**2.3. Characterization**

In this stage, the ear prosthesis was characterized by means of applying different stimuli such as pressure, heat, and cold (imitating human skin receptors) and also acoustic waves and light, that is, temperature, sound, pressure, and light (TSPL) stimuli, in order to register different responses of the PVDF prosthesis as a multisensory unit. **Figure 1** shows the general diagram of the experiments that are mentioned above.

**Figure 1.** Diagram of different applied stimuli TSPL to the PVDF prosthesis.

#### *2.3.1. Hysteresis loop*

In order to measure ferroelectric hysteresis loop, the Sawyer–Tower circuit was implemented for this study (**Figure 2**). By measuring voltage (*VL*) across a capacitor (*CL* = 0.15 μF) in series with the PVDF prosthesis (*CF*), the charge on the ferroelectric can be determined since *QF* = *CL* × *VL*. A sine wave was applied to the circuit with a function generator, Rigol model DG4062; X channel was measured from an X–Y trace using an oscilloscope, Tektronix model MSO 3014.

**Figure 2.** Measure of ferroelectric hysteresis loop using the Sawyer–Tower circuit.

#### *2.3.2. Acoustic response*

In this experiment, a sound source was used as a sound emitter (a) excited by an audio generator at 60 Hz. As the receiver, it was used a face of the PVDF prosthesis which was metalized (mirror finish) (b) by in situ thermal evaporation of aluminum in a vacuum chamber at 10−4 mmHg. A sound source was situated 5 mm from the speaker; at the same time, it was reflected a red laser beam of helium–neon at 633 nm, JDSU model 1145AP (c) which is detected by a light detector, light-to-frequency converter TSL230 from Texas Instruments (d); thus obtaining two electric signals, one from PVDF and another from the light detector in the oscilloscope, Tektronix model MSO3014, (e) displayed. **Figure 3** shows a schematic diagram of the experiment mentioned above.

**Figure 3.** (a) Sound source, (b) section of PVDF (mirror finish), (c) laser HeNe 633 nm, (d) light detector, and (e) oscillo‐ scope.

#### *2.3.3. Photopyroelectric response*

**Figure 1.** Diagram of different applied stimuli TSPL to the PVDF prosthesis.

**Figure 2.** Measure of ferroelectric hysteresis loop using the Sawyer–Tower circuit.

In order to measure ferroelectric hysteresis loop, the Sawyer–Tower circuit was implemented for this study (**Figure 2**). By measuring voltage (*VL*) across a capacitor (*CL* = 0.15 μF) in series with the PVDF prosthesis (*CF*), the charge on the ferroelectric can be determined since *QF* = *CL* × *VL*. A sine wave was applied to the circuit with a function generator, Rigol model DG4062; X channel was measured from an X–Y trace using an oscilloscope, Tektronix model MSO 3014.

In this experiment, a sound source was used as a sound emitter (a) excited by an audio generator at 60 Hz. As the receiver, it was used a face of the PVDF prosthesis which was

*2.3.1. Hysteresis loop*

146 Piezoelectric Materials

*2.3.2. Acoustic response*

The interest of recording photopyroelectric current (Y-axis) is to show that the PVDF used in the manufacture of hearing aid responds to light, PVDF was stimulated with a modulated laser light (electronically chopped by the internal oscillator) with a maximum power of 150 mW through an optical fiber with a wave length λ of 650 nm (laser BWTEK model BWF-650-15E/ 55369). Two frequency sweeps were carried out, one from 0 to 10 Hz and the other from 0 to 100 Hz [frequency (X-axis)] were used. This experimental arrangement is based on Balderas-Lopez and Mandelis "New Technique for Precise Measurements of the thermal Effusivity of Transparent Liquids," 2003 [11] and Mandelis and Wang "A Novel PVDF Thin-Film Photo‐ pyroelectric Thermal-Wave Interferometry," 2000 [12]. Our group have compared and discussed the use of PVDF with the ferroelectric ceramic PLZT as a pyroelectric sensor, "Comparative performance of PLZT and PVDF Sensors Used Phyroelectric to the Thermal Characterization of Liquid Samples," 2013 [13].

In order to evaluate photopyroelectric response of the printed PVDF, the experimental arrangement of **Figure 4** was done. The printed PDVF prosthesis (c) was excited with a laser system, BWTEK model BWF-650-15E/55369 λ = 650 nm (e). The laser beam was modulated under two ranges: from 0 to 100 Hz and from 0 to 10 Hz. The intensity of the laser is a function of the emitter current. To register the photopyroelectric response of the printed PVDF prosthesis, a current preamplifier, Standford Research Systems model SR570 and an oscillo‐ scope, Tektronix model MSO3014, were used.

**Figure 4.** Experimental arrangement of the photopyroelectric response of printed PVDF. (a) Oscilloscope, (b) current preamplifier, (c) printed PVDF prosthesis, (d) fiber optic, (e) laser system, and (f) function generator.

#### *2.3.4. Pressure and temperature characterization*

The prosthesis was tested as a pressure sensor with different pressure loads between 0 and 16.35 kPa using a certified weight set from OHAUS. The prosthesis was set in horizontal position, and weights between 25 g and 3 kg were placed over the printed ear as shown in **Figure 5**.

**Figure 5.** Example of pressure characterization.

From Newton's second law (Eq. 1), we have a relation between force (F) and mass (m).

Polymeric Prosthesis as Acoustic, Pressure, Temperature, and Light Sensor Fabricated by Three-Dimensional Printing http://dx.doi.org/10.5772/63074 149

$$f = ma\tag{1}$$

where

prosthesis, a current preamplifier, Standford Research Systems model SR570 and an oscillo‐

**Figure 4.** Experimental arrangement of the photopyroelectric response of printed PVDF. (a) Oscilloscope, (b) current

The prosthesis was tested as a pressure sensor with different pressure loads between 0 and 16.35 kPa using a certified weight set from OHAUS. The prosthesis was set in horizontal position, and weights between 25 g and 3 kg were placed over the printed ear as shown in

From Newton's second law (Eq. 1), we have a relation between force (F) and mass (m).

preamplifier, (c) printed PVDF prosthesis, (d) fiber optic, (e) laser system, and (f) function generator.

scope, Tektronix model MSO3014, were used.

*2.3.4. Pressure and temperature characterization*

**Figure 5.** Example of pressure characterization.

**Figure 5**.

148 Piezoelectric Materials

F = force (N)

m = mass (kg)

a = 9.81 m/s2

Equation (2) was used in order to determine the pressure generated over the ear prosthesis.

$$P = \frac{F}{A} \tag{2}$$

where

F = force (N)

A = area of the surface of the ear on contact with the weights (m2 ).

Regarding to temperature characterization, it has been performed laying the ear prosthesis in an ice bath from 5 to 25°C and in a chamber furnace from 25 to 90°C (in 5°C intervals). The characterization of temperature was done up to 150°C; however, responses between 90° and 150°C remained unchanged, and a fact that was preliminary confirmed by Davis in "Piezo‐ electric and Pyroelectric Polymers," 1993 [14] who recommends that the maximum tempera‐ ture of operation of the PVDF must be 80°C (melting point of around 177°C) [14]. Moreover, for the ear prosthesis of PVDF, the temperature response is in the range of normal temperature of human be [36.19°C (97°F) to 37.2°C (99°F)].

Concerning low temperatures (under 0°C), our research group tested the response of the 3D printed ear (poled and unpoled) at temperatures from −160 to 5°C, giving as a result a minimum temperature of −43.15°C for the unpoled prosthesis and −47 for the poled prosthesis. In this context, manufacturers of PVDF reported the minimum operating temperature at −35°C. It has been also reported in the literature and technical notes that PVDF has a glass transition temperature (Tg) of about −35°C where PVDF is typically 50–60% crystalline and the lowest operating temperature is −50°C (Hylar® Kynar®).

Moreover, PVDF can be used at temperatures from −80° to 300°F (−62° to 149°C). Polyvinyli‐ dene fluoride (PVDF) is a fluorocarbon classified as "self-extinguishing, group 1″ Underwrit‐ ers Laboratories, Inc. It is not affected by prolonged exposure to sunlight or other sources of ultraviolet radiation. It retains its properties under high vacuum and gamma radiation, and it is also resistant to most acids and alkalis (Porex Corporation). Additionally, the company Boveag indicates a temperature range of −30 to 150°C. Goodfellow reports thermal properties of PVDF, minimum operating temperature: −40°C, and maximum operating temperature: 135– 150°C.

The basic circuit used for detecting pressure and temperature of the PVDF prosthesis is shown in **Figure 6**. The circuit is a relaxation oscillator (Astable Circuit Operation LM555, Texas instrument). Changes in PVDF prosthesis capacitance are observed according to the variations in temperature and pressure.

**Figure 6.** Astable multivibrator circuit LM555 to register changes of temperature and pressure of the PVDF prosthesis.

The circuit shown in **Figure 6** generates a clock signal, where the frequency of oscillation depends on 2 resistors, RA, RB, and one capacitor, C, as shown in Eq. (3). Instead of C, the PVDF prosthesis was set and the frequency response was observed.

$$f = \frac{1.44}{(RA + 2RB)C} \tag{3}$$

We have also used the circuit of **Figure 6** as a moisture sensor [8]. Moreover, pressure and temperature measurements were done to 3D-printed PVDF samples (FV307910), noting that if thickness and/or dimensions change, the response also changes but the trend still remains. This is clearly explained by the Eq. (4):

$$C = \frac{\varepsilon\_r \varepsilon\_0 A}{d} \tag{4}$$

where C is the capacitance, εr is the relative permittivity of the dielectric, ε0 is the vacuum permittivity (ε0 = 8.854 × 10−12 F/m), A is the area of the PVDF or capacitor plates, and d is the PVDF thickness or the distance between the electrodes or plates (see Equation (5) of Ref. [16]). Equation (4) describes the relationship between permittivity, capacity, and physical dimen‐ sions of the ferroelectric PVDF when it is subjected to pressure (see standard weights **Figure 5**) or it is deformed see **Figure 3** article our group "Polyvinylidene Flouride in an Applied Polymer Intraocular Pressure Sensor," 2005 [6].

The basic circuit used for detecting pressure and temperature of the PVDF prosthesis is shown in **Figure 6**. The circuit is a relaxation oscillator (Astable Circuit Operation LM555, Texas instrument). Changes in PVDF prosthesis capacitance are observed according to the variations

**Figure 6.** Astable multivibrator circuit LM555 to register changes of temperature and pressure of the PVDF prosthesis.

The circuit shown in **Figure 6** generates a clock signal, where the frequency of oscillation depends on 2 resistors, RA, RB, and one capacitor, C, as shown in Eq. (3). Instead of C, the

1.44

We have also used the circuit of **Figure 6** as a moisture sensor [8]. Moreover, pressure and temperature measurements were done to 3D-printed PVDF samples (FV307910), noting that if thickness and/or dimensions change, the response also changes but the trend still remains.

where C is the capacitance, εr is the relative permittivity of the dielectric, ε0 is the vacuum permittivity (ε0 = 8.854 × 10−12 F/m), A is the area of the PVDF or capacitor plates, and d is the PVDF thickness or the distance between the electrodes or plates (see Equation (5) of Ref. [16]).

*<sup>r</sup>* <sup>0</sup>*<sup>A</sup> <sup>C</sup> d* e e

( 2) *<sup>f</sup> RA RB C* <sup>=</sup> <sup>+</sup> (3)

= (4)

PVDF prosthesis was set and the frequency response was observed.

This is clearly explained by the Eq. (4):

in temperature and pressure.

150 Piezoelectric Materials

Thermal behavior of PVDF dielectric response changes can be widely seen in the following articles: Casar et al. "Electrical and thermal properties of vinylidene fluoride–trifluoroethy‐ lene-based polymer System with coexisting ferroelectric and relaxor states," 2013 [15]; Jafer et al. "The Use of PE/PVDF Pressure and Temperature Sensors in Smart Wireless Sensor Network for Environmental Monitoring System Developed," 2008 [16]. And Jia et al. "Simulation and Experiment of PVDF temperature sensor," 2013 [17].

Finally, using Eqs. (3) and (4), the frequency of the circuit LM555 where PVDF was set instead of C is; we have, 1.44/(RA + 2RB) *f* =(*εrε*0*A*)/ *d*; where

$$f = \frac{(\text{l.44})\text{d}}{\varepsilon\_r \varepsilon\_0 A (\text{RA} + 2\text{RB})} \tag{5}$$

Demonstrating that any alteration of PVDF in its intrinsic permittivity and/or physical dimensions changes *f* by action of pressure and temperature in the ear made with PVDF shown in the following results of **Figure 7** (*f* vs. P) and **Figure 8** (*f* vs. T). Incidentally by a frequencyto-voltage converter may be plotting voltage vs. pressure or temperature.

**Figure 7.** Response of the prosthesis of PVDF as a pressure sensor from 0 to 16350 Pa.

**Figure 8.** Thermal responses of prostheses made of PVDF from 2 to 90°C.
