“When provoked, Notaden bennetti frogs secrete an exudate which rapidly forms a tacky elastic solid (‘frog glue’). This protein-based material acts as a promiscuous pressure-sensitive adhesive that functions even in wet conditions. We conducted macroscopic tests in air to assess the tensile strength of moist glue (up to 78 + 8 kPa) and the shear strength of dry glue (1.7 + 0.3 MPa). We also performed nanomechanical measurements in water to determine the adhesion (1.9-7.2 nN or greater), resilience (43-56%), and elastic modulus (170-1035 kPa) of solid glue collected in different ways. Dry glue contains little carbohydrate and consists mainly of protein. The protein complement is rich in Gly (15.8 mol %), Pro (8.8 mol %), and Glu/Gln (14.1 mol %); it also contains some 4-hydroxyproline (4.6 mol %) but no 5-hydroxylysine or 3,4- dihydroxyphenylalanine (L-Dopa). Denaturing gel electrophoresis of the glue reveals a characteristic pattern of proteins spanning 13-400 kDa. The largest protein (Nb-1R, apparent molecular mass 350-500 kDa) is also the most abundant, and this protein appears to be the key structural component. The solid glue can be dissolved in dilute acids; raising the ionic strength causes the glue components to self-assemble spontaneously into a solid which resembles the starting material. We describe scattering studies on dissolved and solid glue and provide microscopy images of glue surfaces and sections, revealing a porous interior that is consistent with the high water content (85-90 wt %) of moist glue. In addition to compositional similarities with other biological adhesives and well-known elastomeric proteins, the circular dichroism spectrum of dissolved glue is almost identical to that for soluble elastin and electron and scanning probe microscopy images invite comparison with silk fibroins. Covalent cross-linking does not seem to be necessary for the glue to set.” (Graham et al. 2005:3300)

“Current surgical glues and sealants are either protein-based, in which case they exhibit low bond strength, or synthetic, in which case they form rigid and impervious barriers that hinder wound healing. Our results show that the exudate from N. bennetti frogs rapidly and spontaneously forms a proteinaceous pressure-sensitive adhesive that functions well in wet environments. The hydrated solid is highly elastic and consists of a porous mesh that in clinical contexts should allow the diffusion of gases and nutrients. Since some of the cavity radii…equate to pores with diameters in excess of 10 µm, it is likely that the hydrated material will also permit a degree of cellular infiltration. Initial experiments suggest that the glue is highly biocompatible, and it has been used successfully to bond severed cartilage tissue both ex vivo and in vivo.” (Graham et al. 2005:3311)