The biomechanics behind amazing ant strength
May 21, 2014
How can an ant lift objects many times heavier than its own body? Engineers at The Ohio State University combined computational modeling at the Ohio Supercomputer Center (OSC) and lab experiments to find out.
They focused on the ant’s neck — the single joint of soft tissue that bridges the stiff exoskeleton of the ant’s head and thorax. When an ant carries food or any other object, the neck joint supports the full weight of the load.
The researchers reverse-engineered the biomechanics of the neck by developing 3-D models of the of the ant’s internal and external anatomy from X-ray cross-section images (microCT) of ant specimens and loading the data into a modeling program (ScanIPþFE) that assembled the segments and converted them into a mesh frame model of more than 6.5 million elements.
The model then was loaded into a finite element analysis program (Abaqus), an application that creates accurate simulations of complex geometries and forces, and the data was processed on the powerful Oakley Cluster, an array of 8,300 processor cores (Intel Xeon) at the Ohio Supercomputer Center.
Neck joints can withstand loads of about 5,000 times its body weight
The experiments, published in the Journal of Biomechanics, revealed that the neck joints could withstand loads of about 5,000 times the ant’s body weight, and that the ant’s neck-joint structure produced the highest strength when its head was aligned straight, as opposed to turned to either side.
“Loads are lifted with the mouthparts, transferred through the neck joint to the thorax, and distributed over six legs and tarsi that anchor to the supporting surface,” explained Carlos Castro, assistant professor of mechanical and aerospace engineering at Ohio State. “While previous research has explored attachment mechanisms of the tarsi (feet), little is known about the relation between the mechanical function and the structural design and material properties of the ant.”
“Our results accurately pinpoint the stress concentration that leads to neck failure and identify the soft-to-hard material interface at the neck-to-head transition as the location of failure,” said Castro. “The design and structure of this interface is critical for the performance of the neck joint. The unique interface between hard and soft materials likely strengthens the adhesion and may be a key structural design feature that enables the large load capacity of the neck joint.”
The simulations confirmed the joint’s directional strength and, consistent with the experimental results, indicated that the critical point for failure of the neck joint is at the neck-to-head transition, where soft membrane meets the hard exoskeleton.
Optimized exoskeleton design
“The neck joint [of the ant] is a complex and highly integrated mechanical system. Efforts to understand the structure-function relationship in this system will contribute to the understanding of the design paradigms for optimized exoskeleton mechanisms,” said former Ohio State student Vienny N. Nguyen in her 2012 master’s thesis on this research.
Nguyen, now a robotics engineer at Johnson Space Center, earlier worked on the project under the supervision of Blaine Lilly, an associate professor of mechanical and aerospace engineering at Ohio State. Nguyen and Lilly are co-authors with Castro on the journal paper.
“As we look to the future of human-assistive devices and ultra-light robotics,” she said, “the development of 3-dimensional models for visual analysis and loading and kinematic simulation will also serve as tools for evaluating and comparing the functional morphology of multiple species and types of joints.”
Abstract of Journal of Biomechanics paper
Insects have evolved mechanical form and function over millions of years. Ants, in particular, can lift and carry heavy loads relative to their body mass. Loads are lifted with the mouthparts, transferred through the neck joint to the thorax, and distributed over six legs and tarsi (feet) that anchor to the supporting surface. While previous research has explored attachment mechanisms of the tarsi, little is known about the relation between the mechanical function and the structural design and material properties of the ant. This study focuses on the neck – the single joint that withstands the full load capacity. We combine mechanical testing, computed tomography (CT), scanning electron microscopy (SEM), and computational modeling to better understand the mechanical structure–function relation of the neck joint of the ant species Formica exsectoides (Allegheny mound ant). Our mechanical testing results show that the soft tissue forming the neck joint of F. exsectoides exhibits an elastic modulus of 230±140MPa and can withstand ~5000 times the ant’s weight. We developed a 3-dimensional (3D) model of the structural components of the neck joint for simulation of mechanical behavior. Finite element (FE) simulations reveal the neck-to-head transition where the soft membrane material meets the hard exoskeleton as the critical point for failure of the neck joint, which is consistent with our experiments. Our results further indicate that the neck joint structure exhibits anisotropic mechanical behavior with the highest stiffness occurring when the load path is aligned with the axis of the neck.