Bio-inspired surface modification of PEEK through the dual cross-linked hydrogel layers

The biocompatible high-performance material PEEK (polyetheretherketone) is an attractive implant material, however, its hydrophobicity and high friction coefficients severely hinder its biomedical applications. Thus, it is inferred from the recent advances in surface modification technology, achieving the biomimetic natural joint lubrication systems on PEEK still remains a challenge. In view of the above, herein we proposed a novel two-step strategy to fabricate a "soft (dual cross-linked hydrogel) layer-hard (PEEK) substrate" texture that mimics the structure and function of soft cartilage on the hard basal bone in joints. At first, a layer of acrylic acid-co-acryl amide (AA-AM) hydrogel is anchored to the PEEK substrate through UV-initiated polymerization. In the second step, hydrogel coated PEEK substrate is immersed in ferric nitrate solution to create the secondary cross-linkage between Fe³⁺ and -COOH groups in the hydrogel. As a result, the consequential top-coat hydrogel layer not only transforms the surface wettability (hydrophobic to hydrophilic) but also provides scratch resistance to the underlying PEEK substrate. The modified specimens display low friction coefficients in water under different load conditions. In addition, the obtained surface exhibits a certain self-repairing ability due to its unique physically reversible network structure. Therefore, this work provides a promising strategy for broadening the use of PEEK in orthopedic implants.