**Preface**

As an excellent biopolymer material, hydrogel has been widely used in drug delivery, tissue engineering, and disease treatment, among other applications. By combining different polymers through physical or chemical changes, the prepared hydrogels can show better performance and be used in various environments.

At present, the exploration of hydrogel in biomedicine seems to be diversified. To adapt to the complex environment in organisms, higher requirements must put forward for hydrogel. Under the premise that hydrogel is biosafe, it must become tougher or more easily degradable according to the purpose of its application, be able to stay in the body for a longer time to exert its effect, or be easily removed. Moreover, certain hydrogels can respond to changes in pH, temperature, biological indicators, or microorganisms in the body. We are interested in the intelligent properties of hydrogels, and the condition in which these properties can be used to facilitate their applications is undoubtedly a question that researchers need to consider carefully.

This reprint consists of 12 papers, of which 2 are editorials and 10 are research papers. We hope to share the experimental and research results in this field and inspire new ideas for the design and applications of hydrogels.

> **Shige Wang** *Editor*
