Biohybrid microswimmer

One of the most fundamental questions in science is what defines life. Collective motion is one of the hallmarks of life. This is commonly observed in nature at various dimensional levels as energized entities gather, in a concerted effort, into motile aggregated patterns. These motile aggregated events can be noticed, among many others, as dynamic swarms; e.g., unicellular organisms such as bacteria, locust swarms, or the flocking behaviour of birds.

Hydrid swarming coccoliths

Robocolith hybrids combining polydopamine and coccoliths 

EHUX coccolithophores are cultivated for isolation of coccoliths. When coccoliths (asymmetric morphology) are exposed to light, no collective motion is observed. Coccoliths are then mixed gently with dopamine solutions. Thus, polydopamine-coated coccoliths hybrids are obtained as a basis for design of Robocoliths. Light excitation and the asymmetry of Robocoliths generates a thermal flux of heat because of polydopamine’s photothermal properties. Coupling of convection from neighboring Robocoliths transforms their movement into an aggregated collective motion. Robocolith functionalization is also proposed to prevent and control nonspecific attachment of biomacromolecules and possible diminution of the aggregation.

Asymmetric architecture of coccolith morphology 

(A) EHUX coccolithophores were cultivated successfully and visualized by SEM (scale bar, 4 μm).
(B) Following this, we broke and removed the cellular material from EHUX coccolithophores to isolate multiple (top; scale bar, 20 μm) and individual (bottom; scale bar, 1 μm) coccoliths, as visualized by SEM.
(C) AFM image of an individual coccolith. Micrograph size, 4 × 4 μm.
(D) AFM magnification the micrograph of an individual coccolith. Scale bar, 400 nm.
(E) Illustration of a coccolith, depicting its specific morphological parameters.
(F) Typical plotted values of the specific morphological parameters. Data are represented as mean ± SD (n = 55), where n is the number of coccoliths visualized by TEM.

One of the most fundamental questions in science is what defines life. Collective motion is one of the hallmarks of life. This is commonly observed in nature at various dimensional levels as energized entities gather, in a concerted effort, into motile aggregated patterns. These motile aggregated events can be noticed, among many others, as dynamic swarms; e.g., unicellular organisms such as bacteria, locust swarms, or the flocking behaviour of birds. In contrast to what is accomplished individually, multiple entities enable local interactions between each participant to occur in proximity. If we consider each participant in the collective behavior as a (bio)physical transducer, then the energy will be converted from one type into another. The proxemics will then favor enhanced communication between neighboring individuals via transduction of energy, leading to dynamic and complex synergetic behaviors of the composite powered structure.

Over recent years, fascinating nano- and mesoscopic objects have been designed to collectively move through direct inspiration from nature or by harnessing its existing tools. Such robotic swarms were categorized by an online expert panel as among the 10 great unresolved group challenges in the area of robotics. With this in mind, nano- and mesoscopic objects capable of swarming require immediate scientific attention. Although investigation of their underlying mechanism of action is still in its infancy, various systems have been developed that are capable of undergoing controlled and uncontrolled swarming motion by harvesting energy (e.g., light, thermal, etc.). Importantly, this energy should be transformed into a net force for the system to move. When the systems of interest are mainly small (meso- to nanoscopic), then their motion, typically at low Reynolds numbers (Re ≪ 1), becomes a very challenging concept. Nevertheless, this demands breaking the symmetry of the system for the locomotion to occur.14 Furthermore, collective motion requires a coupling effect between the objects that are part of the population.

In this work, the aim is to identify the minimum requirements to design miniature robots (microrobots) with swarming behavior. Therefore, to develop a nano/mesoscopic object capable of swarming behavior, we hypothesize that this object simultaneously fulfills these requirements: (1) it is characterized by broken symmetry with a well-defined morphology, and (2) it is functionalized with a material capable of harvesting energy. If the harvested energy results in a field surrounding the object, and this field can couple with the field of a neighboring object, then the collective behavior will be correlated.

Emiliania huxleyi (EHUX) coccolithophore-derived asymmetric coccoliths stand out as candidates for the choice of a nano/mesoscopic object with broken symmetry and well-defined morphology. Besides the thermodynamical stability because of their calcite composition, the critical advantage of EHUX coccoliths is their distinctive and sophisticated asymmetric morphology. EHUX coccoliths are characterized by several hammer-headed ribs placed to form a proximal and distal disc connected by a central ring. These discs have different sizes but also allow the coccolith to have a curvature, partly resembling a wagon wheel. EHUX coccoliths can be isolated from EHUX coccolithophores, a unique group of unicellular marine algae that are the primary producers of biogenic calcite in the ocean. Coccolithophores can intracellularly produce intricate three-dimensional mineral structures, such as calcium carbonate scales (i.e., coccoliths), in a process that is driven continuously by a specialized vesicle. After the process is finished, the formed coccoliths are secreted to the cell surface, where they form the exoskeleton (i.e., coccosphere). The broad diversity of coccolith architecture results in further possibilities for specific applications in nanotechnology  or biomedicine. Inanimate coccoliths from EHUX live coccolithophores, in particular, can be isolated easily in the laboratory with a low culture cost and fast reproductive rate and have a reasonably moderate surface area (∼20 m2 g−1) exhibiting a mesoporous structure (pore size in the range of 4 nm).

Presumably, if harvesting of energy is done on both sides of the EHUX coccolith, then it will allow generation of a net force, which means movement in a directional manner. Coccoliths have immense potential for a multitude of applications, but to enable harvesting of energy, their surface properties must be finely tuned. Inspired by the composition of adhesive proteins in mussels, dopamine self-polymerization into polydopamine is currently the most versatile functionalization strategy for virtually all types of materials. Because of its surface chemistry and wide range of light absorption properties, polydopamine is an ideal choice for aided energy harvesting function on inert substrates. In this work, we aim to exploit the benefits of polydopamine coating to provide advanced energy harvesting functionalities to the otherwise inert and inanimate coccoliths. Polydopamine (PDA has already been shown to induce movement of polystyrene beads because of thermal diffusion effects between the object and the surrounding aqueous solution of up to 2°C under near-infrared (NIR) light excitation. However, no collective behavior has been reported. Here, we prove, for the first time, that polydopamine can act as an active component to induce, under visible light (300–600 nm), collective behavior of a structurally complex, natural, and challenging-to-control architecture such as coccoliths. As a result, the organic-inorganic hybrid combination (coccolith-polydopamine) would enable design of Robocoliths.

Dopamine polymerization proceeds in a solution, where it forms small colloidal aggregates that adsorb on the surface of the coccoliths, forming a confluent film. This film is characterized by high roughness, which translates into a high specific surface area and enhanced harvesting of energy. Because of the conjugated nature of the polymer backbone, polydopamine can absorb light over a broad electromagnetic spectrum, including the visible region.

As a result, the surface of coccoliths is endowed with a photothermal effect, locally heating and creating convection induced by the presence of PDA. This local convection is coupled with another nearby local convection, which allows coupling between individual Robocoliths, enabling their collective motion (Figure 1).

Therefore, when the light encounters the anisometric Robocoliths, they heat locally because of the photothermal conversion induced by the presence of PDA on their surface. The intense local heating produces convection that is different on either side of the Robocolith, causing its observed movement. Such convection can couple with the convection of a neighboring Robocolith, resulting in a “swarming” motion. In addition, the surface of Robocoliths is engineered to accommodate antifouling polymer brushes and potentially prevent their aggregation. Although a primary light-activated convective approach is taken as a first step to understand the motion of Robocoliths, a multitude of mechanistic approaches are currently being developed to pave the way for the next generation of multifunctional Robocoliths as swarming bio-micromachines.