Wood is light and strong; nacres are hard and resilient; muscles and tendons are soft and tough. These natural materials show a combination of normally contradicting mechanical properties, which is attributed to their hierarchical structures across multiple length scales. In imitation of natural systems, we explore various polymer preparation strategies to create structured hydrogels, to achieve controllable and tunable microstructures and properties, such as the mechanics and diffusivity. The fundamental studies of processing-structure-property correlation enable unprecedented functions such as artificial muscle, anti-freezing and flexible energy storage devices.

Publications:
- Strong Tough Hydrogels via the Synergy of Freeze-casting and Salting-out, Nature 2021, 590,594–599
- PVA hydrogels with broad-range tunable mechanical properties via Hofmeister effect, Advanced Materials 2021, DOI: 10.1002/adma.202007829- 4D Printable Tough and Thermoresponsive Hydrogels, ACS Applied Materials & Interfaces 2020, DOI: 10.1021/acsami.0c17532
- Tunable Sponge-like Hierarchically Porous Hydrogels with Simultaneously Enhanced Diffusivity and Mechanical Properties, Advanced Materials 2021, DOI: 10.1002/adma.201701938
- Rapid and Scalable Fabrication of Ultra-stretchable, Anti-freezing Conductive Gels by Cononsolvency Effect, EcoMat 2021, DOI: 10.1002/eom2.12085

Many living organisms can self-orient to face the sun throughout the day, known as phototropism, while artificial smart materials typically exhibit non-directional, nastic behaviour in response to an external stimulus. We study and develop photo-responsive materials for optics, photonics, waveguides, and photo-harvesting. Particularly, we create synthetic materials that can intrinsically detect and accurately track the direction of the stimulus, to exhibit tropistic behaviour and enhance sunlight harvesting for electricity generation and water purification.

Publications:
- Artificial Phototropism for Omnidirectional Tracking and Harvesting of Light, Nature Nanotechnology 2019, 14, 1048
- Artificial phototropic systems for enhanced light harvesting based on liquid crystal elastomer, Advanced Intelligent Systems, 2021 DOI: 10.1002/aisy.202000234

Our group has three 3D printing systems, capable of printing devices of various sizes. The newest 3D printing system that we are building is able to print hybrid materials. Combining with our stimuli-responsive polymer research, 3D printing techniques will allow us to print out soft robotics, investigate its practical uses and provide the foundation for developing applications of self-regulated and bio-inspired photosensitive materials as well.

Publications:
- 4D Printable Tough and Thermoresponsive Hydrogels, ACS Applied Materials & Interfaces 2020, DOI: 10.1021/acsami.0c17532
- Visualizing morphogenesis through instability formation in 4-D printing, ACS Applied Materials & Interfaces 2019, 11, 50, 47468-47475
- A Room-temperature High-conductivity Metal Printing Paradigm with Visible-light Projection Lithography, Advanced Functional Materials 2018, 1807615

Stimuli-responsive polymers consisting of elastomeric matrices with embedded flexible materials (e.g. cloth, paper, fiber, particles) are of particular interest to the robotics community because they are lightweight, affordable and easily customized to a given application. These polymers can be rapidly fabricated in a multi-step molding process and can achieve combinations of contraction, extension, bending and twisting with simple controls. In our approach is to use new design concepts, fabrication approaches and soft materials to improve the performance of these stimuli-responsive polymers compared to existing designs.

Publications:
- Soft Phototactic Swimmer Based on Self-sustained Hydrogel Oscillator, Science Robotics 2019, 4, eaax7112
- Muscle-inspired high-power-density strong contractile hydrogel by programmable elastic recoil, Science Advances, 2020, 6, eabd2520