Structural design strategies discovered in Elaeocarpus ganitrus endocarp may help create new fracture-resistant biomaterials

The field of biomaterials is multidisciplinary. The design of a new biomaterial necessitates knowledge and ideas from multiple disciplines. It needs the synergistic integration of materials, biology, medicine, mechanical sciences, and chemistry. Indeed there is a continuous need for fracture-resistant and high-strength materials that are also lightweight. Such biomaterials could be used to produce different products. It may also help create materials that may be readily acceptable to the human body and might be superior to metallic plates.

Though enormous progress has been made in the field of nature-based biomaterials, room for improvement remains. Novel biomaterials which outperform current state-of-the-art products in terms of reduced side effects, lower purification or manufacturing costs, and greater therapeutic efficacies will continue to be developed. Researchers continue to search for new natural sources of biomaterials that will lead to more effective biomaterials and new applications. Indeed, many natural materials are really tough and yet light. One such example of natural material that is strong and light is endocarp, an external shell of seeds. It is one of the strategies plants have developed over millions of years of evolution to protect their seeds from various threats. Experts think that studying the endocarps of some of the plants can help understand how to create light and yet fracture-resistant materials.

In a new study conducted by Ph.D. candidate Ashish Ghimire, and Professor Po-Yu Chen from the Department of Materials Science and Engineering at the National Tsing Hua University in Taiwan, the researchers studied the strength of Elaeocarpus ganitrus. There was a reason for choosing Elaeocarpus ganitrus. First, it is a seed of a plant that grows commonly in many parts of South Asia. Secondly, there have been long traditions of using the endocarps as prayer beads to create religious ornaments. These prayer beads are thought to last almost forever due to the high strength of the endocarp of the seed.

The authors noticed that previous studies have used different seeds with some seeds like macadamia shells showing a strength of 1800–3000 N. They thought this magnitude of specific strength is almost comparable to commercial aluminum. Earlier studies have also investigated the strength of walnuts, hazelnuts, and others, though they have much lower strength than macadamia seeds. They for the first time explored the compressive strength of Elaeocarpus ganitrus, which got its attention not only due to its traditional use but also because of the 200 feet height of the tree, which means that the endocarp of the tree must be strong enough not to break after a fall from such a height.

Researchers noticed several intriguing structural designs in Elaeocarpus ganitrus. For example, unlike walnut and other nuts, it has not one suture seam but rather multiple (commonly found: 5, rare ones: up to 21). Despite the so many sutures, it seems that the seed is incredibly difficult to break. The authors said when they found that a 20 mm-sized plant shell having a low density of 1g/cm³ can withstand high loads up to 5000N, they realized they were about to witness previously unseen design strategies. Thus, the researchers used microscopic, tomographic, and nano-mechanical investigations to understand the reason for its high mechanical strength. The structure density and nano-mechanical properties of the endocarp reveal a gradient decline towards the core. It has three layers of different cells with outer densely packed sclereid cells, middle elongated sclereid cells, and an inner layer of sclerenchyma fibers. The seed is harder on the outside and relatively softer on the inside, the soft core assists energy absorption under impact. The number of sutures helps to increase its strength as they help in the dissipation of energy. Additionally, sutures are strengthened by the sclerenchyma fiber bundles to prevent early fracturing.

In a statement to Medicine Innovates, Professor Po-Yu Chen, the lead author said he believes that further understanding of the multi-layer structure, its seams, and their role in providing strength to the endocarp may help create light and yet fracture-resistant materials. This promises to bring new innovative biomaterials for various health applications.