A BIO-INSPIRED FINGERTIP: 3D Printed Surface Patterns and Their Role in Surface Friction: An Experimental Study

In the field of prostheses, significant developments have been accomplished so far in low-cost prosthetic limbs using 3D printing technology. However, when it comes to prosthetic hands, 3D printed prosthetic hands are still limited in their grasping ability, such as the adaptability to the shape of an object and a sufficient pinch force level for practical use. The goal of this experimental study is to engineer a bio-inspired surface structure to improve the grip action of prosthetic hands. The low-cost FDM 3D printing technology in combination with the flexible material, Thermoplastic Polyurethane (TPU) 95A, was evaluated for this purpose. 3D printed surface (deformable) patterns were printed on top of a flat, rigid surface. The 3D printed patterns consisted of pillars or lines with varying thickness <i>d</i>, tip thickness <i>D</i>, wavelength <i>λ</i>, and curvatures <i>α</i> that were combined into different patterns. The frictional characteristics of the 3D printed patterns were assessed for nine different test scenarios, i.e. three different loads <i>F_N</i> against three different countersurfaces. Despite the small differences in the static coefficient of friction <i>μ_s</i> of the 3D printed patterns, some consistent trends were found. First, <i>μ_s</i> increases with increasing thickness <i>d</i>. Second, <i>μ_s</i> increases with increasing wavelength <i>λ</i> up to a point in which the decrease of number density of the 3D printed features decreases the overall friction. Third, <i>μ_s</i> increases for pattern curvatures with peaks in the opposite direction, such as wave or circular patterns. Lastly, <i>μ_s</i> decreases under increasing normal load <i>F_N</i>. The surface patterns were tested on the fingertips of a 3D printed prosthetic hand. The fingertips were assessed using the Box and Blocks Test (BBT), in which the pattern with the highest score displayed an ~70% increase in the number of blocks moved, compared to the original rigid fingertip of the 3D printed prosthetic hand in question. Further research and development are essential, especially for the FMD 3D print process of small dimensional printing in combination with flexible materials. Nevertheless, the proposed fingertip pattern demonstrated a first step towards future improvements of the grip action of low-budget 3D printed prosthetic hands using soft fingertip patterns.<br/>