The ten classes of polymeric phase transitions: their use as models for self-assembly

Mechanical properties of PNIPAM based hydrogels: A review

Materials which adjust their properties in response to environmental factors such as temperature, pH and ionic strength are rapidly evolving and known as smart materials. Hydrogels formed by smart polymers have various applications. Among the smart polymers, thermoresponsive polymer poly(N-isopropylacrylamide)(PNIPAM) is very important because of its well defined structure and property specially its temperature response is closed to human body and can be finetuned as well. Mechanical properties are critical for the performance of stimuli responsive hydrogels in diverse applications. However, native PNIPAM hydrogels are very fragile and hardly useful for any practical purpose. Intense researches have been done in recent decade to enhance the mechanical features of PNIPAM hydrogel. In this review, several strategies including interpenetrating polymer network (IPN), double network (DN), nanocomposite (NC) and slide ring (SR) hydrogels are discussed in the context of PNIPAM hydrogel.

Dissipative particle dynamics study on self-assembled platycodin structures: the potential biocarriers for drug delivery

Platycodin, as a kind of plant based biosurfactants, are saponins which derived from the root of Platycodon grandiflorum A. DC. It has been confirmed that platycodin have the potential to enhance the solubility of hydrophobic drugs and function as the drug carrier, which depends on their micellization over critical micelle concentration (CMC) in aqueous solutions. With the purpose of investigating the effects of influencing factors on the micellization behavior of platycodin and obtaining the phase behavior details at a mesoscopic level, dissipative particle dynamics (DPD) simulations method has been adopted in this study. The simulations reveal that a rich variety of aggregates morphologies will appear with changes of structure or the concentration of saponins, including spherical, ellipse and oblate micelles and vesicles, multilamellar vesicles (MLVs), multicompartment vesicles (MCMs), tubular and necklace-like micelle. They can be formed spontaneously from a randomly generated initial state and the result has been represented in the phase diagrams. Furthermore, deeper explorations have been done on the concentration-dependent structure variation of spherical vesicles as well as the formation mechanism of MLVs. This work provides insight into the solubilization system formed by platycodin, and may serve as guidance for further development and application in pharmaceutical field of platycodin and other saponins.

Smart drug delivery systems: from fundamentals to the clinic

Smart materials can endow implantable depots, targetable nanocarriers and insertable medical devices with activation-modulated and feedback-regulated control of drug release.

From Drug Dosage Forms to Intelligent Drug-delivery Systems: a Change of Paradigm

The design of new drug-delivery systems (DDSs) able to regulate the moment and the rate at which the release should take place, and even to target the drug to specific tissues and cell compartments, has opened novel perspectives to improve the efficacy and safety of the therapeutic treatments. Ideally, the drug should only have access to its site of action and the release should follow the evolution of the disease or of certain biorhythms. The advances in the DDSs field are possible because of a better knowledge of the physiological functions and barriers to the drug access to the action site, but also due to the possibility of having “active” excipients that provide novel features. The joint work in a wide range of disciplines, comprising materials science, biomedical engineering and pharmaceutical technology, prompts the design and development of materials (lipids, polymers, hybrids) that can act as sensors of physiological parameters or external variables, and as actuators able to trigger or tune the release process. Such smart excipients lead to an advanced generation of DDSs designed as intelligent or stimuli-responsive. This chapter provides an overview of how the progress in DDSs is intimately linked to the evolution of the excipients, understood as a specific category of biomaterials. The phase transitions, the stimuli that can trigger them and the mechanisms behind the performance of the intelligent DDSs are analyzed as a whole, to serve as an introduction to the topics that are comprehensively discussed in the subsequent chapters of the book. A look to the future is also provided.

Morphology of a highly asymmetric double crystallizable poly(epsilon-caprolactone-b-ethylene oxide) block copolymer

The morphology of a highly asymmetric double crystallizable poly(epsilon-caprolactone-b-ethylene oxide) (PCL-b-PEO) block copolymer has been studied with in situ simultaneously small and wide-angle x-ray scattering as well as atomic force microscopy. The molecular masses Mn of the PCL and PEO blocks are 24,000 and 5800, respectively. X-ray scattering and rheological measurements indicate that no microphase separation occurs in the melt. Decreasing the temperature simultaneously triggers off a crystallization of PCL and microphase separation between the PCL and PEO blocks. Coupling and competition between microphase separation and crystallization results in a morphology of PEO spheres surrounded by PCL partially crystallized in lamella. Further decreasing temperature induces the crystallization of PEO spheres, which have a preferred orientation due to the confinements from hard PCL crystalline lamella and from soft amorphous PCL segments in different sides. The final morphology of this highly asymmetric block copolymer is similar to the granular morphology reported for syndiotactic polypropylene and other (co-) polymers. This implies a similar underlying mechanism of coupling and competition of various phase transitions, which is worth further exploration.

How does the coupling of secondary and tertiary interactions control the folding of helical macromolecules?

The authors study how the simultaneous presence of short-range secondary and long-range tertiary interactions controls the folding and collapse behavior of a helical macromolecule. The secondary interactions stabilize the helical conformation of the chain, while the tertiary interactions govern its overall three-dimensional shape. The authors have carried out Monte Carlo simulations to study the effect of chain length on the folding and collapse behavior of the chain. They have calculated state diagrams for four chain lengths and found that the physics is very rich with a plethora of stable conformational states. In addition to the helix-coil and coil-globule transitions, their model describes the coupling between them which takes place at low temperatures. Under these conditions, their model predicts a cascade of continuous, conformational transitions between states with an increase in the strength of the tertiary interactions. During each transition the chain shrinks, i.e., collapses, in a rapid and specific manner. In addition, the number of the transitions increases with increasing chain length. They have also found that the low-temperature regions of the state diagram between the transition lines cannot be associated with specific structures of the chain, but rather, with ensembles of various configurations of the chain with similar characteristics. Based on these results the authors propose a mechanism for the folding and collapse of helical macromolecules which is further supported by the analysis of configurational, configurational, and thermodynamic properties of the chain.

Symmetry, equivalence, and molecular self-assembly

Molecular self-assembly at equilibrium is fundamental to the fields of biological self-organization, the development of novel environmentally responsive polymeric materials, and nanofabrication. Our approach to understanding the principles governing this process is inspired by existing models and measurements for the self-assembly of actin, tubulin, and the ubiquitous icosahedral shell structures of viral capsids. We introduce a family of simple potentials that give rise to the self-assembly of linear polymeric, random surface ("membrane"), tubular ("nanotube"), and hollow icosahedral structures that are similar in many respects to their biological counterparts. The potentials involve equivalent particles and an interplay between directional (dipolar, multipolar) and short-range (van der Waals) interactions. Specifically, we find that the dipolar potential, having a continuous rotational symmetry about the dipolar axis, gives rise to chain formation, while particles with multipolar potentials, having discrete rotational symmetries (square quadrupole or triangular ring of dipoles or "hexapole"), lead to the self-assembly of open sheet, nanotube, and hollow icosahedral geometries. These changes in the geometry of self-assembly are accompanied by significant changes in the kinetics of the organization.

Coupling between helix-coil and coil-globule transitions in helical polymers

We explore the coupling between the helix-coil and coil-globule transitions of a helical polymer using Monte Carlo simulations. A very rich state diagram is found. Each state is characterized by a specific configuration of the chain which could be a helix, a random coil, an amorphous globule, or one of various other globular states which carry residual helical strands. We study the boundaries between states and provide further insight into the physics of the system with a detailed analysis of the order parameter and other properties.

Computer simulation of polypeptide translocation through a nanopore

A simplified model of polypeptide chains was designed and studied by means of computer simulations. Chains were represented by a sequence of united atoms located at the positions of the alpha-carbons. A further assumption was the lattice approximation for the chains. We used a (310) lattice, which was found useful for studying properties of proteins. The force field used consisted of a long-range contact potential between amino-acid residues and a local preference for forming alpha-helical states. The chain consisted of two kinds of residues: hydrophilic (P) and hydrophobic (H) ones forming model helical septets--HHPPHPP--in a sequence. The chains were placed near an impenetrable surface with a square hole in it. The size of the hole was comparable or smaller than the size of a chain. The properties of these model chains were determined using the Monte-Carlo simulation method. During the simulations, translocation of the chain through the hole in the wall was observed. The influence of the chain length, the temperature differences on both sides of the wall and the force field on the chain properties were investigated. It was shown that the translocation time scales as N(2.2) and it was found that the presence of the local helical potential significantly slows down the process of translocation. [Figure: see text]. The snapshots of typical chain's conformation obtained during the simulation for chain consisted of N = 60. The values of the local potential epsilon(loc) = -8.

Translocation of rodlike polymers through membrane channels

A theory of channel-facilitated transport of long rodlike macromolecules through thin membranes under the influence of a driving force of arbitrary strength is developed. Analytic expressions are derived for the translocation probability and the Laplace transform of the probability density of time that a macromolecule spends in the channel. We also derive expressions for the (conditional) probability densities of time spent in the channel by translocating and nontranslocating (returning back) macromolecules. These results are used to study how the distribution of the macromolecule lifetime in the channel depends on a polymer chain length and the driving force. It is shown that depending on the values of the parameters, the lifetime probability density may have one or two peaks. Our theory is a generalization of the theory developed by Lubensky and Nelson, who were inspired by recent experiments on driven translocation of single-stranded RNA and DNA molecules through single channels in narrow membranes.