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Research Vision and Mission
Our human are mostly made of biological hydrogels. They are active, responsive, and have the ability to self-heal or self-recover after injuries. In addition to these biological functionalities, people am highly impressed by their unprecedented mechanical properties, such as high strength, high fracture toughness and anti-fatigue properties while maintaining superior compliance and high water content. This is a schematic of human knee joint autonomy. When a human is walking, both cartilage and meniscus need to sustain nearly the whole human weight with stress as high as MPa. They are soft when contact at small deformations but stiffen drastically at moderate deformations, giving anti-wearing property. Unlike combined compressive-shear loading in meniscus and cartilage, tendon serves to connect muscle with bone for extensional tensile loading, which is a more severe loading stress state to break. For American, data shows that the walking step per day is 3000 – 4000 on average, which is corresponding to millions of steps per year. |
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On the other hand, although various molecular and macromolecular engineering approaches have replicated parts of biological muscles’ characteristics, none of them can synergistically replicate combinational attributes (i.e., high strength, high toughness, high water content, superior compliance) in one single material system as natural tissues. The well-known tough hydrogels are heterogeneous in chain size but quite uniform in macro scale, which is drastically different from that of biological tissues that exhibit heterogeneity and hierarchical alignment across length scales.
The central goal through my PhD research is to design tissue-like hydrogels by understanding extreme mechanics in soft materials, for transforming into various engineering applications. Understanding nature, applying mechanics, designing materials for engineering applications are the central threads governing my PhD work.
1, How nature designs tough tissues? (Bio-mechanics) 2, How to design tissue-like hydrogels? (Material Science) 3, What's the principle of design anti-fatigue-fracture hydrogels: fracture beyond atomic bonds or amorphous chains? (Mechanics) 4, How to transform tissue-like hydrogels for versertile applications? (Design, Fabrication, and Manufacturing) |
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Understand Extreme Mechanics in Soft Materials
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Design of Anti-fatigue-fracture Hydrogels
Existing tough hydrogels still suffer from fatigue fracture under multiple cycles of mechanical loads, because the resistance to fatigue-crack propagation after prolonged cycles of loads is the energy required to fracture a single layer of polymer chains (i.e., the intrinsic fracture energy of the hydrogel), which is unaffected by the additional dissipation mechanisms introduced in tough hydrogels. The reported fatigue thresholds of various tough hydrogels are in the order of 1-100 Joul per meter square. In our recent work, we demonstrate that the controlled introduction of nanocrystalline domains (Science Advances, 2019) and nanofibrils (Proceedings of the National Academy of Sciences, 2019) in hydrogels can significantly enhance their fatigue thresholds, since the process of fracturing crystalline domains and nanofibrils for fatigue-crack propagation requires much higher energy than fracturing a single layer of polymer chains. |
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Harnessing Elastic Instabilities in Confined Layers
Soft elastic layers can be commonly observed in biological glues and joints such as mussel thread, barnacle on the rock and tendon, ligament on the bone. The stressed elastic layer for engineering applications includes hydrogel glue on glass, suture-less adhesive on bioloigical tissues and stressed rubber layers in bearing joints. They undergo severe mechanical tensile loading and may form undulation instabilities at the interface. In our recent work, we 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 fringes portion. We further systematically study the formation, transition, interaction and co-existence of fringe, fingering and cavitation (Journal of the Mechanics and Physics of Solids, 2017). The initial occurrence mode of instability can be determined by both geometry and mechanical properties of the elastic layer through two non-dimensional parameters: layer's lateral dimension over its thickness and elastocapillary length over the defect size. Both fringe instability and fingering instability can be delayed and even suppressed by the strain stiffening of soft materials (International Journal of Solids and Structures, 2018). |
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Design Hydrogels with Unprecedented Properties
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Design Hydrogels with High Toughness: Build Dissipation into Stretchy Networks
Our first work is to propose a general design principle for designing tough hydrogels: build dissipation into stretchy network (Soft Matter, 2014). The principle is when crack propagates, the scission of long polymer chains give intrinsic toughening at nano meter scale, meanwhile the materials around crack tip at mm scale undergoes loading unloading process, which gives large portion of additional dissipation to retard crack propagation as extrinsic toughening. We further develop a numerical model and derive a scaling equation for this principle (Extreme Mechanics Letter, 2015). The model, equation, and design principle is universal and can be applied to various polymer candidates across wide length scales (e.g., fiber reinforced hydrogel composites) (Soft Matter, 2014). Design Hydrogels with High Resilience: Delayed Dissipation Resilience is a property that is seemingly contradictory with high toughness because high resilience means a material has the ability to absorb energy when deformed and release the same amount of energy upon unloading. We propose a bio-inspired delayed dissipation principle for designing hydrogels with both high resilience and high toughness (Extreme Mechanics Letter, 2014). |
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3D Printing of Tough and Stretchable Hydrogels into Cellularized Structures
Living tissues usually have high fracture toughness in order to withstand substantial internal and external mechanical loads. The high toughness of tissues challenges researchers to design hydrogels capable of achieving similar toughness in order to withstand physiological mechanical loads. Despite recent success in developing tough hydrogels, the fabrication of these hydrogels often involves toxic chemicals and/or harsh reactions, limiting their capability to encapsulate cells. In addition, it is desirable to fabricate cell-embedded hydrogels with macroporous architecture conducive to generation of complex tissues. While 3D printing offers rapid prototyping and can print hydrogels into complex 3D structures for functions such as vascular networks and aortic valves, it has not been possible to print tough hydrogels into complex structures other than simple and flat ones such as dog-bone sample. Our work demonstrate a tough hydrogel comprised of PEG and sodium alginate that can be used for cell encapsulation (Advanced Materials, 2015), which has a fracture toughness above 1500 J m−2, making it tougher than natural cartilage and yet with water content (≈77.5 wt%) that is tunable and within the physiologically acceptable range. We further develop a new method and material system capable of 3D-printing hydrogel inks with programed bacterial cells as responsive components into large-scale (3 cm), high-resolution (30 μm) living materials, where the cells can communicate and process signals in a programmable manner (Advanced Materials, 2018). Novel living devices are further demonstrated, enabled by 3D printing of programed cells, including logic gates, spatiotemporally responsive patterning, and wearable devices. |
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Explore Engineering Applications of Tissue-like Hydrogels
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Ingestible Hydrogel Gastro-retentive Devices
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Stretchable Hydrogel Electronics
Owing to the weak and brittle nature of common synthetic hydrogels, existing hydrogel electronics and devices mostly suffer from the limitation of low mechanical robustness and low stretchability. On the other hand, while hydrogels with extraordinary mechanical properties, or so-called tough hydrogels, have been recently developed, it is still challenging to fabricate tough hydrogels into stretchable electronics and devices capable of novel functions. The design of robust, stretchable, and biocompatible hydrogel electronics and devices represents a critical challenge in the emerging field of soft materials, electronics, and devices. In this work, we report a set of new materials and methods to integrate stretchable conductors, rigid electronic components, and drug-delivery channels and reservoirs into biocompatible, and tough hydrogel matrices that contain significant amounts of water (e.g., 70–95 wt%) (Advanced Materials, 2015). |
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© 2018 Shaoting Lin. All right reserved.
