Motility of Enzyme-Powered Vesicles

Chemomechanical Communication between Liposomes Based on Enzyme Cascades

Communication between cells is crucial to the survival of both uni- and multicellular organisms. The primary mode of communication involves chemical cues. There is great current interest in mimicking this behavior in synthetic cells to understand the physical basis of intercellular communication and design collective functional behavior. Using liposomal cell mimics, we demonstrate how a chemical input can elicit a mechanical response (enhanced motility). We employed a single substrate to trigger enzyme cascade-induced control of the diffusion of up to three different liposome populations. Furthermore, substrate competition allows temporal control over enhanced diffusion. The use of enzyme cascades to propagate chemical signals provides a robust and efficient mechanism for diverse populations of protocells to coordinate their motion in response to signals from each other.

Activity-induced diffusion recovery in crowded colloidal suspensions

We show that the forces generated by active enzyme molecules are strong enough to influence the dynamics of their surroundings under artificial crowded environments. We measured the behavior of polymer microparticles in a quasi-two-dimensional system under aqueous environment, at various area fraction values of particles. In the presence of enzymatic activity, not only was the diffusion of the suspended particles enhanced at shorter time-scales, but the system also showed a transition from subdiffusive to diffusive dynamics at longer time-scale limits. Similar observations were also recorded with enzyme-functionalized microparticles. Brownian dynamics simulations have been performed to support the experimental observations.

Enzymatic micro/nanomotors in biomedicine: from single motors to swarms

Micro/nanomotors (MNMs) have evolved from single self-propelled entities to versatile systems capable of performing one or multiple biomedical tasks. When single MNMs self-assemble into coordinated swarms, either under external control or triggered by chemical reactions, they offer advantages that individual MNMs cannot achieve. These benefits include intelligent multitasking and adaptability to changes in the surrounding environment. Here, we provide our perspective on the evolution of MNMs, beginning with the development of enzymatic MNMs since the first theoretical model was proposed in 2005. These enzymatic MNMs hold immense promise in biomedicine due to their advantages in biocompatibility and fuel availability. Subsequently, we introduce the design and application of single motors in biomedicine, followed by the control of MNM swarms and their biomedical applications. In the end, we propose viable solutions for advancing the development of MNM swarms and anticipate valuable insights into the creation of more intelligent and controllable MNM swarms for biomedical applications.

Lipid vesicle-based molecular robots

A molecular robot, which is a system comprised of one or more molecular machines and computers, can execute sophisticated tasks in many fields that span from nanomedicine to green nanotechnology. The core parts of molecular robots are fairly consistent from system to system and always include (i) a body to encapsulate molecular machines, (ii) sensors to capture signals, (iii) computers to make decisions, and (iv) actuators to perform tasks. This review aims to provide an overview of approaches and considerations to develop molecular robots. We first introduce the basic technologies required for constructing the core parts of molecular robots, describe the recent progress towards achieving higher functionality, and subsequently discuss the current challenges and outlook. We also highlight the applications of molecular robots in sensing biomarkers, signal communications with living cells, and conversion of energy. Although molecular robots are still in their infancy, they will unquestionably initiate massive change in biomedical and environmental technology in the not too distant future.

Motility of Synthetic Cells from Engineered Lipids

Synthetic cells are artificial systems that resemble natural cells. Significant efforts have been made over the years to construct synthetic protocells that can mimic biological mechanisms and perform various complex processes. These include compartmentalization, metabolism, energy supply, communication, and gene reproduction. Cell motility is also of great importance, as nature uses elegant mechanisms for intracellular trafficking, immune response, and embryogenesis. In this review, we discuss the motility of synthetic cells made from lipid vesicles and relevant molecular mechanisms. Synthetic cell motion may be classified into surface-based or solution-based depending on whether it involves interactions with surfaces or movement in fluids. Collective migration behaviors have also been demonstrated. The swarm motion requires additional mechanisms for intercellular signaling and directional motility that enable communication and coordination among the synthetic vesicles. In addition, intracellular trafficking for molecular transport has been reconstituted in minimal cells with the help of DNA nanotechnology. These efforts demonstrate synthetic cells that can move, detect, respond, and interact. We envision that new developments in protocell motility will enhance our understanding of biological processes and be instrumental in bioengineering and therapeutic applications.

An acoustically controlled helical microrobot

As a next-generation toolkit, microrobots can transform a wide range of fields, including micromanufacturing, electronics, microfluidics, tissue engineering, and medicine. While still in their infancy, acoustically actuated microrobots are becoming increasingly attractive. However, the interaction of acoustics with microstructure geometry is poorly understood, and its study is necessary for developing next-generation acoustically powered microrobots. We present an acoustically driven helical microrobot with a length of 350 μm and a diameter of 100 μm that is capable of locomotion using a fin-like double-helix microstructure. This microrobot responds to sound stimuli at ~12 to 19 kHz and mimics the spiral motion of natural microswimmers such as spirochetes. The asymmetric double helix interacts with the incident acoustic field, inducing a propulsion torque that causes the microrobot to rotate around its long axis. Moreover, our microrobot has the unique feature of its directionality being switchable by simply tuning the acoustic frequency. We demonstrate this locomotion in 2D and 3D artificial vasculatures using a single sound source.

Advances in giant unilamellar vesicle preparation techniques and applications

Giant unilamellar vesicles (GUVs) are versatile and promising cell-sized bio-membrane mimetic platforms. Their applications range from understanding and quantifying membrane biophysical processes to acting as elementary blocks in the bottom-up assembly of synthetic cells. Definite properties and requisite goals in GUVs are dictated by the preparation techniques critical to the success of their applications. Here, we review key advances in giant unilamellar vesicle preparation techniques and discuss their formation mechanisms. Developments in lipid hydration and emulsion techniques for GUV preparation are described. Novel microfluidic-based techniques involving lipid or surfactant-stabilized emulsions are outlined. GUV immobilization strategies are summarized, including gravity-based settling, covalent linking, and immobilization by microfluidic, electric, and magnetic barriers. Moreover, some of the key applications of GUVs as biomimetic and synthetic cell platforms during the last decade have been identified. Membrane interface processes like phase separation, membrane protein reconstitution, and membrane bending have been deciphered using GUVs. In addition, vesicles are also employed as building blocks to construct synthetic cells with defined cell-like functions comprising compartments, metabolic reactors, and abilities to grow and divide. We critically discuss the pros and cons of preparation technologies and the properties they confer to the GUVs and identify potential techniques for dedicated applications.

Enzymatic Janus Liposome Micromotors

A liposome-based micromotor system that utilizes regional enzymatic conversion and gas generation to achieve directional motion in water is presented. Constituted mainly of a low-melting lipid and a high-melting lipid together with cholesterol, these liposomes maintain stable Janus configuration at room temperature as a result of lipid liquid-liquid phase separation. Local placement of enzymes such as horseradish peroxidase is realized via affinity binding between avidin and biotin, the latter as a lipid conjugate sorted specifically into one domain of these Janus liposomes as a minor component. In the presence of the substrate, hydrogen peroxide, these enzyme-decorated Janus liposomes undergo directional motion, yielding velocities exceeding thermal diffusion by three folds in some cases. Experimental details on liposome size control, motor assembly, and substrate distribution are presented; effects of key experimental factors on liposome motion, such as substrate concentration and liposome Janus ratio, are also examined. This work thus provides a viable approach to building asymmetrical lipid-assembled, enzyme-attached colloids and, in addition, stresses the importance of asymmetry in achieving particle directional motion.

Micromotor Based Mini-Tablet for Oral Delivery of Insulin

Diabetes is a metabolic disorder characterized by hyperglycemia due to defective insulin secretion or its biological dysfunction. However, frequent subcutaneous injection of insulin often results in discomfort and local tissue infection. Herein, we demonstrate the successful fabrication of a mini-tablet system based on self-propelled micromotors with biocompatibility and biodegradability for oral colon administration of insulin. The insulin layer is first constructed onto the surface of a magnesium based micromotor via electrostatic interactions, followed by a tableting process. The resulting mini-tablets are then coated with esterified starch with colonic degradation capability, thus achieving controlled release of the embedded micromotors in the colon region. In the meantime, autonomous movement of the released micromotors with a speed up to 76.22 μm·s^-1 further results in enhanced colonic uptake and absorption of insulin, realizing long-term control of blood glucose for more than 5 h. Our micromotor based mini-tablet system can not only broaden the biomedical applications of emerging self-propelled micromotors but also offer an appealing strategy for oral administration of biomacromolecular drugs represented by insulin.

Enzyme-Free Liposome Active Motion via Asymmetrical Lipid Efflux

As a class of biocompatible, water-dispersed colloids, liposomes have found widespread applications ranging from food to drug delivery. Adding mobility to these colloids, i.e., liposome micromotors, represents an attractive approach to next-generation liposome carriers with enhanced functionality and effectiveness. Currently, it remains unclear as to the scope of material features useful for building liposome micromotors or how they may differ functionally from their inorganic/polymer counterparts. In this work, we demonstrate liposome active motion taking advantage of mainly a pair of intrinsic material properties associated with these assemblies: lipid phase separation and extraction. We show that global phase separation of ternary lipid systems (such as DPPC/DOPC/cholesterol) within individual liposomes yields stable Janus particles with two distinctive liquid domains. While these anisotropic liposomes undergo pure Brownian diffusion in water, similar to their homogeneous analogues, adding extracting agents, cyclodextrins, to the system triggers asymmetrical cholesterol efflux about the liposomes, setting the latter into active motion. We present detailed analyses of liposome movement and cholesterol extraction kinetics to establish their correlation. We explore various experimental parameters as well as mechanistic details to account for such motion. Our results highlight the rich possibility to hierarchically design lipid-based artificial motors, from individual lipids, to their organization, surface chemistry, and interfacial mechanics.

Enzyme-powered micro- and nano-motors: key parameters for an application-oriented design

Nature has inspired the creation of artificial micro- and nanomotors that self-propel converting chemical energy into mechanical action. These tiny machines have appeared as promising biomedical tools for treatment and diagnosis and have also been used for environmental, antimicrobial or sensing applications. Among the possible catalytic engines, enzymes have emerged as an alternative to inorganic catalysts due to their biocompatibility and the variety and bioavailability of fuels. Although the field of enzyme-powered micro- and nano-motors has a trajectory of more than a decade, a comprehensive framework on how to rationally design, control and optimize their motion is still missing. With this purpose, herein we performed a thorough bibliographic study on the key parameters governing the propulsion of these enzyme-powered devices, namely the chassis shape, the material composition, the motor size, the enzyme type, the method used to incorporate enzymes, the distribution of the product released, the motion mechanism, the motion media and the technique used for motion detection. In conclusion, from the library of options that each parameter offers there needs to be a rational selection and intelligent design of enzymatic motors based on the specific application envisioned.

Confined Motion: Motility of Active Microparticles in Cell-Sized Lipid Vesicles

Active materials can transduce external energy into kinetic energy at the nano and micron length scales. This unique feature has sparked much research, which ranges from achieving fundamental understanding of their motility to the assessment of potential applications. Traditionally, motility is studied as a function of internal features such as particle topology, while external parameters such as energy source are assessed mainly in bulk. However, in real-life applications, confinement plays a crucial role in determining the type of motion active particles can adapt. This feature has been however surprisingly underexplored experimentally. Here, we showcase a tunable experimental platform to gain an insight into the dynamics of active particles in environments with restricted 3D topology. Particularly, we examined the autonomous motion of coacervate micromotors confined in giant unilamellar vesicles (GUVs) spanning 10–50 μm in diameter and varied parameters including fuel and micromotor concentration. We observed anomalous diffusion upon confinement, leading to decreased motility, which was more pronounced in smaller compartments. The results indicate that the theoretically predicted hydrodynamic effect dominates the motion mechanism within this platform. Our study provides a versatile approach to understand the behavior of active matter under controlled, compartmentalized conditions.

Fluorescent Microswimmers Based on Cross-β Amyloid Nanotubes and Divergent Cascade Networks

Shaped through millions of years of evolution, the spatial localization of multiple enzymes in living cells employs extensive cascade reactions to enable highly coordinated multimodal functions. Herein, by utilizing a complex divergent cascade, we exploit the catalytic potential as well as templating abilities of streamlined cross-β amyloid nanotubes to yield two orthogonal roles simultaneously. The short peptide based paracrystalline nanotube surfaces demonstrated the generation of fluorescence signals within entangled networks loaded with alcohol dehydrogenase (ADH). The nanotubular morphologies were further used to generate cascade-driven microscopic motility through surface entrapment of sarcosine oxidase (SOX) and catalase (Cat). Moreover, a divergent cascade network was initiated by upstream catalysis of the substrate molecules through the surface mutation of catalytic moieties. Notably, the resultant downstream products led to the generation of motile fluorescent microswimmers by utilizing the two sets of orthogonal properties and, thus, mimicked the complex cascade-mediated functionalities of extant biology.

Enzyme Purification Improves the Enzyme Loading, Self-Propulsion, and Endurance Performance of Micromotors

Enzyme-powered micro- and nanomotors make use of biocatalysis to self-propel in aqueous media and hold immense promise for active and targeted drug delivery. Most (if not all) of these micro- and nanomotors described to date are fabricated using a commercially available enzyme, despite claims that some commercial preparations may not have a sufficiently high degree of purity for downstream applications. In this study, the purity of a commercial urease, an enzyme frequently used to power the motion of micro- and nanomotors, was evaluated and found to be impure. After separating the hexameric urease from the protein impurities by size-exclusion chromatography, the hexameric urease was subsequently characterized and used to functionalize hollow silica microcapsules. Micromotors loaded with purified urease were found to be 2.5 times more motile than the same micromotors loaded with unpurified urease, reaching average speeds of 5.5 μm/s. After comparing a number of parameters, such as enzyme distribution, protein loading, and motor reusability, between micromotors functionalized with purified vs unpurified urease, it was concluded that protein purification was essential for optimal performance of the enzyme-powered micromotor.

Perspective: a stirring role for metabolism in cells

Based on recent findings indicating that metabolism might be governed by a limit on the rate at which cells can dissipate Gibbs energy, in this Perspective, we propose a new mechanism of how metabolic activity could globally regulate biomolecular processes in a cell. Specifically, we postulate that Gibbs energy released in metabolic reactions is used to perform work, allowing enzymes to self‐propel or to break free from supramolecular structures. This catalysis‐induced enzyme movement will result in increased intracellular motion, which in turn can compromise biomolecular functions. Once the increased intracellular motion has a detrimental effect on regulatory mechanisms, this will establish a feedback mechanism on metabolic activity, and result in the observed thermodynamic limit. While this proposed explanation for the identified upper rate limit on cellular Gibbs energy dissipation rate awaits experimental validation, it offers an intriguing perspective of how metabolic activity can globally affect biomolecular functions and will hopefully spark new research.

Displacement Statistics of Unhindered Single Molecules Show no Enhanced Diffusion in Enzymatic Reactions

Recent studies have sparked debate over whether catalytic reactions enhance the diffusion coefficients D of enzymes. Through high statistics of the transient (600 μs) displacements of unhindered single molecules freely diffusing in common buffers, we here quantify D for four enzymes under catalytic turnovers. We thus formulate how ∼ ±1% precisions may be achieved for D, and show no changes in diffusivity for catalase, urease, aldolase, and alkaline phosphatase under the application of wide concentration ranges of substrates. Our single-molecule approach thus overcomes potential limitations and artifacts underscored by recent studies to show no enhanced diffusion in enzymatic reactions.

Synthetic cells in biomedical applications

Synthetic cells are engineered vesicles that can mimic one or more salient features of life. These features include directed localization, sense-and-respond behavior, gene expression, metabolism, and high stability. In nanomedicine, many of these features are desirable capabilities of drug delivery vehicles but are difficult to engineer. In this focus article, we discuss where synthetic cells offer unique advantages over nanoparticle and living cell therapies. We review progress in the engineering of the above life-like behaviors and how they are deployed in nanomedicine. Finally, we assess key challenges synthetic cells face before being deployed as drugs and suggest ways to overcome these challenges. This article is categorized under: Therapeutic Approaches and Drug Discovery > Emerging Technologies Biology-Inspired Nanomaterials > Lipid-Based Structures.

Synthetic Cells: From Simple Bio-Inspired Modules to Sophisticated Integrated Systems

Bottom‐up synthetic biology is the science of building systems that mimic the structure and function of living cells from scratch. To do this, researchers combine tools from chemistry, materials science, and biochemistry to develop functional and structural building blocks to construct synthetic cell‐like systems. The many strategies and materials that have been developed in recent decades have enabled scientists to engineer synthetic cells and organelles that mimic the essential functions and behaviors of natural cells. Examples include synthetic cells that can synthesize their own ATP using light, maintain metabolic reactions through enzymatic networks, perform gene replication, and even grow and divide. In this Review, we discuss recent developments in the design and construction of synthetic cells and organelles using the bottom‐up approach. Our goal is to present representative synthetic cells of increasing complexity as well as strategies for solving distinct challenges in bottom‐up synthetic biology.

Construction of protocell-based artificial signal transduction pathways

The maintenance of an orderly and controllable multicellular society depends on the communication and signal regulation between various types of biological cells. How to replicate complicated signal transduction pathways in synthetic protocellular communities remains a key challenge in bottom-up synthetic biology. Herein, we review recent advances in the design and construction of interactive protocell communities, or protocell communities and biological communities, and explore the ways of designing and constructing artificial paracrine-like signaling pathways and juxtacrine-like signaling pathways. Key molecules involved in the signaling pathways that can be used to connect two or more spatially separated communities, and diverse signal outputs generated by the communication are summarized. We also propose the limitations, challenges and opportunities in this field.

Engineering transient dynamics of artificial cells by stochastic distribution of enzymes

Random fluctuations are inherent to all complex molecular systems. Although nature has evolved mechanisms to control stochastic events to achieve the desired biological output, reproducing this in synthetic systems represents a significant challenge. Here we present an artificial platform that enables us to exploit stochasticity to direct motile behavior. We found that enzymes, when confined to the fluidic polymer membrane of a core-shell coacervate, were distributed stochastically in time and space. This resulted in a transient, asymmetric configuration of propulsive units, which imparted motility to such coacervates in presence of substrate. This mechanism was confirmed by stochastic modelling and simulations in silico. Furthermore, we showed that a deeper understanding of the mechanism of stochasticity could be utilized to modulate the motion output. Conceptually, this work represents a leap in design philosophy in the construction of synthetic systems with life-like behaviors.

Here the authors develop a coacervate micromotor that can display autonomous motion as a result of stochastic distribution of propelling units. This stochastic-induced mobility is validated and explained through experiments and theory.

Enzymatic/Magnetic Hybrid Micromotors for Synergistic Anticancer Therapy

Micro/nanomotors (MNMs), which propel by transforming various forms of energy into kinetic energy, have emerged as promising therapeutic nanosystems in biomedical applications. However, most MNMs used for anticancer treatment are only powered by one engine or provide a single therapeutic strategy. Although double-engined micromotors for synergistic anticancer therapy can achieve more flexible movement and efficient treatment efficacy, their design remains challenging. In this study, we used a facile preparation method to develop enzymatic/magnetic micromotors for synergetic cancer treatment via chemotherapy and starvation therapy (ST), and the size of micromotors can be easily regulated during the synthetic process. The enzymatic reaction of glucose oxidase, which served as the chemical engine, led to self-propulsion using glucose as a fuel and ST via a reduction in the energy available to cancer cells. Moreover, the incorporation of Fe₃O₄ nanoparticles as a magnetic engine enhanced the kinetic power and provided control over the direction of movement. Inherent pH-responsive drug release behavior was observed owing to the acidic decomposition of drug carriers in the intracellular microenvironment of cancer cells. This system displayed enhanced anticancer efficacy owing to the synergetic therapeutic strategies and increased cellular uptake in a targeted area because of the improved motion behavior provided by the double engines. Therefore, the demonstrated micromotors are promising candidates for anticancer biomedical microsystems.

Macromolecular Crowding Effect on the Activity of Liposome-Bound Alkaline Phosphatase: A Paradoxical Inhibitory Action

The cytoplasm of a cell is extremely crowded, with 20-30% being large biomolecules. This crowding enforces a significant amount of the physical and chemical barrier around biomolecules, so understanding any biomolecular event within the cellular system is challenging. Unsurprisingly, enzymes show a diverse kind of catalytic behavior inside a crowded environment and thus have remained an area of active interest in the last few decades. The situation can become even more complex and exciting in the case of understanding the behavior of a membrane-bound enzyme (almost 25-30% of enzymes are membrane-bound) in such a crowded environment that until now has remained unexplored. Herein, we have particularly investigated how a membrane-bound enzyme (using liposome-bound alkaline phosphatase) can behave in a crowded environment comprising polymer molecule-like poly(ethylene glycol) (PEG) of different weights (PEG400, PEG4000, and PEG9000) and Ficoll 400. We have compared the activity using a polymer microbead conjugated enzyme and have found that liposome-bound alkaline phosphatase had much higher activity in crowded environments, showing the importance and superiority of soft-deformable particles (i.e., vesicles) over hard spheres in macro-molecularly crowded media. Interstingly, we have found a paradoxical behavior of inhibitors in terms of both their extent and pathway of inhibitory action. For instance, phosphates, known as competitive inhibitors in buffer, behave as uncompetitive inhibitors in liposome-bound enzymes in crowded media with an ∼5-fold less inhibitory effect, whereas phenyl alanine (an uncompetitive inhibitor in buffer) did not show any inhibitory potential when the enzyme was membrane-bound and in crowded media containing PEG9000 (30 wt %). Overall, this demonstration elucidates aspects of membrane-bound enzymes in crowded media in terms of both catalytic behavior and inhibitory actions and can lead to further studies of the understanding of enzymatic behavior in such complex crowded environments having a dampening effect in regular diffusive transport.

Surface adhesion of viruses and bacteria: Defend only and/or vibrationally extinguish also?! A perspective

Coronaviruses COVID-19, SARS-CoV and NL63 use spikes in their corona to bind to angiotensin converting enzyme 2 (ACE2) sites on cytoskeletal membranes of host cells to deliver their viral payload. While groups such as disulfides in ACE2's zinc metallopeptidase, and also in COVID-19's spikes, facilitate such binding, it is worth exploring how similar complementary sites on materials such as polymers, metals, ceramics, fabrics, and biomaterials promote binding of viruses and bacteria and how they could be further engineered to prevent bioactivity, or to act as agents to collect viral payloads in filters or similar devices. In that vein, this article offers a perspective on novel tools and approaches for chemically and topologically modifying most utilitarian surfaces via defensive topological vibrational engineering to either prevent such adhesion or to enhance adhesion and elicit vibrational characteristics/'musical signatures' from the surfaces so that the structure of the binding sites of viruses and bacteria is permanently altered and/or their cellular machinery is permanently disabled by targeted chemical transformations.

Graphic abstract

Supplementary Information

The online version contains supplementary material available at 10.1557/s43580-021-00079-0.

Interconnectivity between Surface Reactivity and Self-Assembly of Kemp Elimination Catalyzing Nanorods

Understanding the fundamental facts behind dynamicity of catalytic processes has been a longstanding quest across disciplines. Herein, we report self-assembly of catalytically active gold nanorods that can be regulated by tuning its reactivity towards a proton transfer reaction at different pH. Unlike substrate-induced templating and co-operativity, the enhanced aggregation rate is due to alteration of catalytic surface charge only during reactivity as negatively charged transition state of reactant (5-nitrobenzisoxazole) is formed on positively charged nanorod while undergoing a concerted E2-pathway. Herein, enhanced diffusivity during catalytic processes might also act as an additional contributing factor. Furthermore, we have also shown that nanosized hydrophobic cavities of clustered nanorods can also efficiently accelerate the rate of an aromatic nucleophilic substitution reaction, which also demonstrates a catalytic phenomenon that can lead to cascading of other reactions where substrates and products of the starting reactions are not directly involved.

Chemically Propelled Nano and Micromotors in the Body: Quo Vadis?

The active delivery of drugs to disease sites in response to specific biomarkers is a holy grail in theranostics. If successful, it would greatly diminish the therapeutic dosage and reduce collateral cytotoxicity. In this context, the development of nano and micromotors that are able to harvest local energy to move directionally is an important breakthrough. However, serious hurdles remain before such active systems can be employed in vivo in therapeutic applications. Such motors and their energy sources must be safe and biocompatible, they should be able to move through complex body fluids, and have the ability to reach specific cellular targets. Given the complexity in the design and deployment of nano and micromotors, it is also critically important to show that they are significantly superior to inactive "smart" nanoparticles in theranostics. Furthermore, receiving regulatory approval requires the ability to scale-up the production of nano and micromotors with uniformity in structure, function, and activity. In this essay, the limitations of the current nano and micromotors and the issues that need to be resolved before such motors are likely to find theranostic applications are discussed.

Connecting primitive phase separation to biotechnology, synthetic biology, and engineering

One aspect of the study of the origins of life focuses on how primitive chemistries assembled into the first cells on Earth and how these primitive cells evolved into modern cells. Membraneless droplets generated from liquid-liquid phase separation (LLPS) are one potential primitive cell-like compartment; current research in origins of life includes study of the structure, function, and evolution of such systems. However, the goal of primitive LLPS research is not simply curiosity or striving to understand one of life's biggest unanswered questions, but also the possibility to discover functions or structures useful for application in the modern day. Many applicational fields, including biotechnology, synthetic biology, and engineering, utilize similar phaseseparated structures to accomplish specific functions afforded by LLPS. Here, we briefly review LLPS applied to primitive compartment research and then present some examples of LLPS applied to biomolecule purification, drug delivery, artificial cell construction, waste and pollution management, and flavor encapsulation. Due to a significant focus on similar functions and structures, there appears to be much for origins of life researchers to learn from those working on LLPS in applicational fields, and vice versa, and we hope that such researchers can start meaningful cross-disciplinary collaborations in the future.

Enzyme-driven micro/nanomotors: Recent advances and biomedical applications

Micro/nanomotors (MNMs), both self-propelled actuators and external fields-promoted machines, have joined forces in the past decade to accomplish versatile tasks such as precise detection and targeted cargo delivery with adequate propulsion and desirable locomotion. Amongst, enzyme-driven MNMs have been able to differentiate themselves from others owing to their distinct characteristics, such as absence of chemical fuel, enhanced cellular uptake and the possibility to be easily conjugated with many therapeutics, including both small molecules and biologics, displaying superior efficacy, enhanced specificity and diminished side effects. In the present review, we aim to highlight and summarize recent advances in enzyme-driven MNMs, particularly to provide an in-depth discussion focusing on the enzyme linking approaches onto those MNMs and motion control strategies of such MNMs with advantages and limitations thereof. Conclusions and future perspectives are also provided in brief.

Fundamentals and applications of enzyme powered micro/nano-motors

Micro/nanomotors (MNMs) are miniaturized machines that can convert many kinds of energy into mechanical motion. Over the past decades, a variety of driving mechanisms have been developed, which have greatly extended the application scenarios of MNMs. Enzymes exist in natural organisms which can convert chemical energy into mechanical force. It is an innovative attempt to utilize enzymes as biocatalyst providing driving force for MNMs. The fuels for enzymatic reactions are biofriendly as compared to traditional counterparts, which makes enzyme-powered micro/nanomotors (EMNMs) of great value in biomedical field for their nature of biocompatibility. Until now, EMNMs with various shapes can be propelled by catalase, urease and many others. Also, they can be endowed with multiple functionalities to accomplish on-demand tasks. Herein, combined with the development process of EMNMs, we are committed to present a comprehensive understanding of EMNMs, including their types, propelling principles, and potential applications. In this review, we will introduce single enzyme that can be used as motor, enzyme powered molecule motors and other micro/nano-architectures. The fundamental mechanism of energy conversion process of EMNMs and crucial factors that affect their movement behavior will be discussed. The current progress of proof-of-concept applications of EMNMs will also be elaborated in detail. At last, we will summarize and prospect the opportunities and challenges that EMNMs will face in their future development.

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Clear classification and description of different enzyme-powered micro/nanomotors (EMNMs).

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Discussion of the fundamental mechanism of energy conversion process of EMNMs and their movement influence factors.

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Introduction of the current progress of proof-of-concept applications of EMNMs.

Photoactivated Polymersome Nanomotors: Traversing Biological Barriers

Synthetic nanomotors are appealing delivery vehicles for the dynamic transport of functional cargo. Their translation toward biological applications is limited owing to the use of non-degradable components. Furthermore, size has been an impediment owing to the importance of achieving nanoscale (ca. 100 nm) dimensions, as opposed to microscale examples that are prevalent. Herein, we present a hybrid nanomotor that can be activated by near-infrared (NIR)-irradiation for the triggered delivery of internal cargo and facilitated transport of external agents to the cell. Utilizing biodegradable poly(ethylene glycol)-b-poly(d,l-lactide) (PEG-PDLLA) block copolymers, with the two blocks connected via a pH sensitive imine bond, we generate nanoscopic polymersomes that are then modified with a hemispherical gold nanocoat. This Janus morphology allows such hybrid polymersomes to undergoing photothermal motility in response to thermal gradients generated by plasmonic absorbance of NIR irradiation, with velocities ranging up to 6.2±1.10 μm s-1 . These polymersome nanomotors (PNMs) are capable of traversing cellular membranes allowing intracellular delivery of molecular and macromolecular cargo.

Single-molecule diffusometry reveals no catalysis-induced diffusion enhancement of alkaline phosphatase as proposed by FCS experiments

Recent experiments have suggested that the energy released by a chemical reaction can propel its enzyme catalyst (for example, alkaline phosphatase). However, this topic remains controversial, partially due to the indirect and ensemble nature of existing measurements. Here, we used recently developed single-molecule approaches to monitor directly the motions of individual proteins in aqueous solution and find that single alkaline phosphatase enzymes do not diffuse faster under catalysis. Instead, we demonstrate that interactions between the fluorescent dye and the enzyme’s substrate can produce the signature of apparent diffusion enhancement in fluorescence correlation spectroscopy, the standard ensemble assay currently used to study enzyme diffusion and indicate that single-molecule approaches provide a more robust means to investigate diffusion at the nanoscale.

Theoretical and experimental observations that catalysis enhances the diffusion of enzymes have generated exciting implications about nanoscale energy flow, molecular chemotaxis, and self-powered nanomachines. However, contradictory claims on the origin, magnitude, and consequence of this phenomenon continue to arise. To date, experimental observations of catalysis-enhanced enzyme diffusion have relied almost exclusively on fluorescence correlation spectroscopy (FCS), a technique that provides only indirect, ensemble-averaged measurements of diffusion behavior. Here, using an anti-Brownian electrokinetic (ABEL) trap and in-solution single-particle tracking, we show that catalysis does not increase the diffusion of alkaline phosphatase (ALP) at the single-molecule level, in sharp contrast to the ∼20% enhancement seen in parallel FCS experiments using p-nitrophenyl phosphate (pNPP) as substrate. Combining comprehensive FCS controls, ABEL trap, surface-based single-molecule fluorescence, and Monte Carlo simulations, we establish that pNPP-induced dye blinking at the ∼10-ms timescale is responsible for the apparent diffusion enhancement seen in FCS. Our observations urge a crucial revisit of various experimental findings and theoretical models––including those of our own––in the field, and indicate that in-solution single-particle tracking and ABEL trap are more reliable means to investigate diffusion phenomena at the nanoscale.

Biomimicry of Cellular Motility and Communication Based on Synthetic Soft-Architectures

Cells, sophisticated membrane-bound units that contain the fundamental molecules of life, provide a precious library for inspiration and motivation for both society and academia. Scientists from various disciplines have made great endeavors toward the understanding of the cellular evolution by engineering artificial counterparts (protocells) that mimic or initiate structural or functional cellular aspects. In this regard, several works have discussed possible building blocks, designs, functions, or dynamics that can be applied to achieve this goal. Although great progress has been made, fundamental-yet complex-behaviors such as cellular communication, responsiveness to environmental cues, and motility remain a challenge, yet to be resolved. Herein, recent efforts toward utilizing soft systems for cellular mimicry are summarized-following the main outline of cellular evolution, from basic compartmentalization, and biological reactions for energy production, to motility and communicative behaviors between artificial cell communities or between artificial and natural cell communities. Finally, the current challenges and future perspectives in the field are discussed, hoping to inspire more future research and to help the further advancement of this field.

Mechanobiology of dynamic enzyme systems

This Perspective paper advances a hypothesis of mechanosensation by endothelial cells in which the cell is a dynamic crowded system, driven by continuous enzyme activity, that can be shifted from one non-equilibrium state to another by external force. The nature of the shift will depend on the direction, rate of change, and magnitude of the force. Whether force induces a pathophysiological or physiological change in cell biology will be determined by whether the dynamics of a cellular system can accommodate the dynamics and magnitude of the force application. The complex interplay of non-static cytoskeletal structures governs internal cellular rheology, dynamic spatial reorganization, and chemical kinetics of proteins such as integrins, and a flaccid membrane that is dynamically supported; each may constitute the necessary dynamic properties able to sense external fluid shear stress and reorganize in two and three dimensions. The resulting reorganization of enzyme systems in the cell membrane and cytoplasm may drive the cell to a new physiological state. This review focuses on endothelial cell mechanotransduction of shear stress, but may lead to new avenues of investigation of mechanobiology in general requiring new tools for interrogation of mechanobiological systems, tools that will enable the synthesis of large amounts of spatial and temporal data at the molecular, cellular, and system levels.

Enzyme catalysis powered micro/nanomotors for biomedical applications

This review provides insights on enzyme powered motors using fuels present in biological environments for biomedical applications.

Positive and negative chemotaxis of enzyme-coated liposome motors

The ability of cells or cell components to move in response to chemical signals is critical for the survival of living systems. This motion arises from harnessing free energy from enzymatic catalysis. Artificial model protocells derived from phospholipids and other amphiphiles have been made and their enzymatic-driven motion has been observed. However, control of directionality based on chemical cues (chemotaxis) has been difficult to achieve. Here we show both positive or negative chemotaxis of liposomal protocells. The protocells move autonomously by interacting with concentration gradients of either substrates or products in enzyme catalysis, or Hofmeister salts. We hypothesize that the propulsion mechanism is based on the interplay between enzyme-catalysis-induced positive chemotaxis and solute-phospholipid-based negative chemotaxis. Controlling the extent and direction of chemotaxis holds considerable potential for designing cell mimics and delivery vehicles that can reconfigure their motion in response to environmental conditions.

Self-Propulsion Strategies for Artificial Cell-Like Compartments

Reconstitution of life-like properties in artificial cells is a current research frontier in synthetic biology. Mimicking metabolism, growth, and sensing are active areas of investigation; however, achieving motility and directional taxis are also challenging in the context of artificial cells. To tackle this problem, recent progress has been made that leverages the tools of active matter physics in synthetic biology. This review surveys the most significant achievements in designing motile cell-like compartments. In this context, strategies for self-propulsion are summarized, including, compartmentalization of catalytically active particles, phoretic propulsion of vesicles and emulsion droplet motion driven by Marangoni flows. This work showcases how the realization of motile protocells may impact biomedical engineering while also aiming at answering fundamental questions in locomotion of prebiotic cells.