High Internal Phase Emulsion

High Internal Phase Emulsion

The presence of pores, holes, cavities or channels in bulk, technically known as free-space, distinguished the material as porous material. A number of natural porous materials can be witnessed and commendable efforts have been researched to develop the synthetic porous structures, mimicking the porous morphology that of naturally occurring materials. The advent of advanced reaction-chemistries has originated a different kind of materials commonly known as Plastics i.e. a Polymer, highly exploited in routine to industrial applications. Abundant availability, advantageous physico-chemical properties, ease of surface modification through chemical or physical processes to suit a particular function and simplicity of thermal and/or solution processing to transform polymers into an intricate shape porous structures, provoke that polymers are an attractive choice of building materials. Moreover, being a synthetic material, some of the polymers such as aliphatic polyesters like PLLA, PGA, PCL and their co-polymers are bio-degradable and bio-compatible, therefore, favoring their suitability in various disciplines of bio-medical science. A number of classical and modern techniques have been described to develop porous polymers through melt and/or solution processing, except, emulsion templating, which creates porous polymers using the native monomers as the precursor-material.

            Emulsions are the colloidal systems, where one of the liquids is dispersed into another liquid, with the help of a surfactant and/or Pickering stabilizer, and could be of water-in-oil, oil-in-water, CO₂-in-water or non-aqueous type. If the droplet or internal phase of emulsion is accounting ≥ 74% of emulsion volume, then such emulsions are categories as the high internal phase emulsions (HIPEs). The removal of dispersed phase in purification step after solidification of continuous organic phase of HIPE comprising the polymerizable monomers, results in development of an interconnected porous or open-cell material i.e. PolyHIPE, first coined in 1960s. Monoliths, beads, rods and complex structures have been crafted through association of emulsion templating with techniques like microfluidics or three-dimensional (3D) printing. Low density, high interconnected porosity, tailor-made pore-morphology and mechanical strength, uniform in- or ex-situ functionalization are some of the unique characteristics of polyHIPEs making them highly suitable as adsorbents, filtration, support for catalysis and immobilization, template for porous carbons and scaffolds for tissue engineering, to name a few.

In this respect, we at Fibre Science Laboratory are researching to develop emulsion templated porous structures of bio-compatible, bio-degradable, bio-resorbable, FDA approved and CE mark registered aliphatic polyester i.e. PCL, highly recommended as tissue engineering scaffolds. Initially, we developed the pristine or nano-functionalized PCL matrices by electrospinning of emulsifier free oil-in-water emulsions comprising the PCL solution as the internal phase and aqueous PVA solution as the external phase. Later, water-in-oil Pickering emulsions of PCL were processed to develop electrospun-matrices with nano-inorganic functionalities. However, the above approaches were based on the solution processing of pre-synthesized PCL. In contrast to above routes, at present, we are practicing the non-aqueous HIPEs of monomer, ε-caprolactone (CL) and suitable crosslinker to fabricate crosslinked PCL polyHIPE monoliths via single step HIPE-ROP (high internal phase emulsion-ring opening polymerization). All these electrospun matrices and polyHIPE monolith, when assessed in cell-culture experiments demonstrated high cytocompatibility as good adhesion and proliferation of cells on the porous matrix was observed. Further, the non-aqueous HIPEs of CL and crosslinker were employed as feed material in micro-fluidic set-up to develop porous beads of crosslinked PCL.