Silica shell microcapsules

Scheme 5.6 Assembly of hybrid microcapsules. Oppositely charged layers of protamine and silica precursor are alternately deposited on an enzyme-containing CaC03 bead, producing a core-shell microcapsule...

In 2010, Wang s and Fang s teams in China independently showed that PCM with enhanced thermal conductivity and phase-change performance can be successfully fabricated by sol-gel microencapsulation of wax such as n-octadecane and paraffin in silica microspheres obtained from TEOS polycondensation. The thermal conductivity of the microencapsulated n-octadecane is also significantly enhanced due to the presence of the high thermally conductive silica shell. However, the silica microcapsules prepared from TEOS only have poor mechanical properties, with the brittle shell of the microencapsulated PCM easily cracking. 

Rapid and facile generation of capsules from tandem assembly in aqueous media is amenable to encapsulation of water-soluble compounds. Encapsulation of ICG dye within PAH/H2PO4 aggregates was shown by Yu et al. Enzyme encapsulation and the feasibility of capsules to serve as reaction vessels was demonstrated by Rana et al. In their study, they encapsulated acid phosphatase enzyme in PLL-citrate-silica sols and suspended the spheres in a solution containing fluorescein diphosphate. Fluorescence increased in intensity within the shell walls as fluorescein was formed by enzymatic cleavage of phosphate groups. This study showed that microcapsules could serve as reaction vessels that allow enzymatic action to take place in a protective environment and allow for reactants and/or products to diffuse through permeable shell walls. 

FIGURE 18.3 SEM images of shell thickness of broken silica microcapsules doped with aqueous glycerol, obtained from [water + glycerol]/TEOS ratio = 10/1 (a) and 1/1 (b). Aqueous glycerol is homogeneously micro-encapsulated in the capsules core. (Reproduced from Galgali, G. et al Mater. Res. Bullet., 46,2445,2011.)... 

The sol-gel process to make doped silica-based materials has evolved from encapsulates in irregular Si02 xerogel particles, to sophisticate core-shell particles capable to encapsulate high amounts of functional organic species, and effectively release the entrapped species under small load. Sol-gel microcapsule and microparticle delivery systems will soon be introduced by numerous industries. 

Scanning Electron Microscopy - Silica microcapsules are studied by ESEM. The spherical particles have average diameters ranging from 1-30 pm, shown in - Figure 15. The shells of microparticles present a smooth surface. 

Jiang group developed silica/poly(divinylbenzene)-based polymeric microcapsules (PMCs) modified with three kinds of functional groups as carboxylic acid (PMC-C), sulfonic acid (PMC-S), and pyridyl groups (PMC-N), about which the PMCs displayed enhanced water retention capability even under low RH of 20%. The PMCs were well designed to have core-shell structure with controllable shell thickness, and then composite polyelectrolytes were fabricated from PMCs and chi-tosan (CS). Figure 9.9 displays the TEM images of the functionalized PMCs. 

The geometry of the particles could drastically change their properties. Recent findings indicate that the shape could influence the cellular uptake, endothelial targeting in the vasculature, the rate of endocytosis and lysosomal transport within endothelial cells [50, 51]. Hence, capsules templated on specifically shaped particles would be beneficial for certain applications. Furthermore, the flexibility of the polyelectrolyte molecules allows reproducing of the template shape after removal of the solid core [52-54]. CaC03 particles of different shape (spherical, elliptic, and cubic) have been used for fabrication of polyelectrolyte capsules, which were found to replicate the shape of the initial particles [55]. The properties of microcapsule shells assembled on cubic CdC03 particles were compared to the same shells assembled on spherical silica [56]. 

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