In SituMechanical Testing of Hydrated Biological Nanofibers Using a Nanoindenter Transducer

Nanoscale biological fibers, such as collagen, keratin or elastin, serve as building blocks for a wide variety of biological tissues (for example, bone, skin and hair). As such, the elasticity, strength and damage tolerance of these fibers largely control the mechanical performance of tissues at the macroscale. While there is a large body of experimental data for tests on whole biological tissues at the macroscale, mechanical tests on individual biological fibers are scarcer because of their small size (400 nm diameter or less). Isolating, imaging, handling and testing these fibers in hydrated conditions are significant challenges. The AFMbased and MEMS-based techniques developed in the past to test such fibers offer high displacement and load resolution, but they lack the stroke and force capability required to fracture strong and highly extensible fibers such as collagen fibrils. In this work, a microscale mechanical testing platform capable of measuring the tensile stress–strain response of individual type I collagen fibers and fibrils was developed and validated. The platform is composed of a capacitancebased, nanoindenter transducer, an optical microscope to monitor the deformation of the sample in situ and a set of micromanipulators to isolate and handle individual fibers and fibrils. Our preliminary results on type I collagen demonstrate the feasibility of monotonic and cyclic tensile tests under the optical microscope and in hydrated conditions. The setup can be used to study the elasticity, strength and damage tolerance of type I collagen fibers and fibrils (using cyclic tests), and our preliminary data are consistent with existing experiments and predictions from numerical models. This setup offers the advantage of being composed from relatively standard components (optical microscope, nanoindenter) which means that it can be easily duplicated in laboratories that already possess these instruments. This technique can be used to assess the effect of environment, genetic diseases or therapeutic drugs on biological tissues at a fundamental level.