Review on Mechanical Behaviours of Beetle Elytra-Inspired Structures

Abstract

Bio-inspired structures have been a fascinating field of research in recent years, as scientists and engineers have been inspired by natural structures to develop new materials and structures with unique properties. Beetle elytra, the hardened forewings of beetles, have been a prominent source of inspiration for researchers due to their remarkable properties such as lightweight, toughness, and impact resistance. This review paper provides an overview of the bio-inspired structures based on beetle elytra, including their materials and properties, design and applications, and experimental methods. we aim to uncover the key design principles responsible for the mechanical behaviour observed in beetle elytra and femurs by studying their structure-property relationships. Additive manufacturing techniques will be used to fabricate beetle-inspired structures by mimicking these design principles. The biomimetic structures will be tested to evaluate their mechanical performance compared to regular structures without biomimetic features. The paper also discusses the potential applications and future directions of these structures.

Introduction

I. INTRODUCTION

Survival is the most inherent response of any organism that has ever grown on Earth. Survival is very well-suited if it is followed by extraneous adaptations and biological evolutions, Therefore living species tend to transform into more unique biological structures having specialised biological functions. This process of evolutionary adaptation for survival even took generations like in the case of the quasi-social species like Homo sapiens sapiens who started their evolutionary journey from Australophithecus to Homo-Habilis then Homo-Errectus and Homo-Neanderthalensis(Homo sapiens) or miniature Insects like Beetle. The evolutionary period of beetle is quite long and varied because insects are one of the oldest phylogenetic groupings of species on earth. Nature has evolved over millions of years to develop complex hierarchical structures and materials that provide innovative solutions to various challenges around lightweight design, optical properties, adhesion, and more. The variation of about 6,00,000 different kinds of species over a period of more than 480 million years[1]. The concept of bio-inspiration has been widely researched in recent years due to its potential to develop new materials and structures with unique properties. Bio-inspired structures are designed by mimicking natural structures found in living organisms. Beetle elytra have been a prominent source of inspiration for scientists and engineers in developing new materials and structures due to their remarkable properties such as lightweight, toughness, and impact resistance. Many scientists and engineers are turning toward nature for inspiration and biomimicry – the imitation of nature's designs and processes to solve human problems. One area that has garnered much interest is the study of insect exoskeletons, which provide a protective outer casing for insects to survive in a variety of environments. Insects have evolved lightweight yet strong exoskeletons that allow them to inhabit harsh surroundings, fly at high speeds, jump, and display other feats of mobility, all with a high strength-to-weight ratio[2,3].         

This degree of variation provides researchers with a vast number of possible sources for inspired biomimetic designs. The beetle is one of the most ideal models for biomimetic objects epitomised by lightweight functional materials[3-4]. The most awestruck evolution of the beetle is the conversion of forewings into hardened covers, often referred to as elytra. It was a ground-breaking morphological adaptation which has also contributed to the evolutionary success of beetles[5-7]. But, the knowledge of these structures' functional and mechanical aspects is still fragmentary and scattered due to very high variation in beetles worldwide. Bio-inspired structures and materials have garnered significant attention in recent years due to their potential to address a wide range of engineering challenges[8-9]. By mimicking the natural properties and functions of living organisms, bio-inspired designs can offer novel solutions to problems such as energy efficiency, durability, and adaptability. One such biological structure that has inspired researchers is the elytra of beetles[10-13]. Beetle elytra are the hardened forewings of beetles, which serve as protective covers for their delicate hindwings and abdomen. These structures exhibit remarkable mechanical properties, such as strength, toughness, and impact resistance, making them an ideal model for bio-inspired materials and structures.

Furthermore, elytra are lightweight and exhibit multifunctional features, such as self-healing and energy absorption, which make them particularly attractive for a range of engineering applications. The elytra of different beetle species have a diversity of surface structures, from smoothly convex to serrated or crenellated[14]. Increasingly, these structures are being studied and even biologically mimicked or replicated for applications as diverse as water collection, antibacterial surfaces, and aerodynamics.

The elytra of beetles like the ironclad beetle, Japanese rhinoceros beetle, and grapevine beetle have been the focus of many studies. Their highly corrugated and interlocking structures lead to exceptional strength-to-weight ratios and impact resistance, while still retaining flexibility. Analyses of the elytra of the ironclad beetle, in particular, have revealed a highly organized, layered structure with features at the nanoscale, microscale, and structural scale that work together to provide durability[15-19]. Surface structures of beetle elytra have also been studied for properties like hydrophobicity, air retention, and antibacterial activity. The elytra of the cactus longhorn beetle and certain species of diving beetles possess hierarchical structures of pits and ridges that render the surface superhydrophobic and capable of trapping an air layer[20-24]. Some ground beetles have elytral surfaces covered with zinc-based antimicrobial structures, which have spurred interest in developing antibacterial surfaces for applications like medical equipment and food processing facilities. The streamlined and aerodynamic shape of beetle elytra has also made them a model for efficient airfoil design[25]. Studies of elytra from weevils, ground beetles, and certain species of leaf beetles have revealed how microscopic ridges and troughs on their surface reduce drag and create extra lift forces during flight. These microstructures are now being applied to improve the design of wind turbines, aircraft, and vehicles. In all these cases, while biologically inspired or mimicked structures cannot replicate the elegance and efficiency of natural structures, they have spurred innovation in materials, engineering, and design[26-28]. Continued study of the diverse micro-and nanoscale features of beetle elytra is likely to yield more insights and applications in the future.

The aim of this research paper is to provide a comprehensive review of the current state of knowledge on beetle elytra-inspired structures, focusing on their structure, properties, and potential applications. By examining the various design strategies that have been employed to mimic the properties of beetle elytra, this paper seeks to identify the key challenges and future research directions in the field of bio-inspired materials and structures[29]. In the following sections, we will delve into the anatomy and function of beetle elytra, explore the various bio-inspired design strategies that have been developed based on their properties, discuss the potential applications of these structures in various industries, and identify the main challenges and opportunities for future research in this rapidly evolving field. The insights from this study can provide blueprints for the design of lightweight protective structures, energy absorbers, and mobility mechanisms for application in various engineering fields. Overall, this research will demonstrate how bio-inspired design can be used to advance new technologies.

II. BEETLE ELYTRA: STRUCTURE, FUNCTION, AND PROPERTIES

Beetle elytra are hardened forewings that cover and protect the membranous hind wings and abdomen of beetles. They have a complex anatomy and nanostructured surface that endow them with specialized functions and remarkable properties. Beetle elytra represent remarkable examples of form and function coming together at the nano-macro interface in biological materials. Their complex structural design, material composition and mechanical attributes provide valuable inspiration for developing next-generation bio-inspired composites, textiles, coatings and impact-resistant materials.

A. Anatomy of Beetle Elytra

1. Each elytron is a thickened and hardened plate attached to the mesothorax segment of the beetle and overlapping the next elytron at the base.
2. They have vein-like ridges that run longitudinally and cross-veins that provide support and flexibility. The exact patterning of these veins varies among beetle species.
3. The surface of elytra is sculpted with microscopic scales, bumps, grooves and pits that create a rough texture. These surface features span nano to micron scales.
4. Elytra have a waxy epicuticle layer that limits moisture loss and prevents sticking of the elytra. Glands also secrete fluids that allow self-healing of minor damage.

B. Functions of Beetle Elytra

1. The primary function of elytra is protection - they cover and safeguard the more delicate hind wings and soft abdominal segments of the beetle.
2. Elytra protect the hind wings while the beetle is at rest. During the flight, the elytra slide apart to allow unfolding and flapping of the hind wings.
3. The textured surfaces and vein patterns of elytra propagate sound towards acoustic receptors on the beetle's body, enabling communication between individuals.
4. The rough scales and pores on elytra are involved in gas exchange, moisture regulation and scent-releasing functions during certain beetle behaviours.
5. In some beetles, the iridescent patterns, contours and colours of elytra play important roles in visual signalling, camouflage and mating behaviours.

C. Mechanical Properties of Elytra

1. Elytra are lightweight yet extremely durable, capable of withstanding forces many times their own weight.
2. The combination of rigid vein-like ridges and flexible cross-veins gives Elytra the ability to deform under load and recover their original shape - making them both tough and resilient.
3. The waxy layer, hierarchical textures and ability to self-seal small cracks allow elytra to absorb and dissipate impact energy, maintaining mechanical integrity even after repeated impacts.
4. Elytra are anisotropic in mechanical behaviour - they exhibit different strength, stiffness and deformability along different directions due to their veined and textured geometry.

D. Material Composition and Microstructure

1. Elytra are mainly composed of chitin - a tough, fibrous polysaccharide also found in insect exoskeletons and crustacean shells.
2. The chitin microfibrils in elytra are arranged in organized layers and oriented at different angles to provide strength in multiple directions.
3. Proteins form a matrix between the chitin layers, binding them together and endowing elytra with flexibility and impact resistance.
4. Other substances like wax, phenolic compounds and pigments are impregnated into or deposited on the elytra, further modifying their material properties and functions.

III. BIO-INSPIRED DESIGN STRATEGIES BASED ON BEETLE ELYTRA

Beetle elytra exhibit several amazing properties that provide inspiration for designing advanced materials and systems. Beetle elytra provide a rich source of inspiration for bio-inspired design strategies - from hierarchical structuring, multifunctionality and self-healing to enhanced impact resistance and energy absorption. By studying the nanoscale, microscopic and macroscopic features of elytra and their interactions, researchers hope to replicate and even surpass the remarkable properties of these biological materials through technological designs and fabrication techniques. Some key bio-inspired design strategies enabled by beetle elytra include:

A. Hierarchical Structures

Beetle elytra have a multi-scale hierarchical structure with nanoscale to macroscale features. At the nanoscale, they have ridges, platelets and pores that contribute to properties like wetting and iridescence. At larger scales, they have textured veins, slopes and overlapped edges.

This hierarchical arrangement of structural elements across scales enables synergistic combinations of mechanical, optical and functional traits. Researchers can mimic this hierarchical structuring through multi-step fabrication approaches involving lithography, self-assembly and 3D printing. Hierarchical materials inspired by elytra could combine the best properties of different structural levels to achieve enhanced performance.

B. Multifunctionality

Beetle elytra do not just provide protection for the insect; they enable multiple functions simultaneously. They reflect light to display iridescent colours, act as channels for sound propagation, allow gas exchange and even trap moisture for drinking.

This multi-functionality of elytra arises from their complex hierarchical geometry, chemical composition and arrangement of microtextures. Materials designers can learn from this integrated approach to imparting multiple properties and functionalities in a single material or structure. Combining multiple bio-inspired design features can lead to multi-purpose elytra-like materials.

C. Self-Healing Mechanisms

Beetle elytra can self-repair minor damage through the accumulation of wax and fluids secreted by glands within the insect. When cracks occur, these secretions fill the gaps and reinforce the elytra, restoring functionality. Bio-inspired self-healing materials could utilize a similar approach by incorporating micro-containers of healing agents within the material design. Upon damage, these capsules rupture and release the agents that seal cracks, replenish lost material and re-establish structural integrity. Researchers can optimize hierarchical textures, porosity and chemical compositions of elytra-mimetic materials to facilitate self-healing behaviors.

D. Energy Absorption and Impact Resistance

Beetle elytra exhibit remarkable impact resistance and energy absorption due to their complex microscopic surface structures. The ridged and textured surfaces help dissipate impact forces and displace absorbed strain energy more evenly throughout the material. Materials scientists can mimic these design strategies through hierarchical patterns, crumpled geometries and tailored porosity within impact-resistant materials. Local deformation, buckling, crack deflection and other energy absorption mechanisms observed in beetle elytra offer inspiration for developing stronger and tougher bio-inspired impact-resistant materials.

IV. APPLICATIONS OF BEETLE ELYTRA-INSPIRED STRUCTURES

Beetle elytra, the protective forewing shells of beetles, provide key insights for engineered design through their multifunctional mechanical properties, energy absorption capabilities, and damage tolerance. Their natural hierarchical structures, laminated microarchitecture, and composition optimize strength, flexibility, and toughness within minimal weight. Translating these bio-inspired features to synthetic materials and macroscale structures presents exciting opportunities across various fields.

In the aerospace and automotive sectors, lightweight armor and impact-resistant shells inspired by beetle elytra can enhance crashworthiness and survivability. The elytra’s corrugated, interlocking patterns can be adapted into shell structures to promote controlled deformation and folding. This increases energy absorption during collisions while reducing peak stresses. Elytra-inspired hybrid laminates with alternating fiber-reinforced polymers can also optimize mechanical properties and distribute impact forces across scales. Integrating such biomimetic materials into aircraft fuselages or car bodies could prevent catastrophic failures. Their damage tolerance further enables continued operation after minor impacts.

Civil engineering infrastructure can leverage beetle elytra designs for better resilience and sustainability. Bridges, buildings and tunnels embedded with elytra-inspired crumple zones may withstand seismic activity or other natural disasters. Shock-absorbing facades and shielding inspired by elytra could mitigate damage during earthquakes. The elytra's layered architecture can guide earthquake-resistant sandwich composites with strong facings and soft cores to prevent catastrophic collapses. Their micro-corrugations and riblet patterns can also reduce wind loading on buildings. Adapting these bioinspired features could create more durable, damage-tolerant infrastructure.

In architecture, beetle elytron organization and composition offer sustainable building insulation. Overlapping scales on elytra prevent heat loss, inspiring thermal insulators made of stacked tiles or panels. Elytral waxy coatings also resist water, suggesting hydrophobic, moisture-proofing architectural materials. Chitin-protein elytral composites are extremely lightweight yet rigid, guiding the design of roofing elements that minimize dead loads while providing strength. Such nature-inspired materials could enable energy-efficient, resilient architecture.

For personal protective equipment, the impact resistance, flexibility and light weight of elytra can be adapted to design better helmets, body armor and clothing. Helmet shells and pads mimicking the pronounced ridges and corrugations on elytra may mitigate traumatic brain injuries during collisions. Layered elytra-inspired fabrics using stiff and flexible components could enable lightweight bulletproof jackets. Elytra-inspired materials can also reinforce clothing to prevent cuts and abrasions without restricting movement during extreme sports or military use. Such safety gear takes key advantages from elytral structure and mechanics for user protection. Lastly, beetle-inspired elytra mechanisms hold promise for robotics applications like flapping micro-air vehicles  or terrestrial rescue robots that need to traverse rubble. Elytron-inspired flexible joints and shell flaps could enable micro-drone wings to fold neatly backwards. Impact-resistant elytra coatings may also allow robotic crawlers to squeeze through debris after disasters. Adhesive surfaces inspired by beetle elytra could further allow robots to walk up walls and ceilings. Such integration of elytral features could permit robot locomotion and durability in challenging situations.

In summary, the multifunctional properties of beetle elytra provide wide-ranging bioinspiration to enhance engineered materials, structures and robots. Their natural strength, flexibility, damage tolerance and energy absorption can be replicated across scales to create innovations within aerospace, automotive, civil, architectural, personal protection and robotic systems. Continued research and design based on beetle elytra promise even more diverse applications in the future.

V. CHALLENGES AND FUTURE RESEARCH DIRECTIONS

Beetle elytra are fascinating biomimetic structures with mechanical and optical properties that offer great potential for applications in architecture, textiles, microfluidics and biomaterials. However, scaling up production, overcoming environmental impacts and integrating elytra-inspired designs with other materials pose significant challenges.

A. Scaling up Production

Current fabrication methods for beetle elytra-like nanostructures are limited to small lab-scale experiments using techniques like etching, lithography and 3D printing. In order to commercialize and produce elytra-inspired materials at an industrial scale, low-cost mass replication techniques need to be developed. Possible approaches could involve roll-to-roll nanoimprint lithography, colloidal self-assembly of nanoparticles, and electrohydrodynamic spray coating. However, research is needed to optimize these techniques for producing large-area, defect-free elytra-mimetic surfaces.

B. Environmental and Sustainability Considerations

Most beetle elytra fabrication methods require the use of various chemicals and solvents that may pose environmental risks. The end products also contain synthetic polymers that are difficult to biodegrade. There is a need for more sustainable methods that utilize eco-friendly materials, minimize waste, and produce reusable and recyclable structures. Bio-based polymers, bio-templating from actual elytra, and green chemistry processes could help reduce the environmental footprint of scaling up elytra-inspired production. Research in life cycle assessment and durability modeling can quantify the environmental impacts and inform more sustainable designs.

C. Integration with Other Bio-inspired Materials

Current research on beetle elytra mostly focuses on replicating their individual properties. However, future biomimetic structures could integrate elytra-inspired designs with other bio-inspired materials to create hybrid systems with multi-functional capabilities. For example, elytra nanostructures could be combined with:

1. Cellulose and chitin fibers to produce high-performance biomimetic textiles with enhanced strength, wettability and optical properties.
2. Hydrogel scaffolds to develop tissue engineering platforms with optimized cell adhesion, growth and differentiation for regenerative medicine applications.
3. Graphene and carbon nanotubes to create multifunctional composites with electrical conductivity, thermal management and mechanical reinforcement.
4. Microfluidic channels and actuators to develop adaptive moisture harvesting systems and lab-on-chip devices with tunable wetting properties.

Such hybrid bio-inspired materials systems could unlock novel functionalities and performances not feasible with traditional materials or individual bio-mimetics. However, fundamental research is needed to determine optimal interfacial interactions, integration methods and structure-property relationships for different hybrid combinations involving beetle elytra designs.

D. Integration with Other Biological Inspiration

In nature, beetle elytra rarely function in isolation; they work together with other organismal systems and adaptations. Future research could thus integrate elytra-based designs with broader biological inspiration from beetle wing microvenation patterns, iridescent coloration mechanisms and whole-body motions. This multifaceted biomimetics approach has the potential to produce materials with properties optimized for specific functions within larger systems and applications contexts. However, systematic analysis is needed to uncover the synergies between elytra nanostructures and other aspects of beetle biology, morphology and behavior. Integrating such diverse learnings from beetles into rational materials designs remains a significant challenge and opportunity for biologically inspired engineering.

In summary, while beetle elytra offer many promising avenues for novel functional materials, overcoming the challenges of scaling up production, accounting for environmental impacts, integrating with other bio-inspired designs and embracing more holistic biological inspiration represent significant future research directions. Addressing these challenges through interdisciplinary collaboration between materials scientists, biologists, chemists, and engineers could help fully realize the potential of beetle elytra-inspired sustainable materials and systems for applications in the green economy.

Conclusion

In summary, the elytra or hardened forewings of beetles represent a rich source of inspiration for scientists and engineers due to their exceptional mechanical and surface properties. From the ironclad beetle\'s highly impact-resistant elytra to the superhydrophobic and aerodynamic elytra of other species, beetle elytra offer diverse models of lightweight yet durable structures, water-repelling surfaces, antibacterial coatings, and aerodynamic shapes. Bio-inspired structures based on beetle elytra have shown great potential to develop new materials and structures with unique properties. The remarkable properties of beetle elytra have led to the development of bio-inspired structures with applications in various fields. While biological structures are still far more sophisticated and efficient than the materials and designs they have inspired so far, continued study of beetle elytra is likely to yield more insights into how their ordered, multi-scale structures achieve remarkable properties using relatively simple components. As we gain a deeper understanding of how different beetle elytra are adapted to meet the particular needs of each species, they may inspire further innovation in fields from aerospace engineering to medicine. The diverse surface morphologies and mechanical properties of beetle elytra have enabled these insects to inhabit nearly every ecological niche on the planet, an adaptability that we have only begun to mimic in engineered systems. With their multifunctional and efficient designs, beetle elytra represent a rich source of inspiration for future technological development. Future research in this field should focus on the development of more efficient and sustainable manufacturing methods.

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Copyright

Copyright © 2023 Abhay Dwivedi, J. Bhaskar, Anand Kumar. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.