The woodpecker does not suffer head/eye impact injuries while drumming on a tree trunk with high acceleration (more than 1000×g) and high frequency. The mechanism that protects the woodpecker's head has aroused the interest of ornithologists, biologists and scientists in the areas of mechanical engineering, material science and electronics engineering. This article reviews the literature on the biomechanisms and materials responsible for protecting the woodpecker from head impact injury and their applications in engineering and human protection. Traumatic brain injury as a consequence of head impact injury has been a leading cause of morbidity and death in war, aviation and road accidents and sports collisions [1][2][3]. However, the woodpecker repeatedly strikes its head against trees without suffering head injury when drumming a trunk continually at a speed of 6-7 m s 1 and acceleration of ~1000×g [4][5][6][7]. The woodpecker rhythmically drums surfaces such as dead tree limbs and metal poles with its beak to catch worms to eat, attract a mate or announce its territorial boundaries [7,8]. The woodpecker's resistance to head impact injury is a prime example of adaptive natural evolution over millions of years [9], and has been of interest not only to ornithologists and biologists but also to researchers in the fields of mechanical engineering, medical engineering, material science and electronics engineering [4][5][6][7][10][11][12][13].Researchers have explored the mechanism of how a woodpecker avoids head impact injury [4][5][6]14,15] and searched for clues that will help in developing a bionics shock-absorbing system or device for engineering purposes or human protection [10][11][12][13]. This paper presents an overview of the biomechanism that prevents the woodpecker from suffering head impact injury and its applications.

Natural biological materials such as bone, teeth and nacre are nano-composites of protein and mineral frequently exhibit highly superior strength for self-assembly and nanofabrication. Mineral mass and microstructure/nanostructure of bone are susceptible to stimulation by mechanical loads, ensuring that its mechanical behavior and strength are adapted to environmental changes. Woodpeckers repeatedly drum tree trunks at a speed of 6-7 m s−1and acceleration of ~1000 g with no head injuries. The uneven distribution of spongy bone has been founded on woodpecker's skull in our previous study. More knowledge of the distribution of the shock-absorbing spongy bone could be incorporated into the design of new safety helmets, sports products, and other devices that need to be able to resist the impact. In this study, the effect of microstructure of spongy bone in different parts on woodpecker’s skull compared with other birds was observed and analyzed. It was found that the unique coordinate ability of micro-parameters in different parts of woodpecker’s skull could be one of the most important roles of its resistance to impact injury. Better understanding of the materials would provide new inspirations of shock-absorbing composite materials in engineering.

A new triply periodic minimal surface (TPMS) structure mimicking the microstructure of Great Spotted Woodpecker's cranial bone was designed, fabricated and tested in this study. It was found that the designed structures acquired better mechanical performance compared to the unit structures applied in heterogeneous porous scaffolds. The wall thickness mimicking the woodpecker's spongy bone and the TPMS surface structure were two contributors to the good mechanical performance of new designed bionic structures.

The uneven distributed microstructure featured with plate-like spongy bone in woodpecker's skull has been found to further help reduce the impact during woodpecker's pecking behavior. Therefore, this work was to investigate the micro-mechanical properties and composition on different sites of Great Spotted woodpecker's (GSW) skull. Different sites were selected on forehead, tempus and occiput, which were also compared with those of Eurasian Hoopoe (EH) and Lark birds (LB). Micro structural parameters assessed from micro computed tomography (μCT) occurred significantly difference between GSW, EH and LB. The micro finite element (micro-FE) models were developed and the simulation was performed as a compression process. The maximal stresses of GSW's micro-FE models were all lower than those of EH and LB respectively and few concentrated stresses were noticed on GSW's trabecular bone. Fourier transform infrared mapping suggesting a greater organic content in the occiput of GSW's cranial bone compared with others. The nano-hardness of the GSW's occiput was decreasing from forehead to occiput. The mechanical properties, site-dependent hardness distribution and special material composition of GSW's skull bone are newly found in this study. These factors may lead to a new design of bulk material mimicking these characteristics.