Abstract
The dynamics of the human fingertip enable haptic sensing and the ability to manipulate objects in the environment. Here we describe a wearable strain sensor, associated electronics, and software to detect and interpret the kinematics of deformation in human fingernails. Differential forces exerted by fingertip pulp, rugged connections to the musculoskeletal system and physical contact with the free edge of the nail plate itself cause fingernail deformation. We quantify nail warpage on the order of microns in the longitudinal and lateral axes with a set of strain gauges attached to the nail. The wearable device transmits raw deformation data to an off-finger device for interpretation. Simple motions, gestures, finger-writing, grip strength, and activation time, as well as more complex idioms consisting of multiple grips, are identified and quantified. We demonstrate the use of this technology as a human-computer interface, clinical feature generator, and means to characterize workplace tasks.
Highlights
The ungula or nail plate is the visible part commonly thought of as the fingernail
When the fingertip pad was pressed against a test surface with normal force, the distal phalangeal bone pulled the center of the nail downwards, and the fingertip pulp moved around the distal phalangeal bone to push up against the nail at the lateral folds, deforming the edges upward
These results show that when a strain gauge is glued to the nail it is considerably stiffer and less responsive
Summary
The ungula or nail plate is the visible part commonly thought of as the fingernail. Three layers of cornified onychocytes make up the ungula. The anatomical structure and interactions between the nail plate, distal phalangeal bone, musculoskeletal attachments, and fingertip pulp causes nails to deform in complex but repeatable ways during unloaded movement and in interactions with objects. Humans use their fingers extensively in activities of daily living. The most comprehensive fingertip instrumentation effort was carried out by Reid et al.[18] in the context of extravehicular activity glove design for astronauts They studied blood perfusion, galvanic skin response, longitudinal and transverse fingernail strain and pressure relative to force plate and dynamometer data during a battery of static and dynamic tasks. This work produced curves relating fingernail strain in the longitudinal and transverse axes during fingertip compression using three nail-mounted strain gauges
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