Probiotic based Gastroretentive In Situ Gel Systems: A Review

 

 

Figure 1: Schematic representation of various gastroretentive drug delivery systems

 

 

Figure 2: Mechanism of in situ gelling system after oral administration

4.3 Polymers Used in Floating In Situ Gel Systems

• Sodium Alginate : Sodium alginate is one of the most widely used polymers in floating in situ gel formulations because of its excellent biocompatibility, biodegradability, non-toxicity, and ion-sensitive gelation properties [5]. It is a naturally occurring polysaccharide obtained from brown seaweed and consists mainly of guluronic acid and mannuronic acid residues. In acidic gastric conditions, calcium ions interact with guluronic acid blocks of sodium alginate to form a stable three-dimensional gel network through ionic crosslinking [21]. Sodium alginate-based formulations are highly suitable for probiotic delivery because gelation occurs under mild conditions that do not significantly affect microbial viability. In addition, alginate gels provide sustained release characteristics and improve gastric retention due to their floating capability. Alginate also acts as a protective barrier against acidic gastric conditions and improves probiotic survival during gastrointestinal transit [7].
• Gellan Gum : Gellan gum is an anionic polysaccharide produced by Sphingomonas elodea and is widely used in oral controlled-release formulations because of its excellent gel-forming ability [5]. It undergoes gelation in the presence of mono- or divalent cations such as calcium ions present in gastric fluid. Gellan gum-based formulations exhibit good viscosity, excellent floating capability, and prolonged gastric retention [30]. The polymer forms strong transparent gels capable of sustaining drug release over extended durations. Due to its low toxicity and high stability, gellan gum is considered highly suitable for probiotic-based gastroretentive systems [5].
• Chitosan : Chitosan is a cationic biopolymer obtained by deacetylation of chitin and possesses excellent mucoadhesive and biodegradable properties [20]. It is widely used in gastroretentive formulations because it enhances adhesion to gastric mucosa and prolongs gastric residence time. Chitosan also improves probiotic encapsulation efficiency and protects microorganisms from acidic degradation [6]. Due to its positive charge, chitosan interacts with negatively charged mucosal surfaces, thereby enhancing retention and controlled release behavior. In addition, chitosan exhibits mild antimicrobial activity and biocompatibility, making it suitable for oral probiotic formulations [5].
• Hydroxypropyl Methylcellulose (HPMC) : Hydroxypropyl methylcellulose is a semi-synthetic hydrophilic polymer extensively used as a viscosity enhancer and release retardant in controlled-release formulations. HPMC increases gel viscosity and improves mechanical strength, thereby enhancing sustained release behavior [28]. The polymer also helps maintain gel integrity during prolonged gastric retention and reduces burst release of probiotics [5]. Different grades of HPMC are selected depending on the desired viscosity and release profile. Due to its non-toxic and inert nature, HPMC is widely employed in gastroretentive floating systems [11].
• Carbopol : Carbopol is a synthetic high molecular weight polymer widely used in pH-sensitive gel formulations [5]. It exhibits extensive swelling and viscosity enhancement after ionization of carboxylic groups at higher pH values [20]. Carbopol improves gel consistency, mucoadhesion, and sustained release characteristics [5]. In probiotic formulations, carbopol-containing gels provide prolonged gastric retention and better microbial protection by forming highly viscous gel matrices [30].

4.4 Floating Mechanism in In Situ Gel Systems

Floating behavior is essential for prolonged gastric retention [10]. Floating systems work on the principle of gas generation from sodium bicarbonate reacting with gastric acid to produce CO? [27]. The gas becomes entrapped in the gel matrix, reducing density and enabling buoyancy [10]. This floating behavior ensures prolonged gastric residence and improved drug absorption [17,18].

Mechanism

Step 1: Administration

The formulation is swallowed as a liquid.

Step 2: Carbon Dioxide Generation

Sodium bicarbonate reacts with gastric acid:

NaHCO3+HCl→NaCl+CO2+H2O

This generated carbon dioxide becomes entrapped within the gel matrix and decreases density, allowing the system to float [10].

Step 3: Gel Formation

Calcium ions initiate polymer crosslinking [5].

Step 4: Floating

Entrapped CO? decreases density, causing the gel to float [10].

4.5 Formulation Components

Floating in situ gel formulations contain several important ingredients that work together to produce gelation, flotation, and sustained release. Sodium alginate acts as the primary gel-forming polymer, while calcium carbonate serves as a source of calcium ions required for crosslinking [5]. Sodium bicarbonate functions as the gas-generating agent responsible for floating behavior. Chitosan is used as a mucoadhesive polymer. HPMC is added to regulate viscosity and sustain release, and probiotics serve as the active therapeutic component. Stabilizers and preservatives are important for maintaining probiotic viability during storage [9].

4.6 Optimization of Formulation

Optimization is necessary to achieve ideal formulation characteristics such as appropriate viscosity, gel strength, floating lag time, and sustained release behavior. Polymer concentration strongly influences gel viscosity and mechanical stability [4]. Calcium ion concentration affects gel formation and crosslinking efficiency, while gas-generating agents determine buoyancy and floating duration [5]. Statistical optimization techniques such as factorial design, Box–Behnken design, and response surface methodology are commonly used to evaluate interactions between formulation variables and determine optimal compositions [4,23].

V. Evaluation Parameters

• Physical Appearance: The formulation should be clear, homogeneous, and free from particulate matter [5].
• pH Measurement: The pH should be suitable for oral administration and probiotic stability [9].
• Viscosity Study: Viscosity affects pourability and gelation efficiency [5]. Instrument : Brookfield viscometer.
• Floating Lag Time: Time required for the formulation to float after contact with gastric fluid [10]. Ideal floating lag time: Less than 1 minute.
• Total Floating Duration: Time during which the gel remains buoyant [10]. ideal duration: More than 12 hours.
• Gel Strength: Measures mechanical integrity of the formed gel [5].Strong gels: Resist gastric motility , Provide sustained release
• Drug Content Uniformity: Ensures equal distribution of probiotics [9].
• In Vitro Drug Release: Determines release pattern over time [5].Common dissolution medium: Simulated gastric fluid (0.1 N HCl)

VI. Drug Release Kinetics

Drug release kinetics play an important role in understanding how probiotics are released from gastroretentive in situ gel systems over time [5]. These models help in determining whether the release is diffusion-controlled, erosion-controlled, or follows mixed mechanisms, which is essential for optimizing sustained release behavior [28].

6.1 Zero Order Kinetics

In zero-order kinetics, drug release occurs at a constant rate independent of concentration. This is ideal for maintaining uniform probiotic delivery over extended gastric residence time [28].

Q_t  = Q₀  + K₀ t

 where:

• Q_t = amount of drug released at time t
• Q₀? = initial amount of drug
• K₀ = zero-order release constant
• t = time

Zero-order kinetics are generally observed in systems with strong matrix control or reservoir-type formulations that maintain a constant release rate.

6.2  First Order Kinetics

In first-order kinetics, the release rate depends on the concentration of the drug remaining in the system. As concentration decreases, release rate slows down [5].

Log C = Log C₀ – kt/2.303

where:

• C = concentration of drug remaining at time t
• C₀  = initial concentration of drug
• k = first-order rate constant
• t = time

This model is commonly observed in water-soluble drug formulations and porous matrix systems.

6.3 Higuchi Model

Q = K_H√t

This model explains drug release by diffusion from a matrix system [5].

• Q= amount of drug released
• K_H = Higuchi constant
• t = time

6.2 Korsmeyer-Peppas Model

M_t /M_∞ = Ktⁿ

This model is used to identify the mechanism of drug release such as: diffusion, erosion, swelling [5].

Where:

• Mt /M_∞ = fraction of drug released at time t
• k = release constant
• n = release exponent indicating release mechanism

VII. Characterization Techniques

Characterization studies are essential for evaluating physicochemical properties, compatibility, morphology, thermal stability, and release behavior of probiotic-loaded in situ gel systems [5]. These studies help ensure formulation quality, stability, and reproducibility.

7.1 FTIR Spectroscopy

FTIR is used to identify functional groups and detect possible interactions between probiotics and polymers [5]. It ensures chemical compatibility of formulation components.

7.2 DSC (Differential Scanning Calorimetry)

DSC evaluates thermal behavior and compatibility between drug and excipients [5]. It helps confirm stability of probiotics during formulation processing [14].

7.3 SEM Analysis

SEM is used to study surface morphology and internal structure of gel systems [6]. It helps visualize probiotic entrapment within polymer networks.

7.4 XRD Analysis

XRD is used to determine crystalline or amorphous nature of components. Reduced crystallinity often improves drug release and stability [5].

7.5 Rheological Studies

Rheology evaluates viscosity and flow behavior of formulations [5]. It is critical for determining pourability before administration and gel strength after administration.

VIII. Stability Studies

Stability studies are essential for determining the shelf life and viability of probiotic formulations [9]. Since probiotics are highly sensitive organisms, their survival depends on temperature, humidity, oxygen exposure, and formulation matrix protection [26].

Key parameters include:

• Viable cell count
• Gel integrity
• pH stability
• Storage temperature effect

Encapsulation and polymer protection significantly improve stability compared to free probiotic cells [25].

Stability Testing Conditions

Stability studies are generally performed according to ICH guidelines under accelerated and long-term storage conditions [9]. Formulations are stored at controlled temperature and humidity conditions, and samples are analyzed at regular intervals.

IX. Recent Advances

Nanoencapsulation improves probiotic protection against acidic gastric conditions and enhances targeted delivery [6]. Nanoparticles provide large surface area, improved encapsulation efficiency, and sustained release behavior.

Polymeric nanoparticles and lipid-based carriers are increasingly investigated for oral probiotic delivery [14].

9.1 Smart Hydrogels

Smart hydrogels are stimuli-responsive systems capable of responding to pH, temperature, or ionic changes [5]. These systems release probiotics selectively under specific physiological conditions and improve targeted delivery.

9.2 Synbiotic Systems

Synbiotic systems combine probiotics with prebiotics to improve microbial survival and colonization [2]. Prebiotics act as nutritional substrates that promote probiotic growth within the intestine.

9.3 3D Printing Technology

Three-dimensional printing technology enables fabrication of personalized gastroretentive dosage forms with precise geometry and controlled release characteristics [14]. This technology offers significant potential for individualized probiotic therapy.

FUTURE PERSPECTIVES

Future research in probiotic gastroretentive systems is mainly focused on improving microbial stability, targeted delivery, and patient-specific therapy [14]. Artificial intelligence and machine learning approaches are increasingly being explored for formulation optimization and prediction of release behavior.Development of advanced biopolymers with improved mucoadhesion and stimuli-responsive properties may further enhance gastric retention and probiotic survival [5]. Personalized microbiome therapy is another emerging area in which probiotic formulations may be designed according to individual microbial profiles [15]. Clinical translation and commercialization of probiotic gastroretentive systems require large-scale manufacturing methods capable of maintaining probiotic viability and formulation reproducibility [9]. Future advancements in polymer science, nanotechnology, and biotechnology are expected to improve therapeutic effectiveness and commercial feasibility of these systems [14].

CONCLUSION

Probiotic-based floating in situ gel systems represent a highly effective gastroretentive drug delivery strategy for overcoming limitations associated with oral probiotic administration [4]. These systems improve gastric residence time, protect probiotics from acidic degradation, enhance viability, and provide sustained release behavior [5,10]. Polymers such as sodium alginate, gellan gum, and chitosan play a central role in gel formation, floating ability, and microbial protection [21]. Floating mechanisms based on carbon dioxide entrapment ensure prolonged gastric retention and improved therapeutic efficiency [27]. Recent advancements in nanotechnology, smart hydrogels, synbiotic systems, and 3D printing have significantly improved formulation potential [6,14]. Despite challenges in stability and scale-up, these systems show strong promise for future clinical probiotic therapy [25]

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