Biohybrid microswimmer

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.).

Over the past decade, biohybrid microrobots, in which living mobile microorganisms are physically integrated with untethered artificial structures, have gained growing interest to enable the active locomotion and cargo delivery to a target destination.

In addition to the motility, the intrinsic capabilities of sensing and eliciting an appropriate response to artificial and environmental changes make cell-based biohybrid microrobots appealing for transportation of cargo to the inaccessible cavities of the human body for local active delivery of diagnostic and therapeutic agents.

[2][3] 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.

[4] 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.

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.

First, an essential concept, popularized by Osborne Reynolds, is that the relative importance of inertia and viscosity for the motion of a fluid depends on certain details of the system under consideration.

This cannot be effective for microscale swimmers like bacteria: due to the large viscous damping, the inertial coasting time of a micron-sized object is on the order of 1 μs.

Purcell concluded that only forces that are exerted in the present moment on a microscale body contribute to its propulsion, so a constant energy conversion method is essential.

As a further consequence of the continuous dissipation of energy, biological and artificial microswimmers do not obey the laws of equilibrium statistical physics, and need to be described by non-equilibrium dynamics.

[11][12][3] Over the past decade, biohybrid microrobots, in which living mobile microorganisms are physically integrated with untethered artificial structures, have gained growing interest to enable the active locomotion and cargo delivery to a target destination.

[13][14][15][16] In addition to the motility, the intrinsic capabilities of sensing and eliciting an appropriate response to artificial and environmental changes make cell-based biohybrid microrobots appealing for transportation of cargo to the inaccessible cavities of the human body for local active delivery of diagnostic and therapeutic agents.

[17][18][19] Active locomotion, targeting and steering of concentrated therapeutic and diagnostic agents embedded in mobile microrobots to the site of action can overcome the existing challenges of conventional therapies.

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.

The proxemics will then favour enhanced communication between neighbouring individuals via transduction of energy, leading to dynamic and complex synergetic behaviours of the composite powered structure.

[67] 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.).

[72] 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.

[75] 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) exhibiting a mesoporous structure (pore size in the range of 4 nm).

[78] 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.

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.

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.

[61] Biohybrid microswimmers, mainly composed of integrated biological actuators and synthetic cargo carriers, have recently shown promise toward minimally invasive theranostic applications.

[86][87][88][22] Various microorganisms, including bacteria,[23][28] microalgae,[89][19] and spermatozoids,[90][91] have been utilised to fabricate different biohybrid microswimmers with advanced medical functionalities, such as autonomous control with environmental stimuli for targeting, navigation through narrow gaps, and accumulation to necrotic regions of tumor environments.

[99][100][101][102][103][104] The presence of different bacteria species in the human body, such as on the skin and the gut microenvironment, has promoted their use as potential theranostic agents or carriers in several medical applications.

[111][112][113][114][115] For instance, decreased recognition of drug-loaded particles by immune cells was shown when attached to membranes of the RBCs prior to intravenous injection into mice.

[117] It was also reported that the half-life of Fasudil, a drug for pulmonary arterial hypertension, inside the body increased approximately sixfold to eightfold when it was loaded into nanoerythrosomes.

Basic features of an in vivo microrobot [ 2 ]
In a biohybrid approach, all three of these basic features can be realised either biologically by a microorganism, or artificially by synthetic attachments. Blue indicates biological entities (flagellated or target cells), red indicates artificial structures (attached tubes, helices, particles, or external devices). Arrows in the upper left panel indicate the motile actor, wave lines in the upper right panel indicate signal pathways. The lower panel shows how functionalities can be carried out based on cell-cell interactions or by synthetic cargo (red particles).
Biohybrid Chlamydomonas reinhardtii microswimmers [ 31 ]
Top: Schematics of production steps for biohybrid C. reinhardtii .
Bottom: SEM images of bare microalgae (left) and biohybrid microalgae (right) coated with chitosan-coated iron oxide nanoparticles (CSIONPs). Images were pseudocolored. A darker green color on the right SEM image represents chitosan coating on microalgae cell wall. Orange-colored particles represents CSIONPs.
Robocolith hybrids combining polydopamine and coccoliths [ 61 ]
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 [ 61 ]
(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.
Emiliania huxleyi protected with asymmetric coccoliths
Biohybrid bacterial microswimmers [ 83 ]
Biohybrid diatomite microswimmer drug delivery system
Diatom frustule surface functionalised with photoactivable molecules (orange spheres) linked to vitamin B-12 (red sphere) acting as a tumor-targeting tag. The system can be loaded with chemotherapeutic drugs (light blue spheres), which can be selectively delivered to colorectal cancer cells. In addition, diatomite microparticles can be photoactivated to generate carbon monoxide or free radicals inducing tumor cell apoptosis. [ 84 ] [ 85 ]