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Double-network (DN) hydrogels, consisting of two contrasting and interpenetrating polymer networks, are considered as perhaps the toughest soft-wet materials. Current knowledge of DN gels from synthesis methods to toughening mechanisms almost exclusively comes from chemically-linked DN hydrogels by experiments. Molecular modeling and simulations of inhomogeneous DN structure in hydrogels have proved to be extremely challenging. Herein, we developed a new multiscale simulation platform to computationally investigate the early fracture of physically-chemically linked agar/polyacrylamide (agar/PAM) DN hydrogels at a long timescale. A “random walk reactive polymerization” (RWRP) was developed to mimic a radical polymerization process, which enables to construct a physically-chemically linked agar/PAM DN hydrogel from monomers, while conventional and steered MD simulations were conducted to examine the structural-dependent energy dissipation and fracture behaviors at the relax and deformation states. Collective simulation results revealed that energy dissipation of agar/PAM hydrogels was attributed to a combination of the pulling out of agar chains from the DNs, the disruption of massive hydrogen bonds between and within DN structures, and the strong association of water molecules with both networks, thus explaining a different mechanical enhancement of agar/PAM hydrogels. This computational work provided atomic details of network structure, dynamics, solvation, and interactions of a hybrid DN hydrogel, and a different structural-dependent energy dissipation mode and fracture behavior of a hybrid DN hydrogel, which help to design tough hydrogels with new network structures and efficient energy dissipation modes. Additionally, the RWRP algorithm can be generally applied to construct the radical polymerization-produced hydrogels, elastomers, and polymers.

 

Development of a universal and stable surface coating, irrespective of surface chemistry or material characteristics, is highly desirable but has proved to be extremely challenging. Conventional coating strategies including the commonly used catechol surface coating are limited to either a certain type of substrates or weak and unreliable surface bonding. Here, a simple, robust, and universal surface coating method capable for attaching any stimuli‐responsive glycidyl methacrylate (GMA)‐based copolymer, consisting of one surface‐adhesive moiety of epoxy groups and one stimuli‐responsive moiety, to any type of hydrophobic and hydrophilic surfaces via a one‐step ring‐opening reaction is proposed and demonstrated. The resultant GMA‐based copolymers are not only strongly adhered on different substrates (e.g., silicon, polypropylene, polyvinyl chloride, indium tin oxide, polyethylene terephthalate, aluminum, glass, polydimethylsiloxane, and even polyvinylidene fluoride with low surface energy), but also are possessed distinct thermal‐, pH‐, and salt‐responsive functions of bacterial killing, bacterial releasing, tunable multicolor fluorescence emission, and heavy metal detection. This coating method is also compatible with the directional quaternization of GMA‐based copolymers for further improving surface adhesion and functionality. This study provides a simple yet universal coating method to solve the long‐standing challenge of robust integration of stimuli‐responsive polymers with strong adhesion between various polymers and substrates.

 

Development of highly stretchable and sensitive soft strain sensors is of great importance for broad applications in artificial intelligence, wearable devices, and soft robotics, but it proved to be a profound challenge to integrate the two seemingly opposite properties of high stretchability and sensitivity into a single material. Herein, we designed and synthesized a new fully polymeric conductive hydrogel with an interpenetrating polymer network (IPN) structure made of conductive PEDOT:PSS polymers and zwitterionic poly(HEAA- co -SBAA) polymers to achieve a combination of high mechanical, biocompatible, and sensing properties. The presence of hydrogen bonding, electrostatic interactions, and IPN structures enabled poly(HEAA- co -SBAA)/PEDOT:PSS hydrogels to achieve an ultra-high stretchability of 4000–5000%, a tensile strength of ∼0.5 MPa, a rapid mechanical recovery of 70–80% within 5 min, fast self-healing in 3 min, and a strong surface adhesion of ∼1700 J m −2 on different hard and soft substrates. Moreover, the integration of zwitterionic polySBAA and conductive PEDOT:PSS facilitated charge transfer via optimal conductive pathways. Due to the unique combination of superior stretchable, self-adhesive, and conductive properties, the hydrogels were further designed into strain sensors with high sensing stability and robustness for rapidly and accurately detecting subtle strain- and pressure-induced deformation and human motions. Moreover, an in-house mechanosensing platform provides a new tool to real-time explore the changes and relationship between network structures, tensile stress, and electronic resistance. This new fully polymeric hydrogel strain sensor, without any conductive fillers, holds great promise for broad human-machine interface applications. 

Stimuli-responsive hydrogel strain sensors that synergize the advantages of both soft-wet hydrogels and smart functional materials have attracted rapidly increasing interest for exploring the opportunities from material design principles to emerging applications in electronic skins, health monitors, and human–machine interfaces. Stimuli-responsive hydrogel strain sensors possess smart and on-demand ability to specifically recognize various external stimuli and convert them into strain-induced mechanical, thermal, optical, and electrical signals. This review presents an up-to-date summary over the past five years on hydrogel strain sensors from different aspects, including material designs, gelation/fabrication methods, stimuli-responsive principles, and sensing performance. Hydrogel strain sensors are classified into five major categories based on the nature of the stimuli, and representative examples from each category are carefully selected and discussed in terms of structures, response mechanisms, and potential medical applications. Finally, current challenges and future perspectives of hydrogel strain sensors are tentatively proposed to stimulate more and better research in this emerging field.