We report the formation of mesoporous organohydrogels from oil-in-water nanoemulsions containing an end-functionalized oligomeric gelator in the aqueous phase. The nanoemulsions exhibit an abrupt thermoreversible transition from a low-viscosity liquid to a fractal-like colloidal gel of droplets with mesoscale porosity and solid-like viscoelasticity with moduli approaching 100 kPa, possibly the highest reported for an emulsion-based system. We hypothesize that gelation is brought about by temperature-induced interdroplet bridging of the gelator, as shown by its dependence on the gelator chemistry. The use of photocrosslinkable gelators enables the freezing of the nanoemulsion's microstructure into a soft hydrogel nanocomposite containing a large fraction of dispersed liquid hydrophobic compartments, and we show its use in the encapsulation and release of lipophilic biomolecules. The tunable structural, mechanical and optical properties of these organohydrogels make them a robust material platform suitable for a wide range of applications.
Polymer hydrogels and microgels have been widely exploited for the controlled storage, delivery and detection of active compounds, including small molecules and biologics. [ 1 , 2 ] However, due to their intrinsically hydrated microenvironment, the development of hydrogels for encapsulation and/or release of poorly water-soluble cargos remains a persistent challenge. [ 3 ] As such, the development of novel hydrogels with well-controlled hydrophobic compartments remains important to a number of delivery applications including pharmaceuticals, [ 3 , 4 ] cosmetics, [ 5 ] foods, [ 6 ] imaging, [ 7 ] and sensors. [ 8 ] Recent strategies to overcome this challenge include modifi cation of the hydrogel polymer network by co-polymerizing or grafting hydrophobic units to create hydrophobic domains within the polymer matrix, thereby increasing solubility of poorly-soluble actives, [ 9 ] or by direct conjugation of such actives to the polymer itself. [ 10 ] However, these approaches are limited by relatively low loading capacities and the need for designer polymers. [ 3 ] An alternative approach is to incorporate hydrophobic colloidal species into the hydrogel matrix, including nanoparticles, [11][12][13] vesicles, [ 14 ] micelles, [ 15 ] emulsions [ 16 ] and microemulsions. [ 17 ] Although such hydrogel composites have shown some success, material systems that can simultaneously achieve high loading and tunable release for a range of poorly-soluble actives are lacking. [ 7 ] Recently, we have demonstrated the formation of crosslinkable nanoemulsions with controlled droplet size, loading, and stability in which nanoscale oil droplets are suspended in a hydrogel pre-cursor. [ 18 ] The encapsulation of nanoemulsions within a hydrogel matrix affords these materials several advantages compared with other advanced materials for encapsulation of hydrophobic compounds. The ability to produce nanoemulsions at high volume fractions allows for precise control of loading within the hydrogel, while the ability to tune the hydrogel network allows for a controlled barrier to release. [ 3 , 19 ] Compared to ordinary emulsions, nanoemulsions also exhibit increased surface-to-volume ratio, which enhances release kinetics and bioavailability, and greater kinetic stability, which improves shelf life. [ 19 ] Furthermore, the inherent control and homogeneity of droplet properties such as their size [ 20 ] distinguishes them from polymeric micellar [ 21 ] and vesicular [ 22 ] cargos, which are inherently polydisperse and can be adversely affected by a polymer matrix. [ 23 ] In previous work, we tested the ability to process crosslinkable nanoemulsions using fl ow lithography (FL) for the synthesis of structured composite microgel particles. [ 18 ] Flow lithography [ 24 , 25 ] has been used to generate microgels with sophisticated architecture, including shape and chemical anisotropy, [ 26 , 27 ] and tunable diffusion [ 28 ] and degradation [ 29 ] profi les. As such, the potential to combine crosslinkable nanoemulsions with fl ow lithogra...
Multiarm hydrogel microparticles with varying geometry are fabricated to specifically capture cells expressing epithelial cell adhesion molecule. Results show that particle shape influences cell‐capture efficiency due to differences in surface area, hydrodynamic effects, and steric constraints. These findings can lead to improved particle design for cell separation and diagnostic applications.
We report a microfluidic approach for lithographically photo-patterning compartmentalized microparticles with any 2D-extruded shape, down to the cellular length scale (~10 microns). The prepolymer solution consists of a UV crosslinkable perfluorodecalin-in-water nanoemulsion stabilized by Pluronic(®) F-68. The nanoemulsions are generated using high-pressure homogenization and are osmotically stabilized by the trapped species method. The presence of PFC droplets increases the solubility and diffusivity of oxygen in the prepolymer solution, thereby enhancing the rate of O2 inhibition during microparticle synthesis. We develop a simple model that successfully predicts the augmented O2 mass transport, which agrees well with experimental data. Informed by our analytical results, cell-sized composite microgels are generated by controlling the oxygen environment around the polydimethylsiloxane (PDMS) microfluidic synthesis device. These nanoemulsion composites are functionally similar to red blood cells as oxygen carriers. Such bio-inspired polymeric particles with controlled physical properties are promising vehicles for drug delivery and clinical diagnostics.
We report a simple approach to fabricate custom-shape microcapsules using hydrogel templates synthesized by stop flow lithography. Cargocontaining microcapsules were made by coating hydrogel particles with a single layer of poly-L-lysine followed by a one-step core degradation and capsule crosslinking procedure. We determined appropriate coating conditions by investigating the effect of pH, ionic strength, and prepolymer composition on the diffusion of polyelectrolytes into the oppositely charged hydrogel template. We also characterized the degradation of the templating core by tracking the diffusivity of nanoparticles embedded within the hydrogel. Unlike any other technique, this approach allows for easy fabrication of microcapsules with internal features (e.g., toroids) and selective surface modification of Janus particles using any polyelectrolyte. These soft, flexible capsules may be useful for therapeutic applications as well as fundamental studies of membrane mechanics.
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