Production, deformation and mechanical investigation of magnetic alginate capsules

Abstract

In this article we investigated the deformation of alginate capsules in magnetic fields. The sensitivity to magnetic forces was realised by encapsulating an oil in water emulsion, where the oil droplets contained dispersed magnetic nanoparticles. We solved calcium ions in the aqueous emulsion phase, which act as crosslinking compounds for forming thin layers of alginate membranes. This encapsulating technique allows the production of flexible capsules with an emulsion as the capsule core. It is important to mention that the magnetic nanoparticles were stable and dispersed throughout the complete process, which is an important difference to most magnetic alginate-based materials. In a series of experiments, we used spinning drop techniques, capsule squeezing experiments and interfacial shear rheology in order to determine the surface Young moduli, the surface Poisson ratios and the surface shear moduli of the magnetically sensitive alginate capsules. In additional experiments, we analysed the capsule deformation in magnetic fields. In spinning drop and capsule squeezing experiments, water droplets were pressed out of the capsules at elevated values of the mechanical load. This phenomenon might be used for the mechanically triggered release of water-soluble ingredients. After drying the emulsion-filled capsules, we produced capsules, which only contained a homogeneous oil phase with stable suspended magnetic nanoparticles (organic ferrofluid). In the dried state, the thin alginate membranes of these particles were rather rigid. These dehydrated capsules could be stored at ambient conditions for several months without changing their properties. After exposure to water, the alginate membranes rehydrated and became flexible and deformable again. During this swelling process, water diffused back in the capsule. This long-term stability and rehydration offers a great spectrum of different applications as sensors, soft actuators, artificial muscles or drug delivery systems.

Strong Deformation of Ferrofluid-Filled Elastic Alginate Capsules in Inhomogenous Magnetic Fields

“…Figure 5 shows the expected linear dependence of ∆D from the parameter ∆(∆ρω 2 R 3 0 ) according to eq 9. The deviations at higher rotation frequencies are caused by water leakage from the capsule, which was also observed in previous studies [32]. For the calculation of the elastic moduli, the slope ∆D/∆(∆ρω 2 R 3 0 ) is determined from the initial linear regime (blue curve in Figure 5).…”

“…The capsules are washed with water to stop the polymerisation and placed in saturated sodium alginate solution. This lowers the elastic moduli of alginate systems and thus lead to capsules that are easier to deform [32]. For mechanical characterization of the capsule shells we use two methods, compression between parallel plates and deformation in a spinning drop apparatus.…”

“…To reduce this effect oil-soluble magnetic particles can be used, which are encapsulated in an oil in water emulsion, where calcium is solved in the aqueous emulsion phase [26]. In previous work, we showed that encapsulation of a stable ferrofluid is possible with this technique and that a significant deformation in magnetic fields can be achieved [32]. The deformation was, however, limited to approximately 10% because magnetic nanoparticles are only contained in the oil emulsion phase, whereas a certain amount of unmagnetizable aqueous emulsion phase is needed to solve calcium ions.…”

Characterization of Core-Shell Alginate Capsules

“…This swelling process would be considerably favored in presence of sodium salt compare to distilled water in particular due to the competition of sodium and calcium ions for the guluronic acid residues on alginate chains favoring the large decrease of cross-linking density in the alginate network and the huge increase of water uptake within the membrane. Authors recently clearly demonstrated, using spinning drop techniques, capsule squeezing experiments and interfacial shear rheology, changes in the mechanical properties of alginate membrane after storage of capsules in calcium or sodium salt solutions [38]. For instance, surface Young modulus determined by compression experiments decreased from 41 to 6.0 N/m when capsules were stored in calcium and sodium salt solutions, respectively.…”

Spheroidal and conical shapes of ferrofluid-filled capsules in magnetic fields

“…IV). For a typical ferrofluid-filled capsule with χ = 21, R 0 = 1 mm [21,69], and γ = 0.01 N/m, these Bond numbers correspond to magnetic field strengths H between 0 and about 500 kA/m (or fields B = µ 0 H between 0 and 0.5 T). We consider dimensionless Young moduli Y 2D /γ from 10 −2 (nearly no elasticity) to 100 (elastically dominated) and the purely elastic limit Y 2D /γ = ∞ (where the definition of B m is not useful anymore).…”