A new manufacturing method, detailed in a report in the journal AIP Advances, creates microcapsules with a liquid core that are ideal for the storage and delivery of oil-based materials in skin care products. They also show promise in some applications as tiny bioreactors.
The current methods for producing microcapsules use emulsions, which typically require surfactants in order to secure stability of the layer between the inner liquid and the outer shell.
Microfluidics is one method used to create microcapsules, which is done by wetting tiny channels with the liquids used to make droplets. However, this technique also uses surfactants, which are problematic because they can negatively impact the liquids.
“Surfactants may affect chemical processes, such as enzymatic activities in the droplet. This makes them unsuitable for certain enzyme-related research. Liquid marbles, a liquid droplet coated with hydrophobic powder, are also constrained by their relatively large size. On the contrary, smaller microcapsules can be obtained through electro-spraying with a strong electric field.” The authors point out that these methods require the use of additional equipment for electric field generation.
Now, researchers have found a way to utilize the microfluidics technique sans surfactants.
“Addressing all these bottlenecks, we present here a method for generating surfactant-free, ultraviolet (UV) light curable core–shell microcapsules in a fully hydrophilic PDMS microfluidic device. Using a UV-curable polymer instead of an oil-based liquid, we avoid the need for dual surface treatment,” explained the authors.
The authors said this new method can churn out as many as 100 microcapsules per second, adding the output could be even larger at higher flow rates.
To produce the surfactant-free core–shell microcapsules, the researchers designed a device by etching tiny channels into hard plastic. An oily liquid for the core and a different liquid for the shell were injected into the channels.
As the liquids were pumped through, droplets formed when the emulsified liquids came into contact. A third liquid helped to separate the liquids from each other. The last step involves ultraviolet light that causes the outer shell to polymerize and solidify, trapping the liquid core.
The finished droplets were left to dry overnight at a high temperature. The authors reported no evaporation or shrinkage, demonstrating that the microcapsules can be safely stored without rupturing. They noted this makes them ideal for a variety of applications, for instance, microfluidic technology can be used to miniaturize a fish oil capsule from its normal size to the down to the size of a printed dot in a book.
“This feature makes the microcapsules reported here a highly practical and extremely attractive option….Our current proof-of-concept experiments can produce 100 microcapsules per second at low flow rates, which is predicted to increase with higher flow rates,” noted the authors.
Source: AIP Advances
10, 065101 (2020); https://doi.org/10.1063/5.0004736
“Surfactant-free, UV-curable core–shell microcapsules in a hydrophilic PDMS microfluidic device”
Authors: A. Teo et al.