Artificial Spores - Cell Surface Engineering: Fabrication of Functional Nanoshells - page 157

biotechnological fields. For example, functional shells can be utilized for
cell-based sensors, high-throughput screening, and cell therapy. 39 The en-
capsulation of individual living cells also provides a useful model system for
studying the biology of single cells.

d n 8 y 4 n g | 2

8.5.1 Functional Artificial Shells for Biotechnological
Applications
Forming functionalizable/functional shells on living cells allows the cells to
be incorporated into materials or onto surfaces, which is a crucial step in the
development of cell-based sensors, high-throughput screening, and cell
therapy. It is particularly important for single-cell-based sensors, where the
encapsulated cells are immobilized at designated sites on the surfaces. One
way to achieve this is to endow the shell itself with specific functionality
analogs to that of exosporium in natural endospores.
A major obstacle to conferring functionalities onto inorganic shells is their
chemical inertness and/or the harsh reaction conditions for functionaliza-
tions. For example, the well-known silane chemistry for silica shells is lethal
to living cells, and the ionic crystals are not amenable to functionalization.
A bioinspired silicification process involving changes in the silica pre-
cursors suggests that functionalizable groups can be incorporated under
biocompatible conditions. Thiol groups (SH) was incorporated into biomi-
metic silica shells (SiO 2 SH ) by using (3-mercaptopropyl)trimethoxysilane
(MPTMS) as the silica precursor. 40 MPTMS was selected as the model
additive, because it was reported to be polycondensed simultaneously with
silicic acid under physiologically mild conditions. The utility of individual
yeast cells within SiO 2 SH shells (yeast@SiO 2 SH ) was thus extended, because
thiol groups reacted with a variety of maleimide derivatives under cyto-
compatible conditions. For example, the fluorescent dye fluorescein was
introduced into the SiO 2 SH shell with a fluorescein-linked maleimide. Biotin-
linked maleimide was also introduced into the SiO 2 SH shell, allowing
the biospecific interaction with streptavidin. The biospecific interaction led
to the site-specific immobilization of living cells onto a surface: the
biotin-functionalized yeast@SiO 2 SH was immobilized on defined areas
of poly(polyethyleneglycol methacrylate)

.

(poly[PEGMA]) films that were

patterned with streptavidin (Figure 8.10a).
Another approach to the functionalization of inorganic shells is to identify
and utilize the inherent reactivity of materials, as in the case of photo-
catalytic titania (TiO 2 ). Individual Chlorella cells were encased in TiO 2 shells:
the resulting TiO 2 shells were further functionalized using the specific
interaction between TiO 2 and the 1,2-hydroxybenzyl group (Figure 8.10b). 18
Organic shells can also give a possibility of chemical functionalizability.
For example, yeast@PD reacted with the amine or thiol groups via 1,4-
conjugate addition under slightly basic conditions. 14
In the practical application of cell-based sensors, the replacement and
removal of immobilized cells is desirable for the fabrication of reusable