Enzymes go big: surface hydrolysis and functionalisation of synthetic polymers Georg M. Guebitz 1 and Artur Cavaco-Paulo 2 1 Department of Environmental Biotechnology, Graz University of Technology, Petersgasse 12, 8010 Graz, Austria 2 Department of Textile Engineering, University of Minho, 4800 Guimaraes, Portugal Enzyme technology has progressed from the biotransformation of small substrates to biotransform- ation of synthetic polymers. Important breakthroughs have been the isolation and design of novel enzymes with enhanced activity on synthetic polymer substrates. These were made possible by efficient screening pro- cedures and genetic engineering approaches based on an in-depth understanding of the mechanisms of enzymes on synthetic polymers. Enhancement of the hydrophilicity of synthetic polymers is a key requirement for many applications, ranging from electronics to functional textile production. This review focuses on enzymes that hydrolyse polyalkyleneterephthalates, polyamides or polyacrylonitriles, specifically on the poly- mer surface thereby replacing harsh chemical processes currently used for hydrophilisation. Introduction Biocatalytic processes are well established for synthetic biotransformation of small molecules. In 2004, economists predicted a 25-fold increase in turnover for polymers pro- cessed by biotechnological methods up till 2010, compared with only a seven-fold turnover increase for fine chemicals [1,2]. Recent launches of new commercial products, such as enzymes for processing polyester (Inotex Ltd; http://www. inotex.cz/) and patents by major industrial players (Genencor, http://www.genencor.com/; Novozymes, http:// www.novozymes.com/en; CIBA, http://www.cibasc.com/; Henkel, http://www.henkel.com/; [3–6]) indicate that enzymes are going ‘big’ in terms of their substrates. Limited surface hydrolysis of polyalkyleneterephtha- lates (PAT), polyamides (PA) and polyacrylonitriles (PAN) by enzymes increases their hydrophilicity, which is a key requirement for many applications, including gluing, painting, inking, anti-fogging, filtration, textile production, electronics and applications in the biomedical field (Box 1) [2,7–10]. Synthetic polymers are coated with bioactive com- pounds for many applications, including applications in textile manufacturing, microelectronics, bioprocessing and medical and food packaging. For example, biocoating of PET can lead to biocompatible and/or haemocompatible materials and antimicrobial surfaces, and is also used in tissue engineering [11]. Surface hydrophilisation is an important step in the biocoating process. PET is also used in cardiovascular implants such as artificial heart valve sewing-rings and artificial blood vessels. Enhanced hydro- philicity of PET (i.e. 158 lower contact angle) in these applications has led to reduced bacterial adhesion, thereby reducing the risk of infection [12]. PET shows excellent properties for use as a transparent cover layer in Flexible Electronic Devices (FEDs) (e.g. displays or photovoltaic cells), including mechanical stability and resistance to oxygen and water vapour. Again, the PET surface must be rendered more hydrophilic for increased adhesion of the subsequent functional layers, which have a major affect on the FED performance, efficiency and lifetime [9]. Ultrafiltration is used in many processes, including water purification and/or desalination, wastewater treat- ment and separations in the food, dairy, paper, textile, chemical and biochemical industries. Membrane fouling by proteins and other biomolecules increases the energy demand for filtration and requires cleaning with aggres- sive chemicals or replacement of the membrane. Ultrafil- tration and reversed osmosis devices based on polyamide or polyacrylonitrile can be rendered more hydrophilic by grafting poly(ethylene glycol) (PEG) to the devices, or by polymerisation of acrylate monomers to the devices, which increases resistance to fouling [10,12–15]. Textile materials made of PA and PET are uncomfor- table to wear because perspiration cannot penetrate the materials and evaporate. This poor water permeability is due to the hydrophobicity of synthetic polymers, which also leads to static cling and stain retention during laundering. A variety of different plasma treatments had been inves- tigated to increase hydrophilicity for PET, PA and PAN fabrics and films [16–20]. Chemical finishers, for example those based on hydrophilic carboxyl-containing polymers, are widely used to increase hydrophilicity of synthetic textiles and are continuously being improved, as evidenced by numerous patents filed [21]. In addition to these benefits to potential users of the relevant products, increased hydrophilicity also makes polymer processing (e.g. dying) more efficient [7]. Alkaline treatment of polye- ster can improve texture and hydrophilicity, and reduce pilling. However, extremely high weight losses from 10– 30% have been reported for this treatment [22]. Similarly, alkaline hydrogen peroxide or concentrated strong acid treatments for hydrolysing nitrile groups of PAN are diffi- cult to control and have a negative impact on the environ- ment [16]. By contrast, enzyme hydrolysis is targeted to the surface of the polymers while the bulk properties of the polymers remain un-changed. Review Corresponding author: Guebitz, G.M. (guebitz@tugraz.at). 32 0167-7799/$ – see front matter ß 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.tibtech.2007.10.003 Available online 26 November 2007