**4. Dielectric elastomer generators with integrated soft electronics**

The DEG membrane is essentially a soft capacitor and one can take advantage of this, using the DEG to provide the energy storage function of the SPC. This means the generator can be fabricated with its external circuitry consisting solely of a few diodes [16, 20]. One configuration that has been used to integrate the SPC storage function into the DEG membrane is given in Figure 4 where DEG1 is electrically configured as an SPC [16]. Because the generator's elastomer membranes are integrated into the self-priming circuit, this system has been referred to as the integrated self-priming circuit [21]. The generator was configured into two DEG membranes which were deformed so that as one membrane was stretched the other was relaxed. When the voltage of DEG1A and DEG1B connected electrically in parallel exceeded the voltage of DEG2, energy was transferred from the SPC to DEG 2 through the path shown in Figure 4c; when the voltage of DEG2 exceeded that of DEG1A and DEG1B connected in series, energy was transferred onto the SPC from DEG2 through the path shown in Figure 4d.

176 Smart Actuation and Sensing Systems – Recent Advances and Future Challenges

eliminates the need for these typically high cost components.

stored on the generator and priming circuit (From [19]).

a generator that can be fabricated entirely from soft elastomers.

traditionally been supplied by a high voltage power supply or converter, so the SPC

**Figure 3.** The output voltage waveform of a self-priming generator mechanically deformed at 3 Hz. The voltage climbs from cycle to cycle because the generated energy accumulates in the form of extra charge

Although the SPC is low cost, low complexity, and autonomous it still adds considerable mass and stiffness to the DEG system. The SPC consists of diodes and capacitors. The function of the capacitors is to store priming charges and the diodes control the transfer of charge to and from the DEG, so that an appropriate generation cycle is achieved. We will now discuss how these functions have been integrated onto the DEG membrane to produce

The DEG membrane is essentially a soft capacitor and one can take advantage of this, using the DEG to provide the energy storage function of the SPC. This means the generator can be fabricated with its external circuitry consisting solely of a few diodes [16, 20]. One configuration that has been used to integrate the SPC storage function into the DEG membrane is given in Figure 4 where DEG1 is electrically configured as an SPC [16]. Because the generator's elastomer membranes are integrated into the self-priming circuit, this system has been referred to as the integrated self-priming circuit [21]. The generator was configured into two DEG membranes which were deformed so that as one membrane was stretched the other was relaxed. When the voltage of DEG1A and DEG1B connected electrically in parallel exceeded the voltage of DEG2, energy was transferred from the SPC to DEG 2 through the path shown in Figure 4c; when the voltage of DEG2 exceeded that of

**4. Dielectric elastomer generators with integrated soft electronics** 

**Figure 4.** A segmented generator which consists of two membranes that are antagonistically deformed when the inner hub displaces up and down (a). The top membrane DEG1 can be electrically interconnected with diodes to form a self-priming circuit (b). The paths along which current flows off (c) and onto (d) the SPC are also illustrated.

For a small scale portable and potentially wearable generator the diodes of the SPC can represent a significant mass. For instance, the prototype of the generator shown in Figure 4 consisted of DEG membranes with a combined mass of 0.35 grams, whereas the diodes weighed 0.63 grams. If we could remove the diodes, 64% of the total mass (ignoring the mass of the frame) could be eliminated.

The function of the diodes is to control the charge transfer between the DEG and SPC. The diodes simply allow current to flow along one path when one diaphragm is stretched, and along an alternative path when that diaphragm is relaxed. This means that the diodes can be replaced by switches that are toggled at the appropriate time. Since this timing is dependent on the material stretch state, the diodes can be replaced by stretch-sensitive switches coupled to the DEG.

Stretch sensitive electronics called Dielectric Elastomer Switches (DES) have been used to integrate the functionality of the diodes into the DEG membrane. DES consist of piezoresistive electrodes fabricated directly onto a highly stretchable dielectric elastomer membrane. They exhibit very large changes in resistance with stretch. O'Brien et al. first presented the concept [22] and characterized M-shaped DES as illustrated in Figure 5 [23]. These DES had a resistance of several MΩ in their rest state, which increased to several GΩ when they were stretched to approximately 1.4 times their original length (see Figure 5b) [23].

**Figure 5.** Carbon powder-based DES applied to a dielectric elastomer diaphragm (black "M" and "V" shaped tracks) for characterisation by O'Brien et al. (a), and a plot of measured DES resistance versus approximate radial stretch ratio showing that the resistance climbed several orders of magnitude when the DES were stretched (b). Images taken from O'Brien [23].

The generator illustrated in Figure 4 was redesigned by placing DES onto the membrane to replace the hard diodes as illustrated in Figures 6 and 7. The SPC capacitors were fabricated onto diaphragm 1. When diaphragm 1 was relaxed the switches Q1 and Q2 were also relaxed and therefore conducted, configuring the SPC capacitors into a parallel topology (high charge form). When diaphragm 1 was stretched so too were Q1 and Q2, so they no longer conducted, but diaphragm 2 was simultaneously relaxed, causing Q3 to conduct, connecting the SPC capacitors in series. This generator can be fabricated entirely from soft elastomers, so we refer to it as the soft generator.

The output voltage of a prototype soft generator is given in Figure 8. When compared to the output of the self-priming circuit given in Figure 3, the soft generator's voltage climbs very rapidly. But the most profound advantages are the ability to produce DEG that maintain the advantages of high flexibility, softness, low volume, low cost, low component count, and low mass at the system level.

178 Smart Actuation and Sensing Systems – Recent Advances and Future Challenges

[23].

Stretch sensitive electronics called Dielectric Elastomer Switches (DES) have been used to integrate the functionality of the diodes into the DEG membrane. DES consist of piezoresistive electrodes fabricated directly onto a highly stretchable dielectric elastomer membrane. They exhibit very large changes in resistance with stretch. O'Brien et al. first presented the concept [22] and characterized M-shaped DES as illustrated in Figure 5 [23]. These DES had a resistance of several MΩ in their rest state, which increased to several GΩ when they were stretched to approximately 1.4 times their original length (see Figure 5b)

**Figure 5.** Carbon powder-based DES applied to a dielectric elastomer diaphragm (black "M" and "V" shaped tracks) for characterisation by O'Brien et al. (a), and a plot of measured DES resistance versus approximate radial stretch ratio showing that the resistance climbed several orders of magnitude when

The generator illustrated in Figure 4 was redesigned by placing DES onto the membrane to replace the hard diodes as illustrated in Figures 6 and 7. The SPC capacitors were fabricated onto diaphragm 1. When diaphragm 1 was relaxed the switches Q1 and Q2 were also relaxed and therefore conducted, configuring the SPC capacitors into a parallel topology (high charge form). When diaphragm 1 was stretched so too were Q1 and Q2, so they no longer conducted, but diaphragm 2 was simultaneously relaxed, causing Q3 to conduct, connecting the SPC capacitors in series. This generator can be fabricated entirely from soft

The output voltage of a prototype soft generator is given in Figure 8. When compared to the output of the self-priming circuit given in Figure 3, the soft generator's voltage climbs very rapidly. But the most profound advantages are the ability to produce DEG that maintain the

the DES were stretched (b). Images taken from O'Brien [23].

elastomers, so we refer to it as the soft generator.

**Figure 6.** A schematic of the soft generator. The switches Q1-Q3 control the charge flow within the selfpriming circuit in a similar manner to the diodes in the integrated self-priming circuit. The two diaphragms in (a) are connected together to form the antagonistic pair shown in (b). (From [24]).

**Figure 7.** A photograph of a soft generator prototype. The large black regions are the generator electrodes and the thin "M-shaped" electrodes are the piezo-resistive switches.

**Figure 8.** The output voltage waveform of a soft generator initially primed to 10 Volts then mechanically deformed at 3 Hz showing that the generator rapidly boosts its operating voltage through accumulation of generated energy (From [24]).

The total mass of biomechanical energy harvesters has a great influence on the metabolic cost of their use and can be easily compared in a quantitative manner. In Figure 9 we compare the energy density of each DEG system described in this chapter with the mass of their associated electronics included in the calculations (see equation 5).

$$EnergyDensity = \frac{EnergyGenerated}{Mass\_{dielectric} + Mass\_{External Circuit}} \tag{5}$$

The energy and energy densities produced by similarly sized DEG membranes (~0.3 grams) mechanically cycled at a rate of 3 Hz and operating at 2 kV are compared in Figure 9. The soft generator energy density was superior to both the integrated and external SPC generators because their respective external circuit masses are approximately 0 grams, 0.6 grams, and 13.4 grams. The soft generator's energy density of 9.5 mJ/g is highly competitive with the predicted practical maxima of electromagnetics (4 mJ/g) and piezoelectrics (17.7 mJ/g) at the ~1cm3 scale [25], demonstrating the utility of DEG for small-scale energy harvesters.

The recent developments discussed in this chapter have provided progress towards wearable, soft power generators becoming a reality, but there are still issues that need to be addressed:

1. The switching technology used in the soft generator, DES, are in their infancy. Material and process developments are required to create more reliable DES with resistances that can be tuned to their application.

**Figure 8.** The output voltage waveform of a soft generator initially primed to 10 Volts then

their associated electronics included in the calculations (see equation 5).

accumulation of generated energy (From [24]).

that can be tuned to their application.

harvesters.

addressed:

mechanically deformed at 3 Hz showing that the generator rapidly boosts its operating voltage through

The total mass of biomechanical energy harvesters has a great influence on the metabolic cost of their use and can be easily compared in a quantitative manner. In Figure 9 we compare the energy density of each DEG system described in this chapter with the mass of

The energy and energy densities produced by similarly sized DEG membranes (~0.3 grams) mechanically cycled at a rate of 3 Hz and operating at 2 kV are compared in Figure 9. The soft generator energy density was superior to both the integrated and external SPC generators because their respective external circuit masses are approximately 0 grams, 0.6 grams, and 13.4 grams. The soft generator's energy density of 9.5 mJ/g is highly competitive with the predicted practical maxima of electromagnetics (4 mJ/g) and piezoelectrics (17.7 mJ/g) at the ~1cm3 scale [25], demonstrating the utility of DEG for small-scale energy

The recent developments discussed in this chapter have provided progress towards wearable, soft power generators becoming a reality, but there are still issues that need to be

1. The switching technology used in the soft generator, DES, are in their infancy. Material and process developments are required to create more reliable DES with resistances

*EnergyGenerated EnergyDensity Mass Mass* <sup>=</sup> <sup>+</sup>

*dielectric ExternalCircuit*

(5)

**Figure 9.** The energy and energy density produced by the external SPC, integrated SPC, and soft generators.


With these challenges comes an exciting future: the emergence of smart, wearable soft devices. The DEG membranes are multifunctional; they can be operated as actuators or generators and simultaneously sense strain [12]. Furthermore DES have been used to fabricate rubber-based NAND-gates and memory elements, the two primitives required to build a computer [30-32]. Perhaps the future will not only include soft generators to power portable electronic devices, but it may also include a new breed of smart, multifunctional self-powered portable soft devices. These possibilities open the door for smart devices to be integrated directly into clothing. So soft wearable generators will not only revolutionize the use of today's portable devices, but they will be an integral part of a distributed body-area

network of sensors and smart devices that will improve future prosthetic devices, sports monitors, and video gaming interfaces.
