Responsive polymer materials can adapt to surrounding environments, regulate transport of ions and molecules, change wettability and adhesion of different species on external stimuli, or convert chemical and biochemical signals into optical, electrical, thermal and mechanical signals, and vice versa. These materials are playing an increasingly important part in a diverse range of applications, such as drug delivery, diagnostics, tissue engineering and 'smart' optical systems, as well as biosensors, microelectromechanical systems, coatings and textiles. We review recent advances and challenges in the developments towards applications of stimuli-responsive polymeric materials that are self-assembled from nanostructured building blocks. We also provide a critical outline of emerging developments.

Emerging applications of stimuli-responsive polymer materials

Block copolymer (BCP) self-assembly has attracted considerable attention for many decades because it can yield ordered structures in a wide range of morphologies, including spheres, cylinders, bicontinuous structures, lamellae, vesicles, and many other complex or hierarchical assemblies. These aggregates provide potential or practical applications in many fields. The present tutorial review introduces the primary principles of BCP self-assembly in bulk and in solution, by describing experiments, theories, accessible morphologies and morphological transitions, factors affecting the morphology, thermodynamics and kinetics, among others. As one specific example at a more advanced level, BCP vesicles (polymersomes) and their potential applications are discussed in some detail.

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Self-assembly of block copolymers

Interaction of spherical particles with cells and within animals has been studied extensively, but the effects of shape have received little attention. Here we use highly stable, polymer micelle assemblies known as filomicelles to compare the transport and trafficking of flexible filaments with spheres of similar chemistry. In rodents, filomicelles persisted in the circulation up to one week after intravenous injection. This is about ten times longer than their spherical counterparts and is more persistent than any known synthetic nanoparticle. Under fluid flow conditions, spheres and short filomicelles are taken up by cells more readily than longer filaments because the latter are extended by the flow. Preliminary results further demonstrate that filomicelles can effectively deliver the anticancer drug paclitaxel and shrink human-derived tumours in mice. Although these findings show that long-circulating vehicles need not be nanospheres, they also lend insight into possible shape effects of natural filamentous viruses.

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Shape effects of filaments versus spherical particles in flow and drug delivery.

Since their discovery in the 1960s, liposomes have been studied in depth, and they continue to constitute a field of intense research. Liposomes are valued for their biological and technological advantages, and are considered to be the most successful drug-carrier system known to date. Notable progress has been made, and several biomedical applications of liposomes are either in clinical trials, are about to be put on the market, or have already been approved for public use. In this review, we briefly analyze how the efficacy of liposomes depends on the nature of their components and their size, surface charge, and lipidic organization. Moreover, we discuss the influence of the physicochemical properties of liposomes on their interaction with cells, half-life, ability to enter tissues, and final fate in vivo. Finally, we describe some strategies developed to overcome limitations of the "first-generation" liposomes, and liposome-based drugs on the market and in clinical trials.

https://www.dovepress.com/getfile.php?fileID=23533

Liposomes as nanomedical devices

Polymer-based nanocapsules for drug delivery

Embodiments of this invention relate to a method of removing debris from a wellbore including contacting debris in a wellbore with a treatment fluid, wherein the fluid contains degradable fibers, collecting at least a portion of the treatment fluid, and allowing the degradable fibers to degrade. Embodiments of this invention also relate to a method of removing debris from a wellbore including contacting debris in a wellbore with a treatment fluid, wherein the fluid contains degradable fibers, collecting the treatment fluid, and exposing the fluid to a composition to encourage the degradable fibers to degrade.

Wellbore debris cleanout with coiled tubing using degradable fibers

Liposomes were first described in literature more than 30 years ago by Bangham (1). They have since been used widely in many fundamental studies of amphiphilic systems as well as in applications to extend and control the delivery of a broad variety of drugs (Fig. 1A). Thousands of publications, reviews, and monographs (e.g., (2)) now exist on liposomes. A handful of the documents have expressed ideas that emerge as the clearest backdrop for outlining the motivation, formation, properties, and compatibility of novel polymer-based vesicle platforms called “polymersomes” (3) (Fig. 1B).

Polymersomes: A New Platform for Drug Targeting

Vesicle contents-mixing assays were used to study the fusion of a lipid membrane system that was designed to model the sperm plasma membrane. Comparisons were made to a simpler membrane system that was composed of an equimolar mixture of neutral and anionic phospholipids (phosphatidylcholine (POPC) and phosphatidylserine (POPS)). For both membrane systems, fusion was induced by calcium and was significantly enhanced by the presence of poly(ethylene glycol) (PEG) at a low concentration that was insufficient to cause depletion interactions. In particular, the polymer serves to increase the initial burst and subsequent kinetics of calcium-induced fusion. In comparison with the simpler membrane system, the more-complex sperm mimic vesicles are more resistant to fusion, requiring a higher threshold of calcium concentration. This difference in behavior is attributed to the large fraction of cholesterol (30%) in the sperm mimic membranes. We examined the influence of PEG on ion-binding to membranes using a syste...

Polymer Enhanced Fusion of Model Sperm Membranes as Induced by Calcium

CD47 on erythrocytes has been shown previously to act as a "marker of self" with SIRP-alpha on macrophages. This study examines the functional role of CD47 in the erythrocyte membrane with respect to both cytoskeletal attachment and association with other surface proteins. CD47 was shown to have a fractional, but significant cytoskeletal connectivity on normal and Rh/sub nun/ cells. Additionally, CD47 was shown to colocalize with Rh proteins but not with other cytoskeletally attached proteins, Glycophorin C and Band 3. This suggests that CD47 is independently connected to the underlying cytoskeleton, ensuring that an immobile fraction of CD47 remains equally distributed peripherally at all times. The unconnected fraction could then be recruited to reinforce or amplify this "self" signal. In parallel studies, CD47's IgV domain, which is hypothesized to be the key signaling moiety, was expressed on the surface of yeast. Upon contact with neutrophils in plasma, wild type yeast were readily phagocytosed, whereas IgV-CD47+ yeast show dramatically reduced neutrophil activation and phagocytosis. These fundamental insights into CD47's integration into the membrane as well as its role in immune system avoidance can further be utilized for biomimetic, proactively stealth drug delivery systems.

"Marker of self" CD47: from erythrocyte to drug delivery

The biocompatibility and controlled release profiles of polymersome were investigated. These hyperstable self-assembled block copolymers were injected into rats to elucidate in vivo circulation lifetimes, t-half. For these synthetic vesicles, t-half is 20-30 hours, which is comparable to stealth liposomes. Clearance appears to be primarily mediated by opsonization in spite of the heavily PEGylated brush arising from the hydrophilic fractions of all these block copolymers. Nonetheless, the t-half of these polymersomes compares well with any stealth liposome system. Therefore, novel controlled release polymersomes were prepared by blending the hydrolysable block copolymer poly (ethyleneoaide)-poly(lactic acid) (PEO-PLA) with the diblock copolymer poly(ethyleneoaide)-poly(butadiene) (PEO-PBD). The average time, t-release, for encapsulant release in vitro is obtained from exponential decays in the intact vesicle population and ranges from tens of hours to days or longer. Release kinetics showed minimal difference in plasma at 37/spl deg/C and the rate depends linearly on the mean molar ratio of PEO-PLA incorporated into the vesicle membrane. We propose a mechanism of membrane poration wherein partially hydrolyzed PLA chains congregate and achieve the desired spontaneous membrane curvature by forming a pore and rupturing the membrane, synonymous with detergent-mediated poration, release, and degradation. Emerging studies probe the encapsulant release dynamics in vivo of systems with t-release = t-half.

Peter J. Photos

Papers

Wellbore debris cleanout with coiled tubing using degradable fibers

Polymersomes: A New Platform for Drug Targeting

Polymer Enhanced Fusion of Model Sperm Membranes as Induced by Calcium

"Marker of self" CD47: from erythrocyte to drug delivery

Controlled release and stealthy circulation of polymersomes