Relationship between processing, surface energy and bulk properties of ultrafine silk particles
Graphical abstract
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], [6], [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 radiation 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 particles retained much of the original composition and structures of parent fibres [12], [13].
The processing and applications of silk powders require a good understanding of their bulk properties such as cohesiveness, flowability, spreadability, aggregation, and dispersion. For example, flow characteristics are important for their prospective processing and applications such as drug delivery via dry formulations, fluidisation in a coater, filling into a powder container or tablet dies from a hopper, or even dispersion in a liquid. Similarly, avoiding aggregation is critical in particle production stages such as wet grinding and spray drying. Good compressibility and flowability will be useful for their applications in moulding or 3D printing. Many bulk properties of powders are dependent on their surface free energy. The surface energy of a particle is the analogue of the surface tension of a liquid. A high surface energy means a more reactive surface, which has important implications in processes involving interfacial interaction, as happens in wetting, coating, mixing, compaction, cohesion and adhesion [14], [15]. In other words, surface energy determines the interactions between particles themselves as well as with other surfaces such as binders, polymers, and tissues [16]. However these properties of silk particles have not been well understood. For the first time we have reported quantitative measurements of these properties using two advanced techniques, namely inverse gas chromatography and powder rheology. We have measured surface energy of silk particles to understand how this energy is influenced by the particle size, shape and particle fabrication techniques. Such understanding is relevant for fabricating particles to suit different application needs.
Measurements of surface energy of particles involve determination of long and short-range intermolecular forces, which are commonly described as London dispersive and acid/base interactions respectively. Unlike flat surfaces of materials in which surface energy can be determined by contact angle measurements, in powder materials such measurements are difficult to perform. Compressing powder into pellets for contact angle measurements may change their surface properties. In this work, we have used a Surface Energy Analyser (SEA), which works on the principle of Inverse Gas Chromatography (IGC), a suitable technique for measurement of surface energy of powders. SEA can determine long range dispersive (non-polar) interactions such as van der Waals interactions using non-polar (series of n-alkanes) gaseous probes. By using polar gaseous probes, the specific short range (polar) interactions can be determined, which involve charge redistribution as exemplified by the formation of weak chemical bonds, such as the hydrogen bonds [17].
Apart from measurement of surface energy in static conditions, powder bulk properties under dynamic conditions have been measured using powder rheometry. These measurements provide data on compressibility, flow energy and frictional energy (shear forces) during particle movement, which relate to particle flowability, and the degree of particle cohesion. Powder rheometer measurements were obtained in both consolidated and unconsolidated forms. Understanding bulk properties and the ability to predict cohesion and flow properties of powders are important for two reasons. First, such knowledge helps in designing particles for specific applications. Second, it helps process control during powder production such as minimising batch to batch variations. Finally, the relationships between the bulk and surface properties were analysed by interpreting dynamic properties measured by the powder rheometer using the surface energy data form SEA.
Section snippets
Silk degumming
Eri silk cut cocoons were purchased from Fabric Plus Ltd. (India). Eri silk is less popular for textile applications as its cocoons are not reelable into continuous filaments. However Eri is more disease resistant than other commercial silkworm silk varieties, and its host plants are available in wide climatic conditions. Therefore it is an attractive silk for biomaterials applications. Moreover Eri is relatively easy to mill into powders compared to other silkworm silk varieties [18]. Hence we
Particle morphology and surface area
Table 2 contains data on d50 of silk particles measured using a volume based distribution from laser diffraction studies and BET specific surface areas from SEA tests. D50 and surface area of AM particles are 5 μm and 18.57 m2·g− 1 respectively. The BET surface area measured in this study by SEA is very close to the BET surface area of 18.4 ± 0.30 m2·g−1 measured earlier by the nitrogen adsorption method [23]. Assuming that there is no change in density of eri silk during milling, i.e. 1.31 g·cm− 3 the
Conclusions
The surface energy results showed dominance of dispersive surface energy and presence of more basic groups compared to acid groups on the surfaces of all forms of silk materials. The dispersive surface energy increased after milling and the result could explain the reduction in moisture absorption by powders as a result of milling observed in earlier studies. Results from this study confirmed that chemical processing altered the surface energy and associated flow characteristics of silk
Acknowledgement
We wish to acknowledge the financial support from the Australian Research Council under its Discovery grant scheme (Project DP1094979).
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