Nucleator induced highly oriented crystalline structure of poly(lactic acid) fiber enables superior intrinsic piezoelectric and antibacterial effect†

Wenjun He^a, Chenglong Yan^b, Xiaowen Zhao^a, Yuanchun Zhang*^b and Lin Ye*^a
aState Key Laboratory of Polymer Materials Engineering, Polymer Research Institute of Sichuan University, Chengdu 610065, China. E-mail: yelinwh@126.com; Tel: +86-28-85408802
^bPanzhihua Steel City Group Co. Ltd, Panzhihua 617000, Sichuan, China. E-mail: zyc34088@163.com; Tel: +86 13548214630

First published on 14th August 2024

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1. Introduction

Infectious diseases, such as Corona Virus Disease (COVID) and Ebola virus disease, pose severe threats to human health, killing millions of people worldwide annually, due to their emergence, reemergence, and persistence.^1–5 Thus, novel personal protective equipment (PPE), including face masks and protective clothing, is in urgent demand. However, conventional PPE is mainly made of polyethylene (PE), polypropylene (PP), polyethylene terephthalate (PET), etc., causing serious environmental pollution, owing to their non-biodegradability and the improper disposal of PPE.^6–8 Moreover, antimicrobial PPE is usually achieved by adding antimicrobial agents, such as Ag, ZnO, Cu, or CuO, which are toxic to human beings, non-eco-friendly, and easily lost in use.⁹ Therefore, the development of safe, biodegradable and eco-friendly PPE with an intrinsic antimicrobial property is very important for its application.

Recently, piezoelectric antibacterial materials have been widely reported, which can generate electrical signals with mechanical strain, to generate reactive oxygen species (ROS), and induce disturbance of the normal functions of the pathogen by oxidizing membrane lipids and amino acids in proteins, cross-linking of proteins and oxidative damage to nucleic acids, thus achieving antibacterial effects.^10,11 Most piezoelectric antibacterial materials are prepared by adding piezoelectric nanoparticles, such as barium titanate (BaTiO₃) or FeWO₄, and the piezoelectric effects could be triggered and then generate ROS effectively, exhibiting superior antibacterial efficiency.^12–15 Poly(lactic acid) (PLA), as a biodegradable aliphatic polyester, derived from renewable agricultural resources, such as corn, starch and sugarcane, is considered the most promising sustainable substitute for petroleum-based polymers, due to its superior thermoplastic processability, mechanical properties, biocompatibility and biodegradability.^16–18 Besides, PLA exhibits weak antibacterial properties, due to the weak acidic property of lactic acid and carbon dioxide produced by its hydrolysis. Furthermore, in the 1950s, Fukada first discovered that PLA also presented piezoelectric effects by stretching orientation, due to the presence of chiral carbon in the repeating units of PLA molecules and the enhanced asymmetric arrangement of –CO bonds in helical chains.^19–21 Afterwards, a PLA-based piezoelectric system and antibacterial effects were reported successively. Hyun Ju Oh et al. prepared piezoelectric PLA/BaTiO₃ fibers through a melt-spinning process.²² With increasing draw ratio (DR), the output voltage increased from 800 mV at a DR of 1 to 1.46 V at a DR of 3, but it decreased at a DR of 3.5. Tianyi Zheng et al. composited piezoelectric Ca/Mn co-doped BaTiO₃ (CMBT) nanofibers with PLLA, which were then subjected to polarization, after which the piezoelectric coefficient d₃₃ and inhibition rate against S. aureus bacteria reached 3.5 pCN and ∼50% compared with PLLA samples (0.38 pCN and ∼0%).²³ Qingjie Liu et al. fabricated piezoelectric wound dressings using fibrous composites of PLLA, poly(ethylene glycol) (PEG) and barium titanate (BT) by electrospinning.¹¹ With the incorporation of 10 wt% PEG and 8 wt% BT, the composite film showed excellent piezoelectricity with an output voltage of ∼6 V and an inhibition rate of 92% against S. aureus bacteria. However, the incorporation of a large amount of inorganic piezoelectric nanoparticles not only damaged the mechanical properties of PLA, but also weakened its biodegradability and biodegradability.

Self-assembling nucleators could aggregate into special frameworks by themselves upon cooling by dissolving them in a polymer melt, inducing crystallization of the polymer with different kinds of crystalline morphology. A hydrazide nucleating agent (HNA) was found to self-assemble into fibrillar networks, inducing crystallization of PLA on the surface, forming a “shish-kebab” fibrillar crystalline material with high crystallinity, as reported by our previous work.²⁴ Herein, in this work, HNA was introduced into PLA through melt blending, and PLA–HNA fibers with different DR were prepared by melt spinning–hot drawing dual processes, to enhance the oriented crystalline structure and obtain PLA–HNA fibers with superior mechanical, intrinsic piezoelectric and antibacterial properties. The PLA–HNA fibers were knitted into a plain fabric. The effect of HNA nucleators on the formation of highly oriented crystalline PLA fibers was studied, while the piezoelectric and antibacterial effects of the PLA–HNA fibers and fabric were further explored.

2. Experimental section

2.1 Materials

Poly(lactic acid) (PLA, grade 4032D) was purchased from Natureworks LLC, with melt flow index and density of 7 g/10 min and 1.24 g cm⁻³, respectively. The hydrazide nucleating agent (HNA) was provided by Shanxi Provincial Institute of Chemical Industry, China.

2.2 Sample preparation

The procedure for preparing PLA–HNA fibers was as follows. First, PLA granules were mixed evenly with 0.7 wt% HNA and melt extruded at a temperature of 165–185 °C. The granulated PLA–HNA samples were dried at 90 °C for 12 h; then the as-spun PLA–HNA fibers were produced by melt spinning at a temperature of 170–190 °C, and further hot drawn at a temperature of 90–110 °C. The draw ratio (DR) of PLA fibers was calculated with eqn (1):

DR = vlast/vfirst	(1)

where v_first and v_last are the roller speed of the first heating roller and the last heating roller, respectively, as shown in Fig. 1(a).

The PLA–HNA-6 fiber was knitted into a plain weave fabric, where the yarn twist density was 45 turns/10 cm, and the knitted density of the fabric along the longitudinal and latitudinal directions was 256 strands/10 cm and 460 strands/10 cm, respectively.

2.3 Measurements

The morphologies of the PLA fibers were observed with a JEOL JSM-5900LV scanning electron microscope (SEM) (Japan). The fibers were etched with a methanol–water mixture solution (1:2 by volume) containing 0.25 mol L⁻¹ sodium hydroxide to selectively remove both HNA nucleators and amorphous regions from the crystals. The crystallization behavior of the PLA fibers was analyzed with a D8 Discover two-dimensional wide-angle X-ray diffractometer (2D-WAXD) from Bruker AXS Co. (Germany), a two-dimensional small-angle X-ray scattering (2D-SAXS) instrument from Xenocs SA (France) and a 204 Phoenix differential scanning calorimeter (DSC) from Netzsch-Gerätebau GmbH (Germany). For DSC measurement, a 7–10 mg fiber was heated from 25 °C to 250 °C at a heating rate of 10 °C min⁻¹, and the crystallinity (X_c) was calculated with eqn (2):²⁵

	(2)

where ΔH_m is the melting enthalpy of the sample and ΔH⁰_m is the melting enthalpy of 100% crystallized PLA, which is 93 J g⁻¹. The mechanical properties of the PLA fibers were measured on an XQ-1A electronic monofilament strength tester (Shanghai New Fiber Instrument Co., Ltd, China) with stretching rate of 20 mm min⁻¹ and gauge length of 20 mm at room temperature (25 °C).

In order to test the piezoelectricity of the PLA fibers, 3 strands of PLA fiber bundles (∼1200 filaments in each bundle) were braided with a fixed length of 10 cm (Fig. S1†); then conductive copper foil was attached on both sides and the sample was packaged with polyimide tapes (Fig. S2(a)†). The piezoelectric output signal was tested through a liner motor testing system, including a linear motor (HS01-37×166, NTIAG, USA), an electrometer (6514, Keithley, USA) and a low-noise preamplifier (SR750, SRS, USA), which were used to collect the output voltage and current signals of the samples under a certain acceleration (5 m s⁻²), and a periodic external force was applied by a linear motor using the Labview system (Fig. S2(b)†). To test the piezoelectric response under ultrasonication, an ultrasonic transmitter was incorporated into the testing system under different transmitting distances from the PLA samples, as shown in Fig. 7(a).

Antibacterial experiments were carried out as follows. First, a 1.8 wt% nutrient broth for culture media was mixed with deionized water to prepare the liquid medium, and 2 wt% agar powder was further mixed with the liquid medium to prepare a solid medium. Both the solid and liquid media were sterilized at 120 °C for 1 h before use. Second, a single strain resuscitated E. coli colony was put into 10 mL of liquid medium and shaken at 37 °C for 18 h, and 200 μL of E. coli solution (1st generation) was diluted with 10 mL of liquid media containing PLA samples, while the liquid medium without a PLA sample was set as a control sample (the weight of all PLA samples was 0.2 g), and the mixed bacterial solution (2nd generation) was cocultured in an ultrasound environment at 37 °C for 2 h. Optical density values at 630 nm (OD₆₃₀) of the bacterial solution (2nd generation) were tested before and after ultrasonic coculture. Finally, the bacterial solutions (2nd generation) were further diluted 1000 times and 100 μL of diluted solutions were coated onto the solid medium, and the samples were incubated at 37 °C for 18 h. Then the colony forming units (CFU) of all the samples were determined by plate counting, and the inhibition rate was calculated with eqn (3):

	(3)

where CFU (sample) and CFU (control) are the statistical CFU counts of the PLA sample and control sample, respectively.

To detect the generation of singlet oxygen (¹O₂) for the PLA fiber and fabric samples under ultrasonication, an electron paramagnetic resonance (EPR) spectrometer from Bruker (EMXplus, Germany) was employed to obtain the EPR spectra, in the presence of an ¹O₂ scavenger (2,2,6,6-tetramethylpyridine, TEMP, 100 mmol L⁻¹) in deionized water.

3. Results and discussion

3.1 Preparation of PLA–HNA fibers with high mechanical properties

PLA–HNA fibers were prepared by melt spinning–hot drawing dual processes, as illustrated in Fig. 1(a). First, the PLA–HNA granules were prepared by melt blending PLA and HNA as nucleators; then the as-spun PLA–HNA fiber was obtained by melt spinning. Finally, PLA–HNA-x fibers with different draw ratios (DR, x) were prepared through a hot drawing process.

The SEM images of the surfaces and cross-sections of PLA–HNA fibers with different DR are shown in Fig. 1(b–d). When the DR was 2, lots of elongated textures appeared on the surface of the drawn fiber along the drawing direction, because the fiber suffered relatively low tensile stress under insufficient roller speed difference during the hot drawing process, especially when the speed difference between the first (v_first) and the second (v_second) rollers was extremely small, resulting in rough textures in the fiber sample. However, above a DR of 4, the elongated textures disappeared, and the surface of the fiber became smooth by being prepared under a relatively high roller speed difference and tensile stress during the hot drawing process. With increasing DR, the fiber diameter decreased gradually, from 47.4 μm for PLA–HNA-2 fiber to 27.2 μm for PLA–HNA-6 fiber. Moreover, compared with neat PLA-6 fiber, the PLA–HNA-6 fiber showed lower fiber diameter distribution, indicating that the incorporation of HNA improved the spinnability and uniformity of the PLA fibers.

The stress–strain curves and mechanical properties of the PLA fibers are shown in Fig. 2(a–c), respectively. For PLA–HNA-2, the stress first increased sharply with strain, and then yielded, followed by a much longer elastic deformation plateau with ductile fracture characteristics. With increasing DR, the elastic deformation plateau of the stress–strain curves disappeared, exhibiting obvious strain-hardening behavior immediately after yielding. Meanwhile the tensile strength and modulus of the stretched PLA–HNA fibers increased remarkably, while the elongation at break decreased. With a DR of 6, the tensile strength and modulus increased from 120.30 MPa and 4.85 GPa for PLA–HNA-2 fiber to 645.27 MPa and 8.87 GPa, respectively, and the elongation at break decreased from 123.25% to 35.31%, which were also higher than those of neat PLA-6 fiber. Compared with PLA-based fibers reported in the literature, as shown in Fig. 2(d), PLA–HNA-6 fiber showed significantly improved tensile strength and modulus, exhibiting excellent mechanical properties.^26–34

3.2 HNA nucleators induced highly oriented crystalline structure of PLA–HNA fibers

The DSC curves, melting temperature (T_m) and crystallinity (X_c) of PLA–HNA fibers are shown in Fig. 3(a and b). For all as-spun fibers, three distinct peaks appeared, corresponding to the glass transition at 65 °C, cold crystallization at 85–105 °C, and a melting point around 170 °C. The as-spun fibers showed slow crystallization when cooling from the melt, and thus, imperfectly crystalline structures formed. For fibers with different DR, the glass transition and cold crystallization peaks disappeared. With increasing DR, the melting peak intensity increased, while the T_m and X_c increased from 170.1 °C and 26.5% for as-spun PLA–HNA fiber to 172.2 °C and 64.8% for PLA–HNA-6 fiber, respectively, indicating the stress-induced crystallization of PLA–HNA fiber by stretching orientation. Moreover, the PLA–HNA-6 sample exhibited higher T_m and X_c relative to PLA-6 (T_m: 171.7 °C; X_c (DSC): 60.3%).

2D-WAXD patterns of the PLA fibers are shown in Fig. 3(c). By melt spinning, all as-spun fiber samples presented weak and uniform Debye–Scherrer diffraction rings, indicating low X_c and orientation degree. By hot drawing, when DR was 2, the (200)/(110) reflection of the α/α′ crystal appeared as two strong diffraction arcs, while the (203) reflection of the α crystal appeared as weaker long diffraction arcs. However, as the DR increased to 4, the bright diffraction arc patterns of the α/α′ crystals became narrower, forming diffraction spots. When DR increased to 6, the PLA-6 and PLA–HNA-6 samples both showed more concentrated diffraction spots, indicating a stress-induced enhanced oriented crystalline structure under high DR. Moreover, compared with PLA-6, the diffraction spots of PLA–HNA-6 were even more concentrated, indicating that the introduction of HNA further promoted the formation of a highly oriented crystalline structure of PLA fibers.

The orientation factor of the crystalline regions (f_c) of the fiber was calculated by integrating the azimuthal angle (θ) with Herman's orientation function:

	(4)

	(5)

where I(θ) is the diffraction intensity. In the I(θ)–θ curves in Fig. 3(d) for the as-spun fibers, the peak intensities of the crystal plane were low, indicating low orientation degree in the sample. With increasing DR, the peak intensity increased, and the orientation degree of the crystalline lamellae increased perpendicular to the drawing direction, while the peak intensity of PLA–HNA-6 was also higher than that of PLA-6. The orientation factor of the amorphous regions (f_a) was also calculated, and the relative peak areas (A_amor) vs. θ curves for calculating f_a were obtained via peak separation and fitting through Gaussian functions (Fig. 3(e)), as described in ESI.† In Fig. 3(f), by melt spinning, all as-spun fibers showed low f_a, while f_c was not available. By hot drawing, with increasing DR, the orientation factor increased significantly from 0.080 for as-spun PLA–HNA to f_c = 0.473 and f_a = 0.709 for PLA–HNA-6. Moreover, the PLA–HNA-6 sample exhibited higher orientation degree relative to PLA-6 (f_c: 0.432; f_a: 0.653).

1D-WAXD spectra of PLA fibers are shown in Fig. 3(g). For as-spun fibers, no diffraction peak was observed. By hot drawing, a strong peak at 2θ of 16.5° was observed, corresponding to the diffraction of the (200)/(110) plane of the α crystal of the sample. With increasing DR, the crystallization peak of the α crystal became stronger, indicating enhanced crystallization for PLA–HNA fibers. The X_c and grain size (L_hkl) of the PLA granules and fibers were calculated with the following equations:

	(6)

	(7)

where I_c and I_a are the integral peak areas of the crystalline and amorphous regions, L_hkl refers to the grain size perpendicular to the hkl planes, λ is the incident X-ray wavelength (nm), θ is the Bragg angle, β is the half width of the crystalline peak, and k is the Scherrer shape factor.

X_c and L_hkl for PLA granules and fibers are shown in Fig. 3(h). With increasing DR, X_c increased, while L_hkl decreased, indicating that the crystallization of the fiber became more perfect, while the large-sized lamellae were broken and slipped under stress to form small-sized lamellae. Compared with PLA–HNA-2 fiber, the X_c and L_hkl of the PLA–HNA-6 fiber changed from 53.66% and 244.2 Å to 64.47% and 116.5 Å, respectively. Moreover, PLA–HNA-6 fiber also exhibited higher X_c values compared with PLA-6 (X_c (1D-WAXD): 56.12%).

2D-SAXS patterns of PLA fibers are shown in Fig. 4(a). For all as-spun fibers, a narrow strip distribution along the equatorial direction was observed, while a microfibrillar structure formed. By hot drawing, when DR was 2, a two-point pattern along the meridian direction was observed, indicating that the lamellae stacked in an orderly manner, while the streak pattern along the equator stands for the shish structures, suggesting that the shish-kebab crystalline structure formed at low DR. However, the pair of spots along the meridian disappeared for PLA–HNA-4 fiber, indicating the destruction and reorganization of lamellae under stress.³⁵ When DR further increased to 6, the streak pattern along the equator was enhanced, indicating that the orientation degree of the sample increased.

The electron density correlation function K(z) was as follows:³⁶

	(8)

where z is the correlation distance perpendicular to the lamellae and I(q) is obtained by integrating along the equator at different scattering vectors q. K(z) curves of PLA fibers are shown in Fig. 4(b). The fitted straight line of the declining region of the K(z) curves had an intersection with the horizontal baseline of the minimum point, corresponding to the average thickness of the lamellar layers (L_c) and amorphous layers (L_a) (X_c < 50%: L_c; X_c > 50%: L_a). Moreover, the long period (L_ac) was obtained from the z values at the peak of the K(z) curves (marked by arrows), and the L_a and L_c values were calculated according to L_ac = L_a + L_c. From the scattering intensity (I(q_y)) distribution along the q_y direction (Fig. 4(c)), the half-height width of the resulting Lorentz function (Δq_y) was related to the lateral size of the lamellae (L_lateral) by the relation L_lateral = 2π/Δq_y. In Fig. 4(d and e), by hot drawing, with increasing DR, for PLA–HNA fibers, L_c and L_ac increased, while L_a and L_lateral first decreased and then increased, and PLA–HNA-6 fiber showed higher L_c and L_lateral and lower L_a compared with as-spun fibers and even with PLA-6 fiber, suggesting the formation of a dense lamellar structure under stress by the hot drawing process.

The oriented crystalline morphology of etched PLA fibers was further investigated with SEM analysis (Fig. 4(f)). For PLA–HNA-2 fiber with low DR, shish-kebab crystalline structures were observed. With increasing DR, a large number of fibrillar crystalline structures appeared, forming compact microfibrillar structures along the hot drawing direction. The SEM result of etched PLA fibers was consistent with the 2D-SAXS results.

3.3 Intrinsic piezoelectric and antibacterial effect of PLA–HNA fibers with highly oriented crystalline structure

Fig. 5(a and b) show the open circuit voltage (V_oc) and current signals of PLA sheet, as-spun fibers and drawn fibers, and the peak-to-peak voltage (V_pp) and short-circuit current (I_sc) were further obtained. For isotrophic PLA sheet and as-spun PLA–HNA fiber, the V_pp and I_sc were both as low as 0.81 V and 9.5 nA, and 0.76 V and 9.0 nA, respectively. For drawn PLA–HNA fibers, with increasing DR, the V_pp and I_sc values rose from 0.78 V and 28.1 nA for PLA–HNA-2 fiber to 1.73 V and 35.6 nA for PLA–HNA-6 fiber samples, respectively. Compared with PLA-6 fiber (V_pp: 1.11 V; I_sc: 18.6 nA), incorporation of HNA improved the V_pp and I_sc of the fiber sample significantly, due to the enhanced oriented crystalline structure. The V_oc of PLA sheet and PLA–HNA-6 fiber under different external applied frequences is shown in Fig. S3 in ESI.† The V_pp first increased and then remained stable with increasing applied frequency, reaching 0.81 V and 1.73 V at a frequency of about 1.62 Hz for both samples.

Fig. 5(a and d) show the values of V_pp and response time for PLA fibers with different DR under diverse acceleration. The V_pp of all fiber samples increased gradually with increasing acceleration, and the fitting correlation coefficients (R²) were relatively high (>0.98), showing apparent linear relationships. The slope of the fitting lines, representing the piezoelectric sensitivity, increased with increasing DR, while the response time decreased. The PLA–HNA-6 fiber sample reached the highest V_pp of 1.96 V at an acceleration of 6 m s⁻², and the sensitivity increased from 0.097 for PLA–HNA-2 to 0.284, while the response time decreased from 156 ms to 136 ms, indicating that the PLA fibers with much perfect oriented crystalline structure, was more sensitive to external forces. In addition, the PLA–HNA-6 fiber samples were tested for ∼2000 cycles, and the V_oc is shown in Fig. 5(e). During the first 1000 cycles, V_pp showed a slight increase perhaps due to a triboelectric effect, and afterwards, V_pp tended to be stable, up to ∼2.5 V.

We knitted the PLA–HNA-6 fiber into plain fabric, and the V_oc and current signals of the fabric are shown in Fig. 5(f and g). Compared with the PLA–HNA-6 fiber sample, the PLA–HNA-6 fabric showed much higher V_pp and I_sc, which were 9.17 V and 210 nA, respectively. The fibers were twisted and tightly interweaved in the fabric (Fig. 5(h)), and during the test process where the sample was impacted by the linear motor, many more fibers in the fabric were squeezed and sheared each other, producing remarkably higher deformation and stronger piezoelectric signals than the fiber bundles. Compared with PLA-based fibers, films, or generators reported in the literature (Fig. 5(i)), the PLA–HNA-6 fabric showed significantly high V_pp values, exhibiting excellent piezoelectric properties.^37–42

Fig. 6(a) shows the power density of the PLA–HNA-6 fiber/fabric samples under different external resistances loads, for which the effective force areas were 1.5 cm² and 4 cm². With increasing external resistance, the output power density of both the fiber and fabric samples showed a tendency to first increase and then decrease. The fiber sample exhibited a maximum power density of 0.68 mW m⁻² at a resistance load of 20 MΩ, which reached as high as 5.44 mW m⁻² at a resistance load of 30 MΩ for the fabric sample. As shown in Fig. 6(b), we set the fiber/fabric samples under external forces as a power supply; then converted the generated AC signals into DC signals through a rectifier bridge and charged a 1 μF capacitor, while the charging–discharging behaviors were monitored within ∼900 seconds (discharging the capacitor once it was charged to 5 V). The time for a single charge–discharge cycle was ∼180 s for the fiber sample, while it was ∼90 s for fabric, which indicated that the fabric showed much higher charging efficiency than the fiber bundles. Furthermore, the charged capacitor (10 μF) could drive the working of a commercial LED and electronic watch (Fig. 6(c)).

To realize a piezoelectric antibacterial effect, an ultrasonic transmitter was incorporated into the piezoelectric testing system, and a schematic diagram of the measurement of the piezoelectric signals for the PLA samples under ultrasonication is shown in Fig. 7(a). The V_oc signals varied when switching the ultrasonication on or off (Fig. 7(b)), and with decreasing transmission distance, V_pp increased from 40.0 mV at a distance of 5 cm to 151 mV at a distance of 1 cm for the PLA–HNA-6 fabric sample (Fig. 7(c)). Meanwhile, the PLA–HNA-6 fiber and fabric samples showed much higher V_pp (89.3 and 95.8 mV, respectively) than those of PLA sheet and fiber samples (54.2 and 64.5 mV) at a distance of 3 cm (Fig. 7(d)). Such a piezoelectric response for PLA samples under ultrasonication made piezoelectric antibacterial applications possible.

The antibacterial properties of the PLA samples are shown in Fig. 8. The optical density at 630 nm (OD₆₃₀ values) of bacterial solutions without/with ultrasonic coculture are shown in Fig. 8(a). The OD₆₃₀ values before coculture were ∼0.030 for all samples, while after coculture without ultrasonication, for the control and isotrophic PLA sheet samples, the OD₆₃₀ increased significantly to 0.092 and 0.082, respectively, while the fiber and fabric samples showed less increase (less than 0.060). However, by coculture with ultrasonication at a distance of 3 cm, the OD₆₃₀ values for all samples decreased, and the PLA–HNA-6 fiber and fabric samples showed much lower OD₆₃₀ values (as low as 0.038 and 0.037, respectively) than those of control, PLA sheet, and PLA-6 samples (0.086, 0.050, 0.042), indicating significantly enhanced antibacterial properties under ultrasonic coculture. Moreover, the diluted E. coli bacterial solutions were coated onto the solid agar medium to count the statistical colony forming units (CFU) values (Fig. 8(b)). By coculture without ultrasonication, all PLA samples exhibited weak antibacterial abilities, while by ultrasonic coculture, compared with the control/PLA sheet sample with a dense colony, many fewer colonies were observed for the PLA fiber samples, while only a few colonies were found for the PLA–HNA-6 fiber and fabric samples. In Fig. 8(c), both the PLA–HNA-6 fiber and fabric samples inhibited E. coli bacteria almost completely, reaching an inhibition rate of more than 99% (99.22% and 99.71%, respectively), much higher than that of the PLA-6 fiber sample (84.32%), and exhibited excellent antibacterial properties. It should be noted that under an ultrasonic environment, the PLA fibers and fabric with highly oriented crystalline structure and twisted or tightly interweaved fibers could release strong piezoelectric signals to induce a significant antibacterial effect by deformation. Furthermore, EPR analysis was employed to detect the generation of singlet oxygen (¹O₂) for PLA–HNA-6 fiber and fabric without/with ultrasonication. In Fig. 8(d), by ultrasonication, the representative triple ¹O₂ peaks (3493G, 3510G, 3528G) were detected for these two samples, demonstrating that piezoelectric PLA fiber and fabric achieved antibacterial effects through generating ROS.

Based on the above analysis, the improvement mechanism of the intrinsic piezoelectric and antibacterial effect of PLA–HNA fiber/fabric is proposed in Fig. 9. PLA fibers with high DR (PLA-6) prepared by melt spinning–hot drawing dual processes, exhibited a high orientation factor and crystallinity (f_c: 0.432; f_a: 0.653; X_c (DSC): 60.3%). The incorporation of HNA nucleators further improved the orientation crystallization degree of the PLA fiber (f_c: 0.473; f_a: 0.709; X_c(DSC): 64.8%), endowing the PLA–HNA-6 fiber with significantly enhanced intrinsic piezoelectric and antibacterial effects (V_pp: 1.73 V (89.3 mV under ultrasonication (US)); I_sc: 35.6 nA; inhibition rate for E. coli bacteria: 99.22%) relative to PLA-6 fiber (V_pp: 1.11 V (64.5 mV under US); I_sc: 18.6 nA; inhibition rate for E. coli bacteria: 84.32%). Meanwhile, with increasing draw ratio of the PLA–HNA fiber, the X_c, f_c, and f_a of the fiber all increased, leading to an obvious improvement in the piezoelectric output and antibacterial effects. The formation of a highly oriented crystalline structure facilitated an enhanced asymmetric arrangement of –CO bonds in helical PLA chains, while the generated strong piezoelectric signals under an external force (even under ultrasonication) triggered the production of singlet oxygen (¹O₂), thus achieving excellent antibacterial effects.^43,44 Moreover, the knitted PLA–HNA-6 fabric with twisted and tightly interweaved fibers even exhibited significantly enhanced piezoelectric and antibacterial effects (V_pp: 9.17 V (95.8 mV under US); I_sc: 210 nA; inhibition rate for E. coli bacteria: 99.71%), by the fibers squeezing and shearing each other in the fabric under an external force, producing much higher deformation and stronger piezoelectric signals than the fiber bundles.

4. Conclusions

Self-assembling HNA was introduced into PLA through melt blending, and PLA–HNA fibers with different DR were prepared by melt spinning–hot drawing dual processes. Incorporation of HNA improved the spinnability and uniformity of the fibers, and enhanced the oriented crystalline structure under high DR (as high as 6), exhibiting higher orientation degree in both crystalline and amorphous phases (f_c: 0.473; f_a: 0.709) and X_c (64.8%) relative to neat PLA fibers (f_c: 0.432; f_a: 0.653; X_c: 60.3%). With increasing DR, the shish-kebab crystalline structures of the fibers (DR = 2) changed to fibrillar crystalline structures (DR > 2), forming compact microfibrillar structures along the hot drawing direction, and the tensile strength and modulus increased remarkably, reaching as high as 645.27 MPa and 8.87 GPa for PLA–HNA-6 fiber. Meanwhile, the V_pp, I_sc and piezoelectric sensitivity values of the PLA–HNA fibers increased from 0.78 V, 28.1 nA, and 0.097 for PLA–HNA-2 fiber to 1.73 V, 35.6 nA, and 0.284 for PLA–HNA-6 fiber, respectively, and exhibited high cycling stability during a ∼2000 cycle test. Moreover, the knitted PLA–HNA-6 fabric with twisted and tightly interweaved fibers exhibited far more enhanced piezoelectric properties (V_pp: 9.17 V; I_sc: 210 nA), and showed higher power density and charging efficiency than PLA–HNA-6 fiber, which could drive a commercial LED and electronic watch to work under external forces as a power supply. Furthermore, both the PLA–HNA-6 fiber and fabric samples inhibited E. coli bacteria almost completely by ultrasonic coculture, reaching an inhibition rate of more than 99% (99.22% and 99.71%, respectively), exhibiting excellent intrinsic antibacterial properties and promising application potential as safe, biodegradable and eco-friendly personal protective equipment.

Data availability

The data supporting this article have been included as part of the ESI.†

Author contributions

Wenjun He: conceptualization, methodology, software, investigation, writing – original draft. Chenglong Yan: conceptualization, methodology, software, investigation, writing – original draft. Xiaowen Zhao: conceptualization, investigation, supervision, writing – review & editing, funding acquisition. Yuanchun Zhang: conceptualization, methodology, software, investigation, writing – original draft. Lin Ye: conceptualization, investigation, supervision, writing – review & editing, funding acquisition.

Conflicts of interest

This work has no conflict of interest.

Acknowledgements

This work was supported by National Key R&D Program of China (No. 2023YFE0105600), National Natural Science Foundation of China (No.51933007), Natural Science Foundation of Sichuan Province (No. 2024NSFSC0196), and Project of Engineering Characteristic Team of Sichuan University. We also appreciate Wang Hui from the Analytical & Testing Center of Sichuan University for her help in SEM characterization.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4ta04657a

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