Sodium Alginate and applications: a review

Written by Remigijus Vasliauskas
Published August 18th 2020
Contact: partnership@elvesys.com, Elvesys SAS, 172 Rue de Charonne 75011 Paris

What is sodium alginate?

Fig.1 Molecular structure of sodium alginate [2]

Sodium alginate is one of the best-known members of the hydrogel group. The hydrogel is a water-swollen, and cross-linked polymeric network produced by the simple reaction of one or more monomer [1]. The ability of hydrogels to absorb water arises from hydrophilic functional groups attached to the polymeric backbone, while their resistance to dissolution arises from cross-links between network chains.

Sodium alginate is a naturally occurring anionic polymer typically obtained from brown seaweed, it consists of mannuronic (M) and guluronic (G) acids arranged in different combinations (fig. 1) such as blocks rich in either M or G units, or blocks of alternating G and M units [3]. In the presence of divalent Ca²⁺ cations, the guluronic acids from nearby chains form ionic crosslinks resulting in alginate hydrogel. The ratio of M and G units defines the physicochemical properties of the hydrogel [4].

Alginate-based hydrogels are highly promising candidates for use as drug delivery systems [7] and as biomedical implants [8] as they are structurally similar to the macromolecular-based components in the body, and can often be delivered into the body via minimally invasive administration [9]. Alginate is an excellent candidate for delivery of protein drugs, since proteins can be incorporated into alginate-based formulations under relatively mild conditions that minimize their denaturation, and the gels can protect them from degradation until their release [5], [10].

Sodium alginate gels are increasingly being utilized as a model system for mammalian cell culture in biomedical studies. These gels can be readily adapted to serve as either 2-D or more physiologically relevant 3-D culture systems. The lack of mammalian cell receptors for alginate, combined with the low protein adsorption to alginate gels allows these materials to serve in many ways as an ideal blank slate, upon which highly specific and quantitative modes for cell adhesion can be incorporated. Further, basic findings uncovered with in vitro studies can be readily translated in vivo, due to the biocompatibility and easy introduction of alginate into the body [11].

Sodium alginate gelation process

Alginate hydrogels can be prepared by various cross-linking methods, and their structural similarity to extracellular matrices of living tissues allows wide applications. The most common method to prepare hydrogels from an aqueous alginate solution is to combine the solution with ionic cross-linking agents, such as divalent cations (i.e., Ca2+).

Calcium chloride (CaCl2) is one of the most frequently used agents to ionically cross-link alginate. However, it typically leads to rapid and poorly controlled gelation due to its high solubility in aqueous solutions. One approach to slow and control gelation is to utilize a buffer containing phosphate (e.g., sodium hexametaphosphate), as phosphate groups in the buffer compete with carboxylate groups of alginate in the reaction with calcium ions, and delay gelation. Calcium sulfate (CaSO4) and calcium carbonate (CaCO3), due to their lower solubilities, can also slow the gelation rate and widen the working time for alginate gels. The gelation rate is a critical factor in controlling gel uniformity and strength when using divalent cations, and slower gelation produces more uniform structures and greater mechanical integrity [12]. Finally, thermo-sensitive hydrogels have been widely investigated to date in many drug delivery applications, due to their adjustable swelling properties in response to temperature changes, leading to on-demand modulation of drug release from the gels [13].

Alginate micro particles

Sodium alginate-based particles have emerged as one of the most searched drug delivery platforms due to their inherent properties, including good biocompatibility and biodegradability for improved delivery, stabilization and prolonged release of encapsulated drugs [14], [15]. They are also extensively used for the encapsulation of living cells in pharmaceutical research, tissue engineering, and regenerative medicine. Such microgels act as micrometer-sized 3D culturing units, allowing individual cells to be independently monitored or manipulated, for example to study the role of confinement on cell fate or to deliver cells for the repair of damaged tissue [16].

Fig 2. Alginate microparticles produced using microfluidic technique.

There have been a lot of methods utilized in the literature to produce sodium alginate microparticles [17]. The most widely applied methods are: Simple dripping [18], Electrostatic potential [19], Vibrating nozzle [20], Jet cutting [21], Spinning disk [22], Spinning nozzle [23], Spray nozzle [24], Emulsification [25] and Microfluidics. While other systems can produce higher amounts of particles, microfluidic systems enable precise control of the microgel droplet size and allows the production of monodisperse microgels with a defined size distribution for the encapsulation of biomaterials in the field of molecular biology, pharmaceutical, health, food, and cosmetics.

Indeed, much effort has been dedicated to producing different types of alginate hydrogel droplets and microparticles (fig. 2). For example, Huang et al., showed alginate droplet production using T-junction geometry but the resultant bimodal distribution required an additional step of separation [26]. Capretto et al., generated alginate droplets on-chip while inducing an “external gelation” process in bulk [27]. Others have employed capillary devices to produce alginate-based double emulsions [28], [29], Janus particles [30] and hydrogel beads carrying encapsulated cells [31]. A lot of research has been performed on controlled gelation of microparticles using of calcium carbonate (CaCO₃) nanoparticles [25], [32]. Here, the water-insoluble particles are dispersed in the alginate solution and can be dissolved under acidic conditions after drop formation. Premature gelation is avoided and monodisperse particles result. However, the dissolution of solid calcium salt particles causes a heterogeneous distribution of calcium ions inside the droplets and diminishes the homogeneity of the resulting particles. Additionally, clogging of small microfluidic channels in the presence of particle aggregates limits the range of accessible microgel dimensions. Other techniques involve the initiation of the crosslinking process by the delivery of calcium chlorides or acetate particles through the oil phase which are subsequently dissolved in the sodium alginate emulsion droplet and releases Ca ions that initiate gelation [33], [34]. However, this method can suffer from the same problems, namely inhomogeneous calcium distribution or clogging issues. Alternatively, the generation of sodium alginate microgels via coalescence of separate droplets containing alginate and calcium chloride has been tried [17]. However, mixing inside the coalesced droplets still results in heterogeneous particles since crosslinking takes place before a homogenous distribution of calcium ions can be achieved. Additionally, coalescence generally results in a volume increase of the final, cross-linked alginate microgels.

Fig 3. Design and operation of microfluidics chip [35]. The device consists of three inlets for, (1) polymer precursor (e.g. alginate), (2) water phase, (3) initiator (e.g. CaCl₂), (4) continuous phase and (5) collection outlet. The channels for dispersed and continuous phases merge into a single channel (red square), where droplet generation takes place.

To produce pancake shaped microgel particles and encapsulate antibodies, Mazutis et al. [35], have designed special chip where a stream of sodium alginate solution is separated from Ca solution with a stream of water (fig. 3). All these three streams are mixed in the droplets where the gelation process takes place. This method produces very uniform pancake shaped particles. However, it is very difficult to arrange all three flows in a proper manner to avoid gelation in the channels, before droplet production.

To fabricate monodisperse sodium alginate micro-particles with structural homogeneity and encapsulate mammalian cells, Utech et al. [16] have suggested a new method to deliver calcium ions to the alginate droplets in the form of a water-soluble calcium-ethylenediaminetetraacetic acid (calcium–EDTA) complex. By chelating the calcium ions with EDTA, the ions remain in solution but are inaccessible to the alginate chains. Here, the homogenous microdroplets with mixture of alginate and the chelated calcium ions are created in microfluidic channels. Only with the addition of acetic acid to the continuous phase, the reaction is triggered with the release of calcium ions. The free ions react with the alginate chains in a highly controlled fashion forming alginate microparticles with excellent structural homogeneity. The separation of drop formation and crosslinking allows to eliminate clogging issues, and generates alginate microparticles particles with narrow size distributions without the need of special design microfluidic chips. Using this approach, Elveflow has designed an application note for fast and straightforward alginate microparticle production.

Conclusions

Sodium alginate is a natural non-toxic water-swollen crosslinked polymeric material that can be used in a lot of different applications ranging from additives to food and beverages up to scientific applications. The possibility to control the gelation process by changing the way Ca ions interact with alginate polymers allows the production of specifically shaped and still high uniformity alginate particles. Which can then be used for cell encapsulation or 3D structure build as well as drug encapsulation and controlled release. Elveflow developed an easy-to-use microfluidic pack for high monodispersity alginate beads generation.

References