The line between life and non-life is becoming increasingly blurred. We use biological molecules as molecular machines and even as a base of computing. On the other side of the spectrum, inorganic materials are constructed to display a cell-like behaviour.
But there’s a gap between these organisation levels and the most primitive living cells capable of matter and energy exchange with the environment (metabolism) and reproduction. Experiments that bridge the gap between complex inorganic and a living cell brings us closer to understanding how life came to be.
Two consequent of optical microscopy images showing spontaneous transfer of silica colloidosome (red object, dotted line) into a magnetic droplet through a fatty acid stabilized aperture. Scale bar = 100 µm. (Image by University of Bristol).
Magnetite + organic solvent =
The authors of a recent article in Nature materials (Rodrigues-Arco et al. (2017), DOI: 10.1038/NMAT 4916) tried to bridge the gap between inorganic and organic. They mixed magnetic particles of iron oxide (magnetite) with droplets of an organic solvent and water. The particles with diameter of 500 ± 250 µm self-assembled on the surface of the solvent and were stable for several weeks.
Applying a magnetic field to the magnetic droplets opened the spheres along the surface, but they didn’t lose structural integrity and returned to the spherical shape.
Magnetite droplets open and close on magnetic field application (Image by Fr4zer via Wikimedia Commons)
Increasing pH of the water phase to 10.5 and oleate concentration led to the creation of Matryoshka structures: parts of droplets remained covered by magnetic nanoparticles, and the rest of the structure was bordered by a monolayer of organic molecules. At an optimal concentration, 3/4 magnetite with 1/4 water surface droplets resembled the hero of the classic game, Packman in appearance and behaviour.
Under the same conditions, silica particles form smaller spheres, 50 ± 20 µm in diameter. Silica colloids mixed with magnetic spheres do not interact in the absence of oleic acid. However, applying a magnetic field to the mix opened apertures in the magnetic spheres led to their self-propelled movement and random engulfment of colloidosomes. Only spheres with apertures – Packmen – were able to move. The authors call this the engulfment ‘phagocytic-like behavior’ after ameba-like white blood cells that eat bacteria.
Particles movement explained by Marangoni effect – a movement due to surface tension gradient because of the uneven distribution of oleate on the surface of magnetic particles. As the oleate gradient dissipate, the droplets moved only for several seconds.
The authors proposed a model of ‘phagocytosis’. Non-magnetite covered surface Packman aperture covered by oleic acid molecules acts as a single layer proto-membrane. The silica colloidosomes have this layer as well. Fusion of molecular layers on the surface of Packmen aquatic opening and release of colloidosomes into the inner space creates semi-double membrane particles.
See the pictures and models on Nature web-site. If by some miracle you have access to the original paper, do look into the supplemental videos of Packmen moving, Packmen ‘eating’, it’s mesmerising.
What is it good for
The authors propose using composite droplets mixtures for development of new material and nanoscale engineering approach, for example in microfluidics and delivering reagents for spatially controlled reactions. This sounds plausible and the author’s intention to mimic predation and chemical communication even more interesting.
However, the scientists also call droplets ‘protocells’ and talk about ‘populations’ and their ‘collective behaviour’. In my opinion, this is a bridge too far. Life is characterised by sustained metabolism and ability of self-propagation. Magnetite and silica droplets display neither. The reports about creating synthetic life is overhyping, the chronic disease of modern science.