Applying Cyclical Loading Parameters for In-Vitro Neo-Tendon Development

Abstract

INTRODUCTION: Annually, over 545,000 rotator cuff tear repairs occur in the USA. Current surgical methods have varied success, due to factors including tear size and patient age. The fibrotic tissue that results from repair is susceptible to re-tear: retear rate following repair is up to 94% for large tears. There is a need for better treatment. Tissue engineering is a potential solution. Poly(lactic acid) meltblown scaffolds produce neotendon but cannot withstand physiological strains. Here, we evaluate how cyclical tensile loading of poly(ε-caprolactone) (PCL) meltblown scaffolds affects neo-tendon development. METHODS: We seeded PCL with human adipose stem cells and cultured for 28 days with cyclic loading to 6% strain three times per week for 0, 125, 5,000, and 10,000 cycles/day using a tensile bioreactor. We characterized the viscoelastic mechanical properties using this protocol: preconditioning, hysteresis loops and 10 min stress relaxations at 1, 3, 5, and 7% strain, each followed by frequency sweeps at 0.1, 1, and 5 Hz, and a ramp-to-failure. We determined various parameters using custom MATLAB code. Data were evaluated for effect of loading using ANOVA with Tukey HSD post-hoc tests (n=3-5, α=0.05). RESULTS: Loading at 5,000 cycles (Fig. 1) improved sample linear modulus and yield stress compared to other loading groups, while increased loading led to decreased yield stretch. Samples loaded at 5,000 cycles had higher phase shifts at lower frequencies, suggesting greater stress dissipation. Stress relaxation decreased from 3% strain to 5% and 7% strain. There was no effect of loading on stress relaxation. Loading up to 5,000 cycles tended to increase energy storage and secant stiffness but dropped when increased to 10,000 cycles. CONCLUSION: Tendon viscoelastic properties are essential for their mechanical stability and function; scaffolds with similar mechanical function could improve repair. Tensile loading during culture improves mechanical function up to 5,000 cycles/day but loading at 10,000 cycles/day seems to cause damage to the fibers, reducing modulus, yield stretch, yield stress, stiffness, and energy storage at higher strains. Our data will establish if meltblown scaffolds are viable for preclinical studies and could inform rehabilitation protocols for engineered tendon development.