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Piezoelectricity is a well-established property of biological materials, yet its functional role has remained unclear. Here, we demonstrate a mechanical effect of piezoelectric domains resulting from collagen fibril organisation, and describe its role in tissue function and application to material design. Using a combination of scanning probe and nonlinear optical microscopy, we observed a hierarchical structuring of piezoelectric domains in collagen-rich tissues, and explored their mechanical effects in silico. Local electrostatic attraction and repulsion due to shear piezoelectricity in these domains modulate fibril interactions from the tens of nanometre (single fibril interactions) to the tens of micron (fibre interactions) level, analogous to modulated friction effects. The manipulation of domain size and organisation thus provides a capacity to tune energy storage, dissipation, stiffness and damage resistance.

The objective of this work is to covalently attach bacteriorhodopsin (BR) to a gold surface via genetic substitution of cysteine for serine (S35C) at the 35th amino acid position. Samples of BR-containing purple membrane (PM) on gold were evaluated using atomic force microscopy (AFM) and x-ray photoelectron spectroscopy (XPS). AFM images reveal a surface coverage of S35C-containing PM fragments of approximately 25%. XPS measurements reveal a small excess of sulfur for S35C-containing PM on gold, and a much larger excess of sulfur on wildtype-containing PM on gold. In both cases, the excess sulfur is covalently bound to the gold surface and appears to originate from dissociated methyl mercaptan groups from methionine residues on the external surfaces of BR. We conclude that the quantity of excess sulfur is smaller for S35C than for wildtype because the S35C’s sulfhydryl binds some PM fragments to the surface, reducing the quantity of methyl mercaptan that can bind to the surface. It then follows that the rather low coverage of S35C-containing PM fragments on gold is due to interference of the methyl mercaptan groups with the binding of S35C-containing PM fragments to the surface. Coverage might be increased by immobilizing S35C-containing fragments on a functionalized surface using a heterobifunctional cross-linker, thereby preventing dissociation of methyl mercaptan groups from BR, and by using smaller PM fragments.

This study investigated whether increased loading (representing obesity) in the extended knee and flexed knee led to increased stresses in areas of typical medial and lateral osteoarthritis cartilage lesions, respectively. We created two paired sets of subject-specific finite element models; both sets included models of extended knees and of flexed knees. The first set represented normal loading; the second set represented increased loading. All other variables were held constant. The von Mises stresses and contact areas calculated on the tibial cartilage surfaces of the paired models were then compared. In the extended knee models, applying a larger load led to increased stress in the anterior and central regions of the medial tibial cartilage. These are the typical locations of medial osteoarthritis cartilage lesions. Therefore, the results support that increased loading in the extended knee may result in medial osteoarthritis. In the flexed knee models, applying a larger load increased stress in the anterior and central regions of the lateral tibial cartilage. Lateral osteoarthritis cartilage lesions typically occur centrally and posteriorly. Therefore, these results do not support our hypothesis. Shear stress was increased in areas of typical lateral lesions, however, and should be investigated in future studies.