Recent strategies to develop pH-sensitive injectable hydrogels

Recent advances in injectable hydrogel therapies for periodontitis

Periodontitis is an immune-inflammatory disease caused by dental plaque, and deteriorates the periodontal ligament, causes alveolar bone loss, and may lead to tooth loss. To treat periodontitis, antibacterial and anti-inflammation approaches are required to reduce bone loss. Thus, appropriate drug administration methods are significant. Due to their "syringeability", biocompatibility, and convenience, injectable hydrogels and associated methods have been extensively studied and used for periodontitis therapy. Such hydrogels are made from natural and synthetic polymer materials using physical and/or chemical cross-linking approaches. Interestingly, some injectable hydrogels are stimuli-responsive hydrogels, which respond to the local microenvironment and form hydrogels that release drugs. Therefore, as injectable hydrogels are different and highly varied, we systematically reviewed the periodontal treatment field from three perspectives: raw material sources, cross-linking methods, and stimuli-responsive methods. We then discussed current challenges and opportunities for the translation of hydrogels to clinic, which may guide further injectable hydrogel designs for periodontitis.

Hydrogel Coatings of Implants for Pathological Bone Repair

Hydrogels are well-suited for biomedical applications due to their numerous advantages, such as excellent bioactivity, versatile physical and chemical properties, and effective drug delivery capabilities. Recently, hydrogel coatings have developed to functionalize bone implants which are biologically inert and cannot withstand the complex bone tissue repair microenvironment. These coatings have shown promise in addressing unique and pressing medical needs. This review begins with the major functionalized performance and interfacial bonding strategy of hydrogel coatings, with a focus on the novel external field response properties of the hydrogel. Recent advances in the fabrication strategies of hydrogel coatings and their use in the treatment of pathologic bone regeneration are highlighted. Finally, challenges and emerging trends in the evolution and application of physiological environment-responsive and external electric field-responsive hydrogel coatings for bone implants are discussed.

Soft Materials with Time-Programmed Changes in Physical Properties through Lyotropic Phase Transitions Induced by pH-Changing Reactions

We present the development of time-programmable functional soft materials. The materials undergo reversible phase transitions between lyotropic phases with different topologies and symmetries, which in turn have very different physical properties: viscosity, diffusion, and optical transparency. Here, this behavior is achieved by combining pH-responsive lyotropic phases made from the lipid monoolein doped with 10% oleic acid, with chemical reactions that have well-defined controllable kinetics: autocatalytic urea-urease and methyl formate hydrolysis, which increase and decrease pH, respectively. In this case, we use small-angle X-ray scattering (SAXS) and optical imaging to show temporally controlled transitions between the cloudy hexagonal phase, which is a two-dimensional (2D) array of cylindrical inverse micelles, and the transparent, highly viscous three-dimensional (3D) bicontinuous cubic phases. By combining these into a single reaction mixture where the pH increases and then decreases again, we can induce a sequential transformation cycle from hexagonal to cubic and back to hexagonal over several hours. The sample therefore changes from cloudy to transparent and back again as a proof-of-concept demonstration for a wider range of soft materials with time-programmable changes in physical properties.

Biomedical applications of stimuli-responsive "smart" interpenetrating polymer network hydrogels

In recent years, owing to the ongoing advancements in polymer materials, hydrogels have found increasing applications in the biomedical domain, notably in the realm of stimuli-responsive "smart" hydrogels. Nonetheless, conventional single-network stimuli-responsive "smart" hydrogels frequently exhibit deficiencies, including low mechanical strength, limited biocompatibility, and extended response times. In response, researchers have addressed these challenges by introducing a second network to create stimuli-responsive "smart" Interpenetrating Polymer Network (IPN) hydrogels. The mechanical strength of the material can be significantly improved due to the topological entanglement and physical interactions within the interpenetrating structure. Simultaneously, combining different network structures enhances the biocompatibility and stimulus responsiveness of the gel, endowing it with unique properties such as cell adhesion, conductivity, hemostasis/antioxidation, and color-changing capabilities. This article primarily aims to elucidate the stimulus-inducing factors in stimuli-responsive "smart" IPN hydrogels, the impact of the gels on cell behaviors and their biomedical application range. Additionally, we also offer an in-depth exposition of their categorization, mechanisms, performance characteristics, and related aspects. This review furnishes a comprehensive assessment and outlook for the advancement of stimuli-responsive "smart" IPN hydrogels within the biomedical arena. We believe that, as the biomedical field increasingly demands novel materials featuring improved mechanical properties, robust biocompatibility, and heightened stimulus responsiveness, stimuli-responsive "smart" IPN hydrogels will hold substantial promise for wide-ranging applications in this domain.

A Comprehensive Review of Hydrogel-Based Drug Delivery Systems: Classification, Properties, Recent Trends, and Applications

As adaptable biomaterials, hydrogels have shown great promise in several industries, which include the delivery of drugs, engineering of tissues, biosensing, and regenerative medicine. These hydrophilic polymer three-dimensional networks have special qualities like increased content of water, soft, flexible nature, as well as biocompatibility, which makes it excellent candidates for simulating the extracellular matrix and promoting cell development and tissue regeneration. With an emphasis on their design concepts, synthesis processes, and characterization procedures, this review paper offers a thorough overview of hydrogels. It covers the various hydrogel material types, such as natural polymers, synthetic polymers, and hybrid hydrogels, as well as their unique characteristics and uses. The improvements in hydrogel-based platforms for controlled drug delivery are examined. It also looks at recent advances in bioprinting methods that use hydrogels to create intricate tissue constructions with exquisite spatial control. The performance of hydrogels is explored through several variables, including mechanical properties, degradation behaviour, and biological interactions, with a focus on the significance of customizing hydrogel qualities for particular applications. This review paper also offers insights into future directions in hydrogel research, including those that promise to advance the discipline, such as stimuli-responsive hydrogels, self-healing hydrogels, and bioactive hydrogels. Generally, the objective of this review paper is to provide readers with a detailed grasp of hydrogels and all of their potential uses, making it an invaluable tool for scientists and researchers studying biomaterials and tissue engineering.

N-Sulfonyl amidine polypeptides: new polymeric biomaterials with conformation transition responsive to tumor acidity

Manipulation of pH responsiveness is a frequently employed tactic in the formulation of trigger-responsive nanomaterials. It offers an avenue for "smart" designs capitalizing on distinctive pH gradients across diverse tissues and intracellular compartments. However, an overwhelming majority of documented functional groups (>80%) exhibit responsiveness solely to the heightened acidic milieu of intracellular pH (about 4.5-5.5). This scenario diverges markedly from the moderately acidic extracellular pH (∼6.8) characteristic of tumor microenvironments. Consequently, systems predicated upon intracellular pH responsiveness are unlikely to confer discernible advantages concerning targeted penetration and cellular uptake at tumor sites. In this study, we elucidated the extracellular pH responsiveness intrinsic to N-sulfonyl amidine (SAi), delineating a method to synthesize an array of SAi-bearing polypeptides (SAi-polypeptides). Notably, we demonstrated the pH-dependent modulation of SAi-polypeptide conformations, made possible by the protonation/deprotonation equilibrium of SAi in response to minute fluctuations in pH from physiological conditions to the extracellular milieu of tumors. This dynamic pH-triggered transition of SAi-polypeptides from negatively charged to neutrally charged side chains at the pH outside tumor cells (∼6.8) facilitated a transition from coil to helix conformations, concomitant with the induction of cellular internalization upon arrival at tumor sites. Furthermore, the progressive acidification of the intracellular environment expedited drug release, culminating in significantly enhanced site-specific chemotherapeutic efficacy compared with free-drug counterparts. The distinct pH-responsive attributes of SAi could aid the design of tumor acidity-responsive applications, thereby furnishing invaluable insights into the realm of smart material design.

pH-responsive scaffolds for tissue regeneration: In vivo performance

A myriad of pH-sensitive scaffolds has been reported in recent decades. Information on their behaviour in vitro under conditions that mimic the pH changes that occur during tissue regeneration is abundant. Differently, the in vivo demonstration of the advantages of pH-responsive systems in comparison with non-responders is more limited. The in vivo scenario is very complex and the intricate relationship between the host response, the overall pathological conditions of the patient, and the risk of colonization by microorganisms is very difficult to imitate in in vitro tests. This review aims to shed light on how the changes in pH between healthy and damaged states and also during the healing process have been exploited so far to develop polymer-based scaffolds that actively contribute in vivo to the healing process avoiding chronification. The main strategies so far tested to prepare pH-responsive scaffolds rely on (i) changes in ionization of natural polymers, ionizable monomers and clays, (ii) reversible cross-linkers, (iii) coatings, and (iv) production of CO2 gas. These strategies are analysed in detail in this review with the description of relevant examples of their performance on specific animal models. The versatility of the techniques used to prepare biocompatible and environment-friendly pH-responsive scaffolds that have been implemented in the last decade may pave the way for a successful translation to the clinic. STATEMENT OF SIGNIFICANCE: We report here on the most recent advances in pH-responsive polymer-based scaffolds that have been demonstrated in vivo to be suitable for wound and bone healing. pH is a critical variable in the tissue regeneration process, and small changes can speed up or completely stop the process. Although there is still a paucity of information on the performance in the complex in vivo environment, recently reported achievements using scaffolds endowed with pH-responsiveness through ionic natural polymers, ionizable monomers and clays, reversible cross-linkers, coatings, or formation of CO2 ensure a promising future towards clinical translation.