DNA possesses characteristics such as excellent biocompatibility, biodegradation, molecular recognition ability, nanoscale controllability and programmability. In recent decades, DNA has taken on an assortment of diverse roles, not only as the central genetic molecule in biological systems but also as generic materials for nanoscale engineering. DNA hydrogel combines the characteristics both from DNA and hydrogel, such as controllable shape, high mechanical strength, materials delivery. The hydrogel formation includes chemical method based formation via covalent bond and physical method based formation via non covalent bond. DNA hydrogel can combine with perssad, molecule or DNA sequence which can be sensitive to incitant stimulating factor in order to expand the application of DNA hydrogel. DNA hydrogel can be sensitive to stimuli-response, such as pH, light, temperature and small molecules. These novel DNA hydrogels provide a natural bridge between nanotechnology and biotechnology, and this also leads DNA hydrogel to far-ranging real-world applications. Because of this, DNA hydrogel as a smart material has been widely used in biosensor, drug delivery, and three-dimensional cell culture. In this review, we summarize the classification of DNA hydrogel and then give the stimuli-response DNA hydrogel and its biological application. Also, the development prospect of DNA hydrogel is demonstrated.

Contents
1 Introduction
2 Formation mechanism of DNA hydrogel
2.1 Chemical method based DNA hydrogel formation
2.2 Physical method based DNA hydrogel formation
3 Stimuli-responsive DNA hydrogel
3.1 pH-responsive DNA hydrogel
3.2 Photo-responsive DNA hydrogel
3.3 Temperature-responsive DNA hydrogel
3.4 Molecule-responsive DNA hydrogel
4 Applications
4.1 Biosensor
4.2 Drug delivery
4.3 3D cell culture
5 Conclusion and outlook