What could these tough materials have in common? www.dkimages.com/.../previews/913/55027588.JPG www.hannanpak.com/.../images/surlyn_hannan.jpg Mussel byssal thread Surlyn www.lclark.edu/~autumn/PNAS/ Gecko feet Surlyn: ionic, polymer with electrically neutral and ionic units, behaves as a thermoplastic material, conducts electricity Gecko feet: Van der Waals, noncovalent, gecko feet can withstand enormous forces Mussel byssal thread: organo-metalic, allow mussel to move Bacteriorhodopsin: hydrophobic, integral membrane protein in archaea which is used to pump protons out of cells Titin: hydrogen bond, in muscle fibers www.bio-pro.de/.../rnd/titin.jpeg www.daviddarling.info/images/bacteriorhodpsin.jpg Titin Bacteriorhodopsin Amanda Morris

…Sacrificial Bonds & Hidden Length Georg Fantner, et al. Biophysical Journal. 2006. Sacrificial bonds & hidden length = toughening mechanism to prevent backbone breakage, hidden length doesn’t take on force because constrained from stretching by sacrificial bond Entropic and enthalpic elasticity: when sac. Bond intact, only black part contributes to entropic energy. At sacrifical bond fracture force (~300 nN), red hidden length released and contributes to entropy  drop in force. Further stretching  work is large (red area). Self-healing: mechanism could be reversible. Entropy collapses polymer and sac. Bonds reform. Rupture force = force that bond breaks at Sawtooth profile: multiple bonds create multiple peaks in force-extension curve. AFM = used to understand molecular construction in materials, but can be complicated! So this study presents some characteristic features.

Simulated cases of sacrificial bonds Worm-like chain model: Simple cases of mechanism to form basis for interpreting real force curves. Modeled on MATLAB with wormlike chain model: Lp = persistence length, Lc = contour length, kb = Boltzmann, T = absolute temp, x = pulling distance, F = stretching force. 1: bonds break according to relative bond strengths 2: bonds break in sequence 3: hidden length equals distance between two neighboring sites on one molecule plus on other, cf, DNA unzipping 4: all break at once when force is larger than combined force, no hidden length set free

Simulations for multi-strand connections A: different contour lengths, one molecule breaks first B: one behaves as exponential spring (background forces) C: breaks at same distributed force but occurs at different pulling lengths D: not equally spaced, depends on sequence of molecules whether peak is followed by higher or lower peak

Actual pulling curves Aggrecan-Hyaluronic acid Wheat gluten A  parallel sacrifical bonds and exponential spring B  multiple strands in parallel with increasing Lc C  both Agg-HA: High occurrence of decreasing bond ruptures could be because of branched nature of Agg-HA Pulling performed with SiN cantilevers Stepwise decreasing bond rupture peaks

First derivative of force spectra Single molecule with multiple domains or multiple molecules in parallel? Noise added Lc and binding F chosen so that rupture peaks occur at same pulling distance and rupture force. Single molecules with two domains has steeper incline than three molecules in parallel. Can be seen clearly when first derivative of pulling spectra is taken. Difference is slope can be used as criteria to differentiate between whether single multiple with multiple domains or several molecules in parallel. Figure B shows same experiment with low noise cantilevers (add white noise then apply a low pass filter)  difference can still be resolved. Single molecule with domains held by sacrificial bonds stiffer than parallel molecules.

Effects of sacrificial bonds and hidden length on material properties Increased initial stiffness and fracture toughness Measure of stiffness is initial slope of force curve. Increased gain in toughness represented by increased energy dissipated when deforming material with sacrificial molecules (shaded area). Requirement – hidden length shielded from applied force (multiple domain structure or parallel network of difft lengths) Common patterns can help distinguish (stepwise decreasing force ruptures or whether single or multiple molecules involved)  basis for interpretation.