The construction of bioactive micro/nano polyetheretherketone (PEEK) arrays for antibacterial study

Introduction: The antibacterial behavior found on some natural surfaces such as insect wings has promoted the research of antibacterial materials without the use of additional chemicals [1]. The nanopatterns can impose stress on the bacterial membrane, causing rupture and death. However, the effects of different types of micro-nano structures and related mechanisms are still unclear. Polyetheretherketone (PEEK) is a biocompatible polymer but the poor antibacterial and bioactive properties have impeded its orthopedic application. The construction of bioinspired structures on PEEK surface is supposed to make it a better candidate for orthopedics. 
Materials and Methods: The pillar-like and cone-like PEEK mirco/nano-arrays were fabricated by colloidal lithography and plasma etching, with controllable shape and dimension by regulating the plasma sources and etching steps. The polystyrene spheres with diameters of 320 nm and 1.5 µm were used to prepare a monolayer on PEEK surface as the mask. The dimension and morphology of the arrays were measured by atomic force microscope (AFM) and scanning electron microscopy (SEM). The surface properties were characterized by Infrared spectroscopy (IR), X-ray photoelectron spectroscopy (XPS), nano indenter and contact angle measurement. The antibacterial behaviors of the arrays were evaluated by colony-forming unit (CFU) counting method and the measurement of intracellular species, and analyzed by SEM and adhesion force. The osteoblast viability and morphology were also obtained to confirm the bioactivity of the arrays. 
Results and Discussion: Nano pillar-like (NP), nano cone-like (NC), micro pillar-like (MP), and micro cone-like (MC) arrays were prepared. The nanoarrays had a bacteria-killing mechanism similar to that on cicada wings, through a penetrating effect. With a sharper and larger interspace, NC killed E. coli more effectively, but also triggered a self-adjusting effect of the bacteria. However, this problem was solved by the microarrays, due to the hybrid micro-submicron structure, their slight changed mechanical properties, and increasing surface energy. They mechanically stretched the bacteria during the initial attachment and subsequently distorted and killed the bacteria using the microarray and the sub-micrometer hybrid structures on the microarrays (Figure 1). The bioactivity was also enhanced because of the improved toughness and surface free energy of the arrays (Figure 2) [2]. 
Conclusions: The antibacterial effect and behavior of the PEEK arrays varied from micrometer to nanometer. For the microarrays, a stretching distortion bacterial killing mechanism was proposed, which due to their special structures and surface properties. Moreover, with these properties, the microarrays greatly improved the proliferation of osteoblasts, providing a promising strategy for designing new antibacterial biomaterials.