Polycaprolactone (PCL) nanofibers are a promising material for biomedical applications due to their biocompatibility, slow degradation rate, and thermal stability. We electrospun PCL fibers onto a striated substrate with 12 μm wide ridges and grooves and determined their mechanical properties in an aqueous solution with a combined atomic force/inverted optical microscopy technique. Fiber diameters, D, ranged from 27 to 280 nm. The hydrated PCL fibers had an extensibility (breaking strain), ε_max, of 137%. The Young's modulus, E, and tensile strength, σ_T, showed a strong dependence on fiber diameter, D; decreasing steeply with increasing diameter, following empirical equations E(D)=(4.3∙10³∙e^-D51nm+1.1∙10²) MPa and σ_T(D)=(2.6∙10³∙e^-D55nm+0.6∙10²) MPa. Incremental stress-strain measurements were employed to investigate the viscoelastic behavior of these fibers. The fibers exhibited stress relaxation with a fast and slow relaxation time of 3.7 ± 1.2 s and 23 ± 8 s and these experiments also allowed the determination of the elastic and viscous moduli. Cyclic stress-strain curves were used to determine that the elastic limit of the fibers, ε_elastic, is between 19% and 36%. These curves were also used to determine that these fibers showed small energy losses (<20%) at small strains (ε < 10%), and over 50% energy loss at large strains (ε > 50%), asymptotically approaching 61%, as E_loss=61%·(1-e^-0.04*ε). Our work is the first mechanical characterization of hydrated electrospun PCL nanofibers; all previous experiments were performed on dry PCL fibers, to which we will compare our data.

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