Fibroin Membrane Preparation and Stabilization by Polyethylene Glycol Diglycidyl Ether Piyarut Moonsri, 1 Ruangsri Watanesk, 1 Surasak Watanesk, 1 Hataichanoke Niamsup, 1 Richard L. Deming 2 1 Department of Chemistry, Faculty of Science, Chiang Mai University, Chiang Mai 50200, Thailand 2 Department of Chemistry and Biochemistry, California State University, Fullerton, California 92833 Received 22 June 2007; accepted 15 October 2007 DOI 10.1002/app.27528 Published online 23 January 2008 in Wiley InterScience (www.interscience.wiley.com). ABSTRACT: Membranes prepared by drying aqueous Bombyx mori silk fibroin (SF) solution and modified silk fibroin (MSF) solutions, prepared by adding the low molecular weight crosslinking agent, polyethylene glycol diglycidyl ether (PEGDE) MW 526, 0–10% w/w, were investigated by Scanning Electron Microscopy (SEM), Fou- rier Transform Infrared (FTIR) spectroscopy, and UV–vis spectroscopy. Weight gain in aqueous solutions and their mechanical properties (tensile strength, elongation, and Young’s modulus) were then characterized. SEM measure- ments revealed greater porosity in MSF membranes. IR spectra showed transformation from the largely a-helical/ random coil structures in SF membranes to predominantly b-sheet in MSF membranes. Results from UV–vis spectro- scopy showed that the MSF membranes were largely inso- luble within the pH range of 4–10. Water absorbability of the MSF membranes improved with increasing the amounts of cross-linker, up to 4%. The MSF membranes showed greater pliability and tenacity, but lower tensile strength, with increasing PEGDE concentrations. In the wet condition, PEGDE levels up to 4% can improve both tensile strength and tenacity of the MSF membrane, but higher levels (up to 10%) did not significantly change these properties. Ó 2008 Wiley Periodicals, Inc. J Appl Polym Sci 108: 1402–1406, 2008 Key words: silk fibroin; modified silk fibroin; PEGDE crosslinking agent INTRODUCTION Silk, derived from the silkworm Bombyx mori, is composed of two proteins: fibroin (about 70%) form- ing the central core of the fiber and imparting strength, and sericin (30%), the outer glue-like pro- tein coating that helps protect the fiber against water and other environmental factors (microorganisms, insects, etc.). 1 Sericin can be removed by chemical treatment, such as boiling in 5% aqueous sodium carbonate. The resulting silk fibroin (SF) can be dis- solved by treatment with reagents such as CaCl 2 - EtOH-H 2 O, Ca(NO 3 ) 2 -MeOH-H 2 O or aqueous LiBr, 2 and purified by dialysis against deionized water. It can be incorporated into a wide range of materials that have high biocompatibility, thermal stability, microbial resistance, and oxygen permeability. These materials have been utilized for wound protection, 3 as substrates for cell cultures, 4 in drug-release agents, 5 and as substrates for enzyme immobilization in biological sensors. 6 The primary structure of SF consists of the repeat- ing amino-acid sequence Gly-Ala-Gly-Ala-Gly-X, where X is Ala (64%), Ser (22%), Tyr (10%), and other amino acids (4%). 7,8 Two crystalline poly- morphs are observed for SF: Silk I, containing a- helical and random coil structures, and Silk II, con- taining the antiparallel b-sheet structure. 9 In aqueous solution, the SF molecules exhibit the random coil conformation with an average hydrodynamic radius of 139 nm and a molecular weight of  25 kDa. 10 Silk fibroin membranes can be prepared by casting SF solutions onto different surfaces, such as polyeth- ylene, polytetrafluoroethylene, polystyrene, or glass. Silk I membranes are flexible, but the fibroin can dis- solve when in contact with water or solvents. A sta- ble SF membrane which contains high proportions of the Silk II can be prepared by (a) casting at room temperature, followed by soaking in ‡ 75% methanol in water; 11 (b) casting at temperatures above 608C; 12 or (c) dissolving silk fibroin in 98% formic acid prior to casting and drying. 13 While these treatments are useful in stabilizing the SF membrane against water, the b-sheet-rich membranes are generally rigid and brittle in the dry state, causing difficulty in practical applications. Chemical modification by crosslinking or chemical blending results in improvements in Correspondence to: R. Watanesk (scirwtns@chiangmai. ac.th). Contract grant sponsors: Phetchabun Rajabhat Univer- sity Fund, The Graduate School of Chiang Mai University, The Center for Innovation in Chemistry: Postgraduate Education and Research Program in Chemistry (PERCH- CIC), Thailand. Journal of Applied Polymer Science, Vol. 108, 1402–1406 (2008) V V C 2008 Wiley Periodicals, Inc.