Inkjet fabrication and characterization of calcium alginate microcapsules Jiří Dohnal, František Štěpánek ⁎ Department of Chemical Engineering, Institute of Chemical Technology, Prague, Technická 5, 166 28 Praha 6, Czech Republic abstract article info Article history: Received 15 January 2010 Received in revised form 22 February 2010 Accepted 23 February 2010 Available online 1 March 2010 Keywords: Calcium alginate Drop-on-demand Microencapsulation Diffusion Droplet impact The present work describes a method for producing calcium alginate hydrogel microcapsules in the size range of 50–70 μm by means of a piezoelectric drop-on-demand inkjet device. Particles were prepared by emitting droplets of 0.5% and 1% (w/w) sodium alginate solutions into a magnetically stirred pool of CaCl 2 solution of variable viscosity ranging from 1 to 100 mPas. The effect of viscosity on the morphology of the resulting micro- capsules was systematically investigated — lower viscosity of the receiving solution has lead to the formation of elongated particles, medium viscosities lead to spherical capsules, and for higher viscosities flattened particles were obtained. The applied voltage used for driving the piezoelectric inkjet print-head was found to be the most significant parameter for influencing the droplet size. The duration of the voltage pulse or the droplet viscosity had only minor effects. The release rates of two model substances (methylene blue dye and vitamin B12) from alginate beads were measured and their effective diffusion coefficients determined as function of alginate concentration. © 2010 Elsevier B.V. All rights reserved. 1. Introduction Alginate is a naturally occurring substance obtainable from sea algae. Chemically it is a block co-polymer consisting of D-mannuronic and L- guluronic acid units. While sodium alginate is soluble in water, bi-valent ions such as Ca 2+ or positively charged polymers such as chitosan can form gels by ionically cross-linking alginate polymer chains [1]. Alginate-based gels are often used for the encapsulation of various substances including pharmaceuticals, enzymes [2] or living cells [3]. Alginate has the advantage of being bio-compatible and it is generally regarded as safe (it is used as a food additive). The formation of beads or particles from alginate gel can be achieved by several techniques that generally consist of two steps — dispersion of sodium alginate solution in the form of droplets, followed by contact with a suitable gelling agent, such as Ca 2+ ions. Alginate particles can subsequently be modified e.g. by the formation of a protective shell layer [4]. Commonly used techniques for the dispersion step include mechanical agitation [2,5], ejection of droplets from a capillary or a nozzle [6,7], as well as the use of microfluidic devices [8]. While techniques suitable for the production of relatively larger alginate beads (100's of micrometers and above) are well established, reproducible formation of uniform alginate gel particles in the 10's of micrometers range still remains a challenge. When alginate microparticles are to be used for controlled release or delivery of active substances, close control of the particle size and mor- phology is crucial. A suitable way of generating uniform droplets can be found in inkjet technology [9]. While this technique is well established for use in desktop computer printers, it has only relatively recently been adopted for other diverse applications including printed electronics fabrication, controlled deposition of fluids onto substrates [10], formulation devel- opment and high-throughput screening [11], or microparticle formation [12]. Inkjet technology is based on emitting liquid droplets from a print- head by the action of suitable impulses that can be thermal, acoustic, or piezoelectric [13–15]. The typical sizes of droplets ejected from inkjet devices are in the range of 10's of micrometers [16]. The main advantage of inkjet technology is good uniformity of the droplet size and relatively easy scale-up (by replication). The main limitation is that highly viscous or shear-thickening fluids can be difficult to process. The aim of the present work was to investigate the use of inkjet technology for the production of calcium alginate microcapsules, to map the effect of operating characteristics such as applied voltage and its duration on the properties of the resulting particles, and to systemat- ically investigate the effect of the viscosity of the receiving liquid pool on the morphology of the resulting microcapsules. The rate of diffusion of two model substances from alginate beads was also investigated and a functional dependence of the effective diffusion coefficient of each sub- stance on alginate concentration has been obtained. 2. Materials and methods 2.1. Materials Sodium alginate, CaCl 2 , glycerol, methylene blue dye and vitamin B12 were purchased from Sigma-Aldrich. Deionised water (conductivity 1.1 μS/cm) was prepared by a two-stage ionex demi water generator Powder Technology 200 (2010) 254–259 ⁎ Corresponding author. Tel.: + 420 220 443 236; fax: + 420 220 444 320. E-mail address: frantisek.stepanek@vscht.cz (F. Štěpánek). 0032-5910/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.powtec.2010.02.032 Contents lists available at ScienceDirect Powder Technology journal homepage: www.elsevier.com/locate/powtec