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Preparation and characterization of photo-responsive microcapsules based on azobenzene derivatives, for controlled delivery of active agents under visible light

Final Report Summary - PHOTOCAP (Preparation and characterization of photo-responsive microcapsules based on azobenzene derivatives, for controlled delivery of active agents under visible light.)

The proposed work is expected to have a significant positive impact on advancing knowledge in the field of product engineering, which will benefit science, the economy and citizens’ daily lives. Moreover PhotoCap project will have a significant positive impact on advancing knowledge in the field of sustainable growth protecting the environment, by developing new green technologies and production methods.The ability of industry to develop cost effective delivery systems will permit active systems to be developed that will require much less materials to be used in a wide range of applications, which will minimize environmental impact. Moreover the PhotoCap project will have a high impact on health protection by development and testing of new photo-controlled release microcapsules for innovative breakthrough applications with encapsulation and smart release of, for instance: (1) protective substances (i.e. used as a main component of sun protection creams) only in appropriate time – during sun light illumination, in order to minimize their side-effect on human body; (2) pharmaceutics or supplements in order to protect their properties from external environment. PhotoCap project deals with the obtainment of visible light-sensitive microcapsules for different final applications, such as personal and home care products.
Goal 1: Synthesis and characterization of photo-responsive materials
During the first two years, first the synthesis of a monomer based on an ortho-substituted azobenzene was optimized; afterwards, the obtained compound was characterized as refers to its photoisomerization: remarkably, the presence of electron-donor substituents determined that the trans/cis isomerization resulted activated solely by the use of visible light and there was no need for UV light.
Goal 2: Kinetics of the interfacial polymerization, influence of physico-chemical parameters on microcapsules size and morphology
In a following step, polymeric microcapsules based on the mentioned photoisomerizable moiety, with spacers of different lengths and two different kinds of crosslinkers were prepared by interfacial polymerization. Microcapsules contained a perfume which is used in commercial applications and was provided by P&G. The interfacial polymerization reaction was carefully optimized with respect to: reaction time, monomers concentration and ratio, temperature, stirring speed both during the preliminary emulsification step and following polymerization, in order to get the best final product as far as yield, size, morphology, and mechanical properties are concerned.
Goal 3: Photo-responsive microcapsules preparation
According to the conditions optimized in the previous goal, 15 different types of photo-sensitive microcapsules based on synthesized AMC were prepared by interfacial polymerization. The microcapsules were prepared by using spacers with three different lengths, 2 types of crosslinkers and two types of perfumes.
Goal 4: Microcapsules characterization. Controlled release of encapsulated perfume
First, microcapsules mean size was determined by Laser Diffraction Particle Size Analysis (LS) and optical microscopy (OM), both before and after irradiation with visible light. Results showed that the size is not affected by the type of cross-linker, perfume or diamine used for microcapsule preparation, and it depends on the overhead stirrer speed used for the oil in water dispersion preparation. After 3 h of irradiation, the microcapsule size decreased approximately 16 % (LS) or 11% (OM), respectively; after 15 hours in the dark, they recoverd 90% initial size. By cryo-SEM analysis, we could see that in all cases microcapsules appeared well-formed, with internal and external surfaces relatively homogeneous and smooth. Dimension change after photo-irradiation could be justified after extensive computational studies on surface models, which showed that the cis-isomer polymers forming the shell occupy less space and are approximately 20 ±1% shorter than the trans- polymers, assuming total isomeric purity. Concerning the release, it was investigated by (1) a Gas Chromatograph equipped with mass spectrometer (GCMS), and (2) an olfactive panel, during sample irradiation with light of various defined wavelengths. Results showed that: 1) the nature of the crosslinker affects the first release steps; 2) the perfume permeability through the microcapsule depends on the length of the spacer present in the polymeric shell. Moreover, we found that the shells are not selective and all perfume raw components passed through them independent of their cLogP. Finally, release experiments performed while irradiating with different wavelengths, results suggested that the trans-cis photo-isomerization of substituted azobenzene moiety in the solid state is driven by the light with a wavelength value of 550 nm and depends on its intensity.
Perfume release characterizations by an olfactive panel clearly demonstrate that: 1) encapsulated perfume release occurs after photo-irradiation exclusively; 2) the encapsulated perfume shows delayed release and is noticeable up to 7 hours while the non-encapsulated perfume is not noticeable by the normal consumer after 3 hours.

Goal 5: Develop a model of leakage/release of a capsule
In an effort to show the squeezing release mechanism (numerical modeling), we created 3-D models using periodic boundary conditions by applying an Amorphous cell module of the Materials studio platform. By comparing structure density before and after irradiation, it seems that the irradiated microcapsule structures are much denser than prior to light exposure. This is due to polymer length decrease caused by the trans-cis isomerization of azobenzene moiety. Moreover, estimation of polymer properties by empirical and semi-empirical methods showed that the shell permeability to several gases decreases on decreasing the spacer length used for the photo-sensitive moiety connection.

Goal 6: Application of the microcapsules to real products and stabilization test
The most promising microcapsules were formulated into 3 real products provided by P&G: Product#1 – liquid product without surfactants; Product#2 - liquid product containing surfactants; Product#3 - gel product without surfactants. Microcapsules stability tests showed that the immersed microcapsules are only homogeneously dispersed in Product# 2 and #3. Obtained results demonstrated that perfume leakage does not occur from microcapsules mixed with the Product# 3. Perfume release tests put into evidence that approximately 50 wt% of encapsulated perfume is squeezed from the capsule shells after 4h of visible light irradiation. The percentage of released perfume is not affected by concentration of the final products.
Obtained results indicate that the photo-sensitive microcapsules have future potential applications in: consumer goods (household and personal care products), medicine, catalysis, electronics, textile, chemical industry, and others.

Goal 7: Dissemination of Photo-triggered microcapsules in EU community.
See attached document

Project website:
Relevant contact details:
Dr. Bartosz Tylkowski – IOF fellow
Dr. Marta Giamberini – IOF supervisor