FUDGE, D.S.*; GOSLINE, J.M.: Mechanical and Optical Properties of Hagfish Slime Threads
Hagfishes are unique in their ability to produce vast quantities of fiber-reinforced slime when they are provoked. In order to understand the material properties of hagfish slime, we investigated the properties of isolated slime threads from the Pacific hagfish (Eptatretus stouti). The threads are manufactured within specialized cells called Gland Thread Cells (GTCs), which are found within the numerous slime glands. Each GTC produces a single, continuous, intricately coiled protein thread that unravels upon contact with seawater. We found GTCs to exhibit a bi-directional taper, being on average 3.0 � 0.4 mm in diameter in the middle, and 1.0 � 0.2, and 1.5 � 0.2 mm in diameter at the two (distinct) ends. Slime threads were on average 12 � 0.44 cm long in the unstrained state, but can be as long as 34 � 1.2 cm when strained to the breaking point. Tensile tests of the threads in seawater revealed them to be quite extensible and strong, and therefore tough. Average extensibility (DELTAL/Lo) in water was 1.8 � 0.11, strength was 170 � 24 MPa, and toughness was 100 � 19 MJ/m3. Threads tested in air were stronger, stiffer, tougher, and less extensible than those tested in seawater. Average extensibility in air was 1.0 � 0.10, strength was 520 MPa, and toughness was 270 � 17 MJ/m3. Tensile tests performed in chaeotropic agents such as guanidine HCl suggest that the IF proteins that make up the threads are held together by non-covalent intermolecular forces. Birefringence data and evidence from Congo Red staining of strained and unstrained threads suggest that the low modulus plateau region of the stress-strain curves corresponds to a transformation of alpha-helical and/or globular domains of the constituent IFs to a more highly-ordered beta-sheet crystalline state.
