Chemical gradients as directional controls for autonomous protocell motors

Enzyme-coated liposomal motors show either positive or negative chemotaxis depending on the interplay between enzyme catalysis and solute–phospholipid interactions.

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Motility is ubiquitous in biology: The ability to move towards or away from chemical signals is critical for the survival of living organisms. Directional migration enables living beings to move towards food, escape away from toxins, coordinate collective behavior, and transport cargo. As we approach the micro and the nanoscale within our body, we observe the importance of transport and migration in chemical/ biochemical processes that underlie cellular functions. Most of the transport that arises is due to gradients of chemical species and is termed chemotaxis. Inspired by nature and biology, scientists have created a variety of synthetic materials that can exhibit directional migration based on inorganic (metals) and biochemical (enzymes) catalysts. These are called “nanomotors” since they are very small and they convert energy from one form to another (e.g., chemical energy into mechanical motion). Previous studies on nanomotors, have shown transport of catalysts, as well as catalyst-coated particles in the positive chemotactic direction (i.e., towards specific chemicals).

In our new article in Nature Nanotechnology, along with positive chemotaxis, we also show negative chemotaxis by enzyme-coated soft nanomotors, away from specific chemicals. Soft materials, such as liposomes have recently come under the spotlight as motors due to their high biocompatibility. They have the potential to be cargo delivery vehicles within the body and being structurally similar to membranes of natural cells, they provide platforms to study cellular functions and biochemical pathways in confined environments. We show the positive chemotaxis of catalase-coated liposomes, negative chemotaxis of urease-coated liposomes and tunable chemotaxis of ATPase-bound liposomes.  

In trying to explain why different enzyme-attached liposomes behave differently, we examined various potential transport mechanisms for the observed negative chemotaxis. In doing so, we ruled out existing mechanisms of particle transport, paving the way for a new and previously unrecognized extension of the Hofmeister series: solute-phospholipid interaction-based negative chemotaxis. More details about the mechanism can be found in our paper “Positive and negative chemotaxis of enzyme-coated liposome motors” published in Nature nanotechnology, DOI: 10.1038/s41565-019-0578-8.

We believe this work represents a major step in providing an additional level of control on the direction of transport of autonomous nanomotors. Soft nanocapsules with programmable directional migration can serve as cell mimics and cargo delivery vehicles. Autonomous nano and microscale machines and self-propelled vehicles have long captured human imagination. We hope we are getting closer and closer to be able to finally “swallow the surgeon” as suggested by physicist Richard Feynman.

Ambika Somasundar

PhD candidate, Pennsylvania State University