Response-Retaliation Behavior in Synthetic Protocell Communities

Continuous Transformation from Membrane-Less Coacervates to Membranized Coacervates and Giant Vesicles: Toward Multicompartmental Protocells with Complex (Membrane) Architectures

The membranization of membrane-less coacervates paves the way for the exploitation of complex protocells with regard to structural and cell-like functional behaviors. However, the controlled transformation from membranized coacervates to vesicles remains a challenge. This can provide stable (multi)phase and (multi)compartmental architectures through the reconfiguration of coacervate droplets in the presence of (bioactive) polymers, bio(macro)molecules and/or nanoobjects. Herein, we present a continuous protocell transformation from membrane-less coacervates to membranized coacervates and, ultimately, to giant hybrid vesicles. This transformation process is orchestrated by altering the balance of non-covalent interactions through varying concentrations of an anionic terpolymer, leading to dynamic processes such as spontaneous membranization of terpolymer nanoparticles at the coacervate surface, disassembly of the coacervate phase mediated by the excess anionic charge, and the redistribution of coacervate components in membrane. The diverse protocells during the transformation course provide distinct structural features and molecular permeability. Notably, the introduction of multiphase coacervates in this continuous transformation process signifies advancements toward the creation of synthetic cells with different diffusible compartments. Our findings emphasize the highly controlled continuous structural reorganization of coacervate protocells and represents a novel step toward the development of advanced and sophisticated synthetic protocells with more precise compositions and complex (membrane) structures.

Tuning interfacial fluidity and colloidal stability of membranized coacervate protocells

The cell membrane not only serves as the boundary between the cell's interior and the external environment but also plays a crucial role in regulating fundamental cellular behaviours. Interfacial membranization of membraneless coacervates, formed through liquid-liquid phase separation (LLPS), represents a reliable approach to constructing hierarchical cell-like entities known as protocells. In this study, we demonstrate the capability to modulate the interfacial membrane fluidity and thickness of dextran-bound coacervate protocells by adjusting the molecular weight of dextran or utilizing dextranase-catalyzed hydrolysis. This modulation allows for rational control over colloidal stability, interfacial molecular transport and cell-protocell interactions. Our work opens a new avenue for surface engineering of coacervate protocells, enabling the establishment of cell-mimicking structures and behaviours.

Optical Control over Liquid–Liquid Phase Separation

Liquid-liquid phase separation (LLPS) is responsible for the emergence of intracellular membrane-less organelles and the development of coacervate protocells. Benefitting from the advantages of simplicity, precision, programmability, and noninvasiveness, light has become an effective tool to regulate the assembly dynamics of LLPS, and mediate various biochemical processes associated with LLPS. In this review, recent advances in optically controlling membrane-less organelles within living organisms are summarized, thereby modulating a series of biological processes including irreversible protein aggregation pathologies, transcription activation, metabolic flux, genomic rearrangements, and enzymatic reactions. Among these, the intracellular systems (i.e., optoDroplet, Corelet, PixELL, CasDrop, and other optogenetic systems) that enable the photo-mediated control over biomolecular condensation are highlighted. The design of photoactive complex coacervate protocells in laboratory settings by utilizing photochromic molecules such as azobenzene and diarylethene is further discussed. This review is expected to provide in-depth insights into phase separation-associated biochemical processes, bio-metabolism, and diseases.

Exploring Emergent Properties in Enzymatic Reaction Networks: Design and Control of Dynamic Functional Systems

The intricate and complex features of enzymatic reaction networks (ERNs) play a key role in the emergence and sustenance of life. Constructing such networks in vitro enables stepwise build up in complexity and introduces the opportunity to control enzymatic activity using physicochemical stimuli. Rational design and modulation of network motifs enable the engineering of artificial systems with emergent functionalities. Such functional systems are useful for a variety of reasons such as creating new-to-nature dynamic materials, producing value-added chemicals, constructing metabolic modules for synthetic cells, and even enabling molecular computation. In this review, we offer insights into the chemical characteristics of ERNs while also delving into their potential applications and associated challenges.

Controlled exchange of protein and nucleic acid signals from and between synthetic minimal cells

Synthetic minimal cells are a class of bioreactors that have some, but not all, functions of live cells. Here, we report a critical step toward the development of a bottom-up minimal cell: cellular export of functional protein and RNA products. We used cell-penetrating peptide tags to translocate payloads across a synthetic cell vesicle membrane. We demonstrated efficient transport of active enzymes and transport of nucleic acid payloads by RNA-binding proteins. We investigated influence of a concentration gradient alongside other factors on the efficiency of the translocation, and we show a method to increase product accumulation in one location. We demonstrate the use of this technology to engineer molecular communication between different populations of synthetic cells, to exchange protein and nucleic acid signals. The synthetic minimal cell production and export of proteins or nucleic acids allows experimental designs that approach the complexity and relevancy of natural biological systems. A record of this paper's transparent peer review process is included in the supplemental information.

Construction of Membraneless and Multicompartmentalized Coacervate Protocells Controlling a Cell Metabolism-like Cascade Reaction

In recent years, there has been growing attention to designing synthetic protocells, capable of mimicking micrometric and multicompartmental structures and highly complex physicochemical and biological processes with spatiotemporal control. Controlling metabolism-like cascade reactions in coacervate protocells is still challenging since signal transduction has to be involved in sequential and parallelized actions mediated by a pH change. Herein, we report the hierarchical construction of membraneless and multicompartmentalized protocells composed of (i) a cytosol-like scaffold based on complex coacervate droplets stable under flow conditions, (ii) enzyme-active artificial organelles and a substrate nanoreservoir capable of triggering a cascade reaction between them in response to a pH increase, and (iii) a signal transduction component based on the urease enzyme capable of the conversion of an exogenous biological fuel (urea) into an endogenous signal (ammonia and pH increase). Overall, this strategy allows a synergistic communication between their components within the membraneless and multicompartment protocells and, thus, metabolism-like enzymatic cascade reactions. This signal communication is transmitted through a scaffold protocell from an "inactive state" (nonfluorescent protocell) to an "active state" (fluorescent protocell capable of consuming stored metabolites).

Signal Transduction in Artificial Cells

In recent years, significant progress has been made in the emerging field of constructing biomimetic soft compartments with life-like behaviors. Given that biological activities occur under a flux of energy and matter exchange, the implementation of rudimentary signaling pathways in artificial cells (protocells) is a prerequisite for the development of adaptive sense-response phenotypes in cytomimetic models. Herein, recent approaches to the integration of signal transduction modules in model protocells prepared by bottom-up construction are discussed. The approaches are classified into two categories involving invasive biochemical signals or non-invasive physical stimuli. In the former mechanism, transducers with intrinsic recognition capability respond with high specificity, while in the latter, artificial cells respond through intra-protocellular energy transduction. Although major challenges remain in the pursuit of a sophisticated artificial signaling network for the orchestration of higher-order cytomimetic models, significant advances have been made in establishing rudimentary protocell communication networks, providing novel organizational models for the development of life-like microsystems and new avenues in protoliving technologies.

Synthetic-Cell-Based Multi-Compartmentalized Hierarchical Systems

In the extant lifeforms, the self-sustaining behaviors refer to various well-organized biochemical reactions in spatial confinement, which rely on compartmentalization to integrate and coordinate the molecularly crowded intracellular environment and complicated reaction networks in living/synthetic cells. Therefore, the biological phenomenon of compartmentalization has become an essential theme in the field of synthetic cell engineering. Recent progress in the state-of-the-art of synthetic cells has indicated that multi-compartmentalized synthetic cells should be developed to obtain more advanced structures and functions. Herein, two ways of developing multi-compartmentalized hierarchical systems, namely interior compartmentalization of synthetic cells (organelles) and integration of synthetic cell communities (synthetic tissues), are summarized. Examples are provided for different construction strategies employed in the above-mentioned engineering ways, including spontaneous compartmentalization in vesicles, host-guest nesting, phase separation mediated multiphase, adhesion-mediated assembly, programmed arrays, and 3D printing. Apart from exhibiting advanced structures and functions, synthetic cells are also applied as biomimetic materials. Finally, key challenges and future directions regarding the development of multi-compartmentalized hierarchical systems are summarized; these are expected to lay the foundation for the creation of a "living" synthetic cell as well as provide a larger platform for developing new biomimetic materials in the future.

Coacervate Microdroplets as Synthetic Protocells for Cell Mimicking and Signaling Communications

Synthetic protocells are minimal systems that mimic certain properties of natural cells and are used to research the emergence of life from a nonliving chemical network. Currently, coacervate microdroplets, which are formed via liquid-liquid phase separation, are receiving wide attention in the context of cell biology and protocell research; these microdroplets are notable because they can provide liquid-like compartment structures for biochemical reactions by creating highly macromolecular crowded local environments. In this review, an overview of recent research on the formation of coacervate microdroplets through phase separation; the design of coacervate-based stimuli-responsive protocells, multichamber protocells, and membranized protocells; and their cell mimic behaviors, is provided. The simplified protocell models with precisely defined and tunable compositions advance the understanding of the requirements for cellular structure and function. Efforts are then discussed to establish signal communication systems in protocell and protocell consortia, as communication is a fundamental feature of life that coordinates matter exchanges and energy fluxes dynamically in space and time. Finally, some perspectives on the challenges and future developments of synthetic protocell research in biomimetic science and biomedical applications are provided.

Plant Cell-Inspired Membranization of Coacervate Protocells with a Structured Polysaccharide Layer

The design of compartmentalized colloids that exhibit biomimetic properties is providing model systems for developing synthetic cell-like entities (protocells). Inspired by the cell walls in plant cells, we developed a method to prepare membranized coacervates as protocell models by coating membraneless liquid-like microdroplets with a protective layer of rigid polysaccharides. Membranization not only endowed colloidal stability and prevented aggregation and coalescence but also facilitated selective biomolecule sequestration and chemical exchange across the membrane. The polysaccharide wall surrounding coacervate protocells acted as a stimuli-responsive structural barrier that enabled enzyme-triggered membrane lysis to initiate internalization and killing of Escherichia coli. The membranized coacervates were capable of spatial organization into structured tissue-like protocell assemblages, offering a means to mimic metabolism and cell-to-cell communication. We envision that surface engineering of protocells as developed in this work generates a platform for constructing advanced synthetic cell mimetics and sophisticated cell-like behaviors.

Coacervates Forming Coexisting Phases on a Mineral Surface

Minerals played a crucial role in the chemical evolution of small molecules into biopolymers. Yet, it is still not clear how the minerals are related to the formation and the evolution of protocells on early Earth. In this work, using the coacervate formed by quaternized dextran (Q-dextran) and single-stranded oligonucleotides (ss-oligo) as the protocell model, we systematically studied the phase separation of Q-dextran and ss-oligo on the muscovite surface. Serving as rigid and 2D polyelectrolytes, the muscovite surface can be treated by Q-dextran to become negatively charged, neutral, or positively charged. We observed that Q-dextran and ss-oligo form uniform coacervates on naked and neutral muscovite surfaces, while they form biphasic coacervates containing Q-dextran-rich phases and ss-oligo-rich phases on positively or negatively charged muscovite surfaces that were pretreated by Q-dextran. The evolution of the phases is caused by the redistribution of the components as the coacervate touches the surface. Our study indicates that the mineral surface could be a potential driving force for the formation of protocells with hierarchical structures and desirable functions on prebiotic Earth.

Programmatically Dynamic Microcompartmentation in Coacervate-in-Pickering Emulsion Protocell

The desire for exploration of cellular functional mechanisms has substantially increased the rapid development of artificial cells. However, the construction of synthetic cells with high organizational complexity remains challenging due to the lack of facile approaches ensuring dynamic multi-compartments of cytoplasm and stability of membranes in protocells. Herein, a stable coacervate-in-Pickering emulsion protocell model comprising a membraneless coacervate phase formed by poly-l-lysine (PLys) and adenosine triphosphate (ATP) encapsulated in Pickering emulsion is put forward only through simple one-step emulsification. The dynamic distribution of intracellular components (coacervates in this protocell model) can be manipulated by changes in temperature or pH. This coacervate-in-Pickering emulsion protocell system exhibits repeatable cycle stability in response to external stimuli (at least 24 cycles for temperature and 3 cycles for pH). By encapsulating antagonistic enzymes into coacervates, glucose oxidase (GOx) and urease as an example, the control of local enzyme concentration is achieved by introducing glucose and urea to adjust the pH value in Pickering emulsion droplets. This hybrid protocell model with programmatically dynamic microcompartmentation and sufficient stability is expected to be further studied and applied in cellular biology, facilitating the development of lifelike systems with potential in practical applications.

A Synthetic Protocell-Based Heparin Scavenger

Heparin is a commonly applied blood anticoagulant agent in clinical use. After treatment, excess heparin needs to be removed to circumvent side effects and recover the blood-clotting cascade. Most existing heparin antidotes rely on direct heparin binding and complexation, yet selective compartmentalization and sequestration of heparin would be beneficial for safety and efficiency. However, such systems have remained elusive. Herein, a semipermeable protein-based microcompartment (proteinosome) is loaded with a highly positively charged chitosan derivative, which can induce electrostatics-driven internalization of anionic guest molecules inside the compartment. Chitosan-loaded proteinosomes are subsequently employed to capture heparin, and an excellent heparin-scavenging performance is demonstrated under physiologically relevant conditions. Both the highly positive scavenger and the polyelectrolyte complex are confined and shielded by the protein compartment in a time-dependent manner. Moreover, selective heparin-scavenging behavior over serum albumin is realized through adjusting the localized scavenger or surrounding salt concentrations at application-relevant circumstances. In vitro studies reveal that the cytotoxicity of the cationic scavenger and the produced polyelectrolyte complex is reduced by protocell shielding. Therefore, the proteinosome-based systems may present a novel polyelectrolyte-scavenging method for biomedical applications.

Synthetic Protocell as Efficient Bioreactor: Enzymatic Superactivity and Ultrasensitive Glucose Sensing in Urine

It is believed that membraneless cellular condensates play a critical role in accelerating various slow and thermodynamically unfavorable biochemical processes. However, the exact mechanisms behind the enhanced activity within biocondensates remain poorly understood. Here, we report the fabrication of a high-performance integrated cascade bioplatform based on synthetic droplets for ultrasensitive glucose sensing. Using a horseradish peroxidase (HRP) and glucose oxidase (GOx) cascade pair, we report an unprecedented enhancement in the catalytic activity of HRP inside the synthetic membraneless droplet. Liquidlike membraneless droplets have been prepared via multivalent electrostatic interactions between adenosine triphosphate (ATP) and poly(diallyldimethylammonium chloride) (PDADMAC) in an aqueous medium. Compartmentalized enzymes (GOx/HRP@Droplet) exhibit high encapsulation efficiency, low leakage, prolong retention of activity, and exceptional stability toward protease digestion. Using an HRP@Droplet composite, we have shown that the enzymatic reaction within the droplet follows the classical Michaelis-Menten model. Our findings reveal remarkable enhancement in the catalytic activity of up to 100- and 51-fold for HRP@Droplet and GOx/HRP@Droplet, respectively. These enhanced activities have been explained on the basis of increased local concentrations of enzymes and substrates, along with altered conformations of sequestered enzymes. Furthermore, we have utilized highly efficient and recyclable GOx/HRP@Droplet composite to demonstrate ultrasensitive glucose sensing with a limit of detection of 228 nM. Finally, the composite platform has been exploited to detect glucose in spiked urine samples in solution and filter paper. Our present study illustrates the unprecedented activity of the compartmentalized enzymes and paves the way for next-generation composite bioreactors for a wide range of applications.

Modulating liquid-liquid phase separation of FUS: mechanisms and strategies

Liquid-liquid phase separation (LLPS) of biomolecules inspires the construction of protocells and drives the formation of cellular membraneless organelles. The resulting biomolecular condensates featuring dynamic assembly, disassembly, and phase transition play significant roles in a series of biological processes, including RNA metabolism, DNA damage response, signal transduction and neurodegenerative disease. Intensive investigations have been conducted for understanding and manipulating intracellular phase-separated disease-related proteins (e.g., FUS, tau and TDP-43). Herein, we review current studies on the regulation strategies of intracellular LLPS focusing on FUS, which are categorized into physical stimuli, biochemical modulators, and protein structural modifications, with summarized molecular mechanisms. This review is expected to provide a sketch of the modulation of FUS LLPS with its pros and cons, and an outlook for the potential clinical treatments of neurodegenerative diseases.

Chemical Communication in Artificial Cells: Basic Concepts, Design and Challenges

In the past decade, the focus of bottom-up synthetic biology has shifted from the design of complex artificial cell architectures to the design of interactions between artificial cells mediated by physical and chemical cues. Engineering communication between artificial cells is crucial for the realization of coordinated dynamic behaviours in artificial cell populations, which would have implications for biotechnology, advanced colloidal materials and regenerative medicine. In this review, we focus our discussion on molecular communication between artificial cells. We cover basic concepts such as the importance of compartmentalization, the metabolic machinery driving signaling across cell boundaries and the different modes of communication used. The various studies in artificial cell signaling have been classified based on the distance between sender and receiver cells, just like in biology into autocrine, juxtacrine, paracrine and endocrine signaling. Emerging tools available for the design of dynamic and adaptive signaling are highlighted and some recent advances of signaling-enabled collective behaviours, such as quorum sensing, travelling pulses and predator-prey behaviour, are also discussed.

Macromolecular Crowding-Induced Unusual Liquid-Liquid Phase Separation of Human Serum Albumin via Soft Protein-Protein Interactions

Macromolecular crowding has a profound impact on the conformational dynamics and intermolecular interactions of biological macromolecules. In this context, the role of inert synthetic crowders in the protein-protein interactions of globular proteins is poorly understood. Here, using native human serum albumin (HSA) under physiological conditions, we show that macromolecular crowding induces liquid-liquid phase separation (LLPS) via liquid-like membrane-less droplet formation in a concentration- and time-dependent manner. Circular dichroism measurements reveal significant alteration in the secondary structure of HSA inside the droplet during aging. In contrast, at a high protein concentration, a liquid-to-solid-like phase transition has been observed upon maturation. Our findings reveal that the LLPS of HSA is mainly driven by enthalpically controlled intermolecular protein-protein interactions via hydrophobic contacts involving aromatic and/or nonaromatic residues. Moreover, modulation of LLPS of HSA has been demonstrated upon denaturation and ligand binding. This study highlights the importance of soft protein-protein interactions of globular proteins in a crowded cellular environment in driving the LLPS.

Triggerable Protocell Capture in Nanoparticle-Caged Coacervate Microdroplets

Controlling the dynamics of mixed communities of cell-like entities (protocells) provides a step toward the development of higher-order cytomimetic behaviors in artificial cell consortia. In this paper, we develop a caged protocell model with a molecularly crowded coacervate interior surrounded by a non-cross-linked gold (Au)/poly(ethylene glycol) (PEG) nanoparticle-jammed stimuli-responsive membrane. The jammed membrane is unlocked by either exogenous light-mediated Au/PEG dissociation at the Au surface or endogenous enzyme-mediated cleavage of a ketal linkage on the PEG backbone. The membrane assembly/disassembly process is used for the controlled and selective uptake of guest protocells into the caged coacervate microdroplets as a path toward an all-water model of triggerable transmembrane uptake in synthetic protocell communities. Active capture of the guest protocells stems from the high sequestration potential of the coacervate interior such that tailoring the surface properties of the guest protocells provides a rudimentary system of protocell sorting. Our results highlight the potential for programming surface-contact interactions between artificial membrane-bounded compartments and could have implications for the development of protocell networks, storage and delivery microsystems, and microreactor technologies.

Nanoprogrammed Cross-Kingdom Communication Between Living Microorganisms

The engineering of chemical communication at the micro/nanoscale is a key emergent topic in micro/nanotechnology, synthetic biology, and related areas. However, the field is still in its infancy; previous advances, although scarce, have mainly focused on communication between abiotic micro/nanosystems or between microvesicles and living cells. Here, we have implemented a nanoprogrammed cross-kingdom communication involving two different microorganisms and tailor-made nanodevices acting as "nanotranslators". Information flows from the sender cells (bacteria) to the nanodevice and from the nanodevice to receiver cells (yeasts) in a hierarchical way, allowing communication between two microorganisms that otherwise would not interact.

Fatty Acid-Based Coacervates as a Membrane-free Protocell Model

Membrane-less scenarios that involve liquid-liquid phase separation (coacervation) provide clues for how protocells might emerge. Here, we report a versatile approach to construct coacervates by mixing fatty acid with biomolecule dopamine as the protocell model. The coacervate droplets are easily formed over a wide range of concentrations. The solutes with different interaction characteristics, including cationic, anionic, and hydrophobic dyes, can be well concentrated within the coacervates. In addition, reversible self-assemblies of the coacervates can be controlled by concentration, pH, temperature, salinity, and bioreaction realizing cycles between compartmentalization and noncompartmentalization. Through in situ dopamine polymerization, the stability of coacervate droplets is significantly improved, leading to higher resistance toward external factors. Therefore, the coacervates based on fatty acid and dopamine could serve as a bottom-up membrane-less protocell model that provides the links between the simple (small molecule) and complex (macromolecule) systems in the process of cell evolution.

Controlled evaporation-induced phase separation of droplets containing nanogels and salt molecules

Droplets without protection from surfactants or surfactant-like objects normally experience merging or a coalescence process since it is thermodynamically favored. However, division or replication of droplets is thermodynamically unfavored and comparably more difficult to realize. Herein, we demonstrate that a population of droplets that are composed of nanogels and salt spontaneously undergo a separation process under a slow solvent evaporation condition. Each individual droplet underwent changes in size, shape and eventually developed into two domains, which was caused by the screening effect due to the increased salt concentration as a result of solvent evaporation. The two domains gradually separated into nanogel-rich and salt-rich parts. These two parts eventually evolved into nanogel aggregates and branched structures, respectively. This separation was mainly due to the salting out effect and dewetting. Comparison studies indicate that both the nanogels and salt are indispensable ingredients for the phase separation. These discoveries may have profound applications in the fields of biomimetics and offer new routes for self-replication systems.

An individual droplet containing nanogels and salts can evolve into gel-rich and salt-rich two separate parts upon evaporation.

Chemical communication at the synthetic cell/living cell interface

Although the complexity of synthetic cells has continued to increase in recent years, chemical communication between protocell models and living organisms remains a key challenge in bottom-up synthetic biology and bioengineering. In this Review, we discuss how communication channels and modes of signal processing can be established between living cells and cytomimetic agents such as giant unilamellar lipid vesicles, proteinosomes, polysaccharidosomes, polymer-based giant vesicles and membrane-less coacervate micro-droplets. We describe three potential modes of chemical communication in consortia of synthetic and living cells based on mechanisms of distributed communication and signal processing, physical embodiment and nested communication, and network-based contact-dependent communication. We survey the potential for applying synthetic cell/living cell communication systems in biomedicine, including the in situ production of therapeutics and development of new bioreactors. Finally, we present a short summary of our findings.

Nanocatalysis under Nanoconfinement: A Metal-Free Hybrid Coacervate Nanodroplet as a Catalytic Nanoreactor for Efficient Redox and Photocatalytic Reactions

Nature utilizes cellular and subcellular compartmentalization to efficiently drive various complex enzymatic transformations via spatiotemporal control. In this context, designing of artificial nanoreactors for efficient catalytic transformations finds tremendous importance in recent times. One key challenge remains the design of multiple catalytic centers within the confined space of a nanoreactor without unwanted agglomeration and accessibility barrier for reactants. Herein, we report a unique blend of nanoscience and chemical catalysis using a metal-free hybrid synthetic protocell as a catalytic nanoreactor for redox and photocatalytic transformations, which are otherwise incompatible in bulk aqueous medium. Hybrid coacervate nanodroplets (NDs) fabricated from 2.5 nm-sized carbon dots (CDs) and poly(diallyldimethyl)ammonium chloride have been utilized toward reductive hydrogenation of nitroarenes in the presence of sodium borohydride (NaBH₄). It has been found that the reduction mechanism follows the classical Langmuir-Hinshelwood (LH) model at the surface of embedded CDs inside the NDs via the generation of reactive surface hydroxyl groups. These NDs show excellent recyclability without any compromise on reaction kinetics and conversion yield. Importantly, spatiotemporal control over the hydrogenation reaction has been achieved using two mixed populations of coacervates. Moreover, efficient visible light-induced photoredox conversion of ferricyanide to ferrocyanide and artificial peroxidase-like activity have also been demonstrated inside these catalytic NDs. Our findings indicate that the individual polymer-bound CD inside the NDs acts as the catalytic center for both the redox and photocatalytic reactions. The present study highlights the unprecedented catalytic activity of the metal-free CD-based coacervate NDs and paves the way for next-generation catalytic nanoreactors for a wide range of chemical and enzymatic transformations.

A three-tiered colloidosomal microreactor for continuous flow catalysis

Integrative colloidosomes with hierarchical structure and advanced function may serve as biomimetic microreactors to carry out catalytic reactions by compartmentalizing biological species within semipermeable membranes. Despite of recent progress in colloidosome design, integration of biological and inorganic components into tiered structures to tackle the remaining challenges of biocatalysis is highly demanded. Here, we report a rational design of three-tiered colloidosomes via the Pickering emulsion process. The microreactor consists of crosslinked amphiphilic silica-polymer hybrid nanoparticles as the semipermeable shell, an enzyme-incorporated catalytic sub-layer, and a partially-silicified adsorptive lumen. By leveraging confinement and enrichment effect, we demonstrate the acceleration of lipase-catalyzed ester hydrolysis within the microcompartment of organic-inorganic hybrid colloidosomes. The catalytic colloidosomes are further assembled into a closely packed column for enzymatic reactions in a continuous flow format with enhanced reaction rates. The three-tiered colloidosomes provide a reliable platform to integrate functional building blocks into a biomimetic compartmentalized microreactor with spatially controlled organization and high-performance functions.

From vesicles to materials: bioinspired strategies for fabricating hierarchically structured soft matter

Certain organisms including species of mollusks, polychaetes, onychophorans and arthropods produce exceptional polymeric materials outside their bodies under ambient conditions using concentrated fluid protein precursors. While much is understood about the structure-function relationships that define the properties of such materials, comparatively less is understood about how such materials are fabricated and specifically, how their defining hierarchical structures are achieved via bottom-up assembly. Yet this information holds great potential for inspiring sustainable manufacture of advanced polymeric materials with controlled multi-scale structure. In the present perspective, we first examine recent work elucidating the formation of the tough adhesive fibres of the mussel byssus via secretion of vesicles filled with condensed liquid protein phases (coacervates and liquid crystals)-highlighting which design principles are relevant for bio-inspiration. In the second part of the perspective, we examine the potential of recent advances in drops and additive manufacturing as a bioinspired platform for mimicking such processes to produce hierarchically structured materials. This article is part of the theme issue 'Bio-derived and bioinspired sustainable advanced materials for emerging technologies (part 1)'.

Directing Transition of Synthetic Protocell Models via Physicochemical Cues-Triggered Interfacial Dynamic Covalent Chemistry

As the preliminary synthetic analogs of living cells, protocells with life-like features serve as a versatile platform to explore the origin of life. Although protocells constructed from multiple components have been developed, the transition of primitive cellular compartments toward structural complexity and advanced function remains a scientific challenge. Herein, a programmable pathway is established to exploit a simple chemistry to construct structural transition of protocell models from emulsion droplets, nanocapsules to molecularly crowded droplets. The transitional process toward distinct cell-like compartments is driven by interfacial self-assembly of simple components and regulated by physicochemical cues (e.g., mechanical force, solvent evaporation, acid/base equilibrium) triggered dynamic covalent chemistry. These protocell models are further studied by comparing their compartmentalization behavior, sequestration efficiency, and the ability to enrich biomolecules (e.g., enzyme and substrate) toward catalytic reaction or biological activity within the compartments. The results showcase physiochemical cues-driven programmable transition of life-like compartments toward functionalization, and offer a new step toward the design of living soft materials.

Engineering chemical communication between micro/nanosystems

Chemical communication, based on the exchange of molecules as messengers, allows different entities to share information, cooperate and orchestrate collective behaviors. In recent years, the development of strategies of chemical communication between micro/nanosystems is becoming a key emergent topic in micro/nanotechnology, biomimicry and related areas. In this tutorial review, we provide a general perspective of the concepts used on the topic of chemical communication, and the advances made using different approaches that include nanomaterials, synthetic biology and information-processing tools. Although studies in this direction are very recent, they can be divided in two main categories: (i) communication between abiotic systems and (ii) communication between living and abiotic systems. Using illustrative examples, we give an overview of the ongoing progress, potential applications in different areas and current challenges. The engineering of chemical communication between micro/nanosystems represents a paradigm shift and may open a myriad of new concepts, applications and new technological possibilities in the near future in a number of research fields.

Bottom-Up Synthesis of Artificial Cells: Recent Highlights and Future Challenges

The bottom-up approach in synthetic biology aims to create molecular ensembles that reproduce the organization and functions of living organisms and strives to integrate them in a modular and hierarchical fashion toward the basic unit of life-the cell-and beyond. This young field stands on the shoulders of fundamental research in molecular biology and biochemistry, next to synthetic chemistry, and, augmented by an engineering framework, has seen tremendous progress in recent years thanks to multiple technological and scientific advancements. In this timely review of the research over the past decade, we focus on three essential features of living cells: the ability to self-reproduce via recursive cycles of growth and division, the harnessing of energy to drive cellular processes, and the assembly of metabolic pathways. In addition, we cover the increasing efforts to establish multicellular systems via different communication strategies and critically evaluate the potential applications.

Membrane-confined liquid-liquid phase separation toward artificial organelles

As the basic unit of life, cells are compartmentalized microreactors with molecularly crowded microenvironments. The quest to understand the cell origin inspires the design of synthetic analogs to mimic their functionality and structural complexity. In this work, we integrate membraneless coacervate microdroplets, a prototype of artificial organelles, into a proteinosome to build hierarchical protocells that may serve as a more realistic model of cellular organization. The protocell subcompartments can sense extracellular signals, take actions in response to these stimuli, and adapt their physicochemical behaviors. The tiered protocells are also capable of enriching biomolecular reactants within the confined organelles, thereby accelerating enzymatic reactions. The ability of signal processing inside protocells allows us to design the Boolean logic gates (NOR and NAND) using biochemical inputs. Our results highlight possible exploration of protocell-community signaling and render a flexible synthetic platform to study complex metabolic reaction networks and embodied chemical computation.

Interfacing Living and Synthetic Cells as an Emerging Frontier in Synthetic Biology

The construction of artificial cells from inanimate molecular building blocks is one of the grand challenges of our time. In addition to being used as simplified cell models to decipher the rules of life, artificial cells have the potential to be designed as micromachines deployed in a host of clinical and industrial applications. The attractions of engineering artificial cells from scratch, as opposed to re-engineering living biological cells, are varied. However, it is clear that artificial cells cannot currently match the power and behavioural sophistication of their biological counterparts. Given this, many in the synthetic biology community have started to ask: is it possible to interface biological and artificial cells together to create hybrid living/synthetic systems that leverage the advantages of both? This article will discuss the motivation behind this cellular bionics approach, in which the boundaries between living and non-living matter are blurred by bridging top-down and bottom-up synthetic biology. It details the state of play of this nascent field and introduces three generalised hybridisation modes that have emerged.

Interfacing Living and Synthetic Cells as an Emerging Frontier in Synthetic Biology

The construction of artificial cells from inanimate molecular building blocks is one of the grand challenges of our time. In addition to being used as simplified cell models to decipher the rules of life, artificial cells have the potential to be designed as micromachines deployed in a host of clinical and industrial applications. The attractions of engineering artificial cells from scratch, as opposed to re-engineering living biological cells, are varied. However, it is clear that artificial cells cannot currently match the power and behavioural sophistication of their biological counterparts. Given this, many in the synthetic biology community have started to ask: is it possible to interface biological and artificial cells together to create hybrid living/synthetic systems that leverage the advantages of both? This article will discuss the motivation behind this cellular bionics approach, in which the boundaries between living and non-living matter are blurred by bridging top-down and bottom-up synthetic biology. It details the state of play of this nascent field and introduces three generalised hybridisation modes that have emerged.

A chemical circular communication network at the nanoscale

In nature, coordinated communication between different entities enables a group to accomplish sophisticated functionalities that go beyond those carried out by individual agents. The possibility of programming and developing coordinated communication networks at the nanoscale-based on the exchange of chemical messengers-may open new approaches in biomedical and communication areas. Here, a stimulus-responsive circular model of communication between three nanodevices based on enzyme-functionalized Janus Au-mesoporous silica capped nanoparticles is presented. The output in the community of nanoparticles is only observed after a hierarchically programmed flow of chemical information between the members.

Leveraging synthetic particles for communication: from passive to active systems

Communication is one of the most remarkable behaviors in the living world. It is an important prerequisite for building an artificial cell which can be considered as alive. Achieving complex communicative behaviors leveraging synthetic particles will likely fill the gap between artificial vesicles and natural counterpart of cells and allow for the discovery of new therapies in medicine. In this review, we highlight recent endeavors for constructing communication with synthetic particles by revealing the principles underlying the communicative behaviors. Emergent progress using active particles to achieve communication is also discussed, which resembles the dynamic and out-of-equilibrium properties of communication in nature.

Interface Engineering in Multiphase Systems toward Synthetic Cells and Organelles: From Soft Matter Fundamentals to Biomedical Applications

Synthetic cells have a major role in gaining insight into the complex biological processes of living cells; they also give rise to a range of emerging applications from gene delivery to enzymatic nanoreactors. Living cells rely on compartmentalization to orchestrate reaction networks for specialized and coordinated functions. Principally, the compartmentalization has been an essential engineering theme in constructing cell-mimicking systems. Here, efforts to engineer liquid-liquid interfaces of multiphase systems into membrane-bounded and membraneless compartments, which include lipid vesicles, polymer vesicles, colloidosomes, hybrids, and coacervate droplets, are summarized. Examples are provided of how these compartments are designed to imitate biological behaviors or machinery, including molecule trafficking, growth, fusion, energy conversion, intercellular communication, and adaptivity. Subsequently, the state-of-art applications of these cell-inspired synthetic compartments are discussed. Apart from being simplified and cell models for bridging the gap between nonliving matter and cellular life, synthetic compartments also are utilized as intracellular delivery vehicles for nuclei acids and nanoreactors for biochemical synthesis. Finally, key challenges and future directions for achieving the full potential of synthetic cells are highlighted.

Invasion and Defense Interactions between Enzyme-Active Liquid Coacervate Protocells and Living Cells

The design and construction of mutual interaction models between artificial microsystems and living cells have the potential to open a wide range of novel applications in biomedical and biomimetic technologies. In this study, an artificial form of invasion-defense mutual interactions is established in a community of glucose oxidase (GOx)-containing liquid coacervate microdroplets and living cells, which interact via enzyme-mediated reactive oxygen species (ROS) damage. The enzyme-containing coacervate microdroplets, formed via liquid-liquid phase separation, act as invader protocells to electrostatically bind with the host HepG2 cell, resulting in assimilation. Subsequently, the glucose oxidation in the liquid coacervates initiates the generation of H₂ O₂ , which serves as an ROS resource to block cell proliferation. As a defense strategy, introduction of catalase (CAT) into the host cells is exploited to resist the ROS damage. CAT-mediated decomposition of H₂ O₂ leads to the ROS scavenging and results in the recovery of cell viability. The results obtained in the current study highlight the remarkable opportunities for the development of mutual interacting communities on the interface of artificial protocells/living cells. They also provide a new approach for engineering cellular behaviors through exploiting artificial nonliving microsystems.

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.

Bioinspired Protein-Based Assembling: Toward Advanced Life-Like Behaviors

The ability of living organisms to perform structure, energy, and information-related processes for molecular self-assembly through compartmentalization and chemical transformation can possibly be mimicked via artificial cell models. Recent progress in the development of various types of functional microcompartmentalized ensembles that can imitate rudimentary aspects of living cells has refocused attention on the important question of how inanimate systems can transition into living matter. Hence, herein, the most recent advances in the construction of protein-bounded microcompartments (proteinosomes), which have been exploited as a versatile synthetic chassis for integrating a wide range of functional components and biochemical machineries, are critically summarized. The techniques developed for fabricating various types of proteinosomes are discussed, focusing on the significance of how chemical information, substance transportation, enzymatic-reaction-based metabolism, and self-organization can be integrated and recursively exploited in constructed ensembles. Therefore, proteinosomes capable of exhibiting gene-directed protein synthesis, modulated membrane permeability, spatially confined membrane-gated catalytic reaction, internalized cytoskeletal-like matrix assembly, on-demand compartmentalization, and predatory-like chemical communication in artificial cell communities are specially highlighted. These developments are expected to bridge the gap between materials science and life science, and offer a theoretical foundation for developing life-inspired assembled materials toward various applications.