Introduction:
Hydrogels have been used extensively in the development of smart drug delivery systems. A hydrogel is a network of hydrophilic polymers that can swell in water and hold a large amount of water while maintaining the structure. A three-dimensional network is formed by crosslinking polymer chains either by covalent bonds, hydrogen bonding, van der Waals interactions or physical entanglements. Hydrogels can control drug release by changing the gel structure in response to environmental stimuli such as temperature, electric fields, solvent composition, light, pressure, sound and magnetic fields, pH.
Temperature-sensitive hydrogels:
Temperature-sensitive hydrogels are probably the most commonly studied class of environmentally sensitive polymer systems. The phenomenon of transition from a solution to a gel is commonly referred to as sol-gel transition. Some hydrogels exhibit a separation from solution and solidification above certain temperature. This threshold is defined as the lower critical solution temperature (LCST). Below the LCST, the polymers are soluble. Above the LCST, they become increasingly hydrophobic and insoluble, leading to gel formation (see inverse temperature dependent gels below). In contrast, hydrogels that are formed upon cooling of a polymer solution have an upper critical solution temperature (UCST). The sol-gel transition of thermosensitive hydrogels can be experimentally verified by a number of techniques such as spectroscopy, differential scanning calorimetry (DSC) and rheology (Klouda, “Thermoresponsive hydrogels in biomedical applications” E.J.Phar and Biophar, 68 (2008).
Of the many temperature sensitive polymers PNIPAAm is probably the most extensively used. PDEAAm is also widely used because of its lower critical solution temperature (LCST) in the range of 25-32°C, close to body temperature. Certain types of block copolymers made of poly-(ethylene oxide) (PEO) and poly(propylene oxide) (PPO) also possess an inverse temperature sensitive property. Because their LCST at around the body temperature, they have been used in controlled delivery based on the sol-gel phase conversion at the body temperature.
Microparticle Encapsulation and Delivery:
Temporal intervention with microparticle encapsulation and delivery (TIMED):
Wang “Temperal intervention with microparticle encapsulation and delivery -A programmed release system for post-myocardial infarction therapy” Cell Biomaterial 2, 2026) discloses TIMED, an implantable polymeric system, which combines spatially patterned microparticles within a hydrogel patch. TIMED achieves a programmable multi-phasic release profile. In a rodent MI model, TIMED treatment improved survival, reduced cardiac injury markers and infarct size, and restored heart function beyond what was achieved with equivalent dosing delivered by conventional intravenous therapy. The hybrid polymeric system combines various drug-loaded microparticles within a tough hydrogel, with excellent mechanical performance and controlled release at dynamic surgical sites such as the heart. The treatment regimen involved precise delivery of multiple molecuels at designed time points. Each molecule was selected fro precilinical studies highlighting their temperoal effeccts in cardiac repar; NRG1, known for its cardioprotective effects, which is often adinsitered acutely post MI to minimize immediate cardiac apoptosis, VEGF delivered 1 week post MI during the peak of the angiogenic pahse to support vessel formation and tissue repair and GW788388, a small molecule inhibitor to TGF-beta usually intorduced around week 2 to mitigate fibrosis, aligning with the transition from beneficial to detremiental TGF-beta signaling in post-MI would healing. Polylactic-co-glycolic acid (PLGA), a polymer recognized for its biocompatibility was used as the substrate for microparticle fabrication. PLGA films were molded into base configurations with a central reservoir for drug loading, and then thermally bonded with PLGA caps to form hermetically sealed particles. Each of the therapeutic. compounds was encapsulated within distinct segmetns of the PLGA MP arrays, all housed within the same patch. To secure placement of PLGA microparticlesat the tissue site, a tough hydrogel network was engineered to pvoide a matrix that effectively immobilized the micro particles, preventing their migraiton and enabling localized delivery over an extended time-frame. A double network hydrogen composed of alginate and HMW poly(ethylee glycol) diacrylate (PEGDA) formulation was optimized for its exceptional mechnical strenght, stretchability, biocompatiility, and rapid UV corsslinking. To envelop the PLGA MPs within the hydrogel matrix, a bilayer composit encapsulation technique as used which prevented particle deteachment druing implantation and long term placement, avoiding adverse foreign reactions and payload loss.
Reversible Gelling Compositions See outline
Gelatin: See outline