Figure 1
Needle insertion into a body tissue. (a) Insertion through a single tissue layer Fin = insertion force, Fstiff = surface stiffness force, Ffric = friction force and Fcut = cutting force. (b) Insertion through two tissue layers. \({{\bf{F}}}_{{\rm{f}}{\rm{r}}{\rm{i}}{{\rm{c}}}_{1}}\) is the friction force of tissue layer 1, and \({{\bf{F}}}_{{\rm{s}}{\rm{t}}{\rm{i}}{\rm{f}}{{\rm{f}}}_{2}}\), \({{\bf{F}}}_{{\rm{f}}{\rm{r}}{\rm{i}}{{\rm{c}}}_{2}}\) and \({{\bf{F}}}_{{\rm{c}}{\rm{u}}{{\rm{t}}}_{2}}\) are the surface stiffness force, the friction force, and the cutting force of tissue layer 2, respectively. The axis (x) represents the depth of the needle inside the tissue. x = 0 is the start of the (first) tissue layer, x = d1 is the end of the (first) tissue layer and the start of the second tissue layer, and x = d2 is the end of the second tissue layer.
