Responsive microcapsules from complex emulsions

Engineered microcompartments and capsules that respond to multiple external stimuli and partly replicate key features of the fascinating dynamic response of living cells have attracted growing interest in academia and industry. In this talk, I will present our efforts to create a library of chemically- and mechanically-responsive microcompartments that are able to release cargo molecules on-demand through different triggering mechanisms. To obtain microcapsules with unprecedented functionalities, we use complex emulsions made in microfluidic devices as soft templates. Conversion of soft double emulsions into functional microcapsules is accomplished by a polymerization reaction or dissolution of the oil phase into the continuous medium, thus generating polymer-based compartments or colloidosomes with predictable size, shell thickness, mechanical behavior and shell microstructure. The resulting microcapsules can be designed to undergo one-time release or can be made sufficiently robust to enable multiple release events without impairing the compartment’s mechanical integrity. Release is triggered by a variety of external stimuli, including pH, temperature or magnetic fields. Proof-of-concept experiments are shown to illustrate the potential of these microcompartments in modifying on-demand the mechanical response of organic or inorganic matrices in capsule-loaded composite materials

Facile synthesis of self-healing microcapsules

In nature biological materials self-heal and adapt repeatedly to stresses caused by the environment. So far, major efforts have been made to create engineered microcapsules that can, upon rupturing, release a healing agent. To mimic the dynamic biological function, we create functional microcapsules that release self-healing agents, but may also themselves be healed, allowing for multiple release events. Currently there are many limitations in synthesizing microcapsules with self-healing hydrogel shells. We address these challenges with a facile strategy for synthesizing monodisperse hydrogel microcapsules by the deprotection and aqueous solubilization of an initially water-insoluble polymer shell. We use a microfluidic approach to produce w/o/w emulsions as a template for microcapsules [1], where the monomer is in the oil phase. Using such a technique one can prepare poly(acrylic acid) shell microcapsules by the deprotection of a poly(tert-butyl acrylate) shell microcapsule through hydrolysis [2]. Hydrophobic comonomers and water insoluble interpenetrating polymers may be included with the tert-butyl acrylate monomer in order to form microcapsules with self-healing shell materials such as semi-interpenetrating hydrogels or hydrophobic association hydrogels [3,4]. To stabilize self-healing microcapsules we used particle armoring as self-healing hydrogels posses sticky surfaces and tend to aggregate [5]. With this work we demonstrate an easy approach to produce microcapsules with self-healing shells. These capsules will open up the possibility of repeated release from microcapsules, taking a step closer to reproducing self-healing processes seen in nature. [1] Utada, A. S.; Lorenceau, E.; Link, D. R.; Kaplan, P. D.; Stone, H. A.; Weitz, D. A. Science 2005, 308, 537–541. [2] Heise, A.; Hedrick, J. L.; Trollsås, M.; Miller, R. D. … 1999. [3] Hou, C.; Huang, T.; Wang, H.; Yu, H.; Zhang, Q.; Li, Y. Sci Rep 2013, 3, 3138. [4] Jiang, G.; Liu, C.; Liu, X.; Chen, Q.; Zhang, G.; Yang, M.; Liu, F. Polymer 2010. [5] Chen, R.; Pearce, D. J. G.; Fortuna, S.; Cheung, D. L.; Bon, S. A. F. J. Am. Chem. Soc. 2011, 133, 2151–2153. Please click Additional Files below to see the full abstract

3D Printed Architectured Silicones with Autonomic Self-healing and Creep-resistant Behavior

Self-healing silicones that are able to restore the functionalities and extend the lifetime of soft devices hold great potential in many applications. However, currently available silicones need to be triggered to self-heal or suffer from creep-induced irreversible deformation during use. Here, we design and print silicone objects that are programmed at the molecular and architecture levels to achieve self-healing at room temperature while simultaneously resisting creep. At the molecular scale, dioxaborolanes moieties are incorporated into silicones to synthesize self-healing vitrimers, whereas conventional covalent bonds are exploited to make creep-resistant elastomers. When combined into architectured printed parts at a coarser length scale, layered materials exhibit fast healing at room temperature without compromising the elastic recovery obtained from covalent polymer networks. A patient-specific vascular phantom is printed to demonstrate the potential of architectured silicones in creating damage-resilient functional devices using molecularly designed elastomer materials

Stimuli sensitive microcapsules with macroporous polymer shells

Porous microcapsules are of great interest in diverse applications, ranging from encapsulation for controlled release, to catalyst support to filtration and purification systems in analytical science. Here, we demonstrate a novel method to obtain porous microcapsules with polymer shells whose macroporosity and mechanical properties can be tuned within a wide range. Microcapsules are produced by microfluidics, using a co-flow flow-focusing glass capillary device to make water-oil-water (W/O/W) double emulsion templates. A mixture of acrylate monomers (glycidyl methacrylate and ethylene glycol dimethacrylate) and porogens (phthalate-based, alkanes or linear alcohols) is used as oil phase. Heterogeneous polymerization of the acrylate monomers leads to a biphasic structure in the capsule shell, in which a network of polymer beads is permeated by the liquid porogen. In the presence of hydrophobic porogens, the formation of a thin and tight polymer skin is observed on the inner and outer surfaces of the shell. This leads to sealed pores within the shell of the microcapsules, which can be used for the storage of chemicals in addition to the main encapsulant in the capsule core. As a proof of concept of such co-encapsulation of reactive compounds, we produced capsules loaded with separately stored monomers commonly used for two-components epoxy resins. Such capsules provide a rich platform for the design of solid adhesive and self-healing materials. Furthermore, the utilization of porogens with low boiling point, such as a short alkanes, leads to thermosensitive capsules that explosively release their content within seconds. Combining these capsules with magnetic particles heated by magnetic hyperthermia, we achieved a magnetic release of the capsules content within seconds and without over-heating the surrounding matrix. Incorporation of glycidyl methacrylate monomers results in polymer capsules with epoxy-functionalized surfaces, which can be further reacted with amine-based functional compounds. Exploiting such epoxy groups as anchors for grafting of sensitive polymers and for covalently attaching nanoparticles, we prepared multi-functional capsules with tailored shell structure and surface chemistries. Please click Additional Files below to see the full abstract

Strong and ductile platelet-reinforced polymer films inspired by nature: Microstructure and mechanical properties

The unique structure and mechanical properties of platelet-reinforced biological materials such as bone and seashells have motivated the development of artificial composites exhibiting new, unusual mechanical behavior. On the basis of designing principles found in these biological structures, we combined high-performance artificial building blocks to fabricate platelet-reinforced polymer matrix composites that exhibit simultaneously high tensile strength and ductility. The mechanical properties are correlated with the underlying microstructure of the composites before and after mechanical loading using transmission electron microscopy. The critical role of the strength of the platelet-polymer interface and its dependence on the platelet surface chemistry and the type of matrix polymer are studied. Thin multilayered films with highly oriented platelets were produced through the bottom-up layer-by-layer assembly of submicrometer-thin alumina platelets and either polyimide or chitosan as polymer matrix. The tensile strength and strain at rupture of the prepared composites exceeded that of nacre, whereas the elastic modulus reached values similar to that of lamellar bones. In contrast to the brittle failure of clay-reinforced composites of similar or higher strength and stiffness, our composites exhibit plastic deformation in the range of 2-90% before failure. In addition to the high reinforcing efficiency and ductility achieved, several toughening mechanisms were identified in fractured composites, namely friction, debonding, and formation of microcracks at the platelet-polymer interface, as well as plastic deformation and void formation within the continuous polymeric phase. The combination of high strength, ductility, and toughness was achieved by selecting platelets that exhibit an aspect ratio high enough to carry significant load but small enough to allow for fracture under the platelet pull-out mode. At high concentrations of platelets, the ductility gets lost because of out-of-plane misalignment of the platelets and incorporation of voids in the microstructure during processing. The designing principles applied in this study can potentially be extended to other types of platelets and polymers to obtain new, hybrid materials with tunable mechanical propertie

Powder-based processing of highly-loaded platelet-reinforced composites

Conventional processes commonly used for the fabrication of composites with high volume fraction of reinforcing elements usually require the infiltration of monomers that are subsequently consolidated into a continuous polymer matrix. Such infiltration step often leads to long processing times and limits the choices of materials that can be used as soft polymer matrices. In this work, we present a new infiltration-less route in which a co-suspension of organic/inorganic powders is assembled through vacuum-assisted magnetic alignment and the resulting composite consolidated by uniaxial hot pressing at temperatures close to the melting point of the polymer phase. [1] We demonstrate that the fabrication process of thermoset- and thermoplastic-reinforced composites containing up to 50% in volume of aligned reinforcing platelets can be significantly simplified using this infiltration-less method (Figure 1a). Consolidation of thermosets matrices through hot pressing of assembled powder mixtures is achieved by employing polymers containing dynamic covalent bonds as crosslinking points in their molecular structure. As illustrated in Figure 1b, incorporation of 50% in volume of reinforcing platelets within dynamic polymer matrices enhances the flexural modulus and flexural strength by 14-fold and 3-fold as compared to the pure polymer, reaching values as high as 13 GPa and 90 MPa, respectively. As expected, the strain at rupture decreases from 3.0% to 0.8% upon addition of brittle ceramic platelets. These results demonstrate the potential of using infiltration-less routes to enable the fabrication of high-performance platelet-reinforced composites with high volume fraction of reinforcing ceramic particles and polymer matrices that are difficult to be infiltrated using conventional methods. Please click Additional Files below to see the full abstract

Drying of complex suspensions

We investigate the 3D structure and drying dynamics of complex mixtures of emulsion droplets and colloidal particles, using confocal microscopy. Air invades and rapidly collapses large emulsion droplets, forcing their contents into the surrounding porous particle pack at a rate proportional to the square of the droplet radius. By contrast, small droplets do not collapse, but remain intact and are merely deformed. A simple model coupling the Laplace pressure to Darcy's law correctly estimates both the threshold radius separating these two behaviors, and the rate of large-droplet evacuation. Finally, we use these systems to make novel hierarchical structures.Comment: 4 pages, 4 figure