Cyclic Uniaxial Tensile Strain Enhances the Mechanical Properties of Engineered, Scaffold-Free Tendon Fibers.

Abstract

The treatment of injured tendon is an ever-increasing clinical and financial burden, for which tissue-engineered replacements have shown great promise. Recently, there has been growing interest in a more regenerative approach to tissue engineering, in which the cells' abilities to self-assemble and create matrix are harnessed to create tissue constructs without the use of a scaffold. Herein, utilizing our scaffold-free technique to engineer tendon at the single fiber level, we study how applied mechanical loading, namely cyclic uniaxial strain, influences the mechanical properties and nuclear alignment of developing tendon fiber constructs. Engineered fibers were subjected to 1, 3, and 7 days of intermittent uniaxial loading (0.0-0.7% sinusoidal strain), and then characterized mechanically by constant-rate elongation to failure to obtain tensile properties and histologically to examine cytoskeletal arrangement and nuclear shape, and characterized using real-time polymerase chain reaction to measure the expression of tendon-specific makers, scleraxis and tenomodulin. Fiber peak stress, elastic modulus, toughness, and nuclear aspect ratio increased with the presence and duration of loading, while failure strain, toe-in strain, and nuclear area were unchanged. These biomechanical results suggest that cyclic strain promotes matrix deposition in a manner that increases the fiber resistance to stretch, but preserves fiber extensibility over the 7-day loading period. Over 7 days of loading, the scleraxis and tenomodulin expression increased drastically. Histologically, while there was no immediate difference in nuclear area with the addition of loading, nuclear aspect ratio significantly increased with loading duration, such that nuclei became progressively more elongated to the long axis of the fiber. Together with our biomechanical findings, such nuclear deformation suggests that cyclic strain elicits a mechanotransductive response, particularly one that modulates gene expression to promote matrix deposition during fiber development.