Medical grade polyethylene terephthalate (polyester, PET) yarns and fibers used for diverse medical applications, including vascular and ligament prosthetic grafts, are distinct in several aspects from PET used for other non-medical textile applications. Notably, PET strength and fiber structures are deliberately altered for implantable medical applications to produce high strength, high modulus and low elongation materials as devices. Polymer technology, textile and spinning techniques, drawing methods and processing machinery have been critical to enabling new PET fiber and fabric processing that yield fibers with these requisite properties [1–3].
Given current broad use of PET fibers and fabrics for medical devices applications , the PET ligament prosthesis holds a unique place. Despite clinical prominence of PET woven vascular prostheses for many years, recognized as the most reliable solution for replacing diseased vessels , PET ligament prostheses are more recently introduced and clinically important . Ligament tears are increasingly common injuries in pivot sports, requiring stabilization of the knee by surgical ligament reconstruction to prevent osteoarthritis. Basically, these prostheses facilitate tissue re-connection of the torn injured ligament component, involving reconstruction of new ligament through host cellular re-attachment and healing [7, 8]. This requires extended rehabilitation time. Nonetheless, textile ligament prostheses use remains limited due to poor control of in vivo degradation risk factors of implanted prostheses and performance that is sub-standard versus clinical standards of care. Improved ligament prostheses designs are sought.
After disastrous clinical results for the first generation of artificial ligaments, largely based on PET textile biomaterials , second- and third- generation PET ligament prostheses have yielded improvements in both materials and implant designs. Second-generation PET implants, developed by LARS laboratories (France), focused on a biomaterial prosthesis structure mimicking that of the natural ligament . Third generation designs exploited further surface functionalizing of the PET ligament prosthesis with grafted bioactive polymers to improve biocompatibility and tissue “bio-integration”, i.e. increased host cell adhesion, proliferation and signaling to stabilize implant integration [10, 11]. Migonney et al. showed that poly(sodium 4-styrene sulfonate) (PNaSS) grafting onto PET surfaces improves both adhesion and functions of fibroblast cells that constitute ligament and tendon endogenous cells [10–14].
PET ligament prostheses comprise complex knitted and woven fabrics made from commercial PET fibers previously coated with an added manufacturing oil by design . The process called “spin-finish” is performed using FDA-approved synthetic or natural oil for biomedical applications . Indeed, spin finish oil is coated to protect fibers, reduce fiber and fabric adhesion and stickiness, improve handling and assembly, and provide optimal properties that enable extrusion of precision calibrated PET fiber dimensions. It is well-recognized that fiber spin finish oil allows PET fabrics to be easily woven, handled and safely stored [15, 16]. Spin finish treatments of fibers dedicated to medical devices use oils such as soybean oil containing fatty acids ester and epoxide groups for antioxidant properties [17–21]. However, depending on storage conditions (i.e. short or long term, ambient or controlled atmosphere, and strictly controlled or uncontrolled temperature), these spin finishing (or “sizing”) oils either remain inert or degrade, generating oxidized species and radicals that can affect polymer fiber integrity and impede their further functionalization and properties. Prior to surface functionalization of polymers fibers or fabrics, a surface pre-treatment called spin finish removal (SFR) is usually performed using Soxhlet solvent extraction to remove the protective spin finish oil . SFR is required for PNaSS “grafting from” reactions on clean PET fiber surfaces through radical grafting polymerization processes [10–14].
Rahman and East reported spin finish effects on the hydrolysis of medical-grade PET fibers used to produce antithrombotic fabric vascular grafts . They demonstrated that spin-finish oil reduced PET degradation by ambient oxidation and hydrolysis. However, no reports describe any effect of finish oil and storage on SFR pre-treatment of PET for surface functionalization.
This study was designed to determine the influence of spin-finish treatment and long-term storage on PET fabric degradation and functionalization required for production of PET ligament prostheses. SFR was first achieved using diethyl ether (DE) as solvent for oil extraction in a Soxhlet system. Further experiments compared the extraction efficiencies of other solvents (DE vs. n-hexane, and tetrahydrofuran, THF) and the spin finish oil and resulting PET fiber alterations. After these assays, PET fabric functionalization was assessed following PNaSS radical “grafting from” polymer coating techniques in order to evaluate the efficiency and effects of the SFR process on polymer surface grafting. Results demonstrate that spin finish oil instability over time and extended storage conditions alters both SFR, surface functionalization and PET fiber/fabric degradation under long-term storage. A mechanism is proposed that correlates the significant and deleterious interactions between spin finish oil and PET fabric interfaces during storage, degrading PET surfaces and reducing PNaSS GR after SFR and grafting functionalization processing.