Determination of the Spatial Anisotropy of the Surface MicroStructures of Different Implant Materials: An Atomic Force Microscopy Study

Abstract

Many biomaterials’ surfaces exhibit directional properties, i.e., possess spatial anisotropy on a range of spatial scales spanning from the domain of the naked eye to the sub-micrometer level. Spatial anisotropy of surface can influence the mechanical, physicochemical, and morphological characteristics of the biomaterial, thus affecting its functional behavior in relation, for example, to the host tissue response in regenerative processes, or to the efficacy of spatially organized surface patterns in avoiding bacterial attachment. Despite the importance of the availability of quantitative data, a comprehensive characterization of anisotropic topographies is generally a hard task due to the proliferation of parameters and inherent formal complications. This fact has led so far to excessive simplification that has often prevented researchers from having comparable results. In an attempt to overcome these issues, in this work a systematic and multiscale approach to spatial anisotropy is adopted, based on the determination of only two statistical parameters of surface, namely the texture aspect ratio Str and the roughness exponent H, extracted from atomic force microscopy images of the surface. The validity on this approach is tested on four commercially available implant materials, namely titanium alloy, polyethylene, polyetheretherketone and polyurethane, characterized by textured surfaces obtained after different machining. It is found that the “two parameters” approach is effective in describing the anisotropy changes on surfaces with complex morphology, providing a simple quantitative route for characterization and design of natural and artificial textured surfaces at spatial scales relevant to a wide range of bio-oriented applications.

Near-Surface Nanomechanics of Medical-Grade PEEK Measured by Atomic Force Microscopy

“…As a result, indentation becomes an irreversible process, or, in other words, residual indent is found on the sample, and elastic–plastic deformation regime should be assumed to fit the data [ 38 ]. In our work, the usage of nanometrically sharp tips for better spatial resolution implies h ~ R , therefore the modified Hertz model of Section 2.2 can be used to fit the data [ 41 , 42 , 43 ]. This allowed to numerically process nanometric indentations into PEEK, while the usage of low-stiffness cantilevers allows investigating the polymer mechanical response to small applied forces.…”

“…The PEEK samples were medical-grade sheets (10 × 10 × 6 mm height; Direct Plastics Ltd., Sheffield, UK) with semimachined surfaces, as described previously [ 43 ]. AFM measurements were performed by a NT-MDT (Moscow, Russia) system equipped with an upright optical microscope.…”

“…Indentation maps obtained by AFM were processed using an assembly of different algorithms implemented as a Python module [ 46 ] that could be adapted to a user’s computational needs, as described previously [ 43 ]. In brief, the software processes the F-d curves, aligning them and separating loading and unloading curves to generate F-h curves.…”

“…The Poisson’s ratio of PEEK ( ν = 0.38) and the radius R enters into calculation of E . Following previous studies [ 33 , 43 ], an uncertainty of 10% on these quantities was propagated. Then, the uncertainty Δ E and the correlation coefficient of the fitting r 2 were computed.…”

Fractal dimension analysis: A link between porous polystyrene films patterns and cancer cell patterns

“…The surface of porous polystyrene films produced with 50% PS/50% PBMA presents the highest degree of isotropy, 90.34%; followed by the 40/60 weight ratio with the isotropy degree of 77.54%, a surface is isotropic if it presents identical characteristics regardless of the measurement direction, such as the case of surfaces with a random surface texture without any texture that stands out. Gambardella et al [ 33 ] found that anisotropy tends to decrease towards small length scales because downscaling gives way to smaller features contributing to the distribution of asperities within the topography. For films formed with a 40/60 weight ratio, the polar spectrum shows the angles for the first direction (90°), second direction (63.49°), and third direction (33.72°), and the privileged texture direction corresponds to 90°.…”