Non-equilibrium structural and dynamic behaviors of active polymers in complex and crowded environments.

Active matter systems, which convert internal chemical energy or energy from the environment into directed motion, are ubiquitous in nature and exhibit a range of emerging non-equilibrium behaviors. However, most of the current works on active matter have been devoted to particles, and the study of active polymers has only recently come into the spotlight due to their prevalence within living organisms. The intricate interplay between activity and conformational degrees of freedom gives rise to novel structural and dynamical behaviors of active polymers. Research in active polymers remarkably broadens diverse concepts of polymer physics, such as molecular architecture, dynamics, scaling and so on, which is of significant importance for the development of new polymer materials with unique performance. Furthermore, active polymers are often found in strongly interacting and crowded systems and in complex environments, so that the understanding of this behavior is essential for future developments of novel polymer-based biomaterials. This review thereby focuses on the study of active polymers in complex and crowded environments, and aims to provide insights into the fundamental physics underlying the adaptive and collective behaviors far from equilibrium, as well as the open challenges that the field is currently facing.

Programmable Potentials Choreograph Defects in a Colloidal Crystal Shell.

Crystallization on spherical surfaces is obliged by topology to induce lattice defects. But controlling the organization of such defects remains a great challenge due to the long-range constraints of the curved geometry. Here, we report on DNA-coated colloids whose programmable interaction potentials can be used to regulate the arrangement of defects and even achieve perfect icosahedral order on a sphere. Combined simulations and theoretical analysis show how the potential can be tuned by changing the temperature, thereby controlling the number of defects. An explicit expression for the effective potential is derived, allowing us to distinguish the effects of entropic repulsion and enthalpic attraction. Altogether, the present findings provide insights into the physics of crystallization on curved spaces and may be used for designing desired crystal geometries.

Unconventionally fast transport through sliding dynamics of rodlike particles in macromolecular networks.

Transport of rodlike particles in confinement environments of macromolecular networks plays crucial roles in many important biological processes and technological applications. The relevant understanding has been limited to thin rods with diameter much smaller than network mesh size, although the opposite case, of which the dynamical behaviors and underlying physical mechanisms remain unclear, is ubiquitous. Here, we solve this issue by combining experiments, simulations and theory. We find a nonmonotonic dependence of translational diffusion on rod length, characterized by length commensuration-governed unconventionally fast dynamics which is in striking contrast to the monotonic dependence for thin rods. Our results clarify that such a fast diffusion of thick rods with length of integral multiple of mesh size follows sliding dynamics and demonstrate it to be anomalous yet Brownian. Moreover, good agreement between theoretical analysis and simulations corroborates that the sliding dynamics is an intermediate regime between hopping and Brownian dynamics, and provides a mechanistic interpretation based on the rod-length dependent entropic free energy barrier. The findings yield a principle, that is, length commensuration, for optimal design of rodlike particles with highly efficient transport in confined environments of macromolecular networks, and might enrich the physics of the diffusion dynamics in heterogeneous media.

Hydrodynamic Anisotropy of Depletion in Nonequilibrium.

Active colloids in a bath of inert particles of smaller size cause anisotropic depletion. The active hydrodynamics of this nonequilibrium phenomenon, which is fundamentally different from its equilibrium counterpart and passive particles in an active bath, remains scarcely understood. Here we combine mesoscale hydrodynamic simulation as well as theoretical analysis to examine the physical origin for the active depletion around a self-propelled noninteractive colloid. Our results elucidate that the variable hydrodynamic effect critically governs the microstructure of the depletion zone. Three characteristic states of anisotropic depletion are identified, depending on the strength and stress of activity. This yields a state diagram of depletion in the two-parameter space, captured by developing a theoretical model with the continuum kinetic theory and leading to a mechanistic interpretation of the hydrodynamic anisotropy of depletion. Furthermore, we demonstrate that such depletion in nonequilibrium results in various clusters with ordered organization of squirmers, which follows a distinct principle contrary to that of the entropy scenario of depletion in equilibrium. The findings might be of immediate interest to tune the hydrodynamics-mediated anisotropic interactions and active nonequilibrium organizations in the self-propulsion systems.

The entropy-controlled strategy in self-assembling systems.

Self-assembly of various building blocks has been considered as a powerful approach to generate novel materials with tailorable structures and optimal properties. Understanding physicochemical interactions and mechanisms related to structural formation and transitions is of essential importance for this approach. Although it is well-known that diverse forces and energies can significantly contribute to the structures and properties of self-assembling systems, the potential entropic contribution remains less well understood. The past few years have witnessed rapid progress in addressing the entropic effects on the structures, responses, and functions in the self-assembling systems, and many breakthroughs have been achieved. This review provides a framework regarding the entropy-controlled strategy of self-assembly, through which the structures and properties can be tailored by effectively tuning the entropic contribution and its interplay with the enthalpic counterpart. First, we focus on the fundamentals of entropy in thermodynamics and the entropy types that can be explored for self-assembly. Second, we discuss the rules of entropy in regulating the structural organization in self-assembly and delineate the entropic force and superentropic effect. Third, we introduce the basic principles, significance and approaches of the entropy-controlled strategy in self-assembly. Finally, we present the applications where this strategy has been employed in fields like colloids, macromolecular systems and nonequilibrium assembly. This review concludes with a discussion on future directions and future research opportunities for developing and applying the entropy-controlled strategy in complex self-assembling systems.

Configurational Entropy-Enabled Thermostability of Cell Membranes in Extremophiles: From Molecular Mechanism to Bioinspired Design.

Understanding physicochemical interactions and mechanisms related to the cell membranes of lives under extreme conditions is of essential importance but remains scarcely explored. Here, using a combination of computer simulations and experiments, we demonstrate that the structural integrity and controllable permeability of cell membranes at high temperatures are predominantly directed by configurational entropy emerging from distorted intermolecular organization of bipolar tethered lipids peculiar to the extremophiles. Detailed simulations across multiple scales─from an all-atom exploration of molecular mechanism to a mesoscale examination of its universal nature─suggest that this configurational entropy effect can be generalized to diverse systems, such as block copolymers. This offers biomimetic inspiration for designing heat-tolerant materials based on entropy, as validated by our experiments of synthetic polymers. The findings provide new insight into the basic nature of the mechanism underlying the adaptation of organisms to extreme conditions and might open paths for designed materials inspired by entropic effects in biological systems.

Studies on the Synergistic Effect of Tandem Semi-Stable Complementary Domains on Sequence-Defined DNA Block Copolymers.

Tandem semi-stable complementary domains play an important role in life, while the role of these domains in the folding process of nucleic acid molecules has not been systematically studied. Here, we designed a clean model system by synthesizing sequence-defined DNA-OEG copolymers composed of ssDNA fragments with palindromic sequences and orthogonal oligo(tetraethylene glycol) (OEG) linkers. By altering the lengths of DNA units (6-12 nt) and OEG linkers (Xn = 0-4) separately, we systematically studied how stabilities of tandem complementary domains and connecting flexibilities affect the assembly topology. Combining experimental methods and coarse-grained molecular simulation analysis, distributions of multiple assembled conformations (mainly monomers, dimers, and clusters) were characterized. Both results indicated that tandem semi-stable complementary domains tend to form homogeneous closed circular dimers instead of larger clusters due to the synergistic enhancement effect, and the distributions of each conformation highly depend on flexibilities.

"Shutter" Effects Enhance Protein Diffusion in Dynamic and Rigid Molecular Networks.

Hydrogels have been widely applied to understand the fundamental functions and mechanism of a natural extracellular matrix (ECM). However, revealing the high permeability of ECM through synthetic hydrogels is still challenged by constructing analogue networks with rigid and dynamic properties. Here, in this study, taking advantage of the rigidity and dynamic binding of DNA building blocks, we have designed a model hydrogel system with structural similarity to ECM, leading to enhanced diffusion for proteins compared with a synthetic polyacrylamide (PAAm) hydrogel. The molecular diffusion behaviors in such a rigid and dynamic network have been investigated both in experiments and simulations, and the dependence of diffusion coefficients with respect to molecular size exhibits a unique transition from a power law to an exponential function. A "shutter" model based on the rigid and dynamic molecular network has been proposed, which has successfully revealed how the rigidity and dynamic bond exchange determine the diffusion mechanism, potentially providing a novel perspective to understand the possible mechanism of enhanced diffusion behaviors in ECM.

Understanding Interfacial Nanoparticle Organization through Simulation and Theory: A Review.

Understanding the behaviors of nanoparticles at interfaces is crucial not only for the design of novel nanostructured materials with superior properties but also for a better understanding of many biological systems where nanoscale objects such as drug molecules, viruses, and proteins can interact with various interfaces. Theoretical studies and tailored computer simulations offer unique approaches to investigating the evolution and formation of structures as well as to determining structure-property relationships regarding the interfacial nanostructures. In this feature article, we summarize our efforts to exploit computational approaches as well as theoretical modeling in understanding the organization of nanoscale objects at the interfaces of various systems. First, we present the latest research advances and state-of-the-art computational techniques for the simulation of nanoparticles at interfaces. Then we introduce the applications of multiscale modeling and simulation methods as well as theoretical analysis to explore the basic science and the fundamental principles in the interfacial nanoparticle organization, covering the interfaces of polymer, nanoscience, biomacromolecules, and biomembranes. Finally, we discuss future directions to signify the framework in tailoring the interfacial organization of nanoparticles based on the computational design. This feature article could promote further efforts toward fundamental research and the wide applications of theoretical approaches in designing interfacial assemblies for new types of functional nanomaterials and beyond.

Topology mediates transport of nanoparticles in macromolecular networks.

Diffusion transport of nanoparticles in confined environments of macromolecular networks is common in diverse physical systems and regulates many biological responses. Macromolecular networks possess various topologies, featured by different numbers of degrees and genera. Although the network topologies can be manipulated from a molecular level, how the topology impacts the transport of nanoparticles in macromolecular networks remains unexplored. Here, we develop theoretical approaches combined with simulations to study nanoparticle transport in a model system consisting of network cells with defined topologies. We find that the topology of network cells has a profound effect on the free energy landscape experienced by a nanoparticle in the network cells, exhibiting various scaling laws dictated by the topology. Furthermore, the examination of the impact of cell topology on the detailed behavior of nanoparticle dynamics leads to different dynamical regimes that go beyond the particulars regarding the local network loop. The results might alter the conventional picture of the physical origin of transport in networks.

Entropic control of nanoparticle self-assembly through confinement.

Entropy can be the sole driving force for the construction and regulation of ordered structures of soft matter systems. Specifically, under confinement, the entropic penalty could induce enhanced entropic effects which potentially generate visually ordered structures. Therefore, spatial confinement or a crowding environment offers an important approach to control entropy effects in these systems. Here, we review how spatial confinement-mediated entropic effects accurately and even dynamically control the self-assembly of nanoscale objects into ordered structures, focusing on our efforts towards computer simulations and theoretical analysis. First, we introduce the basic principle of entropic ordering through confinement. We then introduce the applications of this concept to various systems containing nanoparticles, including polymer nanocomposites, biological macromolecular systems and macromolecular colloids. Finally, the future directions and challenges for tailoring nanoparticle organization through spatial confinement-mediated entropic effects are detailed. We expect that this review could stimulate further efforts in the fundamental research on the relationship between confinement and entropy and in the applications of this concept for designer nanomaterials.

Superstretchable, yet stiff, fatigue-resistant ligament-like elastomers.

Ligaments are flexible and stiff tissues around joints to support body movements, showing superior toughness and fatigue-resistance. Such a combination of mechanical properties is rarely seen in synthetic elastomers because stretchability, stiffness, toughness, and fatigue resistance are seemingly incompatible in materials design. Here we resolve this long-standing mismatch through a hierarchical crosslinking design. The obtained elastomer can endure 30,000% stretch and exhibit a Young's modulus of 18 MPa and toughness of 228 MJ m-3, outperforming all the reported synthetic elastomers. Furthermore, the fatigue threshold is as high as 2,682 J m-2, the same order of magnitude as the ligaments (~1,000 J m-2). We reveal that the dynamic double-crosslinking network composed of Li+-O interactions and PMMA nanoaggregates allows for a hierarchical energy dissipation, enabling the elastomers as artificial ligaments in soft robotics.

De Novo Design of Entropy-Driven Polymers Resistant to Bacterial Attachment via Multicomponent Reactions.

Nonbactericidal polymers that prevent bacterial attachment are important for public health, environmental protection, and avoiding the generation of superbugs. Here, inspired by the physical bactericidal process of carbon nanotubes and graphene derivatives, we develop nonbactericidal polymers resistant to bacterial attachment by using multicomponent reactions (MCRs) to introduce molecular "needles" (rigid aliphatic chains) and molecular "razors" (multicomponent structures) into polymer side chains. Computer simulation reveals the occurrence of spontaneous entropy-driven interactions between the bacterial bilayers and the "needles" and "razors" in polymer structures and provides guidance for the optimization of this type of polymers for enhanced resistibility to bacterial attachment. The blending of the optimized polymer with commercially available polyurethane produces a film with remarkably superior stability of the resistance to bacterial adhesion after wear compared with that of commercial mobile phone shells made by the Sharklet technology. This proof-of-concept study explores entropy-driven polymers resistant to bacterial attachment via a combination of MCRs, computer simulation, and polymer chemistry, paving the way for the de novo design of nonbactericidal polymers to prevent bacterial contamination.

Entropy-Driven Unconventional Crystallization of Spherical Colloidal Nanocrystals Confined in Wide Cylinders.

The densest packings of identical spherical colloidal nanocrystals in a thin cylinder generally give rise to confinement-induced chiral ordering. Here, we demonstrate that entropy can invalidate Pauling's packing rules for the nanocrystals confined in wide cylinders and novel ordered phases, where chiral ordering is broken, emerge. The nucleation and growth of spherical colloidal nanocrystals in the wide cylinders exhibit unique mechanisms which are distinctly different from that of thin ones. Furthermore, theoretical models which capture the essential physics of the ordering transitions are developed to reproduce the achiral ordering and reveal that the ordered phases are thermodynamically stable and stabilized through confinement-mediated entropic effect. These findings demonstrate that entropy arising from thermal motion can invalidate Pauling's packing rules of spherical colloidal nanocrystals confined in cylinders, which provides new insights into confinement physics of colloidal particles and might inspire nonintuitive design rules for the fabrication of novel ordered phases through confinement.

Selective Permeable Lithium-Ion Channels on Lithium Metal for Practical Lithium-Sulfur Pouch Cells.

Lithium metal batteries are considered a promising candidate for high-energy-density energy storage. However, the strong reducibility and high reactivity of lithium lead to low Coulombic efficiency when contacting oxidants, such as lithium polysulfide caused by the serious "shuttle effect" in lithium-sulfur batteries. Herein we design selectively permeable lithium-ion channels on lithium metal surface, which allow lithium ions to pass through by electrochemical overpotential, while the polysulfides are effectively blocked due to the much larger steric hindrance than lithium ions. The selective permeation of lithium ions through the channels is further elucidated by the molecular simulation and visualization experiment. Consequently, a prolonged cycle life of 75 cycles and high Coulombic efficiency of 99 % are achieved in a practical Li-S pouch cell with limited amounts of lithium and electrolyte, confirming the unique role the selective ion permeation plays in protecting highly reactive alkali metal anodes in working batteries.

Enhanced Heterogeneous Diffusion of Nanoparticles in Semiflexible Networks.

The transport of nanoparticles in semiflexible networks, which form diverse principal structural components throughout living systems, is important in biology and biomedical applications. By combining large-scale molecular simulations as well as theoretical analysis, we demonstrate here that nanoparticles in polymer networks with semiflexible strands possess enhanced heterogeneous diffusion characterized by more evident hopping dynamics. Particularly, the hopping energy barrier approximates to linear dependence on confinement parameters in the regime of moderate rigidity, in contrast to the quadratic dependence of both its soft and hard counterparts. This nonmonotonic feature can be attributed to the competition between the conformation entropy and the bending energy regulated by the chain rigidity, captured by developing an analytical model of a hopping energy barrier. Moreover, these theoretical results agree reasonably well with previous experiments. The findings bear significance in unraveling the fundamental physics of substance transport confined in network-topological environments and would provide an explanation for the dynamics diversity of nanoparticles within various networks, biological or synthetic.

Entropy at Bio-Nano Interfaces.

Entropy, one of the central concepts of thermodynamics, can be a predominant contribution to structural formation and transition. Although it is well-known that diverse forces and energies can significantly contribute to the structures and activities at bio-nano interfaces, the potential entropic contribution remains less well understood. Therefore, this review article seeks to provide a conceptual framework demonstrating that entropy can be exploited to shape the physicochemical properties of bio-nano interfaces and thereby regulate the structures, responses, and functions of biological systems. We introduce the typical types of entropy that matter at bio-nano interfaces. Moreover, some key characteristics featuring entropy at bio-nano interfaces, such as the difference between entropic force and energetic interaction and the associated implications for biomimetic research, are discussed. We expect that this review could stimulate further effort in the fundamental research of entropy in biology and in the biological applications of entropic effects in designer biomaterials.

Cellular Uptake of Active Particles.

Active particles are widely recognized to potentially revolutionize technologies in numerous biomedical applications. However, the physical origin behind cellular uptake of these particles in the nonequilibrium state remains scarcely understood. Here we combine Brownian dynamics simulation as well as theoretical analysis to provide the criterion for cellular uptake of active particles, related to various physical attributes. Upon enhancing the activity, the uptake efficiency for the active particles with tilted orientation is examined to be nonmonotonic, in stark contrast to the monotonic dependence for active particles orientated normally to the membrane. This can be attributed to the interplay between membrane adhesion energy and kinetic energy of active particles, resulting in unique kinetic pathways. Furthermore, a theoretical model that captures the essential physics of the cellular endocytosis process is developed to reproduce this nonmonotonic feature. The results are of immediate interest to understand and tune activity-mediated cellular interaction and internalization of such emerging colloids.

Diffusive transport of nanoscale objects through cell membranes: a computational perspective.

Diffusion is an essential and fundamental means of transport of substances on cell membranes, and the dynamics of biomembranes plays a crucial role in the regulation of numerous cellular processes. The understanding of the complex mechanisms and the nature of particle diffusion have a bearing on establishing guidelines for the design of efficient transport materials and unique therapeutic approaches. Herein, this review article highlights the most recent advances in investigating diffusion dynamics of nanoscale objects on biological membranes, focusing on the approaches of tailored computer simulations and theoretical analysis. Due to the presence of the complicated and heterogeneous environment on native cell membranes, the diffusive transport behaviors of nanoparticles exhibit unique and variable characteristics. The general aspects and basic theories of normal diffusion and anomalous diffusion have been introduced. In addition, the influence of a series of external and internal factors on the diffusion behaviors is discussed, including particle size, membrane curvature, particle-membrane interactions or particle-inclusion, and the crowding degree of membranes. Finally, we seek to identify open problems in the existing experimental, simulation, and theoretical research studies, and to propose challenges for future development.

C18:1 Improves the Freeze-Drying Resistance of Lactobacillus plantarum by Maintaining the Cell Membrane.

Increasing knowledge about lactic acid bacteria as fermentation starters and probiotics to improve health has led to a growing awareness of their application potential. Despite a long history of applying cryoprotectants, the maintenance of probiotic viability is still a major challenge. In this study, we implemented a strategy and explored its mechanisms in detail. We found that the survival rates after freeze-drying were positively correlated with the relative concentration of the octadecenoic acid (C18:1) and with the ratio of unsaturated to saturated FAs (U/S ratio). The addition of C18:1 significantly improved the survival of L. plantarum after freeze-drying. Contrary to the most commonly used cryoprotectants, the addition of C18:1 did not affect the glass transition temperature or collapse temperature. We predicted that the cell membrane characteristics would be significantly degraded during the drying stage, but C18:1 can effectively maintain the cell membrane integrity and fluidity. Our experiments confirmed those predictions, and simultaneously found that the enzyme activities of key enzymes of glucose metabolism were increased compared with the control group. These finding indicate that C18:1 might serve as a lyoprotectant to maintain the cell membrane integrity and fluidity, and thereby increasing the survival rate of L. plantarum after freeze-drying. This study constitutes a strategy to safeguard bacterial viability.