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The two-tailed anionic surfactant, AOT is well-known to form spherical reverse micelles in organic solvents such as cyclohexane and n-alkanes. Here, we report that trace amounts (e.g., around 1 mM) of the dihydroxy bile salt, sodium deoxycholate (SDC) can transform these dilute micellar solutions into self-supporting, transparent organogels. Gels can be obtained at a total AOT + SDC concentration as low as 6 mM or about 2 mg mL−1. Among all the bile salts studied, SDC is the only one that is capable of inducing organogels. The structure and rheology of these organogels is reminiscent of the self-assembled networks formed by proteins such as actin in water. In particular, both classes of gels exhibit the remarkable property of strain-stiffening, where the gel stiffness (modulus) increases with strain amplitude. Structurally, both gels are based on entangled networks of long, cylindrical filaments. We propose that SDC forms hydrogen bonds with AOT headgroups, transforming some of the spherical AOT micelles into semiflexible filaments. The average diameter of these filaments has been measured by small-angle neutron scattering (SANS), and suggests that SDC molecules are stacked together in the filament core.

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.

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.

This work describes a new method to selectively capture liposomes and other vesicle entities in the patterned pores of breath-figure polymer films. The process involves the deposition of a hydrophobe containing biopolymer in the pores of the breath figure, and the tethering of vesicles to the biopolymer through hydrophobic interactions. The process is versatile, can be scaled up and extended to the deposition of other functional materials in the pores of breath figures.

This communication describes a novel strategy to achieve programmable shape transformation of hybrid hydrogel sheets by modulating both the in-plane and out-of-plane mismatches in mechanical properties. Both our experimental and computational results demonstrate that the shape transformation of hybrid hydrogel sheets shows rich features (e.g., rolling direction, axis, chirality, etc.) and versatile tunability (e.g., via various external stimuli, material properties, pattern geometry, etc.). This work can provide guidance for designing soft materials that are able to undergo more precise and complex shape transformation.

We examine the gelation of alginate undergoing ionic crosslinking upon ultraviolet (UV) irradiation using in situ dynamic rheology. Hydrogels are formed by combining alginate with calcium carbonate (CaCO3) particles and a photoacid generator (PAG). The PAG is photolyzed upon UV irradiation, resulting in the release of free calcium ions for ionic crosslinking. The viscous and elastic moduli during gelation are monitored as a function of the UV irradiation intensity, exposure time, alginate concentration, and the ratio between alginate and calcium carbonate. Gel time decreases as irradiation intensity increases because a larger concentration of PAG is photolyzed. Interestingly, dark curing, the continuing growth of microstructure in the absence of UV light, is observed. In some instances, the sample transitions from a solution to a gel during the dark curing phase. Additionally, when exposed to constant UV irradiation after the dark curing phase, samples reach the same plateau modulus as samples exposed to constant UV without dark curing, implying that dark curing does not affect the gelation mechanism. We believe the presence of dark curing is the result of the acidic environment persisting within the sample, allowing CaCO3 to dissociate, thereby releasing free Ca(2+) ions capable of binding with the available appropriate ionic blocks of the polymer chains. The growth of microstructure is then detected if the activation barrier has been crossed to release sufficient calcium ions. In this regard, we calculate a value of 30 J that represents the activation energy required to initiate gelation.

Liposomal coatings are formed on films of a biopolymer, hydrophobically modified chitosan (hm-chitosan), containing dodecyl groups as hydrophobes along the polymer backbone. The alkyl groups insert themselves into the liposome bilayer through hydrophobic interactions and thus tether liposomes, leading to a densely packed liposome layer on the film surface. Such liposomal surfaces exhibit effective lubrication properties due to their high degree of hydration, and reduce the coefficient of friction to the biologically-relevant range. The compliancy and robustness of these tethered liposomes allow retention on the film surface upon repeated applications of shear. Such liposome coated films have potential applications in biolubrication.

Self-assembly of lipids in water is well-known to result in nanostructures such as vesicles in dilute solution. In contrast, self-assembly in nonpolar organic solvents (oils) is much less established and there are hardly any known routes to forming structures such as reverse vesicles. Here, we build such structures based on our surprising recent discovery that salts can influence self-assembly of lipids in oil. We induce the self-assembly of nanoscale multilamellar vesicles (“onions”) in cyclohexane and toluene by combining the saturated phospholipid, 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC), with salts of di- or trivalent cations like calcium (Ca2+) or gadolinium (Gd3+) in the absence of water. DMPC–Gd3+ onions can be seen by a transmission electron microscope (TEM) without additional staining. They have sizes between ∼100 and 400 nm and have 6–18 concentric bilayer shells (lamellae) that are uniformly spaced. The presence of lamellae is further confirmed by small-angle X-ray scattering (SAXS), from which we find the inter-lamellar spacing to be 6.0 nm. The formation of reverse onions is driven by the binding of these multivalent cations with the lipid headgroups, which in turn brings adjacent lipids close and causes a tighter packing of the lipid tails. Evidence for such binding is provided by the cation-induced lowering of the lipid melting temperature.

We describe the creation of monodisperse microbeads of polydimethylsiloxane (PDMS) via a microfluidic approach. Using a flow-focusing configuration, a PDMS precursor solution is dispersed into microdroplets within an aqueous continuous phase. These droplets are collected and thermally cured off-chip into solid microbeads. Our microfluidic technique allows for direct integration of payloads into the PDMS microbeads. Specifically, we integrate an oxygen-sensitive porphyrin dye into the beads and show that the resulting structures can function as non-invasive and real-time oxygen microsensors utilizing a simple optical readout at the single-particle level.

We describe a simple way to create patterns of “soft” biomolecular nanostructures such as vesicles on “hard” surfaces such as gold. The key to our approach is the use of an amphiphilic biopolymer as an “interconnect” or tether. The polymer is hydrophobically modified chitosan (hm-chitosan), which is obtained by covalently attaching alkyl tails to the backbone of chitosan. We electrodeposit films of hm-chitosan onto microscale gold cathodes formed by lithography on a silicon wafer. Subsequently, the hm-chitosan films are used to spontaneously capture vesicles from solution; this is demonstrated both for surfactant as well as lipid vesicles (liposomes). Vesicles remain strongly bound to the hm-chitosan to a much greater extent than to native chitosan. This suggests that the mechanism for vesicle capture involves non-covalent binding of hydrophobes from hm-chitosan chains to the hydrophobic portions of vesicle bilayers. Importantly, the vesicles captured by hm-chitosan films are intact—this is shown both by direct visualization of captured vesicles (via optical and cryo-transmission electron microscopy) as well as through the capture and subsequent disruption of dye-filled vesicles. Various microscale patterns of immobilized vesicles are created and the vesicles are demonstrated to be capable of sensing a reporter molecule from the external solution.

Many small molecules can self-assemble by non-covalent interactions into fibrous networks and thereby induce gelation of organic liquids. However, no capability currently exists to predict whether a molecule in a given solvent will form a gel, a low-viscosity solution (sol), or an insoluble precipitate. Gelation has been recognized as a phenomenon that reflects a balance between solubility and insolubility; however, the distinction between these regimes has not been quantified in a systematic fashion. In this work, we focus on a well-known gelator, 1,3:2,4-dibenzylidene sorbitol (DBS), and study its self-assembly in various solvents. From these data, we build a framework for DBS gelation based on Hansen solubility parameters (HSPs). While the HSPs for DBS are not known a priori, the HSPs are available for each solvent and they quantify the solvent's ability to interact via dispersion, dipole–dipole, and hydrogen bonding interactions. Using the three HSPs, we construct three-dimensional plots showing regions of solubility (S), slow gelation (SG), instant gelation (IG), and insolubility (I) for DBS in the different solvents at a given temperature and concentration. Our principal finding is that the above regions radiate out as concentric shells: i.e., a central solubility (S) sphere, followed in order by spheres corresponding to SG, IG, and I regions. The distance (R0) from the origin of the central sphere quantifies the incompatibility between DBS and a solvent—the larger this distance, the more incompatible the pair. The elastic modulus of the final gel increases with R0, while the time required for a super-saturated sol to form a gel decreases with R0. Importantly, if R0 is too small, the gels are weak, but if R0 is too large, insolubility occurs—thus, strong gels fall within an optimal window of incompatibility between the gelator and the solvent. Our approach can be used to design organogels of desired strength and gelation time by judicious choice of a particular solvent or a blend of solvents. The above framework can be readily extended to many other gelators, including those with molecular structures very different from that of DBS. We have developed a MATLAB program that will be freely available (upon request) to the scientific community to replicate and extend this approach to other gelators of interest.

Biology is a master of mesoscale science, possessing unprecedented capabilities for fabricating components with nano-scale precision and then assembling them over a hierarchy of length scales. Biology's fabrication prowess is well-recognized and there has been considerable effort to mimic these capabilities to create materials with diverse and multiple functions. In this review, we pose the question – why mimic, why not directly use the materials and mechanisms that biology provides to biofabricate functional materials? This question seems especially relevant when considering that many of the envisioned applications – from regenerative medicine to bioelectronics – involve biology. Here, we provide a sampling to illustrate how self-assembly, enzymatic-assembly and the emerging tools of modern biology can be enlisted to create functional soft matter. We envision that biofabrication will provide a biocompatible approach to mesoscale science and yield products that are safe, sustainable and potentially even edible.

Recently, there has been much interest in photorheological (PR) fluids, i.e., fluids whose rheological properties can be tuned by light. In particular, there is a need for simple, low-cost PR fluids that can be easily created using inexpensive, commercially available ingredients and that show substantial, reversible changes in rheology upon exposure to different wavelengths of light. Towards this end, we report a class of photoreversible PR fluids prepared by combining the azobenzene derivative 4-azobenzene carboxylic acid (ACA) (in its salt form) with the cationic surfactant erucyl bis(2-hydroxyethyl)methyl ammonium chloride (EHAC). We show that certain aqueous mixtures of EHAC and ACA, which are low-viscosity solutions at the outset, undergo nearly a million-fold increase in viscosity when irradiated with UV light. The same solutions revert to their initial viscosity when subsequently exposed to visible light. Using an array of techniques including UV-vis and NMR spectroscopies, small-angle neutron scattering (SANS) and cryo-transmission electron microscopy (cryo-TEM), we have comprehensively characterized these PR fluids at the molecular, nanostructural, and macroscopic scales. Initially, EHAC–ACA are self-assembled into unilamellar vesicles, which are discrete container structures and give the sample a low viscosity. Upon exposure to UV light, ACA undergoes a trans to cis photoisomerization, which alters the geometry of the EHAC–ACA complex. In turn, the molecules self-assemble into a different structure, viz. wormlike micelles, which are long, entangled chains and impart a high viscosity to the sample. The above changes in viscosity are repeatable, and the sample can be reversibly cycled back and forth between low and high viscosity states. Our photoreversible PR fluids can be easily replicated in any industrial or academic lab, and it is hoped that these “smart” fluids will eventually find a host of applications.

Vesicles are self-assembled nanocontainers that are used for the controlled release of cosmetics, drugs, and proteins. Researchers have been seeking to create photoresponsive vesicles that could enable the triggered release of encapsulated molecules with accurate spatial resolution. While several photoresponsive vesicle formulations have been reported, these systems are rather complex as they rely on special light-sensitive amphiphiles that require synthesis. In this study, we report a new class of photoresponsive vesicles based on two inexpensive and commercially available amphiphiles. Specifically, we employ p-octyloxydiphenyliodonium hexafluoroantimonate (ODPI), a cationic amphiphile that finds use as a photoinitiator, and a common anionic surfactant, sodium dodecylbenzenesulfonate (SDBS). Mixtures of ODPI and SDBS form “catanionic” vesicles at certain molar ratios due to ionic interactions between the cationic and anionic headgroups. When irradiated with ultraviolet (UV) light, ODPI loses its charge and, in turn, the vesicles are converted into micelles due to the loss of ionic interactions. In addition, a mixture of these photoresponsive vesicles and a hydrophobically modified biopolymer gives a photoresponsive vesicle-gel. The vesicle-gel is formed because hydrophobes on the polymer insert into vesicle bilayers and thus induce a three-dimensional network of vesicles connected by polymer chains. Upon UV irradiation, the network is disrupted because of the conversion of vesicles to micelles, with the polymer hydrophobes getting sequestered within the micelles. As a result, the gel is converted to a sol, which manifests as a 40 000-fold light-induced drop in sample viscosity.

Self-assembled nanocontainers that can penetrate the protective barrier of skin could prove useful for the needle-free delivery of chemicals, drugs or vaccines into the skin. The main examples of these are the so-called ‘‘flexible liposomes’’, which are mixtures of lipids and detergents. In vitro experiments by numerous researchers have confirmed the skin-penetrating ability of these structures, which is believed to involve the squeezing of flexible bilayers through the pores in the stratum corneum. Here, we reexamine the structure of ‘‘flexible’’ liposome formulations and show that these are actually mixtures of liposomes and micelles. These findings are reinforced through a combination of small-angle neutron scattering (SANS) and cryo-transmission electron microscopy (cryo-TEM). We believe that our new findings necessitate a new mechanism for the penetration of soft assemblies into the skin. Several possibilities in this regard are considered, including one in which micelles facilitate the dynamic exchange of amphiphiles between liposomes and/or lipid bilayers.

The term gel is central to many scientific fields, including polymer science, biophysics, and supramolecular chemistry. In polymer science, a gel is said to be formed when polymer chains are linked into a permanent three-dimensional (3-D) network by cross-links that are either chemical bonds or strong physical associations. Linear chains in the absence of such cross-links are expected to form a network that is only defined by topological (entanglement) interactions between the chains; accordingly, such a network is expected to show viscoelastic rather than gel-like, rheology. On the other hand, many systems consisting of extended nanoscale fibers/chains (e.g., supramolecular organo- and hydro-gels, wormlike micelles, and protein filaments like F-actin) do exhibit the rheology of permanent gel networks, even in the absence of putative cross-links. We argue here that linear fibers can indeed form gels through their topological interactions alone, i.e., without cross-links, provided the fibers are sufficiently long (contour length much larger than cross-sectional size), sufficiently stiff (worm-like or rod-like) and temporally persistent, i.e., “unbreakable”. This hypothesis is supported by recent experimental and theoretical studies. In particular, we review recent experiments on wormlike micelles that show a smooth rheological transition from viscoelastic (relaxing) to gel (non-relaxing) behavior upon varying temperature.

We describe the remarkable capabilities of polymer–nanoparticle hybrids for cation binding and separation. The materials investigated here are hydrogels of N-isopropylacrylamide (NIPA) prepared using nanoparticles of the synthetic clay, Laponite® RD, as the crosslinkers. When immersed into an aqueous solution containing a cationic amine-based dye (methylene blue), the gel rapidly soaks up the dye from solution. Additionally, the gel is able to selectively extract the above cationic dye from a mixture of cationic and anionic dyes. These ion-exchange properties are driven by the strong binding affinity of certain cations for the anionic surfaces of the clay nanoparticles within the gel matrix. A comparison of the nanostructured gel with a typical cation-exchange resin (polystyrene-sulfonate) shows that the former is quicker and more efficient at extracting cationic species from solution. We also show that the solute adsorbed within the gel can be further concentrated by exploiting the shrinking property of NIPA gels when heated. Additionally, we demonstrate the disassembly of Laponite-crosslinked gels by exposure to an organic solvent or a hydrophilic oligomer, which allows the adsorbed solute to be released and thereby recovered.

We report the creation of biopolymer capsules that show a colorimetric response to changes in pH and temperature. Polymerized diacetylenic (PDA) vesicles are embedded within capsules formed by complexing the cationic biopolymer, chitosan, with an anionic surfactant. The PDA vesicles impart their colorimetric properties to the capsules while remaining embedded within the capsule lumen. Accordingly, the capsules show a blue color at low pH (∼6) or temperature (∼25 °C) whereas the color changes to purple at medium pH (∼8) or temperature (∼45 °C) and finally to red at high pH (∼10) or temperature (∼60 °C). These capsule-based sensors are easy to prepare, low-cost, and can be easily tailored for various applications.

The gelation of hydrophobically modified chitosan solutions can be accomplished through the incorporation of uniform carbon microspheres. The carbon particles act as nodes in the gel network where hydrophobic alkyl groups, attached to the polysaccharide backbone, interact with multiple carbon microspheres to form a three-dimensional matrix. Rheological characterizations show significant increases in elastic moduli upon incorporation of the carbon microspheres.

Traditional microfabrication has tremendous capabilities for imparting order to hard materials (e.g., silicon wafers) over a range of length scales. However, conventional microfabrication does not provide the means to assemble pre-formed nano-scale components into higher-ordered structures. We believe the aminopolysaccharide chitosan possesses a unique set of properties that enable it to serve as a length-scale interconnect for the hierarchical assembly of nano-scale components into macro-scale systems. The primary amines (atomic length scale) of the glucosamine repeating units (molecular length scale) provide sites to connect pre-formed or self-assembled nano-scale components to the polysaccharide backbone (macromolecular length scale). Connections to the backbone can be formed by exploiting the electrostatic, nucleophilic, or metal-binding capabilities of the glucosamine residues. Chitosan's film-forming properties provide the means for assembly at micron-to-centimetre lengths (supramolecular length scales). In addition to interconnecting length scales, chitosan's capabilities may also be uniquely-suited as a soft component-hard device interconnect. In particular, chitosan's film formation can be induced under mild aqueous conditions in response to localized electrical signals that can be imposed from microfabricated surfaces. This capability allows chitosan to assemble soft nano-scale components (e.g., proteins, vesicles, and virus particles) at specific electrode addresses on chips and in microfluidic devices. Thus, we envision the potential that chitosan may emerge as an integral material for soft matter (bio)fabrication.