In the recently published paper by the Department of Biology at College of Charleston and at Valdosta State University, researchers measured the material properties of hagfish skin and compared histological data from the skins of four hagfish species. Skin samples were subjected to uniaxial and biaxial tensile testing to study their strength, stiffness, and extensibility. ADMET eXpert 8600 Planar Biaxial testing system was used to conduct planar biaxial tests. To read all methods and conclusions, access the full paper here.
Hagfish exhibits whole-body flexibility and can tie its body into a knot. The rigid knotted part of the body slides over the head to apply an opposing force to the force generated by the protractible dental plates that function like lower jaws and procure food items. The opposable knotted body and dental plates articulate with one another at the site of the hagfish feeding apparatus, which consists of connective tissues and muscle fibers. When muscles contact, the reactor complex of the feeding apparatus creates a compression-resistant joint between the teeth and the opposing body. These actions transform the jawless feeding system of a hagfish into a true biting system.
The skin’s looseness allows hagfish to not only feed using the unique jawless feeding system explained above but also to extract prey, remove excess slime from the body surface, and to maneuver and escape from predators. Authors of the paper go into further detail and compare the morphology and material properties such as strength and stiffness of hagfish skins relative to those of others. Previous research suggests a possible adaptation for knot-tying and maneuverability, yet, this conclusion is based on an examination of one species.
There are 78 species of hagfishes and approximately 90% of them belong to one of two major subfamilies: the Eptatretinae and Myxininae. The kinematics of knotting, and the styles of knots created, can vary across species of hagfish.
The objective of this study was to determine if the skins of hagfish are morphologically or functionally different across species. Methods included: uniaxial and synchronized equibiaxial testing, gross dissection, and histological approaches.
The primary reason for running biaxial tests, as opposed to running standard tensile tests, is to determine the material properties in different points and analyze the stress and strain distribution. To do so, a planar biaxial testing system with multiple actuators that can also be configured with different strokes and speeds is used. Each actuator represents the different axes of the planar biaxial test and moves in equal or opposite directions so that the sample center point remains stationary.
Method: Biaxial Tensile Testing
One of the methods followed to measure material properties was biaxial testing. Biaxial tensile tests were conducted on square-shaped skin specimens as seen in the figure below.
Hagfish skin sample clamped in the biaxial testing rig during mechanical failure (E.B.L. Kennedy et al.)
Prior to starting testing, initial length of each test specimen at each axis was recorded. Specimens were simultaneously strained in longitudinal and circumferential axes at the specified rates until failure and the force-displacement data were sampled, at 100Hz. The authors also converted the force-displacement curves to stress-strain curves, as described in the published paper and shown below, in order to determine the biaxial stiffness.
Representative stress-strain curves acquired from a biaxial test performed on a skin sample from Pacific hagfish Eptatretus stoutii (E.B.L. Kennedy et al.)
Planar biaxial tests were conducted on an ADMET eXpert 8600 planar biaxial testing machine with MTESTQuattro® controller and software. The load cell capacity was 250N. The two-axis system included two fast-acting servo driven motors that could achieve speed rates up to 250 mm/min along both axes. The machine’s crossheads used twin ball-screw actuators to move symmetrically about the center lines of the test space and each axis was programmed for coordinated motion in MTESTQuattro®. Biaxial tensile grips were used with the addition of 80-grit sandpaper fitted on the grip jaws.