A Novel Approach for the Sustainable Synthesis of Carbon Fibers Using Light Induced Dielectrophoresis of Bacteria
Authors: Keck Devin Monroe , Duncan Josie , Islam Monsur , Martinez-Duarte Rodrigo 
Published: February 2020

Presented here are preliminary results on the use of the cellulose-extruding bacteria Gluconoacetobacter xylinus as a sustainable tool for manufacturing carbon fibers. G. xylinus has the natural ability to extrude highly crystalline cellulose nanofibers with higher purity and superior mechanical properties compared to cellulose extracted from plants [1]. Traditionally, the movement of the bacteria within a cellulose culture is random and leads to a porous cellulose scaffold with no apparent order. Light induced dielectrophoresis (LiDEP), or the movement of electrically polarized cells in response to a non-uniform electric field created by incident light, will permit controllable cell movement.
LiDEP will be utilized to selectively position G. xylinus at specified locations. A microfluidic chamber will be used to perfuse the trapped G. xylinus in a continuous flow of nutrient-rich media, allowing for the continuous extrusion of straight cellulose fibers (figure 1). These fibers are carbonized using an optimized pyrolysis protocol at 900 oC in a nitrogen atmosphere. This technology is important because it utilizes G. xylinus as the enabling tool for fiber extrusion and thus eliminates the need for expensive machinery and its’ associated processes [2-3]. Air, water, and soil pollution created by oil refineries and diminishing supplies of petroleum has sparked a push towards the development of sustainable approaches for manufacturing carbon fibers. Our end goal is to develop such an approach.
To the authors best knowledge, no technique to pattern a single bacterial cellulose fiber exists. However, wet spinning and drawing processes have been used to align and assemble bacterial cellulose bundles into high performance macro fibers [3]. Cellulose has also been cultivated on an agarose film with honeycomb microgrooves to form a cellulose bundle with honeycombed architecture [4].
In this work, we present the initial characterization of the response of G. xylinus to electrical stimuli of varying frequencies as well as the heat treatment protocol developed for carbonizing bacterial cellulose. The dielectrophoretic response was characterized using AC signals from 1-20MHz frequencies at a constant voltage of 20 . The results indicate a positive DEP response across the spectrum of 7.5-20 MHz with the largest DEP force at a frequency of 20 MHz (figure 2). Frequencies above 20 MHz were not investigated because the voltage generator used in this experiment is limited to generating frequencies below 20 MHz.
Additional work focused on the carbonization of bacterial cellulose nanofibers. Amorphous carbon was successfully obtained through heat treatment of bacterial cellulose in a nitrogen atmosphere to 900⁰C with 5 ⁰C/min heating ramp. A dwell time of 30 minutes at 300 ⁰C was included to enable the release of oxygen from the sample and prevent burning at higher temperatures. Such a step is also pertinent because the removal of the hydroxyl groups stabilizes the carbon polymer by producing conjugated double bonds, thus forming an aromatic structure [5]. An optimal heating rate was determined to be 5 ⁰C/min to achieve high carbon yield without making the process prohibitively long. Although we observed cellulose to carbonize at around 400 ⁰C, 900 ⁰C was used to ensure a high degree of carbonization [6].
Ongoing work is on developing the infrastructure to reliably position individual G. xylinus bacteria, as well as characterizing the fiber properties and dimensions depending on the suspending media and its flow rate over the cells. The goal is to understand how the conditions in the microfluidic chamber lead to the tailoring of the fiber properties and a maximized throughput.
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Figure 1