Capturing local and anisotropic behaviour in surface topography

Capturing functionally relevant characteristics of surface geometry is a challenging task. Currently, 15 or more parameters have been defined for the specification of an aggregate measure of surface roughness. Some of these parameters have elementary descriptions, others much less so, and roughness is only one attribute of a surface that we may wish to quantify. The National Physical Laboratory is currently involved in a number of projects for which the assessment of surface geometry attributes is important. One such project is concerned with protein and cell adhesion on bio-implants. For this application, the relevant properties of the surface need to be related to the scales defined by the biological material with which the surface is interacting. The project aims to define a relationship between the observed adherence patterns and a description of the local behaviour of the surface. One difficulty is that the extent to which a surface fulfils its intended functionality can be assessed only obliquely, e.g., through an estimate of the dimensions of adhering cells. In this paper, we describe a number of surface analysis methods that seek to capture the local and anisotropic behaviour of surfaces, building on some of the concepts arising in the assessment of surface topography.