Tension tests on mammalian collagen fibrils

Abstract

A brief overview of isolated collagen fibril mechanics testing is followed by presentation of the first results testing fibrils isolated from load-bearing mammalian tendons using a microelectromechanical systems platform. The in vitro modulus (326 ± 112 MPa) and fracture stress (71 ± 23 MPa) are shown to be lower than previously measured on fibrils extracted from sea cucumber dermis and tested with the same technique. Scanning electron microscope images show the fibrils can fail with a mechanism that involves circumferential rupture, whereas the core of the fibril stays at least partially intact.

Keywords: collagen fibril; mechanics; nanoscience; nanotechnology; tendon.

Figure 1.

(a) Bright field optical microscopy image of a MEMS device for testing collagen fibrils. (b) Higher magnification optical microscopy image of the sample loading area of a MEMS device (indicated with the box in (a)), showing a single collagen fibril loaded and fixed with three micro epoxy droplets. (c) Schematic of a MEMS device. The grey part indicates the base of the device. The black parts are connected to the base. The white part (movable pad) is suspended above the base with four tether beams connected to the anchor pad. Collagen fibrils samples are loaded across the fixed pad and the strain gauge pad. Upon testing, the movable pad is pulled towards the direction indicated with the arrow labelled ‘tensile displacement’, with a needle inserted to the pinhole, applying a tensile force to the specimen. The force is reflected by the deformation of the force gauge beams. The diameter of the specimen is measured above the window on the fixed pad using SEM. Contraction of the fibril is accounted for as described previously [35]. (d) SEM image of the window region of a MEMS device (indicated by the box in (b)), showing a region of the collagen fibril in the same state of load as the gauge region prior to application of any tensile stress. (Online version in colour.)

Figure 2.

SEM images of two fibrils at the locations of tensile fracture (scale bars represent 1 µm in a,b and 200 nm in c,d). (a) In this particular fibril, tensile fracture occurs in the middle of the strain gauge region. (b) In this particular fibril, tensile fracture occurs close to the surface of the MEMS device. (c) A higher magnification image of the fibril in the box in (a). (d) A higher magnification image of the fibril in the box in (b). (e) In both fibrils, fracture occurs at angles of about 45° to the axial direction of the fibrils. (f) In both fibrils, one of the fracture edges appears to be sharp, while the other edge appears to be blunt and irregularly shaped. (Online version in colour.)

Figure 3.

(a) Stress–strain curve of a typical collagen fibril tested in this study. The stress–strain ratio at the low strain linear region (less than 12%) is 315 MPa, maximum tensile strength is 90 MPa and fracture stress is 75 MPa with associated fracture strain of 86%. Fracture toughness is 47 × 10⁶ J m⁻³. (b) Stress–strain curves for all 12 fibrils tested are shown. For reference, the particular curve in (a) is indicated with an asterisk. (Online version in colour.)