Fabricating hierarchical micro and nano structures on implantable Co–Cr–Mo alloy for tissue engineering by one-step laser ablation

Highlights

• Hierarchical micro and nano (HMN) structured surfaces were fabricated.

• HMN surfaces exhibited enhanced osteoblast adhesion and improved cell proliferation.

• Osteoblast cells were highly oriented and aligned on HMN surfaces.

Abstract

Surface texturing is one of the effective strategies to improve bioactivity of implantable materials. In this study, hierarchical micro and nano structure (HMN) were fabricated on Co–Cr–Mo alloy substrate by a movable picosecond laser irradiation. Respectively, microgrooves with nano ripples and islands were produced on Co–Cr–Mo alloy by low and high laser power density. X-ray diffraction apparatus (XRD) phase analysis illustrated that substrate was in the phase of γ- face-centered cubic structure (FCC) before laser treatment, while it was in ε-hexagonal closest packing structure (HCP) phase dominant after laser treatment. Cell adhesion and proliferation studies showed that the HMN surface exhibits enhanced adhesion of MC3TC-E1 osteoblast and promoted cell activity. Analyzing of the morphology of osteoblast cells indicated cells were in high ratio of elongation on the HMN surface, while they mainly kept in round shape on the polished surface. Results indicated the formation of hierarchical structure on Co–Cr–Mo alloy was able to improve biological performances, suggesting the potential application in cobalt based orthopedic implants.

Introduction

Nowadays, the number of patients who are suffering from trauma, congenital malformation and degenerative diseases is increasing. Fortunately, new biological therapeutics applications, such as regenerative medicine and tissue engineering, are promising ways to treat these diseases. It was reported that biomaterials played a pivotal role to guide new tissue formation in tissue engineering. Fabricating the substance that can provide best cellular microenvironment/interface for cell accommodation is the final goal [1], [2]. The cobalt and its alloys, commonly used Co–Cr–Mo alloy, are ideal biomaterials for orthopedic and dental implants in recent years. It is owing to their high performance of wear and corrosion resistance [3], [4], [5], [6], [7]. However, the bulk ingredient of Co–Cr–Mo alloy is chemical inertness, which makes it more difficult to be modified. When these implants were transplanted in the human body, problems in inducing new bone/tissue formation have been aroused a lot of attentions. It has been found aseptic loosening was attributed to the main mechanism of hip joint replacement failure over 35 years, but no satisfactory solution has been found to date. Due to the recent attempts of less wear on articulating surfaces, focusing on the implant/bone interface has been increased [8]. In the previous study, layer-by-layer deposition technique was used to graft chitosan molecules on Co–Cr–Mo alloys to improve the biocompatibility [9]. The fabricating process was complicated and several steps were necessary. Designing and fabricating the micro and nano scale topography, called surface texturing, was an alternative way and has tremendous technological importance. The surface area was enhanced for the formation of various micro and nano structures. The textured surface has broad applications, such as improving cell growth and cell proliferation for biomedical implants [10], enhancing light trapping behaviors in optical usage and altering the surface wetting properties for self-cleaning performance [11], [12].

Specially, increasing studies have been showed that surface texturing can be effective and mentioned as a promising way for solving loosening failure and the long-term success of the implant [13]. The final cell adhesion was depended on initial interaction between biomolecule and substrate, which played an important role in the fate of cells. So far several types of surface texture, such as fibers, grooves and pits at the nano and micro scale, were fabricated on various materials. The cell adhesion, shape, gene expression and differentiation were investigated by several researchers [14], [15], [16], [17]. Amongst, Lu showed that the micro grooves (groove width from 4 to 40 μm), in the range of 0.5–2 fold of cell size, are more effective in guiding the cell orientation [17]. However, others showed that surfaces structured with 40–160 nm diameter nano-dots or nano grooves had better proliferation of osteoblast-like cells and an increase of focal adhesions [18], [19]. Thus, hierarchical micro and nano (HMN) structuring orthopedic implants at two dimensions may be a fascinating strategy for achieving fast and stable fixation due to the synergetic effect of micro- and nano-scale surface roughness with surrounding tissues. However, few researches reported the effect of micro and nano structures on cell behaviors in implantable cobalt alloys. It was mainly restrained by the fabrication techniques for inducing defined patterns on the cobalt alloy surfaces. The laser fabrication provides both suitable surface topography and less surface contamination as compared with other methods. Another advantage of laser technique is texturing implants with more complicated patterns [20]. More recently, with advent of ultrafast laser techniques, femto/picosecond lasers have become an advanced tool for texturing materials. Periodic nanostructures were successfully produced on metals and semiconductors by picosecond laser irradiation [21], [22]. The periodic structures self-form at the laser focusing spot and are perpendicular to the laser electric field polarization vector. The periodicity can be adjusted by the output of the laser. Although the mechanism of formatting the structure remained unclear, nanoplasma or surface instability that was generated by the femtosecond laser would be involved in the process [22], [23]

In the present in-vitro study, the one-step laser ablation technique was explored to rapidly fabricate HMN structures on the commercial cobalt alloy using a picosecond Q-switched laser system. Various HMN structures were obtained on Co-Cr-Mo depending on the laser energy. Surface topographies were analyzed using a scanning microscope and 3D laser confocal microscope. The composites were evaluated by XRD and EDS. Behaviors of osteoblast cells on the HMN structured surfaces were investigated to evaluate the biocompatibility.