Christian Eichler1, Joseph Lomakin2, Yasuyuki Arakane3, Karl J. Kramer4, Michael R. Kanost3, and Stevin H. Gehrke5. (1) Chemical and Petroleum Engineering, University of Kansas, 1530 W 15th Street, Lawrence, KS 66045-7609, (2) Chemical and Petroleum Engineering, Kansas University, The University of Kansas, 4132 Learned Hall 1530 W. 15th Street, Lawrence, KS 66045, (3) Department of Biochemistry, Kansas State University, 104 Willard Hall, Manhattan, KS 66506, (4) Grain Marketing and Production Research Center, Agricultural Research Service, US Department of Agriculture, 1515 College Avenue, Manhattan, KS 66502, (5) Departments of Chemical & Petroleum Engineering and Pharmaceutical Chemistry, University of Kansas, 1530 W 15th Street, Lawrence, KS 66045-7609
Development of cuticle properties during sclerotization was hypothesized to be the result of catechol-mediated cross-linking of cuticle proteins in the presence of chitin fibers. These interactions were examined via dynamic mechanical and ultimate property tests. The relative importance of water loss during tanning was also to be determined. This was done by testing native cuticle, at several different stages throughout its sclerotization process, as well as developing model networks. Elytra (wing covers) from Tribolium castaneum and Tenebrio molitor were the insect cuticle samples tested. Frequency sweeps, maintaining both a linear and non-linear frequency dependence in untanned and hydrated as well as untanned and dried elytra, showed that that water loss upon tanning plays a smaller role in the improvement of cuticular mechanical properties compared to the contribution of covalent cross-linking, which lead to frequency independent frequency spectra upon tanning completion. It was thus demonstrated that water loss alone is not responsible for the superior mechanical properties of fully tanned insect cuticle. Recent literature illustrated the potential of such composite material motifs (protein and chitin) in synthetic polymer systems. PAMPS/ PAAm - poly(2-acrylamido-2-methylpropanesulfonic acid)/ poly acrylamide - as well as agarose/PHEMA (poly(2-hydroxyethyl methacrylate)) are two IPN and semi-IPN gels that were investigated as synthetic model analogs to insect cuticle. Agarose thermally gels by physical interactions; the other networks were formed by photoinitiated copolymerization/ cross-linking reactions. Tensile test results show that a dramatic increase in fracture stress (10 and 4x), strain (10x) and toughness (80 and 30x) is apparent at high levels of hydration in the semi-IPN (agarose/PHEMA) gel. The values in parenthesis refer to the changes in properties relative to the primary and secondary network. Increases in fracture stress (40 and 25x), and toughness (70 and 9x) were found for the IPN (PAMPS/PAAm) hydrogels. A novel semi-IPN based on biocompatible polymers compatible with cell incropration for tissue engineering has also been developed with an increase of modulus nearly an order of magnitude greater than the precursor gel networks.