PIEZOELECTRIC DEVICE COMPRISING A MEMBRANE COMPRISING FIBRES OF A POLYHYDROXYALKANOATE

Abstract

A piezoelectric device includes a membrane having fibres of a polyhydroxyalkanoate (PHA), having average diameter between 100 nm and 2000 nm, and at least one oxide having piezoelectric properties in a subdivided form having at least one average size between 1 nm and 100 nm. Preferably, the PHA fibres are produced by electrospinning. Advantageously, demonstrating good piezoelectric properties and considering that the piezoelectric device includes PHA, a biodegradable and biocompatible material, the device can be used in biological systems, for example in flexible micro-actuator systems for drug delivery and in the engineering of biological tissues (such as, for example, in pacemaker devices).

Claims

1. A piezoelectric device comprising a membrane comprising fibres of a polyhydroxyalkanoate (PHA), having average diameter comprised between 100 nm and 2000 nm, and at least one oxide with piezoelectric properties in a subdivided form having at least one average size comprised between 1 nm and 100 nm.

2. The device according to claim 1, wherein the PHA is a polymer containing repeating units of formula (I):
—O—CHR.sub.1—(CH.sub.2).sub.n—CO—  (I) where: R.sub.1 is selected from: C.sub.1-C.sub.12 alkyls, C.sub.4-C.sub.16 cycloalkyls, C.sub.2-C.sub.12 alkenyls, optionally substituted with at least one group selected from: halogen (F, Cl, Br), —CN, —OH, —OOH, —OR, —COOR (R=C.sub.1-C.sub.4 alkyl, benzyl); n is zero or is an integer from 1 to 6.

3. The device according to claim 2, wherein the PHA is a polyhydroxybutyrate (PHB).

4. The device according to claim 1, wherein the oxide is selected from: barium titanate (BaTiO.sub.3), zinc oxide (ZnO), aluminium nitride (AlN), gallium nitride (GaN), cadmium sulfide (CdS), and is used in a subdivided form, preferably in the form of spherical nanoparticles, nanowires, nanotubes, nanobelts or nanorods.

5. The device according to claim 1, wherein the oxide is present in a concentration comprised between 1% and 30% by weight, the % being expressed with respect to the overall weight of the membrane.

6. The device according to claim 1, wherein the oxide has at least one binding agent on its surface.

7. The device according to claim 6, wherein the binding agent is present in a concentration comprised between 10% and 30%, the % being expressed with respect to the total weight of the oxide bound to the binding agent.

8. The device according to claim 6, wherein the binding agent is a hydrophilic binder, selected from: hydrocaffeic acid, polyethylene glycol derivatives, amino acids, proteins, vitamins, carbohydrates and lignin derivatives.

9. The device according to claim 6, wherein the binding agent is a lipophilic binder, selected from: N-(3,4-dihydroxyphenethyl)dodecanamide, hydroxamic acids, amides, acetates, carboxylic acids with aliphatic chains with a number of atoms of carbon comprised between 6 and 16.

10. A method for producing a piezoelectric device according to claim 1, the method including the following steps: solubilizing a polyhydroxyalkanoate (PHA) in an organic solvent and dispersing in the solution thus obtained at least one oxide having piezoelectric properties in a subdivided form having at least an average size comprised between 1 nm and 100 nm, such to prepare a spinning solution, and subjecting the spinning solution to an electrospinning process by means of a spinneret and a rotating support placed substantially perpendicular to the spinning direction, so as to obtain a membrane.

11. The process according to claim 10, wherein in the spinning solution, the PHA has a concentration comprised between 1% and 20% p/v.

12. The process according to claim 10, wherein the oxide is present in the spinning solution in a concentration comprised between 0.05% and 15% p/v.

13. Use of the piezoelectric device according to claim 1 in piezoelectric energy transducers.

Description

DETAILED DESCRIPTION OF THE DISCLOSURE

[0052] The following embodiment examples are provided for the sole purpose of illustrating the present disclosure and are not to be understood in a sense limiting the scope of protection defined by the appended claims.

Example 1

[0053] a) Functionalization of BaTiO.sub.3 with Hydrocaffeic Acid.

[0054] 0.577 g of BaTiO.sub.3 was dispersed in 50 ml of absolute ethanol in a 250 ml beaker. The suspension was sonicated for 1 minute. 0.860 g of hydrocaffeic acid was added to the suspension, which was sonicated again for 1 minute. The suspension thus obtained was poured into a round-bottomed flask and stirred overnight at 60° C. Subsequently the suspension was purified by repeated centrifugation (15 min at 6000 rpm) and washes with a solution of water and ethanol (1:1), until a colourless supernatant was obtained.

[0055] b) Preparation of the Spinning Solution.

[0056] The purified PHB with 99.5% purity was dissolved in HFIP (1,1,1,3,3,3,3-hexafluoro-2-propanol) so as to obtain a spinning solution with a concentration of PHB equal to 4% p/v. The suspension of BaTiO.sub.3 functionalized as described above was added to this solution. The solution was stirred on a plate at 800 rpm at room temperature, until complete dissolution of the polymer.

[0057] Subsequently, the solution was cooled, poured into a 10 mL plastic syringe and placed inside the electrospinning system in order to be processed. The deposition of the fibres of PHB and BaTiO.sub.3 bound to hydrocaffeic acid was carried out on a cylindrical support (spindle) in steel placed in rotation by an electric motor.

[0058] The following parameters were used for the electrospinning process:

[0059] spinning solution: purified PHB dissolved in HFIP at 4% p/v;

[0060] processed solution volume: 1 mL;

[0061] flow rate of the solution: 8 mL/hour;

[0062] translational speed of the spinneret along the spindle: 0 mm/s;

[0063] distance between the tip of the spinneret and the spindle (gap): 30 cm;

[0064] voltage applied to the electrodes: 10-30 kV;

[0065] spindle rotation speed: 2100 rpm;

[0066] diameter of the spinneret exit hole: 2.15 mm (AWG 12);

[0067] spindle diameter: 15 cm;

[0068] FIG. 1 shows the image of the membrane, obtained by scanning electron microscopy (SEM).

Example 2

[0069] a) Functionalization of BaTiO.sub.3 with N-(3,4-Dihydroxyphenethyl)Dodecanamide.

[0070] 0.577 g of BaTiO.sub.3 was dispersed in 50 ml of absolute ethanol in a 250 ml beaker. The suspension was sonicated for 1 minute. 1.15 g of N-(3,4-dihydroxyphenethyl)dodecanamide was added to the suspension, which was sonicated again for 1 minute. The suspension thus obtained was poured into a round-bottomed flask and stirred overnight at 60° C. Subsequently the suspension was purified by repeated centrifugation (15 min at 6000 rpm), double washes with ethanol and then with hexane until a colourless supernatant was obtained.

[0071] b) Preparation of the Spinning Solution.

[0072] The purified PHB and the BaTiO.sub.3 functionalized as described above were dissolved in HFIP (1,1,1,3,3,3-hexafluoro-2-propanol) so as to obtain a spinning solution with a concentration of PHB equal to 4% p/v. The solution was stirred on a plate at 800 rpm at room temperature until the complete dissolution of the polymer.

[0073] Subsequently, the solution was cooled, poured into a 10 mL plastic syringe and placed inside the electrospinning system in order to be processed. The deposition of the PHB and BaTiO.sub.3 fibres bound to N-(3,4-dihydroxyphenethyl)dodecanamide was carried out on a steel cylindrical support (spindle) placed in rotation by an electric motor.

[0074] The following parameters were used for the electrospinning process:

[0075] spinning solution: purified PHB dissolved in HFIP at 4% p/v;

[0076] processed solution volume: 1 mL;

[0077] flow rate of the solution: 8 mL/hour;

[0078] translational speed of the spinneret along the spindle: 0 mm/s;

[0079] distance between the tip of the spinneret and the spindle (gap): 30 cm;

[0080] voltage applied to the electrodes: 10-30 kV;

[0081] spindle rotation speed: 2100 rpm;

[0082] diameter of the spinneret exit hole: 2.15 mm (AWG 12);

[0083] spindle diameter: 15 cm;

Example 3—Comparative Example

[0084] The purified PHB was dissolved in HFIP (1,1,1,3,3,3-hexafluoro-2-propanol) so as to obtain a spinning solution with a concentration of 4% p/v. The solution was stirred on a plate at 800 rpm at room temperature, until complete dissolution of the polymer.

[0085] Subsequently, the solution was cooled, poured into a 10 mL plastic syringe and placed inside the electrospinning system in order to be processed. The deposition of the PHB fibres was carried out on a steel cylindrical support (spindle) placed in rotation by an electric motor.

[0086] The following parameters were used for the electrospinning process:

[0087] spinning solution: purified PHB dissolved in HFIP at 4% p/v;

[0088] processed solution volume: 1 mL;

[0089] flow rate of the solution: 8 mL/hour;

[0090] translational speed of the spinneret along the spindle: 0 mm/s;

[0091] distance between the tip of the spinneret and the spindle (gap): 30 cm;

[0092] voltage applied to the electrodes: 10-30 kV;

[0093] spindle rotation speed: 2100 rpm;

[0094] diameter of the spinneret exit hole: 2.15 mm (AWG 12);

[0095] spindle diameter: 15 cm;

Example 4

[0096] The purified PHB was dissolved in HFIP (1,1,1,3,3,3-hexafluoro-2-propanol) so as to obtain a spinning solution with a concentration of 4% p/v. The solution was stirred on a plate at 800 rpm at room temperature, until complete dissolution of the polymer. A solution of BaTiO.sub.3 at 5% in 1 mL of HFIP was added to the solution.

[0097] Subsequently, the solution thus obtained was cooled, poured into a 10 mL plastic syringe and placed inside the electrospinning system in order to be processed. The deposition of the PHB fibres was carried out on a steel cylindrical support (spindle) placed in rotation by an electric motor.

[0098] The following parameters were used for the electrospinning process:

[0099] spinning solution: purified PHB dissolved in HFIP at 4% p/v;

[0100] processed solution volume: 1 mL;

[0101] flow rate of the solution: 8 mL/hour;

[0102] translational speed of the spinneret along the spindle: 0 mm/s;

[0103] distance between the tip of the spinneret and the spindle (gap): 30 cm;

[0104] voltage applied to the electrodes: 10-20 kV;

[0105] spindle rotation speed: 2100 rpm;

[0106] diameter of the spinneret exit hole: 2.15 mm (AWG 12);

[0107] spindle diameter: 15 cm;

Example 5: Measurement of Piezoelectric Properties

[0108] A sample of about 2 cm.sup.2 of each of the membranes obtained in Examples 3 and 4 was inserted between two aluminium sheets. The electric contacts for the connection to the voltage measurement instrument (voltmeter) were made by connecting the two aluminium sheets to two electric copper wires, through conductive silver paste or conductive epoxy resin.

[0109] A thin film of polydimethylsiloxane (PDMS) was placed over each sample in order to avoid the sliding of the upper aluminium sheet.

[0110] The measurement of the piezoelectric property was carried out by applying, on the upper aluminium sheet of each sample, a dynamic force exerted by a piston of about 7 mm.sup.2 in diameter and a frequency of about 1.5 Hz. This force was applied along a perpendicular direction with respect to the upper aluminium sheet.

[0111] Following the application of this force, the electric charges of opposite sign accumulated on the two surfaces of the membrane. These charges led to the formation of a corresponding flow of charges through the external circuit (aluminium sheets, electric contacts and voltmeter), thereby restoring the electrostatic balance.

[0112] In detail, the measurements were taken in “open circuit” mode, i.e. measuring the voltage generated by the sample when subjected to compression by the piston. Table 1 shows the results of the above measurements.

TABLE-US-00001 TABLE 1 PHB (Example 3) PHB/BaTiO.sub.3 (Example 4) V.sub.pp (V) 2.7 3.2 V.sub.pp +18.5% Increment

[0113] The parameter “V.sub.pp” is the measured peak-to-peak voltage value. While the “increment” is the percentage difference between the measured V.sub.pp parameters.

[0114] As can be seen in Table 1, the addition of BaTiO.sub.3 to the PHB leads to an increase in the measured V.sub.pp parameter. This indicates a corresponding improvement of the piezoelectric properties. This improvement is confirmed by the “increment” parameter which shows an increase of about 18% in the measured peak-to-peak voltage value for the sample of Example 4, compared to the comparative sample of Example 3.