Continuously graded composite scaffolds for tendon-bone interface regeneration

Rotator cuff tears are the most common injuries of the shoulder, with over 250,000 repairs performed anually in the U.S., only. In Turkey, up to 26% of the adult population suffer from shoulder related problems, and rotator cuff tears constitute %70 of this population. Tendon tissue has limited self-regenerative capacity due to poor vasculature and wide range of motion ability. Tendon injuries generally exist at the tendon-bone interface, and current clinical approach for tendon repair is mechanical fixation, which leads to as high as 90% recurrence rates. In addition, repaired tendon site is unable to form the hierarchical physical and biological structure of the native tissue. Native tendon-bone interface is composed of three different yet continuous layers of tendon, fibrocartilage (unmineralized and mineralized) and bone tissues. Therefore, this study aims at i) fabricating electrospun polycaprolactone scaffolds also enriched with hydroxyapatite and growth hormones in a graded manner, and ii) investigating human stem cell behavior on these scaffolds. This type of graded scaffold demonstrates significant deviation from conventional multi-layered scaffold approach. Graded scaffolds fabricated in this study were demonstrated to contain linearly changing hydroxyapatite concentration as a function of scaffold thickness, a distribution typically observed in native tendon-bone interface. Mixing polycaprolactone and hydroxyapatite nanoparticles (20% by weight) led to an increase in fiber diameter from 361±9 nm to 459±21 nm, and a decrease in contact angle from 120±3° to 115±1°. In biomechanical tests, maximum loads were found as 7.5±1.7N for polycaprolactone only scaffolds, and 11.6±1.3N for scaffolds with 16% hydroxyapatite. Strain at break values were found to be 0.12±0.02 mm/mm and 0.18±0.02 mm/mm, respectively. Release studies showed a continuous release of connective tissue growth factor (CTGF) and transforming growth factor-β3 (TGF- β3) over a period of 28 days. At the end of day 28, %27±7 of CTGF and %81±25 of TGF-β3 were released from nanofiber scaffolds. No effect of scaffold type was observed for proliferation of and collagen production by mesenchymal stem cells. These in-vitro findings are expected to provide significant inputs for biologic graft fixation.