Hyuntaek Oh

Autonomous self-healing of poly(acrylic acid) hydrogels induced by the migration of ferric ions

A facile and versatile strategy was developed for the preparation of self-healing hydrogels containing double networks of both physically and chemically cross-linked polymers. The autonomous self-healing of the hydrogel was achieved through the dynamic bonding of physical cross-linking and the migration of ferric ions.

G4-Quartet·M + Borate Hydrogels

The ability to modulate the physical properties of a supramolecular hydrogel may be beneficial for biomaterial and biomedical applications. We find that guanosine (G 1), when combined with 0.5 equiv of potassium borate, forms a strong, self-supporting hydrogel with elastic moduli >10 kPa. The countercation in the borate salt (MB(OH)4) significantly alters the physical properties of the hydrogel. The gelator combination of G 1 and KB(OH)4 formed the strongest hydrogel, while the weakest system was obtained with LiB(OH)4, as judged by (1)H NMR and rheology. Data from powder XRD, (1)H double-quantum solid-state magic-angle spinning (MAS) NMR and small-angle neutron scattering (SANS) were consistent with a structural model that involves formation of borate dimers and G4·K(+) quartets by G 1 and KB(OH)4. Stacking of these G4·M(+) quartets into G4-nanowires gives a hydrogel. We found that the M(+) cation helps stabilize the anionic guanosine-borate (GB) diesters, as well as the G4-quartets. Supplementing the standard gelator mixture of G 1 and 0.5 equiv of KB(OH)4 with additional KCl or KNO3 increased the strength of the hydrogel. We found that thioflavin T fluoresces in the presence of G4·M(+) precursor structures. This fluorescence response for thioflavin T was the greatest for the K(+) GB system, presumably due to the enhanced interaction of the dye with the more stable G4·K(+) quartets. The fluorescence of thioflavin T increased as a function of gelator concentration with an increase that correlated with the systems gel point, as measured by solution viscosity.

A simple route to fluids with photo-switchable viscosities based on a reversible transition between vesicles and wormlike micelles

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.

Biopolymer-connected liposome networks as injectable biomaterials capable of sustained local drug delivery.

Biopolymers bearing hydrophobic side-chains, such as hydrophobically modified chitosan (hmC), can connect liposomes into a gel network via hydrophobic interactions. In this paper, we show that such liposome gels possess an attractive combination of properties for certain drug delivery applications. Their shear-thinning property allows these gels to be injected at a particular site, while their gel-like nature at rest ensures that the material will remain localized at that site. Moreover, drugs can be encapsulated in the interior of the liposomes and delivered at the local site for an extended period of time. The presence of two transport resistances - from the liposomal bilayer and the gel network - is shown to be responsible for the sustained release; in turn, disruption of the liposomes both weakens the gel and causes a faster release. We have monitored release kinetics from liposome gels of a cationic anticancer drug doxorubicin (Dox) encapsulated in liposomes. Sustained release of Dox from these gels and the concomitant cytotoxic effect could be observed for over a week.

Photoreversible micellar solution as a smart drag-reducing fluid for use in district heating/cooling systems.

A photoresponsive micellar solution is developed as a promising working fluid for district heating/cooling systems (DHCs). It can be reversibly switched between a drag reduction (DR) mode and an efficient heat transfer (EHT) mode by light irradiation. The DR mode is advantageous during fluid transport, and the EHT mode is favored when the fluid passes through heat exchangers. This smart fluid is an aqueous solution of cationic surfactant oleyl bis(2-hydroxyethyl)methyl ammonium chloride (OHAC, 3.4 mM) and the sodium salt of 4-phenylazo benzoic acid (ACA, 2 mM). Initially, ACA is in a trans configuration and the OHAC/ACA solution is viscoelastic and exhibits DR (of up to 80% relative to pure water). At the same time, this solution is not effective for heat transfer. Upon UV irradiation, trans-ACA is converted to cis-ACA, and in turn, the solution is converted to its EHT mode (i.e., it loses its viscoelasticity and DR) but it now has a heat-transfer capability comparable to that of water. Subsequent irradiation with visible light reverts the fluid to its viscoelastic DR mode. The above property changes are connected to photoinduced changes in the nanostructure of the fluid. In the DR mode, the OHAC/trans-ACA molecules assemble into long threadlike micelles that impart viscoelasticity and DR capability to the fluid. Conversely, in the EHT mode the mixture of OHAC and cis-ACA forms much shorter cylindrical micelles that contribute to negligible viscoelasticity and effective heat transfer. These nanostructural changes are confirmed by cryo-transmission electron microscopy (cryo-TEM), and the photoisomerization of trans-ACA and cis-ACA is verified by (1)H NMR.

Shedding light on helical microtubules: real-time observations of microtubule self-assembly by light microscopy.

Helical tubules are a fascinating and an intriguing class of self-assemblies. They occur frequently in biology and are believed to be intermediates in formation of gallstones. The pathway by which amphiphiles transform from an initial state of vesicles or micelles into such tubules has puzzled soft matter physicists, and it has raised important questions about the interplay between molecular chirality and self-assembly. Here, for the first time, we demonstrate direct, real-time observations by light microscopy of the pathway to helical microtubules from an initial solution of nanoscale vesicles. The tubules are formed in aqueous mixtures of the single-tailed diacetylenic surfactant, 10,12-pentacosadiynoic acid (PCDA), and a short-chain alcohol. The stepwise process involves nucleation of thin helical microribbons from the vesicle solution. These ribbons then thicken, rearrange, and fold into closed tubules. Subsequently, most tubules further rearrange into plate-like structures, and once again, we are able to visualize this process in real time. A notable aspect of the above system is that the precursors are achiral; yet, the tubules are formed from helical ribbons. Our study provides new insights into tubule formation that will be valuable in clarifying and refining theoretical models for these fascinating structures.

Light-induced transformation of vesicles to micelles and vesicle-gels to sols

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.

Reversible gelation of cells using self-assembling hydrophobically-modified biopolymers: towards self-assembly of tissue

Polymer hydrogels have long been used to hold and culture biological cells within their three-dimensional (3-D) matrices. Typically, in such cases, the cells are passively entrapped in a mesh of polymer chains. Here, we demonstrate an alternate approach where cells serve as active structural elements (crosslinks) within a polymer gel network. The polymers used in this context are hydrophobically modified (hm) derivatives of common biopolymers such as chitosan and alginate. We show that hm-polymers rapidly transform a liquid suspension of cells into an elastic gel. In contrast, the native biopolymers (without hydrophobes) do not cause such gelation. Gelation occurs because the hydrophobes on the polymer get embedded within the hydrophobic interiors of cell bilayer membranes. The polymer chains thus connect the cells into a 3-D sample-spanning network, with the cells serving as the junctions in the network. We demonstrate that a variety of cell types, including blood cells, endothelial cells, and breast cancer cells can be gelled by this approach. Cells gelled by hm-alginate are shown to remain viable within the network. Also, since the crosslinking mechanism is based on hydrophobic interactions, we show that the addition of supramolecules with hydrophobic binding pockets can reverse the gelation and release the cells. Cell-gels can be employed as injectable biomaterials since the connections in the network are susceptible to shear, but recover rapidly once shear is stopped. The overall approach provides a simple route towards the directed assembly of cell clusters and potentially to living tissue.

A new method for centrifugal separation of blood components: Creating a rigid barrier between density-stratified layers using a UV-curable thixotropic gel

Current gels used in blood separation tubes create an imperfect barrier between the blood components because of their physical and thixotropic nature. As a result, blood components tend to leak into the gel layer or vice versa during transport and storage. To overcome these problems, we demonstrate the use of a UV-curable thixotropic gel composed of a sorbitol-based gelator in a diacrylate oligomer. Initially, the sample is a physical gel composed of weak, non-covalent bonds, and its thixotropic nature allows it to flow under centrifugation and form a barrier between the density-stratified layers of blood. Immediately afterward, the gel is chemically crosslinked by short exposure to UV light for 10–30 s. This results in a rigid, impenetrable barrier that is freeze-thaw stable. The gel is compatible with blood, allowing blood samples to be stored in the tube and analyzed over long times. We believe the present method is a significant advance in the practice of blood analysis for medical purposes.

Gelation of Oil upon Contact with Water: A Bioinspired Scheme for the Self-Repair of Oil Leaks from Underwater Tubes

Molecular organogelators convert oils into gels by forming self-assembled fibrous networks. Here, we demonstrate that such gelation can be activated by contacting the oil with an immiscible solvent (water). Our gelator is dibenzylidene sorbitol (DBS), which forms a low-viscosity sol when added to toluene containing a small amount of dimethyl sulfoxide (DMSO). Upon contact with water, DMSO partitions into the water, activating gelation of DBS in the toluene. The gel grows from the oil/water interface and slowly envelops the oil phase. We have exploited this effect for the self-repair of oil leaks from underwater tubes. When a DBS/toluene/DMSO solution flows through the tube, it forms a gel selectively at the leak point, thereby plugging the leak and restoring flow. Our approach is reminiscent of wound-sealing via blood-clotting: there also, inactive gelators in blood are activated at the wound site into a fibrous network, thereby plugging the wound and restoring blood flow.