Deformation and Failure of Mechanically Adaptive Cellular Materials

Man-made structures with a significant load-bearing function require laborious and costly design efforts in order to ensure safe and efficient performance. In contrast, load-bearing structures found in the natural world evolve and grow into suitable configurations because they are composed of adaptive materials, such as bone and wood, that self-configure according to the mechanical loads they carry. Familiar examples of this mechanical adaptability are the higher bone density observed in the heavily loaded racket-arm of tennis players, the loss of bone mass in astronauts due to a low gravity environment, or variation in annual tree ring thicknesses that reflect environmental stress. As a result of mechanical adaptability to the external environment, bone and wood exhibit excellent load-carrying capacity, are lightweight, and require little in the way of resources to produce (i.e. little energy and raw materials) relative to synthetic materials.

This project will evaluate a bio-inspired strategy for fabricating synthetic materials that adapt to their mechanical environment. The strategy will employ a nano-fabrication technique to deposit a structurally robust nanocomposite coating onto a porous foam substrate. Under appropriate processing conditions, it is hypothesised that thicker coatings can be deposited in regions experiencing higher loading, resulting in adaptive nanocomposite-coated foams with mechanically tailored microstructures. A program of testing will be conducted in order to establish the mechanical behaviour and properties of these materials, which will play an integral role in establishing the interrelationship between the processing, structure, and properties of uniform and adaptive foams.

The results will enable an assessment of the mechanical adaptability of nanocomposite-coated foams, and the suitability of these materials for anticipated engineering applications. Theoretical predictions and preliminary results suggest a combination of exceptional mechanical properties (e.g. strength and stiffness) with high porosity, low density, and multi-functionality is achievable, making these materials excellent candidates for use as artificial tissue scaffolds in biomedical applications, and as lightweight load-bearing materials in weight-critical structures.

The goal of this nascent program of research is a laboratory proof-of-principal for mechanically adaptive materials. However, the PI envisages that, once proven, over a span of decades adaptive materials have the potential to revolutionise the way in which load-bearing structures are designed and produced. Current technology and practices see structures designed to endure predicted mechanical conditions and manufactured accordingly. Under the proposed new paradigm, adaptive materials would be subjected to representative or actual in-service mechanical conditions during their manufacture, and naturally evolve into a structure with more material located where loading is concentrated.

The economic impacts of this step-change in the design and build of load-bearing structures would include beneficiaries that extend along a generic supply chain, from suppliers of materials/parts, to manufacturers of products, and end-users including service providers and consumers. At the supplier level, commercialisation and exploitation of adaptive material processing will enable the production of high-value materials that are customised for application-specific conditions. The reduced waste associated with more optimum material placement will enhance environmental sustainability. Manufacturers will benefit from offering products with higher performance via the use of mechanically customised materials and the additional wealth generated by material-level customisation will come without the extra cost of developing prescribed designs. Replacing prescribed design with adaptive materials may also help to avert failures caused by errors or oversights in the design process, which are common occurrences in complex structures and can be very costly to redress.

End-users will derive further economic benefits and enhanced environmental sustainability from the improved performance of products made from adaptive materials, including reduced weight, increased porosity, and increased mechanical properties. Striking examples of the potential to reduce costs due to weight savings in aerospace applications are the American Airlines' estimate that 400,000 gallons of jet fuel will be saved annually by replacing 35-pound pilot kitbags with Apple iPads, or the $10,000 (USD) cost per pound of space launches into Earth orbit. Weight reductions have also enabled the rapid increase in the size of wind turbine towers and blades (nearly double from 2008-13) and the commensurate drop in the cost of the energy produced (33% reduction from 2008-13). High porosity is required in tissue scaffold materials to enable the in-growth of cells and biological tissue, the exchange of nutrients, and vascularisation. It is estimated that over 1,000,000 bone-grafting procedures are performed globally every year in order to repair bone defects caused by trauma and disease (e.g. osteoporosis, bone metastases). The shortage in musculoskeletal donor tissue used in these procedures has created a market for bone graft substitutes totalling $1.3 billion in 2010 and expected to increase to $2.2 billion by 2017 in the USA alone. In addition to this huge potential for wealth generation, the health and wellbeing of patients would be improved by the enhanced characteristics of tissue scaffolds composed of adaptive materials with higher mechanical properties and porosity.

The exciting potential outcomes and the relatable analogy of the research strategy to ubiquitous processes occurring in natural biological materials provide a strong impetus for attracting public interest. The bio-inspired approach and potential medical applications are aspects that are likely to help attract females into materials science and related fields, since the tendency for females to prefer "organic" subjects (e.g. biology and medicine) is a recognised factor in the under-representation of females in mathematics-intensive fields [1].

[1] Ceci SJ, Williams WM (2011) Proc Nat Acad Sci USA 108:3157-3162