Dielectrophoresis Response Research Articles

Combined Dielectric-Optical Characterization of Single Cells Using Dielectrophoresis-Imaging Flow Cytometry

In this paper, we present a microfluidic flow cytometer for simultaneous imaging and dielectric characterization of individual biological cells within a flow. Utilizing a combination of dielectrophoresis (DEP) and high-speed imaging, this system offers a dual-modality approach to analyze both cell morphology and dielectric properties, enhancing the ability to analyze, characterize, and discriminate cells in a heterogeneous population. A high-speed camera is used to capture images of and track multiple cells in real-time as they flow through a microfluidic channel. A wide channel is used, enabling analysis of many cells in parallel. A coplanar electrode array perpendicular to cell flow is incorporated at the bottom of the channel to perform dielectrophoresis-based dielectric characterization. A frequency-dependent voltage applied to the array produces a non-uniform electric field, translating cells to higher or lower velocity depending on their dielectric polarizability. In this paper, we demonstrate how cell size, obtained by optical imaging, and DEP response, obtained by particle tracking, can be used to discriminate viable and non-viable Chinese hamster ovary cells in a heterogeneous cell culture. Multiphysics electrostatic-fluid dynamics simulation is used to develop a relationship between cell incoming velocity, differential velocity, size, and the cell’s polarizability, which can subsequently be used to evaluate its physiological state. Measurement of a mixture of polystyrene microspheres is used to evaluate the accuracy of the cytometer.

Dielectrophoresis microbial characterization and isolation of Staphylococcus aureus based on optimum crossover frequency.

Characterization of antibiotic-resistant bacteria is a significant concern that persists for the rapid classification and analysis of the bacteria. A technology that utilizes the manipulation of antibiotic-resistant bacteria is key to solving the significant threat of these pathogenic bacteria by rapid characterization profile. Dielectrophoresis (DEP) can differentiate between antibiotic-resistant and susceptible bacteria based on their physical structure and polarization properties. In this work, the DEP response of two Gram-positive bacteria, namely, Methicillin-resistant Staphylococcus aureus (MRSA) and Methicillin-susceptible S. aureus (MSSA), was investigated and simulated. The DEP characterization was experimentally observed on the bacteria influenced by oxacillin and vancomycin antibiotics. MSSA control without antibiotics has crossover frequencies ( ) from 6 to 8MHz, whereas MRSA control is from 2 to 3MHz. The changed when bacteria were exposed to the antibiotic. As for MSSA, the decreased to 3.35MHz compared to MSSA control without antibiotics, MRSA, increased to 7MHz when compared to MRSA control. The changes in the DEP response of MSSA and MRSA with and without antibiotics were theoretically proven using MyDEP and COMSOL simulation and experimentally based on the modification to the bacteria cell walls. Thus, the DEP response can be employed as a label-free detectable method to sense and differentiate between resistant and susceptible strains with different antibiotic profiles. The developed method can be implemented on a single platform to analyze and identify bacteria for rapid, scalable, and accurate characterization.

Dielectrophoretic characterization of macrophage phenotypes.

Different macrophage phenotypes play important roles in diverse biological processes and diseases. In this study, we have characterized the dielectrophoretic responses of human monocytes and macrophage phenotypes: nonactivated (M0), pro-inflammatory (M1), and pro-healing (M2a). Dielectrophoretic responses of cells change as a function of frequency of the applied electric field. We measured the crossover frequency at which cells transition from negative to positive dielectrophoresis (DEP) or vice versa using interdigitated electrodes. For these characterization experiments, we also developed a new low-conductivity media formulation that retained 100% of the initial viability for 1h. Human THP1 monocytes showed a distinguishable DEP response from mature macrophages. M1 macrophages also showed a distinct DEP response compared to M0 and M2a macrophages. No clear distinction could be drawn between M0 and M2a. The median values of the crossover frequencies of monocytes, M0, M1, and M2a were 38, 21, 11, and 23kHz, respectively. Membrane capacitances of these cells were calculated consequently, and the values were 0.0111, 0.0128, 0.0244, and 0.0117F/m2 for monocytes, M0, M1, and M2a, respectively. These results show how bioelectric properties are influenced by changes in macrophage phenotype.

Separation of microalgae cells in a microfluidic chip based on AC Dielectrophoresis

AbstractBACKGROUNDMicroalgae is an important natural resource with extensive applications in various fields, such as health products, animal feed, biopharmaceuticals, and clean energy. However, before the research and utilization of microalgae resources, the microalgae cells of interest must be separated from hybrid strains and bacteria. The traditional method of microalgae separation requires manual labor under the microscope that is time‐consuming and labor‐intensive.RESULTSHerein, a novel microalgae cell separation method based on the dielectrophoresis (DEP) technique was proposed. A unique DEP separation chip with a 3D electrode was designed. Chlorella and Closterium were used as the experimental samples to verify the effectiveness of the proposed method because they have similar volumes and very different electrical properties. Within a certain frequency range, Chlorella suffers from negative dielectrophoresis (nDEP) force and Closterium suffers from positive dielectrophoresis (pDEP) force, so their separation can be achieved. The separation efficiency exceeded 90%. In addition, the DEP response of the Chlorella and Closterium were studied, and the DEP spectra of the two microalgae cells were obtained. Moreover, the effects of DEP force on cell viability under different voltages and frequencies were experimentally analyzed.CONCLUSIONThese findings will contribute to DEP technology, to the portable and harmless separation of microalgae, and to research on the dielectric properties of microalgae cells. © 2022 Society of Chemical Industry (SCI).

P-015 Automated, efficient sorting of sperm using dielectrophoresis (DEP), utilizing the electrical properties of the tail as a biomarker of the viability, even for immotile cells

Abstract Study question Developing a simple and automated method and device that sorts semen cell by cell for eventual application to retrieving rare sperm from severely oligozoospermic samples. Summary answer Over 90% of the viable cells in a sample can be sorted out from non-viable sperm cells and other debris using this method. What is known already The varying electrical properties of different cell types can result in unique frequency dependent dielectrophoretic (DEP) behavior when they are exposed to alternating current (AC). They are either repelled or attracted depending on their properties and the field frequency. This has been applied to sorting rare cells such as circulating tumor cells from blood. We have previously shown that the head and tail of human sperm have different electrical properties. As a result, at certain frequencies of an AC electric field the tails of viable cells (even immotile ones) are attracted to electric field gradients while the heads are distanced. Study design, size, duration Semen samples from several patients at Rambam medical center were collected and cryopreserved. Later the thawed semen samples were then tested in our sorting device. At least one hundred live cells were evaluated in each sample. The percentage of live cells that were successfully sorted out of the original mixture of live sperm cells, dead sperm cells and other debris was assessed. Participants/materials, setting, methods After thawing the samples were stained using CFDA and PI for visual live/dead identification (in actual practice no staining is necessary). The device consists of a polydimethylsiloxane (PDMS) microchannel on a glass slide patterned with Indium Tin Oxide (ITO) transparent electrodes. In the device the sperm are transferred to a low conductivity buffer. A curved electrode then sorts them out of the stream of the cell mixture to an alternate exit. Main results and the role of chance We have successfully demonstrated a new biomarker of live sperm (even immotile ones). The tails of live sperm have a positive (attractive) DEP response while dead sperm react negatively and are repelled from field gradients. At the specific frequencies used the head is simultaneously distanced from the electric fields helping to prevent damage to the DNA during the sorting. We also showed that this effect could be used to sort sperm with over 90% efficiency from a sample with dead sperm and other debris. Since each cell passes the electrodes and is automatically sorted it could possibly be used for severely oligozoospermic and cryptozoospermic samples to find sperm and make more sperm available for more discriminatory sperm selection in such samples. Nowadays such samples are searched manually and the minute number of sperm present is often missed leading to a mistaken azoospermic diagnosis and possibly unnecessary surgery. The components are relatively low cost and the chips disposable allowing for wider implementation. Significant progress was also made in preventing sperm from sticking to the substrates using coatings and a vibrational motor. Additionally, a proof of concept parallelization of the electrodes to increase the throughput was also demonstrated. Limitations, reasons for caution The inherent difficulty with microfluidic technologies is low throughput. Further work is needed to effectively parallelize and optimize the system. Effective pre-processing to clean the sample may also help to increase the throughput. Wider implications of the findings This biomarker can potentially be useful in determining an immotile cell’s viability for ICSI. This technology can also be of potential use for testicular tissue samples where there are many other cell types and debris and it is difficult to find rare sperm. Advanced sperm selection can also be incorporated. Trial registration number not applicable

Purification of pluripotent embryonic stem cells using dielectrophoresis and a flow control system.

Pluripotent stem cells (PSCs) such as embryonic stem cells and induced PSCs can differentiate into all somatic cell types such as cardiomyocytes, nerve cells, and chondrocytes. However, PSCs can easily lose their pluripotency if the culture process is disturbed. Therefore, cell sorting methods for purifying PSCs with pluripotency are important for the establishment and expansion of PSCs. In this study, we focused on dielectrophoresis (DEP) to separate cells without fluorescent dyes or magnetic antibodies. The goal of this study was to establish a cell sorting method for the purification of PSCs based on their pluripotency using DEP and a flow control system. The dielectrophoretic properties of mouse embryonic stem cells (mESCs) with and without pluripotency were evaluated in detail, and mESCs exhibited varying frequency dependencies in the DEP response. Based on the variance in DEP properties, mixed cell suspensions of mESCs can be separated according to their pluripotency with an efficacy of approximately 90%.

Protein Dielectrophoresis: A Tale of Two Clausius-Mossottis-Or Something Else?

Standard DEP theory, based on the Clausius–Mossotti (CM) factor derived from solving the boundary-value problem of macroscopic electrostatics, fails to describe the dielectrophoresis (DEP) data obtained for 22 different globular proteins over the past three decades. The calculated DEP force appears far too small to overcome the dispersive forces associated with Brownian motion. An empirical theory, employing the equivalent of a molecular version of the macroscopic CM-factor, predicts a protein’s DEP response from the magnitude of the dielectric β-dispersion produced by its relaxing permanent dipole moment. A new theory, supported by molecular dynamics simulations, replaces the macroscopic boundary-value problem with calculation of the cross-correlation between the protein and water dipoles of its hydration shell. The empirical and formal theory predicts a positive DEP response for protein molecules up to MHz frequencies, a result consistently reported by electrode-based (eDEP) experiments. However, insulator-based (iDEP) experiments have reported negative DEP responses. This could result from crystallization or aggregation of the proteins (for which standard DEP theory predicts negative DEP) or the dominating influences of electrothermal and other electrokinetic (some non-linear) forces now being considered in iDEP theory.

Preliminary dielectrophoresis study: Manipulation of protein albumin and electrical quantification by using cyclic voltammetry technique

Purpose The purpose of this paper is to visualize protein manipulation using dielectrophoresis (DEP) as a substantial perspective on being an effective protein analysis and biosensor method as DEP is able to be used as a means for manipulation, fractionation, pre-concentration and separation. This research aims to quantify DEP using an electrochemical technique known as cyclic voltammetry (CV), as albumin is non-visible without any fluorescent probe or dye. Design/methodology/approach The principles of DEP were generated by an electric field on tapered DEP microelectrodes. The principle of CV was analysed using different concentrations of albumin on a screen-printed carbon electrode. Using preliminary data from both DEP and CV methods as a future prospect for the integration of both techniques to do electrical quantification of DEP forces. Findings The size of the albumin is known to be 0.027 µm. Engineered polystyrene particle of size 0.05 µm was selected to mimic the DEP actuation of albumin. Positive DEP of the sample engineered polystyrene particle was able to be visualized clearly at 10 MHz supplied with 20 Vpp. However, negative DEP was not able to be visualized because of the limitation of the apparatus. However, albumin was not able to be visualized under the fluorescent microscope because of its translucent properties. Thus, a method of electrical quantification known as the CV technique is used. The detection of bovine serum albumin (BSA) using the CV method is successful. As the concentration of BSA increases, the peak current obtained from the voltammogram decreases. The peak current can be an indicator of DEP response as it correlates to the adsorption of the protein onto the electrodes. The importance of the results from both CV and DEP shows that the integration of both techniques is possible. Originality/value The integration of both methods could give rise to a new technique with precision to be implemented into the dialyzers used in renal haemodialysis treatment for manipulation and sensing of protein albumin.

A Two-Stage Separation of Circulating Tumor Cells Based on Deterministic Lateral Displacement and Dielectrophoresis Techniques

Circulating tumor cells (CTCs) are cancer cells that fall off from cancer lesions and enter the blood of human extracorporeal circulation. They are the early basis of judging cancer metastasis. However, CTCs are extremely rare, there is a great challenge to create an effective platform to isolate these cells. Herein, a two-stage CTCs separation method that combines deterministic lateral displacement (DLD) and dielectrophoresis (DEP) technology based on microfluidic chips was proposed. In order to verify the effectiveness of the proposed method, the human non-small cell lung cancer cells (H322) were spiked into diluted human blood as the samples. Firstly, a designed DLD chip was used, and the overwhelming majority of RBCs and the platelets were removed based on size. However, due to the size overlapping, the separation of WBCs and H322 cells cannot be achieved by DLD technology. In the second stage, a novel DEP chip was used to separate the mixed cells (H322 cells, WBCs and few residual RBCs) according to their difference dielectric properties. The experimental results show that after the two-stage separation, H322 cells with high purity can be obtained. The separation efficiency of H322 cells was 91%, and the purity was 80.7%, and maintains its activity. In addition, the DEP response of H322 cells under various frequencies was investigated. Those findings will provide a useful reference to the separation and enrichment of CTCs in peripheral blood.

Dielectrophoresis Response of Water-in-Oil-in-Water Double Emulsion Droplets with Singular or Dual Cores.

Microfluidic technologies have enabled generation of exquisite multiple emulsion droplets, which have been used in many fields, including single-cell assays, micro-sized chemical reactions, and material syntheses. Electrical controlling is an important technique for droplet manipulation in microfluidic systems, but the dielectrophoretic behaviors of multiple emulsion droplets in electrical fields are rarely studied. Here, we report on the dielectrophoresis response of double emulsion droplets in AC electric fields in microfluidic channel. A core-shell model is utilized for analyzing the polarization of droplet interfaces and the overall dielectrophoresis (DEP) force. The water-in-oil-in-water droplets, generated by glass capillary devices, experience negative DEP at low field frequency. At high frequency, however, the polarity of DEP is tunable by adjusting droplet shell thickness or core conductivity. Then, the behavior of droplets with two inner cores is investigated, where the droplets undergo rotation before being repelled or attracted by the strong field area. This work should benefit a wide range of applications that require manipulation of double emulsion droplets by electric fields.

Protein dielectrophoresis: Key dielectric parameters and evolving theory.

Globular proteins exhibit dielectrophoresis (DEP) responses in experiments where the applied field gradient factor ∇E2 appears far too small, according to standard DEP theory, to overcome dispersive forces associated with the thermal energy kT of disorder. To address this a DEP force equation is proposed that replaces a previous empirical relationship between the macroscopic and microscopic forms of the Clausius-Mossotti factor. This equation relates the DEP response of a protein directly to the dielectric increment δε+ and decrement δε- that characterize its β-dispersion at radio frequencies, and also indirectly to its intrinsic dipole moment by way of providing a measure of the protein's effective volume. A parameter Γpw , taken as a measure of cross-correlated dipole interactions between the protein and its water molecules of hydration, is included in this equation. For 9 of the 12 proteins, for which an evaluation can presently be made, Γpw has a value of ≈4600±120. These conclusions follow an analysis of the failure of macroscopic dielectric mixture (effective medium) theories to predict the dielectric properties of solvated proteins. The implication of a polarizability greatly exceeding the intrinsic value for a protein might reflect the formation of relaxor ferroelectric nanodomains in its hydration shell.

Characterization of the Dielectrophoretic Response of Different Candida Strains Using 3D Carbon Microelectrodes.

Bloodstream infection with Candida fungal cells remains one of the most life-threatening complications among hospitalized patients around the world. Although most of the cases are still due to Candida albicans, the rising incidence of infections caused by other Candida strains that may not respond to traditional anti-fungal treatments merits the development of a method for species-specific isolation of Candida. To this end, here we present the characterization of the dielectrophoresis (DEP) response of Candida albicans, Candida tropicalis and Candida parapsilosis. We complement such characterization with a study of the Candida cells morphology. The Candida strains exhibited subtle differences in their morphology and dimensions. All the Candida strains exhibited positive DEP in the range 10–500 kHz, although the strength of the DEP response was different for each Candida strain at different frequencies. Only Candida tropicalis showed positive DEP at 750 kHz. The current results show potential for manipulation and enrichment of a specific Candida strain at specific DEP conditions towards aiding in the rapid identification of Candida strains to enable the effective and timely treatment of Candida infections.

Examination of the dielectrophoretic spectra of MCF7 breast cancer cells and leukocytes.

The detection of circulating tumor cells (CTCs) in blood is crucial to assess metastatic progression and to guide therapy. Dielectrophoresis (DEP) is a powerful cell surface marker-free method that allows intrinsic dielectric properties of suspended cells to be exploited for CTC enrichment/isolation from blood. Design of a successful DEP-based CTC enrichment/isolation system requires that the DEP response of the targeted particles should accurately be known. This paper presents a DEP spectrum method to investigate the DEP spectra of cells without directly analyzing their membrane and cytoplasmic properties in contrast to the methods in literature, which employ theoretical assumptions and complex modeling. Integrating electric field simulations based on DEP theory with the experimental data enables determination of the DEP spectra of leukocyte subpopulations, polymorphonuclear and mononuclear leukocytes, and MCF7 breast cancer cells as a model of CTC due to their metastatic origin over the frequency range 100kHz-50MHz at 10 Vpp . In agreement with earlier findings, differential DEP responses were detected for mononuclear and polymorphonuclear leukocytes due to the richness of the cell surface features and morphologies of the different leukocyte types. The data reveal that the strength of the DEP force exerted on MCF7 cells was particularly high between 850kHz and 20MHz. These results illustrate that the proposed technique has the potential to provide a generic platform to identify DEP responses of different biological particles.

Distinct and independent dielectrophoretic behavior of the head and tail of sperm and its potential for the safe sorting and isolation of rare spermatozoa.

Often, in semen samples with minute amounts of sperm, even the single spermatozoon required to fertilize an oocyte cannot be found in the ejaculate. This is primarily because currently, sperm is generally searched for manually under a microscope. In this study, dielectrophoresis (DEP) was investigated as an alternative automated technique for sorting sperm cells. Using a quadrupolar electrode array it was shown that the head and tail of the sperm had independent and unique crossover frequencies corresponding to the transition of the DEP force from repulsive (negative) to attractive (positive). These surprising results were further analyzed, showing that the head and tail have their own distinct electrical properties. This significant result allows for the sperm's head, which contains the DNA, to be distanced from potentially damaging high electric fields using negative DEP while simultaneously manipulating and sorting the sperm using the positive DEP response of the tail. A proof of concept sorting chip was designed and tested. The low crossover frequency of the tail also allows for the use of a higher conductivity, and thus more physiological, medium than the conventional DEP solutions. Although more research is required to design and optimize an efficient, user-friendly, and high-throughput device, this research is a proof of concept that DEP has the potential to automate and improve the processing of semen samples, especially those containing only rare spermatozoa.

Dielectrophoresis velocities response on tapered electrode profile: simulation and experimental

PurposeThe purpose of this paper is to use a particle velocity measurement technique on a tapered microelectrode device via changes of an applied voltage, which is an enhancement of the electric field density in influencing the dipole moment particles. Polystyrene microbeads (PM) have used to determine the responses of the dielectrophoresis (DEP) voltage based on the particle velocity technique.Design/methodology/approachAnalytical modelling was used to simulate the particles’ polarization and their velocity based on the Clausius–Mossotti Factor (CMF) equation. The electric field intensity and DEP forces were simulated through the COMSOL numerical study of the variation of applied voltages such as 5 V p-p, 7 V p-p and 10 V p-p. Experimentally, the particle velocity on a tapered DEP response was quantified via the particle travelling distance over a time interval through a high-speed camera adapted to a high-precision non-contact depth measuring microscope.FindingsThe result of the particle velocity was found to increase, and the applied voltage has enhanced the particle trajectory on the tapered microelectrode, which confirmed its dependency on the electric field intensity at the top and bottom edges of the electrode. A higher magnitude of particle levitation was recorded with the highest particle velocity of 11.19 ± 4.43 µm/s at 1 MHz on 10 V p-p, compared to the lowest particle velocity with 0.62 ± 0.11 µm/s at 10 kHz on 7 V p-p.Practical implicationsThis research can be applied for high throughout sensitivity and selectivity of particle manipulation in isolating and concentrating biological fluid for biomedical implications.Originality/valueThe comprehensive manipulation method based on the changes of the electrical potential of the tapered electrode was able to quantify the magnitude of the particle trajectory in accordance with the strong electric field density.

(Invited) Dielectrophoresis Studies of Mammalian Cells at High Frequencies (10 ~ 400 MHz)

Most reported studies of the dielectrophoresis (DEP) of biological cells have employed voltage signals of frequency that extend from 10 kHz to around 10 MHz. In this range the DEP response is determined by the dielectric properties of the suspending medium, as well as the size and shape of the cell and the dielectric properties (capacitance, conductivity) of its plasma membrane. An important parameter to determine is the so-called cross-over frequency f x01, at which a transition from negative to positive DEP occurs on increasing the frequency. Theoretical and experimental evaluations [e.g., 1-5] of f xo1 have been extensive and exploited in practical applications of DEP, such as the manipulation, separation, and isolation of target cells from mixtures in suspension. DEP measurements up to 400 MHz have recently been reported [6] for murine myeloma cells. This provided, for the first time, the ability to determine under controlled conditions the value of the cross-over frequency fx02, where the transition from positive back to negative DEP occurs. In agreement with theory, it was found that fx02 is independent of the cell parameters that control fx01, and is predominantly determined by the conductivity of the intracellular medium. The myeloma cells were suspended in buffer media of different osmolarities, with measurements taken of cell volume as well as both fx01 and fx02. Flow cytometry was employed with a potassium-sensitive fluorophore to monitor relative changes in the intracellular potassium concentration. This revealed that the value of fx02 was highly correlated to the concentration of the most dominant cytoplasmic ion, namely potassium. The implications of these new high-frequency results will be discussed, particularly with regards to the potential of DEP being adopted as a biomarker-free method for sorting and characterizing cells based upon their dielectric properties. Lei U., Lo, Y. J., IET Nanobiotechnology 2011, 5, 86-106.Cetin, B., Li, D., Electrophoresis 2011, 32, 2410-2427.Gagnon, Z. R., Electrophoresis 2011, 32, 2466-2487.Vahey, M. D., Pesudo, L. Q., Svenssson, J. P., Samson, L. D., Voldman, J., Lab Chip 2013, 2754-2763.Pethig, R., Dielectrophoresis: Theory, Methodology and Biological Applications, John Wiley & Sons, Chichester 2017.Chung, C., Pethig, R., Smith, S., Waterfall, M., Electrophoresis, doi: 10.1002/201700433

Single cell dielectrophoresis study of apoptosis progression induced by controlled starvation

Nutrient depletion in fed-batch cultures and at the end of batch cultures is among the main causes of stress on cells and a trigger of apoptosis. In this study, we investigated changes in the cytoplasm conductivity of Chinese hamster ovary (CHO) cells under controlled starvation. Employing a single-cell dielectrophoresis (DEP) cytometer, we measured the DEP response of CHO cells incubated in a medium without glucose and glutamine over a 48-h period. Using the measured data in conjunction with numerical simulations, we determined the cytoplasm conductivity of viable and apoptotic cell subpopulations. The results show that a small subpopulation of apoptotic cells emerges after 24 to 36 h of starvation and increases rapidly over a short period of time, <12 h. The apoptotic cells have a dramatically lower cytoplasm conductivity, ∼0.05 S/m, than viable cells, ∼0.45 S/m. Viability of starvation cultures was measured by fluorescent cytometry, DEP cytometry, and trypan blue exclusion assays. DEP, Annexin V, caspase-8, and 7-AAD assays show a similar decline in viability after 36 h of starvation and indicate a very low viability after 48 h. Trypan blue exclusion assay fails to detect early-stage viability decline and estimates a much higher viability after 48 h.

A novel dielectrophoresis potential spectroscopy for colloidal nanoparticles.

Dielectrophoresis (DEP) has been widely used to manipulate nanoparticles in microfluidic applications. However, determination of the DEP force of nanoparticles by theoretical models is not easy due to complications caused by the polarization of electrical double layer. Additionally, there is a lack of suitable experimental techniques to quantify the DEP force of nanoparticles. This article reports a statistical mechanics-based experimental method to determine the DEP potential energy of a single particle by measuring the equilibrium number density of particles in a DEP force field. Results show that at high frequencies, the measured potentials agree with the Maxwell-Wagner-O'Konski (MWO) theory. At frequencies lower than the crossover frequency (ωco ), the measured potential values are larger than MWO theory's predictions. When an effective particle radius (particle radius plus Debye length) is used to replace the particle radius, MWO theory fits better with the measured potentials on both sides of ωco . Also, the measured ωco was found inversely proportional to the effective particle radius, which agrees with MWO theory. The new DEP potential spectroscopy is not limited to the size or shape of particles, opening doors to investigate the DEP response functions of quantum dots and proteins in an alternating current electric field.

A Novel Approach to the Sustainable Synthesis of Carbon Fibers

Presented here are preliminary experiments facilitating the use of the cellulose-extruding bacteria Gluconoacetobacter xylinus as a sustainable tool for the production of carbon fibers. G. xylinus has the natural ability to convert different kinds of sugars from its environment into highly crystalline cellulose nanofibrils. When grown in culture, the random movement of the bacteria leads to a porous cellulosic scaffold with no apparent order. Compared to the cellulose extracted from plants, bacterial cellulose has higher purity and exhibits superior mechanical properties [1]. Results presented last year in this same conference showed how heat treatment of these cellulose fibers that were functionalized with metal precursors yielded a porous tungsten carbide. Here, we focus on the positioning of G. xylinus cells using Dielectrophoresis (DEP) to grow cellulose fibers from specific locations, which we then carbonize by pyrolysis. DEP refers to the movement of electrically polarized cells in response to a non-uniform electric field of specific frequency and magnitude. Specifically, positive DEP will be used here to position the G. xylinus cells on the electrodes while they are continuously perfused with nutrient-rich media, allowing for the continuous extrusion of straight fibers. The ultimate goal is to develop a scalable platform for the continuous production of cellulose nanofibers that can then be converted into carbon. This can become a more sustainable alternative to current practices of cellulose production by eliminating the cost and energy required for forest management and cellulose extraction and purification. Most importantly, it eliminates the use of oil in the production of carbon fibers. In this work, we present initialcharacterization of the response of G. xylinus to electrical stimuli of varying frequencies as well as the optimized protocol to convert bacterial cellulose into carbon. Dielectrophoresis experiments were completed using AC signals of 1, 5, 10, 15, and 20MHz frequencies at a constant voltage of 20Vpp. The results indicate a positive DEP response across the spectrum of 1-20MHz with the highest trapping at a frequency in the range of 15-20 MHz (figure 1 and 2). Additional work focused on the carbonization of bacterial cellulose nanofibers. Amorphous carbon was successfully achieved (figure 3) through heat treatment of bacterial cellulose to 900 ºC in a nitrogen atmosphere following a 5 ºC/min heating ramp. A dwell time of 30 minutes at a temperature of 300 ºC was included in the heat treatment to induce as much oxygen release from the sample as possible to prevent burning the piece when increasing the temperature. Such step is also necessary because the removal of the hydroxyl groups stabilizes the carbon polymer by producing conjugated double bonds, forming an aromatic structure [2]. An optimal heating rate was determined to be 5 ºC/min to achieve high carbon yield without making the process prohibitively long. Although we observed cellulose to carbonize at around 400 ºC, 900 ºC was used as the final temperature to ensure a high degree of carbonization [3]. Ongoing work is on characterizing the fiber properties and dimensions depending on the suspending media and its perfusion rate over the cells. The goal is to understand how the conditions in the microreactor lead to the tailoring of the fiber properties and a maximized throughput. ******INSERT FIGURE 1 HERE******* *****CAPTION****Figure 1: Characterizing the response of G. xylinus to electrical stimuli using Carbon DEP ******INSERT FIGURE 2 HERE******* *****CAPTION****Figure 2: A 2D carbon electrode showing indications of inducing a positive dielectrophoretic force on the G. xylinus bacterium (A.) Electrode is electrically polarized (B.) Same electrode immediately after electric field is switched off ******INSERT FIGURE 3 HERE******* *****CAPTION****Figure 3: Carbonized Cellulose Nanofibrils

A dielectrophoresis-impedance method for protein detection and analysis

Dielectrophoresis (DEP) has increasingly been used for the assessment of the electrical properties of molecular scale objects including proteins, DNA, nanotubes and nanowires. However, whilst techniques have been developed for the electrical characterisation of frequency-dependent DEP response, biomolecular study is usually limited to observation using fluorescent markers, limiting its applicability as a characterisation tool. In this paper we present a label-free, impedance-based method of characterisation applied to the determination of the electrical properties of colloidal protein molecules, specifically Bovine Serum Albumin (BSA). By monitoring the impedance between electrodes as proteins collect, it is shown to be possible to observe multi-dispersion behaviour. A DEP dispersion exhibited at 400 kHz is attributable to the orientational dispersion of the molecule, whilst a second, higher-frequency dispersion is attributed to a Maxwell-Wagner type dispersion; changes in behaviour with medium conductivity suggest that this is strongly influenced by the electrical double layer surrounding the molecule.