Controlling the Synthesis of Polyurea Microcapsules and the Encapsulation of Active Diisocyanate Compounds

The encapsulation of active components is currently used as common methodology for the insertion of additional functions like self-healing properties on a polymeric matrix. Among the different approaches, polyurea microcapsules are used in different applications. The design of polyurea microcapsules (MCs) containing active diisocyanate compounds, namely isophorone diisocyanate (IPDI) or hexamethylene diisocyanate (HDI), is explored in the present work. The polyurea shell of MCs is formed through the interfacial polymerization of oil-in-water emulsions between the highly active methylene diphenyl diisocyanate (MDI) and diethylenetriamine (DETA), while the cores of MCs contain, apart from IPDI or HDI, a liquid Novolac resin. The hydroxyl functionalities of the resin were either unprotected (Novolac resin), partially protected (Benzyl Novolac resin) or fully protected (Acetyl Novolac resin). It has been found that the formation of MCs is controlled by the MDI/DETA ratio, while the shape and size of MCs depends on the homogenization rate applied for emulsification. The encapsulated active compound, as determined through the titration of isocyanate (NCO) groups, was found to decrease with the hydroxyl functionality content of the Novolac resin used, indicating a reaction between NCO and the hydroxyl groups. Through the thorough investigation of the organic phase, the rapid reaction (within a few minutes) of MDI with the unprotected Novolac resin was revealed, while a gradual decrease in the NCO groups (within two months) has been observed through the evolution of the Attenuated Total Reflectance-Fourier-Transform Infrared (ATR-FTIR) spectroscopy and titration, due to the reaction of these groups with the hydroxyl functionalities of unprotected and partially protected Novolac resin. Over longer times (above two months), the reaction of the remaining NCO groups with humidity was evidenced, especially when the fully protected Acetyl Novolac resin was used. HDI was found to be more susceptible to reactions, as compared with IPDI.

Thiol-Ene Cross-linking of Poly(ethylene glycol) within High Internal Phase Emulsions: Degradable Hydrophilic PolyHIPEs for Controlled Drug Release

Macroporous polymer monoliths prepared from high internal phase emulsions (HIPEs) can be found in various biomedical applications. While typically water-in-oil HIPEs are applied for polyHIPE preparation, they are not suitable for hydrophilic polyHIPE preparation. Herein, direct oil-in-water emulsions based on water-soluble poly(ethylene glycol)diacrylate or poly(ethylene glycol)dimethacrylate were developed. Furthermore, the incorporation of a hydrophilic water-miscible thiol, ethoxylated trimethylolpropane tris(3-mercaptopropionate) (ETTMP) was reported for the first time within thiol–ene polyHIPEs. Due to the transparency of the emulsions, rapid curing via photopolymerization was feasible. The average pore diameters of the resulting polyHIPEs ranged between 1.2 and 3.6 μm, and porosity of up to 90% was achieved. The water uptake of the materials reached up to 1000% by weight. Drug loading and release were demonstrated, employing salicylic acid as a model drug. Porous profile and biodegradability add to the usefulness of the material for biomedical applications.

Porous Polymers from High Internal Phase Emulsions as Scaffolds for Biological Applications

High internal phase emulsions (HIPEs), with densely packed droplets of internal phase and monomers dispersed in the continuous phase, are now an established medium for porous polymer preparation (polyHIPEs). The ability to influence the pore size and interconnectivity, together with the process scalability and a wide spectrum of possible chemistries are important advantages of polyHIPEs. In this review, the focus on the biomedical applications of polyHIPEs is emphasised, in particular the applications of polyHIPEs as scaffolds/supports for biological cell growth, proliferation and tissue (re)generation. An overview of the polyHIPE preparation methodology is given and possibilities of morphology tuning are outlined. In the continuation, polyHIPEs with different chemistries and their interaction with biological systems are described. A further focus is given to combined techniques and advanced applications.

Colloidal systems toward 3D cell culture scaffolds

Three-dimensional porous scaffolds are essential for the development of tissue engineering and regeneration, as biomimetic supports to recreate the microenvironment present in natural tissues. To successfully achieve the growth and development of a specific kind of tissue, porous matrices should be able to influence cell behavior by promoting close cell-cell and cell-matrix interactions. To achieve this goal, the scaffold must fulfil a set of conditions, including ordered interconnected porosity to promote cell diffusion and vascularization, mechanical strength to support the tissue during continuous ingrowth, and biocompatibility to avoid toxicity. Among various building approaches to the construction of porous matrices, selected strategies afford hierarchical scaffolds with such defined properties. The control over porosity, microstructure or morphology, is crucial to the fabrication of high-end, reproducible scaffolds for the target application. In this review, we provide an insight into recent advances toward the colloidal fabrication of hierarchical scaffolds. After identifying the main requirements for scaffolds in biomedical applications, conceptual building processes are introduced. Examples of tissue regeneration applications are provided for different scaffold types, highlighting their versatility and biocompatibility. We finally provide a prospect about the current state of the art and limitations of porous scaffolds, along with challenges that are to be addressed, so these materials consolidate in the fields of tissue engineering and drug delivery.

Basic Principles of Emulsion Templating and Its Use as an Emerging Manufacturing Method of Tissue Engineering Scaffolds

Tissue engineering (TE) aims to regenerate critical size defects, which cannot heal naturally, by using highly porous matrices called TE scaffolds made of biocompatible and biodegradable materials. There are various manufacturing techniques commonly used to fabricate TE scaffolds. However, in most cases, they do not provide materials with a highly interconnected pore design. Thus, emulsion templating is a promising and convenient route for the fabrication of matrices with up to 99% porosity and high interconnectivity. These matrices have been used for various application areas for decades. Although this polymer structuring technique is older than TE itself, the use of polymerised internal phase emulsions (PolyHIPEs) in TE is relatively new compared to other scaffold manufacturing techniques. It is likely because it requires a multidisciplinary background including materials science, chemistry and TE although producing emulsion templated scaffolds is practically simple. To date, a number of excellent reviews on emulsion templating have been published by the pioneers in this field in order to explain the chemistry behind this technique and potential areas of use of the emulsion templated structures. This particular review focusses on the key points of how emulsion templated scaffolds can be fabricated for different TE applications. Accordingly, we first explain the basics of emulsion templating and characteristics of PolyHIPE scaffolds. Then, we discuss the role of each ingredient in the emulsion and the impact of the compositional changes and process conditions on the characteristics of PolyHIPEs. Afterward, current fabrication methods of biocompatible PolyHIPE scaffolds and polymerisation routes are detailed, and the functionalisation strategies that can be used to improve the biological activity of PolyHIPE scaffolds are discussed. Finally, the applications of PolyHIPEs on soft and hard TE as well as in vitro models and drug delivery in the literature are summarised.

Preparation of fluoropolymer materials with different porous morphologies by an emulsion template method using supercritical carbon dioxide as a medium

The choice of a suitable surfactant is key to the formation of a stable water-in-CO2 (W/C) or CO2-in-water (C/W) emulsion. It is even more critical in stabilization of the emulsion containing carbon dioxide (CO2). In this study, the successful preparation of W/C emulsion was achieved by using the amphiphilic block polymer poly(ethylene glycol) methyl ether-b-poly(trifluoroethyl methacrylate) (mPEG45-b-(TFEMA) n ) as a surfactant, in which CO2 was used as a solvent for the fluoromonomer, trifluoroethyl methacrylate (TFEMA). In the case of the W/C emulsion, CO2 and TFEMA were used as the continuous phase and water as the internal phase of the emulsion system. It has been found that in the length of the block polymer mPEG45-b-(TFEMA) n , the fluorine-containing chain end has a significant effect on the morphology of the polymer and the type of emulsion formed. The morphology of the polymer was observed by scanning electron microscopy which confirmed the type of emulsion formed. With the fluorine-containing end segment, the morphology of the polymer changes from a small hollow sphere in a large hollow sphere to a hollow spherical to a porous structure. Correspondingly, it could be concluded that the type of emulsion could go through the process from water-in-CO2-in-water-in-CO2 (W/C/W/C) emulsion to water-in-CO2-in-water (W/C/W) emulsion to water-in-CO2 (W/C) emulsion. Also, suitable co-surfactants were identified in this study. Investigations were also attempted to check the effect of the amount of surfactant, cross-linker and water/CO2 ratio on the type of emulsion formed as well as the morphology of the resultant polymer.

Highly porous and chemical resistive P(TFEMA–DVB) monolith with tunable morphology for rapid oil/water separation

A facile preparation for a series of porous poly(2,2,2-trifluoroethylmethacrylate–divinylbenzene) P(TFEMA–DVB) foams is discussed in this paper. The foams have adjustable morphology utilizing a suitable commercial surfactant, Hypermer B246, as stabilizer, and were compared with traditional organic surfactants or macromolecular block-polymers. Combining the porous properties and advantages of fluorine atoms, this type of fluoropolymer exhibited superb chemical stability and hydrophobicity performances with high porosity. These porous fluoro-monoliths preserved their regular porous structure without any degradation after immersion into strong acidic or basic solution for three days, hence demonstrating an excellent potential to deal with environmental pollution caused by oil spillages in severe environments. The tunable morphology (open and closed pores) and pore sizes were achieved by investigating various parameters like surfactant concentration, amount of external crosslinker, and aqueous phase volume. Droplet sizes of HIPEs were characterized using an optical microscope under different experimental conditions. The influence of pore structure and surface properties of polyHIPE on water contact angle and oil adsorption capacity was also explored. The results indicated that the porous material has an excellent oleophilicity and hydrophobicity, with water contact angles (WCA) up to 146.4°. Additionally, the results presented a noticeable adsorption with a very fast rate towards organic oils from either a water surface or bottom with adsorption saturation achieved in about 120 s. The prepared polyHIPEs showed a good recycling ability; even after 10 adsorption–centrifugation experiments, the adsorption capacity was still more than 85%.

A facile preparation for a series of porous poly(2,2,2-trifluoroethylmethacrylate–divinylbenzene) P(TFEMA–DVB) foams is discussed in this paper.

Macroporous poly(ionic liquid)/ionic liquid gels via CO2-based emulsion-templating polymerization

A high internal phase emulsion composed of ionic liquids and CO₂ serves as a template for producing unprecedented macroporous poly(ionic liquid) gels.