Protocells programmed through artificial reaction networks

DNA-empowered synthetic cells as minimalistic life forms

Cells, the fundamental units of life, orchestrate intricate functions - motility, adaptation, replication, communication, and self-organization within tissues. Originating from spatiotemporally organized structures and machinery, coupled with information processing in signalling networks, cells embody the 'sensor-processor-actuator' paradigm. Can we glean insights from these processes to construct primitive artificial systems with life-like properties? Using de novo design approaches, what can we uncover about the evolutionary path of life? This Review discusses the strides made in crafting synthetic cells, utilizing the powerful toolbox of structural and dynamic DNA nanoscience. We describe how DNA can serve as a versatile tool for engineering entire synthetic cells or subcellular entities, and how DNA enables complex behaviour, including motility and information processing for adaptive and interactive processes. We chart future directions for DNA-empowered synthetic cells, envisioning interactive systems wherein synthetic cells communicate within communities and with living cells.

Biomimetic Vesicles with Designer Phospholipids Can Sense Environmental Redox Cues

Cell-like materials that sense environmental cues can serve as next-generation biosensors and help advance the understanding of intercellular communication. Currently, bottom-up engineering of protocell models from molecular building blocks remains a grand challenge chemists face. Herein, we describe giant unilamellar vesicles (GUVs) with biomimetic lipid membranes capable of sensing environmental redox cues. The GUVs employ activity-based sensing through designer phospholipids that are fluorescently activated in response to specific reductive (hydrogen sulfide) or oxidative (hydrogen peroxide) conditions. These synthetic phospholipids are derived from 1,2-dipalmitoyl-rac-glycero-3-phosphocholine and they possess a headgroup with heterocyclic aromatic motifs. Despite their structural deviation from the phosphocholine headgroup, the designer phospholipids (0.5-1.0 mol %) mixed with natural lipids can vesiculate, and the resulting GUVs (7-20 μm in diameter) remain intact over the course of redox sensing. All-atom molecular dynamics simulations gave insight into how these lipids are positioned within the hydrophobic core of the membrane bilayer and at the membrane-water interface. This work provides a purely chemical method to investigate potential redox signaling and opens up new design opportunities for soft materials that mimic protocells.

Proto-neural networks from thermal proteins

Proteinoids are synthetic polymers that have structural similarities to natural proteins, and their formation is achieved through the application of heat to amino acid combinations in a dehydrated environment. The thermal proteins, initially synthesised by Sidney Fox during the 1960s, has the ability to undergo self-assembly, resulting in the formation of microspheres that resemble cells. These microspheres have fascinating biomimetic characteristics. In recent studies, substantial advancements have been made in elucidating the electrical signalling phenomena shown by proteinoids, hence showcasing their promising prospects in the field of neuro-inspired computing. This study demonstrates the advancement of experimental prototypes that employ proteinoids in the construction of fundamental neural network structures. The article provides an overview of significant achievements in proteinoid systems, such as the demonstration of electrical excitability, emulation of synaptic functions, capabilities in pattern recognition, and adaptability of network structures. This study examines the similarities and differences between proteinoid networks and spontaneous neural computation. We examine the persistent challenges associated with deciphering the underlying mechanisms of emergent proteinoid-based intelligence. Additionally, we explore the potential for developing bio-inspired computing systems using synthetic thermal proteins in forthcoming times. The results of this study offer a theoretical foundation for the advancement of adaptive, self-assembling electronic systems that operate using artificial bio-neural principles.

Ultrasound-Stimulated "Exocytosis" by Cell-Like Microbubbles Enhances Antibacterial Species Penetration and Immune Activation Against Implant Infection

Host immune systems serving as crucial defense lines are vital resisting mechanisms against biofilm-associated implant infections. Nevertheless, biofilms hinder the penetration of anti-bacterial species, inhibit phagocytosis of immune cells, and frustrate host inflammatory responses, ultimately resulting in the weakness of the host immune system for biofilm elimination. Herein, a cell-like construct is developed through encapsulation of erythrocyte membrane fragments on the surface of Fe3 O4 nanoparticle-fabricated microbubbles and then loaded with hydroxyurea (EMB-Hu). Under ultrasound (US) stimulation, EMB-Hu undergoes a stable oscillation manner to act in an "exocytosis" mechanism for disrupting biofilm, releasing agents, and enhancing penetration of catalytically generated anti-bacterial species within biofilms. Additionally, the US-stimulated "exocytosis" by EMB-Hu can activate pro-inflammatory macrophage polarization and enhance macrophage phagocytosis for clearance of disrupted biofilms. Collectively, this work has exhibited cell-like microbubbles with US-stimulated "exocytosis" mechanisms to overcome the biofilm barrier and signal macrophages for inflammatory activation, finally achieving favorable therapeutic effects against implant infections caused by methicillin-resistant Staphylococcus aureus (MRSA) biofilms.

Spontaneous transfer of small peripheral peptides between supported lipid bilayer and giant unilamellar vesicles

Vesicular trafficking facilitates material transport between membrane-bound organelles. Membrane protein cargos are trafficked for relocation, recycling, and degradation during various physiological processes. In vitro fusion studies utilized synthetic lipid membranes to study the molecular mechanisms of vesicular trafficking and to develop synthetic materials mimicking the biological membrane trafficking. Various fusogenic conditions which can induce vesicular fusion have been used to establish synthetic systems that can mimic biological systems. Despite these efforts, the mechanisms underlying vesicular trafficking of membrane proteins remain limited and robust in vitro methods that can construct synthetic trafficking systems for membrane proteins between large membranes (>1 μm2) are unavailable. Here, we provide data to show the spontaneous transfer of small membrane-bound peptides (∼4 kD) between a supported lipid bilayer (SLB) and giant unilamellar vesicles (GUVs). We found that the contact between the SLB and GUVs led to the occasional but notable transfer of membrane-bound peptides in a physiological saline buffer condition (pH 7.4, 150 mM NaCl). Quantitative and dynamic time-lapse analyses suggested that the observed exchange occurred through the formation of hemi-fusion stalks between the SLB and GUVs. Larger protein cargos with a size of ∼77 kD could not be transferred between the SLB and GUVs, suggesting that the larger-sized cargos limited diffusion across the hemi-fusion stalk, which was predicted to have a highly curved structure. Compositional study showed Ni-chelated lipid head group was the essential component catalyzing the process. Our system serves as an example synthetic platform that enables the investigation of small-peptide trafficking between synthetic membranes and reveals hemi-fused lipid bridge formation as a mechanism of peptide transfer.

Engineering status of protein for improving microbial cell factories

With the development of metabolic engineering and synthetic biology, microbial cell factories (MCFs) have provided an efficient and sustainable method to synthesize a series of chemicals from renewable feedstocks. However, the efficiency of MCFs is usually limited by the inappropriate status of protein. Thus, engineering status of protein is essential to achieve efficient bioproduction with high titer, yield and productivity. In this review, we summarize the engineering strategies for metabolic protein status, including protein engineering for boosting microbial catalytic efficiency, protein modification for regulating microbial metabolic capacity, and protein assembly for enhancing microbial synthetic capacity. Finally, we highlight future challenges and prospects of improving microbial cell factories by engineering status of protein.

Cell-free expression of RuBisCO for ATP production in the synthetic cells

Recent advances in bottom-up synthetic biology have made it possible to reconstitute cellular systems from non-living components, yielding artificial cells with potential applications in industry, medicine and basic research. Although a variety of cellular functions and components have been reconstituted in previous studies, sustained biological energy production remains a challenge. ATP synthesis via ribulose-1,5-diphosphate carboxylase/oxygenase (RuBisCO), a central enzyme in biological CO2 fixation, holds potential as an energy production system, but its feasibility in a cell-free expression system has not yet been tested. In this study, we test RuBisCO expression and its activity-mediated ATP synthesis in a reconstituted Escherichia coli-based cell-free translation system. We then construct a system in which ATP is synthesized by RuBisCO activity in giant vesicles and used as energy for translation reactions. These results represent an advance toward independent energy production in artificial cells. Graphical Abstract.

Direct assessment of nitrative stress in lipid environments: Applications of a designer lipid-based biosensor for peroxynitrite

Lipid membranes and lipid-rich organelles are targets of peroxynitrite (ONOO-), a highly reactive species generated under nitrative stress. We report a membrane-localized phospholipid (DPPC-TC-ONOO-) that allows the detection of ONOO- in diverse lipid environments: biomimetic vesicles, mammalian cell compartments, and within the lung lining. DPPC-TC-ONOO- and POPC self-assemble to membrane vesicles that fluorogenically and selectively respond to ONOO-. DPPC-TC-ONOO-, delivered through lipid nanoparticles, allowed for ONOO- detection in the endoplasmic reticulum upon cytokine-induced nitrative stress in live mammalian cells. It also responded to ONOO- within lung tissue murine models upon acute lung injury. We observed nitrative stress around bronchioles in precision cut lung slices exposed to nitrogen mustard and in pulmonary macrophages following intratracheal bleomycin challenge. Results showed that DPPC-TC-ONOO- functions specifically toward iNOS, a key enzyme modulating nitrative stress, and offers significant advantages over its hydrophilic analog in terms of localization and signal generation.

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.

Double Emulsion Droplets as a Plausible Step to Fatty Acid Protocells

It is assumed that life originated on the Earth from vesicles made of fatty acids. These amphiphiles are the simplest chemicals, which can be present in the prebiotic soup, capable of self-assembling into compartments mimicking modern cells. Production of fatty acid vesicles is widely studied, as their growing and division. However, how prebiotic chemicals require to further yield living cells encapsulated within protocells remains unclear. Here, one suggests a scenario based on recent studies, which shows that phospholipid vesicles can form from double emulsions affording facile encapsulation of cargos. In these works, water-in-oil-in-water droplets are produced by microfluidics, having dispersed lipids in the oil. Dewetting of the oil droplet leaves the internal aqueous droplet covered by a lipid bilayer, entrapping cargos. In this review, formation of fatty acid protocells is briefly reviewed, together with the procedure for preparing double emulsions and vesicles from double emulsion and finally, it is proposed that double emulsion droplets formed in the deep ocean where undersea volcano expulsed materials, with fatty acids (under their carboxylic form) and alkanols as the oily phase, entrapping hydrosoluble prebiotic chemicals in a double emulsion droplet core. Once formed, double emulsion droplets can move up to the surface, where an increase of pH, variation of pressure and/or temperature may have allowed dewetting of the oily droplet, leaving a fatty acid vesicular protocell with encapsulated prebiotic materials.

Investigation of Compatibility between DNA Replication, Transcription, and Translation for in Vitro Central Dogma

Recent advances in in vitro synthetic biology have made it possible to reconstitute various cellular functions in a test tube. However, the integration of these functions remains a major challenge. This study aimed to identify a suitable condition to achieve all three reactions that constitute the central dogma: transcription, translation, and DNA replication. Specifically, we investigated the effect of the concentrations of 11 nonprotein factors required for in vitro transcription, translation, and DNA replication on each of these reactions. Our results indicate that certain factors have opposing effects on the three reactions. For example, while dNTP is necessary for DNA replication, it inhibited translation, and both rNTP and tRNA, which are essential for transcription and translation, inhibited DNA replication with several DNA polymerases. We also found that these opposing effects were partially alleviated by optimizing the magnesium concentration. Using this knowledge, we successfully demonstrated transcription/translation-coupled DNA replication with higher levels of transcription and translation while maintaining a certain level of DNA replication. These findings not only provide useful insights for the development of a complex artificial system with the central dogma but also raise the question of how natural cells overcome the incompatibility between the three reactions.

A Practical Guide to Preparation and Applications of Giant Unilamellar Vesicles Formed via Centrifugation of Water-in-Oil Emulsion Droplets

Giant vesicles (GVs), which are closed lipid bilayer membranes with a diameter of more than 1 μm, have attracted attention not only as model cell membranes but also for the construction of artificial cells. For encapsulating water-soluble materials and/or water-dispersible particles or functionalizing membrane proteins and/or other synthesized amphiphiles, giant unilamellar vesicles (GUVs) have been applied in various fields, such as supramolecular chemistry, soft matter physics, life sciences, and bioengineering. In this review, we focus on a preparation technique for GUVs that encapsulate water-soluble materials and/or water-dispersible particles. It is based on the centrifugation of a water-in-oil emulsion layered on water and does not require special equipment other than a centrifuge, which makes it the first choice for laboratory use. Furthermore, we review recent studies on GUV-based artificial cells prepared using this technique and discuss their future applications.

In Vitro Transcription/Translation-Coupled DNA Replication through Partial Regeneration of 20 Aminoacyl-tRNA Synthetases

The in vitro reconstruction of life-like self-reproducing systems is a major challenge in in vitro synthetic biology. Self-reproduction requires regeneration of all molecules involved in DNA replication, transcription, and translation. This study demonstrated the continuous DNA replication and partial regeneration of major translation factors, 20 aminoacyl-tRNA synthetases (aaRS), in a reconstituted transcription/translation system (PURE system) for the first time. First, we replicated each DNA that encodes one of the 20 aaRSs through aaRS expression from the DNA by serial transfer experiments. Thereafter, we successively increased the number of aaRS genes and achieved simultaneous, continuous replication of DNA that encodes all 20 aaRSs, which comprised approximately half the number of protein factors in the PURE system, except for ribosomes, by employing dialyzed reaction and sequence optimization. This study provides a step-by-step methodology for continuous DNA replication with an increasing number of self-regenerative genes toward self-reproducing artificial systems.

Artificial receptor-mediated phototransduction toward protocellular subcompartmentalization and signaling-encoded logic gates

Engineering artificial cellular systems capable of perceiving and transmitting external signals across membranes to activate downstream targets and coordinate protocellular responses is key to build cell-cell communications and protolife. Here, we report a synthetic photoreceptor-mediated signaling pathway with the integration of light harvesting, photo-to-chemical energy conversion, signal transmission, and amplification in synthetic cells, which ultimately resulted in protocell subcompartmentalization. Key to our design is a ruthenium-bipyridine complex that acts as a membrane-anchored photoreceptor to convert visible light into chemical information and transduce signals across the lipid membrane via flip-flop motion. By coupling receptor-mediated phototransduction with biological recognition and enzymatic cascade reactions, we further develop protocell signaling-encoded Boolean logic gates. Our results illustrate a minimal cell model to mimic the photoreceptor cells that can transduce the energy of light into intracellular responses and pave the way to modular control over the flow of information for complex metabolic and signaling pathways.

Colony-like Protocell Superstructures

We report the formation, growth, and dynamics of model protocell superstructures on solid surfaces, resembling single cell colonies. These structures, consisting of several layers of lipidic compartments enveloped in a dome-shaped outer lipid bilayer, emerged as a result of spontaneous shape transformation of lipid agglomerates deposited on thin film aluminum surfaces. Collective protocell structures were observed to be mechanically more stable compared to isolated spherical compartments. We show that the model colonies encapsulate DNA and accommodate nonenzymatic, strand displacement DNA reactions. The membrane envelope is able to disassemble and expose individual daughter protocells, which can migrate and attach via nanotethers to distant surface locations, while maintaining their encapsulated contents. Some colonies feature "exocompartments", which spontaneously extend out of the enveloping bilayer, internalize DNA, and merge again with the superstructure. A continuum elastohydrodynamic theory that we developed suggests that a plausible driving force behind subcompartment formation is attractive van der Waals (vdW) interactions between the membrane and surface. The balance between membrane bending and vdW interactions yields a critical length scale of 236 nm, above which the membrane invaginations can form subcompartments. The findings support our hypotheses that in extension of the "lipid world hypothesis", protocells may have existed in the form of colonies, potentially benefiting from the increased mechanical stability provided by a superstructure.

Signal-processing and adaptive prototissue formation in metabolic DNA protocells

The fundamental life-defining processes in living cells, such as replication, division, adaptation, and tissue formation, occur via intertwined metabolic reaction networks that process signals for downstream effects with high precision in a confined, crowded environment. Hence, it is crucial to understand and reenact some of these functions in wholly synthetic cell-like entities (protocells) to envision designing soft materials with life-like traits. Herein, we report on all-DNA protocells composed of a liquid DNA interior and a hydrogel-like shell, harboring a catalytically active DNAzyme, that converts DNA signals into functional metabolites that lead to downstream adaptation processes via site-selective strand displacement reactions. The downstream processes include intra-protocellular phenotype-like changes, prototissue formation via multivalent interactions, and chemical messenger communication between active sender and dormant receiver cell populations for sorted heteroprototissue formation. The approach integrates several tools of DNA-nanoscience in a synchronized way to mimic life-like behavior in artificial systems for future interactive materials.

Programming Molecular Systems To Emulate a Learning Spiking Neuron

Hebbian theory seeks to explain how the neurons in the brain adapt to stimuli to enable learning. An interesting feature of Hebbian learning is that it is an unsupervised method and, as such, does not require feedback, making it suitable in contexts where systems have to learn autonomously. This paper explores how molecular systems can be designed to show such protointelligent behaviors and proposes the first chemical reaction network (CRN) that can exhibit autonomous Hebbian learning across arbitrarily many input channels. The system emulates a spiking neuron, and we demonstrate that it can learn statistical biases of incoming inputs. The basic CRN is a minimal, thermodynamically plausible set of microreversible chemical equations that can be analyzed with respect to their energy requirements. However, to explore how such chemical systems might be engineered de novo, we also propose an extended version based on enzyme-driven compartmentalized reactions. Finally, we show how a purely DNA system, built upon the paradigm of DNA strand displacement, can realize neuronal dynamics. Our analysis provides a compelling blueprint for exploring autonomous learning in biological settings, bringing us closer to realizing real synthetic biological intelligence.

Identifying and Manipulating Giant Vesicles: Review of Recent Approaches

Giant vesicles (GVs) are closed bilayer membranes that primarily comprise amphiphiles with diameters of more than 1 μm. Compared with regular vesicles (several tens of nanometers in size), GVs are of greater scientific interest as model cell membranes and protocells because of their structure and size, which are similar to those of biological systems. Biopolymers and nano-/microparticles can be encapsulated in GVs at high concentrations, and their application as artificial cell bodies has piqued interest. It is essential to develop methods for investigating and manipulating the properties of GVs toward engineering applications. In this review, we discuss current improvements in microscopy, micromanipulation, and microfabrication technologies for progress in GV identification and engineering tools. Combined with the advancement of GV preparation technologies, these technological advancements can aid the development of artificial cell systems such as alternative tissues and GV-based chemical signal processing systems.

DNA-Mediated Protein Shuttling between Coacervate-Based Artificial Cells

The regulation of protein uptake and secretion is crucial for (inter)cellular signaling. Mimicking these molecular events is essential when engineering synthetic cellular systems. A first step towards achieving this goal is obtaining control over the uptake and release of proteins from synthetic cells in response to an external trigger. Herein, we have developed an artificial cell that sequesters and releases proteinaceous cargo upon addition of a coded chemical signal: single‐stranded DNA oligos (ssDNA) were employed to independently control the localization of a set of three different ssDNA‐modified proteins. The molecular coded signal allows for multiple iterations of triggered uptake and release, regulation of the amount and rate of protein release and the sequential release of the three different proteins. This signaling concept was furthermore used to directionally transfer a protein between two artificial cell populations, providing novel directions for engineering lifelike communication pathways inside higher order (proto)cellular structures.

Nucleic acid-based molecular computation heads towards cellular applications

Nucleic acids, with the advantages of programmability and biocompatibility, have been widely used to design different kinds of novel biocomputing devices. Recently, nucleic acid-based molecular computing has shown promise in making the leap from the test tube to the cell. Such molecular computing can perform logic analysis within the confines of the cellular milieu with programmable modulation of biological functions at the molecular level. In this review, we summarize the development of nucleic acid-based biocomputing devices that are rationally designed and chemically synthesized, highlighting the ability of nucleic acid-based molecular computing to achieve cellular applications in sensing, imaging, biomedicine, and bioengineering. Then we discuss the future challenges and opportunities for cellular and in vivo applications. We expect this review to inspire innovative work on constructing nucleic acid-based biocomputing to achieve the goal of precisely rewiring, even reconstructing cellular signal networks in a prescribed way.

An Electrocatalytic Reaction As a Basis for Chemical Computing in Water Droplets

Aqueous droplets covered with amphiphilic Janus Au/Fe₃O₄ nanoparticles and suspended in an organic phase serve as building blocks of droplet-based electronic circuitry. The electrocatalytic activity of these nanoparticles in a hydrogen evolution reaction (HER) underlies the droplet's ability to rectify currents with typical rectification ratios of ∼10. In effect, individual droplets act as low-frequency half-wave rectifiers, whereas several appropriately wired droplets enable full-wave rectification. When the HER-supporting droplets are combined with salt-containing "resistor" ones, the resulting ensembles can act as AND or OR gates or as inverters.

DNA-Based Artificial Signaling System Mimicking the Dimerization of Receptors for Signal Transduction and Amplification

Transmembrane signal transduction is of profound significance in many biological processes. The dimerization of cell-surface receptors is one prominent mechanism by which signals are transmitted across the membrane and trigger intracellular cascade amplification reactions. Recreating such processes in artificial systems has potential applications in sensing, drug delivery, bioengineering, and providing a new route for a deep understanding of fundamental biological processes. However, it remains a challenge to design artificial signal transduction systems working by the receptor dimerization mechanism in a predictable and smart manner. Here, benefitting from DNA with features of programmability, controllability, and flexible design, we use DNA as a building material to construct an artificial system mimicking dimerization of receptors for signal transduction and cascade amplification. DNA-based membrane-spanning receptor analogues are designed to recognize external signals, which drives two receptors into close spatial proximity to activate DNAzymes inside the cell-mimicking system. The activation of the DNAzyme initiates the catalyzed cleavage of encapsulated substrates and leads to the release of fluorescent second messengers for signal amplification. Such an artificial signal transduction system extends the range of biomimetic DNA-based signaling systems, providing a new avenue to study natural cell signaling processes and artificially regulate biological processes.

Protocell arrays for simultaneous detection of diverse analytes

Simultaneous detection of multiple analytes from a single sample (multiplexing), particularly when done at the point of need, can guide complex decision-making without increasing the required sample volume or cost per test. Despite recent advances, multiplexed analyte sensing still typically faces the critical limitation of measuring only one type of molecule (e.g., small molecules or nucleic acids) per assay platform. Here, we address this bottleneck with a customizable platform that integrates cell-free expression (CFE) with a polymer-based aqueous two-phase system (ATPS), producing membrane-less protocells containing transcription and translation machinery used for detection. We show that multiple protocells, each performing a distinct sensing reaction, can be arrayed in the same microwell to detect chemically diverse targets from the same sample. Furthermore, these protocell arrays are compatible with human biofluids, maintain function after lyophilization and rehydration, and can produce visually interpretable readouts, illustrating this platform's potential as a minimal-equipment, field-deployable, multi-analyte detection tool.

Fe(III)-complex mediated bacterial cell surface immobilization of eGFP and enzymes

We report a facile and reversible method to immobilize a broad range of His6-tagged proteins on the E. coli cell surface through Fe(iii)-metal complexes. A His6-tagged eGFP and four His6-tagged enzymes were successfully immobilized on the cell surface. Additionally, a hydrogel sheath around E. coli cells was generated by immobilized His6-tagged HRP.

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.

Shape changes and budding of giant vesicles induced by an internal chemical trigger: an interplay between osmosis and pH change

Shape transformation and budding of phospholipid/fatty acid giant hybrid vesicles can be induced by an internal chemical stimulus (pH change) when coupled with an osmotic shock. In particular, an autocatalytic enzymatic reaction set (urea-urease system), confined in the lumen of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC)/oleic acid (HOA) vesicles, can force the budding of the hosting vesicle, when properly fed by a trans-membrane substrate infusion. Herein, we elucidate the budding mechanism by simulating the shape changes of a vesicle during the enzymatic reaction. The area-difference-elasticity (ADE) theory is thus implemented to minimize the surface elastic energy and obtain the equilibrium shape at different values of the reduced volume and different values of the reduced preferred area difference (Δa0). Simulations, together with control experiments, unambiguously show that to obtain an effective vesicle shape transformation, the osmotic stress and the pH change in the lumen of the vesicle must act in synergy at the same timescale. Osmotic pressure induces a vesicle deflation (volume loss), while the pH change affects the preferred area difference between the outer and the inner membrane leaflets.

Stimuli-responsive metal-organic framework nanoparticles for controlled drug delivery and medical applications

Stimuli-responsive metal-organic framework nanoparticles, NMOFs, provide a versatile platform for the controlled release of drugs and biomedical applications. The porous structure of NMOFs, their biocompatibility, low toxicity, and efficient permeability turn the NMOFs into ideal carriers for therapeutic applications. Two general methods to gate the drug-loaded NMOFs and to release the loads were developed: by one method, the loaded NMOFs are coated or surface-modified with stimuli-responsive gates being unlocked in the presence of appropriate chemical (e.g., ions or reducing agents), physical (e.g., light or heat), or biomarker (e.g., miRNA or ATP) triggers. By a second approach, the drug-loaded NMOFs include encoded structural information or co-added agents to induce the structural distortion or stimulate the degradation of the NMOFs. Different chemical triggers such as pH changes, ions, ATP, or redox agents, and physical stimuli such as light or heat are applied to degrade the NMOFs, resulting in the release of the loads. In addition, enzymes, DNAzymes, and disease-specific biomarkers are used to unlock the gated NMOFs. The triggered release of drugs for cancer therapy, anti-blood clotting, and the design of autonomous insulin-delivery systems ("artificial pancreas") are discussed. Specifically, multi-drug carrier systems and functional NMOFs exhibiting dual and cooperative therapeutic functions are introduced. The future perspectives and applications of stimuli-responsive particles are addressed.

Cell mimicry as a bottom-up strategy for hierarchical engineering of nature-inspired entities

Artificial biology is an emerging concept that aims to design and engineer the structure and function of natural cells, organelles, or biomolecules with a combination of biological and abiotic building blocks. Cell mimicry focuses on concepts that have the potential to be integrated with mammalian cells and tissue. In this feature article, we will emphasize the advancements in the past 3-4 years (2017-present) that are dedicated to artificial enzymes, artificial organelles, and artificial mammalian cells. Each aspect will be briefly introduced, followed by highlighting efforts that considered key properties of the different mimics. Finally, the current challenges and opportunities will be outlined. This article is categorized under: Nanotechnology Approaches to Biology > Nanoscale Systems in Biology.

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.

Near-infrared light-activated membrane fusion for cancer cell therapeutic applications

The spatiotemporal stimulation of liposome-liposome or liposome-membrane fusion processes attracts growing interest as a means to mimic cell-cell interactions in nature and for using these processes for biomedical applications. We report the use of o-nitrobenzyl phosphate functionalized-cholesterol tethered nucleic acid-modified liposomes as functional photoresponsive units for inducing, by NIR-irradiation, spatiotemporal liposome-liposome or liposome-membrane fusion processes. The liposomes are loaded with upconversion nanoparticles (UCNPs) and their NIR irradiation (λ = 980 nm) yields luminescence at λ = 365 nm, providing a localized light-source to deprotect the o-nitrobenzyl phosphate groups and resulting in the fragmentation of the nucleic acid structures. In one system, the NIR-triggered fusion of two liposomes, L1 and L2, is exemplified. Liposome L1 is loaded with UCNPs and Tb3+ ions, and the liposome boundary is functionalized with a cholesterol-tethered, o-nitrobenzyl phosphate caged hairpin nucleic acid structure. Liposome L2 is loaded with 2,6-pyridinedicarboxylic acid, DPA, and its boundary is modified with a cholesterol-tethered nucleic acid, complementary to a part of the caged hairpin, associated with L1. NIR-irradiation of the L1/L2 mixture resulted in the photocleavage of the hairpin structure, associated with L1, and the resulting fragmented nucleic acid associated with L1 hybridized with the nucleic acid linked to L2, leading to the fusion of the two liposomes. The fusion process was followed by dynamic light scattering, and by monitoring the fluorescence of the Tb3+-DPA complex generated upon the fusion of the liposomes and their exchange of contents (fusion efficiency 30%). In a second system, the fusion of the liposomes L1, loaded with UCNPs and doxorubicin (DOX), with HeLa cancer cells functionalized with nucleic acid tethers, complementary to the hairpin units associated with the boundary of L1, and linked to the MUC-1 receptor sites associated with the HeLa cells, through a MUC-1 aptamer unit is exemplified. The effect of DOX-loaded L1/HeLa cell fusion on the cytotoxicity towards HeLa cells is addressed. The NIR UCNP-stimulated cleavage of the o-nitrobenzyl phosphate caged hairpin units associated with L1 leads to the fragmentation of the hairpin units and the resulting nucleic acid tethers hybridize with the nucleic acid-modified HeLa cells, resulting in the liposome-HeLa cell fusion and the release of DOX into the HeLa cells. Selective spatiotemporal cytotoxicity towards HeLa cells is demonstrated (ca. 40% cell killing within two days). The study presents a comprehensive stepwise set of experiments directed towards the development of NIR-driven liposome-liposome or liposome-membrane fusion processes.

A minimalistic catalytically-active cell mimetic made of a supra-molecular hydrogel encapsulated into a polymersome

A minimalistic multicomponent cell mimetic is constructed from a catalytic low molecular weight fibrillar network and a polymersome compartment.