Production of submicron silk particles by milling Mehdi Kazemimostaghim a , Rangam Rajkhowa a , Takuya Tsuzuki a , Xungai Wang a, b, ⁎ a Australian Future Fibres Research and Innovation Centre, Deakin University, Geelong, VIC 3217, Australia b School of Textile Science and Engineering, Wuhan Textile University, Wuhan, China abstract article info Article history: Received 30 December 2012 Received in revised form 15 February 2013 Accepted 1 March 2013 Available online 13 March 2013 Keywords: Silk powder Particle size Bead milling Spray drying Bead milling was used to produce nano silk particles from silk fibres. Silk fibres were first milled to volume median particle size d(0.5) of 7 μm using an attritor mill. It was followed by bead milling to prepare submicron particles. Several processing parameters including milling speed, milling time, bead load, and pH were examined to reduce the particle size in the bead mill. It was found that, at pH 10 and using optimized milling parameters, d(0.5) could be reduced from 7 μm down to about 200 nm, with a narrow particle size distribution. © 2013 Elsevier B.V. All rights reserved. 1. Introduction As a textile fibre, silk has outstanding properties such as lustre, dye and moisture adsorption as well as high strength, which differentiate it from other natural and synthetic fibres used in the textile industry [1]. Silk fibre is made of two fibroin protein filaments which are surrounded by a glue-like protein named sericin [2]. Silk fibre has crystallinity of about 60% due to a high amount of crystallisable amino acids with small side chains [3]. In solutions, fibroin is in the random coil and α-helix forms with a crankshaft pleated structure, whereas the fibroin in silk fibre is in an oriented β-form structure which has an anti-parallel pleated structure [4]. Due to its good mechanical strength and biocompatibility, silk fibre has been used as sutures for many years [5]. High wet strength, good re- sistance to enzymatic breakdown, good oxygen permeability and drug binding make silk a suitable biomaterial [6]. Silk fibroin has been used for bone regeneration [7], blood vessels [8], tissue scaffold [9], drug deliv- ery [10], enzyme immobilization [11], wound healing [12], and cosmetics [13]. For these applications, various silk morphologies including powders, films, foams, nanofibres, composites have been used. It is relatively difficult to convert silk fibre directly to powdered silk, as silk is flexible and has a high mechanical strength [14]. There- fore, silk powder is commonly made using chemical processes (bottom-up). In the chemical methods, silk particles are prepared by regenerating the silk dissolved in solvents that have the ability to break down the strong intermolecular forces between β-sheets [10,15–22]. Highly concentrated solutions of chaotropic salts, ionic liquids, and fluorinated solvents can be used for this purpose. Alco- hols (usually methanol), or kosmotropic salts such as potassium phosphate may be used to stimulate the regeneration of β-sheet, which alter the mechanical properties and make regenerated silk water insoluble [23]. The drawback of chemical methods is that the regenerated silk protein is de-natured since the chemicals used change the native structures of silk. Furthermore, the chemicals should be removed from the silk particles by dialysis, which is a lengthy process. In the past, some attempts have been made to use mechanical methods to produce silk powders. In these top-down methods, silk fibres were broken to small pieces by crushing or milling. Fibres were pre-treated with alkali [13,24], acid [16], steam[25], or radiation [26] to decrease the strength prior to milling [24,27,28]. However, production of sub-micron silk particles with a narrow size distribution has been dif- ficult. In this work, we produced submicron-sized silk particles with a rel- atively narrow size distribution, using a combination of attritor milling and bead milling processes. The effects of milling speed, slurry concentra- tion, milling time, pH and drying methods on mean particle size were investigated. 2. Experimental 2.1. Material preparation Eri silk was selected as the raw material, as it has lower mechanical strength in comparison with Mulberry silk [24,29]. Eri silk was sourced from northeast India. All chemicals were laboratory grade, and deionised water was used in all experiments. Powder Technology 241 (2013) 230–235 ⁎ Corresponding author at: Australian Future Fibres Research and Innovation Centre, Deakin University, Geelong, VIC 3217, Australia. Tel.: +61 3 5227 2894. E-mail address: xungai.wang@deakin.edu.au (X. Wang). 0032-5910/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.powtec.2013.03.004 Contents lists available at SciVerse ScienceDirect Powder Technology journal homepage: www.elsevier.com/locate/powtec