As the oldest and largest living creatures on earth, trees have been pivotal both ecologically and economically in supporting the prosperity of humankind for millennia. Although the invention of petroleum-based plastics saw the prominence of forest products fade, over the past decade we have witnessed a resurgence in the popularity as concerns around plastic pollution increase and the benefits of a circular economy are advanced.
With this resurgence, we are seeing the range of forest products expand beyond conventional wood, pulp, and paper products to include high-performance cellulose-based materials. My lab is focused on the latter. We design high-performance materials by mimicking nature’s processes and using simple and abundantly available elements and materials.
Nature has created numerous smart and sophisticated designs – from butterfly wings to beetle shells – reflecting millions of years of evolution. It is always inspiring to learn how nature can manipulate these sophisticated designs with less than 10 out of the 118 elements in the periodic table.
Structural materials that are lightweight, strong, and tough are highly desirable in many fields including building and construction, transportation, and aerospace. In general, strength and ductility are considered two opposite characteristics that cannot be found in a single material. However, strong and ductile materials have existed in nature for thousands of years. One excellent example is the abalone nacre which consists of no more than six elements, and the only adhesive is a hydrogen bond, the same force holding the DNA double helices together.
Inspired by the simplicity of nature, my lab investigates how cellulose – the most abundant polymer on Earth – can be used to develop materials with the strength and toughness suitable for engineering applications. Cellulose is one of the major components that can be found in all lignocellulosic biomass, including trees, herbaceous plants, agricultural residues, and marine plants. From the molecular level, a cellulose chain consists of hundreds to thousands of glucose units strung together. When 30-40 cellulose chains pack together in parallel by hydrogen bonds, a very strong nanomaterial, nanocellulose, can be formed with the mechanical strength comparable to some existing engineered materials such as Kevlar, carbon fibres, and steel. However, it is a tremendous challenge to transfer the nano-scaled mechanical properties to the bulk materials, such as wood and other wood products, owing to both the vascular structure and the limited hydrogen bond networks in the wood cell walls due to the presence of other heterogeneous polymers such as lignin and hemicellulose.
My lab recently published a paper in the Journal of Materials Chemistry A outlining our research to address this challenge and make strong and tough materials out of cellulose using simple processes found in nature. In order to resume the hydrogen bond, networks among cellulose elementary fibrils, lignin, and hemicellulose are first removed by chemical treatments. This enables additional hydrogen bonds to form by compressing the wood under different moisture contents, where water molecules can serve as structural components to bridge the neighbouring elementary fibrils by forming 3D hydrogen bond networks.
Our key finding is this “water-glued wood” showed significantly improved tensile strength and toughness, more than six and 10 times greater than the original wood, respectively. Due to the low density of cellulose, the specific strength of this “water-glued wood” is much higher than concrete, stainless steel, and superlight aluminum alloy, manifesting a new type of material inspired from nature.
Delicate architecture design represents another research direction in my lab inspired by nature. Paper wasps can be considered as a pioneer in house design using cellulosic materials. Wasps diligently gather fibres from wood and produce pulp using saliva which they reconstruct into a honeycomb structure. By learning from wasps, we have developed a 3D printing technology – a high-resolution computer-aided design and fabrication technology that enables production of complicated and customized 3D objects in a controlled manner – that can produce a similar structure using cellulose fibres.
Once again hydrogen bonding plays a critical role in binding cellulose fibres together, owing to the large surface area created by breaking down large cellulose fibres into nanofibrils or even molecular level by nanotechnology. By comparing 20 micrometre wide cellulose fibres and 2 nm nanofibrils, the surface area of the latter is approximately 10,000 times greater than the former, meaning more hydrogen bonds can be formed between these cellulose nanofibrils.
By using this technology, we have printed a lightweight honeycomb structure with excellent mechanical properties. It is light enough to stand on top of a dandelion, yet strong enough to withstand over 15,000 times of its own weight or the weight of a kettle ball standing on top of the cellulose honeycomb structure. When wet, this structure also demonstrates superior flexibility that can be bent, rolled, and twisted in all directions. It is envisioned that this type of cellulose honeycomb structure could be used in the future as a lightweight structural component, thermal insulation layers, or for other consumer products.
In my lab, we are trying to push the limits of 3D printing of cellulosic materials by printing different types of cellulose, exploring varied geometry and complexity, tuning the mechanical performance, and incorporating other ingredients for improved functionalities such as fireretardancy, water treatment, and biomedical applications. In addition to the nacre-mimicry and 3D printing technologies, our team is also developing technologies including high-yield and high-performance lignocellulosic nanomaterials that can be used for making commodity products, thermal insulation, ionic conducting materials, fire-retardant wood products, polymer emulsions, and thermal-regulating fibres and foams for infrared shielding and thermal comfort.