Characterisation of membrane surfaces: direct measurement of biological adhesion using an atomic force microscope W. Richard Bowen * , Nidal Hilal, Robert W. Lovitt, Chris.J. Wright Biochemical Engineering Group, Centre for Complex Fluids Processing, Department of Chemical and Biological Process Engineering, University of Wales Swansea, Singleton Park, Swansea, SA2 8PP, UK Received 2 September 1998; received in revised form 9 September 1998; accepted 9 September 1998 Abstract An atomic force microscope (AFM) in conjunction with coated colloid probe and cell probe techniques has been used to measure directly the adhesive force between both the protein bovine serum albumin (BSA) and a yeast cell at two different membranes. These were polymeric ultra®ltration membranes of similar MWCO (4000 Da) but of different materials (ES 404 and XP 117, PCI Membrane Systems, UK). The XP 117 membrane is made from a mixture of polymers chosen with the aim of achieving low fouling. The BSA was adsorbed on a 5 mm silica colloid probe formed from a tipless V-shaped AFM cantilever. The cell probe was created by immobilising a single yeast cell on such a tipless cantilever. Measurements were made in 10 2 M NaCl solution. It was found for both protein and cell systems that the adhesive force at the ES 404 membrane was greater than that at the XP 117 membrane. The paper shows that coated colloid probe and cell probe techniques can provide useful means of directly quantifying the adhesion of biological materials to membrane surfaces. # 1999 Elsevier Science B.V. All rights reserved. Keywords: Membrane surfaces; Biological adhesion; Atomic force microscope 1. Introduction Membrane separation is an important operation in a range of biotechnological processes. Many compo- nents of such processes, such as macromolecules and biological cells, have a high af®nity for surfaces. Certain biological macromolecules prefer to reside at an interface due to their high molecular weight and amphipathic character [1]. Cell adhesion is a highly complicated phenomenon with many cells favouring interaction to a surface [2]. Hence, an inherent problem of membrane separations in biotech- nological processes is that of fouling caused by the adhesion of the biological macromolecules or cells to the membrane surface. The ability to measure directly the adhesion of such macromolecules or cells to different membrane surfaces would be of great use to the membrane technologist. Not only would this aid the development and testing of new membrane mate- rials but this would also allow optimisation of the choice of membrane when designing a biotechnolo- gical membrane separation process. The atomic force microscope (AFM) is an extre- mely useful tool for the membrane technologists [3]. The AFM's power is that it can provide topographical Journal of Membrane Science 154 (1999) 205±212 *Corresponding author. Tel.: +44-1792-295862; fax: +44-1792- 295862; e-mail: r.bowen@swansea.ac.uk 0376-7388/99/$ ± see front matter # 1999 Elsevier Science B.V. All rights reserved. PII: S0376-7388(98)00287-7