The biology-nano-objects interface

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The rapid growth of nanotechnologies is increasing the likelihood of contact between nanomaterials and human beings and their environment. Nanoparticles (NPs) can interact with proteins, membranes, cells, DNA and organelles. It is therefore important to establish a better understanding of the interface between nanoparticles and biological components. These nanoparticle/biological component interfaces depend on colloidal forces as well as numerous dynamic biophysicochemical interactions. These interactions lead to the formation of protein crowns enveloping the particles, resulting in modifications to intracellular absorption phenomena and biocatalytic processes. These modifications can have undesirable biological effects or alter biocompatibility. Biomolecules, for their part, can induce phase transformations, restructuring and dissolution of nanomaterial surfaces. Probing and analyzing these different interfaces and their dynamics enables the development of predictive relationships between the structure and activity of NPs. These relationships are determined by nanomaterial properties such as size, shape, surface chemistry, roughness and coatings present on their surface. It is very important to establish this knowledge reliably, as it should enable nanomaterials to be used more safely.

It is in this context, at the interfaces between nanomaterials and biological systems, that the bio-organic and synthetic worlds merge into a new science, which studies not only the parameters for the safe use of nanotechnologies, but also the design of tailor-made nanomaterials for biological applications. The "nano-bio" interface encompasses the dynamic physico-chemical interactions, kinetics and thermodynamics of matter exchange between nanomaterial surfaces and the surfaces of biological components (proteins, membranes, phospholipids, endocytosis vesicles, other organelles, DNA and all biological fluids). To enable this field to evolve, we need to understand the dynamic forces and molecular components that shape these interactions. Today, it is still impossible to describe with certainty all the biophysicochemical interactions at play at the nano-bio interface. However, the unification of partial knowledge generated by different disciplines should provide us with a conceptual framework to guide this exploration. Within this framework, we explore these interfaces from the point of view of the elementary forces that govern colloidal chemistry and the adaptations that occur at biological interfaces. After nanoparticles have been suspended in tissue or biological media, and after interaction with cells (membrane surfaces of endosomal compartments, organelles and cytoplasm), we analyze nanoparticle-biocomponent interfaces and discuss methods for probing the "nano-bio" interface. Determining the interactions of processes such as protein corona formation, cell-cell contact, particle packaging on the cell surface, endocytosis and intracellular biocatalysis, and probing these different interfaces require new ideas and imaging techniques. A better understanding of these mechanisms should enable us to make more informed decisions on how to design nano-objects.

After evaluating over a hundred different nanoparticle families, including fullerenes, metal oxides, polymers, liposomes, dendrimers, quantum dots and gold colloids, it appears that hydrophobicity, size and surface charge are the main parameters influencing nanoparticle biocompatibility.

In vivo, the half-lives of hydrophobic nanoparticles are generally short (a few seconds to a few minutes) because they are rapidly captured and removed from circulation by the cellular elements of the reticuloendothelial system, ending up mainly in the liver and spleen. Although this problem may limit the bioavailability and usefulness of nanoparticles as carriers of active ingredients, some in vivo applications rely on rapid uptake by macrophages to transport nanoparticles to lymph nodes or other sites of inflammation (e.g. for magnetic resonance imaging of the liver using paramagnetic iron nanoparticles). The importance of particle size is corroborated by data revealing that the lungs, gastrointestinal tract and skin are highly effective barriers to nanoparticle absorption and diffusion. For example, metal oxides and polystyrene nanoparticles do not penetrate healthy human skin, while quantum dots can penetrate damaged skin and lodge in other tissues and organs. When nanoparticles are deliberately injected into the bloodstream, their size has a significant effect on the body's clearance rates and pathways. For example, the kidneys cannot excrete particles smaller than 8 nm. On the other hand, via specialized macrophages, the liver and spleen can trap particles larger than 200 nm. Although the liver can a priori capture any particle larger than around 200 nm, clearance from the bloodstream does not necessarily mean excretion, as the biliary tract can only eliminate nanoparticles smaller than 30 nm in diameter. Kupffer cells retain the particles until the liver can break them down. Nanoparticles of sizes between 30 nm and a few hundred nanometers can undergo passive accumulation at the tumor site thanks to the enhanced penetration associated with the EPR(Enhanced Permeability and Retention) effect. This phenomenon refers to the increased permeability of tumor vasculature, which, when coupled with decreased lymphatic drainage, allows nanomaterials to accumulate in the vicinity of the leaky cancer vasculature. Finally, the importance of particle charge was illustrated by in vivo and in vitro studies of a series of nanoparticles of relatively constant size, but varying their surface charge in sign and absolute value. Cationic particles are more cytotoxic and more likely to induce aggregation of red blood cells and platelets than neutral or anionic nanoparticles. This trend is confirmed by studies of the ability of cationic polystyrene particles to induce cytotoxicity, vascular leakage and inflammatory infiltration in rat and mouse lungs. This mechanism of toxicity may also explain the onset of acute pulmonary edema and bronchiolitis in humans exposed to cationic nanoparticle aerosol paint.

This analysis can be summarized and illustrated using a three-dimensional phase diagram proposed by Scott McNeil (ref. 1, 2).

This diagram perfectly summarizes the main qualitative trends in nanoparticle biocompatibility revealed after in vivo screening of around 130 nanoparticles for a variety of uses, in particular as vectors in biology and medicine. The main independent variables determining the in vivo biocompatibility of nanoparticles (color spectrum) are size, zeta potential (surface charge) and dispersibility (in particular the hydrophobic effect). This diagram clearly shows the areas in which NPs are expected to behave either toxically (red areas) or biocompatibly (blue and green areas). Optimum use of nanoparticles in biomedical applications therefore requires perfect control not only of their intrinsic chemical properties, composition and structure, but also of their surface. To reach a target for therapeutic purposes, nano-vectors need to be cleverly camouflaged so that they are not too quickly recognized and thus eliminated by the reticuloendothelial system. One lesson was devoted entirely to the study of the different and principal camouflage methods used. The small size of silica NPs combined with pegylation of their surface, control of NP charge and dispersion stability even after formation of the protein corona, masking of certain functionalities, and the presence of a controlled number of molecules capable of targeting biocomponents overexpressed by cancer cells are among the main camouflage and targeting strategies used. More recently, the preliminary use of NPS endocytosis and exocytosis phenomena to construct new biogenic vectors is opening up new perspectives. These microvesicular biohybrid compartments, which can exchange and vectorize active ingredients or probes with a variety of functionalities, should open up interesting prospects in nanomedicine.