Extreme Properties of Soft Materials by Polymer-Network Design

towards Long-term Healthcare and Sustainable Water Harvesting

    While soft materials have their unique features of softness, wetness, and bioactivity, they typically suffer from limited performances due to their fragility, nonlinearity, and complexity. Fragility refers to low fracture and fatigue resistance, nonlinearity refers to hyper-elasticity, and complexity refers to multi-scale and multi-mechanism properties. My research goal, at the intersection of polymer science, solid mechanics, and advanced manufacturing, is to achieve extreme properties of soft materials, including but not limited to fracture, fatigue, and sorption via unconventional polymer network design, thereby developing high-performing soft machines that can work in extreme environments while performing tasks that are challenging for conventional machines. I have been working on three areas towards my research goal: 1) polymer mechanics, 2) soft materials by design, and 3) soft machines.

    Soft materials, including hydrogels, elastomers, and organogels, have been widely used in many applications. In many applications, the mechanical and physical properties of soft materials play an important role. As tissue adhesives for wound closure, for example, soft materials need to form strong and tough adhesions to tissues to resist dynamic motions of the human body. In soft robotics, soft materials must respond to external stimuli, including temperature, light, electrical or magnetic fields, for controlled shape-morphing. In addition, the responsiveness of soft materials must be predictable and persistent. More recently, hydrogels show their unique potentials in regulating and retaining water molecules as alternative ways for water harvesting. In all these applications, understanding soft materials’ mechanical and physical properties is crucial for designing soft materials with desired properties.

    My research mission is to design extreme properties of soft materials via conventional polymer network design. Classical models are mostly based on conventional polymer networks, which neglect non-ideal features such as topological defects, chain entanglements, crystalline domains, and micro-/nano fibers. However, real soft materials such as biological tissues in animals and plants are mostly made of unconventional polymer networks, featuring topological defects, chain entanglements, crystalline domains, and micro-/nano- fibers. Common unconventional polymer networks include but are not limited to interpenetrating polymer networks, polymer networks with slidable crosslinks, polymer networks with high-functionality cross-links, and nano-/micro- fibrous polymer networks. My research goal is to understand how these non-ideal features affect the mechanical and physical properties of soft materials, thereby pushing their limits through unconventional polymer network design. Given the designed soft materials with extreme properties, my research strives to develop high-performing soft machines as biosensors, soft electronics, soft robots, and water harvesting. 

Polymer Mechanics

Fracture and Fatigue of Soft Materials

The project is to use solid mechanics and polymer physics to rationally guide the design of extreme properties for soft materials (Chem. Rev., 2021). Soft materials have enabled diverse modern technologies, but their practical deployment is usually limited by their mechanical failures. Fracture and fatigue of polymer networks are two important causes of mechanical failures of soft materials. While fracture and fatigue have been extensively studied in randomly crosslinked elastomers and gels, they have not been comparatively studied in polymer networks with well-controlled architectures and defects. In my recent works, I study fracture and fatigue of ideal polymer networks with controlled densities of topological defects. I develop a defect-network model to theoretically explain the intrinsic fracture energy of polymer networks (Phys. Rev. E, 2020). We also use A-B type tetra arm PEG hydrogel as an experimental platform to understand polymer mechanics including fracture, fatigue, and elasticity of soft materials (In press). 

Elastic Instabilities in Hydrogel Adhesions

This project is to develop soft wet adhesives to achieve extremely high mechanical strength and toughness by harnessing elastic instabilities (funded by NSF award 1661627). Soft wet adhesives are widely found in nature and have broad engineering applications. However, most synthetic soft adhesives are severely limited by their own mechanical strength and toughness. I exploit various modes of elastic instabilities (fringe, fingering, and cavitation instabilities) when a stressed soft adhesive undergoes tensile loading and form undulation instabilities at the interface. I discover a new mode of instability, fringe instability (Soft Matter, 2016), which forms at the layer's exposed surfaces but is localized at the constrained fringe portions. We also construct a phase diagram to understand the formation and interaction among different modes of elastic instabilities (J. Mech. Phys. Solids, 2017), guiding the design of soft wet adhesives to achieve extraordinary interfacial properties by exploiting mechanical instabilities in soft materials.

Selected Publications

Soft Materials by Design

Fatigue-resistant Hydrogels

This project is to use bioinspiration and mechanics as tools to enhance soft materials' fatigue thresholds through nanostructural engineering (iMechanica Journal Club, July 2020). Existing tough hydrogels still suffer fatigue failures when subject to multiple cycles of mechanical loadings. The reported fatigue thresholds of various tough hydrogels are on the order of 10-100 Joule per meter square. Inspired by load-bearing biological tissues, we demonstrate that the controlled introduction of nanocrystalline domains (Sci. Adv., 2019) and nanofibrils (Proc. Natl. Acad. Sci. U.S.A, 2019) in hydrogels can significantly enhance their fatigue thresholds to 1000 Joule per meter square. We further explore nanostructured interfaces between tendons/ligaments/cartilages and bones and report that bonding ordered nanocrystalline domains of synthetic hydrogels on engineering materials can give a fatigue-resistant adhesion with an interfacial fatigue threshold of 800 Joule per meter square (Nat. Com., 2020).

Tough yet Resilient Hydrogels

This project is to use fundamental principles to rationally design hydrogels with high toughness and high resilience. The central principle for high toughness is to build dissipations into stretchy polymer networks. Following this principle, I use 3D printing to construct stiff, tough, and stretchy hydrogel composites via nanoscale hybrid crosslinking and macroscale fiber reinforcement (Soft Matter, 2014). We further develop a coupled cohesive-zone and Mullins-effect model and report a scaling theory to decipher the toughening mechanisms in soft materials (Extreme Mech. Lett., 2015). Resilience is a property that is seemingly contradictory with high toughness. To reconcile high toughness and high resilience, we propose a resilient domain for hydrogels' deformation, below which hydrogels are deformed with low mechanical dissipation, but above which the deformation is highly dissipative (Extreme Mech. Lett., 2014).

Selected Publications

Soft Machines

Gastro-retentive Hydrogel Devices

A nascent field named hydrogel machines rapidly evolves, exploiting hydrogels as key components for devices and machines (Mater. Today, 2020). Achieving gastric residency with hydrogels requires the hydrogels to swell very rapidly and to withstand gastric mechanical forces over time. However, high swelling ratio, high swelling speed, and long-term robustness do not coexist in existing hydrogels. We introduce a hydrogel robot that can swell rapidly into a large soft sphere and maintain robustness under repeated mechanical loads in the stomach for up to 30 days (Nat. Com., 2019). Large animal tests support the exceptional performance of the ingestible hydrogel device for long-term gastric retention and physiological monitoring.

A Soft Neuroprosthetic Hand

This project is to develop a soft, low-cost, and lightweight hand for helping amputees' regain real-time tactile control. There are more than 5 million individuals with upper-limb amputations worldwide. The heavy weights (>400g) and high cost (>US$10,000) of the existing neuroprosthetic hands severely limit their broad use for individuals with amputations. In this work, we report the design, fabrication, and performance of a lightweight (292g) and potentially low-cost (cost of components of less than US$500) soft neuroprosthetic hand that is capable of simultaneous myoelectric control and tactile feedback for amputations (Nature Biomedical Engineering, 2021). I developed a computer model to relate a finger’s desired position to the corresponding pressure a pump would have to apply to achieve that position. Using this model, we developed a controller that directs the pneumatic system to inflate the fingers, in positions that mimic five common grasps. 

Stretchable Anti-fogging Tapes

Surface wetting prevents surface fogging on transparent materials by facilitating filmwise condensation with specific chemistry but suffers from material and geometry selectivity. In this project, we report a stretchable anti-fogging tape that can be adhered to versatile transparent materials with various curvatures for persistent fogging prevention, enabling new applications including fog-free eyeglasses, protective goggles, and efficient solar-powered freshwater production (Adv. Funct. Mater., 2021).

Selected Publications