Design Article
Silicon nanomembranes enable fingertip electronics
9/28/2012 1:00 PM EDT
Results and discussion (Page 3)
Figure 5 shows a set of straight, uniformly doped Si NMs as strain gauges addressed with interconnects in a mesh geometry. The FEM calculations summarized in figure 5 reveal the strain profiles in a 1 x 4 array of gauges (vertical strips; the yellow dashed box in the upper inset highlights an individual device) on Ecoflex, under a uniaxial in-plane strain of 10 percent. These results show that the overall strain is mostly accommodated by changes in the shapes of the serpentine interconnects and, of course, the Ecoflex itself. The Si NM gauges experience strains (about 10-3) that are ten times lower than the applied strain, as shown in the inset in figure 5(a).
The ability to use Si NMs as high performance strain gauges in stretchable forms results from the strong piezoresistance properties of Si, combined with serpentine layouts. These characteristics, taken together, determine the fractional change in resistance per applied strain. The associated effective gauge factor (GFeff) can be related to the intrinsic gauge factor of a silicon gauge, GFSi = ΔR (RεSi), where ΔR is the change in resistance, R is the initial resistance, and εSi is the strain in the silicon, by the following expression: GFeff = GFSi (εSi/εapp), where εapp is the strain applied to the overall, integrated system.
The designs reported here yield values of εSi/εapp that are much smaller than one, specifically to avoid fracture-inducing strains in the Si during fabrication, mounting and use over physiologically relevant ranges of strain. Figure 5(b) shows experimentally measured values of ΔR/R (evaluation at 1 V, using an Agilent 4155C semiconductor parameter analyzer) as a function of εapp, which corresponds to GFeff ∼1. By fitting the experimental and FEM results to figure 5(b), the GFSi is ∼95, consistent with a recent report on Si NM strain gauges, with otherwise similar designs, on flexible sheets of plastic [26]. We emphasize that the device parameters, such as the size of the gauge and the dimensions of the serpentine interconnects, enable engineering control over GFeff, from values as large as GFSi to those that are much smaller, with a correspondingly increased range of strains over which measurements are possible.
Figure 5(c) shows a strain gauge array on a finger-tube located near the knuckle region of the thumb, in straight (I) and bent (II) positions. Upon bending, the gauges experience tensile strain, resulting in an increase in resistance, as shown for three bending cycles in figure 5(d). The relative resistance changes suggest that the strain associated with bending reaches ∼6 percent. As expected, side-to-side motions induce no changes. Figure 5(e) highlights a similar array on a thin sheet of Ecoflex, mounted near the metacarpal region of the thumb. Here, the device adheres to the skin by van der Waals interactions, similar to the mechanisms observed in epidermal electronic systems [13]. The images in figure 5(e) correspond to the thumb in straight (III) and sideways deflected (VI) positions. The changes in resistance for the two gauges on opposite ends of the 1 x 4 array for three side-to-side cycles of motion appear in figure 5(f). For each cycle, the change in resistance of the rightmost gauge indicates compressive strain; the leftmost indicates the corresponding tensile strain. The results suggest that arrays of gauges can be used to identify not only the magnitude but also the type of motion.
Figure 5 shows a set of straight, uniformly doped Si NMs as strain gauges addressed with interconnects in a mesh geometry. The FEM calculations summarized in figure 5 reveal the strain profiles in a 1 x 4 array of gauges (vertical strips; the yellow dashed box in the upper inset highlights an individual device) on Ecoflex, under a uniaxial in-plane strain of 10 percent. These results show that the overall strain is mostly accommodated by changes in the shapes of the serpentine interconnects and, of course, the Ecoflex itself. The Si NM gauges experience strains (about 10-3) that are ten times lower than the applied strain, as shown in the inset in figure 5(a).
Figure 5: Detection of finger motion with arrays of stretchable Si NM strain gauges.
(a) FEM results of the maximum principal strain for a 1 x 4 array of gauges (straight, vertical structures near the top of the serpentine interconnect mesh) due to an overall 10 percent strain applied along the longitudinal (y) direction. The upper inset shows the strains in the gauge highlighted by the yellow dashed box. The lower inset provides an image of a fabricated device with a layout that matches that of the FEM results. (b) Experimentally measured and analytically calculated changes in resistance for a representative Si NM strain gauge as a function of applied strain along the longitudinal direction. The inset provides an SEM image of a portion of the device, with the Si NM gauge located in the dashed box.
(c) Images of a strain gauge array on a finger-tube mounted on the thumb, in straight (I) and bent (II) positions.
(d) Change in resistance of a representative gauge during three bending cycles (black) and side-to-side motion (red).
(e) Images of a strain gauge array on a thin, elastomeric sheet laminated onto the metacarpal region of the thumb in straight (III) and sideways deflected (IV) positions.
(f) Change in resistance of gauges at two ends of the array during three cycles of side-to-side motion.
The ability to use Si NMs as high performance strain gauges in stretchable forms results from the strong piezoresistance properties of Si, combined with serpentine layouts. These characteristics, taken together, determine the fractional change in resistance per applied strain. The associated effective gauge factor (GFeff) can be related to the intrinsic gauge factor of a silicon gauge, GFSi = ΔR (RεSi), where ΔR is the change in resistance, R is the initial resistance, and εSi is the strain in the silicon, by the following expression: GFeff = GFSi (εSi/εapp), where εapp is the strain applied to the overall, integrated system.
The designs reported here yield values of εSi/εapp that are much smaller than one, specifically to avoid fracture-inducing strains in the Si during fabrication, mounting and use over physiologically relevant ranges of strain. Figure 5(b) shows experimentally measured values of ΔR/R (evaluation at 1 V, using an Agilent 4155C semiconductor parameter analyzer) as a function of εapp, which corresponds to GFeff ∼1. By fitting the experimental and FEM results to figure 5(b), the GFSi is ∼95, consistent with a recent report on Si NM strain gauges, with otherwise similar designs, on flexible sheets of plastic [26]. We emphasize that the device parameters, such as the size of the gauge and the dimensions of the serpentine interconnects, enable engineering control over GFeff, from values as large as GFSi to those that are much smaller, with a correspondingly increased range of strains over which measurements are possible.
Figure 5(c) shows a strain gauge array on a finger-tube located near the knuckle region of the thumb, in straight (I) and bent (II) positions. Upon bending, the gauges experience tensile strain, resulting in an increase in resistance, as shown for three bending cycles in figure 5(d). The relative resistance changes suggest that the strain associated with bending reaches ∼6 percent. As expected, side-to-side motions induce no changes. Figure 5(e) highlights a similar array on a thin sheet of Ecoflex, mounted near the metacarpal region of the thumb. Here, the device adheres to the skin by van der Waals interactions, similar to the mechanisms observed in epidermal electronic systems [13]. The images in figure 5(e) correspond to the thumb in straight (III) and sideways deflected (VI) positions. The changes in resistance for the two gauges on opposite ends of the 1 x 4 array for three side-to-side cycles of motion appear in figure 5(f). For each cycle, the change in resistance of the rightmost gauge indicates compressive strain; the leftmost indicates the corresponding tensile strain. The results suggest that arrays of gauges can be used to identify not only the magnitude but also the type of motion.
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