# Biomimicry and the Built Environment, Learning from Nature's Solutions
## 3 Biomimicry in Structural Engineering
### Concept of Biomimetics and Structural Engineering Design Process
#### 3.1.1 Materials  
Materials are the smallest, indistinguishable building blocks in the structure. Natural materials have always been in use. The first tools our ancestors used were little more than sticks or stones picked up and used to hammer open food. In the modern era of engineering, where specific properties are needed for calculations or factors of safety, natural materials have become less popular in structural uses and not obsolete. Biological materials are elegant and practical in the engineering field. They provide sufficient strength and other special characteristics while remaining relatively light in weight. Most of the natural materials are biodegradable, which increases their value in an era of sustainability. Biomaterials have two main classes: elastic-tensile biomaterials and hard rigid biomaterials. Tensile materials are mainly composed of protein, whereas rigid materials are formed by combining the protein with minerals (primarily calcium or silica) [39]. For example, natural silks have been found to have excellent strength and extensibility [40; 41; 42]. Spider silks have low density, tensile strength exceeding 1000 MPa, and extensibility of approximately 0.27, which is way beyond the yield strain of steel (0.0025). In short, natural protein silks are one of the best structural bio-materials made by nature. They have incredible tensile capacity and impressive extensibility, not to mention their low density compared with traditional steel wire. If scientists and engineers can find a way to mass-produce natural silk with a big diameter, the size of concrete reinforcement or cables can be substantially reduced. Another particular type of protein worth discussing is resilin and abduction, which do not have extremely high tensile strength or elasticity. However, the two types have a special ability to store energy and release it back with high efficiency. Abalone shell is another great example of nature's wisdom in building construction by using the nearby environment to minimize energy use. Besides its amazing growth mechanism, the abalone has outstanding mechanical properties, as its average fracture strength is 185 MPa [43]. Another type of biomaterials that are also mineralized is bones. Bones are the essential component of our body, performing mechanical, chemical, and biological functions. They are a highly hierarchical structure and have incredible mechanical properties. Bones have two types: cortical (or compact) and cancellous (or trabecular). Bones have a highly hierarchical order. The main components of bones are bone crystals, collagen, and water. The mechanical properties mainly depend on individual bone porosity, degree of mineralization, and bone age [44]. As a result, similar to most biomaterials, their mechanical properties are varied. Enamel and dentine, (which are known as the stiffest biomaterial in the human body) are mainly found in human teeth, form the other type of bio-materials. Enamel is the stiffest biomaterial in the human body. It has a yield stress of 330 MPa and a Young modulus of 83 GPa [45]. Therefore, the yield stress of enamel is comparable to steel. Furthermore, compared with other metals, such as steel, enamel displays metallic-like behavior in a stress-strain relationship and crack initiation even though most enamel is made up of brittle hydroxyapatite crystallites [46]. This finding shows nature's ability to achieve metallic mechanical properties from brittle ingredients using hierarchical order or special arrangement. These examples show that using natural materials does not necessarily mean a compromise in performance and can indeed be of significant benefit. Thus, why are raw materials not used more today? One major problem of applying the discovery directly to the structural engineering field is that most bio-materials found in nature cannot be mass-produced, and their durability is relatively low.