**1.1.2 Electrophoretic display**

Electrophoretic system inherited some basic concepts from Gyricon. As depicted in Figure 2, electrophoretic system further reduced the beads' sizes and increased their density. The main difference between electrophoretic beads and Gyricon beads is there's only one color on one electrophoretic bead. This change makes electrophoretic system possible to show higher resolution and contrast. Furthermore, since beads' sizes (approximately 1m) are smaller than Gyricon's, gray scale display can be divided into finer details. Similar to Gyricon, an outside electric field plays the critical role on controlling the charged beads' movement in a liquid environment. But different from Gyricon, where beads have to rotate inside the liquid and stabilize after long time; the electrophoretic beads don't have to wait for stabilization since the beads are with only one color. This makes the electrophoretic system response faster than Gyricon. Likewise, electrophoretic system does not require any maintenance voltage after operation. Some main advantages and disadvantages of electrophoretic system are listed below:

#### Advantage


#### Disadvantage


Fig. 2. Concept of electrophoretic system.

Possibilities for Flexible MEMS:Take Display Systems as Examples 373

Gyricon and electrophoretic systems are using physical control by electric field outside on colored particles and electrowetting system is also using physical control but the main material contains only liquid. Here, the electrochromic system is using chemical concept to change material's charging condition in dielectric electrolyte in order to change its light absorption band as shown in Figure 5. The electrochromic material can be either dissolved in the electrolyte or coated on the electrode substrate. Typical materials in electrochromic system are WO3 and TiO2. WO3 had been reported with capability to change its colors between transparent under oxidation state and blue under reduction state; TiO2 had been reported with capability to change its colors between transparent under reduction state and white under oxidation state. Thus, using only one electrochromic material can realize a twocolor system and a combination of two electrochromic materials can realize a multiple color system. When the electrochromic material is thinly coated on the electrode, the whole structure will be bendable; when the electrochromic material is dissolved in electrolyte, the whole structure will also be bendable. Thus the electrochromic system can be used as a flexible color filtering device. Some main advantages and disadvantages of electrowetting

All previous described technologies are supposed to be applicable on flexible substrate since all Gyricon, electrophoretic, electrowetting, and electrochromic (if material dissolves in electrolyte) are using liquid as intermediate or main material. But this also implies a reliability concern under critical operation conditions such as: high/low temperature, vibration or shock, and gravity influence, not mention to the jeopardy when the whole

After reviewing these technologies from their basic operations, a summary can be made: Both physical and chemical concepts make the system slim and simple, thus the whole system can be fabricated on a thin substrate for flexible applications while reliability is a special concern. In contrary, stable, predictable, reliable, and reproducible mechanical

Fig. 4. Switching colors by controlling liquid's electrowetting performance.

**1.1.4 Electrochromic display** 

system are listed below:

1. Low response time,

3. No black color.

2. Low operation voltage,

1. Possible combinations for various colors,

3. Can be designed for either transmissive or reflective.

2. Color purity (depends on material's natural characteristic),

system is breaking and the chemical or electrolyte is leaking.

Advantage

Disadvantage

#### **1.1.3 Electrowetting display**

Compared to particle based Gyricon and electrophoretic display systems, electrowetting system uses all liquid material for color modification. This kind of concept takes advantage of Young-Lippmann's equation (Equation 1) to modify different surface energies between the droplet and the substrate underneath, which in turn changes the contact angle between the droplet and the substrate. Here, *γLG* is the surface tension between liquid and gas, *γSG* is the surface tension between solid and gas, *γSL* is the surface tension between solid and liquid, *θ* is the contact angle, *V* is the applied voltage, and *C* is the electric capacitance per unit area in the region of contact between a metal surface and the electrolyte drop.

$$
\gamma\_{\rm LG} \cos \theta = \gamma\_{\rm SG} - \gamma\_{\rm SL} + \frac{\rm CV}{2} \tag{1}
$$

According to Equation 1 and Figure 3, the applied voltage will change the droplet's contact angle. A smaller contact angle represents a larger droplet diameter while a larger contact angle represents a smaller droplet diameter. By following this concept, an electrowetting display system was designed: The intermediate liquid was dyed for different colors while the water was kept transparent. Under normal (OFF) condition, the intermediate color liquid is laying under transparent water thus the reflective light shows intermediate liquid's color. Under operation (ON) condition, the intermediate color liquid is pressed into a specific corner of a pixel, leaving the rest area only with transparent water. Thus the reflection light with background color appears. By switching this system ON and OFF, two different colors can be switched for display purpose as shown in Figure 4. Since electrowetting system is using all liquid material for color modification, it is supposed to be flexible for display application. Some main advantages and disadvantages of electrowetting system are listed below:

#### Advantage


Disadvantage


Fig. 3. Contact angle change by applied voltage is the basic of electrowetting.

Fig. 4. Switching colors by controlling liquid's electrowetting performance.
