**3. Results**

**Figure 9a** shows the design of a 3D human ear model according to anthropometric parameters [18–20] and created with a computer-aided design (CAD) software. It was exported as a stereolithography file in order to print it using a 3D printer.

**Figure 9.** (a) Human ear created with a 3D CAD program. (b) Ear prosthesis printed of PVDF.

The ear was printed in polyvinylidene fluoride as it exhibits ferroelectric properties. **Figure 9b** shows the 3D-printed ear prosthesis. The dimensions of the printed ear are 60.1 mm wide by 34.74 mm long and has an average thickness of 6.88 mm. Electrodes were painted over the printed ear with silver paint from SPI Supplies. The area of these electrodes is 6.26 mm2 , and the distance between them is 41 mm (**Figure 10**). Dimensions were measured with a Starrett Vernier with 0.02 mm precision.

**Figure 10.** Dimensions of ear prosthesis printed of PVDF.

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

stereolithography file in order to print it using a 3D printer.

**Figure 9.** (a) Human ear created with a 3D CAD program. (b) Ear prosthesis printed of PVDF.

**Figure 9a** shows the design of a 3D human ear model according to anthropometric parameters [18–20] and created with a computer-aided design (CAD) software. It was exported as a

**3. Results**

152 Piezoelectric Materials

Sawyer–Tower circuit (**Figure 2**) was used to measure ferroelectric properties such as hyste‐ resis. **Figure 11** shows P–E hysteresis loop of the printed PVDF, and X axis shows the electric field (E) in kV/cm and Y axis polarization (P) in μC/cm2 .

**Figure 11.** Printed PVDF hysteresis loop.

The acoustic response using a metalized printed PVDF sample is presented in **Figure 12**. The electrical signal from the light detector was recorded after PVDF reflected the laser beam. Variations of the laser intensity correspond to fluctuations of amplitude and frequency in the electrical signal.

**Figure 12.** Reference signal for printed PVDF (Ch1) and the signal obtained from a commercial light detector (Ch2).

Results of photopyroelectric response of printed PVDF prosthesis at 10 and 100 Hz are shown in **Figure 13a, b**, respectively.

**Figure 13.** (a) Printed PVDF sample photopyroelectric response (100 Hz), (b) printed PVDF sample photopyroelectric response (10 Hz).

The results in **Figure 13a, b** showed an asymptotic behavior, similar to the ones described in the theory in the case of frequency modulation of laser stimulation. References amply illustrate the use of PVDF in the photopyroelectric technique [11–13].

The prosthesis was also tested as a pressure sensor, applying pressure loads between 0 and 16.35 kPa.

With regard to the characterization of PVDF, ß-phase was reached by means of corona poling. **Figures 7** and **8** show the difference between unpoled and poled PVDF (Tests 1, 2, and 3); the average of these three characterizations (mean) and the line adjustment (fit line).

**Figure 7** shows the response of the ear prosthesis of PVDF as a pressure sensor using Eq. (3).

Equation (6) fits a line through the points at 0–16,350 Pa for the corona-poled PVDF prosthesis with correlation coefficient of 0.9670.

$$\mathbf{y} = -0.4687\mathbf{x} + 217.16\tag{6}$$

The temperature characterization of the ear prosthesis has been performed from 5 to 90°C in 5°C intervals, and **Figure 8** illustrates the response of the PVDF ear prosthesis as a temperature sensor using Eq. (5). After 90°C, no changes were observed as Davis reported [14].

Equation (7) fits a line through the points at 5 and 90°C for corona-poled PVDF prosthesis with correlation coefficient of 0.9940.

$$y = -2.5132x + 199.34\tag{7}$$
