Fabrication of electrically conductive and antibacterial biocomposite materials

13 January 2017
Mohammad Taghi Satoungar, Saeid Fattahi, Mohammad Khajeh Mehrizi, and Hamed Azizi
A direct electrospinning process was used to produce polylactic acid/silver nanowire fibers that exhibit a core–shell structure.

In the past, the development of electrically and thermally conductive polymers has been focused mainly on intrinsically conductive polymers such as polyaniline. As an alternative, several different methods—including the use of nanofiller materials in less-conductive polymers—have also been studied.1–7 Indeed, the use of nanofillers in biomaterials has become popular in various technologies. In particular, polylactic acid (PLA) is a good polymer for use in biomedical applications and the packaging industry because it is both biocompatible and biodegradable (whereas polyaniline is harmful in some applications).8 Likewise, 1D metal nanostructures—including silver nanowires (AgNWs)—have gained a large amount of recent attention because of their high electrical and thermal conductivity properties.9–13 To date, however, there has been no investigation into the electrical properties of PLA/AgNW biocomposites.

Although the use of NWs as nanoelectronics and nanosensors has previously been studied, there is still substantial scope for further investigations of AgNWs in biopolymer composites. For example, the antibacterial properties of several different PLA composites have recently been investigated. These included PLA/chitosan composite nanofibers8 and PLA nanofibrous webs that incorporated triclosan/cyclodextrin complexes.14 In addition, it has been found that powerful antibacterial activity can be produced with silver nanoparticle/PLA nanofiber composites when the size of the silver particles was increased from the microscale to the nanoscale.15

In our work we have therefore used a direct electrospinning synthesis method—in which a polymer solution is charged to a high voltage to produce fibers—to fabricate PLA/AgNW biocomposite fibers for the first time and to investigate their physical, electrical, and antimicrobial properties.16 In the first step of our synthesis process we prepared two different solutions. In the first solution, AgNWs were dispersed in chloroform and subjected to supersonic agitation for 30 minutes. We produced the second solution by dissolving 0.4g of PLA in 4.5ml of chloroform and 1.2ml of dimethylformamide and then heating at 50°C for two hours. In the next stage, we added the first solution (containing different concentrations of AgNWs) to the second solution and then stirred the mixed solution for 24 hours at room temperature. We were then able to perform electrospinning on the final homogenized suspension.16

We obtained field emission scanning electron microscope (FESEM) and transmission electron microscope (TEM) images of our synthesized AgNWs to characterize their morphology and physical structure. From the FESEM results—see Figure 1(a)—we find that the average diameter of the AgNWs is in the 65–85nm range and that their average length is about 10mm. In addition, the TEM images—see Figure 1(b)—illustrate the physical uniformity of our AgNW samples. The FESEM images of our composites also show that almost all the AgNWs are located in the central bulk of the nanofibers rather than on their surface: see Figure 2(a). Our TEM images allow us to confirm this ‘core–shell’ distribution of fibers within the polymer matrix: see Figure 2(b). Overall, these imaging results indicate that the AgNWs were successfully incorporated into the PLA matrix during our electrospinning process.


(a) Field emission scanning electron microscope (FESEM) image of silver nanowire (AgNW) particles. (b) Transmission electron microscope (TEM) image of a single AgNW.


(a) FESEM image of the polylactic acid/AgNW biocomposite containing 5wt% AgNWs. This image illustrates that the majority of the AgNWs are located within the central part of the sample's polylactic acid (PLA) matrix, rather than on the surface. This ‘core–shell’ structure of the composite is also seen in a TEM image of the same sample (b).

To quantitatively evaluate the antibacterial capability of our PLA/AgNW composite nanofibers, we tested (using the AATCC 100-1003 method) them against Staphylococcus aureus and E. coli. Our results show that the PLA/AgNW nanofibers offer strong antibacterial activity against both types of bacteria. In fact, we find that in our 3 and 5wt% AgNW samples the bacteria completely lost their culturing potential. We believe this increase in the amount of the high-surface-area AgNWs strengthens the electrostatic attractive forces that disrupt the surface charge of the bacteria, which leads to their deactivation.

In the final part of our study, we measured the electrical resistivity of our PLA/AgNW composites by applying different currents (556, 1.122, 2.266, and 4.532mA). The resistivity of the pure AgNWs and the pure PLA are 22 and 1050Ω (at 556mA), respectively. Our results (see Figure 3) show that the nanofibers became more conductive upon the addition of increasing AgNW concentrations. Indeed, we observe an approximately 50% decrease in electrical surface resistance in the 7wt% AgNW sample compared with the pure PLA and 0.9wt% samples. The high electrical potential of our AgNWs thus makes them suitable for use in many polymer applications where conductivity is required.


Electrical resistivity of the pure PLA and PLA/AgNW composite nanofibers, measured at four different current values.

In summary, we have demonstrated a simple and innovative electrospinning method for the fabrication of nanowire biocomposite fibers. In particular, we have shown that PLA/AgNW nanofibers exhibit useful antibacterial and electrical properties. Such biocomposites may find application in stretchable and wearable electrochromic devices, elastomer nanocomposites, highly conductive flexible paper, flexible touch panels, and as antibacterial textiles for medical purposes. In our future work we hope to develop our biocomposites as smart filters.


Authors

Mohammad Taghi Satoungar
Yazd University

Saeid Fattahi
Yazd University

Mohammad Khajeh Mehrizi
Yazd University

Hamed Azizi
Department of Plastics, Iran Polymer and Petrochemical Institute


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DOI:  10.2417/spepro.006856