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Posted: Nov 28, 2012
Artificial spores - protecting living cells with tough artificial shells
(Nanowerk Spotlight) Spores are reproductive structures that have developed in nature to preserve genetic information and protect cellular components in harsh conditions and against external stresses – nutrient deprivation, desiccation, high temperatures, radiation, and caustic chemicals. Spores form part of the life cycles of many bacteria and plants. The cellular components of a spore are protected against the environment by a very robust hierarchical shell structure that allows it to survive for many – in some cases, millions of – years under hostile conditions found naturally that can easily and quickly kill normal cells.
Fabricating artificial spores by selectively coating a living cell could lead to single-cell based biosensors where the cell can be kept alive without division for a long time.
Artificial spores: a cell is encapsulated individually within the thin, tough artificial shell, the minimum required-properties of which are mechanical stability, selective permeability, and chemical functionalizability. The shell degrades in response to an externally applied stimulus in the controlled manner (controlled degradability). (Reprinted with permission from Wiley-VCH Verlag)
At first they discuss cytocompatible approaches to shell formation of artificial spores. The strategies that have been developed for this purpose include layer-by-layer (LbL) self-assembly, bioinspired mineralization, and mussel-inspired polymerization.
Next, they describe potential applications of these artificial spores that can be achieved by functionalization of the shells' surfaces. This functionalization can be achieved during or after the fabrication process. Depending on the modifications, this could lead to whole-cell sensors, bioreactors, cellular therapeutic agents, and microfluidic devices.
"Cell coating and encapsulation offer a feasible way of providing cells with new properties that are not intrinsic to the native cells," writes Choi. "The LbL coating process has intensively been utilized for deposition of various nanomaterials onto cells via electrostatic interactions, such as nanoparticles (silica, gold, silver, and iron), carbon nanotubes, and graphene oxides."
An alternative to the decoration of cells by pre made nanomaterials, in situ generation of functional nanomaterials on cell surfaces has also been demonstrated, based on the extension of bioinspired mineralization to non-biogenic materials, such as TiO2 or ZrO2.
Notwithstanding the great potential of artificial spores, Choi cautions that some issues remain to be resolved for their realization and their related structures; these include: "1) stimulus-responsive degradation of artificial shells: shells should be degradable by external signals and at pre-determined times; and 2) individual encapsulation of mammalian cells: mammalian cells are more mortality prone than microorganisms in the presence of harsh conditions, but are important entities for many technological and medical applications, as well as in single-celled processes."
The team also touches upon a few potential application areas of artificial spores, such as biochemical sensors, biocatalysis, cell therapy, and regenerative medicine.
For instance, in single-cell-based sensors, the individual cell acts as a detection entity, in that the biological signal-amplification process does not require additional cumbersome amplification steps. One of the problems to be solved for realization of single-cell-based sensors is the long-term stability of the cells, as they are easily affected by outside environments. That's where the artificial spore comes in – the encapsulation by an artificial shell preserves the cell's viability.
In conclusion, Choi and his team note that the field still is in its infancy, but holds great promise for the understanding of fundamental cell processes at the single-cell level, as well as for the development of many cell-based applications.
"The demonstration of enhanced viability, control of cell division, protection against foreign aggression, and chemical functionalization, is the basis for the realization of artificial spores and their related structures, and for further developments involving chemical manipulation and control of cellular metabolism at the single-cell level."