Artificial morphogen-mediated differentiation in synthetic protocells

Synthetic cells in tissue engineering

Tissue functions rely on complex structural, biochemical, and biomechanical cues that guide cellular behavior and organization. Synthetic cells, a promising new class of biomaterials, hold significant potential for mimicking these tissue properties using simplified, nonliving building blocks. Advanced synthetic cell models have already shown utility in biotechnology and immunology, including applications in cancer targeting and antigen presentation. Recent bottom-up approaches have also enabled synthetic cells to assemble into 3D structures with controlled intercellular interactions, creating tissue-like architectures. Despite these advancements, challenges remain in replicating multicellular behaviors and dynamic mechanical environments. Here, we review recent advancements in synthetic cell-based tissue formation and introduce a three-pillar framework to streamline the development of synthetic tissues. This approach, focusing on synthetic extracellular matrix integration, synthetic cell self-organization, and adaptive biomechanics, could enable scalable synthetic tissues engineering for regenerative medicine and drug development.

Programmed Fabrication of Vesicle-Based Prototissue Fibers with Modular Functionalities

Multicellular organisms have hierarchical structures where multiple cells collectively form tissues with complex 3D architectures and exhibit higher-order functions. Inspired by this, to date, multiple protocell models have been assembled to form tissue-like structures termed prototissues. Despite recent advances in this research area, the programmed assembly of protocells into prototissue fibers with emergent functions still represents a significant challenge. The possibility of assembling prototissue fibers will open up a way to a novel type of prototissue subunit capable of hierarchical assembly into unprecedented soft functional materials with tunable architectures, modular and distributed functionalities. Herein, the first method to fabricate freestanding vesicle-based prototissue fibers with controlled lengths and diameters is devised. Importantly, it is also shown that the fibers can be composed of different specialized modules that, for example, can endow the fiber with magnetotaxis capabilities, or that can work synergistically to take an input diffusible chemical signals and transduce it into a readable fluorescent output through a hosted enzyme cascade reaction. Overall, this research addresses an important challenge of prototissue engineering and will find important applications in 3D bio-printing, tissue engineering, and soft robotics as next-generation bioinspired materials.

Engineering the acoustic field with a Mie scatterer for microparticle patterning

The utilization of acoustic fields offers a contactless approach for microparticle manipulation in a miniaturized system, and plays a significant role in medicine, biology, chemistry, and engineering. Due to the acoustic radiation force arising from the scattering of the acoustic waves, small particles in the Rayleigh scattering range can be trapped, whilst their impact on the acoustic field is negligible. Manipulating larger particles in the Mie scattering regime is challenging due to the diverse scattering modes, which impacts the local acoustic field. The rapid movement of free-moving Mie scatterers in an acoustic standing wave field makes it difficult to study the interaction between a sound field and a Mie scatterer in an engineering context. Here, a combined approach that integrates theoretical analysis and experimental investigation was developed to explore the influence of a Mie scatterer on the acoustic field by fabricating an acoustic trapping device featuring a fixed Mie scatterer at its center. We demonstrate that an insonified Mie scatterer can operate as an acoustic emitter in water, enabling dynamic and versatile modulation of the total acoustic field. Such a scatterer can interact with one or multiple incident propagating acoustic waves, leading to the generation of a localized standing wave field in the vicinity of the scatterer. This local field can be controlled by the relative location of the scatterer with respect to the incident field leading to control over the transformation from an incident 1D acoustic field into a 2D acoustic field. This control paves the way for localized and multi-scale micro-object manipulation.

Spontaneous Emergence of Lipid Vesicles in a Coacervate-Based Compartmentalized System

The spontaneous emergence of lipid vesicles in the absence of evolved biological machinery represents a major challenge for bottom-up synthetic biology. We show that coacervate microdroplets could create a compartmentalized environment that enriches lipid molecules and facilitates their spontaneous assembly into lipid vesicles. These vesicles can escape from the coacervate microdroplets in a continuous process under non-equilibrium conditions, resembling a constant production process akin to a "primitive enzyme" factory assembly line. These findings significantly extend our understanding of the intricate interaction between lipid molecules and coacervate microdroplets, shedding light on the emergence of cellular systems and offering a new perspective on the conditions necessary for the development of life on Earth.

Adaptive metal ion transport and metalloregulation-driven differentiation in pluripotent synthetic cells

Pluripotent cells can yield different cell types determined by the specific sequence of differentiation signals that they encounter as the cell activates or deactivates functions and retains memory of previous inputs. Here, we achieved pluripotency in synthetic cells by incorporating three dormant apo-metalloenzymes such that they could differentiate towards distinct fates, depending on the sequence of specific metal ion transport with ionophores. In the first differentiation step, we selectively transported one of three extracellular metal ion cofactors into pluripotent giant unilamellar vesicles (GUVs), which resulted in elevation of intracellular pH, hydrogen peroxide production or GUV lysis. Previously added ionophores suppress transport with subsequent ionophores owing to interactions among them in the membrane, as corroborated by atomistic simulations. Consequently, the addition of a second ionophore elicits a dampened response in the multipotent GUV and a third ionophore results in no further response, reminiscent of a terminally differentiated GUV. The pluripotent GUV can differentiate into five final fates, depending on the sequence in which the three ionophores are added.

Positional Information-Based Organization of Surfactant Droplet Swarms Emerging from Competition Between Local and Global Marangoni Effects

Positional information is key for particles to adapt their behavior based on their position in external concentration gradients, and thereby self-organize into complex patterns. Here, position-dependent behavior of floating surfactant droplets that self-organize in a pH gradient is demonstrated, using the Marangoni effect to translate gradients of surface-active molecules into motion. First, fields of surfactant microliter-droplets are generated, in which droplets floating on water drive local, outbound Marangoni flows upon dissolution of surfactant and concomitantly grow myelin filaments. Next, a competing surfactant based on a hydrolysable amide is introduced, which is more surface active than the myelin surfactant and thereby inhibits the local Marangoni flows and myelin growth from the droplets. Upon introducing a pH gradient, the amide surfactant hydrolyses in the acidic region, so that the local Marangoni flows and myelin growth are reestablished. The resulting combination of local and global surface tension gradients produces a region of myelin-growing droplets and a region where myelin growth is suppressed, separated by a wave front of closely packed droplets, of which the position can be controlled by the pH gradient. Thereby, it is shown how "French flag"-patterns, in synthetic settings typically emerging from reaction-diffusion systems, can also be established via surfactant droplet systems.

Matching Together Living Cells and Prototissues: Will There Be Chemistry?

Scientific advancements in bottom-up synthetic biology have led to the development of numerous models of synthetic cells, or protocells. To date, research has mainly focused on increasing the (bio)chemical complexity of these bioinspired micro-compartmentalized systems, yet the successful integration of protocells with living cells remains one of the major challenges in bottom-up synthetic biology. In this review, we aim to summarize the current state of the art in hybrid protocell/living cell and prototissue/living cell systems. Inspired by recent breakthroughs in tissue engineering, we review the chemical, bio-chemical, and mechano-chemical aspects that hold promise for achieving an effective integration of non-living and living matter. The future production of fully integrated protocell/living cell systems and increasingly complex prototissue/living tissue systems not only has the potential to revolutionize the field of tissue engineering, but also paves the way for new technologies in (bio)sensing, personalized therapy, and drug delivery.

Dynamically Reconfigurable Micro-Patterned Hydrogels Based on Magnetic Pickering Emulsion Droplets

Reconfigurability within hydrogels has emerged as an attractive functionality that can be used in information encryption, cargo/delivery, environmental remediation, soft robotics, and medicine. Here micro-patterned polymer hydrogels capable of temperature-dependent reconfigurability are fabricated. For this, the hydrogels are provided with micron-sized Pickering emulsion droplets stabilized by magnetic particles, which are capable of harnessing energy from external force fields. The droplets can both migrate under magnetic field gradients and heat the environment when laser irradiated. These functions not only affect a single compartment but have higher-order effects on the mesoscale, thanks to the temperature-responsiveness of the polymeric network. This double responsiveness is exploited to control the spatial organization of hundreds of droplets within the hydrogel matrix and form predesigned and sophisticated patterns. Furthermore, pattern self-reconfiguration driven by the droplets themselves upon laser irradiation is induced. Finally, we show that due to their internal liquid phase, the droplets can be used as reservoirs of hydrophobic nutrients for living cells (i.e., Yarrowia lipolytica yeast) in the solid-like environment of the polymeric network, and demonstrate communication between the droplets and the cells to facilitate nutrient uptake. Altogether, the results provide opportunities for the development of stimuli-sensitive polymer hydrogels with post-synthesis reprogrammable response using micro-compartments as building blocks.

Engineering the Spatial Distribution of Amphiphilic Molecule within Complex Coacervate Microdroplet via Modulating Charge Strength of Polyelectrolytes

The investigation of the interplay between complex coacervate microdroplets and amphiphilic molecules offers valuable insights into the processes of prebiotic compartmentalization on the early Earth and presents a promising avenue for future advancements in biotechnology. Herein, the interaction between complex coacervate microdroplets and amphiphilic molecule (decanoic acid) is systematically investigated by varying charge strengths of negatively charged polyelectrolytes (DNA and PAA) and positively charged polyelectrolytes (PDDA and DEAE-Dextran). It is found that the interaction between amphiphilic molecule and complex coacervate microdroplets depended on the delicate balance between the interaction between decanoic acid and polyelectrolyte and the interaction between two polyelectrolytes. The different spatial distribution of amphiphilic molecule can result in differences in the internal microenvironment, which can further alter the uptake or exclusion of small molecules and biomolecules with different charges and polarities and functional biological process.

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.

Self-assembly of stabilized droplets from liquid-liquid phase separation for higher-order structures and functions

Dynamic microscale droplets produced by liquid-liquid phase separation (LLPS) have emerged as appealing biomaterials due to their remarkable features. However, the instability of droplets limits the construction of population-level structures with collective behaviors. Here we first provide a brief background of droplets in the context of materials properties. Subsequently, we discuss current strategies for stabilizing droplets including physical separation and chemical modulation. We also discuss the recent development of LLPS droplets for various applications such as synthetic cells and biomedical materials. Finally, we give insights on how stabilized droplets can self-assemble into higher-order structures displaying coordinated functions to fully exploit their potentials in bottom-up synthetic biology and biomedical applications.

Superstructural ordering in self-sorting coacervate-based protocell networks

Bottom-up assembly of higher-order cytomimetic systems capable of coordinated physical behaviours, collective chemical signalling and spatially integrated processing is a key challenge in the study of artificial multicellularity. Here we develop an interactive binary population of coacervate microdroplets that spontaneously self-sort into chain-like protocell networks with an alternating sequence of structurally and compositionally dissimilar microdomains with hemispherical contact points. The protocell superstructures exhibit macromolecular self-sorting, spatially localized enzyme/ribozyme biocatalysis and interdroplet molecular translocation. They are capable of topographical reconfiguration using chemical or light-mediated stimuli and can be used as a micro-extraction system for macroscale biomolecular sorting. Our methodology opens a pathway towards the self-assembly of multicomponent protocell networks based on selective processes of coacervate droplet-droplet adhesion and fusion, and provides a step towards the spontaneous orchestration of protocell models into artificial tissues and colonies with ordered architectures and collective functions.

Ultrasound-Induced Enrichment of Ultra-Trace miRNA Biosensing in Nanoliter Samples

The rapid and accurate analysis of micro-samples is a crucial foundation for precision medicine, particularly for early screening and monitoring of cancer, where it holds significant importance. Ultrasound-based multifunctional biocompatible manipulation techniques have been extensively applied in a variety of biomedical fields, providing insights for the development of rapid, cost-effective, and accurate biomarker detection strategies. In this chapter, we combine ultrasound-based gradient pressure fields with functionalized microsphere enrichment to develop a biosensing method for ultra-trace miRNA enrichment in nanoliter samples without PCR. This system relies on inexpensive capillaries, enabling simultaneous visual imaging and trace sample detection.

Ultrasound-enhanced catalytic hairpin assembly capable of ultrasensitive microRNA biosensing for the early screening of Alzheimer's disease

Catalytic hairpin assembly (CHA) is a promising enzyme-free, isothermal signal amplification strategy, but the relatively time-consuming strand replacement limits its application scenarios. Here, we developed an ultrasound-enhanced catalytic hairpin assembly (UECHA) biosensing platform for early screening of Alzheimer's disease by introducing a portable acoustic-drive platform with functionalized microspheres for effective biomarkers enrichment and fluorescence enhancement. By constructing a gradient ultrasonic field in a microcavity, the platform concentrates the functionalized microspheres in a central position, accompanied by an enhanced fluorescence signal with a specific release. In addition, the programmable frequency modulation can also modify the acoustic potential well and effectively promote non-equilibrium chemical reactions such as CHA (25 min). Compared with the conventional catalytic hairpin assembly (CHA), UECHA allows for direct and quantitative measurement of AD miRNAs down to 3.55 × 10-15 M in 1 μL samples. This visual analysis of ultra-trace biomarkers based on acoustic enrichment and promotion provides a new perspective for the rapid and highly sensitive clinical detection of Alzheimer's disease.

Cholesterol-Mediated Anchoring of Phospholipids onto Proteinosomes for Switching Membrane Permeability

Modulated membrane functionalization is a necessary and overarching step for hollow microcompartments toward their application as nanoreactors or artificial cells. In this study, we show a way to generate phospholipid hybrid proteinosomes that could show superposed virtues of liposomes and proteinosomes. In comparison to pure proteinosomes, both the membrane fluidity and permeability are improved obviously after forming the phospholipid hybrid proteinosomes. Specifically, the integration of phospholipids also endows the hybrid proteinosomes demonstrating a stepwise release of the encapsulants of FITC-dextran (70 and 150 kDa) triggered sequentially by phospholipase and protease, and then a modulated cascaded enzymatic reaction between two different populations of proteinosomes are achieved. Therefore, it is anticipated that such constructed phospholipid hybrid proteinosomes could be employed as an improved microcompartmental model for further advanced artificial cell design toward achieving logic signal communication within the various artificial cellular populations as well as potential applications in the field of microreactors.

Hierarchical Structuration in Protocellular System

Spatial control is one of the ubiquitous features in biological systems and the key to the functional complexity of living cells. The strategies to achieve such precise spatial control in protocellular systems are crucial to constructing complex artificial living systems with functional collective behavior. Herein, the authors review recent advances in the spatial control within a single protocell or between different protocells and discuss how such hierarchical structured protocellular system can be used to understand complex living systems or to advance the development of functional microreactors with the programmable release of various biomacromolecular payloads, or smart protocell-biological cell hybrid system.

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.

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.

Interface-mediated protein aggregation

The aggregation of proteins at interfaces has significant roles and can also lead to dysfunction of different physiological processes. The interfacial effects on the assembly and aggregation of biopolymers are not only crucial for a comprehensive understanding of protein biological functions, but also hold great potential for advancing the state-of-the-art applications of biopolymer materials. Recently, there has been remarkable progress in a collaborative context, as we strive to gain control over complex interfacial assembly structures of biopolymers. These biopolymer structures range from the nanoscale to mesoscale and even macroscale, and are attained through the rational design of interactions between biological building blocks and surfaces/interfaces. This review spotlights the recent advancements in interface-mediated assembly and properties of biopolymer materials. Initially, we introduce the solid-liquid interface (SIL)-mediated biopolymer assembly that includes the inorganic crystalline template effect and protein self-adoptive deposition through phase transition. Next, we display the advancement of biopolymer assembly instigated by the air-water interface (AWI) that acts as an energy conversion station. Lastly, we discuss succinctly the assembly of biopolymers at the liquid-liquid interface (LLI) along with their applications. It is our hope that this overview will stimulate the integration and progression of the science of interfacial assembled biopolymer materials and surfaces/interfaces.

Regulation of species metabolism in synthetic community systems by environmental pH oscillations

Constructing a synthetic community system helps scientist understand the complex interactions among species in a community and its environment. Herein, a two-species community is constructed with species A (artificial cells encapsulating pH-responsive molecules and sucrose) and species B (Saccharomyces cerevisiae), which causes the environment to exhibit pH oscillation behaviour due to the generation and dissipation of CO2. In addition, a three-species community is constructed with species A' (artificial cells containing sucrose and G6P), species B, and species C (artificial cells containing NAD+ and G6PDH). The solution pH oscillation regulates the periodical release of G6P from species A'; G6P then enters species C to promote the metabolic reaction that converts NAD+ to NADH. The location of species A' and B determines the metabolism behaviour in species C in the spatially coded three-species communities with CA'B, CBA', and A'CB patterns. The proposed synthetic community system provides a foundation to construct a more complicated microecosystem.

Peptide-Based Coacervate Protocells with Cytoprotective Metal-Phenolic Network Membranes

Protocells have garnered considerable attention from cell biologists, materials scientists, and synthetic biologists. Phase-separating coacervate microdroplets have emerged as a promising cytomimetic model because they can internalize and concentrate components from dilute surrounding environments. However, the membrane-free nature of such coacervates leads to coalescence into a bulk phase, a phenomenon that is not representative of the cells they are designed to mimic. Herein, we develop a membranized peptide coacervate (PC) with oppositely charged oligopeptides as the molecularly crowded cytosol and a metal-phenolic network (MPN) coating as the membrane. The hybrid protocell efficiently internalizes various bioactive macromolecules (e.g., bovine serum albumin and immunoglobulin G) (>90%) while also resisting radicals due to the semipermeable cytoprotective membrane. Notably, the resultant PC@MPNs are capable of anabolic cascade reactions and remain in discrete protocellular populations without coalescence. Finally, we demonstrate that the MPN protocell membrane can be postfunctionalized with various functional molecules (e.g., folic acid and fluorescence dye) to more closely resemble actual cells with complex membranes, such as recognition molecules, which allows for drug delivery. This membrane-bound cytosolic protocell structure paves the way for innovative synthetic cells with structural and functional complexity.

Programmable-Modulated Ultrasonic Transducer Array for Contactless Detection of Viral RNAs

The current polymerase chain reactions-based nucleic acid tests for large-scale infectious disease diagnosis are always lab-dependent and generate large amounts of highly infectious plastic waste. Direct non-linear acoustic driven of microdroplets provide an ideal platform for contactless spatial and temporal manipulation of liquid samples. Here, a strategy to programmable-manipulate microdroplets using potential pressure well for contactless trace detection is conceptualized and designed. On such contactless modulation platform, up to seventy-two piezoelectric transducers are precisely self-focusing single-axis arranged and controlled, which can generate dynamic pressure nodes for effectively contact-free manipulating microdroplets without vessel contamination. In addition, the patterned microdroplet array can act as contactless microreactor and allow multiple trace samples (1-5 µL) biochemical analysis, and the ultrasonic vortex can also accelerate non-equilibrium chemical reactions such as recombinase polymerase amplification (RPA). The results of fluorescence detection indicated that such programmable modulated microdroplet achieved contactless trace nucleic acid detection with a sensitivity of 0.21 copy µL-1 in only 6-14 min, which is 30.3-43.3% shorter than the conventional RPA approach. Such a programmable containerless microdroplet platform can be used for toxic, hazardous, or infectious samples sensing, opening up new avenues for developing future fully automated detection systems.

Enabling technology and core theory of synthetic biology

Synthetic biology provides a new paradigm for life science research ("build to learn") and opens the future journey of biotechnology ("build to use"). Here, we discuss advances of various principles and technologies in the mainstream of the enabling technology of synthetic biology, including synthesis and assembly of a genome, DNA storage, gene editing, molecular evolution and de novo design of function proteins, cell and gene circuit engineering, cell-free synthetic biology, artificial intelligence (AI)-aided synthetic biology, as well as biofoundries. We also introduce the concept of quantitative synthetic biology, which is guiding synthetic biology towards increased accuracy and predictability or the real rational design. We conclude that synthetic biology will establish its disciplinary system with the iterative development of enabling technologies and the maturity of the core theory.

Membranous and Membraneless Interfaces-Origins of Artificial Cellular Complexity

Living cell architecture is based on the concept of micro-compartmentation at different hierarchical levels [...].

Non-interfacial self-assembly of synthetic protocells

BACKGROUND

Protocell refers to the basic unit of life and synthetic molecular assembly with cell structure and function. The protocells have great applications in the field of biomedical technology. Simulating the morphology and function of cells is the key to the preparation of protocells. However, some organic solvents used in the preparation process of protocells would damage the function of the bioactive substance. Perfluorocarbon, which has no toxic effect on bioactive substances, is an ideal solvent for protocell preparation. However, perfluorocarbon cannot be emulsified with water because of its inertia.

METHODS

Spheroids can be formed in nature even without emulsification, since liquid can reshape the morphology of the solid phase through the scouring action, even if there is no stable interface between the two phases. Inspired by the formation of natural spheroids such as pebbles, we developed non-interfacial self-assembly (NISA) of microdroplets as a step toward synthetic protocells, in which the inert perfluorocarbon was utilized to reshape the hydrogel through the scouring action.

RESULTS

The synthetic protocells were successfully obtained by using NISA-based protocell techniques, with the morphology very similar to native cells. Then we simulated the cell transcription process in the synthetic protocell and used the protocell as an mRNA carrier to transfect 293T cells. The results showed that protocells delivered mRNAs, and successfully expressed proteins in 293T cells. Further, we used the NISA method to fabricate an artificial cell by extracting and reassembling the membrane, proteins, and genomes of ovarian cancer cells. The results showed that the recombination of tumor cells was successfully achieved with similar morphology as tumor cells. In addition, the synthetic protocell prepared by the NISA method was used to reverse cancer chemoresistance by restoring cellular calcium homeostasis, which verified the application value of the synthetic protocell as a drug carrier.

CONCLUSION

This synthetic protocell fabricated by the NISA method simulates the occurrence and development process of primitive life, which has great potential application value in mRNA vaccine, cancer immunotherapy, and drug delivery.

Autonomic Integration in Nested Protocell Communities

The self-driven organization of model protocells into higher-order nested cytomimetic systems with coordinated structural and functional relationships offers a step toward the autonomic implementation of artificial multicellularity. Here, we describe an endosymbiotic-like pathway in which proteinosomes are captured within membranized alginate/silk fibroin coacervate vesicles by guest-mediated reconfiguration of the host protocells. We demonstrate that interchange of coacervate vesicle and droplet morphologies through proteinosome-mediated urease/glucose oxidase activity produces discrete nested communities capable of integrated catalytic activity and selective disintegration. The self-driving capacity is modulated by an internalized fuel-driven process using starch hydrolases sequestered within the host coacervate phase, and structural stabilization of the integrated protocell populations can be achieved by on-site enzyme-mediated matrix reinforcement involving dipeptide supramolecular assembly or tyramine-alginate covalent cross-linking. Our work highlights a semi-autonomous mechanism for constructing symbiotic cell-like nested communities and provides opportunities for the development of reconfigurable cytomimetic materials with structural, functional, and organizational complexity.

Acoustic Trapping: An Emerging Tool for Microfabrication Technology

The high throughput deposition of microscale objects with precise spatial arrangement represents a key step in microfabrication technology. This can be done by creating physical boundaries to guide the deposition process or using printing technologies; in both approaches, these microscale objects cannot be further modified after they are formed. The utilization of dynamic acoustic fields offers a novel approach to facilitate real-time reconfigurable miniaturized systems in a contactless manner, which can potentially be used in physics, chemistry, biology, as well as materials science. Here, the physical interactions of microscale objects in an acoustic pressure field are discussed and how to fabricate different acoustic trapping devices and how to tune the spatial arrangement of the microscale objects are explained. Moreover, different approaches that can dynamically modulate microscale objects in acoustic fields are presented, and the potential applications of the microarrays in biomedical engineering, chemical/biochemical sensing, and materials science are highlighted alongside a discussion of future research challenges.

Membranized Coacervate Microdroplets: from Versatile Protocell Models to Cytomimetic Materials

Although complex coacervate microdroplets derived from associative phase separation of counter-charged electrolytes have emerged as a broad platform for the bottom-up construction of membraneless, molecularly crowded protocells, the absence of an enclosing membrane limits the construction of more sophisticated artificial cells and their use as functional cytomimetic materials. To address this problem, we and others have recently developed chemical-based strategies for the membranization of preformed coacervate microdroplets. In this Account, we review our recent work on diverse coacervate systems using a range of membrane building blocks and assembly processes. First, we briefly introduce the unusual nature of the coacervate/water interface, emphasizing the ultralow interfacial tension and broad interfacial width as physiochemical properties that require special attention in the judicious design of membranized coacervate microdroplets. Second, we classify membrane assembly into two different approaches: (i) interfacial self-assembly by using diverse surface-active building blocks such as molecular amphiphiles (fatty acids, phospholipids, block copolymers, protein-polymer conjugates) or nano- and microscale objects (liposomes, nanoparticle surfactants, cell fragments, living cells) with appropriate wettability; and (ii) coacervate droplet-to-vesicle reconfiguration by employing auxiliary surface reconstruction agents or triggering endogenous transitions (self-membranization) under nonstoichiometric (charge mismatched) conditions. We then discuss the key cytomimetic behaviors of membranized coacervate-based model protocells. Customizable permeability is achieved by synergistic effects operating between the molecularly crowded coacervate interior and surrounding membrane. In contrast, metabolic-like endogenous reactivity, diffusive chemical signaling, and collective chemical operations occur specifically in protocell networks comprising diverse populations of membranized coacervate microdroplets. In each case, these cytomimetic behaviors can give rise to functional microscale materials capable of promising cell-like applications. For example, immobilizing spatially segregated enzyme-loaded phospholipid-coated coacervate protocells in concentrically tubular hydrogels delivers prototissue-like bulk materials that generate nitric oxide in vitro, enabling platelet deactivation and inhibition of blood clot formation. Alternatively, therapeutic protocells with in vivo vasoactivity, high hemocompatibility, and increased blood circulation times are constructed by spontaneous assembly of hemoglobin-containing cell-membrane fragments on the surface of enzyme-loaded coacervate microdroplets. Higher-order properties such as artificial endocytosis are achieved by using nanoparticle-caged coacervate protocell hosts that selectively and actively capture guest nano- and microscale objects by responses to exogenous stimuli or via endogenous enzyme-mediated reactions. Finally, we discuss the current limitations in the design and programming of membranized coacervate microdroplets, which may help to guide future directions in this emerging research area. Taken together, we hope that this Account will inspire new advances in membranized coacervate microdroplets and promote their application in the development of integrated protocell models and functional cytomimetic materials.

Bottom-up assembly of viral replication cycles

Bottom-up synthetic biology provides new means to understand living matter by constructing minimal life-like systems. This principle can also be applied to study infectious diseases. Here we summarize approaches and ethical considerations for the bottom-up assembly of viral replication cycles.

Artificial Cells: Past, Present and Future

Artificial cells are constructed to imitate natural cells and allow researchers to explore biological process and the origin of life. The construction methods for artificial cells, through both top-down or bottom-up approaches, have achieved great progress over the past decades. Here we present a comprehensive overview on the development of artificial cells and their properties and applications. Artificial cells are derived from lipids, polymers, lipid/polymer hybrids, natural cell membranes, colloidosome, metal-organic frameworks and coacervates. They can be endowed with various functions through the incorporation of proteins and genes on the cell surface or encapsulated inside of the cells. These modulations determine the properties of artificial cells, including producing energy, cell growth, morphology change, division, transmembrane transport, environmental response, motility and chemotaxis. Multiple applications of these artificial cells are discussed here with a focus on therapeutic applications. Artificial cells are used as carriers for materials and information exchange and have been shown to function as targeted delivery systems of personalized drugs. Additionally, artificial cells can function to substitute for cells with impaired function. Enzyme therapy and immunotherapy using artificial cells have been an intense focus of research. Finally, prospects of future development of cell-mimic properties and broader applications are highlighted.

Integrated Acoustic Chip for Culturing 3D Cell Arrays

Three-dimensional (3D) cell arrays provide an in vitro platform for clinical drug screening, but the bulky culture devices limit their application scenarios. Here, we demonstrate an integrated portable device that can realize contact-free construction of 3D cell spheroids. The interaction between the ultrasound generated by the portable device and the capillary results in periodic pressure nodes or anti-nodes, which lead to form a 3D cell array for cell culture. Such a 3D cell array pattern can be constructed in seconds and requires only 1 μL of cell samples. We further assessed the spheroids formed by the portable device and the impact of the acoustic field on spheroids and demonstrated the drug screening with assembled spheroids. More importantly, the integrated acoustic device can be further integrated with other components for more complex cell culture and all-round analysis. This portable and effective integrated device provides a new avenue for clinical biomedicine.

Temporal Stimulus Patterns Drive Differentiation of a Synthetic Dipeptide-Based Coacervate

The fate of living cells often depends on their processing of temporally modulated information, such as the frequency and duration of various signals. Synthetic stimulus-responsive systems have been intensely studied for >50 years, but it is still challenging for chemists to create artificial systems that can decode dynamically oscillating stimuli and alter the systems' properties/functions because of the lack of sophisticated reaction networks that are comparable with biological signal transduction. Here, we report morphological differentiation of synthetic dipeptide-based coacervates in response to temporally distinct patterns of the light pulse. We designed a simple cationic diphenylalanine peptide derivative to enable the formation of coacervates. The coacervates concentrated an anionic methacrylate monomer and a photoinitiator, which provided a unique reaction environment and facilitated light-triggered radical polymerization─even in air. Pulsed light irradiation at 9.0 Hz (but not at 0.5 Hz) afforded anionic polymers. This dependence on the light pulse patterns is attributable to the competition of reactive radical intermediates between the methacrylate monomer and molecular oxygen. The temporal pulse pattern-dependent polymer formation enabled the coacervates to differentiate in terms of morphology and internal viscosity, with an ultrasensitive switch-like mode. Our achievements will facilitate the rational design of smart supramolecular soft materials and are insightful regarding the synthesis of sophisticated chemical cells.

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.

Programmable Fusion and Differentiation of Synthetic Minimal Cells

Synthetic cells can mimic the intricate complexities of live cells, while mitigating the level of noise that is present natural systems; however, many crucial processes still need to be demonstrated in synthetic cells to use them to comprehensively study and engineer biology. Here we demonstrate key functionalities of synthetic cells previously available only to natural life: differentiation and mating. This work presents a toolset for engineering combinatorial genetic circuits in synthetic cells. We demonstrate how progenitor populations can differentiate into new lineages in response to small molecule stimuli or as a result of fusion, and we provide practical demonstration of utility for metabolic engineering. This work provides a tool for bioengineering and for natural pathway studies, as well as paving the way toward the construction of live artificial cells.

Building a community to engineer synthetic cells and organelles from the bottom-up

Employing concepts from physics, chemistry and bioengineering, 'learning-by-building' approaches are becoming increasingly popular in the life sciences, especially with researchers who are attempting to engineer cellular life from scratch. The SynCell2020/21 conference brought together researchers from different disciplines to highlight progress in this field, including areas where synthetic cells are having socioeconomic and technological impact. Conference participants also identified the challenges involved in designing, manipulating and creating synthetic cells with hierarchical organization and function. A key conclusion is the need to build an international and interdisciplinary research community through enhanced communication, resource-sharing, and educational initiatives.

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.

Dendrimer-Based Polyion Complex Vesicles: Loops Make Loose

Associations of amphiphiles assume their various morphologies according to the so-called packing parameter under thermodynamic control. However, one may raise the question of whether polymers can always relax fast enough to obey thermodynamic control, and how this may be checked. Here, a case of polyion complex (PIC) assemblies where the morphology appears to be subject to kinetic control is discussed. Poly (ethylene oxide)-b-(styrene sulfonate) block copolymers are combined with cationic PAMAM dendrimers of various generations (2-7). The PEO-PSS diblocks, and the corresponding PSS-PEO-PSS triblocks should have nearly identical packing parameters, but surprisingly creat different assemblies, namely core-shell micelles and vesicles, respectively. Moreover, the micelles are very stable against added salt, whereas the vesicles are not only much more sensitive to added salt, but also appear to exchange matter on relevant time scales. The small and largely quenched early-stage precursor complexes are responsible for the morphological and dynamic differences, implying that kinetic control may also be a way to obtain particles with well-defined and useful properties. The exciting new finding that triblocks produce more "active" vesicles will hopefully trigger the exploration of more pathways, and so learn how to tune PICsomes toward specific applications.

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)'.

A review of TNP-ATP in protein binding studies: benefits and pitfalls

We review 50 years of use of 2',3'-O-trinitrophenyl (TNP)-ATP, a fluorescently tagged ATP analog. It has been extensively used to detect binding interactions of ATP to proteins and to measure parameters of those interactions such as the dissociation constant, K_d, or inhibitor dissociation constant, K_i. TNP-ATP has also found use in other applications, for example, as a fluorescence marker in microscopy, as a FRET pair, or as an antagonist (e.g., of P2X receptors). However, its use in protein binding studies has limitations because the TNP moiety often enhances binding affinity, and the fluorescence changes that occur with binding can be masked or mimicked in unexpected ways. The goal of this review is to provide a clear perspective of the pros and cons of using TNP-ATP to allow for better experimental design and less ambiguous data in future experiments using TNP-ATP and other TNP nucleotides.

Construction of Hybrid Bi-microcompartments with Exocytosis-Inspired Behavior toward Fast Temperature-Modulated Transportation of Living Organisms

Inspired by the unique characteristics of living cells, the creation of life-inspired functional ensembles is a rapidly expanding research topic, enabling transformative applications in various disciplines. Herein, we report a facile method for the fabrication of phospholipid and block copolymer hybrid bi-microcompartments via spontaneous asymmetric assembly at the water/tributyrin interface, whereby the temperature-mediated dewetting of the inner microcompartments allowed for exocytosis to occur in the constructed system. The exocytosis location and commencement time could be controlled by the buoyancy of the inner microcompartment and temperature, respectively. Furthermore, the constructed bi-microcompartments showed excellent biocompatibility and a universal loading capacity toward cargoes of widely ranging sizes; thus, the proliferation and temperature-programmed transportation of living organisms was achieved. Our results highlight opportunities for the development of complex mesoscale dynamic ensembles with life-inspired behaviors and provide a novel platform for on-demand transport of various living organisms.

Synthesis of lipid membranes for artificial cells

A major goal of synthetic biology is to understand the transition between non-living matter and life. The bottom-up development of an artificial cell would provide a minimal system with which to study the border between chemistry and biology. So far, a fully synthetic cell has remained elusive, but chemists are progressing towards this goal by reconstructing cellular subsystems. Cell boundaries, likely in the form of lipid membranes, were necessary for the emergence of life. In addition to providing a protective barrier between cellular cargo and the external environment, lipid compartments maintain homeostasis with other subsystems to regulate cellular processes. In this Review, we examine different chemical approaches to making cell-mimetic compartments. Synthetic strategies to drive membrane formation and function, including bioorthogonal ligations, dissipative self-assembly and reconstitution of biochemical pathways, are discussed. Chemical strategies aim to recreate the interactions between lipid membranes, the external environment and internal biomolecules, and will clarify our understanding of life at the interface of chemistry and biology.

Droplet Microfluidics for Tumor Drug-Related Studies and Programmable Artificial Cells

Anticancer drug development is a crucial step toward cancer treatment, that requires realistic predictions of malignant tissue development and sophisticated drug delivery. Tumors often acquire drug resistance and drug efficacy, hence cannot be accurately predicted in 2D tumor cell cultures. On the other hand, 3D cultures, including multicellular tumor spheroids (MCTSs), mimic the in vivo cellular arrangement and provide robust platforms for drug testing when grown in hydrogels with characteristics similar to the living body. Microparticles and liposomes are considered smart drug delivery vehicles, are able to target cancerous tissue, and can release entrapped drugs on demand. Microfluidics serve as a high-throughput tool for reproducible, flexible, and automated production of droplet-based microscale constructs, tailored to the desired final application. In this review, it is described how natural hydrogels in combination with droplet microfluidics can generate MCTSs, and the use of microfluidics to produce tumor targeting microparticles and liposomes. One of the highlights of the review documents the use of the bottom-up construction methodologies of synthetic biology for the formation of artificial cellular assemblies, which may additionally incorporate both target cancer cells and prospective drug candidates, as an integrated "droplet incubator" drug assay platform.

Synthetic tissue engineering with smart, cytomimetic protocells

Synthetic protocells are rudimentary origin-of-life versions of natural cell counterparts. Protocells are widely engineered to advance efforts and useful accepted outcomes in synthetic biology, soft matter chemistry and bioinspired materials chemistry. Protocells in collective symbiosis generate synthetic proto-tissues that display unprecedented autonomy and yield advanced materials with desirable life-like features for smart multi-drug delivery, micro bioreactors, renewable fuel production, environmental clean-up, and medicine. Current levels of protocell and proto-tissue functionality and adaptivity are just sufficient to apply them in tissue engineering and regenerative medicine, where they animate biomaterials and increase therapeutic cell productivity. As of now, structural biomaterials for tissue engineering lack the properties of living biomaterials such as self-repair, stochasticity, cell synergy and the sequencing of molecular and cellular events. Future protocell-based biomaterials provide these core properties of living organisms, but excluding evolution. Most importantly, protocells are programmable for a broad array of cell functions and behaviors and collectively in consortia are tunable for multivariate functions. Inspired by upcoming designs of smart protocells, we review their developmental background and cover the most recently reported developments in this promising field of synthetic proto-biology. Our emphasis is on manufacturing proto-tissues for tissue engineering of organoids, stem cell niches and reprogramming and tissue formation through stages of embryonic development. We also highlight the exciting reported developments arising from fusing living cells and tissues, in a valuable hybrid symbiosis, with synthetic counterparts to bring about novel functions, and living tissue products for a new synthetic tissue engineering discipline.