This paper describes the sensor, electronics, software, modeling, and characterization of a fingernail-mounted RF-connected wearable strain sensor system that measures nail deformation from finger movement. Applications to health monitoring and human computer interfaces in homes, hospitals, and workplaces are discussed. The mechanical deformation of a fingertip pressed or drawn against a plate is demonstrated using a three-dimensional finite-element linear-elastic model to predict the signal level, optimum sensor locations and the type and location of deformation expected for different finger motions. The 3D finite-element linear elastic model is derived from X-ray images of a human finger but generalized and parameterized to allow new models to be created by scaling internal and external parameters such as skin thickness and nail and finger shape to predict sensor system performance for a more general human population. Our analysis finds that a single sensor mounted in the center of the nail will respond to typical grip pressures on the fingertip with readily detectible strain amplitudes but that a multi-sensor array will be sensitive to more general haptic phenomena such as the direction and magnitude of frictional loads and loading of the distal phalangeal joint. It is shown that depending on finger use and loading the nail exhibits shifts in direction, location and sign of strain over the fingernail surface. Measurement data from a simple multi-sensor array is shown to be useful in distinguishing between load conditions, however additional sensors are required for full determination.