Polycaprolactone (PCL) is a biodegradable polyester widely used in tissue engineering when slow degradation is desired: this material can remain in vivo for several months without significant loss of its properties, while offering tunable mechanical characteristics, easy processing, and the possibility of being functionalized. With a view to an application in anterior cruciate ligament replacement, this study investigated the hydrolytic degradation of PCL surfaces, either pristine or rendered bioactive by grafting of poly(sodium styrene sulfonate) (pNaSS), a functionalization strategy that introduces sulfonate groups intended to mimic the functionality of glycosaminoglycans and to enhance cellular interaction.
To this end, the authors monitored PCL films and fiber bundles placed in phosphate buffer solution at 25 and 37 °C over a period of up to 120 weeks, i.e., nearly two years. Mass and surface degradation were characterized using a set of complementary techniques—mechanical analysis, contact angle measurement, X-ray photoelectron spectroscopy (XPS), Fourier-transform infrared spectroscopy (FTIR), and atomic force microscopy. The biological response was assessed using primary fibroblasts isolated from the sheep anterior cruciate ligament, examining their spreading, morphology, and expression of collagen genes.
The results confirm that PCL degrades through a so-called "bulk" mode, with accelerated kinetics for the grafted samples: at 37 °C, the rate constant of grafted PCL is approximately three times higher than that of pristine PCL at 25 °C. This acceleration, attributed to the grafting process and to the hydrophilic nature of the sulfonate groups, which promote surface swelling and smoothing, is maintained up to 48 weeks before reaching a steady state. Beyond this point, pNaSS is largely eliminated from the outermost surface, and the behaviors of the grafted and pristine materials converge. FTIR, however, which probes more deeply than XPS, detects the persistent presence of pNaSS after prolonged degradation. From a mechanical standpoint, despite the accelerated degradation, the properties remain sufficiently stable during the first six months; after 96 weeks, the ultimate stress of the grafted fibers remains higher than that of the native human ligament.
On the cellular side, the degradation products, including those derived from the grafted surfaces, exhibit no cytotoxicity. The grafted surfaces promote better initial spreading of fibroblasts, an advantage that tends to fade with degradation time while remaining perceptible up to nearly two years. The ligament cells, moreover, retain their phenotype, with high and stable expression of type I and type III collagens and an absence of type II collagen expression, which is associated with the cartilaginous lineage.