Autonomic Behaviors in Lipase‐Active Oil Droplets

Quorum Sensing in Emulsion Droplet Swarms Driven by a Surfactant Competition System

Quorum sensing enables unicellular organisms to probe their population density and perform behavior that exclusively occurs above a critical density. Quorum sensing is established in emulsion droplet swarms that float at a water surface and cluster above a critical density. The design involves competition between 1) a surface tension gradient that is generated upon release of a surfactant from the oil droplets, and thereby drives their mutual repulsion, and 2) the release of a surfactant precursor from the droplets, that forms a strong imine surfactant which suppresses the surface tension gradient and thereby causes droplet clustering upon capillary (Cheerios) attraction. The production of the imine-surfactant depends on the population density of the droplets releasing the precursor so that the clustering only occurs above a critical population density. The pH-dependence of the imine-surfactant formation is exploited to trigger quorum sensing upon a base stimulus: dynamic droplet swarms are generated that cluster and spread upon spatiotemporally varying acid and base conditions. Next, the clustering of two droplet subpopulations is coupled to a chemical reaction that generates a fluorescent signal. It is foreseen that quorum sensing enables control mechanisms in droplet-based systems that display collective responses in contexts of, e.g., sensing, optics, or dynamically controlled droplet-reactors.

Motility of Synthetic Cells from Engineered Lipids

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

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

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

Hybrid photoresponsive/biocatalytic micro- and nano-swimmers

Micro/nano biomimetic systems that convert energy from the surroundings into mechanical motion have emerged as promising tools to enhance the efficiencies of different biomedical and environmental processes. The inclusion of multiple engines into the same device has become a promising strategy to achieve dual/triple stimuli responses. Such hybrid micro/nanoswimmers combining different propulsion forces exhibit advanced motion behaviors and different physical features that are interesting not only to achieve strong propulsion capabilities in complex environments but also to modulate their movement according to the intended use. The development of hybrid systems that can be actuated by both light and biocompatible fuels is of particular interest. This minireview covers the main types of photoactive/biocatalytic micro/nanoswimmers developed so far. Their main photoresponsive and enzymatic components are discussed along with the most representative designs. The applicability of such hybrid machines for analyte sensing, antibacterial and therapeutical uses are also described. The remaining challenges and opportunities are then explored.

Enzyme-Powered Micro/Nanomotors for Cancer Treatment

The incidence and lethal rate of cancers are rapidly rising recently, however current treatments of cancers, such as surgical resection, radiotherapy, chemotherapy and targeted therapy, usually require long treatment period and have more side effects and high recurrence rate. Enzyme-powered micro/nanomotors (EMNMs), with powerful self-propulsion, enhanced permeability and good biocompatibility, have shown great potential in crossing biological barrier and targeted drug transportation for cancer treatment; moreover, advanced approaches based on EMNMs such as photothermal therapy and starvation therapy have also been widely explored in cancer treatment. Although there are several review works discussing the progress of micro/nanomotors for biomedical applications, there is not one review paper with the focus on the cancer treatment based on EMNMs. Therefore, in this review, we try to concisely and timely summarize the recent progress of cancer treatment based on enzyme-driven micro/nanomotors, such as brain tumors, bladder cancer, breast cancer and others. Finally, the challenges and outlook of cancer therapy based on EMNMs are discussed, hoping to provide fundamental guidance for the future development.

Trends in lipase engineering for enhanced biocatalysis

Lipases, also known as triacylglycerol hydrolases (E.C.No. 3.1.1.3), are considered as leading biocatalysts in the lipid modification business. With properties like ease of availability, capability to work in heterogeneous media, stability in organic solvents, property of catalyzing at the lipid-water interface and even in nonaqueous conditions, have made them a versatile choice for applications in the food, flavor, detergent, pharmaceutical, leather, textile, cosmetic, and paper industries [1]. The increasing alertness toward sustainable technologies, lesser waste generation and solvent usage and minimization of energy input has brought light toward the production and usage of recombinant/improved lipases. For example, Novozym 435, a broadly used recombinant lipase isolated from Candida antarctica, dominates the lipase industry and has even created a supplier bias in the market. This shows that there is a desperate need for novel, low-cost lipases with better properties. For this, mining of existing extremophilic genomes seems more rewarding. But considering the diversity of industrial requirements such as types of solvents used or carrier systems employed for enzyme immobilization, tailor-designed enzymes are an unrealized pressing priority. Therefore, protein engineering strategies in collaboration with the discovery of new lipases can serve as a vital tool to obtain tailor-made enzymes with specific characteristics.

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.

Polymeric Micro/Nanocarriers and Motors for Cargo Transport and Phototriggered Delivery

Smart polymer-based micro/nanoassemblies have emerged as a promising alternative for transporting and delivering a myriad of cargo. Cargo encapsulation into (or linked to) polymeric micro/nanocarrier (PC) strategies may help to conserve cargo activity and functionality when interacting with its surroundings in its journey to the target. PCs for cargo phototriggering allow for excellent spatiotemporal control via irradiation as an external stimulus, thus regulating the delivery kinetics of cargo and potentially increasing its therapeutic effect. Micromotors based on PCs offer an accelerated cargo-medium interaction for biomedical, environmental, and many other applications. This review collects the recent achievements in PC development based on nanomicelles, nanospheres, and nanopolymersomes, among others, with enhanced properties to increase cargo protection and cargo release efficiency triggered by ultraviolet (UV) and near-infrared (NIR) irradiation, including light-stimulated polymeric micromotors for propulsion, cargo transport, biosensing, and photo-thermal therapy. We emphasize the challenges of positioning PCs as drug delivery systems, as well as the outstanding opportunities of light-stimulated polymeric micromotors for practical applications.

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.

Propelling microdroplets generated and sustained by liquid-liquid phase separation in confined spaces

Flow transport in confined spaces is ubiquitous in technological processes, ranging from separation and purification of pharmaceutical ingredients by microporous membranes and drug delivery in biomedical treatment to chemical and biomass conversion in catalyst-packed reactors and carbon dioxide sequestration. In this work, we suggest a distinct pathway for enhanced liquid transport in a confined space via propelling microdroplets. These microdroplets can form spontaneously from localized liquid-liquid phase separation as a ternary mixture is diluted by a diffusing poor solvent. High speed images reveal how the microdroplets grow, break up and propel rapidly along the solid surface, with a maximal velocity up to ∼160 μm s-1, in response to a sharp concentration gradient resulting from phase separation. The microdroplet propulsion induces a replenishing flow between the walls of the confined space towards the location of phase separation, which in turn drives the mixture out of equilibrium and leads to a repeating cascade of events. Our findings on the complex and rich phenomena of propelling droplets suggest an effective approach to enhanced flow motion of multicomponent liquid mixtures within confined spaces for time effective separation and smart transport processes.

Speeding up biphasic reactions with surface nanodroplets

Biphasic chemical reactions compartmentalized in small droplets offer advantages, such as streamlined procedures for chemical analysis, enhanced chemical reaction efficiency and high specificity of conversion. In this work, we experimentally and theoretically investigate the rate for biphasic chemical reactions between acidic nanodroplets on a substrate surface and basic reactants in a surrounding bulk flow. The reaction rate is measured by droplet shrinkage as the product is removed from the droplets by the flow. In our experiments, we determine the dependence of the reaction rate on the flow rate and the solution concentration. The theoretical analysis predicts that the life time τ of the droplets scales with Peclet number Pe and the reactant concentration in the bulk flow c_re,bulk as τ∝ Pe^-3/2c_re,bulk^-1, in good agreement with our experimental results. Furthermore, we found that the product from the reaction on an upstream surface can postpone the droplet reaction on a downstream surface, possibly due to the adsorption of interface-active products on the droplets in the downstream. The time of the delay decreases with increasing Pe of the flow and also with increasing reactant concentration in the flow, following the scaling same as that of the reaction rate with these two parameters. Our findings provide insight for the ultimate aim to enhance droplet reactions under flow conditions.

Dynamic Behaviour in Microcompartments

Compartmentalisation is recognised to be a primary step for the assembly of non-living matter towards the construction of life-like microensembles. To date, a host of hollow microcompartments with various functionalities have been widely developed. Within this respect, given that dynamic behaviour is one of the fundamental features to distinguish living ensembles from those that are non-living, the design and construction of microcompartments with various dynamic behaviours are attracting considerable interest from a wide range of research communities. Significantly, the created dynamic microcompartments could also be widely used as chassis for further bottom-up design towards building protocell models by integrating and booting up necessary biological information. Herein, strategies to install the various motility behaviours into microcompartments, including haptotaxis, chemotaxis and gravitaxis, are summarized in the anticipation of inspiring more designs towards creating various advanced active microcompartments, and contributing new techniques to the ultimate goal of constructing a basic living unit entirely from non-living components.

Self-assembly of colloids based on microfluidics

Self-assembly of colloids provides a powerful way for the construction of complex multi-scale materials. Microfluidic techniques possess great potential to precisely control the assembly of micro- and nano-scale building blocks via the rational design of various microfluidic environments. In this review, we first discuss the self-assembly of colloids without templates by using the laminar microfluidic technique. The self-assembly of colloids based on a droplet as a template was subsequently summarized and discussed via droplet microfluidic technique. Moreover, the evaporation-driven self-assembly of colloids in microfluidic channels has been discussed and analysed. Finally, the representative applications in this field have been pointed out. The aim of this review is to summarize the state-of-art on the self-assembly of colloids based on various microfluidic techniques, exhibit their representative applications, and point out the current challenges in this field, hoping to inspire and guide future work.