A method of monitoring the concentration of an analyte in a host or portion thereof over a given period of time. The thin first contact charging layer includes a first material that has a first rating on a triboelectric series. In another embodiment, a quarter-wave matching layer which forms another mass component and a second passive compliant member component, is interposed between the head mass and the second end. The bend part surrounds at least two adjacent lateral sides of the bottom of the key cap to constitute a support plane.