Adaptive and Dissipative Hierarchical Population Crowding of Synthetic Protocells through Click-PISA under Gradient Energy Inputs.

The ability of living objects to respond rapidly en masse to various stimuli or stress is an important function in response to externally applied changes in the local environment. This occurs across many length scales, for instance, bacteria swarming in response to different stimuli or stress and macromolecular crowding within cells. Currently there are few mechanisms to induce similar autonomous behaviors within populations of synthetic protocells. Herein, we report a system in which populations of individual objects behave in a coordinated manner in response to changes in the energetic environment by the emergent self-organization of large object swarms. These swarms contain protocell populations of approximately 60 000 individuals. We demonstrate the dissipative nature of the hierarchical constructs, which persist under appropriate UV stimulation. Finally, we identify the ability of the object populations to change behaviors in an adaptive population-wide response to the local energetic environment.

Growth model interpretation of planet size distribution.

The radii and orbital periods of 4,000+ confirmed/candidate exoplanets have been precisely measured by the Kepler mission. The radii show a bimodal distribution, with two peaks corresponding to smaller planets (likely rocky) and larger intermediate-size planets, respectively. While only the masses of the planets orbiting the brightest stars can be determined by ground-based spectroscopic observations, these observations allow calculation of their average densities placing constraints on the bulk compositions and internal structures. However, an important question about the composition of planets ranging from 2 to 4 Earth radii (R⊕) still remains. They may either have a rocky core enveloped in a H2-He gaseous envelope (gas dwarfs) or contain a significant amount of multicomponent, H2O-dominated ices/fluids (water worlds). Planets in the mass range of 10-15 M⊕, if half-ice and half-rock by mass, have radii of 2.5 R⊕, which exactly match the second peak of the exoplanet radius bimodal distribution. Any planet in the 2- to 4-R⊕ range requires a gas envelope of at most a few mass percentage points, regardless of the core composition. To resolve the ambiguity of internal compositions, we use a growth model and conduct Monte Carlo simulations to demonstrate that many intermediate-size planets are "water worlds."

Synchronization of Belousov-Zhabotinsky oscillators with electrochemical coupling in a spontaneous process.

A passive electrochemical coupling approach is proposed to induce spontaneous synchronization between chemical oscillators. The coupling exploits the potential difference between a catalyst redox couple in the Belousov-Zhabotinsky (BZ) reaction, without external feedback, to induce surface reactions that impact the kinetics of the bulk system. The effect of coupling in BZ oscillators under batch condition is characterized using phase synchronization measures. Although the frequency of the oscillators decreases nonlinearly over time, by a factor of 2 or more within 100 cycles, the coupling is strong enough to maintain synchronization. In such a highly drifting system, the Gibbs-Shannon entropy of the cyclic phase difference distribution can be used to quantify the coupling effect. We extend the Oregonator BZ model to account for the drifting natural frequencies in batch condition and for electrochemical coupling, and numerical simulations of the effect of acid concentration on synchronization patterns are in agreement with the experiments. Because of the passive nature of coupling, the proposed coupling scheme can open avenues for designing pattern recognition and neuromorphic computation systems using chemical reactions in a spontaneous process.

Intermittent regimes as a synchronization phenomenon in two sets of nonlinear chemical oscillators.

Systems of nonlinear chemical oscillators can exhibit a large diversity of non-trivial states depending on the parameters that characterize them. Among these, a synchronization phenomenon is of special interest due to its direct link with chemical and biological processes in nature. We carry out numerical experiments for two different sets of chemical oscillators with different properties and immersed in a Belousov-Zhabotinsky solution. We document the emergence of different states of synchronization that depend on the parameters characterizing the solution. We also show that, in the interface regions, this system generates a stable dynamics of intermittency between the different synchronization states where interesting phenomena, such as the "devil's staircase," emerge. In general, the added complexity introduced with the additional set of oscillators results in more complex non-trivial synchronization states.

Self-Organizing Microdroplet Protocells Displaying Light-Driven Oscillatory and Morphological Evolution.

The development of synthetic systems that enable the sustained active self-assembly of molecular blocks to mimic the complexity and dynamic behavior of living systems is of great interest in elucidating the origins of life, understanding the basic principles behind biological organization, and designing active materials. However, it remains a challenge to construct microsystems with dynamic behaviors and functions that are connected to molecular self-assembly processes driven by external energy. Here, an active self-assembly of microdroplet protocells with dynamic structure and high structural complexity through living radical polymerization under constant energy flux is reported. The active microdroplet protocells exhibit nonlinear behaviors including oscillatory growth and shrinkage. This relies on the transient stabilization of molecular assembly, which can channel the inflow of energy through noncovalent interactions of pure synthetic components. The intercommunication of microdroplet protocells through stochastic fusion leads to the formation of a variety of dynamic and higher-order biomimetic microstructures. This work constitutes an important step toward the realization of autonomous and dynamic microsystems and active materials with life-like properties.

Autonomous Low-Reynolds-Number Soft Robots with Structurally Encoded Motion and Their Thermodynamic Efficiency.

Soft low-Reynolds-number robotics hold the potential to significantly impact numerous fields including drug delivery, sensing, and diagnostics. Realizing this potential is predicated upon the ability to design soft robots tailored to their intended function. In this work, we identify the effect of different geometric and symmetry parameters on the motion of soft, autonomous robots that operate in the low-Reynolds-number regime and use organic fuel. The ability to power low-Reynolds-number soft robots using an organic fuel would provide a new avenue for their potential use in biomedical applications, as is the use of a polymeric biocompatible material as is done here. We introduce a simple and cost-effective 3D-printer-assisted method to fabricate robots of different shapes that is scalable and widely applicable for a variety of materials. The efficiency of chemical energy to mechanical energy conversion is measured in soft low-Reynolds-number robots for the first time, and their mechanism of motion is assessed. Motion is a result of a periodic and oscillatory change in the charge state of the gel. This work lays the groundwork for the structure-function design of soft, chemically operated, and autonomous low-Reynolds-number robots.

Polymerization-Induced Self-Assembly for Artificial Biology: Opportunities and Challenges.

The study of the origin of life and current undergoing efforts to produce artificial chemical systems mimicking the behavior of natural living systems have emerged as a hot topic at the interfaces among disciplines. In these two problems, the spontaneous generation of free-energy gradients by means of material interfaces plays a central role and, until recently, hindered progress. Polymerization-induced self-assembly (PISA) is a promising strategy for the formation of polymeric vesicles from a homogeneous mixture which, in the form of artificial biology, may reflect and inform the generation of vesicular structures in primitive Earth. In the past few years, PISA has been used for the construction of biomimetic vesicles or artificial protocells in artificial biology. These not only give inspiration for decoding some aspects of the origin of life in arbitrary environments but also offer potential for building innovative functional systems with a wide variety of applications. In this review, a brief summary of some of the unique possibilities offered by PISA and the development of PISA in exploration of artificial biology is provided, while some of the allied current challenges, limitations, and opportunities in this exciting field are highlighted.

Resonant Behavior in a Periodically Forced Nonisothermal Oregonator.

Nonisothermal chemical oscillators are poorly studied systems because chemical oscillations are conventionally studied under isothermal conditions. Coupling chemical reactions with heat generation and removal in a nonisothermal oscillatory system can lead to a highly nontrivial nonlinear dynamic behavior. For the current study, we considered the three-variable Oregonator model with the temperature incorporated as a variable (not a parameter), thus adding an energy balance to the set of equations. The effect of temperature on reaction rates is included through the temperature-dependent reaction rate coefficients (Arrhenius law). To model a continuous operation in a laboratory environment, the system was subjected to external forcing through the coolant temperature and infrared irradiation. By conducting numerical simulations and parametric studies, we found that the system is capable of a resonant behavior exhibiting induced oscillations. Our findings indicate that an external source of heat (e.g., via an infrared light emitting diode) can be used to induce a Hopf bifurcation under resonant conditions in an experimental Belousov-Zhabotinsky reactor.

Direct Synthesis of Polymer Vesicles on the Hundred-Nanometer-and-Beyond Scale Using Chemical Oscillations.

The direct synthesis of block copolymer vesicles on the scale of tens to hundreds of nanometers using reversible addition-fragmentation chain transfer (RAFT) dispersion polymerization as an effect of chemical oscillations is reported. RAFT polymerization is successfully accomplished between polyethylene glycol containing a RAFT agent (PEG-CTA) and ethyl acrylate monomer in the presence of the Belousov-Zhabotinsky (B-Z) reaction in oscillatory mode. The self-assembly of poly(ethylene glycol)-b-poly(ethyl acrylate) unimers gives rise to spherical micelles. The self-assembled micelles reorganize and transform to vesicles. All the chemistry of polymerization, self-assembly and self-organization, of macromolecules takes place in a single pot using only a few simple raw materials in aqueous solution.

Osmotically Induced Membrane Fission in Giant Polymer Vesicles: Multilamellarity and Effect of the Amphiphilic Block Lengths.

Amphiphilic block co-polymers are used to form large spherical vesicles. A sudden change in the osmotic pressure across the polymer membrane is used to induce the fission of the polymer vesicle. The membrane area to volume ratio, as expected, is observed to be a parameter suitable to describe the process and even mark the critical points along this transition. The effect of the length of the hydrophobic and hydrophilic chains on the fission process is analyzed. The effects of membrane lamellarity and initial polydispersity are thoroughly analyzed from the experimental data following mathematical models, and the phenomenon of fission in these polymer vesicles is understood via measurements characterizing the membrane, i.e., area stretch modulus.

Microfluidics Fabrication of Self-Oscillating Microgel Clusters with Tailored Temperature-Responsive Properties Using Polymersomes as "Microreactors".

Poly(N-isopropylacrylamide)-based microgel clusters were successfully prepared using polymersomes as "microreactors", which were fabricated through microfluidics. The clusters were formed from the cross-linking reaction between ruthenium/amino group dual functionalized poly(N-isopropylacrylamide) microgels and linear poly(N-isopropylacrylamide)-r-(N-acryloxysuccinimide)-based polymer linkers under neutral pH conditions. By simply adjusting the ratio of N-isopropylacrylamide to N-acryloxysuccinimide in the polymer cross-linkers, the internal structures of the clusters can be controlled; hence, the temperature response of the clusters can be regulated. It was demonstrated that these different microgel clusters showed various degrees of chemomechanical oscillations when the clusters were exposed to a catalyst-free solution containing Belousov-Zhabotinsky reaction substrates.

Controlled Synthesis of Uniform, Micrometer-Sized Ruthenium-Functionalized Poly(N-Isopropylacrylamide) Gel Particles and their Application to the Catalysis of the Belousov-Zhabotinsky Reaction.

Ruthenium-functionalized poly(N-isopropyl acrylamide)-based chemically oscillating microgels with diameters between 1 and 6 µm are synthesized by a modified precipitation polymerization approach. It is found that the initial amount of N-isopropyl acrylamide (NIPAAm) can significantly affect the final sizes of the microgels. 2.5 g of initial NIPAAm results in microgels with maximum average diameter of ≈6 ± 0.5 µm. Making use of their fluorescence due to their ruthenium contents and their larger sizes compared to microgels prepared using other traditional methods, the impact of changes in the NaBrO3 concentrations on their microscopic behavior is studied using a combination of fluorescence microscopy and dynamic light scattering techniques. When increasing the concentration of NaBrO3 in a solution, the microgels first experience a decrease in size followed by aggregation that leads to the loss of colloidal stability. Finally, the redox potential behavior and optical performance of the Belousov-Zhabotinsky reaction catalyzed by these microgels are studied by electrochemical and spectroscopic means.

The thermodynamic efficiency of computations made in cells across the range of life.

Biological organisms must perform computation as they grow, reproduce and evolve. Moreover, ever since Landauer's bound was proposed, it has been known that all computation has some thermodynamic cost-and that the same computation can be achieved with greater or smaller thermodynamic cost depending on how it is implemented. Accordingly an important issue concerning the evolution of life is assessing the thermodynamic efficiency of the computations performed by organisms. This issue is interesting both from the perspective of how close life has come to maximally efficient computation (presumably under the pressure of natural selection), and from the practical perspective of what efficiencies we might hope that engineered biological computers might achieve, especially in comparison with current computational systems. Here we show that the computational efficiency of translation, defined as free energy expended per amino acid operation, outperforms the best supercomputers by several orders of magnitude, and is only about an order of magnitude worse than the Landauer bound. However, this efficiency depends strongly on the size and architecture of the cell in question. In particular, we show that the useful efficiency of an amino acid operation, defined as the bulk energy per amino acid polymerization, decreases for increasing bacterial size and converges to the polymerization cost of the ribosome. This cost of the largest bacteria does not change in cells as we progress through the major evolutionary shifts to both single- and multicellular eukaryotes. However, the rates of total computation per unit mass are non-monotonic in bacteria with increasing cell size, and also change across different biological architectures, including the shift from unicellular to multicellular eukaryotes.This article is part of the themed issue 'Reconceptualizing the origins of life'.

Noise-Induced and Control of Collective Behavior in a Population of Coupled Chemical Oscillators.

Synchronization of intercommunicating individual oscillators is an important form of collective behavior used in nature as a mechanism to face dangers, act collectively, and communicate. The involvement of the medium where oscillators exist is an important ingredient. Because of their nature and their multiple different components, the medium and the environment are often perceived as stochastic relative to the deterministic nature of the individuals on some scale. This injects energy/matter into the system in ways that can enhance or de-enhance communication in a stochastic manner. Here we experimentally consider a large number of coupled nonlinear-chemical oscillators under the effect of a controlled normally distributed noise. Experiments show that the collective behavior of the oscillator is triggered by this stochastic perturbation, and we observe the dependence on the noise parameters. Our results point to the potential use of environmental fluctuations in determining the emergence and properties of collective behaviors in complex systems.

Facile One-Pot Synthesis of Functional Giant Polymeric Vesicles Controlled by Oscillatory Chemistry.

We introduce a novel application of an oscillatory chemical reaction to the synthesis of block copolymers. The Belousov-Zhabotinsky (B-Z) reaction is coupled with the polymerization of an amphiphilic block copolymer. Radicals generated in the B-Z reaction initiate the polymerization between a polyethylene glycol (PEG) macroreversible addition-fragmentation chain-transfer agent and butyl acrylate monomers. The attachment of a hydrophobic block on PEG leads to self-assembly and formation of spherical micelles. The nanoscale micelles transform into submicrometer vesicles and grow to giant vesicles as a consequence of the oscillatory behavior of the B-Z reaction. The one-pot synthesis of an amphiphilic di-block copolymer and retention of oscillatory behavior for the B-Z reaction with the formation of giant vesicles bring a new insight into possible pathways for the synthesis of active functional microreactors in the range from hundreds of nanometers to tens of micrometers.

Unusual kinetics of poly(ethylene glycol) oxidation with cerium(IV) ions in sulfuric acid medium and implications for copolymer synthesis.

The cerium(IV)-alcohol couple in an acidic medium is an example of a redox system capable of initiating free radical polymerization. When the alcohol has a polymeric nature, the outcome of such a process is a block copolymer, a member of a class of compounds possessing many useful properties. The most common polymer with a terminal -OH group is poly(ethylene glycol) (PEG); however, the detailed mechanism of its reaction with cerium(IV) remains underexplored. In this paper, we report our findings for this reaction based on spectrophotometric measurements and kinetic modeling. We find that both the reaction order and the net rate constant for the oxidation process depend strongly on the nature of the acidic medium used. In order to account for the experimental observations, we postulate that protonation of PEG decreases its affinity for some of the cerium(IV)-sulfate complexes formed in the system.

Periodic perturbation of chemical oscillators: entrainment and induced synchronization.

It is well known that a large number of catalyst-carrying beads immersed in an oscillatory chemical medium (Belousov-Zhabotinsky reaction) display collective oscillatory behavior. Using a light sensitive version of BZ, we show that this collective behavior can be entrained to an external light source with an oscillatory intensity. Thus, the emerging collective behavior can be controlled by an external perturbation.

Straightforward synthesis route to polymersomes with simple molecules as precursors.

We demonstrate an easy-to-implement experimental emulsion polymerization protocol whose outcome is an amphiphilic copolymer capable of forming vesicles in an aqueous phase. The protocol does not require prior purification of chemicals or the exclusion of oxygen. Using n-butyl acrylate as the monomer, we employ a redox initiation system composed of cerium(IV) ions and poly(ethylene glycol) (PEG), optimizing the performance of this redox couple such that the reaction can be conducted in air. The PEG-based chain radicals produced during initiation attack the monomer molecules, resulting in an amphiphilic product, which brings the synthesis of a vesicle-forming polymer to a level where no complicated equipment is required and may have implications for origins of life research.

Internal composite bound states in deterministic reaction diffusion models.

By identifying potential composite states that occur in the Sel'kov-Gray-Scott (GS) model, we show that it can be considered as an effective theory at large spatiotemporal scales, arising from a more fundamental theory (which treats these composite states as fundamental chemical species obeying the diffusion equation) relevant at shorter spatiotemporal scales. When simulations in the latter model are performed as a function of a parameter M=λ-1, the generated spatial patterns evolve at late times into those of the GS model at large M, implying that the composites follow their own unique dynamics at short scales. This separation of scales is an example of dynamical decoupling in reaction diffusion systems.

Effect of noise correlation on noise-induced oscillation frequency in the photosensitive Belousov-Zhabotinsky reaction in a continuous stirred tank reactor.

We report on the experimental study of noise-induced oscillations in the photosensitive Ru(bpy)3(2+)-catalyzed Belousov-Zhabotinsky reaction in a continuous stirred tank reactor (CSTR). In the absence of deterministic oscillations and any external periodic forcing, oscillations appear when the system is perturbed by stochastic fluctuations in light irradiation with sufficiently high amplitude in the vicinity of the bifurcation point. The frequency distribution of the noise-induced oscillations is strongly affected by noise correlation. There is a shift of the noise-induced oscillation frequency toward higher frequencies for an intermediate range of the noise correlation exponent, indicating the occurrence of coherence resonance. Our findings indicate that, in principle, noise correlation can be used to direct chemical reactions toward certain behavior.