For over 100 years, R&D&I efforts have focused on optimizing drug delivery. In recent years, cells, micro- and nano-particles, and combinations thereof as carriers have emerged, mainly for facilitating superior encapsulation and controlled release pharmacokinetics. Indeed, most cells and drugs require an appropriate formulation for their successful application as pharmaceuticals. Herein, the use of polymer-based hydrogel biomaterials for the delivery of drugs and recruitment of cells to promote tissue regeneration in the body is of growing interest and currently a hot research topic in pharmacology, pharmaceuticals, and bio-material science; a multi-/intra-disciplinary research field. For example, hydrogels are cross-linked three-dimensional polymeric networks with unique properties such as affinity for biological fluids, tunable porosity, high water content and uptake, ease of preparation, flexibility, malleability, biocompatibility, and biodegradability. Hence, while biomimetic hydrogels have been popular for tackling pharmacological, biomedical and clinical limitations of the existing drug delivery systems, injectable hydrogels have been recently gaining more attention due to the potential spatio-temporal control, tunability and stimuli-responsiveness capacities, offering significant innovative solutions in the delivery controlled and targeted delivery of therapeutics such as cells, genes, proteins and drug molecules, amongst others, with versatile platform applications in tissue engineering, regenerative medicine, implantology, functional and aesthetic surgical interventions, cancer, disease prevention and/or treatment and beyond.

Despite recent advances that continue accruing, including that several injectable hydrogels have already received US-FDA approval, are under study in the clinical trial stage and/or are already commercially available in the market, limitations exist. Such, whether based on natural or synthetic (or hybrid) polymers, include challenges in structural chemistry, phase transition, synthesis, cross-linking, mechanical stability and visco-elasticity, in situ load release profile and pharmacokinetics over time, biodegradability, immunological compatibility, host-immune responses, lack of molecular-level studies; formulation costs, administration route, data analysis from quality clinical trials, GMP (good manufacturing practices) scale-up processes or strategies and/or application-specific regulatory obstacles; topics that still require more extensive research.

This Special Issue aims to address such challenges and attract research that comprehensively demonstrates and analyzes the presently-ongoing development of several important types or classes of injectable hydrogels. Hence, this Special Issue will invite original research and review articles focusing primarily on the principles and strategies employed for the design, structural chemistry, formulation, synthesis, characterization, analysis, evaluation, and translation of novel injectable hydrogels for pharmaceuticals, tissue engineering, oncology and wound healing applications, via reviewing and/or presenting recent innovative advances in polymers, the building blocks and engineering chemistries for their formation and release pharmaco-kinetics, whether by chemical cross-linking reactions or physical bonding interactions. A discussion of the obstacles and prospects, including regulatory affairs, will be advocated.

Potential topics include but are not limited to the following:

• Recent advances in hydrogels designed as cell and drug delivery systems
• Biocompatibility and biodegradability of hydrogels under different conditions
• Assessing hydrogel pharmacokinetics and load release profile overtime
• Advances in photo-crosslinkable hydrogels for biomedical applications
• Role of stimuli-responsive polymers and polymerization in hydrogels
• Inter-penetrating and homopolymer network formation in hydrogels
• Advances in formulating injectable hydrogels and clinical use
• Potential of natural-synthetic hybrid synthesis of hydrogels
• Hydrogel sol-gel transition characterization and properties
• Effect of cell and drug encapsulation in novel nanogels
• Advances in pH-induced or -responsive hydrogels
• Novel hydrogel-based vaccine delivery systems
• Thermal-responsive and -sensitive hydrogels
• Discussing hydrogel-based dosage forms and regulatory affairs
• Hydrogel-based dosage forms, scale-up strategies, and GMP processes