Synchronous Sound Recognition and Energy Harvesting by Flexible Piezoelectric PLLA/VB2 Composites

In the present study, poling−free PLLA/VB2 piezoelectric composites are fabricated to achieve synchronous sound recognition and energy harvesting. The addition of VB2 can interact with PLLA by intermolecular hydrogen bonding, inducing the dipole orientation of C=O in PLLA. Meanwhile, VB2 can promote crystallization of PLLA through heterogeneous nucleation. The combination of the two strategies significantly improves the piezoelectric performance of PLLA/VB2 composites. The PLLA/VB2 can detect the sound frequency with an accuracy of 0.1% in the range of 0–20 kHz to recognize characteristic sounds from a specific source. PLLA/VB2 can also convert sound into electrical energy synchronously with an energy density of 0.2 W/cm−3 to power up LEDs. Therefore, PLLA/VB2 shows great potential in the field of information and energy synchronous collection.


Introduction
The increasing demand for energy supplies of portable terminals and wearable devices has prompted research on ambient energy harvesting [1][2][3][4][5].The ambient energy harvesting technology allows to collect and convert ambient mechanical, thermal, and electromagnetic energy to electricity to achieve a clean and sustainable energy supply [6][7][8][9][10].Among them, harvesting of ambient mechanical energy is the most investigated due to its wide availability and ease of collection.By using piezoelectric generators (PEGs), the low and irregular mechanical energy can be efficiently converted to electrical power due to the piezoelectric mechanism [11][12][13].
Currently, most research on harvesting ambient mechanical energy is related to human body motions such as breathing, walking and running due to its high power density [14][15][16][17][18].However, such an approach is limited by specific application scenarios.In contrast, one of ambient mechanical energy or sound energy is ignored due to its relatively low power density even though they are widely available without application scenarios limitations.As PEG is prepared by materials with piezoelectric properties, the efficiency of electricity generation of PEG is strongly dependent on the piezoelectricity of the material used.This means materials with higher piezoelectricity are of great concern when designing PEG for harvesting sound energy.
Piezoelectric polymers, such as poly (vinylidene fluoride) (PVDF), offer natural flexibility and a low weight that makes them suitable for energy harvesting in portable and wearable applications compared to inorganic piezoelectric materials.PVDF has a high piezoelectric output for mechanical energy harvesting [19,20], but it requires additional poling to form the piezoelectric β−phase crystal [21,22].Meanwhile, the possible biotoxicity and high cost due to large amounts of F elements in the backbone have limited further applications.Another flexible piezoelectric material attracting attention is poly(L−lactide) (PLLA) due to its low cost and biocompatibility.The permanent dipole of the C-O bond in PLLA eliminates the need for electrical poling treatment [23], but its relatively weak piezoelectricity cannot provide sufficient output for sound energy harvesting.Mature strategies for improving the piezoelectric output include co−polymerization [24,25], the use of additives [26,27], and the application of nanostructures [28,29].Among these methods, the use of additives is a simple and effective way to enhance the piezoelectric properties.However, for the case of PLLA, the requirements for additives are not only the improvement of piezoelectricity but also the maintenance of its low cost and biocompatibility.
In this paper, based on our previous work, we prepared a polling−free piezoelectric composite by blending PLLA and Vitamin B 2 (VB 2 ).The addition of biocompatible VB 2 significantly enhanced the piezoelectric properties of PLLA through hydrogen bonding and nucleation effects and maintained the low cost and good biocompatibility of PLLA/VB 2 composites.The PLLA/VB 2 composites present a constant piezoelectric response to sound in the range of 0 to 20 kHz with an accuracy of 0.1% to recognize the specific sound source and energy density of 0.2 W/cm −3 by harvesting sound energy.The synchronous realization of sound recognition and energy harvesting by PLLA/VB 2 provides great potential for ambient energy harvesting and information collection.

Materials
The PLLA (2003D) used in this study was sourced from NatureWorks (Minneapolis, MN, USA), while VB 2 and chloroform were procured from Shanghai Aladdin Biochemical Technology Co., Ltd.(Chengdu, China).

Preparation of PLLA/VB 2 Composites
The PLLA/VB 2 composites are prepared by dissolving VB 2 and PLLA in chloroform and stirring for 8 h at 25 • C. The content of VB 2 is varied from 0 wt% to 20 wt% to PLLA.The target solution was coated with a copper film to form a film with a thickness of 20 µm.The film was then dried under vacuum at 25 • C and annealed at 140 • C. Subsequently, a Ag electrode was coated onto the PLLA/VB 2 film.Typically, the PLLA/VB 2 composites with 15 wt% VB 2 was named PLLA/VB 2 −15.

Characterization of PLLA/VB 2 Composites
The Fourier transform infrared spectroscopy (FTIR) was recorded on a Bruker Tensor 27 spectrometer with a range from 4000 cm −1 to 500 cm −1 and a resolution of 4 cm −1 .The X−ray diffraction (XRD) experiments were carried out on a D/Max2500 VB2t/PC X−ray diffractometer (Rigaku, Tokyo, Japan) with a 2θ range of 5-50 • .The differential scanning calorimetry (DSC) curves are recorded on Mettler Toledo DSC 3 instruments (TA Instruments, Shanghai, China) under a N 2 atmosphere and a heating speed of 10 • C/min.The piezoelectric signals were collected using NI−9238 voltage acquisition (National Instruments, Austin, TX, USA).The cytotoxicity is investigated according to CCK−8 methods.

Characterization of PLLA/VB 2 Composites
The FTIR spectra of PLLA/VB 2 composites are provided in Figure 1A.The vibration of the PLLA ester bond corresponds to the wave number of 1740 cm −1 .As the VB 2 content increases, the characteristic peak shifts towards the lower frequency region due to the possible interaction between the C=O dipoles in the ester bond of PLLA and the multiple functional groups including carboxyl, carbonyl, and amino of VB 2 .Furthermore, a broad hydrogen bond peak between 3200 cm −1 and 3600 cm −1 was observed, suggesting the existence of hydrogen bonds, which simultaneously formed between the PLLA intermolecular chains and between C=O dipoles in PLLA and functional groups in VB 2 .Such hydrogen bond and dipole interaction may promote the orientation and further enhances the piezoelectricity of PLLA/VB 2 .
Polymers 2024, 16, 1071 3 of 11 existence of hydrogen bonds, which simultaneously formed between the PLLA intermolecular chains and between C=O dipoles in PLLA and functional groups in VB2.Such hydrogen bond and dipole interaction may promote the orientation and further enhances the piezoelectricity of PLLA/VB2.In Figure 1B, the XRD curves of pure PLLA show several sharp diffraction peaks located at 2θ = 15°, 17°, and 19°, respectively, corresponding to the (010), (110)/(200), and (203) planes in PLLA.The addition of VB2 leads to the appearance of a new diffraction peak at 2θ = 30.8°,which belongs to the (003) plane of the β-phase crystal in PLLA [26][27][28].In addition, with the increasing content of VB2, the diffraction peak intensity of the (003) plane increases accordingly.This phenomenon can be attributed to the heterogeneous nucleation promotion of VB2 on the crystalline behavior of PLLA, which is a benefit of the improvement of piezoelectricity.Thus, the introduction of VB2 not only helps the formation of hydrogen bonds between PLLA and VB2, but also promotes the content of polar β-phase crystals.As the hydrogen bond can promote the orientation of C=O dipoles in the easter bonds of PLLA, and the increasing of polar β-phase is benefit to the dipole orientation as reported in the PVDF-based piezoelectric polymer [30], the introduction of VB2 can improve the piezoelectricity of PLLA.
Figure 1C,D provides the SEM images of PLLA/VB2 composites with different VB2 content.The PLLA and VB2 are presented as light and dark phases, respectively.VB2 distributes uniformly as a rod shape in the PLLA matrix at a content of 15 wt%, while at a higher content (20 wt%) of VB2, some agglomerations are observed.Meanwhile, the edge between PLLA and VB2 seems indistinct, indicating a strong interaction as evidenced by the hydrogen bond in FITR spectra.
The DSC heating and cooling curves are provided in Figure 2A,B, respectively.The heating curves show that all the samples have similar thermal transition peaks.The peak at 62 °C is attributed to the glass transition of the samples, and the peak near 125 °C is due to their cold crystallization.The melting peaks of β-crystals and α-crystals are observed at In addition, with the increasing content of VB 2 , the diffraction peak intensity of the (003) plane increases accordingly.This phenomenon can be attributed to the heterogeneous nucleation promotion of VB 2 on the crystalline behavior of PLLA, which is a benefit of the improvement of piezoelectricity.Thus, the introduction of VB 2 not only helps the formation of hydrogen bonds between PLLA and VB 2 , but also promotes the content of polar β-phase crystals.As the hydrogen bond can promote the orientation of C=O dipoles in the easter bonds of PLLA, and the increasing of polar β-phase is benefit to the dipole orientation as reported in the PVDF-based piezoelectric polymer [30], the introduction of VB 2 can improve the piezoelectricity of PLLA.
Figure 1C,D provides the SEM images of PLLA/VB 2 composites with different VB 2 content.The PLLA and VB 2 are presented as light and dark phases, respectively.VB 2 distributes uniformly as a rod shape in the PLLA matrix at a content of 15 wt%, while at a higher content (20 wt%) of VB 2 , some agglomerations are observed.Meanwhile, the edge between PLLA and VB 2 seems indistinct, indicating a strong interaction as evidenced by the hydrogen bond in FITR spectra.
The DSC heating and cooling curves are provided in Figure 2A,B, respectively.The heating curves show that all the samples have similar thermal transition peaks.The peak at 62 • C is attributed to the glass transition of the samples, and the peak near 125 • C is due to their cold crystallization.The melting peaks of β-crystals and α-crystals are observed at 162 • C and 167 • C, respectively, and these melting peaks are partially overlapped.In addition, the melting enthalpy of β-crystals increased with the increasing VB 2 content, indicating that the crystallinity of β-crystals in the samples was improved by the addition of VB 2 .Meanwhile, the cold crystallization temperatures of the samples decreased with increasing VB 2 content, indicating an increase in the crystallinity of the β-crystals and a decrease in the crystallinity of the α-crystals.According to the calculation results, the crystallinity of β-crystals in PLLA increased from 91.93% to 93.97% with increasing VB 2 content from 5wt% to 15wt%.This phenomenon is attributed to the interaction between the ester bonds in PLLA and the hydroxyl, carbonyl, and amine groups of VB 2 , which is beneficial to promoting the orientation of the PLLA chains and improves the crystallinity of the β-crystals.
162 °C and 167 °C, respectively, and these melting peaks are partially overlapped.In addition, the melting enthalpy of β-crystals increased with the increasing VB2 content, indicating that the crystallinity of β-crystals in the samples was improved by the addition of VB2.Meanwhile, the cold crystallization temperatures of the samples decreased with increasing VB2 content, indicating an increase in the crystallinity of the β-crystals and a decrease in the crystallinity of the α-crystals.According to the calculation results, the crystallinity of β-crystals in PLLA increased from 91.93% to 93.97% with increasing VB2 content from 5wt% to 15wt%.This phenomenon is attributed to the interaction between the ester bonds in PLLA and the hydroxyl, carbonyl, and amine groups of VB2, which is beneficial to promoting the orientation of the PLLA chains and improves the crystallinity of the β-crystals.

Piezoelectric Performance of PLLA/VB2 Composites
The output performances of the PLLA/VB2 composites are investigated using a linear motor system to generate a shear force which drives the PLLA/VB2 composites to realize a linear reciprocating motion, the electrical signals of PLLA/VB2 piezoelectric devices are sampled by voltage acquisition cards (NI-9238).The experimental procedure is shown in Figure 3B, and the output voltages are shown in Figure 3C.The peak voltage of the pure PLLA materials was only 1.45 V.However, as the VB2 content in the PLLA/VB2 composite increased in the range of 0-15 wt%, the peak voltage of the PLLA/VB2-15 composites increased, reaching a maximum output voltage of 4.42 V, which is three times that of the pure PLLA.This increase in output voltage is attributed to the addition of VB2 into PLLA, promoting the dipole orientation of the PLLA molecular chains and improving the crystallinity of PLLA, which in turn improves the piezoelectricity of PLLA.However, the peak voltage of the PLLA/VB2-20 composites was lower than that of the PLLA/VB2-15 composite, which is attributed to the excess content of VB2 in PLLA resulting in a non-uniform distribution.Therefore, the PLLA/VB2-15 composite seems to have optimum output performance.

Piezoelectric Performance of PLLA/VB 2 Composites
The output performances of the PLLA/VB 2 composites are investigated using a linear motor system to generate a shear force which drives the PLLA/VB 2 composites to realize a linear reciprocating motion, the electrical signals of PLLA/VB 2 piezoelectric devices are sampled by voltage acquisition cards (NI-9238).The experimental procedure is shown in Figure 3B, and the output voltages are shown in Figure 3C.The peak voltage of the pure PLLA materials was only 1.45 V.However, as the VB 2 content in the PLLA/VB 2 composite increased in the range of 0-15 wt%, the peak voltage of the PLLA/VB 2 -15 composites increased, reaching a maximum output voltage of 4.42 V, which is three times that of the pure PLLA.This increase in output voltage is attributed to the addition of VB 2 into PLLA, promoting the dipole orientation of the PLLA molecular chains and improving the crystallinity of PLLA, which in turn improves the piezoelectricity of PLLA.However, the peak voltage of the PLLA/VB 2 -20 composites was lower than that of the PLLA/VB 2 -15 composite, which is attributed to the excess content of VB 2 in PLLA resulting in a nonuniform distribution.Therefore, the PLLA/VB 2 -15 composite seems to have optimum output performance.
In Figure 3A, according to the previous FTIR, XRD and DSC results, the addition of VB 2 into PLLA leads to the formation of hydrogen bonds and increasing content of β-phase crystal.The hydrogen bonds between the C=O dipoles in PLLA and the function group in VB 2 may promote the orientation of C=O dipoles [30,31].Furthermore, the polar β-phase crystal may further improve and maintain the orientation of C=O dipoles.Thus, the piezoelectricity of PLLA is obviously improved.
We compared our PLLA/VB 2 composites with recent references regarding their sound recognition and energy harvesting performance.Compared to rigid piezoelectric ceramics such as PZT, AlN, and ZnO, our PLLA/VB 2 material offers several advantages, As shown in Table 1.It is lightweight, flexible, has a low Young's modulus, and is easy to fabricate and process.Although its piezoelectric coefficient and energy density are lower, PLLA/VB 2 has the added benefit of reducing noise due to its low speed of sound while converting sound energy into electrical energy [32].In daily life, low-frequency sound waves primarily exert a shear force on objects.Our PLLA/VB 2 is sensitive to this shear force due to the piezoelectric constant along the d 14 direction, making it more prone to vibration or deformation and thus generating electrical energy [33].Simultaneously, PLLA/VB 2 possesses the benefits of being non-toxic, naturally degradable, and not requiring polarization, which is absent in traditional rigid piezoelectric ceramics [34][35][36].As a result, it has a wider potential for large-scale applications in real-world settings.In Figure 3A, according to the previous FTIR, XRD and DSC results, the addition of VB2 into PLLA leads to the formation of hydrogen bonds and increasing content of βphase crystal.The hydrogen bonds between the C=O dipoles in PLLA and the function group in VB2 may promote the orientation of C=O dipoles [30,31].Furthermore, the polar β-phase crystal may further improve and maintain the orientation of C=O dipoles.Thus, the piezoelectricity of PLLA is obviously improved.
We compared our PLLA/VB2 composites with recent references regarding their sound recognition and energy harvesting performance.Compared to rigid piezoelectric ceramics such as PZT, AlN, and ZnO, our PLLA/VB2 material offers several advantages, As shown in Table 1.It is lightweight, flexible, has a low Young's modulus, and is easy to fabricate and process.Although its piezoelectric coefficient and energy density are lower, PLLA/VB2 has the added benefit of reducing noise due to its low speed of sound while converting sound energy into electrical energy [32].In daily life, low-frequency sound waves primarily exert a shear force on objects.Our PLLA/VB2 is sensitive to this shear force due to the piezoelectric constant along the d14 direction, making it more prone to vibration or deformation and thus generating electrical energy [33].Simultaneously, PLLA/VB2 possesses the benefits of being non-toxic, naturally degradable, and not requiring polarization, which is absent in traditional rigid piezoelectric ceramics [34][35][36].As a result, it has a wider potential for large-scale applications in real-world settings.

Piezoelectric Acoustical Performance of PLLA/VB 2 Composites
The piezoelectric acoustical response of PLLA/VB 2 composites is performed using a speaker as a sound source driven by a function generator to play sound samples with different distances and frequencies (as provided in Figure 4A), and the piezoelectric voltage of PLLA/VB 2 composites is recorded.In Figure 4B, under fixed sound distance and frequency, the PLLA/VB 2 with increasing VB 2 content from 0 wt% to 15 wt% shows an increasing piezoelectric output from 0.67 mV to 1.10 mV, benefitting from promotion in the dipole orientation and crystalline formation by the addition of VB 2 .With a higher content of VB 2 (20 wt%), the piezoelectric output decreases to 0.83 mV, which is due to the excessive content of VB 2 in PLLA, resulting in the uneven distribution of VB 2 in PLLA.The age of PLLA/VB2 composites is recorded.In Figure 4B, under fixed sound distance and frequency, the PLLA/VB2 with increasing VB2 content from 0 wt% to 15 wt% shows an increasing piezoelectric output from 0.67 mV to 1.10 mV, benefitting from promotion in the dipole orientation and crystalline formation by the addition of VB2.With a higher content of VB2 (20 wt%), the piezoelectric output decreases to 0.83 mV, which is due to the excessive content of VB2 in PLLA, resulting in the uneven distribution of VB2 in PLLA.The maximum piezoelectric output is observed for PLLA/VB2-15; thus, it is selected for further investigation on piezoelectric acoustical performance.As sound is one of the mechanical waves, it leads to surface vibration and deformation when it propagates to the surface PLLA/VB 2 -15.Meanwhile, piezoelectric charges are generated in response to the change in the degree of polarization.In this view, shorter distance and higher amplitude intensify this process, leading to higher piezoelectric voltage.Meanwhile, PLLA/VB 2 -15 show higher piezoelectric output to sound with higher frequency ranging from 0 to 20kHz, indicating frequency-dependent properties within the human auditory range.
The frequency-dependent response of PLLA/VB 2 -15 enables its capacity to recognize sound frequency.The voltage-time curves can be converted to amplitude-frequency curves by using a fast Fourier transform algorithm.Thus, a standard sound sample with a fixed frequency is used to investigate the detected error of PLLA/VB 2 -15.In Figure 4E, the PLLA/VB 2 -15 shows an average error of less than 0.1% at a frequency from 0 to 20 kHz, indicating a high accuracy of sound frequency recognition.

Sound Recognition and Energy Harvesting Performance of PLLA/VB 2 Composites
We asked two students (one male and one female) to say "hello" to the PLLA/VB 2 -15 film samples.Upon acoustic excitation, PLLA/VB 2 -15 generates voltage signals.The voltage acquisition card collects and records the output voltage signal of PLLA/VB 2 -15.
The sound samples of the male and female saying "hello" are used to investigate the sound recognition performances of PLLA/VB 2 -15 composites.According to the FFT curves (Figure 5C,D), which were converted from the voltage-time curves (Figure 5A,C), the frequency of male voice samples is mainly concentrated in the low-frequency region from 400 to 800 Hz, while that of the female voice samples is concentrated in the highfrequency region from 1000 to 1400 Hz.By comparing and analyzing the frequency spectral characteristics of voice, we can achieve sound recognition of certain keywords or specific vocabulary.Additionally, the amplitude and distance of the sound sample are fixed at 50 db and 30 cm, respectively, to evaluate the energy harvesting performances of PLLA/VB 2 -15 composites.By recording the piezoelectric voltage and current in Figure 5E,F, the average piezoelectric voltage and current are 0.25 V and 30 µA, respectively.Thus, the energy density of the PLLA/VB 2 -15 composite is calculated to be 0.2 W/cm −3 .With the help of a rectifying circuit, this device can drive low-power devices such as LEDs (Figure S1).In our daily lives, sound recognition technology is used to judge or distinguish the speaker by capturing the speaker's voice and extracting the voiceprint feature signal.Since the PLLA/VB2-15 piezoelectric device has good sound recognition capability, it is used as an acoustic sensor.
Five different people successively shouted the same seven Chinese characters based on their own speaking habits when they were 40 cm away from the PLLA/VB2-15 acoustic sensor, and the PLLA/VB2-15 acoustic sensor generated piezoelectric voltage under the sound stimulations.The piezoelectric voltage of the PLLA/VB2-15 acoustic sensor was recorded by the voltage acquisition card.The experimental procedure is shown in Figure 6A.In Figure 6B, the performance of the time interval of spitting words is different and the  Considering the variable speaking frequency and loudness when people are in ferent surroundings, the FFT algorithm is employed to further improve the recogn accuracy of the PLLA/VB2-15 acoustic sensor.The FFT algorithm can convert the volt time curves from PLLA/VB2-15 acoustic sensor to intensity-frequency curves, which help to better distinguish the speakers.As shown in Figure 6C, differences in the quency of the same word spoken by each student were found because the frequenc each person's articulation is based on the characteristics of their vocal cords.Thes tracting the characteristic information of pronunciation frequency from the frequency main signal can further distinguish the speakers.

Cytotoxicity and Biocompatibility of PLLA/VB2 Composites
CCK-8 (Cell Counting Kit-8) is a commonly used cell counting kit for evaluating viability and proliferation [39].CCK-8 is widely used in cell biology research, pharm ogy research, and drug screening because it is easy to use, does not require the hand of toxic solutions, and the test results can be measured directly on a microplate re [40].
The CCK-8 methods are used to evaluate the possible cytotoxicity of PLLA/VB2 c posites.With increasing incubating time up to 72 h, the relative growth rate of PLLA composites (Figure 7A) is higher than 80%, which belongs to "grade 0", regarded a cytotoxicity.In Figure 7B, the incubated cells present various shapes such as rod and s tle, indicating they grow in good conditions.Thus, the PLLA/VB2 composites pre good biocompatibility and no cytotoxicity.Considering the variable speaking frequency and loudness when people are in different surroundings, the FFT algorithm is employed to further improve the recognition accuracy of the PLLA/VB 2 -15 acoustic sensor.The FFT algorithm can convert the voltagetime curves from PLLA/VB 2 -15 acoustic sensor to intensity-frequency curves, which may help to better distinguish the speakers.As shown in Figure 6C, differences in the frequency of the same word spoken by each student were found because the frequency of each person's articulation is based on the characteristics of their vocal cords.These extracting the characteristic information of pronunciation frequency from the frequency domain signal can further distinguish the speakers.

Cytotoxicity and Biocompatibility of PLLA/VB 2 Composites
CCK-8 (Cell Counting Kit-8) is a commonly used cell counting kit for evaluating cell viability and proliferation [39].CCK-8 is widely used in cell biology research, pharmacology research, and drug screening because it is easy to use, does not require the handling of toxic solutions, and the test results can be measured directly on a microplate reader [40].
The CCK-8 methods are used to evaluate the possible cytotoxicity of PLLA/VB 2 composites.With increasing incubating time up to 72 h, the relative growth rate of PLLA/VB 2 composites (Figure 7A) is higher than 80%, which belongs to "grade 0", regarded as no cytotoxicity.In Figure 7B, the incubated cells present various shapes such as rod and shuttle, indicating they grow in good conditions.Thus, the PLLA/VB 2 composites present good biocompatibility and no cytotoxicity.

Figure 1 .
Figure 1.(A) FTIR, (B) XRD spectra and SEM images of (C) PLLA/VB 2 -15, (D) PLLA/VB 2 -20.In Figure1B, the XRD curves of pure PLLA show several sharp diffraction peaks located at 2θ = 15 • , 17 • , and 19 • , respectively, corresponding to the (010), (110)/(200), and (203) planes in PLLA.The addition of VB 2 leads to the appearance of a new diffraction peak at 2θ = 30.8• , which belongs to the (003) plane of the β-phase crystal in PLLA[26][27][28].In addition, with the increasing content of VB 2 , the diffraction peak intensity of the (003) plane increases accordingly.This phenomenon can be attributed to the heterogeneous nucleation promotion of VB 2 on the crystalline behavior of PLLA, which is a benefit of the improvement of piezoelectricity.Thus, the introduction of VB 2 not only helps the formation of hydrogen bonds between PLLA and VB 2 , but also promotes the content of polar β-phase crystals.As the hydrogen bond can promote the orientation of C=O dipoles in the easter bonds of PLLA, and the increasing of polar β-phase is benefit to the dipole orientation as reported in the PVDF-based piezoelectric polymer[30], the introduction of VB 2 can improve the piezoelectricity of PLLA.Figure1C,D provides the SEM images of PLLA/VB 2 composites with different VB 2 content.The PLLA and VB 2 are presented as light and dark phases, respectively.VB 2 distributes uniformly as a rod shape in the PLLA matrix at a content of 15 wt%, while at a higher content (20 wt%) of VB 2 , some agglomerations are observed.Meanwhile, the edge between PLLA and VB 2 seems indistinct, indicating a strong interaction as evidenced by the hydrogen bond in FITR spectra.The DSC heating and cooling curves are provided in Figure2A,B, respectively.The heating curves show that all the samples have similar thermal transition peaks.The peak at 62 • C is attributed to the glass transition of the samples, and the peak near 125 • C is due to their cold crystallization.The melting peaks of β-crystals and α-crystals are observed at 162 • C and 167 • C, respectively, and these melting peaks are partially overlapped.In addition, the melting enthalpy of β-crystals increased with the increasing VB 2 content, indicating that the crystallinity of β-crystals in the samples was improved by the addition of VB 2 .Meanwhile, the cold crystallization temperatures of the samples decreased with increasing VB 2 content, indicating an increase in the crystallinity of the β-crystals and a decrease in the crystallinity of the α-crystals.According to the calculation results, the crystallinity of β-crystals in PLLA increased from 91.93% to 93.97% with increasing VB 2

Figure 3 .
Figure 3. (A) Schematic diagram of VB 2 -enhanced PLLA piezoelectric performance, (B) schematic, actual diagram platform of output performance of PLLA/VB 2 composites and (C) output voltage of PLLA/VB 2 nanogenerators with different VB 2 contents.
is observed for PLLA/VB 2 -15; thus, it is selected for further investigation on piezoelectric acoustical performance.

Figure 4 .
Figure 4. (A) Experimental platform for piezoelectric acoustical performance investigation.(B) Piezoelectric output of PLLA/VB2 composites with different VB2 content.(C) Piezoelectric output of PLLA/VB2-15 under different sound distance.(D) Piezoelectric output of PLLA/VB2-15 under different sound frequency.(E) Measurement error between input and detected sound frequency.

Figure 4 .
Figure 4. (A) Experimental platform for piezoelectric acoustical performance investigation.(B) Piezoelectric output of PLLA/VB 2 composites with different VB 2 content.(C) Piezoelectric output of PLLA/VB 2 -15 under different sound distance.(D) Piezoelectric output of PLLA/VB 2 -15 under different sound frequency.(E) Measurement error between input and detected sound frequency.

Figure
Figure4C,D show the piezoelectric output of PLLA/VB 2 -15 with variated sound distance and frequency.The output voltage of PLLA/VB 2 -15 increases with the decrease in sound distance (which equals to increase in sound amplitude).As sound is one of the mechanical waves, it leads to surface vibration and deformation when it propagates to the surface PLLA/VB 2 -15.Meanwhile, piezoelectric charges are generated in response to the change in the degree of polarization.In this view, shorter distance and higher amplitude intensify this process, leading to higher piezoelectric voltage.Meanwhile, PLLA/VB 2 -15 show higher piezoelectric output to sound with higher frequency ranging from 0 to 20kHz, indicating frequency-dependent properties within the human auditory range.The frequency-dependent response of PLLA/VB 2 -15 enables its capacity to recognize sound frequency.The voltage-time curves can be converted to amplitude-frequency curves by using a fast Fourier transform algorithm.Thus, a standard sound sample with a fixed frequency is used to investigate the detected error of PLLA/VB 2 -15.In Figure4E, the PLLA/VB 2 -15 shows an average error of less than 0.1% at a frequency from 0 to 20 kHz, indicating a high accuracy of sound frequency recognition.

Figure 5 .
Figure 5. Piezoelectric output of PLLA/VB 2 -15 stimulated by (A) male and (C) female sound sample and (B,D) FFT curves; (E) piezoelectric voltage and (F) current of PLLA/VB 2 -15 stimulated by sound sample.In our daily lives, sound recognition technology is used to judge or distinguish the speaker by capturing the speaker's voice and extracting the voiceprint feature signal.Since the PLLA/VB 2 -15 piezoelectric device has good sound recognition capability, it is used as an acoustic sensor.Five different people successively shouted the same seven Chinese characters based on their own speaking habits when they were 40 cm away from the PLLA/VB 2 -15 acoustic

Table 1 .
Comparative data between PLLA/VB 2 and other piezoelectric materials.