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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.
By developing the concept of artificial spores, researchers have been developing strategies to coat single cells with a hard, protective layer of a hard thin shells. This has involved processes of layer-by-layer self-assembly, bio inspired mineralization (see for instance: "Researchers are getting closer to making artificial nacre"), and mussel-inspired polymerization (see for instance: "Nanotechnology inspired by mussels and seashells").
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
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)
In a concept article in the November 2, 2012 online edition of Small ("Artificial Spores: Cytocompatible Encapsulation of Individual Living Cells within Thin, Tough Artificial Shells"), Insung S. Choi, a professor in the Department of Chemistry at KAIST, and his team, review various chemical approaches to individually encapsulate living cells within thin and tough shells with the aim of creating artificial spores. They discuss current status and future prospects of this emerging field.
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."
He describes an example where, to utilize the properties of the coating materials, a carbon nanotube coating has been utilized for studying the viability of yeast cells, using voltammetry and electrochemical impedance measurements ("Polyelectrolyte-Mediated Assembly of Multiwalled Carbon Nanotubes on Living Yeast Cells").
In another example, the electroconductivity of graphene has also been utilized, when interfaced with yeast cells ("You have free access to this content Interfacing Living Yeast Cells with Graphene Oxide Nanosheaths"; open access).
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."
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