iMechanica - H-bond - Comments

Re: Tensile testing for a fibrillar material like collagen

Xiaodong Li — Wed, 13 Feb 2008 12:49:46 -0500

Thanks for the post. Indeed, the small dimensions of a fibrillar material like collagen impose a great challenge. Calibration is the key in performing such a novel test. My group has some papers that may be of interest to you.

1. Xiaodong Li, Xinnan Wang, Wei-Che Chang, Yuh J. Chao and Ming Chang, "Effect of Tensile Offset Angles on Micro/Nanoscale Tensile Testing," Review of Scientific Instruments, 76 (2005) 033904-5.

2. Xinnan Wang, Xiaodong Li and Michael J. Yost, "Microtensile Testing of Collagen Fibril for Cardiovascular Tissue Engineering," Journal of Biomedical Materials Research Part A, 74A (2005) 263-268.

3. Xinnan Wang, Yongda Yan, Michael J. Yost, Shen Dong, and Xiaodong Li, "Nanomechanical Characterization of Micro/nanofiber Reinforced Type I Collagens," Journal of Biomedical Materials Research Part A, 83 (2007) 130-135.

Tensile testing for a

ColinGrant — Wed, 13 Feb 2008 09:38:11 -0500

Tensile testing for a fibrillar material like collagen makes much more intuitive sense as the monomeric molecules run in the same direction as the fibril axis. Also, collagenous tissues such as tendons (bundles of collagen fibres) are anatomically arranged so that most of the applied work done is in the same direction as the fibril axis.

However, I do feel that there is a need to examine the transverse properties as this reflects how the material, at fibrillar and tissue level, reacts due to impact and compressions that may occur say during sport (eg being kicked on ankle - Achilles tendon)

Indentation and tensile of silks

Xiaodong Li — Fri, 08 Feb 2008 22:28:18 -0500

Thanks a lot for this interesting discussion. I think that indentation of cross section and in situ imaging will uncover some structural feature and the mechanical properties across the section. Another cross checking test will help to validate the results - like tensile test using a nanotensile tester. I think that Mother Nature has already created an excellent recipe for making composite fibers - silks. This may inspire us to make silk -like composite fiber materials. There are still a lot we need to learn from nature. Again, thanks for this nice J-Club theme .

Hi Michelle, this is an

Sinan Keten — Wed, 06 Feb 2008 17:34:39 -0500

Hi Michelle, this is an excellent comment. Transverse experiments have come up primarily in amyloid studies as far as I know. Amyloid structure is quite intriguing because the weak H-bonds are oriented in the direction of the fibril axis and the beta-sheet strands are stacked up such that the covalently bonded polypeptide chains lie in transverse direction. So the analogous loading conditions to single molecule pulling experiments would be transverse loading in amyloids, where H-bonds are sheared between beta-strands. Protein structures such as Ig domains in titin have a shear topology that makes them resistant to external forces since H-bonds are uniformly sheared in parallel. Transversely loaded amyloid fibrils hypothetically exhibit similar mechanical features governed by the rupture of interstrand H-bonds. The most interesting observation would be that the bending stiffness and elastic moduli calculated from these experiments are significantly high, comparable to steel or dragline silk, despite the weak hydrogen bonding. It is very intriguing how a material made primarily out of weak bonds can exhibit such high a modulus and strength.

Indenting proteins

MichelleLOyen — Wed, 06 Feb 2008 12:14:06 -0500

I have been very curious to see recent work on AFM indentation of proteins or protein bundles, especially in the context of fibrillar proteins such as collagen. I can understand what is being measured in the single-molecule protein unfolding experiments using AFM, but the indentation experiments are also being done. Fibrillar protein materials have very low transverse stiffness due to very little covalent bonding between individual molecules, and would be very different in transverse compression compared with in tension. The tensile behavior, in which the strong covalent bonds are more or less aligned in the tensile direction, would seem more obvious to me for study using say optical tweezers experiments rather than indentation. I have thus had a difficult time parsing what is being measured in an indentation on a transversely-lying fibril; some longitudinal measurements have been made in resin-embedded samples in cross section (hair, silk) and those I can understand much better than the transverse ones. Perhaps someone has an idea of what is being sought in measuring transverse fibrils by indentation.

Thanks for giving an

Jing Zhou — Tue, 05 Feb 2008 17:40:03 -0500

Thanks for giving an introduction to this interesting topic.

I am not familiar with this field, but just remember recently I saw a paper provide a general model for silk mechanical properites based on linear viscoelasticity in PRL (Igor Krasnov, Imke Diddens, Nadine Hauptmann, Gesa Helms, Malte Ogurreck, Tilo Seydel, Sérgio S. Funari, and Martin Müller, 2008, PRL, "Mechanical Properties of Silk: Interplay of Deformation on Macroscopic and Molecular Length Scales").

Hope this might be helpful.

Dr. Eom, thanks for your

Sinan Keten — Sun, 03 Feb 2008 16:15:26 -0500

Dr. Eom, thanks for your post regarding spider silk properties and pointing out to the interesting article by Hansma and coworkers. To the best of my knowledge, this is one of the few models developed for spider capture silk, along with the hierarchical chain model developed by Zhou et al. whereas Termonia's model focused on the spider dragline silk. I'd be curious to know if there is any molecular structure based model that can explain both dragline and capture silk's mechanical signature, using for instance different input parameters.

Elasticity of spider silk and microtubule

Kilho Eom — Fri, 01 Feb 2008 02:41:23 -0500

Keten, Thanks for your writing up the jClub article for Feb 1st.

As you mentioned about Termonia's paper, it is conceived that beta-sheet is responsible for elasticity. However, Termonia's viewpoint may not be sufficient to represent the elasticity of spider silk protein, To my knowledge, Hansma and coworkers published the elasticity of spider silk protein based on single-molecule AFM experiments at Nature Materials (Click Here). In their work, they remarkably found that the spider silk protein has the heirarchical molecular structures based on their observation that the elastic responses of bulk spider silk and spider silk protein are qualitatively comparable to each other. In that paper, Dima Makarov and Hellen Hansma suggested the hierarhical spring model for spider silk protein.

There are recent works on AFM experiments for finding the mechanical properties of biomolecules such as amyloid fibril and microtubule. If one is interested in the paper 3 (Welland's paper in Science), then the paper by Florin will be also interesting (Click Here). In Florin's paper, the relationship between persistence length (representing the bending rigidity) and the contour length for microtubule fiber is suggested based on simple elastic beam model (Timoshenko beam model).