capillaries. The intradiffusion process is limited by the transfer of molecules in the sorbent phase. The third and final linear region corresponds to the interaction of B₁₂ with the binding sites of the PMSSO-hydrogel: the B₁₂ molecule has a cyano group and a cobalt center, which can react with hydroxy groups on the sorbent surface by forming coordination bonds. At this point, an equilibrium of B₁₂ concentration has been established between the solid and liquid phases [27]. Hydroxyl groups can form hydrogen bonds with hydrogen or oxygen atoms in the B₁₂ molecule, providing a strong binding of the molecule to the surface of the sorbent.

Similar results were recently obtained for crystal violet and eosin sorption in the organic carries [28]. The results of mathematical processing using the Weber–Morris model for each site are presented in

Table 2. Internal diffusion parameters according to the Weber–Morris model (p = 0.95, n = 3).

Step	${\mathbf{k}}_{\mathbf{id}}$, mg/(g × min1/2)	c, mg/g	R2
1	0.216	0.08	1
2	0.059 ± 0.008	0.45 ± 0.04	0.96
3	0.045 ± 0.002	0.52 ± 0.01	0.99

.

To assess the influence of the chemical stage on the sorption process in a solid-liquid phase system, it is convenient to use pseudo-first and pseudo-second-order models in linear form [29]. The results of processing according to these models are shown in Figure 3. According to the obtained data, the pseudo-first-order kinetic equation describes experimental data at the initial and intermediate stages of the process, and the pseudo-second-order model refers to the entire time interval. The coefficients for each model are 0.967 and 0.998, respectively. The correspondence of the experimental sorption data to the pseudo-second-order kinetic equation indicates that the interaction between B₁₂ molecules and the binding sites of the sorbent follows pseudo-second-order kinetics, where the rate of interaction is proportional to the product of the concentrations of the two reactants.

Discussing the obtained data, we would like to put forward a hypothesis about the possible mechanism of B₁₂ sorption in PMSSO-hydrogel. PMSSO-hydrogels are well-known organo-silicon carriers for various types of compounds [30]. Initial PMSSO-hydrogels do not contain special binding cites and the main mechanism of sorption could be considered as non-specific [13]. However, after certain modifications, one could achieve higher sorption effectiveness, as we have demonstrated recently in [31]. Here we consider initial non-modified PMSSO-hydrogel as a biocompatible and safe carrier, while for modified hydrogels one must prove it before even preclinical studies. As a potential binding site, we could consider hydroxyl groups of PMSSO capable to bind with carboxyl groups and amides in B₁₂ structure. Thus, according to our data, the sorption of B₁₂ could be described as a combination of two processes: non-specific sorption via weak interactions and sorption supported by hydrogen bonds formation. This combination provides considerable values of loading efficacy.

Thus, mathematical processing of the integral kinetic curve of B₁₂ sorption into the PMSSO-hydrogel showed that during the sorption process, the combined effect of diffusion and chemical reaction is observed, and the sorption occurs in a diffusion mode.

3.3. B₁₂ Vitamin Release from PMSSO-Hydrogel in the Digestive Medium

The release curve of B₁₂ from PMSSO-hydrogel in buffer solutions imitating the gastric, small, and large intestines is demonstrated in Figure 7.

Firstly, there is a rapid release of the vitamin in the gastric medium till 90% of loaded into PMSSO. The remaining 10% release at the slightly alkaline environment of the small intestine. According to Figure 7, about 69 μg B₁₂ is released in an acidic environment and 7.6 μg in a slightly alkaline environment. This behavior of the release curve can be explained by the H⁺ interaction with the hydroxyl groups of the PMSSO-hydrogel displacing the B₁₂ associated with gel. In a slightly alkaline environment, the stability of the PMSSO-hydrogel decreases—hydroxyl groups from the solution contribute to the gradual destruction of the siloxane skeleton of the hydrogel and its dissociation. As a result, we observed the complete dissolution of PMSSO. From the point of view of vitamin B₁₂ delivery, the result is an advantage, since the degradation of the hydrogel produces safe products, and the drug is completely released.

Many factors, such as the type of dosage form and its solubility, the crystallinity of the matrix, the size of the particles, the acidity of the medium, etc., influence the release behavior [16]. Therefore, mathematical models are used to study the kinetics and establish the mechanism of drug release from drug delivery systems [32].

For the delivery systems based on solid-liquid phase systems, where polymer carriers act as a matrix, it is most convenient to use zero- and first-order models, as well as the Higuchi and Korsmeyer–Peppas models. The zero-order release model is used to describe osmotic systems, low-soluble release matrix systems, transdermal systems, and drug-coated systems. In this model there is a linear relationship between the degree of drug dissolution and time, and release is a diffusion process. The first-order release model reflects the process of dissolution of solid particles in a liquid medium and characterizes the release process as a diffusion process according to Fick’s first law, which governs most polymer carrier delivery systems [33].

The Korsmeyer–Peppas release model is a semi-empirical model based on diffusion mechanisms in polymeric systems [34]. In case diffusion is the primary mechanism of drug release, the graph of the amount of substance released from the square root of time is represented by a straight line, and if the mechanism of release is different from Fick’s law, then a straight-line relationship will be observed depending on the equation above.

The Higuchi model describes a diffusion release that obeys Fick’s first law and is linearly dependent on the square root of time. To use the Higuchi model, several conditions must be met: the swelling and dissolution of the matrix are limited or insignificant, the initial concentration of the released substance in the matrix exceeds its solubility, and the size of the system is much larger than the size of the particles (molecules) released from it [33]. Among other things, the immersion condition must always be achieved in the environment.

The B₁₂ molecule release curve can be divided into three parts: two ascending sections, one of which is steeper and faster (in an acidic medium), and the other, more uniform and straight, has a smaller slope (slightly alkaline medium), and the remaining part of the profile is a plateau. The upward sections of the release profile were processed according to the above mathematical models of release kinetics. The results of mathematical processing for each of the two ascending sections of the curve are shown in

Table 3. B₁₂ release constants from PMSSO-hydrogel for different models of release kinetics (p = 0.95, n = 3).

${\mathbf{Q}}_{\infty }$, µg	Step	Zero-Order Release Model			Korsmeyer-Peppas Release Model
76.6		${\mathbf{Q}}_{\infty }$, µg	${\mathbf{K}}_{\mathbf{0}}$, min−1	R2	K, min−1	n	R2
	1	53.3 ± 3.6	0.318 ± 0.096	0.85	32.1 ± 0.3	0.25 ± 0.04	0.98
	2	70.6 ± 0.2	0.062 ± 0.003	0.992	52.6 ± 0.1	0.12 ± 0.01	0.99
		First-Order Release Model			Higuchi Release Model
		${\mathbf{Q}}_{\infty }$, µg	${\mathbf{K}}_{\mathbf{1}}$, min−1	R2	${\mathbf{K}}_{\mathbf{H}}$, min−1/2		R2
	1	69.8 ± 0.4	0.062 ± 0.002	0.998	5.78 ± 1.28		0.98
	2	76.3 ± 0.7	0.025 ± 0.002	0.88	1.95 ± 0.09		0.99

. The differences between some R² values are insignificant, which necessitates further consideration of the features of the organosilicon matrix of the hydrogel and the physical laws governing release kinetics, as typically described using the Higuchi and Korsmeyer–Peppas models.

Thus, the Higuchi and Korsmeyer–Peppas models (n < 0.5) allow us to consider the process of release of cyanocobalamin molecules from PMSSO-hydrogel as a diffusion process obeying Fick’s first law in an acidic and slightly alkaline environment. The release exhibits delayed kinetics at the final stages in a slightly alkaline environment and rapid kinetics in an acidic medium, which follows from the obtained values of the release and dissolution constants. Considering hydrogen bond formation and non-specific sorption as the main forces supporting B₁₂ loading onto the hydrogel, acidic medium could indeed lead to enhanced release.

The key processes of vitamin B₁₂ metabolism in the human body occur in the stomach, namely, the binding of cyanocobalamin molecules to the Castle factor for their subsequent absorption into the blood in the upper small intestine via receptor-mediated endocytosis [35]. Comparing the release curve with the mechanism of metabolism indicates that the cyanocobalamin delivery system based on PMSSO-hydrogel is effective, since most of the vitamin is released in the stomach.

4. Conclusions

This study examined the feasibility of using PMSSO-hydrogel as a B₁₂ carrier to support and/or treat the vitamins deficiency. UV-VIS spectroscopy served as a main analytical technique for quantifying the vitamin’s content. This method exhibits appropriate sensitivity and precision for low concentrations. The detection limit of cyanocobalamin in its solutions is 2 ± 1 μg/mL. The measurable concentration range was from 6 ± 1 μg/mL to 56 ± 3 μg/mL.

We successfully optimized the conditions for the sorption of cyanocobalamin in PMS-SO-hydrogel by determining the process temperature and the contact duration of the polymer matrix with the B₁₂ solution at 37 °C. The hydrogel exhibits mixed-diffusion mode sorption of B₁₂. The examination of the cyanocobalamin release profile from the PMSSO-hydrogel in a system mimicking the human gastrointestinal tract indicated that the release transpires through a diffusion process adhering to Fick’s first law and is optimally characterized using the Korsmeyer–Peppas and Higuchi models. A fast release of the active molecule occurs in an acidic medium, linked to the protonation of the hydrogel’s active sites. In a mildly alkaline environment, the process progressively decelerates and becomes linear, while the hydrogel eventually disintegrates due to the dissociation resulting from the degradation of the siloxane framework.

The developed system could be considered as a perspective vitamin B₁₂ delivery system with a prolonged release profile.