Monday  |  Salon 8  |  02:10 PM–02:30 PM

#15833, Micromechanics of Collagen Networks Under Impact Loading

During impact loading to the brain, e.g., such as during a traumatic brain injury event, high-rate deformations are being carried through a complex microstructure formed by neural cells and their extracellular matrix in the brain. In order to fully capture the injury response and its associated mechanical loading conditions deterministically, a quantitative understanding of how deformations and forces are being transmitted through such a microstructure needs to be developed. Recent investigations by us and others have shown that characteristic length scales and physical parameters of cellular and acellular features in native brain tissue can be mimicked, at least in terms of their mechanical behavior, by fibrillar-architected materials such as collagen gels at certain concentrations.

To this end, we have developed an in-situ mechanical characterization platform that can subject 3D collagen networks to finite deformations over 6 decades of strain rate (~10-4 to 102 1/s). To approximate the amount of force experienced by the networks, a custom, fiber-optic and PDMS-based flexure load cell is designed as commercially available load cells are not readily capable to detect such small forces in hydrogels. This custom load cell is connected in-line with the collagen sample and the linear voicecoil actuator. Use of a fiber optic sensor, to measure the deflection of the PDMS beam proportional to the applied force, affords the capability to conduct these measurements in a liquid environment and in real-time at high strain rate. Simultaneously, the microstructure of the type-I, fluorescently-labeled collagen is captured via two-photon excited fluorescent microscopy. Collagen gels used here are molded into dogbone-shaped samples, approximately 2 mm in thickness, providing nominally uniform global loading conditions. Three-dimensional, in-situ fiber angle changes and distributions are quantified to reveal the local kinematics of the applied deformation. Detailed discussions about how stress and strain affect local microstructure will be presented as well as developmental steps towards a microstructurally informed, finite deformation and rate-dependent constitutive model for collagen networks.

Sinan Candan   University of Wisconsin-Madison
Luke Summey   University of Wisconsin-Madison
Jing Zhang   University of Wisconsin-Madison
Jacob Notbhom   University of Wisconsin-Madison
Christian Franck   University of Wisconsin-Madison