Pacman goes Forth

by Victoria Doronina Labtimes 04/2017

The line between life and non-life is becoming increasingly blurred. Biological molecules are used, e.g. as molecular machines. On the other side of the spectrum, inorganic materials are constructed to display cell-like behaviour.

Pacman-like MPE-droplets ingest­ ­silica particles. Photo: Man lab

A recent article in Nature Materials from Stephan Man’s team at the University of Bristol continues a long established tradition in biophysical science of using a combination of inorganic materials, for example, water and intra-lipid or polystyrene beads to create ‘phantoms’, designed to mimic optical or radiological properties of living tissues (Rodrigues-Arco et al., DOI: 10.1038/NMAT 4916).

But the colloidal mixtures of Man’s co-workers do more than just acting as a passive stand in. Rodrigues-Arco et al. created composite colloidal mixtures that, in their opinion, not only mimic the behaviour of phagocytes but “exhibit collective behavior.”

The group prepared so called Magnetic Pickering Emulsion (MPE) droplets with the help of magnetic Fe3O4 (magnetite) particles that self-assembled on the interface between water and an organic phase, consisting of dodecane. MPE are spherical shells of oleate-covered magnetite with a mean diameter of approx. 500 micrometres that depend on water/magnetite concentration.

Application of a magnetic field to MPE caused them to open along the surface but they didn’t lose structural integrity and returned to the spherical shape. The apertures allowed the introduction of various molecules into the interior of the spheres such as proteins and polysaccharides. The spheres aligned along magnetic field lines creating unlined chains.

Oleate-stabilised aperture

Increasing the pH of the water phase to 10.2 and adding oleic acid resulted in heterogeneous, more complicated structures: parts of droplets remained covered by magnetic nano-particles and the rest of the structure was bordered by a monolayer of oleic acid molecules. Increasing the oleate concentration below 1 mg/ml, decreased the magnetite-covered MPE surface and enlarged oleate-stabilised apertures. At oleic acid concentrations of 1 mg/ml and above, about 40 percent of the spherical MPE droplet surface was uncoated by magnetic particles, leading to MPE droplets, which resembled the hero of the classic game, Pacman, in appearance and behaviour.

To test the phagocytosis-like behaviour of MPE droplets, the researchers added a few MPE droplets to a suspension of cross-linked silica particles (colloidosomes), dispersed in oleic acid-containing dodecane with diameters of approx. 50 micrometres.

Mixing the silica colloids with MPE did not lead to any interactions in the absence of oleic acid. Applying a magnetic field to the combination of MPE and silica colloidosomes, however, opened apertures in the MPE droplets, allowing their self-propelled, non-directional movement and random engulfment of colloidosomes.

The group explained the MPE droplets spontaneous movement by the Marangoni effect: a movement of particles due to a surface tension gradient on the uneven distribution of oleate on the surface of MPE particles. Due to the gradient dissipation, the droplets were able to move only for several seconds. Only ‘Pacmen’, i.e, MPE with apertures, were able to move.

Both MPE and cross-linked colloidosomes retained their integrity after ‘phagocytosis’. The crossing took several seconds. Within a minute, eight percent of colloidosomes were engulfed by MPE. Using a magnetic field to guide MPE toward colloidosomes doubled the percentage of engulfed particles.

Based on their findings, the Man group proposes the following model of artificial ‘phagocytosis’: The non-magnetite covered surface aperture of the ‘pacman’ is coated by oleic acid molecules as a single layer proto-membrane. Colloidosomes have this layer as well. Fusion of molecular layers on the surface of MPE aquatic apertures and colloidosomes creates semi-double membrane particles that facilitate the translocation of the colloidosomes through the oleate-stabilised apertures into the Pacman’s interior.

Ingested colloidal particles retained large biological macromolecules, such as FITC-labelled BSA, but small molecules, like calcein, diffuse into the water phase of MPE. Non-cross-linked colloidosomes spontaneously disassembled in the interior of MPE, releasing fluorescent proteins or ­micrometre-sized ­polystyrene beads. Preparing MPE containing a substrate for enzyme alkaline phosphatase and loading colloidosomes with the enzyme resulted in fluorescence after the ‘phagocytosis’, indicating a reaction between the enzyme and its substrate inside the MPE.

The authors propose to apply these composite colloidal mixtures for development of new material and nanoscale engineering approaches. This sounds plausible, with the group’s intention to mimic predation and chemical communication being even more interesting.

Calling emulsion droplets ‘protocells’, and talking about ‘populations’ and their ‘collective behaviour’ is perhaps a bridge too far. Life is characterised by sustained metabolism and ability of self-propagation. MPE and colloidosomes display neither.

But whatever your interests and opinions about the origins of life may be, a look at the article’s supplementary materials, containing videos of the MPE behaviour, is highly recommended – you’ll be fascinated.

Last Changed: 26.06.2017