Relationship between processing, surface energy and bulk properties of ultrafine silk particles Rangam Rajkhowa a , Abdullah Kafi b , Qi Tony Zhou c , Anett Kondor d , David A.V. Morton e , Xungai Wang a,f, ⁎ a Australian Future Fibres Research and Innovation Centre, Institute for Frontier Materials, Deakin University, Waurn Ponds, Victoria 3217, Australia b Sabic SPADC Centre, Riyadh Techno Valley, Saudi Arabia c Faculty of Pharmacy, The University of Sydney, Sydney, NSW 2006, Australia d Surface Measurement System Ltd., 5 Wharfside Rosemont Road, HA0 4PE London, UK e Drug Delivery, Disposition and Dynamics, Monash Institute of Pharmaceutical Sciences, Monash University, 381 Royal Parade, Parkville, Victoria 3052, Australia f School of Textile Science and Engineering, Wuhan Textile University, Wuhan, China abstract article info Article history: Received 2 August 2014 Received in revised form 27 September 2014 Accepted 1 October 2014 Available online 18 October 2014 Keywords: Silk Surface energy Shear stress Powder Flowability Silk particles of different sizes and shapes were produced by milling and interactions with a series of polar and non-polar gaseous probes were investigated using an inverse gas chromatography technique. The surface energy of all silk materials is mostly determined by long range dispersive interactions such as van der Waals forces. The surface energy increases and surface energy heterogeneity widens after milling. All samples have amphoteric surfaces and the concentration of acidic groups increases after milling while the surfaces remain predominantly basic. We also examined powder compression and flow behaviours using a rheometer. Increase in surface energy, surface area, and static charges in sub-micron air jet milled particles contributed to their aggregation and there- fore improved flowability. However they collapse under large pressures and form highly cohesive powder. Alka- line hydrolysis resulted in more crystalline fibres which on milling produced particles with higher density, lower surface energy and improved flowability. The compressibility, bulk density and cohesion of the powders depend on the surface energy as well as on particle size, surface area, aggregation state and the testing conditions, notably the consolidated and unconsolidated states. The study has helped in understanding how surface energy and flowability of particles can be changed via different fabrication approaches. © 2014 Elsevier B.V. All rights reserved. 1. Introduction Silk has a long history of use not only in luxury textiles, but also as a suture material [1]. In recent years, powders from silk fibres have been used in cosmetic applications due to their moisturising, UV absorbing, antibacterial, and antioxidant properties [2,3]. Silk powder has potential applications in coating textiles and other material surfaces. It can be also used as a filler in synthetic fibres and polymeric products. Incorporation of silk powder improves moisture management, handling, dyeing and functional properties in such products [4]. More recently, various forms of silk materials have received considerable interests for potential biomedical, biotechnological, and healthcare applications, thanks to their good biocompatibility, biodegradability and biomechanical properties [5–7]. Among biomedical applications of silk, powdered silk can be used as a resorbable vehicle for biomolecules for diagnostic and tissue engineering applications [7]. Particles have also been used as fillers in composite scaffolds for growing bone tissues [8,9]. Particles may be used as smart sorbents due to their ability to rapidly bind dyes and transition metal ions at ambient temperature [10]. Silk powder can be produced either by dissolving silk fibres followed by liquid–solid phase transfer or by a top-down approach of milling. There are prohibitive challenges associated with the bottom up approach of regeneration due to slow production rate, difficulty in scaling-up and use of harmful chemicals and extent of silk degradation [11]. The top-down approach of milling overcomes many such problems and commercial silk powders are therefore produced mostly by milling. However, as viscoelastic silk fibres are difficult to mill into fine particles, pre-treatments such as chemical hydrolysis, exposure to thermal or ra- diation energy are often needed to reduce fibre strength and impart brittleness to facilitate milling. In contrast, we have used a combined wet milling/spray drying approach and demonstrated that ultrafine silk particles could be produced without pre-treatments and the parti- cles retained much of the original composition and structures of parent fibres [12,13]. The processing and applications of silk powders require a good un- derstanding of their bulk properties such as cohesiveness, flowability, spreadability, aggregation, and dispersion. For example, flow character- istics are important for their prospective processing and applications such as drug delivery via dry formulations, fluidisation in a coater, filling Powder Technology 270 (2015) 112–120 ⁎ Corresponding author at: Australian Future Fibres Research and Innovation Centre, Institute for Frontier Materials, Deakin University, Waurn Ponds, Victoria 3217, Australia. Tel.: +61 3 5227 2894. E-mail address: xungai.wang@deakin.edu.au (X. Wang). http://dx.doi.org/10.1016/j.powtec.2014.10.004 0032-5910/© 2014 Elsevier B.V. All rights reserved. Contents lists available at ScienceDirect Powder Technology journal homepage: www.elsevier.com/locate/powtec