Antibacterial, Flexible, and Conductive Membrane Based on MWCNTs/Ag Coated Electro-Spun PLA Nanofibrous Scaffolds as Wearable Fabric for Body Motion Sensing

In the present study, flexible and conductive nanofiber membranes were prepared by coating PLA nanofibrous scaffolds with carbon nanotubes and silver nanoparticles. The morphology and structure of the prepared membrane was characterized, as well as its mechanical properties, electrical sensing behavior during consecutive stretching-releasing cycles and human motion detecting performance. Furthermore, the antibacterial properties of the membrane was also investigated. Due to the synergistic and interconnected three-dimensional (3D) conductive networks, formed by carbon nanotubes and silver nanoparticles, the membrane exhibited repeatable and durable strain-dependent sensitivity. Further, the prepared membrane could accurately detect the motions of different body parts. Accompanied with promising antibacterial properties and washing fastness, the prepared flexible and conductive membrane provides great application potential as a wearable fabric for real-time body motion sensing.


Preparation of PLA Nanofibrous Scaffolds
The PLA nanofibrous scaffolds were prepared using an electrospinning method, described as follows. Typically, 1.5 g of PLA pellets were first dissolved in 10 mL of dimethyl formamide (DMF)/tetrahydrofuran (THF) mixed solution in which the volume ratio of DMF/THF was 1/9. After the PLA was fully dissolved, the PLA solution was then transferred into a 20 mL syringe, equipped with a constant speed syringe pump. For the electrospinning process, a constant distance of 20 cm between the syringe tip and rotating collector was applied, during which the jet rate of the PLA solution was set at 1.5 mL/h, and the applied voltage was set at 20.0 kV. After electrospinning, the PLA nanofibrous scaffolds were maintained at 60 • C for 24 h in a vacuum oven before further processing.

Preparation of MWCNTs/Ag/PLA Nanofibrous Membrane (MAPNM)
The MWCNTs were first pre-acidified by concentrated sulfuric acid and nitric acid, according to the previous study. At the same time, 50 mg of AgNO 3 was dissolved in 50 mL of distilled water (H 2 O). Afterwards, 100 mg of acidified MWCNTs was added into the above AgNO 3 solution, and the mixture was sonicated for 2 h to obtain a homogeneous dispersion. Above-dried PLA nanofibrous scaffolds in the amount of 0.5 g were then put into the MWCNTs/Ag dispersion. After being soaked in this dispersion for 1 h with continuous magnetic stirring (200 r/min), the coated PLA nanofibrous scaffolds were separated, washed with H 2 O for 3 times to wash the uncoated MWCNTs and AgNO 3 residues, dried at 60 • C for 24 h in a vacuum oven, and the MWCNTs/Ag/PLA nanofibrous membrane (MAPNM) was finally obtained. The procedures were schematically shown in Figure 1. For comparison, the pure acidified MWCNTs coated PLA nanofirous membrane (MPNM) without the introduction of AgNO 3 was also prepared following similar preparation procedures.
Polymers 2019, 11, x FOR PEER REVIEW 3 of 12 (H2O). Afterwards, 100 mg of acidified MWCNTs was added into the above AgNO3 solution, and the mixture was sonicated for 2 h to obtain a homogeneous dispersion. Above-dried PLA nanofibrous scaffolds in the amount of 0.5 g were then put into the MWCNTs/Ag dispersion. After being soaked in this dispersion for 1 h with continuous magnetic stirring (200 r/min), the coated PLA nanofibrous scaffolds were separated, washed with H2O for 3 times to wash the uncoated MWCNTs and AgNO3 residues, dried at 60 °C for 24 h in a vacuum oven, and the MWCNTs/Ag/PLA nanofibrous membrane (MAPNM) was finally obtained. The procedures were schematically shown in Figure 1. For comparison, the pure acidified MWCNTs coated PLA nanofirous membrane (MPNM) without the introduction of AgNO3 was also prepared following similar preparation procedures.

Mechanical and Electrical Tests of the Nanofibrous Membrane
The mechanical properties of the membranes were measured by a universal tensile testing machine (Instron 5966, Instron Corporation, Canton, OH, USA) with a 500 N cell at room temperature. The membranes were cut into a 30 × 5 mm 2 rectangular shape with the thickness of ~0.4 mm. The extension rate was 5 mm/min and the gauge length was 20 mm. The resistance of the prepared nanofibrous membrane was tested by a universal multimeter (Fluke, F115C, Fluke Corporation, Everett, WA, USA). The resistance change of the MAPNM during the stress-strain test was conducted by the Instron 5966 universal testing machine and recorded by the Keithley 2000E multimeter (Tektronix, Inc, Beaverton, OR, USA). The resistance data of the samples was acquired simultaneously as a function of the applied strain. Two conductive silver wires connecting to the multimeter were linked to each end of the samples before testing.
The washing fastness of the MAPNM was conducted by measuring the electrical resistance change (R/R0) and mechanical strength of one piece of 30 × 5 × 0.4 mm 3 MAPNM sample after 1-5 washing cycles. In each washing cycle, the sample was put into 100 mL of 40 °C H2O, stirred mechanically at 600 r/min for 20 min, taken out and dried at 60 °C.

Mechanical and Electrical Tests of the Nanofibrous Membrane
The mechanical properties of the membranes were measured by a universal tensile testing machine (Instron 5966, Instron Corporation, Canton, OH, USA) with a 500 N cell at room temperature. The membranes were cut into a 30 × 5 mm 2 rectangular shape with the thickness of~0.4 mm. The extension rate was 5 mm/min and the gauge length was 20 mm. The resistance of the prepared nanofibrous membrane was tested by a universal multimeter (Fluke, F115C, Fluke Corporation, Everett, WA, USA). The resistance change of the MAPNM during the stress-strain test was conducted by the Instron 5966 universal testing machine and recorded by the Keithley 2000E multimeter (Tektronix, Inc, Beaverton, OR, USA). The resistance data of the samples was acquired simultaneously as a function of the applied strain. Two conductive silver wires connecting to the multimeter were linked to each end of the samples before testing. The washing fastness of the MAPNM was conducted by measuring the electrical resistance change (R/R 0 ) and mechanical strength of one piece of 30 × 5 × 0.4 mm 3 MAPNM sample after 1-5 washing cycles. In each washing cycle, the sample was put into 100 mL of 40 • C H 2 O, stirred mechanically at 600 r/min for 20 min, taken out and dried at 60 • C.

Strain Sensing and Motion Sensing Tests of the Nanofibrous Membrane
The resistance change of MAPNM during consecutive stretching/release cycles was also recorded by the Instron 5966 and Keithley 2000E. The sample was stretched to 3% of elongation and released to initial state for 100 cycles. The shape of the MAPNM, gauge length were in accordance with the stress-strain test. The extension speed was set at 40 mm/min. The resistance changes during consecutive body motions, including finger, inner elbow, knee joints and forehead, with different bending angles of the MAPNM were also recorded by Keithley 2000E. During the test, the MAPNM sample, with the shape based on the dimensions, 30 × 5 × 0.4 mm 3 , was tightly attached to the respect part, and each part was bended and released to initial state for 50 time.

Antibacterial Test
The antibacterial properties of the prepared MAPNM and MPNM membranes against Escherichia coli (E. coli) and Staphylococcus aureus (S. aureus) were evaluated by both inhibition zone and co-culture methods, and described as follows [21]. First, 100 µL of E. coli (ATCC8739) or S. aureus (ATCC6538) mother liquor was first grown in 900µL of fluid nutrient medium, and cultivated at 37 • C for 12 h to obtain a saturated bacterial suspension of 10 8 -10 9 CFU/mL. The bacterial suspension was then diluted to 10 5 -10 6 CFU/mL. For inhibition zone method, the diluted bacterial suspension was coated evenly on the surface of the autoclaved culture dishes. After the prepared membranes (10 × 10 mm 2 ) were placed on each bacteria suspension surface, the dishes were then incubated at 37 • C for 18 h. The inhibition zone around each sample was measured for the evaluation of the antibacterial properties of the prepared membranes. For co-culture method, 100 µL of diluted bacterial suspension, 900 µL of bacteria-free phosphate buffer saline solution and 5 mg of smashed membrane sample were mixed together and shock co-cultured at 37 • C for 12 h. The co-cultured suspension was then coated evenly on the surface of the autoclaved culture dishes, and cultivated at 37 • C for another 24 h. The antibacterial properties of the prepared membranes were evaluated by observing the growing status of the bacteria on the culture dish. For comparison, the co-culture of the bacteria, without the incorporation of the prepared membrane samples, was also conducted.

Results and Discussion
The morphology of the MAPNM was investigated first in terms of SEM and TEM shown in Figure 2. It could be seen from Figure 2a,b that the PLA nanofibrous scaffolds had a hierarchical 3D network structure in which the nanofibers interconnect with each other. Moreover, the PLA nanofibers had a smooth and uniform appearance with an average diameter of~600 nm. After MWCNTs and AgNO 3 were coated, it was clearly observed from Figure 2c that the MAPNM had a rough nanofiber surface. From the magnified SEM image shown in Figure 2d, it was seen that the MWCNTs were tightly anchored on the surface of the PLA nanofiber surface, which constructed a continuous and integrated conductive pathway. It was notable that the existence of AgNPs could not be observed visually from the SEM image since the AgNPs were rather finite in size. Thus, the energy dispersive spectrum (EDS) mapping of the Ag element was then conducted with the results shown in Figure 2e,f. From the selected region, it could be clearly observed that the AgNPs were sufficiently and uniformly dispersed on the MAPNM surface. The distribution of AgNPs in MAPNM could be also be directly observed through TEM, with the results shown in Figure 2g,h. It could be seen From Figure 2g that the PLA nanofibers entangled with each other, in which fiber-shaped MWCNTs and AgNPs could be observed. From the magnified TEM image shown in Figure 2h, it could be clearly observed that the The structural information of the MAPNM was further investigated by FT-IR and XRD. Figure  3a shows the FT-IR spectra of PLA nanofibrous scaffolds, MPNM and MAPNM. It was seen that pure PLA had a characteristic peak in 1747 cm −1 , which was ascribed to the stretching vibration of its C=O ester groups within PLA skeleton [22]. When the MWCNTs and AgNPs was adhere onto the PLA matrix, this peak in both MPNM and MAPNM shifted a little to 1751 and 1753 cm −1 , which was might because the oxygen containing groups in acidified MWCNTs interacted with the groups in PLA through hydrogen bonding and/or ionic bonding [23]. Moreover, a series of new peaks at ~1550 cm −1 appeared in MPNM and MAPNM, which was ascribed to the vibrations of the benzene rings in MWCNTs [24].  Figure 3b shows the XRD patterns of PLA nanofibrous scaffolds, MPNM and MAPNM. It was observed that PLA had two characteristic peaks located at ~17° and 30°, which were ascribed to the α phase, and α' phase, of the PLA matrix, respectively [23]. When MWCNTs and AgNPs were coated onto the PLA surface, the characteristic peaks of MWCNTs at ~24° for MPNM and AgNPs at 37° and 43° for MAPNM could be observed from their XRD patterns [25]. For both MPNM and MAPNM, these two peaks could also be observed, which became broader and weaker, thereby, indicating a lower degree of crystallinity of the PLA in the composites. This was ascribed to the incorporation of The structural information of the MAPNM was further investigated by FT-IR and XRD. Figure 3a shows the FT-IR spectra of PLA nanofibrous scaffolds, MPNM and MAPNM. It was seen that pure PLA had a characteristic peak in 1747 cm −1 , which was ascribed to the stretching vibration of its C=O ester groups within PLA skeleton [22]. When the MWCNTs and AgNPs was adhere onto the PLA matrix, this peak in both MPNM and MAPNM shifted a little to 1751 and 1753 cm −1 , which was might because the oxygen containing groups in acidified MWCNTs interacted with the groups in PLA through hydrogen bonding and/or ionic bonding [23]. Moreover, a series of new peaks at 1550 cm −1 appeared in MPNM and MAPNM, which was ascribed to the vibrations of the benzene rings in MWCNTs [24]. The structural information of the MAPNM was further investigated by FT-IR and XRD. Figure  3a shows the FT-IR spectra of PLA nanofibrous scaffolds, MPNM and MAPNM. It was seen that pure PLA had a characteristic peak in 1747 cm −1 , which was ascribed to the stretching vibration of its C=O ester groups within PLA skeleton [22]. When the MWCNTs and AgNPs was adhere onto the PLA matrix, this peak in both MPNM and MAPNM shifted a little to 1751 and 1753 cm −1 , which was might because the oxygen containing groups in acidified MWCNTs interacted with the groups in PLA through hydrogen bonding and/or ionic bonding [23]. Moreover, a series of new peaks at ~1550 cm −1 appeared in MPNM and MAPNM, which was ascribed to the vibrations of the benzene rings in MWCNTs [24].  Figure 3b shows the XRD patterns of PLA nanofibrous scaffolds, MPNM and MAPNM. It was observed that PLA had two characteristic peaks located at ~17° and 30°, which were ascribed to the α phase, and α' phase, of the PLA matrix, respectively [23]. When MWCNTs and AgNPs were coated onto the PLA surface, the characteristic peaks of MWCNTs at ~24° for MPNM and AgNPs at 37° and 43° for MAPNM could be observed from their XRD patterns [25]. For both MPNM and MAPNM, these two peaks could also be observed, which became broader and weaker, thereby, indicating a lower degree of crystallinity of the PLA in the composites. This was ascribed to the incorporation of  Figure 3b shows the XRD patterns of PLA nanofibrous scaffolds, MPNM and MAPNM. It was observed that PLA had two characteristic peaks located at~17 • and 30 • , which were ascribed to the α phase, and α' phase, of the PLA matrix, respectively [23]. When MWCNTs and AgNPs were coated onto the PLA surface, the characteristic peaks of MWCNTs at~24 • for MPNM and AgNPs at 37 • and Polymers 2020, 12, 120 6 of 11 43 • for MAPNM could be observed from their XRD patterns [25]. For both MPNM and MAPNM, these two peaks could also be observed, which became broader and weaker, thereby, indicating a lower degree of crystallinity of the PLA in the composites. This was ascribed to the incorporation of the conductive nanomaterials, which restrained the mobility and affected the regularity of the polymer chain [26]. These two peaks shifted into smaller 2θ values, indicating the formation of interactions between the nanoparticles and matrix, which was in accordance with the FT-IR results.
The mechanical properties of the prepared samples were then studies through stress-strain test with the results shown in Figure 4a. Figure 4b shows the detailed mechanical stress and strain values of the respect sample. As could be seen, the PLA nanofibrous scaffolds showed a classic plastic mechanical behavior with the tensile stress of 4.35 MPa, and breaking elongation of 4.86%. After the conductive nanoparticles were coated on the surfaces of the PLA nanofibers, both MPNM and MAPNM showed an obvious increase in the tensile stress and a decrease in the elongation at break. Specifically, the tensile stress and tensile strain for MAPNM were 5.01 MPa, and 4.44%, respectively. This was ascribed to the interactions between the rigid nanoparticles and the PLA matrix, which resulted in the mechanical strength enhancement of the composites compared with the pure PLA membrane.
Polymers 2019, 11, x FOR PEER REVIEW 6 of 12 the conductive nanomaterials, which restrained the mobility and affected the regularity of the polymer chain [26]. These two peaks shifted into smaller 2θ values, indicating the formation of interactions between the nanoparticles and matrix, which was in accordance with the FT-IR results. The mechanical properties of the prepared samples were then studies through stress-strain test with the results shown in Figure 4a. Figure 4b shows the detailed mechanical stress and strain values of the respect sample. As could be seen, the PLA nanofibrous scaffolds showed a classic plastic mechanical behavior with the tensile stress of 4.35 MPa, and breaking elongation of 4.86%. After the conductive nanoparticles were coated on the surfaces of the PLA nanofibers, both MPNM and MAPNM showed an obvious increase in the tensile stress and a decrease in the elongation at break. Specifically, the tensile stress and tensile strain for MAPNM were 5.01 MPa, and 4.44%, respectively. This was ascribed to the interactions between the rigid nanoparticles and the PLA matrix, which resulted in the mechanical strength enhancement of the composites compared with the pure PLA membrane. The electrical response of the MPNM and MAPNM, during the stress-strain test, were also studied. Figure 4c shows the relative resistance change of the MPNM and MAPNM as a function of the applied strain. It was clearly observed that, due to the excellent electrical conductivity of the MWCNTs and the AgNPs, both MPNM and MAPNM exhibited promising electrical properties, which showed a monotonic increase in their R/R0 values, and preserved their electrical properties until the membranes were stretched to break. Moreover, it was also seen that during the whole stretching test process, the MAPNM exhibited a near-linear response to the applied strain. On the contrary, the MPNM showed more drastic change in its resistance during the stress-strain test. It is generally known that the resistance of the conductive polymer composite increases with the increase of the applied strain, which is ascribed to the disconnection of the conductive pathways while the polymer composite is stretched [27]. Many previous studies have shown that, when designing a conductive polymer composite, the incorporation of multi-dimensional conductive particles composed of one 0D component and one 1D component could form synergetic and well-connected conductive pathways within the polymer matrix, through the confinement effect and volume exclusion effect, leading to much stronger 3D conductive networks of the prepared polymer composite [28,29]. As schematically shown in Figure 4d, for MAPNM prepared in the present study, the hybrid MWCNTs/AgNPs formed synergistic and interconnected networks on the PLA nanofiber  The electrical response of the MPNM and MAPNM, during the stress-strain test, were also studied. Figure 4c shows the relative resistance change of the MPNM and MAPNM as a function of the applied strain. It was clearly observed that, due to the excellent electrical conductivity of the MWCNTs and the AgNPs, both MPNM and MAPNM exhibited promising electrical properties, which showed a monotonic increase in their R/R 0 values, and preserved their electrical properties until the membranes were stretched to break. Moreover, it was also seen that during the whole stretching test process, the MAPNM exhibited a near-linear response to the applied strain. On the contrary, the MPNM showed more drastic change in its resistance during the stress-strain test. It is generally known that the resistance of the conductive polymer composite increases with the increase of the applied strain, which is ascribed to the disconnection of the conductive pathways while the polymer composite is stretched [27]. Many previous studies have shown that, when designing a conductive polymer composite, the incorporation of multi-dimensional conductive particles composed of one 0D component and one 1D component could form synergetic and well-connected conductive pathways within the polymer matrix, through the confinement effect and volume exclusion effect, leading to much stronger 3D conductive networks of the prepared polymer composite [28,29]. As schematically shown in Figure 4d, for MAPNM prepared in the present study, the hybrid MWCNTs/AgNPs formed synergistic and interconnected networks on the PLA nanofiber surfaces which provided 3D conductive pathways during the MAPNM was stretched, resulting a linear response of its resistance to the applied strain.
For a strain sensor, the longtime-use stability and conductive reconstruction after the applied mechanical deformation is released are two vital factors to be considered. On this basis, the cyclic mechanical strength and electrical response were investigated. Figure 5 shows 100 stress-strain cycles of the MAPNM with the strain of 3%, and the resistance response of the MAPNM during the 100 cycles. As was observed in Figure 5a, the MAPNM could recover to its initial point quickly with relatively stable hysteresis loop when the applied mechanical stimulus was removed, indicating a promising mechanical stability and durability of the prepared MAPNM [30]. It was also observed from Figure 5b, which recorded the dynamic resistance change during 100 stress-strain cycles that the R/R 0 responded simultaneously from 1.0 to 3.3 with the applied strain. Moreover, the R/R 0 barely changed, even after 100 cycles of stretching-releasing times, which indicated high cyclic repeatability and stability. surfaces which provided 3D conductive pathways during the MAPNM was stretched, resulting a linear response of its resistance to the applied strain. For a strain sensor, the longtime-use stability and conductive reconstruction after the applied mechanical deformation is released are two vital factors to be considered. On this basis, the cyclic mechanical strength and electrical response were investigated. Figure 5 shows 100 stress-strain cycles of the MAPNM with the strain of 3%, and the resistance response of the MAPNM during the 100 cycles. As was observed in Figure 5a, the MAPNM could recover to its initial point quickly with relatively stable hysteresis loop when the applied mechanical stimulus was removed, indicating a promising mechanical stability and durability of the prepared MAPNM [30]. It was also observed from Figure 5b, which recorded the dynamic resistance change during 100 stress-strain cycles that the R/R0 responded simultaneously from 1.0 to 3.3 with the applied strain. Moreover, the R/R0 barely changed, even after 100 cycles of stretching-releasing times, which indicated high cyclic repeatability and stability. To further prove the application potential of the prepared MAPNM as the wearable strain sensor for use in real-life body detection, the MAPNM was adhered to different body parts, including finger joint, inner elbow, knee, and forehead to detect the real-time body motion signals of these parts. Figure 6 shows the resistance change signals as a function of the folding and unfolding of the motions of the above-mentioned body parts for 50 consecutive cycles. As can be seen, for all conducted body motions, the MAPNM could respond rapidly, accurately, and consistently. For example, it could be seen from Figure 6a that, when the finger folded and unfolded repeatedly, the R/R0 of the MAPNM increased simultaneously and periodically with the finger folding, and decreased to its original resistance value when the finger was unfolded. For all 50 folding-unfolding cycles, the R/R0 of the MAPNM was able to keep its value from 1.0 to 1.62. Similar responses of the MAPNM could be also detected when the MAPNM was attached to inner elbow, knee, and forehead, as shown in Figure 6bd. As a result, the prepared MAPNM had promising potentials as the real-time body motion sensing detector. To further prove the application potential of the prepared MAPNM as the wearable strain sensor for use in real-life body detection, the MAPNM was adhered to different body parts, including finger joint, inner elbow, knee, and forehead to detect the real-time body motion signals of these parts. Figure 6 shows the resistance change signals as a function of the folding and unfolding of the motions of the above-mentioned body parts for 50 consecutive cycles. As can be seen, for all conducted body motions, the MAPNM could respond rapidly, accurately, and consistently. For example, it could be seen from Figure 6a that, when the finger folded and unfolded repeatedly, the R/R 0 of the MAPNM increased simultaneously and periodically with the finger folding, and decreased to its original resistance value when the finger was unfolded. For all 50 folding-unfolding cycles, the R/R 0 of the MAPNM was able to keep its value from 1.0 to 1.62. Similar responses of the MAPNM could be also detected when the MAPNM was attached to inner elbow, knee, and forehead, as shown in Figure 6b-d. As a result, the prepared MAPNM had promising potentials as the real-time body motion sensing detector.
The antibacterial properties are also significant to a wearable strain sensor, given the strain sensor is always exposed to daily-life environments, encountered frequently with various bacteria. Thus, the E. coli which represented the Gram-negative bacteria, and S. aureus which represented the Gram-positive bacteria, were employed to investigate the antimicrobial performance of the prepared nanofibrous membranes. Figure 7 shows the growth of inhibition zones of E. coli and S. aureus treated with PLA nanofibrous scaffolds and MAPNM for 18 h at 37 • C. As was shown, the PLA nanofibrous scaffolds did not show any antibacterial effect to both E. coli and S. aureus. Comparatively, MAPNM Polymers 2020, 12, 120 8 of 11 had a very promising antibacterial performance whose inhibition mean zones were 0.48 cm against E. coli, and 0.45 cm against S. aureus, respectively. This was because when MAPNM incubated with the bacteria, the Ag + ions could be released from the MAPNM surface, which were able to kill adjacent bacteria quickly through electrostatic adsorption and protein coagulation [31]. Furthermore, the reason why the inhibition zone of MAPNM against S. aureus was smaller than that of E. coli, as the Gram-positive had a relatively thicker cell wall with peptidoglycan layers, which were more difficult to penetrate by the Ag+ ions, when compared with Gram-nagative E. coli [32]. The antibacterial properties are also significant to a wearable strain sensor, given the strain sensor is always exposed to daily-life environments, encountered frequently with various bacteria. Thus, the E. coli which represented the Gram-negative bacteria, and S. aureus which represented the Gram-positive bacteria, were employed to investigate the antimicrobial performance of the prepared nanofibrous membranes. Figure 7 shows the growth of inhibition zones of E. coli and S. aureus treated with PLA nanofibrous scaffolds and MAPNM for 18 h at 37 °C. As was shown, the PLA nanofibrous scaffolds did not show any antibacterial effect to both E. coli and S. aureus. Comparatively, MAPNM had a very promising antibacterial performance whose inhibition mean zones were 0.48 cm against E. coli, and 0.45 cm against S. aureus, respectively. This was because when MAPNM incubated with the bacteria, the Ag + ions could be released from the MAPNM surface, which were able to kill adjacent bacteria quickly through electrostatic adsorption and protein coagulation [31]. Furthermore, the reason why the inhibition zone of MAPNM against S. aureus was smaller than that of E. coli, as the Gram-positive had a relatively thicker cell wall with peptidoglycan layers, which were more difficult to penetrate by the Ag+ ions, when compared with Gram-nagative E. coli [32]. The bacteria co-culture approach was also introduced to further prove the antibacterial performance of the prepared MAPNM with the results, as shown in Figure 8. It could be clearly seen that after 24 h of cultivation, still a large amount of E. coli and S. aureus were able to survive and grow in the presence of the PLA nanofibrous scaffolds. Meanwhile, the MAPNM could kill almost all the bacteria, indicating, again, a promising antibacterial property of the MAPNM against both Grampositive and Gram-negative bacteria. The bacteria co-culture approach was also introduced to further prove the antibacterial performance of the prepared MAPNM with the results, as shown in Figure 8. It could be clearly seen that after 24 h of cultivation, still a large amount of E. coli and S. aureus were able to survive and grow in the  The bacteria co-culture approach was also introduced to further prove the antibacterial performance of the prepared MAPNM with the results, as shown in Figure 8. It could be clearly seen that after 24 h of cultivation, still a large amount of E. coli and S. aureus were able to survive and grow in the presence of the PLA nanofibrous scaffolds. Meanwhile, the MAPNM could kill almost all the bacteria, indicating, again, a promising antibacterial property of the MAPNM against both Grampositive and Gram-negative bacteria. As a wearable conductive fabric, property stability after several washing cycles also needs to be taken into consideration. Figure 9 shows the washing speed of the MAPNM for 7 washing cycles. It could be seen that after the MAPNM was washed for 7 cycles, the tensile stress of the sample decreased a little, which was because a small number of conductive nanoparticles were removed in the initial washing cycles. After 4 washing cycles, both the R/R0 and tensile stress became relatively stable. This was because most of the conductive nanoparticles adhered to the PLA matrix tightly, with strong interactions through hydrogen bonding and/or ionic bonding, which was in accordance with the FT-IR and XRD results. As a wearable conductive fabric, property stability after several washing cycles also needs to be taken into consideration. Figure 9 shows the washing speed of the MAPNM for 7 washing cycles. It could be seen that after the MAPNM was washed for 7 cycles, the tensile stress of the sample decreased a little, which was because a small number of conductive nanoparticles were removed in the initial washing cycles. After 4 washing cycles, both the R/R 0 and tensile stress became relatively stable. This was because most of the conductive nanoparticles adhered to the PLA matrix tightly, with strong interactions through hydrogen bonding and/or ionic bonding, which was in accordance with the FT-IR and XRD results. The bacteria co-culture approach was also introduced to further prove the antibacterial performance of the prepared MAPNM with the results, as shown in Figure 8. It could be clearly seen that after 24 h of cultivation, still a large amount of E. coli and S. aureus were able to survive and grow in the presence of the PLA nanofibrous scaffolds. Meanwhile, the MAPNM could kill almost all the bacteria, indicating, again, a promising antibacterial property of the MAPNM against both Grampositive and Gram-negative bacteria. As a wearable conductive fabric, property stability after several washing cycles also needs to be taken into consideration. Figure 9 shows the washing speed of the MAPNM for 7 washing cycles. It could be seen that after the MAPNM was washed for 7 cycles, the tensile stress of the sample decreased a little, which was because a small number of conductive nanoparticles were removed in the initial washing cycles. After 4 washing cycles, both the R/R0 and tensile stress became relatively stable. This was because most of the conductive nanoparticles adhered to the PLA matrix tightly, with strong interactions through hydrogen bonding and/or ionic bonding, which was in accordance with the FT-IR and XRD results.

Conclusions
To conclude, we prepared conductive and antibacterial MAPNM, using PLA nanofibrous scaffolds, coated with MWNCTs and AgNPs. The MWCNTs and AgNPs was attached on the PLA nanofiber surface tightly with strong interactions. The incorporation of the MWCNTs and AgNPs obviously enhanced the mechanical properties of the prepared MAPNM. Moreover, since the hybrid conductive nanoparticles integrated by MWCNTs, and AgNPs formed a 3D network with abundant conductive pathways, the MAPNM showed a quick and accurate electrical response to the applied strain during the stress-strain test. In addition, the MAPNM exhibited promising stability and repeatability in its mechanical strength and electrical response after 100 times of consecutive stretching-releasing cycles. The MAPNM could be further utilized to precisely detect the human motions of different body parts like finger, elbow, knee, and forehead. Furthermore, the MAPNM showed promising antibacterial properties to both Gram-negative E. coli and Gram-positive S. aureus. Further, the MAPNM showed good washing speed, which could preserve its properties after several times of washing. The MAPNM prepared in the present study showed great promise as the wearable conductive fabric to monitor the human activities in real life.
Author Contributions: Writing-review and editing, L.G.; investigation, A.G.; methodology, Y.W., L.W.; formal analysis, X.F.; supervision, L.X.; project administration, C.M. All authors have read and agreed to the published version of the manuscript.
Funding: This work was supported by Natural Science Foundation of Jiangsu Province, China (BK20160938), Natural Science Foundation of China (51708297).