Development of Low‐Viscosity Silicone Rubber With Variable Hardness for Precise Microfabrication

ABSTRACT Silicone rubbers, such as polydimethylsiloxane (PDMS), have been widely used in biotechnology and medical science because of their excellent properties. However, microfabrication of silicone rubbers remains to be problematic because of the high viscosity and adhesiveness (tackiness) of silicone rubber, making it difficult to fill molds and reproduce shapes during soft lithography. Also, the patterns made of soft silicone rubber are prone to pattern collapse. To address these, we designed molecules that can be modified by radiation and whose hardness can be adjusted after microfabrication. By combining materials with different chemical structures and molecular weights, we developed a silicone rubber with low viscosity and adhesiveness suitable for radiation modification. The synthesized silicone rubbers had a high filling ability in micropatterned molds and low adhesiveness, which prevented pattern collapse. Moreover, the elastic modulus of silicone rubbers can be controlled using high‐energy electron beam (EB) irradiation. The radiation‐modified silicone rubber sheet retained its high transparency to visible light, high oxygen permeability, and low drug adsorption and adhesion. By utilizing these characteristics, a new technique was developed that produces microfabricated structures with different hardness and hydrophilicity using imprinting technology followed by EB irradiation. Soft lithography was then performed on the developed silicone rubber to transfer the micropillar structure. Pillar shapes were accurately fabricated with a diameter of 1.0 μm or less along the mold, which is difficult to fabricate with conventional PDMS. This new silicone rubber overcomes the problems associated with conventional silicone rubber and is a promising material for high‐performance medical microdevices.