3. Results and discussion
The device designs described previously have the advantage that they are conformal to the finger, in a way that naturally presses the electronics on the interior surface of the fingertube (in this case the electrotactile stimulating electrodes) into intimate contact with the skin. The flipping-over process represents a critical step, enabled by careful design of the mechanics in the device mesh. Quantitative mechanics modeling provides important insights. The finger-tube can be approximated as a self-equilibrated, axisymmetric tube with two-dimensional symmetry. Energy minimization using linear elastic shell theory determines the resulting shapes.
Figure 3: Mechanics modeling of the ‘flipping-over’ process and application to arrays of electrotactile stimulators multiplexed with Si NM diodes.
(a) Calculated (analytical and FEM) profiles of an Ecoflex finger-tube during bending associated with flipping the tube inside out, showing the relationship between the radius (Rradial = 7.5 mm) of the tube and the minimum bending radius (Raxial).
(b) FEM results for maximum strains on the inner and outer surfaces during this process.
(c) Schematic illustration of a multiplexed electrotactile array with serpentine mesh interconnects, with a magnified diagram (right top) and an image (right bottom) of a PIN Si NM diode (after flipping over).
(d) Schematic cross sectional illustrations of two regions of the device, with the position of the NMP indicated by a dashed red line, and analytical results for the maximum strains during the flipping-over process.
(e) I–V characteristics of a Si NM diode before and after flipping over.
(f) Maximum strain in the Si NM diode and hNMP (the offset between the neutral mechanical plane and the lower surface of the Si NM) as a function of the thickness of the Si NM.
shows analytical and FEM results for an Ecoflex cylinder with a radius (Rradial
) of 7.5 mm and a thickness of 500 µm when bent back on itself, at a midway point during the flipping-over process. The minimum axial radius of curvature (Raxial
) of 596 µm, as indicated in figure 3(a)
, defines the location of maximum induced strain as the tube is flipped over.
The maximum strains on the inner and outer surfaces in this configuration, as shown in the color map of figure 3(b)
, are ∼ 30–40 percent. The device mesh structures must, therefore, be able to accommodate strains in this range. This requirement is non-trivial for systems like the ones described here, due to their incorporation of brittle materials such as silicon (fracture strain ∼1 percent).