Encapsulation stability and temperature-dependent release kinetics from hydrogel-immobilised liposomes Martin Ullrich, Jaroslav Hanuš, Jir ˇí Dohnal, František Šte ˇpánek ⇑ Chemical Robotics Laboratory, Department of Chemical Engineering, Institute of Chemical Technology Prague, Technicka 5, 166 28 Prague 6, Czech Republic article info Article history: Received 23 September 2012 Accepted 7 November 2012 Available online 29 November 2012 Keywords: Liposomes Hydrogel Alginate Controlled release Diffusion abstract Composite microparticles consisting of a calcium alginate gel matrix with embedded liposomes made from cholesterol:DPPC (dipalmitoylphosphatidylcholine) mixtures were considered. Factors affecting the encapsulation stability of liposomes during the gel formation by ionic cross-linking – namely temper- ature and the cholesterol:DPPC ratio – were systematically investigated. The liposomes were found to be tolerant to Ca 2+ ions during cross-linking of the gel and stable in the hydrogel matrix for extended periods of time when cholesterol was present in the phospholipid bilayer and temperature was kept sufficiently below the phase transition. The temperature-controlled release rate of encapsulated fluorescent dye was quantified. It is shown that a defined quantity of encapsulated substance can be repeatedly released from the embedded liposomes ‘‘on-demand’’ by short temperature pulses of suitably chosen duration and amplitude. This makes the hydrogel–liposome composites potential candidates for applications such as controlled drug delivery or study of reaction–diffusion phenomena in compartmentalised systems. Ó 2012 Elsevier Inc. All rights reserved. 1. Introduction One of the main differences between chemical and biochemical reactions that take place within the cells of living systems and those carried out in laboratory or industrial reactors is compart- mentalisation. Laboratory and industrial reactors tend to be well- mixed vessels in which local fluctuations in the concentration of reactants and products are generally undesirable. In contrast, the separation of reaction components (substrates, enzymes, reaction products) by internal compartments within a cell is one of the essential conditions for their functionality. The breach of internal or external membrane integrity within a cell and mixing at the intracellular level would lead to loss of homeostasis and eventually cell death. In order to study reaction–diffusion processes in micro- scopically compartmentalised systems under controlled conditions not necessarily represented by actual cells, it would be desirable to possess artificially made structures with the ability to store and release reaction components from embedded compartments. One possible realization of such structures can be the combina- tion of a hydrogel matrix with internal compartments represented by liposomes. Liposomes are spherical vesicles formed by a phospholipid bilayer and the methods for their preparation as well as basic physico-chemical properties are well described in the lit- erature [1]. One of the characteristic features of liposomes is their ability to encapsulate solutions and release them to the external environment in response to temperature. Below the phase transi- tion temperature of the phospholipid, the bilayer is solid-like and has a low permeability for the diffusion of polar molecules. Once heated above its phase transition temperature, the bilayer changes to fluid-like and its permeability can substantially increase [2]. This property can be used for controlled release (or uptake) from liposomes in applications such as medicine or cosmetics [3,4]. However, relatively little is known about the factors that influ- ence the encapsulation stability of liposomes when they are embedded in a solid or a gel matrix, which is a pre-requisite to the formation of the composite structures mentioned above. A few studies on liposome–gel composites can be found in the liter- ature, such as the immobilization of liposomes into PNIPAM [5], chitosan [6] or collagen [7] matrices. Since the physical conditions of gel formation and the nature of the polymer (e.g., charge, func- tional groups, etc.) can differ substantially from one material to another, knowledge about the stability of liposomes has to be ob- tained on a case by case basis. In the present work, liposomes in an alginate hydrogel are considered. Alginate is a biocompatible and biodegradable polysaccharide obtained from sea algae, consisting of mannuronate and guluronate homopolymer blocks. While so- dium alginate is water soluble, it rapidly forms a gel with multi-va- lent cations such as Ca 2+ . This property is used for the microencapsulation of various compounds including enzymes [8] and whole cells [9]. Alginate is also used as a wound dressing with haemostatic properties [10]. The aim of the present work was to investigate the stability of dye encapsulation into liposomes consisting of a mixture of DPPC 0021-9797/$ - see front matter Ó 2012 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.jcis.2012.11.016 ⇑ Corresponding author. Fax: +420 220 444 320. E-mail address: Frantisek.Stepanek@vscht.cz (F. Šte ˇpánek). Journal of Colloid and Interface Science 394 (2013) 380–385 Contents lists available at SciVerse ScienceDirect Journal of Colloid and Interface Science www.elsevier.com/locate/jcis