Friction Property of Gel Surface Modified By Laser Processing
Authors: Wada Masato , Kameyama Toshiki , Yoshida Kazunari , Khosla Ajit , Kawakami Masaru , Furukawa Hidemitsu 
Published: February 2020

Friction and lubrication are very important concept in surface science and various kinds of engineering fields. To date, they have been widely studied since friction bereaves a lot of energy and expenditure. Atomic scale friction of graphite surface was, for instance, investigated by using molecular dynamics (MD) simulation (Matsushita et al 2005). Such simulation revealed that cause of low frictional coefficients of graphite is the honeycomb structure of each layer, not only the weak interlayer interaction. The graphite is a representative carbon material possessing the low frictional properties and is beneficial as a one of the solid lubricants (Ravindran et al., 2013).
Bio-friction and -lubrication have been studied for many years in addition to the friction research of hard solid materials (Jin et al., 2013). Many research groups have focused on friction of several biological systems such as eye (contact lenses, Rennie et al., 2005) and oral cavity (Ranc et al., 2006). Since biological systems originally possess low-frictional properties, it is beneficial to clarify the mechanisms of low friction of such systems for several applications such as biomimetic soft robotics. Hence, similar systems to biological lubrication methods is commonly focused and is paid attention.
Double network hydrogel (DN gel) is one of the representative materials for biomimetic applications because such material possesses biocompatible and robust properties (Gong et al., 2003). DN gel has inter-penetrating structure of two types of polymer networks. The first network is rigid and brittle, while the second one is soft and high ductile. The origin of robustness of DN gel lies on such inter-penetrating structure. So far, various kind of DN gel has been developed, such as thermo- and pH-responsive gels (Zhiqiang et al., 2013) and three-dimensional (3D) printable DN gels (Hidema, et al, 2011; Muroi, et al, 2013) because of such beneficial property.
Hydrogel friction has been studied since 1990s (Gong et al., 1997) by Gong and co-authors. In addition, the frictional properties of DN gel have been studied using experimental system of ball on plate (Wada et al., 2013; Wada et al., 2016; Wada et al., 2017). The friction force of hydrogel surface is expressed as 𝐹∝𝐴𝑃𝛼, where 𝐴 is the apparent contact area, 𝑃=𝑊/𝐴 is averaged pressure, and α (0≤𝛼≤1) is derived value from experiments (Gong et al., 1999). Hence, frictional properties depend on the apparent contact area (𝐴) in the case of hydrogels. In this way, Various gels have been studied in the last 10 to 20 years. They have been developed as useful materials in various scenes with high strength, and the attention is increasing more and more. The gel is building an excellent low friction system of a biomechanical joint. Its friction coefficient shows low friction of 0.003 - 0.01. Utilizing the low friction properties of these high strength gels, the range of application as materials is expanded.
In this study, we show the novel method of low-frictional processing of DN gel using laser and sandpaper, which increases the roughness of gel surface to reduce the 𝐴. This technique is expected to be applied to biomimetic devise (Green et al., 2016) using 3D gel printing technique.