Results and discussion (page 2)
Circuit layouts, guided by theory, can be identified to satisfy these requirements. As an example, figure 3(c)
provides a diagram of a multiplexed electrotactile array in a mesh configuration with narrow, serpentine interconnects. The orange and blue regions correspond to Au layers separated by layers of PI, respectively; the red regions indicate Si NM (300 nm thick) diodes in a PIN (p-doped/intrinsic/ndoped) configuration. The short dimensions of the diodes lie parallel to the flipping-over direction, to minimize the strains in the Si during this process. These optimizations lead to maximum calculated strains that are only 0.051 percent, 0.10 percent, and 0.040 percent for the Au, PI, and the Si, respectively (see figure 3(d)
and supplementary file available at. The computed position of the NMP also appears in figure 3(d)
. Since the moduli of the device layers are several orders of magnitude larger than that of Ecoflex, the location of the NMP plane is largely independent of the Ecoflex. Appropriate selection of the thicknesses of the PI layers allows the NMP to be positioned at the location of the Si NMs, thereby minimizing the induced strains in this brittle material [21, 22
]. The thicknesses of the Si NM diodes influence the maximum strains that they experience, as shown in the analytical calculations of figure 3(f)
. A minimum occurs at the thickness that places the NMP at the shortest distance from the Si NM diode (i.e. hNMP
). The position of this minimum can also be adjusted by changing the thicknesses of the PI layers, for example. Further reductions in strain can be realized by reducing the lengths of the devices. Implementation of designs that incorporate these considerations and together with the use of interconnects with optimized serpentine layouts ensure robust device behavior throughout the fabrication sequence. For example, figure 3(e)
shows negligible change in the I–V
characteristics (Agilent 4155C semiconductor parameter analyzer) of a Si NM diode before and after the flipping-over process.
The experimental results demonstrate the expected functionality in the electrotactile arrays. Figure 4(a)
shows the perception of touch on a dry human thumb as a function of voltage and frequency, applied between the inner dot and outer ring electrodes (figure 3(c))
. The stimulation used a monophasic, square wave with 20 percent duty cycle, generated using a custom setup. The inset provides an image of a device, with connection to external drive electronics via a flexible anisotropic conductive film (ACF). The required voltage for sensation decreases with increasing frequency, consistent with equivalent circuit models of skin impedance that involve resistors and capacitors connected in parallel. The absolute magnitudes of these voltages depend strongly on the skin hydration level, electrode design, and stimulation waveform [23
]. Figure 4(b)
characteristics of an electrotactile electrode pair while in contact with a hydrated human thumb, measured through a multiplexing diode. At high positive voltages, the resistance of the diode is negligible compared to the skin; here, the slope of the I–V
characteristics yield an estimate of the resistance of the skin–electrode contact plus the skin. The value (∼40 kΩ) is in a range consistent with measurements using conventional devices [24, 25
]. The diode is stable to at least 20 V, corresponding to currents of 0.25 mA, which is sufficient for electrotactile stimulation on the skin and tongue [2, 6, 7
These diodes enable multiplexed addressing, according to an approach that appears schematically in figure 4(c)
. Each unit cell consists of one diode and one electrotactile electrode pair. Figure 4(d)
presents a table of the inputs required to address each of the six electrotactile channels. For example, channel SDA
can be activating by applying a high potential (+5 V) to inputs A and E and a low potential (0 V) to inputs B, C, and D, thereby yielding a +5 V bias across the outer ring (+5 V) and inner ring electrodes (0 V) of this channel. This configuration forward biases the Si NM diode, which results in a stimulation current, as shown in figure 4(b).
Figure 4: Mechanics and electrical characteristics of a 2 x 3, multiplexed electrotactile array on a finger-tube.
(a) Voltage required for electrotactile sensation as a function of stimulation frequency; inset: electrotactile array on a human finger during experiments.
(b) I–V characteristics of multiplexed electrotactile electrodes in contact with a human thumb.
(c) Circuit diagram of the diode multiplexing scheme.
(d) Function table showing inputs for addressing each of the six channels (H D high; L D low).
At the same time, channels SEB and SEC experience a bias of -5 V across the electrodes but in these cases the Si NM diodes are reverse biased, thus preventing a stimulating current. Channels SDB, SDC, and SEA have the same potentials on the inner and outer electrodes, resulting in zero bias. Electrical isolation of adjacent channels is a consequence of inner to outer electrode separations (250 µm) that are small compared to the distances between channels (6000 µm). Advanced multiplexing schemes that use several diodes per stimulation channel, or active transistors, are compatible with the fabrication process and design principles outlined here.