Electrokinetics in a Uniform Electric Field

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Chapter 2. Background & Significance

Directed Motion of Bacteria and Calcium Ions to Create a Meniscus

What is Bacterial Cellulose (BC)?

In the Philippines, a gel-like dessert exists, Nata de Coco, which is produced in coconut water from bacteria cultures. The gel-like substance is referred to as bacterial cellulose and can be generated by the species Acetobacter xylinum. The production of cellulose begins with the movement of millions of bacteria cells in an acidic liquid at the air interface. The oxygen present at the air-liquid interface allows for continuous cellulose production. Cellulose nanofibrils form from cell metabolic secretions of glucose chains that entangle with other cellulose nanofibrils, resulting in a complex network of fibers that forms a blanket of hydrophilic material [8, 9]. The hydrophilic pellicle has been found to have great mechanical properties compared to other polymers [10] and proven to be biocompatible [11]. BC can easily be molded into many shapes if unagitated, therefore, has potential in medical applications including skin substitutes [12],artificial blood vessels [13], bone[12, 14], and other tissue engineering projects. Some include manipulating the bacterial cellulose porosity with wax particles to induce cell proliferation [15].

Alignment of Fibers Using Electric Fields

In the tissue engineering aspect, top-down fabrication of materials are limited on their feature size and therefore clinical translations of the products are limited due to their inability to reproduce on the nano or micro level. Complications with feature size arise when cells cannot infiltrate scaffolds leading to cell proliferation failure. Bacterial cellulose is a potential scaffolding material but is limited in its application due to their porous structure. The cellulose naturally forms a tight, random network of fibers creating small pores that are not large enough for cells to infiltrate. Furthermore, top-down fabrication techniques on bacterial cellulose such as cutting it into a desired shape compromises the mechanical integrity. Therefore, any bottomup approach to fabricating larger pores and/or controlling fiber orientation would increase the applications for bacterial cellulose in the medical field.Sano et al. have been able to approach manufacturing issues by creating a bottom-up fabrication method that controls the motion of the cellulose-producing bacteria. The Acetobacter xylinum are one or two microns in size and therefore can feel the effects of electrokinetics [16]. Their size assists with electrokinetics having a stronger effect than Brownian motion and negligible effects from gravity [17]. Furthermore, since Acetobacter xylinum thrive in acidic solutions, their net charge is negative [18]. The phenomena of electrophoresis, or motion of a charged particle relative to a fluid under a uniform field, can play a dominant role having precise control over bacteria cells and has been utilized to align bacterial cellulose fiber orientation. Using low DC fields in the range of 0.25 V/cm to 1 V/cm, bacterial cells continued to secrete cellulose nanofibrils and result in fiber alignment with their electrophoretic mobility estimated to be 4.68×10-9 m 2 Vs -1. The ability to direct movement of bacteria potentially offers controlled fiber alignment and pore size while maintaining mechanical integrity.


Deposition of Ions onto Bacterial Cellulose Fibers

The bacterial cellulose fiber network presents a good template to deposit or mineralize nanoparticles for several biomedical applications. These include creating antimicrobial wound dressings which are impregnated with silver nanoparticles [19] and bone regeneration by creating a composite of cellulose and hydroxyapatite [12, 14]. Methods described for creating these composites involve coating after the bacterial cellulose pellicle is formed using immersion techniques. For bone mimics, the cellulose is first immersed in calcium solution then immersed in a phosphorus solution to create a hydroxyapatite structure. Concerns arise when the porosity of the cellulose limits the infiltration of the fluid into the pellicle resulting in solely surface deposition.Previous experiments in the Bioelectromechanical systems laboratory has shown to simplify deposition processes. Using similar experimental protocols for aligning fibers, the deposition of materials onto cellulose fibers using applied DC fields was also discovered. Altering electrode materials and media constituents allowed deposition of free ions on to the cellulose fibers [20]. Deposition on individual cellulose fibers was seen using FESEM analysis and EDS verified deposition of the following electrode materials: copper, graphite, aluminum, and platinum. Phosphorus, a component not needed in the liquid medium, was added to the system before the application of an electric field and FESEM verified the deposition of phosphorus. The electric fields used during these experiments ranged from 0.15 V/cm to 1 V/cm. With this technique, one can grow a BC pellicle while depositing desired materials onto individual fibers, thereby, simplifying the fabrication process.

Table of Contents
List of Figures
List of Tables 
Chapter 1. Introduction 
Chapter 2. Background & Significance 
2.1 Directed Motion of Bacteria and Calcium Ions to Create a Meniscus
2.2 Separation of Breast Cancer Cells for Personalized Medicine
Chapter 3. Electrokinetic Theory
3.1 Electrokinetics in a Uniform Electric Field
3.2 Electrokinetics in a Non-uniform Electric Field 
3.3 Clausius-Mossotti (CM) Factor
Chapter 4. Methods and Materials
4. 1 Microdevice Fabrication 
4. 2 Cell Culture Protocol
Chapter 5. Bacterial Cellulose Studies
5.1 Experimental Procedures
5.2 Results and Discussion
5.3 Future Work
Chapter 6. Separation of Breast Cancer by Stage 
6.1 Experimental Procedures
6.2 Results and Discussion
6.3 Future Work 
Chapter 7. Conclusions
Appendix A: MATLAB Code to Analyze Results from High Frequency cDEP Experiments
Appendix B: MATLAB Code to Analyze Results from Low Frequency cDEP Experiments
Appendix C: MATLAB Code for Plotting the Clausius-Mossotti Curve

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