Synthesis and Applications of Elastomeric Polymerized High Internal Phase Emulsions (PolyHIPEs)

Polymer foams (PFs) are among the most industrially produced polymeric materials, and they are found in applications including aerospace, packaging, textiles, and biomaterials. PFs are predominantly prepared using gas-blowing techniques, but PFs can also be prepared from templating techniques such as polymerized high internal phase emulsions (polyHIPEs). PolyHIPEs have many experimental design variables which control the physical, mechanical, and chemical properties of the resulting PFs. Both rigid and elastic polyHIPEs can be prepared, but while elastomeric polyHIPEs are less commonly reported than hard polyHIPEs, elastomeric polyHIPEs are instrumental in the realization of new materials in applications including flexible separation membranes, energy storage in soft robotics, and 3D-printed soft tissue engineering scaffolds. Furthermore, there are few limitations to the types of polymers and polymerization methods that have been used to prepare elastic polyHIPEs due to the wide range of polymerization conditions that are compatible with the polyHIPE method. In this review, an overview of the chemistry used to prepare elastic polyHIPEs from early reports to modern polymerization methods is provided, focusing on the applications that flexible polyHIPEs are used in. The review consists of four sections organized around polymer classes used in the preparation of polyHIPEs: (meth)acrylics and (meth)acrylamides, silicones, polyesters and polyurethanes, and naturally occurring polymers. Within each section, the common properties, current challenges, and an outlook is suggested on where elastomeric polyHIPEs can be expected to continue to make broad, positive impacts on materials and technology for the future.

A simple, robust and scalable route to prepare sub-50 nm soft PDMS nanoparticles for intracellular delivery of anticancer drugs

Soft nanoparticles (NPs) have recently emerged as a promising material for intracellular drug delivery. In this regard, NPs derived from polydimethylsiloxane (PDMS), an FDA approved polymer can be a suitable alternative to conventional soft NPs due to their intrinsic organelle targeting ability. However, the available synthesis methods of PDMS NPs are complicated or require inorganic fillers, forming composite NPs and compromising their native softness. Herein, for the first time, we present a simple, robust and scalable strategy for preparation of virgin sub-50 nm PDMS NPs at room temperature. The NPs are soft in nature, hydrophobic and about 30 nm in size. They are stable in physiological medium for two months and biocompatible. The NPs have been successful in delivering anticancer drug doxorubicin to mitochondria and nucleus of cervical and breast cancer cells with more than four-fold decrease in IC50 value of doxorubicin as compared to its free form. Furthermore, evaluation of cytotoxicity in reactive oxygen species detection, DNA fragmentation, apoptosis-associated gene expression and tumor spheroid growth inhibition demonstrate the PDMS NPs to be an excellent candidate for delivery of anticancer drugs in mitochondria and nucleus of cancer cells.

A Buoyant, Microstructured Polymer Substrate for Photocatalytic Degradation Applications

Microbubble fabrication of poly(dimethylsiloxane) (PDMS) beads with incorporated TiO2 provides a low-density, microstructured photocatalyst that is buoyant in water. This approach surmounts many of the challenges traditionally encountered in the generation of buoyant photocatalysts, an area which is critical for the implementation of widespread environmental cleaning of organic pollutants in water resources. Because the incorporation into the polymer bead surface is done at low temperatures, the crystal structure of TiO2 is unaltered, ensuring high-quality photocatalytic activity, while PDMS is well-established as biocompatible, temperature stable, and simple to produce. The photocatalyst is shown to degrade methylene blue faster than other buoyant, TiO2-based photocatalysts, and only an order of magnitude less than direct suspension of an equivalent amount of photocatalyst in solution, even though the photocatalyst is only present at the surface of the solution. The reusability of the TiO2/PDMS beads is also strong, showing no depreciation in photocatalytic activity after five consecutive degradation trials.

High-permeability functionalized silicone magnetic microspheres with low autofluorescence for biomedical applications

Functionalized magnetic microspheres are widely used for cell separations, isolation of proteins and other biomolecules, in vitro diagnostics, tissue engineering, and microscale force spectroscopy. We present here the synthesis and characterization of a silicone magnetic microsphere which can be produced in diameters ranging from 0.5 to 50 μm via emulsion polymerization of a silicone ferrofluid precursor. This bottom-up approach to synthesis ensures a uniform magnetic concentration across all sizes, leading to significant advances in magnetic force generation. We demonstrate that in a size range of 5-20 μm, these spheres supply a full order of magnitude greater magnetic force than leading commercial products. In addition, the unique silicone matrix exhibits autofluorescence two orders of magnitude lower than polystyrene microspheres. Finally, we demonstrate the ability to chemically functionalize our silicone microspheres using a standard EDC reaction, and show that our folate-functionalized silicone microspheres specifically bind to targeted HeLa and Jurkat cells. These spheres show tremendous potential for replacing magnetic polystyrene spheres in applications which require either large magnetic forces or minimal autofluorescence, since they represent order-of-magnitude improvements in each. In addition, the unique silicone matrix and proven biocompatibility suggest that they may be useful for encapsulation and targeted delivery of lipophilic pharmaceuticals.

Magnetic, Fluorescent, and Copolymeric Silicone Microspheres

Silicone microspheres are exceedingly difficult to make. Here, polydimethylsiloxane microspheres (≈1 μm diameter) are synthesized using ultrasonic spray pyrolysis, the first demonstration of a scalable synthetic procedure for crosslinked silicone microspheres. This continuous, aerosol process is also used to directly produce fluorescent, magnetic, and copolymeric derivatives; the potential biomedical applications of these microspheres are explored.

Preparation of novel silicone multicompartment particles by multiple emulsion templating and their use as encapsulating systems

Multicompartment poly(dimethylsiloxane) particles were produced for the first time using water-in-oil-in-water (W1/O/W2) emulsions as templates. Multiple silicone W1/O/W2 emulsions were successfully prepared by using silicone precursors with a low viscosity. Several formulation parameters were studied to determine their effect on the properties of emulsions and derived particles. It was observed that the mass fraction of the inner aqueous phase (φ(W1)) and the concentration of both the hydrophobic and hydrophilic surfactants played a crucial role in the morphology and stability of the emulsions. Thus, the derived silicone porous particles also showed different characteristics depending on the emulsion formulation because of the templating effect. At low φ(W1) or high concentrations of the hydrophobic surfactant, particles showed smaller pore sizes as a result of more stable inner droplets. On the other hand, high concentrations of the hydrophobic surfactant resulted in an increase in the size of the derived particles, whereas high concentrations of the hydrophilic surfactant caused the opposite effect. In addition, fluorescein was encapsulated into the hydrophobic particles during the synthesis process and released in a controlled manner. The possibility to encapsulate simultaneously but independently two different hydrophilic components inside the same globule was also tested. On the basis of these results, the obtained silicone porous particles are envisioned to have applications in several advanced fields, for instance, as hydrophobic delivery systems.

Fabrication of novel silicone capsules with tunable mechanical properties by microfluidic techniques

A novel approach for the synthesis of silicone capsules using double W/O/W emulsions as templates is introduced. The low viscosity of the silicone precursors enables the use of microfluidic techniques to accurately control the size and morphology of the double emulsion droplets, which after cross-linking result in the desired monodisperse silicone capsules. Their shell thickness can be finely tuned, which in turn allows control over their permeability and mechanical properties; the latter are particularly important in a variety of practical applications where the capsules are subjected to large external forces. The potential of these capsules for controlled release is also demonstrated using a model hydrophilic substance.

Reaction-diffusion phenomena in a PDMS matrix can modify its topography

Various reagents and solvents can be absorbed into polydimethylsiloxane (PDMS), which may be a concern for many applications. We hypothesize that these absorbed reagents can also react with each other within the elastomer matrix. Here we demonstrate this phenomenon and use it as a means to physically modify the surface topography of the PDMS by generating wrinkles or pores.

Polymer-lipid microbubbles for biosensing and the formation of porous structures

Polymer-lipid microbubbles (PLBs) are generated by microfluidic flow-focusing devices to form a new class of long-lasting hybrid particles. The specific PLB construct developed is an elastic gas-filled microsphere with a polydimethylsiloxane (PDMS) shell containing phospholipids conjugated to functionalized polyethyleneglycol (PEG). Digital "droplet-based" microfluidics technology enables control of particle composition, size, and polydispersity (sigma<10%). Use of PDMS as a shell component improves the functionality and stability (lifetime>6 months) of the hybrid particles due to the thermally maneuverable solidification process. With a gas core, they serve as a template material for creating three-dimensional porous structures and surfaces, requiring no cumbersome post-processing removal steps. By adding biotinylated PEG-lipid derivatives that offer targeting capabilities, we demonstrate the immobilization of fluorescent IgG antibodies on stationary PDMS-lipid microbubbles through biotin-avidin interactions and on-chip trapping for immunoassays. A PDMS-lipid composition offers several advantages such as biocompatibility and biodegradability for future in vivo use as porous engineered scaffolds, packing materials, or delivery (e.g. therapeutic) agents with cell targeting capability.