Gel microdroplets

Although this gel microdroplet method is still new (even after 10 years) and its potential applications relatively untested, it has been described here because it can teach us certain general lessons. It serves to remind us that cytometry, despite its name, does not necessarily involve the flow analysis of cells particles of many sorts will do just as well. It is also of interest as a method that has, in fact, institutionalized the formation of clumped cells that most workers try so hard to avoid. In addition, it has provided us with a way to make a small cell into a larger (flow-friendly) particle. Finally, it has given us inspiration by exemplifying the way in which lateral thinking can extend the impact of flow cytometry in new directions. 

Fig. 11.11. Illustration of the use of gel microdroplets for sensing growth at the level of one cell growing into a two-cell microcolony. By staining the cells within microdroplets with, for example, a DNA-specific fluorochrome, a small subpopulation of cells dividing more or less rapidly than most could be detected in a flow histogram of microdroplet fluorescence. From Weaver (1990).

Fig. 11.12. The use of gel microdroplets and flow cytometry to assay drug sensitivity of bacterial cells. The figure shows side scatter and green fluorescence contour plots of gel microdroplets (GMDs) containing E. coli cells that have been stained with fluorescein isothiocyanate for total protein. The microdroplets have been analyzed in the flow cytometer either at time 0 or 2 h after incubation in control medium (left plots) or medium containing penicillin (right plots). A model system was created by mixing two strains of bacteria (susceptible or resistant to penicillin). The data show that a small subpopulation of resistant cells could be detected within 2 h because of its rapid growth in comparison to susceptible cells. From Weaver et al. (1991).

The gel microdroplet technique is described in articles by Powell KT, Weaver JC (1990). Gel microdroplets and flow cytometry Rapid determination of antibody secretion by individual cells within a cell population. Bio/Technology 8 333 337 and Weaver JC, Bliss JG, Powell KT, et al. (1991). Rapid clonal growth measurements at the single cell level. Bio/ Technology 9 873. 

Fig. 11.12. Reprinted from Weaver JC, et al. (1991). Rapid clonal growth measurements at the single cell level gel microdroplets and flow cytometry. Bio/Technology 9 873.

Figure 1.20 Encapsulation of microdroplets of liquid crystals in ORMOSIL matrices results in materials with better transparency and thermal stability than polymer-dispersed liquid crystals. Gel-glass dispersed liquid crystal device switched between the OFF and ON state (thickness 10 pm, 4 x 2 cm, Fp p = 90V). (Reproduced from ref. 45, with permission.)...

FIGURE 7. Optical microscope picture with crossed polarizers of the microdroplets showing a liquid-crystal state filling the pores of a GDLC thin film. The orientations in microdroplets can be observed as the result of the specific sol-gel processing (parallel polarizers gave Maltese crosses of LC microdroplets and a black image of the silica-gel substrate)... 

During the preparation of macroporous materials by crosslinking copolymerization in the presence of precipitants, phase separation takes place within a relatively short period of time. This fast phase separation naturally results in the formation of microdroplets of the rejected porogen. Since the conversion of comonomers at that moment is very low, the rapidly growing polymeric network fixes in the gel the emerging liquid droplets. The nonequilibrium microsyneresis thus transforms into the stable form of phase separation within a heterogeneous system. Thus, the fast arrival at the unstable local polymer-solvent relationship, as compared with the slow rates of solvent macrosyneresis and of network relaxation, leads to the formation of gel-included microdroplets and, finally, to a permanent macroporous structure of copolymers. 

Finally, it is worth mentioning that PVA cryogels are able to incorporate different soluble and insoluble additives inside the pores to obtain composite materials of both scientific and applied interest [4—6]. In particular, PVA-based cryogels have been obtained that include inside the porous structure nanoparticles of solid crystalline compounds, co-elastic gels, microorganisms, gas bubbles, or microdroplets of liquids immiscible with PVA solutions [4—6, 92-98]. 

Formation of molecular assemblies in ionic liquids provide a new dimension in the design of functional supramolecular polymers. It introduces new supramolecular interfaces in ionic liquids, which hold a key to the development of new functions. The introduction of microscopic interfaces also expands the area of ionic liquid research. For example, one-step sol-gel synthesis of hollow Ti02 microspheres was achieved in ionic liquids by the use of organic microdroplets as template [129]. Now biomolecules, molecular assemblies, organic, and inorganic polymers can be handled in the engineered solvents. A rapid growth of supramolecular chemistry in these particular media is expected. 

---