Hydrogels are polymeric or supramolecular networks crosslinked by physical or chemical means. Owing to their great biocompatibility, they have been widely used in many biomedical fields, such as tissue engineering, regenerative medicine, stem cell and cancer research, and immunotherapy. In all these applications, the mechanical properties of hydrogels are important, as they provide not only mechanical support but also biophysical cues that regulate cell proliferation, spreading, and differentiation. Many methods have been developed recently to engineer synthetic hydrogels with mechanical properties similar to different tissues.

The macroscopic mechanical properties are related to many microscopic factors. First, network density and topology play an important role. Generally, the elasticity of hydrogels increases alongside polymer concentration. The toughness and extensibility of double-network hydrogels are higher than single-network ones. Second, the physically crosslinked hydrogels show time-dependent dynamic mechanical properties. It is possible to use different metal-coordination bonds or protein-protein interactions to fine-tune the dynamic mechanical response of hydrogels. Third, stimuli-responsive motives can be implemented for hydrogels to endow them with tunable macroscopic mechanical properties. Many light- or electric-responsive hydrogels are engineered in this direction.

Many new mechanical responsive polymers have been employed as the building blocks for hydrogels. For example, folded proteins with well-defined mechanical properties have been used for the construction of hydrogels with predictable mechanical features. Metal-coordination polymers have also been used to program the dynamic mechanical response of hydrogels.

Many new characterization techniques, such as microrheology and traction force microscopy, have been developed to allow a more quantitative understanding of how local mechanical environments impact the behaviors of cells. These technological developments will certainly further advance this field.

This Research Topic aims to highlight high-quality original research and review articles covering the design and application of hydrogels with desirable mechanical properties. In particular, this Research Topic will feature new developments in the synthesis of hydrogels with diverse mechanical properties, cutting-edge characterization and simulation techniques, and biological applications of the hydrogels, including:

• mechanochemistry of polymers
• mechanically adaptive hydrogels
• tough hydrogels
• strong hydrogels
• adhesive hydrogels
• new characterization techniques
• simulations
• viscoelastic properties
• stress-relaxation
• strain-stiffening

New directions for addressing significant challenges in the field will also be covered. However, studies that only report the synthesis and applications of hydrogels without emphasizing the mechanical properties will be not included in this Research Topic.

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Frontiers in Chemistry

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