Novel Soft Materials with Nonlinear Mechanical Response

Abstract

As the population ages, the number of elderly people walking outside is expected to increase. The weakening of muscle strength by aging leads to a decrease in walking speed and increases the risk of falling. [1,2] In addition, Soft materials have been used in a wide scope of fields such as soft robotics, medicine, and welfare, because they are safer and closer to biological characteristics than conventional metallic materials [3,4]. The purpose of this study is to develop a soft material with a nonlinear mechanical response by Hybrid structural design using multiple materials, which can be applied as a shoe bottom material that supports keeping walk speed while reducing the risk of falling, and robot hands that resemble human fingers. In this research, we focus on giving an extra-viscosity to silicone rubber by embedding a flow channel of viscous fluid [5,6].Experiments: A water-soluble flow channel model (15mm cubic, Parallel crosses structure) was 3D printed with an FDM 3D printer (QIDITECH X-pro) using polyvinyl alcohol (PVA) filament. The channel model was fixed in the center of a mold (20 mm cubic inside dimension), then silicone resin (Ecoflex00-30, Smooth-on) was poured and cured. After the curing, the silicone part is removed from the mold and dipped in water to dissolve the PVA flow channel model. The dissolved hollow part in the silicone was filled with highly viscous fluid (hydroxyl propyl cellulose (HPC, Wako) in water). The fluid was colored with black pigment so that the fluid can be visually recognized through the translucent silicone part. As a Material evaluation, we measured the viscosity frequency dependence of the HPC aqueous solution was measured using a rheometer (Anton Paar, MCR302), And then we measured the dynamic viscoelasticity of the fabricated samples using a DMA tester (RSA-G2 (TA Instruments) or homemade tester).Results: Fig.1 shows the fabricated silicone sample. To dissolve the embedded PVA part in the silicone part, the silicone part had to be sliced into two pieces to make the edge of the PVA part exposed to water for smooth dissolution and diffusion. To infuse the viscous fluid into the hollow of the silicone part, we used a manual centrifuge machine to apply high gravity for smooth entering of the viscous fluid and degassing. Fig.2 shows the Measurement results of DMA test. In this results, we verified the change in tangent loss tan δ due to thixotropy of the HPC aqueous solution. Figure 2 shows that the sample filled with the viscous fluid has a lower tan δ than the simple silicone. From this result, we considered that the deformation (about 1.8 mm) was small for the sample size (20 mm cubic) injected with viscous fluid. Therefore, we attempted to utilize a 3D printer to create a DMA testing machine capable of large deformation at low frequencies. In this conference, we will report the fabrication method and evaluation results of the fabricated samples. Figure 1