PerspectiveMaterials Science

Exploring the Interface of Graphene and Biology

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Science  18 Apr 2014:
Vol. 344, Issue 6181, pp. 261-263
DOI: 10.1126/science.1246736

Graphene is highly conductive, flexible, and has controllable permittivity and hydrophilicity, among its other distinctive properties (1, 2). These properties could enable the development of multifunctional biomedical devices (3). A key issue for such applications is the determination of the possible interactions with components of the biological milieu to reveal the opportunities offered and the limitations posed. As with any other nanomaterial, biological studies of graphene should be performed with very specific, well-designed, and well-characterized types of materials with defined exposure. We outline three layers of complexity that are interconnected and need to be considered carefully in the development of graphene for use in biomedical applications: material characteristics; interactions with biological components (tissues, cells, and proteins); and biological activity outcomes.

Graphene has now been developed in many different forms in terms of shapes, sizes, chemical modifications, and other characteristics that can produce dramatically different results when studied biologically. Methods for producing graphene include direct exfoliation in organic liquids (4, 5), reduction of graphene oxide (GO) (6), and epitaxial growth by CVD (chemical vapor deposition) on copper (7) or epitaxial growth on silicon carbide (8). The three aspects of this layer of structural complexity—the thickness, the lateral extent, and the surface functionalization of graphene—are illustrated in panel A of the figure and show how the materials produced by different methods fall in very different parts of this parameter space. These different physical and chemical characteristics dictate the suitability of a material for specific biomedical applications.

These wide discrepancies between the available graphene types will crucially determine the second layer of complexity, that of interactions of graphene with living cells and their compartments. In panel B of the figure, we show in a schematic fashion some of the possible cell-graphene interactions.

Graphene materials and their biological interactions.

(A) A parameter space for the most widely used graphene materials can be described by the dimensions and surface functionalization of the material, the latter defined as the percentage of the carbon atoms in sp3 hybridization. Green squares represent epitaxially grown graphene; yellow, mechanically exfoliated graphene; red, chemically exfoliated graphene; blue, graphene oxide. Note that a number of other graphene-related materials (such as graphene quantum dots and graphene nanoribbons) are also being used in experiments. (B) Possible interactions between graphene-related materials with cells (the graphene flakes are not to scale). (a) Adhesion onto the outer surface of the cell membrane. (b) Incorporation in between the monolayers of the plasma membrane lipid bilayer. (c) Translocation of membrane. (d) Cytoplasmic internalization. (e) Clathrin-mediated endocytosis. (f) Endosomal or phagosomal internalization. (g) Lysosomal or other perinuclear compartment localization. (h) Exosomal localization. The biological outcomes from such interactions can be considered to be either adverse or beneficial, depending on the context of the particular biomedical application. Different graphene-related materials will have different preferential mechanisms of interaction with cells and tissues that largely await discovery.

CREDIT: ADAPTED FROM CONRAD GESNER

Nanoflakes of chemically exfoliated graphene, micrometer-size flakes of GO, or substrate-bound CVD graphene will have dramatically different interactions and effects (if any) on live cells and tissue that can result in contradicting conclusions. Even experiments on similar, but not well-defined, materials can produce puzzling results. For instance, recently published papers on pulmonary inflammation after exposure to graphene platelets found no effects after 6 weeks (9) but some degree of acute (24 hours) inflammatory response (10).

The consequence of interactions at the cellular level will determine the third layer of complexity, that of overall biological activity outcomes. These outcomes can be adverse to the cell or tissue (e.g., fibrosis, membrane damage, or accumulation) or beneficial (e.g., facilitating intracellular transport of therapeutic or diagnostic agents, or providing antimicrobial or protective shielding). Some outcomes, such as cell activation or apoptosis, can be harmful or beneficial depending on the cell type and the intended use (in cancer therapy, harm to cells may be good; in vaccine design, activation of some parts of the immune system may be desired).

Both adverse and beneficial outcomes have been reported recently by different groups, even for similar graphene materials and cellular interactions. For example, cell internalization has been shown both as a mechanism that can lead to cell intoxication in some studies (11, 12) and as a means to transport therapeutic agents intracellularly without ensuing damage in others (13). In another example, some recent computational and experimental studies have demonstrated that specific forms of graphene can directly interact with plasma membranes, which suggests that graphene may cause cell membrane damage (14, 15). However, other studies demonstrated that interaction and binding of various graphene material types onto the mammalian plasma membrane can lead to a potentially beneficial enhancement of cell growth (16) or shielding effect (17, 18) with no cell damage. Lastly, some types of graphene materials have been shown to physically adsorb and wrap around bacterial cell membranes, suggesting possible antibacterial activity (1921), but this result has not been confirmed by others (16).

Also, the safety prof ile of graphene materials on interaction with living biological matter cannot be directly drawn from that for other carbon-based materials (graphitic platelets, amorphous carbon, and diamond-like carbon that have been studied for decades). These materials have properties very different from either graphene, bilayer graphene, or even few-layer graphene, and so will be their biological outcomes. Furthermore, despite some (very vague) similarities between graphene and carbon nanotubes, the former is generally not fiber-shaped, so fiber toxicology paradigms are not directly applicable (22). The limited number of available in vivo studies suggest that flat graphitic structures are not able to trigger the adverse (inflammatory) reaction associated with fibrous asbestos or long, rigid carbon nanotubes (23).

Another biological process of great importance is the biodegradation of graphene that will determine the safety profile of graphene materials from its residence time and persistence within tissues. Additionally, the kinetics of graphene degradation will define the limitations posed in relation to specific biomedical applications that may require long-term integration within the biological milieu (e.g., orthopedic or neuronal implants, catheters, wound healing agents, and corneal devices). The biodegradability of different graphene types will vary, as will the products of any biodegradation process. Some initial experimental evidence suggests that graphene can be enzymatically degradable by the oxidation activity of horseradish peroxidase (24) or macrophage-mediated degradation in vivo (25).

The development of graphene-based technologies for biomedical applications, either in the form of a device or an administered substance for therapeutic or diagnostic purposes, will be thoroughly scrutinized by the existing regulatory and approval framework implemented by national and international agencies. In the meantime, we urge very careful characterization and rational selection of the graphene materials to be studied in specific biological models, based on a hypothesis-driven intended biomedical purpose. Only rational, well-designed studies of graphene interactions with cells, tissues, and organisms will help guide the best choices for the use of this exciting family of materials.

References and Notes

  1. Acknowledgments: The collaboration was supported by EC-FET European Graphene Flagship.
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