Development of micro-structural units in a silk fibroin (SF) thin film deposited on Si (100) by using 1064 nm pulsed laser beam was examined. The smallest units ranging ca 10-20 nm were always observed in more or less uniformly dispersed states, which we coined as smallest protein units. At the same time, we frequently observed much larger chunks. Formation of such chunks was suppressed by choosing smaller fluence, shorter deposition time or by sputtering intermittently. When the laser fluence does not appreciably exceed the empirically determined threshold of 1.7 j/cm(2), the basic secondary structure of SF, i.e. anti-parallel beta-sheet, was well preserved, as confirmed by Fourier transform infrared spectroscopy. Since the target material exhibits only very small extent of absorption of 1064 nm beam, we suspected that formation of radicals and free electrons and subsequent optical breakdown are mainly responsible for the deposition process, although a number of mechanistic questions remain open. The present near-infrared pulsed laser deposition technique seems attractive since it is free from appreciable damage of the protein secondary structure and solubility of the protein species, and a compact, relatively inexpensive laser source suffices. (c) 2006 Elsevier B.V. All rights reserved.