The contact force of a finger has been measured using various methods for applications in biomechanics and user interface, due to its importance for object manipulation and body motion. However, in most of the previous methods, the motion of the user is restricted in a limited space due to the sensing device structure. This research focused on the changes in blood flow due to deformation of blood vessels caused by mechanical stress on the body. Furthermore, this report proposes a novel method to estimate the contact force of a fingertip using a photoplethysmogram (PPG) measured at the proximal part of the finger. The proposed method detects non-pulsatile and pulsatile components of PPG signals, extracts feature values related to the contact force, and estimates the contact force at the fingertip by multiple regression. In the validation experiments, the participants wore a PPG detector at the proximal part of the index finger and applied a contact force. Then, the relationship between the contact force and PPG feature values was investigated. The estimated error was found to be 15% of the maximum contact force for passive touch. Furthermore, the experiment indicated that the contact force could be estimated for active touch regardless of the finger angle.

One of the most distinctive features of transthoracic admittance (or impedance) cardiography is its capability of non-invasive, continuous and convenient monitoring of stroke volume (SV) or cardiac output (CO) with a very compact design, which allows ambulatory measurement. Although accurate calibration of the time derivative of the admittance (or impedance) signals is required, no appropriate and practical method is available. In the present study, we developed a new electrical thoracic simulator using light-emitting diodes and photoresistors, which is capable of generating a wide range of admittance signals, and proposed a standardized calibration procedure optimized for dynamic calibration using this simulator. We applied the proposed method to a previously developed admittance cardiograph and derived the calibration formula. We then compared this formula with that obtained by conventional method based on calculation using circuit constants. The formulae obtained by the two calibration methods were significantly different. Considering the errors in the actual circuit elements, we recommend using the proposed dynamic calibration method with the novel simulator, rather than the conventional method, to obtain an accurate calibration formula for an admittance (impedance) cardiograph.