Hydrogels are natural or synthetic polymer networks that exhibit high water absorbent capacities and have been used as scaffolds for tissue engineering or as delivery carriers for therapeutic agents and cells. Owing to their tunable physicochemical properties, hydrogels can provide spatial and temporal control over the release of loaded therapeutic agents, including chemotherapeutic drugs, proteins or cells. In particular, in situ-forming injectable hydrogels, the state-of-the-art clear free flowing polymer solutions that transform to viscoelastic gels upon exposure to stimuli including pH, temperature, light, enzymes and magnetic field, have been widely studied as delivery carriers for therapeutic agents. Therapeutic agents can be easily mixed with the free flowing polymer solutions and injected into the subcutaneous tissue or target site that could form a viscoelastic gel and act as therapeutic agents release depot. Hence, injectable hydrogels paid attention as sustained delivery vehicles. In this review, we systematically summarize the development of biocompatible, biodegradable, and pH- and temperature-responsive injectable hydrogels for sustained release of therapeutic agents. The key factors responsible for in situ gelation, interaction between polymers and therapeutic agents, and controlling the degradation of hydrogel matrix, are discussed. Advantages and perspectives of pH- and temperature-responsive injectable hydrogels in sustained therapeutic agents release are highlighted.

水凝胶是具有高吸水能力的天然或合成聚合物网络，已被用作组织工程的支架或用作治疗剂和细胞的输送载体。由于其可调节的理化特性，水凝胶可提供对负载的治疗药物（包括化学治疗药物，蛋白质或细胞）释放的时空控制。特别是原位形成可注射的水凝胶，最先进的透明自由流动的聚合物溶液，在暴露于包括pH，温度，光，酶和磁场的刺激下转变成粘弹性凝胶，已被广泛用作治疗剂的输送载体。治疗剂可以轻松地与自由流动​​的聚合物溶液混合，然后注射到皮下组织或目标部位，形成粘弹性凝胶并充当治疗剂释放库。因此，可注射水凝胶作为持续递送的载体而受到关注。在这篇综述中，我们系统地总结了生物相容性，生物可降解性以及pH和温度响应型可注射水凝胶的发展，以持续释放治疗剂。负责原地的关键因素讨论了凝胶化，聚合物与治疗剂之间的相互作用以及控制水凝胶基质的降解。强调了pH和温度响应型可注射水凝胶在持续治疗剂释放中的优势和前景。