Abstract
Tendon regeneration is complex since the scaffold has to bear high loads and stress concentrations, while providing suitable deformability. Previous studies demonstrated a physiological orientation of the fibers and good cell adhesion on electrospun polymeric scaffolds [1]. The aims of this work were to: (i) prepare and characterize electrospun resorbable scaffolds with different compositions and (ii) develop a process to produce a multiscale bundle assembly to mimic the hierarchical structure and biomechanical properties of a real tendon.
We produced fibrous scaffolds made of blends of poly-L-lactic acid (PLLA) and collagen (Coll):
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Pure PLLA;
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PLLA/Coll 75/25 w/w;
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PLLA/Coll 50/50 w/w.
In order to prepare 3D bundles made of aligned fibres, we used a high-speed rotating collector. The electrospun nanofibers were deposited tangentially onto the drum, the electrospun layer was manually rolled transversely along the drum and then removed. The bundles were approximately 150 mm long and 300–450 mm in diameter. Five specimens were prepared and tested for each blend.
To evaluate the mechanical properties of the bundles a tension test was applied with capstan grips on a testing machine with a 100N load cell, under the following conditions:
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Gauge length: 20 mm.
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Monotonic ramp to break detection.
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Actuator speed 5 mm/min.
For all the bundles, the stress-strain curve showed an initial non-linear part (toe region), similar to the laxity of the tendon at rest. The mechanical analysis confirmed the outstanding ductility and toughness of pure PLLA. Increasing the percentage of collagen resulted in a reduction of ductility. The PLLA/Coll 50/50 had a rather brittle behaviour.
The values of mechanical properties found for the different compositions were slightly lower but of the same order of magnitude as tendon fibers (Failure stress: 33.7±19.2 MPa; Failure strain: 21.0±9.1 %; Young Modulus: 257±101 MPa [2]). The bundles made of pure PLLA had a failure stress of 13.2±0.8 MPa; failure strain of 84.7±9.4%; Young Modulus of 78.6±7.5 MPa. The bundles made of PLLA/Coll 50/50 had: failure stress of 10.5±1.5 MPa; failure strain of 21.4±2.7%, Young Modulus of 65.7±9.8 MPa. The most promising composition was the PLLA/Coll 75/25, with a failure stress of 14.0±0.7 MPa; failure strain of 40.3±2.2 %, Young Modulus of 98.6±12 MPa.
We also tested bundles mechanical properties after aging samples in phosphate buffer at 37 °C for 48 hours, 7 and 14 days. After ageing, stress and strain values were progressively lower, while the toughness increased, compared to the dry samples.
The promising results found in this work for the electrospun PLLA-collagen blends confirm their potential use for tendon tissue regeneration. This is a starting point for developing multiscale scaffolds mimicking the structure of tendon tissue, which can potentially be used in human regenerative medicine both as bioresorbable prosthesis, or inserted in a bioreactor for in vitro production of tendon tissue.
