Nanometres to micrometres: three-dimensional surface measurement in bio-engineering

Abstract

Traditionally, quantitative measurement of surface texture has been carried out using contacting stylus instruments yielding purely two-dimensional topographical information. Recently, there has been a great expansion in this field through developments in instrumentation and computers which now make three-dimensional surface measurement a routine procedure. The latest instruments measure roughness ranges from nanometres up to millimetres. In conjunction with the development of these new techniques, a new range of numerical parameters is currently being developed in an attempt to establish a basis for new international three-dimensional surface measurement standards avoiding the “parameter rash” problems encountered in the development of two-dimensional surface measurement. Primarily, the new measurement techniques can be divided into contract measurement (employing styli) or non-contact measurement (employing mostly interferometers or focus detection systems) regimes. Despite the commercial availability of many of these new systems, they all possess certain important drawbacks as far as their wide applicability is concerned. True quantitative three-dimensional surface measurement has meant that the field of applications of these techniques has expanded enormously, from the traditional two-dimensional area of quality monitoring of machined surfaces into three-dimensional measurement fields ranging from ultra-fine thin film measurement to dermatological cancer studies. A relatively new and developing field of application of three-dimensional analysis is that of bio-engineering where the surface topography is of extreme importance in areas such as joint prosthesis surface replacement, heart valve seal quality and contact lens quality. The present study presents a review of a number of the new measuring systems available and their associated numerical characterization methods. A number of bio-engineered surfaces are given as examples covering four decades of vertical measurement range in order to illustrate the data sampling techniques, measurement strategies, range to resolution and instrument selection issues.