Tensile test of tissue-engineered tendon constructs using a dynamic load frame

Abstract

Fibrin is a biopolymer with properties ideal for supporting cell seeding and mimicking native tissue environments, and it has gained significant interest in musculoskeletal tissue engineering, which has recently also concerned tendon repair. This work investigates the mechanical properties of fibrin ring constructs produced for tissue engineering applications, focusing on developing a testing protocol and experimental setup to study how cyclic and stress relaxation loading affects cell-free and cell-ring-shaped constructs and aims to provide a comprehensive understanding of the stresses the constructs experience during mechanical testing and also during mechanical stimulation in the MagneTissue bioreactor. Previous studies apply displacements and strains to the ring constructs while in the MagneTissue bioreactor; however, previous works fail to address the stress the rings experience while in the bioreactor. A protocol is developed using a dynamic load frame equipped with adapters similar to the ones used in a specific bioreactor. The testing is conducted in a water bath containing \ac{PBS} to ensure the rings are hydrated. The cyclic and stress relaxation loading mimics the loading applied to the ring constructs in the bioreactor, which are loaded to 10\% or 20\% strain, depending on the experiment. The experimental findings show that for both cyclic loading and stress relaxation, constructs subjected to 20\% strain experienced higher maximum stress values than those at 10\% strain. Statistical analysis indicated significant differences in maximum stress between the 20\% strain 2-cell and cell-free groups, suggesting that the presence of cells influences the mechanical response of fibrin constructs. Significant differences were also noted between the 10\% and 20\% pooled data for the maximum and minimum stress values. This clearly supports the idea that the strain level impacts the stress the constructs experience and, in constructs without cells, leads to permanent deformation of the constructs. While this setup replicates and mimics the mechanical environment within the bioreactor, it provides an understanding of the stresses experienced by the constructs without having to perform multiple day-long experiments.