Highly lubricious SPMK-g-PEEK implant surfaces to facilitate rehydration of articular cartilage

To enable long lasting osteochondral defect repairs which preserve the native function of synovial joint counter-face, it is essential to develop surfaces which are optimised to support healthy cartilage function by providing a hydrated, low friction and compliant sliding interface. PEEK surfaces were modified using a biocompatible 3-sulfopropyl methacrylate potassium salt (SPMK) through UV photo-polymerisation, resulting in a 350 nm thick hydrophilic coating rich in hydrophilic anionic sulfonic acid groups. Characterisation was done through Fourier Transformed Infrared Spectroscopy, Focused Ion Beam Scanning Electron Microscopy, and Water Contact Angle measurements.

Using a Bruker UMT TriboLab, bovine cartilage sliding tests were conducted with real-time strain and shear force measurements, comparing untreated PEEK, SPMK functionalised PEEK (SPMK-g-PEEK), and Cobalt Chrome Molybdenum alloy. Tribological tests over 2.5 h at physiological loads (0.75 MPa) revealed that SPMK-g-PEEK maintains low friction (

0.024) and minimises equilibrium strain, significantly reducing forces on the cartilage interface. Post-test analysis showed no notable damage to the cartilage interfacing against the SPMK functionalised surfaces.

The application of a constitutive biphasic cartilage model to the experimental strain data reveals that SPMK surfaces increase the interfacial permeability of cartilage in sliding, facilitating fluid and strain recovery. Unlike previous demonstrations of sliding-induced tribological rehydration requiring specific hydrodynamic conditions, the SPMK-g-PEEK introduces a novel mode of tribological rehydration operating at low speeds and in a stationary contact area. SPMK-g-PEEK surfaces provide an enhanced cartilage counter-surface, which provides a highly hydrated and lubricious boundary layer along with supporting biphasic lubrication.

Soft polymer surface functionalisation of orthopaedic implant surfaces are a promising approach for minimally invasive synovial joint repair with an enhanced bioinspired polyelectrolyte interface for sliding against cartilage. These hydrophilic surface coatings offer an enabling technology for the next generation of focal cartilage repair and hemiarthroplasty implant surfaces.