Prepolymer Droplets Research Articles

Photografting of Surface-assembled Hydrogel Prepolymers to Elastomeric Substrates for Production of Stimuli-Responsive Microlens Arrays.

Hydrogels have emerged as prototypical stimuli-responsive materials with potential applications in soft robotics, microfluidics, tissue engineering, and adaptive optics. To leverage the full potential of these materials, fabrication techniques capable of simultaneous control of microstructure, device architecture, and interfacial stability, i.e., adhesion of hydrogel components to support substrates, are needed. A universal strategy for the microfabrication of hydrogel-based devices with robust substrate adhesion amenable to use in liquid environments would enable numerous applications. This manuscript reports a general approach for the facile production of covalently attached, ordered arrays of microscale hydrogels (microgels) on silicone supports. Specifically, silicone-based templates were used to: i) drive mechanical assembly of prepolymer droplets into well-defined geometries and morphologies, and ii) present appropriate conjugation moieties to fix gels in place during photoinitiated crosslinking via a "graft from" polymerization scheme. Automated processing enabled rapid microgel array production for characterization, testing, and application. Furthermore, the stimuli-responsive microlensing properties of these arrays, via contractile modulated refractive index, were demonstrated. This process is directly applicable to the fabrication of adaptive optofluidic systems and can be further applied to advanced functional systems such as soft actuators and robotics and 3D cell culture technologies.

Bulk Nanoencapsulation of Phase Change Materials (PCMs) via Spontaneous Spreading of a UV-Curable Prepolymer.

Phase change materials (PCMs) have received considerable attention for various latent heat storage systems for efficient thermal energy utilization. Herein, a facile and fast method for the bulk nanoencapsulation of organic PCMs is proposed, based on the thermodynamically spontaneous spreading phenomenon of three immiscible liquid phases. In this approach, a complete engulfing of PCM nanodroplets (core phase) by immiscible prepolymer droplets (coating phase), both of which are bulk-dispersed in another immiscible medium (continuous phase), is thermodynamically driven by the relation between the surface energies of the core, coating, and continuous phases. To demonstrate the proposed method, melted n-docosane (PCM, core phase) nanodroplets are completely engulfed within a couple of minutes by immiscible polyethylene glycol diacrylate (PEGDA, coating phase) in an aqueous poly(vinyl alcohol) solution (continuous phase), and the PEGDA layer quickly cross-linked upon UV irradiation to form a rigid shell protecting the PCM core. As-produced PCM nanocapsules display promising heat storage and release performances as well as high durability in repeated heating-cooling cycles in both dry and wet states. The proposed process may serve as a useful platform for bulk production of PCM nanocapsules with various core and shell compositions in a facile, fast, and scalable way.

Self-Healing Epoxy Coating Modified by Double-Walled Microcapsules Based Polyurea for Metallic Protection

The effectiveness of preploymer and 1,6-Hexamethylene diamine encapsulated by double-walled microcapsules based polyurea (PUA) was explored for healing the cracks generated in epoxy coatings. Double-walled microcapsules were systhesized by interfacial polymerization at the interface between the prepolymer droplets and the 1,6-Hexamethylene diamine droplets to form the polyurea shell. The effect of synthetic stirring speed on the morphology of the microcapsules was observed by scanning electronmicroscopy (SEM) and optical microscopy (OM). The chemical structure as well as the thermal properties and the core content were characterized by Fourier transform infrared spectroscopy (FTIR) and Thermogravimetric analyzer (TGA) respectively. Electrochemical impedance spectroscopy (EIS) studies of the artificial scratched area showed that the coating containing 2wt% and 5wt% microcapsules could effectively prevent further corrosion of the coating with high corrosion resistance efficiencies of 61.61% and 45.99% after immersing for 144h in seawater.

Self-assembled polymer gravel array in prepolymer-doped nematic liquid crystals

We demonstrate a new method to fabricate a large-area polymer gravel array. The formation of the polymer gravel array on the substrates of the liquid crystal (LC) cell during polymerization is attributed to the surface tension interaction among the LC, prepolymer, and substrate surface. The gravel can be oriented along the rubbing direction of the substrate. Moreover, the diameter and pitch of the oriented gravel array can be controlled by means of UV curing intensity and supplied voltage under curing because of the competition between the dipole and quadrupole interactions of the colloidal prepolymer droplets. The estimated anchoring energy given by the oriented polymer gravel array can align LCs. The possible application of the formed periodical gravel array on optical devices is explained in this paper.

Constructing 3D heterogeneous hydrogels from electrically manipulated prepolymer droplets and crosslinked microgels.

Formation of multifunctional, heterogeneous, and encoded hydrogel building blocks, or microgels, by crosslinking and assembly of microgels are two essential steps in establishing hierarchical, complicated, and three-dimensional (3D) hydrogel architectures that recapitulate natural and biological structures or originate new materials by design. However, for the variety of the hydrogel materials crosslinked differently and for the varied scales of microgels and architectures, the formation and assembly processes are usually performed separately, which increases the manufacturing complexity of designed hydrogel materials. We show the construction of hydrogel architectures through programmable formation and assembly on an electromicrofluidic platform, adopting two reciprocal electric manipulations (electrowetting and dielectrophoresis) to manipulate varied objects (i) in multiple phases, including prepolymer liquid droplets and crosslinked microgels, (ii) on a wide range of scales from micrometer functional particles or cells to millimeter-assembled hydrogel architectures, and (iii) with diverse properties, such as conductive and dielectric droplets that are photocrosslinkable, chemically crosslinkable, or thermally crosslinkable. Prepolymer droplets, particles, and dissolved molecules are electrically addressable to adjust the properties of the microgel building blocks in liquid phase that subsequently undergo crosslinking and assembly in a flexible sequence to accomplish heterogeneous and seamless hydrogel architectures. We expect the electromicrofluidic platform to become a general technique to obtain 3D complex architectures.

An automated system for high-throughput generation and optimization of microdroplets.

Microdroplets have been widely used in various biomedical applications. During droplet generation, parameters are manually adjusted to achieve the desired size of droplets. This process is tedious and time-consuming. In this paper, we present a fully automated system for controlling the size of droplets to optimize droplet generation parameters in a microfluidic flow-focusing device. The developed system employed a novel image processing program to measure the diameter of droplets from recorded video clips and correspondingly adjust the flow rates of syringe pumps to obtain the required diameter of droplets. The system was tested to generate phosphate-buffered saline and 8% polyethylene (glycol) diacrylate prepolymer droplets and regulate its diameters at various flow rates. Experimental results demonstrated that the difference between droplet diameters from the image processing and manual measurement is not statistically significant and the results are consistent over five repetitions. Taking the advantages of the accurate image processing method, the size of the droplets can be optimized in a precise and robust manner via automatically adjusting flow rates by the feedback control. The system was used to acquire quantitative data to examine the effects of viscosity and flow rates. Droplet-based experiments can be greatly facilitated by the automatic droplet generation and optimization system. Moreover, the system is able to provide quantitative data for the modelling and application of droplets with various conditions in a high-throughput way.

Integrated fabrication-conjugation approaches for biomolecular assembly and protein sensing with hybrid microparticle platforms and biofabrication - A focused minireview

Controlled manufacturing of polymeric hydrogel microparticles is crucial, yet challenging, for rapid and sensitive detection of biomacromolecules in biodiagnostics and biosensing applications. Our approach is an integrated fabrication-conjugation strategy utilizing a simple and robust micromolding technique and biofabrication with a potent aminopolysaccharide chitosan as an efficient conjugation handle for high-yield bioorthogonal conjugation reactions. We present a concise overview of our recent findings in the controlled fabrication of shape-encoded or core-shell structured microparticles consisting of poly(ethylene glycol) (PEG) and short single-stranded (ss) DNA or chitosan, and their utility in the covalent conjugation and nucleic acid hybridization-based assembly of target ssDNAs, proteins and viral nanotemplates. Particularly, two novel routes to achieve substantially improved protein conjugation capacity and kinetics are presented from our recent reports: tobacco mosaic virus (TMV) as a high capacity nanotubular template and polymerization-induced phase separation (PIPS) of pre-polymer droplets for controlled core-shell structure formation. We envision that our fabrication-conjugation approaches reported here, combined with our current and future endeavors in improved fabrication and design of controlled structures with chemical functionalities, should permit a range of manufacturing strategies for advanced functional microscale materials and platforms in a wide array of applications.

Encapsulated PDMS Microspheres with Reactive Handles

Cured poly(dimethyl siloxane) microspheres are prepared by an emulsion polymerization reaction of silicone droplets in a continuous aqueous phase. The commonly used PDMS elastomer, Sylgard 184 from Dow Corning, is used as the dispersed phase. PDMS is polymerized and cross‐linked by reacting vinyl end‐terminated poly(dimethyl siloxane) oligomers with dimethylmethylhydrogen siloxane cross‐linkers via the hydrosilylation reaction using platinum catalyst and heat. Weight ratios of 10:1, 20:1, and 25:1 of the PDMS mixtures are used and emulsified in water using two water‐soluble surfactants as stabilizers (sodium dodecyl sulphate and polyvinylalcohol). The temperature is subsequently increased to accelerate the rate of cross‐linking and prevent the prepolymer droplets from coalescing. The particle size distribution of cured PDMS microspheres is determined by Mastersizer (laser diffraction). Finally, cured PDMS microspheres are coated with poly(methyl methacrylate) using a chemical process (solvent evaporation technique). Three solvents are used in three different experiments: dichloromethane, tetrahydrofuran, and acetone. The composition and morphology of the cured PDMS microspheres and PMMA coated cured PDMS microspheres are characterized by differential scanning calorimetry, Fourier transform infrared spectroscopy in attenuated‐total‐reflection mode, optical microscopy, and thermogravimetric analysis. Curing profiles of PDMS elastomer with different ratios between the silicone elastomer base and the silicone elastomer curing agent are obtained. The reactivity of cured PDMS microspheres and PMMA coated cured PDMS microspheres are measured by rheology to evaluate the efficiency of the PMMA coating.

Preparation of Monodisperse Poly(pentaerythritol triacrylate) Microspheres using a T-junction Microfluidic Chip

This article described the preparation of monodisperse poly(pentaerythritol triacrylate) (PPETA) microspheres by using microfluidic T-junction geometries coupled with photopolymerization technique. Based on sheath effect in T-junction microfluidic channels, the uniform PPETA prepolymer droplets were controlled very well by altering the relative sheath/sample flow rate ratios in silicon oil. After photopolymerization under UV exposure, the monodisperse PPETA microspheres with diameters from 150 μm to 600 μm were obtained.

Preparation of Monodisperse PEG Microspheres by a T-Junction Microfluidic Chip

Monodisperse polyethylene glycol (PEG) microspheres were prepared using microfluidic chips coupled with photopolymerization technique. Based on sheath effect in T-junction microfluidic channels, dispersions of uniform PEG prepolymer droplets in silicon oil are formed. The diameters of the formed PEG prepolymer droplets in the dispersions were controlled very well by altering the relative sheath/sample flow rate ratios. After photopolymerization under UV exposure, the uniform PEG prepolymer droplets isolated by silicon oil underwent photocrosslinking and became monodisperse PEG microspheres.

Microreactor-Based Synthesis of Molecularly Imprinted Polymer Beads Used for Explosive Detection

Molecularly imprinted polymer (MIP) beads based on acrylate formulations have been successfully synthesized in two different types of microreactors capable of generating spherical prepolymer droplets of controlled size in continuous segmented flow processes. After polymerizing and curing the droplets monodisperse acrylate particles have been obtained. Particle sizes have been adjusted in a wide range from 10 to 140 μm by choosing appropriate flow rates of the continuous and the disperse phase. By adding trinitrotoluene (TNT) as a template and PEG as a coporogen to the disperse phase MIP beads with TNT-selective sites and a sufficiently high porosity have been synthesized, as required, for explosive detection techniques.

Surface Free Energy and Wettability Determination of Various Fullerene Derivative Films on Amorphous Carbon Wafer

The surface energy of a polished amorphous carbon (a-C) wafer spin-coated with fullerene derivatives is evaluated by the contact angle measurements of test liquids. The contact angles of the epoxy prepolymer droplets are also measured to investigate the effects of fullerene derivatives on the adhesiveness between the epoxy resin and the a-C wafer surface. A marked increase in surface energy is observed when the wafer is spin-coated with the fullerene derivatives having a polar group. The three surface energy components, dispersive, polar, and hydrogen bonding forces, are determined from the contact angles by an expanded Fowkes method. The total surface energy thus obtained varies from 36.2 to 50.4 mN/m, depending on the nature of fullerene derivatives. The major factor, which increases the surface total energy, is found to be the polar component of the fullerene derivatives. Microscopic structures of the thin films of fullerenes on the wafer have no essential effects on the surface energy and the wettability between the a-C wafer and the epoxy resin. In the present study, we demonstrate that the fullerene derivative with a polar group acts as an adhesive in the fixation between the epoxy resin and the a-C wafer: a polar group of the fullerene derivative has large affinity to the epoxy resin, and the fullerene core, to the a-C surface.