Complexity in a Simple Self-Assembling System: Lecithin-Water-Ethanol Mixtures Exhibit a Re-Entrant Phase Transition and a Vesicle-Micelle Transition (VMT) on Heating.

We report surprising results for the self-assembly of lecithin (a common phospholipid) in water-ethanol mixtures. Lecithin forms vesicles (∼100 nm diameter) in water. These vesicles are transformed into small micelles (∼5 nm diameter) by a variety of destabilizing agents such as single-tailed surfactants and alcohols. In a surfactant-induced vesicle-micelle transition (VMT), vesicles steadily convert to micelles upon adding the surfactant─thereby, the turbidity of the solution drops monotonically. Instead, when an alcohol like ethanol is added to lecithin vesicles, we find a new, distinctive pattern in phase behavior as the ethanol fraction feth in water is increased. The turbidity first decreases (from feth = 0 to 37%), then rises sharply (feth = 37 to 50%), and then eventually decreases again (feth > 55%). Concomitant with the turbidity rise, the vesicles separate into two phases around feth = 50% before a single phase reappears at higher feth─in other words, there is a "re-entrant" phase transition from 1-phase to 2-phase and back to 1-phase with increasing feth. Vesicles near the phase boundary (∼feth = 45%) also show a VMT upon heating. Similar patterns are seen with other alcohols such as methanol and propanol. We ascribe these complex trends to the dual role played by alcohols: (a) first, alcohols reduce the propensity for flat lipid bilayers to bend and form closed spherical vesicles; and (b) second, alcohols diminish the tendency of lipids to self-assemble in the solvent mixture. At low alcohol fractions, (a) dominates, causing the initially unilamellar vesicles to grow into multilamellar vesicles (MLVs), which eventually phase-separate. Thereafter, (b) dominates, and the vesicles convert into micelles. Support for our hypothesis comes from scattering (SANS) and microscopy (cryo-TEM). Thus, we have uncovered a general paradigm for lipid self-assembly in solvent mixtures, and this may even have physiological relevance.

Cell-Like Capsules with "Smart" Compartments.

Eukaryotic cells have inner compartments (organelles), each with distinct properties and functions. One mimic of this architecture, based on biopolymers, is the multicompartment capsule (MCC). Here, MCCs in which the inner compartments are chemically unique and "smart," i.e., responsive to distinct stimuli in an orthogonal manner are created. Specifically, one compartment alone is induced to degrade when the MCC is contacted with an enzyme while other compartments remain unaffected. Similarly, just one compartment gets degraded upon contact with reactive oxygen species generated from hydrogen peroxide (H2 O2 ). And thirdly, one compartment alone is degraded by an external, physical stimulus, namely, by irradiating the MCC with ultraviolet (UV) light. All these specific responses are achieved without resorting to complicated chemistry to create the compartments: the multivalent cation used to crosslink the biopolymer alginate (Alg) is simply altered. Compartments of Alg crosslinked by Ca2+ are shown to be sensitive to enzymes (alginate lyases) but not to H2 O2 or UV, whereas the reverse is the case with Alg/Fe3+ compartments. These results imply the ability to selectively burst open a compartment in an MCC "on-demand" (i.e., as and when needed) and using biologically relevant stimuli. The results are then extended to a sequential degradation, where compartments in an MCC are degraded one after another, leaving behind an empty MCC lumen. Collectively, this work advances the MCC as a platform that not only emulates key features of cellular architecture, but can also begin to capture rudimentary cell-like behaviors.

Chemically Fueled Dissipative Cross-Linking of Protein Hydrogels Mediated by Protein Unfolding.

Cells assemble dynamic protein-based nanostructures far from equilibrium, such as microtubules, in a process referred to as dissipative assembly. Synthetic analogues have utilized chemical fuels and reaction networks to form transient hydrogels and molecular assemblies from small molecule or synthetic polymer building blocks. Here, we demonstrate dissipative cross-linking of transient protein hydrogels using a redox cycle, which exhibit protein unfolding-dependent lifetimes and mechanical properties. Fast oxidation of cysteine groups on bovine serum albumin by hydrogen peroxide, the chemical fuel, formed transient hydrogels with disulfide bond cross-links that degraded over hours by a slow reductive back reaction. Interestingly, despite increased cross-linking, the hydrogel lifetime decreased as a function of increasing denaturant concentration. Experiments showed that the solvent-accessible cysteine concentration increased with increasing denaturant concentration due to unfolding of secondary structures. The increased cysteine concentration consumed more fuel, which led to less direction oxidation of the reducing agent and affected a shorter hydrogel lifetime. Increased hydrogel stiffness, disulfide cross-linking density, and decreased oxidation of redox-sensitive fluorescent probes at a high denaturant concentration provided evidence supporting the unveiling of additional cysteine cross-linking sites and more rapid consumption of hydrogen peroxide at higher denaturant concentrations. Taken together, the results indicate that the protein secondary structure mediated the transient hydrogel lifetime and mechanical properties by mediating the redox reactions, a feature unique to biomacromolecules that exhibit a higher order structure. While prior works have focused on the effects of the fuel concentration on dissipative assembly of non-biological molecules, this work demonstrates that the protein structure, even in nearly fully denatured proteins, can exert similar control over reaction kinetics, lifetime, and resulting mechanical properties of transient hydrogels.

Phase-Selective Gelation of the Water Phase in an Oil-Water Mixture: An Approach Based on Oil-Activated Nanoparticle Assembly in Water.

Phase-selective gelation refers to the selective gelation of one phase in an immiscible mixture. Thus far, all such examples have involved a molecular gelator forming nanofibers in (and thus gelling) the oil phase in an oil/water mixture. Here, for the first time, we report the counterpart to the above phenomenon, i.e., selective gelation of the water phase in an oil/water mixture (while leaving the oil undisturbed). This has been a challenging problem because moieties that gel water tend to be either amphiphilic or oil-soluble; thus, if combined with an oil/water mixture, they invariably form an emulsion. Our approach solves this problem by exploiting the tunable self-assembly of laponite (LAP) nanoparticles. Initially, LAP nanoparticles (25 nm disks) are dispersed in water, where they remain unaggregated due to the steric stabilization provided by a triblock copolymer (Pluronic P123) adsorbed on their surface. Thus, the dispersion is initially a low-viscosity sol. When an immiscible oil such as hexadecane is introduced above the sol, the mixture remains biphasic, and both phases remain unaffected. Next, an organic acid such as butanoic acid (BA) is added to the oil. The BA is oil-soluble but also has limited solubility in the water. Over about 30 min, some of the BA enters the water, whereupon it "activates" the self-assembly of LAP particles into a three-dimensional "house-of-cards" network. Ultimately, the water phase is converted into a homogeneous gel with a sufficient yield stress: the aqueous gel holds its weight in the inverted vial while the oil phase remains a thin liquid that can be poured out of the vial. On the whole, the concept advanced here is about activating nanoparticle assembly in water through an adjacent, immiscible phase. This concept could prove useful in conducting certain separations or reactions in the laboratory as well as in enhanced oil recovery.

Spontaneous Formation of Stable Vesicles and Vesicle Gels in Polar Organic Solvents.

The self-assembly of lipids into nanoscale vesicles (liposomes) is routinely accomplished in water. However, reports of similar vesicles in polar organic solvents like glycerol, formamide, and ethylene glycol (EG) are scarce. Here, we demonstrate the formation of nanoscale vesicles in glycerol, formamide, and EG using the common phospholipid lecithin (derived from soy). The samples we study are simple binary mixtures of lecithin and the solvent, with no additional cosurfactants or salt. Lecithin dissolves readily in the solvents and spontaneously gives rise to viscous fluids at low lipid concentrations (∼2-4%), with structures ∼200 nm detected by dynamic light scattering. At higher concentrations (>10%), lecithin forms clear gels that are strongly birefringent at rest. Dynamic rheology confirms the elastic response of gels, with their elastic modulus being ∼20 Pa at ∼10% lipid. Images from cryo-scanning electron microscopy (cryo-SEM) indicate that concentrated samples are "vesicle gels," where multilamellar vesicles (MLVs, also called "onions"), with diameters between 50 and 600 nm, are close-packed across the sample volume. This structure can explain both the elastic rheology as well as the static birefringence of the samples. The discovery of vesicles and vesicle gels in polar solvents widens the scope of systems that can be created by self-assembly. Interestingly, it is much easier to form vesicles in polar solvents than in water, and the former are stable indefinitely, whereas the latter tend to aggregate or coalesce over time. The stability is attributed to refractive index-matching between lipid bilayers and the solvents, i.e., these vesicles are relatively "invisible" and thus experience only weak attractions. The ability to use lipids (which are "green" or eco-friendly molecules derived from renewable natural sources) to thicken and form gels in polar solvents could also prove useful in a variety of areas, including cosmetics, pharmaceuticals, and lubricants.

Using Microemulsion Phase Behavior as a Predictive Model for Lecithin-Tween 80 Marine Oil Dispersant Effectiveness.

Marine oil dispersants typically contain blends of surfactants dissolved in solvents. When introduced to the crude oil-seawater interface, dispersants facilitate the breakup of crude oil into droplets that can disperse in the water column. Recently, questions about the environmental persistence and toxicity of commercial dispersants have led to the development of "greener" dispersants consisting solely of food-grade surfactants such as l-α-phosphatidylcholine (lecithin, L) and polyoxyethylenated sorbitan monooleate (Tween 80, T). Individually, neither L nor T is effective at dispersing crude oil, but mixtures of the two (LT blends) work synergistically to ensure effective dispersion. The reasons for this synergy remain unexplained. More broadly, an unresolved challenge is to be able to predict whether a given surfactant (or a blend) can serve as an effective dispersant. Herein, we investigate whether the LT dispersant effectiveness can be correlated with thermodynamic phase behavior in model systems. Specifically, we study ternary "DOW" systems comprising LT dispersant (D) + a model oil (hexadecane, O) + synthetic seawater (W), with the D formulation being systematically varied (across 0:100, 20:80, 40:60, 60:40, 80:20, and 100:0 L:T weight ratios). We find that the most effective LT dispersants (60:40 and 80:20 L:T) induce broad Winsor III microemulsion regions in the DOW phase diagrams (Winsor III implies that the microemulsion coexists with aqueous and oil phases). This correlation is generally consistent with expectations from hydrophilic-lipophilic deviation (HLD) calculations, but specific exceptions are seen. This study then outlines a protocol that allows the phase behavior to be observed on short time scales (ca. hours) and provides a set of guidelines to interpret the results. The complementary use of HLD calculations and the outlined fast protocol are expected to be used as a predictive model for effective dispersant blends, providing a tool to guide the efficient formulation of future marine oil dispersants.

Multilayer tubes that constrict, dilate, and curl in response to stimuli.

Tubular structures in nature have the ability to respond to their environment-for example, blood vessels can constrict or dilate, thereby regulating flow velocity and blood pressure. These tubes have multiple concentric layers, with each layer having a distinct composition and properties. Inspired by such natural structures, we have synthesized responsive multilayer tubes in the laboratory without resorting to complex equipment such as a 3-D printer. Each layer of our tubes is a polymer gel formed by free-radical polymerization of water-soluble monomers. We can precisely control the inner diameter of the tube, the number of layers in the tube wall, and the thickness and chemistry of each layer. Tubes synthesized in this manner are robust, flexible, and stretchable. Moreover, our technique allows us to incorporate stimuli-responsive polymers into distinct regions of these tubes, and the resulting tubes can change their shape in response to external stimuli such as pH or temperature. In the case of laterally patterned tubes, the tube can be made to constrict or dilate over a particular segment-a behavior that is reminiscent of blood vessels. In the case of longitudinally patterned tubes, a straight tube can be induced to systematically curl into a coil. The versatility of our technique is further shown by constructing complex tubular architectures, including branched networks. On the whole, the polymeric tubes shown in this paper exhibit remarkable properties that cannot be realized by other techniques. Such tubes could find utility in biomedical engineering to construct anatomically realistic mimics of various tissues.

Light-Triggered Rheological Changes in a System of Cationic Wormlike Micelles Formulated with a Photoacid Generator.

"Smart" fluids displaying large changes in their rheological properties in response to external stimuli have been of great interest in recent years. For example, "smart" wormlike micelles (WLMs) that respond to pH can be readily formulated by combining a cationic surfactant such as cetyltrimethylammonium bromide (CTAB) with an aromatic compound such as 1,2-dihydroxybenzene (DHB). Here, we show that a pH-responsive aqueous formulation as mentioned above can be simultaneously made responsive to ultraviolet (UV) light by incorporating a photoacid generator (PAG) into the system. A commercially available PAG, diphenyliodonium-2-carboxylate, is used here. Upon exposure to UV light, this PAG irreversibly photolyzes into iodobenzene (IB) and benzoic acid (BA), with the formation of BA, leading to a drop in pH. WLMs formed by mixtures of CTAB, DHB, and the PAG are systematically characterized before and after UV irradiation. As the PAG photolyzes, an increase in the viscosity of WLMs occurs by a factor of 1000. We show that the ratio of the zero-shear viscosity η0 (after UV/before UV) depends on the initial pH of the sample. The UV-induced increase in η0 can be attributed to the growth of WLMs in solution, which in turn is influenced by both the ionization state of DHB and the presence of IB and BA.

How Do Amphiphilic Biopolymers Gel Blood? An Investigation Using Optical Microscopy.

Amphiphilic biopolymers such as hydrophobically modified chitosan (hmC) have been shown to convert liquid blood into elastic gels. This interesting property could make hmC useful as a hemostatic agent in treating severe bleeding. The mechanism for blood gelling by hmC is believed to involve polymer-cell self-assembly, i.e., insertion of hydrophobic side chains from the polymer into the lipid bilayers of blood cells, thereby creating a network of cells bridged by hmC. Here, we probe the above mechanism by studying dilute mixtures of blood cells and hmC in situ using optical microscopy. Our results show that the presence of hydrophobic side chains on hmC induces significant clustering of blood cells. The extent of clustering is quantified from the images in terms of the area occupied by the 10 largest clusters. Clustering increases as the fraction of hydrophobic side chains increases; conversely, clustering is negligible in the case of the parent chitosan that lacks hydrophobes. Moreover, the longer the hydrophobic side chains, the greater the clustering (i.e., C12 > C10 > C8 > C6). Clustering is negligible at low hmC concentrations but becomes substantial above a certain threshold. Finally, clustering due to hmC can be reversed by adding the supramolecule α-cyclodextrin, which is known to capture hydrophobes in its binding pocket. Overall, the results from this work are broadly consistent with the earlier mechanism, albeit with a few modifications.

The Unusual Rheology of Wormlike Micelles in Glycerol: Comparable Timescales for Chain Reptation and Segmental Relaxation.

Wormlike micelles (WLMs) are polymer-like chains formed by surfactant self-assembly in water. Recently, we have shown that WLMs can also be self-assembled in polar organic liquids like glycerol using a cationic surfactant and an aromatic salt. In this work, we focus on the dynamic rheology of the WLMs in glycerol and demonstrate that their rheology is very different from that of WLMs in water. Aqueous WLMs that are entangled into transient networks exhibit the rheology of a perfect Maxwell fluid having a single relaxation time tR-thereby, their elastic modulus G' and viscous modulus G″ intersect at a crossover frequency ωc = 1/tR. WLMs in glycerol also form entangled networks, but they are not Maxwell fluids; instead, they exhibit a double-crossover of G' and G″ (at ωc1 and ωc2) within the ω-window accessible by rheometry (10-2 to 102 rad/s). The first crossover at ωc1 (∼1 rad/s) corresponds to the terminal relaxation time (i.e., the timescale for chains to disentangle from the transient network and relax by reptation). At the other extreme, at frequencies above ωc2 (which is ∼10 rad/s), the rheology is dominated by the segmental motion of the chains. This "breathing regime" has rarely been accessed via experiments for aqueous WLMs because it falls around 105 rad/s. We believe that glycerol, a solvent that is much more viscous than water, exerts a crucial influence in pushing ωc2 to 1000-fold lower frequencies. On the basis of the rheology, we also hypothesize that WLMs in glycerol are shorter and weakly entangled compared to WLMs in water. Moreover, we suggest that WLMs in glycerol are "unbreakable" chains-i.e., the chains remain mostly intact instead of breaking and re-forming frequently-and this polymer-like behavior explains why the samples are quite unlike Maxwell fluids.

Liposomes Entrapped in Biopolymer Hydrogels Can Spontaneously Release into the External Solution.

Hydrogels of biopolymers such as agar and gelatin are widely used in many applications, and in many cases, the gels are loaded with nanoparticles. The polymer chains in these gels are cross-linked by physical bonds into three-dimensional networks, with the mesh size of these networks typically being 10-100 nm. One class of "soft" nanoparticles are liposomes, which have an aqueous core surrounded by a lipid bilayer. Solutes encapsulated in the liposomal core can be delivered externally over time. In this paper, we create liposomes with diameters ∼150 nm from an unsaturated phospholipid (lecithin) and embed them in agar gels (the aqueous phase also contains 0-50% of glycerol, which is an active ingredient in cosmetic products). Upon placing this gel in quiescent water, we find that the liposomes release out of the gel into the water over a period of 1-3 days, even though the gel remains intact. This is a surprising result that runs contrary to our expectation that the liposomes would simply remain immobilized in the gel. We show that the release rate of liposomes can be tuned by several variables: for example, the release rate increases as the agar concentration is lowered and the rate increases steadily with temperature. In addition to agar, release of liposomes also occurs out of other physical gels including those of agarose and gelatin. However, liposomes made from a saturated phospholipid do not release out of any gels. We discuss a possible mechanism for liposomal release, which involves intact liposomes deforming and squeezing through transient large pores that arise in physical networks such as agar. Our findings have relevance to transdermal delivery: they suggest the possibility of systematically delivering liposomes loaded with actives out of an intact matrix.

Catalyst-Loaded Capsules that Spontaneously Inflate and Violently Eject their Core.

We present a design for polymer capsules that exhibit a range of unusual autonomous behaviors when exposed to a chemical fuel. The capsules have a physically gelled core (alginate-Ca2+) loaded with catalytic (silver) particles and a shell composed of a chemically cross-linked gel. In the presence of the fuel (H2O2), a catalytic reaction occurs, which generates oxygen (O2) gas. The gas collects in a zone between the core and the shell, and the resulting gas pressure causes the elastic shell to stretch. This makes the capsule inflate in a process reminiscent of a swelling pufferfish. As the capsule inflates, the polymer chains in the shell continue to stretch until a breaking point is reached, whereupon the shell ruptures. Three rupture modes are documented: gentle, moderate, and violent. The latter involves the gelled core being forcefully ejected out of the shell in a manner similar to the ejection of needles out of nematocysts on jellyfish. The extent and duration of inflation can be tuned by altering the core and shell composition; for example, shells that are more densely cross-linked swell less and rupture faster. Also, instead of a catalytic reaction, capsule inflation can be achieved by combining reactants, one in the capsule and the other in the external solution, that together generate a different gas (e.g., CO2).

Wormlike Micelles of a Cationic Surfactant in Polar Organic Solvents: Extending Surfactant Self-Assembly to New Systems and Subzero Temperatures.

Wormlike micelles (WLMs) are long, flexible cylindrical chains formed by the self-assembly of surfactants in semidilute solutions. Scientists have been fascinated by WLMs because of their similarities to polymers, while at the same time, the viscoelastic properties of WLM solutions have made them useful in a variety of industrial applications. To date, most studies on WLMs have been performed in water (i.e., a highly polar liquid), while there are a few examples of "reverse" WLMs in oils (i.e., highly nonpolar liquids). However, in organic solvents with lower polarity than water such as glycerol, formamide, and ethylene glycol, there have been no reports of WLMs thus far. Here, we show that it is indeed possible to induce a long-tailed cationic surfactant to assemble into WLMs in several of these solvents. To form WLMs, the surfactant is combined with a "binding" salt, i.e., one with a large organic counterion that is capable of binding to the micelles. Examples of such salts include sodium salicylate and sodium tosylate, and we find self-assembly to be maximized when the surfactant and salt concentrations are near-equimolar. Interestingly, the addition of a simple, inorganic salt such as sodium chloride (NaCl) to the same surfactant does not induce WLMs in polar solvents (although it does so in water). Thus, the design rules for WLM formation in polar solvents are distinct from those in water. Aqueous WLMs have been characterized at temperatures from 25 °C and above, but few studies have examined WLMs at much lower (e.g., subzero) temperatures. Here, we have selected a surfactant with a very low Krafft point (i.e., the surfactant does not crystallize out of solution upon cooling due to a cis-unsaturation in its tail) and a low-freezing solvent, viz. a 90/10 mixture of glycerol and ethylene glycol. In these mixtures, we find evidence for WLMs that persist down to temperatures as low as -20 °C. Rheological techniques as well as small-angle neutron scattering (SANS) have been used to characterize the WLMs under these conditions. Much like their aqueous counterparts, WLMs in polar solvents show viscoelastic properties, and accordingly, these fluids could find applications as synthetic lubricants or as improved antifreezing fluids.

Does the Solvent in a Dispersant Impact the Efficiency of Crude-Oil Dispersion?

Dispersants, used in the mitigation of oil spills, are mixtures of amphiphilic molecules (surfactants) dissolved in a solvent. The recent large-scale use of dispersants has raised environmental concerns regarding the safety of these materials. In response to these concerns, our lab has developed a class of eco-friendly dispersants based on blends of the food-grade surfactants, soy lecithin (L) and Tween 80 (T), in a solvent. We have shown that these "L/T dispersants" are very efficient at dispersing crude oil into seawater. The solvent for dispersants is usually selected based on factors like toxicity, volatility, or viscosity of the overall mixture. However, with regard to the dispersion efficiency of crude oil, the solvent is considered to play a negligible role. In this paper, we re-examine the role of solvent in the L/T system and show that it can actually have a significant impact on the dispersion efficiency. That is, the dispersion efficiency can be altered from poor to excellent simply by varying the solvent while keeping the same blend of surfactants. We devise a systematic procedure for selecting the optimal solvents by utilizing Hansen solubility parameters. The optimal solvents are shown to have a high affinity for crude oil and limited hydrophilicity. Our analysis further enables us to identify solvents that combine high dispersion efficiency, good solubility of the L/T surfactants, a low toxicity profile, and a high flash point.

Freestanding organogels by molecular velcro of unsaturated amphiphiles.

A simple amphiphile, N-cardanyltaurine amide (NCT) with different degrees of cis-unsaturation in its tail resulted in the formation of strong organogels. Interestingly, this is in contrast to the commonly accepted notion that introducing unsaturation in alkyl chains enhances fluidity in lipid assemblies. The physico-chemical and first-principles DFT calculations confirmed the pegging of 'kinked' unsaturated side chains, where the hydrophobic interlocking as in Velcro fasteners leads to a network of cylindrical micelles, resulting in self-standing organogels. Textural profile analysis and spectroscopic details substantiated the dynamic assembly to resemble a 3D network of gelators rather than being a cross-linked or polymerized matrix of monomers.

Expanding Hydrophobically Modified Chitosan Foam for Internal Surgical Hemostasis: Safety Evaluation in a Murine Model.

BACKGROUND: A novel injectable expanding foam based on hydrophobically modified chitosan (HM-CS) was developed to improve hemostasis during surgeries. HM-CS is an amphiphilic derivative of the natural biopolymer chitosan (CS); HM-CS has been shown to improve the natural hemostatic characteristics of CS, but its internal safety has not been systematically evaluated. The goal of this study was to compare the long-term in vivo safety of HM-CS relative to a commonly used fibrin sealant (FS), TISSEEL (Baxter). METHODS: Sixty-four Sprague-Dawley rats (275-325 g obtained from Charles River Laboratories) were randomly assigned to control (n = 16) or experimental (n = 48) groups. Samples of the test materials (HM-CS [n = 16], CS [n = 16], and FS [n = 16]) applied to a nonlethal liver excision (0.4 ± 0.3 g of the medial lobe) in rats were left inside the abdomen to degrade. Animals were observed daily for signs of morbidity and mortality. Surviving animals were sacrificed at 1 and 6 wk; the explanted injury sites were microscopically assessed. RESULTS: All animals (64/64) survived both the 1- and 6-wk time points without signs of morbidity. Histological examination showed a comparable pattern of degradation for the various test materials. FS remnants and significant adhesions to neighboring tissues were observed at 6 wk. Residual CS and HM-CS were observed at the 6 wk with fatty deposits at the site of injury. Minimal adhesions were observed for CS and HM-CS. CONCLUSIONS: The internal safety observed in the HM-CS test group after abdominal implantation indicates that injectable HM-CS expanding foam may be an appropriate internal use hemostatic candidate.

Incorporating LsrK AI-2 quorum quenching capability in a functionalized biopolymer capsule.

Antibacterial resistance is an issue of increasing severity as current antibiotics are losing their effectiveness and fewer antibiotics are being developed. New methods for combating bacterial virulence are required. Modulating molecular communication among bacteria can alter phenotype, including attachment to epithelia, biofilm formation, and even toxin production. Intercepting and modulating communication networks provide a means to attenuate virulence without directly interacting with the bacteria of interest. In this work, we target communication mediated by the quorum sensing (QS) bacterial autoinducer-2, AI-2. We have assembled a capsule of biological polymers alginate and chitosan, attached an AI-2 processing kinase, LsrK, and provided substrate, ATP, for enzymatic alteration of AI-2 in culture fluids. Correspondingly, AI-2 mediated QS activity is diminished. All components of this system are "biofabricated"-they are biologically derived and their assembly is accomplished using biological means. Initially, component quantities and kinetics were tested as assembled in microtiter plates. Subsequently, the identical components and assembly means were used to create the "artificial cell" capsules. The functionalized capsules, when introduced into populations of bacteria, alter the dynamics of the AI-2 bacterial communication, attenuating QS activated phenotypes. We envision the assembly of these and other capsules or similar materials, as means to alter QS activity in a biologically compatible manner and in many environments, including in humans.

Cation-induced folding of alginate-bearing bilayer gels: an unusual example of spontaneous folding along the long axis.

The spontaneous folding of flat gel films into tubes is an interesting example of self-assembly. Typically, a rectangular film folds along its short axis when forming a tube; folding along the long axis has been seen only in rare instances when the film is constrained. Here, we report a case where the same free-swelling gel film folds along either its long or short axis depending on the concentration of a solute. Our gels are sandwiches (bilayers) of two layers: a passive layer of cross-linked N,N'-dimethylyacrylamide (DMAA) and an active layer of cross-linked DMAA that also contains chains of the biopolymer alginate. Multivalent cations like Ca2+ and Cu2+ induce these bilayer gels to fold into tubes. The folding occurs instantly when a flat film of the gel is introduced into a solution of these cations. The likely cause for folding is that the active layer stiffens and shrinks (because the alginate chains in it get cross-linked by the cations) whereas the passive layer is unaffected. The resulting mismatch in swelling degree between the two layers creates internal stresses that drive folding. Cations that are incapable of cross-linking alginate, such as Na+ and Mg2+, do not induce gel folding. Moreover, the striking aspect is the direction of folding. When the Ca2+ concentration is high (100 mM or higher), the gels fold along their long axis, whereas when the Ca2+ concentration is low (40 to 80 mM), the gels fold along their short axis. We hypothesize that the folding axis is dictated by the inhomogeneous nature of alginate-cation cross-linking, i.e., that the edges get cross-linked before the faces of the gel. At high Ca2+ concentration, the stiffer edges constrain the folding; in turn, the gel folds such that the longer edges are deformed less, which explains the folding along the long axis. At low Ca2+ concentration, the edges and the faces of the gel are more similar in their degree of cross-linking; therefore, the gel folds along its short axis. An analogy can be made to natural structures (such as leaves and seed pods) where stiff elements provide the directionality for folding.

Liposomes: Clinical Applications and Potential for Image-Guided Drug Delivery.

Liposomes have been extensively studied and are used in the treatment of several diseases. Liposomes improve the therapeutic efficacy by enhancing drug absorption while avoiding or minimizing rapid degradation and side effects, prolonging the biological half-life and reducing toxicity. The unique feature of liposomes is that they are biocompatible and biodegradable lipids, and are inert and non-immunogenic. Liposomes can compartmentalize and solubilize both hydrophilic and hydrophobic materials. All these properties of liposomes and their flexibility for surface modification to add targeting moieties make liposomes more attractive candidates for use as drug delivery vehicles. There are many novel liposomal formulations that are in various stages of development, to enhance therapeutic effectiveness of new and established drugs that are in preclinical and clinical trials. Recent developments in multimodality imaging to better diagnose disease and monitor treatments embarked on using liposomes as diagnostic tool. Conjugating liposomes with different labeling probes enables precise localization of these liposomal formulations using various modalities such as PET, SPECT, and MRI. In this review, we will briefly review the clinical applications of liposomal formulation and their potential imaging properties.

Amphiphilic Polypeptoids Serve as the Connective Glue to Transform Liposomes into Multilamellar Structures with Closely Spaced Bilayers.

We report the ability of hydrophobically modified polypeptoids (HMPs), which are amphiphilic pseudopeptidic macromolecules, to connect across lipid bilayers and thus form layered structures on liposomes. The HMPs are obtained by attaching hydrophobic decyl groups at random points along the polypeptoid backbone. Although native polypeptoids (with no hydrophobes) have no effect on liposomal structure, the HMPs remodel the unilamellar liposomes into structures with comparable diameters but with multiple concentric bilayers. The transition from single-bilayer to multiple-bilayer structures is revealed by small-angle neutron scattering (SANS) and cryo-transmission electron microscopy (cryo-TEM). The spacing between bilayers is found to be relatively uniform at ∼6.7 nm. We suggest that the amphiphilic nature of the HMPs explains the formation of multibilayered liposomes; i.e., the HMPs insert their hydrophobic tails into adjacent bilayers and thereby serve as the connective glue between bilayers. At higher HMP concentrations, the liposomes are entirely disrupted into much smaller micellelike structures through extensive hydrophobe insertion. Interestingly, these small structures can reattach to fresh unilamellar liposomes and self-assemble to form new two-bilayer liposomes. The two-bilayer liposomes in our study are reminiscent of two-bilayer organelles such as the nucleus in eukaryotic cells. The observations have significance in designing new nanoscale drug delivery carriers with multiple drugs on separate lipid bilayers and extending liposome circulation times with entirely biocompatible materials.