Showing papers in "Soft Matter in 2010"

Why are double network hydrogels so tough

Abstract: Double-network (DN) gels have drawn much attention as an innovative material having both high water content (ca. 90 wt%) and high mechanical strength and toughness. DN gels are characterized by a special network structure consisting of two types of polymer components with opposite physical natures: the minor component is abundantly cross-linked polyelectrolytes (rigid skeleton) and the major component comprises of poorly cross-linked neutral polymers (ductile substance). The former and the latter components are referred to as the first network and the second network, respectively, since the synthesis should be done in this order to realize high mechanical strength. For DN gels synthesized under suitable conditions (choice of polymers, feed compositions, atmosphere for reaction, etc.), they possess hardness (elastic modulus of 0.1–1.0 MPa), strength (failure tensile nominal stress 1–10 MPa, strain 1000–2000%; failure compressive nominal stress 20–60 MPa, strain 90–95%), and toughness (tearing fracture energy of 100∼1000 J m−2). These excellent mechanical performances are comparable to that of rubbers and soft load-bearing bio-tissues. The mechanical behaviors of DN gels are inconsistent with general mechanisms that enhance the toughness of soft polymeric materials. Thus, DN gels present an interesting and challenging problem in polymer mechanics. Extensive experimental and theoretical studies have shown that the toughening of DN gel is based on a local yielding mechanism, which has some common features with other brittle and ductile nano-composite materials, such as bones and dentins.

In pursuit of propulsion at the nanoscale

Abstract: This review describes recent developments in self-propelling nano- and micro-scale swimming devices. The ability of these devices to transport nano-scale components in a fluidic environment is demonstrated. Furthermore, the adaptations needed for these devices to meet biological transport challenges such as targeted drug delivery are highlighted. Particular emphasis is placed on describing autonomously powered devices driven by asymmetrical chemical reactions. Methods to control the speed and direction of such swimming devices using external fields are described, and contrasted to recent demonstrations of statistical autonomous migrations and organisations driven by chemical gradients, inter swimmer interactions and external photo-stimulus. Finally the challenges and advantages of converting other nature inspired swimming mechanisms into realistic artificial self-powered devices are considered.

Synthesis and characterisation of zein–curcumin colloidal particles

Abstract: Zein–curcumin composite colloidal particles were synthesized using an antisolvent precipitation method The average particle size could be controlled down to 100–150 nm, depending on the solvent system and the ratio of zein and curcumin used in the synthesis In all cases, spherical particles were obtained, as confirmed by transmission electron microscopy Depending on the preparation conditions, curcumin load and encapsulation efficiency varied from 16 to 41% and 711 to 868%, respectively Solid state characterization by differential scanning calorimetry and X-ray diffraction indicated the amorphous nature of entrapped curcumin An UV irradiation study confirmed enhanced photostability of curcumin due to the entrapment of curcumin in the biopolymeric matrix The particles were also found to have good colloidal stability at a broad range of physiologically relevant pH (12, 45, 67 and 74) and in simulated gastro-intestinal conditions Results from an in vitro mucoadhesion study showed retention of more than 60% curcumin for 150 minutes The mucoadhesion property was further confirmed from a mucin association study carried out on Caco-2 cells

Mechanical performance tailoring of tough ultra-high porosity foams prepared from cellulose I nanofiber suspensions

Abstract: Low-density structures of mechanical function in plants, arthropods and other organisms, are often based on high-strength cellulose or chitin nanofibers and show an interesting combination of flexibility and toughness. Here, a series of plant-inspired tough and mechanically very robust cellular biopolymer foams with porosities as high as 99.5% (porosity range 93.1–99.5%) were therefore prepared by solvent-free freeze-drying from cellulose I wood nanofiber water suspensions. A wide range of mechanical properties was obtained by controlling density and nanofiber interaction in the foams, and density–property relationships were modeled and compared with those for inorganic aerogels. Inspired by cellulose–xyloglucan (XG) interaction in plant cell walls, XG was added during preparation of the toughest foams. For the cellulose–XG nanocomposite foams in particular, the mechanical properties at comparable densities were superior to those reported in the literature for clay aerogel/cellulose whisker nanocomposites, epoxy/clay aerogels, polymer/clay/nanotube aerogels, and polymer/silica aerogels. The foam structure was characterized by high-resolution field-emission scanning electron microscopy and the specific surface area was measured by nitrogen physisorption. Dynamic mechanical thermal analysis and uniaxial compression tests were performed. The foam was thermally stable up to 275 °C where cellulose started to degrade.

Magnetorheology: materials and application

Abstract: As one of the most important field-responsive intelligent and smart soft matter materials, magnetorheological (MR) fluids, consisting of magneto-responsive magnetizable particles suspended in non-magnetic fluids, have drawn a lot of attentions in both academia and industry as their physical and rheological characteristics can be controlled with external magnetic field strength. In this highlight, preparation methods and MR properties of various magnetic composites with soft magnetic particles and polymers are reviewed. In addition, some industrial applications, such as a MR dampers and a MR polishing, are briefly summarized.

Thermo-sensitive, injectable, and tissue adhesive sol–gel transition hyaluronic acid/pluronic composite hydrogels prepared from bio-inspired catechol-thiol reaction

Abstract: Hyaluronic acid (HA) hydrogels are widely pursued as tissue regenerative and drug delivery materials due to their excellent biocompatibility and biodegradability. Inspired by mussel adhesion, we report here a novel class of thermo-sensitive and injectable HA/Pluronic F127 composite tissue-adhesive hydrogels applicable for various biomedical applications. HA conjugated with dopamine (HA-DN) was mixed with thiol end-capped Pluronic F127 copolymer (Plu-SH) to produce a lightly cross-linked HA/Pluronic composite gel structure based on Michael-type catechol-thiol addition reaction. The HA/Pluronic hydrogels exhibited temperature-dependent sol–gel phase transition behaviors different from Pluronic hydrogels. Rheological studies showed that the sol–gel transitions were rapid and reversible in response to temperature. The HA/Pluronic hydrogels could be injected in vivo in a sol state at room temperature using a syringe, but immediately became a robust gel state at body temperature. The in situ formed hydrogels exhibited excellent tissue-adhesion properties with superior in vivo gel stability and are potentially useful for drug and cell delivery.

A robust superhydrophobic and superoleophobic surface with inverse-trapezoidal microstructures on a large transparent flexible substrate

Abstract: A robust superhydrophobic and superoleophobic surface is demonstrated. On a large-size template of the transparent polydimethylsiloxane (PDMS) elastomer surface, perfectly ordered microstructures with an inverse-trapezoidal cross section are fabricated with two consecutive PDMS replication processes and a three-dimensional diffuser lithography technique. The hydrophobicity and transparency were improved by additional coating of a fluoropolymer layer. The robustness of superhydrophobicity was confirmed by the water droplet impinging test. This transparent, flexible, and superhydrophobic surface provides a new approach for self-cleaning surfaces.

A theory of constrained swelling of a pH-sensitive hydrogel†‡

Abstract: Many engineering devices and natural phenomena involve gels that swell under the constraint of hard materials. The constraint causes a field of stress in a gel, and often makes the swelling inhomogeneous even when the gel reaches a state of equilibrium. This paper develops a theory of constrained swelling of a pH-sensitive hydrogel, a network of polymers bearing acidic groups, in equilibrium with an aqueous solution and mechanical forces. The condition of equilibrium is expressed as a variational statement of the inhomogeneous field. A free-energy function accounts for the stretching of the network, mixing of the network with the solution, and dissociation of the acidic groups. Within a Legendre transformation, the condition of equilibrium for the pH-sensitive hydrogel is equivalent to that for a hyperelastic solid. The theory is first used to compare several cases of homogenous swelling: a free gel, a gel attached to a rigid substrate, and a gel confined in three directions. To analyze inhomogeneous swelling, we implement a finite element method in the commercial software ABAQUS, and illustrate the method with a layer of the gel coated on a spherical rigid particle, and a pH-sensitive valve in microfluidics.

Effect of cholesterol on the rigidity of saturated and unsaturated membranes: fluctuation and electrodeformation analysis of giant vesicles

Abstract: Two different methods for measuring the bending stiffness of lipid membranes are used and further developed: fluctuation analysis and vesicle electrodeformation. For this purpose, fast camera imaging was employed minimizing the experimental effort. The methods were applied to study the effect of cholesterol on the bending stiffness of two types of membranes. We explored giant vesicles prepared from dioleoylphosphatidylcholine–cholesterol and sphingomyelin–cholesterol mixtures. The results show that the effect of cholesterol on the bending stiffness is quite different and lipid-specific. While the bending stiffness of dioleoylphosphatidylcholine membranes does not change significantly, sphingomyelin membranes become more flexible with the addition of cholesterol. Finally, we report data on vesicles prepared from lipid extracts of the plasma membrane of human red blood cells and investigate the influence of naturally present transmembrane peptides. The latter molecules do not alter the membrane stiffness significantly.

Injectable solid hydrogel: mechanism of shear-thinning and immediate recovery of injectable β-hairpin peptide hydrogels

Abstract: β-Hairpin peptide-based hydrogels are a class of injectable hydrogel solids with significant potential use in injectable therapies. β-hairpin peptide hydrogels can be injected as preformed solids, because the solid gel can shear-thin and consequently flow under a proper shear stress but immediately recover back into a solid on removal of the stress. In this work, hydrogel behavior during and after flow was studied in order to facilitate fundamental understanding of how the gels flow during shear-thinning and how they quickly recover mechanically and morphologically relative to their original, pre-flow properties. While all studied β-hairpin hydrogels shear-thin and recover, the duration of shear and the strain rate affected both the gel stiffness immediately recovered after flow and the ultimate stiffness obtained after complete rehealing of the gel. Results of structural analysis during flow were related to bulk rheological behavior and indicated gel network fracture into large (>200 nm) hydrogel domains during flow. After cessation of flow the large hydrogel domains are immediately percolated which immediately reforms the solid hydrogel. The underlying mechanisms of the gel shear-thinning and healing processes are discussed relative to other shear-responsive networks like colloidal gels and micellar solutions.

Hydrogel nanocomposites: a review of applications as remote controlled biomaterials

Abstract: In the past few years, there has been increased interest in the development and applications of hydrogel nanocomposites, specifically as a new class of biomaterials. In some cases, the nanoparticles (e.g., gold, magnetic, carbon nanotubes) can absorb specific stimuli (e.g., alternating magnetic fields, near-IR light) and generate heat. This unique ability to remotely heat the nanocomposites allows for their remote controlled (RC) applications, including the ability to remotely drive the polymer through a transition event (e.g., swelling transition, glass transition). This review highlights some of the recent studies in the development of the RC hydrogel nanocomposites. In particular, some of the important applications of RC nanocomposites as RC drug delivery devices, as RC actuators, and in cancer treatment are discussed.

Evaporation of macroscopic sessile droplets

Abstract: This review is aimed at presenting the evaporation of macroscopic sessile droplets on inert substrates in normal atmosphere in simple cases, as a basis for more complex analyses.

Liquid crystalline polymer cantilever oscillators fueled by light

Abstract: We report on the laser and sunlight driven, fast and large oscillation of cantilevers composed of photoresponsive liquid crystal polymer materials. The oscillation frequency, driven with a focused 100 mW laser of multiple wavelengths (457, 488, 514 nm), is as high as 270 Hz and is shown to be strongly correlated to the physical dimensions of the cantilever. The experimental frequency response is accurately described by the calculated natural resonant frequency for a non-damped cantilever. To further understand the conversion efficiency of light energy to mechanical work in the system, the oscillatory behavior of a 2.7 mm × 0.7 mm × 0.04 mm cantilever was examined at pressures ranging from 1 atm to 0.03 atm. A large increase in amplitude from 110° at STP to 250° at low pressure was observed. A first approximation of the system efficiency was calculated at 0.1%. The large increase in amplitude at low pressure indicates strong hydrodynamic loss and thus, the material efficiency is potentially much greater. Using a simple optical setup, oscillatory behavior was also demonstrated using sunlight. This work indicates the potential for remotely triggered photoactuation of photoresponsive polymer cantilevers from long distances with lasers or focused sunlight.

Aerogels from nanofibrillated cellulose with tunable oleophobicity

Abstract: The formation of structured porous aerogels of nanofibrillated cellulose (NFC) by freeze-drying has been demonstrated. The aerogels have a high porosity, as shown by FE-SEM and nitrogen adsorption/desorption measurements, and a very low density (<0.03 g cm−3). The density and surface texture of the aerogels can be tuned by selecting the concentration of the NFC dispersions before freeze-drying. Chemical vapor deposition (CVD) of 1H,1H,2H,2H-perfluorodecyltrichlorosilane (PFOTS) was used to uniformly coat the aerogel to tune their wetting properties towards non-polar liquids. An XPS analysis of the chemical composition of the PFOTS-modified aerogels demonstrated the reproducibility of the PFOTS-coating and the high atomic fluorine concentration (ca. 51%) in the surfaces. The modified aerogels formed a robust composite interface with high apparent contact angles (θ* ≫ 90°) for castor oil (γlv = 35.8 mN m−1) and hexadecane (γlv = 27.5 mN m−1).

Spatially controlled hydrogel mechanics to modulate stem cell interactions

Abstract: Local control of the stem cell microenvironment with biomaterial design is of critical importance for tissue engineering. Matrix mechanics is one aspect of biomaterial design that has received considerable attention recently due to the effect of mechanics on stem cell proliferation, morphology, and differentiation. In order to investigate the effect of locally controlled mechanics on human mesenchymal stem cells (hMSCs), a sequentially crosslinked hyaluronic acid hydrogel system was developed that permits spatial patterning of mechanics (distinct patterns and gradients). Methacrylated hyaluronic acid was synthesized to allow for crosslinking via both Michael-type addition using a dithiol and radical polymerization using light. By varying the initial methacrylate consumption through addition crosslinking, restricting UV light to specified regions, and varying UV exposure time, a wide range of mechanics (from ∼3 kPa to ∼100 kPa) was possible in both uniform and patterned hydrogels. hMSCs exhibited increased spreading and proliferation on stiffer gels compared to cells cultured on softer gels. Furthermore, cells grown on gels with patterned mechanics exhibited spreading and proliferation behavior that correlated with the local mechanics. This method to spatially control matrix mechanics represents a novel hydrogel system to tune the stem cell microenvironment.

Dynamic polymeric micelles versus frozen nanoparticles formed by block copolymers

Abstract: In selective solvents the solvophobic blocks of block copolymers associate leading to the formation of aggregates. If exchange of polymer chains between aggregates is rapid the system reaches equilibrium and the aggregates are dynamic. However, in many cases kinetically frozen aggregates are formed. In this highlight we review experimental and theoretical work focused on the kinetics of the formation of aggregates by block copolymers and on the rate of exchange of polymers between aggregates at steady state. We will illustrate the importance of the exchange dynamics by their effect on gels and ordered phases formed by block copolymers in solution.

Immersed superhydrophobic surfaces: Gas exchange, slip and drag reduction properties

Abstract: Superhydrophobic surfaces combine high aspect ratio micro- or nano-topography and hydrophobic surface chemistry to create super water-repellent surfaces. Most studies consider their effect on droplets, which ball-up and roll-off. However, their properties are not restricted to modification of the behaviour of droplets, but potentially influence any process occurring at the solid-liquid interface. Here, we highlight three recent developments focused on the theme of immersed superhydrophobic surfaces. The first illustrates the ability of a superhydrophobic surface to act as a gas exchange membrane, the second demonstrates a reduction in drag during flow through small tubes and the third considers a macroscopic experiment demonstrating an increase in the terminal velocity of settling spheres.

Responsive reversible hydrogels from associative “smart” macromolecules

Abstract: The present review highlights the recent developments on the reversible hydrogels which are formed through self-assembling and association mechanisms of well-defined macromolecular entities and which are able to respond to external stimuli like temperature, pH, light and chemical triggers. Thanks to the great progress on macromolecular engineering, especially the enormous development of the “living”/controlled polymerization methods, together with the ability to create artificial peptide-based macromolecules through the recombinant DNA methods, novel “smart” macromolecules capable to form responsive transient 3D networks have been designed and evaluated. Nowadays it has been made possible to create macromolecules with tunable macromolecular architectural characteristics, like chain length of low polydispersity, block topology, hydrophobic/hydrophillic balance and specific functionality, that self assemble in specific environments forming tailor made hydrogels with tunable gel properties, such as injectability and responsiveness (i.e. precise sol to gel transitions triggered by one or more stimuli) mesh size, mechanical strength and dynamics.

Molecular dynamics simulations of glassy polymers

Abstract: We review recent results from computer simulation studies of polymer glasses, from the chain dynamics around the glass transition temperature Tg to the mechanical behaviour below Tg. These results clearly show that modern computer simulations are able to address and give clear answers to some important issues in the field, in spite of the obvious limitations in terms of length and time scales. In the present review we discuss the cooling rate effects, and the dynamic slowing down of different relaxation processes when approaching Tg for both model and chemistry-specific polymer glasses. The impact of geometric confinement on the glass transition is discussed in detail. We also show that computer simulations are very useful tools to study structure and mechanical response of glassy polymers. The influence of large deformations on mechanical behaviour of polymer glasses in general, and strain hardening effect in particular are reviewed. Finally, we suggest some directions for future research, which we believe will be soon within the capabilities of state of the art computer simulations, and correspond to problems of fundamental interest.

Microstructural regimes of colloidal rod suspensions, gels, and glasses

Abstract: We review the diverse range of materials made up of rod-shaped colloids. A common feature of such suspensions is the strong and efficient contribution of rods to the material's solid-like rheological properties such as elastic modulus and yield stress. Colloidal rod suspensions span from biomaterials such as f-actin and fd virus to inorganic materials such as boehmite and hematite, and to commercial fibers such as cellulose. We argue that, depending on the strength of pair potential interactions, such rod suspensions form microstructures that vary between the two limits of heterogeneous fractal clusters and homogeneous fiber networks. The volume fraction range for transition between these two limiting cases is strongly aspect ratio dependent. The two limiting microstructures can be distinguished by differences in the scattering vector dependence of their structure factors, as long as the range of scattering vector probed is sufficient to span regimes both above and below qL ≈ 1. Here q is the scattering vector and L is the rod length. Theories of the Brownian dynamics of fractal clusters and fiber networks show that the two types of microstructure can lead to the arrested dynamics of gelation and the glass transition, respectively. The volume fraction and aspect ratio dependences of the dynamical slowing down of these two cases differ significantly; we suggest that probing these differences is a convenient way to distinguish between gels and glasses of colloidal rods. This distinction is important to clarify because these microstructures are determinants of rheological properties such as elasticity and yielding. By combining these structural and dynamical ideas about rods, we classify a set of literature measurements on more than fifteen different colloidal materials and thereby distinguish between regimes of gelation and vitrification. We conclude by suggesting directions for future research in the arrested dynamics, the non-linear rheology, and the absolute lower limit of gelation in colloidal rod suspensions.

Jamming of frictional spheres and random loose packing

Abstract: The role of the friction coefficient, μ, on the jamming properties of disordered, particle packings is studied using computer simulations Compressed, soft-sphere packings are brought towards the jamming transition—the point where a packing loses mechanical stability—by decreasing the packing fraction The values of the packing fraction at the jamming transition, ϕμc, gradually decrease from the random close packing point for zero friction, to a value coincident with random loose packing as the friction coefficient is increased over several orders of magnitude This is accompanied by a decrease in the coordination number at the jamming transition, zμc, which varies from approximately six to four with increasing friction Universal power law scaling is observed in the pressure and coordination number as a function of distance from the generalised, friction-dependent jamming point Various power laws are also reported between the ϕμc, zμc, and μ Dependence on preparation history of the packings is also investigated

Responsive microcapsule reactors based on hydrogen-bonded tannic acid layer-by-layer assemblies

Abstract: We explore responsive properties of hollow multilayer shells of tannic acid assembled with a range of neutral polymers, poly(N-vinylpyrrolidone) (PVPON), poly(N-vinylcaprolactam) (PVCL) or poly(N-isopropylacrylamide) (PNIPAM). We found that properties of the nanoscale shells fabricated through hydrogen-bonded layer-by-layer (LbL) assembly can be tuned changing the interaction strength of a neutral polymer with tannic acid, and by a change in counterpart hydrophobicity. Unlike most hydrogen-bonded LbL films with two polymer components, the produced tannic acid-based multilayer shells are extremely stable in the wide pH range from 2 to 10. We demonstrate that gold nanoparticles can be grown within tannic acid-containing shell walls under mild environmental conditions paving the way for further modification of the capsule walls through thiol-based surface chemistry. Moreover, these shells show reversible pH-triggered changes in surface charge and permeability towards FITC-labeled polysaccharide molecules. The permeability of these LbL containers can be controlled by changing pH providing an opportunity for loading and release of a functional cargo under mild conditions.

Structure and dynamics of cross-linked actin networks

Abstract: The actin cytoskeleton, a network of protein-polymers, is responsible for the mechanical stability of cells. This biopolymer network is also crucial for processes that require spatial and temporal variations in the network structure such as cell migration, division and intracellular transport. The cytoskeleton therefore has to combine structural integrity and mechanical stability with the possibility of fast and efficient network reorganization and restructuring. Cells meet this challenge by using proteins to link filamentous actin (F-actin) and construct complex networks. The molecular properties of the cross-linking proteins determine to a large extent the (micro)structure, viscoelastic properties and dynamics of the resulting networks. This review focuses on the structural polymorphism that can be induced by cross-linking proteins in reconstituted F-actin networks and summarizes recent results on how the molecular properties of cross-linking proteins dictate the ensuing viscoelastic properties.

Solvent induced shape recovery of shape memory polymer based on chemically cross-linked poly(vinyl alcohol)

Abstract: A shape memory polymer based on poly(vinyl alcohol) (SM-PVA) chemically cross-linked with glutaraldehyde exhibits good temperature responsive shape memory behavior. In the present investigation, solvent-induced shape memory behavior is observed by immersing this kind of SM-PVA in good or poor solvents (including water, DMF, and EG etc.) for PVA. A significant indication of shape memory is the decrease of the glass transition temperature (Tg), which is caused by PVA swelling in certain solvents. Differential scanning calorimetry (DSC), dynamic mechanical analysis (DMA), swelling equilibrium and bending tests are carried out to find the mechanism behind this interesting phenomenon. Factors that influence the permeation of solvents in the polymer depend on the rate and degree of swelling and whether swelling will occur, which determines the shape recovery ratio in different solvents. Water can induce shape recovery in a shorter time than organic solvents. The structure of water and PVA, and the interaction between them, contribute to the shorter recovery time. SM-PVA has an excellent shape recovery ratio even after several test cycles. Using solvents as a stimulus for shape memory behavior will extend applications of SMPs, especially in the field of medical devices, where stimulus instead of heat is highly desired.

Conductive shape memory nanocomposites for high speed electrical actuation

Abstract: A new shape memory nanocomposite that exhibits rapid electrical actuation capabilities is fabricated by incorporating continuous, non-woven carbon nanofibers (CNFs) into an epoxy based SMP matrix. The fiber morphology and nanometre size provide a percolating conductive network with a large interfacial area. This not only gives high electrical conductivity but also simultaneously enhances heat transfer and recovery stress.

Mechanisms of the multi-shape memory effect and temperature memory effect in shape memorypolymers

Abstract: As recently demonstrated, after programming, thermo-responsive shape memorypolymers can exhibit the multi-shape memory effect (SME) upon heating. In addition, it is confirmed that the temperature corresponding to the maximum recovery stress in constrained recovery is roughly the temperature at which pre-deformation is conducted, a phenomenon known as the temperature memory effect (TME). In this paper, we propose a framework to investigate the underlying mechanisms behind both effects and provide the conditions for the TME. According to this framework, we can achieve fully controllable shape recovery following a very complicated sequence in a continuous manner.

Liposomes as ‘smart’ pharmaceutical nanocarriers

Abstract: Since the discovery of liposomes, these phospholipid ‘bubbles’ have received enormous attention to be recognized as ‘smart’ pharmaceutical nanocarriers. Recently, much effort has been directed to the development of so-called ‘smart’ stimuli-sensitive liposomes that will respond to certain internal or external stimuli, such as, pH, temperature, redox potential or magnetic field. These programmable delivery systems can also be made ‘multifunctional’ so as to expose certain functions in an orchestrated manner which can be readily modulated by the stimulus. In this article, the evolution of liposomes with emphasis on the recent advances in stimuli-sensitive liposomes has been reviewed.

PEG/clay nanocomposite hydrogel: a mechanically robust tissue engineering scaffold

Abstract: Nanocomposite hydrogels with enhanced mechanical properties could have tremendous biomedical applications. Here we describe synthesis and characterizations of biocompatible poly(ethylene glycol) diacrylate (PEGDA)/Laponite nanocomposite (NC) hydrogels that can support both two- and three-dimensional (2D and 3D) cell cultures. The PEGDA/Laponite NC hydrogels with enhanced mechanical properties were developed by harnessing the ability of PEGDA oligomers to simultaneously form chemically crosslinked networks while interacting with Laponite nanoparticles through secondary interactions. Incorporation of Laponite nanoparticles significantly enhanced both the compressive and tensile properties of PEGDA hydrogels, which were dependent on both the molecular weight of PEG, and concentrations of Laponite nanoparticles. Unlike PEGDA hydrogels, PEGDA-NC hydrogels supported cell adhesion and their subsequent spreading in a 2D culture. In addition to supporting the 2D cell growth, the PEGDA NC hydrogels supported 3D cell encapsulation similar to that of widely used PEGDA hydrogel systems. Such nanocomposite hydrogels with enhanced mechanical properties could have potential applications as 3D scaffolds for tissue engineering. Additionally, the ability of PEGDA NC hydrogels to support 3D culture of encapsulated cells makes them an ideal injectable system with minimally invasive strategies for in vivo applications.

Nematic phases of bent-core mesogens

Abstract: Bent-core mesogens derived from 4-cyanoresorcinol with terminal alkyl chains have been synthesized and investigated by polarizing microscopy, XRD and electro-optical methods. Short chain compounds have exclusively nematic phases which can be cooled to ambient temperature. These nematic phases are similar to ordinary nematic phases with only nearest neighbour correlation (N) whereas long chain compounds form SmC-type cybotactic clusters and these cybotactic nematic phases (NcybC) can be regarded as strongly fragmented SmC phases. The chain length dependent as well as temperature dependent structural transition from N to NcybC is continuous and associated with a change of the position and intensity of the small angle scattering in the XRD patterns. Moreover, a temperature dependent stepwise transition from cybotactic nematic phases to different types of non-polar and tilted smectic phases (SmC(I) and SmC(II)) is observed with a mesophase composed of elongated, but not yet fused cybotactic clusters (CybC) as an intermediate state of this transition. This improves the understanding of the nature and special properties of the nematic phases formed by bent-core molecules as well as their transition to smectic phases and it paves the way to new materials with spontaneous or field-induced biaxial nematic phases at ambient temperatures.

Viscosity and interfacial properties in a mussel-inspired adhesive coacervate

Abstract: The chemistry of mussel adhesion has commanded the focus of much recent research activity on wet adhesion. By comparison, the equally critical adhesive processing by marine organisms has been little examined. Using a mussel-inspired coacervate formed by mixing a recombinant mussel adhesive protein (fp-151-RGD) with hyaluronic acid (HA), we have examined the nanostructure, viscosity, friction, and interfacial energy of fluid-fluid phase-separated coacervates using the surface forces apparatus and microscopic techniques. At mixing ratios of fp-151-RGD:HA resulting in marginal coacervation, the coacervates showed shear-thickening viscosity and no structure by cryo-transmission electron microscopy (cryo-TEM). However, at the mixing ratio producing maximum coacervation, the coacervate showed shear-thinning viscosity and a transition to a bicontinuous phase by cryo-TEM. The shear-thinning viscosity, high friction coefficient (>1.2), and low interfacial energy (<1 mJ m−2) observed at the optimal mixing ratio for coacervation are promising delivery, spreading and adhesion properties for future wet adhesive and coating technologies.