2.3. Matching Medium Preparation

BaTiO3 is a dielectric ceramic material that possesses piezoelectric, ferroelectric, dielectric and thermometric properties. It has a wide range of industrial applications such as multilayer ceramic capacitors, thermistors, microwave absorbers, transducers and electro-optic devices. The properties of BaTiO3 are dependent on the particle size, purity and crystalline phase resulting from the preparation method [21,22]. Its dielectric properties strongly depend on the grain size, and a very dense ceramic of BaTiO3 can have relative permittivity values as high as 7000 at 1 kHz [23]. It is reported in [24] that pure BaTiO3 shows values of relative permittivity around 105 in the X-band, but mixing BaTiO3 with other materials like PANI drastically reduces the values of complex permittivity and loss tangent of the mixture (adding only 5% of PANI reduces the relative permittivity to 26). Such studies show that BaTiO3 permittivity values can drop heavily when mixed with other materials, and this is what we also observed in our experiments in Section 3.1. BaTiO3 is also bio-compatible and thus suitable for direct contact with the human body in imaging and therapy systems. Consequently, BaTiO3 nanoparticles have been used for drug delivery and as label-free imaging probes [25].

Polydimethylsiloxane (PDMS) is a widely used silicon-based organic polymer. It is optically clear, inert, non-toxic, and non-flammable as well as biocompatible and gas-permeable. Unmodified PDMS is hydrophobic, however surface treatment with oxygen plasma can make the surface temporarily hydrophilic [26]. Aqueous solvents will not infiltrate and swell the material, but most organic solvents (with exception of some alcohols) will diffuse into the material and cause it to swell. High oxygen and carbon dioxide permeability allow cell respiration, therefore PDMS is suitable for microfluidic cell cultures applications [27]. PDMS applications range from soft lithography, microfluidics chips and microelectromechanical systems to medical devices and cosmetics [27,28].

To prepare the matching medium samples, we mixed BaTiO3 powder with particle size less than 3 μm with PDMS in various mixing ratios. Both materials were purchased from Sigma-Aldrich. To take into account the duration of the degassing step and ensure the mixture is completely degassed before it solidifies, we chose a PDMS with long setting time. Previous research has stated that particle size around 1 micron is a good choice for high permittivity [23].

We now describe the steps involved in the process of making the composite. First, we measure the weight of the BaTiO3 powder and the PDMS to achieve the desired mass ratio. Then, we mix the PDMS with its curing agent, using a 10:1 ratio, according to the manufacturer instructions. This ratio can be adjusted to achieve different material stiffness, noting that hardness and flexibility are mostly influenced by the amount of the BaTiO3 powder. Then, PDMS is placed on a flat surface and we add part of the powder on top. Using an elastic steel or a plastic spatula, we press the powder into the silicone and mix it thoroughly. The procedure of pressing and mixing the material is repeated until there are no clumps. Powder is gradually added until the entire amount of powder is used. The mixing step is critical for removing the clumps of powder and to obtain a homogeneous material. Next, for the degassing step, we use a low pressure vacuum with a pressure of 0.1 bar. If the pressure is lowered too rapidly, the mixture will start to foam and can leak out of the mold. Using slow setting PDMS is crucial as degassing can take up to 4 h at 0.1 bar. The mixture is being checked for bubbles after the container is repressurized by scraping off the surface and observing if any gas is still trapped under it. The degassing step is necessary as many gas bubbles can make the sample unusable. Finally, after the mixture is set, we observe that it is stiffer than pure PDMS. However, it can be cut and machined easily to create molds. Figure 3 shows a snapshot of the mixing process as well as the used materials.

Photos of the experimental procedure: (a) the materials used to make the composite, and (b) snapshot of the mixing process.