Smart soaps: stimulus responsive soap–hydrogel bead composites for controlled dissolution and release of actives

Abstract

We designed pressure responsive soap–hydrogel bead composites by incorporating agar hydrogel beads of different size distributions within a molten soap matrix at various volume fractions. Upon cooling, the combined suspension of hydrogel beads into the molten soap was set into a composite of soap matrix. We demonstrate pressure driven syneresis of water from the soap–hydrogel bead composites upon compression. This allowed a release of active components embedded in the hydrogel beads upon application of pressure on these “smart” soap composites. We found that the dissolution rate of these composites generally increases with the volume percentage of hydrogel beads. We achieved a composite dissolution rate approximately 2.8 times higher than the soap control sample without hydrogel beads. However, the composite dissolution rate was independent of the size of the embedded hydrogel beads. We studied the release rates of active components encapsulated within the hydrogel beads used to prepare the composites. It was found that the release rate can be controlled in three different ways: varying the hydrogel beads size, using different concentrations of the gelling polymer used to make the hydrogel and also by co-encapsulating an oppositely charged polyelectrolyte along with the active encapsulated species. We found that the composites compressional strength decreased with an increasing volume percentage of hydrogel beads incorporated within the soap composite. Young's modulus showed a maximum when 7.5% by volume of hydrogel beads were used for composite preparation. These fast-dissolving soap–hydrogel composites contain significantly less raw materials and would reduce the pollution of waste water with surface active components. We envisage that soap–hydrogel bead composites could improve the sustainability of the soap-producing industry and could find their application within the hotel business, where they could reduce costs and the waste of millions of partially used soap bars discarded on a daily basis.