Biohybrid microswimmer

Biohybrid microswimmers can be defined as microswimmers that consist of both biological and artificial constituents, for instance, one or several living microorganisms attached to one or various synthetic parts.

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.

Bacterial biohybrids

See also: Protist locomotion § Biohybrid microswimmers

Bacteria-driven biohybrid microswimmers with a spherical body 

(a) SEM images showing 2 µm diameter polystyrene microbeads, each attached by a few E. coli bacteria
(b) An illustration of the forces and torques exerted on the spherical microbead by its attached bacteria, where the force and the motor reaction torque of each bacterium are state dependent.

Biohybrid microswimmers can be defined as microswimmers that consist of both biological and artificial constituents, for instance, one or several living microorganisms attached to one or various synthetic parts. The pioneers of this field, ahead of their time, were Montemagno and Bachand with a 1999 work regarding specific attachment strategies of biological molecules to nanofabricated substrates enabling the preparation of hybrid inorganic/organic nanoelectromechanical systems, so called NEMS. They described the production of large amounts of F1-ATPase from the thermophilic bacteria Bacillus PS3 for the preparation of F1-ATPase biomolecular motors immobilized on a nanoarray pattern of gold, copper or nickel produced by electron beam lithography. These proteins were attached to one micron microspheres tagged with a synthetic peptide. Consequently, they accomplished the preparation of a platform with chemically active sites and the development of biohybrid devices capable of converting energy of biomolecular motors into useful work.

Recent publications of biohybrid microswimmers include the use of sperm cells, contractive muscle cells, and bacteria as biological components, as they can efficiently convert chemical energy into movement, and additionally are capable of performing complicated motion depending on environmental conditions. In this sense, biohybrid microswimmer systems can be described as the combination of different functional components: cargo and carrier. The cargo is an element of interest to be moved (and possibly released) in a customized way. The carrier is the component responsible for the movement of the biohybrid, transporting the desired cargo, which is linked to its surface. The great majority of these systems rely on biological motile propulsion for the transportation of synthetic cargo for targeted drug delivery/ There are also examples of the opposite case: artificial microswimmers with biological cargo systems.

Artificial micro and nanoswimmers are small scale devices that convert energy into movement. Since the first demonstration of their performance in 2002, the field has developed rapidly in terms of new preparation methodologies, propulsion strategies, motion control, and envisioned functionality. The field holds promise for applications such as drug delivery, environmental remediation and sensing. The initial focus of the field was largely on artificial systems, but an increasing number of "biohybrids" are appearing in the literature. Combining artificial and biological components is a promising strategy to obtain new, well-controlled microswimmer functionalities, since essential functions of living organisms are intrinsically related to the capability to move. Living beings of all scales move in response to environmental stimuli (e.g., temperature or pH), to look for food sources, to reproduce, or to escape from predators. One of the more well-known living microsystems are swimming bacteria, but directed motion occurs even at the molecular scale, where enzymes and proteins undergo conformational changes in order to carry out biological tasks.

Swimming bacterial cells have been used in the development of hybrid microswimmers. Cargo attachment to the bacterial cells might influence their swimming behavior. Bacterial cells in the swarming state have also been used in the development of hybrid microswimmers. Swarming Serratia marcescens cells were transferred to PDMS-coated coverslips, resulting in a structure referred to as a "bacterial carpet" by the authors. Differently shaped flat fragments of this bacterial carpets, termed "auto-mobile chips", moved above the surface of the microscope slide in two dimensions. Many other works have used Serratia marcescens swarming cells, as well as E. coli swarming cells  for the development of hybrid microswimmers. Magnetotactic bacteria have been the focus of different studies due to their versatile uses in biohybrid motion systems.

Swarming coccolith biohybrids

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.