A novel approach to measuring the overall and layer-by-layer thickness of in vivo skin tissue based on near infrared absorbance spectra is described. The spring contact structure is configured for releasing electrostatic charges of the electronic device, and includes a fixing portion, a clamping portion and a grounding portion. The wheels may be separated by a wheel distance when the crossbow is in an un-cocked position. Similarly, user profile information for a document may be determined by determining initial user profile information for the document, inferring user profile information for the document, and determining the user profile information for the document using both the initial user profile information and the inferred user profile information. Both ends of a complete rolling path and both ends of a complete loaded path are connected to both ends of two complete return paths to complete circulation paths.