Thermo-reversible gelatin hydrogels: mechanisms, rheological behavior, formulation strategies, and pharmaceutical application

Glycine, proline, and hydroxyproline-rich polypeptide sequences constitute a major part of gelatin, a denatured collagen-derived biopolymer. It is produced by the partial hydrolysis of collagen and has many biological advantages, such as low immunogenicity, biodegradability, and biocompatibility (1).  Native gelatin shows thermoreversible gelation, whereas synthetic gels are irreversibly crosslinked. Gelatin when dissolved in hot water it produces a sol, and when it cools, its collagen-like triple helices partially renaturate to form an elastic gel (2). Owing to its reversible characteristics, which enable processing as a liquid followed by moderate gel setting, gelatin is used in capsules, wound dressings, scaffolds, injectable systems, microneedles, and drug reservoirs (3, 4). However, rapid dissolution, low mechanical strength, and thermal softening at physiological temperatures are common problems associated with natural gelatin hydrogels. According to one study, gelatin hydrogels are physically stable below their gelation temperature of 23 °C, indicating the need for stability for use at temperatures below the body temperature (5). To address these issues and increase the medicinal potential of thermoreversible gelatin systems, more formulation improvement is required. This review's goals are to critically analyse the mechanisms underlying thermoreversible gelation in gelatin hydrogels, provide an overview of their rheological behavior, talk about formulation techniques like polymer blending and crosslinking for improved performance, and highlight their present and future pharmaceutical uses in injectable systems, drug delivery, tissue engineering, and 3D printing.

MECHANISMS OF THERMOREVERSIBILITY

Reversible conformational changes in polymer chains control the thermoreversible properties of gelatin. At high temperatures, the chains appeared as haphazard coils scattered across the water. Intermolecular association and the formation of triple-helical junction zones, which are primarily stabilized by hydrogen bonds, are encouraged by cooling (1).  Obas et al. used DSC to analyse gelatin high-solids gels and observed two thermal events: an endothermic peak linked to the helix-coil transition and a glass transition. They concluded that denaturation, not only melting, is what the thermal event refers to. Up to 22 °C, the amount of triple-helix structure that reformed during cooling was highly dependent on the renaturation temperature, after which it declined. The helical structure loss and recovery processes were both time-dependent (1).  Reheating causes the hydrogen bonds to break, junction zones to separate, and gel to revert to a flowing sol. Although the final structure is dependent on the cooling rate and thermal history, this reversible cycle can recur.

RHEOLOGICAL BEHAVIOR

Rheology is crucial for assessing gelatin hydrogels. The loss modulus (G′) indicates a viscous response, whereas the storage modulus (G′) increases when an elastic structure is formed during cooling. All samples in the gelatin–hemp protein hydrogels showed G′ > G′ at low temperatures, indicating gel-like activity (6).  Owing to the gel-sol transition, both moduli drastically decreased during heating and then substantially increased during cooling. Thermal hysteresis was demonstrated for pure gelatin (HP0), where the gel-sol transition occurred at approximately 30–35 °C during heating, and network recovery occurred 4-8 °C lower during cooling. Following the addition of hemp protein, the elastic stiffness was significantly enhanced. The storage modulus (G′) increased from 72.38 Pa for pure gelatin (HP0) to 2534.35 Pa for the HP20 formulation at 18 °C. Concurrently, the loss modulus increased from 2.06 Pa to 334.21 Pa (6). At 5 °C, the storage modulus of the gelatin-alginate binary systems was higher than that of pure gelatin gels, increasing by 9% and 24%, depending on the composition (7). Tagami et al. demonstrated clear shear-thinning behavior in gelatin-HPMC printer inks, where the viscosity decreased with increasing shear rate, which is a desirable characteristic for extrusion-based pharmaceutical printing. While lamotrigine loading had little effect, increasing the content of either gelatin or HPMC improved the perceived viscosity (8).

FORMULATION STRATEGIES

Polymer Blending

One important method for improving the mechanics, swelling behavior, and heat stability is by blending gelatin with synthetic or natural polymers. According to Sun et al., the addition of gelatin enhanced the denaturation temperature and elastic modulus of the PVA scaffold matrices. Certain physical characteristics were weakened by more than 10% gelatin was added (8). Pal et al. created PVA-gelatin hydrogel membranes and discovered that they were hemo compatible, strong, and capable of storing water for biomedical applications. Salicylic acid has a diffusion coefficient of 1.32 × 10?¹ cm²/s across the membrane (4).

Composite Reinforcement

The water absorption capacity of the gelatin-hemp protein hydrogels improved systematically from 95.0% (HP0) to 98.31% (HP20), demonstrating extremely effective water retention following protein incorporation. Researchers observed a shift from the thermoreversible behavior of classical gelatin to reinforced composite networks dominated by the hemp-protein structure at higher hemp concentrations (15–20%) (6).

Chemical Crosslinking

Controlled chemical crosslinking can overcome the mechanical fragility of native gelatin while maintaining certain physical network behavior. Pullulan dialdehyde-crosslinked gelatin hydrogels were made by Zhang et al. According to reports, the improved formulation's compressive stress of 5.80 MPa at 80% strain was 152 times greater than that of the pure gelatin hydrogel. As the concentration of the crosslinker increased, the swelling and enzymatic degradation decreased (5).

PHARMACEUTICAL APPLICATIONS

Drug Delivery Systems

Thermoreversible gelatin hydrogels, which can be prepared as liquids and set after administration, enable the localized distribution of medications, proteins, or bioactives. PVA-gelatin membranes exhibited regulated salicylic acid diffusion behavior, indicating their potential use in implantable or transdermal systems (4).  Additionally, methotrexate loaded with gelatin/PVA hydrogels were created as pH-sensitive delivery matrices, where drug release is contingent upon pH, crosslinking density, and polymer concentration (9).

Tissue Engineering

For tissue engineering applications requiring stronger matrices, porous gelatin/PVA scaffolds have demonstrated appropriate mechanical behavior and degradability (8).

Additionally, gelatin promotes cell adhesion and proliferation, making it a desirable scaffold biomaterial (5).

Injectable and Thermosensitive Platforms

Thermoreversible hydrogels are useful for minimally invasive systems because they can be injected as precursor liquids and gels after a temperature shift. This idea is frequently applied to scaffold-filling and depot systems (10).

3D printing

Lamotrigine-containing gelatin-HPMC gummy formulations were successfully 3D printed into a variety of forms and colors in a pediatric dosage-form trial. For rapid-release chewable medications, most formulations release approximately 85% of the drug within 15 min (8).

Table 1- PHARMACEUTICAL APPLICATIONS OF THERMOREVERSIBLE GELATIN HYDROGELS

(Ref.4,9,10,11)

LIMITATIONS AND FUTURE PERSPECTIVES

Despite their major advantages, native gelatin hydrogels still exhibit the following disadvantages:

• weak mechanics
• low stability near body temperature
• rapid aqueous dissolution
• batch variability
• dependence on animal sources

Future strategies should focus on the following:

• hybrid reinforced gelatin systems
• smart dual-responsive hydrogels
• microneedle reservoirs
• 3D printed gelatin biomaterials
• recombinant or non-animal gelatin
• AI-guided formulation optimization

Reverse thermoresponsive gelatin systems that gel upon warming rather than cooling may also broaden the scope of injectable biomedical applications. Injectable biomedical applications may also be expanded using reverse thermoresponsive gelatin systems that gel upon warming instead of cooling (8).

Table 2 - Formulation Strategies to Improve Native Gelatin Hydrogel Limitations (Ref.5-11)