Contents I. Introduction
Fingertips are the primary agents of human interaction with the physical world. We use them extensively for tap-ping, grasping and probing the environment. They help us explore a rich variety of haptic information like textures, sliding and contact to shape (curvature), size and stiffness (hardness/softness). No wonder, they are the most sensitive region of the human hand [1]. The high density of sensitive mechanoreceptors in the fingertips gives them a spatial tactile resolution in the sub-millimeter range (in the absence of movement or applied vibrations) [2], [3]. The threshold for tiniest feature detection in dynamic touch (presence of movement and vibrations) can be as small as 300 nm [4], [5]. Furthermore, fingertips can also sense a large range of forces (both normal and shear) [6]. In addition, coarse features (e.g., a braille cell) are encoded with spatially distributed signals, while fine textures (e.g., fine-grit sandpaper) are encoded by temporal frequencies [7].
