Microbubble Fabrication of Concave-porosity PDMS Beads

Summary

Procedures used to generate microstructured concave-porosity polydimethylsiloxane beads are presented. Effects of electrolyte concentration and identity within the aqueous phase are particularly emphasized.

Abstract

Microbubble fabrication (by use of a fine emulsion) provides a means of increasing the surface-area-to-volume (SAV) ratio of polymer materials, which is particularly useful for separations applications. Porous polydimethylsiloxane (PDMS) beads can be produced by heat-curing such an emulsion, allowing the interface between the aqueous and aliphatic phases to mold the morphology of the polymer. In the procedures described here, both polymer and crosslinker (triethoxysilane) are sonicated together in a cold-bath sonicator. Following a period of cross-linking, emulsions are added dropwise to a hot surfactant solution, allowing the aqueous phase of the emulsion to separate, and forming porous polymer beads. We demonstrate that this method can be tuned, and the SAV ratio optimized, by adjusting the electrolyte content of the aqueous phase in the emulsion. Beads produced in this way are imaged with scanning electron microscopy, and representative SAV ratios are determined using Brunauer–Emmett–Teller (BET) analysis. Considerable variability with the electrolyte identity is observed, but the general trend is consistent: there is a maximum in SAV obtained at a specific concentration, after which porosity decreases markedly.

Introduction

Polydimethylsiloxane (PDMS) is one of the most widely used silicone compounds. Its biocompatibility has led to widespread use in implant and other biomedical engineering structures^1,2. It is trivially cross-linked into elastic structures using an organosilyl compound (such as triethoxysilane), a simple and reliable procedure which has made it useful for cast polymer applications where some flexibility is required³. Once cross-linked, PDMS is largely inert, particularly in biological conditions, and is therefore useful for a variety of food and medical applications^4,5. Ease of casting, chemical inertness, and hydrophobicity have made it....

Protocol

1. Preparation of Emulsion

1. Emulsion Contents
   1. Mass an appropriate amount of salt to produce 10 ml of 0.03-M solution. For platinum(IV) chloride measure 0.101 g, for zinc(II) chloride (ZnCl₂) measure 0.032 g, and sodium chloride (NaCl) measure 0.018 g.
   2. In individual test tubes, dissolve each salt into 10 ml of DI water. Set aside for later use.
   3. Use a 20-ml, sealable glass vial for the contents throughout this procedure. Tare a balance to the glass vial.
   4. Weigh vinyl-terminated polydimethylsiloxane by slowly pouring it over a stir rod and into the glass vial resting on the zeroed scale. Weigh out 1.02 g (....

Results

Representative SEM images of beads arising from emulsions with different electrolyte conditions are shown in Figure 1. Figure 1A shows a bead similar to those obtained by DuFaud, et al.¹³, produced using our procedures, without the addition of any electrolyte. Beads shown in Figure 1B-D, resulting in different morphologies for each metal ion. For all images shown, 300 μl of 0.03-M electrolyte solutions were used in place of 300 μl of the DI water for .......

Discussion

The beads produced using this protocol (and by adjusting the electrolyte concentration and identity) are fundamentally different from those produced with a low-ionic strength emulsion, as seen by comparison of Figure 1A to the other SEM images in Figure 1. Our initial report used PtCl₄ with the intention of further catalyzing the polymerization cross-linking at the aqueous-aliphatic interface¹⁴. In that report, large concave indentations were seen. Since that report.......

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
Poly(dimethylsiloxane), vinyl terminated	Sigma-Aldrich	68083-19-2
n-Heptane	Sigma-Aldrich	142-82-5	Flammable
Triethoxysilane	Sigma-Aldrich	998-30-1	Flammable, Accutely Toxic
Sorbitan Monoleate (Span-80)	Fluker	1338-43-8
Platinum(IV) Chloride	Sigma-Aldrich	13454-96-1	Accutely Toxic
Zinc(II) Chloride	Sigma-Aldrich	7646-85-7
Sodium Chloride	Sigma-Aldrich	7647-14-5
2.8 L Water Bath Sonicator	VWR	97043-964