Induced helicity in biopolymer networks under stress

Soft, strong, tough, and durable bio-hydrogels via maximizing elastic entropy

Load-bearing soft tissues are soft but strong, strong yet tough. These properties can only be replicated in synthetic hydrogels, which do not have the biocomplexity required by many biomedical applications. By contrast, natural hydrogels, although retaining the native complexity, are weak and fragile. Here we present a thermomechanical casting method to achieve the mechanical capabilities of synthetic materials in biopolymer hydrogels. The thermomechanical cast and chemically crosslinked biopolymer chains form a short-range disordered but long-range ordered structure in water. Upon stretch, the disordered structure transforms to a hierarchically ordered structure. This disorder-order transformation resembles the synergy of the disordered elastin and ordered collagen in load-bearing soft tissues. As entropy drives a reverse order-disorder transformation, the hydrogels can resist repeated cycles of loads without deterioration in mechanical properties. Gelatin hydrogels produced by this method combine tissue-like tunable mechanical properties that outperform the gelatin prepared by synthetic approaches, and in vivo biocomplexity beyond current natural systems. Unlike polymer engineering approaches, which rely on specific crosslinks provided by special polymers, this strategy utilizes the entropy of swollen chains and is generalizable to many other biopolymers. It could thus significantly accelerate translational success of biomaterials.

Polymer crystallization under external flow

The general aspects of polymer crystallization under external flow, i.e., flow-induced crystallization (FIC) from fundamental theoretical background to multi-scale characterization and modeling results are presented. FIC is crucial for modern polymer processing, such as blowing, casting, and injection modeling, as two-third of daily-used polymers is crystalline, and nearly all of them need to be processed before final applications. For academics, the FIC is intrinsically far from equilibrium, where the polymer crystallization behavior is different from that in quiescent conditions. The continuous investigation of crystallization contributes to a better understanding on the general non-equilibrium ordering in condensed physics. In the current review, the general theories related to polymer nucleation under flow (FIN) were summarized first as a preliminary knowledge. Various theories and models, i.e., coil-stretch transition and entropy reduction model, are briefly presented together with the modified versions. Subsequently, the multi-step ordering process of FIC is discussed in detail, including chain extension, conformational ordering, density fluctuation, and final perfection of the polymer crystalline. These achievements for a thorough understanding of the fundamental basis of FIC benefit from the development of various hyphenated rheometer, i.e., rheo-optical spectroscopy, rheo-IR, and rheo-x-ray scattering. The selected experimental results are introduced to present efforts on elucidating the multi-step and hierarchical structure transition during FIC. Then, the multi-scale modeling methods are summarized, including micro/meso scale simulation and macroscopic continuum modeling. At last, we briefly describe our personal opinions related to the future directions of this field, aiming to ultimately establish the unified theory of FIC and promote building of the more applicable models in the polymer processing.

Soft-Mechanochemistry: Mechanochemistry Inspired by Nature

Cells and bacteria use mechanotransduction processes to transform a mechanical force into a chemical/biochemical response. The area of chemistry where chemical reactions are induced by mechanical forces is called mechanochemistry. Over the last few years, chemists developed force-induced reactions affecting covalent bonds in molecules under tension which requires high energy input and/or high intensity forces. In contrast, in nature, mechanotransduction processes take place with forces of much weaker intensity and much less demanding energy. They are mainly based on protein conformational changes or changes in supramacromolecular architectures. Mechanochemistry based on such low-energy-demanding processes and which does not affect chemical bonds can be called soft-mechanochemistry. In this feature article, we first discuss some examples of soft-mechanochemistry processes encountered in nature, in particular, cryptic sites, allowing us to define more precisely the concepts underlying soft-mechanochemistry. A series of examples, developed over the past few years, of chemomechanoresponsive systems based on soft-mechanochemistry principles are given. We describe, in particular, cryptic site surfaces, enzymatically active films whose activity can be modulated by stretching and films where stretching induces changes in their fluorescence properties. Finally, we give our view of the future of soft-mechanochemistry.

Stretch-Induced Helical Conformations in Poly(l-lysine)/Hyaluronic Acid Multilayers

We investigate the effect of stretching on the secondary structure of cross-linked poly(l-lysine)/hyaluronic acid (PLL/HA) multilayers. We show that stretching these films induces changes in the secondary structure of PLL chains. Our results suggest that not only α- but also 310-helices might form in the film under stretching. Such 310-helices have never been observed for PLL so far. These changes of the secondary structure of PLL are reversible, i.e., when returning to the nonstretched state one recovers the initial film structure. Using molecular dynamics simulations of chains composed of 20 l-lysine residues (PLL20), we find that these chains never adopt a helical conformation in water. In contrast, when the end-to-end distance of the chains is restrained to values smaller than the mean end-to-end distance of free chains, a distance domain rarely explored by the free chains, helical conformations become accessible. Moreover, the formation of not only α- but also 310-helices is predicted by the simulations. These results suggest that the change of the end-to-end distance of PLL chains in the stretched film is at the origin of the helix formation.

Molecular basis of the mechanical hierarchy in myomesin dimers for sarcomere integrity

Myomesin is one of the most important structural molecules constructing the M-band in the force-generating unit of striated muscle, and a critical structural maintainer of the sarcomere. Using molecular dynamics simulations, we here dissect the mechanical properties of the structurally known building blocks of myomesin, namely α-helices, immunoglobulin (Ig) domains, and the dimer interface at myomesin's 13th Ig domain, covering the mechanically important C-terminal part of the molecule. We find the interdomain α-helices to be stabilized by the hydrophobic interface formed between the N-terminal half of these helices and adjacent Ig domains, and, interestingly, to show a rapid unfolding and refolding equilibrium especially under low axial forces up to ∼ 15 pN. These results support and yield atomic details for the notion of recent atomic-force microscopy experiments, namely, that the unique helices inserted between Ig domains in myomesin function as elastomers and force buffers. Our results also explain how the C-terminal dimer of two myomesin molecules is mechanically outperforming the helices and Ig domains in myomesin and elsewhere, explaining former experimental findings. This study provides a fresh view onto how myomesin integrates elastic helices, rigid immunoglobulin domains, and an extraordinarily resistant dimer into a molecular structure, to feature a mechanical hierarchy that represents a firm and yet extensible molecular anchor to guard the stability of the sarcomere.

Electrospinning of gelatin for tissue engineering--molecular conformation as one of the overlooked problems

Gelatin is one of the most promising materials in tissue engineering as a scaffold component. This biopolymer indicates biocompatibility and bioactivity caused by the existence of specific amino acid sequences, being preferred sites for interactions with cells, with high similarity to natural extracellular matrix. The present paper does not aspire to be a full review of electrospinning of gelatin and gelatin containing nanofibers as scaffolds in tissue engineering. It is focused on the still open question of the role of the higher order structures of gelatin in scaffold's bioactivity/functionality. Gelatin molecules can adopt various conformations depending on temperature, solvent, pH, etc. Our review indicates the potential ways for formation of α-helix conformation during electrospinning and the methods of further structure stabilization. It is intuitively expected that the native α-helix conformation appearing as a result of partial renaturation of gelatin can be beneficial from the viewpoint of bioactivity of scaffolds, providing thus a much cheaper alternative approach as opposed to expensive electrospinning of native collagen.

Competition for hydrogen-bond formation in the helix-coil transition and protein folding

The problem of the helix-coil transition of biopolymers in explicit solvents, such as water, with the ability for hydrogen bonding with a solvent is addressed analytically using a suitably modified version of the Generalized Model of Polypeptide Chains. Besides the regular helix-coil transition, an additional coil-helix or reentrant transition is also found at lower temperatures. The reentrant transition arises due to competition between polymer-polymer and polymer-water hydrogen bonds. The balance between the two types of hydrogen bonding can be shifted to either direction through changes not only in temperature, but also by pressure, mechanical force, osmotic stress, or other external influences. Both polypeptides and polynucleotides are considered within a unified formalism. Our approach provides an explanation of the experimental difficulty of observing the reentrant transition with pressure and underscores the advantage of pulling experiments for studies of DNA. Results are discussed and compared with those reported in a number of recent publications with which a significant level of agreement is obtained.

Universal kinetics of helix-coil transition in gelatin

By covering a much wider concentration range than previous studies we find a very unusual exponential dependence of the rate of helix formation on concentration of gelatin in water and ethylene glycol solutions. By applying a procedure of concentration-temperature superposition we build a master curve describing the initial renaturation rates in both solvents. The growth of the normalized helical fraction chi(t) is a first-order process, with a rate constant consistent with cis-trans isomerization, in most situations. We propose that association of three separate chains to form a triple helical nucleus occurs rapidly and contributes less to the helical onset than previously thought. The measured helix content is a result of lengthening of the triple helix after nucleation, by zipping from the associated nuclei.

Solvent control of crack dynamics in a reversible hydrogel

The resistance to fracture of reversible biopolymer hydrogels is an important control factor of the textural characteristics of food gels (such as gummy candies and aspic preparations). It is also critical for their use in tissue engineering, for which mechanical protection of encapsulated components is needed. Its dependence on loading rate and, recently, on the density and strength of crosslinks has been investigated. But, so far, no attention has been paid to solvent or to environment effects. Here we report a systematic study of crack dynamics in gels of gelatin in water/glycerol mixtures. We show in this model system that increasing solvent viscosity slows down cracks; moreover soaking with solvent markedly increases gel fragility; finally tuning the viscosity by adding a miscible liquid affects crack propagation through diffusive invasion of the crack tip vicinity. The results highlight the fact that fracture occurs by viscoplastic chain pull-out. This mechanism, as well as the related phenomenology, should be common to all reversibly crosslinked (physical) gels.