**5.3. Flexible capacitive sensor**

Highly sensitive pressure sensors are expected to be applied in humanoid robots and medical instruments to detect tactile sensation, which would enable safe physical interaction with the environment, including human contact. MEMS-based capacitive sensors that have simple structures composed of electrodes and a dielectric component have been widely studied, due to good compatibility with MEMS fabrication technologies. Capacitive sensors require not only high sensitivity, but also flexibility to detect the pressure applied to curved surfaces. Siliconbased MEMS capacitive sensors have been developed; however, silicon is brittle, which makes it difficult for the sensors to conform to a curved surface. Therefore, polymer-based flexible sensors have been proposed and demonstrated. Polymer-based flexible sensors are typically used to maintain flexibility with air as the dielectric; however, air has a relatively low dielectric constant. A solid dielectric may enhance the sensitivity, but impairs the flexibility of the sensor. Therefore, a polymer-based capacitive sensor that uses a dielectric liquid has been proposed, as depicted in Figure 8 [23]. DI water and glycerin have high relative dielectric constants of approximately 80.4 and 47; therefore, the proposed sensor with such liquids can have high sensitivity while maintaining flexibility. The capacitance of the electrodes increases when pressure is applied to the device. PDMS is used as a structural material in this device. An escape reservoir is designed to allow an incompressible liquid, such as DI water and glycerin, to move from the cavity between the electrodes when pressure is applied to the sensor, which allows the flexible sensors to deform and vary the capacitance. The proposed microsensor has been fabricated, and both high sensitivity and flexibility have been experimentally demonstrated.

### **5.4. Dye-sensitized photovoltaic cell**

Dye-sensitized photovoltaic cells are currently attracting widespread scientific and techno‐ logical interest as a high efficiency, low-cost, and transparent alternative to inorganic solar cells. Figure 9 shows a schematic illustration of the structure and operation principle of the dye-sensitized photovoltaic device. The cell consists of two electrodes and an encapsulated liquid electrolyte that contains iodide and triiodide ions. The cathode is a highly porous nanocrystalline semi-conductive titanium dioxide (TiO2) layer, in many cases consisting of TiO2 nanoparticles, deposited on a transparent electrically conductive glass. TiO2 absorbs only UV light; therefore, dye is adsorbed onto the TiO2 layer to utilize the light with a wider range of wavelength. The counter electrode (anode) is a transparent electrically conductive glass with a platinum catalyst. The device is transparent and is colored according to the dye employed.

The HDAM shown in Figure 7(b) was combined with piezoelectric actuators and applied to develop large-displacement MEMS-actuators, with a particular aim to application in MEMSbased tactile displays [21,22,40]. When applied to a vibrational Braille code display, it was experimentally verified that the large-displacement HDAM could display Braille codes more efficiently than a static display. This is because both fast and slow adaptive tactile receptors could be used to detect the displayed patterns when individual cells were vibrated at several tens of hertz [40]. When the actuation of the large displacement MEMS actuators was controlled both spatiallyandtemporary,differentsurfacetextures,suchasroughandsmooth,couldbedisplayed.

52 Advances in Micro/Nano Electromechanical Systems and Fabrication Technologies

Highly sensitive pressure sensors are expected to be applied in humanoid robots and medical instruments to detect tactile sensation, which would enable safe physical interaction with the environment, including human contact. MEMS-based capacitive sensors that have simple structures composed of electrodes and a dielectric component have been widely studied, due to good compatibility with MEMS fabrication technologies. Capacitive sensors require not only high sensitivity, but also flexibility to detect the pressure applied to curved surfaces. Siliconbased MEMS capacitive sensors have been developed; however, silicon is brittle, which makes it difficult for the sensors to conform to a curved surface. Therefore, polymer-based flexible sensors have been proposed and demonstrated. Polymer-based flexible sensors are typically used to maintain flexibility with air as the dielectric; however, air has a relatively low dielectric constant. A solid dielectric may enhance the sensitivity, but impairs the flexibility of the sensor. Therefore, a polymer-based capacitive sensor that uses a dielectric liquid has been proposed, as depicted in Figure 8 [23]. DI water and glycerin have high relative dielectric constants of approximately 80.4 and 47; therefore, the proposed sensor with such liquids can have high sensitivity while maintaining flexibility. The capacitance of the electrodes increases when pressure is applied to the device. PDMS is used as a structural material in this device. An escape reservoir is designed to allow an incompressible liquid, such as DI water and glycerin, to move from the cavity between the electrodes when pressure is applied to the sensor, which allows the flexible sensors to deform and vary the capacitance. The proposed microsensor has been fabricated, and both high sensitivity and flexibility have been experimentally demonstrated.

Dye-sensitized photovoltaic cells are currently attracting widespread scientific and techno‐ logical interest as a high efficiency, low-cost, and transparent alternative to inorganic solar cells. Figure 9 shows a schematic illustration of the structure and operation principle of the dye-sensitized photovoltaic device. The cell consists of two electrodes and an encapsulated liquid electrolyte that contains iodide and triiodide ions. The cathode is a highly porous nanocrystalline semi-conductive titanium dioxide (TiO2) layer, in many cases consisting of TiO2 nanoparticles, deposited on a transparent electrically conductive glass. TiO2 absorbs only UV light; therefore, dye is adsorbed onto the TiO2 layer to utilize the light with a wider range of wavelength. The counter electrode (anode) is a transparent electrically conductive glass with a platinum catalyst. The device is transparent and is colored according to the dye employed.

**5.3. Flexible capacitive sensor**

**5.4. Dye-sensitized photovoltaic cell**

**Figure 8.** a) Cross-sectional view of a capacitive sensor with encapsulated liquid dielectric to enhance the sensitivity while maintaining flexibility. (b) Fabricated capacitive sensor.

When light passes through the electrically conductive transparent glass electrode, the dye molecules are excited and transfer an electron to the semiconducting TiO2 layer via electron injection. The electron is then transported through the TiO2 layer and collected by the con‐ ductive layer on the glass. The mediator (I-/I3-) undergoes oxidation and regeneration in the electrolyte. Electrons lost by the dye molecules to the TiO2 layer are replaced by electrons from the iodide and triiodide ions in the electrolyte, thereby generating iodine or triiodide, which in turn obtains electrons at the counter electrode, culminating in a current flow through the external electrical load. This is the mechanism for the conversion of light energy received by the device to electricity [41]. This device has an interesting feature in that it reacts strongly to light that enters through the TiO2 layer.

The dye-sensitized photovoltaic cell has been conventionally studied as a solar cell, where miniaturization was not considered. However, when the cells are microfabricated and arrayed, they can be used as a transparent optical sensor. Shigeoka et al. proposed to microfabricate a transparent optical sensor on eyeglasses, which could detect the pupil position by detecting reflection from the eye, as shown in Figure 10 [42]. The light reflected from the pupil is considered to be smaller than that from the white. The sensor reacts strongly to light from the TiO2 electrode side, i.e., when the TiO2 layer electrode is faced towards the eyes, it detects only the light reflected from the pupil and white of the eye, without being affected by the light incident on the device from the environment.

Figure 11 shows a schematic of the processes used for fabrication of this device. The most critical part is encapsulation of the electrolyte. The conductive layer (ITO) is firstly patterned on the glass substrate using photolithography. TiO2 nanoparticles are patterned on the cathode using a lift off process. The device is subsequently annealed in air at 450 °C for 60 min and then dipped in a ruthenium-containing dye solution for 60 min to ensure the dye is adsorbed onto the TiO2 nanoparticles. The two glass substrates are bonded via a hot melt film and application of 600-800 kPa at 100 °C. Lastly, the liquid electrolyte is flowed from the inlet hole into a channel between the two electrodes, and then the inlet and outlet holes are covered by end seals. The dyes used are UV sensitive; therefore, BiLT was not applicable to this liquid encapsulation process, and the fill and seal approach was used instead. However, the filling rate of the electrolyte was quite high and no interfusion of air between the electrodes was observed. An array of the dye-sensitized photovoltaic devices successfully detected the pupil position. The line-of-sight (LOS) was successfully deduced [42, 43] from the obtained pupil position and the front image of the subject, acquired using a CCD camera on the eyeglasses.

**Figure 9.** Structure and operation principle of the dye-sensitized photovoltaic device.

Liquid Encapsulation Technology for Microelectromechanical Systems http://dx.doi.org/10.5772/55514 55

considered to be smaller than that from the white. The sensor reacts strongly to light from the TiO2 electrode side, i.e., when the TiO2 layer electrode is faced towards the eyes, it detects only the light reflected from the pupil and white of the eye, without being affected by the light

Figure 11 shows a schematic of the processes used for fabrication of this device. The most critical part is encapsulation of the electrolyte. The conductive layer (ITO) is firstly patterned on the glass substrate using photolithography. TiO2 nanoparticles are patterned on the cathode using a lift off process. The device is subsequently annealed in air at 450 °C for 60 min and then dipped in a ruthenium-containing dye solution for 60 min to ensure the dye is adsorbed onto the TiO2 nanoparticles. The two glass substrates are bonded via a hot melt film and application of 600-800 kPa at 100 °C. Lastly, the liquid electrolyte is flowed from the inlet hole into a channel between the two electrodes, and then the inlet and outlet holes are covered by end seals. The dyes used are UV sensitive; therefore, BiLT was not applicable to this liquid encapsulation process, and the fill and seal approach was used instead. However, the filling rate of the electrolyte was quite high and no interfusion of air between the electrodes was observed. An array of the dye-sensitized photovoltaic devices successfully detected the pupil position. The line-of-sight (LOS) was successfully deduced [42, 43] from the obtained pupil position and the front image of the subject, acquired using a CCD camera on the eyeglasses.

incident on the device from the environment.

54 Advances in Micro/Nano Electromechanical Systems and Fabrication Technologies

**Figure 9.** Structure and operation principle of the dye-sensitized photovoltaic device.

**Figure 10.** a) Array of dye sensitized photovoltaic cells patterned onto eyeglasses to detect the pupil position. The electrolyte was encapsulated between the electrodes. (b) Photograph of the sensor when worn by a subject.

**Figure 11.** Fabrication process to produce an array of dye-sensitized photovoltaic devices. The fill and seal approach was employed to encapsulate the electrolyte (g).
