Engineering inorganic and cartilage surfaces by topologically different poly(2-oxazolines)

Abstract

Surface modification by polymer grafting has represented a revolution in material science, enabling the fabrication of stabilizers for colloids, biopassive coatings for sensors, drug delivery systems and model lubricants for technologically relevant applications. In this Thesis, I investigate the role of polymer topology in determining the interfacial physicochemical properties of surface-grafted assemblies, especially focusing on linear and cyclic polymer brushes. Cyclic polymers have been a scientific curiosity for decades, due to their intrinsic features, such as smaller hydrodynamic radius, higher thermal stability and unfavorable entanglement when compared to their linear analogues. In this work, cyclic polymers are applied for the first time as surface modifiers and the biopassive and lubricating properties of the derived films are studied. In particular, linear and cyclic poly(2-ethyl-2-oxazolines) (PEOXA) or poly(2-methyl-2-oxazolines) (PMOXA) are synthesized and coupled to specific functional moieties, which allow their anchoring to different surfaces. Cyclic and linear PEOXA are post-modified with catechols and chemisorbed on metal oxide surfaces, i.e. TiO2. The smaller hydrodynamic radius of cyclic polymers, combined with the absence of chain ends, determines the formation of denser films and provides enhanced steric stabilization to the surface when compared with linear polymers. Moreover, two opposite cyclic polymers-coated surfaces sliding against each other under load provide extremely low coefficient of friction, due to their highly unfavorable interdigitation. Cyclic and linear PMOXA are applied as side chains within graft-copolymers. The structure of these macromolecules resembles that of natural lubricants of articular joints and includes a poly(glutamic acid) (PGA) backbone, coupled to linear or cyclic PMOXA chains and to aldehyde-bearing segments (HBA). Since articular joint diseases, such as osteoarthritis (OA), are associated with a partial loss of natural lubricants and with a subsequent increase in friction, there is an emerging need for engineered friction-reducers for articular cartilage. PGA-PMOXA-HBA are designed to fulfill this need and to further stop or delay OA progression. Specifically, aldehyde groups spontaneously react with the degraded cartilage via Schiff-base formation and PMOXA chains form a biopassive and lubricious brush layer at the surface. Analogously to cyclic PEOXA on TiO2, graft-copolymers bearing cyclic PMOXA chains form denser films on degraded cartilage with respect to those having linear PMOXA. Furthermore, the replacement of linear PMOXA chains with cyclic ones results in enhanced biopassivity and in lubricating properties comparable to those of the healthy tissue.