The core formulation contains an oligomer, poly(ethylene glycol)-diacrylate (PEG-DA, g·mol−1, density kg·m−3, Merck, Darmstadt, Germany), a photo-initiator (HMP, 2-hydroxy-2-methylpropiophenone, g·mol−1, density kg·m−3, Merck, Darmstadt, Germany, also known as Darocur 1173) and ultra-pure water.
To this formulation, we add two ingredients: (1) Bare micronized calcite (CaCO3
) particles that will act as a solid sacrificial porogen: Microcarb 60, kindly provided by Reverté Productos Minerales, Spain, volume-averaged diameter ≈1.3 μm, distributed between 0.2 and 10 μm, molar mass
; (2) A stabilizer: we studied two different polymeric stabilizers for the calcite: poly(sodium 4-styrenesulfonate) (PSS,
, Merck, Darmstadt, Germany) known to disperse calcite particles during calcium salt precipitation [8
] or poly(vinyl alcohol) (PVA,
, fully hydrolyzed, density
, Merck, Darmstadt, Germany) known as a highly hydrophilic compound due to hydroxyl groups yet with somewhat amphiphilic properties [10
In practice, we prepare several stock solutions (a polymeric batch of PEG-DA/HMP and aqueous batches of PSS or PVA solutions); we add calcite to the aqueous batches in specific quantities, and finally add the polymeric batch in order to match the mass ratio we seek and to vary the volume fraction of calcite particles. We ensure that the final formulation contains PEG-DA/HMP/water in mass proportions 1/0.1/1.1 and the concentration of calcite particles will be given as a volume fraction in the final formulation. Thorough mechanical mixing followed by ultra-sound fragmentation is used before any post-treatment such as photo-polymerization.
After photo-polymerization of the formulation, calcite is removed through acidic treatment [4
] using hydrochloric acid (HCl solution, 1 mol·L−1
, Fisher Scientific, United States).
Silane are used to enhance adhesion of the formulation on glass slide (3-(trimethoxysilyl)propyl acrylate from Merck, Germany) or to reduce it (Aquapel, a commercial fluoroalkylsilane purchased online).
The poly(dimethylsiloxane) (PDMS) used for microfluidics is the Sylgard 184 Silicone Elastomer Kit.
The oil used for emulsification is the Fluorinert FC-40 purchased from Merck, Germany, and microfluidic and batch emulsions are stabilized with Pico-Surf purchased from Dolomite, United Kingdom. The microfluidic chips are made fluorophilic with Aquapel after plasma treatment and thorough oxygen degassing and argon regassing.
Flat and shallow monoliths are fabricated using frontal photo-polymerization (FPP) [11
] performed as follows: about 2 mL of the formulation is pipetted and squeezed in between two glass slides (thickness 1 mm, fluorophilic) spaced with a 1 mm-thick spacer. The sample is then exposed to ultra-violet (UV) radiation using a collimated LED working at 365 nm (a UV-KUB1 from Kloé, France, delivering a tunable power density up to
). The photo-polymerization yields a hydrogel monolith which turns from transparent to opaque as the calcite load is increased.
Concerning the systematic study of photo-polymerization, the main outcome is the thickness as a function of UV dose; the thickness is measured sidewise from a cross-section of a cut sample using stereo-microscopy or optical microscopy depending on the size of the sample.
Eventually, we also measured systematically the absorbance of all species in solution (except microparticles) and found that, at 365 nm, only the photo-initiator absorbs light and we measured the corresponding Beer–Lambert law as a function of concentration in water. From these measurements, we estimate that, in a typical formulation with no particle, the absorbance reads , where l is the optical path and mm.