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Technical Review Article | Open Access | Published 29th September 2026
In Situ Forming Injectable Gels: A Novel Approach for Controlled Drug Delivery—Polymers, Mechanisms, and Pharmaceutical Applications
Sweta S Koka, Sumeet Dwivedi, G. N. Darwhekar - Acropolis Institute of Pharmaceutical Education and Research, India. | EJPPS | 313 (2026)| https://doi.org/10.37521/ejpps31307
Abstract
In situ forming injectable gels have emerged as a promising platform in advanced drug delivery systems, offering the ability to transform from a liquid state into a semi-solid gel upon administration. This unique sol–gel transition enables the formation of a localized drug depot, facilitating sustained and controlled release of therapeutic agents. These systems are triggered by physiological stimuli such as temperature, pH, ionic strength, and solvent exchange. A wide range of natural and synthetic polymers, including poloxamers, chitosan, sodium alginate, and poly(lactic-co-glycolic acid), are utilized in their formulation. The present review provides a comprehensive discussion on the mechanisms of gel formation, polymer selection, formulation strategies, characterization techniques, and diverse pharmaceutical applications. Furthermore, key challenges such as burst release, sterility concerns, scale-up limitations, and regulatory barriers are critically analyzed. Emerging trends, including smart polymers, nanogel integration, and artificial intelligence-driven formulation design, are also highlighted. Overall, in situ forming gels represent a versatile and innovative approach for enhancing therapeutic efficacy and patient compliance in parenteral drug delivery.
Keywords: In situ forming gel, Injectable gel, Controlled drug delivery, Sol–gel transition, Thermosensitive polymers, pH-sensitive polymers, Ion-activated gelation, Poly(lactic-co-glycolic acid), Parenteral drug delivery, Sustained release systems
Introduction
Controlled drug delivery systems have revolutionized modern therapeutics by enabling the administration of drugs at predetermined rates, thereby maintaining drug concentrations within the therapeutic window for extended durations. This approach not only enhances therapeutic efficacy but also minimizes adverse effects associated with fluctuating drug levels ¹. Despite their widespread use, conventional injectable formulations are often associated with several limitations, including rapid drug clearance, short biological half-life, and the necessity for repeated administration. These drawbacks can lead to poor patient compliance and increased risk of complications such as tissue irritation and infection ².
In response to these challenges, in situ forming injectable gels have gained considerable attention as an advanced drug delivery strategy. These systems are administered as low-viscosity solutions and undergo phase transition into gels upon exposure to physiological conditions ³. The resulting gel acts as a reservoir that allows for controlled and sustained drug release over prolonged periods. This innovative approach combines the advantages of both liquid and solid dosage forms, offering ease of administration along with prolonged therapeutic action. The growing interest in minimally invasive and patient-friendly drug delivery systems has further accelerated research in this field. ⁴
Mechanism of In Situ Gel Formation
The formation of in situ gels is governed by physicochemical interactions between polymers and physiological stimuli. These mechanisms are crucial in determining the performance, stability, and drug release characteristics of the system ⁵.
Temperature-Induced Systems
Temperature-sensitive systems are based on polymers that exhibit a phase transition at physiological temperatures. These formulations remain in a liquid state at room temperature, allowing easy administration through injection. Upon exposure to body temperature (~37°C), hydrophobic interactions among polymer chains increase, leading to micelle formation and aggregation, ultimately forming a three-dimensional gel network. Poloxamers are widely used in such systems due to their reversible thermogelling properties ⁶,⁷.
pH-Triggered Systems
pH-sensitive systems rely on polymers containing ionizable functional groups that respond to environmental pH changes. For example, chitosan remains soluble in acidic conditions due to protonation of amino groups. At physiological pH (~7.4), deprotonation occurs, reducing solubility and promoting polymer chain aggregation, resulting in gel formation. This mechanism enables site-specific drug delivery, particularly in environments with distinct pH variations ⁸,⁹.
Ion-Activated Systems
Ion-sensitive gels form through ionic crosslinking between polymer chains and multivalent ions. Sodium alginate is a commonly used polymer that forms gels in the presence of calcium ions (Ca²⁺). The interaction between guluronic acid residues and calcium ions leads to the formation of a structured network known as the egg-box model, resulting in a stable three-dimensional gel matrix capable of sustained drug release ¹⁰.
Solvent Exchange Systems
In solvent exchange systems, gelation occurs due to the diffusion of solvent from the formulation into surrounding biological fluids, accompanied by influx of water into the system. Polymers such as poly(lactic-co-glycolic acid) (PLGA) are dissolved in organic solvents prior to administration. Upon injection, solvent diffusion and polymer precipitation (phase inversion) occur, resulting in the formation of a solid or semi-solid depot that enables prolonged drug release ¹¹,¹².
These mechanisms demonstrated how in situ gels function as stimuli-responsive systems, transforming from a liquid to a gel under physiological conditions. This transition enables the formation of a localized drug reservoir, ensuring controlled, sustained, and targeted drug delivery, which is essential for advanced parenteral formulations ¹².
Polymers Used in In Situ Gels ¹³
Polymers constitute the structural and functional backbone of in situ forming gel systems, governing critical attributes such as gelation mechanism, mechanical strength, biodegradation, drug loading capacity, and release kinetics. The selection of polymers is a key determinant of formulation performance, especially for parenteral applications, where biocompatibility, sterility, and reproducibility are essential. Based on origin and functionality, polymers used in in situ gels can be broadly classified into natural polymers, synthetic polymers, and stimuli-responsive (smart) polymers.
Table 1: Natural Polymers Used in In Situ Gels ¹⁴
Polymer | Source | Mechanism of Gelation | Key Properties | Advantages | Applications |
Chitosan | Derived from chitin (marine source) | Deprotonation of amino groups at physiological pH leading to polymer aggregation | Mucoadhesive, antimicrobial, biodegradable | Enhances drug permeability and retention | Injectable gels, nasal delivery, ocular systems |
Sodium Alginate | Brown seaweed | Ionic crosslinking with divalent cations (Ca²⁺); egg-box model formation | Biocompatible, mild gelation, non-toxic | Gentle gelation conditions, safe | Injectable depots, wound healing systems |
Gellan Gum | Microbial (Sphingomonas elodea) | Gelation in presence of cations (Na⁺, Ca²⁺) | Transparent, stable, elastic gels | Good gel clarity and stability | Ophthalmic and injectable formulations |
Xanthan Gum | Microbial (Xanthomonas campestris) | Acts as viscosity enhancer (not primary gel former) | High viscosity, stabilizing ability | Improves gel consistency | Used with other polymers in gels |
Table 2: Advantages and Limitations of Natural Polymers ¹⁵
Category | Details |
Advantages | Biocompatible and biodegradable; Low toxicity and immunogenicity; Environmentally sustainable |
Limitations | Batch-to-batch variability; Lower mechanical strength; Risk of microbial contamination |
Table 3: Synthetic Polymers Used in In Situ Gels ¹⁶
Polymer | Composition | Mechanism | Key Properties | Advantages | Applications |
Poloxamers (Pluronics) | Polyethylene oxide–polypropylene oxide–polyethylene oxide (PEO–PPO–PEO) | Thermosensitive gelation (liquid → gel at body temperature) | Reversible gelation, non-toxic | Easy administration, temperature-responsive | Injectable, topical, ocular delivery |
Poly(lactic-co-glycolic acid) (PLGA) | Copolymer of lactic acid and glycolic acid | Solvent exchange leading to polymer precipitation | Biodegradable via hydrolysis | Controlled degradation, regulatory approval | Long-acting injectable depot systems |
Polyethylene Glycol (PEG) | Hydrophilic polymer | Crosslinking to form hydrogels | Improves solubility and stability | Reduces immunogenicity, enhances compatibility | Hydrogel systems, drug carriers |
Polycaprolactone (PCL) | Semicrystalline polymer | Slow degradation via hydrolysis | Long degradation time | Suitable for prolonged release | Implantable and injectable systems |
Table 4: Advantages and Limitations of Synthetic Polymers ¹⁵
Category | Details |
Advantages | High reproducibility; Tunable physicochemical properties; Strong mechanical stability; Better control over drug release |
Limitations | Possible toxicity of degradation products; Comparatively lower biocompatibility than natural polymers |
Table 5: Stimuli-Responsive (Smart) Polymers ¹⁷
Feature | Description |
Definition | Polymers that respond to environmental stimuli for controlled drug delivery |
Types of Stimuli | Temperature, pH (potential of hydrogen), ionic strength, enzymes, light, magnetic field |
Key Features | On-demand drug release; Site-specific targeting; Improved therapeutic efficiency |
Examples | Dual-responsive hydrogels (temperature + pH); Enzyme-sensitive biodegradable systems |
Table 6: Comparative Overview of Polymers ¹⁸
Parameter | Natural Polymers | Synthetic Polymers |
Source | Biological | Chemical |
Biocompatibility | Excellent | Good |
Reproducibility | Variable | High |
Mechanical Strength | Moderate | High |
Regulatory Acceptance | Moderate | High |
Formulation Strategies ¹⁹
The formulation of in situ forming injectable gels requires a systematic and rational optimization of multiple formulation variables to achieve the desired performance in terms of injectability, gelation, stability, and controlled drug release. Since these systems are administered via parenteral routes, additional considerations such as sterility, safety, and regulatory compliance are critical.
Polymer Concentration Optimization ²⁰,²¹
Polymer concentration is one of the most critical factors influencing the physicochemical and mechanical properties of in situ forming gels. It directly affects gelation behaviour, viscosity, injectability, and drug release kinetics. The ability of the system to undergo sol–gel transition largely depends on the polymer concentration; insufficient amounts may result in weak or incomplete gel formation, whereas excessively high concentrations can lead to premature gelation or highly viscous formulations that are difficult to administer. From an injectability perspective, lower polymer concentrations facilitate easy syringeability, while higher concentrations increase resistance during injection and may cause discomfort to the patient. Furthermore, polymer concentration plays a key role in drug release behaviour, where lower concentrations often lead to rapid drug diffusion and burst release, whereas higher concentrations form a dense polymeric network that enables sustained and controlled release. Therefore, an optimal balance must be achieved between maintaining low viscosity prior to administration, ensuring sufficient gel strength after gelation, and achieving the desired release kinetics.
Drug–Polymer Compatibility ²²,²³
Drug–polymer compatibility is essential for ensuring the stability, efficacy, and uniform distribution of the drug within the formulation. Incompatibility between the drug and polymer can result in undesirable outcomes such as drug degradation, altered release profiles, and reduced bioavailability. These interactions may occur through mechanisms such as hydrogen bonding, ionic interactions, or hydrophobic associations, which can influence the overall performance of the formulation. To assess compatibility, several analytical techniques are employed. Fourier Transform Infrared Spectroscopy is used to identify potential chemical interactions by analyzing functional group changes, while Differential Scanning Calorimetry provides insight into thermal behaviour and possible incompatibilities. X-ray Diffraction helps determine changes in crystallinity that may occur due to drug–polymer interactions. Ensuring compatibility is crucial for maintaining chemical stability and achieving predictable and reproducible drug release profiles, which are necessary for regulatory approval and therapeutic effectiveness.
Use of Co-solvents and Additives ²⁴,²⁵
Co-solvents and additives play a significant role in enhancing the solubility, stability, and overall performance of in situ gel formulations. Co-solvents such as ethanol, polyethylene glycol, and propylene glycol are commonly used to improve the solubility of poorly water-soluble drugs and to facilitate uniform drug distribution within the formulation. In addition to co-solvents, various additives are incorporated to optimize formulation characteristics. Stabilizers help prevent drug degradation caused by oxidation or hydrolysis, while surfactants enhance drug solubilization and dispersion. Plasticizers improve the flexibility of the gel matrix and reduce brittleness, and buffering agents maintain the optimal pH required for stability and gelation. However, the selection of these excipients must be carried out with caution, ensuring that they are biocompatible, non-toxic, and suitable for parenteral administration. Additionally, they should not interfere with the gelation mechanism and must comply with regulatory standards.
Sterility Considerations for Injectable Systems ²⁶
Sterility is a critical and non-negotiable requirement for in situ injectable gels, as any microbial contamination can lead to severe infections and compromise patient safety. These formulations must be prepared under strictly controlled aseptic conditions using sterile raw materials, equipment, and environments such as cleanrooms and laminar airflow units. Depending on the nature of the formulation, appropriate sterilization techniques must be employed. Membrane filtration is commonly used for heat-sensitive formulations, whereas autoclaving is suitable for thermostable systems. Gamma irradiation may also be applied in specific cases where applicable. In addition to sterility, the formulation must be free from pyrogens (endotoxins), and sterile containers and closures must be used to maintain product integrity. Validation of sterilization processes and adherence to Good Manufacturing Practices are essential to ensure consistent product quality and compliance with regulatory requirements.
Characterization of In Situ Gels
Comprehensive characterization of in situ forming gels is essential to ensure their quality, performance, safety, and reproducibility, particularly for parenteral applications. These systems must be evaluated for their ability to undergo efficient sol–gel transition, maintain structural integrity, deliver drugs in a controlled manner, and remain stable throughout their shelf life. A combination of physicochemical, mechanical, and biological parameters is therefore assessed ²⁷.
Gelation Temperature and Gelation Time
Gelation temperature is defined as the temperature at which the formulation transitions from a liquid (sol) to a semi-solid (gel) state, while gelation time refers to the time required for complete gel formation. These parameters are particularly important for thermosensitive systems, where the formulation should remain liquid at room temperature for ease of administration and rapidly form a gel at physiological temperature (~37°C). Common evaluation methods include the vial inversion method and rheological analysis. An ideal system exhibits rapid gelation at body temperature without premature gel formation during handling ²⁸.
Rheological Properties
Rheological evaluation provides insights into the flow behaviour and viscoelastic characteristics of in situ gels. Parameters such as viscosity, shear-thinning behavior, and viscoelastic moduli—namely storage modulus (G′) and loss modulus (G″)—are critical. Prior to administration, the formulation should exhibit low viscosity and shear-thinning properties to facilitate injection. After gelation, a higher storage modulus indicates the formation of a strong gel network. Rheological studies help in determining the gelation point and mechanical stability of the formulation ²⁹.
Injectability
Injectability refers to the ease with which the formulation can be administered through a syringe and needle. It is an important parameter for patient comfort and clinical applicability. Injectability is typically evaluated by measuring the force required to expel the formulation using instruments such as a texture analyzer or universal testing machine. An ideal in situ gel formulation should require minimal force for injection, ensuring ease of administration without compromising gelation performance ³⁰.
Drug Content Uniformity
Drug content uniformity ensures that the drug is evenly distributed throughout the formulation, which is essential for accurate dosing and therapeutic consistency. This parameter is typically assessed using analytical techniques such as Ultraviolet–Visible Spectroscopy or High-Performance Liquid Chromatography. Uniform drug distribution indicates proper mixing and formulation homogeneity, which are critical for reproducible drug delivery ²⁸.
In Vitro Drug Release Studies:
In vitro drug release studies are conducted to evaluate the rate and mechanism of drug release from the gel matrix. These studies are commonly performed using methods such as the dialysis membrane technique or the Franz diffusion cell. The release data obtained are used to determine cumulative drug release and to fit into kinetic models such as zero-order, first-order, Higuchi, or Korsmeyer–Peppas models. An ideal system should demonstrate a controlled and sustained release profile with minimal initial burst effect ³¹,³².
Mechanical Strength and Gel Integrity
Mechanical strength refers to the ability of the gel to maintain its structural integrity after formation. It is evaluated using techniques such as texture analysis and compression testing. Adequate gel strength is necessary to ensure that the gel remains intact at the site of administration and provides sustained drug release over time ³³.
Swelling and Degradation Studies
Swelling studies assess the water uptake capacity of the gel, which influences drug diffusion and release behaviour. Degradation studies evaluate the rate at which the polymer matrix breaks down under physiological conditions. These parameters are critical for determining the duration of drug release and biocompatibility of the system ³⁴.
Stability Studies
Stability studies are conducted to evaluate the physical, chemical, and microbial stability of the formulation over time. These studies are performed according to International Council for Harmonisation guidelines, under conditions such as accelerated (40°C ± 2°C / 75% relative humidity ± 5%) and long-term storage (25°C ± 2°C / 60% relative humidity ± 5%). Parameters such as appearance, viscosity, drug content, and sterility are monitored to ensure product quality and shelf life ³⁵.
Applications
In situ gels have diverse applications in pharmaceutical and biomedical fields. In parenteral delivery, they are used for cancer therapy and long-acting formulations, providing sustained drug release and improved therapeutic outcomes. In ophthalmic delivery, these systems enhance drug retention and bioavailability by forming gels in the ocular environment. Periodontal applications involve localized drug delivery within periodontal pockets, ensuring prolonged antimicrobial action. Additionally, in situ gels are being explored for central nervous system targeting, enabling drug delivery across or bypassing the blood–brain barrier ²⁸.
Challenges
Despite their advantages, in situ gel systems face several challenges. The initial burst release of drugs can lead to toxicity and reduced therapeutic duration. Maintaining sterility throughout formulation and manufacturing processes is critical yet challenging. Scale-up from laboratory to industrial production requires careful optimization to ensure reproducibility. Regulatory approval processes for such novel systems are complex and require extensive validation and clinical data ³⁷.
Future Perspectives
Future research in in situ gels is focused on the development of smart polymers capable of responding to multiple stimuli, enabling precise and controlled drug delivery. The integration of nanotechnology has led to the emergence of nanogel systems with enhanced drug loading and targeting capabilities. Personalized medicine approaches are also gaining attention, allowing customization of formulations based on patient-specific needs. Furthermore, the application of artificial intelligence in formulation design is expected to revolutionize drug delivery by enabling predictive modeling and optimization ³⁸.
Conclusion
In situ forming injectable gels represent a significant advancement in controlled drug delivery systems, offering sustained release, targeted delivery, and improved patient compliance. Their versatility and adaptability make them suitable for a wide range of therapeutic applications. However, challenges related to formulation, sterility, and regulatory approval must be addressed to ensure successful clinical translation. Continued research and technological innovation are essential to unlock the full potential of these systems and establish them as a cornerstone of modern pharmaceutical development.
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Authors
Corresponding Author: Dr. Sweta S Koka
Professor
Acropolis Institute of Pharmaceutical Education and Research, Indore
9893106061,
Email: swetaskoka@acropolis.edu.in
Authors: Sweta S Koka*, Sumeet Dwivedi, G. N. Darwhekar
Acropolis Institute of Pharmaceutical Education and Research, Indore, 453771 Madhya Pradesh, India.
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